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A new ferment in the mirror world of antimatter/an
Citation-> Popular Science, July 1986 v229 p54(7)
COPYRIGHT Times Mirror Magazines Inc. 1986
New ferment in the mirror world of antimatter/ antigravity
Grim new find at UC.'
That was the headline in the Berkeley Gazette after physicists at the
University of California in Berkeley announced the discovery of the
antiproton in 1955. Why grim? The reporter covering the story had heard
that if antiprotons were to collide with him or his newspaper, they would
blow up.
The incident was recalled last November by Nobel laureate Luis Alvarez at a
meeting in Berkeley commemorating the fateful discovery 30 years before.
The discoverers were sure, he says, that antiprotons did indeed exist,
although he doubted that they could be observed. Other physicists didn't
believe in them at all.
Today, of course, there are no more disbelievers concerning the
antiproton--or antimatter in general. There is evidence that small amounts
of antimatter are being generated at the center of our galaxy. And
physicists are beginning to create quantities of antimatter large enough--
though still minute--to serve as experimental probes, opening whole new
avenues of research.
Perhaps most fascinating: In an experiment being designed at Los Alamos
National Laboratory, in New Mexico, which will be carried out at CERN, the
European Center for Nuclear Research, in Geneva, Switzerland, antiprotons
will provide the means for testing the existence of a long-hypothesized but
never observed force: antigravity. Contrariwise, the experiment could
reveal a different manifestation that could be called supergravity. And
finally, there are proposals to use antimatter as a propulsion source for
difficult missions in space--proposals under serious study by the U.S. Air
Force.
The concept of antimatter originated at the climax of a period of intense
intellectual ferment that saw the birth of a revolution in physics. This
activity resulted in one of the most powerful descriptions of nature ever
conceived --quantum mechanics, which has been called by Murray Gell-Mann
"that magnificent and confusing discipline.'
At the core of quantum physics is the idea of the quantum. In the atomic
world, energies do not exist over a continuous range, but at definite
levels, like the rungs of a ladder. In 1900, Max Planck theorized that
radiant energy could be emitted and absorbed only in discrete bundles,
called quanta. In 1905, Einstein extended the concept to light, and thus to
all electromagnetic radiation. The bundles, or quanta, of light energy are
called photons.
Reconciling principles
By the late 1920s, a shy young British-born physicist named Paul Adrien
Maurice Dirac was working to reconcile Einstein's principle of special
relativity with quantum mechanics. Dirac was writing an equation that would
quantitatively describe various properties of the electron. He had only to
plug in the charge and mass of the electron: Its spin, associated magnetic
moment, and behavior in the hydrogen atom followed mathematically.
But there was a time bomb concealed in the solutions to Dirac's
mathematics--one that was to overturn particle physics. It seemed to be
nothing more than an innocent minus sign. But its implication was
shattering: that there existed another electronlike particle, with a charge
the same magnitude as the electron's but of opposite sign: a positive
charge.
Dirac at first recoiled from this result, and questioned whether he had made
some blunder. In 1928, only a handful of players made up the known particle
world: the photon, electron, and proton. It would somehow not have been
cricket to disturb this cozy array.
But Dirac eventually became convinced that his mathematics demanded a
positively charged particle with a mass identical to that of the electron.
Finally, in 1931, he predicted the existence of an "antielectron,' the first
intimation that a whole new world of antimatter lay as yet unexplored.
The stunning prediction was, in the words of physicist Victor Weisskopf,
"one of the most dramatic developments of the intellectual penetration of
nature by man.' It changed forever the view of matter as being fundamental
and immutable, for it meant that not only the electron but every atomic
particle must have a doppelg anger, an antiparticle with identical mass,
spin, and other properties, but with opposite charge (and those properties
related to charge, such as magnetic moment).
Dirac's equation had a further ramification: If a particle encountered an
antiparticle, both would be annihilated --flashed into a spray of pure
energy. For an electron-positron annihilation, the energy would be in the
form of gamma rays. And the amount of energy released would be exactly that
predicted by Einstein's equation for the conversion of mass into energy: E
= mc2.
