In article ,
dlzc writes:
Individually, no. But in groups, with a center of momentum frame,
[photons] do have rest mass.

So the sum of a bunch of zeroes is non-zero? That's new physics.

I have no idea what "with a center of momentum frame" is supposed to
mean.

Standard physics says photons have momentum and energy but zero rest
mass. Photons react to gravity and (in principle, but I don't think
it has been measured) create gravity, but neither of those properties
requires rest mass.

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