Thursday, January 12, 2012

Ray laser 1985: Physics


1985: Physics

Archives consist of articles that originally appeared in Collier's Year Book (for events of 1997 and earlier) or as monthly updates in Encarta Yearbook (for events of 1998 and later). Because they were published shortly after events occurred, they reflect the information available at that time. Cross references refer to Archive articles of the same year.
1985: Physics
Superstrings.
Physicists have long believed that the most elementary constituents of matter are 'point' particles that do not occupy any volume in space. While such particles may have properties such as mass and electric charge, they would not have a length, width, or height. Considerable discussion this year, however, surrounded a new theory that has increasingly captured the fancy of theoretical physicists. It conjectures that the most elementary particles—the quarks (which combine to make protons, neutrons, mesons, and the like) and the leptons (such as electrons and neutrinos)—are not, in fact, points. Based on an older idea that has been revived by Michael Green of Queen Mary College in London and John Schwartz of the California Institute of Technology in Pasadena, this 'superstring' theory holds that the elementary particles are one-dimensional entities; that is, they have a single dimension, length, in space—hence the idea of a string. Not only do the particles become miniature strings in this theory, but the universe they inhabit is no longer the familiar four-dimensional one with three space dimensions and one time dimension. Instead, the universe becomes a ten-dimensional object.
How could something so contrary to everyday experience be true? Compactification is part of the answer. In this little-understood process, the six dimensions that we do not experience get curled up to such a small size that they are not experimentally detectable. Although not a result of the compactification process, the length of a string representing an elementary particle is also very short, 10-33 centimeter, another undetectably small distance.
Bizarre as it all sounds, it is possible that the superstring approach will evolve into a unified theory explaining all the forces of nature—a theory uniting in a single mathematical framework gravity, electromagnetism, and the two nuclear forces called strong and weak (which show up in the behavior of atomic nuclei and their components). Past attempts by physicists to construct such a unified quantum field theory were based on pointlike particles and were plagued with problems. So far, the superstring theory appears to be the best hope for overcoming these obstacles.
Albert Einstein tried for years to unify gravity and electromagnetism but failed, as did several other physicists. The hope for unification through superstrings traces back to a 60-year-old idea of the German-Polish mathematician Theodor Kaluza and the Swedish physicist Oskar Klein. They suggested that gravity could be mathematically unified with electromagnetism if the universe were regarded as consisting of five dimensions. Through the superstring theory, it is hoped, gravity can be unified with all three other fundamental forces in a universe of ten dimensions where six are compactified. Theoretical work by researchers has already shown that compactification makes it possible to reproduce some of the features of quantum field theories of these three other forces.
Local Lorentz Invariance.
Physicists require that any physical law be independent of the velocity of the frame of reference in which measurements are done. This principle, called local Lorentz invariance, is part of the more general Einstein equivalence principle, which says that physical laws are the same at all times and everywhere in the universe. The validity of the equivalence principle cannot be proved, but it would be difficult to construct useful models of the universe, such as the superstring theory, without it. Scientists have been able to carry out experiments establishing limits on the magnitude of any possible violation of local Lorentz invariance. In 1985, experiments setting tighter limits than ever on how large the violations could be were reported by John Prestage and colleagues at the Boulder, Colo., laboratories of the U.S. National Bureau of Standards and Blayne Heckel, Frederick Raab, and coworkers at the University of Washington at Seattle.
Local Lorentz equivalence could be violated if not all frames of reference are equivalent—that is, if there exists some 'preferred' frame of reference in the universe. The idea behind the NBS and Washington experiments was that if there is such a preferred frame of reference, the frequency (or wavelength) of light absorbed or emitted by an atom might depend on the orientation of the atom relative to its motion through that frame of reference. (Atoms of an element absorb or emit only light of certain frequencies.) If a change in frequency were observed, this would mean that the laws of electromagnetism that govern light depend on the motion of the atoms with respect to the preferred frame of reference—which would be a violation of local Lorentz invariance.
