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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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