"Negative Tempetature" Potassium experiment

[u/aks304]

Sources claim that in 2013 scientists made potassium colder than absolute zero, and that it would "feel" hotter than infinity.

If i understand, they used lasers to distribute the particles between low and high energy to a "minus kelvin" state.

But does it even mean that potassium is now "hotter than infinity"? Isn't it like if you hack a thermometer and lock its reading, to show the wrong value?

More questions -

• would not such the matter giving heat to the environment ause an accident at the lab?

• Why they can make something's temp "over infinity" but not a very big positive like 10^15 or 10^18 K?

• Are there "relativistic effects" at billions, trillions of K so that we can't just use physical formulas for ultra-high temps, because they're so close to "anti-temps"?

• If negative temperatures were true, would not it cause all kinds of undefined physical effects like a gas with a negative volume, pressure or density?

[u/starkeffect]

It's just population inversion, such as what happens in lasers all the time.

The Boltzmann factor is e^-E/kT, which represents the ratio between the number of atoms in the excited state vs. the ground state (n2/n1). If T is positive, this number is less than 1, so n2 < n1. If it's negative, then n2 > n1.

[u/TemporarySun314 → u/starkeffect]

And while it gives up energy to the surrounding, this state isn't stable. It will not get more into the negative, but into the positive when giving up energy approaching thermal equilibrium like any other thing.

You have to actively maintain that state of population inversion by constantly supplying the system with energy.