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uncommonsense
2010-Mar-16, 10:15 PM
Probably an elementary relativity question but,
From the relative perspective of a particle moving at the speed of light, is distance a constant?

grav
2010-Mar-16, 10:31 PM
Probably an elementary relativity question but,
From the relative perspective of a particle moving at the speed of light, is distance a constant?
It is not clear what you are asking, but as long as the speed of a particle remains constant and thereby inertial without any acceleration applied, distances will remain constant in the particle's frame also, yes, although different to what would be measured in other frames of reference for speeds under c.

macaw
2010-Mar-16, 10:34 PM
Probably an elementary relativity question but,
From the relative perspective of a particle moving at the speed of light, is distance a constant?

The distance beween two points in frame F is d.
According to special relativity an observer moving at speed v wrt frame F measures the distance between the two points to be d'=d*sqrt(1-(v/c)^2).
This is the so-called "length contraction". To answer your question, make v=c.

uncommonsense
2010-Mar-16, 10:45 PM
It's hard to imagine, but yes, I was referring to a particle that has obtained c. Difficult to understand because at that instant, all places in the universe are at an equal distance from the particle - as if the particle was, relatively, at the center of a universe where all points in the universe were located on the surface of a spherical shell and the particle was at the exact center of the sphere. The particle can "move" to all places with same "effort". So then, how does it "choose" where to go?

grav
2010-Mar-16, 10:58 PM
It's hard to imagine, but yes, I was referring to a particle that has obtained c. Difficult to understand because at that instant, all places in the universe are at an equal distance from the particle - as if the particle was, relatively, at the center of a universe where all points in the universe were located on the surface of a spherical shell and the particle was at the exact center of the sphere. The particle can "move" to all places with same "effort". So then, how does it "choose" where to go?Consider it from our point of view of a ship that travels away at c. The time that passes for the clocks and passengers on the ship has frozen from our perspective, so they are in stasis as far as we are concerned, and we can see that the ship can travel from point A to point B in some time on our own clock, but no time has passed for the clocks and passengers on the ship, so from their perspective they have travelled instantly. Since the relative speed cannot be greater than c to the passengers of the ship either, so cannot be infinite, then the distance measured to have been travelled is zero also from the perspective of the ship observers, whereas zero distance divided by zero time is any real value for the speed, which is c in this case. SR puts things in a better perspective for speeds under c, though, rather than working through infinities, so it is best when actually attempting to do thought experiments and work through the math to use some speed that is perhaps close but still less than c.

uncommonsense
2010-Mar-16, 11:16 PM
Consider it from our point of view of a ship that travels away at c. The time that passes for the clocks and passengers on the ship has frozen from our perspective, so they are in stasis as far as we are concerned, and we can see that the ship can travel from point A to point B in some time on our own clock, but no time has passed for the clocks and passengers on the ship, so from their perspective they have travelled instantly. Since the relative speed cannot be greater than c to the passengers of the ship either, so cannot be infinite, then the distance measured to have been travelled is zero also from the perspective of the ship observers, whereas zero distance divided by zero time is any real value for the speed, which is c in this case. SR puts things in a better perspective for speeds under c, though, rather than working through infinities, so it is best when actually attempting to do thought experiments and work through the math to use some speed that is perhaps close but still less than c.

Thank you. I guess i'm hung up on what happens from the perspective of any particle reaching c. It seems that any consideration of a particle going speed of c leads to imagining that the particle, from it's perspective, is no longer moving, as distance is a constant and time is 0, and so the particle is at all places at once, as is the universe. Have I imagined too far out?

grav
2010-Mar-16, 11:18 PM
One might also wonder how time can pass for us as a particle such as a photon travels from point A to point B when from the particle's perspective, our clocks are also frozen. A photon sees all clocks always frozen at the same time they were when the photon was emitted at c, but due to the relativity of simultaneity, observers still say it took some time for the photon to travel from one position to another. For instance, using speeds just under c for better clarity, all passengers on a train will say that all of their clocks are synchronized, but an observer standing on a platform watching the train whip by at near c will say that their clocks are simultaneity shifted to greater times for the readings on the clocks at the back of the train and lesser times on the clocks at the front of the train, although due to the relative speed, the platform observer will say that all of the clocks are barely ticking at all, pretty much frozen. But although the clocks are frozen and the platform observer considers himself to be standing still, as the train goes by each clock he reads on the train directly in front of him will read a greater time as the train passes due to the relativity of simultaneity for the readings on the clocks along the length of the train. So to the passengers on the train, it is the platform observer's time that is nearly frozen, but their own clocks will read greater and greater times as each clock passes the platform observer directly, same as the platform observer observed directly for the clocks that coincided in front of him as the train passed, but to the passengers, time is passing normally as the clocks in the back read a greater time when they see the platform observer pass than the clocks in the front when the platform observer passed those.

grav
2010-Mar-16, 11:31 PM
Thank you. I guess i'm hung up on what happens from the perspective of any particle reaching c. It seems that any consideration of a particle going speed of c leads to imagining that the particle, from it's perspective, is no longer moving, as distance is a constant and time is 0, and so the particle is at all places at once, as is the universe. Have I imagined too far out?Everybody wonders about the perception of a photon eventually, but one has to be very careful when working directly with infinities in this way, since it is very easy to get caught up, so the best way to go about it is to work with some speed that is just under c to help visualize what is taking place and then slowly push it toward the limit.

DrRocket
2010-Mar-16, 11:34 PM
It's hard to imagine, but yes, I was referring to a particle that has obtained c. Difficult to understand because at that instant, all places in the universe are at an equal distance from the particle - as if the particle was, relatively, at the center of a universe where all points in the universe were located on the surface of a spherical shell and the particle was at the exact center of the sphere. The particle can "move" to all places with same "effort". So then, how does it "choose" where to go?

That is not quite right.

Length contraction occurs only along the axis of motion. Distances along an axis transverse to velocity vector are unchanged. That is all part of the Lorentz transformation.

