AFTER years of struggling to revive the excitement of the space age鈥檚
pioneering years, NASA may have found the answer to its prayers. The agency has
announced that from 2001, the international space station Alpha will be home to
the first space-based antiparticle detector. There鈥檚 a chance that it will make
a truly sensational discovery: of entire stars and galaxies ruled by
antimatter,
the curious enemy of normal matter that makes it disappear in a flash.
The idea of antimatter worlds is not as strange as it sounds. Every
subatomic
particle in nature has an associated antiparticle which has the opposite
properties. For example, the negatively charged electron has a positively
charged twin known as the positron. Antiparticles are easily made in particle
accelerators. If they have always existed naturally in the Universe,
antiprotons
and antineutrons should have come together to form antinuclei. These would
combine with positrons to make anti-atoms, which would clump into stars and
galaxies, just as normal matter has done.
Long shot
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If the Alpha Magnetic Spectrometer (AMS) succeeds in finding
antigalaxies and
antistars, physicists will have to take all their most treasured notions about
the Universe back to the drawing board. 鈥淚t might appear a long shot,鈥� says
Samuel Ting of CERN, the European particle physics laboratory in Geneva,
and the
Massachusetts Institute of Technology. 鈥淗owever, there is no compelling reason
why the Universe should be made of matter rather than an equal mix of
matter and
补苍迟颈尘补迟迟别谤.鈥�
Ting, who is the driving force behind the AMS, shared the 1976 Nobel Prize
for Physics with Burton Richter for the discovery of the J/psi, the first
subatomic particle containing a charmed quark. The AMS project was born two
years ago, shortly after the US government pulled the plug on the
Superconducting Supercollider, a giant particle accelerator that was to have
been built in Texas. 鈥淲hen the SSC was closed down, I got together with some
friends,鈥� says Ting. 鈥淲e thought that maybe we could do a different type of
particle experiment鈥攐ne that didn鈥檛 involve thousands of people and cost
hundreds of millions of dollars.鈥�
Ting and his colleagues came up with plans for a small particle experiment
鈥攖he first in space鈥攖hat could run on the space station for a cost
of only $20 million. The detector will identify the atomic nuclei and
electrons in cosmic rays, the hailstorm of particles that continually bombard
the Earth. Most come from the Sun, others from the remnants of exploded
stars in
our Galaxy. But the most energetic of all come from unknown sources beyond the
Milky Way. The AMS will sift through the incoming cosmic rays, searching
for the
nuclei of anti-atoms (see 鈥淒etecting antimatter鈥�). 鈥淚t鈥檚 a very clever
experiment indeed,鈥� says astrophysicist David Schramm of the University of
Chicago and Fermilab. 鈥淚t鈥檚 very exciting.鈥�
Finding antigalaxies is not an easy task. Unfortunately, antiphotons emitted
by anti-atoms would be indistinguishable from photons given out by atoms. So
antigalaxies would look no different from ordinary galaxies. However, there are
some signs that would be a dead giveaway. If antimatter were nearby, we would
see the intense gamma rays produced when matter and antimatter collide and
annihilate each other. Such collisions are inevitable, as material is
constantly
exchanged between stars as they explode into interstellar space, or as entire
galaxies collide.
Because such bursts of gamma rays have never been seen, we know for certain
that there are no patches of antimatter in our own Galaxy. Similarly, we can
rule out the existence of antimatter in the Local Group, the small cluster of
galaxies that includes the Milky Way. So we can be sure that antimatter almost
certainly does not exist in the space between us and the nearest cluster, more
than 30 million light years away.
Violation
Yet if there is no antimatter in the Universe, this is difficult to explain.
In theory, equal amounts of matter and antimatter should have been created in
the big bang. So how could it be that no antimatter survives today? The
conventional explanation, suggested by Soviet physicist Andrei Sakharov in
1967,
is that the imbalance is due to a slight asymmetry in the laws of physics,
technically known as CP violation.
This asymmetry would have come into play in the first split second after the
big bang. In the earliest moments, according to a widely favoured Grand Unified
Theory, all the forces that we now find in nature were merged into a single
force, carried by the so-called X-particle. X-particles would have been
extraordinarily heavy, with a mass of almost a million million times the mass of
a lead nucleus.
Experiments in particle accelerators suggest that when the Universe cooled
and the X-particle decayed, the asymmetry in the laws of physics would
have left
a tiny fraction more particles than antiparticles鈥攔oughly a billion and
one particles for every billion antiparticles. Physicists assume that
these were
thoroughly mixed together, and since particles and antiparticles
annihilate each
other in a puff of gamma rays when they meet, this would explain why today鈥檚
Universe contains no antimatter. It would also explain the brilliance of the
background radiation that fills the Universe鈥攁bout a billion photons for
every particle of matter. This would be the afterglow of all these
matter-antimatter annihilations.
