Although we've answered this question before, it comes in so often that it seems time to try to put a compact, easily searchable version of the answer up again. As a brief preview of the more complete answer, a photon has energy, which is equivalent to mass, and therefore interacts via gravity with everything else.
Much of the confusion arises because the word "mass" has been used in two different ways in physics. The "m" in E=mc2
is (as the equation makes clear) just another symbol for energy, expressed in different units. This same "m" also appears in the equation for momentum p
, where v
is velocity. Light has energy and momentum, so it has "m" in this sense. This m is the same thing that appears in General Relativity (or even Newtonian gravity) as the source of gravitational effects. So light is definitely affected by gravity. Since light has energy, it is also a source
of gravitational effects on other objects, although not a very strong one under ordinary circumstances.
Now when people are describing the "mass" of different objects, including particles, it's much more convenient to talk about the rest mass, also called the "invariant mass", the mass a particle has in a frame in which its momentum is zero. That way you don't have to ask what reference frame you're using, and can just give a specific mass for each object. In that sense, the mass of a photon is zero. However, that's not the term that enters into the gravitational equations.
Although the "invariant mass" is indeed invariant under choice of reference frame, it is not invariant under choice of how to group things into objects. For example, take two similar blips of light traveling opposite directions. Each one has energy E, momentum |p
|=E/c, and an invariant mass of zero. Since the momenta are opposite, we are already using the reference frame in which the momentum of the two-blip object is zero. The invariant mass of the two-blip object is then 2E/c2
, not zero. Even when things have no interaction, the invariant mass of the sum is not the sum of the invariant masses
. A big box of photons has energy, zero average momentum, and thus has some invariant mass. It acts gravitationally just like anything else with the same energy and no momentum.
(published on 01/09/2011)