It is important to look at the complete group of Lorentz transformations and see what they imply. The elementary description in terms of length contractions and time dilations can become quite confusing unless you are used to them.

The bottom line is that applying the Lorentz transformations to the "reference frame of a photon" results in singularities because of terms that then have a zero denominator. The reference frame of a photon is not an acceptable inertial reference frame.

uncommonsense
2010-Mar-16, 11:36 PM
One might also wonder how time can pass for us as a particle such as a photon travels from point A to point B when from the particle's perspective, our clocks are also frozen. A photon sees all clocks always frozen at the same time they were when the photon was emitted at c, but due to the relativity of simultaneity, observers still say it took some time for the photon to travel from one position to another. For instance, using speeds just under c for better clarity, all passengers on a train will say that all of their clocks are synchronized, but an observer standing on a platform watching the train whip by at near c will say that their clocks are simultaneity shifted to greater times for the readings on the clocks at the back of the train and lesser times on the clocks at the front of the train, although due to the relative speed, the platform observer will say that all of the clocks are barely ticking at all, pretty much frozen. But although the clocks are frozen and the platform observer considers himself to be standing still, as the train goes by each clock he reads on the train directly in front of him will read a greater time as the train passes due to the relativity of simultaneity for the readings on the clocks along the length of the train. So to the passengers on the train, it is the platform observer's time that is nearly frozen, but their own clocks will read greater and greater times as each clock passes the platform observer directly, same as the platform observer observed directly for the clocks that coincided in front of him as the train passed, but to the passengers, time is passing normally as the clocks in the back read a greater time when they see the platform observer pass than the clocks in the front when the platform observer passed those.

Thank you. I can grasp relativity of simulaneity. It makes sense.

I am trying to self study relativity but I can't quite grasp the distance issues I mentioned above. Most discussions and explanations on relativity center upon time. Time is a rate. Obviously, f a rate is relative it is due to the relativity of its components - so I am looking closer at distance and space(kinda synonymous).

If we accept that a particle can reach c, then we must understand how that particle deals with a universe with all points in space at a single place, and therefore time is 0, or the other way around, any thoughts?

grav
2010-Mar-16, 11:43 PM
Thank you. I can grasp relativity of simulaneity. It makes sense.

I am trying to self study relativity but I can't quite grasp the distance issues I mentioned above. Most discussions and explanations on relativity center upon time. Time is a rate. Obviously, f a rate is relative it is due to the relativity of its components - so I am looking closer at distance and space(kinda synonymous).

If we accept that a particle can reach c, then we must understand how that particle deals with a universe with all points in space at a single place, and therefore time is 0, or the other way around, any thoughts?DrRocket answered that one well in the last post. From the perspective of the photon, the universe is a flat plane, so still exists in two dimensions.

uncommonsense
2010-Mar-16, 11:45 PM
That is not quite right.

Length contraction occurs only along the axis of motion. Distances along an axis transverse to velocity vector are unchanged. That is all part of the Lorentz transformation.

It is important to look at the complete group of Lorentz transformations and see what they imply. The elementary description in terms of length contractions and time dilations can become quite confusing unless you are used to them.

The bottom line is that applying the Lorentz transformations to the "reference frame of a photon" results in singularities because of terms that then have a zero denominator. The reference frame of a photon is not an acceptable inertial reference frame.

That make sense, but now I am imagining that the particle experiences O timke and distance, and space for that matter, only along one axis, and so it now exists in a 2 dimentional universe???

ETA I was posting at same time as last response

uncommonsense
2010-Mar-17, 12:01 AM
Thank you. Excellent responses. I am fascinated.....

DrRocket
2010-Mar-17, 02:51 AM
That make sense, but now I am imagining that the particle experiences O time and distance, and space for that matter, only along one axis, and so it now exists in a 2 dimensional universe???

ETA I was posting at same time as last response

You are trying to impose the prejudices of everyday experience and language on special relativity. That is not only difficult and confusing, it misses the major point of Einstein's theory, and that is that you must deal not with space and time but with a merger of the two - space time.

One observer's time is to another observer a mixture of both space and time. You cannot cleanly separate space from time, except in the reference frame of a single observer. General relativity tells us that you cannot separate them even for a single observer, except as a local approximation.
Special relativity is the local approximation to general relativity.

All of this requires some detailed study of the Lorentz group, the group of linear transformations that preserves the Minkowski metric or quadratic form. There is a nice treatment of this subject in the book The Geometry of Minkowski Space time by Gregory Naber.

In a good treatment of the geometry of special relativity you will find an invariant treatment of the transformations and see the subtle way in which space and time are woven together in a single entity. It helps to overcome the confusion that arises from looking at "time dilation" and "length contraction" alone. They are merely coordinate effects that come from what is really preserved, the Lorentzian length of an interval.

One sign of confusion is the problem that you are having with a photon. You simply cannot apply the transformations in the frame of a photon, for there is no such thing. No experiment can be performed, even in principle, in such a frame. The theory does not address it either, as the transformations would be singular/undefined.

DrRocket
2010-Mar-17, 02:56 AM
If we accept that a particle can reach c, then we must understand how that particle deals with a universe with all points in space at a single place, and therefore time is 0, or the other way around, any thoughts?

Only particles of 0 rest mass can reach c, and they cannot travel at any other speed.

You do not have to understand how that particle deals with the universe. The theory does not address that issue. It predicts the outcome of experiments conducted in reference frames in which experiments can be conducted.

forrest noble
2010-Mar-17, 03:18 AM
uncommonsense,

Distances at c

Probably an elementary relativity question but, from the relative perspective of a particle moving at the speed of light, is distance a constant?

Thank you. Excellent responses. I am fascinated.....

Maybe by your last response your question has been answered. Thought I'd mention a couple of points. Using the word particle implies that the entity has mass. however you could mean particle of light meaning a photon.

