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Science

What 'Negative Temperature' Really Means 204

Posted by Soulskill
from the not-a-car-analogy dept.
On Friday we discussed news of researchers getting a quantum gas to go below absolute zero. There was confusion about exactly what that meant, and several commenters pointed out that negative temperatures have been achieved before. Now, Rutgers physics grad student Aatish Bhatia has written a comprehensible post for the layman about how negative temperatures work, and why they're not actually "colder" than absolute zero. Quoting: "...you first need to engineer a system that has an upper limit to its energy. This is a very rare thing – normal, everyday stuff that we interact with has kinetic energy of motion, and there is no upper bound to how much kinetic energy it can have. Systems with an upper bound in energy don’t want to be in that highest energy state. ...these systems have low entropy in (i.e. low probability of being in) their high energy state. You have to experimentally ‘trick’ the system into getting here. This was first done in an ingenious experiment by Purcell and Pound in 1951, where they managed to trick the spins of nuclei in a crystal of Lithium Fluoride into entering just such an unlikely high energy state. In that experiment, they maintained a negative temperature for a few minutes. Since then, negative temperatures have been realized in many experiments, and most recently established in a completely different realm, of ultracold atoms of a quantum gas trapped in a laser."
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What 'Negative Temperature' Really Means

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  • Layman (Score:4, Insightful)

    by Anonymous Coward on Saturday January 05, 2013 @08:08PM (#42491705)

    I do not think this word means what you think it means.

  • Re:Uhhhh (Score:5, Insightful)

    by cwebster (100824) on Saturday January 05, 2013 @08:22PM (#42491807)

    All quantum means is that energy can only have specific values. Imagine a stereo with a volume knob that clicks between values, ie it can be 1, 2, 3, n, but cannot be anything inbetween those numbers. Now you have a quantum volume knob.

    Temperature is a statistical property of matter that only exists once we consider things as a continuum. At scales where we consider quantum mechanics, a molecule has energies (kinetic, rotational, vibrational, electrical, etc) which can only take on specific values (quantized) and these values are specific to the atom/molecule to some degree (atom makeup, radiative properties, etc).

    That probably doesnt help wtih the sub-0 part of the article, but perhaps it will help with the quantum part.

  • by Iamthecheese (1264298) on Saturday January 05, 2013 @08:30PM (#42491845)
    Give up a little precision and rigor for ease of understanding you snob.
  • by dispersionrelation (2534290) on Saturday January 05, 2013 @08:46PM (#42491945)
    I majored in Physics and am currently in grad school and I have no problem with that wording. In fact we Physicist often anthropomorphize when talking amongst ourselves, so what the hell is your problem? Grow up and realize that language is simply a tool used to convey ideas, no one with half a brain reads that statement and actually thinks the particles in the system have needs or desires. Instead they will realize by the wording and context that the particle(s) are simply less likely to be in the higher energy states for reasons that the author doesn't want to go into. If you disagree you're wrong.
  • by VortexCortex (1117377) <VortexCortex@@@project-retrograde...com> on Saturday January 05, 2013 @09:06PM (#42492075)

    Here! [wikipedia.org]

    a system with a truly negative temperature in absolute terms on the Kelvin scale is hotter than any system with a positive temperature. If a negative-temperature system and a positive-temperature system come in contact, heat will flow from the negative- to the positive-temperature system.

    That a system at negative temperature is hotter than any system at positive temperature is paradoxical if absolute temperature is interpreted as an average internal energy of the system. The paradox is resolved by understanding temperature through its more rigorous definition as the tradeoff between energy and entropy, with the reciprocal of the temperature, thermodynamic beta, as the more fundamental quantity. Systems with positive temperature increase in entropy as one adds energy to the system. Systems with negative temperature decrease in entropy as one adds energy to the system.

    You add more energy, but the entropy doesn't increase. Gods damn that moronic blogger and his useless "tricks" metaphor. You don't "trick" shit you stupid fuck. You wouldn't say gunpowder "tricks" a lead projectile to scurry from the gun barrel if you were explaining a gun. We're not idiots, we just need to have the terms defined because some of us hadn't heard the term before in relation to absolute zero.

    Protip: Next time you want to submit or vote up a "follow-up" fucking read the damn thing, and compare it to the wiki. Unless it's significantly more useful than the damn wikipedia article, don't fucking submit or vote it up.

  • Re:Uhhhh (Score:5, Insightful)

    by hairyfeet (841228) <bassbeast1968@@@gmail...com> on Saturday January 05, 2013 @09:53PM (#42492341) Journal

    That still doesn't explain how in the fuck you get below zero movement, how can you move less than none? For those that haven't seen it I suggest the excellent PBS documentary "The search for absolute zero" which is easy enough to find on the web where the second half deals with nothing but the attempts to reach absolute zero. in that video the scientists explain quite plainly that the reason its so damned hard to get those last couple of degrees out of the system is because you ALL movement from the medium has to be removed, not a single atom can move because movement is energy and absolute zero is the absolute absence of ALL energy.

    So sorry, still don't get it, its not like you can magically remove something from nothing. Absolute zero is absolute nothing, no energy left it the system at all, so how in the fuck are you gonna get less than nothing?

  • by sydneyfong (410107) on Saturday January 05, 2013 @11:37PM (#42492823) Homepage Journal

    You, being a physicist don't generally run into these people. I, on the other hand, have to deal with them daily.

    You blame a wording used to more conveniently convey a meaning, because you surround yourself with idiots.

    It's not a physicists problem that you end up with uncool friends. Give it up, no amount of "correct" wording is going to make sane people out of crackpots. Your attempts to teach them logic are going to be futile no matter what (hey, you called them crackpots, and you're _still_ arguing with them). Just give it up dude.

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