So like refrigerators and freezers and things like that use the Carnot Cycle. And I understand how that cools things down. But I’ve also heard that with conventional methods the biggest temperature difference you can make is about 100⁰C. So how do we get something like liquid nitrogen at -195⁰C? Are we putting a freezer inside a freezer inside a freezer? Or how do we get stuff that cold?
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This is actually really complicated and several Nobel prizes have been awarded for work in this area.
To the best of my knowledge the most advanced method of cooling is a process referred to as laser cooling.
The first thing to keep in mind is that temperature is a measurement of energy content in a substance. When something is hot, the molecules move quickly, when it is cold they move relatively more slowly. When you cool something you are basically taking energy from it and putting it elsewhere.
Laser cooling involves precisely shooting a specially designed and carefully positioned laser (or multiple lasers) at an object. The atoms in the object scatter the light. The cooling effect comes from the fact that incoming photons from the laser are absorbed and re-emitted in a different direction. Overall the re-emitted photon will (on average based on the many, many photons) shift to the blue. That is the wavelength will slightly change. This small shift means that the re-emitted photon will have (therefore remove) slightly more energy than the incoming photon. The impact of this is that the object being bombarded will have less energy.
This method has been used to cool objects to very close to Absolute Zero (although Absolute Zero has not been achieved).
There are many other advanced methods of cooling but I believe laser cooling is the main technique that has been used to achieve the lowest temperatures that we have been able to achieve.
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