A Quantum Gas Drops Below Absolute Zero

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Someone is going to have to explain this one to me. -SG

From Nature magazine via Scientific American:

It may sound less likely than hell freezing over, but physicists have created an atomic gas with a sub-absolute-zero temperature for the first time. Their technique opens the door to generating negative-Kelvin materials and new quantum devices, and it could even help to solve a cosmological mystery.

Lord Kelvin defined the absolute temperature scale in the mid-1800s in such a way that nothing could be colder than absolute zero. Physicists later realized that the absolute temperature of a gas is related to the average energy of its particles. Absolute zero corresponds to the theoretical state in which particles have no energy at all, and higher temperatures correspond to higher average energies.

However, by the 1950s, physicists working with more exotic systems began to realise that this isn't always true: Technically, you read off the temperature of a system from a graph that plots the probabilities of its particles being found with certain energies. Normally, most particles have average or near-average energies, with only a few particles zipping around at higher energies. In theory, if the situation is reversed, with more particles having higher, rather than lower, energies, the plot would flip over and the sign of the temperature would change from a positive to a negative absolute temperature, explains Ulrich Schneider, a physicist at the Ludwig Maximilian University in Munich, Germany.

Peaks and valleys
Schneider and his colleagues reached such sub-absolute-zero temperatures with an ultracold quantum gas made up of potassium atoms. Using lasers and magnetic fields, they kept the individual atoms in a lattice arrangement. At positive temperatures, the atoms repel, making the configuration stable. The team then quickly adjusted the magnetic fields, causing the atoms to attract rather than repel each other. “This suddenly shifts the atoms from their most stable, lowest-energy state to the highest possible energy state, before they can react,” says Schneider. “It’s like walking through a valley, then instantly finding yourself on the mountain peak.”

Read the rest of this article here.

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2 Responses to A Quantum Gas Drops Below Absolute Zero

  1. Mike Dziekan says:

    Sheldon –

    It sounds as if he has created a Bose-Einstein Condensate, and it using a magnetic field to flip it out of its singular quantum state. I can’t see how anyone can make a “below absolute zero” enery state unless our definition of absolute zero is wrong. Maybe there is a “false” absolute zero and an “acual” absolute zero … kind of like a “false vacuum.”


  2. KRA5H says:

    Curiouser and curiouser!

    Also from the article, “Rosch and his colleagues have calculated that whereas clouds of atoms would normally be pulled downwards by gravity, if part of the cloud is at a negative absolute temperature, some atoms will move upwards, apparently defying gravity.”


    “Another peculiarity of the sub-absolute-zero gas is that it mimics ‘dark energy’, the mysterious force that pushes the Universe to expand at an ever-faster rate against the inward pull of gravity. Schneider notes that the attractive atoms in the gas produced by the team also want to collapse inwards, but do not because the negative absolute temperature stabilises them. “It’s interesting that this weird feature pops up in the Universe and also in the lab,” he says. “This may be something that cosmologists should look at more closely.””

    Rosch seems to have stepped through the looking glass–down is up, white is black…. I recommend skepticism and patience until other scientists can reproduce these experiments and confirm their findings.

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