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Christine112
2002-Feb-15, 07:07 PM
I can understand why this works. For the person traveling in the spaceship near the speed of light, time has slowed down so they age less than the one one Earth. But, to the person on the spaceship, time is running normal and the people on Earth appear to be moving more slowly. So wouldn't they expect to come back to Earth to find a younger twin?

Wiley
2002-Feb-15, 07:42 PM
Our own Great and Angry Grape has written about this. Try the short version (http://mentock.home.mindspring.com/twins.htm) or the long version (http://mentock.home.mindspring.com/twinrdux.htm). Also there are helpful links to FAQ's at the bottom of those pages.

dx
2002-Feb-15, 07:53 PM
The Twin "paradox" always seems to cause confusion. I've found that one thing that helps is to not think of people in the spaceship "seeing things on earth". This thinking of one being able to check simultaneously what is happening in the spaceship and what is happening on earth is completely against SR. It is easier to think of the total journey of one observer then compare it to the other observer upon return.

SeanF
2002-Feb-15, 08:16 PM
On 2002-02-15 14:07, Christine112 wrote:
I can understand why this works. For the person traveling in the spaceship near the speed of light, time has slowed down so they age less than the one one Earth. But, to the person on the spaceship, time is running normal and the people on Earth appear to be moving more slowly. So wouldn't they expect to come back to Earth to find a younger twin?

Christine,

While the rocket is moving away from the Earth, both observers say the other's clock is running slow. While the rocket is moving back towards the Earth, both observers say the other's clock is running slow.

However, the rocket stops and changes direction, while the Earth does not. At that time, the two observers do not need to have reciprocity on what they see the other's clock as doing.

Think about it with two rockets instead of a rocket and a planet (lets call them Rocket A and Rocket B). Now, let's envision three distinct scenarios:

1) Rocket A moves away from Rocket B, then stops and moves back.

2) Rocket B moves away from Rocket A, then stops and moves back.

3) Rocket A moves away from Rocket B, then stops. Rocket B moves towards Rocket A.

It would be possible to describe all three of these situations as "Rocket A and B moved apart, then moved back together", but they are different because there are differences in which rocket(s) changed inertial frames and when. Do you see?

For what it's worth, in scenario 1, Observer A is younger at the end. In scenario 2, Observer B is younger at the end. In scenario 3, they are the same age at the end.

Kaptain K
2002-Feb-15, 08:35 PM
As SeanF said, it is a matter of who changed inertial frames. In the original scenario, the rocket accelerates away from Earth, decelerates to a stop, accelerates back to Earth, and decelerates to a stop. Note that the rocket is the one that changed inertial frame every time. The Earth continued merrily on its way. So the twin on the rocket comes back younger than the twin who stayed on Earth.

_________________
TANSTAAFL!

<font size=-1>[ This Message was edited by: Kaptain K on 2002-02-15 15:37 ]</font>

GrapesOfWrath
2002-Feb-15, 08:35 PM
Thanks for the nod, Wiley. Here is my GR version of the twin paradox (http://mentock.home.mindspring.com/twin2.htm), also.

On 2002-02-15 14:53, dx wrote:
This thinking of one being able to check simultaneously what is happening in the spaceship and what is happening on earth is completely against SR.
::Jake on::
Why would you say that? One of the principles of SR, as delineated in the original paper (http://www.fourmilab.ch/etexts/einstein/specrel/www/), was that you could synchronize clocks across the entire inertial reference frame.
::Jake off::

Christine112
2002-Feb-16, 02:54 AM
On 2002-02-15 15:35, Kaptain K wrote:
As SeanF said, it is a matter of who changed inertial frames. In the original scenario, the rocket accelerates away from Earth, decelerates to a stop, accelerates back to Earth, and decelerates to a stop. Note that the rocket is the one that changed inertial frame every time. The Earth continued merrily on its way. So the twin on the rocket comes back younger than the twin who stayed on Earth.

_________________
TANSTAAFL!

<font size=-1>[ This Message was edited by: Kaptain K on 2002-02-15 15:37 ]</font>

But why does the short period of time that the rocket decelerates and stops make such a difference in their aging? For most of the trip, they are moving (or seeing the Earth/ Rocket B moving). And what if the rocket does not stop, but say orbits around a planet and continues back home, without changing their velocity?

Peter B
2002-Feb-16, 03:11 AM
Christine

You ask why a short period of decelleration would make such a difference. The answer is that the decelleration wouldn't be for a short period of time, but for half the journey. With these sorts of questions, the effect doesn't arise to any noticeable extent when you're travelling at the speed an Apollo spacecraft travels at (which was a few kilometres a second), but at relativistic speeds (tens of thousands of kilometres a second) it does arise.

Therefore, with regard to your second question, decellerating merely to orbital speed means decellerating to such a slow speed compared to what you were travelling before that you might as well be stationary compared to the Earth.

GrapesOfWrath
2002-Feb-16, 11:16 AM
On 2002-02-15 22:11, Peter B wrote:
You ask why a short period of decelleration would make such a difference. The answer is that the decelleration wouldn't be for a short period of time, but for half the journey.
I disagree, I think. The GR variation of the twin paradox link I gave earlier is my attempt to deal with that aspect of it, although there are other approaches that you can find at the sci.physics faq links.

2002-Feb-17, 02:23 AM
On 2002-02-15 14:07, Christine112 wrote:
I can understand why this works. So wouldn't they expect to come back to Earth to find a younger twin?

The one in the spaceship is not in an inertial frame during the turn around. Therefore, on the way back, he finds that the universe that was far away from him during the turn around "speeded up."
According to Einstein's 1905 paper where he defined SR (more or less) he defines a "stationary frame is one where both Newtonian mechanics and Maxwell's equations are true." An inertial frame was one which "moved at a constant velocity relative to the stationary frame." From this, one deduces that an inertial frame is any frame where both Newtonian mechanics and Maxwell's equations hold true. Not all frames are inertial frames.

Well, during the turn around, Newtonian mechanics does not strictly hold true for the guy in the turn around spaceship. He feels a gravity-like force, yes. This force is a type of "action." However, there is no force corresponding to a "reaction" to this force. Therefore, Newton's Third Law does not hold for the gravity-like force.

The guy who stayed behind, who never is in a rocket ship, always has all the Laws of Newtonian mechanics (and Maxwell's equations) working for him. Therefore, for him, the time scale of distant objects do speed up. However, the speed up in time increases with distance in the direction of the acceleration. Therefore, he does not see a speed up in time in his nearby suroundings. Furthermore, he has to wait until light from distant objects gets to him before he can deduce that part of the rest of the universe speeded up.

The constancy of the speed of light comes from Maxwell's equations that apply in every inertial frame. The speed of light is a consequence of a solution to Maxwell's equations. Maxwell's equations apply to electric fields, magnetic fields, and optics. However, without SR, Maxwell's equations and Newtonian mechanics contradict each other.

In 1922, Einstein changed his definition slightly to, "An inertial frame is one where the laws of physics are simplest." This allows SR to apply to nonNewtonian subjects (quantum mechanics, thermodynamics, etc.) Of course, this implies that an observer who is accelerating (i.e., in a noninertial frame) has laws that are more complicated than those in inertial frames.

The thing that is confusing you is Einstein's law of reciprocity: "The laws of physics are the same in all inertial frames." Note that this is not a corrollory of the definition. In principal, having two frames with different but equally simple laws would be possible but for the law of reciprocity. You think that the guy in the rocket and the guy who is stationary should have the same simple law, that he sees time in a moving space ship slow down. However, during the turn around, the guy in the spaceship is NOT in an inertial frame. He will soon deduce that the universe in the direction of his acceleration speed up, and that the universe in the direction opposite his acceleration slowed down. The effect increases with distance.

SeanF
2002-Feb-18, 01:51 PM
Really, it's not the acceleration (or deceleration) of the spacecraft that is the issue, it's simply the fact that it is now moving in a different direction.

Let's say you're receiving broadcast clock signals from a distant planet.

If the planet is motionless relative to you and is right now 100 light years away, you are receiving a signal that left the distant planet 100 years ago.

If the planet is moving away from you at .6c and is right now 100 light years away, you are receiving a signal that left the distant planet only 62.5 years ago.

If the planet is moving toward you at .6c and is right now 100 light years away, you are receiving a signal that left the distant planet a full 250 years ago.

So, what if we have two spacecraft, one travelling towards Earth and one travelling away from Earth, that pass each other 100 light years from Earth? Let's say that when they pass each other, both of their clocks read "1200 hours on February 18, 2100." They would both receive the exact same clock signal (let's say "1200 hours on February 18, 2002") from Earth at that moment. However, one would say that signal left Earth 250 years ago, and the other would say that signal left Earth only 62.5 years ago!

Therefore, one says that Earth's clock read Feb 2002 when his own clock read Aug 2037 (62.5 years ago), and the other says that Earth's clock read Feb 2002 when his own clock read Feb 1850 (250 years ago). Both spaceship clocks see Earth's clock as running at the same slower rate relative to their own (at .6c, the dilation would be 80%), so since they disagree on how long ago it was 2002 on Earth, they disagree on what year it is on Earth right now. One says 2052, the other says 2200.

If we have an observer who is riding the out-bound rocket, and he "leaps" onto the in-bound rocket at the moment they pass, what changes for him? His "current time" stays at 1200 on Feb 18, 2100. His rate of aging (relative to Earth) stays the same. The clock signal he's receiving from Earth stays at "1200 on Feb 18, 2002." His measurement of Earth's rate of aging remains at 80% of his own. However, his measurement of Earth's current time immediately jumps from 2052 to 2200!

He would have left Earth when his clock said 1933 and Earth's clock said 1919. He will arrive at Earth on the new ship when his clock says 2266 and Earth's clock says 2334. He says it took 166 years out and 166 years back. He says Earth's clock counted off 133 years during the trip out and 133 years during the trip back. But that 148-year jump in "Earth time" when he changed directions is what makes Earth older.

This is not a result of deceleration and reacceleration. It is only a result of the synchronicity difference in SR which is direction dependent, unlike the time dilation and spatial contraction which are direction independent.

I'm probably just making this even more confusing than it already was . . . but I think I'll go ahead and submit this post anyways! /phpBB/images/smiles/icon_smile.gif

GrapesOfWrath
2002-Feb-18, 02:03 PM
On 2002-02-18 08:51, SeanF wrote:
Really, it's not the acceleration (or deceleration) of the spacecraft that is the issue, it's simply the fact that it is now moving in a different direction.
But SeanF, that is acceleration.

At least, that is the way that we define acceleration in this context (modern physics). Constant speed but a change in direction is an acceleration.

SeanF
2002-Feb-18, 02:15 PM
On 2002-02-18 09:03, GrapesOfWrath wrote:

On 2002-02-18 08:51, SeanF wrote:
Really, it's not the acceleration (or deceleration) of the spacecraft that is the issue, it's simply the fact that it is now moving in a different direction.
But SeanF, that is acceleration.

At least, that is the way that we define acceleration in this context (modern physics). Constant speed but a change in direction is an acceleration.

All right, technicality point granted.

However, if you have two spaceships that are moving at the same constant speed but in different directions relative to Earth, and those two spaceships measure the current time on Earth differently, would you argue that that difference is a result of acceleration?

GrapesOfWrath
2002-Feb-18, 02:49 PM
On 2002-02-18 09:15, SeanF wrote:
However, if you have two spaceships that are moving at the same constant speed but in different directions relative to Earth, and those two spaceships measure the current time on Earth differently, would you argue that that difference is a result of acceleration?
I would need a few more specifics of the setup here. How are they moving differently? And when?

If two spaceships start out in opposite directions at the same speed, would they necessarily see Earth anyway differently at all?

SeanF
2002-Feb-18, 03:09 PM
On 2002-02-18 09:49, GrapesOfWrath wrote:

On 2002-02-18 09:15, SeanF wrote:
However, if you have two spaceships that are moving at the same constant speed but in different directions relative to Earth, and those two spaceships measure the current time on Earth differently, would you argue that that difference is a result of acceleration?
I would need a few more specifics of the setup here. How are they moving differently? And when?

If two spaceships start out in opposite directions at the same speed, would they necessarily see Earth anyway differently at all?

You're asking if two spaceships start out at Earth and move off in opposite directions? In that case, there would be no way to define any simultaneity between the two spacecraft, so trying to discuss whether they see Earth the same is kind of meaningless.

However, see the situation I described in my post above. At the moment the two spacecraft pass each other 100 light-years from Earth (at which time they can have an agreement with each other on simultaneity), they measure different times for the current time on Earth, and that difference is necessary for the infamous "twin paradox" to work out.

It comes down, basically, to the simultaneity issue. Observers on the two spacecraft would disagree as to what "event" (or "time") on Earth was simultaneous with the "event" of the two spacecraft passing each other. Actually, if we throw in an observer on Earth, we can have three distinct Earth-bound "events" that are seen to be simultaneous with the ships' passing, one by each of the three different observers.

So, is that difference in simultaneity a result of acceleration?

(As an aside - Grapes, I absolutely love having SR discussions with you. You have an uncanny capability of asking just exactly the right questions to make me clarify my thoughts in my own mind and on screen!)

2002-Feb-18, 03:11 PM
<a name="20020218.8:34"> page 20020218.8:34 aka Graphic thoughts / HUb'
.1 Vertical scan rate {60} arb units
...2 Horizantal . RATE .of. SCAN TBA
...... anyway I was trying to delete some
POSTs today in the Apogee to Perigee thread.
Not that this matters .. and also trying to
consieve of a TV picture beinng sent to a
TV set on board an orbiting space station
about EARTH aproaching Texas & leaving Dallas2
I guess its about the same story as GPU or is it GPI maybe GiP?

GrapesOfWrath
2002-Feb-18, 07:28 PM
SeanF

That's exactly why I'm here. (No, not for your clarification, for mine.)

On 2002-02-18 10:09, SeanF wrote:
However, see the situation I described in my post above. At the moment the two spacecraft pass each other 100 light-years from Earth (at which time they can have an agreement with each other on simultaneity), they measure different times for the current time on Earth, and that difference is necessary for the infamous "twin paradox" to work out.
Check out my webpage (http://mentock.home.mindspring.com/twins.htm) that Wiley referenced. The bottom paragraph talks about "Carl" who passes Bob and returns to Earth. For all intents, I believe that satisfies your scenario (except I have their meeting occur 6 lightyears from Earth).

