Physics

Wednesday, January 01, 2003

70. Japanese Move DNA With Laser


A molecule of DNA is extremely difficult to manipulate, simply because it is so small. So when biologists experimenting with gene therapy want to insert a single strand into a cell, they often wind up injecting dozens. What researchers really need is a tiny pair of tweezers—and soon they may get it. Researchers in Japan recently developed a way to use lasers to grip a DNA molecule at any point along its length, move it to a desired location, and then let it go.
    The technique, developed by physicist Ken Hirano and his colleagues at the University of Tokushima and the Toyohashi University of Technology, suspends DNA molecules in a water-based solution of microscopic polystyrene beads. To grab the DNA, Hirano focuses a laser beam on a point on the strand. The beam creates a slight electrical field that attracts a cluster of about 40 of the beads to that spot on the molecule, much as lint sticks to a polyester jacket. Move the laser, and everything else—the electrical field, the cluster, and the whole strand of DNA—moves with it. Turn off the laser, and the beads simply float away. Hirano hopes these laser tweezers will allow much finer control and analysis of gene-therapy tests. "Maybe we can even knit with DNA molecules," he jokes.
— Jeffrey Winters

2. Neutrino Mystery Solved


Ultraviolet images from the SOHO spacecraft reveal activity in the sun's atmosphere. Today's neutrino detectors can probe much deeper, to the sun's core.
Courtesy of SOHO.

For physicists, 2002 may go down as the Year of the neutrino. In October Raymond Davis Jr. of the University of Pennsylvania and Brookhaven National Laboratory shared a Nobel Prize for detecting solar neutrinos and discovering that the sun emits far fewer than expected of these ghostly subatomic particles—a finding that exposed a serious flaw in our understanding of fundamental natural laws. Last spring an international team of physicists conducted an elegant experiment that finally solved the enigma Davis had uncovered nearly 30 years earlier.
    Standard models of how the sun shines tell exactly how many neutrinos should be created by nuclear reactions in the solar core. Checking those models proved quite a challenge. Neutrinos are so inert, they mostly pass right through Earth, but Davis managed to capture and count a few in an enormous underground detector. He was shocked: He found just one-third as many as theory had predicted. Repeated tallies have since confirmed the solar neutrino deficit. More recently, physicists at the Sudbury Neutrino Observatory in Ontario, Canada, and at the Super-K detector in Japan have provided a possible explanation. Neutrinos are known to exist in three varieties, called flavors, each of which is associated with another subatomic particle. Until recently, physicists could effectively detect only one flavor, the electron-neutrino. According to theory, that is the kind that should be generated by the nuclear fusion of hydrogen in the sun. Some physicists have speculated, however, that certain solar neutrinos might transform en route into the other flavors, making them extremely difficult to find.
    Last year preliminary evidence from the Sudbury and Super-K detectors showed tentative hints of such neutrino transformations but with limited statistical accuracy. Then in April, physicists working at Sudbury announced the results of a challenging new study that compared the total flux of all three neutrino types with the flux of the electron-neutrinos alone. The data showed conclusively that the bulk of the neutrinos had transformed to one or both of the other flavors, known as muon-neutrinos and tau-neutrinos. "We observed clearly that there are significantly more than just electron-neutrinos reaching Earth," says Art McDonald, project director at Sudbury.
    The results imply that, contrary to physicists' assumptions, neutrinos are not massless; otherwise, such transformations would not be possible. That finding is forcing researchers to revamp the standard model of physics, which describes the interactions of all the fundamental particles in the universe. "This is the first major extension of the model in more than 20 years," says Kevin Lesko, a physicist at the Lawrence Berkeley National Laboratory in California. Mass-bearing neutrinos would also account for some of the invisible matter thought to hold galaxies and galaxy clusters together. "Neutrinos are a mystery that we are just beginning to understand," Lesko says.
— Maia Weinstock

