Einstein's second postulate (one-way speed of light)

[ghwellsjr]

I have been engaged in several different threads, all relating to comments that I have posted regarding Einstein's second postulate that the one-way speed of light is c in all inertial frames. Rather than continue different aspects of these conversations in these different threads, I have decided to start this new thread with my position stated as clearly as possible.

Prior to Einstein's paper in 1905, virtually all scientists believed in an absolute aether which propagated light at the speed of c. You can think of this as a preferred reference frame in Special Relativity (SR), although no one knew where it was or how to define it. They just thought it existed somewhere. As a result, when they did their experiments, such as MMX which attempted to measure the Earth's motion through the aether, and they got a null result, they believed that their apparatus was experiencing length contraction when the light paths were aligned with the aether wind and they believed that the time it took for light to traverse the distance of the apparatus was different in one direction than in the other. They also believed in time dilation to explain why they couldn't measure variations in the round-trip speed of light. In fact, they were coming to realize that there was no experiment they could do that would reveal the stationary frame of the aether.

What Einstein did was postulate that in all cases, no matter what the constant motion, the time it takes for light to traverse in one direction is that same as it is in the other direction. Note that there is no way to measure the actual times it takes for light to make these two trips, because if there were, then we could identify the state of the aether. If the two times were different in any particular frame of reference, we could make an adjustment in our travel speed or direction until the two times were equal and we would be at rest in the aether frame. Measuring the one-way speed of light turns out to be as impossible as measuring the aether wind, nature just won't let us see it.

There was nothing wrong or inconsistent with what the scientists believed prior to SR, they were basing it on a different postulate which is that light propagates at one speed in only one frame of reference in which the aether is at rest, it's just that they couldn't decide which frame it was, but they could have picked one, at random, or with some rationale behind it and ended up with a consistent, albeit clumsy, science.

Instead, they opted for the "simple" theory (as Einstein called it) where things behave as though they are at rest with respect to the aether, in any reference frame.

 

[HallsofIvy]

I have been engaged in several different threads, all relating to comments that I have posted regarding Einstein's second postulate that the one-way speed of light is c in all inertial frames. Rather than continue different aspects of these conversations in these different threads, I have decided to start this new thread with my position stated as clearly as possible.

Prior to Einstein's paper in 1905, virtually all scientists believed in an absolute aether which propagated light at the speed of c. You can think of this as a preferred reference frame in Special Relativity (SR), although no one knew where it was or how to define it. They just thought it existed somewhere. As a result, when they did their experiments, such as MMX which attempted to measure the Earth's motion through the aether, and they got a null result, they believed that their apparatus was experiencing length contraction when the light paths were aligned with the aether wind and they believed that the time it took for light to traverse the distance of the apparatus was different in one direction than in the other. They also believed in time dilation to explain why they couldn't measure variations in the round-trip speed of light. In fact, they were coming to realize that there was no experiment they could do that would reveal the stationary frame of the aether.

This isn't true. Accoridng to the "absolute aether" theory, the Michelson-Morley experiment of 1887 should have given a non-null result- the motion of the Earth relative to the "absolute aether". It didn't. That was why relativity was needed.

What Einstein did was postulate that in all cases, no matter what the constant motion, the time it takes for light to traverse in one direction is that same as it is in the other direction. Note that there is no way to measure the actual times it takes for light to make these two trips, because if there were, then we could identify the state of the aether. If the two times were different in any particular frame of reference, we could make an adjustment in our travel speed or direction until the two times were equal and we would be at rest in the aether frame. Measuring the one-way speed of light turns out to be as impossible as measuring the aether wind, nature just won't let us see it.

There was nothing wrong or inconsistent with what the scientists believed prior to SR, they were basing it on a different postulate which is that light propagates at one speed in only one frame of reference in which the aether is at rest, it's just that they couldn't decide which frame it was, but they could have picked one, at random, or with some rationale behind it and ended up with a consistent, albeit clumsy, science.

Instead, they opted for the "simple" theory (as Einstein called it) where things behave as though they are at rest with respect to the aether, in any reference frame.

 

[Dale]

I am not very strong on the history here, but I believe that the various aether theories prior to 1887 were indeed as HallsOfIvy describes, but that sometime between 1887 and 1905 Lorentz developed his aether theory which included the Lorentz transforms. Then SR came in 1905 essentially as a new derivation of the Lorentz transforms and therefore was experimentally indistinguishable from Lorentz's aether theory as ghwellsjr describes.