[lightarrow]

While I am absolutely convinced that there is a speed difference in the one-way speed of light, it is also apparently very difficult to measure.  While they could have measured the one way speed of light, it is apparently not possible to do so using their methods.

Light goes the same speed in all directions.

If light went different speeds in different directions, it would be easily shown by measuring distances; if some light was going at an angle to the Earth's motion measured against an ether you would get very different distances in one direction than another.

That's basically how the Michelson-Morley experiment worked.

Yes, but what CliffordK intended to say is something that is not well specified in relativity: every time we talk about the speed of light in one direction ("direction" is one of the infinite straight lines passing by the origine, with arbitrary angle) we always intend "go and return" speed: the one-way speed of light is not measurable with standard techniques (and I don't know if it's possible at all and, in case, how).

[yor_on]

I don't think it is possible Lightarrow. A 'one way measure' of the speed of light seems more and more like a pipe-dream Although I wondered abut it recently.

[wolfekeeper]

Nope.

In relativity, the one way and two way and three way and... speed of light in a vacuum is the same in all directions at all times.

You can synchronise clocks, and slowly separate them, and then measure the one-way speed of light.

[lightarrow]

Nope.

In relativity, the one way and two way and three way and... speed of light in a vacuum is the same in all directions at all times.

You can synchronise clocks, and slowly separate them, and then measure the one-way speed of light.

The problem is that, whatever way you use to synchronise clocks, you have to use the speed of light (or the method used, even the slow transport of physical clocks, is equivalent to using a light signal for synchronisation).
http://plato.stanford.edu/entries/spacetime-convensimul/#3

http://en.wikipedia.org/wiki/One-way_speed_of_light

[wolfekeeper]

I wondered if you'd argue that.

In that case, if you think that nature is conspiring to make it impossible to distinguish the one way speed of light from the two way, then it's completely equivalent to the speed of light being the same in all directions.

[lightarrow]

If it were as you say, that is if nature really did this, I would agree with you.
But consider this: imagine that you are in a region of spacetime which is almost uniformly dragged by GR effects (for example the Lense-Thirring effect of a very far and massive object); at all practical effects it would be an inertial frame of reference. But light speed would be different in the two opposite directions along the drag.

[yor_on]

"It is easily demonstrated that if two clocks are brought together and synchronized, then one clock is moved rapidly away and back again, the two clocks will no longer be synchronized.

If however one clock is moved away slowly and returned the two clocks will be very nearly synchronized when they are back together again. The clocks can remain synchronized to an arbitrary accuracy by moving them sufficiently slowly. If it is taken that, if moved slowly, the clocks remain synchronized at all times, even when separated, this method can be used to synchronize two spatially separated clocks. In the limit as the speed of transport tends to zero, this method is experimentally and theoretically equivalent to the Einstein convention."

And how do you compensate for gravitational time dilations, and Lorentz contractions? Nist has already showed time dilations here on earth, and for that your 'relative motion' doesn't matter at all. Eh, I mean, the gravitational potentials will be there, and the 'slower' your relative motion, the longer the time you will be (locally) in those potentials.

[yor_on]

I know, a gravitational 'Lorentz contraction' is arguable, but the equivalence must be there as I see it.

[Geezer]

Surely all you need is three clocks. They all start off at the same position, then you move two of them away in opposite directions at the same speed for an equal distance. Now measure the delay for light to travel from the one in the middle to the other two.

What am I missing?  []
 

[imatfaal (OP)]

Surely all you need is three clocks. They all start off at the same position, then you move two of them away in opposite directions at the same speed for an equal distance. Now measure the delay for light to travel from the one in the middle to the other two.

What am I missing?  []
 

Or why not measure the speed from the two that went in opposite directions - forces and accelerations will have been same magnitude  (opposite sign) - so any dilation would have been the same; surely the clocks must remain synchronized.

I don't know what you are missing - but I expect I am missing it too.

