Attn: chi meson re. explination of bose-einstein condensation

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Chi Meson


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This might be jumping the topic, but...Other areas: The proof of Bose-Einstein condensate. Old theory, but refined physics? Isn't this "evolution"?...

how is bose einstein condensation explained? do you have any handy links?
 

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  • #2
Chi Meson
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OK.

UMM.

"explain Bose-Einstein condensate." Umm.

The simplest, and most direct thing I can say is:

When certain materials are brought to an extremely low temperature, they completely lose their mass-like nature (other people, jump right in, I'm out of my league here). THis means that two things can occupy the same space at the same time. In other words, you can stack matter, not on top of one another, but into one another.

I haven't previewed any good links, but try a google search with "bose-einstein" in the search bar. I'll look now.

Others: please help.
 
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I was under the impression that people thought b-i condensation might be explained by the process you allude to of a high proportion of the particles in the liquid all going into the same (lowest) state, but it was by no means certain. However this is what I was taught last summer term and I was led to believe there was a lot of recearch going on in the field.
 
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Chi Meson
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Originally posted by anilrapire
I was under the impression that people thought b-i condensation might be explained by the process you allude to of a high proportion of the particles in the liquid all going into the same (lowest) state, but it was by no means certain. However this is what I was taught last summer term and I was led to believe there was a lot of recearch going on in the field.
OK. No joke. If you were taught about B-I (not "B-E"?) last summer, you should be answering my questions. I've only skimmed the subject, and I keep meaning to study it in deeper detail, but I haven't yet done so. I do know that it is really intriguing, and the possibilities are fantastic, but this is really a question for someone else.

I'm quite sure (or did I dream it) that some guys just got the Nobel for sucessfully achieving the condition.

Hey! Others! Help!
 
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yeah ok peace. isn't it cool that Bose got his name attatched to half the particles in the universe?
 
  • #6
chroot
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There are two kinds of particles in the world: bosons and fermions. Bosons have integral spin, and follow Bose statistics. Fermions have half-integral spin, and follow Fermi statistics.

Fermions obey the so-called Pauli exclusion principle. It is impossible for two fermions to occupy the exact same quantum mechanical state. If you have one fermion in a state, the amplitude to put another particle in the same state is exactly zero.

Bosons, on the other hand, do not obey any exlcusion principle. If you have one boson in a state, the amplitude to put another in the same state is non-zero. Futhermore, the amplitude to put another boson into the same state already occupied by two bosons is even larger. In general, bosons "try" to occupy the same state.

Two familiar examples: electrons are fermions. The exclusion principle they follow in atoms leads to the periodic table and all the rest of chemistry and biology. Photons are bosons. You can put as many photons into the same state as you want. Lasers do this, with great numbers of photons.

Now -- what is BCS condensation? It's a mechanism by which fermions can pretend they're bosons by grouping up. The classic example is liquid helium-3. A helium-3 atom is a fermion. When you reduce the temperatures to very close to absolute zero, the helium atoms begin to interact with one another, and form loosely-associated pairs. (At high temperatures, these pairs are disrupted by thermal excitation.) A pair of fermions (each of half-integral spin) acts like a boson (of integral spin). In effect, the two helium atoms act together as a single particle, a boson.

As you decrease the temperature of He-3, all of a sudden you cross a critical point, and the atoms undergo Bose-Einstein condensation. Suddenly, rather than being unable to be in the same state, the pairs seek strongly to be in the same state. In fact, they quickly all enter the same state. This is the "condensation."

Once condensed, the atoms will try very hard to stay that way. The condensed, so-called superfluid, helium flows without viscosity, can climb up the side of containers, and flow easily through the tiniest pores in a container. Why? Because in order to the change the quantum-mechanical state of one helium atom-pair, you have to change the state of all of them. This takes a great deal of energy. Normally, fluids have viscosity because collisions with the walls knocks the fluid particles around willy-nilly. Superfluid helium simply doesn't allow this -- the atoms are so strongly interested in being in the same state that they just don't interact with the walls at all.

Does this help?

- Warren
 
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  • #7
Chi Meson
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That's what I was going to say!

You know I never made that connection: is super-fluid helium a Bose-Einstein condensate?

And it was NOT a dream: Cornell, Ketterle, and Wieman, 2001 Nobel Prize. ANd they did achieve the state. But Anilrapire said: [as of last summer] "it was by no means certain." SO I have no idea what's not certain or not proven about the state.
 
  • #8
chroot
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Superfluid helium is indeed a Bose-Einstein condensate. A BEC is simply a material whose constituent atoms seek the same quantum-mechanical state. The photons in a laser resonant cavity are also a Bose-Einstein condensate.

I just realized I had a brain fart when I was writing the previous post: Helium-4 atoms are bosons, and don't need to undergo any pairing to Bose condense. Helium-3 atoms require pairing, however, and Helium-3 becomes superfluid and much lower temperatures than does Helium-4. Sorry for the mistake!

- Warren
 
  • #10
Chroot would you give a short rundown on why some atoms are bosons and some are fermions? For me, it seems weird that not all atoms are the same "type", if you wish.
 
  • #11
Chi Meson
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Originally posted by AndersHermansson
Chroot would you give a short rundown on why some atoms are bosons and some are fermions? For me, it seems weird that not all atoms are the same "type", if you wish.
I know this one! Helium 4 is nessesary because it has four particles in its nucleus each with a spin of 1/2. To be a boson, the particle must have a full integer spin. The protons and neutrons of Helium 4 (at a very cold state) will "combine" spins to act like a full-integer spin boson (I'm not sure if they add up to "2" or to "0"). Helium 3 will still be a 1/2 integer spin particle and will continue to be excluded by Pauli.

And now... what did I leave out?
 

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