Unable to see how light's one-way speed can be c experimentally

[BluMuun]

It has been said that special relativity calls for light's one-way speed to be exactly c per all inertial coordinate systems. I have been trying to picture how this can happen experimentally, but cannot come up with a scenario. Perhaps someone here can help?

 

[ghwellsjr]

It has been said that special relativity calls for light's one-way speed to be exactly c per all inertial coordinate systems. I have been trying to picture how this can happen experimentally, but cannot come up with a scenario. Perhaps someone here can help?

Nope, no one can help. Light's one-way speed of exactly c is Einstein's second postulate on which Special Relativity is based. It's consistent with all experiments but it cannot come from an experiment.

 

[Nugatory]

It has been said that special relativity calls for light's one-way speed to be exactly c per all inertial coordinate systems. I have been trying to picture how this can happen experimentally, but cannot come up with a scenario. Perhaps someone here can help?

Can you turn this around? Give us a scenario in which the experimental results would not be consistent with the speed of light being exactly c in all inertial coordinate systems?

If you're saying that you can't come up with an experiment that shows that one-way speed of light is c in all inertial coordinate systems, you're absolutely right. The one-way speed of light is a postulate and cannot be proven by experiment (of course it could be disproven, bu that hasn't happened).

 

[Dale]

Here is an excellent review of the various experimental tests of SR. It includes information about tests of the postulates in section 3.

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

 

[BluMuun]

[2nd try at posting this]

Thanks for the replies, guys.

Nugatory asked about the opposite scenario, and luckily for me Einstein had already been there and done that, as follows:

During the creation of relativity (SR), Einstein noted that prior to SR it was possible for light's one-way speed to be c for one observer but c - v for another. It was this scenario that gave Einstein a major pain and caused him to create SR on the spot. http://www.bartleby.com/173/7.html

Einstein's cause for concern was his notion that the above one-way results conflicted with the principle of relativity. (Einstein's w = c - v is not a closing velocity - such a velocity would not even apparently conflict with the PR.)

As for the postulate part, it seems to me that both Nugatory and Wells are slightly confusing a math axiom or postulate with a physical or scientific postulate.

A scientific postulate is a guess, a supposition, a hunch, or a hypothesis about the nature of nature.

Indeed, as Nugatory noted, all scientific postulates must be experimentally testable.

So this leads to the question How can SR's one-way invariance be tested?

 

[Dale]

See the reference in my post above.

 

[BluMuun]

Oddly enough, given all the info at that site that you cited and I sighted, one would think that the direct experiment would be mentioned somewhere. By "direct" I mean using two mutually-at-rest clocks that are not rotating and have not been transported (because such clocks run slow).

SR says that light's one-way, two-clock speed (with the caveats above) must be c in all inertial frames, but I am still unable to see how it can be c in any inertial frame.

Indeed, according to John A. Wheeler (in his book _Spacetime Physics_), clocks are simply set by definition to get the chosen value "c." There is really no way to falsify a definition or to even test one.

Color me confused.

 

[ghwellsjr]

Oddly enough, given all the info at that site that you cited and I sighted, one would think that the direct experiment would be mentioned somewhere. By "direct" I mean using two mutually-at-rest clocks that are not rotating and have not been transported (because such clocks run slow).

SR says that light's one-way, two-clock speed (with the caveats above) must be c in all inertial frames, but I am still unable to see how it can be c in any inertial frame.

Indeed, according to John A. Wheeler (in his book _Spacetime Physics_), clocks are simply set by definition to get the chosen value "c." There is really no way to falsify a definition or to even test one.

Color me confused.

If you are "unable to see how it can be c in any inertial frame", then it should be easy for you to falsify the definition, shouldn't it?

If light propagated at c only in a single absolute ether rest state, and if the Principle of Relativity were not true, then it would be very easy to falsify Einstein's second postulate, wouldn't it?

 

[Vanadium 50]

Be careful - most articles about the one-way speed of light are written by crackpots.

Professionals don't worry about this because it's an underconstrained system. There are two things you need to know - the clock synchronization convention and the one-way speed of light. You can't determine two things with a single measurement.

A similar example: the sun is directly overhead. What is your longitude?

 

[Dale]

Oddly enough, given all the info at that site that you cited and I sighted, one would think that the direct experiment would be mentioned somewhere. By "direct" I mean using two mutually-at-rest clocks that are not rotating and have not been transported (because such clocks run slow).

How would you suggest synchronizing the clocks in such an experiment?

 

[Nugatory]

SR says that light's one-way, two-clock speed (with the caveats above) must be c in all inertial frames, but I am still unable to see how it can be c in any inertial frame.

I asked a while back in this thread, and I'm asking again... What is a situation in which you find this assumption problematic? That is, can you show us an example of how it's hard to see that the one-way speed of light could be c in all inertial frames? (It's easy to find situations in which if you assume that the one-way speed of light is not c in all inertial frames you get horrible complications).

Indeed, according to John A. Wheeler (in his book _Spacetime Physics_), clocks are simply set by definition to get the chosen value "c."

That's true. We do that because if we set clocks any other way, we get various horrible complications, or internal contradictions, or results that don't match experiment. Thus, even though it is not possible to measure the one-way speed of light (dig deep into any experiment that appears to measure the constancy of the one-way speed of light and you'll find that the clocks used to measure the travel time were synchronized using some procedure that assumes what we're trying to prove) it is very convenient to assume that the one-way speed of light is constant.

There is really no way to falsify a definition or to even test one.

If the definition leads to a contradiction it's falsified. If the definition leads to predictions that don't match experimental results, it's falsified. The one-way speed of light assumption has held up just fine against challenges of this sort and there are no alternatives that do so as well.

 

[ghwellsjr]

Indeed, according to John A. Wheeler (in his book _Spacetime Physics_), clocks are simply set by definition to get the chosen value "c."

Could you please supply the page number from which you took this quote?

 

[harrylin]

[..]
A scientific postulate is a guess, a supposition, a hunch, or a hypothesis about the nature of nature.

Indeed, as Nugatory noted, all scientific postulates must be experimentally testable.

So this leads to the question How can SR's one-way invariance be tested?

A simple, direct test is completely useless, insofar as two-way light speed has been tested already. That's due to the synchronization convention which makes the one-way speed of light equal to the (average) two-way speed of light. As a matter of fact, that's already implied in the first section of Einstein's 1905 paper : http://www.fourmilab.ch/etexts/einstein/specrel/www/
Thus the one-way speed is made equal to the two-way speed.

The self-consistency can be tested by indirect measurements, such as for example a measurement of the speed of light after clock transport. For example one synchronizes two clocks that one next moves apart very slowly or at equal speeds relative to the lab. A light signal that is next sent one way should be recorded with those clocks as taking the same time as a signal that is sent the other way. Similarly, one could rotate the setup (or wait for the lab to rotate), and again the measured speeds should be c in both directions.