42. Physics Beyond the Speed of Light
According to relativity, communicating faster than light is impossible: Your message would arrive before it was sent. So two Canadian physicists raised some eyebrows in January when they announced they had created a signal that traveled at nearly four times the speed of light.
    Alain Haché, a physicist at the University of Moncton in New Brunswick, Canada, transmitted a radio-frequency pulse through a 500-foot cable made of alternating wire segments with different levels of impedance—a measure of resistance to electric current. As the pulse crossed into each segment, some of its wave components reflected backward and slowed down, while other parts reflected forward and sped up. The leading edge of the pulse kept getting faster, eventually attaining super-luminal speed.
    No laws of physics were broken, says Haché: "The pulse is highly distorted, so the peak of the pulse arrives faster than the speed of light. But since most of the pulse's energy is absorbed by the cable, the entire pulse velocity never exceeds that of light." The research could nonetheless get the world moving more quickly. Signals through today's ubiquitous coaxial cables move at about two-thirds the speed of light. Haché's approach might bump them all the way up to the limit of normal physics.
— Jeffrey Winters

65. Evidence Emerges Heisenberg Attempted An A-Bomb for Hitler


Bohr (left) and Heisenberg in happier times, taking a skiing holiday in 1932.
Courtesy of Niels Bohr Archive, Copenhagen.

During World War II, Germany failed to develop an atomic bomb. After the war, the project's lead physicist, Nobel laureate Werner Heisenberg, suggested he had moral qualms about delivering the weapon to the Nazis. That idea, which inspired the hit play Copenhagen, has been called into question by letters released in February from the Niels Bohr Archive in Denmark. They indicate Heisenberg would have developed an atomic bomb for Hitler if he had known how.
    The letters document an enigmatic 1941 meeting between Heisenberg and Danish physicist Niels Bohr. In his memoirs, Heisenberg wrote that he used the meeting to suggest to Bohr that physicists should "ask themselves whether they should work in this field at all." Soon after, the German atomic effort turned from bomb making to nuclear-reactor design, leading some historians to believe that Heisenberg helped maneuver the research toward more peaceful purposes.
    Years later, Bohr labored over a letter he never sent to Heisenberg, apparently intending to set the record straight about their 1941 conversation. "I am greatly amazed to see how much your memory has deceived you," Bohr writes in one draft. "[You gave] me the firm impression that, under your leadership, everything was being done in Germany to develop atomic weapons and that you . . . had spent the past two years working more or less exclusively on such preparations."
    Heisenberg biographer David Cassidy of Hofstra University in Hempstead, New York, says this revelation debunks the revisionist view of the German scientist: "He's not quite as innocent as he'd been portrayed."
— Jeffrey Winters

4. Antimatter Harvested for Experiments


Researchers at CERN, the joint European physics lab in Geneva, have created the first significant quantity of antimatter atoms. By November they had synthesized several hundred thousand atoms of antihydrogen, enough to begin testing whether the laws of physics apply equally to matter and antimatter. The results could help explain a longstanding puzzle: Why is matter ubiquitous, while its mirror twin is exceedingly rare?
    An atom of antihydrogen consists of an antiproton and a positron, the antimatter versions of a proton and an electron. At CERN's Athena experiment, a team of physicists creates the antiprotons by shooting high-speed protons at a chunk of metal. To accumulate positrons, the researchers use a new device that collects them from a radioactive source. Antiprotons and positrons are both charged particles, so they can be slowed and trapped in a magnetic bottle. The particles then combine, forming antihydrogen. These neutral atoms cannot be confined by a magnetic field, so they fly off, collide with surrounding matter and, within 1/10,000 of a second, are annihilated.
    Still, that may be long enough to study the properties of antihydrogen. The next step is to illuminate the antiatoms using a laser. The experiment should produce a signature spectrum of light from the antihydrogen. "Then we'll compare that to the spectrum of hydrogen," says physicist Jeffrey Hangst of the University of Aarhus in Denmark, who coordinates the Athena experiment. Theory predicts the two spectra should look the same. Any discrepancies would provide insight into the physical processes that favor matter over antimatter.
    Further in the future, Hangst and his colleagues would like to examine how antihydrogen responds to gravity's pull. Studying gravitational effects on positrons and antiprotons is extremely difficult because of the particles' electrical charge. Electromagnetism is so much more powerful than gravity that, even with heavy shielding, stray charges tend to muddy the results. Again, the behaviors of matter and antimatter should be identical. "If they aren't, then something's very wrong with our understanding of nature," Hangst says.
— Jeffrey Winters


RELATED WEB SITES:

"Evidence Emerges Heisenberg Attempted an A-bomb for Hitler." Find more information about last year's release of documents pertaining to the 1941 meeting of Bohr and Heisenberg: www.nbi.dk/NBA/webpage.html.

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