Then, how did the one going towards Earth (Carl) originally establish "current time on Earth?" Don't you have your simultaneity problem there? I assume the one going away (Bob) established it when he left Earth?

So, is that difference in simultaneity a result of acceleration?

SeanF
2002-Feb-18, 08:12 PM
Yes, that's essentially the same thought experiment on your page . . .

Einstein's original 1905 paper established how one determines synchronicity with distant clocks (I know you know about it, GoW! /phpBB/images/smiles/icon_wink.gif ), and his method of determining that synchronicity is not limited to clocks which are stationary relative to one another. Both outgoing and incoming observers can use the signals received from Earth to determine how far away Earth is and how fast (and in what direction) it is moving, as well as what "time" it is on Earth. To be more accurate, the observers can only determine what Earth's status was at the time the signal was sent -- However, continuous monitoring will allow both to determine "after the fact" what "Earth time" was at the moment they passed each other. While the outgoing observer could have established "Earth time" at the moment of passing, he would still need to continue to monitor the signal to maintain that knowledge.

Now, I would like to take this opportunity to re-phrase my previous objection to "acceleration." I probably should have been objecting to the terms "non-inertial frame" or "not in an inertial frame."

Consider the thought experiment wherein we have the Earth and the two spaceships already travelling in opposite directions. Our single observer starts out on Earth and "leaps" onto the first spaceship as it passes. He then "leaps" from the first spaceship to the second as they pass each other, and then again from the second spaceship to Earth (before we get any futher, let me acknowledge that it is physically impossible in the real world to have this kind of instantaneous change in inertial frame - but this is just a thought experiment, so please bear with me).

The practical upshot of this is that the observer spends exactly zero time "in a non-inertial frame" or "not in an inertial frame." None. Never. This is similar to the situation Einstein was describing in his 1905 paper with the observer in the "closed loop of constant velocity" that brought him back to his starting point - the observer was constantly changing from one inertial frame to another, but was never "not in an inertial frame" (or so I believe Einstein intended).

Now, since neither our traveling observer nor the one left on Earth ever spent any time in non-inertial frames, why does the traveling observer wind up younger? Because the Earth-bound observer spent the entire duration of the experiment in the same inertial frame, whereas the traveling observer did not. Although the traveling observer "changed inertial frames," he never spent any time in a "non-inertial" or "accelerating" frame, which is why I asserted that "acceleration" is not responsible for the time dilation.

I also once described a thought experiment where two rockets experienced identical accelerations, decelerations, and re-accelerations, with one merely spending more time cruising in the final inertial frame before decelerating and turning around. Although the two spacecraft's experience with acceleration and non-inertial frames would be identical (albeit not simultaneous nor colocated), the traveler who gets more "cruising time" would experience a greater time dilation.

While General Relativity predicts consequences of "acceleration" or "non-inertial frames," Special Relativity does not and SR's "twin paradox" is not dependent on them.

Roy Batty
2002-Feb-18, 08:53 PM
On 2002-02-18 15:12, SeanF wrote:
(before we get any futher, let me acknowledge that it is physically impossible in the real world to have this kind of instantaneous change in inertial frame - but this is just a thought experiment, so please bear with me).

I admit i'm lost, but doesnt this have some bearing on the matter ie thought experiment or no we are talking about physical laws & wouldnt such an instantaneous jump of inertial frames still involve an acceleration of sorts?
Please feel free to ignore this, I probably dont know what i'm talking about but finding it interesting! /phpBB/images/smiles/icon_smile.gif

SeanF
2002-Feb-18, 09:21 PM
On 2002-02-18 15:53, Roy Batty wrote:

On 2002-02-18 15:12, SeanF wrote:
(before we get any futher, let me acknowledge that it is physically impossible in the real world to have this kind of instantaneous change in inertial frame - but this is just a thought experiment, so please bear with me).

I admit i'm lost, but doesnt this have some bearing on the matter ie thought experiment or no we are talking about physical laws & wouldnt such an instantaneous jump of inertial frames still involve an acceleration of sorts?
Please feel free to ignore this, I probably dont know what i'm talking about but finding it interesting! /phpBB/images/smiles/icon_smile.gif

Roy,

It does have bearing on the matter - in actual practice, the acceleration involved in actually doing an experiment like this would have an effect on the observer. However, SR predicts that the simple relative motion as described would create time dilation "above and beyond" any effects caused by the acceleration, which is why I tried to describe a situation where we can ignore the acceleration-induced effects and focus on the others.

Another way to think about is to just have separate observers on Earth and the two ships. Both ship observers say the Earth clock is running more slowly than their own. The observer on the outgoing ship says x time passed between his ship passing Earth and the two ships passing each other; the observer on the incoming ship says x time passed between the two ships passing each other and his ship passing Earth, but the observer on Earth says more than 2x time passed between the first ship passing Earth and the second.

Same apparent paradox, no acceleration involved . . .

_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2002-02-18 16:23 ]</font>

GrapesOfWrath
2002-Feb-19, 11:38 AM
On 2002-02-18 15:12, SeanF wrote:
While General Relativity predicts consequences of "acceleration" or "non-inertial frames," Special Relativity does not and SR's "twin paradox" is not dependent on them.
Well....

As near as I can tell, that jumping from one inertial frame to another (see Twins Redux link (http://mentock.home.mindspring.com/twinrdux.htm)) avoids the acceleration of physical objects (why have the observers change frames--just pass notes somehow to each other as they pass) but clearly it is an acceleration of some sort.

So, in Einstein's 1905 paper, he cobbled together a finite set of such paths, where the inertial paths were linked at endpoints--"avoiding" acceleration, as you say. However, that is a consequence of acceleration, so I'd hesitate to say that SR does not predict such consequences.

In fact, it's just that sort of thing that led Einstein to General Relativity. It is Special Relativity taken to the limit. You can do it mathematically, if you assume that the limit is valid. In his 1905 paper, he said he didn't know why it wouldn't be, but he didn't go farther than that, in the paper.

So, one of the consequences of SR does involve acceleration, and Einstein even made that point in his first paper.

<font size=-1>[ This Message was edited by: GrapesOfWrath on 2002-02-19 06:40 ]</font>

SeanF
2002-Feb-19, 01:05 PM
We may be just arguing semantics and definitions, but I still think I need to disagree. The "acceleration," such as it is, is necessary in order to get the observer back to his original starting point, but I still don't think it contributes to the time dilation.

New thought experiment! /phpBB/images/smiles/icon_smile.gif

You have two stationary clocks separated by six light-minutes' distance and synchronized. A spaceship moving at 0.6c passes first by one clock and then by the other. The spaceship observer says it only takes eight minutes from when he passes one clock to when he passes the second, but the two stationary clocks show a difference of ten minutes.

Now put the two stationary clocks in the same place (or replace them with a single clock) and just send the spaceship out and back on a "constant" 0.6c that lasts eight minutes of spaceship time, and the stationary clock(s) will have ticked off ten minutes.

Now, there's no difference in the results, even though the second experiment involves acceleration and non-inertial frames while the first does not. So how can those results be "due to" acceleration and/or non-inertial frames?

GrapesOfWrath
2002-Feb-19, 02:37 PM
On 2002-02-19 08:05, SeanF wrote:
New thought experiment!
We're going to fry each others brains.

Now, with your new experiment, I need one more piece of information. Since everything appears to be in inertial reference frames, how do you account for the symmetry between them? What do the observers stationed with each set of clocks see, on their clocks and the others clocks?

SeanF
2002-Feb-19, 02:56 PM
On 2002-02-19 09:37, GrapesOfWrath wrote:

On 2002-02-19 08:05, SeanF wrote:
New thought experiment!
We're going to fry each others brains.

Mmmmm . . . did you ever see "Hannibal"?

Now, with your new experiment, I need one more piece of information. Since everything appears to be in inertial reference frames, how do you account for the symmetry between them? What do the observers stationed with each set of clocks see, on their clocks and the others clocks?

Observers at either of the two relatively stationary clocks see the two clocks as being synchronized, per Einstein's definition in his 1905 paper.

In other words, an observer at one clock will be receiving signals from the other that are exactly six minutes behind his own. By bouncing his own signal off the other clock, he can determine that the other clock is motionless with respect to himself and is synchronized with his own. An observer at the other stationary clock would see exactly the same thing.

Both of those observers can use the same method to measure the spaceship clock, although it requires a bit more math on their parts to compensate for Doppler. They would both recognize that the spaceship clock is running slow (by a factor of .8 ) compared to their own clocks.

The spaceship observer, using essentially the same math, would also recognize that the two "stationary" clocks are running slow (by a factor of .8 ) compared to his own clock. However, he would conclude that the two stationary clocks are not synchronized, but that the "second" clock is set three minutes and 36 seconds ahead of the "first" clock. This is due to the simultaneity factor of SR.

[Edited to correct auto-"smilies"]

_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2002-02-19 09:57 ]</font>

GrapesOfWrath
2002-Feb-19, 03:37 PM
On 2002-02-19 09:56, SeanF wrote:
Now, there's no difference in the results, even though the second experiment involves acceleration and non-inertial frames while the first does not. So how can those results be "due to" acceleration and/or non-inertial frames?
Sorry it took so long to answer this question. The key seems to be in that first sentence. You claim that there is no difference in the results, but

On 2002-02-19 09:56, SeanF wrote:
The spaceship observer, using essentially the same math, would also recognize that the two "stationary" clocks are running slow (by a factor of .8 ) compared to his own clock. However, he would conclude that the two stationary clocks are not synchronized, but that the "second" clock is set three minutes and 36 seconds ahead of the "first" clock. This is due to the simultaneity factor of SR.
Notice that in the second experiment of your new thought experiment, the spaceship observer would not be allowed to conclude this--because the first clock is the same clock as the "second" clock.

The only way to break the symmetry (and come up with truly different results) is by acceleration.

SeanF
2002-Feb-19, 05:32 PM
What the spaceship observer sees during the experiment is not the "results" of the experiment, is it?

The experiments are not identical - if they were, there'd be no point. There're two clocks in the first version, so the observer can see both clocks simultaneously. There's only one clock in the second version, so he can't. Obviously, then, they're going to see different things. However, the first observer's observations of the first clock during the first half of the experiment will be identical to the second observer's observations of his clock during the first half of his experiment (while he's moving away from it). Second clock, second half, same observations. It would not be incorrect to say that in the single-clock experiment, the clock is not (or does not remain) synchronized with itself as viewed from the spaceship.

Bottom line is that in both cases, the spaceship spends eight minutes travelling at 0.6c relative to the "stationary frame" and ends up two minutes behind it.

The idea of physically separating the "stationary" observer into two so the spaceship doesn't have to come back is mere convenience. An "inertial frame" is not a point, or a spaceship, or a planet - it's a coordinate frame that extends throughout spacetime and can be thought of as a whole.

I once gave JW an analogy of a mountain. The air is thinner at the top, so a person will have a harder time breathing at the top than at the bottom. The difference in breathing is a result of being at the top rather than being at the bottom; but is it correct to say that the difference in breathing is caused by the climbing or the moving up? I don't think so. Likewise, the difference in that single clock is a result of the spaceship being in a different inertial frame, but it's not correct to say it's caused by the acceleration.

GrapesOfWrath
2002-Feb-19, 06:10 PM
On 2002-02-19 12:32, SeanF wrote:
Likewise, the difference in that single clock is a result of the spaceship being in a different inertial frame, but it's not correct to say it's caused by the acceleration.
But the breaking of symmetry is. The point is that in the first half of your experiment, if you had another observer with your clocks, both observers would see each other grow old and die while they stayed young--or they would infer that from the clocks they were passing. Totally different outcome, as far as their experience.

SeanF
2002-Feb-19, 08:26 PM
I feel like we may be going in circles here . . . I'm not arguing that the turn-around is irrelevent, just that it doesn't contribute to the time dilation. Really, my problem is that I think the wording of some Relativity explanations is misleading, but I don't know that my own wording is any better! I would make a horrible teacher . . .

The classic SR "twin paradox" is a spaceship that accelerates, cruises, decelerates, turns around, accelerates, cruises, decelerates, and stops back at Earth, right? The astronaut ends up younger than his twin left behind on Earth.

If you leave all the accelerating and decelerating the same but increase the time spent "cruising" (both ways, of course), you will end up with an even bigger difference in age. If you reduce the "cruising" time, you end up with less difference in age.

My concern is when I read things that suggest that it's something that happens during acceleration that causes the time dilation.

The fact that only one twin changes direction is, as you point out, what breaks the symmetry postulate of SR, but it's really not what causes the twins to age at different rates.

GrapesOfWrath
2002-Feb-20, 10:09 AM
Yes, but without the turnaround, it happens to both of them.

That's the genesis of what appears to be a paradox--if SR is a symmetric effect, and the dilation depends upon relative velocity, how does an asymmetry develop when they meet back up again? That's why the turnaround is basic to the twins paradox.

SeanF
2002-Feb-20, 12:56 PM
Yes, it's true that the turnaround time is important in the paradox, which I mentioned in my first post in this thread:

However, the rocket stops and changes direction, while the Earth does not. At that time, the two observers do not need to have reciprocity on what they see the other's clock as doing.

But then later posts by others said things like:

You ask why a short period of decelleration would make such a difference. The answer is that the decelleration wouldn't be for a short period of time, but for half the journey.

and:

The one in the spaceship is not in an inertial frame during the turn around.

And it was really those types of statements to which I was intending to "object" (although in re-reading, I have less objection to that second one than I did initially).

GrapesOfWrath
2002-Feb-20, 01:03 PM
But the short period of deceleration does make all the difference! Otherwise, they both see each other withering away and dying while they themselves stay young. Why does one not? Because of the short period of deceleration, apparently.

If you don't include the deceleration, you're not really talking about the twins paradox--it's just SR symmetric time dilation.

SeanF
2002-Feb-20, 01:39 PM
On 2002-02-20 08:03, GrapesOfWrath wrote:
But the short period of deceleration does make all the difference! Otherwise, they both see each other withering away and dying while they themselves stay young. Why does one not? Because of the short period of deceleration, apparently.

If you don't include the deceleration, you're not really talking about the twins paradox--it's just SR symmetric time dilation.

I guess what I'm seeing is that the fact that the spaceship changed direction affects simultaneity, and that's where the break in symmetry comes from, not from acceleration affecting time dilation.