It followed logically that antimatter could be created by the reverse
process, converting energy into a pair of antiparticles, and that the energy
level required would be at least equal to the sum of the masses (when
converted into energy) of the particles involved.
Dirac once said that "It is more important to have beauty in one's equations
than to have them fit experiment.' The Dirac equation was indeed beautiful,
but no experiment had turned up even an inkling of an antielectron.
That was soon to change.
Tracing the particles
In 1932, a young graduate student named Carl Anderson was working on a
project that was originated by his mentor, the noted physicist Robert
Millikan. Millikan was interested in measuring the energy of cosmic rays,
and he asked Anderson to build a unique apparatus--a cloud chamber placed in
a powerful magnetic field--to carry out the study. A cloud chamber is an
instrument filled with moist, supersaturated air or some other gas under
pressure. When a charged particle moves through the chamber under just the
right conditions, the particle creates a trail of ions. Droplets of
condensed water form along this path, tracing out a visible track of the
invisible particle, much as a jet plane leaves a vapor trail in the sky.
And the tracks could be photographed.
Using a cloud chamber, an experimenter could determine not only the mass of
a particle, but its charge. The curvature of the path of a lighter particle
as it was deflected by the magnetic field would be greater than that of a
heavier particle. If the particle were positive, it would curve in one
direction as it traveled across the magnetic field; if negative, in the
opposite direction.
On August 2, 1932, Anderson peered at a frame of film that showed the thin,
white trail of what was clearly an electron, judging from the large
curvature of the track. Yet the direction of its path showed just as
clearly that it had a positive rather than a negative charge.
Impossible! The graduate student who had developed the film fretted.
Perhaps the film negative had somehow been reversed, both right to left and
upside down. But Anderson knew the picture had not been reversed, and that
the track was the spoor of a strange, never-before-seen particle--an
antielectron. Six months later, Anderson dubbed it the "positron,' a
contraction of "positive electron.'
The first bit of antimatter had been found, exactly as Dirac had predicted.
In 1933, Dirac was awarded a Nobel Prize for his epochal work. And in 1936,
the Nobel went to 31-year-old Anderson, who had to borrow $500 from Millikan
to pay for his ticket to Sweden.
Anderson's discovery was not merely a vindication of Dirac's theory. It
launched physicists on a new and more difficult quest for another
hypothetical particle--a search that was to take some 25 years. That
particle was the antiproton, the mirror image of the proton, with the same
mass and spin but opposite charge and magnetic moment.
Why did it take so long to detect? The way to make antiprotons (and other
antiparticles) is to create them out of the energy of particle collisions.
But the energy has to be higher than the combined masses of the
particle-antiparticle pair desired. And the proton has a much higher mass
than the electron: about 1,840 times higher. That converts to an energy of
about two GeV (two billion electron-volts. For years, however, no
accelerator was capable of the necessary energies--not until the completion
of the proton synchrotron at the University of California in Berkeley in
1955.
The machine was called the Bevatron, from the unit BeV, the
billion-electron-volt energies it was designed to produce. (The
abbreviation BeV later was supplanted by the now universally used GeV. The
G is from "giga,' now the prefix denoting billion.) University of California
physicists calculated that because of the nature of particle collisions, it
would take not two GeV but at least six GeV to produce antiprotons.
In October 1955, the Bevatron whipped protons with an energy of six GeV into
a stationary copper target. A complex detector sifted through the intense
spray of particles spewed from the target. The job was difficult because
for every antiproton predicted, some 40,000 other particles with the same
momentum would be ejected. But the antiproton had a distinctive signature,
and the detector sorted it out. Sixty antiprotons were recorded, about four
for each hour of the Bevatron's operation. For this culmination of a
25-year search, Owen Chamberlain, designer of the detector, and Emilio
Segre, the experiment's leader, were awarded the Nobel Prize in 1959.
Accelerated smashings
Today, not scores but trillions of antiprotons are produced in accelerators.
They are, says Caltech physicist John Schwarz, "the bread and butter of
high-energy experimental physics.' When protons and antiprotons collide,
they can sizzle into sprays of energy from which new particles arise.
