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New Theories Dispute the Existence of Black Holes



New Theories Dispute the Existence of Black Holes
January 17, 2002 08:00 CDT



Two U.S. scientists have questioned the existence of black holes and
suggested, in their place, the existence of an exotic bubble of superdense
matter, an object they call a gravastar. The two are pointing out that
physicists have swept some "humiliating" problems with black holes under the
carpet. By confronting these problems, they claim to 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 gravastars are
cold, dense shells supported by a springy, weird space inside. They'd 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 "outstandingly brilliant" to "unlikely." What's
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's 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 -- not even photons -- could
escape its grasp.

Einstein shared the view of most physicists of that 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 "singularity" with infinite density, where the known laws of
physics break down. The singularity's gravity would be so powerful it would
be cloaked in an "event horizon", a boundary beyond which matter or light
couldn't escape.

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.

"So far, there is no direct observational evidence to show that any of the
things astronomers call black holes have event horizons or central
singularities," 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 centers of galaxies. These black hole candidates give themselves away
because hot stars, gas and dust spiraling toward them emit bright X-rays. But
that doesn't mean there's a cataclysmic black hole in the vicinity; it could
simply be a very massive object. The debate petered out decades ago but
there's still no ironclad proof that black holes exist.

There 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're to produce a proper description of the Universe we live
in, Einstein's classical theories need to be meshed together with what we
know about the quantum laws governing the behavior 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 "quantum fluctuations" 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. "Like 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," Mottola
said.

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-such as black holes. So, they reasoned, when early black
hole theorists ignored quantum effects they were creating an unreal
space-time.

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's 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 --
its chemical composition, for instance -- appears to be squashed out of
existence. Yet current theory suggests black holes have enormous entropy -- a
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. "Where are all
these zillions of states hiding in a black hole?" says Mottola. "It is quite
literally incomprehensible."

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's entropy might be hidden in other universes.

Mottola doesn't buy these "esoteric assumptions" and concludes that black
holes are a bag of contradictions that don't 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 "super-atom," 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's 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's equations. What's exciting, they say, is that gravastars don't
suffer any of the mathematical ailments of black holes.

There's no riotous singularity where the laws of physics break down. There's
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 kilometers in diameter. But it would be around just 10-35 meters thick.
Just a teaspoonful of the material would weigh about 100 million tons. 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't emit any
radiation, making it as black as any black hole would be.

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

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

Astronomers aren't 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 Scientist, 11 April 1998, p 26). Many physicists ascribe this
acceleration to a mysterious "dark energy" 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 big cosmic gravastar: a giant shell trapping the
Milky Way and all the other galaxies we see. "We might be able to entertain
the really radical notion that we -- and everything we see in the Universe --
could be inside such an object," Mottola speculates.

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

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 -- after all, in isolation, they're 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 said.
"It is the only way to convince the skeptical-including ourselves-that nature
really behaves this way." Yet physicists aren't 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's not clear why it's 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's a better explanation for that
extra energy. The "black hole" 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.

"In 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," says Cornish. "Mazur and Mottola talk about a star shedding
entropy in some way to make the formation of a gravastar possible, but I
don't think that is a likely scenario." 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'll 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? "What happens if a gravastar has accreting matter showered upon
it? Won't it collapse to a black hole?" he says.

"The gravastar is stable," 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't 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. "We must have a better idea of how this phase
transition actually occurs in the gravitational collapse process," 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's violent death -- and whether gravastars could merge to
form the heavyweight objects that sit at the center of galaxies. They are
encouraging others to join the investigation. "There are many unanswered
questions and we are really just opening a new direction for future
research," says Mottola.

But if gravastars can weather the controversy, then maybe there'll no longer
be any need for black holes -- maybe they really are pure fantasy. It
wouldn't be the first time that Einstein's dazzling intuition has been proved
correct.

Source: New Scientist