How can photon have energy, but no mass if “m=E/c^2” (E= mc^2)
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The equation is E^2 =(mc^2 )^2 +(pc)^2 where m is the *proper mass*
This E=mc^2 is only correct if the thing is at rest in the frame of reference (so that p=0), or if m is interpreted as relativistic mass.
Both p and E can depend on the frame of reference. The thing that is constant is m, also known as proper mass. The proper mass of photon is 0.
In that past, physicists put emphasis on the concept called “relativistic mass”, which is a different m that is *defined* by E=mc^2 , in other word, m is defined to be E/c^2 so that the equation is always true. Later, physicists realized this isn’t a good concept, but this concept is still remain in popular science. The relativistic mass of photon can be non-zero.
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