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  1. #1
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    Quantum gas goes below absolute zero

    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 time1. 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.”

    At positive temperatures, such a reversal would be unstable and the atoms would collapse inwards. But the team also adjusted the trapping laser field to make it more energetically favourable for the atoms to stick in their positions. This result, described today in Science1, marks the gas’s transition from just above absolute zero to a few billionths of a Kelvin below absolute zero.

    Wolfgang Ketterle, a physicist and Nobel laureate at the Massachusetts Institute of Technology in Cambridge, who has previously demonstrated negative absolute temperatures in a magnetic system2, calls the latest work an “experimental tour de force”. Exotic high-energy states that are hard to generate in the laboratory at positive temperatures become stable at negative absolute temperatures — “as though you can stand a pyramid on its head and not worry about it toppling over,” he notes — and so such techniques can allow these states to be studied in detail. “This may be a way to create new forms of matter in the laboratory,” Ketterle adds.

    If built, such systems would behave in strange ways, says Achim Rosch, a theoretical physicist at the University of Cologne in Germany, who proposed the technique used by Schneider and his team3. For instance, 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 gravity4.

    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.”
    http://www.nature.com/news/quantum-g...e-zero-1.12146

  2. #2
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    No ****ing way. That's fascinating!


  3. #3
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    If 0K means the object has zero energy, would this mean that the particles having negative energy and something to provide to other particles?
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  4. #4
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    Not really sure how to answer that question.

    Having "zero energy" is probably a bit of a misnomer, anyway. All that that's really saying is that there isn't a transfer of energy between particles - they're in their theoretically lowest energy state. Without a transfer of energy, you don't have heat.

    When I finish thermodynamics courses, I'll get back to you


  5. #5
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    Quote Originally Posted by debo View Post
    Not really sure how to answer that question.

    Having "zero energy" is probably a bit of a misnomer, anyway. All that that's really saying is that there isn't a transfer of energy between particles - they're in their theoretically lowest energy state. Without a transfer of energy, you don't have heat.

    When I finish thermodynamics courses, I'll get back to you
    I too am in the middle of my thermodynamics courses.

    But what I understand zero energy to mean is that there is no vibration in the particles and also (obviously) no energy transfer because there is no energy to transfer.

    So I have trouble grasping how there could be less than none unless they are actively giving away energy that the particles have. Sort of an equal but opposite thing. But again classes don't start up for a couple weeks...then we'll understand it all and laugh at these peons.
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  6. #6
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    From what the article says, they're not less energetic. Rather, they're at a higher energy state. It sounds like they're manipulating the particles to move them from lowest to highest energy states, and that process yields temperatures below 0K.


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    Thermodynamically, temperature is defined as the inverse of the rate of change of entropy wrt internal energy. So a positive temperature would be correspond to a state such that an increase in internal energy would result in an increase in entropy, and analogously, a negative temp would correspond to a state where an increase in internal energy would result in a decrease in entropy. Remember that equilibrium is reached when a system reaches maximum entropy.

    Intuitively speaking, temperature is the tendency of an object to spontaneously release energy to its surroundings.

  8. #8
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    I learned that 0 Kelvin was the temperature at which all motion stops. So this is pretty fascinating...

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    0K has no energy movement whatsoever, though.. How could you have less than none?

  10. #10
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    Quote Originally Posted by Steel Curtain View Post
    I learned that 0 Kelvin was the temperature at which all motion stops. So this is pretty fascinating...
    Quote Originally Posted by Fly View Post
    0K has no energy movement whatsoever, though.. How could you have less than none?
    You guys are referring to the macroscopic temperature of the system.

    The stuff the article talks about relates to the quantum mechanical state of the system, as they use lasers to stimulate atoms (more specifically their spin states) from a "ground state" to an "excited state" (more energy). Energy is shared equally between the translational (left to right) motions of the atom as well as its rotational motion .

    Energy increases as the upper states begin to be filled; this corresponds to an increase in entropy (see my previous post). The system achieves maximum entropy when spins are evenly distributed between ground and excited states.

    Basically the point of maximum entropy is where all hell breaks loose. The mathematics is a bit too involved for our current purpose, but essentially there is a singular (blows up to infinity/negative infinity) point in the temperature function when maximum entropy is reached. Meanwhile, the derivative of the entropy function switches from zero to negative at this point. Since the temperature is the inverse of the derivative of the entropy, giving more energy causes the spin states (NOT the macroscopic system) to formally exhibit a negative temperature.

    This was already theoretically proven a long time ago. Most people who work on particle physics are familiar with this stuff already. What's fascinating is that now people were actually able to exhibit this phenomena in a lab.

  11. #11
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    How long until Deepak Chopra is referring to quantum gas?
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    "Glad the GOP finally came out with an Obamacare alternative. Can't wait to see their alternative to the Iraq War." - @LOLGOP

  12. #12
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    Quote Originally Posted by lakers4sho View Post
    You guys are referring to the macroscopic temperature of the system.

    The stuff the article talks about relates to the quantum mechanical state of the system, as they use lasers to stimulate atoms (more specifically their spin states) from a "ground state" to an "excited state" (more energy). Energy is shared equally between the translational (left to right) motions of the atom as well as its rotational motion .

    Energy increases as the upper states begin to be filled; this corresponds to an increase in entropy (see my previous post). The system achieves maximum entropy when spins are evenly distributed between ground and excited states.

    Basically the point of maximum entropy is where all hell breaks loose. The mathematics is a bit too involved for our current purpose, but essentially there is a singular (blows up to infinity/negative infinity) point in the temperature function when maximum entropy is reached. Meanwhile, the derivative of the entropy function switches from zero to negative at this point. Since the temperature is the inverse of the derivative of the entropy, giving more energy causes the spin states (NOT the macroscopic system) to formally exhibit a negative temperature.

    This was already theoretically proven a long time ago. Most people who work on particle physics are familiar with this stuff already. What's fascinating is that now people were actually able to exhibit this phenomena in a lab.
    Thanks for the clarification!

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