That's a lot of questions, but they aren't inane and we'll try to answer most of them.
Collections of photons do indeed have mass. For a big collection, with zero net momentum (going all different directions) the mass is just the energy divided by c^2.
That doesn't have much to do with the expanding unverse, since there are much bigger contributors to the mass density and the pressure than the photons. You can think of the bending of light as related to its mass, but the source of the gravity that makes it bend is almost entirely other things, not light itself.
We usually don’t say that ’heat’ is something an object has, but a property of the energy transfer between objects. Photons can have ’thermal energy’, which is what is often informally meant by ’heat’. The cosmic microwave background consists of thermal photons, so all of its energy is thermal and that of course can be described numerically.
Lasers are an example in which photon density is high, but beyond that I don’t follow the question about lasers or the photoelectric question.
Mike W.
(published on 10/22/2007)
