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Q & A: cooling below 3K

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Most recent answer: 01/04/2011
Q:
If, due to background radiation, everything is warmer than 3K then where do scientists find material cooler than 3K to suck the heat out of nitrogen to bring it close to 0K without violating the law of entropy?
- Mehran (age 60)
Miami, Florida
A:
It's not hard to get decent thermal isolation from the background radiation, which is at about 2.7K. Any shiny metallic box does a good job of reflecting the long wavelength thermal radiation.

So then the issue becomes how to pump heat out of a cold region into a hotter one, just like with any air conditioner or refrigerator. Of course that requires the input of low-entropy energy. You end up heating the hot region more than you cool the cold region, by enough so that net entropy increases.

To give about the simplest way of cooling below the background radiation temperature: Take some liquid helium (at 4.2K) in a well-insulated dewar. Pump on it with a vacuum pump. You can cool it down pretty easily to around 2K. You don't even notice anything interesting happen as you cool past the 2.7K mark. Fancier methods are required to get much colder. Of course the power that the pump draws gets dumped in the room, increasing total entropy despite the cooling of the remaining helium

Mike W.

(published on 01/04/2011)

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