In any case, atoms can be oriented with respect to an applied magnetic field by the absorption of light from a laser or other high-intensity light source. Since the atoms are in a fixed orientation to the magnetic field, as the earth rotates (carrying the magnetic field and the collection of atoms with it), the orientation of the atoms with respect to the putative preferred frame of reference changes. The recent tests consisted in searching for changes in frequency over the course of a 24-hour day.
The NBS and Washington groups used somewhat different techniques in testing for frequency shifts, but the results were the same: no measurable shift was found. The limit set by the NBS researchers on how large a possible frequency shift could be was much smaller (5 x 10-15 the size) than the equivalent result of a famous 1887 experiment by the American physicists Albert Michelson and Edward Morley. (By failing to find a change in the travel time of light through the arms of an interferometer device as the earth rotated, Michelson and Morley helped disprove the existence of the ether, a hypothetical ethereal substance that physicists in those days thought filled the universe and whose only role was to carry electromagnetic waves.) The Washington physicists set even tighter limits, some 20 times smaller still.
Laser Cooling.
A technique called laser cooling was a key part of the NBS experiment, which used ions (atoms with a net electric charge) of beryllium. As a result of absorbing and reradiating thousands of photons from laser light of a suitable wavelength, each ion was cooled to an effective temperature of 0.1 kelvin (a tenth of a degree Celsius above absolute zero). The low temperature meant that the ions moved slowly, which made them a nearly ideal sample for measurements of their spectra—that is, the frequencies at which they absorb or emit light. Because of their electric charge, ions can be trapped with relative ease by an appropriate configuration of electric and magnetic fields. These act as a kind of bottle to hold the particles, which are then cooled.
Trapping neutral atoms, which are often preferred over ions as subjects for experiments, is much harder because they have no net electric charge to interact with the trapping fields. In 1985, however, two groups reported that they had confined sodium atoms for as long as one second—long enough for many kinds of measurements. Both groups began with beams of sodium atoms traveling at a high velocity (several hundred meters per second). In contrast to ions, where trapping precedes cooling, atoms must be cooled before confinement. For both groups laser cooling played a key part in slowing the atoms down.
The first group (William Phillips and colleagues at the National Bureau of Standards at Gaithersburg, Md. and the State University of New York at Stony Brook) cooled the sodium atoms until they were stopped inside the trap, which consisted of two coils of wire that generated a weak magnetic field. The interaction between the field and the tiny magnetic field of a spinning atom is just enough to trap a slowly moving atom or repel it, depending on its orientation. In the magnetic field of the trap, the atoms all have the same orientation.
The second group (Steven Chu and coworkers at AT&T Bell Laboratories in Holmdel N.J.) tried a somewhat different approach. Having 'precooled' the sodium atoms to a certain velocity, the Bell Labs scientists then turned on a laser system comprising six light beams, one for each of the two opposite directions along three mutually perpendicular axes. The six beams do not trap the atoms, but they have dramatic effects. One is to cool the atoms to a very low temperature of 0.00024 kelvin. The second is to make it very difficult for the atoms to escape from the region irradiated by the laser beams. No matter in what direction the atoms try to move away, they are pushed back. The researchers, comparing the situation to trying to walk through a viscous fluid, call their method optical molasses. Eventually, however, the atoms diffuse out—in a fraction of a second in this experiment.
X-Ray Lasers.
Lasers for cooling sodium atoms emit light in the visible region of the spectrum, where a good deal of the spectroscopic study of atoms is carried out. For other applications, scientists have dreamed for years of an X-ray laser. Such a device emitting long-wavelength, or 'soft,' X-rays of about 30 angstroms would, if powerful enough, make possible X-ray holographic images of biological molecules within living organisms, thereby giving three-dimensional structural information not now obtainable. (By comparison, the wavelengths of visible light range from about 7,400 angstroms, or 7.4 x 10-7 meter, to 4,000 angstrom, or 4 x 10-7 meter.) X-ray lasers are also under consideration as a means of defense against intercontinental ballistic missiles in the so-called Star Wars system. Scientists at the Lawrence Livermore National Laboratory in Livermore, Calif., have tested an X-ray laser with this goal in mind. Because a low-yield nuclear bomb powers the laser, the project is classified and few details about it are known. A report of technical failures surfaced in late 1985.