ADDENDUM: those are tests of the constancy of the one-speed of light - that is, using the same reference system. For such a test one doesn't need to re-synchronize the clocks, as they should remain in sync with each other wrt the lab. However, in order to find that the one-way speed of light is invariant (between inertial frames that are accelerated to different velocities) one must in general first re-synchronize each pair of clocks after acceleration.

 

[stevendaryl]

Could you please supply the page number from which you took this quote?

I don't have Wheeler's book, but I think that the original poster is talking about the Einstein synchronization convention: You set two distant, comoving clocks so that the one-way time for a light-signal is half the round-trip time. This makes light-speed isotropic by definition.

This doesn't make SR into a tautology, because there is more than one way to synchronize clocks. For example, slow clock transport: You synchronize two clocks, then slowly (at a speed much less than the speed of light) separate them. SR says that two clocks synchronized by this definition will also be synchronized by the Einstein synchronization convention.

 

[stevendaryl]

It has been said that special relativity calls for light's one-way speed to be exactly c per all inertial coordinate systems. I have been trying to picture how this can happen experimentally, but cannot come up with a scenario. Perhaps someone here can help?

Conceptually, the empirical meaning of the constancy of the one-way speed of light is given by:

1. Pick an inertial reference frame F. Assume that we have a large, flat platform at rest in F to do our experiments on.
2. Draw a straight line on the platform, and mark two spots on the line a distance L apart (as measured by meter sticks at rest in F).
3. Take two identical clocks, bring them together at one of the marked spots and synchronize them.
4. Move one of the clocks at constant speed to the other spot, and then bring it back to rest in frame F. Let v be the speed that the clock was transported (as computed by v=L/\delta t, where \delta t is the elapsed time on the transported clock.
5. At time t_0 according to the first clock, send a light signal toward the second clock.
6. Let t_1 be the time the signal arrives, according to the second clock.
7. In the limit of slow clock transport (v \rightarrow 0), the ratio L/(t_1 - t_0) will approach a constant c.
8. This constant is found to be the same in every inertial reference frame F.

I think it's clear how it could work this way in a single frame. Then the question is: How could work this way in EVERY frame?

Well, the Lorentz transformations relating inertial coordinates in different rest frames show how it could work in every frame.

 

[stevendaryl]

Conceptually, the empirical meaning of the constancy of the one-way speed of light is given by:

1. Pick an inertial reference frame F. Assume that we have a large, flat platform at rest in F to do our experiments on.
2. Draw a straight line on the platform, and mark two spots on the line a distance L apart (as measured by meter sticks at rest in F).
3. Take two identical clocks, bring them together at one of the marked spots and synchronize them.
4. Move one of the clocks at constant speed to the other spot, and then bring it back to rest in frame F. Let v be the speed that the clock was transported (as computed by v=L/\delta t, where \delta t is the elapsed time on the transported clock.
5. At time t_0 according to the first clock, send a light signal toward the second clock.
6. Let t_1 be the time the signal arrives, according to the second clock.
7. In the limit of slow clock transport (v \rightarrow 0), the ratio L/(t_1 - t_0) will approach a constant c.
8. This constant is found to be the same in every inertial reference frame F.

I think it's clear how it could work this way in a single frame. Then the question is: How could work this way in EVERY frame?

Well, the Lorentz transformations relating inertial coordinates in different rest frames show how it could work in every frame.

Note that this operational meaning of lightspeed does NOT make it true by definition that light has one-way speed c.

 

[Dale]

SR says that light's one-way, two-clock speed (with the caveats above) must be c in all inertial frames, but I am still unable to see how it can be c in any inertial frame.

If you synchronize the two clocks using Einstein's synchronization convention then it is guaranteed to be c. Do you understand why?

Indeed, according to John A. Wheeler (in his book _Spacetime Physics_), clocks are simply set by definition to get the chosen value "c." There is really no way to falsify a definition or to even test one.

I agree. In the reference I posted above, that is what they are talking about when they say:

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”).

 

[BluMuun]

GHWells noted:
>If you are "unable to see how it can be c in any inertial frame", then it should be easy >for you to falsify the definition, shouldn't it?

>If light propagated at c only in a single absolute ether rest state, and if the Principle of >Relativity were not true, then it would be very easy to falsify Einstein's second >postulate, wouldn't it?

You cannot falsify a definition because it is a given (given by man, not nature). All two-clock measurements in relativity are given by definition (the synchronization definition), including light's one-way speed and the transformation equations. As long as we are given Einstein's clocks, we are stuck with his "c invariance."

Also note that the principle of relativity does not call for c invariance, but only for the same laws. It does not say that frames are indistinguishable, but that their laws are indistinguishable. For example, if the one-way, two-clock light speed law is c ± v (as in Einstein's own example ref'd above), then all frames share the same law, but they are distinguishable (and can detect their absolute motion).

But not only is c invariance "given" by definition, it cannot happen even on paper as far as I can see as long as two mutually-at-rest clocks are used. (No transported or rotated clocks allowed because they run slow.)

For Wells: Wheeler's stuff came from Wheeler & Taylor's book
_Spacetime Physics_, 1963 edition, page 18.

Dalespam asked:
"How would you suggest synchronizing the clocks in such an experiment?"

The same way clock were set on paper for 100's of years prior to Einstein. Can you tell me why Einstein decided to get rid of such clocks?

Nugatory asked re no experiment showing c invariance:
"I asked a while back in this thread, and I'm asking again... What is a situation in which you find this assumption problematic? That is, can you show us an example of how it's hard to see that the one-way speed of light could be c in all inertial frames? (It's easy to find situations in which if you assume that the one-way speed of light is not c in all inertial frames you get horrible complications)."

To put the ball back in your court, please show on paper any frame getting c for the one-way, two-clock speed of light (not using slowed clocks as in clock transport or in the case of rotated clocks). As I see it, this must be possible or the c invariance of SR is not scientific.

And I am very interested in the "horrible complications" of not getting c? Can you cite one please?

Nug also wrote:
"Thus, even though it is not possible to measure the one-way speed of light (dig deep into any experiment that appears to measure the constancy of the one-way speed of light and you'll find that the clocks used to measure the travel time were synchronized using some procedure that assumes what we're trying to prove) it is very convenient to assume that the one-way speed of light is constant."

What does "the one-way light speed is constant" mean if it is not an experimental (or a possible experimental) result? Of what use is it to physics?

Why is it convenient to assume that which cannot happen?

DaleSpam quoted:
"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."

Sorry, but this is not true. As Einstein said, a theory that contains truly synchronous clocks predicts a variable one-way, two-clock light speed, unlike SR.
http://www.bartleby.com/173/7.html

For all of those who mentioned clock transport: I said up front that this is not allowed because moving clocks run slow, no matter how slowly they move. (I also disallowed rotating clocks for the same reason.)

In other words, I am looking for a legit experiment that shows a two-clock one-way light speed of c in any inertial frame, much less many.

