How do we get things *really* cold?

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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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Basically yes, usually this is done with multi-level refrigeration. The same basic concept, stacked like Russian nesting dolls with different refrigerants that work over different temperature ranges.

Use your basic household R-22 coolant or whatever to get *cold*, and then a second stage with a cryogenic coolant that works over a much lower temp range will kick in. Then maybe even a third or fourth stage until you get down to liquid helium as a working refrigerant.

Liquid nitroget temperatures can be reached via compression and expansion.

When gas is compressed with pressure it heats up. When gas expands due to lack of pressure it cools down.

So you take gas and compress it greatly. It gets hot. You let it cool down (or cool it with anoter refrigerator).
Now you let this cool pressurized gas expand back to normal pressure. It cools down.

With big enough pressure difference you can cool down enough to turn gas into liquid.

Fridges and AC units also do their cooling (and heating) with the same principle of compressing and expanding gases. The pressure differences they use are just much smaller.

Here is a video on making liquid nitrogen: [Veritasium: Making Liquid Nitrogen From Scratch!](https://youtu.be/dCXkaQa53QQ)

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.

Liquid nitrogen is made by fractional distillation of air. In general, air is compressed a lot, this heats it up, but it is then allowed to cool back to room temperature while staying compressed. When it’s room temp again it is allowed to expand, which cools it down a lot. Repeat the process til you’ve separated out the different gasses in air and eventually you’ll have liquid nitrogen.

There are alternate architectures like Stirling engines, which can get down to -200 deg C in a single stage. They’re commonly used in things like infrared detectors, where you have to cool the detectors down to liquid nitrogen-like temps to reduce noise.

https://en.wikipedia.org/wiki/Applications_of_the_Stirling_engine#Stirling_cryocoolers