Last October 13, at 3:30 a.m., after weeks of day-and-night effort,
physicists at the Tevatron at Fermilab (Fermi National Accelerator
Laboratory) in Batavia, Ill., bashed a beam of antiprotons headlong into a
counter-circulating beam of protons at a record high energy: 1.6 trillion
electron-volts (or TeV--terra being the prefix for trillion--whence
Tevatron). When the Tevatron is fully operational, it will yield collision
energies of two TeV, three times the magnitude of any other accelerator in
the world.
Why the rush to higher and higher energies? The more massive a particle,
the more energy it takes to create it. And the still-elusive particles that
physicists need to find to make theoretical progress are ever more massive
["Cosmic Order,' PS, May '83].
"What we have with the Tevatron,' Leon Lederman, Fermilab's director, told
me on a recent visit, "is the ability to explore a whole new energy domain
where no one has ever looked before. There may well be great surprises in
store.'
Testing gravity
One of the greatest of all surprises could come from an experiment in which
antimatter will play a different, though central, role. Conceived by a group
of physicists at Los Alamos National Laboratory, and scheduled to start in
1988, the experiment is designed to test whether gravity has the same effect
on antimatter as on normal matter. If gravity does not, some fundamental
ideas about the way the universe works will crumble.
It all began in 1982, when two Los Alamos theoreticians, Terry Goldman and
Michael Nieto, authored an article in Physics Letters titled "Experiments to
Measure the Gravitational Acceleration of Antimatter.' In the article, they
pointed out that currently accepted theories of gravity, based on Einstein's
work, absolutely precluded the existence of a force called (in science
fiction and otherwise) antigravity --that is, that antimatter would be
gravitationally repelled by matter. The catch, they said, was that this
foreclosure had never been experimentally proved. No one had ever shown
that, for example, antiprotons would not fall up in Earth's gravitational
field. Goldman and Nieto suggested an experiment that could test such a
theory, although neither really believed a surprising result was likely.
"If you believe Einstein's theory of gravity,' Nieto said, "antimatter will
fall exactly like matter.' Any other outcome would be earthshaking.
The challenge was taken up by a team of experimentalists at Los Alamos, led
by Michael Hynes. Their plan is to obtain a source of low-energy
antiprotons (which everyone involved calls p-bars because the symbol for an
antiproton is a p with a bar over it), decelerate the antiprotons and cool
them to still lower energies, trap them in a magnetic bottle that isolates
them from normal matter (so they don't blow up), and then release them into
a 10-foot-high vacuum tube where their flight up or down can be measured.
The experimenters will obtain the antiprotons at CERN's low-energy
antiproton ring (LEAR), Hynes explained as we toured the Los Alamos
facility, perched on a dun-colored plateau in New Mexico's high country. "We
envision doing the antigravity experiment as a ballistic trial, firing an
antiproton up or down a tube and measuring how long it takes, and from that
calculating what the acceleration of gravity is. But to get enough accuracy
in timing over a reasonable distance, the particles can't be going very
fast. Otherwise the whole experiment would be over before you knew it. We'd
like the particle to take a reasonable amount of time to go a few meters.
It's essentially a measurement problem.
"What we need to do is cool this antimatter down to the temperature of
liquid helium--four degrees Kelvin [about 450 degrees F below zero]. At
that average energy the antiprotons will travel the length of the deift tube
slowly enough to allow the small effect of gravity to be measured. But at a
hundred degrees Kelvin the antiprotons are going too fast, and the gravity
effect gets swamped.'
The antiprotons will be decelerated, or cooled, in a device called a
radio-frequency quadrupole (RFQ), originally proposed by Soviet scientists,
which has previously been used as an accelerator for low-energy particles.
Roughly speaking, the acceleration process will be reversed. Finally, the
antiprotons will be still further cooled, to four degrees Kelvin, in an ion
trap of a particular design, called a Penning trap, which uses both
electrostatic and magnetic fields to confine antiprotons.
The experimental group has now grown to dozens of participants from both the
U.S. and abroad. One version of the ion trap is being built at Los Alamos.
The final version may be built elsewhere, perhaps at Texas A&M University.
At any rate, the trap and associated equipment at Los Alamos looked as
shinily anonymous as similar stainless-steel assemblages in any other
advanced physics laboratory.
New wrinkles
At the time of my visit to Los Alamos, last November, physicists Hynes and
Alan Picklesimer were saying that although "something totally unexpected
could happen,' the betting was that the experiment would simply confirm the
general conviction that gravity affected matter and antimatter in the same
way.
Then, in March, I got an urgent telephone message from Hynes. Something
radical had changed in the theorists' thinking, he said. And what it meant
was that they now believed antimatter would fall, but at an acceleration
considerably greater than for normal matter--up to 300 times greater.
Theorist Richard Hughes of Los Alamos told me that the radical revision had
been stimulated by a paper written by Ephraim Fischbach, a physicist at the
University of Washington, Seattle. Fischbach had reanalyzed a 70-year-old
gravitational experiment by the Hungarian L[or[ant (Roland) E otv os, and
believed that he had found evidence for a "fifth force' that would be a kind
of antigravity ["Science Newsfront,' PS, April]. More recently,
measurements made deep in Australian mines by Frank Stacey have also
uncovered gravitational anomalies.
Supergravity?
When the Los Alamos theoreticians examined the old and new data, Hughes
said, they came to a different conclusion. Both sets of data fit a modern
non-Einsteinian theory of gravity called supergravity, one of the recent
attempts to nudge gravity under the quantum umbrella in a so-called "grand
unification' of all the forces of nature. For reasons too technical to
elaborate here, supergravity implies that gravity has effects that are both
repulsive and attractive, and that these effects depend on both the mass and
the composition of the matter involved. Surprise! A cannonball and an
equal mass of water may not necessarily fall with the same acceleration,
according to this theory, although the discrepancies may be too small to be
observable under normal circumstances.
But the theory does have one unequivocal prediction: Antimatter will be
attracted to matter with a greater force than matter-to-matter attaction.
And antiprotons in the CERN experiment should therefore undergo a greater
than normal acceleration due to gravity. This result would mean that
gravitation is far less simple than in Einstein's elaboration of Newtonian
mechanics; much that is now held sacred in physics would have to be
scrapped.
Hynes follows what he calls "the trend line' in antimatter production, much
as a technical analyst follows a stock's performance on Wall Street. "We
started producing p-bars in 1955 --sixty of them when they were first
discovered. . . . The slope of the trend line now is about an order of
magnitude every two-and-a-half years. If you extrapolate to the future,
we'll produce about a gram in 2007.
"Now, a gram of antiprotons is about 6 X 10(23) p-bars, and that represents
a substantial amount of energy-- about six megawatt-years.'
Why so much? Antiproton-proton annihilation releases far more energy per
gram of material than either nuclear fission or fusion. In fission, using
uranium or plutonium, the amount of mass converted is a mere parts per
thousand. In fusion, using hydrogen or deuterium, the fraction is almost
one percent; that's why a hydrogen bomb is so much more powerful than a
fission bomb. But in antimatter annihilation, the amount is 100 percent.
Costly antimatter
That fantastic energy potential is undoubtedly what has given rise to
speculation about antimatter bombs. But any time I mentioned the
possibility to physicists who are involved with antimatter, the notion was
met with ridicule because of the enormous cost of generating antimatter, the
difficulties in handling it, and the fact that it would make a "lousy' bomb.
The nature of the explosion is not a bang, says one physicist, but a "poom.'
"An antimatter bomb of twenty kilotons yield, the same size as the Hiroshima
bomb, would probably cost at least $10 billion, about one percent of the
national budget,' says Robert L. Forward. "You'd go broke trying to destroy
an enemy with them, and they wouldn't even make good bombs.'
Forward is a senior scientist at the Hughes Research Laboratories in Malibu,
Calif., high in the hills overlooking Malibu beach and the Pacific Ocean.
He is an impressive figure, tall and portly, with a mop of pure white hair.
When I met him, he was resplendent in a white-on-white shirt, white-on-white
bow tie, and a vest striped in many more colors than are in the spectrum.
His career has been equally multifaceted, ranging, in 31 years, from gravity
wave detectors (a design he worked on is in the Smithsonian Institution, in
Washington, D.C.) to space science.
Antimatter rockets
If antimatter doesn't make sense for bombs, Forward says, it does make sense
for something else: space propulsion. The reason is the ferocious cost of
getting any kind of fuel into orbit, even low Earth orbit.
"It costs up to $5 million a ton to lift the fuel into space, beyond what it
costs to manufacture. And a milligram of antimatter will replace something
on the order of twelve tons of the best chemical fuels we have, liquid
oxygen and hydrogen.'
Although Forward is a successful science fiction writer (and despite the
fact that Star Trek ships used antimatter propulsion drives), the concept is
being treated seriously by the U.S. government. Last year, Forward
completed a hefty report, titled "Antiproton Annihilation Propulsion,' for
the Air Force Rocket Propulsion Laboratory, awarded to him on a personal
consulting contract. (Studies in antimatter propulsion have also been
conducted at NASA's Jet Propulsion Laboratory, Lawrence Livermore
Laboratory, and a number of aerospace contractors.)
Plasma thrust
Forward's studies concluded that one feasible scenario for antimatter
propulsion would go like this: Antiprotons would be captured in a magnetic
ring, cooled, and slowed down. Positrons would be added to make
antihydrogen, which in turn would be controlled by laser beams and cooled
into a gas. The spacecraft would carry about twice its dry weight in
propellant-- hydrogen, say, or perhaps something heavier--and a small
amount, only a few milligrams, of antihydrogen. The antimatter would ignite
the propellant and heat it to a hot plasma that would give the engines their
thrust.
This scenario could become an actuality, Forward calculates, if the cost of
antimatter came down to about $10 million per milligram, a level he believes
is possible if "antiproton factories' were designed exclusively for that
purpose rather than just for research.
The real payoff, he believes, would be to bring now-impossible space
ventures within our grasp. These are journeys that require such high
velocities that they can never carry enough chemical fuel to achieve
them--the fuel simply adds too much mass to be accelerated. Such trips
include something as apparently simple as leaving an orbiting space station
to visit a satellite traveling in the opposite direction, and then returning
to the original base; and something as obviously difficult as descending
deep within the rings of Saturn and being able to extricate the craft from
Saturn's gravity well.
If Forward is right, tomorrow may see accomplished what are now impossible
missions to the planets. And next, perhaps, the stars?
Photo: An aerial view of Fermilab National Accelerator Laboratory (above) is
dominated by the outline of the main accelerator ring, four miles in
circumference. A tunnel lying 30 ft. below ground houses particle-guiding
magnets for the two accelerators: an upper ring of magnets for the original
400-GeV accelerator, and, installed below them, a ring of superconducting
magnets for the Tevatron, where protons and antiprotons collide. The
16-story Robert Wilson Hall for offices and research labs punctuates the
Illinois plain at left. The partially completed Collider Detector Fermilab,
or CDF, appears in the photo below. The first proton-antiproton collisions
generated in the Tevatron occurred in a beam tube at the center, and were
recorded by complex electronic systems in the surrounding components. When
completed, the two-story-high CDF will weigh 4,500 tons.
Photo: Two critical devices for the so-called antigravity experiment being
planned at Los Alamos National Laboratory: Far left, a radio-frequency
quadrupole (RFQ) decelerator; at left, an antiproton "bottle' called a
Penning ion trap. The experiment will test whether gravity acts on
antimatter in the same way it does on matter. Antiprotons from the
low-energy antiproton ring (LEAR) at CERN in Geneva will be slowed first in
the RFQ, and eventually cooled to 450~ below zero F in the ion trap. Then
the antiprotons will be released up a 10-ft. drift tube, traveling slowly
enough so that their time of flight can be accurately measured and any
variance in the action of gravity detected.
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