Asri-unix.889
net.space
utcsrgv!utzoo!decvax!ucbvax!ARPAVAX:C70:sri-unix!ES@MIT-MC
Wed Mar 3 23:40:12 1982
angle and energy shifts of moving objects
Let K be the blueshift factor of an approaching object. Then the
angular size of the approaching object is reduced by a factor 1/K, and
the solid angle is reduced by a factor 1/K^2. I don't know an obvious
proof of this; you can work out the Lorentz transformation. However,
the energy per photon is increased by factor K (the Doppler shift), and
the number of recieved photons per unit time increases by factor K
(again a sort of Doppler shift), so the total received power increases
by a factor K^2. If the object subtends an observable nonzero solid angle,
i.e. is not a point source, the its brightness (Watt/stearadian) increases
by a factor of K^4. For this reason I would expect a blue shifted quasar
to be more readily detected than a similar red shifted one.
The above results apply regardless of the angle of approach, as
long as K is the blueshift factor.
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