Theos-Talk Archives (November 2006 Message tt00472)

Thought you might be interested
Cass


Tue Nov 7, 2006 12:46 am (PST) 
http://www.newscien tistspace. com/article/ mg19225751.
200-the-elephant - 
and-the-event- horizon.html
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What happens when you throw an elephant into a black hole? It sounds 
like a bad joke, but it's a question that has been weighing heavily 
on Leonard Susskind's mind. Susskind, a physicist at Stanford 
University in California, has been trying to save that elephant for 
decades. He has finally found a way to do it, but the consequences 
shake the foundations of what we thought we knew about space and 
time. If his calculations are correct, the elephant must be in more 
than one place at the

In everyday life, of course, locality is a given. You're over there, 
I'm over here; neither of us is anywhere else. Even in Einstein's 
theory of relativity, where distances and timescales can change 
depending on an observer's reference frame, an object's location in 
space-time is precisely defined. What Susskind is saying, however, is 
that locality in this classical sense is a myth. Nothing is what, or 
rather, where it seems.

This is more than just a mind-bending curiosity. It tells us 
something new about the fundamental workings of the universe. Strange 
as it may sound, the fate of an elephant in a black hole has deep 
implications for a "theory of everything" called quantum gravity, 
which strives to unify quantum mechanics and general relativity, the 
twin pillars of modern physics. Because of their enormous gravity and 
other unique properties, black holes have been fertile ground for 
researchers developing these ideas.

It all began in the mid-1970s, when Stephen Hawking of the University 
of Cambridge showed theoretically that black holes are not truly 
black, but emit radiation. In fact they evaporate very slowly, 
disappearing over many billions of years. This "Hawking radiation" 
comes from quantum phenomena taking place just outside the event 
horizon, the gravitational point of no return. But, Hawking asked, if 
a black hole eventually disappears, what happens to all the stuff 
inside? It can either leak back into the universe along with the 
radiation, which would seem to require travelling faster than light 
to escape the black hole's gravitational death grip, or it can simply 
blink out of existence.

Trouble is, the laws of physics don't allow either possibility. 
"We've been forced into a profound paradox that comes from the fact 
that every conceivable outcome we can imagine from black hole 
evaporation contradicts some important aspect of physics," says Steve 
Giddings, a theorist at the University of California, Santa Barbara.

Researchers call this the black hole information paradox. It comes 
about because losing information about the quantum state of an object 
falling into a black hole is prohibited, yet any scenario that allows 
information to escape also seems in violation. Physicists often talk 
about information rather than matter because information is thought 
to be more fundamental.

In quantum mechanics, the information that describes the state of a 
particle can't slip through the cracks of the equations. If it could, 
it would be a mathematical nightmare. The Schrödinger equation, which 
describes the evolution of a quantum system in time, would be 
meaningless because any semblance of continuity from past to future 
would be shattered and predictions rendered absurd. "All of physics 
as we know it is conditioned on the fact that information is 
conserved, even if it's badly scrambled," Susskind says.

For three decades, however, Hawking was convinced that information 
was destroyed in black hole evaporation. He argued that the radiation 
was random and could not contain the information that originally fell 
in. In 1997, he and Kip Thorne, a physicist at the California 
Institute of Technology in Pasadena, made a bet with John Preskill, 
also at Caltech, that information loss was real. At stake was an 
encyclopedia - from which they agreed information could readily be 
retrieved. All was quiet until July 2004, when Hawking unexpectedly 
showed up at a conference in Dublin, Ireland, claiming that he had 
been wrong all along. Black holes do not destroy information after 
all, he said. He presented Preskill with an encyclopedia of baseball.

What inspired Hawking to change his mind? It was the work of a young 
theorist named Juan Maldacena of the Institute for Advanced Study in 
Princeton, New Jersey. Maldacena may not be a household name, but he 
contributed what some consider to be the most ground-breaking piece 
of theoretical physics in the last decade. He did it using string 
theory, the most popular approach to understanding quantum gravity.

In 1997, Maldacena developed a type of string theory in a universe 
with five large dimensions of space and a contorted space-time 
geometry. He showed that this theory, which includes gravity, is 
equivalent to an ordinary quantum field theory, without gravity, 
living on the four-dimensional boundary of that universe. Everything 
happening on the boundary is equivalent to everything happening 
inside: ordinary particles interacting on the surface correspond 
precisely to strings interacting on the interior.

This is remarkable because the two worlds look so different, yet 
their information content is identical. The higher-dimensional 
strings can be thought of as a "holographic" projection of the 
quantum particles on the surface, similar to the way a laser creates 
a 3D hologram from the information contained on a 2D surface. Even 
though Maldacena's universe was very different from ours, the 
elegance of the theory suggested that our universe might be something 
of a grand illusion - an enormous cosmic hologram (New Scientist, 27 
April 2002, p 22).

The holographic idea had been proposed previously by Susskind, one of 
the inventors of string theory, and by Gerard't Hooft of the 
University of Utrecht in the Netherlands. Each had used the fact that 
the entropy of a black hole, a measure of its information content, 
was proportional to its surface area rather than its volume. But 
Maldacena showed explicitly how a holographic universe could work 
and, crucially, why information could not be lost in a black hole.