DaleSpam wrote:
>If you synchronize the two clocks using Einstein's synchronization convention then it is >guaranteed to be c. Do you understand why?

But it is not via experiment; the clocks are merely manually forced to get "c," as Wheeler said.

Nugatory noted:
"If the definition leads to a contradiction it's falsified. If the definition leads to predictions that don't match experimental results, it's falsified. The one-way speed of light assumption has held up just fine against challenges of this sort and there are no alternatives that do so as well."

How can a definition lead to any contradictions if it has been assumed that any and all results of the definition are acceptable? For example, if two same-frame SR clocks are
compared with a passing clock, then the result is that "the passing clock ran slow." This may or may not be true when it comes to the intrinsic atomic rhythm of the clock, but it is accepted in SR as a valid and meaningful result. (However, it does not seem to be all that meaningful when you consider the fact that SR also has the other frame's clock running slower so that two clocks both run slower than each other. Is this really physics?)

I am not trying to be argumentative, but just trying to see how any frame's observers could use two mutually-at-rest clocks to get a one-way light speed of c experimentally. Of course, one of you said this:

stevendaryl noted:
"This doesn't make SR into a tautology, because there is more than one way to synchronize clocks. For example, slow clock transport: You synchronize two clocks, then slowly (at a speed much less than the speed of light) separate them. SR says that two clocks synchronized by this definition will also be synchronized by the Einstein synchronization convention."

Everyone agrees that using slowed clocks can result in approximate c invariance, but what I am wondering about is what happens using two mutually-at-rest clocks because SR says that this will also result in c (exactly c) experimentally. To me, these are two entirely different things (slow transport & true c invariance).

Harrylin wrote:
>A simple, direct test is completely useless, insofar as two-way light speed has been >tested already.

But SR did not predict two-way invariance - this was given prior to SR via a direct experiment. This left only the one-way case for SR to consider, and SR claims that light's one-way speed is c in all frames. Is this via experiment or merely given by definition? Can it even happen on paper?

If c invariance cannot happen, then it would seem to be of no real use to physics. This is the only reasonable conclusion that I see.

 

[Nugatory]

And I am very interested in the "horrible complications" of not getting c? Can you cite one please?

One such problem is that we can calculate the speed of light from Maxwell's laws of electricity and magnetism, and Maxwell's equations make no allowance whatsoever for the velocity of the observer. Thus, if we don't accept that the one-way speed of light is c for all observers, we are obliged to construct an internally consistent theory in which observers moving relative to one another experience different laws of electricity and magnetism, or to reject Maxwell's equations.

Rejecting Maxwell's equations doesn't work. There's just too much experimental evidence from the past few centuries of studying electricity and magnetism.

Different laws of electricity and magnetism for people moving at different speeds would imply that our observations on Earth would change with the seasons because of the Earth's motion around the sun. They don't, to the limits of accuracy of some very precise experiments.

Some history: Maxwell's laws were discovered in 1861. Reconciling them with the intuitive notion that if you're moving at speed $u$ relative to me and I measure the speed of a light signal moving in the same direction as $c$, you will measure the speed as $c-u$ was one of the great challenges of the second half of the 19th century. That's why when Einstein published his classic paper in 1905 resolving this problem by proposing a consistent and experimentally confirmed theory that assumed a constant one-way speed of light he titled it "On the electrodynamics of moving bodies".

 

[ghwellsjr]

Indeed, according to John A. Wheeler (in his book _Spacetime Physics_), clocks are simply set by definition to get the chosen value "c."

Could you please supply the page number from which you took this quote?

For Wells: Wheeler's stuff came from Wheeler & Taylor's book
_Spacetime Physics_, 1963 edition, page 18.

I have the 1966 edition which you can see http://www.eftaylor.com/pub/spacetime/STP1stEdThruP20.pdf.

I cannot find anything even remotely resembling your quote. Could you please find about where it got edited out?

 

[PAllen]

Dalespam asked:
"How would you suggest synchronizing the clocks in such an experiment?"

The same way clock were set on paper for 100's of years prior to Einstein. Can you tell me why Einstein decided to get rid of such clocks?

That's a total non-answer. Again: how do you think this was done? Tell us how you think clocks were synchronized between e.g. London and Glasgow in the 1800s?

 

[ghwellsjr]

GHWells noted:
>If you are "unable to see how it can be c in any inertial frame", then it should be easy >for you to falsify the definition, shouldn't it?

>If light propagated at c only in a single absolute ether rest state, and if the Principle of >Relativity were not true, then it would be very easy to falsify Einstein's second >postulate, wouldn't it?

You cannot falsify a definition because it is a given (given by man, not nature).

Suppose instead of Einstein's convention, we use one that says that the outbound propagation of light is twice as fast as the inbound. Don't you think that would easily be falsifiable?

All two-clock measurements in relativity are given by definition (the synchronization definition), including light's one-way speed and the transformation equations. As long as we are given Einstein's clocks, we are stuck with his "c invariance."

Also note that the principle of relativity does not call for c invariance, but only for the same laws. It does not say that frames are indistinguishable, but that their laws are indistinguishable. For example, if the one-way, two-clock light speed law is c ± v (as in Einstein's own example ref'd above), then all frames share the same law, but they are distinguishable (and can detect their absolute motion).

But not only is c invariance "given" by definition, it cannot happen even on paper as far as I can see as long as two mutually-at-rest clocks are used. (No transported or rotated clocks allowed because they run slow.)

I don't understand why you say it cannot happen even on paper. I draw spacetime diagrams all the time that illustrate how light propagates at c in any Inertial Reference Frame (IRF) and how the coordinates of all the events can be transformed to any other IRF moving with respect to the original one and yet light still propagates at c and all observers continue to see and measure exactly the same things in all IRF's.

Is it that you have never seen how this happens on paper because you don't understand Special Relativity or is it that you thoroughly understand Special Relativity but still find fault with it?

 

[harrylin]

[..]
Harrylin wrote:
>A simple, direct test is completely useless, insofar as two-way light speed has been tested already.

But SR did not predict two-way invariance - this was given prior to SR via a direct experiment. This left only the one-way case for SR to consider, and SR claims that light's one-way speed is c in all frames. Is this via experiment or merely given by definition? Can it even happen on paper?

If c invariance cannot happen, then it would seem to be of no real use to physics. This is the only reasonable conclusion that I see.

I think to see a slight but perhaps important misunderstanding here. The purpose of SR was to match two apparently contradictory facts that were concluded from observations:

1. The principle of relativity is also valid for light propagation (=>invariance of the two-way speed of light)
2. Maxwell's model of light propagation is valid (=>the speed of light is independent of that of the source)

Those two constraints together led to strong predictions for the two-way speed of light and many other things such as electrons and clocks. If the two-way speed of light is not found to be the same c in different inertial frames then also the one-way speed of light can't be c both ways. As by definition the one one-way speed is made equal to the two-way speed, c invariance means experimentally the invariance of the two-way speed. And that is obviously very useful to physics.

