Parallel Worlds (19 page)

ROTATING BLACK
HOLES

In 1963,
however, this view began to change, when New Zealand mathematician Roy Kerr
found an exact solution of Einstein's equation describing perhaps the most
realistic dying star, a spinning black hole. Because of the conservation of
angular momentum, as a star collapses under gravity, it spins faster. (This is
the same reason why spinning galaxies look like pinwheels, and why skaters spin
faster when they bring their arms in.) A spinning star could collapse into a
ring of neutrons, which would remain stable because of the intense centrifugal
force pushing outward, canceling the inward force of gravity. The astonishing
feature of such a black hole was that if you fell into the Kerr black hole, you
would
not
be crushed to
death. Instead, you would be sucked completely through the Einstein- Rosen
bridge to a parallel universe. "Pass through this magic ring
and—presto!—you're in a completely different universe where radius and mass
are negative!" Kerr exclaimed to a colleague, when he discovered this
solution.

The frame of
Alice's looking glass, in other words, was like the spinning ring of Kerr. But
any trip through the Kerr ring would be a one-way trip. If you were to pass
through the event horizon surrounding the Kerr ring, the gravity would not be
enough to crush you to death, but it would be sufficient to prevent a return
trip back through the event horizon. (The Kerr black hole, in fact, has two
event horizons. Some have speculated that you might need a second Kerr ring,
connecting the parallel universe back to ours, in order to make a return trip.)
In some sense, a Kerr black hole can be compared to an elevator inside a
skyscraper. The elevator represents the Einstein-Rosen bridge, which connects
different floors, where each floor is a different universe. In fact, there are
an infinite number of floors in this skyscraper, each one different from the
others. But the elevator can never go down. There is only an "up"
button. Once you leave a floor, or universe, there would be no turning back
because you would have passed an event horizon.

Physicists are
divided about how stable a Kerr ring would be. Some calculations suggest that
if one tried to pass through the ring, the person's very presence would
destabilize the black hole, and the gateway would close. If a light beam, for
example, were to pass into the Kerr black hole, it would gain enormously in
energy as it fell toward the center and become blue-shifted—that is, it would
increase in frequency and energy. As it approached the horizon, it would have
so much energy that it would kill anyone trying to pass through the
Einstein-Rosen bridge. It would also generate its own gravitational field,
which would interfere with the original black hole, perhaps destroying the
gateway.

In other words,
while some physicists believe that the Kerr black hole is the most realistic of
all black holes, and could indeed connect parallel universes, it is not clear
how safe it would be to enter the bridge or how stable the doorway would be.

OBSERVING BLACK HOLES

Because of the
bizarre properties of black holes, as late as the early i990s their existence
was still considered science fiction. "Ten years ago, if you found an
object that you thought was a black hole in the center of a galaxy, half the
field thought you were a little nuts," remarked astronomer Douglas
Richstone of the University of Michigan in i998. Since then, astronomers have
identified several hundred black holes in outer space via the Hubble space
telescope, the Chandra X-ray space telescope (which measures X-ray emissions
from powerful stellar and galactic sources), and the Very Large Array Radio
Telescope (which consists of a series of powerful radio telescopes in New
Mexico). Many astronomers believe, in fact, that most of the galaxies in the
heavens (which have central bulges at the center of their disks) have black
holes at their centers.

As predicted,
all of the black holes found in space are rotating very rapidly; some have been
clocked by the Hubble space telescope rotating at about a million miles per
hour. At the very center, one can see a flat, circular core often about a
light-year across. Inside that core lies the event horizon and the black hole
itself.

Because black
holes are invisible, astronomers have to use indirect means to verify their
existence. In photographs, they try to identify the "accretion disk"
of swirling gas that surrounds the black hole. Astronomers have now collected
beautiful photographs of these accretion disks. (These disks are almost
universally found for most rapidly spinning objects in the universe. Even our
own Sun probably had a similar disk surrounding it when it formed 4.5 billion
years ago, which later condensed into the planets. The reason these disks form
is that they represent the lowest state of energy for such a rapidly spinning
object.) By using Newton's laws of motion, astronomers can calculate the mass
of the central object by knowing the velocity of the stars orbiting around it.
If the mass of the central object has an escape velocity equal to the speed of
light, then even light itself cannot escape, providing indirect proof of the
existence of a black hole.

