Moontanman Posted January 4, 2013 Posted January 4, 2013 Researchers have achieved temperatures below absolute zero and may have opened up a new realm of possibilities in heat engines.. http://www.livescience.com/25959-atoms-colder-than-absolute-zero.html
timo Posted January 4, 2013 Posted January 4, 2013 Until I read the hype today I was under the impression that a population inversion of states was important for certain types of lasers to work. So it's a bit strange that what I considered decade-old technology is suddenly sold as the scientific breakthrough of the century. 1
swansont Posted January 4, 2013 Posted January 4, 2013 Until I read the hype today I was under the impression that a population inversion of states was important for certain types of lasers to work. So it's a bit strange that what I considered decade-old technology is suddenly sold as the scientific breakthrough of the century. My reaction is similar. The method of the inversion is novel and there may be interesting things to study with such systems, but there's a lot of hype having to do with the negative temperature that is IMO unwarranted.
D H Posted January 4, 2013 Posted January 4, 2013 Researchers have achieved temperatures below absolute zero and may have opened up a new realm of possibilities in heat engines.. http://www.livescience.com/25959-atoms-colder-than-absolute-zero.html The headline is extremely misleading, "atoms colder than absolute zero". That's wrong. Negative temperatures are hotter than hot rather than colder than cold. The concept of negative temperatures arises from the thermodynamic definition of temperature, [latex]\frac 1 T = \frac{\partial S}{\partial E}[/latex] . Typically, entropy increases as energy is added to a system. What if the ground state is overpopulated? Now adding energy decreases entropy, making 1/T (and hence T) negative. This result has received a huge amount of hype. It's not new. It's older than me. E. M. Purcell and R. V. Pound, A Nuclear Spin System at Negative Temperature, Phys. Rev. 81, 279 - 280 (1951) 3
Moontanman Posted January 4, 2013 Author Posted January 4, 2013 While i obviously don't understand the nuances of this I was puzzled when I came across this article. I had been under the impression that colder than 0 K was a nonsensical idea. But now I've seen it across the net in several places. DH saying it was "discovered" in 1951 is especially puzzling taking into consideration that it is being hyped so loudly... I do remember an old SciFi novel called the cloak of Aesir that used technology based on negative temps to fight a group of aliens who had taken over the Earth, 1
alpha2cen Posted January 4, 2013 Posted January 4, 2013 This is 0 law of thermodynamics. Does this experiment obey this rule?
swansont Posted January 5, 2013 Posted January 5, 2013 While i obviously don't understand the nuances of this I was puzzled when I came across this article. I had been under the impression that colder than 0 K was a nonsensical idea. But now I've seen it across the net in several places. DH saying it was "discovered" in 1951 is especially puzzling taking into consideration that it is being hyped so loudly... Being hyped by pop-sci journalists, not by scientists (beyond the typical announcing of an interesting experiment).
imatfaal Posted January 7, 2013 Posted January 7, 2013 Moon - Physics Faq has a section on negative temperature I am not sure it is up to date regarding experimental ability to do this but it explains the concept in a way I could understand. 1
CarbonCopy Posted January 17, 2013 Posted January 17, 2013 Wait... Doesn't this go against everything that we know of thermodynamics ? We can't possibly go below 0 Kelvin can we ? I'm confused Some explain this ?
swansont Posted January 17, 2013 Posted January 17, 2013 Wait... Doesn't this go against everything that we know of thermodynamics ? We can't possibly go below 0 Kelvin can we ? I'm confused Some explain this ? No, it's more like exploiting a loophole that exists because of how temperature is defined; normally you add energy and the entropy increases, and there is no limit to the energy of the system. But when there is, and maximum entropy occurs at some energy below the maximum value, you get this situation. D H explained this, and the faq linked to by imatfaal gives an example. Temperature as defined by kinetic theory, and applied to that situation, will never be negative. 2
SamBridge Posted January 17, 2013 Posted January 17, 2013 Smething seems wrong with this article, and usually the media is often behind. You can't go "above" infinity, that's inpossible by the defanition of infiity. What's more likely is that the media finally realized that more people are interested in complex mechanics that can be put into terms of words that people understand, and that really what happened is atoms have been cooled to the lowest possible energy state, using liquid helium, which creates a friction-less, entangled fluid. which was done at least 5 years ago. They are behind in many other scientific news items, such as cold fusion, nature, discovery of planets, all sorts of things. Because of the fact that the helium is at the lowest possible energy state, the uncertainty in it's particles actually "accidentally" create random amounts of energy, in the same manner that the uncertainty of thermal radiation in a black hole can exceed the boundaries of the event horizon. "The Wacky Physics Experiment" is something I saw at least 5 years ago and scientists won a Nobel Prize for it. -1
imatfaal Posted January 17, 2013 Posted January 17, 2013 Smething seems wrong with this article, and usually the media is often behind. You can't go "above" infinity, that's inpossible by the defanition of infiity. What's more likely is that the media finally realized that more people are interested in complex mechanics that can be put into terms of words that people understand, and that really what happened is atoms have been cooled to the lowest possible energy state, using liquid helium, which creates a friction-less, entangled fluid. which was done at least 5 years ago. They are behind in many other scientific news items, such as cold fusion, nature, discovery of planets, all sorts of things. Because of the fact that the helium is at the lowest possible energy state, the uncertainty in it's particles actually "accidentally" create random amounts of energy, in the same manner that the uncertainty of thermal radiation in a black hole can exceed the boundaries of the event horizon. "The Wacky Physics Experiment" is something I saw at least 5 years ago and scientists won a Nobel Prize for it. Nope - thats not it. Have a read of DH's explanation way up the thread and I posted a link to Baez's site which hosts the physics faq and has a nice write up that is comprehensible to a lay man like me. As SwansonT said a few posts ago the nub of the matter is the different definitions of temperature. In an ideal gas the temperature is related to the average kinetic energy of the atoms - this leads to one definition in which the temperature is proportionally related to the average kinetic energy. However the temperature can be examined thermodynamically and is inversely related to the change of entropy with respect to energy. You will not find systems which have a negative temperature according to the first definition - but you can create systems that when you put energy in the entropy falls. In this situation, by the thermodynamical definition of temperature, you have a system where T is negative
timo Posted January 17, 2013 Posted January 17, 2013 Smething seems wrong with this article, and usually the media is often behind. You can't go "above" infinity, that's impossible by the definition of infinity.Strictly speaking, the article does not say "above" infinity, it says "hotter" than infinity. If you interpret "infinity" as "any/every positive temperature" and "hotter" as "higher energy" then what the article states is -at least in this respect- correct.
SamBridge Posted January 17, 2013 Posted January 17, 2013 Strictly speaking, the article does not say "above" infinity, it says "hotter" than infinity. If you interpret "infinity" as "any/every positive temperature" and "hotter" as "higher energy" then what the article states is -at least in this respect- correct. Still not a correct physical interpretation, infinity isn't a number that can be computed, therefore they cannot have scientifically confirmed an infinite temperature from any scientific mathematics, it appears that the media is just really trying to hpe it up to make it sound as exciting as possible.
swansont Posted January 17, 2013 Posted January 17, 2013 Still not a correct physical interpretation, infinity isn't a number that can be computed, therefore they cannot have scientifically confirmed an infinite temperature from any scientific mathematics, it appears that the media is just really trying to hpe it up to make it sound as exciting as possible. Well, if we look at [latex]\frac 1 T = \frac{\partial S}{\partial E}[/latex], what happens when entropy is at a maximum? The slope goes to zero, meaning the temperature tends to infinity. However, the energy has not become a maximum. Here "hotter" is being used to mean more thermal energy, when it normally it simply means higher temperature — once again, exploiting the loophole that you have from multiple definitions of temperature (cue Obi-Wan: "This isn't the definition you're looking for"). So I think it's a matter of being both a correct physical interpretation and media hype, because they're playing switcharoo with the definition and not saying anything (probably because they don't know)
SamBridge Posted January 17, 2013 Posted January 17, 2013 (edited) Well, if we look at [latex]\frac 1 T = \frac{\partial S}{\partial E}[/latex], what happens when entropy is at a maximum? The slope goes to zero, meaning the temperature tends to infinity. However, the energy has not become a maximum. The temperature indefinitely approaches infinity, it never attains the value of infinity itself. And I am guessing that there is not an infinite amount of thermal energy, or we would know right away, but I suppose there could be an indefinitely increasing temperature because it's at the lowest energy state. Edited January 17, 2013 by SamBridge -1
swansont Posted January 17, 2013 Posted January 17, 2013 The temperature indefinitely approaches infinity, it never attains the value of infinity itself. And I am guessing that there is not an infinite amount of thermal energy, or we would know right away, but I suppose there could be an indefinitely increasing temperature because it's at the lowest energy state. But that wasn't the issue. It was whether it could be hotter — in the sense of having more energy — than the state of infinite temperature, i.e. the state where dS/dE is zero. And the answer to that is "yes". The flaw is in the equivocation of using two different definitions, not the physical interpretation.
michel123456 Posted January 18, 2013 Posted January 18, 2013 Two different definitions and 2 different results: isn't that there are 2 "temperatures"?
swansont Posted January 18, 2013 Posted January 18, 2013 Two different definitions and 2 different results: isn't that there are 2 "temperatures"? No, not really. If you apply the more comprehensive definition to a purely kinetic system you should get consistent answers.
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