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PITY the quarks. Fifteen billion years ago, they ruled Creation, together
with their energy-bearing accomplices, the gluons. Then, suddenly, just 10
microseconds after the big bang, their reign was over. As the Universe cooled,
they were locked up inside nuclear dungeons鈥攑rotons, neutrons and more
exotic particles鈥攆rom which two generations of experimental physicists
have striven in vain to free them.

Yet quarks can break free. In theory, if you take nuclear particles and
squeeze them together tightly enough, the quarks they contain will burst out to
form their ancient quark-gluon plasma once again. For decades, physicists have
wondered whether this still happens anywhere in the Universe. To find out, they
have been studying the Universe鈥檚 densest lumps of matter鈥攁t the heart of
stars that have collapsed under the force of their own gravity, crushing most of
their material into neutrons. There, if anywhere, quarks may still roam free. If
they do, they will offer scientists a clearer picture of the life cycle of some
of the most bizarre objects in the heavens, and help to tidy up a particularly
messy corner of the theory of matter.

How iffy is that 鈥渋f鈥�? No one knows. For astrophysicists, the innards of
neutron stars are too extreme for comfort鈥攔egions where gravity distorts
matter so wildly that conventional nuclear physics no longer works. Quark
theorists, meanwhile, have the opposite problem: for them, neutron stars aren鈥檛
extreme enough. The rules governing quark behaviour (an elegant theory known as
quantum chromodynamics, or QCD) work fine at densities tens or even hundreds of
times higher than those inside neutron stars. Unfortunately, QCD says next to
nothing about what happens inside neutron stars themselves, and nothing at all
about how quarks could be set free there鈥攁 process that physicists
gratingly refer to as 鈥渄econfinement鈥�.

Now it looks as if the two camps are converging. Astrophysicists are starting
to come up with observational tests for 鈥渜uark matter鈥� in neutron stars. If
found, these telltale quirks would strongly suggest that free quarks have the
run of their interiors. Theorists, meanwhile, are labouring to bridge the gap
between the ideal matter that QCD can handle and the real stuff that stars are
made of. Late last year, physicists at the Institute for Advanced Study in
Princeton, New Jersey, and the Massachusetts Institute of Technology in
Cambridge, Massachusetts, announced a new approach that they believe gives the
first realistic sketch of the conditions under which quarks break free.

To find out whether neutron stars harbour quark matter, scientists need to
know what conditions are needed to make it, and whether these conditions exist
inside a neutron star. Energy and density are the crucial factors: how hot a
particular patch of matter is, and how tightly it is packed together. To
describe how a substance behaves over a wide range of conditions, physicists
draw its phase diagram. This is like a map with energy as latitude and density
as longitude, on which different regions represent the substance as a solid,
liquid, gas or some more subtle incarnation. If you had a complete phase diagram
for matter, determining whether quarks roam free inside neutron stars would be
as simple as finding your street on a regular map. You would simply put your
finger on the point representing conditions at the star鈥檚 core, and see if you
are touching an ordinary-matter continent or a quark-matter one.

The problem is that nobody knows where the continents are or where you should
point your finger. To know where to point, you need to know the conditions
inside the star. By measuring the orbital periods of binary neutron stars,
astrophysicists have calculated that most are between 1.4 and 1.5 times as
massive as the Sun, and they suspect that this mass is crammed into bodies about
10 kilometres in diameter. As in all stars and planets, the density is lowest at
the surface and increases toward the centre.

Squeezed from above and warped by the neutron star鈥檚 powerful magnetic field,
matter assumes a variety of forms鈥攃rystalline solids, liquids, gases,
superfluids and more outlandish substances. 鈥淵ou begin to get what we call
`pasta phases鈥�,鈥� says Gordon Baym, a physicist at the University of Illinois in
Urbana-Champaign. 鈥淪paghetti rods as nuclei, lasagne sheets as nuclear matter.
Then the matter tends to turn inside out, and eventually the whole thing merges
together to form a liquid, which is primarily neutron matter. That鈥檚 the inner
肠辞谤别.鈥�

The inner core is where quark matter may or may not exist. Everything hinges
on how dense the core is. Physicists suspect that protons and neutrons in an
ordinary atomic nucleus let loose their quarks somewhere around the point at
which the nuclear particles are squeezed so tightly together that there is no
space left between them. Because a nucleus contains roughly one-third particles
and two-thirds empty space, quarks probably break free at a density of around
three times that of a standard nucleus.