If I say, "You have to include the fact that the spaceship has turned around, but you don't have to include the actual act of turning around," does that make any sense at all?

GrapesOfWrath
2002-Feb-20, 02:32 PM
On 2002-02-20 08:39, SeanF wrote:
I guess what I'm seeing is that the fact that the spaceship changed direction affects simultaneity, and that's where the break in symmetry comes from, not from acceleration affecting time dilation.
Then why the big difference for the "stay-at-home" twin?

I'm pretty sure everyone agrees that they both see each other as time dilated, and that's caused by their relative velocity. The whole issue--or the crux of it--is why the effect apprently disappears for the stay-at-home twin. That's the basis for the "paradox," and the part that needs explaining. Ignoring it by saying that time dilation is not affected by the acceleration (in SR) because you can isolate it, is essentially skipping over the interesting part of the problem.

SeanF
2002-Feb-20, 05:37 PM
On 2002-02-20 09:32, GrapesOfWrath wrote:

On 2002-02-20 08:39, SeanF wrote:
I guess what I'm seeing is that the fact that the spaceship changed direction affects simultaneity, and that's where the break in symmetry comes from, not from acceleration affecting time dilation.
Then why the big difference for the "stay-at-home" twin?

I'm pretty sure everyone agrees that they both see each other as time dilated, and that's caused by their relative velocity. The whole issue--or the crux of it--is why the effect apprently disappears for the stay-at-home twin. That's the basis for the "paradox," and the part that needs explaining. Ignoring it by saying that time dilation is not affected by the acceleration (in SR) because you can isolate it, is essentially skipping over the interesting part of the problem.

The big difference is the simultaneity issue, and I certainly don't feel like I'm "skipping over" interesting stuff here! /phpBB/images/smiles/icon_smile.gif

Any specific event which occurs somewhere in space occurs in the past, present, or future, correct? In SR, you've got to take relative motion into account.

There was an entire sequence of events on Earth that were in the "future" for the spaceship before it changed direction, but in the "past" afterwards. That's why the Earth twin ends up older.

In much the same way as the time dilation effect of SR is separate-yet-connected to the Lorentz contraction, so too is the simultaneity difference separate-yet-connected.

We have three inertial frames in this experiment. Frame A (that the left-behind clock stayed in) and Frames B and C (that the spaceship used).

Frame A's relationships to Frames B and C are the same in terms of time-dilation and Lorentz contraction, but they are different ("opposite", in a manner of speaking) in terms of simultaneity.

And that is what causes the break in symmetry.

_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2002-02-20 12:38 ]</font>

GrapesOfWrath
2002-Feb-20, 06:19 PM
I'll email you.

Roy Batty
2002-Feb-20, 06:46 PM
On 2002-02-20 13:19, GrapesOfWrath wrote:
I'll email you.

Doh.. & just when I thought i might be getting a (v.slight) grip on it! /phpBB/images/smiles/icon_smile.gif
Way i see it at the moment is that SeanF, you think time dilation effects can be measurable during a period when all frames are not undergoing acceleration & independantly of any such previous acceleration?
& GoW you think that the initial acceleration that you both agree must of occurred actually still has an impact on the argument because theres no way of establishing a common base ground without taking it into account?

I'm so far behind on the math/theory of SR at the moment but am I completely wrong about the nature of this discussion?
Interesting stuff.
Anyway if you reach any agreement feel free to let me in on it /phpBB/images/smiles/icon_smile.gif

SeanF
2002-Feb-20, 07:03 PM
Roy, I think you're pretty close.

Grapes and I agree on a lot here. The time dilation of SR is symmetrical - that is, if two observers are moving relative to each other, they will both see the other as aging more slowly (and thus staying younger) than themselves.

If we then envision a scenario wherein the two observers pass by each other twice (by having one change direction or whatever), we find that when they pass each other the second time, one will be younger and one will be older.

So, we have Observer A who sees Observer B aging more slowly and ends up older than Observer B (as he should), but we also have Observer B who sees Observer A aging more slowly but ends up younger than Observer A (which would be a paradox).

All of that, we agree on (at least I hope so!) Where we're disagreeing is on the terminology of answering why Observer B encounters this paradox and Observer A does not.

Is "acceleration" an appropriate answer or not? I say it's not, Grapes seems to think it is . . .

Now I just hope I'm not mis-stating Grapes' position on this whole thing! /phpBB/images/smiles/icon_smile.gif

GrapesOfWrath
2002-Feb-20, 08:16 PM
On 2002-02-20 13:46, Roy Batty wrote:
Doh.. & just when I thought i might be getting a (v.slight) grip on it!

OK, ok, we're BACK by popular demand!!!

Way i see it at the moment is that SeanF, you think time dilation effects can be measurable during a period when all frames are not undergoing acceleration & independantly of any such previous acceleration?

I believe that too though, as far as it goes

& GoW you think that the initial acceleration that you both agree must of occurred actually still has an impact on the argument because theres no way of establishing a common base ground without taking it into account?

Initial acceleration? I wouldn't have used those words myself. Which initial acceleration is that? /phpBB/images/smiles/icon_smile.gif

<font size=-1>Thank you, thank you, we're here till Tuesday

[Fixed quotes]</font>

<font size=-1>[ This Message was edited by: GrapesOfWrath on 2002-02-20 15:18 ]</font>

Wiley
2002-Feb-20, 10:50 PM
On 2002-02-20 08:39, SeanF wrote:
I guess what I'm seeing is that the fact that the spaceship changed direction affects simultaneity, and that's where the break in symmetry comes from, not from acceleration affecting time dilation.

If I say, "You have to include the fact that the spaceship has turned around, but you don't have to include the actual act of turning around," does that make any sense at all?

At the risk of getting beaten up (metaphorically, of course), I shall enter the fray. I'm going to have to agree with Grapes and say that acceleration is the key to unlocking the twin's paradox. All this is provided I understand y'all's reasoning.

Consider the one-way trip. Say you have an observer A stuck on Earth and another observer B stationary with respect to the A. For a given event, like the life of a twin, both observers A and B will measure the event as lasting the same amount of time. Now let's throw spaceman C into the picture.

Spaceman C travels from A to B but not at a constant velocity. C starts from rest at A, accelerates to some really fast speed, and eventually decelerates and comes to rest at B. How long does the event last for spaceman C? Obviously between A and B when C is zipping along, he will measure the event lasting a shorter time than either A or B. But as C slows and approaches B and eventually comes to rest at B, his perspective must come into alignment with B's. So because during acceleration phases, C's perspective is changing and symmetry breaking.

Since the two-way trip is just two one-way trips. And at the end of the two-way trip C's perspective must be the same as A's perspective.

GrapesOfWrath
2002-Feb-21, 09:56 AM
On 2002-02-20 14:03, SeanF wrote:
So, we have Observer A who sees Observer B aging more slowly and ends up older than Observer B (as he should), but we also have Observer B who sees Observer A aging more slowly but ends up younger than Observer A (which would be a paradox).

I had to read that three times before I was sure that all the cases were in order. The first time I was sure that they weren't. :)

All of that, we agree on (at least I hope so!) Where we're disagreeing is on the terminology of answering why Observer B encounters this paradox and Observer A does not.

Is "acceleration" an appropriate answer or not? I say it's not, Grapes seems to think it is . . .

I've gone back and forth on this, but now that you've extracted the essense, I'm convinced that acceleration is the appropriate answer. Would there be the "paradox" without it?

SeanF
2002-Feb-21, 01:45 PM
On 2002-02-21 04:56, GrapesOfWrath wrote:

On 2002-02-20 14:03, SeanF wrote:
So, we have Observer A who sees Observer B aging more slowly and ends up older than Observer B (as he should), but we also have Observer B who sees Observer A aging more slowly but ends up younger than Observer A (which would be a paradox).

I had to read that three times before I was sure that all the cases were in order. The first time I was sure that they weren't. /phpBB/images/smiles/icon_smile.gif

I re-wrote it about three times before I was sure I had the grammar and pronoun relationships clear, without getting too wordy! /phpBB/images/smiles/icon_smile.gif

All of that, we agree on (at least I hope so!) Where we're disagreeing is on the terminology of answering why Observer B encounters this paradox and Observer A does not.

Is "acceleration" an appropriate answer or not? I say it's not, Grapes seems to think it is . . .

I've gone back and forth on this, but now that you've extracted the essense, I'm convinced that acceleration is the appropriate answer. Would there be the "paradox" without it?

Depends on how you look at it. You can't get the twin back to the starting point without acceleration, so you can say there'd be no paradox without it, but that's not the same as saying the acceleration caused the paradox.

Let me ask this. With the two "stationary" clocks and a single spacecraft already moving past them . . . an observer sitting at the first clock says the two clocks are synchronized, an observer on the ship says they're not. Is that "a result of acceleration" or just "a result of relative motion"?

If we have a single observer who starts out at the clock and "leaps" onto the ship as it passes (or who "instantaneously accelerates"), his perception of the clocks changes from synchronized to non-synchronized. Is that "a result of acceleration" or just "a result of relative motion"? If it's different than the first case, why does it have to be?

I think that in our original twin paradox, the simultaneity discrepancy is a result of being in different inertial frames, and the change in inertial frames is a result of acceleration. But I think to say that the simultaneity discrepancy is a result of acceleration is indirect and therefore misleading (and [dare I say it?] ignoring the interesting stuff!).

DJ
2002-Feb-21, 04:02 PM
The problem is the observation.

The truth lies in whether the cesium atom vibrates at a different frequency for the individual approaching the speed of light vs. the individual standing still.

Time is a measurement of elemental decay. For there to be a true time paradox, we must determine of there is a change in the rate of elemental decay. Bending or dilating light cones is not necessarily an illusion, but creates what we call "relativity."

DJ

<font size=-1>[ This Message was edited by: DJ on 2002-02-21 11:08 ]</font>

Wally
2002-Feb-21, 04:51 PM
DJ, observation is not the problem. That's the fundamental gyst of SR. The cesium atom does NOT slow down for that traveller in the same inertial frame as the atom (the one travelling near the speed of light, if you wish). It really DOES slow down for the individual in the relatively "stantionary" frame. I don't see this as a problem at all. . . just nature working as designed (aka WAD, in my occupation's varnicular) according to SR.

SeanF. I like what your saying better than GoW (sorry Grapes!). It's the introduction of the 3rd inertial frame that causes one to be younger than the other, not the accelleration one must experience to switch to the 3rd frame.

GrapesOfWrath
2002-Feb-21, 07:22 PM
No apologies necessary!

I dunno about that. I use three inertial frames when I explain the twins paradox--at least one version anyway--but I note that it could just as well be done with three different observers. Is it still a twins paradox if you have to have three observers?

SeanF
2002-Feb-21, 07:55 PM
On 2002-02-21 14:22, GrapesOfWrath wrote:
No apologies necessary!

I dunno about that. I use three inertial frames when I explain the twins paradox--at least one version anyway--but I note that it could just as well be done with three different observers. Is it still a twins paradox if you have to have three observers?

Hmm . . . the new-and-improved Special Relativity Triplets Paradox! No, actually, that wouldn't work anyway since you'd have disagreement on whether the three observers were born simultaneously. /phpBB/images/smiles/icon_smile.gif

Any "normal" (whatever that means!) depiction of the Twins Paradox requires two (and only two) observers, who cross paths with each other twice. This is impossible with only two inertial frames (they'd pass once and then just keep moving apart), so you need at least three.

(Of course, if the universe actually is analgous to JW's favorite balloon, the observers would eventually come back together from the other side, and . . . okay, that's really making my head hurt - forget I said it)

So, I guess the three-observer example is not technically a "Twins Paradox," but it does demonstrate the same time dilation and simultaneity discrepency factors of SR - and it does so without accelerating anything, which is kind of why I feel that acceleration shouldn't be considered a contributing factor in the normal Twins Paradox to begin with.

Wiley
2002-Feb-21, 08:02 PM
I see what you're getting at. My initial thoughts are that acceleration and the third perspective are tantamount. To account for acceleration you need three observers, and you would only need three observers to account for acceleration. Is there any other SR thought experiment that requires three observers? I think we have a necessary and sufficient condition.

<font size=-1>[ This Message was edited by: Wiley on 2002-02-21 15:04 ]</font>

GrapesOfWrath
2002-Feb-21, 08:15 PM
On 2002-02-21 14:55, SeanF wrote:

On 2002-02-21 14:22, GrapesOfWrath wrote:
No apologies necessary!

I dunno about that. I use three inertial frames when I explain the twins paradox--at least one version anyway--but I note that it could just as well be done with three different observers. Is it still a twins paradox if you have to have three observers?

Hmm . . . the new-and-improved Special Relativity Triplets Paradox! No, actually, that wouldn't work anyway since you'd have disagreement on whether the three observers were born simultaneously. :)

Any "normal" (whatever that means!) depiction of the Twins Paradox requires two (and only two) observers, who cross paths with each other twice. This is impossible with only two inertial frames (they'd pass once and then just keep moving apart), so you need at least three.

(Of course, if the universe actually is analgous to JW's favorite balloon, the observers would eventually come back together from the other side, and . . . okay, that's really making my head hurt - forget I said it)

So, I guess the three-observer example is not technically a "Twins Paradox," but it does demonstrate the same time dilation and simultaneity discrepency factors of SR - and it does so without accelerating anything, which is kind of why I feel that acceleration shouldn't be considered a contributing factor in the normal Twins Paradox to begin with.

I wrestled with this when I wrote up Twins Paradox (http://mentock.home.mindspring.com/twins.htm), Bride of Twins Paradox (http://mentock.home.mindspring.com/twin2.htm), and Son of Twins Paradox (http://mentock.home.mindspring.com/twinrdux.htm). You may have noticed that the first one says "A lot of explanations of the twin paradox have claimed that it is necessary to include a treatment of accelerations, or involve General Relativity. Not so."

So, I don't. <font size=+1>But</font> (and this is a big but), the "peculiar consequence" is only produced by changing from one inertial reference frame to another. That is an acceleration, whether or not a material object actually accelerates.

Our missing friend used to insist that the time dilation was just acceleration, and that it was just a sort of mechanical problem--clocks stuck when pushed, or something. But it is pretty well modeled as a mathematical transformation--and what do you do with objects in free fall? They're subject to "forces" but they're weightless, too.

