One hundred years ago, some physicists began to suspect that electromagnetic radiation was packaged - or
"quantized" - rather than being a continuous stream. This followed Max Planck's discovery that the spectrum of
light from a hot object could be explained only if the radiators sat in discrete energy states. By 1905 Albert
Einstein concluded that the radiation itself was emitted as bursts of energy - light quanta - later to be called
photons. Einstein's key explanation earned him the 1921 Nobel Prize for Physics.
In 1924 Satyendra
Nath Bose from Dacca University, in what was then India, wrote to Einstein asking for his help in
getting a paper published. Bose had already sent it to the Philosophical Magazine, where it had been
turned down. The paper showed how Planck's distribution law for photons could be derived from first
principles. Duly impressed, Einstein translated it into German, and the paper was published in 1924 in
Zeitschrift für Physik.
As a result, Einstein temporarily turned away from his dogged but
unsuccessful search for a unified theory of gravitation and electromagnetism and started work on the quantum
theory of radiation. Thus was born the concept of "Bose-Einstein" statistics for quanta ("bosons") carrying an
integer value of intrinsic angular momentum (spin). There is no limit to the number of bosons that can
simultaneously occupy any one quantum state.
Einstein noted that if the number of such particles is
conserved, even totally non-interacting particles should undergo a change of behaviour at low enough
temperatures - Bose-Einstein condensation. Bose had not predicted this because he was looking at photons,
which can simply disappear when the energy of the system is decreased.
The condensation that Einstein
predicted derives from the fact that the number of states available at very low energy becomes exceedingly
small. With less and less room for all of the particles when the temperature is decreased, they accumulate
(condense) in the lowest possible (ground) energy state.
Superbehaviour and its effects
Even before
this was going on, the liquefaction of helium by cryogenic pioneer H Kamerlingh Onnes (1913 Nobel Prize for
Physics) opened up new areas of physics study. Materials cooled by liquid helium to within a few degrees of
absolute zero showed bizarre behaviour. However, it took time for the real nature of these effects, which are
now known as superconductivity and superfluidity, to become clear. Superconductivity is the virtual
disappearance of electrical resistance at liquid-helium temperatures, and superfluidity is the virtual disappearance
of viscosity as we know it. Superfluid helium flows without resistance, as if no internal frictional forces act in
the liquid. (Appropriately, these properties are being exploited to the full in the cryogenics for CERN's new
LHC collider, the superconducting magnets of which will be cooled by superfluid helium at 1.9 K).
In
1938 Fritz London suggested that superfluidity could be caused by the bosonic condensation of helium-4
atoms, which have integer spin. This was supported by the fact that no similar effect had then been seen with
the rarer helium-3 isotope, the atoms of which do not have integral spin (see below).
In the 1950s, O
Penrose and L Onsager related superfluidity to the long-range order displayed by a highly correlated bosonic
system. This gave an estimate of the amount of condensed atoms in the liquid - only about 8%, because strong
interactions in liquid helium make it deviate significantly from the ideal non-interacting gas.
Superfluid
helium flows without resistance, as if no internal frictional forces act in the liquid. This was explained by a
phenomenological theory devised by L D Landau in 1941, eventually earning him the 1962 Nobel Prize for
Physics. In this theory, superfluidity derives from the fact that when the available energy is low enough, only
long-wavelength phonons (the vibration quanta of the medium) can be excited. Although superconductivity was first seen in 1911, reaching a full theoretical explanation took nearly 50
years. In 1957 J Bardeen, L N Cooper and J R Schrieffer ("BCS") proposed a theory based on phonon-mediated
interactions between the electrons of the metal. This earned them the 1972 Nobel Prize for Physics.
The
BCS theory showed that superconductivity is due to strong correlations between electrons of opposite spin.
This creates a highly coherent state that is insensitive to perturbations, hence the lack of electrical resistance.
