Highlights of the year
Dec 20, 2001
From the first fraction of a second in the universe to brand new materials, physicists have this year peered deeper into nature than ever before. PhysicsWeb selects ten success stories – and a few tragic tales – from 2001.
1. Magnesium diboride makes its debut
2. Light grinds to a halt
3. Life in the fast lane: ultrashort laser pulses
4. Origins of the universe come to into focus
5. Particles prove theories correct
6. Fine structure constant shifts
7. Condensates claim their crown
8. Superconductors and magnets go organic
9. Nanotubes: from novelty to nanoelectronics
10. Physics gets out and about
11. And now for the bad news
Japanese physicists kicked-started the year with the discovery that a surprisingly simple compound – magnesium diboride – is a superconductor up to temperatures of 39 kelvin, almost double the temperature of any other intermetallic superconductor. This particular combination of elements was apparently overlooked by scientists searching for superconductors among the transition metals in the 1950s.
The breakthrough sparked an explosion of research into the underlying mechanisms of this superconductivity and possible applications for magnesium diboride.
We usually think of light as travelling at 300 million metres per second, but this year two groups of physicists in the US managed to stop laser pulses in their tracks. A kind of "quantum interference pattern" traps the light pulses in a gas of cold atoms, from which they can be made to re-emerge several milliseconds later. Although the light pulses used in the experiments would be several kilometres long in free space, the gas-filled cell in which they were trapped was just half a millimetre across. This achievement comes two years after physicists successfully reduced the speed of light to 17 metres a second.
As the boundaries of optics are pushed back, new and unexpected phenomena are appearing that could be exploited in fields as diverse as quantum information and cosmology. On a more practical level, future developments in fibre-optic communications depend upon our understanding of the fundamental behaviour of light.
Once upon a time, super-fast phenomena – such as the progress of chemical reactions – were invisible to us because even the best probes could not detect changes on sufficiently short timescales. But with the advent of light pulses lasting just femtoseconds – that is 10 -15 seconds - scientists have opened a window on the world of biological and chemical processes.
Even this is too slow for some physical effects: for example, the transitions of tightly bound electrons within atoms occur in just attoseconds – that is 10 -18 seconds. But in November, light pulses lasting hundreds of attoseconds were reported. These pulses were used to the track an ultrafast electronic process for the first time – the ionization of krypton gas.
Physicists also showed this year that the oscillation of visible light in a femtosecond pulse can be used as the pendulum in an 'optical clock', which is around seven times more accurate than the best existing atomic clocks.
Results announced in April of three independent cosmological experiments gave the strongest indication yet that the so-called inflationary model of the universe is correct. The Boomerang, DASI and Maxima projects measured variations in the temperature of the 'cosmic microwave background' with unprecedented accuracy. NASA's Microwave Anisotropy Probe – which was launched in June – is expected to make even more precise measurements of a larger portion of the sky.
Cosmologists believe that the universe underwent a period of extremely rapid expansion when it was less than a second old. As the hot plasma cooled down to form the first light elements, it released an intense flux of photons. Over time – and with the subsequent expansion of the universe – the wavelengths of these photons were stretched to microwave wavelengths, which can be observed today. Fluctuations in the temperature of microwaves across the sky are thought to correspond to the distribution of matter in the early universe.
Astronomers solved a long-standing puzzle in solar physics this year with conclusive proof that neutrinos can change flavour – or 'oscillate' – as they travel from the Sun to Earth. Observations had long shown that the Sun appeared to produce fewer neutrinos than the so-called standard solar model predicted. But the discovery that a fraction of the neutrinos change to a flavour that cannot be detected by existing instruments accounts for the shortfall perfectly.
Particle physicists, meanwhile, reported the first evidence for an effect known as charge-parity violation in B-mesons. Physicists believe that the Big Bang created equal amounts of matter and antimatter particles, and that charge-parity (CP) violation is needed to explain why matter dominates the universe today.
According to results published in August, the fine structure constant – which determines the strength of interactions between charged particles and electromagnetic fields – may not be so constant after all. The effects of the fine structure constant can be seen in the splitting of energy levels in atoms, so a comparison of the spectra of light from quasars of different ages should reveal any change over time.
