Relativity of Simultaneity

jtbell

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

Austin0

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.


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.

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.



If everything contracts equally why wouldn't they make accurate measurements??

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.


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.

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.

pervect

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.

ghwellsjr

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:
http://physicsforums.com/showthread.php?t=656924
You might also want to study the wikipedia article on The One-Way Speed of Light.

Once you understand that the one-way speed of light cannot be measured, you can see how Einstein's second postulate arbitrarily resolves that dilemma and provides a simple and consistent way to coordinate all the measurements that we can make. It works by the way he defines an Inertial Reference Frame (IRF) and how we can use the Lorentz Transformation to convert all the coordinates of the events in the first IRF into coordinates in any other IRF we choose. I have posted numerous diagrams for different scenarios that illustrate how this works. In every case, the one-way speed of light is always c, and yet everything that every observer sees or measures remains the same.

Here is a thread that shows a scenario where travelers go to a distant star that I think might help you:

http://physicsforums.com/showthread.php?t=659389
You can do a search for "diagram" under my name and find more.