How an argument with Hawking suggested the Universe is a hologram
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 black 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.
