杏吧原创

NOBODY has ever seen a black hole. Yet, despite this lack of direct evidence,
most scientists believe that a massive star at the end of its life can implode
to form an object so dense that nothing鈥攏ot even light鈥� can
escape.

They may be about to change their minds, however. Two
researchers in the US are pointing out that physicists have swept some
鈥渉umiliating鈥� problems with black holes under the carpet. By confronting these
problems, they say, they have found an alternative fate for a collapsing star.
Emil Mottola of the Los Alamos National Laboratory in New Mexico and Pawel Mazur
of the University of South Carolina in Columbia think it might turn into an
exotic bubble of superdense matter, an object they call a gravastar.

According to Mottola and Mazur, gravastars are cold, dense shells
supported by a springy, weird space inside. They鈥檇 look like black holes, lit
only by the material raining down onto them from outside. In fact, they seem to
fit all the observational evidence for the existence of black holes.

So far, however, physicists have mixed feelings about the
idea of gravastars. Their verdicts range from 鈥渙utstandingly brilliant鈥� to
鈥渦nlikely鈥�. What鈥檚 certain is that gravastars will rekindle a great debate of
the early 20th century: are black holes fact or fantasy?

The idea of black holes dates back to the First World War, when German
astronomer Karl Schwarzschild solved the equations of Einstein鈥檚 newborn theory
of gravity while serving on the Russian front. He showed that space-time around
any massive star would be curved. Squeeze a large enough star into a tiny enough
space and its density would become infinite and the curvature of space-time
would spiral out of control. The gravity near one of these objects would be so
strong that nothing鈥攏ot even photons鈥攃ould escape its grasp.

Einstein shared the view of most physicists of the time that such objects,
later dubbed black holes, were too outrageous to exist. He argued that it was
all academic anyway, since stars never shrink this small. But scientists
gradually became convinced that they do. If a star is very massive, it will
blast apart in a supernova explosion at the end of its life and if a core twice
as heavy as the Sun remains, no known force can prevent gravity squeezing it to
a point.

The result is a 鈥渟ingularity鈥� with infinite density, where the known laws of
physics break down. The singularity鈥檚 gravity would be so powerful it would be
cloaked in an 鈥渆vent horizon鈥�, a boundary beyond which matter or light couldn鈥檛
get out. The dramatic idea of a black hole, which would rip to shreds anyone
caught inside it, fired the imaginations of scientists, artists and writers
alike. But no one has ever rooted the drama in fact. 鈥淪o far, there is no direct
observational evidence to show that any of the things astronomers call black
holes have event horizons or central singularities,鈥� says Neil Cornish, an
astrophysicist at the University of Montana in Bozeman.

We know there are compact objects millions of times as heavy as the Sun that
hog the centres of galaxies. These black hole candidates give themselves away
because hot stars, gas and dust spiralling towards them emit bright X-rays. But
that doesn鈥檛 mean there鈥檚 a cataclysmic black hole in the vicinity; it could
simply be a very massive object. The debate petered out decades ago but there鈥檚
still no ironclad proof that black holes exist.

But never mind the lack of physical evidence鈥攖here are enough problems
in black-hole theory itself to make their existence seem implausible to say the
least. These problems stem from the fact that our Universe is actually very
different from the one that Schwarzschild considered. If we鈥檙e to produce a
proper description of the Universe we live in, Einstein鈥檚 classical theories
need to be meshed together with what we know about the quantum laws governing
the behaviour of fundamental particles and fields.

Mazur and Mottola have been thinking about quantum gravity for nearly a
decade. They began by examining the nature of 鈥渜uantum fluctuations鈥� in space,
time and even in energy fields. Empty space, for example, is never really empty.
On the tiniest scales, little particles are popping in and out of existence all
the time, creating a seething, fluctuating fluid. 鈥淟ike a fish in a calm pond,
who is not aware of all the incessant jiggling of the water molecules, we are
usually not aware of the quantum medium we are immersed in,鈥� says Mottola.

And they have found that quantum fluctuations in the electromagnetic fields
that describe tiny things like photons can influence gravitational phenomena on
the large scale鈥攕uch as black holes. So, they reasoned, when early
black-hole theorists ignored quantum effects they were creating an unreal
space-time.

