Colder than the coldest cold

Absolute zero is supposed to be the coldest cold — 0 Kelvin, the point where atoms stop moving.

But researchers at the University of Munich say it's possible to get colder than that, an idea they've demonstrated experimentally. But what does it mean to be colder than cold? Here's the scientists' totally unhelpful explanation:

another way to look at these negative temperatures is to consider them hotter than infinity, researchers added.

Cool. Thanks, guys. Luckily, journalist Charles Q. Choi makes this strange idea make a whole lot more sense. Read his explanation at LiveScience.

Image: Frozen Peas Corn Carrots IMG_1000, a Creative Commons Attribution (2.0) image from stevendepolo's photostream


  1. “colder than the coldest cold” and “hotter than infinity” should be lyrics in a song. 

  2. I am going to put this in the pile with descriptions of dark matter, and multidimensional time stuff.  I call that pile the “we have no idea what the fuck we are talking about” pile of sorta science.

  3. This is standardly awful science journalism.

    Negative temperatures on the Kelvin scale are nothing new.   It is, in a number of systems, more useful to deal with the reciprocal of temperature, 1/T.  In thermodynamics, this value is called beta, and negative values show up in a lot of places.

    If you have a system where the constitutent particles can only be in one of a few energy states, then if you have more particles in a higher energy state than there are particles in a lower energy state, you have negative beta, which implies T is also negative.  This doesn’t mean this can “lead to new engines that could technically be more than 100 percent efficient”; that’s utter nonsense.  Nor is this a “new advance” (except in magnitude, qualitatively it’s not new).  Just about every single laser out there reaches a state where beta is negative, it’s a population inversion that’s necessary for the laser to operate at all.  This is only “bizarre” if you insist upon thinking of temperature as an average internal energy of the system. 

    The Wikipedia article on this is far better than the LiveScience one:

    The three plots on the right side of the article illustrate why beta is a more useful concept in such systems: you avoid that nasty singularity at T=0.

    1. I certainly hope this is terrible journalism, because I was left thinking either that or some con artists bamboozled Science magazine.  It ended up sounding like phlogiston.

      1. That, or some universe programmer used a signed variable when he should have used a unsigned one…

      2.  It is for comments such as yours that I continue to return to boingboing.
        I fear too many other sites are breathing heavily de-phlogisticated air.
        Well done, Boundegar, well done.

    2. Very cool.  I knew about the population inversion of energy states being the basis of lazing (?) for the more common varieties of laser but I did not realize that such a state would have a “negative temperature”.  I guess I need to study up on my thermo.

      Thanks for the explanation!

    3. Thank you for that.  I knew something was badly awry when I read “At the physically impossible-to-reach temperature of zero kelvin, …atoms would stop moving”, which is a common misconception about absolute zero.  The paragraph on negative temperatures in the Wikipedia article on absolute zero is both more concise and more informative than this piece. 

  4. This is all well and good, but I would like Mr Choi to explain why sometimes my half-open bags of frozen peas sometimes develop little jaggedy shards of ice crystals all over them? Also, I’d like to know whether it’s better to defrost an old refrigerator on a cold or hot day (or does it even matter?)

  5. Help me out here: I’m not aware of anyone bringing matter to zero Kelvin before, so that would seem like a pretty major accomplishment in itself, but I also don’t see any suggestion in the article that this happened: rather they have a lattice of atoms that behaves like it’s at a few nanokelvins…then they wave their hands a bit and say that the atoms behave like they’re at a few nanokelvins, but actually this is because they are at a few negative nanokelvins…and therefore we can conclude that negative nanokelvins remarkably impart properties similar to positive kelvins, and then step 2 and then profit.

    Negative temperatures could be used to create heat engines — engines that convert heat energy to mechanical work, such as combustion engines — that are more than 100-percent efficient, something seemingly impossible.


    1. It’s simple, you just push the rules of the universe to the near breaking point, and then everything runs on nonsenseium because god is out to lunch.

    2. See Phanatic’s comment above.

      I don’t think zero molecular movement is possible (measuring the temperature requires probing it with a sensor; probing will require an exchange of energy, and this energy exchange will jiggle the molecules), but apparently it is possible in principle to have a collection of molecules whose beta value is zero.  Since temperature is the reciprocal of beta, though, such a substance would have an undefined (not zero) temperature. 

      Based on Phanatic’s comment and my own limited understanding of this issue, substances with zero or negative beta values will actually be “hotter” (more energetic) than substances with similar positive temperatures, but don’t take my word on it.

  6. “Luckily, journalist Charles Q. Choi makes this strange idea make a whole lot more sense.”
    I think that’s being a bit charitable.

  7. The more than 100% efficiency comes the equation for Carnot efficiency of a heat engine. The efficiency is calculated using the temperature of the working fluid entering and leaving the heat engine, and if the temperature leaving is less than absolute zero then the efficiency will be greater than 100%. However, this is only possible if you have a heat sink at below absolute zero capable of absorbing infinite amounts of energy without changing temperature. Science journalism strikes again.

    1. So…it’s possible to generate that metric, just not possible to do anything useful with it?

  8. From the perspective of my admittedly abject ignorance, this just sounds like a new description of the classic Bose-Einstein Condensate… dissolution of bosons into clouds of probability so that they can be said to statistically occupy all possible quantum states.

