What is a critical temperature?How do experiments approach absolute zero?

[KingNothing]

You know those experiments where they cool something down to almost absolute zero...you know, to like +0.005 degrees aove it?

Well, I was wondeirng how they did that, so I asked my physics/chem teacher, and he said 'its probably something like what with refrigerators use with freon...as its boiling it 'sucks' heat.

Well, that explains a little. Assuming that's correct...
How would they set it up in such a way that the boiling freon could suck heat, but yet not lose any heat to whatever substance they are cooling?

 

[LURCH]

Evaporative cooling is indeed the method used to make Bose-Einstein Condensates in the lab. However, the evaporative cooling comes only after a laser cooling. Laser cooling is a very interesting topic in itself, and you might want to look into it.

The evaporative cooling is not accomplished by use of a medium. Rather, the substance being pulled (usually rubidium) is confined by a magnetic field, and that field is adjusted to such a a level of strength that the atoms with the highest energy are able to escape, carrying energy away from the system with them. In this way, the process does not resemble a refrigerator so much as a cup of coffee left sitting out.

By this method, temps of < 50 billionths of a degree Kelvin.

 

[Kurdt]

Also laser cooling is being employed to reach even lower levels of temperature. I have read this but only superficially.

 

[swansont]

Also laser cooling is being employed to reach even lower levels of temperature. I have read this but only superficially.

Laser cooling is the first step, as Lurch said, but "only" gets you down to a microKelvin or so. BEC's are down at ~ a nanoKelvin.

http://www.colorado.edu/physics/2000/bec/evap_cool.html is a neat java applet that demonstrates evaporative cooling in a magnetic trap.

 

[LURCH]

Love that applet! Thanks, Swansont.

 

[KingNothing]

Hey, that's cool. I get that now. Is that why we can't cool something to absolute zero? Because the coldest molecules would have to be absolute zero to begin with? Is the proper term 'molecule' or 'atom'? I think its molecule...molecule is what, the smallest quantity of a compound you can have whilst still having the same compound? I don't know, I've never known what a molecule is.

 

[swansont]

Hey, that's cool. I get that now. Is that why we can't cool something to absolute zero? Because the coldest molecules would have to be absolute zero to begin with? Is the proper term 'molecule' or 'atom'? I think its molecule...molecule is what, the smallest quantity of a compound you can have whilst still having the same compound? I don't know, I've never known what a molecule is.

You can't do it in general, but the stumbling block with evaporation is that you're relying on collisions to rethermalize the remaining atoms. Even if you got them to collide and the remaining ones somehow be at rest, fluctuations in the magnetic trap would mean that they could not be at 0 K - there would still be some motion. (the temperature of only a few atoms really isn't well-defined anyway)

"Atom" is what you're looking for. A molecule is two or more atoms bound together. If the constituent atoms are different, it might also be referred to as a compound.

 

[ben_kneale]

hehe, good thing I am studying this stuff in school presently,

actually to cool something down to absolute zero is impossible by many phyiscians theories, including Einstein. The way they cooled it though, according to my book, is a magnetic 'bottle'. But another question is...what would happen if we got an atom to stop moving completey (reach absolute zero)?

 

[KingNothing]

I'm guessing it'd be one heck of a conductor...or at least some kinds of atoms would. A superconductor methinks.

 

[ZapperZ]

I'm guessing it'd be one heck of a conductor...or at least some kinds of atoms would. A superconductor methinks.

Not quite. Keep in mind that a "perfect conductor" is not the same as a "superconductor". In principle, one can cool down copper, for example, to 0K and have it become a perfect conductor, but it still won't become a superconductor. A superconductor requires a phase transition, where there is an abrupt change of "order" at the critical temperature. Also, a perfect conductor doesn't show the Meissner effect when field-cooled, unlike a superconductor (at least a Type I, or Type II below H_c1).

Another problem here is that the HUP will kick in as soon as the atom approaches a "non-moving" state at very low temperatures, resulting in another form of zero-point energy. This has been clearly seen via the deBoer parameter in Nobel gasses.

Zz.

 

[KingNothing]

Sorry ZapperZ...I don't quite understand a few things...maybe someone could help me out.

What is a phase transition? Does that mean the atoms take on a mostly or totally different nature when it reaches the "critical temperature"? I'm not quite sure...any diagrams maybe?

What is the Meissner effect?

HUP = Heisenberg Uncertainty Principle = what? maybe that means that we basically can't be sure what would happen?

What is zero-point energy?

What is a deboer parameter?