Contradiction in Relativistic Simultaneity in Taylor-Wheeler Spacetime Physics?

[GregAshmore]

What do you think that purpose was? I would say the purpose was to show what conclusions we can draw about simultaneity given the two basic postulates, which he had already discussed in sections 5 and 8 of the book where he discussed the train/lightning scenario.

I've read sections 5 through 9 on multiple occasions, over a period of four years. I read through them again just now. I have once again found it difficult to fully accept the truth of this statement:

That light requires the same time to traverse the path A->M as for the path B->M is in reality neither a supposition nor a hypothesis about the physical nature of light, but a stipulation which I can make of my own free will in order to arrive at a definition of simultaneity.

I therefore do not agree that the assertion in the next paragraph is, as Einstein asserts, "clear". That is, it is not clear to me that this definition can be used to give an exact meaning to two events.

Having suspended judgment as to the validity of the proposition (or stipulation), I come to this statement in section 9:

Are two events which are simultaneous with reference to the railway embankment also simultaneous relative to the train? We shall show directly that the answer must be in the negative.

A few paragraphs later he presents his proof:

If an observer sitting at M' in the train did not possesses this velocity , then he would remain permanently at M...and the light flashes would reach him simultaneously. Now in reality considered with reference to the railway embankment he is hastening toward the light coming from B, whilst he is riding on ahead of the beam coming from A. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A.

I don't see that the conclusion necessarily follows from the evidence, even if one accepts the stipulation that c is a physical constant for all inertial observers. Thinking about it, I have sometimes been able to convince myself that the conclusion is wrong, given the stipulation of c. When I run across this sort of problem at work--which happens on a regular basis--I build a system, create the conditions, and measure the results. This usually clears things up.

In this case, I would want to do exactly what Taylor-Wheeler suggest. I'd put six clocks in the apparatus, three on the ground and three in the train. I'd create two sparks simultaneously on the ground. Then I'd record the time at which the flashes are seen at each of the six positions.

Of course, that is much easier said than done, given the precision required. So far as I know, we have never constructed a rigid frame, equipped it with clocks, and moved it in one direction at any significant fraction of light speed.

 

[ghwellsjr]

Greg, I sure hope you're not thinking that Einstein's postulate, that the one-way speed of light is c in all inertial frames (one at at time, please) is something that can be proved or even measured. It cannot, just like the idea that the one-way speed of light is c in only one frame, an assumed absolute ether rest frame, cannot be proven or measured. Once you accept the experimental evidence that the measured round-trip speed of light is always c for any inertial observer (independent of any assumed frame) and that it is impossible for any such observer to know if the time for light to travel both halves of that round trip are equal or not, then you will be on your way to understanding what Special Relativity is all about. It is simply about declaring that those two times are equal for any inertial observer and building a frame of reference around that declaration.

 

[JesseM]

I don't see that the conclusion necessarily follows from the evidence, even if one accepts the stipulation that c is a physical constant for all inertial observers.

What part of the logic don't you agree with? Do you agree that if we start from the stipulation that light travels at c in all inertial frames, then if in the ground frame the two lightning strikes happen simultaneously, it follows that a detector midway between the positions of the strikes will get the light from them at the same time, while a detector at some position other than the midpoint will not?

Thinking about it, I have sometimes been able to convince myself that the conclusion is wrong, given the stipulation of c.

How so? Try presenting a numerical example where you give both the position and times of the strikes and the position and time of each detector receiving the light from them, using the coordinates of the ground frame where the strikes happened at the same time-coordinate.

In this case, I would want to do exactly what Taylor-Wheeler suggest. I'd put six clocks in the apparatus, three on the ground and three in the train. I'd create two sparks simultaneously on the ground. Then I'd record the time at which the flashes are seen at each of the six positions.

Of course, that is much easier said than done, given the precision required. So far as I know, we have never constructed a rigid frame, equipped it with clocks, and moved it in one direction at any significant fraction of light speed.

This isn't an experimental question about whether light actually does move at c in all inertial frames, it's a mathematical question of what conclusions would follow if the two postulates of SR were valid. Logically the two postulates do imply the relativity of simultaneity, so any failure of your experiment could only mean that one of the postulates was in fact false. Best to try to master the basic logic before going on to ask about whether the postulates actually hold in reality.

