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. ----------------------------------------------------------------- gopher://quux.org/ conversion by John Goerzen of http://communication.ucsd.edu/A-News/ This Usenet Oldnews Archive article may be copied and distributed freely, provided: 1. There is no money collected for the text(s) of the articles. 2. The following notice remains appended to each copy: The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996 Bruce Jones, Henry Spencer, David Wiseman.