# Bose-Einstein Condensation

#### PRodQuanta

I grasp the idea. The only thing I don't get, is why do the atoms have to be frozen? How does the lack of heat effect the state of which the atoms are in? And once the state of superfluidity is achieved, why must the liquid not have a viscosity?
wanderingand asking aimlessly to gain knowledge.

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Well, generally heat is a type of energy. The less energy available to something, the less it moves, and so the more it tends to fall to the lowest state possible due to the 1st law of thermodynamics. If the object was hotter than the surrounding environment, it has a tendancy to radiate away the heat, and hence lose energy, thereby going to the lowest state it can.

#### chroot

Staff Emeritus
Gold Member
Thermal excitation gives some of the atoms enough energy to be in states above the ground state.

In addition, the term "frozen" refers to the solid state of matter. Superfluids like liquid He-4 are not frozen; they are just very cold.

- Warren

#### heumpje

A BEC can not have viscosity because most of the atoms are in the absolute groundstate. Therefore they can not loose energy anymore, and as you know viscosity arises because of friction/dissipation. Something that can't loose energy can't dissipate.

#### nocturn

superfluid with finite speed

As we all know,material in superfluid phase has a critical speed,Vc,under which superfluid can be achieved.Particles constituting the material are all in groundstate.They are almost independent without any effect on others.But if their kinetic energy is over m*Vc^2/2,maybe they will feel others' influence,which is the origin of viscosity.

#### anilrapire

^ when an object in a superfluid exceeds the Vc (which, I think, is determined by the properties of the superfluid), the phonon generated by its movement has sufficient energy to excite the particles in the superfluid into various non-ground states. So, the superfluid state breaks down.

#### jammieg

And if that doesn't blow your mind's fuses, light slows down to car speeds as it passes through one of those miniature blackholes or whatever they call them.

#### neutroncount

Originally posted by jammieg
And if that doesn't blow your mind's fuses, light slows down to car speeds as it passes through one of those miniature blackholes or whatever they call them.
They are not miniature blackholes. The reason they can appearently slow light down is that the laser imparts its properties onto the chilled atoms and those atoms can be coaxed into releasing photons identical in wavelength and energy by using another laser as a primer to change the spin conditions of the atoms.

#### jammieg

To 30mph? Seems a bit much to assume "black holes" I admit perhaps I should have said it is something that can do almost what a black hole does in one respect.

#### neutroncount

Originally posted by jammieg
To 30mph? Seems a bit much to assume "black holes" I admit perhaps I should have said it is something that can do almost what a black hole does in one respect.
You misunderstand. The propagation of the signal representing light inside the BEC is moving at 30 mph. I'll say it again. Light doesn't slow down! It's appearent velocity in the BEC is. But that is just the effect of having the signal that represents the photons' properties move slower...or completely stopped.

#### jammieg

I agree neutroncount that light isn't actually being slowed and I make the same mistake as others, but manipulated would be a better word and it's difficult to communicate that becuase of all the physics involved I forgot exactly how light works and so I agree that a less appropriate word was choosen and is misleading. To a layperson it wouldn't make any difference if one said slowed or compressed because it's still kind of true, as if someone compressed millions of miles of air into a tiny spot of gas it is giving the illusion of slowing light but actually it is more increasing the distance light has to travel through this process of refraction and photons absorbtions and reimmisions all adding to the illusion of lagging light but during some processes light isn't really light, but how it so dramatically increases these processes still seems mysterious to me, and no doubt Bose around 100 years ago seemed like a fool to most people at the time telling them that light could be manipulated in such ways. My point is black holes are black because they don't let much of anything go, and it seems that very high refractive indexes do to, and such high indexes are related to extremely cold atoms, but still a somewhat wild deducing on my part, and without explaining myself it's the sort of thing that contributes to the plague of overhyping.

Here are some sources I looked over:

http://news.bbc.co.uk/1/hi/sci/tech/655518.stm
http://ee.tamu.edu/Publications/Hemmer/CrystalStopsLight.pdf[/URL]
[url]http://cua.mit.edu/ketterle_group/Introduction_to_BEC.htm[/url]
[url]http://www.nature.com/nsu/990225/990225-5.html[/url]
[url]http://www.fenrir.com/free_stuff/columns/science/sci-119.htm[/url]

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