# Speed of light

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everywhere i look the statment "the speed of light is constant" is sited, yet in experiments with BOSS EINSTEIN condensates the speed of light has been measured at mere meters per second, this from WIKIPEDIA, so am i missing somthing?

Mentor
Originally posted by wolram
everywhere i look the statment "the speed of light is constant" is sited, yet in experiments with BOSS EINSTEIN condensates the speed of light has been measured at mere meters per second, this from WIKIPEDIA, so am i missing somthing?
Yes. What you are missing is the other half of the statement.

"The speed of light is constant....IN A VACUUM"

MrCaN
Actually what your missin is that the speed of light is constant everywhere. What happens in material is that light interacts with matter, i.e. is absorbed and emitted, making it appear to have slown down, but between those interactions it is still moving at c.

maximus

Originally posted by russ_watters
Yes. What you are missing is the other half of the statement.

"The speed of light is constant....IN A VACUUM"

uhhh... no. it's the constant everywhere. that's why it's called a "constant".

and it is called the constant (i suppose) because nothing known can travel faster than it.

oh, and "mere meters a second" is technically true, but may be a bit of an understatment.

Mentor

[quotee]Actually what your missin is that the speed of light is constant everywhere. What happens in material is that light interacts with matter, i.e. is absorbed and emitted, making it appear to have slown down, but between those interactions it is still moving at c.[/quote]
Originally posted by maximus
uhhh... no. it's the constant everywhere. that's why it's called a "constant".
Ok, maybe what I used is the high school definition, but its a very common one. Here is a college physics class website that words it that way.

I understand the distinction though.
and it is called the constant (i suppose) because nothing known can travel faster than it.
Well its *A* constant because it never changes. Its constant.

I guess which you use depends on if you are talking macro or microscopically. Relativity or Quantum Mechanics. Thats why while talking about the speed of light in a medium, even though there really is no such thing as "the speed of light through a medium" there is an APPARANT speed of light through a medium - people generally don't make the distinction.

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off to the T.P. forum this goes!

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yes it is an understatment maximus, i have seen a report that said,"light stoped in its tracks" aphoton is directed into the condensate and dosent reappear untill a second photon is sent in then the firt photon reappears traveling at c, i didnt quote this because i canot recall where i found it, its hard to imagine a photon traveling at c in what is only a tiny volume.

The speed of light does depend on what it is travelling through. Thats how you can seperate light into its different freqencies.

I've read about them slowing the speed of light to somsething like 38Miles per hour, which is alot slower than the 3x10^8 m/s that it travels in a vacuum, but I dunno how that fits into everything.

KLscilevothma
Why does refraction occur when light travels from air to a glass block? It is because the speed of light in air and in the glass block is different.

Mentor
Kl and Armed, again, it depends on your frame of reference. These other guys are right when in the micro scale - light DOES always travel at C in a vacuum. Light travels from one molecule to the next at C, then is either absorbed and re-emitted or reflected. The effect is that it APPEARS the light has been slowed down.

In the macro scale, it is simply easier to say it has slowed down. The math is much simpler that way.