Electron pairs can be considered as bosonic particles and the superconductivity transition is similar to
Bose-Einstein condensation.
Earlier, V L Ginsburg and Landau had suggested a phenomenological theory.
Although the implications of this approach emerged only slowly, it did lead to new developments in
spontaneous symmetry breaking, which turned out to be crucial for particle physics in what is now known as
the "Higgs mechanism".
Helium-3 atoms, which have half-integer spin, are not bosons and should not
condense like helium-4 to become superfluid. However, in the same way that electron pairs make materials
superconducting at low temperatures, the BCS mechanism also opens up the possibility of superfluid
helium-3. The discovery of superfluid helium-3 earned the 1996 Nobel Prize for Physics for David Lee,
Douglas Osheroff and Robert Richardson.
Pairing effects, this time between nucleons rather than
electrons, can also play a role in nuclear physics.
The ultimate candidates for Bose-Einstein condensation
were atoms. However, the experimental challenges were formidable and had to await the development of
suitable trapping and cooling techniques to confine and groom the atomic states.
In 1995, some 70 years
after Einstein's original prediction, those who went on to earn this year's Nobel laureates succeeded in achieving
this extreme state of matter. Cornell and Wieman produced a pure condensate of about 2000 rubidium atoms at
20 nK. Independently, Ketterle performed experiments with sodium atoms. His condensates contained more
atoms and could therefore be used to investigate the phenomenon further. Making two separate condensates
merge into one another, he obtained very clear interference patterns, showing that the condensate contained
coherent atoms. Ketterle also produced a stream of small drops that fell under the action of gravity - a primitive
"laser beam" using matter rather than light.
To achieve the very low temperatures needed for
Bose-Einstein condensation, physicists have to exploit laser cooling, in which atoms lose energy via the
continued absorption and emission of photons of radiation. Steven Chu, Claude Cohen-Tannoudji and William
Phillips were awarded the 1997 Nobel Prize for Physics for their development of these techniques.
Since
these pioneer experiments, Bose-Einstein condensation has been achieved in a variety of chemical elements (see
http://amo.phy.gasou.edu/bec.html). One of the latest
developments is a Bose-Einstein condensate on a microelectronic chip (see Cold atoms promise versatile atomic chips). These
achievements are a tribute to the ingenuity and perseverance of experimenters, and they demonstrate the subtle
interplay of many new scientific techniques.
Satyendra Nath Bose 1894-1974
Satyendra Nath
Bose was born in Calcutta, the son of a railway worker. An outstanding physics student, he also had a talent
for languages and translated milestone physics material from French and German into English for local
publication. One of his efforts was a text by Einstein on General Relativity, the English-language rights for
which had meanwhile been acquired by a London publisher. At Bose's request, Einstein himself intervened and
allowed the Bose translation to be used inside India.
It was this episode that gave Bose, working in
Dacca, the confidence to approach Einstein again in 1924 with the new derivation of the Planck radiation law:
"Respected Sir, I have ventured to send you the accompanying article for your perusal and opinion." Einstein
was impressed: "The Indian Bose has given a beautiful derivation of Planck's law." Soon physics history was
made. Bose and Einstein met in Berlin in 1925. Bose returned to Dacca and in 1945 moved to Calcutta, where
he spent the remainder of his career.
His name is now enshrined in physics. A "boson" is a particle of
integer spin that obeys Bose-Einstein statistics and is the counterpart of a "fermion", which is a particle of
half-integer spin that obeys Fermi-Dirac statistics. Unlike Dirac, Einstein and Fermi, Bose did not achieve a
Nobel prize. However, in 1930 Venkata Raman, also of Calcutta, earned the Nobel Prize for Physics for the
light-scattering effect that bears his name. He was the first scientist from outside Europe and the US to earn the
coveted award.
What I have heard about Bose-Einstein condensation makes it sound really weird. What is it really, and how did someone think of it? 
In the early 1920s 