Mentor

## Relativity of Simultaneity

 Quote by klyde If, as you say, the clocks are not synchronized (in the 2nd view), then why not? (How did they become asynchronous?) (Did they become asynchronous due to the given assumption of the invariance of light's one-way speed? If so, then you are merely assuming that which you are trying to prove.)
My example is not an attempt to prove that the speed of light is invariant, merely to give an example of how distance and time measurements turn out in the two frames, in such a way as to make the ratio Δx/Δt come out to be the same in both, for a light pulse. I mistakenly thought that's what you were asking for.

As you're probably aware, the invariance of the speed of light is a fundamental postulate of special relativity, and as such cannot be logically proved or disproved from first principles, at least not in the usual derivation of SR from Einstein's two postulates. It can only be verified or disconfirmed by experiment, either by measurements of the speed of light itself, or by other tests of the entire theory that has been built on that principle.

The theory as a whole has been extensively confirmed experimentally, as described here:

http://www.edu-observatory.org/physi...periments.html

 Quote by klyde If, as you say, the clocks are not synchronized (in the 2nd view), then why not? (How did they become asynchronous?) (Did they become asynchronous due to the given assumption of the invariance of light's one-way speed? If so, then you are merely assuming that which you are trying to prove.)
 Quote by klyde And if the clocks of any observer are asynchronous, then how can they correctly measure time spans, including light's one-way travel time?
They don't correctly measure light's one-way time . This is fundamentally unmeasurable.
And as you correctly noted above this is simply a convention.The implementation of an unproved assumption.
They correctly measure time spans because those time intervals as indicated by those conventionally "unsynchronous"clocks correctly correspond to the spacetime at the given velocity and correspond to the physical values of mechanics and electrodynamics etc in that frame.

 Quote by klyde Are asynchronous clocks the cause of said length contraction? (If so, then said contraction is bogus.)
I will give you my take which may not reflect the consensus but may still be helpful.
Length contraction has two separate causal sources.
1) A mechanical source due to the lightspeed interactions of atomic tensile and nuclear forces within the atomic structure of bodies. AN"actual" physical contraction.
2) Purely kinematic contraction due to clock desynchronization.
Individually both these concepts are asserted by different people and different circumstances. My feeling is that both are operative to varying indeterminate degrees in virtually all cases. But like so much in our relative universe there is no way to quantify or evaluate in any real sense which is the cause and to what extent in any particular case.
In neither case is it bogus.

 Quote by klyde Are asynchronous clocks the cause of time dilation? (If so, then said dilation is bogus.)
Likewise for dilation. Clearly there is actual dilation which has nothing to do with clock synch EG. The returning twin.
But in a case of a single inertial clock traveling between two points and clocks in another frame then the ratio of this clocks proper time to the the coordinate delta t of those two clocks does involve consideration of relative synchronization between those clocks.

 Quote by klyde If the observers' rulers actually contract, then how can they make correct measurements?
If everything contracts equally why wouldn't they make accurate measurements??

 Quote by klyde Why has no actual experiment ever shown the invariance of light's one-way speed?
FOr the mentioned reasons. It is unmeasurable.
The 2nd postulate does not actually state that the measured one-way speed of light is invariant. It says that the speed of light is independe3nt of the motion of the source and is everywhere constant in vacuum.
It is measured as invariant by convention which is no real measurement at all.

 Quote by klyde And as he later says on page 87, this forced "clock synchronization" actually causes the clocks in each frame to NOT be absolutely or truly synchronous.
yes that is obviously true. This is an implicit axiom of SR, that system clocks are only operationally synchronous with no implication of actuality or absoluteness.. The only absolute or actual simultaneity is in the case of co-located occurrence of events.

 Quote by klyde Now we can clearly see why no experiment has shown one-way light speed invariance. It simply cannot happen experimentally because light's one-way speed actually varies with frame velocity, as would readily be seen if absolutely synchronous clocks were used.
Of course all experiments I.e.actual one way measurements of light speed do show invariance so your first statement here is not really correct but I would certainly agree that the actual one-way speed of light does vary with the velocity (unknown) of the frame. so those measurements have no real meaning . And so the actual relative velocity of the light or velocity of the measuring frame is also completely indeterminable.
 Recognitions: Science Advisor Staff Emeritus I haven't seen the point I'm about to make in the literature, (at least not very directly) but I think it's a good one. There is one, and only one, way of synchronzing clocks that will make Newton's laws work correctly. To see this, just imagine having two equal masses with two equal velocities collide and then come to a complete stop - perfectly standard, and routine. Now, tweak the clock synchronization you used to measure the velocities and repeat. The velocities you measure are no longer equal. But the colliding masses still stop. They don't care how you synchronize your clocks. Thus, there is one and only one clock synchronization that will make Newton's laws work. One way of viewing relativity is to say that this method is always equivalent to Einstein's much simpler method, using light signals.

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Quote by klyde
 Quote by ghwellsjr Relativity of simultaneity is the result of both the principal of relativity (Einstein's first postulate) and his second postulate, that light propagates at speed c in any and all inertial frames. {quote paraphrased}
That 2nd postulate is giving me fits. Supposedly, it says that light's speed from point A to point B (in any inertial frame) is c, but I can't for the life of me see how this can happen. Would someone please provide an example using one or two inertial frames?
 Quote by klyde ...May I humbly suggest that Mr. ghwellsjr himself provide an example? (He seems to have faded away for some reason!) ... My problem is that I do not see how it is possible for light's one-way speed to be measured as c (relative to the Frame A observers). Can anyone show me how this can happen, if only on paper??
No one can show you how to measure the one-way speed of light because it is impossible to do. Here is a thread that deals directly with that topic: