In article ,
dlzc writes:
Quite old, actually. I first heard of it in "Spacetime Physics" by
Taylor and Wheeler.

I don't have that book on my shelf, but I can probably get it from
the library. What page are you looking at (and in what edition)?

You can read about it he
http://physics.stackexchange.com/que...less-particles

Most of the "answers" there look wrong to me, though parts of some of
them are correct.

... or just realize for two oppositely-directed, equal-energy photons:
E^2 = (sigma( p1, p2) = 0)^2 + (mc^2)^2

What are the symbols supposed to represent? And where does the
equation come from?

If the photons have non-zero energy (E=/=0), their momenta cancel,
and they *must* have rest mass.

For the system you describe, momentum is zero, energy is non-zero,
and rest mass is zero.

Gravitational mass, is actually the question,

There's no such thing as "gravitational mass" in relativity unless
you want to define it redundantly as E/c^2. All energy contributes
to gravitation (or bends spacetime, if you prefer).

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