A supernova is the explosion of a massive star that is cooling and collapsing because it has literally "run out of gas" by consuming all of its fuel for nuclear fusion. The delicate balance between heat pressure and gravity that holds the surface of a star in place has been upset, and it is avalanching in on itself. This collapse continues until the star reaches a "bounce zone" of nuclear density, where nuclear resistance to further collapse is encountered. There some of the infalling nuclei reverse directions and explode outward, driving the supernova explosion. The remnant that was compressed inward by the bounce becomes either a neutron star or a black hole, depending on the mass of the original star. The whole process is powered by gravity acting on the infalling matter, converting part of the star's mass into energy. (For more details, see my columns on supernovas in the Dec-87 and Feb-89 issues of Analog.)
However, this column is not about stars and supernovas. It is about a mysterious new physics result discovered in the behavior of Bose-Einstein condensates that bears a striking resemblance to a small-scale supernova, including the collapse, explosion, and neutron star formation. I'll begin with a review of Bose-Einstein condensation and quantum spin statistics.
If you turn yourself by 360o, the world you observe when you finish appears to be the same as before you rotated. It is reasonable to think that when an object like an atom is rotated by 360o, it should also return to its original state. All atomic-scale particles have the property of "spin" or intrinsic angular momentum, behaving like tiny tops that must always spin around some axis and never stop. In quantum mechanics the condition of returning to the same state after a 360o rotation requires that spin must be "quantized" into values that are always integer multiples of h/2p, where h is Planck's constant and is equal to 6.626 × 10-34 joule-seconds. Photons of light, for example, obey this rule by having one h/2p spin unit (spin 1) that must always point either along or against the photon's direction of travel.
In the 1920s, therefore, it came as a big surprise when experimental physicists discovered that the electron breaks this quantum rule by having a spin angular momentum that is ½ of an h/2p unit (i.e., spin ½). This means that electrons (along with quarks, neutrinos, protons, neutrons, and many atoms) are NOT the same after a 360o rotation. They must be rotated through two full turns or 720o before they return to their original states. This peculiar spin behavior is not well understood, but it is nevertheless an accepted fact of quantum physics.
The distinction between particles with integer spin (like photons, mesons, and 4He nuclei) and half-integer spin (like electrons, quarks, and protons) is very important in the fields of atomic, nuclear, and particle physics because the two classes of particles have distinctively different behaviors. The particles with half- integer spin have the statistical behavior first described by Enrico Fermi and Paul Dirac and are called "fermions", while integer spin particles are called "bosons" and show a statistical behavior first described by S. N. Bose and Albert Einstein.
Fermions obey the Pauli Exclusion Principle, behaving like highly territorial individualists, with only one particle allowed in each unique quantum state. If one fermion is occupies a particular quantum "box" described by its spin orientation, momentum, and position, then all other identical fermions are excluded from that box.
In contrast, bosons in the same box tend to congregate together, with other identical bosons "invited in" to share the same box. As more bosons pile into the box, the tendency for others to join them becomes stronger and stronger factorially. If a group of identical boson atoms having integer spins are tightly confined in a "bottle", trapped in a particular location by a combination of magnetic fields and intersecting laser beams, and cooled to a temperature of nearly absolute zero, they can form a Bose-Einstein condensate. This is a unique state of matter, predicted in 1924 but not observed until 1995, in which all of the participating atoms share the same quantum mechanical wave function. (For more details on Bose-Einstein condensates, see my column in the March-96 Analog.)
Any group of identical atoms having integer net spin can be used to form a Bose-Einstein condensate. However, those most often used are the alkali-metal atoms that reside in the first column of the periodic table. These are bosons because their half-integer electron and nuclear spins combine to give them an overall integer spin. To form a condensate, such atoms are collected as boil-off from an oven, then cooled in a succession of atomic "holding pens" or traps while the hot atoms are removed by evaporation and the energy of the survivors is reduced with laser cooling. In the experiment described here, the atoms are ultimately held within a cryostat, a super-insulated thermos-bottle that shields them from external heating, at the center of a "baseball coil" trap.
The baseball coil is an electromagnet wound so that the current-carrying wires trace a path like the stitching of a baseball, producing magnetic field lines that come in from two directions along one axis and exit in two perpendicular directions along another axis. Also, an outer solenoidal coil superimposes a uniform field of a few hundred gauss on the twisted field of the baseball coil. The very cold alkali-metal atoms are held near the center of this system, where the baseball coil creates a very large magnetic field gradient. Then, under favorable conditions, perhaps a million atoms of identical boson alkali metals will form themselves into a Bose-Einstein condensate.
