MYSTERIES OF THE BOSE-EINSTEIN CONDENSATE
In 1995, researchers Eric Cornell and Carl Wieman of the Joint Institute for Laboratory Astrophysics at the University of Colorado (JILA) cooled a handful of gaseous rubidium-87 atoms (approximately 2,000) to less than one degree Kelvin. Zero degrees K is absolute zero, at which point all atomic activity ceases and beyond which things can not be colder. The coldest places in the Universe in the deepest regions of deep space are about 3 degrees K. The work of Cornell and Wieman on rubidium atoms and that of Wolfgang Ketterle of Massachusetts Institute of Technology on sodium atoms earned for them the Nobel Prize for Physics in 2001.
The result of the experiment at JILA was that all the atoms that were cooled to that temperature (about 170 nanokelvins) assumed the same quantum state – termed the ground state of the gas – wherein all the atoms behaved in concert. The wave functions of the atoms that make up this Bose-Einstein Condensate (BEC) overlap. The atoms are said to collapse into what physicists call a “super atom”. The properties of a BEC include the complete absence of viscosity, in effect creating a “super-fluid” wherein all the atoms move in the same way therefore losing no energy due to friction when they flow; and the generation of ultra low speed electro-magnetic waves in the optical spectrum, down to about 17 m/s for a sodium BEC. Insomuch as the deBroglie waves of the atoms in a BEC are coherent, they can be compared to laser light wherein the EM waves generated are also coherent.
The creation of a BEC is only possible with particles that have a total spin that is an integer multiple of the Planck constant divided by 2(pi), which defines the boson. The concept of the BEC was developed by Albert Einstein after he reviewed an intuitive paper written by the Indian physicist Satyendra Nath Bose in 1924 which dealt with the behavior of photons in an enclosed space, or black body radiation. Einstein predicted that at sufficiently low temperatures identical atoms or particles would become locked together in the lowest quantum state of the system. This could not be proved until the tools were made available to create the necessary temperatures seventy years later.
The first stage in creating a BEC involves tuning a laser beam array to the exact frequency of the atoms which have been purified by isolation in a chamber. The photon waves that bounce off the atoms acquire velocity in the process, stealing a little energy from them, thereby effectively reducing the atoms’ energy causing them to slow down and cool. The second stage involves confining the already cool atoms in a strong magnetic field configuration analogous to a deep bowl which allows the cooled atoms to collect in a bunch at the bottom and the more active atoms to “evaporate” away from the top. Temperatures to within a few billionths of a degree Kelvin have been obtained this way without the use of cryogenic technology.
Studying and analyzing BEC’s has become a popular pastime among physics researchers. Since the first BEC was created at the University of Colorado in 1995 twenty groups around the world were able to produce BEC’s before the turn of this century. An example of how the condensate takes on the properties of a super atom involves the experiment at MIT wherein a cigar-shaped BEC cloud is induced into quadruple oscillation by modulating the magnetic containment field, causing the cloud to oscillate by contracting along the long direction and expanding along the short direction and vice versa. The fundamental frequency of this oscillation can be predicted based on the parameters of the cloud. Agglomerations of atoms in a BEC cloud about the size of a dime have been achieved.
Interestingly, a complete analogy to the behavior of photon wavelengths in lasers stops when separate clouds of BEC’s interact. It is possible to cross two laser beams without disturbing either beam through their interaction. However, a condensate will offer some resistance to interference from another condensate, acting more like a fluid. On the other hand, Wolfgang Ketterle’s group at MIT has observed a fringing pattern of alternating constructive and destructive wave interference when the waves of two condensates overlap, as occurs when two laser beams cross.
An example of Einstein’s reference to “spooky behavior at a distance” was observed by Harvard researchers when they successfully halted a pulse of laser light in a BEC cloud of sodium atoms and then revived it in a separate BEC cloud a fraction of a millimeter away. This experiment is described in a February 7, 2007 article in Scientific American on-line.
Experiments on BEC’s have for the most part involved the use of isotopes like rubidium-87 which naturally repel one another. They more readily produce BEC’s. In one experiment of note the researchers at JILA altered the naturally attractive forces of rubidium-85 by using a magnetic field sweep to cause spin-flip collisions thereby inducing the Rb-85 atoms to repel one another. They were then able to form those atoms into a stable condensate. When the magnetic field strength was increased still further, the atoms reverted to attraction and the BEC cloud imploded and shrank beyond detection, then spontaneously exploded, releasing about two-thirds of its atoms. About half the atoms that made up the original BEC could not be located either in the remaining condensate or the expanding cloud of gas. Their strange disappearance has not as yet been satisfactorily explained and was not predicted by quantum theory although the physics community is not for want of ideas on the subject.
One might regard the study of Bose-Einstein Condensates to represent the cutting edge of research in the area of quantum physics. Observation of their behavior has for the most part corroborated all the theoretical and proposed quantum principles and speculation to date and further research on BEC’s will most certainly aid in our understanding of the fundamental nature of reality.