Incidentally, it's also true logically that the two postulates will hold as long as the equations of the fundamental laws of physics as expressed in any one frame all have a mathematical property called "Lorentz-invariance" (meaning they are unchanged under the Lorentz transformation). So in that sense you don't actually need to get different observers (or different ruler/clock systems) moving at relativistic speeds relative to one another to test relativity, you can just determine the laws in a single frame and check whether they are Lorentz-invariant. All the most fundamental laws known so far (the equations of quantum field theory for example) have had this property.

 

[GregAshmore]

Greg, I sure hope you're not thinking that Einstein's postulate, that the one-way speed of light is c in all inertial frames (one at at time, please) is something that can be proved or even measured. It cannot, just like the idea that the one-way speed of light is c in only one frame, an assumed absolute ether rest frame, cannot be proven or measured. Once you accept the experimental evidence that the measured round-trip speed of light is always c for any inertial observer (independent of any assumed frame) and that it is impossible for any such observer to know if the time for light to travel both halves of that round trip are equal or not, then you will be on your way to understanding what Special Relativity is all about. It is simply about declaring that those two times are equal for any inertial observer and building a frame of reference around that declaration.

My reticence is not related to the experimental evidence which we have. Nor do I have a problem (any longer) with the concept of relative time. I have a problem with dogmatic statements concerning aspects of the theory which we have not tested directly, such as "time travel" on a cosmological scale, or even the conceptually simple experiment of moving two equivalent rigid bodies past each other at a significant fraction of light speed to directly test the relativity of simultaneity and (perhaps) length contraction.

I might be less of a stickler on these points if we had a better understanding of light itself. It seems to me that while we have learned much about how light interacts with other particles, we know precious little about what goes on in the interval between the creation of a photon and its destruction on our detectors. In my view, this gap in our knowledge (which I understand to be typical of all particles) leaves open the possibility of unexpected behavior as we expand the range of our practical operations.

What I say may sound silly to people who work in this arena every day. Certainly, I respect the understanding which can only come through hands-on experience. But my reservations are sound in principle, and they are informed by the experience of one who knows his own field pretty well, yet has had the humbling experience of discovering that "we don't know what we don't know."

 

[nismaratwork]

My reticence is not related to the experimental evidence which we have. Nor do I have a problem (any longer) with the concept of relative time. I have a problem with dogmatic statements concerning aspects of the theory which we have not tested directly, such as "time travel" on a cosmological scale, or even the conceptually simple experiment of moving two equivalent rigid bodies past each other at a significant fraction of light speed to directly test the relativity of simultaneity and (perhaps) length contraction.

I might be less of a stickler on these points if we had a better understanding of light itself. It seems to me that while we have learned much about how light interacts with other particles, we know precious little about what goes on in the interval between the creation of a photon and its destruction on our detectors. In my view, this gap in our knowledge (which I understand to be typical of all particles) leaves open the possibility of unexpected behavior as we expand the range of our practical operations.

What I say may sound silly to people who work in this arena every day. Certainly, I respect the understanding which can only come through hands-on experience. But my reservations are sound in principle, and they are informed by the experience of one who knows his own field pretty well, yet has had the humbling experience of discovering that "we don't know what we don't know."

If you hadn't started the thread with the notion that this was some flaw in the theory, maybe what you say would fly. As it is, I think your reservations are absurd unless you're really waiting for proof on a cosmological scale. Just because an observation or experiment is grand or massive doesn't make it less accurate or any less compatible with other observations and/or experimental evidence. SR and GR have had decades of challenges, and the areas where it fails to make useful predictions is nowhere NEAR what you're talking about.

It shouldn't take pages to agree what a postulate is, and how that differs from an experimentally verified value. Take some friendly advice and save the critique for after you master the material a bit, when you'll be far less likely to hare off after shadows, and more likely to identify and ponder real problems.

 

[GregAshmore]

If you hadn't started the thread with the notion that this was some flaw in the theory, maybe what you say would fly.

Fair enough. I have already acknowledged that I acted foolishly in asserting a contradiction in the theory.

As it is, I think your reservations are absurd unless you're really waiting for proof on a cosmological scale.

I take issue with those who make smug statements about "time travel" as though it were an unquestionable truth.

But yes, I'm done raising objections until I've learned how to work with the theory, not merely read about it.

 

[nismaratwork]

Fair enough. I have already acknowledged that I acted foolishly in asserting a contradiction in the theory.


I take issue with those who make smug statements about "time travel" as though it were an unquestionable truth.

But yes, I'm done raising objections until I've learned how to work with the theory, not merely read about it.