SeanF
2002-Feb-21, 08:52 PM
That's what I was afraid of. All this discussion, and it boils down to "You say po-tay-to, I say po-tah-to"? Yuck. /phpBB/images/smiles/icon_smile.gif

I must concede that "a change in inertial frame" is a perfectly valid definition of "acceleration," so I cannot deny that <font site=+5>it</font> is a factor in the Twin Paradox. I still say, though, that one should be careful using the word in Relativity discussions, since it seems to immediately suggest that GR is involved. Fair enough?

Now, I wonder if poor Christine, who's initial question started this thread, is still reading it, and if we've succeeded in our nefarious plot to thoroughly confuse her!

_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2002-02-21 15:54 ]</font>

DJ
2002-Feb-21, 09:36 PM
On 2002-02-21 11:51, Wally wrote:
DJ, observation is not the problem. That's the fundamental gyst of SR. The cesium atom does NOT slow down for that traveller in the same inertial frame as the atom (the one travelling near the speed of light, if you wish). It really DOES slow down for the individual in the relatively "stantionary" frame.

Well now, you kind of said exactly what I implied /phpBB/images/smiles/icon_wink.gif But then you disagreed with "us."

To your point, we believe that the cesium atom does not change it's vibration rate, for either observer. If that is the case, then the body continues to age at the same rate for both observers. 1 year is still 1 year, to both, wherever they are. If they could communicate via some hyperlight comm link, they would find nothing amiss.

It is only when one compares one to the other that we get the effect. That effect is based on the observations of one to the other, and the effect is caused by the bending of light cones.

The one thing I can say, however, is that we are unable to observe anything that travels faster than the speed of light... thus our feeling that it's a speed limit. ( I don't agree... it's just an observational limit. )

What do you consider to be the boundary of the inertial frame? What is it's structure (timelike, spacelike, boundary composition, etc.)?

<font size=-1>[ This Message was edited by: DJ on 2002-02-21 16:37 ]</font>

<font size=-1>[ This Message was edited by: DJ on 2002-02-21 16:39 ]</font>

GrapesOfWrath
2002-Feb-23, 06:50 PM
On 2002-02-21 16:36, DJ wrote:
What do you consider to be the boundary of the inertial frame? What is it's structure (timelike, spacelike, boundary composition, etc.)?
I believe that there were no boundary in the original formulation. You imagined it extended, as a reference system, as far as you wanted. And everywhere could be synchronized.

grumium
2002-Feb-24, 02:34 AM
Pardon me for interupting as I am new here, and I don't know the protocol for criticising these great masters of thought, but the answer to your question, Christine112, is- no one knows.There are several logical flaws here but I believe the actual cause of the time dilation is due to momentum. As you approach the speed of light your mass increases and it takes a geometric progression of energy to go faster at a rate increasing linearly.Before you could reach C your mass would approach infinity.As mass increases the velocity of the subatomic particles that make up your frame of reference actually move slower.In other words, to you,time seems normal but is actually slowed down, with respect to time on Earth. But remember, this is all strictly theoretical as it is not currently verifiable. The reason we still call it the THEORY of relativity is because, even after almost 100 years, it remains unproven. And it doesn't help to speculate about "thought experiments" like those outlined above. With all due respect to GoW and SeanF, these require not just advanced technology, but a suspension of physical laws, a state we may now call Hypertheoretical. The few facts we have suggest that the theory is on the right track, but incomplete. Unless and until we can verify these predictions, they will remain unknown and irrelevant.

<font size=-1>[ This Message was edited by: grumium on 2002-02-26 00:11 ]</font>

2002-Aug-11, 02:06 PM
<a name="20020811.5:57"> page 20020811.5:57 aka Dredge p15 4most#
On 2002-02-15 14:07, Christine112 wrote: To? 2-8-11 HUb'
I can understand why this works. For the person traveling in the spaceship near the speed of light, time has slowed down so they age less than the one one Earth. But, to the person on the spaceship, time is running normal and the people on Earth appear to be moving more slowly. So wouldn't they expect to come back to Earth to find a younger twin?
[/quote]
sample dredge form "MOST" # of pages / thread
from page 15
To provide level 3 density {maybe}
and only for a short amount of time {if any}

Dunash
2002-Aug-15, 09:45 PM
A "humorous" attack on Relativity by Polish Geocentrist Pawel Kolasa
http://www.wiser.tv/physics/
http://www.wiser.tv/physics/einstein.html

<font size=-1>[ This Message was edited by: Dunash on 2002-08-15 18:46 ]</font>

David Hall
2002-Aug-15, 10:21 PM
On 2002-08-15 17:45, Dunash wrote:
An interesting atack on Relativity by Polish Geocentrist Pawel Kolasa
http://www.wiser.tv/physics/einstein.html

Interesting attack? LOLOL! Heck, even with my limited understanding of relativity I can see that this clown doesn't understand a thing about the theory he is attacking. This is about the lamest anti-relativity site I've ever seen.

He can't even tell the difference between a real clock and an imaginary timepiece used to illustrate a concept. This page is only interesting when viewed as comic relief.

Pawel Kolasa
2002-Aug-16, 02:29 AM
To David Hall: Your "knowledge" of Relativity is INDEED "limited".

As for calling me names... how old are you? Because you sould like a child.

overrated
2002-Aug-16, 02:47 AM
I dunno... it's pretty easy to be suspicious of an essay that starts out by blasting "the establishment" for not accepting it's views. Sounds like sour grapes to me.

Also, I'd say that a "logical" analysis of a mathematical theory is kind of like evaluating a sight based on its smell.

GrapesOfWrath
2002-Aug-16, 03:10 AM
You're in heady company, David Hall. He calls Einstein childish (http://www.wiser.tv/physics/einstein.html), too.

Rift
2002-Aug-16, 05:28 AM
1. Einstein claims that the time would stop, if he moved away from the clock at the speed of light because the light from the stationary clock would never reach him, and he wouldn't know what time it is.

This is a joke right? This is as far as I got on that website...

What is polish for 'strawman'?

And I don't think David is childish, I think a clown must have written this, wouldn't be nearly as funny other wise.

_________________
"Ignorance has caused more calamity then malignity" H.G. Wells

<font size=-1>[ This Message was edited by: Rift on 2002-08-16 01:30 ]</font>

beskeptical
2002-Aug-16, 09:24 AM
Polish Geocentrist certainly sounds like substantial credentials. /phpBB/images/smiles/icon_rolleyes.gif

All I can say about the website from Pawel Kolasa's profile is that my brain does not interpret the world the same way his(hers?) does.

The website spouts a lot of angry rhetoric towards those who just don't buy the logic. PK, believe whatever you want to but the only name calling I read here is from your response to strong statements of disagreement about your position. From your website I'd say it's the same approach you use in talking about scientists you don't agree with. Are you saying it's OK for you to disparage others but if they say the same about your ideas they 'sound like a child' and have 'limited knowledge of Relativity'?

Not a very convincing statement in support of your ideas.

Andrew
2002-Aug-16, 09:51 AM
Huh? (http://www.wiser.tv/physics/rotation.html)

Is this guy aware of the difference between weight and mass?

David Hall
2002-Aug-16, 10:05 AM
Well, I almost feel flattered at being the target of an attack. /phpBB/images/smiles/icon_biggrin.gif

Perhaps I was a bit hasty with the "clown" remark, and I apologise for that. But I will not back down on my assertion that this site shows a very low level of understanding of relativity. It does not take a complete understanding to see some of the gaping mistakes here, because it even gets a lot of the basic concepts wrong.

A few examples from his list that I managed to figure out on my own:

Number 1: shows a lack of understanding of what a thought experiment is, or frames of reference. And what about this quote: The idea that time depends on the clock, is childish. I seriously doubt Einstein was thinking of physical clocks here.

Number 8: He claims there's an inconsistancy because light always travels at c. Yes, but the example given is not the movement of light, but the movement of the clock (an example of a massive object), which can go slower than light (and in fact, can't ever reach c), so the only paradox is in the author's head.

Number 9. The twin paradox. It ignores the fact that the change in aging is not due to physical processes, but is in fact due to a relative change in the rate of time itself. The physiological processes aren't changed, time itself is changed.

Number 13. The whole crux of the matter. This is exactly what Einstein is saying. C always remains the same, therefore it's the other variables that have to change instead. The whole of his theory is spent in explaining how and why those variables can and do change.

Number 15. Another example of the above. Relativity is quite specific on this point. Velocity cannot be added to the speed of light. Therefore other effects are changed. Time changes, space contracts, and the wavelength of light is shifted, all so that c can remain unchanged. This is Einsteinian relativity at it's most basic, and this guy can't see it.

Number 16. A quote: Now, it is incorrect to say that THE SAME EVENT can have DIFFERENT properties of space and time. Such statement is logically incorrect!
Logic has nothing to do with it. With the idea of spacetime, this is exactly what's happening.

Number 18. He seems to be confusing the total mass energy of an item with it's chemical energy. In truth, equal masses of butter, bacon and apples will have the same mass energy. And they do hold that large number within them. But normal chemical reactions will not release that energy.

Number 20. Is ignoring the fact that the energy increase comes from whatever force is accelerating the object. So no violation of Newton is actually evident. (This isn't even relativity and he get's it wrong).

Well, that's what I get from my incomplete knowledge. Imagine what someone with an actual grounding in relativity physics could do. Oh, and just so you don't think I'm being arrogant and overstepping my bounds, I fully realize I may be wrong myself on some counts. In that case, I expect someone here to come along and correct me.

Jim
2002-Aug-16, 12:02 PM
On 2002-08-15 18:21, David Hall wrote:

On 2002-08-15 17:45, Dunash wrote:
An interesting atack on Relativity by Polish Geocentrist Pawel Kolasa
http://www.wiser.tv/physics/einstein.html

Interesting attack? ...

Which may explain why Dunash edited his original post:

A "humorous" attack on Relativity by Polish Geocentrist Pawel Kolasa

[ This Message was edited by: Dunash on 2002-08-15 18:46 ]

FP
2002-Aug-16, 02:16 PM
I guess we have seen almost every thing now. And no, I don't want to change my homepage to "www.fastseeker.net"

DoctorDon
2002-Aug-16, 02:41 PM
Wow, that guy really doesn't understand relativity. He also doesn't understand that the phenomenal success of relativitiy (not to mention the photoelectric effect) is why Einstein is admired/revered, not vice versa. In case anyone is interested, here's what he gets wrong. I'm following his numbered points.

One general thing he doesn't understand about relativistic thought experiments is how time is measured: it's not just one clock and one observer. The idea is you set up an infinite grid of clocks and observers, they all record times as the moving clock zips by, then they get back together and piece together the information they recorded. (This is, after all, a *thought* experiment.)

1. Relativity claims no such thing. Indeed, in relatvistic thought experiments, the network of stationary observers can always see the clock.

2. Here he's correct, but it's irrelevant, as relativity does not contradict him.

3. Relativity is not unfounded. It's based in solid physics, and it has passed every test it's been subjected to so far.

4. One reason this guy is messing up is he's still thinking of time in the Aristotalean sense. The time in the moving clock goes slower *from* the perspective of the stationary clock, *and* vice versa. And again, his critique of the moving clock illustration fails to understand the reference network of observers.

5. This critique fails to take into account that from the perspective of the stationary clock, the light beam in the moving clock has a horizontal component of velocity.

6. & 7. Time dilation is not the same thing as time delay. Even when you take delay of information arrival into account (which is what that hypothetical infinite array of observers is supposed to alleviate), you *still* get time dilation.

8. No, because it's the *clock* that's moving at different speeds, not the light beam.

9. Here he makes the rather bizarre claim that the ticking of the clock sets the flow rate of time rather than vice versa. I guess when the clock on my desk runs slow, the day really does last longer. /phpBB/images/smiles/icon_smile.gif This is, of course, nonsense, and misses the whole point. If you were on a spaceship approaching the speed of light, you would not perceive yourself to be aging at anything other than the normal rate. You would live you a normal life. The whole point is that people *outside* the ship would think you weren't aging. From the *outside* (stationary) point of view, it *would* look like your blood had stopped flowing. But it would *not* look like that to you.

10. Actually, they are. GR corrections have to be included in GPS units, or they wouldn't work. They've put one of a pair of synchronized clocks on fast ships like the concorde, and when they brought them back together, they were out of synch. This statement about different time units makes no sense.

11. Again, he misses the whole point. It matters not with what wave or mechanism you measure time. The relativistic effects to not depend on the *mechanism* of the clock. The speed of sound is not constant in any reference frame. There are no relativistic effects in moving at or past the speed of sound. Springs would not care at what speed they travel, but stationary observers would see them compressing and stretching at a different rate than an observer moving with the clock.

12. Again, Aristotalean time. His statement about there has to be a constant is sheer nonsense. He's simply defined it that way.

13. Now he's using Galilean transformations, but the whole point (as he should know, see point 22) is that you have to use Lorentz transformations. It's not c+a = c, so to cite that equation and fault relativity for using it is simply false, because the only one using that equation is Mr. Kolasa.

14. No, that is not the "proper equation". Second of all, Mr. Kolasa here makes a specific prediction about an observable, one that is directly contradicted by observation. Doesn't that worry you, Mr. Kolasa?

15. Again, he misses the whole point. Just adding two numbers together proves nothing. I can say "2+2=4, therefore I will have kippers for breakfast this morning", but that doesn't mean that one has anything to do with the other. A space ship moving near the speed of light that turns its (yellow) headlights on would "see" the light beam moving off at the speed of light, but a stationary observer would also see the light beam moving at the speed of light, just ahead of the ship. An observer ahead of the ship would see the wave front from the headlights arrive (at the speed of light) just before he was hit by the ship coming along right behind the beam, but the light would look blue.

16. Mr. Kolasa's claim here about being "logically incorrect" is simply wrong. He does not understand what a frame of reference is. In his diagram, from London's point of view, Moscow is to the right and down, while from Sydney's point of view, Moscow is to the right an up. That *is* a different x,y,z! To say there is only one x,y,z is just saying you're only considering a single reference frame. That doesn't mean there aren't any others.

17. False analogy.

18. False analogy. Chemical bonding energy (how much energy is released in digestion) has nothing to do with E=mc<sup>2</sup>.

