How do we accurately measure temperatures near absolute zero?

480 views
0

How do we accurately measure temperatures near absolute zero?

In: Physics

with a temperature-sensitive probe.
A lot of effects happen at low temperature, such as properties of semiconductors or other quantum effects. You then just measure the effect and know the temperature.

There are certain materials (notably platinum, for this purpose) that change electrical characteristics based on their temperature. Most commonly, they have a lower resistance as they get colder. We can tell how cold that material is by putting a tiny amount of electricity through it and measuring how much of that electricity makes it through — the more that does, the colder the material must be. We have really good graphs that show how much resistance there is at any temperature. We can know the temperature by measuring the resistance and finding that point on the graph. And we can extend that graph to accurately predict what happens at very cold temperatures.

So we put that kind of material in the near-absolute-zero chamber, run a tiny amount of electricity through it (it has to be a small amount or the little bit of heat it makes can be a problem!), and measure the result. Seeing where that result lands on the graph tells us how cold the material is very accurately! And since that material sits in the chamber, we know how cold the chamber is too.

You could just accurately measure the pressure and volume, essentially. Then apply the [ideal gas law.](https://en.wikipedia.org/wiki/Ideal_gas_law)

Why is measuring this more difficult than any other temperature?