Who smugly talked about time travel here? Oh, and remember that asking a question and discussing it isn't the same as objecting... the former is a very good thing.

 

[ghwellsjr]

I don't see that the conclusion necessarily follows from the evidence, even if one accepts the stipulation that c is a physical constant for all inertial observers. Thinking about it, I have sometimes been able to convince myself that the conclusion is wrong, given the stipulation of c. When I run across this sort of problem at work--which happens on a regular basis--I build a system, create the conditions, and measure the results. This usually clears things up.

In this case, I would want to do exactly what Taylor-Wheeler suggest. I'd put six clocks in the apparatus, three on the ground and three in the train. I'd create two sparks simultaneously on the ground. Then I'd record the time at which the flashes are seen at each of the six positions.

Of course, that is much easier said than done, given the precision required. So far as I know, we have never constructed a rigid frame, equipped it with clocks, and moved it in one direction at any significant fraction of light speed.

Greg, I sure hope you're not thinking that Einstein's postulate, that the one-way speed of light is c in all inertial frames (one at at time, please) is something that can be proved or even measured. It cannot, just like the idea that the one-way speed of light is c in only one frame, an assumed absolute ether rest frame, cannot be proven or measured. Once you accept the experimental evidence that the measured round-trip speed of light is always c for any inertial observer (independent of any assumed frame) and that it is impossible for any such observer to know if the time for light to travel both halves of that round trip are equal or not, then you will be on your way to understanding what Special Relativity is all about. It is simply about declaring that those two times are equal for any inertial observer and building a frame of reference around that declaration.

My reticence is not related to the experimental evidence which we have. Nor do I have a problem (any longer) with the concept of relative time. I have a problem with dogmatic statements concerning aspects of the theory which we have not tested directly, such as "time travel" on a cosmological scale, or even the conceptually simple experiment of moving two equivalent rigid bodies past each other at a significant fraction of light speed to directly test the relativity of simultaneity and (perhaps) length contraction.

I might be less of a stickler on these points if we had a better understanding of light itself. It seems to me that while we have learned much about how light interacts with other particles, we know precious little about what goes on in the interval between the creation of a photon and its destruction on our detectors. In my view, this gap in our knowledge (which I understand to be typical of all particles) leaves open the possibility of unexpected behavior as we expand the range of our practical operations.

What I say may sound silly to people who work in this arena every day. Certainly, I respect the understanding which can only come through hands-on experience. But my reservations are sound in principle, and they are informed by the experience of one who knows his own field pretty well, yet has had the humbling experience of discovering that "we don't know what we don't know."

Greg, do you understand the difference between the experimental evidence that the round trip speed of light is always measured to be c and the postulate that the inherently unmeasureable one way speed of light is arbitrarily set to c in any inertial frame in Special Relativity?

You proposed an experiment with six clocks and two simultaneous sparks that you claimed would be difficult to perform because of the precision involved. Since you propose this type of experiment, it means that you do not believe that it is impossible to perform your experiment--you believe that if we had enough technology, we could perform your experiment. Do you realize that if we could perform your experiment, it would provide a way to establish an absolute ether rest frame and that SR would not be valid?

You see, the problem is that in order to put six clocks around and have any credence that we can synchronize the clocks, that is, guarantee that the data we get from them is meaningful, or that we can cause two sparks to go off at the same time when separated in space, is the issue that you are trying to measure. Don't you see that the very act of setting up your experiment requires you to have prior knowledge of that which you are trying to determine from the outcome of your experiment?

Now, you state that you believe that we have "never constructed a rigid frame, equipped it with clocks, and moved it in one direction at any significant fraction of light speed". But we have. That is exactly what the Michelson-Morley Experiment was. The only question is whether the null result of that experiment could be explained, as Michelson orginally did, that the ether was dragged along by the moving earth, just like air is so that we don't actually have an ether wind even though we have an ether OR whether the null result is explained some other way. There have been so many other experiments performed that we have to take some other way.

So the question is: do you accept as fact that any inertial observer will always measure the round trip speed of light to be c? If you don't, is it because of the factors that you outlined, that we don't know enough about quantum mechanics? And if you don't, what do you say about all the previous experiments, including MMX, that have nothing to do with quantum mechanics?

 

[GregAshmore]

Greg, do you understand the difference between the experimental evidence that the round trip speed of light is always measured to be c and the postulate that the inherently unmeasureable one way speed of light is arbitrarily set to c in any inertial frame in Special Relativity?

