1. An eventful year at CERN
2. Tau neutrino slots into place
3. Schrödinger's cat makes an appearance
4. Silicon lasers shape up
5. Laser goes faster than light
6. Make way for organics
7. Universal truths
8. Out of this world
9. Hydrogen goes superfluid
10. Vibrations of the year

1. An eventful year at CERN

Researchers at the CERN particle physics lab will probably remember 2000 as something of a roller-coaster ride. The lab kicked off the year with evidence for quark matter – a new state of matter in which quarks are not confined inside protons and neutrons but free to roam around in a quark-gluon plasma. Such a state of matter is thought to have existed shortly after the Big Bang.

In the summer, CERN's antimatter factory opened for business, delivering antiprotons to three experiments designed to investigate the difference between matter and antimatter. The experiments should also improve our understanding of why the universe is dominated by matter, even though it is thought that equal amounts of matter and antimatter were created in the Big Bang.

The real drama began in autumn, with a tantalizing first glimpse of the Higgs boson – the particle that is thought to give other particles their mass. Physicists have been searching for the Higgs for years and the new evidence arrived just as the LEP collider at CERN was about to be decommissioned. Particle physicists were divided over whether LEP should be granted a reprieve to secure more convincing evidence for the Higgs, which would have delayed the new LHC machine which the lab plans to build in the same tunnel. LEP was given a stay of execution of five weeks to collect more data but the teams searching for the Higgs could not convince the lab's management to keep LEP running for another year. On 13 December, after a remarkable eleven-year history, engineers started to dismantle LEP.

CERN claims quark-gluon first
Antimatter factory opens at CERN
Higgs boson on the horizon
CERN chases the Higgs
End of the line for LEP

2. Tau neutrino slots into place

Particle physicists were also busy on the other side of the Atlantic. An international team of physicists at Fermilab in the US found the first direct evidence for the tau neutrino – the final missing piece of the 'standard model'. The team spotted the signature of the tau neutrino in July after a three-year stint on the DONUT – Direct Observation of the Nu Tau – experiment. All 12 matter particles in the Standard Model – the six quarks, three leptons and three neutrinos – have now been detected. Meanwhile, in June experiments started at the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory. The experiments at RHIC will be able to study the properties of the quark-gluon plasma in detail.

Tau neutrino identified at last
RHIC makes its debut

3. Schrödinger's cat makes an appearance

The ability of a particle to be in two places at the same time is one of the central features of quantum theory -- which celebrated its 100th anniversary this year. One of the most astonishing consequences of quantum theory, first outlined by Erwin Schrödinger, is that a cat could therefore be both dead and alive at the same time. Such a cat is said to be in a quantum superposition of distinct macroscopic states. This year two teams of physicists created such a macroscopic superposition for the first time. Both teams – one at the State University of New York at Stony Brook, the other at the Technical University of Delft – reported evidence for currents of microamps flowing through a superconducting ring in opposite directions at the same time.

Schrödinger's cat comes into view
New life for Schrödinger's cat

4. Silicon lasers shape up

In November, a team of Italian physicists took a crucial step towards creating the first laser made from silicon. The researchers observed optical gain – the prerequisite for laser action – in a device made from silicon 'nanocrystals'. The electronic structure of bulk silicon makes it an inefficient light emitter, so semiconductor lasers are made from materials other than silicon. However, this greatly reduces their compatibility with other silicon-based microelectronics. A silicon laser would revolutionize the semiconductor and telecommunications industries.

First light on silicon laser

5. Laser goes faster than light

Lijun Wang and co-workers at the NEC Research Institute in Princeton made headlines around the world earlier this year when they observed a laser pulse travelling at more than 300 times the speed of light. However, the experiment did not break any of the laws of physics. Wang and colleagues directed the laser pulse into a cell containing caesium gas that had been prepared in an excited state by two other lasers and a magnetic field. In effect the front of the laser pulse 'borrowed' energy from the gas, which was 'repaid' by the tail of the pulse.

Laser smashes light-speed record
No thing goes faster than light

6. Make way for organics

2000 was a year of progress for new organic materials with useful optical and electronic properties, which are easier and cheaper to make than traditional inorganic materials. The year got off to promising start with a new display made from organic semiconductors at Princeton University. The screen – made from thin layers of organic material that emit light when a voltage is applied – is cheap, flat, high-quality and efficient.

