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BLACK HOLES litter our Universe, and loom large in sci-fi nightmares. Yet
they still have physicists pretty much stumped. 鈥淯nderstanding the behaviour of
black holes is really a central puzzle in modern theoretical physics,鈥� says
Andrew Strominger, a theoretical physicist at Harvard University. 鈥淲e鈥檝e been
wrestling with them for nearly 100 years and we still don鈥檛 understand
迟丑别尘.鈥�

A few experiments seem long overdue. But a black hole isn鈥檛 a piece of
equipment you can just knock together in a lab. Not yet, at least.

It is possible that in a few years we will be making black holes to order.
Unlike the monsters that wander the celestial wilderness, these home-made holes
won鈥檛 be heavy enough to swallow the Earth. They鈥檇 be as light as a small
protein molecule, and so frail that they鈥檇 live and die in a split second. But
that brief life could tell us enough to revolutionise our ideas about gravity.
鈥淚t鈥檚 just mind-boggling to think about making a black hole at an accelerator,鈥�
says Michael Turner of the University of Chicago.

Black holes are thought to form in space when a very massive star鈥檚 core
collapses under its own weight鈥攁nd keeps on going until it is crushed to a
point. This point is cloaked by the 鈥渆vent horizon鈥�, a boundary from within
which nothing, not even light, can wriggle out. To make a black hole in this way
you need a vast amount of matter, more than the mass of our Sun, to produce
strong enough gravity to crush itself. That鈥檚 hardly feasible in the lab.

To make a black hole of a more practical size, you have to give gravity a
helping hand by smashing matter together at high speeds to compress it. A
particle accelerator could do the job鈥攅xcept that, according to
conventional physics, the minimum energy required is 10 million billion times as
much as any existing particle accelerator can manage.

All that energy would go to make a black hole weighing just 10 micrograms.
Nothing lighter can make a black hole, it seems. It would have to be compressed
into an impossibly small space鈥攆orbidden by quantum mechanics鈥� in
order for its gravity to get strong enough to swallow light. The problem is that
gravity is terribly weak, many orders of magnitude weaker than the other forces
of nature.

However, gravity鈥檚 feebleness puzzles physicists
(see 鈥淭he edge of infinity鈥�),
and some of them suggest it can be explained if space has extra dimensions that we
can鈥檛 see. These invisible dimensions are peculiar in that only gravity can reach into
them. The idea is that gravity is so weak because it leaks into the extra
dimensions.

If that鈥檚 true, gravity could get a lot stronger very close to a piece of
matter. At a range of far less than a millimetre, it hasn鈥檛 had much chance to
leak sideways out of our Universe
(see Diagram). So the density at the
collision site of a particle accelerator might, after all, be enough to create a
black hole.

The latest word is that these mini black holes might put in an appearance at
the Large Hadron Collider, under construction at CERN, the European laboratory
for particle physics near Geneva. In 2006, the LHC will start smashing protons
and antiprotons together at energies of 14 teraelectronvolts.

Eating the Earth

In two papers posted on the Web in June, Steve Giddings of the University of
California at Santa Barbara and Scott Thomas of Stanford University calculate
that in certain theories with two or more extra dimensions, the LHC will make
about one black hole per second. They would each weigh just 5000 times the mass
of the proton, and be 10^-18 metres across.

Despite media scares that little black holes could grow by sucking in matter
around them, there鈥檚 no danger that these babies will eat the Earth. Cosmic rays
are continually smashing into the Earth鈥檚 atmosphere at even higher energies
than are found in particle accelerators. If it were possible to make a stable
and dangerous black hole that way, it would already have happened鈥攁nd we
wouldn鈥檛 be here.

But how come they are so innocuous? It turns out that small black holes don鈥檛
survive long. In the 1970s, Stephen Hawking showed that the intense
gravitational field of a black hole can make a particle and its antiparticle pop
up near the black hole鈥檚 event horizon. One might escape, while the other falls
in. The result is that black holes gradually lose energy and evaporate.

With big black holes, gradually is putting it mildly. The amount of radiation
escaping from a black hole is inversely proportional to the square of its mass,
so a hole with a mass of 30 times that of the Sun would take 10⁶¹ times the
current age of the Universe to disappear. But the kind of black hole that could
appear in the LHC would vanish in just 10^-24 seconds鈥攆ar too
quickly to gobble anything, let alone a planet.

But it would leave a calling card. Savas Dimopoulos of Stanford University in
California and Greg Landsberg of Brown University in Providence, Rhode Island,
have worked out that a black hole would decay into an unusually wide array of
particles鈥攁 splash of quarks, photons, electrons and muons among others.
鈥淏lack holes are completely democratic鈥攖hey鈥檒l decay into any particle you
know of,鈥� says Landsberg. 鈥淚t would light up a detector like a Christmas
迟谤别别.鈥�

Giddings and Landsberg say that by measuring the energy needed to make black
holes of different sizes, we could work out how many extra dimensions there are
and how tightly they are folded up. 鈥淗igh-energy experimentalists would become
the geographers of the extra dimensions,鈥� says Giddings.

鈥淚t would be a field day for astrophysics,鈥� says Turner. We鈥檇 know that mini
black holes pop up in space every day, from collisions between cosmic rays. They
would also have thrived in the hot, youthful Universe shortly after the big
bang. 鈥淲e鈥檇 have to go back and think about all the different places that tiny
black holes could be made in the Universe, and figure out their consequences,鈥�
says Turner.

Black holes at the LHC could allow scientists to test another idea. Hawking
pointed out that black holes seem to devour and erase information. Regardless of
what falls in, it all ends up as the same anonymous super-dense mush. The
history of the matter that formed the hole has been extinguished for good. But
other scientists suggest that the information is still there, and will be
imprinted on the Hawking radiation that comes out of a black hole. This question
is at the root of the nature of information鈥攊s it a fundamental quantity
in the Universe? Is it conserved or can it be destroyed?

Strominger says that if black holes show up in the LHC we could resolve this
question once and for all. The test would be simple: use accelerators to cook up
lots of black holes with different ingredients but the same mass, and see if the
radiation they emit as they decay looks the same. If it does, black holes really
do destroy information. The perfect antidote, perhaps, to the modern blight of
information overload.