If you are talking about atomic particles, or other massive particles, to continue traveling at close to the speed of light the particle would need continuous acceleration (see correction below). As to distance, according to the BB model, distance in the past and present has the same measure regardless of the perspective of matter moving through it so that distances would remain constant regardless of the motion or perspective of mass within it. Accordingly, if a particle is traveling at a "constant speed" considering both length contraction and time dilation, the distance traveled per unit of time would accordingly remain constant from any single SR perspective, regardless of any constant speed.

WayneFrancis
2010-Mar-17, 04:20 AM
It's hard to imagine, but yes, I was referring to a particle that has obtained c. Difficult to understand because at that instant, all places in the universe are at an equal distance from the particle - as if the particle was, relatively, at the center of a universe where all points in the universe were located on the surface of a spherical shell and the particle was at the exact center of the sphere. The particle can "move" to all places with same "effort". So then, how does it "choose" where to go?

Umm don't think this is right. If you go by the reference frame of the object travelling at c then the universe collapses in the direction of motion to 0 length, Time stops for said object and in essence would then be in a 2D universe. For the particle to move to a point perpendicular to its direction of travel it would have to accelerate in that direction.

DrRocket
2010-Mar-17, 04:36 AM
If you are talking about atomic particles, or other massive particles, to continue traveling at close to the speed of light the particle would need continuous acceleration.

No

Acceleration is a change in velocity. A particle traveling at a fixed speed in a straight line (a fixed direction) does not accelerate.

forrest noble
2010-Mar-17, 04:53 AM
Dr. Rocket,

Thanks; I meant force and velocity (not acceleration); a continuous force to maintain speed/velocity.

uncommonsense
2010-Mar-17, 05:09 AM
I'm getting alot of great information. Another question based upon prior answers: If travel along single axis at c eliminates the experience of that dimention for the traveler, does light "bending" around a planet/sun (whatever), thereby moving on a 2 dimentional path at speed of c, result in the light experiencing the universe in only 1 dimention?

macaw
2010-Mar-17, 05:15 AM
Only particles of 0 rest mass can reach c,

Careful, this has been discovered experimentally not to be the case. See here (http://en.wikipedia.org/wiki/Neutrino#Speed)

forrest noble
2010-Mar-17, 05:41 AM
uncommonsense,

If travel along single axis at c eliminates the experience of that dimention for the traveler, does light "bending" around a planet/sun (whatever), thereby moving on a 2 dimentional path at speed of c, result in the light experiencing the universe in only 1 dimention?

Light traveling past a star or galaxy will bend toward that gravitational influence in a curve within a single plane from a single axis. If the center of gravity of the object changes over the path, such as when traversing or passing by a galaxy, the curve can become multi-plainer, multi-axial, and 3 dimensional over its course.

As the light passes close to any star it would probably change its previous plainer course.

sirius0
2010-Mar-17, 05:54 AM
Careful, this has been discovered experimentally not to be the case. See here (http://en.wikipedia.org/wiki/Neutrino#Speed)

That seems like quite an assertion when the article you linked to makes no mention of rest mass. Exactly the term DrRocket used.

uncommonsense
2010-Mar-17, 06:30 AM
You are trying to impose the prejudices of everyday experience and language on special relativity. That is not only difficult and confusing, it misses the major point of Einstein's theory, and that is that you must deal not with space and time but with a merger of the two - space time.

One observer's time is to another observer a mixture of both space and time. You cannot cleanly separate space from time, except in the reference frame of a single observer. General relativity tells us that you cannot separate them even for a single observer, except as a local approximation.
Special relativity is the local approximation to general relativity.

I don't disagree that space time is at the center of most relativity discussions, and that the major point is merger of space and time.

But, time has no inherrent aspects -rather, it is a rate that compares the movement of one thing thru space to the movement of another. It compares distances. Thats it. So I find it clearer to imagine the merger of movement and distance. Maybe I shouldn't.

macaw
2010-Mar-17, 02:18 PM
That seems like quite an assertion when the article you linked to makes no mention of rest mass. Exactly the term DrRocket used.

You need to understand the article first, comment after. For people knowing physics, the article is quite clear.

Tensor
2010-Mar-17, 03:38 PM
But, time has no inherrent aspects -rather, it is a rate that compares the movement of one thing thru space to the movement of another. It compares distances. Thats it. So I find it clearer to imagine the merger of movement and distance. Maybe I shouldn't.

This may be at the root of your problem. You're thinking of time classically, as something separate from space. In spacetime, a change in time is a movement through spacetime. Each event in spacetime requires four coordinates, three space, one time. As the time coordinate of a partcle changes, that particle has moved through spacetime. It matters not whether the space coordinates have changed.

DrRocket
2010-Mar-17, 04:15 PM
Careful, this has been discovered experimentally not to be the case. See here (http://en.wikipedia.org/wiki/Neutrino#Speed)

??

The link simply says that if neutrinos have 0 rest mass then they travel at c and if they have positive rest mass (which they apparently do according to recent experiment) then they travel at less than c. That is no contradiction.

DrRocket
2010-Mar-17, 04:17 PM
Dr. Rocket,

Thanks; I meant force and velocity (not acceleration); a continuous force to maintain speed/velocity.

No, because if you have force then you have acceleration. Force will change the speed unless it is applied normal to the velocity in which case it merely changes direction (as in circular motion).

Ken G
2010-Mar-17, 04:20 PM
Careful, this has been discovered experimentally not to be the case. See here (http://en.wikipedia.org/wiki/Neutrino#Speed)That article in no way contradicts what DrRocket said. Clearly, the issue in that article is simply that speed can never be accurately enough measured to determine whether or not any given particle has zero rest mass, whenever the rest mass is on the scale of tens of MeV or less. Nevertheless, we expect a particle to require exactly zero rest mass to move at exactly c in a perfect vacuum. As none of those three things are ever establishable in practice, we will always have uncertainties in practice that are nevertheless not built into our best theory, the latter being what DrRocket was commenting on.

