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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