> funkenstein gmail.com> writes:

>>Assume a sun-like stellar wind with 10^31 particles/second.  Assume
>>isotropy, and that all the energy goes to gamma rays, none to heat,
>>other particles, or other electromagnetic channels (unlikely but an
>>upper limit on gamma flux).  Take alpha cen as 4 light years away, so
>>the sphere with alpha cen at the center and us on the edge has a
>>surface area of 6*10^33 meters.  This would give us an expected flux
>>of 511keV gamma photons from alpha cen of  ~0.002 ph/m^2/s.  That's
>>2*10^-7 ph/cm^2/s, well under galactic background.

>
> This reminds me of a puzzle I heard once.  We get in communication with a
> civilization from a distant stellar system.  They want to send a visitor
> to earth.  We reply "Wait - we want to be sure that you aren't made of
> what we call antimatter - and vice versa. You'll have to perform an
> experiment and tell us the results first."  What experiment can they (and
> we) do to tell if they're made of the same matter as us?  Remember, it
> can't be something like "what is the charge on one of your protons?"
> because they have their _own_ definitions of "positive" and "negative",
> we have no idea whether their "positive" charge is the same or different
> from our "positive" charge.  No interchanging of samples or something like
> whether there are lots of 511 keV gammas when they approach our solar wind
> (this is also really interchanging samples)  Also ignore the lags in
> communication time, as if they had a Star Trek type communicator.