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Q & A: photons in the cosmos

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Most recent answer: 10/22/2007
Q:
the last question i asked was not a joke but i was trying to invision the universe 1/100 after big bang. I am not in any science class but trying to gain some knowledge on a fascinating albeit diffucult subject that is seemingly at best speculative.

Any way my question is would it be possible for large quantities of photons to have mass, immeasurable by earth standards. Would that explain the expanding universe and why light bends over long distances. would proof be the photo electric effect where electrons are moved. would lasers be an example of the photon density. and finally is there a heat in photons that is constant and can it be numerically expressed. Forgive me if my questions are inane, but I have no one else to ask. thank you
- devon (age 20)
costa mesa ca
A:
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 donít say that íheatí is something an object has, but a property of the energy transfer between objects. Photons can have íthermal energyí, which is what is often informally meant by íheatí. 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 donít follow the question about lasers or the photoelectric question.

Mike W.

(published on 10/22/2007)

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