Meanwhile a team from Bell Laboratories created a laser based on a crystal of tetracene, an organic material that consists of four linked benzene rings. The device represented a major breakthrough because it was the first time an organic laser had been 'driven' by an electric current – a prerequisite for any practical laser – rather than by light from another laser.
NB: The work at Bell Labs described in this paragraph has since been the subject of an investigation into scientific misconduct and a number of papers have been retracted as a result. More details about the investigation can be found at http://www.lucent.com/press/0902/020925.bla.html. Further information can be found at http://physicsweb.org/article/news/6/9/15

Just a month later, the same team achieved superconductivity in crystals of tetracene and two other related "acene" materials.

Organic laser breakthrough
Organic superconductivity
New light on organic LEDs

7. Universal truths

The field of astrophysics and cosmology had an eventful year 2000, with major discoveries about black holes and the structure of the universe.

In February, physicists at the Gran Sasso laboratory controversially claimed to have found evidence for dark matter particles known as WIMPs – weakly interacting massive particles. The claims were strongly disputed by a rival experiment in the US.

Later in the year two international teams – Boomerang and Maxima – reported conclusive evidence that the universe is 'flat' – that is, the universe contains just enough matter and energy to expand indefinitely without collapsing back in on itself. The evidence came from careful measurements of the cosmic microwave background radiation left over from the Big Bang.

In September astronomers at the University of California at Los Angeles found evidence that a well-known radio source – Sagittarius A* - is actually the black hole at the centre of our galaxy. Shortly afterwards, an international team of astrophysicists detected a medium-sized black hole, which represents the missing link between already-known supermassive black holes and those just a few times more massive than our Sun.

Dark matter claim meets resistance
The universe is flat - official
Milky Way's central black hole located
Astronomers find middleweight black hole

8. Out of this world

Closer to home, astronomers investigating our own solar system made some fascinating discoveries. The Galileo spacecraft found convincing evidence that Europa, one of Jupiter's moons, has oceans of liquid water beneath its icy crust. And according to astronomers at Northern Arizona University, Saturn's largest moon, Titan, has a cloud system similar to the weather on Earth. The Ulysses solar probe gathered valuable data during its second orbit of the Sun this year, during a maximum in its eleven-year cycle of activity. And this month astronomers presented new images of sedimentary rocks on Mars, which strongly suggest that the planet's surface was once dotted with lakes. Just this week, US scientists presented evidence of magnetite crystals on Mars that are startlingly similar to those produced by microscopic bacteria on Earth.

Astronomers have found many planets outside our solar system in recent years – the count stands at over 50 – but in October an international team announced the discovery of 18 planet-like objects that seem to be floating in space rather than orbiting a star. The objects appear to contradict the current theory of planetary formation based on the gravitational influence of a parent star.

Europa: water, water everywhere
Clouds gather on Titan
Ulysses probes the solar maximum
Martian sedimentary rocks suggest a watery past
'Floating planets' challenge theorists

9. Hydrogen goes superfluid

While many materials lose their electrical resistance and become superconducting at low enough temperatures, until earlier this year we only knew of two materials that lose all their viscosity and become superfluid at low temperatures. Now these two substances – helium-3 and helium-4 – have been joined by a new superfluid, hydrogen. Physicists at the Max Planck Institute in Gottingen and the Russian Academy of Science have shown that clusters containing 15 or so molecules of parahydrogen – hydrogen molecules in which the spins of the two protons point in opposite directions – become superfluid at a temperature between 0.38 and 0.15 kelvin. However, it is unlikely that superfluidity will be seen in bulk parahydrogen.

Liquid hydrogen turns superfluid

10. Vibrations of the year

Vibrations – large and small – link three diverse discoveries in physics this year. Researchers at the University of California at Los Angeles identified underground geological structures that can act as acoustic 'lenses'. In September they reported that one such lens focused the vibrations from an earthquake at a location 21 km from the epicentre – resulting in a much higher level of vibration and damage than is usually experienced at such distances.

Also in September, physicists at Hong Kong's University of Science and Technology fabricated 'sonic crystals' just a few centimetres in size that can block out everyday noise. The structure of the crystals allows them to absorb sounds that would normally require a much thicker acoustic shield to stop them.

And finally a physicist at the University of Twente in The Netherlands established that the so-called snapping shrimp stuns and kills its prey using the vibrations caused by cavitating – or collapsing – bubbles. Cavitation is best known for causing damage to ships, but the shrimps create and implode the bubbles between their claws to send out shock waves.

Bad vibrations from acoustic lenses
Sonic crystals make the sound barrier
The bubble bursts for shrimps

Science magazine's top ten highlights of the year include four achievements from the above list: macroscopic quantum superpositions, plastic electronics, possible recent water flow on Mars, and the Boomerang cosmic background radiation experiment.