Far below the foam and the freighters, in the pitch blackness and extreme
pressure of the deep Pacific, ghostly particles stream from the ocean floor.
These particles are high-energy neutrinos: subatomic hints of apocalyptic
events occurring millions of light years away.
And dangling from one of the longest pieces of string in the world,
off the coast of Hawaii, is the first of a new generation of huge telescopes
specially designed to explore the world of high-energy neutrinos. This
is DUMAND2, and it has three brothers and sisters round the world: AMANDA
at the South Pole, NESTOR in the Mediterranean near Greece, and NT-36 in
Siberia’s Lake Baikal.
With their long strings of light-detecting devices, lowered into deep
water or ice to wait for the telltale flashes that signal a neutrino,
none of them looks remotely like a conventional telescope. Each of them
will literally look ‘downwards,’ not skywards, to see evidence of neutrinos.
If they looked upwards, they would be overwhelmed by the steady rain of
neutrinos caused by showers of cosmic rays.
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By looking downwards, they can use the Earth as a filter to shield them
from this radiation. From their location thousands of feet below the surface
of water or ice, these telescopes will see evidence of neutrinos coming
the other way, through the Earth itself. As these neutrinos pass through
the planet, they interact with atoms, creating muons – positive or negative
particles with a mass much greater than an electron’s. If these muons happen,
in turn, to pass through something dense but transparent, such as water
or ice, they emit flashes of light called Cherenkov radiation. When they
do, the four neutrino telescopes will be lying in wait.
The only flashes these particular astronomers are interested in are
those caused by high-energy neutrinos coming from deep space, from active
galactic nuclei (AGNs) and other exotic objects. What distinguishes these
from any other sort is their direction. A neutrino moving upwards from
the Earth’s crust is almost certainly from deep space. This is why the new
telescopes have multiple strings of phototubes, parallel to each other.
A neutrino’s direction can be ‘plotted’ by the ‘line’ it makes as it – or
to be strictly accurate, the muon – passes through the water or ice, interacting
with different phototubes on different strings. The paths of the neutrino
and the muon are expected to be no more than a degree apart, so muons offer
a reliable way to link a particular neutrino to a particular celestial object.
Precise timing of the interactions is essential to know if the interactions
on a line were caused by the same particle, and were not simply part of
the random photon ‘noise’ from interference such as bioluminescence (the
production of light by organisms, here bacteria in the ocean), potassium-40
decay or atmospheric neutrinos. Once the neutrino’s path has been plotted,
the scientists can trace that line back to its point of origin – say, an
AGN. And they hope to ‘map’ AGNs and other high-energy neutrino sources
across the sky by exploiting the Earth’s rotation: and as the Earth rotates,
the neutrino telescope automatically ‘scans’ the sky.
High-energy neutrinos have been detected on Earth before by about a
dozen underground particle detectors. Most have been observed by the Irvine/Michigan/Brookhaven
(IMB) observatory near Cleveland. IMB has ‘seen’ muons caused by neutrino
interactions within the Earth rising at almost the speed of light. But the
real challenge of charting the neutrino sky requires a much larger detector.
IMB covers some 400 square metres; DUMAND2 alone will extend 20 000 square
metres. And, funds permitting, its successor, DUMAND3, is expected to cover
about a square kilometre.
High-energy neutrino telescopes could well extend the horizons of other
scientists, too. Because neutrinos would be the most likely particles to
interact with atoms in high-density parts of the planet, the neutrino flux
from within the Earth should reflect the planet’s internal structure. It
might even be possible to use high-energy neutrinos to ‘CAT scan’ the Earth’s
core and mantle, and perhaps to trace convection currents that drive plate
tectonics.
And particle physicists may want to join the party, too. ‘We’re talking
about neutrinos at energies we have no hope of producing on Earth, particularly
in the post-SSC days,’ says John Learned of the University of Hawaii at
Manoa, who is leading the DUMAND2 project. The Superconducting Super Collider
(SSC), a giant particle accelerator which was ‘killed’ late last year by
the US Congress, would have generated scientifically ‘useful’ neutrino beams
in the range 1 to 2 teraelectronvolts.
