[Stephen VandeCarr]

Stephen VandeCarr asked the Naked Scientists:

Does light ever travel at the speed of light?

The "speed of light" in stated as "in vacuo", but where does a true vacuum exist?  Even deep space had a few atoms per cc. The theoretical concept that time stops at light-speed may never be realized in the real world over intervals larger than a few millimetres.

What do you think?

[graham.d]

Actually, light speed fluctuates slightly above and below "c" according to the late prof. Feynman. It is true that a total vacuum in space is not possible on average but the space between the odd atom is a vacuum, though I am not sure about the effects of quantum fluctuations on this and the effect that would have. I don't think the density is as high as "a few atoms per cc" though. In models of interstellar space, inside the galaxy, it is more typically about 1 Hydrogen atom per cc, though rises significantly towards the galactic centre and falls to about about a tenth of this between the spiral arms (according to models currently used). Intergalactic space is more empty than this though; perhaps 1/10,000,000 times as dense if the universe was not to have more than the critical mass (which it seems not to) - this is the subject of much debate though.

[lyner]

In models of interstellar space, inside the galaxy, it is more typically about 1 Hydrogen atom per cc

I've always understood the overall average to be about 1 proton per m cubed. But  who cares about a factor of a million?
I seem to remember working out the critical density once and getting a similar answer. But then there's the dark stuff, too.

[graham.d]

Actually Sophie, 1 proton/cubic metre was what I remembered. As an undergraduate (a long time ago), I did some work on modelling the galaxy to find the distance of Pulsars and used this number. But research on the web came up with these numbers so I assume that estimates have improved.

[lyner]

That's the trouble with Cosmology. They're shameless about the plus and minuses.

[qazibasit]

well as far as time stopping is concerned it only occurs in space where there is vacume and cant stop on the earth. Second such velocity is only attained when there is a long initiation run up to gain such velocity.