However, Ting says the conventional explanation for the absence of
antimatter
is far from proven. 鈥淐ontrary to what many physicists believe, we cannot rule
out the possibility that somewhere in the Universe antimatter has survived from
the big bang,鈥� he says. For example, the asymmetry in the laws of physics could
have been reversed in certain regions of the big bang fireball, favouring the
creation of antimatter over matter. 鈥淭he result would be a patchwork
Universe in
which regions of matter alternated with regions of 补苍迟颈尘补迟迟别谤.鈥�
Ting admits that no one knows how such a reversal in the asymmetry of the
laws could have happened. But he points out that the whole question of
asymmetry
is far from being understood. 鈥淚t is not even clear that the amount of
asymmetry
we know about from experiments so far is sufficient to account for the matter
dominance in the Universe,鈥� he says.
So in Ting鈥檚 view, there is still a chance that antimatter rules some
galaxies beyond our local cluster, so far away that the telltale gamma rays of
annihilation might be too faint for us to see. 鈥淭he Universe contains about 100
million other clusters,鈥� he says. 鈥淚t鈥檚 impossible to rule out any of these
being made of 补苍迟颈尘补迟迟别谤.鈥� The best clues, he says, will lie in the cosmic rays
from distant stars and galaxies that shoot across the Universe at almost the
speed of light.
Cosmic showers
Studies of these cosmic rays have been under way since early this century.
Ground-based experiments have studied them by catching the showers of particles
created when they collide with nuclei in the upper atmosphere, shattering them
into pieces. These can reveal the energy of the original cosmic ray, but
unfortunately they do not say anything about whether it was matter or
antimatter.
To catch the original cosmic rays, experiments have been flown high in the
atmosphere under balloons. But these have only taken measurements over periods
of a paltry few hours. Ting says that such short-lived experiments would only
have detected antimatter if at least 1 in 10 000 cosmic rays were an
antinucleus. But cosmic ray particles from beyond our local cluster probably
make up only about 1 in 10 billion cosmic rays. 鈥淚t鈥檚 far too premature to
claim, as some have, that such experiments have ruled out the existence of
antimatter in the Universe,鈥� says Ting. 鈥淭hey were simply not sensitive
别苍辞耻驳丑.鈥�
Relentless search
The AMS should have a much better chance of success, he says. Unlike the
ground-based experiments, the orbiting detector will be able to detect the
original cosmic rays in space, before they crash into the Earth鈥檚 atmosphere.
And unlike the balloon experiments, it will be relentless in its search,
registering cosmic rays over a period of around three years. This means that it
will have just the sensitivity it needs, being able to detect one antimatter
particle in 10 billion ordinary particles.
If the AMS detects the telltale signature of antiworlds, it will be one of
the most significant astronomical discoveries in history. 鈥淚f antimatter is
found, particle physicists the world over will be far happier,鈥� says Ting鈥檚
colleague, Ulrich Becker of MIT. 鈥淣o longer will they face the problem of
explaining how a Universe that initially contained equal quantities of matter
and antimatter evolved into one that contains only matter.鈥�
But clearly, the discovery would also bring new problems to solve. 鈥淓arly on
in the Universe, matter and antimatter would have been created in the same
place,鈥� says Nobel prizewinner Steven Weinberg of the University of Texas. 鈥淲e
would have to ask ourselves why they did not annihilate each other.鈥� This view
is echoed by Schramm. 鈥淭he early Universe would have to contain domains of
matter and antimatter,鈥� he says. 鈥淚t would mean all our notions about the early
Universe would have to change.鈥�
Antimatter apart, the AMS will also set its sights on some of the other
questions that have perplexed astronomers for decades. For instance, what
fraction of the cosmic rays that reach Earth have come from galaxies beyond our
own? This will depend partly on how the cosmic rays react to our Galaxy鈥檚
magnetic field, which forces charged particles to move in curved paths. To some
extent, this confines cosmic rays within the Milky Way. If they leak into
intergalactic space fairly quickly, cosmic rays from other galaxies will do the
same and more will eventually arrive on Earth.
A simple measurement of the ratio of two particular cosmic ray
particles鈥攂eryllium-10 and beryllium-9鈥攕hould help to resolve this.
Since beryllium-10 decays with a half-life of about a million years, a high
ratio will mean that most of the cosmic rays are relatively young in
astronomical terms鈥攍ess than a million years old. A low ratio, on the
other hand, would suggest that most of the beryllium-10 has had time to decay
because the cosmic rays have been travelling in the Galaxy for much longer.
The AMS will also measure the relative abundances of other isotopes in the
cosmic rays, helping to pin down the nuclear processes responsible for forging
the elements in stars and in the big bang. Isotopes of the light elements, such
as deuterium, make useful cosmological probes. Their relative abundances are
very sensitive to the conditions in which they formed in the first few minutes
of the Universe. For instance, deuterium would have rapidly transformed into
heavier nuclei if the big bang fireball contained a large concentration of
protons and neutrons to fuse with it. So a low abundance of deuterium would
indicate a relatively high-density Universe when deuterium was forged.
Still further mysteries may be resolved by the AMS. For instance, it will
investigate the unidentified dark matter that makes up at least 90 per cent of
the Universe. 鈥淧ersonally, I think it is the most exciting thing about the
experiment,鈥� says Schramm. One possibility is that the dark matter mainly
comprises as yet undetected particles predicted by the so-called supersymmetry
theory. Supersymmetry is an attempt to unify gravity with the other forces of
nature, and predicts that every particle in nature has a supersymmetric
partner.