But the sky is also a ‘particle accelerator’ that almost certainly swarms
with neutrinos in the gigaelectronvolt and teraeletron-volt ranges – and
beyond, up to energies 100 million times greater than any terrestrial accelerator.
Such high-energy neutrino sources might help physicists test the theory
of neutrino oscillations, which postulates that three types of neutrinos
– the electron, muon and tau neutrino – can turn into each other. Only
the existence of the electron and muon neutrinos has been proven; perhaps
high-energy neutrinos from space will help to verify the tau neutrino,
too.
In December, scientists working on DUMAND2 (less elegantly known as
the Deep Underwater Muon and Neutrino Detector) lowered a 436-metre string
of 24 phototubes into the ocean west of Keahole Point on the Big Island
of Hawaii. The string, which is taller than the Eiffel Tower, floats vertically
in the water, anchored to cables from a ‘junction box’ 4760 metres down
on the ocean floor. The junction box transmits the strings’ observations
– muon flashes – as laser pulses along a 30-kilometre fibreoptic cable to
computers on the island.
The results were patchy: the system gathered 10 hours of optical data
before expiring, apparently because a temperature-regulating device on the
laser failed. But things went well enough to please the project leader,
Learned. By 16 December, the first components of the DUMAND2 array were
in place. ‘With the acoustical data being collected now, the DUMAND2 collaboration
has begun to operate the world’s first deep ocean neutrino telescope,’ Learned
told his colleagues in an e-mail message from the ship’s computer. Assuming
all goes well, they hope to lower the remaining two strings on a further
expedition, by mid-1994.
DUMAND2, which Learned and his colleagues have been planning since the
mid-1970s (DUMAND1, a single prototype short string, was lowered in 1987),
is funded by the US Department of Energy, the US National Science Foundation,
the Swiss National Science Foundation, as well as the Japanese ministry
of education, Mombusho, and various German sources. So far, $6 million of
the projected $10 million total costs has been received. Eventually, the
DUMAND2 array should consist of nine strings with 216 phototubes, arranged
in a vertical octagonal shape and linked to the junction box. According
to one of the DUMAND2 team, Peter Grieder of the University of Bern in
Switzerland, this set-up could generate 6 billion bytes of information
a week even after data filtering and compression. The next generation, DUMAND3,
should start in 1996 if all goes according to schedule, and if the estimated
$80 million funding is available. It will comprise approximately 8000 phototubes
and generate about 40 times as much data as DUMAND2.
Meanwhile, half a world away, other researchers have been pursuing a
radically different approach with the Antarctic Muon and Neutrino Detector
Array (AMANDA). Near the US polar station at zero degrees latitude, where
the wind-chill may be -30 degreesC or below on a ‘summer’ day and often
falls to about -70 degreesC, the Antarctic experimenters fire jets of
hot water into the polar ice to drill deep holes. In December, while their
DUMAND2 counterparts were bobbing on the waves, the AMANDA phototubes were
lowered a thousand metres into ice that has a surface temperature of around
-55 degreesC.
‘The deeper you go, the more transparent the ice becomes,’ says Buford
Price, one of the AMANDA team from the University of California at Berkeley.
This phenomenon occurs because air bubbles – which interfere with clarity
– are squeezed out by the extreme pressures at great depth. ‘At the surface
it’s like looking through snow, then a few metres lower it’s like looking
through ice cubes. And below a few hundred metres, there are literally no
bubbles left. It’s as clear as being in water that depth in Bermuda.’
Pure coincidence
According to Francis Halzen, who joined from the University of Wisconsin
at Madison, it was pure coincidence that the two teams were both assembling
their prototypes in December. ‘We have done very well in avoiding cutthroat
competition,’ he says. There is a sense of competition, but the two teams
exchange research findings with each other and with the groups working on
NESTOR and NT-36.
