(I hope I've chosen the right sub-forum for this question...) Hi folks - I've recently joined here to see if people who are more knowledgeable than me can help me understand some physics issues I have struggled with for a long time. My current question is a pretty basic one about how it is possible for a photon to have no mass. We have the famous equation, "E = mc squared" My math knowledge is very limited, but from what I know - if I assign the value "0" to m, and multiply 0 by c squared, the answer for E should be zero. Yet a photon possesses energy, and is said to have no mass. I can see 3 possibilities: - "E = mc squared" is not a standard algebra equation, and assigning the value "0" to m doesn't result in E being zero. - "E = mc squared" does not apply to photons - something that seems very unlikely to me. - photons do, in fact, have mass, or conversely, have no energy. Can someone help me understand this? I would be very grateful. Thanks!