Evidence for neutrino mass

In June a team of Japanese and American physicists announced they had found evidence for neutrino oscillations in the SuperKamiokande underground detector in Japan. Since neutrinos can only oscillate if they have mass, the result has profound implications for particle physics and cosmology. The results from SuperKamiokande suggest that neutrinos have a mass of 0.1 electron volts or greater. The electron, the lightest of the other fundamental particles, is five million times heavier. According to the Standard Model of particle physics, neutrinos have zero mass, so the SuperKamiokande results could lead to new physics beyond the Standard Model.

Neutrinos come in three types - electron neutrinos, muon neutrinos and tau neutrinos - and only interact very weakly with matter, which makes them extremely difficult to detect. However, experiments to measure neutrinos from the Sun have detected less than half the number of neutrinos predicted by theory. There has also been a shortfall in the number of atmospheric neutrinos - produced when cosmic- rays interact with nuclei in the Earth's atmosphere - detected in experiments. One possible explanation of these results is that electron neutrinos are 'oscillating' into muon and tau neutrinos, and vice versa. However, oscillations are only possible if neutrinos have mass. A non-zero neutrino mass could also explain some of the 'missing mass'in the universe.

SuperKamiokande found evidence for oscillations in atmospheric muon neutrinos. The experiment detected more muon neutrinos from above than below. Neutrinos entering from above would have travelled only tens of kilometres through the atmosphere, while those from below would have had to travel thousands of kilometres through the Earth. The SuperKamiokande team claim that the muon neutrinos oscillate into tau neutrinos - or, possibly, a new type of 'sterile' neutrino - on their journey through the Earth.

Time's arrow

The laws of physics cannot distinguish between the past and the future, yet everyday experience tells us that it is impossible to travel backwards in time. Thermodynamics backs this up: entropy or disorder always increases with time. However the CPLEAR collaboration at CERN in Geneva has found that the symmetry between the past and the future is broken in certain particle decays. The KTeV experiment at Fermilab in the US has also found evidence for the violation of time-reversal symmetry. This is the first time that the violation of time-reversal (T) symmetry has been observed directly in an experiment.

The violation of T symmetry has been expected ever since 1964 when a violation of charge-parity (CP) symmetry in the decay of neutral kaons was detected in experiments at the Brookhaven National Laboratory in the US. This discovery led to a Nobel prize for James Cronin and Val Fitch. The violation of both CP and T symmetry is predicted by theory, but their combined effect - so-called CPT symmetry - is thought to be conserved in all quantum field theories.

Another Fermilab collaboration, the CDF team, has recently found evidence for CP violation in the decay of B-mesons (mesons that contain a bottom quark). This is the first time CP violation has been observed in particles other than kaons.

The accelerating universe

Evidence that the expansion of the universe is speeding up, rather than slowing down as predicted by most theories, has been the biggest surprise in astrophysics this year.

Astronomers have known for decades that the universe is expanding, based on measurements of the "red shift" of light emitted by galaxies. According to the standard cosmological model, there are three possible types of universe: a "closed universe" in which there is enough mass to eventually stop the expansion and cause the universe to collapse in on itself; a "flat universe" in which there is enough mass to slow the expansion, but not enough to cause it to collapse; and an "open universe" which contains so little mass that it will expand for ever.

Results from two international teams of astronomers - the High-Z SN Search and the Supernova Cosmology Project - suggest that we live in an open universe. However, when the teams looked at really distant supernovae, they found that the universe was expanding more slowly in the distant past than it has been in more recent times. One way to explain the results is to include a term called the "cosmological constant" in the general theory of relativity. Einstein once called the cosmological constant his "biggest blunder" but time may prove that he was right in the first place.

The accelerating universe was named as "Breakthrough of the Year" by Science magazine earlier this month.

Bose-Einstein condensates

Bose-Einstein condensation has been one of the hottest topics in physics since it was first observed in 1995. Some 15 groups around the world have now produced condensates by trapping and cooling gases of atoms. Every week brings new advances in both the techniques used to produce the condensate and our understanding of this "fifth state of matter".

For example, condensates have been used to amplify coherent beams of atoms - which could lead to a "laser" in which photons are replaced by atoms - and to observe an analogy of the Josephson effect. Other groups have condensed hydrogen in both two and three dimensions, and developed all-optical methods for producing condensates. After the long wait to produce the first condensate - Bose and Einstein first predicted the condensate in the early 1920s - physicists are now taking full advantage of this unique form of quantum matter.

