TOP TEN PHYSICS STORIES OF THE YEAR
Former presidential science advisor Vannevar Bush referred to science as an endless frontier of new discoveries. So what were the big physics findings for 2008? The following list was chosen by editors and science writers at the American Institute of Physics and the American Physical Society. It winnows a wealth of discoveries into the following ten topic areas, which are listed in no particular order.
What’s new-discovery of an unusual class of materials made from iron and arsenic. Superconductors don’t lose any energy when electricity runs through them, providing they’re chilled to very low temperatures. Superconductors are used in specialty applications where high electrical currents are needed, such as in MRI scanners at hospitals or in the magnets used to steer particles at atom smashers. There are two reasons that superconductors aren’t used more widely, such as for carrying electrical power: superconductors need lots of cooling apparatus and it’s hard to turn the material into miles-long strands of wire. The new iron-arsenic materials are the first relatively high-temperature materials that remain superconducting above a temperature of 50 K that don't contain copper; the copper materials are brittle. Researchers hope that the iron-arsenic version might lead to the more practical manufacture of superconducting wire. Furthermore, having a new class of materials to study should help theorists understand how high-temperature superconductors work in the first place.
Background: A summary of work in this area can be found at Physics Today, May 2008 (http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_61/iss_5/11_1.shtml ); APS survey of topic, http://physics.aps.org/articles/v1/21; overview and text for a specific article in the journal Physical Review Letters (PRL) at http://physics.aps.org/articles/v1/28
LARGE HADRON COLLIDER
What’s new---the LHC, the world’s largest scientific instrument, started operations in September. At this huge particle accelerator, located underground near Geneva, Switzerland, two beams of protons, each traveling at unprecedented speeds will be smashed together. The goal is to create exotic new particles that can’t be observed in any other way except in the tiny fireball created by such violent collisions. These collisions have not yet occurred, but physicists did succeed in sending proton beams in both directions around LHC’s 27-km (16 mile) circular path. Problems with some of the apparatus forced a premature shutdown shortly thereafter. General operations should resume in summer 2009. Background: a summary of the magnet malfunction which brought testing to a halt in September and a timetable for operations are available at http://press.web.cern.ch/press/PressReleases/Releases2008/attachments/CERN_081205_LHCrestart.pdf
What’s new-planets orbiting distant stars have been imaged directly, and a host of interesting results have come back from spacecraft hovering near the planets in our own solar system. Extrasolar planets, planets orbiting far-away stars, had been detected indirectly by watching what happens to the light coming from the star. But now the glare of the star has been blocked sufficiently that the extrasolar planet itself could be imaged. The Gemini, Keck, and Hubble telescopes provided pictures. Background summary at http://blogs.physicstoday.org/update/2008/12/images_of_exoplanets_orbiting.html
In our own solar system, at Mercury, the Messenger spacecraft (which will be the first to orbit the planet) made first-ever maps of large portions of the surface. It also determined that Mercury’s magnetic field is highly symmetric. Background at http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=276 . At Saturn, the Cassini craft found geysers near the south end of the moon Enceladus. Background: http://www.nasa.gov/mission_pages/cassini/multimedia/pia08386.html ). At Mars, measurements made by several craft strengthened evidence in favor of sub-surface glaciers outside the polar regions. Meanwhile, the Venus Express craft recorded pictures at several wavelengths, facilitating, among other things, a better knowledge of clouds on Venus. UV pictures of the planet: http://www.stfc.ac.uk/resources/image/venus1.jpg
What’s new-unusual combinations of quarks were observed for the first time. Physicists believe that an atom consists of one or more electrons orbiting a central nucleus. The nucleus, in turn, is made of protons and neutrons, and these particles are made of something still more elementary-quarks held together by gluons. Most nuclear particles are made from two quark types: “up” and “down” quarks. In addition to this there are four other quark types. One discovery consists of the sighting of nuclear particles containing rare “bottom” quarks. At Fermilab’s D0 experiment, a particle (a bottom-quark version of the omega hyperon) containing two "strange" quarks and one bottom quark was detected; background at Physics Today, Nov 2008 (http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_61/iss_11/20_1.shtml ); results published in PRL. The very lowest-energy state of bottomonium, a family of bound states consisting of a bottom and anti-bottom quark, was observed at SLAC’s Babar detector; background at Physics Today, Sept 2008 (http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_61/iss_9/14_1.shtml ); summary and PRL text at http://physics.aps.org/articles/v1/11 ; press release at http://home.slac.stanford.edu/pressreleases/2008/20080709.htm At the KEK lab in Japan several meson-like particles believed to contain four (not the usual two) quarks were observed in the Belle detector; publication in PRD, background in Physics Today, June 2008 (http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_61/iss_6/18_1.shtml ). Finally, there has been progress in predicting the masses of nuclear particles using computer simulation of quark interactions (see results in Science, 21 Nov and an assessment at Nature magazine, 27 Nov).
