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发信人: zjliu (Robusting), 信区: Physics
标题: Physics in 2002
发信站: 哈工大紫丁香 (Sat Dec 28 09:39:43 2002) , 转信

2002 has been an exciting year for physicists. From the production of
large numbers of anti-atoms at CERN to the first measurement of the
polarization of the cosmic background radiation, there has been no
shortage of important developments. PhysicsWeb selects its top ten
stories of 2002 -- a year that will also be remembered for two high-
profile cases of scientific misconduct.

1. Anti-atoms at CERN
2. Cosmic microwaves reveal polarization
3. New results confirm neutrino oscillation
4. Defying the second law
5. Advances in Superconductivity
6. Ultra-cold atoms research continues to make advances
7. Magnets in nanoscale logic devices
8. Neutrons used to measure quantum gravitational effects
9. First evidence for 'tetra-neutrons'
10. Bright times in optics
11. Low points of the year
12. Hope for the future

1. Anti-atoms at CERN

Large numbers of cold anti-atoms were produced this year for the
first time. In September the ATHENA experiment at CERN made 50 000
cold antihydrogen atoms by combining antiprotons and positrons in a
series of magnetic and electrostatic traps.

Two months later the ATRAP experiment, which is also based at CERN,
produced an estimated 170 000 antihydrogen atoms. This group was also
able to study the internal states of antihydrogen for the first time.
Both groups used similar techniques to make the anti-atoms, but
different detection methods.

The ultimate goal of both experiments is to compare the energy levels
of hydrogen and antihydrogen atoms in detail and perform the most
accurate ever tests of CPT (charge-parity-time) symmetry. Any
violation of CPT symmetry -- which would require new physics beyond
the Standard Model of particle physics -- would appear as a slight
difference in the frequency of electronic transitions between the
ground state and the first excited state in hydrogen and the
corresponding transition in antihydrogen.

2. Cosmic microwaves reveal polarization

Astronomers in the US detected the polarization of the cosmic
microwave background -- the microwave 'echo' of the Big Bang -- for
the first time. The researchers used the Degree Angular Scale
Interferometer (DASI) at the South Pole to make the measurements,
which agree with predictions and provide further support for the
standard Big Bang plus 'inflation' model of cosmology. The results
also confirm that ordinary matter accounts for less than 5% of the
total mass and energy of the universe.

Earlier in the year, the Cosmic Background Imager produced the
sharpest ever images of the cosmic background. These images allowed
astrophysicists to see for the first time the tiny density
fluctuations that became our present-day galaxy clusters.

Ultimately polarization experiments may be able to investigate the
Universe in the very first fractions of a second after the Big Bang --
when it underwent a period of extremely rapid expansion known
as 'inflation'. If the inflation model is correct, then gravitational
waves emitted during this period will leave a signature on the
polarization of the cosmic background.

3. New results confirm neutrino oscillation

In April, physicists at the Sudbury Neutrino Observatory (SNO) in
Canada presented conclusive new evidence that electron neutrinos
oscillate -- or change 'flavour' -- on their way from the Sun to the
Earth.

Previous results from SNO and the Superkamiokande experiment in Japan
had strongly suggested that neutrinos can oscillate. Oscillation can
only occur if neutrinos have mass -- a finding that requires new
physics beyond the Standard Model.

Later in the year, the KamLAND experiment confirmed that electron
anti-neutrinos also oscillate.

This year's Nobel Prize for Physics recognized two astrophysicists
who pioneered the fields of neutrino astrophysics. Ray Davis of the
University of Pennsylvania and Masatoshi Koshiba of the University of
Tokyo shared half the prize. Riccardo Giacconi received the other
half for his contribution to X-ray astronomy.

4. Defying the second law

The second law of thermodynamics says that the entropy or disorder of
an isolated system undergoing a cyclic process will increase or
remain the same. In July, however, Australian researchers showed that
entropy can decrease over short time periods for small systems. This
is the first time that a deviation from the second law has been
demonstrated experimentally.

The researchers state that the discovery could be important in the
design of micromachines, and argue that the probability of
thermodynamic systems running 'in reverse' will increase as they
become smaller. This could have important consequences for
nanotechnology and could even give researchers an insight into how
life itself functions.

5. Advances in Superconductivity

Two more elements joined the ranks of the superconductors this year.
Physicists in Japan and the US found that lithium becomes
superconducting when subjected to extremely high pressures.
Superconductivity was also observed in a plutonium-based material for
the first time. Researchers in the US and Germany discovered that an
alloy of plutonium, cobalt and gallium exhibited superconductivtiy at
temperatures below 18.5 Kelvin. The material also has a large
critical current, which would be of technological importance if it
were not for the hazardous radioactive properties of plutonium.