19. He misses the point. It's not the light wave itself that has anything to do with the potato, it's the speed that does. The universal constant "c" is called the speed of light for historical reasons, but the point is that light moves at "c" precisely because it has no mass. There's the connection between mass and speed right there. To put it another way, the kinetic energy of a body depends on its mass and speed. Therefore, for a given energy, mass and speed *are* related. The speed of light was not chosen as an arbitrary speed limit; the energy of a potato depends on the speed of light because the potato has mass and light does not. The fact that light does not have mass reveals to us what the universal speed limit is, and that is relevant to the potato.

20. The energy comes from whatever is pushing the body to a higher speed.

21. The former inhabitants of Hiroshima and Nagasaki will be most surprised to learn this.

22. But if you describe Superman and then find someone that fits all aspects of the description, I think it would be fair to conclude that you had proven Superman's existance. So far, relativity has passed every test thrown at it. What part of *that* don't you understand?

23. Nope.

It'd be funny, if it weren't so sad. Can someone explain to me the mindset in which a person can say "I don't understand it, so it must be wrong"? Me, I feel like I need to understand something before I can say that it's wrong.

But that's just me, I guess.

Don

Jim
2002-Aug-16, 04:22 PM
On 2002-08-16 10:16, FP wrote:
I guess we have seen almost every thing now. And no, I don't want to change my homepage to "www.fastseeker.net"

Yeah, me either.

<font color=red>Warning! Danger, Will Robinson!</font>

_________________
<font color=000099>Never attribute to malice what can be adequately explained by ignorance or stupidity.</font>
Isaac Asimov

<font size=-1>[ This Message was edited by: Jim on 2002-08-16 12:23 ]</font>

Wiley
2002-Aug-16, 04:40 PM
I think this page has to a joke. If not, this Pawel Kolasa unwittingly did an excellent parody of a crank. To be so vociferous about relativity and know so litte about relativity smacks of parody.

Of course, the pop-up ads and such are just contemptible.

HankSolo
2002-Aug-16, 05:17 PM
Forgive me if I'm way off, but how do we know that speed affects age?

I mean, as far as I know, we only know that speed affects the vibrations of a cesium atom (or whatever it is that an atomic clock actually measures).

Do we have any physical evidence that anything else is affected? And is it time that is affected, or atomic vibrations?

Wiley
2002-Aug-16, 05:39 PM
On 2002-08-16 13:17, HankSolo wrote:
Do we have any physical evidence that anything else is affected? And is it time that is affected, or atomic vibrations?

I'm not sure if this is what you're after, but particle accelerators do this all the time. For instance (doing from memory so numbers may be a bit off) p+ meson at rest has a lifetime of about 18 nS, but at 0.998c it lasts about 285 nS. Of course in meson's frame, it only lasts about 18 nS.

HankSolo
2002-Aug-16, 09:56 PM
I'm not sure of my point, something just doesn't sit well with me regarding the discussion of time in this thread.

In the meson example, it appears that speed slows down the activity causing decay. I can accept that speed slows down particle activity or interaction, though I don't know why, but it's apparent. It's the definition of "time" that I'm confused about. Please bear with me, because I'm no scientist or mathematician, I just have a lot of curiousity.

Here's my take on time, please feel free to correct me where I'm wrong: Time is just a measurement of particle activity. There's no such thing as "time", just as there's no such thing as an "inch" (or a "meter" for you scientists). It's a measurement of activity from the point of view of the observer. In the meson example, a "thinking" meson would be affected by slower brain activity, giving that meson the perception that only 18ns passed by. The gears inside the watch on the meson's wrist would also slow down. Decay slows down so the meson does indeed age more slowly.

But time doesn't slow down. Time is just a figment of our imagination. Particle activity slowed down. Time travel and the paradoxes that go with it, as in the wormhole scenario that I heard described by Hawking, would be impossible unless time was a physical thing and we could affect more than just the traveler, and there was some sort of "universal time" to compare to. Also, going back in time would suddenly add your matter to the matter already present at that time (which already includes your matter), wouldn't it? So you're creating matter in the past, as well as destroying matter in the present, since you've just disappeared into nowhere. There's a law against that, isn't there?

Time cannot be slowed or quickened because it doesn't exist. Time only exists in our head because we need a method of measurement. The only thing that exists is the present moment.

Extreme cold produces the same effect in our mind as near-light-speed travel. If Ted Williams was alive when frozen, and will be thawed out a thousand years from now, he is experiencing the same thing you and I would on a spaceship travelling at near-light speed. Did Ted go into a time-warp?

Also interesting (this is new to me and I'm making it up as I go, so forgive me if I'm repeating common knowledge), if I'm travelling on a spaceship at near-light speed, and keep track of time on my watch, after one year (by my watch) I will have travelled much more than one light year. In my perception, I have travelled faster than light. I can picture this relationship as an exponential curve to infinity. The closer I get to the speed of light in reality, the slower my perception gets. Therefore I can travel an infinite multiple of light-speed in my eyes. In my perception, I can get from here to the edge of the universe (just an expression) in the blink of an eye but nobody would be around when I got back.

I'm still not sure of my point... I just know I have a tough time when I hear people talk about space/time because it seems to me that they're talking about something that actually exists outside of our imaginations. (probably what you all think when I'm talking about ufo's!)

Feel free to enlighten me.

overrated
2002-Aug-16, 10:59 PM
Dr. Don, there's definitely a difference in mental wiring that explains why someone would so passionately defend an factually incorrect view. It mostly, I think, has to do with the person not being able, psychologically speaking, to deal with the actual dangers and problems in the world that they can't control. So they invent something they CAN control (a weird conspiracy, an anti-relativity theory, what have you). Because the human mind is infinitely creative, they can always think around objections to their theories, no matter how convoluted or illogical it gets.

Or that Web site could be a joke. Einstein would know the answer....

Pawel Kolasa
2002-Aug-17, 06:02 PM
[quote]
On 2002-08-15 22:47, overrated wrote:
I dunno... it's pretty easy to be suspicious of an essay that starts out by blasting "the establishment" for not accepting it's views. Sounds like sour grapes to me.

Yes, so?

Also, I'd say that a "logical" analysis of a mathematical theory is kind of like evaluating a sight based on its smell.

Sight is irrelevant to smell. However logic is a CHAPTER of mathematics.

Pawel Kolasa
2002-Aug-17, 06:08 PM
beskeptical: I will change the angry rhetoric. See, I thought that people pretend to be that stupid as not to understand simple arguments. I thought it was malice. Now I realized that they really are that stupid /phpBB/images/smiles/icon_frown.gif

[quote]
On 2002-08-16 05:24, beskeptical wrote:
Polish Geocentrist certainly sounds like substantial credentials. /phpBB/images/smiles/icon_rolleyes.gif

All I can say about the website from Pawel Kolasa's profile is that my brain does not interpret the world the same way his(hers?) does.

The website spouts a lot of angry rhetoric towards those who just don't buy the logic. PK, believe whatever you want to but the only name calling I read here is from your response to strong statements of disagreement about your position. From your website I'd say it's the same approach you use in talking about scientists you don't agree with. Are you saying it's OK for you to disparage others but if they say the same about your ideas they 'sound like a child' and have 'limited knowledge of Relativity'?

Not a very convincing statement in support of your ideas.

Pawel Kolasa
2002-Aug-17, 06:11 PM
On 2002-08-16 05:51, Andrew wrote:
Huh? (http://www.wiser.tv/physics/rotation.html)

Is this guy aware of the difference between weight and mass?

Andrew: you probably think you are the first person to say that? huh?

Tell me: Why in physics in equation "m" stands for MASS, however they put WEIGHT in kilograms ???

Kaptain K
2002-Aug-17, 06:15 PM
Tell me: Why in physics in equation "m" stands for MASS, however they put WEIGHT in kilograms ???
Carelessness!

Pawel Kolasa
2002-Aug-17, 06:32 PM
overrated: you projected exactly what YOU are doing.

[quote]
On 2002-08-16 18:59, overrated wrote:
Dr. Don, there's definitely a difference in mental wiring that explains why someone would so passionately defend an factually incorrect view. It mostly, I think, has to do with the person not being able, psychologically speaking, to deal with the actual dangers and problems in the world that they can't control. So they invent something they CAN control (a weird conspiracy, an anti-relativity theory, what have you). Because the human mind is infinitely creative, they can always think around objections to their theories, no matter how convoluted or illogical it gets.

Andrew
2002-Aug-17, 07:05 PM
"Andrew: you probably think you are the first person to say that? huh?"

I was kinda hoping I wasn't! /phpBB/images/smiles/icon_wink.gif

"Tell me: Why in physics in equation "m" stands for MASS, however they put WEIGHT in kilograms ???"

Weight is measured in newtons if you want to be pedantic. I don't think "in physics" people measure weight in kilograms.

Imagine this: You're in on of these non-sensical rotating space stations that is currently stationary, you attach yourself to what is to become the "floor", the space station starts spinning to the desired speed, you un-attach your self from the space station floor, what happens? Do you just stop moving and the space station continues to rotate without you? Or are you forced to the outside of the space station (the "floor") with a force proportional to your mass multiplied by the square of your velocity and divided by the radius of the space station?

overrated
2002-Aug-17, 09:11 PM
First: Kilograms are not a measure of weight. They're a measure of mass, which does not depend on gravity (as weight does). Thus, "m" in a physics equation SHOULD be measured in kilograms (or milligrams, decagrams, whatever). If someone wrote "weight" instead, it's carelessness, as the Kaptain pointed out.

Second, when I say "logical" analysis of relativity isn't really evaluating it correctly, I mean that it's in some ways a counterintuitive theory, but mathematically, it works exactly as it should. Similarly, scientific observations bear out its predictions. Dr. Don explains how your argument is flawed on a point-by-point basis.

Which brings me to my third point. I don't think I'm being irrational by siding with the vast majority of 20th Century physicists and scientists in general when I say: Relativity isn't bunk. I understand the concepts behind it, and, as I mentioned above, the predictions made by the theory are easily observed. It strikes me that it is YOU who is being irrational by not listening to the people (again, like Dr. Don) who critique your theory and point out where it is flawed.

True scientific endeavor means accepting the results of your research or experiments, even if it's not what you hoped. You seem to have put a lot of thought into disproving relativity, but you won't let it go when people try to show you you're wrong.

DoctorDon
2002-Aug-17, 11:19 PM
On 2002-08-17 14:11, Pawel Kolasa wrote:
Tell me: Why in physics in equation "m" stands for MASS, however they put WEIGHT in kilograms ???

Um, we don't. Anyone that puts weight in kilograms is wrong and would get that marked incorrect on any physics exam.

Don

beskeptical
2002-Aug-18, 08:56 AM
On 2002-08-17 14:08, Pawel Kolasa wrote:
beskeptical: I will change the angry rhetoric. See, I thought that people pretend to be that stupid as not to understand simple arguments. I thought it was malice. Now I realized that they really are that stupid /phpBB/images/smiles/icon_frown.gif

Doesn't it strike you as a little odd that the people you believe are stupid and can't understand 'simple arguments' are designing computers, building and launching satellites, sending spaceships to the Moon & to Mars, building 100+ story buildings, transplanting livers, lungs, & hearts into people, saving infants born weighing less than 14 ounces, and more?

beskeptical
2002-Aug-19, 02:10 AM
Apparently, I have made my point. I see no PK answer yet.

Jim
2002-Aug-19, 12:46 PM
On 2002-08-17 19:19, DoctorDon wrote:

On 2002-08-17 14:11, Pawel Kolasa wrote:
Tell me: Why in physics in equation "m" stands for MASS, however they put WEIGHT in kilograms ???

Um, we don't. Anyone that puts weight in kilograms is wrong and would get that marked incorrect on any physics exam.

Don

In (terrestrial) engineering, we tend to use mass and weight interchangeably. But, we also operate under a system where a=g=1, so mass and weight are the same.

If someone is confusing the two, it indicates that he is passingly (no pun intended) familiar with engineering, but not sufficiently familiar to realize that we know this is a special case.

DoctorDon
2002-Aug-19, 01:50 PM
On 2002-08-19 08:46, Jim wrote:

In (terrestrial) engineering, we tend to use mass and weight interchangeably. But, we also operate under a system where a=g=1, so mass and weight are the same.

If someone is confusing the two, it indicates that he is passingly (no pun intended) familiar with engineering, but not sufficiently familiar to realize that we know this is a special case.

But Mr. Kolasa specifically said "in physics", not "in engineering". Also, I doubt he has a passing acquaintence with engineering, either, as in a different one of his web pages, he says that since people are weightless in space, they will experience no force at all, since m=0, and force equations like F=mv<sup>2</sup>/2 depend on m. Now, if that page isn't a bad joke, I don't think even an engineer would substitute weight for mass in that equation.

Beskeptical wrote:

Apparently, I have made my point. I see no PK answer yet.

And I note that although Mr. Kolasa seems to be quite ready to answer ad hominem attacks related to his web page, direct rebuttals of his arguments such as those made by David Hall and myself go completely ignored.

Don

99homer99
2002-Aug-21, 08:54 PM
I personally believe that whatever speed the first twin travelled at, and whatever distance he travelled, when he returned back to the second twin the total time dilation (if any) would be zero.
Because if the time slowed on the way out it would increase on the way back and therefore would give a total time dilation of zero. Meaning the difference in the twins ages would be no different after the event than before the event.

SpacedOut
2002-Aug-21, 09:06 PM
On 2002-08-21 16:54, 99homer99 wrote:
Because if the time slowed on the way out it would increase on the way back and therefore would give a total time dilation of zero. Meaning the difference in the twins ages would be no different after the event than before the event.

The slowing of time is strictly a function of speed and direction of travel does not factor into it, therefore the twin in motion will age more slowly from the perspective of the twin who is not in motion.

[sp]

<font size=-1>[ This Message was edited by: SpacedOut on 2002-08-21 17:07 ]</font>

DoctorDon
2002-Aug-21, 09:20 PM
On 2002-08-21 16:54, 99homer99 wrote:
I personally believe that whatever speed the first twin travelled at, and whatever distance he travelled, when he returned back to the second twin the total time dilation (if any) would be zero.
Because if the time slowed on the way out it would increase on the way back and therefore would give a total time dilation of zero. Meaning the difference in the twins ages would be no different after the event than before the event.

That is incorrect. Time does not "increase on the way back". The acceleration that the moving twin undergoes breaks the symmetry.

