Skip to comments.Quantum gas goes below absolute zero - Ultracold atoms pave way for negative-Kelvin materials.
Posted on 01/03/2013 11:44:46 PM PST by neverdem
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......
(Excerpt) Read more at nature.com ...
Does this mean absolute zero is lower, is all, or what?
It's a technicality that isn't new. There are certain systems with anomolous "temperatures" because the thermodynamic definition of temperature isn't quite perfectly defined. It's a macroscopic state variable that doesn't completely line up with the microscopic description of reality (and it isn't supposed to, by the way.)
Simplest example I learned in graduate school was: Under the thermodynamic defintion of temperature, a perfect vacuum is at absolute zero because it can provide no heat to any themral reservoir placed in contact with it, no matter how cold. Thus, a vacuum must be "colder" than anything. But at the same time, it must also be at inifinte temperature, because no thermal reservoir, no matter how hot, placed in contact with it can transfer heat to it. So it has to be hotter than anything else. There are a number of these kinds of anomalies.
The absolute zero of temperature is only a point of zero energy in classical physics, which is only an approximation of reality.
In quantum physics it is the point of lowest system energy, which is never zero.
[The simplest, but not always the most correct way to see this is that by the Uncertainty Principle, the ground state of every quantum system that has finite spatial extent (which, for example, descibes anything in our universe) has some non-zero momentum. So "all" motion cannot cease, even at absolute zero.]
They date absolute zero vaguely to the mid-nineteenth century. I don’t think Kelvin lived very far into the last century. It had occurred to me absolute zero predates Einstein and quantum mechanics. I wish they would come out and tell us whether various oldtimey concepts are real, or merely being used to help us rubes follow along.
Or maybe they don’t know, either.
The really important question is: How can I use this information in MY life?
I guess this means cryo-freeze like in “Demolition Man” is possible?
We could have cold bombs or cryo grenades?
I can get motivated knowing I have access to a cryo grenade that will stop molecular function of metals to the point I could shoot a 9mm at a tank and it will shatter like fine crystal.
It appears to be that the good scientists merely redefined Absolute Zero to include some atomic motion thus permitting them to say that they can thus bring a gas to a state of less than Absolute Zero. One can also redefine the pure color Red to include some yellow and thus surprisingly find a state of red that includes some yellow.
But, but science taught us that absolute...that is ABSOLUTE zero is the coldest anything can get! Science is based on facts and logic and HAVE to be right!!! This article must be a religious propaganda piece!
Absolutely relative. Everything is absolutely relative. Except the Bible, which is absolute.
All of science is models of reality. All models break down at some point.
How do you contact a vacuum? There’s nothing to contact.
Exactly, how do you get a Pinto out of this?
That’s a very interesting way of looking at the properties of a vacuum. Thanks. Haven’t we also come to the conclusion that the ‘ether’ as spoken of in previous centuries may actually be more “real” than we were told through most of the 20th century, as there are quantum fluctuations of space-time that seem to have some rather interesting and peculiar properties.
Science is going to have to undergo a paradigm shift.
Mass that exists devoid of energy?
A state yet beyond the complete absence of heat?
The exact value of this absolute zero, relative to Celsius temperature has been measured to within a few microkelvins (it is actually defined as 0K, which is defined to be -273.15 C) It was well established IIRC in the late 19th century by extrapolation. It actually cannot be reached (this is one of several alternative versions of what is called The Third Law of Thermodynamics: "By no finite series of processes is the absolute zero of temperature achievable.") In its strongest formulation, the absolute zero of temperature isn't really defined in terms of temperature, it's defined as the temperature at which the entropy of a perfect crystal is zero.
A much better discussion of what is going on here than is stated in the article is in http://en.wikipedia.org/wiki/Absolute_zero in the section under "negative temperature." It is brief and accessible to the layman.
The beauty of the macroscopic state variables of thermodynamics is that they don't depend on any underlying theory of matter: none of the theorems or results of classical thermodynamics were changed when classical physics was modified by relativity, and none of them were changed with the advent of quantum mechanics. If quantum mechanics was overthrown tomorrow, macroscopic thermodynamics would still be entirely valid and not a single definition, result, or equation would change.
“the ‘ether’ as spoken of in previous centuries may actually be more ‘real’ than we were told”
I thought that’s what bosons and fermions were, or else I have no idea. Not that we’re sure they even exist.
Recently, the Higgs Field (if it exists... some are skeptical) appears to have some of the properties hitherto assigned to "the Luminiferous Aether."
...physicists have created an atomic gas with a sub-absolute-zero temperature for the first time...And when the gullible grad student got his tongue stuck on it, everyone else laughed.
Q. What is the greatest invention in history?
A. The Thermos.
Q. The Thermos? Why?
A. It keeps hot foods hot, it keeps cold foods cold.
Q. And why is that so important?
A. How does it know?
Non-commutation of position and momentum operators.
What's frightening is how well they agree over how wide a range of conditions.
The odds of that must be around 1720, due to the 2nd Law of Thermal Documents /crevo-thread>
“The really important question is: How can I use this information in MY life?”
I don’t really believe you think the only reason for the universe to exist is all about you. That said, while we may not yet know how to make use of this information, we didn’t know in the 1920’s that quantuum mechanics would make possible the iPhone, etc., and in that respect all this type of information is important to learn as much as we can.
You have done a very good job of making this rather mis-named concept of “negative temperature” accessible to people here. Thanks.
If Helen Keller fell down in the woods and no one was around to hear it, would she make a sound?
The luminiferous aether was problematic from the beginning, because it had to have bizarre physical properties. For example, as a general rule, the more rigid a material is, the faster it transmits waves. The speed of sound in air is only about 330 meters/sec at sea level, but it is over 6000 meters per second through steel (and more than twice that through diamond.) So, the aether had to be tremendously rigid, since the speed of light through the supposed aether was 50,000 times faster than the speed of sound through steel.
Despite that requirement, it had to offer no mechanical "drag" through space, because the motions of the planets (for example) were perfectly accounted for by Newton's Laws, which assumed that space was effectively empty.
The natural question then arose: does the speed of light change as the earth moves around the sun? It should, because for one part of the year light transmitted through the aether should be moving with the aether, and for the other half of the year, it should be moving against the aether. In fact, because the earth travels in roughly circular motion around the sun, the speed of light should rise and fall sinusoidally as the earth moves through the aether. In a series of brilliant experiments, two American Physicists, Albert Michelson and Edward Morley, proved that the speed of light never changed, regardless of the orientation of the observer or location of the Earth. It is one of the most famous negative results in history, and spelled the eventual end of the theory of the "luminiferous aether."
Fermions and bosons don't have anything to do with this really. In quantum mechanics, there is a requirement that the wave functions of systems of particles must be either symmetric or antisymmetric when the particles trade places. This must happen because the sum total of the square of the wave function indicates the likelihood of finding a particle somewhere in space, and that squared value must not change when two particles are "swapped." [They still have to be somewhere, even if they trade places!]So, the wave function can only change by a factor of +1 (symmetric) or -1 (antisymmetric) when the particles are interchanged. [AND ... Those are the only possibilities, because -1 and +1 are the only numbers that square to 1.]
Particles which make up systems with antisymmetric wave functions are called fermions. (Named after the Italian/American Physicist, Enrico Fermi; probably the most underrated physicist of the twentieth century: both a brilliant theoretician and equally fantastic experimentalist -- a very rare combination in the last 200 years.) Very loosely, a fermion is any material particle: electrons, protons, neutrons, quarks, ... and their antiparticles.
Particles which make up systems with symmetric wave functions are called bosons (Named for Satyendra Nath Bose and Albert Einstein, who developed some of their statistical properties.) Again very loosely, particles which are either: 1) even number combinations of more basic particles or, more famously, 2) the so-called "gauge bosons" which transmit forces throughout the universe are bosons. Gravitons, gluons, photons, the W and Z particles, and the Higgs boson are all bosons. So is, for example, Helium 4 (because it's made up of an even number of fermions: 2 protons, and 2 neutrons.) But not Helium 3, because it's made up of an odd number of fermions (3 in all, 2 protons, and 1 neutron.)
So ... clearly fermions and bosons exist.
And despite the silly claims of science "journalists" and popularizers that quantum physics "describes things that are very small," the truth is that whether a system is made up of bosons or fermions has a great deal to say about its large-scale behavior. For example, nearly ALL of the properties of metals are attributable to the fact that their free electrons are fermions. (Fermi himself developed this theory.) The behavior of many elements near absolute zero changes dramatically, depending on whether their isotopes are bosons (like He4) or fermions (like He3.)
This universe could not possibly exist in any kind of recognizable form if the photon were not a boson.
You’re welcome. I miss teaching, which shows sometimes. Maybe when I retire I’ll return to it as a second career.
The way I had the existence of one particular type, the Higgs boson, was that in order to unite various particles under the Standard Model they had to posit the existence of clouds of bosons, so to speak, interacting with electrons to give them their mass. Apparently such things come into being through spontaneous symmetry breaking, and I don’t think I’ll ever really understand what that is.
Why I brought it up is that imagining electrons passing through as yet undetected particles to explain their behavior reminded me of what I had learned about the ether through the Michelson-Morely experiment. No doubt the ether theory was much grander and more complex than that. I often misuse scientific concepts. But scientists are always borrowing for their own purposes half remembered bits from the humanities, so it’s a wash.