杏吧原创

ON A clear dark night in October 1991, a spectacular visitor came from outer
space, heading for the deserts of western Utah. It was probably just a tiny
proton, but it had an amazing amount of energy鈥攎ore than a tennis ball
travelling at 300 kilometres per hour. In other words it was going at a cracking
pace. Had it raced a photon of light from one end of the Milky Way to the other,
a distance of about 130 000 light years, the light would have crossed the
finishing line first鈥攂ut only just. The zooming proton would have been
just half a centimetre behind.

Five years on, that event still haunts astronomers who study cosmic rays, the
particles that constantly rain down on the Earth鈥檚 atmosphere. In theory, such a
particle should not exist. While most cosmic rays have relatively low energies
and can safely be attributed to eruptions on the Sun and exploding stars in our
Galactic back yard, high-energy cosmic rays like the one that arrived in Utah
are in a completely different league. 鈥淭here鈥檚 no good reason why they should
exist up there,鈥� says Arnold Wolfendale of the University of Durham.

The unusual visitor was recorded by the Fly鈥檚 Eye detector in the Utah
desert, 75 miles southwest of Salt Lake City. Though the detector is now
defunct, when it was operating, its array of mirrors collected the light emitted
when high-energy cosmic rays collided with molecules in the atmosphere,
setting off a cascade of collisions involving hundreds of subatomic particles.
In such events, a growing shower of particles falls to the ground and excites
the electrons in nitrogen molecules, which glow faintly as they settle back
down. That October night, the Fly鈥檚 Eye picked up the brightest shower yet.

The famous Utah cosmic ray had an energy of 3.2 脳 10²⁰ electronvolts,
several hundred million times more than the protons that pop out of the world鈥檚
most powerful particle accelerator, Fermilab鈥檚 Tevatron near Chicago. It is the
most energetic to date, but it is not alone. Many others with similar energies
have been recorded, and they are becoming increasingly hard to ignore. 鈥淚t was
controversial when there was just one event,鈥� says Alan Watson of the University
of Leeds. 鈥淏ut people are not arguing about these events now.鈥�

Instead, they are worrying about where in the Universe these bizarre cosmic
travellers come from. With energies this high, the speeding particles must have
been born in events of almost unimaginable violence. Astronomers are casting
around for clues, and a new generation of cosmic-ray detectors鈥攕ome as
large as Rhode Island鈥攃ould soon solve the mystery once and for all.

It is easy enough to account for low-energy cosmic rays. Each second
thousands of these bombard the atmosphere from all directions. Detectors flown
on balloons and rockets have shown that most are protons鈥攈ydrogen
nuclei鈥攂ut there are also heavier nuclei, along with the odd electron.
Their average energy is fairly low, roughly a billion (10⁹) electronvolts.

Cosmic rays like these are probably born when giant stars explode as
supernovae at the end of their lives. The explosion sends a supersonic shock
wave ploughing outwards through the remnants of the star鈥檚 atmosphere. The blast
creates a dense band of hot, ionised gas that carries tightly tangled magnetic
fields. Some charged particles would bounce off this magnetic shield, and just
as a tennis ball bouncing between two walls would move faster if the walls were
pushed together, so the charged particles could pick up energy by bouncing
between the gas that envelops the supernova and the shock wave ploughing into
it.

Astronomers are agreed that the shock waves from supernovae tend to lose
energy and disperse after about 1000 years. A bit of number crunching shows that
in that time, charged particles can soak up as much as 10¹⁵ electronvolts of
energy. And while cosmic rays do not appear to come directly from supernovae,
charged particles would curve in the Galaxy鈥檚 magnetic field, disguising their
origins.

But supernovae cannot explain the many cosmic rays with energies of 10¹⁶
electronvolts and above (see
Diagram). Instead, astronomers have turned to
other objects鈥攏eutron stars, for instance. These collapsed stars, which
are roughly 20 to 30 kilometres across, have magnetic fields that are billions
of times stronger than the Earth鈥檚. As they rotate at up to 1000 times a second,
their whirling magnetic fields set up gargantuan electric fields.FIG-20594601.gif

Natural accelerators

In these electric fields, charged particles could be accelerated in much the
same way as they are in accelerators at Fermilab. But the maximum energy of any
accelerator鈥攏atural or not鈥攄epends roughly on the strength of the
magnetic field and the size of the accelerating region. Apply that to a neutron
star and the maximum energy is 10¹⁹ electronvolts at a push. Nothing, it seems,
could send a particle on its way with any more energy than that.

