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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:Comment about this article, ask questions, or add new information about this topic: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
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with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP