Physicists working with Ulrich Schneider and Immanuel Bloch have now realised a gas in which this distribution is precisely inverted: many particles possess high energies and only a few have low energies. This inversion of the energy distribution means that the particles have assumed a negative absolute temperature. The meaning of a negative absolute temperature can best be illustrated with rolling spheres in a hilly landscape, where the valleys stand for a low potential energy and the hills for a high one.
The faster the spheres move, the higher their kinetic energy as well: if one starts at positive temperatures and increases the total energy of the spheres by heating them up, the spheres will increasingly spread into regions of high energy. If it were possible to heat the spheres to infinite temperature, there would be an equal probability of finding them at any point in the landscape, irrespective of the potential energy. If one could now add even more energy and thereby heat the spheres even further, they would preferably gather at high-energy states and would be even hotter than at infinite temperature.
The Boltzmann distribution would be inverted, and the temperature therefore negative. At first sight it may sound strange that a negative absolute temperature is hotter than a positive one.
This is simply a consequence of the historic definition of absolute temperature, however; if it were defined differently, this apparent contradiction would not exist. This inversion of the population of energy states is not possible in water or any other natural system as the system would need to absorb an infinite amount of energy — an impossible feat! However, if the particles possess an upper limit for their energy, such as the top of the hill in the potential energy landscape, the situation will be completely different.
In their experiment, the scientists first cool around a hundred thousand atoms in a vacuum chamber to a positive temperature of a few billionths of a Kelvin and capture them in optical traps made of laser beams. The surrounding ultrahigh vacuum guarantees that the atoms are perfectly thermally insulated from the environment. The laser beams create a so-called optical lattice, in which the atoms are arranged regularly at lattice sites. In this lattice, the atoms can still move from site to site via the tunnel effect, yet their kinetic energy has an upper limit and therefore possesses the required upper energy limit.
Temperature, however, relates not only to kinetic energy, but to the total energy of the particles, which in this case includes interaction and potential energy. The system of the Munich and Garching researchers also sets a limit to both of these. The physicists then take the atoms to this upper boundary of the total energy — thus realising a negative temperature, at minus a few billionths of a kelvin.
I f spheres possess a positive temperature and lie in a valley at minimum potential energy, this state is obviously stable — this is nature as we know it. If the spheres are located on top of a hill at maximum potential energy, they will usually roll down and thereby convert their potential energy into kinetic energy. The energy limit therefore renders the system stable! This does not mean, however, that the law of energy conservation is violated. So absolute zero is colder than the dark reaches of outer space.
And today, you get to be our guinea pig, and touch something cooled down to absolute zero. I know, right? What an amazing opportunity. Would you mind picking that up for us? OK, you can let go of the metal now. Think of it like when you lick a flagpole in winter, but A LOT more painful.
And if you try to remove the piece of metal, the flesh of your hand will rip off with it. Just remember you volunteered for this. Next, blood vessels near your skin will begin to constrict. This will reduce the blood flow to the affected areas, and help keep your vital organs alive.
The problem is, this lack of blood supply and oxygen to your skin can cause damage to your cells. This would create sharp ice crystals, and damage the structure of your skin cells. And brace yourself. It gets worse. You can expect even more damage to the lower layer of your skin, AKA the dermis. OK, maybe we should call it quits on this.
This will eventually lead to gangrene and extensive necrosis. DO NOT google that. Trust me. So besides maiming you, what could we actually do with absolute zero? Well hopefully, the study of absolute zero temperature will lead to discoveries in cryogenic research. Scientists reported in that gases blowing out from a central dying star have expanded and rapidly cooled to 1 kelvin, only one degree warmer than absolute zero.
Usually, gas clouds in space have been warmed to at least 2. If you count artificial satellites, things get chillier still. The space environment, combined with mechanical and cryogenic refrigeration systems using hydrogen and helium, chill the coldest instruments to 0. The lowest temperature ever recorded was back here on Earth in a laboratory. Earlier, scientists at the Helsinki University of Technology in Finland achieved a temperature of 0.
However, this was the temperature for just one particular type of motion — a quantum property called nuclear spin — not the overall temperature for all possible motions.
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