Fair enough. I have already acknowledged that I acted foolishly in asserting a contradiction in the theory.nismaratwork said:If you hadn't started the thread with the notion that this was some flaw in the theory, maybe what you say would fly.
I take issue with those who make smug statements about "time travel" as though it were an unquestionable truth.As it is, I think your reservations are absurd unless you're really waiting for proof on a cosmological scale.
GregAshmore said: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.
GregAshmore said: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, 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?GregAshmore said: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.ghwellsjr said: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.
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."
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.ghwellsjr said: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.
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.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?
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.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?
It doesn't seem the same to me at all. The two arms of the apparatus were at rest in the same frame.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.
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.GregAshmore said: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.
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).Doc Al said: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.
Here's the issue and the reason for the disconnect between us:ghwellsjr said: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?
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 said: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.
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?ghwellsjr said: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.
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.GregAshmore said: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.
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 said: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.
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.JesseM said: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).
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.JesseM said: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.
"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.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.
I'm working on the math. I'm a little slow, but I get it eventually.ghwellsjr said: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.
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?GregAshmore said:I'm working on the math. I'm a little slow, but I get it eventually.
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.GregAshmore said:From what I have read, two separate corrections are applied--one for SR and another for GR.
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.GregAshmore said:"Fundamentally" mistaken? That's a bit too strong, I think.
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.GregAshmore said: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.
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).GregAshmore said: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.
I described and explained Menzel's scene in post #18:JM said:Greg
Where is the scene of Menzel described?
ghwellsjr said: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.
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.JM said: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
JesseM said: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).
bcrowell said:An extreme position. I certainly don't agree.
ghwellsjr said: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.
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.JM said:Thus the startling result is that sumultaneity is not relative!
nismaratwork said: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.
bcrowell said: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.
I agree with this...nismaratwork said: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.
...but not with this.nismaratwork said:I'm not saying that's a bad thing, but it's true, and it makes 'why' something for philosophy or theology.
bcrowell said:I agree with this...
...but not with this.
ghwellsjr said:,it cannot be true that the light arrives at the remote locations simultaneously for two observers with a speed difference between them.Have you read post 55 and the related section of Einsteins 1905 Paper? Do you disagree with something in Einsteins analysis?
JM