# Is it possible for a photon to be at rest? [duplicate]

I know it doesn't really make sense if looking at the photon from the wave point of view, but is there any law of physics which prohibits a photon from stopping completely? Thanks.

• Do you mean, is there an imaginary medium that could slow down a photon to 0? Jul 19, 2013 at 21:54
• @fffred I was thinking if there is some way to apply a force to decelerate the photon.
– Ovi
Jul 19, 2013 at 21:58
• Jul 19, 2013 at 22:16

The photon is a massless particle. Massless particles move at the speed of light. The special theory of relativity postulates that the speed of light is independent of the frame of reference and the maximal possible speed at which a particle can travel. If there existed a reference in which the photon rested, it would be possible to take a lorentz boost into that frame and so the speed of light at which that massless particle travels would depend on the choice of the frame of reference, which is a contradiction to the Einstein postulates of the special relativity. Therefore massless particles like the photon do not have a rest frame.

• "special theory of relativity postulates that the speed of light is independent of the frame of reference" So if you were traveling at 99% of the speed of light (with respect to me) you would still measure the speed of light as 300, 000 m/s (with respect to you)?
– Ovi
Jul 19, 2013 at 21:57
• @Ovi, yes. This is probably the most famous outcome of the theory of relativity. Jul 19, 2013 at 21:59
• "From the Wikipedia article: The Principle of Invariant Light Speed – "... light is always propagated in empty space with a definite velocity [speed] c which is independent of the state of motion of the emitting body." "(from the preface).[1] That is, light in vacuum propagates with the speed c (a fixed constant, independent of direction) in at least one system of inertial coordinates (the "stationary system"), regardless of the state of motion of the light source. Jul 19, 2013 at 22:01
• en.wikipedia.org/wiki/Special_relativity Jul 19, 2013 at 22:02
• @Hansenet But I watched this video and at 5:23 they seem to be saying that the photon can be stopped: youtube.com/watch?v=5r6vyZ2bykg
– Ovi
Jul 19, 2013 at 22:02

Those laws of physics would be Maxwell's Equations. I won't go into too much detail but from those equations you can get a wave equation for light. The speed of the wave is determined by two fundamental constants, $\epsilon_{0}$ and $\mu_{0}$. If the speed of light was variable in anyway then those constants would also be variable. No experiment has ever shown these constants to be different, therefore the speed of light must of constant. Since we know that light is composed of photons then photons must move at light speed.

• By the way I know that $\epsilon$ is the constant of permittivity, but what does $\mu$ stand for again?
– Ovi
Jul 20, 2013 at 2:26
• That would be vacuum permeability which is associated with electric currents. Jul 20, 2013 at 2:29
• Oh ok than you.
– Ovi
Jul 20, 2013 at 2:35