What Happens at Absolute Zero?

Absolute zero? Bose-Einstein condensate at billionths of a degree. Image by NIST
Bose-Einstein condensate at billionths of a degree. Image by NIST
Classical physics suggests that at absolute zero, particles cease all motion. But what about quantum mechanics, the science of the very small? Ah, therein lies the rest of the story.

From Gas to Frigid Solid

It is well established fact that heated gas atoms or molecules move with great vigor. In fact, gas expands as energy increases. The reverse is also true. Cool a gas and it shrinks. Particle motion decreases. The atoms or molecules come closer together. At some point a liquid forms.

Cool the liquid further and the result is a solid. Keep cooling the solid, and in theory it is possible to reach the coldest known temperature. That temperature is absolute zero degrees Kelvin. What happens then? Does all atomic or molecular motion stop?

Absolute Zero Degrees Kelvin

Despite the predictions of classical physics, at absolute zero degrees Kelvin all motion does not cease. The remaining motion is due to the “zero point vibrational energy.” That is defined as the quantum ground state for all matter. Those with a background in mechanics should have been able to predict this. The Heisenberg Uncertainty Principle dictates one cannot simultaneously describe the position and momentum of any particle.

Value of the Zero Point Energy

Momentum is equal to mass times velocity. If the velocity of all particles at absolute zero was zero, the momentum would be zero. The location of each particle would be determined absolutely. This would violate the Heisenberg principle. In fact, the zero point energy is equal to one-half the value of Planck’s constant times the frequency of the vibration of the harmonic oscillator (the particle) at absolute zero. The frequency depends upon the mass of the particle. See the references cited at the end of this article for specifics.

An Interesting Result for Helium-4

The most abundant form of helium is helium-4. The nucleus of helium-4 contains two protons, two neutrons. At 2.17 degrees Kelvin, helium-4 becomes super fluid. However, the zero-point vibrational energy for helium-4 is greater than the energy the substance would have in a solid lattice. So it cannot be converted into a solid, being the only known substance to fall into that category. There is no classical theorem to explain this. Only quantum mechanics describes such behavior. Liquid helium can be frozen if approximately 25 atmospheres of pressure are applied in the process.

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