Such particles would have been created in the big bang alongside ordinary
particles, and if supersymmetry is correct, some may have survived until the
present day.
Astronomers have high hopes that the AMS might be the first instrument to
spot supersymmetric particles if they do actually exist in nature. One example
is the neutralino, the lightest of the stable supersymmetric particles. If it
exists, then it will have an antiparticle, the antineutralino, which might also
have survived. If neutralinos and antineutralinos are meeting somewhere in the
Universe and annihilating, as well as gamma rays they will produce both
positrons and antiprotons, which are the characteristic signature of
supersymmetric particle annihilation. So the AMS will be on the lookout for
these positrons and antiprotons.
The first results might not be far away. Ting and his colleagues are aiming
to launch the AMS for a week-long test flight on the space shuttle Discovery on
28 May 1998. This flight, which will last for about 100 hours at an altitude of
300 kilometres, will test the new detector鈥檚 performance under flight
conditions. It will also give the AMS the chance to make an accurate
measurement
of the number of antiprotons, if any, arriving at a given energy.
If a large proportion of the antiprotons have the same energy, this will be
very powerful evidence that they come from the annihilation of one type of
particle and its antiparticle. This fixed energy would depend only on the
mass of the particle. The most likely candidate is
the neutralino, as the chances are that the lightest of the supersymmetric
particles would be the most common in the Universe. The intensity of the
anti-proton signal would also reveal the density of the neutralinos in the
Universe. From this it would be possible to work out whether these particles
alone were enough to account for all of the mysterious dark matter.
But even tracing the signatures of neutralinos could not eclipse the
sensational discovery of antimatter empires. The physicist Werner Heisenberg
once said: 鈥淚 think that the discovery of antimatter was perhaps the biggest
jump of all the big jumps in physics in our century.鈥� Arguably, the
discovery of
natural anti-atoms, antistars and antigalaxies would be an even bigger jump,
shaking the foundations of astronomy and physics. 鈥淐osmology would be turned
upside down by the discovery of antinuclei,鈥� says Weinberg. 鈥淲e鈥檇 have to
rethink everything.鈥�
* * *
Detecting antimatter
The Alpha Magnetic Spectrometer is scheduled to be delivered to the
international space station on a space shuttle flight in February 2001. The
detector will remain on the station for about three years before being returned
to Earth. Its design and construction will be a truly international effort,
involving almost 100 scientists from Europe, the US, Russia, China and
Taiwan.
Although scientists have been building particle detectors for almost half a
century, they have never built one for space. According to Samuel Ting of CERN,
the main reason for this is that Earth-based detectors rely on powerful magnets
to determine the properties of particles. The way charged particles are
deflected in a magnetic field depends on their charge and mass, allowing
different nuclei to be identified.
On Earth, particle detectors use the most powerful superconducting magnets
available. But these are complex and bulky, as they have to be cooled to just a
few degrees above absolute zero. 鈥淧utting such an elaborate piece of equipment
in space would be extremely difficult and prohibitively expensive鈥攊n the
region of $200 million to $300 million,鈥� says Ting. Instead, his
team intends to take advantage of a new and powerful type of permanent magnet
made from iron, boron and the rare-earth element neodymium. 鈥淚t won鈥檛 be as
powerful as a superconducting magnet, but it鈥檒l cost only $500
000,鈥� says
Ting.
The 2-tonne magnet, a hollow cylinder about a metre high and a metre in
diameter, will create a uniform field of 0.15 tesla. Within the hollow magnet
will be a series of silicon detectors designed to track charged cosmic ray
particles (see
Diagram). Stacked in six layers, they
will track each charged
particle as it flies towards the Earth, registering its position in each layer
to within just 10 micrometres.FIG-20414501.gif
In the magnetic field, a positively charged particle will bend in the
opposite direction to a negatively charged one. To find out the amount of
charge, researchers will use the fact that a particle is slowed down by the
electrical force between itself and the electrons in the material through which
it is passing. This force is proportional to the size of the electrical charge,
so the greater the charge, the faster it loses energy to the material.
The energy loss will be measured in two ways: using the silicon detectors,
and slabs of plastic scintillator above and below them. The scintillators give
out a flash of light with a brightness in direct proportion to the energy of a
particle flying through, so comparing the brightness above and below gives a
measure of the particle鈥檚 energy loss.
The silicon tracker will also pin down the particle鈥檚 momentum, as a
particle
with a large momentum will be deflected less than one with low momentum. Along
with data on the particle鈥檚 velocity, this will allow physicists to work
out its
mass. For Ting and his colleagues, the Holy Grail will be a particle with the
same charge and mass as a carbon nucleus, for instance, but which is deflected
in the opposite direction鈥攖he unmistakable signature of an anticarbon
nucleus.
Most of the technology of the AMS has been tried and tested on Earth for
decades. But the trauma of blastoff is something no particle physics experiment
has ever gone through before. Ting鈥檚 team will sandwich the silicon sensors
between sheets of carbon fibre to keep them rigid during the shuttle
launch.