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

[2clockdude]

How ironic that you accuse someone above of not having read the whole example, when you yourself obviously didn't read the whole section that you are quoting above. The quote comes from a part where he is discussing the state of physics before Relativity and is pointing out what,at the time, seemed a incompatibility between the propagation of light and the principle of Relativity. He then goes on later on the same page to state "in reality there is not the least incompatibility between the principle of relativity and the law of propagation of light, " Put another way, he is giving a little background before going on to present his theory. He is not arguing for the statement you quoted. He hasn't even gotten to presenting his argument yet.

I have read the whole example many times. Of course it is pre-SR. Perhaps this is a way to get you to see the real story here: 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.)

However, the train observer sits at the midpoint of the train, and the lightning strikes hit the end of the train. Due to the invariance of the speed of the light, the flashes approaching from each end of the train have to travel at the same speed relative to him as determined from his frame.

Again, it may help if I pin you down to one question, namely, can you show one-way light speed invariance on paper? (You must use at least two frames, and in order to clearly differentiate them, you must use a single light source, because otherwise you are just repeating a single frame case. One source usage is justified by the fact of light's source independent nature.)

 

[Erland]

I have read the whole example many times. Of course it is pre-SR.

But you wrote before:

But there is evidence from Einstein himself that is not c in all frames when truly or absolutely synchronous clocks are used. Again, I am not here to educate, but look at Einstein's equation w = c - v at http://www.bartleby.com/173/7.html where he said that "The velocity of propagation of a ray of light relative to the carriage thus comes out smaller than c."

Since you now say you understand that it is pre-SR, and thus that Einstein didn't really claim it is true, this example does NOT give any "evidence from Einstein himself that is not c in all frames when truly or absolutely synchronous clocks [whatever that may be] are used."

So, you are contradicting yourself.

can you show one-way light speed invariance on paper?

It cannot be "shown on paper". It follows from the Special Relativity Principle and the assumption that the light speed is constant in at least one inertial frame. Both these are experimentally verified facts.

 

[Erland]

It follows from the Special Relativity Principle and the assumption that the light speed is constant in at least one inertial frame. Both these are experimentally verified facts.

I should add that the Michelson-Morley experiment clearly shows that the measured light speed is independent of the motion of the observer, since the Earth is moving in opposite directions at opposite times of the year.

If the speed of light is frame dependent, how can you then explain the outcome of this famous experiment, 2clockdude?

 

[harrylin]

I've said it before and I say it again: I don't like this example. I only get confused when I think of it. But there is a simpler example which demonstrates the relativity of simultaneity. I quote myself from another post:

"... imagine a light being turned on in the middle of the train. The light reaches the front and the back of the train simultaneously, according to an observer on the train. But according to an observer on the embankment, the light will reach the back of the train before it reaches the front of the train, because according to such an observer, the light that reaches the back of the train will have traveled a shorter distance than the light that reaches the front of the train, since train is moving, according to this observer."

Although I don't find Einstein's example confusing, I fully agree that the variant with one light source in the middle of the train is much easier to follow - very much preferable.

Perhaps the best way to teach this is to first present that simpler variant as problem 1a, and then switch to Einstein's variant as 1b, to check that 1a was fully understood.

 

[2clockdude]

Since you now say you understand that it is pre-SR, and thus that Einstein didn't really claim it is true, this example does NOT give any "evidence from Einstein himself that is not c in all frames when truly or absolutely synchronous clocks [whatever that may be] are used."

So, you are contradicting yourself.

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.

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 for your above, Einstein claimed that the carriage observer got c - v for the departing light ray's speed, whilst the railway observer got simply c, and both of these results were fully in the context of classical physics, where only truly synchronous clocks were used.
Also, as I said, both results were in the pre-SR, pre-E-synch era.

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

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. 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. (Of course, he could not prove the negative that such clocks cannot exist, but he hated such clocks because he knew that they could detect absolute motion by measuring the one-way speed of light. This is why he wanted to discard such clocks, and to replace them with clocks that he hoped would get only c invariantly.)