The event
horizon lies at the center of the accretion disk. (It is unfortunately too
small to be identified with current technology. Astronomer Fulvio Melia claims
that capturing the event horizon of a black hole on film is the "holy
grail" of black hole science.) Not all the gas that falls toward a black
hole passes through the event horizon. Some of it bypasses the event horizon
and is hurled past it at huge velocities and ejected into space, forming two
long jets of gas emanating from the black hole's north and south poles. This
gives the black hole the appearance of a spinning top. (The reason jets are
ejected like this is probably that the magnetic field lines of the collapsing
star, as they become more intense, become concentrated above the north and
south poles. As the star continues to collapse, these magnetic field lines
condense into two tubes emanating from the north and south poles. As ionized
particles fall into the collapsed star, they follow these narrow magnetic lines
of force and are ejected as jets via the north and south polar magnetic
fields.)

Two types of
black holes have been identified. The first is the stellar black hole, in
which gravity crushes a dying star until it implodes. The second, however, is
more easily detected. These are galactic black holes, which lurk at the very
centers of huge galaxies and quasars and weigh millions to billions of solar
masses.

Recently, a
black hole was conclusively identified in the center of our own Milky Way
galaxy. Unfortunately, dust clouds obscure the galactic center; if not for
that, a huge fireball would be visible to us on Earth every night coming from
the direction of the constellation Sagittarius. Without the dust, the center of
our Milky Way galaxy would probably outshine the Moon, making it the brightest
object in the night sky. At the very center of this galactic nucleus lies a
black hole that weighs about 2.5 million solar masses. In terms of its size, it
is about a tenth of the radius of the orbit of Mercury. By galactic standards,
this is not an especially massive black hole; quasars can have black holes that
weigh several billion solar masses. The black hole in our backyard is rather
quiescent at present.

The next closest
galactic black hole lies at the center of the Andromeda galaxy, the closest
galaxy to Earth. It weighs 30 million solar masses, and its Schwarzschild
radius is about 60 million miles. (At the center of the Andromeda galaxy lie at
least two massive objects, probably the leftovers of a previous galaxy that
was devoured by Andromeda billions of years ago. If the Milky Way galaxy eventually
collides with Andromeda billions of years from now, as appears likely, perhaps
our galaxy will wind up in the "stomach" of the Andromeda galaxy.)

One of the most
beautiful photographs of a galactic black hole is the one taken by the Hubble
space telescope of the galaxy NGC 4261. In the past, radio telescope pictures
of this galaxy showed two very graceful jets being shot out of the galaxy's north
and south poles, but no one knew what the engine behind it was. The Hubble
telescope photographed the very center of the galaxy, revealing a beautiful
disk about 400 light-years across. At its very center was a tiny dot containing
the accretion disk, about a light-year across. The black hole at the center,
which could not be seen by the Hubble telescope, weighs approximately 1.2
billion solar masses.

Galactic black
holes like this are so powerful they can consume entire stars. In 2004, NASA
and the European Space Agency announced that they had detected a huge black
hole in a distant galaxy devouring a star in a single gulp. The Chandra X-ray
telescope and the European XMM-Newton satellite both observed the same event: a
burst of X rays being emitted by the galaxy RX J1242-11, signaling that a star
had been gobbled up by the huge black hole at the center. This black hole has
been estimated to weigh i00 million times the mass of our Sun. Calculations
have shown that, as a star comes perilously close to the event horizon of a
black hole, the enormous gravity distorts and stretches the star until it
breaks apart, emitting a telltale burst of X rays. "This star was
stretched beyond its breaking point. This unlucky star just wandered into the
wrong neighborhood," observed astronomer Stefanie Komossa of the Max
Planck Institute in Garching, Germany.

