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starship-design: Antimatter in a Trap
Antimatter in a Trap
by John G. Cramer
Alternate View Column AV-10
Keywords: antimatter, positron, quadrupole, Penning trap, CPT
Published in the December-1985 issue of Analog Science Fiction & Fact
This column was written and submitted 5/3/85 and is copyrighted © 1985,
John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of
"What", the Alchemist asked his new apprentice, "is the Universal Solvent?"
"Master", said the lad, "it's one of the fundamental substances of Alchemy.
It will dissolve any solid material. A drop will dissolve the hardest
steel, the finest glass, the most inert wax."
"Very well", said the old man with a frown. "I am about to make some. Your
assignment is to prepare a bottle in which to put it ..."
This AV Column is about the Universal Solvent of modern physics which we
call antimatter, and about a bottle in which it can be and has been kept.
However, before getting to the hardware I want to talk about antimatter as
it relates to the fundamental symmetries of the universe.
Physicists have, over the years, been able to get a lot of mileage out of a
single nifty idea: Nature is Symmetric. Pioneers like Galileo, Newton,
Maxwell, Einstein, Fermi, and many recent Nobel laureats have based their
work on the notion that nature at the core is basically symmetrical and
even-handed. For example, space has the same properties in all directions.
The laws of physics must be the same in all inertial (constant-speed)
reference frames. An object with certain symmetries produces effects with
the same symmetries. The laws of physics must be the same here and now as
they were long ago in a galaxy far away. And so on ...
And yet as more and more is learned about the inner workings of the
universe we have discovered that the breaking of these fundamental
symmetries of the universe is also important. The major physics
breakthrough of the late 1950's was the revelation that the space symmetry
called "parity" (that nature looks the same in mirror-image) is thoroughly
broken, spindled, and mutilated by the "weak" force acting in radioactive
transformation processes like beta decay. In the mid 1960's it was
discovered that the weak decay of the "strange" KoL meson showed a
"CP-violation" (the rough equivalent of broken time symmetry). And the
recent development of what theoretical physicists modestly call Grand
Unified Theories (GUTs) is based on the symmetry breaking of three of the
fundamental forces (strong, weak, and electromagnetic). This splitting of
one "unified" force into three very different forces happened
"spontaneously" as the universe cooled off in expanding after the Big Bang
(see "Other Universes I", ANALOG, September, 1984). The symmetries of
nature seem made for the breaking.
Some broken symmetries are important for our well-being. Our everyday life
depends on two overwhelmingly important breakings of symmetry at the
macroscopic level: (1) our world is clearly different with time running
forward than it would be with time running backwards, and (2) there is more
matter than antimatter in our local environment. Some SF writers (Brian
Aldiss in Cryptozoic!!, for example) have been able to contemplate a
somewhat time-symmetric world. But no one, to my knowlege, has written
about everyday life in a "C-symmetric" world in which the local environment
was an equal mix of matter and antimatter. Such a literary undertaking
would surely be a short story because the incipient matter-antimatter
annihilation would blow everything to photons and neutrinos in nanoseconds.
And so we have a paradox. The microscopic world is so symmetric that only
with the greatest of difficulty have we been able to find one obscure
physical process, the KoL decay, which shows any preference at all for one
direction of time over another or for matter over antimatter. And yet in
the macroscopic every-day world these time and matter preferences are
everywhere, and we depend on them in our everyday lives. The
matter/antimatter unbalance is not just a local phenomenon. There is now
fairly good observational evidence that there are no large amounts of
antimatter even in more remote parts of the universe in the form of
anti-stars and anti-galaxies. And so we must ask, "How can the macrocosm be
so radically different from the microcosm when it is really only a
summation of all of the microscopic fundamental processes, as viewed from a
distance in space and time?"
The preference of the everyday world for the forward time direction, the
"Arrow of Time" problem, was discussed in one of my recent AV columns
("Light in Reverse Gear II", ANALOG, August, 1985) and that discussion will
do for now. In this AV column I want to consider the questions: "Where did
all of this matter come from, and where did all of the antimatter go?" The
GUTs theorists believe that they have the answer to this question. Their
scenario is that in the primordial soup of the very early universe there
were other heavier particles which, like the KoL meson, had a "CP
violation", a slight preference for decaying into matter particles instead
of antimatter particles. The net result of this is that the early universe
had about 100,000,001 protons for every 100,000,000 antiprotons. In the
cooling after the Big Bang the protons and antiprotons found and destroyed
each other until the slight excess of protons became all the matter there
was (and is). A side-effect of the same CP-violating processes is that
there is also an excess of electrons over positrons. The surviving protons
and electrons, about 100,000 years after the Big Bang, paired off to form
hydrogen atoms which eventually went into business as stars and galaxies.
