This year saw many advances in physics. PhysicsWeb has compiled a list of ten highlights (in no particular order) of 1997.
Physicists at the Massachusetts Institute of Technology made the world’s first atom laser in 1997. The team started by cooling sodium atoms in a magneto-optical trap to form a Bose-Einstein condensate. This is a new state of matter in which all the atoms are in the same quantum state. By applying a short radiofrequency pulse to the trap, a laser-like beam of atoms can be released from the trap.
A crucial feature of any laser is its coherence. The MIT team demonstrated the coherence of the condensate by using an ordinary laser to split it into two, and then recording an interference pattern as the two condensates fell under gravity.
The MIT groups describes its atom laser as rudimentary. An operational atom laser would have applications in atom optics, atom lithography and precision measurements.
1997 has been a great year for planetary science. On Independence Day NASA celebrated its first lander mission in over twenty years with a successful touchdown of the Pathfinder spacecraft on Ares Vallis, the mouth of a suspected outflow channel. The lander contained the “Sojourner” rover which moved across the surface sampling different types of rocks. The mission has special revelance to climatologists as data from the probes indicated that Mars was indeed significantly wetter in its past. The first papers from the mission suggest that rocks visualised by the rover had undergone fluvial erosion, comfirming the belief that Pathfinder had landed on a flood plain. Rocks were also found to be similar to Earth rocks.
The Pathfinder Web site has also entered the record books with more visitors in a single 24 hour period than any other Web site.
Mars Observer, Pathfinder’s sister craft, quietly went into orbit around the red planet with a series of breathtaking aerobraking maneourves. Recently cameras on the spacecraft noticed the start of a dust storm in the southen hemisphere which could provide valuable data on Martian dust storms. Information from both spacecraft have reminded scientists how similar Mars is to Earth, and generated significant public interest about the possibility of life on Mars in the past.
1997 was the year in which researchers got serious about applications of carbon nanotubes. Previously these nanometre-sized tubes of carbon were only available as bundles of tubes with different diameters or as concentric multiwall tubes. The breakthrough came when researchers at CNST in the US developed a reliable way of making single-wall nanotubes with well-defined diameters.
Nanotubes are characterized by two numbers – usually m and n – which determine many of their properties, including their diameter and whether a given tube is metallic or semiconducting. Nanotubes are also expected to behave as one-dimensional quantum wires and to exhibit many unusual electronic properties. Measurements on single nanotubes have confirmed many of these properties and researchers at Berkeley are now investigating the possibility of building all-carbon nanometre-sized electronic devices. Other experiments have shown that nanotubes also have useful mechanical properties.
Recent years have seen exciting progress in a new field of physics that goes by the name of quantum information technology. The field is essentially based on the ability of quantum particles to be in two or more places at the same time. In the mid-1980s it was shown that a quantum computer could, in theory, perform calculations much faster than is possible on a classical computer. Two years ago the first simple quantum logic gates were demonstrated – just one of a series of experimental and theoretical breakthroughs that continued in 1997.
Quantum information technology essentially involves three subjects: quantum computation, quantum cryptography (in which quantum entanglement is exploited to achieve completely secure communication), and quantum communication, in which quantum properties are used to communicate in ways that are not possible classically. The most recent breakthrough in the field is the quantum teleportation of the polarization of a photon at the University of Innsbruck.
In addition to its potential for applications, the techniques being used in quantum information technology are also revolutionizing studies of the fundamentals of quantum theory.
1997 sees the end of the Galileo spacecraft’s primary mission. In the past two years the probe has sent over a gigabyte of data and hundreds of photographs back to Earth. Its most notable achievements were the discovery of a magnetic field around the moon of Ganymede, volcanic ice flows on Europa’s surface (which supports the premise of liquid oceans underneath the icy crust), and the presence of metallic cores inside Europa, Io and Ganymede, but no evidence for one in Callisto.