Astrophysicists suspect that the density of a neutron star core lies near
this value. The exact figure depends on how fiercely the neutron matter fights
the inward pull of its own gravity. If neutron matter is weak and mushy, then
1.4 solar masses of it might be enough to squeeze the core of a neutron star
into quark matter. If it is stiff and springy enough, however, it will be able
to bear its own weight without collapsing into the quark matter phase.

Critical density

Unfortunately, nobody knows how stiff a neutron star really is. The repulsive
forces that keep the star from collapsing even further come from interactions
among the nuclear particles, primarily neutrons, that make up the star.
Physicists have a handle on these nuclear forces where only a few particles are
involved, but they become impossible to calculate in a sea of particles.
Conditions at the core of a neutron star are tantalisingly close to the critical
density. But whether they exceed it or fall short is anyone鈥檚 guess.

In the absence of a good phase map to stick pins in, some astrophysicists are
pinning their hopes on indirect evidence. A star with quark matter in its core,
they reason, ought to look different from one without it鈥攊t鈥檚 just a
matter of figuring out what the differences should be. The clues probably won鈥檛
show up in ordinary radiation from neutron stars, such as light or X-rays. These
come from the outermost layers, which will be the same regardless of what lurks
in the stellar core. Instead, astronomers need to watch for characteristics that
spring from the heart of the star. So far they have two candidates: the way a
star cools, and the way it spins.

Early in its life cycle a neutron star cools off mainly by emitting
neutrinos: swift, shadowy particles that zip into being whenever an electron and
a proton are squeezed together to form a neutron. When a neutron star is hot,
during the first million or so years of its existence, the squeezing is fast and
furious and neutrinos spew from the core. Over time, as the supply of uncombined
protons and electrons runs low, the neutrino cascade starts to dry up and the
star cools mainly by radiating X-rays from its surface.

QCD predicts that quark matter should generate considerably more neutrinos
than ordinary nuclear matter does. As a result, physicists have assumed that a
young neutron star with a quark-matter centre ought to cool more quickly than
one with neutrons through and through. 鈥淨uickly鈥� is relative, of course鈥攖o
watch a single neutron star cool off would take tens of thousands of
years鈥攂ut by measuring the ages and temperatures of many known neutron
stars, astrophysicists have tried to construct general cooling curves for
typical neutron stars. At present the 鈥渃urves鈥� are more like smudges, because
neither the ages nor the temperatures of neutron stars have been calculated with
much precision. But scientists hope that things will improve when new satellites
due to be launched over the next two years start sending back more-sensitive
measurements of radiation from the stars.

The spinning of a neutron star is much easier to measure. Some neutron stars
whirl furiously, whipped by the explosions that gave them birth or by the
gravitational pull of nearby objects, including other neutron stars. Known as
pulsars, these spinning neutron stars give off radio blips so regular that
astronomers can measure their rotation rates to 13 decimal places.

Some scientists hope that changes in these rates might give evidence of a
quark-matter core. Norman Glendenning, a nuclear theorist and astrophysicist at
the Lawrence Berkeley National Laboratory in California, thinks astronomers
should take a close look at millisecond pulsars, neutron stars that spin at
hundreds of rotations a second. A millisecond pulsar starts out as an ordinary
pulsar whirling perhaps one to 30 times per second, but it obtains a new lease
on life by gobbling up matter from nearby companion objects. As a result,
instead of losing momentum and running down as pulsars normally do, it is
boosted to breathtaking speeds. Once the companion has been completely devoured,
the pulsar starts spinning down again, this time permanently鈥� unless
another sacrificial victim turns up.

But if quark matter should start to form in the centre of the star,
Glendenning argues, the pulsar might spin up again even without a fresh source
of matter to feast on. His scenario goes like this: First the slowing rotation
eases the centrifugal tug on the star鈥檚 matter, causing the core to draw more of
its material back into itself and the core becomes denser. If at this point the
quarks stay confined inside their neutrons, the core will shrink by only a few
percent鈥攖oo little to have a noticeable effect on the star鈥檚 rotation. If
the quarks break free, however, the core could shrink by as much as 30 per cent,
because free quarks repel one another more weakly than neutrons do. Like ice
skaters pulling in their arms in mid-spin, the star might then reverse its
spindown and speed up.

