Instigating a Bosenova

[KALSTER]

First a quote from the NIST page on recent Bose-Einstein Condensate experiments:

 

” Working at JILA, physicist Carl Wieman's [pronounced wy-man] team has explored tuning the self-interaction of atoms in a BEC. By making a BEC in a particular isotope of rubidium, rubidium-85, and then changing the magnetic field in which the BEC is sitting, the team is able to adjust the wavefunction's self-interaction between repulsion and attraction. If the self-interaction is repulsive, all the parts of the wavefunction push each other away. If it is attractive, they all pull towards each other, like gravity. Achieving a pure BEC in rubidium-85 required the cloud of atoms to be cooled to about 3 billionths of a degree above absolute zero, the lowest temperature ever achieved.

Making the self-interaction mildly repulsive causes the condensate to swell up in a controlled manner, as predicted by theory. However, when the magnetic field is adjusted to make the interaction attractive, dramatic and very unexpected effects are observed.

The condensate first shrinks as expected, but rather than gradually clumping together in a mass, there is instead a sudden explosion of atoms outward. This "explosion," which actually corresponds to a tiny amount of energy by normal standards, continues for a few thousandths of a second. Left behind is a small cold remnant condensate surrounded by the expanding gas of the explosion. About half the original atoms in the condensate seem to have vanished in that they are not seen in either the remnant or the expanding gas cloud.”

 

They change the properties of the magnetic field trapping the BEC, but how does that affect the forces present in between the constituent atoms? Do they simply tune the field so that it cancels out repulsive forces by destructively interfering with them? Can this only be done with BEC’s because of the uniformity of the wavefunction the atoms collapsed to? Does it mean that the virtual force-carrier particles of each field (van der Waal?) interfere with each other and cancels out? What happens to the energy then, or does it simply revert to the ground zero point energy? What do they mean when they say that “this procedure caused the BEC to implode and shrink beyond detection”?

[swansont]

They change the properties of the magnetic field trapping the BEC, but how does that affect the forces present in between the constituent atoms? Do they simply tune the field so that it cancels out repulsive forces by destructively interfering with them? Can this only be done with BEC’s because of the uniformity of the wavefunction the atoms collapsed to? Does it mean that the virtual force-carrier particles of each field (van der Waal?) interfere with each other and cancels out? What happens to the energy then, or does it simply revert to the ground zero point energy? What do they mean when they say that “this procedure caused the BEC to implode and shrink beyond detection”?

 

The scattering length varies with magnetic field and can change sign (Feshbach resonances) from attractive to repulsive, or vice-versa. I think shrink beyond detection may have a little poetic license, though I can't say for sure. My impression is that in the initial stage of this phenomenon the BEC collapses, i.e. it gets smaller, and they haven't been able to measure that.