Ken G
2010-Mar-17, 04:41 PM
From the relative perspective of a particle moving at the speed of light, is distance a constant?
Let's go back to this, because I think this is the source of your problem. This question is fundamentally inconsistent with itself, and not because of the reference to c-- rather, because of the use of the "relative perspective of a particle" being used along with the word "moving." That's inconsistent, right there. As soon as you talk about the relative perspective of a particle, it is part of that perspective that the particle is not moving. So a moving particle has no relative perspective, it's meaningless-- only stationary particles have "relative perspectives."

So what are you really asking? You are asking about the perspective of a particle that is stationary, that perceives a bunch of other things that are moving, and asking to make a comparison between that particle's perspective, and the perspective of some other moving particle (in the latter particle's frame, where it is of course stationary). So you are asking to compare the frames of reference of two particles, each stationary in their own perspective, but in relative motion-- where the relative motion is c. That situation must exhibit reciprocity, meaning that you cannot tell from the answer to the question which one is "really moving"-- so if one particle sees "contracted space", then so does the other.

This can get confusing. The first problem you face here has been pointed out-- the singularity of c. So don't start with a relative motion of c, use some v very close to c, and just take the limit as v goes to c. Will anything unusual happen during that limiting process? No, it's all the same, at every step along the way, so the limit is not particularly insightful and it is really the steps that tell you what you want to know.

Consider again the reciprocity. Let's imagine two particles approaching each other at some v very close to c, and imagine a football field with lines on it that is at the intermediate speed, meaning that in the frame of the football field, both particles appear to be moving at the same speed. That speed is not v/2, it will be close to c. But more to the point, both particles will see the lines on the football field to be highly contracted in the direction the field is moving, and both will think that is happening not because the particle is moving (it isn't in that perspective), but because the field itself is moving.

So if we have a particle that sees everything in the universe as moving in a single direction at a speed very close to c, then distances between the moving stuff in that universe will be contracted, but not because the observer is moving, but because the stuff in that universe is moving. In other words, that observer may conceptualize a perfectly stationary universe in which distances are not contracted in any way, but within that stationary universe is a bunch of moving stuff, and the moving stuff is very contracted in the direction of motion, because of its motion.

So what about the limit as the motion goes to c? The stuff just gets more contracted, if there is a finite amount of it (if there is an infinite amount, there continues to be an infinite amount, covering an infinite distance at all stages of the limit). So if there is a finite amount of stuff observed, it gets contracted to a very thin shell as v goes to c, but it's all seen as a consequence of the fact that the stuff is moving-- there could also be some stuff perceived as not moving that would not be contracted at all. So it really isn't true that the universe itself contracts for particles that are moving near c relative to us-- all we can say is that the stuff we see in the universe that we don't see as moving all that fast would appear to have a highly compressed density to that other observer, but it would be a manifestation of the motion of the stuff-- that observer would be living in a universe where the stuff in the universe (not the space or time of the universe) would have a curious property of moving very fast and being very contracted. There could still be other stuff that neither. Such an observer would say think "how curious that I'm moving so fast the universe has a bunch of really contracted stuff in it", such an observer would just think "I wonder why that huge amount of superdense stuff is moving so fast that it is so highly contracted relative to the rather low average density that it perceives itself has having?"

And all this is special relativity language-- when talking about the "universe" that we now perceive, one would really have to be using GR language, and I have no idea what that would sound like to an observer moving relative to us at speeds closer and closer to c.

Andrew D
2010-Mar-17, 04:57 PM
DrRocket answered that one well in the last post. From the perspective of the photon, the universe is a flat plane, so still exists in two dimensions.

Are curves on that plane space-like or time-like? My intuition is telling me that the plane defines space-like.

I'm getting alot of great information. Another question based upon prior answers: If travel along single axis at c eliminates the experience of that dimention for the traveler, does light "bending" around a planet/sun (whatever), thereby moving on a 2 dimentional path at speed of c, result in the light experiencing the universe in only 1 dimention?

Light doesn't 'bend' through space-time, just space. It travels on geodesics through spacetime, which are defined by the mass the light is travelling past. The photon doesn't require acceleration on any 'space-time axis' to curve around the planet/star. (Right?)

macaw
2010-Mar-17, 05:03 PM
??

The link simply says that if neutrinos have 0 rest mass then they travel at c and if they have positive rest mass (which they apparently do according to recent experiment) then they travel at less than c. That is no contradiction.
This is not what the article is saying, please read carefully:

"The same observation was made, on a somewhat larger scale, with supernova 1987a. The neutrinos from the supernova were detected within a time window that was consistent with a speed of light for the neutrinos. So far, the question of neutrino masses cannot be decided based on measurements of the neutrino speed."

Clearly, the latest experimental evidence dissociates "can travel at c" from "only if it has zero (rest) mass".

Ken G
2010-Mar-17, 05:11 PM
This is not what the article is saying, please read carefully:

"The same observation was made, on a somewhat larger scale, with supernova 1987a. The neutrinos from the supernova were detected within a time window that was consistent with a speed of light for the neutrinos. So far, the question of neutrino masses cannot be decided based on measurements of the neutrino speed."

We are still unclear on why you see any contradiction there. We can all agree that the rest mass of an object cannot be determined by measuring its speed, whenever that rest mass is below a few tens of MeV. Speed measurements are simply not precise enough. But none of that relates to DrRocket's statement, which was about our best current theory, not about observations. The observations you refer to do not contradict our best current theory, they just point to limitations in how we can test it, and I think DrRocket understands that just fine. He's a mathematician-- they are always more interested in the theory than on how certain we can be that the theory is correct (never as certain as we'd like, basically), so it's natural that his comments would be directed there.

macaw
2010-Mar-17, 05:13 PM
I think DrRocket understands that just fine. He's a mathematician-- they are always more interested in the theory, so it's natural that his comments would be directed there.

Irrelevant, I am an experimental physicist, I am not challenging DrRocket's understanding, I am just pointing out that his statement is not quite correct in the light of the latest experimental discoveries.

macaw
2010-Mar-17, 05:14 PM
We are still unclear on why you see any contradiction there. We can all agree that the rest mass of an object cannot be determined by measuring its speed, whenever that rest mass is below a few tens of MeV. Speed measurements are simply not precise enough.