    1. No, it is not the same thing- it can be well described as a classical phenomenon. Look up “two state paramagnet” in a statistical mechanics textbook.

  9. Here is my hypothesis. Somebody had a lot of fun in Theresienweise last september, and decided to write a paper that same night. When the person woke up the next day, he chuckled and said to himself “Let’s see who buys this”. The rest, as they say, got Boingboinged

  10. This is nothing new. Negative Kelvin temperatures were first created in the lab by Norman Ramsey in the 1950’s. And the quoted scientists are right, it isn’t colder than 0K, it’s hotter than infinite.

    Just like with the supposed wierdness of quantum mechanics, the key insight is that the words don’t mean what you think they do. To a physicist, temperature isn’t “The thing a thermometer measures,” it’s “The derivative of energy with respect to entropy.”

    Energy (traditionally written as U) = just what you think, measured in joules
    Entropy (traditionally written as S) = the logarithm of the number of ways it is possible to divide the available energy up among the degrees of freedom present in a system.
    Temperature = dU/dS

    For most systems, this does what you’d expect. If you add energy to a gas, you can each each additional unit of energy to any molecule you want (making it go faster, vibrate more, ionize, whatever). So entropy always goes up when you add energy – and so dU/dS is always positive.

    But put iron atoms in a magnetic field, and there are only 2 magnetic states – parallel to the field or opposed to it. The lowest energy state is every atom aligned, the highest energy states is every atom opposed. Both of these are states of zero entropy – there is only 1 arrangement of energy that achieves them. So to get from one to the other entropy must go up, then down as you add energy – meaning temperature (dU/dS) is first positive then negative. In practice you make such a system by aligning the magnet to an external field, then rapidly reversing the external field. Not a practical heat in engine, since that takes more work than the “bonus” you’d get out by using the result in a carnot-equivalent cycle. And actually, it isn’t clear that there *is* a way to make a carnot-equivalent engine for such a system.

    So yeah, the Kelvin scale is defined in a way that doesn’t go from negative infinity to positive infinity. It looks qualitatively like a tangent curve: zero to positive infinity, jump to negative infinity, to zero.

    Which, of course, really made me laugh when I played the Sims 3 and earned the “-1 Kelvin refrigerator.”

          1. You didn’t have to remove it, I was happy regardless. Sorry I didn’t include a /s in my reply.

    1. thank you very much.  the linked article didn’t explain it this well, the wikipedia page that i read didn’t explain it this well, and i was left assuming this was utter crank crap (ala: “Lisa, in this house we obey the laws of thermodynamics!”).  your explanation made it ~sensible (for me)

  11. this is indeed either nonsense or bad journalism.  an inverted state occupation happens in lasers, for example, and they have been referred to historically as “negative temperature” systems in textbooks.  also, one earlier physicist noted here that “even” 0K has never been achieved and, even if you could, the act of measuring it would increase it’s temperature.  rubbish reporting and shameless salesmanship by the scientists.

  12.  See your July 2012 issues of Science: for the paper of one cited author; that said, what is this garbage with an unlabeled graph and no citations, Ulrich et al are doing fine taking the temperature of a superfluid and showing energy distribution consistent with having one, but claiming that he has a big-bang sucker (analagous to the dark-sucker lightbulb theory) isn’t quite right. Moreover it ain’t so that molecules at 0K are stopped; it would take more energy to hold them still than to let them be and woggle steadily. Negative temperatures are common enough in thermodynamics that way. In ‘ideal gases’ here though, we see the Boltzman distribution of the laser cooling well mirrored, and a particular kind of A/C shining.

  13. Shouldn’t we suppose any atom will always be moving, even at zero K? I mean, if you “freeze” it in a lab, it will still be travelling across the universe along with Earth, Solar system, Via Lactea, etc… I suppose you could freeze it relative to an hypotetical center of the Universe. It would actually be moving quite fast in relation to Earth. In that sense, is a temperature reading as relative as Einstein’s Relativity? An alien would spot it as glowing? Could an atom be frozen from “center of the universe” point of view, but melting hot from our pov? Is the Sun actually a giant snowball?

    That blows my mind.

    1. “Moving” is not quite as appropriate as “vibrating” in this instance.

      If I’m not mistaken, it would be most correct to say “not moving relative to whatever is making the measurement of its position”.  (This is of course rather problematic, as being able to precisely locate the relative position of an atom would be an uncertainty principle violation.)

  14. Also covered by Nature at – though the journalist there makes the “clustered at the mean energy” mistake.

    Since T = dQ/dS, and S = k ln W, if you can create a system where adding energy reduces W, the number of ways the system can be in a similar configuration, then a positive dQ makes a negative dS.

    Suppose you have a system of 50 magnets in a magnetic field. 49 of them are aligned against the external field, and 1 with. This represents an energized system, with 49Q and 50 possible states. To spin the 50th magnet needs 1 Q, and then there will be only 1 possible configuration. 49Q and 50 ways becomes 50Q and 1 way.

    dQ = Q, and dS = k(ln(1) – ln(50)) = – k ln(50); and T = -Q/(k ln(50)).

Comments are closed.