The existence of
black holes has helped to solve many old mysteries. The galaxy M-87, for
example, was always a curiosity to astronomers because it looked like a
massive ball of stars with a strange "tail" emerging from it. Because
it emitted copious quantities of radiation, at one point astronomers thought
that this tail represented a stream of antimatter. But today, astronomers have
found that it is energized by a huge black hole weighing perhaps 3 billion
solar masses. And that strange tail is now believed to be a gigantic jet of
plasma which is streaming out of, not into, the galaxy.

One of the more
spectacular discoveries concerning black holes occurred when the Chandra X-ray
telescope was able to peer through a small gap in the dust in outer space to
observe a collection of black holes near the edge of the visible universe. In
all, six hundred black holes could be seen. Extrapolating from that, astronomers
estimate there are at least 300 million black holes over the entire night sky.

GAMMA RAY BURSTERS

The black holes
mentioned above are perhaps billions of years old. But astronomers now have the
rare opportunity to see black holes being formed right before our eyes. Some
of these are probably the mysterious gamma ray bursters which release the
largest amount of energy in the universe. Huge gamma ray bursters are second
only to the big bang itself in terms of the energy they release.

Gamma ray
bursters have a curious history, dating back to the Cold War. In the late
i960s, the United States was worried that the Soviet Union or another country
might secretly detonate a nuclear bomb, perhaps on a deserted part of the Earth
or even on the Moon, violating existing treaties. So the United States launched
the Vela satellite to specifically spot "nuke flashes," or
unauthorized detonations of nuclear bombs. Because a nuclear detonation
unfolds in distinct stages, microsecond by microsecond, each nuke flash gives
off a characteristic double flash of light that can be seen by satellite. (The
Vela satellite did pick up two such nuke flashes in the 1970s off the coast of
Prince Edward Island near South Africa, in the presence of Israeli war ships,
sightings that are still being debated by the intelligence community.)

But what
startled the Pentagon was that the Vela satellite was picking up signs of huge
nuclear explosions in space. Was the Soviet Union secretly detonating hydrogen
bombs in deep space, using an unknown, advanced technology? Concerned that the
Soviets might have leapfrogged over the U.S. in weapons technology, top
scientists were brought in to analyze these deeply disturbing signals.

After the
breakup of the Soviet Union, there was no need to classify this information,
so the Pentagon dumped a mountain of astronomical data onto the world of
astronomy, which was overwhelming. For the first time in decades, an entirely
new astronomical phenomenon of immense power and scope had been revealed.
Astronomers quickly realized that these gamma ray bursters, as they were
called, were titanic in their power, releasing within seconds the entire energy
output of our Sun over its entire life history (about 10 billion years). But
these events were also fleeting; once detected by the Vela satellite, they had
dimmed so much that by the time ground telescopes were pointed in their
direction, nothing could be seen in their wake. (Most bursters last between 1
and 10 seconds, but the shortest one lasted 0.01 second, and some lasted as
long as several minutes.)

Today, space
telescopes, computers, and rapid response teams have changed our ability to
spot gamma ray bursters. About three times a day, gamma ray bursters are
detected, setting off a complex chain of events. As soon as the energy from one
is detected by satellite, astronomers using computers rapidly locate its
precise coordinates and aim more telescopes and sensors in its precise
direction.

The data from
these instruments has revealed truly astounding results. At the heart of these
gamma ray bursters lies an object often only a few tens of miles across. In
other words, the unimaginable cosmic power of gamma ray bursters is
concentrated within an area the size of, say, New York City. For years, the
leading candidates for such events were colliding neutron stars in a binary
star system. According to this theory, as the orbit of these neutron stars
decayed over time, and as they followed a death spiral, they would ultimately
collide and create a mammoth release of energy. Such events are extremely
rare, but because the universe is so large, and since these bursters light up
the entire universe, they should be seen several times a day.