### Less Than Nothing

Beth writes:

“Question for you. It is zero degrees here. Does that mean that there is no temperature outside? And if it it is below zero does that mean its less than nothing?”

Beth, this is actually a very interesting question. The Celsius temperature scale is based on the freezing point and boiling point of water (with 0 being the freezing point of pure water and 100 being its boiling point).

The pressure of a gas (like air) in a closed container goes up as the temperature goes up and down as it goes down. The British scientist William Thompson (the Lord Kelvin) was the first to figure that if we cool a gas down far enough, “we should arrive at a point corresponding to the volume of air being reduced to nothing“.

From experiments on different gases, he estimated that this point was about −273 °Celsius, or about -459 °F. The temperature scale that starts at this ‘absolute zero’ temperature and where each degree is the same change in temperature as 1 °C is called the Kelvin scale as a result, and the lowest possible temperature is 0 K.

At this point, a gas has zero volume and molecules have no energy (so they cannot move).

Above absolute zero, the molecules in a gas move at different speeds, but the Maxwell-Boltzmann distribution tells us the portion of molecules that move at each speed. Here are the speeds for Helium and Neon at 25 °C.

You can see that at this temperature, most of the molecules of Neon move at around 500 meters per second and some are traveling more than twice that speed. The shape of the distribution is similar to a bell curve, but it’s not symmetric – There are more molecules moving 900 m/s (that is, 400 m/s above the peak) than are moving 100 m/s (400 m/s below the peak).

All of this has been known for a hundred years or more. The molecules in a gas move at different speeds, but the distribution of speeds depends on the temperature. As you cool a gas down, the speeds (and pressure) drop. At absolute zero (o Kelvin), the molecules don’t move at all, so you can’t get colder than that.

Until last month.

Scientists in Germany published a paper in the journal Science announcing that they made the first gas whose temperature was below absolute zero. Sort of.

Actually what they did was very strange, but creative. They set up a chamber with a grid of laser lights in a regular square pattern, put potassium atoms into the chamber and lowered the temperature so close to absolute zero that the atoms got trapped at the intersection points in the grid.

The atoms had such little energy that they could only move between grid points along the laser lines. The scientists then adjusted the strength and spacing of the lattice so that there were more atoms moving at low speeds than high speeds, the exact inverse of the Maxwell-Boltzmann distribution.

Here are a couple of good articles about what they did from ScienceDaily and Nature, though you can get a PDF version of Braun and Schneider’s paper online too.

Mathematically, this can only happen if the temperature of the gas is negative. “The temperatures we achieved are negative nanokelvin,” one of the scientists said, just a tiny bit below absolute zero.

And the scientists were able to show that the ‘lattice gas’ they created had some strange properties. If a gas has negative temperature then it also has negative pressure, so the atoms attract each other rather than repel. And even though energy normally flows from hotter things to colder things, if the temperature is negative, heat energy will flow from colder things to hotter things.

So, yes, if it is below zero, then the temperature (and pressure) is less than nothing, but only if the zero you are talking about is on the Kelvin scale.