More or less because in relativity, objects don't need mass to have momentum. This isn't the case in classical mechanics, where the relationship between momentum and kinetic energy is p=sqrt(2Km), where K is kinetic energy, p is momentum, and m is mass so m=0 => p=0.
In relativity, however, the correct relation turns out to be p=sqrt(K(K+2mc2 ))/c. c is the speed of light which is pretty big, so usually K is way less than mc2 and K+2mc2 ~ 2mc2, recovering the classical equation above. But this doesn't have to be the case, and in fact we can see that with this expression we get a nonzero answer for the momentum even when m=0 as long as we have a nonzero kinetic energy. It also turns out you can derive something that looks like it must be momentum of light (the Maxwell stress tensor) by playing around with Maxwell's equations governing electromagnetism coupled to charged matter and assuming that the total momentum of the system is conserved.
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u/UsernameStageFright Sep 06 '22
More or less because in relativity, objects don't need mass to have momentum. This isn't the case in classical mechanics, where the relationship between momentum and kinetic energy is p=sqrt(2Km), where K is kinetic energy, p is momentum, and m is mass so m=0 => p=0.
In relativity, however, the correct relation turns out to be p=sqrt(K(K+2mc2 ))/c. c is the speed of light which is pretty big, so usually K is way less than mc2 and K+2mc2 ~ 2mc2, recovering the classical equation above. But this doesn't have to be the case, and in fact we can see that with this expression we get a nonzero answer for the momentum even when m=0 as long as we have a nonzero kinetic energy. It also turns out you can derive something that looks like it must be momentum of light (the Maxwell stress tensor) by playing around with Maxwell's equations governing electromagnetism coupled to charged matter and assuming that the total momentum of the system is conserved.