You proposed an experiment with six clocks and two simultaneous sparks that you claimed would be difficult to perform because of the precision involved. Since you propose this type of experiment, it means that you do not believe that it is impossible to perform your experiment--you believe that if we had enough technology, we could perform your experiment.

The setup for the experiment is taken directly out of Taylor-Wheeler "Spacetime Physics". One of my points in the original post is that the thought experiment which establishes the relativity of simultaneity does not use the experimental setup which T-W say is required, and instead relies on what a remote observer sees with his eyes--something that T-W explicitly say is not permitted.

Do you realize that if we could perform your experiment, it would provide a way to establish an absolute ether rest frame and that SR would not be valid?

We must be misunderstanding each other somehow. The concept of a latticework of identical rods in an inertial frame, with a synchronized clock at each node to record the time of events at that node, is presented by T-W as the ideal way to collect data in spacetime, provided the length of the rods is scaled appropriately. This makes sense, as such a latticework is nothing more than a physical realization of the four-dimensional spacetime modeled by the equation of relativity.

If it is possible to synchronize three recording clocks at known locations on a rigid body, and if it is possible to have two of these bodies pass near each other at high velocity, then it seems to me that the experiment can be performed. The flashes of light, or other suitable events, can be triggered simultaneously by two of the recording clocks on one of the bodies. The receipt of the flashes can be recorded by the clocks on the other body. Of course, there are many details to attend to, not the least of which is determining the cumulative error band of the results. But in principle, I don't see why the experiment can't be performed.

By the way, though I am a skeptic, I would certainly not predict that SR will be proven a failure by the experiment.

You see, the problem is that in order to put six clocks around and have any credence that we can synchronize the clocks, that is, guarantee that the data we get from them is meaningful, or that we can cause two sparks to go off at the same time when separated in space, is the issue that you are trying to measure. Don't you see that the very act of setting up your experiment requires you to have prior knowledge of that which you are trying to determine from the outcome of your experiment?

Synchronization of the clocks depends on c and accurate spacing. In principle, why can't it be done? We would not be trying to verify a particular value of c, only the behavior of bodies and clocks at high relative speeds.

If the experiment can't be done in principle, it seems to me that we are wasting our time in the lab right now. The same factors come into play, I would think: c in an inertial frame, precise clocks, and accurate placement of those clocks.

Now, you state that you believe that we have "never constructed a rigid frame, equipped it with clocks, and moved it in one direction at any significant fraction of light speed". But we have. That is exactly what the Michelson-Morley Experiment was.

It doesn't seem the same to me at all. The two arms of the apparatus were at rest in the same frame.

 

[Doc Al]

The setup for the experiment is taken directly out of Taylor-Wheeler "Spacetime Physics". One of my points in the original post is that the thought experiment which establishes the relativity of simultaneity does not use the experimental setup which T-W say is required, and instead relies on what a remote observer sees with his eyes--something that T-W explicitly say is not permitted.

As was pointed out in post #2, as long as the travel time of light is taken into account, a remote observer's observations are perfectly OK.

 

[JesseM]

As was pointed out in post #2, as long as the travel time of light is taken into account, a remote observer's observations are perfectly OK.

I also pointed this out to Greg in [post=3010539]this post[/post] on another thread. In any case, you are free to think about the train thought-experiment in terms of what would logically have to be recorded by a lattice of rulers and clocks at rest in each frame, measuring the positions and times of the lightning strikes and of the light from each strike hitting each of the observers, under the assumption that the two strikes are assigned the same time-coordinate by the lattice of the ground observer, and that clocks in the lattice are synchronized in such a way as to guarantee that each measures the speed of light to be c. The conclusions are exactly the same, and I would argue that this is really implicit in Einstein's formulation of the problem anyway, since he had already discussed the idea that position coordinates should be interpreted in terms of measurements on a rigid lattice of rulers in section 2, and introduced the idea of assigning time coordinates in terms of clocks synchronized by light signals in section 8 (which is also the section where he introduces the idea of lightning striking either end of a rail car).

 

[ghwellsjr]

Greg, I appreciate that you have answered some of my questions but you have ignored the all-important ones:

Greg, do you understand the difference between the experimental evidence that the round trip speed of light is always measured to be c and the postulate that the inherently unmeasureable one way speed of light is arbitrarily set to c in any inertial frame in Special Relativity?
...
Do you realize that if we could perform your experiment, it would provide a way to establish an absolute ether rest frame and that SR would not be valid?
...
Don't you see that the very act of setting up your experiment requires you to have prior knowledge of that which you are trying to determine from the outcome of your experiment?
...
So the question is: do you accept as fact that any inertial observer will always measure the round trip speed of light to be c? If you don't, is it because of the factors that you outlined, that we don't know enough about quantum mechanics? And if you don't, what do you say about all the previous experiments, including MMX, that have nothing to do with quantum mechanics?