And yet, on that October night in the Utah desert, the Fly鈥檚 Eye detected a
cosmic ray with 30 times as much energy. 鈥淲hen you see something as unbelievable
as that, your first responsibility is to try to shoot it down,鈥� says Pierre
Sokolsky of the Fly鈥檚 Eye team, which found the footprint of the cosmic ray
buried within reams of data about a year after it arrived, and a good while
after the experiment had shut down. 鈥淢y first reaction was `get rid of that
thing鈥�!鈥� But every instrument check in the book failed to find fault with the
figures.

In fact, the Utah sighting has become just one of a growing dossier of
inexplicable cosmic rays that have made their way here鈥攎ore than forty
above 4 脳 10¹⁹ electronvolts, and eight above 10²⁰ electronvolts. These have
shown up at detector arrays in the US, Japan, Britain and Russia. The
observations suggest that far from being impossible, a steady trickle of cosmic
rays of 10²⁰ electronvolts strikes the Earth鈥攁bout one per square
kilometre per century.

One of the few things that does seem clear is that these high-energy cosmic
rays are probably coming from relatively nearby. Otherwise, they would have
fallen prey to the cosmic background radiation, a relic of the big bang
fireball, which fills every orange-sized region of space with about 10 000
photons of microwave radiation. To a particle moving at almost the speed of
light, this radiation appears to have much higher energy, an effect called
Doppler blueshifting. Cosmic ray protons would see a hot sea of gamma rays, and
would interact with them to produce new particles鈥攕prays of pions,
neutrons, gamma rays and protons鈥攍osing their energy in the process.

The upshot is that the highest energy particles must come from within 200
million light years or so of the Earth. Another good bet is that they travel in
a straight line from their sources. The magnetic fields of our Galaxy and others
are probably not strong enough to imprison such fast-moving particles in tangled
orbits. So they probably travel here from other galaxies, their paths pointing
straight back to their birthplaces.

But look for nearby sources in the directions from which high-energy cosmic
rays appeared, and there鈥檚 often nothing there鈥攏ot even a faint galaxy,
never mind some exotic energetic source. 鈥淭here鈥檚 no obvious correlation, and no
nearby sources in sight,鈥� says James Cronin of the University of Chicago.
However, Watson says that the data gathered up till now are starting to show a
slight tendency to coincide with the 鈥渟upergalactic plane鈥�, the part of the sky
in which galaxies in our local cluster congregate. This may be the first hint,
though it鈥檚 far from certain, that the cosmic rays are coming from energetic
galaxies nearby.

Worlds in collision

It鈥檚 possible, for instance, that colliding galaxies could be fuelling the
cosmic rays. Wolfendale says that the magnetic field lines of colliding galaxies
can intertwine and suddenly reconnect into a different pattern, setting up
enormous electric fields. These could well accelerate cosmic rays to the
observed energies, he says. And in a paper to be published in this month鈥檚
Journal of Physics G, Wolfendale and his colleagues report that two of the
highest energy cosmic rays found to date coincide with a pair of colliding
galaxies just 20 million light years away. 鈥淚f I had any money I鈥檇 bet some on
this,鈥� says Wolfendale. 鈥淚 think it is the best idea around at the moment.鈥�

A second possible source is radio galaxies, energetic galaxies that fire
double jets of particles into space. Peter Biermann of the Max Planck Institute
for Radio Astronomy in Bonn says that only the most powerful radio galaxies
could muster the strength. 鈥淏ut there are enough radio galaxies close by to play
the game,鈥� he says. And he is sure that the record-breaking cosmic ray recorded
by the Fly鈥檚 Eye came from the direction of a bright radio galaxy. Estimates of
this galaxy鈥檚 distance are vague because it hides behind the dusty plane of the
Milky Way. But it may be within firing range, as little as 150 million light
years away.

Others argue that attributing the rays to radio galaxies鈥攐r any
familiar objects for that matter鈥攊s pushing the bounds of plausibility too
far. 鈥淚n every case, you really have to stretch everything to the limit to get
up to 10²⁰ electronvolts,鈥� says Cronin. In the real world, he says, protons
would lose energy in collisions with the sea of radiation that usually surrounds
energetic objects. And as they curve in strong magnetic fields, protons that are
moving close to the speed of light would also start to radiate away their energy
as synchrotron radiation.

With no consensus in sight, many astronomers have turned to more exotic
ideas. For instance, could there be a link with gamma ray bursters, baffling
sources of gamma radiation that flash brightly somewhere in the sky about once a
day, then disappear without trace? 鈥淭hese are the most powerful explosions we
know in the Universe,鈥� says Eli Waxman of the Institute for Advanced Study in
Princeton, New Jersey. Because the flashes come from random directions rather
than the regions where matter in our own Galaxy and its neighbours is
concentrated, many astronomers believe that they originate in distant corners of
the Universe. No one has any convincing ideas about what causes them, although
popular candidates include exotica like colliding neutron stars.

