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Constancy of c  second postulate 
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#37
Feb512, 02:04 AM

P: 359

[One of] the consequence[s] of assuming Newtonian [absolute] time, was that clocks in all reference frames would tick at the same rate, wasn't it? This would have meant that a clock on a [moving] train would provided a measurement in units that was meangingful to an observer at rest on earth; such that the 's' in the definition of the speed of light would have been the same. If a clock on a moving train ticks slower, however, it would mean that that a measurement of 300, 000 km/s would not be the same as the same measurement in the earth centred reference frame. 


#38
Feb512, 04:07 AM

P: 359

Because, a sundial is effectively just a means of breaking the daylight period into smaller segments; it effectively just breaks the "arc" of the sun, over a particular location on earth, into hours and minutes, doesn't it? So any measurement, expressed in the units measured by a sundial, could be read as a function of the movement of the sun relative to a an object at rest on earth. If that object were in motion relative to the earth, then the units would be different. The same could be said for measurements expressed in the units measured by observatories plotting the apparent motion of the fixed stars. Equally so, for an atomic clock at rest on earth, but perhaps even a more precise expression of it's location may be necessary. This is more for myself, but I think we can imagine a plane shaped like a plus sign; such that, if the RC planes were both to start from the tail of the plane, and one of the planes turned at the intersection where the wings are, flew out to the end of the wing and then flew the distance to the end of the opposite wing, it would fly the same distance as the other RC plane flying out to the nose of the plane, returning to the midsection turning, and flying to the end of the same wing as the other RC plane; where the the detector determines if they arrived at the same time. Staying with that analogy; what if the RC planes were of such a design (let's say they are made of massless particles) that there would be no wind resistance, they wouldn't need to assume that the plane's length had shortened, would they? Also, if the length of time it took, for both RC planes to complete their respective trips, wasn't actually measured, rather the simple observation of whether they arrived simultaneously, or not was used; could they then conclude, when the planes arrive simultaneously, that someone on the ground would measure the speed of the RC planes to be the same as that measured by a person on the plane? With regard to the MMX, I think what the author suggests is effectively a ballisticlike (not necessarily a ballistic) explanation for the MMX results; namely that the wavelength of the light reflected from the mirrors [in the interferometer] is the same, and so, no fringe shift would be expected. 


#39
Feb512, 04:19 AM

P: 359

In the MMX, there is no motion relative to the source, is that accurate? There is probably something that I am missing, but it seems to suggest that the wavelenght [of the reflected light] is the same from both mirrors. 


#40
Feb512, 04:35 AM

P: 359

If the metre stick contracts and the clock slows down, and the same measurement is noted (using the same instruments) then the light will have traveled a distance shorter than 300,000km, in a period of time longer than a second. Just for the sake of [my own] clarity, when you say the mirrors are arranged at right angles to the direction of motion, I picture them being placed on the floor and ceiling (it could also be sidewalls, I presume). If the train is in motion [near the speed of light], then, just as with the horizontal mirrors (from front to back) the distance between the mirrors is not the actual distance the light travels because the mirrors are traveling also. The time it would take for both light beams to complete a round trip would be the same, wouldn't it? 


#41
Feb512, 04:39 AM

P: 359




#42
Feb512, 04:42 AM

P: 359




#43
Feb512, 04:49 AM

P: 359

If we take the earth centred reference frame for example, which is the frame in which "the second" and "the metre" are defined. If there is a reference frame moving relative to that such that their clock slows down, it means that their clock measures a "second" which is longer than "the second" on earth. If, using that clock, they measure the speed of light to be 300,000km/s, it means that that 300,000km/s isn't the same as the 300,000 km/s on earth  and so the two speeds are, in actuality, different. If they also experience length contraction, such that their metre is shorter than the earth metre, and, along with their slower clock, they measure the speed of light to be 300,000 km/s, it would again mean that the measurement is not the same as the earth measurement. In fact, it would mean that it took light a little over a second to travel a distance shorter than 300,000km. 


#44
Feb512, 05:20 AM

Mentor
P: 17,526

http://www.bipm.org/en/si/base_units/ Please do not repeat this incorrect assertion again. 


#45
Feb512, 05:36 AM

P: 359

the Metre How was the length of the path travelled by light in a vacuum measured, before the metre was defined in terms of the length of the path travlled by light in a vacuum? the Second The atomic clock used to register the oscillations of the caesium atoms e.g. the one in NIST, is that at rest relative to the earth, or in motion relative to it? 


#46
Feb512, 06:06 AM

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P: 22,313

Again, neither of those two questions are relevant.



#47
Feb512, 06:13 AM

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#48
Feb512, 06:15 AM

P: 359




#49
Feb512, 06:55 AM

P: 359

With regard to the atomic clock, the proper second is still defined in terms of an atomic clock at rest on earth, even correcting for the velocity of the atom in the lab  as a matter of interest, the velocity relative to what?  because it isn't defined in terms of the clocks traveling relative to the earth; that is, the clocks used in the HafeleKeating epxeriment can't be said to have counted "the proper second" when they return to rest on earth. 


#50
Feb512, 08:21 AM

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P: 17,526




#51
Feb512, 03:00 PM

P: 3,188

 http://en.wikisource.org/wiki/Influe...ocity_of_Light  http://en.wikisource.org/wiki/On_the...niferous_Ether 


#52
Feb512, 03:25 PM

P: 3,188

Atomic clocks as used by NIST are at rest on the earth, and corrected for such things as altitude, pressure, temperature,... Thanks to a lucky fact of the shape of the earth, it is not necessary to make a correction for the rotational speed: the clock slowdown due to rotation speed is compensated by the rate increase due to the higher potential from the bulging of the earth as a result of that same rotation. 


#53
Feb512, 06:06 PM

P: 359




#54
Feb512, 07:09 PM

P: 359

Just on that point, and this is somewhere I might lack clarity, but if someone uses a slower clock and a smaller ruler (than similar instruments at rest on earth) and if they measure the speed of light to be 300,000 km/s with those instruments, would it not mean that the speed of light in both frames is actually different; because it would mean that the light in the reference frame moving relative to the earth actually took longer than a second to travel a distance shorter than 300,000 km? Are you familiar with the paper by any chance? 


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