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.

The Nobel century

In 1901 the first Nobel prize award ceremony was held at what is now called the Old Royal Academy of Music in Stockholm. In Christiania (now Oslo), the names of the Nobel laureates were announced in the Storting (the Norwegian Parliament - in 1905 Norway reverted to being a separate monarchy).

The winner of the first Nobel prizes were: physics - Wilhelm Röntgen for his discovery of X-rays; chemistry - J H van't Hoff for his work on chemical dynamics; medicine and physiology - Emil Adolf von Behring for his work on serum therapy, especially its application against diphtheria; literature - Sully Prudhomme (René François Armand Prudhomme); peace - Jean Henri Dunant, founder of the Red Cross in Geneva, and Frédéric Passy, founder of the first French peace society.

To commemorate the centennial of the first Nobel awards, all of the living laureates have been invited to participate in a Centennial Week in December. Beginning with lectures at various universities around Sweden and Norway, the week culminates with the Nobel festivities on 10 December in Stockholm and Oslo. Several hundred laureates are expected to participate in the event.