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 ...
Bookmark.
“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.
ping
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?
Cheers!
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>
Cheers!
“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.
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