# What Is Absolute Zero?

## Absolute Zero and Temperature

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 0 K or -273.15°C. This is 0 on the Rankine scale and -459.67°F.

In classical kinetic theory, there should be no movement of individual molecules at absolute zero, but experimental evidence shows this isn't the case. Rather, particles at absolute zero have minimal vibrational motion.

In other words, while heat may not be removed from a system at absolute zero, it does not represent the lowest possible enthalpy state.

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

Robert Boyle was among the first people to discuss the existence of an absolute minimum temperature in his 1665 New Experiments and Observations Touching Cold. The concept was called the primum frigidum.

### Absolute Zero and Temperature

Temperature is used to describe how hot or cold an object it. The temperature of an object depends on how fast its atoms and molecules oscillate. At absolute zero, these oscillations are the slowest they can possibly be. Even at absolute zero, the motion doesn't completely stop.

### Can We Reach Absolute Zero?

It's not possible to reach absolute zero, though scientists have approached it. The NIST achieved a record cold temperature of 700 nK (billionths of a Kelvin) in 1994.

MIT researchers set a new record of 0.45 nK in 2003.

### Negative Temperatures

Physicists have shown that it's possible to have a negative Kelvin (or Rankine) temperature. However, this doesn't mean particles are colder than absolute zero, but that energy has decreased. This is because temperature is a thermodynamic quantity that relates energy and entropy.

As a system approaches its maximum energy, its energy actually starts to decrease. This can lead to a negative temperature, even though energy is added. This only occurs under special circumstances, as in quasi-equilibrium states where spin is not in equilibrium with an electromagnetic field.

Strangely, a system at a negative temperature may be considered hotter than one at a positive temperature. The reason is because heat is defined according to the direction it would flow. Normally, in a positive-temperature world, heat flows from warmer (like a hot stove) to cooler (like 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. Prior to this (2011), Wolfgang Ketterle and his team had demonstrated the possibility of negative absolute temperature in a magnetic system.

The new research into negative temperatures reveals mysterious behavior. For example, Achim Rosch, a theoretical physicist at the University of Cologne in Germany, has calculated that atoms at 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.

Reference

Merali, Zeeya (2013). "Quantum gas goes below absolute zero". Nature.

Medley, P., Weld, D. M., Miyake, H., Pritchard, D. E. & Ketterle, W. "Spin Gradient Demagnetization Cooling of Ultracold AtomsPhys. Rev. Lett. 106, 195301 (2011).