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Synchronized Clocks in Frames boosted by Acceleration 
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#37
Dec912, 03:51 AM

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#38
Dec912, 04:48 AM

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P: 4,793

Ok, good. Now the observer takes one of these cables, along with the light detector on the far end and the electrical signal detector/light generator on the near end, and he swings the cable around so that it can pick up the reflection of the light from the target and send an electrical signal down the cable back towards him.
Now he repeats the experiment. The light flashes at the source. The detector immediately generates an electrical signal that travels down the cable parallel to the flash of light outside the cable. When it gets to the near end of the cable, the electrical signal immediately produces a flash of light coincident with the flash the traveled parallel to the cable in free space and the observer sees both of these at the same time and he starts his timer. Now the flash in free space continues onward toward the target all by itself until it reaches the target. At this point, it reflects off the target and starts back toward the observer and also immediately triggers the detector in the far end of the second cable which generates an electrical signal that travels parallel to the light flash in free space coming back to the observer. After some time, the light flash and the electrical signal arrive at the observer where the detector/generator in the near end of the cable produces a flash at the same time that he also sees the light flash that got to him in free space at which point he stops his timer. Do you understand and agree with this assessment? 


#39
Dec912, 05:56 AM

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Yes George, I think this is equivalent to your original reflection test. I would like to hear why this is equivalent to Don's test.



#40
Dec912, 06:51 AM

PF Gold
P: 4,793

You previously agreed that the observer was measuring the round trip time for the light to go from his location to the target and back to him. Now with the cables in place, you have agreed that the observer is doing exactly the same thing with cables that he was doing with just light.
Now replace the observer with a fast oscilloscope and remove the two detector/light generators at the receiving end and plug the two calibrated cables into the scope. You can start over and put the detector ends of both cables at the source and recalibrate as Don suggested and then move one end of one cable from the source to the target and you have exactly the test that Don devised so do you agree that his test in this case is measuring the round trip time for the light to traverse the last half of the distance from the source to the target? 


#41
Dec912, 12:33 PM

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#42
Dec1112, 12:42 AM

P: 765

His two cables are of arbitrarily, identical lengths, which I define as 1 distance unit and define time so that the 2way speed of light comes out at [itex]c[/itex]. I then assume that the 1way speed of light may be [itex]c[/itex]' (outbound) and [itex]c[/itex]'' (inbound) and that the signal speed in the cables is equivalent to a refraction index [itex]n[/itex], so that (1) [itex]1/c' + 1/c'' = 2/c[/itex] (vacuum) and: [itex]n/c' + n/c'' = 2n/c[/itex] (cables). The time differential that Don measures on his oscilloscope: (2) [itex]\Delta T = 1/c' + n/c''  n/(2c')  n/(2c'') = 1/c'  n/(2c') + n/(2c'') [/itex] This correlates with the 2way speed of light in vacuum only if [itex]n[/itex] = 1; e.g. if [itex]n[/itex] = 2, then [itex]\Delta T = 1/c''[/itex]. Or am I making a wrong assumption somewhere? 


#43
Dec1112, 03:08 AM

Sci Advisor
P: 2,470

SR is built around assumption that oneway speed of light is equal to twoway speed of light. However, it should be possible to construct an equivalent theory in which they are not. Such a theory will have needless complications which simplify for any possible experiment, giving you exactly the same predictions as SR. But that's exactly the thing that tells you that you can't measure oneway speed of light. At least, not within the confines of SR. If you do make a measurement, you prove relativity wrong regardless of whether you got the same or different result. 


#44
Dec1112, 06:01 AM

P: 3,188

Note that SR does not claim that either is "right" (and both would even be contradictory); it claims that if we use that definition then the laws of physics such as that of Maxwell work wrt any inertial frame. 


#45
Dec1112, 10:40 AM

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P: 4,793

That is why I asked you to use cables that propagate an electrical signal the same as light in free space along a parallel path. This is something that can be measured without resorting to clock synchronization or identifying how long that propagation took or what the speed of the light or electrical signal are, just like we can measure that the propagation of light is independent of the speed of the source. See the section called "Experiments that can be done on the oneway speed of light" in the wikipedia article on the oneway speed of light that I referred you to in post #2. That is also why I asked you to not attempt to identify the length of the cables or the distance between the source and the target. All we care about is that the observer is at the midpoint and that the two cables add up to that distance. That is also why I asked you not to attempt to identify the speed of the round trip for light or the electrical signals. Instead, I asked you to only make a measurement of a time interval by a single timing device located at a single position with identifiable stimuli, namely when the observer sees the light and electrical signals after they propagate from the source and when the observer sees the light and electrical signals after they propagate from the target. All I'm trying to get you to recognize, which you already agreed to, is that there is no difference between making the timing measurement with light or with cables. And you already agreed that with light, it is a twoway measurement of the last half of the distance. Now if you want to understand the problem taking advantage of a hundred years of experience with physics then I suggest a different approach. Set up the problem in a single Inertial Reference Frame and establish the coordinate times for each event: the emission of the light at the source, the arrival of the light at the observer when he starts his timer, the arrival of the light at the target when the reflection and return starts, and the arrival of the reflected light back at the observer when he stops his timer. In this IRF, the propagation of light is defined to be c. Now transform the scenario into another IRF moving at some high speed with respect to the first IRF. You will see that the new time coordinates for the same events do not show equal time intervals for each direction. For example, let's say that the source and start of the experiment are at the origin of the IRF's and the observer is located at x=0.5 and the target is at x=1 in the first IRF. Here are the coordinates of the events for the first IRF: Start of light: x=0 t=0 Light reaches observer: x=0.5 t=0.5 Light reaches target: x=1 t=1 Reflected light reaches observer: x=0.5 t=1.5 Now transform the time coordinates of these events into an IRF traveling at 0.6c with respect to the first IRF: Start of light: t=0 Light reaches observer: t=0.25 Light reaches target: t=0.5 Reflected light reaches observer: t=1.5 The difference between the time coordinates of the light reaching the observer and the reflected light reaching the observer is 1.25. This is partitioned into 0.25 for the light to get from the observer to the target and 1.0 for the reflected light to get from the target back to the observer. Now when traveling at 0.6c, gamma is 1.25, so the observer's clock is time dilated by that factor so that when he measures the time for the light to make its trip from him to the target and back, his clock will advance by 1 unit, just like it did in the first IRF. He cannot tell that the light got to the target in one quarter of the time that the light took to get back from the target to himself. Now if you understand this explanation, you can go ahead and repeat it for cables that propagate signals identically to light or at some reduced rate. [George: prepare yourself for the barrage of criticism that you are mixing coordinates from two different IRF's.] Ok, I'm prepared. 


#46
Dec1312, 12:32 AM

P: 765

Thanks for your patience. 


#47
Dec1312, 01:47 AM

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Now, are you going to have another discussion with Don Lincoln? If you do, I would hold out the possibility that he is already well aware that his test is not really measuring the oneway speed of light independent of a synchronization convention but it is an excellent and practical way to follow Einstein's convention to measure the propagation of light in the lab, something that he and his coworkers are probably doing all the time at FermiLab. 


#48
Dec1812, 01:57 AM

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This shows that it is equivalent to having a synchronized clock at each end of the cable; but still, he needed only one clock, the oscilloscope. One clock, two clocks? Probably a moot point. 


#49
Dec1812, 02:51 AM

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And now I see that Don Lincoln used that same word in his description of the test setup. 


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