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Name: Tim H.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 4/1/2004


Question:
Since the speed of light in a material equals the inverse of the square root of the permitivity of the media times the permeability of the media, and since outer space is not homogeneous (i.e. theres cosmic dust etc.), how can we assume that light from distant stars has travelled in a "straight line" from its original source for billions/millions of years without ever encountering a transmission media other than "free vacuum space"?


Replies:
Light does not always get to us in a straight line. In addition to being scattered by dust and "stuff" along the way it can be bent or "lensed" by gravity as it passes massive bodies along the way.

Vince Calder


Tim H.,

We do not know for a fact that this is what has happened. The assumption is based on probability. We have an approximation of the density of material in space. Most of space is empty. Material in space tends to pull together rather than staying evenly distributed. We can say that the probability of a ray of light hitting something as it travels from a distant star to our solar system is very low. Even if it does hit something, the material will most likely be of a very irregular shape. This will cause a spreading out of the light, a blurring, rather than a clear change of direction. We can say that light our telescopes receive has probably traveled in a straight line.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College


I haven't done this, but perhaps you could go thru the numbers of typical interstellar gas density, and multiply by the typical refractive index of a 1-atmosphere gas. If you found there was less gas between the moon and most stars than between earth and low orbit, maybe you'd be convinced.

Dust mostly doesn't count. It scatters or absorbs more than it refracts. We do see it's effects in stellar nebulae and around the galactic core. We don't find many clues of distortion with it. And nebulae are very lumpy places.

We don't see a lot of art-glass distortions when we look out there. I think there would be some image-clues if distortions were present, such as double-images of a spectrographically identifiable single object. We have noticed at least one of those due to gravitaional distortion. Had to look pretty hard to find one. Elimination of short-wavelength light would have been noticed, too, if the light had passed thru that much gas. Come to think of it, the interstellar vacuum would glow noticeably blue, like our sky during the day. And distant stars would, in some slow, broad sense, twinkle. Or be obscured by the hazy or blue glow. Even the best gas isn't a perfectly clear medium. Spectrographically, it's more like a picket fence. Over much of the spectrum it's relatively clear, but at the strongest resonant frequencies it stops more of the light from the star on the other side of it.. Spectroscopy like this is how we measure the interstellar or intergalactic gas density.

So if the light is significantly refracted, it shouldn't be gas that's doing it.

In principle there is some tiny refraction happening. Might be nice to figure out how small we have proof that it is.

Jim Swenson



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