Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.07 Are the QSO's really at their redshift distances? Next Document: H.09 What's the Local Group? See reader questions & answers on this topic! - Help others by sharing your knowledge The apparently faster-than-light motions observed in the jets of some radio-loud quasars have misled a number of people into believing that the speed of light is not really a limit on velocity and that astrophysics has provided a disproof of the theory of relativity. In fact, these motions can be easily understood without any new physics; you just need trigonometry and the idea of the constancy of the speed of light. Consider the situation shown in the diagram below. A blob B of radio-emitting plasma starts at O and moves with velocity v at some angle a to our line of sight. At a time t, B has moved across the sky a distance vt sin a. The light from when it was at O has travelled a distance ct towards us (c is the speed of light). But the light from its position at time t only has to travel an additional distance (ct - vt cos a) to reach us. Thus we measure the time between the two events as (distance / speed of light) = t(1 - (v/c) cos a). If we derive an apparent velocity by dividing the (measurable) transverse motion of the source by the measured time difference, we get vt sin a v sin a v(apparent) = ------------------ = --------------- t(1 - (v/c) cos a) 1 - (v/c) cos a ^ O ^ | |\ | | | \ | | | \ vt cos a | | a \ | ct | \ | | | \ | | | B v | | ^ | | ct - vt cos a v | v \_____I_____/ (Earth, radio telescope) This apparent velocity can clearly be greater than c if a is small and v is close to c. There are other independent reasons for believing that the jets in radio-loud quasars have velocities close to c and are aligned close to the line of sight, so that this explanation is a plausible one. User Contributions:Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.07 Are the QSO's really at their redshift distances? Next Document: H.09 What's the Local Group? Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
Last Update March 27 2014 @ 02:11 PM
|
Comment about this article, ask questions, or add new information about this topic: