7 ms·
Quantum gas goes below absolute zero
- jpdoctor 14y agoThe usual explanation of such things: T is not so important as 1/T. So negative abs temp is achieved by going through +infinity temp and coming back via -infinity temp. Note that this is only allowed in systems which have states that are bounded in energy. The usual example is a laser: When in inversion, its absolute temperature (of the electron population) is negative.
- TylerE 14y agoIf you'll allow a car analogy - isn't this like saying backing up is slower than being parked? Yes, it's a negative number in the context of a forward velocity vector, but it's a higher energy state.
- gus_massa 14y agoNo. The problem is that for historical reasons we choose the wrong scale and it's a little late to change all the books. We choose to measure the temperature in this way (infinityK) ... ->- 10K ->- 1K ->- .1K ->- ... | 0K(impossible) | ... ->-.1K ->- -1K ->- -10K ->- ... (infinityK) Where ->- means hotter In this scale 0K is impossible and impassable. But infinityK is possible (only one unsigned infinity) and some kind of systems when they get hotter they can pass from very "big" positive temperatures to very "big" negative temparatures. And the negative values are hotter than the positive. For example: -.1 is hotter than -10K that is hotter than 10K that is hotter than .1K It's (theoretically) better to measure beta=1/temperature (the usual notation is beta) In this way: (infinity/K) ... -<- 10/K -<- 1/K -<- .1/K -<- ... 0/K(possible) ... -<-.1/K -<- -1K -<- -10K -<- ... (infinityK) Where -<- means hotter In this scale infinity/K is impossible as expected. But 0/K is possible. And the order is the correct order hotter=less_than. Using beta they are ordered more intuitively. Hotter means moving to the right in the line.
- mieubrisse 14y agoThank you; this helped clarify things quite a bit.
- rdfi 14y agoisn't negative absolute temperature an oxymoron?
- aphyr 14y agoNo. Temperature is just the relationship between entropy and energy: ∂S/∂E. This relationship can be negative.
- omegant 14y agoOk, this is at atomical level, it is possible to increase the energy of the system till you freeze the electrons? What would happen then if the particles are organized at subatomical levels?. And below that? Could it be a way to study matter properties?(in place of the cern way)
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- jawns 14y agoI'm not sure I understand this part: "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." If more particles have higher energy, then wouldn't the average energy increase as well? And if the average energy increases, then shouldn't that still keep things in positive, rather than negative, territory?
- jules 14y agoTemperature is not really the average kinetic energy any more. The temperature of a system says something about how much the entropy increases as you add energy. Negative temperature simply means that the entropy decreases as you add energy. This is possible if the number of high energy states is small, because then adding energy will result in the system being in one of a smaller number of high energy states, thus having lower entropy (the entropy of a system is basically the logarithm of the number of states we think the system could be in, so if the number of states is lower, then the entropy is lower). Another way of saying this is that is as follows. For positive temperatures our knowledge of the state of the system would decrease if we added energy (think about a box full of neatly arranged balls and giving it a kick). For negative temperatures our knowledge of the system would increase if we added energy. In the extreme case if the number of possible high energy states is 1, then adding energy can force the system into exactly that state, thus we would know all you can know about the system. In particular in the experiment described in the article, if you removed energy from the system the particles would not be in the lattice arrangement and instead would go around randomly. Thus the entropy would increase if you removed energy.
- fusiongyro 14y agoYour first three sentences are worth more than that entire article. Thanks!
- balsam 14y agoAre any laws of thermodynamics violated here[1]? If not, why is this experiment interesting? As a physics student, let me check: 0. Probably not. 1. Not directly applicable 2. Nope. 3. Nope. [1] http://en.wikipedia.org/wiki/Laws_of_thermodynamics http://en.wikipedia.org/wiki/Laws_of_thermodynamics BTW, I now check HN for physics news too, as a BTW.
- junio 14y agocool.
- 31reasons 14y agoNature's article starting with completely anti-science statement like : "It may sound less likely than hell freezing over" is pretty disappointing.
- polyfractal 14y agoTo be fair, it's a news brief designed for the average layperson, not a research article. I think editorializing the content a bit is fine. Besides, that particular phrase is commonly accepted as an idiom, not a serious reference to a Biblical Hell.
