Gravity and Photons

[circa 1991]

Question:  When gravity acts to bend light, is it because a photon does have 
mass when it has velocity, or does gravity act on the energy of the photon, 
which I understand has no rest mass?
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Well, gravity acts on everything since it really acts on space 
itself!  Yes, a photon has no rest mass, and that means that it is always 
traveling at the speed of light.  The energy of a photon is actually 
proportional to its frequency, y, which does not really have much to do with 
the way gravity acts on it.  However, since bending involves a change of 
momentum, the force that gravity exerts on the photon is proportional to its 
momentum (which is in turn proportional to its energy).  And in fact, it is 
generally the total energy (rest energy plus kinetic energy etc.) that is 
important for gravity, whether the particle in question has a rest mass or 
not.

Arthur Smith
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