[yor_on]

Surely all you need is three clocks. They all start off at the same position, then you move two of them away in opposite directions at the same speed for an equal distance. Now measure the delay for light to travel from the one in the middle to the other two.

What am I missing?  []
 

Are you thinking of using some sort of radial measurement(s) to compensate for gravity?  As being in the spoke sending out one way light beams in all directions to detectors and then count on the differences trying for an average 'one way' speed? A sort of 'weak measuring', well, sort of? Or maybe its me missing the point here totally

For me the point is that as long as we count in 'gravity' it 'fluctuates', and that will influence the clocks as I see it.

The 'two way speed' using a reflecting mirror is isotropic (the same in any direction) in SR, but the one way speed is not invariant with respect to direction (anisotropic), all as I understands it.

"3.2 One-Way Tests of Light-Speed Isotropy

Note that while these experiments clearly use a one-way light path and find isotropy, they are inherently unable to rule out a large class of theories in which the one-way speed of light is anisotropic. These theories share the property that the round-trip speed of light is isotropic in any inertial frame, but the one-way speed is isotropic only in an aether frame. In all of these theories the effects of slow clock transport exactly offset the effects of the anisotropic one-way speed of light (in any inertial frame), and all are experimentally indistinguishable from SR. All of these theories predict null results for these experiments. See Test Theories above, especially Zhang (in which these theories are called “Edwards frames”)."  From What is the experimental basis of Special Relativity?

=

I need to think some more about this one in fact.
Da*', I should get some sleep..

==

Okay, one simple argument.

The easiest to see, for me at least, is that if light speed only is invariant locally for any inertial (uniformly moving) system as Earth, meaning that all measures you do must be done by yourself locally, using your 'wristwatch' and ruler, then it has to follow that the two way experiment is the only one that can fulfill that condition.

At the two way experiment you use a mirror to reflect the light you sent, back to you, and your detector. It also follows from that experiment that all directions are the same (isotropic) as you can turn your contraption (with mirror) around and find it to give the same 'speed' similar to the MM experiment. "The two scientists Michelson and Morley set up an experiment to attempt to detect the ether, by observing relative changes in the speed of light as the Earth changed its direction of travel relative to the sun during the year.  To their surprise, they failed to detect any change in the speed of light."


In a one way experiment there is no way, that I see now, allowing you to send and measure it locally. And if it isn't locally then you will find different clock beats. That as there is no way you can formulate a theory introducing relative motion, and acceleration, as you displace the 'clocks' from each other that will give you a inertial frame. And NIST has shown us that clocks vary with gravity relative a observer even on small scales, and gravity is equivalent to an acceleration. Even if ignoring that we also know that relative motion also will dilate the clock beats.

"In a study published today in the journal Science, researchers at the National Institute of Standards and Technology explain that a one-foot difference in altitude between two clocks caused them to tick at slightly different rates. The optical clocks can even measure changes in the passage of time caused by a 20-mile-per-hour speed difference. The clocks are based on the oscillations of a single aluminum ion that vibrates between two energy levels a million billion times per second. One clock is accurate to within one second in about 3.7 billion years, and the other is almost as accurate, NIST says.


In one experiment, James Chin-Wen Chou and his colleagues placed one clock about 13 inches higher than its counterpart. The higher clock felt less gravity, because it was a teeny bit farther from Earth’s gravitational field. It ticked more slowly — albeit a tiny, tiny bit more slowly. The time difference adds up to about 90 billionths of a second over a 79-year lifetime, according to NIST.

Still, this means that the people who conducted this study, in Boulder, Colo., are apparently aging faster than those of you reading this at sea level. In another experiment, the NIST scientists also observed that time passes more slowly when you move more quickly — a key tenet of relativity — even at very small speed variations. Clocks ticked more slowly at a difference of just 20 miles per hour, they say."

Don't know if it is good enough though We'll see when I wake up.
Sleep..