Until recently, the experimenters had no control over the force that acted between the atoms of the condensate. They had to take whatever interaction force their atoms gave. However, Carl Wieman and his group at the University of Colorado have introduced a new trick. By using the isotope rubidium-85, they have produced a Bose-Einstein condensate for which they can "dial in" the interaction force between atoms. They can even switch the force from repulsive to attractive. This is possible because at just the right energy, atoms of 85Rb happen to have an interaction that is dominated by a magnetic field dependent "Feshbach resonance", a reversible energy-dependent rearrangement of the quantum states of the colliding atoms. Slightly changing the overall magnetic field of the trap in which the Bose-Einstein condensate is confined shifts the energy position of this resonance, thereby changing the strength and the sign of the force between the interacting atoms. The downside of using atoms of 85Rb, however, is that they are difficult to compress and cool. Therefore, their 85Rb condensate contained only about then thousand atoms instead to the million or so atoms they could have achieved with more cooperative atomic species.
The ability to select the strength and sign of the atomic interaction provides remarkable experimental possibilities for studying this Bose-Einstein condensate. It's the equivalent of studying the astrophysical properties of a star while being able to change the strength of gravity and to switch from gravitational attraction to repulsion.
Wieman's group, using a Bose-Einstein condensate with about 10,000 atoms of 85Rb held in a magnetic trap, was able to achieve the super-low temperature of about 3 billionths of a degree above absolute zero (3 nK). This sets a new world's record as the lowest temperature for a group of atoms ever achieved in the laboratory.
They started by setting the overall magnetic field to 162.6 gauss, a field at which the force acting between rubidium atoms was slightly repulsive. When they rapidly changed the overall magnetic field to a value of greater than 166.8 gauss, the force between rubidium atoms became attractive, and they observed a remarkable behavior. The Bose-Einstein condensate first began to collapse, but then it "bounced" and exploded, leaving behind a cool remnant.
The behavior of this Bose-Einstein condensate system bears such a striking resemblance to a supernova that the experimenters christened it a "Bosenova". However, in a supernova explosion the physical processes are relatively well understood. In contract, the most surprising thing about the Bosenova is that the underlying fundamental physical processes behind the explosion are still a mystery. About half of the atoms in the condensate seem to have disappeared from the experiment altogether and are unaccounted for and are not seen either in the cold remnant or the expanding gas cloud.
Moreover, the "bounce" that follows the initial collapse is completely unexpected. Known physical processes that would interfere with the collapse should occur at scales several orders of magnitude smaller than that observed. "Understand," said Carl Wieman, "that atoms have been very well studied. Essentially all the behavior of atoms in general and BECs in particular, we thought were quite well understood, and could be accurately predicted with theoretical calculations. Even for those features that cannot be accurately predicted, the basic physical processes are still qualitatively well understood. But the theoretical calculations of what would happen in this situation predict behaviors that are totally unlike what we've observed, so the basic process responsible for the 'Bosenova' must be something new and different from what has been proposed."
The work raises a number of open questions. How cold is the "neurtron star" remnant left behind by the Bosenova? Its temperature has yet to be measured, but it may be much colder than the world-record 3 nK temperature of the condensate before the collapse was initiated.
The fate of the missing atoms also remains a mystery. The researchers suspect that either these are accelerated so strongly that they escape the atom trap undetected, Or perhaps they form molecules that are invisible to the detection system. The source of the energy that powers the explosion is also mysterious. How can the coldest system of atoms ever produced in the laboratory suddenly find the energy to mount an explosion? What is the source of the energy driving the explosion? Why don't the known laws of atomic physics seem to work for this system?
Watch this column for further developments on the mysteries of Bose-Einstein condensates. We have much more to learn.
First Observation of a Bose-Einstein Condensate:
M. H. Anderson, J. R. Ensher, M. R. Matthews, C. E. Wieman, and E. A. Cornell, Science 269 198 (1995). See also http://jilaweb.colorado.edu/bec/jilabec.html.
"Bosenova" in a Bose-Einstein Condensate:
" Stable 85Rb Bose-Einstein Condensates with Widely Tunable Interactions ", S. L. Cornish, N. R. Claussen, J. L. Roberts, E. A. Cornell, and C. E. Wieman, Physical Review Letters 85 1795-1798 (2000). See also http://xxx.lanl.gov/abs/cond-mat/0004290. and
"Controlled Collapse of a Bose-Einstein Condensate", J. L. Roberts, N. R. Claussen, S. L. Cornish, E. A. Donley, E. A. Cornell, and C. E. Wieman, submitted to Physical Review Letters (2001). See also http://xxx.lanl.gov/abs/cond-mat/0102116 .
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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