Information overload

This traditional approach to black holes has produced strange anomalies
anyway, and these have remained unresolved, Mazur and Mottola claim. There are
problems, for instance, with a black hole鈥檚 entropy, a measure of the amount of
information it holds. An object that contains many possible states has high
entropy, in the same way that a computer with more bits of memory can store more
information. When a star forms a black hole, all the unique information about
the star鈥攊ts chemical composition, for instance鈥攁ppears to be
squashed out of existence. Yet current theory suggests black holes have
enormous entropy鈥攁 billion, billion times that of the star that formed
them. No one can fathom where all this extra entropy comes from or where it
resides. 鈥淲here are all these zillions of states hiding in a black hole?鈥� says
Mottola. 鈥淚t is quite literally incomprehensible.鈥�

Another seemingly impossible feature is that photons falling into a black
hole would gain an infinite amount of energy by the time they reach the event
horizon. But the gravitational effects of this enormous energy are ignored in
the classical theory. Mottola says these problems have forced physicists to
dream up far-fetched excuses. They say, for example, that some of the black
hole鈥檚 entropy might be hidden in other universes. Mottola doesn鈥檛 buy these
鈥渆soteric assumptions鈥�, and concludes that black holes are a bag of
contradictions that don鈥檛 make a good case for their own existence at all.

But is there an alternative? Could it be that when a star collapses,
something happens to prevent a black hole forming? Mazur and Mottola think so.
They have shown that quantum effects can make space-time change into a new and
curious state that would lead to the formation of a strange new object.

That change is a phase transition, like liquid water turning into a solid
block of ice. They believe that in the extreme conditions of a collapsing star,
space-time undergoes a quantum version of a phase transition. The phenomenon is
nothing new. The Nobel Prize for Physics in 2001 was awarded for the observation
of just such an event in the lab: the transformation of a cloud of atoms into
one huge 鈥渟uper-atom鈥�, a Bose-Einstein condensate (BEC). This clump of atoms,
which all share the same quantum state, forms at temperatures within a whisker
of absolute zero.

When an event horizon is about to form around a collapsing star, Mazur and
Mottola believe that the huge gravitational field distorts the quantum
fluctuations in space-time. These fluctuations would become so huge they would
trigger a radical change in space-time, very similar to the formation of a BEC.
This would create a condensate bubble. It would be surrounded by a thin
spherical shell composed of gravitational energy, a kind of stationary shock
wave in space-time sitting exactly where the event horizon of a black hole would
traditionally be. The formation of this condensate would radically alter the
space-time inside the shell. According to Mazur and Mottola鈥檚 calculations, it
would exert an outward pressure. Because of this, infalling matter inside
the shell would do a U-turn and head back out to the shell, while matter outside
the shell would still rain down on it.

In a paper submitted to Physical Review Letters, Mazur and Mottola
have shown that, like classical black holes, gravastars are a stable solution of
Einstein鈥檚 equations. What鈥檚 exciting, they say, is that gravastars don鈥檛 suffer
any of the mathematical ailments of black holes. There鈥檚 no riotous singularity
where the laws of physics break down. There鈥檚 no event horizon to imprison light
and matter. And the entropy of a gravastar would be much lower than that of any
star that might collapse to form it, dodging the problem of excessive entropy
that plagues black holes.

Take a gravastar with a mass 50 times that of the Sun, for example. Like the
event horizon of a black hole with the same mass, the shell would be roughly 300
kilometres in diameter. But it would be around just 10^-35 metres thick.
Just a teaspoonful of the material would weigh about 100 million tonnes. But
Mazur and Mottola have shown it would have a temperature of only about 10
billionths of a degree above absolute zero. And it wouldn鈥檛 emit any radiation,
making it as black as any black hole would be.

Dark energy

Gravastars would be just as much fun for sci-fi buffs鈥攊n fact they鈥檇 be
even more ruthless. Imagine a black hole of a million solar masses, like the one
thought to sit in the centre of our Galaxy. You could cross its event horizon
without feeling a thing: it鈥檚 only as you approached the singularity that you鈥檇
be torn apart by the huge gravity gradient. But if you were drifting towards a
gravastar of the same size, you鈥檇 never get anywhere near its centre. As soon as
you hit the shell you鈥檇 explode into pure gravitational energy.

Marek Abramowicz, an expert on black holes at Gothenburg University in
Sweden, calls the idea of gravastars 鈥渙utstandingly brilliant鈥�. 鈥淭heir unique
and remarkable properties could explain several high-energy astrophysical
phenomena that now are puzzling.鈥� He thinks they might explain gamma-ray
bursts鈥攗ltra-intense flashes of gamma radiation from a distant source that
appear somewhere in the sky about once a day.

Astronomers aren鈥檛 certain what causes gamma-ray bursts. It might be the
formation of a black hole in a supernova explosion, but this process would
struggle to muster enough energy. The birth of a gravastar, on the other hand,
would be extraordinarily violent and might shed enough energy to account for
gamma-ray bursts.