[lightarrow]

Surely all you need is three clocks. They all start off at the same position, then you move two of them away in opposite directions at the same speed for an equal distance. Now measure the delay for light to travel from the one in the middle to the other two.

What am I missing?  []
 

Clock A is the middle one. How do you know where exactly are clock B and clock C at a given time to say "at the same speed for an equal distance"?
 []

[Geezer]

Surely all you need is three clocks. They all start off at the same position, then you move two of them away in opposite directions at the same speed for an equal distance. Now measure the delay for light to travel from the one in the middle to the other two.

What am I missing? 
 

Clock A is the middle one. How do you know where exactly are clock B and clock C at a given time to say "at the same speed for an equal distance"?
 

I think a tape measure would work!

[lightarrow]

Surely all you need is three clocks. They all start off at the same position, then you move two of them away in opposite directions at the same speed for an equal distance. Now measure the delay for light to travel from the one in the middle to the other two.

What am I missing? 
 

Clock A is the middle one. How do you know where exactly are clock B and clock C at a given time to say "at the same speed for an equal distance"?
 

I think a tape measure would work!

But how you know that exactly at 12:00:00 (middle clock time) clock B and clock C are at the same "L" distance from A, if you still have to syncronise them?
I mean, it's not enough to say that clock B and clock C are at 10,000 km from A, even if they stopped there: you have to know when they arrived.

[Geezer]

Surely all you need is three clocks. They all start off at the same position, then you move two of them away in opposite directions at the same speed for an equal distance. Now measure the delay for light to travel from the one in the middle to the other two.

What am I missing? 
 

Clock A is the middle one. How do you know where exactly are clock B and clock C at a given time to say "at the same speed for an equal distance"?
 

I think a tape measure would work!

But how you know that exactly at 12:00:00 (middle clock time) clock B and clock C are at the same "L" distance from A, if you still have to syncronise them?
I mean, it's not enough to say that clock B and clock C are at 10,000 km from A, even if they stopped there: you have to know when they arrived.

Well, I was only thinking of moving them a few meters  []
 
You know the distances between A, B and C. You don't care much when B and C departed and arrived. All you care about is that their accelerations (according to their own times) were the same.
 
When they stop at B and C, they should be equally dilated with respect to A. If they are not, we have a problem!
 
 

[yor_on]

"According to their own time"
hmm?

Did you mean that relative A:s local clock, and ruler, both B and C had the same acceleration and distance done? So that according to A both B & C has an equivalent displacement in 'time' and 'space' as they (finally) stand still relative A?

And for this one we're assuming SR with no gravity, right, a 'flat' SpaceTime?

[Geezer]

Well, see, acceleration is a function of time, so it can only be really meaningful to the clock of the thing that is accelerating, if you see what I mean. An observer can't properly determine the acceleration of another clock, because the observer has no idea what that clock is doing.

[yor_on]

I better put up a warning text here. I've a 'lubricated' brain, after all, it's Friday..

So, two pubs later I 'm totally ready for a debate

No Geezer, that one is the other way around. From the point of relativity all clocks are locally defined, but from the point of the observer (which is the one that counts for locally determine a 'speed' in a one way experiment) it must be A that define both B and C 'relative' time and displacement. It's the 'local observer' doing the experiment that will define both 'distance' and 'times arrow' for what he observes. And he will do it in a very egoistic way By using his wrist watch, and his ruler. After that he will measure the time in distance it takes for that light pulse to 'propagate' from?? to ??.

[yor_on]

What I mean with 'From the point of relativity all clocks are locally defined' is that I think I can see how you mean there. It's the same weird way that I think of it. Somehow that very local definition of 'reality' is 'shared', as the only thing needed to prove that conclusion is to superimpose two 'observers', to  then find them both share the same common 'local time and distance(s)'. So I think that is the way you look at it?

[Airthumbs]

 O well........ back to the drawing board!