Astronomers in Australia and the US did exactly this, and concluded that there is just a 0.001% chance that the fine-structure constant has remained the same since the Big Bang – a result that has enormous implications for models of particle physics and cosmology.
Eric Cornell, Wolfgang Ketterle and Carl Wieman received the Nobel Prize for Physics this year for their creation – just six years ago – of the first Bose-Einstein condensates. Since then, this novel state of matter has yielded new insights into the properties of light and matter almost every month. This year, for example, researchers have made condensates with helium and potassium for the first time.
First predicted to exist in 1924, a Bose-Einstein condensate is an ultra-cold gas in which all the atoms are described by the same wavefunction. This means that a condensate – which is a macroscopic object – has some of the characteristics of a single atom. Physicists believe that Bose-Einstein condensates could form the basis of a whole host of new technologies, ranging from atom circuits to quantum computers.
2001 was the year that polymers added superconductivity and magnetism to their repertoire of properties. For scientists at Bell Labs in the US, the superconducting polymer was the latest in a long line of novel organic materials. Made of thin films of poly(3-hexylthiophene), it offered no resistance to electrical current below a temperature of 2.35 kelvin. The temperature at which carbon-60 can superconduct was also doubled - to 117 kelvin - by expanding its crystal lattice.
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
Physicists at the University of Nebraska created a polymer with both ferromagnetic and antiferromagnetic properties by linking chemical segments together so that the polymer units had alternate strong and weak magnetic moments. The material is about twenty times less 'magnetic' than iron, but about a hundred times more 'magnetic' than the first carbon magnet, also discovered this year. Russian scientists stumbled across the magnetic properties of a polymer made from carbon-60 while searching for signs of superconductivity.
To complete the picture, French scientists found that the ultimate organic material – DNA – appears to become a superconductor at temperatures below 1 kelvin, although the mechanism for the effect is unclear.
Recent research into the properties of carbon nanotubes proves that they are rapidly becoming useful and versatile components, rather than just physical curiosities. The tiny rolled sheets of graphite have been shown to possess surprising electronic characteristics since their debut in the early 1990s, and scientists are confident that nanotubes will play a major role in transition from microelectronics to nanoelectronics.
Along with their superconducting properties, carbon nanotubes are sensitive enough to behave as switches that can be turned on and off by just one electron. Such electrical properties led to the demonstration in October of the first logic circuits made from nanotubes.
Away from the frontiers of physics, it was good to see that familiar phenomena and theories are still earning their keep in the world at large. From medicine and finance to engineering and biology, physics continues to prove its value and versatility in a wide range of environments.
Physics and the stock market: playing with fire
Fibre optics detect drunk drivers
The magnetic attraction of learning
Heavy water tests body water
Lasers illuminate the flight of the bumblebee
Super shock absorber could protect buildings
Physicists go for goal
Canaries sing simple harmonics
Science suffers setbacks every year and 2001 was no exception. Following the discovery in 1999 of element 118 – heralded as evidence for the long-predicted 'island of stability' among the heavy elements – scientists at Lawrence Berkeley National Laboratory in the US found that they could not replicate their results. Re-analysis of the original data showed that the earlier result was ambiguous, and the group was forced to retract its claim in August.
Poor financial planning got CERN into trouble this autumn when magnets for the lab's Large Hadron Collider turned out to be far more expensive than originally anticipated. A 20% hike in the final bill – making a total of SFr3.7bn – left CERN's financial directors wondering how they would foot the bill for the world's next major particle physics facility.
In November, the SuperKamiokande neutrino detector in Japan was dealt a serious blow when routine maintenance led to the explosion of most of the detector's 11 000 photomultiplier tubes. At US$3000 each, it could cost up to US$30m to repair the installation. Following the discovery at SuperKamiokande in 1998 that neutrinos do have mass, directors at the lab are adamant that they will rebuild the detector in order to gather enough data to confirm the initial result.
About the author
Katie Pennicott is Editor of PhysicsWeb