Two lightnings that happen at the same time, a train and a passanger

[yoron]

That is ridiculous. If we find that light only have one speed, as measured locally, then we have the means to synchronize clocks. "Einstein had to admit that he did not possesses the means of measuring time. He did not know how to truly synchronize a pair of clocks."

Where do you find local experiments proving that? I think you mix it with the way gravity influence radiation. If you mean that there can be no two points in SpaceTime having the exact same 'clock' then that is true, but that is a direct result of gravity acting on radiation, not radiation itself.

And that you can compensate for.

 

[Erland]

That is ridiculous. If we find that light only have one speed, as measured locally, then we have the means to synchronize clocks. "Einstein had to admit that he did not possesses the means of measuring time. He did not know how to truly synchronize a pair of clocks."

I thought that 2clockdude with "truly synchronous clocks" meant something like "clocks at different locations showing the same time in all inertial systems". Of course, such clocks cannot exist, since our universe is governed by SR, and of course, neither Einstein nor anybody else could/can "truly synchronize a pair of clocks" in this sense.

For the contradiction issue, if all 2clockdude wanted to say is that before SR, it was generally believed that there was an "absolute" time and "truly synchronous clocks" could exist, and that Einstein knew that, then this is certainly true. The point with pointing that out in the way 2clockdude did, is unclear to me, however.

 

[harrylin]

If someone had just been kind enough to have answered my query "Why or how did the embankment observer get c for light's one-way speed in a pre-SR setting? (To be specific, this was before E-synch.)," then you would know that Einstein himself talked about absolutely synchronous clocks. This is not a forbidden topic.

P-synch:
https://www.physicsforums.com/showthread.php?t=534502

Let me answer my own question. The only way that any observer in any frame could possibly get the result c for light's one-way speed prior to E-synch would be by using absolutely synchronous clocks in a frame that is at absolute rest.

As mentioned in the thread above, astronomers got the result c for light's one-way speed prior to E-synch.

As I said earlier, Einstein even used absolutely synchronous lightning strikes in his train example. This is proved by his phase "able meteorologist," given at the start of the example, where it belongs. http://www.bartleby.com/173/8.html

No. Einstein was familiar with the literature of that time, such as the papers cited in the thread mentioned above. No able meteorologist could establish "absolute synchronization".

Einstein also talked about truly synchronous clocks immediately following the "able" phrase. He was talking about light's one-way speed, and he said this:

“Your definition would certainly be right, if I only knew that the light by means of which the observer at M perceives the lightning flashes travels along the length A —> M with the same velocity as along the length B —> M. But an examination of this supposition would only be possible if we already had at our disposal the means of measuring time. It would thus appear as though we were moving here in a logical circle.”

The pertinent phrase here is "the means of measuring time." This can only mean one thing - truly or absolutely synchronous clocks.

The pertinent phrase is "moving here in a logical circle". He implied the relativity of clock time.

[..] I hate to be the bearer of bad news (for SR proponents)[..] as I can show, this "c invariance" cannot happen either on paper or experimentally. [..]

Sorry to be the bearer of bad news, but "c invariance" happens both on paper and experimentally, thanks to the synchronization convention and the laws of physics.

Harald

 

[harrylin]

That is ridiculous. If we find that light only have one speed, as measured locally, then we have the means to synchronize clocks. "Einstein had to admit that he did not possesses the means of measuring time. He did not know how to truly synchronize a pair of clocks."
Where do you find local experiments proving that?[..]

It's the other way round. Perhaps it's best clarified by Einstein's 1907 reformulation of the light postulate:

"We [...] assume that the clocks can be adjusted in such a way that
the propagation velocity of every light ray in vacuum - measured by
means of these clocks - becomes everywhere equal to a universal
constant c, provided that the coordinate system is not accelerated."

 

[yoron]

To me it builds on a 'postulate' if you like, namely 'c'. That one has been tested and found true in all ' local' measurements I know of. From that one you can draw all kind of conclusions, amongst them the one you refer to Harry:)

To discuss this, refuting it, it seems to me that one first need to prove that 'c' doesn't exist. That means presenting me with a experiment that I can do, and find refuting all other experiments we have proving 'c'. As far as I know we still haven't found such a one, repeatable, experiment.

 

[2clockdude]

P-synch:
https://www.physicsforums.com/showthread.php?t=534502

As mentioned in the thread above, astronomers got the result c for light's one-way speed prior to E-synch.

No. Einstein was familiar with the literature of that time, such as the papers cited in the thread mentioned above. No able meteorologist could establish "absolute synchronization".

The pertinent phrase is "moving here in a logical circle". He implied the relativity of clock time.

Sorry to be the bearer of bad news, but "c invariance" happens both on paper and experimentally, thanks to the synchronization convention and the laws of physics.

Harald

I don't appreciate being given a full thread as a reference, with no specific citing; I assume that you were vaguely referring to this piece of "trash":
"As that 1898 paper points out, astronomers made events simultaneous by assuming, as a postulate, that the speed of light is the same in all directions (that is, wrt the reference frame that they used: the Earth or, I suppose that in fact they used the ECI frame; see also next)."

In his example, Einstein did not "assume" c, he got it via experiment, a proper thought experiment, one that involved the truly synchronous clocks of classical physics. To date, no one has actually used two clocks to measure the one-way speed of light, and this is because no one has yet produced absolute synchronization. As soon as this is done, SR is outta here, pronto.

