🚀 go-pugleaf

> 
> Second, what about Cerenkov(sp?)  radiation?  If the slower than c
> apparant velocity is an effect of the induced radiation from electrons,
> why should a particle moving through the intervening vacuum cause
> radiation?
> 
From: willner@cfa.harvard.edu (Steve Willner)
Path: cfa!willner

Cherenkov (should be spelled as you have, but with hacek over the C;
becomes Ch in English) radiation occurs when a particle exceeds the
speed of light in the current medium of travel.  It obviously cannot
occur for a particle travelling in vacuum, but it does occur for
high energy cosmic rays when they enter the Earth's atmosphere.  The
Smithsonian Astrophysical Observatory operates a telescope on Mt.
Hopkins to observe cosmic gamma rays by Cherenkov radiation of
secondary particles.  That is, the gamma ray enters the atmosphere
and hits an atom, ejecting particles at high energy.  These in turn
hit more atoms, ejecting more particles.  If the gamma ray energy is
high enough (above 10**15 eV or so, if I remember correctly), the
ejected particles will be going faster than the speed of light in air
and will radiate a detectable pulse of light by Cherenkov radiation.
("Detectable" means with a 10-meter diameter collecting dish; you'll
never see anything with your eye.)  
Steve Willner              Phone 617-495-7123        Bitnet: willner@cfa1
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