Here's the issue and the reason for the disconnect between us:

If you accept the experimental evidence that the round trip speed of light is always c for any inertial observer and you realize that this has nothing to do with Special Relativity and you understand that the one-way speed of light cannot be measured and you realize that Special Relativity starts with an arbitrary definition that the time that it takes light to make both halves of its round trip in any inertial frame is the same, then you will not care about any more experiments because they cannot add any more light on what happens in Special Relativity. It cannot be otherwise. There is no point in performing your experient once you use the definition of the one way speed of light because it cannot come out any differently than an analysis based on Special Relativity would predict.

Synchronization of the clocks depends on c and accurate spacing. In principle, why can't it be done? We would not be trying to verify a particular value of c, only the behavior of bodies and clocks at high relative speeds.

Yes, in principle, it can be done as a thought experiment to help illustrate and understand how things work in Special Relativity. It can also be done as an actual experiment but we already know the outcome of the experiment, it can't be otherwise based on our assumptions.

 

[GregAshmore]

Greg, I appreciate that you have answered some of my questions but you have ignored the all-important ones:

Here's the issue and the reason for the disconnect between us:

If you accept the experimental evidence that the round trip speed of light is always c for any inertial observer and you realize that this has nothing to do with Special Relativity and you understand that the one-way speed of light cannot be measured and you realize that Special Relativity starts with an arbitrary definition that the time that it takes light to make both halves of its round trip in any inertial frame is the same, then you will not care about any more experiments because they cannot add any more light on what happens in Special Relativity. It cannot be otherwise. There is no point in performing your experient once you use the definition of the one way speed of light because it cannot come out any differently than an analysis based on Special Relativity would predict.

Yes, in principle, it can be done as a thought experiment to help illustrate and understand how things work in Special Relativity. It can also be done as an actual experiment but we already know the outcome of the experiment, it can't be otherwise based on our assumptions.

I guess I'm from Missouri. It seems to me that if two identical bodies are set up in the lab-- meaning that both are at rest in the same frame, both have clocks which are calibrated to the same tick rate, with identical spacing, and synchronized to account for the spacing--and one of those bodies is then moved at high speed relative to the other, then we are in a position to see whether the results are what we expect. Until then, we don't really know, do we?

I understand the argument based on the lifetime and distance traversed by particles. It is a very powerful argument for the reality of relative time and distance. But it is not the same, it seems to me, as having recording clocks, spatially separated, on both bodies. This is particularly true with respect to length contraction, I think.

When I read claims about differential aging of human beings in spaceships as though it were unquestionably true, yet without even a small-scale test of the concept with a body of significant mass and physical dimensions--well, I think that is a bit of a stretch, to put it mildly.

 

[JesseM]

I understand the argument based on the lifetime and distance traversed by particles. It is a very powerful argument for the reality of relative time and distance. But it is not the same, it seems to me, as having recording clocks, spatially separated, on both bodies.

What about the precision tests of time dilation using atomic clocks? The whole GPS system depends on the validity of relativistic time dilation, for example.

When I read claims about differential aging of human beings in spaceships as though it were unquestionably true, yet without even a small-scale test of the concept with a body of significant mass and physical dimensions--well, I think that is a bit of a stretch, to put it mildly.

Do you understand that for the prediction to be wrong, physicists would have to be fundamentally mistaken about the equations of electromagnetism, since these equations govern the forces that hold atoms together into larger systems like human bodies? The known laws are Lorentz-invariant, which logically guarantees they would work the same in different inertial frames.

 

[GregAshmore]

I also pointed this out to Greg in [post=3010539]this post[/post] on another thread. In any case, you are free to think about the train thought-experiment in terms of what would logically have to be recorded by a lattice of rulers and clocks at rest in each frame, measuring the positions and times of the lightning strikes and of the light from each strike hitting each of the observers, under the assumption that the two strikes are assigned the same time-coordinate by the lattice of the ground observer, and that clocks in the lattice are synchronized in such a way as to guarantee that each measures the speed of light to be c. The conclusions are exactly the same, and I would argue that this is really implicit in Einstein's formulation of the problem anyway, since he had already discussed the idea that position coordinates should be interpreted in terms of measurements on a rigid lattice of rulers in section 2, and introduced the idea of assigning time coordinates in terms of clocks synchronized by light signals in section 8 (which is also the section where he introduces the idea of lightning striking either end of a rail car).