If such spontaneous spinups exist, Glendenning says, they should be easy to
detect. Twenty minutes of radio-telescope observations ought to be enough to
tell whether one is in progress. All astronomers have to do is to point the
telescope at the right kind of star, a millisecond pulsar with no companions to
change its rotation. Because his calculations show that spinups last about 20
million years鈥攁 fiftieth of the time it takes a millisecond pulsar to spin
down鈥擥lendenning predicts that astronomers should observe about one spinup
among every 50 qualified pulsars. Thus far only 25 qualified candidates have
been discovered, so the key piece of evidence could pop up at any time.

First principles

While astrophysicists hunt quark matter in the stars, other investigators are
still trying to pin down its place on the phase diagram. The answer is unlikely
to come from experiments. In principle you could sketch out the phase diagram
using data from particle accelerators, but while most particle experiments climb
ever 鈥渘orthward鈥� into realms of higher and higher energy, neutron stars lie far
to the 鈥渆ast鈥�, in high-density, low-energy regions where experimenters
seldom set foot. So the best hope for mapping may come from theory: models or
equations that colour in vast blocks of the diagram from first principles
alone.

That is what happened two decades ago, when quantum chromodynamics burst onto
the scene and opened up regions of extremely high energies or densities. The
reason is that QCD has a useful property called 鈥渁symptotic freedom鈥�, which
guarantees that its equations get easier and easier to solve as the conditions
they describe grow more and more extreme. Why? Because the forces between them
decrease the closer together quarks are, as if the quarks were connected by
elastic bands. If the quarks are squeezed enough, the forces dwindle until
physicists can approximate them as zero, an assumption that makes mathematical
difficulties melt away. The same thing happens at extremely high energies. As a
result, physicists can map in detail huge far-flung swathes of the phase
diagram鈥攖he Ultima Thule of immense energy and the Far East of super-high
density.

The catch is that most of these regions exist only on paper. They describe
combinations of energy and density more intense than have ever existed anywhere
in the Universe, let alone inside neutron stars or at the deconfinement barrier
where quarks burst out of nuclear particles.

Now a team of physicists, led by Frank Wilczek of the Institute for Advanced
Study in Princeton and Krishna Rajagopal of MIT, think they can bridge the gap.
They have created a mathematical model that predicts how quarks and gluons will
interact at high densities. In nature there are six kinds of quarks, but only
two of them play a role in ordinary matter. When Wilczek and his colleagues
tried to model matter with mathematical stand-ins for those two quarks, the
quark-gluon plasma in the equations gelled into a weird substance unlike any
known form of matter. When they added a third quark, however, things started
looking much more familiar. The model produced a phase change that, in
mathematical terms, bears a striking resemblance to quark confinement. The
result, Gordon Baym says, is the best hope yet for exploring the murky
no-physicist鈥檚-land where quarks burst out of their nuclear cells.

Wilczek stresses that his model will not revolutionise astrophysics
overnight. It still cannot produce the numbers needed to predict at which
energies and densities the freeing of quarks will take place, and it needs some
tinkering to become more realistic. In the current version of the model, all
three quarks have the same mass. In the real world, the third quark, known as
strange, is 20 times as massive as the up and down quarks from which nuclear
matter is made. Rajagopal and two associates are working on a follow-up model to
see whether a more realistic value for mass will still do the trick.

Meanwhile, Wilczek hopes to enlist the help of other scientists to test how
well the model鈥檚 mathematical phase change reflects reality. Experimental
physicists, for example, could use particle accelerators in novel ways to
explore the high-density, low-energy 鈥渆astern鈥� regions that have remained
uncharted until now. 鈥淭he machines are the same; they just have to be operated
differently,鈥� Wilczek says. 鈥淲e鈥檒l have to do some proselytising.鈥�

In the end, Wilczek is confident that his approach will track quark matter to
its lair鈥攚hether in the depths of space or on the connect-the-dots
continents of physical theory. 鈥淏asically,鈥� he says, 鈥渨e鈥檝e stumbled on the Holy
骋谤补颈濒.鈥