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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 360°, 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
360°, 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 360° 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 360°
rotation. They must be rotated through
two full turns or 720° 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 "neutron 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.
John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.
SF Novels by John Cramer: Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106. His new novel, Fermi's Question may be coming soon.
Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .
References:
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 .
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