Quote:
Originally Posted by Dominic
I call BS. How is that possible without melting the laser?
In statistical mechanics, temperature is defined not by how hot something feels, but (short answer) by the accessibility of the particles in question to the various energy states of the system. At 0 K, all particles are in the lowest energy level. This is impossible to fully achieve. As you heat the system up, higher energy levels become accessible to the system. Theoretically, at infinite temperature, all energy states would be equally probable.
But here's the thing. A semiconductor laser, such as that in a laser pointer, requires that
more electrons are in the excited state than are in the ground state. This wouldn't be possible thermally, as infinite temperature means all states are equal. Any less than infinite, and you'd have more electrons in the ground state than in the excited state.
Unless you somehow went beyond infinite temperature! Well, just fitting the pattern of excited states I've been describing, it makes some sense that in order to have more particles in the excited state, you'd have to heat the system beyond infinity, but how could you possibly go beyond the infinite, and what does that mean? It turns out that in physics, we aren't actually concerned with temperature as much as we are with the quantity (-1/T), the negative inverse of the temperature, which physicists label beta. In that sense, it's easy to see why 0 K is unachievable. It's actually negative infinity! Then there's a transition at beta = -/+ 0 (+/- infinite temperature), which then proceeds through the negative temperatures up to -0 K (beta = +infinity), where all the particles would be in the most excited state possible.
Using electricity and the quirky quantum mechanical properties of the atoms in a semiconductor, we can actually drive the electrons in it to a population inversion, where the more excited state is more populated. We didn't need to apply beyond-infinite heat to do this, but the system will still obey the same laws of thermodynamics that things at ordinary temperatures obey provided you treat this system as having a negative temperature, which is, in effect, hotter than infinite temperature.
https://en.wikipedia.org/wiki/Negative_temperature