In short, one-way speed can distract from the experimentally verifiable predictions of SR.

Note also, I'm surprised by the following remark:
"what I am wondering about is what happens using two mutually-at-rest clocks because SR says that this will also result in c (exactly c) experimentally. To me, these are two entirely different things (slow transport & true c invariance)."

I somewhat answered that question of yours in my earlier post, before you asked it. However I provided different answers, depending on the exact test conditions. I even indicated how SR predicts that with a certain procedure one can find a value different from c. Either my answers were not clear, or your question was not clear, but I don't know which. Also your remark that "No transported or rotated clocks allowed because they run slow" seems to contradict your earlier question and my earlier answers. If you like, we can elaborate on that.

 

[Dale]

Hi BluMuun, please use the quote and multi-quote features, particularly in a long post like your previous one where you are responding to multiple people.

Dalespam asked:
"How would you suggest synchronizing the clocks in such an experiment?"

The same way clock were set on paper for 100's of years prior to Einstein.

What way was that? I am unaware of anyone addressing clock synchronization issues prior to Einstein.

DaleSpam quoted:
"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."

Sorry, but this is not true.

It is true and is well known in the literature. See for example:
Mansouri, Sexl. A test theory of special relativity: I. Simultaneity and clock synchronization. General Relativity and Gravitation. July 1977, Volume 8, Issue 7, pp 497-513

If you don't have access to that then there is this arxiv reference, but it is a little math-heavy
http://arxiv.org/abs/physics/0510260

It is even mentioned on Wikipedia: http://en.wikipedia.org/wiki/Test_theories_of_special_relativity

>If you synchronize the two clocks using Einstein's synchronization convention then it is >guaranteed to be c. Do you understand why?

But it is not via experiment; the clocks are merely manually forced to get "c," as Wheeler said.

Good, so you do, in fact, see how the one way speed of light can be c in a frame, you simply "manually force" it to be c.

You seem to think that coordinates are somehow out there in nature, waiting to be experimentally determined. They are not. Coordinates are human-made inventions. You have to "manually force" them to be something since nature doesn't provide them. So you have to choose some simultaneity convention. It is just a convention, so there is no "right" or "wrong" way to do it as long as you are consistent.

Einstein's convention is popular because if you use it then Maxwell's equations take their "textbook" form, and otherwise you have to re-write them. The one way speed of light is governed by Maxwells equations, so it is determined by the synchronization convention chosen.

 

[BluMuun]

I was interested in Nugatory's claim that "horrible complications" occur if we do not get c, so I asked him to list one.

One such problem is that we can calculate the speed of light from Maxwell's laws of electricity and magnetism, and Maxwell's equations make no allowance whatsoever for the velocity of the observer.

Different laws of electricity and magnetism for people moving at different speeds would imply that our observations on Earth would change with the seasons because of the Earth's motion around the sun.

Note that Einstein made no mention of Maxwell's equations during his creation of SR. He talked only about the "apparent" incompatibility of the simple light law with the principle of relativity. This tells us that Maxwell's equations had nothing to do with SR.

Why is this?

It's because the equations do not refer to a coordinate light speed, but merely to light's propagational speed in space. Maxwell used no clocks to measure light's speed. As Einstein said, if the clocks of classical physics were used to measure light's one-way speed, then it would vary with observer velocity. And, again, he made no mention of this being a problem re Maxwell's equations. http://www.bartleby.com/173/7.html

Also note that the laws of mag&elect must necessarily NOT reflect observer velocity because they depend ONLY on *relative* motions of magnets and wires, so they must be the same for all observers.

Can you list a real complication?

Thanks.

 

[BluMuun]

I have the 1966 edition which you can see http://www.eftaylor.com/pub/spacetime/STP1stEdThruP20.pdf.

I cannot find anything even remotely resembling your quote. Could you please find about where it got edited out?

Perhaps it would be best just to give you the quote.

Here is the quote from Wheeler & Taylor's book _Spacetime Physics_, 1963 edition, page 18:

[Wheeler's "latticework" = standard coordinate system]
"... We assume that every clock in the latticework, whatever its construction, has been calibrated in meters of light-travel time." "How are the different clocks in the lattice to be synchronized with one another? As follows: Pick one of the clocks in the lattice as the standard of time and take it to be the origin of an x, y, z coordinate system, Start this reference clock with its pointer at t = 0. At this instant let it send out a flash of light that spreads in all directions. Call this flash of light the reference flash. When the reference flash gets to a clock 5 meters away, we want that clock to read 5 meters of light-travel time. So an assistant sets that clock to 5 meters of time long before the experiment begins, holds it at 5 meters, and releases it only when the reference flash arrives. When [the] assistants at all the clocks in the lattice have followed this procedure (each setting his clock to a time in meters equal to his own distance from the reference clock and starting it when the light flash arrives), the clocks in the lattice are said to be synchronized."

That's it.

 

[BluMuun]

That's a total non-answer. Again: how do you think this was done? Tell us how you think clocks were synchronized between e.g. London and Glasgow in the 1800s?

Perhaps you overlooked my phrase "on paper." Clocks were absolutely synch'd on paper prior to SR. And Einstein used such clocks here: http://www.bartleby.com/173/7.html

 

[PAllen]

Note that Einstein made no mention of Maxwell's equations during his creation of SR. He talked only about the "apparent" incompatibility of the simple light law with the principle of relativity. This tells us that Maxwell's equations had nothing to do with SR.

Check out the whole second part of:

http://www.fourmilab.ch/etexts/einstein/specrel/www/

The FIRST paper on relativity. Also, if you read ANY of the reputable biographies of Einstein, they document the (primary) role played by concerns about EM radiation in in different frames.

 

[PAllen]

Perhaps you overlooked my phrase "on paper." Clocks were absolutely synch'd on paper prior to SR. And Einstein used such clocks here: http://www.bartleby.com/173/7.html

What does synching a clock on paper mean? Are you serious?

Clocks were synchronized in the 1800s. It was done by slow clock transport, later by telegraph, with most people not giving it much thought (though, by the late 1800s at least, it was getting serious thought, e.g. by Poincare before Einstein).

 

[BluMuun]

Suppose instead of Einstein's convention, we use one that says that the outbound propagation of light is twice as fast as the inbound. Don't you think that would easily be falsifiable?

Not if clocks were set that way, just as they are set to get half the round-trip time in standard synch.

I don't understand why you say it cannot happen even on paper. I draw spacetime diagrams all the time that illustrate how light propagates at c in any Inertial Reference Frame (IRF) and how the coordinates of all the events can be transformed to any other IRF moving with respect to the original one and yet light still propagates at c and all observers continue to see and measure exactly the same things in all IRF's.

Is it that you have never seen how this happens on paper because you don't understand Special Relativity or is it that you thoroughly understand Special Relativity but still find fault with it?

