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Number 484, May 11, 2000 by Phillip F. Schewe and Ben Stein
THE SPEED OF LIGHT IS INDEPENDENT of the speed of the light source to within one part in 1020. Kenneth Brecher of Boston University (617-353-3423, reaches this conclusion by studying gamma radiation arriving from distant gamma ray bursters (GRBs). Consider, he says, the gamma production at the GRB: radiation will come to us from the near side of a presumed expanding object, and from the receding far side. Because of expected explosive nature of the GRB engine, its near and far sides might, at least in some cases, be moving apart at a fair fraction of the speed of light. Any differential in the speed of light arising from these two gamma-emitting locations would then result (after a very long extragalactic journey) in a stretched-out gamma pulse upon arriving at Earth. In addition, the emitted gamma rays would scatter off of energetic thermal electrons on leaving the burst sources, further broadening the pulses. From the observed sharpness of the arriving pulses, one can deduce the independence of c from the source speed to be less than a part in 1020, an improvement by a factor of 100 billion, says Brecher, over previous tests of this tenet of relativity theory. At last week's APS meeting in Long Beach, CA, Brecher argued that the speed of light is even more fundamental a concept than light itself since it is related to the intimate relation between space and time. Therefore he urged that c be referred to as "Einstein's constant," in analogy to Planck's constant, which sets the scale of quantum measurements.
WHAT HAPPENS WHEN MATTER MEETS ANTIMATTER? Not content to look for annihilation radiation in the heavens, scientists have manufactured their own proton-antiproton and electron-positron collisions in the lab. But what about hydrogen and antihydrogen? Anti-H atoms have been made fleetingly in collisions at CERN and Fermilab but not in a way which allowed detailed study (Updates 366, 253), but this should change in the next few years when, at CERN's Antiproton Decelerator, slow antiprotons and slow positrons are brought together. To prepare for this eventuality, Piotr Froelich of Uppsala University in Sweden (011-46-18-471-3262; piotr@kvac.uu-se) has studied how the four particles (the proton and electron inside the H atom and the positron and antiproton inside the anti-H atom) come together. While modeling this process, he found that when colliding at low speeds, H and anti-H have a tendency to recombine into protonium (proton plus antiproton, abbreviated Pn) and positronium (electron plus positron, abbreviated Ps) before the particles and antiparticles annihilate. Both Pn and Ps are highly unstable but are slightly longer lived than they would be in the absence of the other pairing; that is, the positronium helps to screen the proton-antiproton interaction. Consideration of the delicate balance between the reactive collisions which lead to annihilation, and the non-reactive ones which leave the hydrogen-antihydrogen pair intact, leads Froelich to believe that atoms and antiatoms might suffer each other's presence longer than was thought possible. This will have a practical impact on the study of anti-H atoms in enclosures maintained at high, but not perfect, vacuum conditions. (Froelich et al., Physical Review Letters, 15 May 2000; Select Article.)
FEMTONEWTON FORCE MEASUREMENTS OF DNA MOLECULES are carried out by Caltech physicists Jens-Christian Meiners and Stephen Quake (626-395-3362, quake@caletch.edu) by attaching beads at either end and holding them in separate "optical tweezers," focused laser beams that trap the beads with radiation pressure (see figure at Physics News Graphics). The resulting graph of the cross-correlations between the beads constitutes a sort of force spectrum, from which mechanical and dynamic properties can be deduced. The new Caltech advance is to make such measurements with femtonewton precision, a hundred times better than previous efforts. And all of this is accomplished in a wet, warm environment typical of biology, not in a chilled vacuum. With the new force resolving power, several longstanding problems in polymers dynamics could be solved. For example, contrary to expectation Meiners and Quake showed that the relaxation time of the molecule actually decreases with greater extension. They also showed that an extended polymer is hydrodynamically equivalent to a rigid rod. (Meiners and Quake, Physical Review Letters, 22 May; Select Article.)
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