The properties of magnesium diboride -- which made its debut as a
superconductor in January 2001 -- were also investigated further this
year. Theorists in the US proposed that its relatively high
transition temperature could be explained by the presence of two
superconducting energy gaps rather than one.

6. Ultra-cold atoms research continues to make advances

Research on ultra-cold atoms has been strong this year, with advances
being made in the study of both Bose-Einstein and degenerate Fermi
gases.

A Bose-Einstein condensate is a novel state of matter in which all
the atoms collapse into the same quantum state. A degenerate Fermi
gas is the equivalent condensation for atoms that obey Fermi-Dirac
statistics.

The year began with the first ever observation of a quantum phase
transition in a condensate. This occurs when the atoms go from all
having the same quantum phase and being able to move about without
friction to one where the atoms are no longer able to move freely.

Physicists also showed that the presence of a Bose-Einstein
condensate could trigger the collapse of a Fermi gas -- raising hopes
that superfluidity could be observed in Fermi gases. Researchers also
reported on the unusual expansion characteristics in an ultra-cold
Fermi gas this month.

Caesium joined the condensates in October when researchers made a
Bose-Einstein condensate with this element for the first time.

7. Magnets in nanoscale logic devices

Physicists in the UK built a nano-metre scale logic gate made
entirely from metal that works at room temperature. In existing
electronic circuits, logic operations are carried out by
semiconductor devices. However, the density of electrons flowing
through a semiconductor is limited and this restricts how small these
devices can be made. Metals have higher electron densities than
semiconductors so a metallic logic gate could be made smaller than a
semiconductor one.

If such devices could be built, they would be ideal for mobile
applications such as phones and smart cards because the data could be
stored without a power source.

8. Neutrons used to measure quantum gravitational effects

The quantum properties of the electromagnetic force are seen in many
phenomena, such as the electronic orbits in atoms and the structure
of nuclei. However, it is extremely difficult to make analogous
observations in gravitational fields because the effect of gravity is
negligible at the atomic scale.

However, physicists at the Institute Laue-Langevin (ILL) observed
quantized states of motion under the influence of gravity for the
first time in 2002. The ILL team used ultra-cold neutrons to make
their measurements, and the findings could be used to study
the 'equivalence principle' and other fundamental aspects of physics.

9. First evidence for 'tetra-neutrons'

It is important to understand the interactions between nucleons to
understand the nucleus. Physicists already know that pairs of
neutrons can exist in an 'almost bound' state, and they have spent
many years trying to find evidence of these and higher numbers of
neutron clusters. However, these experiments are difficult because
neutrons or clusters of neutrons have no charge.

In May researchers working at the GANIL accelerator in France
reported the first evidence for 'tetra-neutrons' -- nuclear clusters
containing four neutrons and no protons. They found six possible
candidates for the four-neutron clusters among fragments of neutron-
rich beryllium nuclei. If confirmed, the findings could greatly help
in our understanding of nuclear forces.

10. Bright times in optics

Optical physicists made progress in many directions in 2002.
Researchers in G€牔鰐tingen used conventional optics to image clumps ofbacteri
a just 33 nanometres across -- equivalent to a mere 1/23 of
the wavelength of light used to illuminate them. The achievement
shows that 'far-field' optical microscopes can operate well beyond
the so-called diffraction limit. Meanwhile physicists in Bielefeld
and Vienna continued last year's revolution in the generation of
attosecond laser pulses by using such pulses to investigate the
dynamics of the electrons in krypton atoms.

There was also a series of firsts in the realm of quantum optics: the
first quantum NOT gates, the storage of two bits of information on a
single photon, and the near-perfect cloning of a photon (perfect
cloning is forbidden by the laws of quantum mechanics). Physicists in
the UK and Germany also set a new distance record for the
transmission of a "quantum key" in free space. Such keys are
essential components of secure communication systems.

11. Low points of the year

Two physicists were fired from prestigious labs in 2002 for
scientific misconduct. Victor Ninov was dismissed by the Lawrence
Berkeley National Laboratory in June after a committee found that he
had fabricated data concerning the discovery of element 118 -- which
would have been the heaviest element ever made. Jan Hendrik Sch€牔鰊 was
sacked by Bell Labs in September after a different committee found
him guilty on 16 out of 24 charges of scientific misconduct. Sch€牔鰊
had published over 100 papers over a five-year period, most of them
on the properties of organic semiconductors.

12. Hope for the future

More than 300 physicists from around the world -- most of them women -
- met in Paris in March for the first International Conference on
Women in Physics. The meeting, which was organized by the
International Union of Pure and Applied Physics, unanimously approved
eight recommendations to enable women to contribute more effectively
to physics. The recommendations were aimed at schools, universities,
industry, government and funding councils. The delegates have now
returned to their home countries, where they hope to turn words into
action.

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