To be honest and straightforward, the only way that you can see what I am talking about is to try to show even two frames' observers getting c via two clocks. You cannot simply use a spacetime diagram that has such stuff built in.

Here is a start:

Frame A
[0]-----------------------[?]
~~>light ray
[0]-----------------------[?]
Frame B

Given a light ray emitted at the origin clocks when both start on zero, please show (on paper) how both frames' observers can get c for the one-way speed of light.

Thanks!

 

[PAllen]

It's because the equations do not refer to a coordinate light speed, but merely to light's propagational speed in space. Maxwell used no clocks to measure light's speed. As Einstein said, if the clocks of classical physics were used to measure light's one-way speed, then it would vary with observer velocity. And, again, he made no mention of this being a problem re Maxwell's equations. http://www.bartleby.com/173/7.html


Thanks.

Is this your only source? How do you misinterpret it so? I believe the part you must be referring to (you don't say) is describing not what would or could happen if clocks were somehow synchronized differently, but what people believed would happen before they had evidence that they were wrong. There is no 'classical clocks' that would allow measurement of varying one way speed of light. Can you tell a reductio ad absurdum argument when you see it?

 

[BluMuun]

I think to see a slight but perhaps important misunderstanding here. The purpose of SR was to match two apparently contradictory facts that were concluded from observations:

1. The principle of relativity is also valid for light propagation (=>invariance of the two-way speed of light)
2. Maxwell's model of light propagation is valid (=>the speed of light is independent of that of the source)

Please see my latest two replies to Nug. (especially the 2nd -- please try to show one-way invariance on paper)

Note that the prin of rel. does not call for light speed invariance, but only for law invariance.

 

[BluMuun]

[Re clock synchronization prior to SR] What way was that? I am unaware of anyone addressing clock synchronization issues prior to Einstein.

As Einstein said, the clocks of classical physics were truly or absolutely synchronized, if only on paper. (That's how Einstein's observers got the results c and c - v for light's one-way speed.) http://www.bartleby.com/173/7.html

I noted that not all theories are indistinguishable from SR, and you deleted my reply, which was the fact that Einstein said that a theory that contains the absolutely synchronous clocks of classical physics would yield c variance. (See above site.) Are you saying that Einstein was wrong here?

Good, so you do, in fact, see how the one way speed of light can be c in a frame, you simply "manually force" it to be c.

But nature cannot be forced by man. As Einstein said, in nature light actually passes frames differently, and given truly synchronous clocks, this fact of nature would be reflected in the clock measurements because they are true measurement, not false one made by absolutely asynchronous clocks.

You seem to think that coordinates are somehow out there in nature, waiting to be experimentally determined. They are not. Coordinates are human-made inventions. You have to "manually force" them to be something since nature doesn't provide them. So you have to choose some simultaneity convention. It is just a convention, so there is no "right" or "wrong" way to do it as long as you are consistent.

As Einstein said, it is not coordinates that we are talking about, but the simple law of the constancy of light's speed in space. (See above URL again.) This law is controlled by one fact, the fact of light's source independency, just as Einstein noted (when he mentioned De Sitter).

Einstein's convention is popular because if you use it then Maxwell's equations take their "textbook" form, and otherwise you have to re-write them. The one way speed of light is governed by Maxwells equations, so it is determined by the synchronization convention chosen.

Maxwell's equations have naught to do with any coordinate speed, especially not the one-way, two-clock light speed. (Maxwell had no clock synchronization definition, and never used clocks to measure light's speed.)

 

[Nugatory]

Note that Einstein made no mention of Maxwell's equations during his creation of SR. He talked only about the "apparent" incompatibility of the simple light law with the principle of relativity. This tells us that Maxwell's equations had nothing to do with SR.

The first six words of the paper in which Einstein introduced the theory of special relativity to his contemporary physicists are "It is known that Maxwell's electrodynamics..."; and as I mentioned above, the title of that paper was "On the electrodynamics of moving bodies.
(There's a translation into English here).

As Einstein said, if the clocks of classical physics were used to measure light's one-way speed, then it would vary with observer velocity. And, again, he made no mention of this being a problem re Maxwell's equations.

You are misunderstanding that passage. Einstein is describing how things would work if we assume that the one-way speed of light were not constant for all observers, thus forcing us to choose between the principle of relativity and Maxwell's electrodynamics; his point in the subsequent discussion is that if if we instead assume that the one-way speed of light is c for all inertial observers we get both.

 

[Nugatory]

P
Note that the prin of rel. does not call for light speed invariance, but only for law invariance.

Yes, but if we have a law that predicts a particular value of light speed, then law invariance applied to that law yields a prediction of light speed that is invariant. And that is exactly the situation that confronted physics between 1861 (Maxwell's electrodynamics) and 1905 (Einstein published).

 

[Dale]

As Einstein said, the clocks of classical physics were truly or absolutely synchronized, if only on paper. (That's how Einstein's observers got the results c and c - v for light's one-way speed.) http://www.bartleby.com/173/7.html

Einstein never mentioned anything about "truly or absolutely synchronized" clocks in the cited material. Furthermore, he never presented a competing method for synchronizing clocks. My original question to you remains completely unanswered:

How would you suggest synchronizing the clocks in your proposed "direct" experiment?

This is a key question. You are surprised that such an experiment has not been done. This is a key part of the experiment, and understanding why may help you understand why the experiment hasn't been done.

I noted that not all theories are indistinguishable from SR

That is true but irrelevant and non-responsive. The reference never claimed that ALL theories are indistinguishable from SR, only that there exists "a large class of theories in which the one-way speed of light is anisotropic" with some specified other characteristics and that "these theories ... are experimentally indistinguishable from SR".

This is important to your question. Think about it a bit. What does the existence of a theory in which the one way speed of light is not c and which is experimentally indistinguishable from SR imply about your desired experiment?

and you deleted my reply, which was the fact that Einstein said that a theory that contains the absolutely synchronous clocks of classical physics would yield c variance. (See above site.) Are you saying that Einstein was wrong here?

I deleted the rest of your reply because I didn't want my response to it to detract from the rest of the exchange, but since you continued to press the issue I will respond.

You must stop misquoting and misattributing statements from Einstein (or anyone else). In the references you have provided Einstein never says what you claim he says. Stop misrepresenting his comments. Such behavior is not tolerated on this forum.

But nature cannot be forced by man.

True, but not relevant. Clock synchronization is not part of nature, it is a purely man-made convention.

As Einstein said, it is not coordinates that we are talking about, but the simple law of the constancy of light's speed in space. (See above URL again.) This law is controlled by one fact, the fact of light's source independency, just as Einstein noted (when he mentioned De Sitter).

Also true but irrelevant. The source independency of the one way speed of light is an experimentally testable fact which does not require clock synchronization. It is a different physical question from the value of the one way speed or its isotropy. The fact that synchronization is not necessary to answer one question in no way implies that synchronization is unnecessary to answer the other.