DUMAND2 and AMANDA have common roots: Learned and Halzen collaborated
on an early proposal for an ice experiment in the late 1980s, though Learned
is no longer directly involved with AMANDA. After initial tests of ice clarity
in Greenland in 1990, AMANDA scientists built the first, small-scale prototype
Antarctic detector in 1991 at the South Pole. Would it reliably transmit
the Cherenkov radiation? The results were ‘very encouraging’, they reported
in Nature back in September 1991. The ice transmitted signals farther than
18 metres. Now, during the 24-hour daylight of the Antarctic summer, they
are installing a larger prototype array near the US base that vaguely resembles
a lunar settlement with a huge geodesic dome surrounded by tractors, scientific
instruments and heated shacks.
The AMANDA scientists are naturally partisan: they reckon their approach
has distinct advantages over DUMAND2 and other sea-based neutrino searches.
Apart from its clarity, they believe that the ice contains virtually no
background radio – activity, unlike the interference in the ocean. The ice
also makes a conveniently solid and stable workbench and servicing ‘shop’
for the scientists – who can escape the difficulties facing the wave-tossed
DUMAND2 researchers thousands of kilometres away with their deeply submerged
electronics. Unlike DUMAND2, they do not have to route the data over many
kilometres of cable. As one DUMAND2 researcher confides: ‘Having just spent
a decade or two doing things in the ocean, I prefer working almost any place
±ð±ô²õ±ð.’
As long ago as 1988, Halzen was envisaging the Antarctic ice sheet as
a giant neutrino telescope: ‘While the South Pole may not be as pleasant
as Hawaii, or as convenient as Arkansas (where other reserachers have speculated
about creating a high – energy neutrino detector in an underground mine),
or even Lake Baikal, it may just turn out to be the best place to initiate
high-energy neutrino astrophysics.’
But DUMAND2’s Grieder sees drawbacks to the ice. AMANDA ‘offers some
intriguing advantages but faces horrendous logistics’, he observes. For
one thing, the location is miserable in human terms – the highest temperature
in summer at the South Pole is, on average, close to – 18 degreesC, and
biting winds can quickly freeze skin.
And these hardships still do not guarantee AMANDA greater scientific
success. The detector will not be as deep as DUMAND2 is in the Pacific,
mainly because it is harder to cut through ice than to lower phototubes
into water. So the polar neutrino observatory is not as well shielded from
atmospheric muons as DUMAND2. Just to make matters worse, the diameter of
the cores could limit the size of the phototubes that can be sunk.
The real debate, however, may be whether the Universe has sources of
high-energy neutrinos which are intense enough to be detected by the experimental
arrays as large as they are expected to become in the next few years, with
muon-intercepting areas of 20 000 square metres. ‘Whether the (DUMAND2)
array will be large enough to do neutrino astronomy is the question,’ says
Henry Crawford, a physicist at the University of California at Berkeley
who is a DUMAND2 associate. ‘We really don’t know how strong the (high-energy
neutrino) sources (in the sky) will be.’
Background ‘noise’ makes it especially difficult to detect high-energy
neutrinos with the current size of the detectors. Still, the worst fears
haven’t come true – yet. Originally, DUMAND2 researchers were concerned
about the background noise of photons emitted by two sources: bioluminescent
creatures and Cherenkov radiation from the beta decay of potassium-40 in
seawater. But background noise from the decay can be largely eliminated
by checking for coincident signals at adjacent phototubes, they say. Bioluminescence
isn’t a big worry, either; in the oceans it is normally triggered by turbulence,
but local sea currents are weak. And any anxieties about contaminants settling
on the phototubes have also evaporated. In a previous study by the DUMAND2
team, one phototube was lost for a year and a half at a depth of 5000 metres,
then recovered in a condition that Learned described as ‘bloody spotless.’
Neutrino watch
Money could help decide whether the ultimate superdetector is based
in water or ice. Learned acknowledges that they might not see anything very
interesting astronomically with DUMAND2. ‘The problem is that nowadays,
funding agencies want to know what you’re going to find before you find
it. Can you imagine what would have happened if Christopher Columbus had
had to fill out a ‘work breakdown structure’ form?’
In the end, both DUMAND2 and AMANDA, and their counterparts NESTOR and
NT-36, may end up being used to create a global high-energy neutrino telescope
– one that, by pooling data from observations at different locations, becomes
the equivalent of a neutrino telescope as big as the Earth. That way, says
Learned, there would be ‘all-sky’ coverage 24 hours a day. And, by precisely
timing when particles arrive at different stations, it would be possible
to locate more precisely the emitting source of, say, a supernova or a millisecond
pulsar. A similar technique has been used to locate gamma-ray sources.
NESTOR, lurking off the Greek coast, and NT-36 in Lake Baikal might
also play a part in this global neutrino watch. The Lake Baikal experiment,
which has been in operation since April 1993, involves a prototype neutrino
detector called NT-36 (it uses 36 phototubes). Participants include the
Institute of Nuclear Research in Moscow and universities in Moscow, Tomsk
and Irkutsk, plus foreign collaborators from Germany and Hungary. The NT-36
phototubes are suspended on three separate strings, supported by a metal
framework which looks like an an umbrella, as deep as 1.1 kilometres. Like
the other neutrino observatories, NT-36 can be expanded in steps. Eventually,
the Russians hope to have eight array strings with a total of 192 phototubes,
a detector that will be known as NT-200.
Each 37-centimetre phototube can detect a flash as short as two-billionths
of a second. It can spot muons moving at relativistic speeds, and can also
look out for magnetic monopoles – hypothetical particles which are postulated
to exist as isolated north or south magnetic poles – and nuggets of ‘strange’
quarks. By last autumn, the Baikal experiment had detected 30 million muons,
most of them from atmospheric neutrino events (see Science, 15 January).
However, about 1 in 10 000 appear to be moving upwards and may be the result
of neutrinos passing through the Earth. That exciting possibility will get
closer attention when the Russians install a 96-photomultiplier array next
spring. But Lake Baikal is only 1.4 kilometres deep, so it does not provide
the shielding the ocean provides for DUMAND2 and NESTOR.
And off the town of Pylos in southwest Greece, NESTOR (Neutrinos from
Supernova and TeV Sources, Ocean Range) is making full use of its advantages.
Leonidas Resvanis, a physicist at the University of Athens, is in charge
of a team of researchers from Athens, Moscow and Florence, which in 1991
started work on installing detectors and using them to measure the atmospheric
muon flux. By 1997, the team plans to build a hexagonal array of seven
towers, each of them about 250 metres tall and about 3800 metres deep.
But what kind of astronomy might these telescopes do? One source of
high-energy neutrinos is intense gamma-ray sources such as those now being
studied by NASA’s Compton Gamma Ray Observatory. Two gamma-ray sources in
the teraelectron-volt range are already known: the AGN Markarian 421 and
the pulsar within the Crab Nebula. Vic Stenger of the University of Hawaii
and co-principal investigator on DUMAND2, has explored the likelihood of
detecting high-energy neutrinos from AGNs, the expanding shells of supernovae
and binary neutron-star systems. The last look least promising because earlier
reports of extremely high-energy gamma rays from binary neutron star systems
such as Cygnus X-3 have not been confirmed. Also, no gamma rays in the teraelectronvolt
or gigalectron – volt ranges were detected from Supernova1987a, so supernovae
may not be so promising either. However, the expanding shell of a supernova
might absorb protons accelerated by any neutron star left over after the
blast, and these proton-cloud collisions might generate high-energy neutrinos.
So the most intriguing possibility remains the detection of neutrinos
from AGNs (see ‘Shine on, brilliant galaxy’, 18 September 1993). Active
galaxies such as quasars and blazars are the brightest known objects in
the Universe, and some are so far away that they date back to the earliest
days of the Universe. Astronomers believe that to appear so brilliant at
such a distance they must contain what Learned calls ‘some special engine’.
This engine must be comparatively small in astronomical terms because the
brightness of active galaxies changes over just a few days. But no one
knows of anything so small that can, at the same time, generate so much
energy – except that rather hypothetical object, a ‘supermassive’ black
hole.
