From: Brian Atkins (brian@posthuman.com)
Date: Fri Sep 12 2003 - 19:06:29 MDT
For those intrigued by the idea of whether it would be possible to
survive within a blackhole...
------------------------------------
The black hole survival guide
New Scientist vol 179 issue 2411 - 06 September 2003, page 26
Falling into a black hole need not spell certain doom. Marcus Chown
looks forward to the ultimate thrill ride
COULD you fall into a black hole and avoid being crushed by its intense
gravity? Because the laws of physics break down under these conditions,
it has widely been considered impossible even to imagine what would
happen deep inside these fearsome objects. But Igor Novikov thought it
was worth trying anyway. And by probing exactly how the laws of physics
break down, he has challenged the conventional view that a trip to a
black hole spells certain death.
Of course, Novikov, who heads the Theoretical Astrophysics Center in
Copenhagen, Denmark, is not planning to visit a black hole. But
physicists still consider the problem worth thinking about; these
calculations can give us insight into what happens where the fabric of
space-time becomes so tremendously warped that it shatters into droplets
or quanta. "Here the general theory of relativity, Einstein's theory of
gravity, is no longer valid," says Eric Poisson of the University of
Guelph in Ontario, Canada. "We are into the mysterious domain of quantum
gravity where no theory yet exists to tell us what to expect." So
looking at the internal structure of black holes might tell us how to
formulate a more consistent theory of the universe.
Black holes - regions of space-time from which nothing can escape - are
rooted in Einstein's general theory of relativity. Soon after Einstein
published his theory describing gravity as warps in space-time, the
German astronomer Karl Schwarzschild used the theory to show that if a
large star were squeezed into a small enough point, it would create such
a strong gravitational field that nothing, not even light, could escape
once it had moved beyond a certain point: the "event horizon".
Inside the event horizon of a black hole, space-time is radically
transformed. Einstein showed that matter or energy stretch space and
time, rather like someone standing on a trampoline. Yet no matter how
much the fabric of space-time warps, space and time always act together
to preserve the speed of light (one of the effects of this is that
clocks run slower in the gravitational field close to Earth than they do
far out in space). The tremendous gravity inside a black hole distorts
space-time so much, that the only way to safeguard the speed of light
travelling in this region is for space and time to swap roles. This has
an extraordinary consequence: instead of being a place, the centre of a
black hole exists in the future and you can no more avoid it than you
can avoid tomorrow. At the end of this journey is the singularity, a
point of infinite density that is widely believed to destroy anyone or
anything that hits it.
The devastation is caused by "tidal" forces: because gravity varies so
quickly near a black hole, the pull on your head would be much greater
than the pull on your feet. Theorists calculate that these tidal forces
increase indefinitely very close to the singularity: matter gets torn
apart, space-time would shred and you would be utterly destroyed.
But it turns out that the existence of this murderous singularity does
not necessarily spell doom. Because stars and galaxies rotate, there is
a good chance that any black hole we encounter would too. In the early
1960s, New Zealand mathematician Roy Kerr worked out that this rotation
drags the space-time surrounding the black hole like a tornado,
profoundly altering its internal structure. His work showed that a
second horizon forms inside a rotating black hole. Novikov has now shown
that this second horizon changes everything
(www.arxiv.org/abs/gr-qc/0304052).
Shortly after a rotating black hole forms, this inner horizon is little
more than a slight wrinkle in space-time whose exact position depends on
the speed of rotation. For the fastest spinning black holes, it sits
halfway between the event horizon and the singularity. But it does not
stay that way for ever. The extreme gravitational field inside the black
hole whips all the in-falling light and matter up to extremely high
energy. The colossal energy of this junk changes the character of the
inner event horizon, warping it so much that the slight wrinkle in
space-time becomes a deep fold. To preserve the speed of light in this
region, time speeds up - it passes so quickly that the inner horizon
concentrates junk from all times, even from events that happen far into
the universe's future.
The result of this is that the inner event horizon concentrates so much
light and matter that it rapidly turns into another singularity, one
that concentrates an infinite amount of energy into a finite volume.
Astrophysicists call it a "mass-inflation" singularity.
And it is this mass-inflation singularity that gets you out of a black
hole alive. As well as junk falling into the black hole, Novikov
believes that the inner event horizon also dredges up stuff scattered
from deep within the black hole. This interacts with the incoming flow
of light and matter, and "gravitational feedback", where the
ever-increasing amount of stuff present causes an ever-increasing
gravitational pull, causes the radius of the inner horizon to shrink.
Eventually the mass-inflation singularity swallows the more dangerous
singularity, leaving a tamer, less dangerous kind of black hole.
Calculations show that the tidal forces around a mass-inflation
singularity may not last long enough to deform an object. Because
space-time is so warped around there, passing near the singularity is
the fastest part of the journey. That gives anyone falling into a black
hole a fighting chance. "Although you would feel an infinite tidal force
as you approached, you would feel it for only a very short time,"
Novikov says. "Consequently, you could pass through without being crushed."
Of course, you will have to choose your black hole carefully: it has
certainly got to be rotating, for a start. It has also got to be old, so
that there has been enough time for the second singularity to form. The
final condition is that your black hole is big - very big.
That's because of the tidal forces. Near the small black holes formed by
collapsing dead stars, they are enough to rip a person limb from limb.
But these forces depend on the mass of the black hole, shrinking by a
factor of 4 each time the mass doubles. So to increase your chances of
survival, it is far better to explore a rotating supermassive black hole
weighing hundreds of millions of times as much as the sun. "The tidal
effect near such a supermassive black hole is far weaker than near a
stellar-mass black hole," Poisson says. "So you would hardly notice as
you slip across the event horizon and into the interior."
