The proponents of string theory seem to think they can provide a more
elegant description of the Universe by adding additional dimensions.
But some other theoreticians think they've found a way to view the
Universe as having one less dimension. The work sprung out of a long
argument with Stephen Hawking about the nature of black holes, which was
eventually solved by the realization that the event horizon could act
as a hologram, preserving information about the material that's gotten
sucked inside. The same sort of math, it turns out, can actually
describe any point in the Universe, meaning that the entire content
Universe can be viewed as a giant hologram, one that resides on the
surface of whatever two-dimensional shape will enclose it.
That was the premise of panel at this summer's World Science Festival,
which described how the idea developed, how it might apply to the
Universe as a whole, and how they were involved in its development.
The whole argument started when Stephen Hawking attempted to describe
what happens to matter during its lifetime in a balck hole. He
suggested that, from the perspective of quantum mechanics, the
information about the quantum state of a particle that enters a black
hole goes with it. This isn't a problem until the black hole starts to
boil away through what's now called Hawking radiation, which creates a
separate particle outside the event horizon while destroying one inside.
This process ensures that the matter that escapes the black hole has
no connection to the quantum state of the material that had gotten
sucked in. As a result, information is destroyed. And that causes a
problem, as the panel described.
As far as quantum mechanics is concerned, information about states is
never destroyed. This isn't just an observation; according to panelist Leonard Susskind,
destroying information creates paradoxes that, although apparently
minor, will gradually propagate and eventually cause inconsistencies in
just about everything we think we understand. As panelist Leonard
Susskind put it, "all we know about physics would fall apart if
information is lost."
Unfortunately, that's precisely what Hawking suggested was happening.
"Hawking used quantum theory to derive a result that was at odds with
quantum theory," as Nobel Laureate Gerard t'Hooft described the situation. Still, that wasn't all bad; it created a paradox and "Paradoxes make physicists happy."
"It
was very hard to see what was wrong with what he was saying," Susskind
said, "and even harder to get Hawking to see what was wrong."
The arguments apparently got very heated. Herman Verlinde,
another physicist on the panel, described how there would often be
silences when it was clear that Hawking had some thoughts on whatever
was under discussion; these often ended when Hawking said "rubbish."
"When Hawking says 'rubbish,'" he said, "you've lost the argument."
t'Hooft described how the disagreement eventually got worked out. It's
possible, he said, to figure out how much information has gotten drawn
in to the black hole. Once you do that, you can see that the total
amount can be related to the surface area of the event horizon, which
suggested where the information could be stored. But since the event
horizon is a two-dimensional surface, the information couldn't be stored
in regular matter; instead, the event horizon forms a hologram that
holds the information as matter passes through it. When that matter
passes back out as Hawking radiation, the information is restored.
Susskind described just how counterintuitive this is. The holograms
we're familiar with store an interference pattern that only becomes
information we can interpret once light passes through them. On a
micro-scale, related bits of information may be scattered far apart, and
it's impossible to figure out what bit encodes what. And, when it
comes to the event horizon, the bits are vanishingly small, on the level
of the Planck scale (1.6 x 10-35 meters). These bits are so
small, as t'Hooft noted, that you can store a staggering amount of
information in a reasonable amount of space—enough to describe all the
information that's been sucked into a black hole.
The price, as Susskind noted, was that the information is "hopelessly scrambled" when you do so.
From a black hole to the Universe
Berkeley's Raphael Bousso
was on hand to describe how these ideas were expanded out to encompass
the Universe as a whole. As he put it, the math that describes how much
information a surface can store works just as well if you get rid of
the black hole and event horizon. (This shouldn't be a huge surprise,
given that most of the Universe is far less dense than the area inside a
black hole.) Any surface that encloses an area of space in this
Universe has sufficient capacity to describe its contents. The math,
he said, works so well that "it seems like a conspiracy."
To him, at least. Verlinde pointed out that things in the Universe
scale with volume, so it's counterintuitive that we should expect its
representation to them to scale with a surface area. That
counterintuitiveness, he thinks, is one of the reasons that the idea has
had a hard time being accepted by many.
When it comes to the basic idea—the Universe can be described using a
hologram—the panel was pretty much uniform, and Susskind clearly felt
there was a consensus in its favor. But, he noted, as soon as you
stepped beyond the basics, everybody had their own ideas, and those
started coming out as the panel went along. Bousso, for example, felt
that the holographic principle was "your ticket to quantum gravity."
Objects are all attracted via gravity in the same way, he said, and the
holographic principle might provide an avenue for understanding why (if
he had an idea about how, though, he didn't share it with the audience).
Verlinde seemed to agree, suggesting that, when you get to objects
that are close to the Planck scale, gravity is simply an emergent
property.
But t'Hooft seemed to be hoping that the holographic principle could
solve a lot more than the quantum nature of gravity—to him, it suggested
there might be something underlying quantum mechanics. For him, the
holographic principle was a bit of an enigma, since disturbances happen
in three dimensions, but propagate to a scrambled two-dimensional
representation, all while obeying the Universe's speed limit (that of
light). For him, this suggests there's something underneath it all, and
he'd like to see it be something that's a bit more causal than the
probabilistic world of quantum mechanics; he's hoping that a
deterministic world exists somewhere near the Planck scale. Nobody else
on the panel seemed to be all that excited about the prospect, though.
What was missing from the discussion was an attempt to tackle one of the
issues that plagues string theory: the math may all work out and it
could provide a convenient way of looking at the world, but is it
actually related to anything in the actual, physical Universe? Nobody
even attempted to tackle that question. Still, the panel did a good job
of describing how something that started as an attempt to handle a
special case—the loss of matter into a black hole—could provide a new
way of looking at the Universe. And, in the process, how people could
eventually convince Stephen Hawking he got one wrong.
Illustration by NASA/WMAP Science Team, R2D2 © Lucasfilm