Question

How is absolute zero achieved?

Answer 1

It cannot be. Physical Chemistry by Levine literally has a section title called "The unattainability of absolute zero" (pg. 168). He mentions the Nernst-Simon formulation of the third law of thermodynamics, which is:

The entropy change associated with any condensed system undergoing a reversible isothermal process approaches zero as the temperature at which it is performed approaches 0 K.

Notice how it only says APPROACHES. Remind you of something? Well, the consequence is stated right below:

It is impossible for any process, no matter how idealized, to reduce the entropy of a system to its absolute-zero value in a finite number of operations.

It should remind you of the definition of a LIMIT, i.e.

`lim_(T->0) S(T) = 0`

Absolute zero is a limit that can never be reached unless a truly reversible process is performed the whole way to absolute zero that (without fail) removes ALL heat from the system (infinitesimally slowly).

But since real-life processes are by definition irreversible (since we know the entropy of the universe always increases!), absolute zero is unattainable.

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Apparently, atoms of silver have been cooled via adiabatic demagnetization (O.V. Lounasmaa, Physics Today, 1989, p. 26) to `2 xx 10^(-9) "K"`. Basically, that relaxes the system in an insulated enclosing to minimal degrees of freedom, decreasing the number of microstates to a very small amount.

That's cool, but... (see what I did there?) that's not `"0 K"`. Although in 2011, Patrick Medley et al. managed `3.50 xx 10^(-10) "K"` using a similar technique, in a Mott insulator using spin gradient demagnetization.