# Speed of light in intergalactic space

1. Jul 14, 2010

### foxpoint11

What is the speed of light at 2.7 degrees K.If its less than C,you know what my next question is.
Empty space is not empty i am told, so we dont have the classical definition of a vacuum so photons
are passing through something that should slow the photons down?
At cryogenic temperatures on earth,photons are as slow as or slower than photons through water.
foxpoint11

2. Jul 14, 2010

### mgb_phys

The temperature of space isn't 2.7K, that's the temperature of the microwave background and so the lowest temperature any material sitting in space can reach in equilibrium.
But more importantly the temperature doesn't affect the speed of light.

Yes if there is any material in the empty space than some of the light will be slowed down by interaction with any gas/dust present.
But it doesn't interact with the empty space - virtual photon pairs and quantum vacuum fluctuations don't give space a refractive index.

That's a fucntion of the material the ligth is passing through, it's not fundamentally because the material is cold. The cold is just necessary to get the material in the right energy state.

3. Jul 14, 2010

### Staff: Mentor

Welcome to PF.

The speed of light in a vacuum, in an inertial (non-accelerating) reference frame is C. Period. Nothing more to say. So you can reference every question you ask back to that reality.

2.7 K is a temperature, not a substance/medium, so I'm not sure what you think it has to do with this issue, but it sounds like you mistakenly think there is a connection. Photons aren't matter. They don't have a temperature except insofar as their frequency can tell you the temperature of the source. So the "at cryogenic temperatures on earth, photons are as slow as or slower than photons through water" isn't just wrong, it's meaningless. It's like saying 'my favorite color is 'car'.'

4. Jul 14, 2010

### mgb_phys

To the OP, the speed of light in a material depends on the atomic structure of the material. In air it's very nearly the same as C, in water it's 75% C in diamond it's slowed to 40% of C.

There are techniques to slow light down to very low speeds, approx 10 billionths of c.
To do this you need some matter in a very weird state - this is only possible at very very low temperatures.
If the popular account emphasizes the very low temperature (because this is the experimental challenge) you could get the idea that this is what causes the light to slow down.

5. Jul 16, 2010

### nkt

Light travels at extremely slow speed at absolute zero.

Light acts like a particle as well as a wave. It's not matter, however experiments prove that like acts much the same way.

6. Jul 16, 2010

### Integral

Staff Emeritus
The speed of light does not depend on the temperature. The properties of matter do however, and the properties of matter do determine how light is transmitted.

For the 3rd time from a 3rd person temperature has no direct effect on the speed of light.

7. Jul 16, 2010

### nkt

My bad, i thought the experiment was done through a vacuum.

8. Jul 25, 2010

### Sakha

Just to add another point, remember that a vacuum doesnt have a temperature. Temperature is a measure of the kinetic energy of particles, in a vacuum, where there are no particles, there is not such thing as a temperature.