Cool molecules

The ability to routinely cool and trap atoms to sub-Kelvin temperatures was a key step on the road to Bose-Einstein condensation (see above). A logical extension of this work is to cool and trap molecules, but many of the optical techniques used to cool atoms do not work with molecules because they have complex internal energy levels. Earlier this year, however, a team at Harvard University in the US successfully trapped molecules below 1 K for the first time with a new magnetic technique.

The calcium hydride molecules trapped at Harvard obey Bose statistics, so it might be possible to produce the molecular equivalent of BEC. And by trapping other molecules that obey Fermi statistics, physicists might be able to create a Fermi degenerate gas for the first time. A group at the US Air Force Academy in Colorado subsequently trapped cold molecules in an all-optical trap for the first time.

Superfluidity on the small scale

Superfluidity - the ability of certain fluids to flow without friction at low enough temperatures - is one of the most intriguing phenomena in physics. But what is the minimum number of atoms needed for superfluidity to occur? Earlier this year a team of physicists from the Max Planck Institute in Gottingen discovered that as few as 60 helium-4 atoms can exhibit superfluidity.

The Gottingen team studied droplets of liquid helium-3 that contained small numbers of helium-4 atoms. Helium-4 becomes superfluid around 2 K, whereas its lighter sibling becomes superfluid at much lower temperatures, around 3 mK. The droplets were produced at a temperature of about 0.1 K and hence droplets of pure helium-3 showed no evidence for superfluidity. However, when as few as 60 atoms of helium-4 were added, superfluid behaviour could be observed.

Quantum information

Quantum information continued to make rapid progress in 1998. The aim of quantum information is to take advantage of the quantum properties of light and matter in communication and computation. Classical information comes in the form of ones and zeroes: a quantum bit, on the other hand, can represent both one and zero at the same time. This makes it possible, in theory, to perform tasks that are simply not possible with classical systems. Quantum cryptography systems have already been demonstrated over tens of kilometres using commercial optical fibres. However, only the very simplest of logic gates for quantum computers have been built in the laboratory.

Quantum information highlights this year have included the first successful demonstration of a quantum algorithm in an experiment, a novel proposal to build a quantum computer based on the silicon technology found in existing computers, the first demonstration of quantum encryption in optical communications through the atmosphere, the quantum teleportation of the quantum state of a whole beam of light, and the development of a technique to reliably and predictably "entangle" ions in a trap.

Quantum information is currently one of the fastest growing and most exciting areas of physics.

SCUBA and planetary debris

Astronomers have been discovering new planets with increasing frequency in recent years. Now a new submillimetre instrument at the James Clerk Maxwell Telescope in Hawaii has provided astronomers with their first glimpse of the clouds of debris from which these planets might have formed. Observations at optical and infrared wavelengths do not reveal debris as the glare from the star tends to obscure any debris.

As planets form, they tend to sweep up all the dust and debris in their path, creating dust- and debris-free regions in the process. Astronomers using the submillimetre common-user bolometer array (SCUBA) noticed such a region around the star Fomalhaut. This dust-free region is the first indication that solar systems similar to our own exist in the universe.

SOHO is safe

The Solar and Heliospheric Observatory (SOHO) satellite caused a lot of sleepless nights in 1998. In June a computer fault caused the satellite to spin out of control and lose contact with ground control. Over the next three months, however, NASA and the European Space Agency carried out one of the most dramatic space rescue attempts ever. First NASA located the position of SOHO by using the Deep Space Network as a giant radar network. Then ground controllers sent a series of commands to SOHO, switching off its instruments and pointing it towards the Sun to recharge its batteries. Finally, the satellite and all its instruments - which had been frozen in the near-zero temperatures of space - had to be slowly thawed. Last month scientists on the project announced that 9 of the 12 instruments were fully operational.

Copenhagen

Our tenth highlight of 1998 might come as a surprise because it is a play rather than a scientific breakthrough. However, Copenhagen translated one of the most intriguing events in the history of physics into a brave and brilliant piece of theatre. In 1941 Werner Heisenberg visited his mentor, Niels Bohr, in Nazi-occupied Copenhagen. What happened during their meeting remains clouded in mystery. One theory is that Heisenberg, who was working on Germany's atomic bomb project at the time, was trying to find out about Allied efforts to build the bomb. Another is that he was trying to inform the Allies, through Bohr, that Germany's bomb programme was not making progress.

Michael Frayn's play reconstructs Heisenberg's visit to Copenhagen from different angles, expertly drawing out the parallels between the fundamental uncertainty of quantum theory and the differing accounts of the meeting. Writing in Physics World, the physicist John Ziman described Copenhagen as "brilliant theatre". The judges of the Evening Standard drama awards in London agreed, voting it the best play of the year.