Re: opposite of relativistic time contraction

Replying to myself, because I think I made a mistake:

> (1) Matter is essentially transparent to gammas of this energy, so a
> collimator won't collimate.

Oops, maybe not. High-energy gamma rays with energies of ~10 MeV
are very penetrating, but I think when we scale the energy up to
10^9 MeV the total cross-section may actually be large, since the
cross-section for e+e- pair production increases with energy, and
you also have lots of other kinds of pair production coming in as
well.