These black holes are massive objects that have collapsed to something
comparatively tiny, perhaps about the size of the Solar System. For years,
astrophysicists’ guiding hypothesis has been that active galaxies are newborn
galaxies with a giant black hole at their core. An accretion disc of matter
swirls around it, and eventually falls inwards. Friction in the in-swirling
gas heats the material so it generates intense radiation. Stenger explains
how a black hole could produce high-energy neutrinos: ‘Protons are (expected
to be) accelerated by shock waves in the accreting matter around the central
black hole, or by some other mechanism. These collide with ultraviolet photons
in the surrounding medium, producing pions and other mesons which decay
to give neutrinos.’ And these high-energy neutrinos could, in turn, provide
astronomers with a way of probing the black hole’s immediate surroundings.
Even if AGNs fail to generate high-energy neutrinos, the new generation
of telescopes might see other sights – perhaps bizarre ones. One possibility
is that high-energy neutrinos may be detected from the transient, puzzling
objects called gamma – ray bursters. An even more exciting possibility is
detecting a hypothetical form of dark matter. Another ‘highly speculative’
possibility, Stenger says, is the detection of high-energy neutrinos generated
by cosmic topological defects, relics of the time just after the big bang.
Perhaps the most startling proposal is to use ‘neutrino tomography’
to map the Earth’s insides. This idea – originally proposed in the 1970s
by Learned and others – was revived in a talk given in December last year
to the American Geophysical Union by Chaincy Kuo. Kuo, a geophysics postgraduate
at the University of California at Berkeley and her colleagues are working
with the DUMAND2 team to explore this possibility.
For decades, seismologists have mapped the Earth’s innards using ‘seismic’
waves. Earthquakes and nuclear blasts trigger acoustic waves that ripple
through the Earth at different speeds through areas of different density.
This is not unlike medical ultrasound in that it uses sound waves to map
internal structures. The trouble is nobody knows for sure the precise mineral
composition of the Earth’s insides, or how that com-position varies from
place to place, or how sound waves move at extremely high pressures in
different places – some pressures are so high that they are difficult to
simulate in the laboratory. So seismology relies at least in part on some
rather theoretical assumptions.
Earth scans
In contrast, because neutrinos interact with matter in ways that are
well understood, they should provide a clear picture of the density variations
throughout the Earth; the greater the density, the greater the chance that
a neutrino will interact with an atom there. The neutrino flux penetrating
Earth might be used to map density variations within it. By pooling data
from different neutrino observatories, the scientists could scan the Earth.
These ‘Earth scans’ would take advantage of Earth’s rotation, mapping the
planetary mass distribution as high-energy neutrino sources ‘moved’ through
the ‘sky’ at the astronomers’ ‘feet’. With luck, such research might shed
light on deep – mantle processes and questions about the core-mantle boundary,
which are currently hot topics among geophysicists and important in understanding
the forces that drive plate tectonics. Still, it’s unclear at present what
advantages neutrino scanning offers over seismology.
Learned suspects it will take ‘a couple of generations’ before neutrino-scanning
of the Earth could work. ‘I’m not sure I’ll be around when it happens.’
Its success would depend heavily on the abundance of high-energy neutrino
sources in the sky, and on how large a detector it would take to detect
them. Learned stresses he’s not sceptical about geo-scanning: ‘It simply
will require observing millions of neutrinos, and with a one-kilometre-square
detector that could take years of observations.’
The high-energy neutrino researchers will not be surprised if they discover
something new. ‘I shall be very surprised if we’re not surprised,’ observes
Hugh Bradner, one of the pioneers of the field and a professor emeritus
of Scripps Institution of Oceanography in La Jolla, California. ‘In this
decade,’ Learned adds, ‘it’s almost certain we’ll see the beginning of high-energy
neutrino astronomy. Even if we don’t manage to ‘CAT-scan’ the Earth, we’re
opening a new window on the Universe. And who knows what wonders will appear?
We’re stepping off into terra incognita.’
Keay Davidson is science writer for the San Francisco Examiner and co-author
(with George Smoot) of Wrinkles in Time, published in Britain by Little,
Brown.