The bottom line remains the question that I posed to you which you have not answered. You are surprised at the lack of a direct experiment, but you are not the first person to be interested in doing this type of experiment. All those before you abandoned the attempt. Why? Because of this one issue. If you want to resolve your surprise, then you must confront the issue of synchronization, not avoid it.

 

[ghwellsjr]

I have the 1966 edition which you can see http://www.eftaylor.com/pub/spacetime/STP1stEdThruP20.pdf.

I cannot find anything even remotely resembling your quote. Could you please find about where it got edited out?

Perhaps it would be best just to give you the quote.

Here is the quote from Wheeler & Taylor's book _Spacetime Physics_, 1963 edition, page 18:

[Wheeler's "latticework" = standard coordinate system]
"... We assume that every clock in the latticework, whatever its construction, has been calibrated in meters of light-travel time." "How are the different clocks in the lattice to be synchronized with one another? As follows: Pick one of the clocks in the lattice as the standard of time and take it to be the origin of an x, y, z coordinate system, Start this reference clock with its pointer at t = 0. At this instant let it send out a flash of light that spreads in all directions. Call this flash of light the reference flash. When the reference flash gets to a clock 5 meters away, we want that clock to read 5 meters of light-travel time. So an assistant sets that clock to 5 meters of time long before the experiment begins, holds it at 5 meters, and releases it only when the reference flash arrives. When [the] assistants at all the clocks in the lattice have followed this procedure (each setting his clock to a time in meters equal to his own distance from the reference clock and starting it when the light flash arrives), the clocks in the lattice are said to be synchronized."

That's it.

That's it? As I said before, there is nothing in there even remotely resembling 'clocks are simply set by definition to get the chosen value "c."'

As DaleSpam just said, "You must stop misquoting and misattributing statements from Einstein (or anyone else)."

 

[harrylin]

Please see my latest two replies to Nug. (especially the 2nd -- please try to show one-way invariance on paper)

Note that the prin of rel. does not call for light speed invariance, but only for law invariance.

Funny enough, you wrote the above in reaction on my clarification that according to SR, the principle of relativity is also valid for light propagation (=>invariance of the two-way speed of light). With that I meant Maxwell's law of light propagation. Obviously the relativity principle on its own is not SR!

One way invariance is perhaps most easy to understand by splitting up the actions of humans from those of nature. One-way invariance, as you now surely realize, is simply two-way invariance plus convenient clock synchronization (this was shown by means of the "Einstein" synchronization equations that I linked to you in post #13).
That leaves us with showing how Kennedy-Thorndike style two-way invariance is accomplished by nature. You can find how that works for example here:
https://en.wikipedia.org/wiki/Kennedy–Thorndike_experiment#Theory

In short, Lorentz contraction + time dilation maintain two-way invariance of the measured speed of light in all directions. Subsequent clock synchronization assures that the measured one-way speeds will equal the measured two-way speeds. Note that in SR, the measured speed of light is commonly called "the speed of light" - SR is void of metaphysics.

 

[ghwellsjr]

I don't understand why you say it cannot happen even on paper. I draw spacetime diagrams all the time that illustrate how light propagates at c in any Inertial Reference Frame (IRF) and how the coordinates of all the events can be transformed to any other IRF moving with respect to the original one and yet light still propagates at c and all observers continue to see and measure exactly the same things in all IRF's.

Is it that you have never seen how this happens on paper because you don't understand Special Relativity or is it that you thoroughly understand Special Relativity but still find fault with it?

To be honest and straightforward, the only way that you can see what I am talking about is to try to show even two frames' observers getting c via two clocks. You cannot simply use a spacetime diagram that has such stuff built in.

Here is a start:

Frame A
[0]-----------------------[?]
~~>light ray
[0]-----------------------[?]
Frame B

Given a light ray emitted at the origin clocks when both start on zero, please show (on paper) how both frames' observers can get c for the one-way speed of light.

Thanks!

I have seen what you are talking about from your very first post. As I said in the very first response to you in this thread "Light's one-way speed of exactly c ... cannot come from an experiment."

And that's what others have been saying in this thread.

We already know that.

Einstein knew that.

Why do you keep asking the same question over and over again when it has already been addressed over and over again?

And you didn't answer my question:

Is it that you have never seen how this happens on paper because you don't understand Special Relativity or is it that you thoroughly understand Special Relativity but still find fault with it?

 

[stevendaryl]

That's it? As I said before, there is nothing in there even remotely resembling 'clocks are simply set by definition to get the chosen value "c."'

I don't understand why you're objecting to BluMuun's characterization. In the passage he quotes, clocks are synchronized by light signals:

When the reference flash gets to a clock 5 meters away, we want that clock to read 5 meters of light-travel time. So an assistant sets that clock to 5 meters of time long before the experiment begins, holds it at 5 meters, and releases it only when the reference flash arrives.

This sure seems to me to be a matter of setting the clocks so that lightspeed has the chosen value. It's saying that at t=0, according to the reference clock, a light signal is sent toward a clock 5 meters away. When the signal reaches that clock, it is set to time t=5/c. With that setting, it's necessarily true that the computed speed of light for that signal will be:

speed = (Distance)/(Elapsed Time) = 5/(5/c) = c

I think BluMuun is missing that the REASON we can use lightsignals to synchronize clocks is because we already know that light has speed c, using pre-relativistic clock synchronization (slow clock transport).

 

[ghwellsjr]

You cannot falsify a definition because it is a given (given by man, not nature).

Suppose instead of Einstein's convention, we use one that says that the outbound propagation of light is twice as fast as the inbound. Don't you think that would easily be falsifiable?

Can you please answer my question?

 

[ghwellsjr]

That's it? As I said before, there is nothing in there even remotely resembling 'clocks are simply set by definition to get the chosen value "c."'

I don't understand why you're objecting to BluMuun's characterization. In the passage he quotes, clocks are synchronized by light signals:

When the reference flash gets to a clock 5 meters away, we want that clock to read 5 meters of light-travel time. So an assistant sets that clock to 5 meters of time long before the experiment begins, holds it at 5 meters, and releases it only when the reference flash arrives.

This sure seems to me to be a matter of setting the clocks so that lightspeed has the chosen value. It's saying that at t=0, according to the reference clock, a light signal is sent toward a clock 5 meters away. When the signal reaches that clock, it is set to time t=5/c. With that setting, it's necessarily true that the computed speed of light for that signal will be:

speed = (Distance)/(Elapsed Time) = 5/(5/c) = c

Wheeler & Taylor never used the phrase or any part of it that BluMuun attributed to them. If BluMuun had attributed that phrase to Einstein, I would have had no problem because he made the point very clear that his synchronization process is an arbitrary definition made of his own free will.

I think BluMuun is missing that the REASON we can use lightsignals to synchronize clocks is because we already know that light has speed c, using pre-relativistic clock synchronization (slow clock transport).