But regardless of what the astronomical ingredients are, says Waxman, the
bursts of gamma rays have the hallmark of the synchrotron radiation that you
would see if electrons were accelerated to close to the speed of light in a hot,
ionised wind. If so, the wind should release just as much energy in high-energy
particles, including protons, as in gamma rays. Waxman adds that the total
energy of all the cosmic rays in the Universe above about 10¹⁹ electronvolts
seems to closely match estimates for the total energy of gamma ray bursts in the
Universe, which is what one would expect if both were caused by the same
events.

Knots in space

Some suggestions are stranger still. In December last year, G眉nter Sigl
and David Schramm of the University of Chicago and their colleagues published a
paper in Science suggesting that two of the highest energy cosmic rays
may have come from the collapse of hypothetical 鈥渢opological defects鈥� in the
fabric of space-time (鈥淐osmic beakers鈥�, New 杏吧原创, 21 September, p
46). These are strange knots that may harbour heavy particles, such as so-called
X particles, that were left over from the first split-second after the big bang.
X particles would have carried the single force that probably existed then, and
they would have been one thousand billion times heavier than a lead nucleus.

An X particle, released in the collapse of a topological defect, would
immediately explode into jets of thousands of particles containing energies of
around 10²⁵ electronvolts. With such gigantic energies, the cosmic rays could
come from well beyond our local cluster, interact with the microwave background
radiation several times on their way to Earth, and still arrive with
top-of-the-range energy. 鈥淚t鈥檚 very exotic and speculative, but at least it
doesn鈥檛 violate any of the basic principles of physics,鈥� says Schramm. 鈥淚鈥檇 give
it good odds.鈥�

鈥淭hat鈥檚 a beautiful fantasy and a very exciting idea, but we think we should
try the cautious ideas first,鈥� says Biermann. In fact, the only thing that
astronomers all agree on is that all the ideas are highly speculative. Solid
theories cannot rest on a handful of observations, they say, and far more data
are needed. Sokolsky even suspects that the highest energy events may be
illusions. 鈥淥ne worries a lot that somehow we鈥檙e just missing some instrumental
effect,鈥� he says.

But these doubts are certain to vanish in the next few years. An upgraded
version of the Fly鈥檚 Eye, known as HiRes, is now being built in Utah. It will
begin collecting data early next year, and should improve on the original Fly鈥檚
Eye tally of high-energy cosmic rays by a factor of ten.

And if all goes according to plan, detector arrays in Argentina and Utah,
covering a total of 6000 square kilometres, could harvest about 60 cosmic rays
with energies above 10²⁰ electronvolts each year. The Pierre Auger Project, as
it is known, has a target starting date of 2001. The detectors will measure the
type, energy and direction of cosmic rays. NASA too is considering a proposal
for a cosmic ray observatory鈥攁 pair of satellites that could fly early
next century (see 鈥淐atching cosmic rays鈥�).

With a sharper view of where the cosmic rays are coming from, it might be
possible to match them with colliding galaxies or powerful radio galaxies. They
may turn out to be more randomly scattered, hinting that gamma ray bursters or
topological defects鈥攐r some astronomical oddities that no one has even
dreamt of yet鈥� could be the culprits. 鈥淚t鈥檚 possible that we鈥檙e going to
discover a little piece of the big bang itself,鈥� says Schramm.

鈥淲e might find that these things don鈥檛 correlate with any darned thing we
know,鈥� says Cronin, who along with Watson is leading the Pierre Auger Project.
鈥淏ut it鈥檚 inconceivable to me that we won鈥檛 see structure of some kind.鈥� The
energies will also help to narrow down the options. Biermann concedes that just
one cosmic ray with 10²² electronvolts of energy, for instance, would put radio
galaxies out of the running.

As well as laying a mystery to rest, these projects could herald the start of
a whole new way of looking at the Universe. 鈥淲e鈥檙e moving into crude astronomy
using charged particles,鈥� says Cronin. 鈥淓ach one is packed with information.鈥�
Just as infrared, visible light and radio waves reveal what鈥檚 going on in stars,
galaxies and the clouds of material in between, high-energy cosmic rays will
tell the tales of their own birthplaces and journeys. For instance, they could
reveal whether the magnetic fields in different regions of our Galaxy and beyond
are strong enough to deflect the rays from their paths by a few degrees. This
would allow astronomers to build up a map of the Galaxy鈥檚 magnetic fields, which
might help to explain how they came into being in the youthful Milky Way.

