Invariance of Light

Jan
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21 years 4 months ago #6425 by Jan

Replied by Jan on topic Reply from Jan Vink

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
[TVF]: No such experiment is possible. The invariance of c for observers is a postulate, not a hypothesis that can be tested. It can always be made true by setting the clocks involved in time and rate so that the measurements will yield c even in the direction of motion.
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Do we really need to synchronise clocks in time? Suppose we manufacture "identical" rulers and clocks, but we only demand that the clock rates are "identical" on Earth. We launch a rocket that is accelerated with a maximum intensity and duration as technologically possible. After some time, having depleted the fuel, the probe should travel with a constant "speed". Then, within the probe, the SOL measurements are taken and labeled as "d". As the results are send back to Earth, we should have "d=c". Moreover, the measurements can be taken at will and at any "time", which emphasises the equal status of the inertial frames. That is, the frames do not need to "know" each others existence.

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tvanflandern
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21 years 4 months ago #6171 by tvanflandern

Replied by tvanflandern on topic Reply from Tom Van Flandern

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>;Jan]: Do we really need to synchronise clocks in time?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Yes, if SR is to be used, clocks must be synchronized by Einstein's method.

Clocks can also be synchronized in other ways or left unsynchronized, in which case the measured speed of light is no longer c. Why is this relevant? -|Tom|-

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Jan
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21 years 4 months ago #6172 by Jan

Replied by Jan on topic Reply from Jan Vink

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
[TVF]: Clocks can also be synchronized in other ways or left unsynchronized, in which case the measured speed of light is no longer c
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I don't quite understand. We only need to measure the time interval between emission and detection of the light pulse over a given length. The initial conditions of the clocks are irrelevant aren't they?. For example, the clocks start when the pulse is emitted and stop when light has been detected, but as long as the clock rates are identical before departure of the two observers. Shouldn't we be interested in time intervals only? When the two clocks start is of no consequence?

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
[TVF]: Why is this relevant?
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I just want to grasp the physical arguments why relativity has thrown away the concept of independent "traversal", and only considers "observers". A wave front first observed in frame A and later observed in frame B both measure "c". This can only make sense if light does not traverse as a single entity between the realms of frame A and frame B, but perhaps travels in a spectrum of speeds. Also, a wave front could have been initiated on the other side of the universe, but its existence is justified by observers only. The notion of independent traversal of space has been completely abandoned in my humble view. Perhaps I just should stop trying to understand the physical aspects of relativity and accept that the invariance of the SOL can only be reconciled by scaling lengths and time variables. If the invariance of the SOL cannot be verified in any way, why is it often said that this is an experimental fact and that Nature behaves in this way?

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Jim
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21 years 4 months ago #6054 by Jim

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Hi Jan, Now this is getting more and more complex rather than simple and that is exactly the opposite of what I hoped for. The models are real events are all mixed up here and no one in the field seems to be at all troubled by that. You have models that work fine as long as you don't rock the boat as they say.

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Jan
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21 years 4 months ago #6288 by Jan

Replied by Jan on topic Reply from Jan Vink

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
[Jim]: You have models that work fine as long as you don't rock the boat as they say.
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Don't get me wrong, I'm certainly _not_ in the position to rock any boat. I'm just an innocent bystander, however, one of my favorite math lecturers once said: "It is never too late to think for yourself."

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
[Jim]: Now this is getting more and more complex rather than simple and that is exactly the opposite of what I hoped for.
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This was not my intention. Relativity tells us that when your are moving, you are actually not moving, and when you think you are at rest, you are actually moving. Relativity says that when you perish, you are resurrected by someone who happens to travel close to "c". On the other hand, you may not even reach your retirement by that same person if he/she changes here direction of motion. Although I have no formal education in physics, please forgive my initial reluctance in accepting the above.

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tvanflandern
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21 years 4 months ago #6289 by tvanflandern

Replied by tvanflandern on topic Reply from Tom Van Flandern

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>[Jan]: I don't quite understand. We only need to measure the time interval between emission and detection of the light pulse over a given length. The initial conditions of the clocks are irrelevant aren't they?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Think about what you just said. The emission and detection events are at two different places and times. To measure the time interval, we must have a clock located at the emission event and another clock located at the detection event. (In SR, because time changes with location, no other clock readings would be relevant.)

If the emission clock says the time is 5 pm at its event, and the reception clock says the time is 3 pm at its event, then the measured time interval is minus two hours. That is obviously meaningless, and simply reflects the fact that the two clocks were never synchronized.

Einstein gave a prescription for synchronizing clocks: exchange of light signals between them. But that method assumes that light travels at a constant speed c with respect to all clocks. Clearly, if we synchronize clocks using the assumption that lightspeed is constant, then use that measured time interval to measure lightspeed -- surprise, surprise -- we get back the measurement that lightspeed is constant.

In case it is not obvious, that was not a real experiment. It was just a check on our assumptions.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>Shouldn't we be interested in time intervals only? When the two clocks start is of no consequence?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Readings on clocks are meaningless unless the clocks are set initially in some meaningful way. So the moment when the clocks start will determine the length of time measured.

Synchronizing clocks already in place requires an assumption about the speed of the signals used to set them. Synchronizing clocks while they are still together, then moving one of them, upsets the synchronization by changing the relative rate of ticking. Once again, the measurements made depend on the assumptions made about the behavior of clocks in motion. If we use all SR assumptions, we cannot claim that we are measuring reality; but we can claim that we are using an internally consistent procedure.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>I just want to grasp the physical arguments why relativity has thrown away the concept of independent "traversal", and only considers "observers".<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

SR is a theory. It is one way to describe nature.

The failure to detect aether was a great frustration in the early 20th century. When Einstein showed a way to make measurements that required no special frame, that was considered a great breakthrough for those times.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>A wave front first observed in frame A and later observed in frame B both measure "c". This can only make sense if light does not traverse as a single entity between the realms of frame A and frame B, but perhaps travels in a spectrum of speeds.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

That is false reasoning. If time itself is frame-dependent and location-within-frame dependent, then the measured lightspeeds are also.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>The notion of independent traversal of space has been completely abandoned in my humble view.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

It is not clear what you mean by this. A speed cannot be measured at an instant. It requires taking the ratio of a finite distance interval and a finite time interval.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>Perhaps I just should stop trying to understand the physical aspects of relativity and accept that the invariance of the SOL can only be reconciled by scaling lengths and time variables.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

I assure you that those scalings are essential to SR.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>If the invariance of the SOL cannot be verified in any way, why is it often said that this is an experimental fact and that Nature behaves in this way?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Because eleven independent experiments that tested the relativity of motion have been declared to have confirmed SR. But the historically accurate truth is that four of those experiments were declared at the time to falsify SR, and were only reconciled with it at a later date.

And of course all this ignores that a competing theory, LR, exists and satisfies the same eleven experiments. No experiment prior to the speed of gravity was able to differentiate between SR and LR. Now, we can, and LR won the competition. -|Tom|-

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