# What Is Absolute Zero in Science?

Absolute zero is defined as the point where no more heat can be removed from a system, according to the absolute or thermodynamic temperature scale. This corresponds to zero Kelvin, or minus 273.15 C. This is zero on the Rankine scale and minus 459.67 F.

The classic kinetic theory posits that absolute zero represents the absence of movement of individual molecules. However, experimental evidence shows this isn't the case: Rather, it indicates that particles at absolute zero have minimal vibrational motion. In other words, while heat may not be removed from a system at absolute zero, absolute zero does not represent the lowest possible enthalpy state.

In quantum mechanics, absolute zero represents the lowest internal energy of solid matter in its ground state.

## Absolute Zero and Temperature

Temperature is used to describe how hot or cold an object is. The temperature of an object depends on the speed at which its atoms and molecules oscillate. Though absolute zero represents oscillations at their slowest speed, their motion never completely stops.

## Is It Possible to Reach Absolute Zero

It's not possible, thus far, to reach absolute zero—though scientists have approached it. The National Institute of Standards and Technology (NIST) achieved a record cold temperature of 700 nK (billionths of a kelvin) in 1994. Massachusetts Institute of Technology researchers set a new record of 0.45 nK in 2003.

## Negative Temperatures

Physicists have shown that it is possible to have a negative Kelvin (or Rankine) temperature. However, this doesn't mean particles are colder than absolute zero; rather, it is an indication that energy has decreased.

This is because temperature is a thermodynamic quantity relating energy and entropy. As a system approaches its maximum energy, its energy starts to decrease. This only occurs under special circumstances, as in quasi-equilibrium states in which spin is not in equilibrium with an electromagnetic field. But such activity can lead to a negative temperature, even though energy is added.

Strangely, a system at a negative temperature may be considered hotter than one at a positive temperature. This is because heat is defined according to the direction in which it flows. Normally, in a positive-temperature world, heat flows from a warmer place such a hot stove to a cooler place such as a room. Heat would flow from a negative system to a positive system.

On January 3, 2013, scientists formed a quantum gas consisting of potassium atoms that had a negative temperature in terms of motion degrees of freedom. Before this, in 2011, Wolfgang Ketterle, Patrick Medley, and their team demonstrated the possibility of negative absolute temperature in a magnetic system.

New research into negative temperatures reveals additional mysterious behavior. For example, Achim Rosch, a theoretical physicist at the University of Cologne, in Germany, has calculated that atoms at a negative absolute temperature in a gravitational field might move "up" and not just "down." Subzero gas may mimic dark energy, which forces the universe to expand faster and faster against the inward gravitational pull.

## Sources

Merali, Zeeya. “Quantum Gas Goes Below Absolute Zero.” Nature, Mar. 2013. doi:10.1038/nature.2013.12146.

﻿Medley, Patrick, et al. "Spin Gradient Demagnetization Cooling of Ultracold Atoms." Physical Review Letters, vol. 106, no. 19, May 2011. doi.org/10.1103/PhysRevLett.106.195301.

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