No, Einstein's second postulate doesn't have (or need) a reason and slow clock transport is just as much an arbitrary definition just like Einstein's. Neither one is the reason for the other.

Furthermore, you seem to think that slow clock transport is an experiment that does measure the one-way speed of light apart from an arbitrary convention of the type that BluMuun is asking for, correct?

 

[BluMuun]

What does synching a clock on paper mean? Are you serious?

Clocks were synchronized in the 1800s. It was done by slow clock transport, later by telegraph, with most people not giving it much thought (though, by the late 1800s at least, it was getting serious thought, e.g. by Poincare before Einstein).

History:

Quote by BluMuun
Oddly enough, given all the info at that site that you cited and I sighted, one would think that the direct experiment would be mentioned somewhere. By "direct" I mean using two mutually-at-rest clocks that are not rotating and have not been transported (because such clocks run slow).

Dalespam asked:
"How would you suggest synchronizing the clocks in such an experiment?"

BluMuun answered:
"The same way clock were set on paper for 100's of years prior to Einstein. Can you tell me why Einstein decided to get rid of such clocks?"

PAllen wrote:
That's a total non-answer. Again: how do you think this was done? Tell us how you think clocks were synchronized between e.g. London and Glasgow in the 1800s?

BluMuun answered:
Perhaps you overlooked my phrase "on paper." Clocks were absolutely synch'd on paper prior to SR. And Einstein used such clocks here: http://www.bartleby.com/173/7.html

PAllen changed the subject from a direct experiment to using slow clock transport, and then called my answer a "total non-answer." He paid no heed to the given that clock transport cannot be involved.

I was of course talking about absolutely synchronous clocks, and since we do not have absolute time, such clocks must necessarily be on paper. This applies to clocks between London and Glasgow as well as all other clocks in this universe.

{re my statement "As Einstein said, if the clocks of classical physics were used to measure light's one-way speed, then it would vary with observer velocity."}
Is this your only source? How do you misinterpret it so? I believe the part you must be referring to (you don't say) is describing not what would or could happen if clocks were somehow synchronized differently, but what people believed would happen before they had evidence that they were wrong. There is no 'classical clocks' that would allow measurement of varying one way speed of light. Can you tell a reductio ad absurdum argument when you see it?

Given your above, I believe that the word "misinterpret" has been gravely misapplied. Re your remark " but what people believed would happen before they had evidence that they were wrong," please be so kind as to tell us what is "wrong" with classical clocks. (I already asked this above by asking why Einstein decided to discard such clocks, but you totally ignored it.)

 

[BluMuun]

BluMuun said:
"I was interested in Nugatory's claim that "horrible complications" occur if we do not get c, so I asked him to list one."

One such problem is that we can calculate the speed of light from Maxwell's laws of electricity and magnetism, and Maxwell's equations make no allowance whatsoever for the velocity of the observer.

I fully explained why this is not a "horrible complication," and asked Nugatory for a real example, but he went on talking about Maxwell. What more can I do re this?

However, I would like to clear up the following:

BluMuun noted:
"As Einstein said, if the clocks of classical physics were used to measure light's one-way speed, then it would vary with observer velocity. And, again, he made no mention of this being a problem re Maxwell's equations."

You are misunderstanding that passage. Einstein is describing how things would work if we assume that the one-way speed of light were not constant for all observers, thus forcing us to choose between the principle of relativity and Maxwell's electrodynamics; his point in the subsequent discussion is that if if we instead assume that the one-way speed of light is c for all inertial observers we get both.

Please give me your opinion of
(i) what the principle of relativity says about light's one-way speed,
and
(ii) what Maxwell's equations say about light's one-way speed.

All I have so far is this:

Yes, but if we have a law that predicts a particular value of light speed, then law invariance applied to that law yields a prediction of light speed that is invariant. And that is exactly the situation that confronted physics between 1861 (Maxwell's electrodynamics) and 1905 (Einstein published).

How did Maxwell give us one-way c invariance? Which experiment gave us c invariance for light's one-way speed?

If c invariance had been a law prior to SR, then there would have been no need for SR.

But, as I said at the start, I cannot see how one-way c invariance can happen experimentally, so I do not see how it can be a law.

 

[BluMuun]

I wrote:
"Oddly enough, given all the info at that site that you cited and I sighted, one would think that the direct experiment would be mentioned somewhere. By "direct" I mean using two mutually-at-rest clocks that are not rotating and have not been transported (because such clocks run slow)."
(I was looking for a test of one-way, two-clock light speed invariance. DaleSpam had pointed me to a bunch of other stuff.)

How would you suggest synchronizing the clocks in your proposed "direct" experiment? [/quote]

I answered your question, as follows:
"The same way clocks were set on paper for 100's of years prior to Einstein. Can you tell me why Einstein decided to get rid of such clocks?"

What way was that? I am unaware of anyone addressing clock synchronization issues prior to Einstein.

"As Einstein said, the clocks of classical physics were truly or absolutely synchronized, if only on paper. (That's how Einstein's observers got the results c and c - v for light's one-way speed.) http://www.bartleby.com/173/7.html"

Einstein never mentioned anything about "truly or absolutely synchronized" clocks in the cited material. Furthermore, he never presented a competing method for synchronizing clocks. My original question to you remains completely unanswered:

He was talking about pre-SR physics, or classical physics. The only time that existed then was absolute time, and the only clock synchronization that existed then was absolute synchronization. Einstein even (elsewhere) defined absolute time as follows:
"The simultaneity of two definite events with reference to one inertial system involves the simultaneity of these events in reference to all inertial systems. This is what is meant when we say that the time of classical mechanics is absolute. According to the special theory of relativity it is otherwise." [Einstein's book on relativity, p. 149]

This is a key question. You are surprised that such an experiment has not been done. This is a key part of the experiment, and understanding why may help you understand why the experiment hasn't been done.

It was you who claimed that there exist tests of SR. You cited a site, but it contained no such tests. Please present a test of one-way two-clock light speed invariance. (SR does not give round-trip invariance, which was given experimentally prior to SR, and neither does SR give mechanical relativity, which also preceded SR.)

You must stop misquoting and misattributing statements from Einstein (or anyone else). In the references you have provided Einstein never says what you claim he says. Stop misrepresenting his comments. Such behavior is not tolerated on this forum.

How else could a pre-SR observer get c for light's one-way speed other than by using the absolutely synchronous clocks of classical physics?

How could any observer get c - v for light's one-way speed other than by using the absolutely synchronous clocks of classical physics?

Simply by giving us these one-way light speeds, Einstein was clearly talking about the use of the absolutely synchronous clocks of classical physics. This is so obvious that saw no need to provide the dreary details. (Please do not accuse me of misrepresenting anyone.)

Good, so you do, in fact, see how the one way speed of light can be c in a frame, you simply "manually force" it to be c.