FARTHEST SEEABLE THING
What’s new-seeing a flash of light from 7 billion light years away. One of the brightest of all celestial objects is gamma-ray bursters, objects that emit immense amounts of gamma radiation, the highest-energy form of light. The brightest-ever gamma ray burster was observed by the Swift satellite-specially designed to detect gamma rays-and other telescopes as well. They deduce that this burst event came form a place in space 7 billion light years away, and was bright enough to have been observable by the naked eye. Since looking out into space is equivalent to looking back in time, this flash would have been coming from a moment when the universe was only half its present age. Publication in Nature, 11 Sept; image at http://swift.gsfc.nasa.gov/docs/swift/results/releases/images/GRB080913/
What’s new-first ever accumulation of molecules in large numbers and at a temperature near absolute zero. Using lasers to slow a gas of particles down to near stillness is by now a standard method for measuring the subtle properties of atoms. Steven Chu, nominated to be the Secretary of Energy, won a Nobel Prize for pioneering this subject. Cooling molecules in this same way is difficult since molecules, made of two or more atoms, have complicated internal motions. But this year several labs succeeded in first cooling atoms and then, at a temperature close to absolute zero, getting them to combine into molecules. Labs at NIST/Colorado (Science, 10 Oct) and at the University of Innsbruck (PRL, 26 Sep) got atoms to pair up into molecules and collect in high densities at very low temperatures inside traps. The NIST experiment produces molecules from rubidium and potassium atoms (publication in Science). Innsbruck researchers first placed rubidium atoms in an optical lattice before condensing them into molecules.
Background at http://www.aip.org/pnu/2008/split/875-1.html; figure http://www.aip.org/png/2008/306.htm; PRL text and overview at http://physics.aps.org/articles/v1/24
What’s new-getting little imperfections in diamond to tell us about how atoms behave like tiny magnets. Diamonds are much favored for their hardness and for their clarity, which makes them popular in jewelry. But they might also be useful in creating a new type of electronic circuit. Diamond is made of a cross-linking of carbon atoms. If one carbon atom is missing from this network, the empty hole, in combination with a stray nitrogen atom, acts as a sort of strange molecule in the middle of all those carbon atoms. This "molecule" can light up like a little LED when you shine laser light in. This in turn, can be used to measure extremely weak magnetism. Possible applications include data storage for computers or high-sensitivity detectors. Manipulating the spin of an electron caught in a vacancy within a diamond sample, scientists from Delft University of Technology and the University of California at Santa Barbara detected the spin of a single electron (Science, April 18); while a Harvard group (Nature, 2 Oct) located the position of a single carbon-11 impurity in diamond to within 1 nanometer through the carbon atom's nuclear spin interactions; see news summary at http://www.aip.org/pnu/2008/split/858-1.html.
What’s new-experiments settle one mystery and uncover others. Cosmic rays are super-high-energy particles whizzing through the cosmos. When they smash into our atmosphere the rays turn out mostly to be ordinary particles, such as protons or electrons, but with energies thousands or millions of times higher than particles speeded up at accelerators on Earth. Here are the new results. (1) One cosmic-ray detector, the Pierre Auger Observatory, now definitely observes a decrease of cosmic rays at the very highest energies (at energies above 4 x 10^19 electron volts); this clears up the mystery associated with some previous observations suggesting an excess of such events; background and PRL article, http://physics.aps.org/articles/v1/9 . (2) The Milagro experiment produced a sky map of incoming cosmic rays. This sets up a new mystery since they observe that some very high energy events (at the level of 10 trillion electron volts) seem to come preferentially come from just a few directions in space (background: summary and PRL text at http://physics.aps.org/articles/v1/37 ). (3) Another mystery pertains to the findings of two detectors held aloft-one by a balloon and one on a satellite-looking for oddities in the number of antiparticles arriving with regular particles among cosmic rays reaching Earth. They see an excess of such particles which some interpret as evidence for “dark matter,” a class of very-weakly-interacting particles not seen before. Scientists associated with the balloon-borne ATIC detector (Nature, 20 Nov) and the satellite PAMELA (http://arxiv.org/abs/0810.4995)
report evidence for an excess of cosmic-ray electrons (and maybe also positrons) at energies of hundreds of GeV. This finding is accounted for in some explanations by the annihilation of heavy dark-matter particles (news item in PT at http://blogs.physicstoday.org/update/2008/11/signs_of_dark_matter.html )
LIGHT PASSES THROUGH OPAQUE MATTER
What's new---getting light to behave in a new way. When light strikes an opaque material like milk most of the radiation is scattered; little of it passes through the sample. But in an experiment at the University of Twente in the Netherlands, much more of the light can be made to traverse the scattering material if beforehand the wavefront of the incoming light is shaped by special filters. Background: summary in Physics Today, Sept 2008 (http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_61/iss_9/20_1.shtml ) and an APS essay and research paper in PRL are available at http://physics.aps.org/articles/v1/20
MACROSCOPIC FEEDBACK COOLING
What's new---Scientists at the AURIGA lab in Padova, Italy have cooled a one-ton aluminum bar to a temperature below 1 milli-kelvin using special electrical circuits. The bar is part of a detector designed to measure passing gravity waves from space. Using sensitive magnetic sensors and feedback coils, the ringing of the bar (which is essentially a large tuning fork) at one characteristic frequency was cooled from an equivalent temperature of 4 K (the temperature of the bath of liquid helium in which the bar sits) to a temperature of about 0.17 mK. Lower temperatures than this have been achieved with this feedback cooling technique but only with much smaller masses. Background: essay and PRL article at http://physics.aps.org/articles/v1/3