Nothing to do with precision.

Ken G
2010-Mar-17, 05:15 PM
Then no one is in disagreement. We have a theory, it says what DrRocket said it says, and we don't know if the theory is completely correct. That is often the case. (And of course it has to do with precision, the article you pointed to is all about precision of the measurement.)

This sidebar is getting nowhere. You are nitpicking an irrelevant point, but we can all agree it is interesting that we don't really know if zero rest-mass particles actually exist or actually move at exactly c, nor will anyone ever know that (which is why it is so irrelevant).

macaw
2010-Mar-17, 05:17 PM
Then no one is in disagreement. We have a theory, it says what DrRocket said it says, and we don't know if the theory is completely correct. That is often the case.

Not exactly, the latest experimental evidence dissociates "can travel at c" from "only if it has zero (rest) mass". This indicates that we need to look into revising the current theory.

Ken G
2010-Mar-17, 05:21 PM
Not exactly, the latest experimental evidence dissociates "can travel at c" from "only if it has zero (rest) mass". This indicates that we need to look into revising the current theory.You are claiming that the current experiment that you cited suggests we need to revise relativity? That's absurd. You must think those people are on their way to the Nobel prize for simply not being able to make a precise enough speed measurement to tell the difference between c and very close to c.

Neutrinos are thought to have rest mass less than 50 MeV, and most physicists expect them to have rest mass less than 1 eV. So when the article says that the speed measurement is not precise enough to tell if it is closer to c than a particle with rest mass less than 50 MeV would have, it means the measurement says nothing at all if we already expect the neutrino to have rest mass less than 1 eV. No revision to relativity is forthcoming.

macaw
2010-Mar-17, 05:26 PM
You are claiming that the current experiment that you cited suggests we need to revise relativity? That's absurd. You must think those people are on their way to the Nobel prize for simply not being able to make a precise enough speed measurement to tell the difference between c and very close to c.

This is not what I am saying. You fail to understand the nuances.

Ken G
2010-Mar-17, 05:27 PM
Then please explain what "revisions to the theory" that you imagine this experiment "suggests". Because where I'm sitting, that experiment calls for a resounding "ho hum." It's fine that they did it, it's a perfectly good state-of-the-art measurement, but it won't be important or interesting until the precision can be increased by at least an order of magnitude.

Disinfo Agent
2010-Mar-17, 05:30 PM
Here's an answer to a question which I suspect is implied by the OP:

I am driving my car at the speed of light and I turn on my headlights. What do I see? (http://edu-observatory.org/physics-faq/Relativity/SpeedOfLight/headlights.html)

Have a look at it, Uncommonsense. The moral of the story seems to be that relativity suggests the question is physically not very meaningful.

macaw
2010-Mar-17, 05:32 PM
Because where I'm sitting, that experiment calls for a resounding "ho hum."

Yes, from where you are sitting. Other might find this interesting and emboldening towards enlarging their horizons.

It's fine that they did it, it's a perfectly good state-of-the-art measurement, but it won't be important or interesting until the precision can be increased by at least an order of magnitude.

You realize that some of the speeds in the measured distribution are greater than c, do you? You realize that the median of the distribution is also shifted towards values greater than c, do you?
Why would I spend my time teaching you experimental physics? You already seem to know it all.

forrest noble
2010-Mar-17, 05:57 PM
Dr. Rocket,

My corrected quote from posting #16 & 19

.....to continue traveling at close to the speed of light the particle would need continuous force to maintain its velocity....

Your quote

No, because if you have force then you have acceleration. Force will change the speed unless it is applied normal to the velocity in which case it merely changes direction (as in circular motion).

From posting #16 I was only discussing mass approaching light speed.

As mass approaches the speed of light continuous force is needed to maintain its speed, hence the amount of force/ energy needed increases approaching infinity the closer to light speed mass travels (according to theory).

Ken G
2010-Mar-17, 05:58 PM
Yes, from where you are sitting. Other might find this interesting and emboldening towards enlarging their horizons.
Well, if all you are going to engage in is vague rhetoric, passing on my challenge that you actually back up your claim that revisions are suggested, then you're not really saying anything at all, are you?

You realize that some of the speeds in the measured distribution are greater than c, do you? Yes, as I can read. I also understand the meaning of experimental precision. You said you are an experimental physicist, I would have thought you would understand that too.

You realize that the median of the distribution is also shifted towards values greater than c, do you? And I also understand experimental precision. If such shifts were meaningful, we could just use centroids and ignore uncertainties. Wouldn't that be nice?

Why would I spend my time teaching you experimental physics? You already seem to know it all.So I'm correct, you are not actually going to say anything here, even when you are asked to. Just a pointless waste of bandwidth, pity.

macaw
2010-Mar-17, 06:02 PM
Well, if all you are going to engage in is vague rhetoric, passing on my challenge that you actually back up your claim that revisions are suggested, then you're not really saying anything at all, are you?

I said what I said, meant for the people that are willing to enlarge their horizons. In your case, it fell on deaf years. Pity.

So I'm correct, you are not actually going to say anything here, even when you are asked to. Just a pointless waste of bandwidth,

Yes, attempting to teach you new things would prove to be a waste of bandwidth since you always act as if you already know it all. So, not.
Others might find the wiki article and the experiment really interesting and mind stimulating, so , in their case, it was worth posting.

DrRocket
2010-Mar-17, 06:06 PM
Dr. Rocket,

My corrected quote from posting #16 & 19

From posting #16 I was only discussing mass approaching light speed.

As mass approaches the speed of light continuous force is needed to maintain its speed, hence the amount of force/ energy needed increases approaching infinity the closer to light speed mass travels.

One more time. No. Not even close.

In order for a mass to increase its speed, that is to accelerate, force is needed and energy must be expended.

No force need be applied and no energy expended to maintain a given velocity. It does not matter how fast it is going.

