Achieving really low temperatures?

[Theophrastus]

Lacking any better category, i've decided to use the General Sciences thread. Anyway, to go straight to the point. In cryogenics, chemistry, and other such endeavours, how are incredibly low temperatures achieved? I understand how high temperatures would work, (simple heating) however how can heat be taken away. I know based on the basic gas laws that increasing the pressure, would work, but how is it actually done? Are there pressurized gases at work, around a main tank, or is it done by some other means? Help would be most appreciated.

Edited May 12, 2009 by Theophrastus
grammatical error

[Klaynos]

You use something like a Joule Thompson valve:

 

http://en.wikipedia.org/wiki/Joule-Thomson_effect

 

To cool a gas until it turns into liquid.

 

For the coldest liquids you first need to cool the gas using some other previously liqufied gas, normally you cool Helium with liquid nitrogen, as j-t effect can form liquid nitrogen at room temperature.

 

So you have a pot of liquid He, which will be around 4K.

 

Now to get even lower you do something called evaporation cooling...

 

http://en.wikipedia.org/wiki/Evaporative_cooler#Evaporative_cooling

 

Using He3 you can get down to about 500mK without too many problems, any more than that and you have to be really quite clever, there are some nice cooler designs around though...

[iNow]

You should check out this thread:

http://www.scienceforums.net/forum/showthread.php?t=30593

 

 

Also, I highly encourage you to watch the PBS Nova special referenced. It's really cool <wink>.

[swansont]

In atomic physics, laser cooling can get you to microKelvin temperatures and, from there, evaporative cooling can get you to nanoKelvin temperatures.

 

Liquid Helium is positively balmy by comparison.

 

(some details are in the link iNow provided)