How is it possible that a force that affects mass (gravity) can affect massless particles like photons?
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In special relativity, Einstein developed a relation between energy and mass based on both the particles momentum and it's rest mass. We often times say that gravity acts on mass, but the effect of gravitational lensing indicates that gravity acts on energy rather than mass (or from a different perspective, on both rest mass and momentum). Thus, since a photon has energy and momentum, it can fall under the effects of gravity. This is why we think of general relativity as a curve in spacetime: because even if a particle has no rest mass, it can still bend due to momentum changes.
Chris
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