Mati Meron wrote:

A photon doesn't have a mass. A set of photons, in general,
will have mass (except when they all propagate in the same
direction). The mass of a system is not the sum of the masses
of its components, in general.

Hmmm. Clues to the answer to a question George Dishman posed
to me a few days ago. Thank you.

Except when they all propagate in in the same direction?
Are you sure about that? If it is so, the mass equivalent of
a bunch of photons depends on the distribution of directions.
Imagine two bunches of photons bouncing between two mirrors.
Sometimes both bunches are moving in the same direction, and
sometimes in opposite directions. Of course the box containing
the mirrors and photons is accelerated whenever a bunch of
photons hits one of the mirrors, and maybe that accounts for
the apparent change in mass of the system?

-- Jeff, in Minneapolis

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