The spacecraft has sent a probe into the Jovian atmosphere, and discovered volcanic activity on Io is more violent than previously thought, with dramatic changes since the Voyager spacecraft first took pictures. It has also discovered Callisto atmosphere contains hydrogen and cardon dioxide.
The Standard Model of particle physics has survived for more than two decades despite regular hints of “cracks in the edifice” from accelerator labs and compelling evidence from theory and astroparticle physics experiments that there is “new physics” beyond the model. Results announced by the DESY Research Centre in Hamburg earlier this year are the strongest evidence to date for such new physics.
In the Standard Model there are two types of matter particles – quarks and leptons – and four forces (gravity, the electromagnetic force and the strong and weak nuclear forces). The matter particles all have half-integer spin (i.e. are fermions) and come in three families. The particles that transmit the forces (photons, gluons and the W and Z particles) all have integer spin (i.e. are bosons).
Collisions between positrons and protons at DESY probe the strong force, which most physicists believe is described by the theory of quantum chromodynamics. However, two multinational experimental teams at DESY – working on the H1 and Zeus detectors – have detected more events under certain conditions than are predicted by the Standard Model. The odds of these results being statistical fluctuations are about one in a thousand. Possible explanations of the results are a new particle called a leptoquark or evidence of substructure in quarks.
The results announced in February were based on data taken between 1994 and 1996. DESY researchers are currently analyzing a run earlier this year which acquired twice as much data.
In the begining of the year, astronomers made the first visible observations of an object causing a gamma-ray burst. The event was still being witnessed by the Hubble Space Telescope some six months after the burst happened. The continued visibility of the object, and its rate of decline over time, supported the theory that the event is from a “relativistic” fireball expanding near the speed of light. This also suggests that such objects occur far outside our own galaxy. The decline in the visible spectrum is as predicted by present fireball theories.
Experiments don’t come any bigger than the Large Hadron Collider (LHC) at CERN, the European particle physics lab in Geneva. And there is no bigger prize in particle physics than the Higgs boson, the particle (or particles) though to be responsible for the generation of mass – and the main raison d’etre of the LHC. But big experiments cost big money and this December the US agreed to contribute $531 million to the construction of the LHC and its two detectors, ATLAS and CMS.
The US contribution will cover about 10 per cent of the costs of the LHC and will also enable the collider to be built by 2005 – three years earlier than would have been possible otherwise. The deal is notable for other reasons: it is the first time that the US has contributed to the construction of a particle physics experiment outside the US, and it should mark the end of the bad feeling that has existed since the cancellation of the US’s Superconducting Super Collider – an even more ambitious experiment to discover the Higgs – in 1993.
Electric charge normally comes in an indivisible unit: the charge of an electron. Indeed, quarks are thought to be the only particles with fractional charge – and they only exist in particles that have a integer charge. But this year, physicists from Weizmann Institute of Science, Israel and the CEA laboratory near Paris revealed the first direct evidence that an electric current can be carried by quasiparticles with fractional charge.
The results agree with a theory which was formulated by Robert Laughlin in 1982 to explain the fractional quantum Hall effect. According to Laughlin, electrons in strong magnetic fields form an exotic new collective state, similar to the way in which collective states form in superfluid helium. A quantum of magnetic flux and an electron exist as a quasiparticle that carries the electric current.
When a group of physicists decided to perform an experiment which, they hoped, would “lead to a wider appreciation of the importance of magnetism in the world around us”, they can hardly have expected to feature on front pages and television bulletins around the world. But that is what happened to a team from the universities of Nijmegen and Nottingham when they levitated a frog in a 16 tesla magnetic field.
The frog floats in mid-air because, like every material and living creature, its possesses molecular diamagnetism. Although this is typically millions of times weaker than ferromagnetism, it means that a frog can be levitated if it is placed in a magnetic field with a high enough gradient. Animal lovers will be pleased to know that the frog was perfectly safe inside the magnet and that afterwards it “returned to its fellow frogs in the biology department.”