Mottola points to another possible connection between gravastars and
astronomical observations. Three years ago, data from distant stellar explosions
suggested that the expansion of the Universe is getting faster all the time
(New 杏吧原创, 11 April 1998, p 26). Many physicists ascribe this
acceleration to a mysterious 鈥渄ark energy鈥� that gives space an outward pressure.
Mottola says that if you scale the size of a gravastar up to around the size of
the visible Universe, the pressure of the vacuum inside roughly matches the
pressure that seems to be accelerating the expansion of the Universe. So our
Universe might be one cosmic gravastar: a giant shell trapping the Milky Way and
all the other galaxies we see. 鈥淲e might be able to entertain the really radical
notion that we鈥攁nd everything we see in the Universe鈥攃ould be inside
such an object,鈥� Mottola speculates.

It鈥檚 a bold claim, and he and Mazur are still working out whether it鈥檚
justifiable. Unlike their hypothetical gravastar, the Universe contains copious
ordinary matter and its visible edge is always ballooning outwards. But they鈥檙e
keen to see what happens when they modify their gravastar model to include these
complications. 鈥淚t is certainly premature at this point, but the seeds of a
possible new cosmological model are contained in the gravastar solution,鈥� says
Mottola.

Fact or fantasy?

In the meantime, they are trying to figure out how they could tell
ordinary-sized black holes and gravastars apart. The differences might be
subtle鈥攁fter all, in isolation, they鈥檙e both dark and the gravitational
fields outside a black hole event horizon and the gravastar shell would be the
same. But a good guess would be that gravastars would shine more brightly, since
matter falling onto one would be turned into radiation. Black holes would gobble
all the matter, but a gravastar would let its energy escape.

The next step is to identify the telltale signs of a gravastar, Mottola says.
鈥淚t is the only way to convince the sceptical鈥攊ncluding
ourselves鈥攖hat nature really behaves this way.鈥� Yet physicists aren鈥檛 even
sure what black holes look like. In October last year, they reported seeing what
appeared to be a heavyweight black hole, but material falling onto it is
emitting far brighter X-rays than theories predict. The excess energy is roughly
equivalent to the output of 10 billion Suns. If it is a black hole, it鈥檚 not
clear why it鈥檚 so bright.

The object may be whirling round and dragging magnetic fields at the event
horizon with it, and these could generate the extra energy by whipping up and
heating nearby gases. But Mazur thinks there鈥檚 a better explanation for that
extra energy. The 鈥渂lack hole鈥� could be a gravastar, he says. Stars, gas and
dust raining down onto its shell would violently dissolve into pure
gravitational energy that might emerge as bright X-rays.

To try to resolve this issue, Mazur is working out what a rotating gravastar
might look like. Like every other compact object in the Universe, a gravastar
would almost certainly be spinning rapidly.

Not all astronomers are as enthusiastic about gravastars. Cornish questions
whether an exploding star could really lose enough entropy to form a gravastar,
given that the second law of thermodynamics says that the entropy of an isolated
object will always tend to increase. 鈥淚n other words, a cup can break into a
thousand pieces, but it is highly unlikely that a thousand shards of pottery
will spontaneously come together to form a cup,鈥� says Cornish. 鈥淢azur and
Mottola talk about a star shedding entropy in some way to make the formation of
a gravastar possible, but I don鈥檛 think that is a likely scenario.鈥� But Mottola
points out that when exploding stars form other remnants, such as neutron stars,
they do shed entropy.

And although Cornish admits that black hole singularities are mathematically
troublesome, he also believes that a satisfactory quantum theory of gravity will
cure this problem. Then there鈥檒l be no need for gravastars, he says. Robert Wald
of Chicago University adds that Mottola and Mazur have put forward no arguments
about how gravastars could form in the devastating collapse of a massive star.
Even if they did form, how would they survive the onslaught of matter raining
down on them? 鈥淲hat happens if a gravastar has accreting matter showered upon
it? Won鈥檛 it collapse to a black hole?鈥� he says.

鈥淭he gravastar is stable,鈥� counters Mottola. He says that matter falling onto
the shell could make it wiggle and radiate away energy, but because the
gravitational pull of the shell balances the force of the springy vacuum inside,
it couldn鈥檛 actually collapse. Any matter that fell onto the shell would simply
become part of it, he says.

All the same, Mottola and Mazur admit there are still unsolved issues with
the formation of gravastars. 鈥淲e must have a better idea of how this phase
transition actually occurs in the gravitational collapse process,鈥� says Mottola.
The exact nature of the exotic stuff inside the gravastar shell is still open to
debate, and they hope to find out whether gravastars can really form in the
mayhem of a star鈥檚 violent death鈥攁nd whether gravastars could merge to
form the heavyweight objects that sit at the centre of galaxies. They are
encouraging others to join the investigation. 鈥淭here are many unanswered
questions and we are really just opening a new direction for future research,鈥�
says Mottola.

But if gravastars can weather the controversy, then maybe there鈥檒l no longer
be any need for black holes鈥攎aybe they really are pure fantasy. It
wouldn鈥檛 be the first time that Einstein鈥檚 dazzling intuition has been proved
correct.