# The temperature of vacuum?

FabianFnas
Posers and Puzzles 12 Nov '06 09:17
1. 12 Nov '06 09:17
What is the temperature of vacuum?

Let's say we want to measure the temperature of the space between the Earth and the Moon. Let's consider it is vacuum there. You go there and stick out a thermometer in the outside of the rocket.
Then, what do you measure? Of course the temp of the thermometer itself, not the actual space.

Has vacuum a temperature? Doesn't we have to have something material, even if it is a gas, to measure? No matter, no temperature?

So has vacuum a temperature, and if so, how do you measure it?
2. XanthosNZ
Cancerous Bus Crash
12 Nov '06 09:47
Originally posted by FabianFnas
What is the temperature of vacuum?

Let's say we want to measure the temperature of the space between the Earth and the Moon. Let's consider it is vacuum there. You go there and stick out a thermometer in the outside of the rocket.
Then, what do you measure? Of course the temp of the thermometer itself, not the actual space.

Has vacuum a temperature ...[text shortened]... No matter, no temperature?

So has vacuum a temperature, and if so, how do you measure it?
Space isn't absolutely empty and therefore isn't a perfect vacuum. The pressure in space is around 10 femtoPascals. So as there are (very few) particles in space they can have a kinetic energy and therefore Space has a temperature. That temperature is 2.7 Kelvin, the background radiation of the universe.
3. 12 Nov '06 10:03
Originally posted by XanthosNZ
Space isn't absolutely empty and therefore isn't a perfect vacuum. The pressure in space is around 10 femtoPascals. So as there are (very few) particles in space they can have a kinetic energy and therefore Space has a temperature. That temperature is 2.7 Kelvin, the background radiation of the universe.
Okay, you're right, there is no empty space, no vacuum anywhere. But between the particles, there is nothing and therefore vacuum. Between the Earth and the Moon, I think (if I recall right) one particle each square centimeter, on average, so there is plenty of vacuum to stick a thermometer in. The question remains.

The 2.7 Kelvin you're talking about is the one of the background radiation, or the average temperature of the Universe as a whole. Of course there is warmer nearby stars.

So how is it possible to measure the temperature of vacuum, if there is any.
4. 12 Nov '06 11:453 edits
Originally posted by XanthosNZ
Space isn't absolutely empty and therefore isn't a perfect vacuum. The pressure in space is around 10 femtoPascals. So as there are (very few) particles in space they can have a kinetic energy and therefore Space has a temperature. That temperature is 2.7 Kelvin, the background radiation of the universe.
2.7 K is the lowest equilibrium temperature, not the average, of the particles in the universe.

The temperature of a random particle between the Earth and the Moon can be millions of degrees if it comes from the Sun, since it can only lose heat by radiation. For this reason, the average temperature can be quite high.

The question whether or not a vacuum has a temperature is a matter of definition.
5. 12 Nov '06 12:07
Originally posted by gambiitti
2.7 K is the lowest equilibrium temperature, not the average, of the particles in the universe.

The temperature of a random particle between the Earth and the Moon can be millions of degrees if it comes from the Sun, since it can only lose heat by radiation. For this reason, the average temperature can be quite high.

The question whether or not a vacuum has a temperature is a matter of definition.
This is interesting, now we're coming somewhere!

If the particles between the Earth and the Moon can have temperatures of millions of degrees - is there all right then to say that the temperature between the two bodies is this temperature?

And back to the question - what is the temperature of vacuum?
What definitions are there about vacuum temperatures?
6. 12 Nov '06 12:46
Temperature is defined as the thermal motion of particles. (more or less). The important thing is it is valid only for particles. If there are no particles, there is no temperature. It is not 0k, it is simply not defined.
7. 12 Nov '06 14:59
Originally posted by ICU2
Temperature is defined as the thermal motion of particles. (more or less). The important thing is it is valid only for particles. If there are no particles, there is no temperature. It is not 0k, it is simply not defined.
So the simple answer is that vacuum has no temperature?
Not zero, not 2.7, not anything else, just no temperature at all?

Is there any definition that gives vacuum any temperature?
8. 12 Nov '06 16:341 edit
Originally posted by FabianFnas
Is there any definition that gives vacuum any temperature?
temperature is a property of a thing

a vacuum is an abscence of "things", it's not a "thing" in it's own right

therefore...
9. 12 Nov '06 17:41
Heat is an energy isnt it? And energy can exist in a vacuum, as far as i know. Why do you think the sun burns dispite there being no oxygen in space.
10. 12 Nov '06 18:13
Originally posted by KingsBishop
Heat is an energy isnt it? And energy can exist in a vacuum, as far as i know. Why do you think the sun burns dispite there being no oxygen in space.
The sun does not actually "burn" as combustion does not occur.

The sun produces electromagnetic radiation as a result of nuclear reaction.

Electromagnetic radiation can travel through a medium or vacuum but does not produce heat unless it coems in contact with a particle.
11. 12 Nov '06 18:18
Temperature is not energy, it measures the "speed" of particles in matter.

The sun burns without oxygen, it uses a nuclear rection invloving helium and hydrogen.
12. PBE6
Bananarama
12 Nov '06 23:07
Actually there are many ways to define temperature, some familiar and some esoteric. If we use the definition that temperature is a measure of the average kinetic energy of a group of particles, it turns out we need about 1x10^6 particles to make sure we have a good average. What if we don't have that many? Well, we could use the a thermodynamic relationship to define temperature, such as:

dS = dQrev/T

where S is entropy, Qrev is the reversible heat flow, and T is temperature. This may not be the easiest way to measure temperature here on Earth, but in space it can come in quite handy when we're dealing with near-vacuum states.
13. 06 Oct '07 08:27
Originally posted by FabianFnas
So the simple answer is that vacuum has no temperature?
Not zero, not 2.7, not anything else, just no temperature at all?

Is there any definition that gives vacuum any temperature?
Vacuum may be empty of matter. But it can't be empty of radiation. Electromagnetic radiation pervades all of interstellar space. And electromagnetic radiation does have temperature which is measurable.
14. wolfgang59