If an entity, such as a photon, has no "mass" : How can it "be" ?
In what domain do we say that it "exist's" ?
A photon has no mass, but it does have energy. We know that photons exist,
because we can detect them (otherwise, we coldn't see). We can measure a
photon's energy, its frequency, its momentum. It obviously exists; it just
doesn't have mass.
Of course, you want a deeper answer than that. Basically, "mass" is a
property that particles can have, but not something they must have. Mass
is just a way to tell how the motion of something will be affected by a
gravitational field (or more properly, the degree to which something curves
space), or by a force acting on it. Photons, which do not have mass, do
not curve space.
Richard Barrans Jr., Ph.D.
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Update: June 2012