How is absolute zero achieved?

Apr 3, 2018

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.

${\lim}_{T \to 0} S \left(T\right) = 0$

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.

Apparently, atoms of silver have been cooled via adiabatic demagnetization (O.V. Lounasmaa, Physics Today, 1989, p. 26) to $2 \times {10}^{- 9} \text{K}$. 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 $\text{0 K}$. Although in 2011, Patrick Medley et al. managed $3.50 \times {10}^{- 10} \text{K}$ using a similar technique, in a Mott insulator using spin gradient demagnetization.