Accelerating for free

There could be rich pickings for particle physicists too. The Auger and OWL
projects (see 鈥淐atching cosmic rays鈥�) will provide an opportunity to watch
nature鈥檚 accelerators in action. Imagine a particle accelerator that comes free
of charge, needs no maintenance, and whips particles up to energies 100 million
times greater than Fermilab鈥檚 Tevatron can manage. Just as the debris of
collisions at Fermilab has brought new particles and forces to light, the ratios
of different high-energy particles speeding across the Universe could do the
same at much higher energy.

鈥淵ou can鈥檛 reach out into the sky and turn a screwdriver,鈥� says Biermann.
鈥淏ut you could check much higher energy interactions, for instance.鈥� The
Universe鈥檚 curious accelerators may be idiosyncratic, but they could take
particle physics into a whole new realm.

* * *

Catching cosmic rays

VERY FEW high-energy cosmic rays reach the Earth, making it nearly impossible
for small, orbiting satellites to catch them directly. Fortunately, larger,
ground-based detectors have a much better chance. When each cosmic ray hits the
Earth鈥檚 atmosphere, it collides with molecules to generate 鈥渁ir
showers鈥濃�攕preading cascades of photons and subatomic particles (electrons
and muons) that rain down over vast areas. A cosmic ray with 10²⁰ electronvolts
of energy, for instance, creates a shower of particles covering about 10 square
kilometres on the ground. Detector arrays can identify and count the particles,
and this gives a measure of the original cosmic ray energy and clues to its
type.

While astronomers have explanations for many of the low-energy cosmic rays,
the really high-energy ones remain a mystery. The only detector currently
searching for high-energy cosmic rays is the AGASA array, southwest of Tokyo.
This collects showers over an area of 100 square kilometres, and will continue
operating until 2000.

But in early 1997 the first of a new generation of detectors is scheduled to
begin collecting data. The old Fly鈥檚 Eye detector in Utah will be dusted down
and upgraded to increase its collecting power by a factor of ten. In this
project, called HiRes, detector arrays will sit at the top of two mountains 13
kilometres apart. Eventually, each will be a compound 鈥渇ly鈥檚 eye鈥� of 54 mirrors,
each 2 metres in diameter.

The two eyes will look for the glow of atmospheric nitrogen molecules, which
fluoresce when struck by particles. With double the collecting area of the
original detector, the new system will be sensitive enough to see the glow of a
4-watt light bulb 20 kilometres away. Also, with two separate arrays the
instrument will have stereo vision, giving a better handle on the precise
direction of the original cosmic rays.

The next new observatory to come on line should be the Pierre Auger Project,
named after the French physicist who discovered air showers in 1938. If all goes
according to plan it will be the largest ever detector array, and will be
completed in 2001. The project leaders hope to raise $100 million for
their array, which would operate for about 20 years.

Each of the two arrays in the Auger project will be made up of 1600 particle
detectors, 3000-gallon tanks containing nothing more high-tech than water. The
tanks will be spaced 1.5 kilometres apart over an area of 3000 square
kilometres. As the high-energy particles stream through the water faster than
the speed of light in water, they excite water molecules which then emit flashes
of light. These concentrate into blue flashes known as Cherenkov
radiation鈥攖he visible equivalent of sonic booms鈥攚hich will register
in sensitive light collectors. A fly鈥檚 eye detector will sit at the centre of
each array to look out for the simultaneous glow of excited nitrogen in the
atmosphere.

The designers say that the combined arrays will pick up about 60 cosmic rays
with energy greater than 10²⁰ electronvolts each year. The times of arrival of
particles at the different detectors will be logged to an accuracy of 10
nanoseconds using signals from the Global Positioning System satellites, and
this will pinpoint the direction of the cosmic ray to an accuracy of about
2掳.

鈥淭he Auger project will push what we can do on Earth to the limits,鈥� says
Pierre Sokolsky of the Fly鈥檚 Eye team. So beyond that, there may be moves into
space. 杏吧原创s at NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland,
have designs on an even more ambitious mission. The OWL (Orbiting Wide-angle
Light collectors) project would send two telescopes, with mirrors about 2 metres
across, into orbit. Looking down on the Earth from space, these would pick up
any fluorescence in the atmosphere.

The effective area surveyed by the OWL would be about ten times that of the
Pierre Auger project. Within a year, it would have pinpointed about 100 events
that are as energetic as the top two to date (more than 2 脳 10²⁰ electronvolts)
to an accuracy of 1掳. The OWL eyes won鈥檛 fly at least until the next
millennium, at an estimated cost of $200 million.