Astronomy.com

### Ask Astro: How can a photon be massless?

How can a photon of light carry energy but also be considered massless? Doesn’t Einstein’s E = mc2 make that impossible?. Einstein’s famous mass-energy equivalence equation, or E = mc2, is actually a special case of a slightly longer formula known as the energy-momentum relation, which is written out as E2 = p2c2 + m2c4.