# Can you reach absolute zero in complete empty space

by mcafej
Tags: absolute, reach, space
 P: 17 Ok, so I know that the laws of physics say reaching absolute zero temperature is impossible, but suppose we took a box that was perfectly insulated in completely empy space, and I took all the particles out of it to create a vacuum. Now, since there are no particles in the box, then wouldn't the temperature inside the box be absolute zero (no heat energy = no temperature, right?)? If I am wrong, what would the temperature inside the box be? Since the average temperature of the universe is only 2.73 kelvin, then it seams like there would be areas in space that are like this....
 Homework Sci Advisor HW Helper Thanks ∞ PF Gold P: 10,913 What does "temperature" measure?
 P: 832 The concept of temperature deals with systems where the number of particles is large enough to approximate the density of states as a continuous function of energy. If you get to small number of particles, then the discrete nature of states becomes important, and temperature ceases to have any meaning.
Homework
HW Helper
Thanks ∞
PF Gold
P: 10,913

## Can you reach absolute zero in complete empty space

I suppose one may choose to attribute a characteristic temperature to the zero-point energy in a volume, by E=kT or whatever, but it is not clear what this would mean, hence the question in post #2.

Focussing on what temperature is should help.
 P: 54 How about colder than absolute zero? http://www.livescience.com/25959-ato...lute-zero.html BTW - 'empty space' is not empty.
Mentor
P: 10,766
 Quote by micky_gta How about colder than absolute zero?
Negative temperatures are hotter than every positive temperature.

 If I am wrong, what would the temperature inside the box be?
The same as the temperature of the box, about 2.7K (there is no perfect heat isolation). You don't need matter to have a temperature, radiation is sufficient.
PF Gold
P: 5,674
 Quote by mcafej Ok, so I know that the laws of physics say reaching absolute zero temperature is impossible, but suppose we took a box that was perfectly insulated in completely empy space, and I took all the particles out of it to create a vacuum. Now, since there are no particles in the box, then wouldn't the temperature inside the box be absolute zero (no heat energy = no temperature, right?)? If I am wrong, what would the temperature inside the box be? Since the average temperature of the universe is only 2.73 kelvin, then it seams like there would be areas in space that are like this....
In addition to the answers above, there is another issue that you might find interesting in this context. Following up on Khashishi's statement "If you get to small number of particles, then the discrete nature of states becomes important, and temperature ceases to have any meaning" --- It is my understanding that there are dust clouds that have a "temperature" of millions of degrees and yet they are a hard vacuum compared to what we can get in a vacuum chamber on earth.
PF Gold
P: 10,990
 Quote by phinds In addition to the answers above, there is another issue that you might find interesting in this context. Following up on Khashishi's statement "If you get to small number of particles, then the discrete nature of states becomes important, and temperature ceases to have any meaning" --- It is my understanding that there are dust clouds that have a "temperature" of millions of degrees and yet they are a hard vacuum compared to what we can get in a vacuum chamber on earth.
But there aren't a small number of particles in a giant dust cloud.
PF Gold
P: 5,674
 Quote by Drakkith But there aren't a small number of particles in a giant dust cloud.
I didn't say there aren't any particles. Reread what I said. Do you think man-made vacuum chambers evacuate every single particle?
Homework
HW Helper
Thanks ∞
PF Gold
P: 10,913
 It is my understanding that there are dust clouds that have a "temperature" of millions of degrees and yet they are a hard vacuum compared to what we can get in a vacuum chamber on earth.
Yes, but so what?
 In addition to the answers above, there is another issue that you might find interesting in this context.
What, in the "this" context of temperature vis-a-vis the limit of zero particle density in a fixed volume, is the interest?

Do you mean that temperature need not decrease with the particle density?
PF Gold
P: 10,990
 Quote by phinds I didn't say there aren't any particles. Reread what I said. Do you think man-made vacuum chambers evacuate every single particle?
You've lost me phinds. I don't know what you are getting at.
 PF Gold P: 5,674 I was just presenting a factoid that I though the OP might find interesting because he was talking about temperature and a hard vacuum. Let me restate it Hm ... I don't see any way to restate it and make it any more clear that what I already said: It is my understanding that there are dust clouds that have a "temperature" of millions of degrees and yet they are a hard vacuum compared to what we can get in a vacuum chamber on earth.
PF Gold
P: 10,990
 Quote by phinds I was just presenting a factoid that I though the OP might find interesting because he was talking about temperature and a hard vacuum. Let me restate it Hm ... I don't see any way to restate it and make it any more clear that what I already said: It is my understanding that there are dust clouds that have a "temperature" of millions of degrees and yet they are a hard vacuum compared to what we can get in a vacuum chamber on earth.
Are you simply saying that a man made vacuum doesn't pump out all the gas? And that these clouds that are millions of degrees are more of a vacuum than what we can produce on Earth?
PF Gold
P: 5,674
 Quote by Drakkith Are you simply saying that a man made vacuum doesn't pump out all the gas? And that these clouds that are millions of degrees are more of a vacuum than what we can produce on Earth?
That is exactly what I am saying.
PF Gold
P: 10,990
 Quote by phinds That is exactly what I am saying.
I thought you were getting at something with the temperature too. My mistake good sir.

 Related Discussions Atomic, Solid State, Comp. Physics 10 General Physics 7 Quantum Physics 4 High Energy, Nuclear, Particle Physics 12 Cosmology 2