Do Atoms Stop Moving at Absolute Zero?

It is a well-established scientific fact that gas atoms or molecules, if heated, move about with greater vigor. In fact, gas expands because of an increase in energy. The reverse is also true. Cool a volume of gas and it shrinks-particle motion decreases. The atoms or molecules eventually approach each other sufficiently that a liquid is produced. Cool the liquid further, and eventually a solid is produced. Keep cooling the solid, and eventually it is theoretically possible to reach absolute zero degrees, as measured on the Kelvin scale. So what happens when that point is reached. Does all atomic or molecular motion disappear?

Absolute Zero Degrees Kelvin

At absolute zero degrees Kelvin, which works out to minus 273.15 degrees Celsius, or minus 459.67 degrees Fahrenheit, all motion does not cease. What remains in effect is what is termed the "zero point vibrational energy." That is defined as, the quantum ground state for all matter. Those who have a little background in mechanics should have been able to predict this, as the Heisenberg Uncertainty Principle states, approximately, that one cannot simultaneously describe both the position and the 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, then the momentum would also be zero. Since the location of each particle could then be determined absolutely, the Heisenberg principle would be violated. In fact, the zero-point energy is equal to one-half the value of Planck's energy times the frequency of the vibration of the harmonic oscillator (the particle) at absolute zero. The frequency depends, in turn, upon the mass of the particle. See the references cited at the end of this article for the mathematical calculation specifics.

An Interesting Result for Helium-4

The most abundant form of helium is the lightest form, helium-4. The nucleus of helium-4 contains two protons and two neutrons. At 2.17 degrees Kelvin, helium-4 becomes a "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 that can explain this-only quantum mechanics can describe such behavior. Liquid helium can, however, be frozen if approximately 25 atmospheres of pressure are applied in the process.

References and Resources:

University of Virginia - Galileo - "The Simple Harmonic Oscillator," by Michael Fowler.

Georgia State University - Quantum Harmonic Oscillator: Energy Minimum from Uncertainty Principle

American Institute of Physics - Quantum Mechanics: The Uncertainty Principle

Calphysics Institute - Zero Point Energy and Zero Point Field