I recognize (now that the relativity of simultaneity has penetrated my thick skull) that the equations of SR are perfectly consistent within themselves. On paper--when one assigns the coordinates and the calibration of clocks--everything works out satisfactorily.

I also recognize that much of the theory has been verified in the lab and in our observations of the cosmos. However, as I pointed out in #43, it doesn't seem to me that we know for sure how things will work out when we put recorders on both bodies, with relative velocity at a significant fraction of light speed.

From this perspective, Einstein does not prove (as he said he would) the relativity of simultaneity in his example of the Train Paradox. His conclusion is the result of his assumptions, as you point out in the bolded text above, and not developed from logically independent evidence. But my complaint here concerns Einstein's pedagogy, not the theory itself.

 

[ghwellsjr]

Well then you don't accept the experimental evidence that the round trip speed of light measures to be the same for all inertial observers.

But you can still understand Special Relativity if you want, even without believing that it provides an accurate depiction of reality, it is simply a set of logical statements based on a couple postulates, just like geometry, if you ever studied that.

I would recommend that you quit trying to find proofs, evidences, or arguments either for or against Special Relativity, and simply try to understand it. It really is quit simple conceptually, the difficulty comes in trying to perform the math, but you are no where near ready for that. If you want me to help you, you need to answer my questions.

 

[GregAshmore]

What about the precision tests of time dilation using atomic clocks? The whole GPS system depends on the validity of relativistic time dilation, for example.

From what I have read, two separate corrections are applied--one for SR and another for GR. That strikes me as rather odd, considering that GR includes SR. Still, I don't know enough to have a valid opinion on the issue.

Do you understand that for the prediction to be wrong, physicists would have to be fundamentally mistaken about the equations of electromagnetism, since these equations govern the forces that hold atoms together into larger systems like human bodies? The known laws are Lorentz-invariant, which logically guarantees they would work the same in different inertial frames.

"Fundamentally" mistaken? That's a bit too strong, I think. No one can deny that we understand electromagnetism very well insofar as we use it in our machines. On the other hand, I would say that we do not have nearly the same level of understanding of what goes on at the subatomic level. In fact, every one of the physicists whom I have read emphasizes our lack of understanding of what we see. We know what goes on, but we don't know why. (And, arguably, we don't really know what goes on, because we can't measure the particles themselves; we can only measure their demise and speculate as to what happens while they live.) Based on that testimony, I'm not sure we can guarantee anything until we've tried it.

 

[GregAshmore]

Well then you don't accept the experimental evidence that the round trip speed of light measures to be the same for all inertial observers.

But you can still understand Special Relativity if you want, even without believing that it provides an accurate depiction of reality, it is simply a set of logical statements based on a couple postulates, just like geometry, if you ever studied that.

I would recommend that you quit trying to find proofs, evidences, or arguments either for or against Special Relativity, and simply try to understand it. It really is quit simple conceptually, the difficulty comes in trying to perform the math, but you are no where near ready for that. If you want me to help you, you need to answer my questions.

I'm working on the math. I'm a little slow, but I get it eventually.

 

[ghwellsjr]

I'm working on the math. I'm a little slow, but I get it eventually.

But do you get the difference between the measured round trip speed of light and the arbitrarily defined one way speed of light and why it matters?

 

[JesseM]

From what I have read, two separate corrections are applied--one for SR and another for GR.

Where'd you read that? They may apply separate corrections for velocity-based time dilation and gravitational time dilation, but both would still be in the context of GR.

"Fundamentally" mistaken? That's a bit too strong, I think.

Well, no it isn't. Do you understand my point about it being logically impossible that Lorentz-invariant laws wouldn't work the same way in all inertial frames? And the equations we have now are Lorentz-invariant, so for there to be a violation of relativity, the equations would have to be wrong.

No one can deny that we understand electromagnetism very well insofar as we use it in our machines. On the other hand, I would say that we do not have nearly the same level of understanding of what goes on at the subatomic level.