Don

99homer99
2002-Aug-22, 09:31 PM
You are changing your time references.
Keep them the same

Going away
1 When you travel away time dilation grows or slows down

Coming back
2 When you travel towards something time dilation shrinks or speeds up

Total dilation there and back=zero

overrated
2002-Aug-22, 09:36 PM
Homer, the direction doesn't matter. It's the speed (near c) that causes the time dilation. So whether the twin is coming or going, the relativistic effects are the same.

Wiley
2002-Aug-22, 10:48 PM
As overrated and Dr. Don have said direction does not matter. Time dilation, length contraction, and all those other funky effects depend on the square of velocity, v<sup>2</sup>. If you change directions but keep your speed the same, the velocity becomes negative (-v). And (-v)<sup>2</sup> = v<sup>2</sup>. (For those who like vectors, it actually depends on v*v.)

GrapesOfWrath
2002-Aug-23, 01:22 AM
On 2002-08-21 16:54, 99homer99 wrote:
I personally believe that whatever speed the first twin travelled at, and whatever distance he travelled, when he returned back to the second twin the total time dilation (if any) would be zero.

Well, that's a personal belief. It doesn't agree with mainstream physics (hence the forum), and it doesn't seem to agree with the data. But there's always a possibility that you can find a way to make it work.

Zathras
2002-Aug-23, 04:00 PM

Take a space where a particular geodesic of travel, with a velocity of c/2, crosses to the same (spatial) position twice. Put one twin at rest at that spatial position and put the second twin travelling along the geodesic at 1/2c. At some point in the future, they will meet again. Time dilation seems to imply that one will age more than the other, but the equivalence principle requires the view of both twins to be the same, so no one ages more than the other.

The difference between this example and the standard Twin Paradox is that no acceleration, and hence no change in inertial frames, occurs in this example. I am assumming, of course, that the "closed in space" geodesic can exist under the GR equations, but I do not know for sure. I would be curious as to people's thoughts on this question.

GrapesOfWrath
2002-Aug-23, 04:56 PM
On 2002-08-23 12:00, Zathras wrote:
The difference between this example and the standard Twin Paradox is that no acceleration, and hence no change in inertial frames, occurs in this example.

Your example seems to be one where space is so curved that the path comes back on itself, even though the object is moving at a speed of c/2--I assume you are thinking of an orbit, perhaps? The gravity would probably be quite strong for such high velocity. If the first one is in orbit, how does the second one maintain its position?

I realize that this is just a thought experiment, but I am trying to point out that your assumption that they are in inertial reference frames may not be valid--so you'll have to be more specific.

Zathras
2002-Aug-23, 05:06 PM
Good question. I was not thinking of an orbit, but perhaps more of a space with a uniform distribution of mass, so that the space has a uniform curvature. This way, space (I think) would have the topology of a hypersphere. Since the distribution of mass is constant, there would not be any gravitational "force."

I am of course thinking of massless twins here (or is it "test twins?")

DoctorDon
2002-Aug-23, 05:24 PM
I don't think you'll be able to find a mass distribution in which the worldline of the moving twin crosses the same point in space without some acceleration being involved.

Don

Zathras
2002-Aug-23, 05:57 PM
I would think that many such universe topologies should be able to admissible. The simplest one is the one I gave, the hypersphere. The 1-dimensional analog is a bead which is constrained to move in a circle. Without friction, it experiences no acceleration in its embedded space (the operative distinction here). If a simultaneous slice of space had the topology of a 3-sphere, it would seem that no acceleration would require an object in motion, relative to one frame, to return to its previous spatial position. Saying that it has the topology of a 3-sphere is not the same as saying it in an actual 3-sphere embedded in a 4-d space, as the topology is instrinsic. In this situation no acceleration is required to return.

DoctorDon
2002-Aug-23, 06:06 PM
Hmmm... Don sees your point. Much apologizings. Don was trapped in linear thinking. Poor Don.

No one listen to poor Don...

Won't talk. Not the One.

Don

99homer99
2002-Aug-23, 09:54 PM
spaced out

you wrote:

The slowing of time is strictly a function of speed and direction of travel does not factor into it, therefore the twin in motion will age more slowly from the perspective of the twin who is not in motion.
-------------------------------------------
I say

due to relativity which twin is moving and which twin is stationary?

anyone would say in this universe - Where is the stationary fixed refence point? - there is none

therefore which twin ages more than the other one

I think I am still correct in thinking when two objects travel towards each other time goes quicker and when two objects travel away from each other time slows
for both in equal amounts

<font size=-1>[ This Message was edited by: 99homer99 on 2002-08-23 17:56 ]</font>

DoctorDon
2002-Aug-24, 02:56 AM
On 2002-08-23 17:54, 99homer99 wrote:
due to relativity which twin is moving and which twin is stationary?
anyone would say in this universe - Where is the stationary fixed refence point? - there is none

This is correct. That's why it's called the twin paradox.

I think I am still correct in thinking when two objects travel towards each other time goes quicker and when two objects travel away from each other time slows
for both in equal amounts

No, you are wrong.

As I and others have been trying to tell you: direction is irrelevant, as the velocity is squared. The symmetry is broken by the acceleration that one twin undergoes to get up to speed and then turn around. That resolves the paradox.

But you don't have to take my word for it; just read a book on relativity.

Don

JS Princeton
2002-Aug-24, 03:59 AM
homer, you are right to feel the way you do. We think that it should be symmetric. This is where the paradox comes into play. Indeed, on the way out the twin on Earth will receive fewer "ticks of the light clock" than the rocket-twin when leaving and more "ticks of the light clock" than the rocket-twin when coming home. However, if you add up all the ticks seen by the twin on Earth and all the ticks seen by the twin on the spaceship, they are not the same! This paradox has, at its root, the fact that we're not dealing with inertial reference frames. You are right, homer, when you feel that there is symmetry in relativity. However, once accleration occurs, the symmetry is broken. This is a profound effect, and is dealt with more carefully in another Equivalence Principle of Einstein's. For more info, homer, please go to the following website:

http://www.physics.hku.hk/~tboyce/modern_physics/topics/twins.html

GrapesOfWrath
2002-Aug-24, 07:12 AM
On 2002-08-23 23:59, JS Princeton wrote:
This paradox has, at its root, the fact that we're not dealing with inertial reference frames.

But, as JW reminded us, the paradox appeared in Einstein's first paper on relativity--using nothing more than his special relativity. The whole paradox can be expressed with nothing more than inertial reference frames.

Zathras
2002-Aug-24, 01:50 PM
Actually, having the twins go in an orbit gives another possibility for my question about a GR twin paradox. Suppose the two twins are in the same circular orbit about a black hole, going in oppposite directions. You can have them going around at r=3m(*G/c^2), so that their speed relative to each other is certainly relativistic. Their perspectives are certainly indistinguishable, so how would time dilation play out in this circumstance?

This problem started badly, and it will likely end badly. At least there is symmetry.
-Zathras

David Hall
2002-Aug-24, 03:29 PM
On 2002-08-24 09:50, Zathras wrote:
Actually, having the twins go in an orbit gives another possibility for my question about a GR twin paradox. Suppose the two twins are in the same circular orbit about a black hole, going in oppposite directions. You can have them going around at r=3m(*G/c^2), so that their speed relative to each other is certainly relativistic. Their perspectives are certainly indistinguishable, so how would time dilation play out in this circumstance?

This problem started badly, and it will likely end badly. At least there is symmetry.
-Zathras

My guess is that since neither of them is accelerating relative to the other, they are both subjected to the same time dialation. There is no paradox in this situation, as all effects are equal.

GrapesOfWrath
2002-Aug-24, 04:17 PM
On 2002-08-24 11:29, David Hall wrote:
My guess is that since neither of them is accelerating relative to the other, they are both subjected to the same time dialation.

I think that's the whole point, the symmetry isn't broken. But I think zathras's question is, aren't they experiencing a relative difference in speed (or velocity), so wouldn't their perception of time be different? And how can those differences be reconciled when they hook up each time around.

Zathras
2002-Aug-26, 03:38 PM
On 2002-08-24 12:17, GrapesOfWrath wrote:

On 2002-08-24 11:29, David Hall wrote:
My guess is that since neither of them is accelerating relative to the other, they are both subjected to the same time dialation.

I think that's the whole point, the symmetry isn't broken. But I think zathras's question is, aren't they experiencing a relative difference in speed (or velocity), so wouldn't their perception of time be different? And how can those differences be reconciled when they hook up each time around.

Right, that is the whole point of my question. How does time dilation work in the examples I gave previously?

Jim
2002-Aug-26, 04:25 PM
Right, that is the whole point of my question. How does time dilation work in the examples I gave previously?

The time dilation calculation uses the square of the velocity, so the direction component essentially drops out. As long as the two are moving at the same velocity they would experience the same time dilation relative to the same reference frame.

t=t'L=t'(1-(v/c)<sup>2</sup>)<sup>-1/2</sup>
where t=reference frame time

If that's true, then they experience no time dilation relative to each other.

(Hit submit too soon.)

<font size=-1>[ This Message was edited by: Jim on 2002-08-26 12:32 ]</font>

GrapesOfWrath
2002-Aug-26, 04:33 PM
On 2002-08-26 12:25, Jim wrote:
The time dilation calculation uses the square of the velocity, so the direction component essentially drops out. As long as the two are moving at the same velocity they would experience the same time dilation relative to the same rest frame.

That's not what zathras is asking. Do they view each other as different? Obviously not, since they experience the same dilation, according to you. Why not?

Wiley
2002-Aug-26, 05:51 PM
Zathras,

First, thanks for the interesting question.

Second, a disclaimer, I am not a astrophysicist but a poor schlub teaching himself general relativity.

This problem has been analyzed for orbits and other cyclic problems. The major problem is that the "moving" twin will not be able to synchronize his or her clock. Observers at the same point in spacetime can have clocks that read different times. The paradox is resolved because the moving twin agrees that he is younger than the stationary twin. All this is tantamount to a "preferred" frame (Lord, I hope the geocentrist are not reading this.) which is set by the geometry of spacetime.

Judging from your previous posts, you are not unaquainted with mathematics. Barrow and Levin did a paper on this: The twin paradox in compact spaces (http://arxiv.org/abs/gr-qc/0101014). I'm still going through the details of this paper so don't ask me any hard questions. /phpBB/images/smiles/icon_smile.gif

MartinM
2002-Aug-26, 06:28 PM
On 2002-08-26 13:51, Wiley wrote:
All this is tantamount to a "preferred" frame (Lord, I hope the geocentrist are not reading this.) which is set by the geometry of spacetime.

An intuitive way of thinking about this is to consider what happens if an observer fires off photons in all directions. Those photons will travel around the compact space and meet up again at their starting point. The observer, whether moving or stationary, will encounter the photons at some point. If he encounters them all together, he knows he is at the same location as when he fired them. Thus establishing an absolute rest frame. I don't think this will help the Geocentrists any, though.

Wiley
2002-Aug-26, 07:03 PM
On 2002-08-26 14:28, MartinM wrote:
Those photons will travel around the compact space and meet up again at their starting point. The observer, whether moving or stationary, will encounter the photons at some point. If he encounters them all together, he knows he is at the same location as when he fired them. Thus establishing an absolute rest frame.

Doesn't this imply that that the space around a "preferred" observer is symmetric? The geodesic path length in each direction is the same? I don't see that.

If an observer emits photons at a constant interval dT. When she recieves the photons, if they are at the same interval spacing, dT, then she is in a "preferred" frame. I can see that.

I don't think this will help the Geocentrists any, though.

You may be surprised at the geocentrist's ability to twist legitmate science to fit his agenda. Read a few of Dunash's and Prince's posts. (shudders)

Zathras
2002-Aug-26, 07:10 PM
On 2002-08-26 15:03, Wiley wrote:

On 2002-08-26 14:28, MartinM wrote:
Those photons will travel around the compact space and meet up again at their starting point. The observer, whether moving or stationary, will encounter the photons at some point. If he encounters them all together, he knows he is at the same location as when he fired them. Thus establishing an absolute rest frame.

Doesn't this imply that that the space around a "preferred" observer is symmetric? The geodesic path length in each direction is the same? I don't see that.

If an observer emits photons at a constant interval dT. When she recieves the photons, if they are at the same interval spacing, dT, then she is in a "preferred" frame. I can see that.

I don't think this will help the Geocentrists any, though.

You may be surprised at the geocentrist's ability to twist legitmate science to fit his agenda. Read a few of Dunash's and Prince's posts. (shudders)

To take the example of the 3-sphere, anyone who stays an inertial frame for the duration of the photons' trip around would see the photons simultaneously. I guess if your space had less symmetry, then there might be only isolated pockets where the above situation might occur.

BTW, thanks for the Barrow article. It looks interesting and on-point, but I haven't taken the time to digest it yet.

dcl
2002-Aug-26, 08:54 PM
A spacetime diagram affords an easy way to visualize the lack of symmetry in the Twin Paradox. It also simplifies things if we treat the spaceship as traveling so close to the speed of light that the difference is undetectable as far as space and time increments are concerned. The fact that the spaceship mass and kinetic energy relative to earth are both eseentially infinite is not relevant to the thought experiment.

Visualize a horizontal x axis extending to the right as the spatial direction of spaceship travel to a point one light year from earth and a vertical upward t axis as the direction toward future time. Let one light year in distance and one year in time be marked off as equal distances along the x and t axes, respectively. Light traveling to the right has a world line that bisects the angles between the x and t axes, that is, slants upward to the right at a 45-degree angle from the x axis. Light traveling in the opposite direction slants upward to the left at a 45-degree angle from the -x axis.

Let the spaceship travel at so close to the speed of light that for practical purposes we can treat it as traveling at exactly the speed of light. Relativity says it would take an infinite amount of energy to accelerate it to that speed, but this only a thought experiment, so this approximation does not affect the validity of the results of the experiment as far as kinematics are concerned.

The world line of the spaceship is therefore indistinguishable from that of a light ray leaving earth in the direction of travel of the space ship simultaneously with the spaceship.

An observer on earth watching the spaceship will see it arrive at the point one light year from earth for practical purposes precisely TWO years after it left earth, ignoring the slight difference between the spaceship's speed and the speed of light. This is because it takes one year for the space ship to reach its turnaround point and another year for light that left the space ship as it's turning around to reach earth. Let the turnaround be instantaneous. From that point, it takes the spaceship another year to return to earth, and it arrives at essentially the same instant as does the light ray that left it as it at the instant when it turned around. Since the observer on earth saw the spaceship traveling at essentially the speed of light both going and returning, time on it appeared to be essentially standing still both going and returning. Thus, two years after the spaceship departed earth, it was back on earth with its occupants essentially the same ages that they were when they started out while the observers on earth are essentially two years older.