Fortunately, such behemoths do exist. No one is quite sure how they
formed but astrophysicists now suspect that supermassive black holes
lurk at the heart of every galaxy, including the Milky Way. Our nearest
black hole, Sagittarius A*, is rotating, supermassive and old: a perfect
candidate for this thrill ride of a lifetime. So, what would the journey
be like?
Although the space-time around you is grossly warped - another way of
saying that gravity is immensely strong - you notice nothing untoward
happening, just as you don't normally notice the effects of Earth's
gravity. To your friends watching you at a safe distance, however,
things seem very different. Everything you do is in slow motion as time
appears to stretch out interminably. Not only that, but you gradually
fade from view as the visible light reflecting off you decreases in
energy and frequency. In effect, gravity stretches the light waves,
stretching or "red shifting" their wavelengths towards the infrared part
of the spectrum.
Fantastic voyage
Immediately ahead of you lies the event horizon, the point of no return
for in-falling light and matter. Here time appears to slow to a
standstill, so your friends see your gradually fading image frozen in
space forever. The truth (for you, anyway) is that you have long gone
over the event horizon and are falling towards the singularity, the
point with infinite density.
Once you are inside, your passage to the mass-inflation singularity
should take only a few hours. Of course, you could prolong your journey
by taking a zigzag route in a rocket through space, but the singularity
is unavoidable. "By steering, you could extend the journey perhaps by a
factor of 10, but no more than that," Poisson says.
And what would you find when you got there? So far, no one knows for
sure. Many physicists believe that when a star's core collapses to form
a black hole, it does not shrink to nothing, but instead spawns a new
region of space-time. Lee Smolin of the Perimeter Institute in Ontario,
Canada, for instance, speculates that black holes can give birth to baby
universes where the fundamental constants of physics are slightly
different. Novikov thinks mass-inflation singularities may act as
portals into these regions; if you fall into a black hole you might
emerge in a different universe, he says.
Amos Ori of the Israel-Technion Institute of Technology in Haifa,
Israel, is exploring an extraordinary scenario that emerges when he
applies general relativity to a simplified black hole model. He
considered a two-dimensional black hole containing a mass-inflation
singularity. "After crossing the mass-inflation singularity, an observer
falling into the black hole will return to the external universe," he
predicts. The bad news is, they will return stretched by an enormous
factor and billions of years after they fell in. "Probably no observer
will survive this," he says.
Unsurprisingly, many physicists are sceptical that anyone might travel
through a black hole unscathed. "We don't know for sure if it is
possible to survive crushing," says Kip Thorne, a theorist at the
California Institute of Technology in Pasadena. "We don't have enough
understanding of the mass-inflation singularity."
And even if you do not get crushed, another danger awaits you. Today's
universe is filled with completely harmless microwave radiation, the
tepid afterglow of the big bang. When these microwaves are sucked into a
black hole, they are accelerated to much higher energies and
frequencies: these innocuous microwaves get transformed into penetrating
gamma radiation. "It is not obvious whether it will be possible to
shield someone against these gamma rays," Ori says, "though there may be
a chance."
If journeying into a black hole sounds too perilous, Novikov has an
ingenious way of looking inside one without venturing too close to the
event horizon. His idea is to use tunnels in space-time called
wormholes, with one end anchored outside the black hole and the other
dangling inside. "Light from the interior could then come out, allowing
us to peer inside," he speculates. The big difficulty would be
distorting space-time enough to make a wormhole in the first place. And
then there is the problem of keeping it open: wormholes tend to snap
shut as soon as they are formed. There has been recent progress this
problem, however (see "Peeping through a wormhole"). But since it is
impossible to change the topology of space-time, we would have to find a
wormhole - possibly one left over from the big bang - then inflate it.
"That's a tall order," Poisson says.
So maybe the best way to learn about the interior of a black hole is to
disregard danger and hurl yourself into one. Of course, whatever you
learned, you would never be able to tell your friends about it - even if
you survived, you would emerge in another universe or at another time.
"But it would be a fascinating trip," Poisson says. Any volunteers?
Peeping through a wormhole
Earlier this year, New Zealand and Indian physicists made a dramatic
discovery: wormholes may be easier to maintain than we thought.
Wormholes are perfectly legitimate solutions of Einstein's equations,
and could help us look inside a black hole without fear of falling in.
But there is a problem: wormholes snap shut in an instant unless held
open by a supply of exotic matter. Unlike the familiar stuff found on
Earth, which always feels the pull of gravity, this exotic matter can
repel gravity, halting the wormhole's collapse. Although recent
measurements of the big bang afterglow suggest that the universe is made
up of a substance with repulsive gravity, no one knows whether it has
the right properties, or even if it exists in the quantities needed to
prise open a wormhole big enough to look through.
But according to Matt Visser of Victoria University in Wellington, New
Zealand, things are not as bad as they initially seemed (Physical Review
Letters, vol 90, p 201102). The empty space of the quantum vacuum, where
fluctuations in energy allow short-lived particles to pop in and out of
existence, shares some of the properties of exotic matter. So it may be
possible to keep open the mouth of wormhole with much smaller amounts of
exotic matter than previously thought. Our chances of a glance inside
black holes just went up - very, very slightly.
Marcus Chown
-- Brian Atkins Singularity Institute for Artificial Intelligence http://www.intelligence.org/
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