MadSci Network: Physics

Re: What is the relative speed of two photons passing each other?

Date: Wed Mar 6 12:19:02 2002
Posted By: Ronald Fisch, Physics, Washington University
Area of science: Physics
ID: 1014364714.Ph

The only proper way of measuring the relative speed of two objects
is to find the speed of object A in a reference frame in which object B is
at rest (or vice versa).  Now, there is no way that we can do this for two
photons.  All real photons in vacuum travel at the speed of light in any
inertial reference frame.  If we change reference frames, the energy and
momentum of each photon will change, but its speed remains the speed of light.
There is no allowed inertial reference frame in vacuum such that a real
photon is at rest.  So there is no proper way to perform this measurement.

Therefore, the relative speed of two photons is not a measurable quantity.
Or, as we say in quantum mechanics, there is no observable which measures
this quantity.

I think you probably will not like this answer, but it is the correct one.

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