- #71
StarThrower
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Tom Mattson said:There is no intertial reference frame in which a photon is at rest. The only way to get a contradiction here is if you assume that there is one. But that is not an assumption made by special relativity.
Assume that there is at least one inertial reference frame in which a photon's speed isn't equal to zero, and refer to this reference frame as F1.
According to the fundamental postulate of the theory of special relativity, the speed of the photon in this inertial reference frame is c, where c = 299792458 m/s.
To Prove: The rest frame of the photon is an inertial reference frame.
Let the photon be moving along the positive x-axis of F1, in the direction of increasing x coordinates of F1.
By Newton's first law, the photon's direction will not change, unless the photon is acted upon by an outside force. Presume that over some period of time, the sum of all forces on this photon is zero. Hence, the photon will move through F1 in a straight line, by Newton's first law.
Now, define reference frame F2 as follows:
The origin of F2 is this photon, and the axes of F2 are not rotating with respect to the axes of F1. Furthermore, let the x-axis of F2 be parallel to the x-axis of F1, and the same for the y,z axes of F2.
Consider the position vector of the origin of F1, as defined in F2.
Let the origin of F1 currently have coordinates (x1,y1,z1) in F2.
The position vector of the origin of F1 as defined using F2 is the vector from the origin of F2 to (x1,y1,z1).
Hence, the position vector of the origin of F1 in F2 is:
(x1-0)i^ + (y1-0) j^ + (z1-0)k^ = <x1,y1,z1>
Now, the position vector of the origin of F1 as defined in F2 is changing in time. Thus, the velocity of the origin of F1 in reference frame F2 is:
V = d/dt [ <x1,y1,z1>] = <d(x1)/dt,d(y1)/dt,d(z1)/dt>
Now, the origin isn't moving in the y or z direction in F2, hence
d(y1)/dt=0 and d(z1)/dt = 0
Thus
V = <d(x1)/dt,0,0>
And since speed is relative,
V=c
And c is constant.
Hence, the origin of F1 moves through F2 in a straight line. And there was no force acting to accelerate F1. Hence F2 is an inertial reference frame.
QED
PS (this was a fast proof) For a more rigorous proof, investigate linear transformations from F1 to F2.
Kind regards,
The Star