1. 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.

2. 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?

3. 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.

4. 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! [img]/phpBB/images/smiles/icon_smile.gif[/img]

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.

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

5. I'll email you.

6. 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! [img]/phpBB/images/smiles/icon_smile.gif[/img]
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 [img]/phpBB/images/smiles/icon_smile.gif[/img]

7. 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! [img]/phpBB/images/smiles/icon_smile.gif[/img]

8. 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? [img]/phpBB/images/smiles/icon_smile.gif[/img]

<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>

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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.

10. 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?

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

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!).

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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>

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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.

14. 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?

15. 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. [img]/phpBB/images/smiles/icon_smile.gif[/img]

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.

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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>

17. 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, Bride of Twins Paradox, and Son of Twins Paradox. 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.

18. 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. [img]/phpBB/images/smiles/icon_smile.gif[/img]

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!

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

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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 [img]/phpBB/images/smiles/icon_wink.gif[/img] 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>

20. 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.

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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>

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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]
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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>

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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.

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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.

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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.

27. You're in heady company, David Hall. He calls Einstein childish, too.

28. 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.

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

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Polish Geocentrist certainly sounds like substantial credentials. [img]/phpBB/images/smiles/icon_rolleyes.gif[/img]

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.

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Huh?

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

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