View Full Version : Question regarding speed of light

2008-Jan-20, 02:45 PM
This question probably is silly with an obvious answer but reading some other speed of light-related threads made me think of it. Since light travels slower than c when passing through a medium, wouldn't our measurements of very distant objects be wrong? Or have we calculated the difference bewteen c and light travelling through interstellar/intergalactic space and used that for our calculations?

I imagine the very tenuous gas that exists between stars/galaxies would permit for light to travel extremely close to c, but given the enormous distances between us and galaxies billions of light years away, wouldn't discrepancies arise?

with regards

2008-Jan-20, 02:57 PM
Simply put, we don't judge distances by the speed of light.

We use a variety of methods, one of which is the measurement of Cepheid variables (http://en.wikipedia.org/wiki/Cepheid_variables). Their absolute brightness and their frequency of pulsation are very reliably linked. So if we see a pulsation of X, and we observe their apparent brightness as Y, then we can estimate how far away they'd need to be to be that dim by distance alone. Other methods back this up, but that (I think) is still the Gold Standard.

Jeff Root
2008-Jan-21, 08:32 AM
And the difference in speed caused by intergalactic gas is too small
to be of any significance. If it were significant, images of distant
objects would be noticably blurred and distorted, especially in color,
because different colors (wavelengths) are affected differently.
That is the cause of rainbows, for example, when different colors
of light are slowed by different amounts as they pass through drops
of water, causing the different wavelengths to be diffracted in
different directions.

-- Jeff, in Minneapolis

2008-Jan-21, 11:32 AM
The difference in distance due to lower speed of light in intergalactic space adds up to a few centimeters for distant galaxies. Space is *very* empty.

2008-Jan-21, 11:44 AM
Cool cool, knew there was something extremely obvious I was missing, ah well. :D

The difference in distance due to lower speed of light in intergalactic space adds up to a few centimeters for distant galaxies. Space is *very* empty.

I remember reading a long time ago in a book that interstellar space is so tenuous that there is on average one atom of H every cubic centimetre (my memory could be wrong, it was ages ago). I imagine that intergalactic space would be even more so. Anyone have any idea how "empty" it is, in comparison to interstellar space?

with regards

2008-Jan-23, 03:43 PM
The effect causes pulses from pulsars to spread out in the frequency domain, with higher frequencies arriving slightly later. You can use this Dispersion Measure to either model the distances to pulsars or to model the electron column density, depending on which is better known. For pulsars of typical distances of a few kpc the DM is only a few milliseconds I believe.

2008-Jan-29, 08:42 AM
That's interesting loglo, more new stuff to me. :D Anyone else have more input?

with regards

Ken G
2008-Jan-29, 10:08 PM
There's another interaction called "Faraday rotation", where if you have electrons spiralling around a magnetic field in the interstellar medium, then light that is circularly polarized in the same sense as the spiralling electrons will interact more strongly and be more slowed, then light with the opposite polarization. Slowing one polarization then acts to rotate the direction of linearly polarized light, which you can see if you realize that linearly polarized light can be treated as though it were equal amounts of oppositely circularly polarized light. The degree of the rotation depends on both the magnetic field and the electron density, a fact we can use to infer them with help from other considerations.