Anyone know the average index of refraction in space? Given all the crap up there, the hydrogen, graphite, and other stuff, it the velocity of waves can't be equal to the theoretical constant of C = 300,000,000 m/s (I say theoretical constant because there's no such thing as a void in nature, and that speed is light in a void) or whatever. Close, but not quite. So, anyone know the index?

PS: This is for a little bit of writing I'm doing, short story.

Since no one else seems to be answering your question, I'll give it a shot.

The index of refraction in a vacuum is 1.0, the index of refraction in air is 1.0003 (I think). Air is much, much more dense than "space" is, dry air at 0 C has a density of 1.275 kilograms per cubic meter. In fact, space on average contains 1 atom per cubic centimeter, or 100 atoms per cubic meter (most likely hydrogen atoms). For this reason, the effects of refraction by these particles can be completely ignored, as it would be somwhere along the lines of 1.00000000000000000000000001, or somewhere around there ;)

Thanks for the repsonse, but I'm pretty sure the one atom per cubic metre thing is a myth. My astronomy book mentions some few things about how much of various materials is out there in space, but doens't give me enough to work out an average refraction index.

It could be, but at this point there is no evidence to suggest it is anything else. Keep in mind though, this is an average. Some areas can contain much more material just as others contain much less. Sorry I can't help you any further though :(

http://www.webelements.com/webelements/properties/text/image-intensity/refractive-index.html

http://www.what-is-the-speed-of-light.com/refractive-index.html

http://www.plasticsusa.com/refract.html

http://www.geology.wisc.edu/~jill/ri.html

Thanks for those. Here are some you may find interesting:

http://sci.esa.int/content/doc/0f/2575_.htm
http://www.astro.virginia.edu/class/hawley/astr124/ism.html
http://hypertextbook.com/facts/2000/DaWeiCai.shtml
http://www.badastronomy.com/bitesize/solar_system/border.html
http://astrobiology.arc.nasa.gov/news/expandnews.cfm?id=1125
http://science.msfc.nasa.gov/headlines/y2000/ast20jun_1.htm
http://astrobiology.arc.nasa.gov/news/expandnews.cfm?id=298
http://www.spaceref.com/news/viewpr.html?pid=5108

Well, I asked through a NASA website, and they sent me this. Very nice and helpful of them.

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Hello,

You are correct that space is not quite empty. However, it's really
close. Most of the gas in space is hydrogen, which has a refractive
index of 1.00014 at standard temperature and pressure. The
difference between the refractive index and one goes as the number
density for gases like hydrogen, and there are 22 liters per mole at
standard temperature and pressure (STP) versus about 1 atom per cubic
centimeter in interstellar space.

So that's 6 x 10^23 atoms / 22 x 10^3 cc = 3 x 10^19 atoms/cc at STP.

And at STP, we have r.i. - 1 = .00014 = 14 x 10^-5.

So divide that by 3 x 10^19 to get

r.i. = 1 + 5 x 10^-24

or in decimal form, 1.000000000000000000000005 (I think I got 23
zeros in there!)

So there's not much difference between that and one. However, you
are correct that the propagation speed through space is not exactly c.

Hope that helps.

--

-Kevin Boyce and Martin Still
for "Ask a High Energy Astronomer"

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