According to his theory, a black hole, like everything else, has an 
alter ego living on the boundary of the universe. Black hole 
evaporation, it turns out, corresponds to quantum particles 
interacting on this boundary. Since no information loss can occur in 
a swarm of ordinary quantum particles, there can be no mysterious 
information loss in a black hole either. "The boundary theory 
respects the rules of quantum mechanics," says Maldacena. "It keeps 
track of all the information. "

Of course, our universe still looks nothing like the one in 
Maldacena's theory. The results are so striking, though, that 
physicists have been willing to accept the idea, at least for now. 
"The opposition, including Hawking, had to give up," says Susskind. 
"It was so mathematically precise that for most practical purposes 
all theoretical physicists came to the conclusion that the 
holographic principle and the conservation of information would have 
to be true."

All well and good, but a serious problem remains: if the information 
isn't lost in a black hole, where is it? Researchers speculate that 
it is encoded in the black hole radiation (see "Black hole 
computers"). "The idea is that Hawking radiation is not random but 
contains subtle information on the matter that fell in," says Maldacena.

Susskind takes it a step further. Since the holographic principle 
leaves no room for information loss, he argues, no observer should 
ever see information disappear. That leads to a remarkable thought 
experiment.

Which brings us back to the elephant. Let's say Alice is watching a 
black hole from a safe distance, and she sees an elephant foolishly 
headed straight into gravity's grip. As she continues to watch, she 
will see it get closer and closer to the event horizon, slowing down 
because of the time-stretching effects of gravity in general 
relativity. However, she will never see it cross the horizon. Instead 
she sees it stop just short, where sadly Dumbo is thermalised by 
Hawking radiation and reduced to a pile of ashes streaming back out. 
>From Alice's point of view, the elephant's information is contained 
in those ashes.

Inside or out?

There is a twist to the story. Little did Alice realise that her 
friend Bob was riding on the elephant's back as it plunged toward the 
black hole. When Bob crosses the event horizon, though, he doesn't 
even notice, thanks to relativity. The horizon is not a brick wall in 
space. It is simply the point beyond which an observer outside the 
black hole can't see light escaping. To Bob, who is in free fall, it 
looks like any other place in the universe; even the pull of gravity 
won't be noticeable for perhaps millions of years. Eventually as he 
nears the singularity, where the curvature of space-time runs amok, 
gravity will overpower Bob, and he and his elephant will be torn 
apart. Until then, he too sees information conserved.

Neither story is pretty, but which one is right? According to Alice, 
the elephant never crossed the horizon; she watched it approach the 
black hole and merge with the Hawking radiation. According to Bob, 
the elephant went through and floated along happily for eons until it 
turned into spaghetti. The laws of physics demand that both stories 
be true, yet they contradict one another. So where is the elephant, 
inside or out?

The answer Susskind has come up with is - you guessed it - both. The 
elephant is both inside and outside the black hole; the answer 
depends on who you ask. "What we've discovered is that you cannot 
speak of what is behind the horizon and what is in front of the 
horizon," Susskind says. "Quantum mechanics always involves replacing 
'and' with 'or'. Light is waves or light is particles, depending on 
the experiment you do. An electron has a position or it has a 
momentum, depending on what you measure. The same is happening with 
black holes. Either we describe the stuff that fell into the horizon 
in terms of things behind the horizon, or we describe it in terms of 
the Hawking radiation that comes out."

Wait a minute, you might think. Maybe there are two copies of the 
information. Maybe when the elephant hits the horizon, a copy is 
made, and one version comes out as radiation while the other travels 
into the black hole. However, a fundamental law called the no-cloning 
theorem precludes that possibility. If you could duplicate 
information, you could circumvent the uncertainty principle, 
something nature forbids. As Susskind puts it, "There cannot be a 
quantum Xerox machine." So the same elephant must be in two places at 
once: alive inside the horizon and dead in a heap of radiating ashes 
outside.

The implications are unsettling, to say the least. Sure, quantum 
mechanics tells us that an object's location can't always be 
pinpointed. But that applies to things like electrons, not elephants, 
and it usually spans tiny distances, not light years. It is the large 
scale that makes this so surprising, Susskind says. In principle, if 
the black hole is big enough, the two versions of the same elephant 
could be separated by billions of light years. "People always thought 
quantum ambiguity was a small-scale phenomenon," he adds. "We're 
learning that the more quantum gravity becomes important, the more 
huge-scale ambiguity comes into play."

All this amounts to the fact that an object's location in space-time 
is no longer indisputable. Susskind calls this "a new form of 
relativity". Einstein took factors that were thought to be invariable 
- an object's length and the passage of time - and showed that they 
were relative to the motion of an observer. The location of an object 
in space or in time could only be defined with respect to an 
observer, but its location in space-time was certain. Now that notion 
has been shattered, says Susskind, and an object's location in space- 
time depends on an observer's state of motion with respect to a horizon.

What's more, this new type of "non-locality" is not just for black 
holes. It occurs anywhere a boundary separates regions of the 
universe that can't communicate with each other. Such horizons are 
more common than you might think. Anything that accelerates - the 
Earth, the solar system, the Milky Way - creates a horizon. Even if 
you're out running, there are regions of space-time from which light 
would never reach you if you kept speeding up. Those inaccessible 
regions are beyond your horizon.

As researchers forge ahead in their quest to unify quantum mechanics 
and gravity, non-locality may help point the way. For instance, 
quantum gravity should obey the holographic principle. That means 
there might be redundant information and fewer important dimensions 
of space-time in the theory. "This has to be part of the 
understanding of quantum gravity," Giddings says. "It's likely that 
this black hole information paradox will lead to a revolution at 
least as profound as the advent of quantum mechanics."