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Constancy of c  second postulate 
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#73
Feb1612, 07:39 AM

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#74
Feb1612, 07:49 AM

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#75
Feb1612, 07:55 AM

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#76
Feb1612, 08:51 AM

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#77
Feb1612, 08:54 AM

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#78
Feb1912, 07:28 PM

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My understanding is that it is a means of translating the coordinates of an event in one reference frame into the coordinates of another. The scaling factor gamma, or Lorentz factor is involved. I don't know the technical details of the formula, but what I've encountered suggests that it can be derived using the Pythgorean theorem  as per the video explanation I posted (in this thread I think it was). 


#79
Mar1012, 12:20 PM

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I think there is only one postulate...that of relativity....that all observers in an inertial frame will find all phenomena to be described by the same equations....from this obviously it follows that the velocity of light has to be constant for all observers...otherwise relativity will not hold...
Hence really there is only ONE postulate...that of relativity...the other (constancy of the velocity of light) is a corollary of it.... 


#80
Mar1012, 05:22 PM

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Hi rjaindia, welcome to PF!



#81
Mar1012, 06:18 PM

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But the reason I asked is because there is no provision for gravity in the Lorentz Transform or in Special Relativity. We pretend like the effects gravity don't exist when we're doing transforms in SR so you don't need to worry about how time is effected by height. 


#82
Mar1012, 06:24 PM

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#83
Mar1112, 04:23 AM

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See my earlier posts in this thread: http://physicsforums.com/showthread.php?p=3742805 #29 http://physicsforums.com/showthread.php?p=3187482 #2 


#84
Mar1112, 05:58 AM

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#85
Mar1112, 12:14 PM

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The second postulate asserts that, within any single inertial frame, the oneway speed of light is a constant value. (So it doesn't depend on the motion of the source or the direction of propagation.) It doesn't assert that the constant value is the same in every frame, but that is something that follows from the first postulate (otherwise you would have a method for distinguishing one frame from another). So to obtain the invariance of the speed of light in all frames you need both postulates.
The second postulate (without the first postulate) implies that, within any single inertial frame,



#86
Mar1212, 02:26 AM

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This was the measurement that lead to the Lorentz Transformation as the basis for the new Principle of Relativity because the old one based on the Galilean Transformation didn't work any more. In the Lorentz Ether Theory, the presumed second postulate was that light propagated at c only in one absolute ether rest state but due to length contraction and time dilation, the measured roundtrip speed of light always came out the same even when the experiment was done in motion through the ether. When Einstein proposed his second postulate, it was a followon to the first one and he noted that it was apparently irreconcilable with the first one because it seemed impossible that light could make both parts of the trip in the same amount of time under differing states of inertial motion in any measurement of the roundtrip speed of light. However, Einstein calls this second postulate "the principle of the constancy of the velocity of light", which I presume is the same as "the invariance of the speed of light", as you called it. I would repeat that the second postulate is a followon to the first one in which the value of the speed of light has already been "determined" (as Einstein says) to be c and so I agree with your twopart summary but I would say that (a) is part of the first postulate and (b) is the second postulate. It is obvious from Einstein's elaboration of the second postulate in section 2 that he is excluding the twoway speed of light from it since he calls it a "ray" of light and defines its velocity as: where the time interval is onehalf of the measured roundtrip interval. 


#87
Mar1212, 03:21 AM

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Recycling in part my post #29, that directly follows from Einstein's 1907 formulation of the second 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." As we discussed in the past, for this to be possible the twoway speed must be the same in all directions. 


#88
Mar1212, 03:34 AM

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See my post #132 here: http://physicsforums.com/showthread....ight=invariant Best regards, Harald 


#89
Mar1212, 07:51 AM

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#90
Mar1212, 11:31 AM

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Harald 


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