- cristianpascu 14y agoWhat you're saying is like "This TV channel is against sports because they aired a theatre show last night." You're mixing metaphysical categories.
- Ensorceled 14y agoYes, science must ignore the Siren call of religion! We welcome your efforts, for it is a truly Herculean task, nay, Sisyphean! You're like modern Prometheus, providing us fire on this Odyssey into the future.
- Xcelerate 14y agoI don't really like the misleading headline, although it seems as though the actual paper does it as well (http://arxiv.org/abs/1211.0545 http://arxiv.org/abs/1211.0545). Temperature is conventionally defined as the average kinetic energy of a collection of particles. However, in statistical mechanics, you derive thermodynamic relations from the principles of quantum mechanics. It turns out there is a term β which for most purposes seems to match up with 1/kT, where k is the Boltzmann constant and T is classical temperature. According to this article, there are some occasions where "conventional" T and quantum mechanical T do not match up. The most famous relationship in statistical mechanics is: S = k ln Ω S is entropy. Ω is the partition function for the microcanonical ensemble, and it corresponds to the number of quantum states available to a system. (In English: Ω is the number of different configurations that a system of particles can possible have.) Another relation that can be derived is: 1/kT = ∂ln(Ω)/ ∂E Substituting the first equation into the second, we get 1/T = ∂S/∂E So what would a negative temperature imply? That increasing energy leads to decreasing entropy. My guess is that these researchers have created some system that does exactly that.
- balsam 14y agoAre any laws of thermodynamics violated here[1]? If not, why is this experiment interesting? As a physics student, let me check: 0. Probably not. 1. Not directly applicable 2. Nope. 3. Nope. [1] http://en.wikipedia.org/wiki/Laws_of_thermodynamics http://en.wikipedia.org/wiki/Laws_of_thermodynamics (double post because I really would like an answer) BTW, I now check HN for physics news too, as a BTW.
- mistercow 14y agoSo your baseline standard for whether an experimental result is interesting is "does this violate the laws of physics?" You must find science so disappointing.
- balsam 14y agoOn the other hand, I find engineering very undisappointing. That's why I'm a scientist, since science has a higher standard for being interesting. Call it hipsterism or whatever you will, I upvote thee. (This is an engineering result, not a science result.)
- ChuckMcM 14y agoOk, mind officially blown. Still wrapping my head around the notion that you can flip the energy signature on atoms at near absolute zero and have that result in mirroring to the other side of the temperature number line. Also intrigued by the notion that 'dark matter' is just 'regular matter' but flipped into the negative temperature side of the scale. Which really makes one wonder about the nature of heat.
- ChuckMcM 14y agoFrom the article: "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." And this: "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.” Just sayin'
- yk 14y agoAs others pointed out, there are two definitions of temprature, usually it is related to average energy per particle and formally it is T=dS/dE (S is the entropy, E the total emergy of the system). What I do not understand, is why is this news? A negative emergy is not very uncommen, for example lasers have negative temperature.
- splat 14y agoI think the new part is that this is very close to absolute zero from the negative side instead of the positive side. It also seems the other interesting part is that the system transitions from positive to negative temperatures very suddenly.
- balsam 14y agoAssuming the second law is not violated, the system of lower (negative) temperature becomes a source of energy (as opposed to sink). If this is right, that is mighty interesting.
- cygx 14y agoNot really - this has been done before and the textbook example are spin systems. The newsworthy part is that the negative temperature is realized via motional degrees of freedom.
- balsam 14y agoJust checked wikipedia: "By contrast, 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." Let's define wikipedia to be the absolute zero of information.
- brudgers 14y agoOnce again I am reminded that half the facts I know are wrong.
- Tycho 14y agoBelow absolute zero is an anti-concept.
- bfe 14y agoMaybe the Universe somehow has a natural form of this effect in the intergalactic medium (or in spacetime) that causes negative pressure and is the mechanism of dark energy? "Negative temperatures imply negative pressures and open up new parameter regimes for cold atoms, enabling fundamentally new many-body states."