Why "subatomic"? I'm talking about how atoms interact with one another in large collections like human bodies, not what goes on inside the atoms themselves (which requires not just electromagnetism but also the strong and weak nuclear forces--and the current equations which are thought to govern these forces are also Lorentz-invariant, of course). That's what determines things like how fast the hands of a mechanical clock move, or how fast cells divide and die.

If by "subatomic" you just meant "microscopic", well, the rules of quantum electrodynamics are understood quite well and have been tested extensively.

In fact, every one of the physicists whom I have read emphasizes our lack of understanding of what we see. We know what goes on, but we don't know why.

Knowing "what goes on" means having equations which accurately predict observed behavior, which is all that is needed to check for Lorentz-invariance and figure out what would the observable behavior would be. "Why" is a question that no theory of physics has ever addressed or could ever address, physics is just about making quantitative predictions using mathematical equations, not about "why" those particular equations are the correct ones. "Why" is a question for philosophy or theology that has nothing to do with science or predictions about the results of empirical experiments (like predictions about how an organism would age if taken on a journey at relativistic speed relative to the Earth).

 

[JM]

Greg
Where is the scene of Menzel described?
The 'two lightning' scene is so poorly described, as in fig 3.1 that its hard to make any clear conclusion. A single flash seems much more straight forward and instructive.
For example, a sound pulse originating at the mid point of train and track reaches the ends of the track segment at the same time, but does not reach the ends of the train at the same time (through the open air). Thus the Relativity of Simultaneaty (ros) is not a new and unique feature of Relativity. Since light is a wave, as sound is, this feature, ros, could be expected to exist for light.
But Relativity says otherwise. A light pulse originating at the mid point also does reach the ends of the track at the same time, but now the light pulse also reaches the end of the train at the same time. These results occur because the light postulate specifies that the speed of light must be the same for all inertial observers. Thus the startling result is that sumultaneity is not relative! How can this be? The answer magically invented by Einstein is to adjust the zero point of time, by adding the second term in the time transfer equation ct=m(cT- vX/c).
To me, this simple, clear example shows the heart of Special Relativity with easily visualized physics and minimum math. Why even get involved with the convolutions of the two-flash picture?
JM

 

[ghwellsjr]

Greg
Where is the scene of Menzel described?

I described and explained Menzel's scene in post #18:

I believe the reason you are having so much problem with this is because you are thinking of the "scene proposed by Menzel" where a single flash of light, set off at the location of when two observers, one stationary and the other moving, produces an expanding sphere of light in which the stationary and the moving observers both observe themselves to be at the center, even though they continue to get farther apart. This is very true but they only tell you half of the story. What they don't tell you is how you observe yourself to be in the center of an expanding sphere of light.

Here's the rest of the story: You cannot see light as it travels away from you unless you reflect it off of something and some of that light comes back to you. You cannot even tell where the light is. So what you do is put up a bunch of mirrors some equal distance from you in all directions so that when the sphere of light hits them they will start their trip back to you and when they arrive at your location, you can see that all of the returned light reflections arrive simultaneously. Now the other observer is doing the same thing except he has a different set of mirrors. Both of you are in the center of your set of mirrors but the traveling one's mirrors are moving with him. Now if you think very carefully about how this experiment could work, you will discover that it is necessary for the moving observer to have his mirrors not really equal distant from him, they are closer to him along the direction of motion. This is the Lorentz contraction. And for him, the light does not arrive at all the mirrors simultaneously but in such a manner as to cause the reflections to arrive simultaneously from all the mirrors at his location. Also, the two sets of reflections, one for the stationary observer and one for the traveling observer do not collapse on their respective observers at the same time. The stationary observer sees the reflections first and then some time later the traveling observer sees his reflections. Once you understand how this works, you will see that in this "scene proposed by Menzel" the light is making a round trip, starting from the co-location of the two observers and ending up after being reflected off of two separate sets of mirrors, on the two observers at different times and at different locations.

In the train situation, there are two flashes of light coming from a single pair of sources that are stationary with one observer. It would be like if the Menzel scene had only one set of mirrors for the stationary observer. It wouldn't work the same. That is why the Menzel scene is not the same as the train scene.

I know this is kind of hard to follow without a visual but it's the direction you're going to have to take to understand what's going on and why the two situations are completely different.