You need to take reading lessons, sir; Einstein did not say that the able meteorologist "established absolute synchronization"; what Einstein did say, and as you should have been able to pick up on, was that the meteorologist had come up with a theoretical result. That is, Einstein was assuming on paper that the strikes occurred absolutely simultaneously, but he well knew that no one had a way to certainly determine this. If there had been such a person, such as that able meteorologist, then Einstein would already have had absolute time.

You don't seem to know what a logical circle problem is, Mr. Harald. It means that Einstein needed to know that light's one-way travel time was the same in both directions in order to use them to synchronize clocks, but he could not ascertain this sans a pair of synchronous clocks, thus the logical circle.

one-way c invariance has never happened experimentally, and never will, because, as I said, it cannot even happen on paper. Why don't you, Sir Harald, boldly step up to the ol' plate, and show us how it can happen, if only on paper, or in theory, or even just in principle?

What I am saying is that no matter what you call it, e.g., a stipulation, a postulate, a hunch, a guess, a wild stab in the dark, a hypothesis, a principle, or whatever - it simply cannot happen.

 

[Erland]

Excuse me for my ignorance, but what is E-synch?

 

[2clockdude]

To me it builds on a 'postulate' if you like, namely 'c'. That one has been tested and found true in all ' local' measurements I know of. From that one you can draw all kind of conclusions, amongst them the one you refer to Harry:)

To discuss this, refuting it, it seems to me that one first need to prove that 'c' doesn't exist. That means presenting me with a experiment that I can do, and find refuting all other experiments we have proving 'c'. As far as I know we still haven't found such a one, repeatable, experiment.

The burden of proof is on the one making the (silly) claim that light's one-way speed is c in all inertial frames, but I can show you a legitimate special relativity case to prove the incorrectness of Einstein's one-way invariance "postulate."

This example comes from a respectable SR text. It shows how Einstein's desired "c invariance" conflicts with reality by improperly demanding that clocks read different times as they are "hit" by a light ray at the same time.

Frame A moves to the right as Frame B moves leftward:
[clocks are in brackets]

Frame A
------[0]--------------- -->
-------S~~>light
---<--[0]--------------------
Frame B

Frame A
------------[?]-----90m-------[300ns] -->
-------S------------------------>light
[?]------------150m-----------[500ns]
Frame B

Given that this example, as I said, came straight from an SR book, the diagram fully conforms to Einstein's belief of c invariance. Indeed, this is the only way to have both frames "getting" the "same" one-way light speed (0.3m/ns). But clocks that are "hit" by the light ray at absolutely the same time* conflict with this fact by reading two different times when hit. To better understand how bad this is, suppose you have two clocks sitting side-by-side on a table, and they read different times; you would instantly know that something is wrong; i.e., you would know that at least one of the clocks must be "lying." In the above diagram, Einstein's clocks in each frame are lying because they are not truly synchronous. Only truly synchronous clocks can correctly measure the light ray's travel time, which, in the given situation, happens to be 400ns. Thus, the distant clocks must both read the time 400ns when they are "hit" by the light ray. The A observers would light's speed to be 90m/400ns = 0.225m/ns, a slower one-way light speed, whereas the B observers would find a one-way speed of 150m/400ns = 0.375m/ns, a faster one-way light speed. (*Bear in mind that everyone agrees that two touching clocks can started absolutely simultaneously; it's only their separation that hurts, but we cannot use two touching clocks for any speed measurements.)

And I can even show the other case, the one where two clocks are forced (by Einstein's definition) to read different the same "start time" when they are in fact started at absolutely different times; here is that case:

The origin clocks start on time zero as the light ray is emitted from source S.

(As shown, B moves to the right relative to A.)

(It is not critical that the two distant clocks be exactly side-by-side as shown; all that matters is that both observers find the same distance x between their own clocks, and this is a given.)

Frame A
clock starts clock (unstarted)
[0]--------x------[x/c]
S~~>light
[0]--------x------[x/c] -->
clock starts clock (unstarted)
Frame B

In order to obtain Einstein's c invariance, we will now let each distant clock start when it is "hit" by the light ray. In our next diagram, we show this happening to A's distant clock:

Frame A clock starts
[?]-------x------[x/c]
S------------------>light
--------[?]-------x------[x/c] -->
Frame B clock (unstarted)

At this point, we see that Einstein's clocks will conflict with reality by being forced to read the same time x/c at absolutely different times. (Light-like events have an absolute before-and-after time order, and these two clock-starting events are light-like.)

As we have seen, Einstein's "c invariance" cannot happen even on paper because it causes clocks to conflict with reality. As we have seen, the clocks will either improperly read the same start time when they are in fact started at absolutely different times, or they will improperly read different times when they should read the same time.

The only way to correctly measure light's one-way speed is (of course) by using truly or absolutely synchronous clocks, and then correcting for clock slowing and ruler shrinkage of the intrinsic sorts.

Whenever this is done, and it will be done, Einstein's ultra-simple light speed equation w = c - v will not be changed much; it will become w = c^2/(c+s). where s is the frame's absolute speed, and the speed is that of a departing light ray.

Special relativity is not a scientific theory. It is merely a ridiculous clock swap, with bad clocks replacing good. We need SR like we need another hole in the head.