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Q & A: Two photons at rest with respect to each other?

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Most recent answer: 02/24/2015
If two photons are travelling parallel to each other then are they at rest with respect to each other?
- harsh (age 20)

The photons do not have a rest frame, i.e. there is no frame of reference in which a photon is at rest. Photon travel always at c in all frames. Therefore such a question is actually not based on an observable situation by anybody. We can try to attempt the problem by taking an object with a velocity < c and take the limit. We would end up with two unpleasant consequences. Firstly, the photon would see everything infinitely contracted to 0 length. Secondly, you would not be able to have any reliable time measurement as we cannot accelerate a clock to c. If we could, a ground observer would say that it will never tick because of infinite time dilation. To claim that two things are at rest would require that you make time and position measurements, but both are problematic.


(published on 02/24/2015)

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