The only situation in which force is needed to maintain speed is a situation in which there is a loss mechanism, friction. That is why you need to keep the accelerator down to drive your car at a fixed speed -- air resistance.

But that is a macroscopic effect that is not relevant to the special theory of relativit. No force is needed to maintain a fixed speed. Newton's first law holds in relativity.

http://en.wikipedia.org/wiki/Newton's_laws_of_motion

DrRocket
2010-Mar-17, 06:18 PM
Not exactly, the latest experimental evidence dissociates "can travel at c" from "only if it has zero (rest) mass". This indicates that we need to look into revising the current theory.

The I suggest that you publish this finding quickly and claim your Prize.

Dissociation of "can travel at c" from "only if it has zero rest mass" would be a major upheaval in the theoretical pillars of physics.

You will either get The Prize or be laughed out of town.

Good luck.

macaw
2010-Mar-17, 06:21 PM
As mass approaches the speed of light continuous force is needed to maintain its speed

If a massive particle approaches the speed of light, you are not maintaining its speed by applying a force, you must accelerate it. So, your sentence contains a contradiction in terms.
Now, if we correct you sentence, we find indeed that the mechanical work expended in trying to accelerate a massive particle to the speed of light is infinite.

This is easily proven by :

Mechanical_Work=Integral_from_0_to_c{Fdx}

where F=dp/dt

p=m_0*v(t)/sqrt(1-(v(t)/c)^2)
m_0=rest mass
Fdx=dp*dx/dt=m_0*v^2(t)/sqrt(1-(v(t)/c)^2)

Integrate from 0 to c and you'll get Mechanical_Work->infinity

Now, photons have 0 rest mass and they are "born" (in vacuum) moving at c, there is no need to accelerate them in order to reach c.
The situation of the neutrinos is more intriguing, they have non-zero rest mass but they appear (at least from what we know) to also be "born" moving at c. We do not understand (yet) why this is true.

macaw
2010-Mar-17, 06:24 PM
Dissociation of "can travel at c" from "only if it has zero rest mass" would be a major upheaval in the theoretical pillars of physics.

This is what the wiki page is saying that well. Neutrinos travel at c, yet they have a non-zero mass. Some real physicists are getting intrigued by this possibility and they are running experiments in order to verify this finding. Pity I wasted my time trying to educate you. I won't do it in the future, I promise.

grav
2010-Mar-17, 06:24 PM
Are curves on that plane space-like or time-like? My intuition is telling me that the plane defines space-like.Events that occur in the frame of a photon would only be light-like, meaning that whatever events occur in the same plane that the photon coincides along the path of travel are the only ones that would be perceived in that frame. Referring back to post #7, if the train passes at c, then taking things at the absolute limit although not generally a good idea when dealing with singularities, all events which occur on the train are completely frozen in time from the platform observer's perspective and that would be all the platform observer would ever see, which would be the same as the photon view in this case, and then of course all events would occur in the same place due to the contraction of the train also so the photon travels instantly from point A to point B from its own perspective, so any events that occur before the photon reaches it from the train's perspective has already occurred before the photon even existed according to the photon's point of view, or not at all, while the readings on the train are simultaneity shifted along its length and remain frozen at those times, so any events that do not directly coincide with the photon across a perpendicular plane along its path of travel as it passes will lie outside of the photon's existence.

macaw
2010-Mar-17, 06:34 PM
Events that occur in the frame of a photon .

There is no such thing as a "frame of a photon", relativity does not allow associating frames of reference to photons.

DrRocket
2010-Mar-17, 06:53 PM
This is what the wiki page is saying that well. Muons travel at c, yet they have a non-zero mass. Some real physicists are getting intrigued by this possibility and they are running experiments in order to verify this finding. Pity I wasted my time trying to educate you. I won't do it in the future, I promise.

The Wiki page to which you linked says nothing about muons. It does talk about muon neutrinos, which are totally different animals.

Moreover, apparently the author of the Wiki article (http://en.wikipedia.org/wiki/Neutrino#Speed)agrees with most of the rest of humanity in not taking the measurements to indicate that neutrinos or muons or anything else is traveling faster than c'.

"This measurement has been repeated using the MINOS detectors, which found the speed of 3 GeV neutrinos to be 1.000051(29) c (http://www.bautforum.com/wiki/Speed_of_light). While the central value is higher than the speed of light, the uncertainty is great enough that it is very likely that the true velocity is not greater than the speed of light. This measurement set an upper bound on the mass of the muon neutrino of 50 MeV at 99% confidence.[15] (http://www.bautforum.com/#cite_note-18)
The same observation was made, on a somewhat larger scale, with supernova 1987a.. The neutrinos from the supernova were detected within a time window that was consistent with a speed of light for the neutrinos. So far, the question of neutrino masses cannot be decided based on measurements of the neutrino speed."

Muons (http://en.wikipedia.org/wiki/Muon) are quite well known not to travel at c, and they are relatively massive (more massive than an electron) so that any such suggestion to the contrary would really be revolutionary. The muon decay from cosmic ray collisions (http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/muon.html) in upper atmosphere was one of the early confirmations of time dilation/length contraction of special relativity (they were called mu mesons back then). The effect is appreciable, but certainly inconsistent with travel at c.

I appreciate the education.

grav
2010-Mar-17, 07:03 PM
There is no such thing as a "frame of a photon", relativity does not allow associating frames of reference to photons.That's generally true, yes, since there is no cause/effect relationship as clocks and events are frozen in time and therefore no experiments can be performed in this frame over zero time, so we should really only be taking the limit as speeds approach c, not at c itself if we want to work through such experiments, but if a photon travels at c, then such a frame does exist, so I am only answering the questions the best I can as they have been asked and according to what the mathematics would say.

macaw
2010-Mar-17, 07:03 PM
The Wiki page to which you linked says nothing about muons. It does talk about muon neutrinos, which are totally different animals.

Thank you for finding a typo. I obviously meant neutrinos, since the discussion is about neutrinos.

Moreover, apparently the author of the Wiki article agrees with most of the rest of humanity in not taking the measurements to indicate that neutrinos or muons or anything else is traveling faster than c'.

