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I was reading Phil Plait's 'Death from the skies' where it describes the fate of the universe. It describes the period from 10^90 years to infinity as the "dark age" when all the of the black holes have disappeared.

In this state the universe consists many particles. If two like-charged particles encounter each other, they are repelled. However, if two unlike-charged particles come close to each other, they will move towards each other and disappear to become light energy.

This wasn't covered in the book, but unless I've misunderstood things (which is quite likely), there will be a point when all of the mass in the universe has been reduced to zero after all the particles have paired up and become photons?

If so, is there an estimate when this event will occurr?

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Photons don't have zero mass. They have zero rest mass. Since they're never at rest, they do have non-zero energy, which is equivalent to mass (E=mc2). (There are other ways to look at it, such as that a photon has zero mass and non-zero energy -- but since mass and energy are fundamentally equivalent, it's pretty much the same thing. (This is where an actual physicist jumps in and tells me I'm wrong.))

For example, the path of a beam of light is actually bent by gravity. We can see this in the gravitational lens phenomenon. And light, because it has mass/energy, exerts a gravitational pull of its own, though it's so small as to be undetectable in any but the most extreme conditions.

If things happen as you describe (all the mass of the universe decaying to photons), then the universe might reach zero rest mass, but the conservation of mass-energy still applies.

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