Incidentally, it will appear to observers on earth that the space ship took TWO years to reach the point ONE light year away, then to make the trip back to earth INSTANTLY -- in zero time!

MartinM
2002-Aug-26, 09:22 PM
On 2002-08-26 15:03, Wiley wrote:
Doesn't this imply that that the space around a "preferred" observer is symmetric? The geodesic path length in each direction is the same? I don't see that.

You're right, that was sloppy.

Consider a closed null geodesic passing through the observers location. Then two photons fired along this geodesic in opposite directions will meet up again in only two locations - the other side of the compact space, and the original starting point. Since the observer cannot reach the other side of the compact space before the photons, the only way he can re-encounter both simultaneously is if he is still at the same location. Similarly for pairs of photons on any other closed null geodesic.

There. I think that's better.

You may be surprised at the geocentrist's ability to twist legitmate science to fit his agenda. Read a few of Dunash's and Prince's posts. (shudders)

Heh. I've spent the last few months debating Creation vs Evolution on a fundie board. I know exactly what you mean /phpBB/images/smiles/icon_biggrin.gif

Jim
2002-Aug-26, 09:30 PM
On 2002-08-26 12:33, GrapesOfWrath wrote:

On 2002-08-26 12:25, Jim wrote:
The time dilation calculation uses the square of the velocity, so the direction component essentially drops out. As long as the two are moving at the same velocity they would experience the same time dilation relative to the same rest frame.

That's not what zathras is asking. Do they view each other as different? Obviously not, since they experience the same dilation, according to you. Why not?

I think they would see each other as aging at the same rate.

They are both in motion wrt the same reference frame; for all relativistic purposes, their motion is the same (the only diference is the velocity vector, but the absolute value is the same).

Zathras
2002-Aug-26, 09:50 PM
On 2002-08-26 17:30, Jim wrote:

On 2002-08-26 12:33, GrapesOfWrath wrote:

On 2002-08-26 12:25, Jim wrote:
The time dilation calculation uses the square of the velocity, so the direction component essentially drops out. As long as the two are moving at the same velocity they would experience the same time dilation relative to the same rest frame.

That's not what zathras is asking. Do they view each other as different? Obviously not, since they experience the same dilation, according to you. Why not?

I think they would see each other as aging at the same rate.

They are both in motion wrt the same reference frame; for all relativistic purposes, their motion is the same (the only diference is the velocity vector, but the absolute value is the same).

This is the result that I would have expected, but the Barrow article referred to earlier states that the result is not the same in the instance of a compact space. It reaches this result even though there would seem to exist a CM frame in which they are both moving in equal but opposite directions.

Now I'm more confused than when I first asked the question. I hope that's a sign of progress.

GrapesOfWrath
2002-Aug-27, 01:39 AM
On 2002-08-26 17:30, Jim wrote:
I think they would see each other as aging at the same rate.

They are both in motion wrt the same reference frame; for all relativistic purposes, their motion is the same (the only diference is the velocity vector, but the absolute value is the same).

That would be the natural conclusion, but what about the usual point of view of one which assumes itself is stationary while the other is moving--and then from the POV of the other? How do you treat those POVs?

2002-Aug-27, 02:24 PM
I did watch TV When the Pz came to PDX to poise 4 City Mayor
Standard Arms race stuff {Yes & nO}
anyway later in the news cast they showed the Governer of the State
{Yes, Yes} ive run for Governor as a LIBERTERIAN
never mind politicks
anyway it looked to me as though the Governer
age like 100 years well at least 10
you know Standard symptoms Gray Hair
wrinkles & just plain aged look
of course i dnt rule out makeup artist in this?

GrapesOfWrath
2002-Aug-27, 03:20 PM
On 2002-08-27 10:24, HUb' wrote:
you know Standard symptoms Gray Hair
wrinkles & just plain aged look
of course i dnt rule out makeup artist in this?

Another conspiracy, so they can play golf more often

Wiley
2002-Aug-27, 04:43 PM
On 2002-08-26 17:22, MartinM wrote:
Consider a closed null geodesic passing through the observers location. Then two photons fired along this geodesic in opposite directions will meet up again in only two locations - the other side of the compact space, and the original starting point. Since the observer cannot reach the other side of the compact space before the photons, the only way he can re-encounter both simultaneously is if he is still at the same location. Similarly for pairs of photons on any other closed null geodesic.

There. I think that's better.

Yeah, that makes sense. The simplest case of an orbit or a 3-sphere that Zathras suggest would have symmetry in all directions. Now does this stationary frame always measure the smallest volume? Thus satisfying Barrow & Levin's definition of a "preferred" frame.

99homer99
2002-Aug-29, 09:49 PM
On 2002-08-23 23:59, JS Princeton wrote:
homer, you are right to feel the way you do. We think that it should be symmetric. This is where the paradox comes into play. Indeed, on the way out the twin on Earth will receive fewer "ticks of the light clock" than the rocket-twin when leaving and more "ticks of the light clock" than the rocket-twin when coming home. However, if you add up all the ticks seen by the twin on Earth and all the ticks seen by the twin on the spaceship, they are not the same! This paradox has, at its root, the fact that we're not dealing with inertial reference frames. You are right, homer, when you feel that there is symmetry in relativity. However, once accleration occurs, the symmetry is broken. This is a profound effect, and is dealt with more carefully in another Equivalence Principle of Einstein's. For more info, homer, please go to the following website:

http://www.physics.hku.hk/~tboyce/modern_physics/topics/twins.html

I'm sorry this web page doesn't prove anything to me
except on the way out Dick receives a signal every three years from Jane

and on the way back Jane receives 3 signals from Dick every year

and it took Dick 15 years to get to a planet that was 20 light years away

at 0.8c it should have taken him 20 devided by 0.8 = 25 years

is it me?

<font size=-1>[ This Message was edited by: 99homer99 on 2002-08-29 17:52 ]</font>

DoctorDon
2002-Aug-30, 03:56 PM
On 2002-08-29 17:49, 99homer99 wrote:

and it took Dick 15 years to get to a planet that was 20 light years away

at 0.8c it should have taken him 20 devided by 0.8 = 25 years

is it me?

It does take Dick 25 years to get there, according to the way Jane measures time. From Dick's point of view, it only took 15 years because the space in Jane's frame is contracted. So the star looks 20 light years away to Jane, but for Dick, moving at 0.8c, it's only 15 light years away. Equivalently, Jane expects Dick to arrive looking 50 years older, but he is only 30 years older, because his time ran slower than hers.

The space-time diagram in that web page shows how this is true, even when you take light travel time into account. In other words, the time dilation is not due to a simple delay of signal propagation due to increasing distance, and it does not "undo itself" when Dick switches direction.

Don

99homer99
2002-Aug-30, 09:22 PM
sorry you can't convince me
that Dick is travelling faster than the light is, when he is travelling at 0.8c

you said:
So the star looks 20 light years away to Jane, but for Dick, moving at 0.8c, it's only 15 light years away.

You are actualy saying that dick gets to the planet before the light from earth has.

(he gets there in fifteen years and the light gets there in 20 years)

SeanF
2002-Aug-30, 09:52 PM
Homer, Dick's not travelling faster than the light. He says it only takes 15 years for him to get to the planet while Jane says it takes 25 years, but Dick says it only takes 12 years for light to get to the planet while Jane says it takes 20 years. In both points-of-view, the light gets there in 4/5 the time it takes Dick to get there.

It took me a while to get it at first, too . . .

DoctorDon
2002-Aug-31, 02:44 AM
On 2002-08-30 17:22, 99homer99 wrote:
sorry you can't convince me
that Dick is travelling faster than the light is, when he is travelling at 0.8c

Good. Cause he isn't.

you said:
So the star looks 20 light years away to Jane, but for Dick, moving at 0.8c, it's only 15 light years away.

You are actualy saying that dick gets to the planet before the light from earth has.

No, I never said that. Look at the diagram in that other web page again. Light emitted at the same time as Dick sets out (the line at the 45&deg; angle) will get to the planet before Dick does.

he gets there in fifteen years and the light gets there in 20 years)

He gets there in 15 years of his time. It's 25 years of Earth time.

From the light's point of view, it takes no time to get there at all, but Jane would observe a 40 year delay for the light to get there and come back.

Don

Wiley
2002-Sep-03, 04:23 PM
Don,

Let me get this straight. Different observers would measure different values: different times, different lengths, different velocities. In other words, it's all relative. (I bet they could come up with a catchy name for this theory.)

/phpBB/images/smiles/icon_smile.gif

AgoraBasta
2002-Sep-03, 05:18 PM
A simple recipe to resolve stupid paradoxes intro'ed by stupid relativity -
1. Build a local "absolute" ref frame by constructing a GPS in the area.
2. Proceed in flat Euclidean space with absolute time (relativists call it "imaginary" - hypocrites).

Wiley
2002-Sep-03, 05:34 PM
On 2002-09-03 13:18, AgoraBasta wrote:
A simple recipe to resolve stupid paradoxes intro'ed by stupid relativity -
1. Build a local "absolute" ref frame by constructing a GPS in the area.
2. Proceed in flat Euclidean space with absolute time (relativists call it "imaginary" - hypocrites).

1.) Realize GPS is based on general relativity.
2.) Pick up cards and build a new house.

AgoraBasta
2002-Sep-03, 06:27 PM
1.) Realize GPS is based on general relativity.
2.) Pick up cards and build a new house.

1. GPS is based on Lorentzian relativity, not even the SR, which (the LR) it agrees completely. Better yet, consider it based on objective reality.
2. You should do your homework a bit better.

<font size=-1>[ This Message was edited by: AgoraBasta on 2002-09-03 14:47 ]</font>

Wiley
2002-Sep-03, 06:36 PM
On 2002-09-03 14:27, AgoraBasta wrote:
1. GPS is based on Lorentzian relativity, not even the SR, which it agrees completely.
2. You should do your homework a bit better.

1.) There is no such thing as Lorentzian relativity. Lorentz's theory is based on a absolute reference frame. Just because the transformations between inertial frames are the same does not mean its relativity.
2.) GPS requires general relativity corrections. Here's your homework (http://www.eftaylor.com/pub/projecta.pdf).

AgoraBasta
2002-Sep-03, 07:05 PM
Wiley,

There is (or was) such thing as LR. At the time of launch, there was no clear understanding if GPS would even work, based on GR/SR. The paper you've linked is a postfactum make-up.
For the matters GPS, I tend to believe Van Flandern, no matter how horrible anathema he seems, for a simple reason that he actually participates in the project.

Notwithstanding all the above chat, GPS delivers a local "absolute" ref frame - that's a fact. For that very reason, analysis in flat space and absolute time is always possible - that's as simple as phylosophy.

MartinM
2002-Sep-03, 09:17 PM
On 2002-09-03 13:18, AgoraBasta wrote:
A simple recipe to resolve stupid paradoxes intro'ed by stupid relativity -
1. Build a local "absolute" ref frame by constructing a GPS in the area.
2. Proceed in flat Euclidean space with absolute time (relativists call it "imaginary" - hypocrites).

Hmph. You don't seem to have grasped GR at all.

1) Spacetime is locally flat, yes. That is precisely what relativity says. No cookie.

2) Euclidian, no. The co-ordinate transformation ct -> ict yields a metric which appears Euclidean. However, the signature of the metric is invariant under such a transformation. Thus, this 'Euclidean' metric is, in fact, still very much Lorentz. Still no cookie.

Try again.

99homer99
2002-Sep-03, 09:31 PM
On 2002-08-30 22:44, DoctorDon wrote:

On 2002-08-30 17:22, 99homer99 wrote:
sorry you can't convince me
that Dick is travelling faster than the light is, when he is travelling at 0.8c

Good. Cause he isn't.

you said:
So the star looks 20 light years away to Jane, but for Dick, moving at 0.8c, it's only 15 light years away.

You are actualy saying that dick gets to the planet before the light from earth has.

No, I never said that. Look at the diagram in that other web page again. Light emitted at the same time as Dick sets out (the line at the 45° angle) will get to the planet before Dick does.

he gets there in fifteen years and the light gets there in 20 years)

He gets there in 15 years of his time. It's 25 years of Earth time.

From the light's point of view, it takes no time to get there at all, but Jane would observe a 40 year delay for the light to get there and come back.

Don

----------------------------------------------------------------------------------------

The Web page did not explain -
where did the 15 years come from?

when the graph shows in my opinion that Dick hits the planet dead on 25 years
and he receives a signal every 5 years from Jane for 25 years and then on the way back he receives 9 signals every 5 years for 25 years etcetera

If the author of the graph is saying that the closer Dick gets to the speed of light the shorter the time until at the speed of light Dick is everywhere instantaneously and from his point of view the universe has stopped and frozen in time

I say the closer you get to the speed of light the closer you get to the speed of light

To help me understand the problem where did the 15 years come from?

<font size=-1>[ This Message was edited by: 99homer99 on 2002-09-03 17:47 ]</font>

99homer99
2002-Sep-03, 09:37 PM
On 2002-02-15 14:07, Christine112 wrote:
I can understand why this works. For the person traveling in the spaceship near the speed of light, time has slowed down so they age less than the one one Earth. But, to the person on the spaceship, time is running normal and the people on Earth appear to be moving more slowly. So wouldn't they expect to come back to Earth to find a younger twin?

Some say at the speed of light you are everywhere at the same time

SEE the post above

<font size=-1>[ This Message was edited by: 99homer99 on 2002-09-03 17:38 ]</font>

AgoraBasta
2002-Sep-03, 10:12 PM
On 2002-09-03 17:17, MartinM wrote:
1) Spacetime is locally flat, yes. That is precisely what relativity says. No cookie.

It can be "flattened" as globally as you wish.

2) Euclidian, no. The co-ordinate transformation ct -> ict yields a metric which appears Euclidean. However, the signature of the metric is invariant under such a transformation. Thus, this 'Euclidean' metric is, in fact, still very much Lorentz. Still no cookie.

Consider time not as a coordinate, but as an order of events, absolutely decoupled from spatial coordinates. This is possible due to definition of GPS instantaneity at any arbirary level of globality (not to mention quantum non-locality). Further, change definitions of length and speed to their "global" GPS meanings, instead of their relativistic local definitions.