Did I say that neutrinos are travelling faster than c? Why do you feel compelled to create a strwaman? What exactly did I try to teach you in the first post to you?

Now, as an aside, SR does not prohibit massive particles from travelling faster than c (if they are "born" at that speed). It only prohibits accelerating them to c (see my post to forrest noble). Google "tachyons", might be an eye opener.

macaw
2010-Mar-17, 07:09 PM
That's generally true.

It is always true.

Ken G
2010-Mar-17, 07:13 PM
I said what I said, meant for the people that are willing to enlarge their horizons. Actually, if you simply look at your posts, it is clear that you said nothing at all except to make a bogus claim that current theory needs revising, which is why your empty remarks enlarged no horizons. What you did was cite a perfectly mundane observation that tells us nothing about neutrinos if their rest mass is much less than 50 MeV, as is now widely expected, nor did the brief Wiki claim to tell us anything about such neutrinos. Now, if you actually do want to say something, then please actually do so: say why you think that observation "suggest revisions", and what revisions you imagine that it suggests. Note you haven't actually said a single useful word yet in that direction, merely you have claimed it would be somehow a waste of your time to actually say something. Otherwise, keep your ATM opinions about "suggested revisions to current theory" to yourself, or to the ATM part of the forum. At least there you could be challenged to actually back up these empty claims using something more than a vague promise of "enlarged horizons," or a misinterpretation of the meaning of measurement uncertainty.

macaw
2010-Mar-17, 08:19 PM
Actually, if you simply look at your posts, it is clear that you said nothing at all except to make a bogus claim that current theory needs revising.

I tried to teach you something, sorry I failed.

Ken G
2010-Mar-18, 01:13 AM
I'm still waiting for you to say what you think needs revision, and why. Tell me that, if you want to say something. So far, all you've done is pointed out that the speed of neutrinos is not known as accurately as accelerators have constrained their rest masses, owing to lack of precision in speed measurements. If you think that requires revising something about neutrino speed, I'm all ears, because it just sounds like you don't understand experimental uncertainty. A more natural interpretation would be the one quoted by DrRocket which you also missed an opportunity to address if you actually had something to teach anyone here.

publius
2010-Mar-18, 01:35 AM
Now, as an aside, SR does not prohibit massive particles from travelling faster than c (if they are "born" at that speed). It only prohibits accelerating them to c (see my post to forrest noble). Google "tachyons", might be an eye opener.

What does a space-like 4-momentum mean? What does the "length", s, along a space-like world line mean? It's sure different from proper time as we think of it. Tachyons must have an *imaginary* rest mass for it to make any sense. And that itself doesn't make much sense. So no, relativity does not allow a particle with real rest mass to travel faster than c.

You can play with that mathematically if you like, but I see nothing of any physical consequence that can come from it. If a tachyon exists, it sure can't interact with the time-like world, because causality would be violated (or relativity is out).

As I understand it, the quantum field theory notion of a tachyon(ic field) is something very different that some simple particle moving faster than c. And it's still a will-o-the-wisp.

-Richard

macaw
2010-Mar-18, 01:55 AM
I'm still waiting

Keep waiting. I already explained but you didn't get it. Not that you'll ever will.

So far, all you've done is pointed out that the speed of neutrinos is not known as accurately as accelerators have constrained their rest masses,

Huh?

1-2.4*10^-5< v/c <1+12.6*10^-5

Ken G
2010-Mar-18, 02:11 AM
Keep waiting. I already explained but you didn't get it. Please cite the post where you "explained" something. As an educator, I have a different standard for what explaining means. Or didn't you notice that neither DrRocket, nor myself, both Ph.D.'s, thought that you explained anything at all.

Huh?

1-2.4*10^-5< v/c <1+12.6*10^-5And the point is, of course, that even 2.4*10^-5 is not good enough precision to say squat that is interesting about neutrino rest masses, compared to what is already expected from the accelerators. This is the part you just keep overlooking, or ignoring. One last time: what, exactly, do you think needs "revision"??

macaw
2010-Mar-18, 05:04 AM
As an educator, I have a different standard for what explaining means.

What educator? you mean teaching high school physics where no equations are needed? Like I said, it is not my fault that you can't learn.

And the point is, of course, that even 2.4*10^-5 is not good enough precision to say squat that is interesting about neutrino rest masses

See? What gives you any idea that 2.4*10^-5 is about rest mass? It is about speed. What does the formula

1-2.4*10^-5<v/c<1+12.6*10^-5 mean?

What does the RHS mean? You don't know.

Ken G
2010-Mar-18, 05:37 AM
What gives you any idea that 2.4*10^-5 is about rest mass? It is about speed.Nothing gives me that idea, I know perfectly well it is about speed. Please note all my comments about speed. Also note the relevance to rest mass. The misunderstanding here is all yours, I know perfectly well what is going on, and I know why your views about that measurement are all ATM. Please take them there, where you can be asked to actually back up what you are claiming.

What does the formula

1-2.4*10^-5<v/c<1+12.6*10^-5 mean?

What does the RHS mean? You don't know.Well, it certainly means that neutrinos might move faster than c, or they might move slower than c. It means they might move at exactly the speed relativity says they should move at, given their energy and rest mass. It means there is no suggestion here that anything needs revising. It means you have no idea what you are talking about. Take it to ATM, as you obviously have no plans of answering any of the challenges I've already given you, and your objections to what DrRocket said are a complete hijack of this thread. Nothing that you've said is interesting yet-- if you have something interesting to say about that observation, please say it. Otherwise, take your unsubstantiated claims to ATM.

macaw
2010-Mar-18, 02:17 PM
your objections to what DrRocket said are a complete hijack of this thread.

Last I checked it was uncommonse's thread about whether distance is invariant or not.

Otherwise, take your unsubstantiated claims to ATM.

Why would I do that? Just because you are unable to understand? No one else seemed to have trouble understanding, you are the only one still struggling with the concept.