The enormous energies from matter-antimatter annihilations of the early
universe cooled with expansion down to 3o K, the present average
temperature of the universe. The electrons and protons around us (and in
us) are the few ragged survivors of the "antimatter wars" of 16 billion
We would like to understand in a more fundamental way why matter was
preferred over antimatter in the early universe. The preference shown by
the KoL meson (a matter-antimatter pairing of a "strange" quark and a
"down" quark) is a tantalizing hint at the matter/antimatter difference,
but we would like to know whether there are other ways in which antimatter
differs from matter. One way of looking for such differences is to compare
all the measurable properties of matter particles (protons and electrons)
with the same properties of antimatter particles (antiprotons and
positrons). This comes down to the experimental problem of how we can weigh
and measure particles of antimatter.
The first problem that we encounter here is that there aren't any
antimatter particles lying around to be used in measurements. They were all
destroyed shortly after the Big Bang. But we are not out of business,
because we can make antimatter. We can make antiprotons with large particle
accelerators. At the LEAR (Low Energy Antiproton Ring) facility at CERN
laboratory in Switzerland, physicists have been able to produce huge
numbers of antiprotons, store them for hours in as they coast in circular
orbits through a ring of magnets, and finally deliver them as a beam of
particles for nuclear reaction studies. Positrons are even easier. Nuclear
reactors make certain isotopes which emit positrons during radioactive
decay, and positrons can also be produced by beams of electrons and
preserved by orbiting in a ring of magnets.
In storage rings measurement of the properties of antimatter particles is
usually not very precise because the very factors which keep the particles
stable in orbit also interfere with measurement precision. Therefore, one
would like to be able to measure the particles "at rest" in the laboratory.
This is related to the alchemist's problem of storing the Universal
Solvent. Since antimatter will annihilate on contact with any matter, what
kind of bottle can hold it?
Fortunately this problem has an experimental solution. A group of
physicists at the University of Washington has developed a bottle for
antimatter called a Penning Trap. It looks rather like a metal hour-glass
with a knob poking into each end. The knobs and hour-glass are given
opposite electrical charges, and the whole thing is placed in a magnetic
field pointing along the axis of the hour glass. Into this apparatus one
can place a single proton or electron, and the particle will stay there,
held in place by the electric and magnetic forces of the trap. One can then
"play games" with the trapped particle, putting it through a routine of
shaking and bouncing and oscillation that determines its mass, charge,
spin, and internal magnetic field to almost unimaginable precision.
A few years ago a single particle of antimatter, a positron, was
successfully captured in such a trap. Positrons from a radioactive source
were slowed and carefully manipulated until one popped into the Penning
Trap. There it was weighed and measured it to see whether it showed any
differences (other than charge) from its equivalent matter particle, the
electron. The same positron stayed in the trap for a number of days. It
represents the first instance of artificially produced antimatter at rest
on Earth lasting for more than a fraction of a second. The measurements
made on the single trapped positron are capable of detecting differences of
one part in a trillion (10-12 ), but even with this remarkable accuracy no
difference between electrons and positrons was detected.
A similar experiment is now being prepared for trapping an antiproton. The
trap apparatus will be taken to the LEAR facility at CERN. There an
antiproton will be carefully slowed and captured in the trap. The
experimenters expect that a proton-antiproton mass difference smaller than
one part in a billion (10-9 ) could be detected. If such a difference
existed, it would be a very significant clue toward solving the mystery of
the matter/antimatter imbalance of the universe.
But beyond the weights-and-measures of anitmatter, the experiment will
represent a demonstration that antimatter can be produced, captured, and
stored at rest for indefinite periods in the laboratory. As Robert W.
Forward has pointed out in the pages of Analog, antimatter is the most
compact way yet devised for storing energy, and it may have enormous
potential as a fuel for starship engines. We have the Universal Solvent and
we have the bottle in which to keep it. The rest is a problem for engineers
... and alchemists.
P. Ekstrom and D. Wineland, Scientific American 243 #2, 105 (August, 1980).
P. B. Schwinberg, R. S. Van Dyck, Jr., and H. G. Dehmelt, Physical Review
Letters 47, 1679 (1981).