# Can you reach absolute zero in complete empty space

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....

## Answers and Replies

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Simon Bridge
Homework Helper
What does "temperature" measure?

Khashishi
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.

Simon Bridge
Homework Helper
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.

mfb
Mentor
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.

phinds
Gold Member
2019 Award
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.

Drakkith
Staff Emeritus
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.

phinds
Gold Member
2019 Award
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?

Simon Bridge
Homework Helper
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?

Drakkith
Staff Emeritus
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.

phinds
Gold Member
2019 Award
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.

Drakkith
Staff Emeritus
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?

phinds
Gold Member
2019 Award
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.

Drakkith
Staff Emeritus