The 'two lightning' scene is so poorly described, as in fig 3.1 that its hard to make any clear conclusion. A single flash seems much more straight forward and instructive.
For example, a sound pulse originating at the mid point of train and track reaches the ends of the track segment at the same time, but does not reach the ends of the train at the same time (through the open air). Thus the Relativity of Simultaneaty (ros) is not a new and unique feature of Relativity. Since light is a wave, as sound is, this feature, ros, could be expected to exist for light.
But Relativity says otherwise. A light pulse originating at the mid point also does reach the ends of the track at the same time, but now the light pulse also reaches the end of the train at the same time. These results occur because the light postulate specifies that the speed of light must be the same for all inertial observers. Thus the startling result is that sumultaneity is not relative! How can this be? The answer magically invented by Einstein is to adjust the zero point of time, by adding the second term in the time transfer equation ct=m(cT- vX/c).
To me, this simple, clear example shows the heart of Special Relativity with easily visualized physics and minimum math. Why even get involved with the convolutions of the two-flash picture?
JM

JM, you have a mixed up view of relativity. If you understand my description of Menzel's scene, then you can see why the light would not arrive simultaneously at equidistant points for both the stationary and traveling observers. No matter what frame of reference you use, light can never behave the way you describe.

 

[bcrowell]

Knowing "what goes on" means having equations which accurately predict observed behavior, which is all that is needed to check for Lorentz-invariance and figure out what would the observable behavior would be. "Why" is a question that no theory of physics has ever addressed or could ever address, physics is just about making quantitative predictions using mathematical equations, not about "why" those particular equations are the correct ones. "Why" is a question for philosophy or theology that has nothing to do with science or predictions about the results of empirical experiments (like predictions about how an organism would age if taken on a journey at relativistic speed relative to the Earth).

An extreme position. I certainly don't agree.

 

[nismaratwork]

An extreme position. I certainly don't agree.

You believe that "why" is a question for philosophy, and "how" one for physics is an extreme view? I hold it, I'll freely admit, and among people from hobbyists to professionals, it seems to be the majority opinion. Physics neither attempts nor is it capable of saying WHY something happens... the concept of an ultimate 'why' already invokes something like a god.

 

[JM]

If you understand my description of Menzel's scene, then you can see why the light would not arrive simultaneously at equidistant points for both the stationary and traveling observers.

See Einsteins 1905 paper, Dover Ed., p.46: A spherical wave is described by the equation X^2+Y^2+Z^2=c^2T^2 relative to the stationary frame K(X,Y,Z,T). The light wave arrives at all points of the sphere at time T, i.e. simultaneously. Transforming this equation by the Lorentz transforms leads to the equation of the sphere relative to the moving frame k( x,y,z,t): x^2+y^2+z^2=c^2t^2. So the light wave arrives at all points of this sphere at time t, i.e. simultaneously. Thats what I said. So what's your question?
Jm

 

[ghwellsjr]

Thus the startling result is that sumultaneity is not relative!

This is not true. The reason you think it's true is because you are switching between frames for the two observers but in anyone frame, it cannot be true that the light arrives at the remote locations simultaneously for two observers with a speed difference between them.

So my question for you is: where did you learn that "sumultaneity is not relative"?

 

[bcrowell]

You believe that "why" is a question for philosophy, and "how" one for physics is an extreme view? I hold it, I'll freely admit, and among people from hobbyists to professionals, it seems to be the majority opinion. Physics neither attempts nor is it capable of saying WHY something happens... the concept of an ultimate 'why' already invokes something like a god.

I never said anything about an ultimate why. But for example, I think there is an answer to the question of why an object that accelerates from rest at 1 m/s2 covers 0.5 m in 1 s.

 

[nismaratwork]

I never said anything about an ultimate why. But for example, I think there is an answer to the question of why an object that accelerates from rest at 1 m/s2 covers 0.5 m in 1 s.

If you take the argument that would ensue to its logical conclusion you would have an 'ultimate why' question. At no point when being asked "why" in physics can you do anything but retreat to a more basic principle until you reach posits. I'm not saying that's a bad thing, but it's true, and it makes 'why' something for philosophy or theology.

Physics can often answer 'how', which is not reducible to some fundamental question about a phantom motive... physics doesn't deal in motives, but methods.

 

[bcrowell]

If you take the argument that would ensue to its logical conclusion you would have an 'ultimate why' question. At no point when being asked "why" in physics can you do anything but retreat to a more basic principle until you reach posits.

I agree with this...

I'm not saying that's a bad thing, but it's true, and it makes 'why' something for philosophy or theology.

...but not with this.

 

[nismaratwork]

I agree with this...


...but not with this.

I admit to being confused and in need of some explanation. How can you accept the first and reject the second?