I was not by any means contradicting myself. The example does give "evidence from Einstein himself that is not c in all frames when truly or absolutely synchronous clocks [whatever that may be] are used."

After using the bogus train example to convince himself that simultaneity was "relative," Einstein then felt like he could at last toss aside those "nasty" truly synchronous clocks, and replace them with his (incredibly-more-nasty-really) asynchronous clocks, which he tried to force to get c invariantly, but this cannot be done, not even on paper.

Here is Einstein's proud abandonment of truly synchronous clocks (a move that he "cleverly" disguised as "we must discard the assumption of absolute time") :
"Now before the advent of the theory of relativity it had always tacitly been assumed in physics that the statement of time had an absolute significance, i.e. that it is independent of the state of motion of the body of reference. But we have just seen that this assumption is incompatible with the most natural definition of simultaneity; if we discard this assumption, then the conflict between the law of the propagation of light in vacuo and the principle of relativity (developed in Section VII) disappears."

Einstein thought that absolutely synchronous clocks (absolute time) would violate the principle of relativity by detecting absolute motion; however, anyone who knows what this principle really says would laugh at Einstein's "violation" claim. The PR allows any and all laws that may be found, and says nothing about what the laws can or cannot be used for. The PR is actually a meta-law, a law about laws, and says that all inertial observers must find the same general laws. Contrary to Einstein (re his c - v example), this is exactly what happens when truly synchronous clocks are used to measure the one-way speed of light; i.e., the one-way law for all observers is w = c +/- v. Of course, as Einstein saw, each frame's observers will get their very own little result, such as w = c - v for the carriage observer, and w = c for the railway observer, whose absolute speed in space was of course zero (v = 0). Note carefully that the one-way law fully satisfies the PR, but can still be used to detect absolute motion. (But this makes perfect sense because truth cannot possibly conflict with truth - that is. truly synchronous clocks cannot possibly conflict with a true principle (the PR). Albert should have known better, but the might Michelson-Morley experiment did him in. He "read" that null result as "null results all the way down!")

I hate to be the bearer of bad news (for SR proponents), but there was never any need for special relativity, none at all. Absolute time (or absolute synchronization) does NOT in any way "violate the principle of relativity"; there was never any need to try to force poor clocks to "always get c"; indeed, as I can show, this "c invariance" cannot happen either on paper or experimentally. On the other hand, as Einstein himself had to admit, if we use absolutely synchronous clocks, then we can have both absolute time and absolute motion detection.

I should add that the Michelson-Morley experiment clearly shows that the measured light speed is independent of the motion of the observer, since the Earth is moving in opposite directions at opposite times of the year.

If the speed of light is frame dependent, how can you then explain the outcome of this famous experiment, 2clockdude?

As John Wheeler pointed out, said experiment did not even close the round-trip case, much less the one-way case. Wheeler noted that physical length contraction (à la Mr. Lorentz) was the proper physical explanation for the MMx null result, but this would not stop the Kennedy-Thorndike experiment from getting a positive result. (This, btw, proves that there was no aether involved, and that the MMx did not "do away with the aether.") Wheeler went on to say that time dilation (physical clock slowing) could explain the KTx result.

Now let's look back at your (Erland's) innocuous-looking phrase "the measured light speed is independent of the motion of the observer," and let's think a bit about that word "measured." Hmmm... Let's combine both round-trip experiments. Now we have a clock that is slowed, and a ruler that is contracted, or at least we have no proof that these distortions are not present during the measurement. If one uses distorted instruments, then one should expect invalid results. Yes, the so-called "null result" was an invalid result - it does not reflect the reality that even light's round-trip speed would vary with frame velocity if undistorted instruments were used.

And now let's look at the one-way case. There are still only two kinds of instruments involved, namely, rulers (or rods) and clocks. However, the addition of a second clock makes all the difference because even tho Nature can cause a null result in the round-trip case by slowing clocks and contracting rulers, She cannot possibly cause a one-way null result because She cannot reach down and set our clocks. Indeed, as I hinted at above, not even man (read "Einstein") can cause a one-way null result, not even on paper.

If you believe that a one-way null result can happen, then show us how. It ain't possible.

 

[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.