Since there is an absolute zero where all molecular movement ceases, is there also an "absolute heat" where molecules are incapable of moving any faster?

There is not. And here's why:

What you're describing is a maximum highest temperature, just as there is a maximum lowest temperature. (The latter is, as you've mentioned, absolute zero.)

However, we now have to consider what temperature actually is. It turns out temperature is not, despite what many people believe, the average kinetic energy of the particles of a material. We can disprove that pretty easily: Take an ice cube at 0° C and some liquid water at 0° C. Yes, they can coexist at that temperature, just as liquid water and steam can coexist at 100° C. Obviously the kinetic energy of the liquid water molecules is much greater than that of the ice water molecules. So that definition comes apart at the seams with just a little picking at the fringes.

It turns out temperature, rigorously defined, is the reciprocal of the derivative of the entropy with respect to energy. That's not too terribly helpful, huh? It only helps a little if I define entropy as the log of the number of energy states. This is the formula if that helps much.

The explanation of how & why that's the temperature is beyond the scope of this conversation, but a decent way to imagine it is an abacus with ten beads and ten wires on it. The beads can only occupy one of two states. Up (where the individual bead's energy = E) or down (where the bead's individual energy = 0). Finally we say it's a closed system, so if a bead drops to the down state another bead has to rise to the up state.

In that system, consider what happens as the system gets more or less energy.

  • At E = 0 there is only 1 state, all beads down.
  • At E = 1 there are 10 states.
  • At E = 2 there are 45 states (10 combination 2)
  • At E = 3 there are 105 states (10 combination 3)
  • At E = 4 there are 210 states
  • At E = 5 there are 252 states

and then it goes back down in the other direction.

Now consider the log of the number of states, which is the entropy as we defined it earlier, S:

  • E=0, S=0
  • E=1, S=1
  • E=2, S=1.65
  • E=3, S=2.02
  • E=4, S=2.32
  • E=5, S=2.40
  • E=6, S=2.32
  • E=7, S=2.02
  • E=8, S=1.65
  • E=9, S=1
  • E=10, S=0

Now we can do a hand-waving determination of the slope, by taking the mean of the slopes on either side. We cannot do that for E=0 or E=10, so we'll skip those. And since we're so close to the reciprocal, I'll include the temperature by the rigorous definition as well:

  • E=1, dS/dE=0.82, T=1.21
  • E=2, dS/dE=0.50, T=2.00
  • E=3, dS/dE=0.335, T=2.99
  • E=4, dS/dE=0.19, T=5.26
  • E=5, dS/dE=0, T=INFINITE
  • E=6, dS/dE=-0.19, T=-5.26
  • E=7, dS/dE=-0.335, T=-2.99
  • E=8, dS/dE=-0.50, T=-2.00
  • E=9, dS/dE=-0.82, T=-1.21

This tells us two things:

  1. That temperature can go to infinity. When it does, you gain no additional entropy in the system by adding energy to the system. Instead you LOSE entropy.

  2. When you have so much energy in the system you're losing entropy, you're in negative temperature.

There's actually a third thing, which isn't immediately apparent because I skipped the E=0 and E=10 states: If we just assume that the derivative dS/dE in those systems is based solely upon the one side we can measure, then dS/dE in those states is not infinite. Therefore in none of those states does the temperature drop to "absolute zero".


But wait! This whole abacus system is bullshit! The universe doesn't work that way!

Actually it does. This whole hand-waving abacus thing? That's the quantum mechanical model of temperature. And it's even valid for negative temperatures. That's how scientists create negative temperature systems, they create macroscopic versions of my abacus model, where when the system gains a quanta of energy, it results in a more negative temperature.

/r/askscience Thread