Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site oddjob.UChicago.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxl!ihnp4!oddjob!matt From: matt@oddjob.UChicago.UUCP (Matt Crawford) Newsgroups: net.physics,net.astro Subject: Re: RE: FLASHING QUASARS (the last word ?) Message-ID: <285@oddjob.UChicago.UUCP> Date: Sun, 10-Jun-84 14:57:58 EDT Article-I.D.: oddjob.285 Posted: Sun Jun 10 14:57:58 1984 Date-Received: Mon, 11-Jun-84 00:33:53 EDT References: <3560@fortune.UUCP> Organization: U. Chicago: Astronomy & Astrophysics Lines: 39 Yes, a system can be made to flash with period P and still be larger than c*P, but only certain observers will see the flashing. As an example consider a square sheet which emits from all points with an intensity that varies periodically with a period P = 1 second. Let the square have an edge of length L = (10 sec) * c = 3*10^9 meters. Let an observer be located a distance d from the center of the square on a line perpendicular to the square. The points on the square farthest from the observer are the corners, at a distance d' = d + L^2/4d + ... . This can be made arbitrarily close to d make making d sufficiently larger than L. To have d'-d < (0.1 sec)*c it suffices to make d > (250 sec)*c = 7.5*10^10 meters. Under these conditions, if the square switches between full intensity and zero intensity with a period of 1 second, the observer will see the pulsation, but with the transition from bright to dark smeared out over a tenth of a second. Consider another observer the same distance from the center of the square but 30 degrees off the perpendicular. The center of the edge nearest to this observer is (247.54 sec)*c away while the center of the farthest edge is (252.54 sec)*c away. This difference of five seconds of light travel time (a bit more for the far corners) will cause the observer not to see simultaneous pulsation but an average brightness, unless the observing instruments are sufficiently powerful to discern the bands of light and dark. Increasing the distance of the second observer will not reduce the time difference to less than 5 seconds. Diagram: (distances in light-seconds) | <- 250 -> 10 |-------------------------------O 1st observer | O 2nd observer If the object is spherical the ideas are the same, except that there are NO positions outside the sphere from which all points on the surface of the sphere are arbitrarily close to being equidistant. ___________________________________________________________ Matt University ARPA: crawford@anl-mcs.arpa Crawford of Chicago UUCP: ihnp4!oddjob!matt