On the unclassified side, 19 physicists headed by Dennis Matthews at the Livermore laboratory reported in late 1984 that they had successfully created a laser emitting radiation of wavelengths 206 and 209 angstroms. Most physicists call this region of the spectrum the extreme ultraviolet or XUV, although it also borders on the soft X-ray region. As a general rule, as the wavelength of laser radiation decreases from the visible range into the ultraviolet and then the X-ray region, it is necessary to pump increasing quantities of energy into the 'lasing' material at ever faster rates. This requirement accounts for the difficulty of making an X-ray laser and for the use of a nuclear bomb in the classified project.
The energy source for the unclassified XUV laser was the giant Novette laser, which could produce light in the infrared and visible ranges and at the time was the world's most powerful laser. In the XUV-laser experiment, Novette's extreme power vaporized a selenium target in the shape of a thin foil, creating a plasma of selenium ions and electrons. Collisions between the ions and electrons pumped more energy into the ions, raising them to very high energy states. Upon relaxing to lower energy levels, the ions gave off the XUV radiation. The intensity of the XUV radiation was not particularly high, but the principle was demonstrated.
A Six-Dimensional Crystal?
When the constituent atoms, ions, or molecules of a solid substance are arranged in a very orderly way, with a basic structure called a unit cell repeating itself continuously in all three dimensions, the substance is a crystal. Only certain regular shapes, however, can serve as a unit cell. One that cannot is the icosahedron, a 20-sided object with triangular faces. Trying to fill up a space with icosahedrons is like trying to cover a floor with tiles in the shape of regular pentagons. It can't be done without leaving some holes between the tiles. A crystal may have a kind of symmetry called rotational: if it is rotated a certain fraction of 360ø, it ends up in exactly the same position as it began. The pentagon has 'fivefold' rotational symmetry. Rotating it by a fifth of 360ø, or 72ø, leaves the pentagon in a position identical to the original. In three dimensions, the icosahedron has fivefold symmetry, and the conventional wisdom among crystallographers has long been that no crystal can exhibit fivefold symmetry. Many were, thus, amazed when the discovery of a substance with precisely this kind of symmetry was reported in late 1984.
The new substance, an alloy of 86 percent aluminum and 14 percent manganese, was found by Dan Shechtman of the Israel Institute of Technology (Technion) in Haifa and colleagues during research at the NBS at Gaithersburg. When prepared by a technique called melt spinning, in which molten metal is splattered onto a cold, spinning disk, the alloy does not assume the usual crystal structure. Instead, it takes on a structure that is still not completely understood but that has a fivefold rotational symmetry.
The structure clearly cannot comprise a single three-dimensional unit cell that is repeated to build a crystal lattice with fivefold rotational symmetry. Whatever the structure is, it does not have the so-called translational periodicity that is characteristic of crystals; that is, it cannot fill space with identical unit cells that leave no voids between them. Instead of being periodic, it is only 'quasiperiodic.'
Several explanations have been proposed for this apparent conflict. One starts with the fact that translational periodicity and fivefold rotational symmetry theoretically are simultaneously possible in crystal lattices of more than three dimensions. Theorists have shown that a structure having both fivefold rotational symmetry and quasi-periodic translational symmetry of the type apparently characterizing the aluminum-manganese alloy can be obtained from a projection in three dimensions of a six-dimensional object. This raises the interesting possibility that other problems in crystallography as well may be solvable by recourse to higher-dimensional spaces.
Particle Accelerators.
There was good news in September for advocates of the construction of the proposed mammoth particle accelerator called the Superconducting Super Collider. A panel of federal scientists selected a design for a key part: the superconducting magnets that produce the magnetic field through which particles are accelerated. The design chosen, a so-called conductor-dominated type of high-field magnet, would result in the accelerator being about 60 miles in circumference. (The alternative, less powerful designs would have resulted in a larger machine.) If approved by Congress, the SSC would be completed by the mid-1990's, at a cost of $3 billion to $6 billion.


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