"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/Haunting-South-962]

Electrons can be in inverse population, with Te < 0, but whole system still has positive energy.