To put it simple, fix GPS clocks in a grid of, say, one GPS-kilometer; then measure position/speed/time by watching those on a fly-by.

But really, it's the existence of instantaneity that kills the basic foundations of GR/SR by introducing absolute time, which effectively removes time from the metric. So 3-space and time are no longer intertwined and the 3-space can be kept flat in presence of gravity while speed of light slows.

Wiley
2002-Sep-03, 10:35 PM
On 2002-09-03 18:12, AgoraBasta wrote:

On 2002-09-03 17:17, MartinM wrote:
1) Spacetime is locally flat, yes. That is precisely what relativity says. No cookie.

It can be "flattened" as globally as you wish.

No. This is just wrong. The size of your locally flat spacetime is set by the curvature (strength of gravity) and the error you're willing to tolerate.

Consider time not as a coordinate, but as an order of events, absolutely decoupled from spatial coordinates.

Well, ya see, ya just can't around decoupling space and time all willy-nilly-like. One of the major implications of relativity is that time is a coordinate and must be treated as such.

This is possible due to definition of GPS instantaneity at any arbirary level of globality (not to mention quantum non-locality).

No. This is just wrong. See response to 1.)

The basic assumption of being able to separate space and time while maintaining accuracy in GPS is flawed. The rest of your arguments is like "Yahoo Serious Film Festival": each word by itself is understandable, but together they make no sense.

AgoraBasta
2002-Sep-03, 11:00 PM
On 2002-09-03 18:35, Wiley wrote:
No. This is just wrong. The size of your locally flat spacetime is set by the curvature (strength of gravity) and the error you're willing to tolerate.

You can always choose as close as your practical need requires. Works like that - oversize the area by a great factor, then cut out your sample area.

Well, ya see, ya just can't around decoupling space and time all willy-nilly-like. One of the major implications of relativity is that time is a coordinate and must be treated as such.

That was not an implied but postulated. You see, if we lived in a EM-only world, that'd be true. But we have instant interactions in 3-space, e.g. non-locality, gravity (instantaneous even at great accelerations), oscillating dipole fields in axial direction (experimentally proved instant in Walker-Dual experiments).

No. This is just wrong. See response to 1.)

Re-read my previous post. Or better yet, dig for some experimental facts elsewhere.
Obviously, your fallacies are not self-induced, you've been taught into them, so you could probably learn out of those.

Wiley
2002-Sep-03, 11:02 PM
On 2002-09-03 15:05, AgoraBasta wrote:
There is (or was) such thing as LR.

There is no such thing as LR and there never was. Lorentz's theory (LT) requires an ether that is stationary with respect to an absolute reference frame. Length contraction in LT is due to a physical force. There were two major objections to LT: 1.)what is the force that causes the contraction? 2.)we can never know our velocity with respect to the absolute frame. LT specifically violates the fundamental principle of (Gallilean or Einstein's) relativity: there is no preferred inertial frame.

At the time of launch, there was no clear understanding if GPS would even work, based on GR/SR. The paper you've linked is a postfactum make-up.

Not quite. The gravitational physicists said that GR was required, but the engineers doubted that. The engineers did it both ways. Initial the GR corrections were neglected, but after month, GPS was off exactly as predicted by GR. A switch was flipped, turning on the GR corrections and that switch has remained on.

For the matters GPS, I tend to believe Van Flandern, no matter how horrible anathema he seems, for a simple reason that he actually participates in the project.

Oh Lord, why would you do that? Van Flandern really does not understand GR nor much else. His "speed of gravity" display a fundamental lack of knowledge not only of GR but of wave mechanics. For thorough debunking of his GPS knowledge see Chris Hillman's analysis (http://math.ucr.edu/home/baez/RelWWW/wrong.html#gps). For a less technical debunk see Salon.com (http://dir.salon.com/people/feature/2000/07/06/einstein/index.html).

Notwithstanding all the above chat, GPS delivers a local "absolute" ref frame - that's a fact.

A local absolute reference frame is a contradiction in terms. Do you mean a locally flat reference frame?

Wiley
2002-Sep-04, 12:15 AM
On 2002-09-03 19:00, AgoraBasta wrote:
You can always choose as close as your practical need requires. Works like that - oversize the area by a great factor, then cut out your sample area.

Oversized area includes the curvature. This does no good.

That was not an implied but postulated. You see, if we lived in a EM-only world, that'd be true. But we have instant interactions in 3-space, e.g. non-locality, gravity (instantaneous even at great accelerations), oscillating dipole fields in axial direction (experimentally proved instant in Walker-Dual experiments).

I see you're a Van Flandern disciple. Instead of mindlessly parroting misinformation, I suggest you do you're own research. For instance you would know that the Walker-Dual experiments never experimentally proved information can be transmitted instantaneously. Phase velocity if very different from group velocity, and it's the latter speed the information is transmitted. They incorrectly derived group velocity: homework problem number 2, find it.

<font size=-1>[ This Message was edited by: Wiley on 2002-09-03 20:19 ]</font>

DoctorDon
2002-Sep-04, 04:30 AM
The Web page did not explain -
where did the 15 years come from?

Let's see if I can find the graph...

...okay, there're two graphs. The first one shows Jane sending out signals every year, but the important one for our purposes is the second, which shows Dick sending out signals once a year:

http://www.physics.hku.hk/~tboyce/modern_physics/topics/images/signal7.jpg

Because Dick is traveling at an angle relative to Jane (moving straight up on the diagram), Dick sends out 15 signals on the outbound trip (which Jane receives over 45 years of her time -- the y-axis). I.e., it takes him 15 years of his time to get to a target that Jane perceives as being 20 light years away (x-axis).

when the graph shows in my opinion that Dick hits the planet dead on 25 years

25 years according to Jane, yes.

and he receives a signal every 5 years from Jane for 25 years and then on the way back he receives 9 signals every 5 years for 25 years etcetera

No. Now we need to look at the first graph:

http://www.physics.hku.hk/~tboyce/modern_physics/topics/images/signal6.jpg

You will note that it points out on the right that Dick receives 1 signal every three years on the way out and 3 signals every year on the way back.

If the author of the graph is saying that the closer Dick gets to the speed of light the shorter the time until at the speed of light Dick is everywhere instantaneously and from his point of view the universe has stopped and frozen in time

... then what? You said "if the author", but never finished the sentence. But this is essentially correct, yes.

I say the closer you get to the speed of light the closer you get to the speed of light

To help me understand the problem where did the 15 years come from?

One way to look at it is that it's just from the length contraction. At v=0.8c, the Lorentz factor (1/sqrt(1-(v/c)<sup>2</sup>)) is 1.667, so 20 light years of distance shortens to 12, and at 0.8c it takes 15 years to cover 12 light years. That's not the only way to think about it, but it's perfectly valid.

Yours,

Don

AgoraBasta
2002-Sep-04, 10:25 AM
On 2002-09-03 20:15, Wiley wrote:
Oversized area includes the curvature. This does no good.

The greater the oversized area, the greater the precision of accounting for the background "curvature" in your sample area. Substitute "curvature" with force field.

I see you're a Van Flandern disciple. Instead of mindlessly parroting misinformation, I suggest you do you're own research. For instance you would know that the Walker-Dual experiments never experimentally proved information can be transmitted instantaneously. Phase velocity if very different from group velocity, and it's the latter speed the information is transmitted. They incorrectly derived group velocity: homework problem number 2, find it.

You are demonstrating classic examples of logical fallacies used for indecent purpose.
Calling me "Van Flandern's disciple" (which I'm not, btw) doesn't prove that I'm wrong or bad. Van Flandern being often wrong doesn't mean he's always wrong; finding a few grave mistakes and several dumb misunderstandings in his works does not invalidate all of his arguments. Wrong is part doesn't mean forever wrong in whole.
Miscalculations in (EM-variation of) Walker-Dual interpretation don't invalidate the experimental result (infinite phase velocity) which, together with zero EM axial component, leaves the instantaneous virtual longitudinal photons as the only possible physical carrier for the field (there's absolutely no real photons on the axis). If you propose finite group velocity of axial field, then modulated signal's energy would radiate in axial dimension primarily, which is not the case for the simple reason that there's no physical carrier for such a process (no real longitudinal photon.)
And what could one do with Guenter Nimtz's results, after all??

GrapesOfWrath
2002-Sep-04, 10:39 AM
On 2002-09-04 06:25, AgoraBasta wrote:
You are demonstrating classic examples of logical fallacies used for indecent purpose.
Calling me "Van Flandern's disciple" (which I'm not, btw) doesn't prove that I'm wrong or bad. Van Flandern being often wrong doesn't mean he's always wrong; finding a few grave mistakes and several dumb misunderstandings in his works does not invalidate all of his arguments. Wrong is part doesn't mean forever wrong in whole.

But it shows a severe bias. Check out the face on Mars discussion (http://www.metaresearch.org/solar%20system/cydonia/proof_files/proof.asp): "Before I studied image processing myself, I worried that the biases of the person doing the processing might contribute significantly to the image seen. Now that I am more familiar with the process, I can see that it uses objective, standardized computer techniques, and does not add features to an image that are not present in the original. The techniques used are more like focusing a camera – they change the camera’s view to one more like what the human eye would see if viewing directly. The exception is the left portion of the east (right-side) eye, which was hidden behind the nose ridge, and for which no data exists other than that in Figure 1. It was therefore filled out artistically by assuming symmetry with the other eye socket."

Just look at the leftmost part of that figure.

AgoraBasta
2002-Sep-04, 11:58 AM
On 2002-09-04 06:39, GrapesOfWrath wrote:
But it shows a severe bias. Check out the face on Mars discussion (http://www.metaresearch.org/solar%20system/cydonia/proof_files/proof.asp):...

So what? That seems to me as an example of wishfull thinking of the author. Still there's a lot of very peculiar features on the Mars surface in need of further investigation. Does it mean the guy is *always* and forever wrong???

GrapesOfWrath
2002-Sep-04, 12:09 PM
On 2002-09-04 07:58, AgoraBasta wrote:
So what? That seems to me as an example of wishfull thinking of the author.

The author is Van Flandern, right?

Still there's a lot of very peculiar features on the Mars surface in need of further investigation. Does it mean the guy is *always* and forever wrong???

No of course not. Check out the footnote at the bottom of that page.

AgoraBasta
2002-Sep-04, 12:34 PM
On 2002-09-03 19:02, Wiley wrote:
For thorough debunking of his GPS knowledge see Chris Hillman's analysis (http://math.ucr.edu/home/baez/RelWWW/wrong.html#gps).

Your link and Van Flandern's writings, when put together, make a great example of a "blind with deaf" dialog. Factually, both parties are right.
The true matter of the question is quite different, i.e. - is it really necessary to use GR/SR formalism to describe reality, or there is possibility to preserve Euclidean metric with an independent absolute time? GR produces good results with a lot of unnecessary effort since it bases on non-physical assumptions. Same results can be obtained upon far more sane basis with greater ease.
It's quite possible to get a GR-equivalent from "quantum aether" of varying density rigidly attached to local gravity.
In fact, it's been shown that GR-like configurations may arise from quite various low-level physical processes, read here http://stacks.iop.org/0264-9381/18/3595.

<font size=-1>[ This Message was edited by: AgoraBasta on 2002-09-04 08:36 ]</font>

AgoraBasta
2002-Sep-04, 01:05 PM
On 2002-09-04 08:09, GrapesOfWrath wrote:
The author is Van Flandern, right?

Right. His "proof of artificiality" doesn't work for me since I can imagine a natural process forming a face-like feature. He is certainly biased, just like every human being of beliefs and wishes. Most of those critisizing him don't distinguish truths from fallacies in his works and simply label him to their taste. That's what is logically fallacious and morally indecent.

Wiley
2002-Sep-04, 07:57 PM
On 2002-09-04 06:25, AgoraBasta wrote:
You are demonstrating classic examples of logical fallacies used for indecent purpose.
Calling me "Van Flandern's disciple" (which I'm not, btw) doesn't prove that I'm wrong or bad. Van Flandern being often wrong doesn't mean he's always wrong; finding a few grave mistakes and several dumb misunderstandings in his works does not invalidate all of his arguments. Wrong is part doesn't mean forever wrong in whole.

True. However I believe my characterization is accurate. You've continually demonstrated your lack of knowledge and have echoed the same mistakes Van Flandern makes regarding the speed of light and the speed of gravity. I stand by my "mindless parroting" description.

To illustrate my point, let's analyze your second paragraph.

Miscalculations in (EM-variation of) Walker-Dual interpretation don't invalidate the experimental result (infinite phase velocity) ...

There are at least three things wrong with this statement.
1.) Walker-Dual experiments do not measure EM anything, they are measuring gravity. An EM derivation is only done as an analogy between EM fields and gravitational fields.
2.) They don't predict infinite phase velocity. They predict a phase velocity of c^3/(2*pi*f*R) which is too fast to measure in a laboratory setting.
3.) Phase velocity is not the speed information is transmitted. It is not unusual to have a phase velocity greater than c; however it is the group velocity that is important. Information is transmitted at the group velocity (if dispersion is low, which it is for this case).

... which, together with zero EM axial component, ...

The axial component of the electric field is non-zero. The field does not radiate (it decays at ~1/R^3), but it is definitely non-zero. The main thrust of the Walker-Dual EM derivation is that it is valid in the near field where the non-radiative fields are important. In fact, it is from the axial component that they derive the phase velocity.

leaves the instantaneous virtual longitudinal photons as the only possible physical carrier for the field (there's absolutely no real photons on the axis).

1.)The process is completely descriped by wave mechanics; we don't need to bring photons into this.
2.)There is neither emission nor absorbtion occurring. Virtual photons are completely irrelevant for this case.

If you propose finite group velocity of axial field ...

Well, I wish I could take credit for this but I can't. A group velocity equal to c inheres to Maxwell's equation (in free space). Electromagnetic fields travel at c; it's just that simple.

...then modulated signal's energy would radiate in axial dimension primarily,

No. Walker-Dual propose a amplitude modulation scheme. So the direction of the acceleration of the charge and consequently the radiative field does not change. (Not that the radiative field is important in this case.)

which is not the case for the simple reason that there's no physical carrier for such a process (no real longitudinal photon.)

I think you need a refresher on the difference between virtual and real photons.