Bose-Einstein Condensates

I’ve been working on a political science post that’s taken a lot more thought than I thought, so in the meantime here’s some info on Bose-Einstein condensation. First, let me remind (or teach) you that all elementary particles are divided into bosons and fermions based on their quantum statistical properties. While protons, electrons, and neutrons are all fermions, they can be combined to make composites that act like bosons; the simplest example is the element Helium. When a bunch of bosons are compressed to sufficient density (usually by cooling them to nearly absolute zero) they form a Bose-Einstein Condensate (BEC). This is a state of matter distinct from solid, liquid, gas, or plasma; here’s its Wikipedia page. The physics of Bose-Einstein condensation is pretty interesting, but it’s covered in detail in any worthwhile statistical physics book and, in my opinion, to focus on the equations distracts from the truly ridiculous chemical and physical properties of this state of matter.

To start with, a BEC is completely frictionless and aviscous. Not low-friction like a ball bearing, or low-viscosity like water; rather, a BEC is literally unaffected by both friction and viscosity. This means that if you have a glass half-full of a BEC, the BEC will, in real time, climb up the walls of the glass and escape in a manner analogous to capillary action. It also means that if you stick a spoon in a bowl of BEC and start it spinning, the resulting vortex would last forever. A zero-friction material is really remarkable and should hopefully be filling your mind with exciting potential applications. The closest technology to which I can compare it is magnetic levitation, which is itself incredibly useful but still subject to friction in the form of air resistance.

However, a BEC can’t easily be poured into a bowl and stirred like a liquid can. condensate1 This is partially because of experimental constraints: the required density is high enough that we can currently only make microscopic BECs and once they warm up slightly past absolute zero they revert back to normal matter. It’s also partially because of that word ‘condensate’ – when BECs form they ‘condense’ down nearly to point sources, bound in volume and velocity space only by the Uncertainty Principle. So they don’t look very exciting; here’s a nice artist’s conception of a Rubidium BEC. 

Finally, there’s also an obscure effect related to BECs that, as far as I can tell, has not yet been satisfactorily explained. This is known as the ‘bosenova’ (pun intended, apparently), and relates to a 2001 experiment wherein a varying magnetic field was applied to a microscopic BEC and caused it to collapse in on itself and then explode, in similar fashion to a Type II Supernova but for completely different (and still mysterious) physical reasons.  This effect is totally baffling to me and I have no idea what its physical significance is, but it has some historical notoriety because of speculation that the collapse of something as dense as a BEC would produce a microscopic black hole, and we all know that a microscopic black hole could SWALLOW THE ENTIRE EARTH; obviously, the official Large Hadron Collider Safety Blog has felt compelled to discuss this issue in some detail, and they’re not worried.

For more information on BECs, I have two links. First, above is a nice YouTube video of superfluid Helium that shows some of the effects I discussed. Superfluid helium actually exists along the transition between a BEC and a liquid, so it exhibits some features of each, but it’s much easier to make and play with than a true BEC. I like to think of it as the liquid/BEC transition’s analogue of the solid/liquid hybrid cornstarch and water mixture. Secondly, this webpage provides a great nonmathematical, elementary-physics-level introduction to BECs, replete with little Java apps and references for more information.

One obvious question that might now be asked is: if BECs occur when bosons are compressed to high density, what happens when fermions are compressed to high density? The answer is something wonderful called degenerate matter, which I will set aside for later.


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