Swift
2010-Mar-18, 03:33 PM
Like I said, it is not my fault that you can't learn.

Ken G and particularly macaw,

If you can not discuss this like civil adults, I will be more than happy to dish out some infractions. Knock off the veiled comments.

Ken G
2010-Mar-18, 05:37 PM
Let me just summarize this aside: experiments have been done on neutrinos that cannot tell if they are moving faster or slower than c, and cannot rule out that they move exactly the way relativity expects them to move, so in no way invalidate DrRocket's summary of what relativity says about neutrinos and the rest mass of particles. The observations show a curious slant toward speeds higher than c, but that is well within the experimental uncertainty so by itself suggests no cause for any revisions of anything (but might be an interesting tidbit to file away in case future observations actually are statistically significant on that issue). That's it, there's macaw's aside, just read the link. Any other claims about that link are ATM and need to be discussed there. Now let's get back to the questions of the thread, if any still persist.

uncommonsense
2010-Mar-18, 09:34 PM
I am still here. I need some time to digest the prior posts. Alot of info to get a handle on. Will reply at later time.

sirius0
2010-Mar-18, 10:32 PM
You need to understand the article first, comment after. For people knowing physics, the article is quite clear.

Yes, well on further reading I, at least, did deserve the dish out above. Wouldn't normally do this; just skim and post. But I saw this new track as really a diversion from the OP. Nevertheless the nuetrino subject is well worth a thread of it's own. Why don't you start one macaw?

macaw
2010-Mar-19, 01:12 AM
Yes, well on further reading I, at least, did deserve the dish out above. Wouldn't normally do this; just skim and post. But I saw this new track as really a diversion from the OP. Nevertheless the nuetrino subject is well worth a thread of it's own. Why don't you start one macaw?

Because it is a very complicated and puzzling subject, some of the experimental results are not yet totally understood by professional experimentalists. There is incontrovertible evidence that neutrinos have non-zero mass (contrary to prior thinking). There is also a non-zero probability that neutrinos move as c. SR does not forbid this per se (in a prior post I demonstrated that SR forbids accelerating massive particles up to c but does not forbid massive particles to be "born" moving at c nor does it forbid motion at >c (see the Tachyon subject) ). For the time being, the neutrino speed is still an open subject. As a footnote, there are perfectly mainstream theories (http://195.96.232.121/workshop/2006/Poenaru.pdf) (google A. Proca (http://www.wolfram-stanek.de/maxwell/maxwell_proca.html)) that allow for non-zero mass for photons. Not only that the theory is mainstream, it is also very famous (Pauli mentioned it in his Nobel prize acceptance address). Amongst other applications, it is the foundation of all experiments (http://pdg.lbl.gov/2006/listings/s000.pdf) that constrain the mass of the photon. I will say no more about this subject, it isn't worth getting insulted for trying to explain advanced topics. It isn't worth posting advanced subjects at all just to see certain members mistaking them for ATMs. I am sorry, I know that you are interested but it isn't worth it for me.

Ken G
2010-Mar-19, 01:32 AM
BAs a footnote, there are perfectly mainstream theories (google N. Proca) that allow for non-zero mass for photons.
You have a mistaken concept of what "mainstream" means. Hans Alfven is a perfectly mainstream physicist who has a non-mainstream cosmology, as is/does Geoffrey Burbidge. Any theory that gives a photon a rest mass, or says that neutrinos with rest mass move at c, is an ATM theory-- not because they are wrong, or contradict any experiments, but simply because they do not reflect our current "best understanding". They have not satisfied the requisite burden of evidence for going against the simplest and most successful descriptions, such theories are quite easy but not very useful until they are needed. Take your speculations to ATM, this is Q&A.

uncommonsense
2010-Mar-19, 06:31 PM
Some good info has been provided - thank you. But..... I am getting a "sense" that there is an overall flavour of "don't worry about it". I feel the concept from the OP begs deeper contemplation, as it is an aspect of relativity and not simply a "red headed step-child" of the same.

TheUFOPilot
2010-Aug-16, 09:06 PM
Hi, 2yrs late but here goes....Regarding light speed and distance at a constant. I think space time is unique for every single place within it, from earth orbit, to an empty zone with no planets or stars near by....in other words, i think gravity is effecting time and light in the said area of space...the person in that said area would not notice any change as to the speed of light, nor time, but from say a person who is looking in on the universe, they could see time tick and light travel slower in different zones of space time. Light seems to run side by side with gravity, and light and time seem to be effected by this...light/gravity/time-speed etc must all run in synchronisation in the particlular area it exists.. Say you had 2 photon clocks, checking the speed of light in two gravity zones with different time speeds, Zone A and B....both clocks would read or calculate the same speed for light, so that must mean, light is in fact traveling at different speeds when .observed from Zone C....your distance would be constant for the particle travelng at the speed of light, even if light slows down, distance/time would also slow down. I Think anyway.

PetersCreek
2010-Aug-16, 09:59 PM
Welcome to the BAUT forums, TheUFOPilot.

Here in the Space and Astronomy Q&A forum, answers to questions must reflect prevailing mainstream understanding. It is not the place for speculation or against-the-mainstream ideas. If you haven't already done so, please familiarize yourself with our rules, linked in my signature below.

forrest noble
2010-Aug-17, 02:25 AM
uncommonsense,

Probably an elementary relativity question but,
From the relative perspective of a particle moving at the speed of light, is distance a constant?

According to Special or General Relativity no particle of matter could reach the speed of light concerning past interpretations. Neutrinos involving C could accordingly do so if they have no mass, but if they do have mass then reaching C is controversial. Large scale distances are calculated by the Hubble formula concerning galactic redshifts and "small-scale" distances can be estimated by different methods which collectively determine the "distance ladder" of calculations. The speed of light is a constant in these formulations but distances, like all measurement, involves comparison to a standard like one Angstrom unit or one light year, for instance. Bottom line is that according to present theory, distances are the same from one time frame to another and unrelated to matter in motion concerning a relative perspective.