Blumuun:
"But nature cannot be forced by man. As Einstein said, in nature light actually passes frames differently, and given truly synchronous clocks, this fact of nature would be reflected in the clock measurements because they are true measurement, not false one made by absolutely asynchronous clocks."

True, but not relevant. Clock synchronization is not part of nature, it is a purely man-made convention.

What I was saying is that light's one-way invariance cannot even be "manually forced" because it is physically impossible - it cannot exist given nature as-is.

But, as I have said, the only way to see this is by trying to show c invariance on paper, as follows:


Frame A
[0]-----------------------[?]
~~>light ray
[0]-----------------------[?]
Frame B

Given a light ray emitted at the origin clocks when both start on zero, please show (on paper) how both frames' observers can get c for the one-way speed of light.

 

[BluMuun]

I don't understand why you say it cannot happen even on paper.

I have seen what you are talking about from your very first post.

First, you say that you do not understand why I say that it cannot happen on paper, and then you say you fully understood it from the start. I have no idea what you are really trying to say. Sorry.

 

[BluMuun]

Can you please answer my question?

I have answered it. I said that given such stuff, it must happen.

 

[BluMuun]

Wheeler & Taylor never used the phrase or any part of it that BluMuun attributed to them. If BluMuun had attributed that phrase to Einstein, I would have had no problem because he made the point very clear that his synchronization process is an arbitrary definition made of his own free will.

As the guy said, it is clear from the context that Wheeler was forcing the value c, i.e., that clocks are simply set by definition to get that value. In fact, Wheeler was just giving his definition of Einstein's definition of synchronization (which Wheeler called "standard synchronization.")

Since Wheeler gave no justification for placing the time on the distant clock, he was clearly just arbitrarily doing so of his own free will.

No, Einstein's second postulate doesn't have (or need) a reason and slow clock transport is just as much an arbitrary definition just like Einstein's. Neither one is the reason for the other.

But Einstein's second postulate must say something about nature, or it is not a scientific postulate. What does it say about nature?

 

[PAllen]

I answered your question, as follows:
"The same way clocks were set on paper for 100's of years prior to Einstein. Can you tell me why Einstein decided to get rid of such clocks?"

They never existed. There was nothing to get rid of. Clocks were set and synchronized using physical procedures.

"As Einstein said, the clocks of classical physics were truly or absolutely synchronized, if only on paper. (That's how Einstein's observers got the results c and c - v for light's one-way speed.) http://www.bartleby.com/173/7.html"

This refers to the false prior belief that when setting and synchronizing clocks using procedures like slow clock transort, an absolute synchronization was achieved. People used real clocks, real procedures, but had in their heads false assumptions about the ideal they were approximating.

It was you who claimed that there exist tests of SR. You cited a site, but it contained no such tests. Please present a test of one-way two-clock light speed invariance. (SR does not give round-trip invariance, which was given experimentally prior to SR, and neither does SR give mechanical relativity, which also preceded SR.)

SR says such an experiment is meaningless. Meaningful is measuring source independence of light speed, and two way isotropy and invariance of lightspeed. Given that that is all that is meaningful, we can make a choice to have a more complex mathematical model (by assuming the right kind of one way anisotropy), or choose a simpler mathematical model by assuming one way isototropy. This is purely a choice. All sane physicists choose the simpler model, but there can' be measurements to select one over the other.

How else could a pre-SR observer get c for light's one-way speed other than by using the absolutely synchronous clocks of classical physics?

Pre-SR observers measured one way light speed all the time using procedures that amounted to slow clock transport. We could do the same today - only now we understand there is no point to it except as a check on how well our clocks are synchronized.

How could any observer get c - v for light's one-way speed other than by using the absolutely synchronous clocks of classical physics?

No one ever got this by measurement.

What I was saying is that light's one-way invariance cannot even be "manually forced" because it is physically impossible - it cannot exist given nature as-is.

But, as I have said, the only way to see this is by trying to show c invariance on paper, as follows:Frame A
[0]-----------------------[?]
~~>light ray
[0]-----------------------[?]
Frame B

Given a light ray emitted at the origin clocks when both start on zero, please show (on paper) how both frames' observers can get c for the one-way speed of light.

Simple:

Frame A sees light go 1 km in approx 1/(300000) seconds.
Frame B sees light go 1 km in approx 1/(300000) seconds.

Frame A sees B's measurement different from how B sees it. Per A, B's 1km is (say) .5 km. Per A, the light takes only 1/600000 seconds to cross B's km. However, per A, the end of B's kilometer is moving such that the the light travels 2 of A's km before reaching the end of B's km. So, per A, the time between B's source flashing, and B's km end receiving it is 2/300000 seconds. But, per A, B's clock is running at 1/2 speed. So it only advances 1/300000 seconds. Thus A sees how B gets a speed of c, just like A does. B sees their own measurement the same way A sees his own measurements. B sees A's measurement the same way A sees B's measurement.

 

[Dale]

He was talking about pre-SR physics, or classical physics. The only time that existed then was absolute time, and the only clock synchronization that existed then was absolute synchronization.

Sure, but my question to you which you continued to avoid rather than address was HOW to synchronize the clocks in your proposed experiment. Whether the synchronization procedure gives absolute or relative synchronization is not the question. The question is experimentally how to synchronize clocks.

You are the one asking for an experiment, so a reply about synchronization "on paper" is simply a transparent attempt to dodge the question.

It was you who claimed that there exist tests of SR.

There are many tests of SR, the "direct" test you are asking for is not one of them, and I never claimed it was.

How else could a pre-SR observer get c for light's one-way speed other than by using the absolutely synchronous clocks of classical physics?

By using clocks synchronized using the Einstein synchronization convention. For such clocks the one way speed of light is guaranteed to be c.

Simply by giving us these one-way light speeds, Einstein was clearly talking about the use of the absolutely synchronous clocks of classical physics. This is so obvious that saw no need to provide the dreary details. (Please do not accuse me of misrepresenting anyone.)

This is a misrepresentation. Einstein clearly defined what he meant by simultaneity in the beginning section of his 1905 paper. He most clearly and emphatically did not mean absolutely synchronous clocks. As I said earlier, such blatant misrepresentation is not permitted on this forum.

What I was saying is that light's one-way invariance cannot even be "manually forced" because it is physically impossible - it cannot exist given nature as-is.

Not only is it not impossible, it is guaranteed under Einsteins synchronization convention. You admitted that it could be done previously, calling it "manually forcing". I don't know why you are backtracking now.

Suppose I have two clocks 10 ft apart. When one clock reads 0 it sends a light pulse to the other which is received and reflected back when the other clock reads 10 ns (c=1 ft/ns), and then the reflection is received at the first clock at 20 ns, then they are synchronized according to Einsteins definition and the one way speed of light is guaranteed to be c. We can always adjust the time on the second clock so that this is true, and since the two way speed of light is c in all frames this process can be done in all frames.