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# Negative Kelvin?!

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When we think of Kelvin temperature, we think only in positives, since zero Kelvin is also absolute zero, the point at which a particle has absolutely no energy, and thus no movement or vibration. Scientists in Germany, however, managed to create the hottest temperatures ever recorded by creating a substance with a negative Kelvin temperature.

How is this possible? Well, in order to understand this bizarre concept, we have to go back to our definition of temperature. In thermodynamics, we typically refer to temperature as the average temperature of the particles in a substance. However, because quantum physics deals with energies as the smallest of small scales, and because quantum physics is, from a mathematical perspective, about probabilities, it makes more sense to define temperature as the distribution of the energies of the particles in a substance. So, for example, a boiling pot of water would obviously have plenty of high energy particles buzzing around, but it would have a few low-energy particles too. We simply would pay them no mind because the average energy of the particles is consistent. To a quantum physicist, however, those few low-energy particles matter, because they form part of the energy distribution of the substance.

By definition, when a substance has a positive Kelvin temperature, the particles start from a minimum temperature (absolute zero) and spread out toward higher energies. The German scientists, however, wanted to create a substance that started at a maximum temperature and spread toward lower energies. By definition, such a substance would have a negative temperature.

Paradoxically, having a negative temperature makes the gas that the scientists created extremely hot. Since the particles start from a maximum temperature and spread to lower temperatures, and since energy flows from hot to cold, heat will always flow away from the negative temperature gas, making it the hottest thing we've ever observed.

One of the other interesting properties of negative temperature gases is that they not only have the hottest temperatures, but negative pressures. Normally, a gas concealed in a container would spread out and apply pressure to all sides of the container. A negative temperature gas, on the other hand, causes the atoms in the container to cave inward, as if everything converges to a single point. Because dark matter is believed to have negative pressure as well, this characteristic of negative temperature gases leads scientists to think that studying them may reveal more to us about the elusive dark matter that is believed to account for a lot of "missing mass" in the universe.

You can read more about the negative temperature gas and the study conducted by the German scientists here:

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How did the German Scientists find out about negative Kelvin temperatures?

They theorized it and then conducted an experiment to confirm their theory.

When it comes to normal substances with positive Kelvin temperatures, we define temperature as the average kinetic energy of the particles in the substance.  However, quantum physicists discovered that the energies of the individual particles vary greatly.  In a pot of boiling water, for example, only a few of the particles have high energies, while most still have low energies, even though to us the pot is painfully hot to the touch.  The distribution of energies in a substance is known as a Boltzmann distribution, and if you look at a Boltzmann distribution for any normal substance, you will see that most of the particles are clustered at the lower end of the energy scale, and only a few actually have high energies.

The German scientists, however, sought to create a new substance that essentially inverted the Boltzmann distribution.  This means that most of the particles would have high energies, while only a few would have lower energies.  In order to create an inverted energy distribution, they needed to enforce a maximum energy for the particles in the gas. Since the total energy of a gas includes its potential and its kinetic energy, they needed to place maxima on both energies.  To illustrate why, consider a set of balls on a hill.  At the top of the hill, the balls have a lot of potential energy, and therefore they want to roll down the hill and convert their potential energy to kinetic energy.  If the kinetic energy of the balls is already maximized, however, then the potential energy cannot be converted, so the balls stay at the top of the hill, and, amazingly, the system remains stable.  Using lasers and magnets, the scientists were able to induce maximum values on the potential and kinetic energies of the particles in the gas, and therefore they inverted the Boltzmann distribution of the substance.  Since the distribution was inverted, it makes sense to define the absolute temperature as negative, since the term "negative" implies an inversion or negation of something.

http://www.mpg.de/6776082/negative_absolute_temperature

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