Archive for the ‘Leonard Susskind’ Category

“Our hypothesis is that the inside of a black hole — it may not be there. Probably that’s the end of space itself. There’s no inside at all.”
– Joe Polchinski, physicist

It could rightly be called the most massive debate of the year: Physicists are locked in an argument over what happens if you fall into a black hole.

On one side are those who support the traditional view from Albert Einstein. On the other, backers of a radical new theory that preserves the very core of modern physics by destroying space itself.

Regardless of who’s right, the new take on black holes could lead to a better understanding of the universe, says Leonard Susskind, a physicist at Stanford University. “This is the kind of thing where progress comes from.”

Black holes are regions of space so dense that nothing, not even light, can escape.

There’s a long-standing view about what would happen if you fell into one of these holes. At first, you’re not going to notice much of anything — but the black hole’s gravity is getting stronger and stronger. And eventually you pass a point of no return.

“It’s kind of like you’re rowing on Niagara Falls, and you pass the point [where] you can’t row fast enough to escape the current,” Susskind says. “Well, you’re doomed at that point. But passing the point of no return — you wouldn’t even notice it.”

Now you can’t get out. And gravity from the black hole is starting to pull on your feet more than your head. “The gravity wants to sort of stretch you in one direction and squeeze you in another,” says Joe Polchinski, a physicist at the University of California, Santa Barbara. He says the technical term for this stretching is spaghettification.

“It’d be kind of medieval,” says Polchinkski. “It’d be like something on Game of Thrones.”

In Einstein’s version of events, that’s the end. But Polchinski has a new version of things: “Our hypothesis is that the inside of a black hole — it may not be there,” he says.

So what’s inside the black hole? Nothing, Polchinski says. Actually even less than that. “Probably that’s the end of space itself; there’s no inside at all.”
This “no inside” idea may sound outrageous, but it’s actually a stab at solving an even bigger problem with black holes.

According to the dominant theory of physics — quantum mechanics — information can never disappear from the universe. Put another way, the atoms in your body are configured in a particular way. They can be rearranged (radically if you happen to slip inside a black hole). But it should always be possible, at least in theory, to look at all those rearranged atoms and work out that they were once part of a human of your dimensions and personality.

This rule is absolutely fundamental. “Everything is built on it,” says Susskind. “If it were violated, everything falls apart.”

For a long time, black holes stretched this rule, but they didn’t break it. People thought that if you fell into a black hole, your spaghettified remains would always be in there, trapped beyond the point of no return.

That is, until the famous physicist Stephen Hawking came along. In the 1970s, Hawking showed that, according to quantum mechanics, a black hole evaporates — very slowly, it vanishes. And that breaks the fundamental rule because all that information that was once in your spaghettified remains vanishes with it.

This didn’t seem to bother Hawking. (“I’m not a psychiatrist, and I can’t psychoanalyze him,” Susskind says.) But it has bothered a lot of other physicists since.

And in the intervening years, work by another theorist — Juan Maldacena, with Princeton’s Institute for Advanced Study — seems to show that Hawking was wrong. Information has to get out of the black hole … somehow. But nobody knows how.

So Polchinski took another look. “We took Hawking’s original argument,” he says, “and very carefully ran it backwards.”

And Polchinski and his colleagues found one way to keep things from vanishing when they fall inside a black hole — they got rid of the inside. By tearing apart the fabric of space beyond the point of no return, the group was able to preserve the information rule of quantum mechanics.

In this version, anything falling into a black hole is instantly vaporized at the point of no return, in a fiery storm of quantum particles. Particles coming from the hole collectively carry away any and all information about the object that’s falling in.

So in Polchinski’s version, when you fall into a black hole, you don’t disappear. Instead, you smack into the end of the universe.

“You just come to the end of space, and there’s nothing beyond it. Terminated,” Susskind says. All the information once contained in your atoms is re-radiated in a quantum mechanical fire.

This new version seems too radical to Susskind. “I don’t think this is true,” he says. “In fact, I think almost nobody thinks this is true — that space falls apart inside a black hole.”

Even Polchinski still feels that black holes should have insides. “My gut believes that the black hole has an interior,” he says. But, he adds, nobody’s been able to disprove his hypothesis that it doesn’t.

“Every counterargument I’ve seen is flawed,” Polchinski says.

Susskind agrees: “Nobody quite knows exactly what’s wrong with their argument — and that’s what makes this so important and interesting.”

And as crazy as it sounds, this is progress. In the year ahead, Susskind hopes someone can find the flaw in Polchinski’s argument, just the way Polchinski found a flaw in Stephen Hawking’s argument. But it will be awhile before we understand black holes inside and out.



At a black hole, Albert Einstein’s theory of gravity apparently clashes with quantum physics, but that conflict could be solved if the Universe were a holographic projection.

A team of physicists have provided what has been described by the journal Nature as the “clearest evidence yet” that our universe is a hologram.

The new research could help reconcile one of modern physics’ most enduring problems : the apparent inconsistencies between the different models of the universe as explained by quantum physics and Einstein’s theory of gravity.

The two new scientific papers are the culmination of years’ work led by Yoshifumi Hyakutake of Ibaraki University in Japan, and deal with hypothetical calculations of the energies of black holes in different universes.

The idea of the universe existing as a ‘hologram’ doesn’t refer to a Matrix-like illusion, but the theory that the three dimensions we perceive are actually just “painted” onto the cosmological horizon – the boundary of the known universe.

If this sounds paradoxical, try to imagine a holographic picture that changes as you move it. Although the picture is two dimensional, observing it from different locations creates the illusion that it is 3D.

This model of the universe helps explain some inconsistencies between general relativity (Einstein’s theory) and quantum physics. Although Einstein’s work underpins much of modern physics, at certain extremes (such as in the middle of a black hole) the principles he outlined break down and the laws of quantum physics take over.

The traditional method of reconciling these two models has come from the 1997 work of theoretical physicist Juan Maldacena, whose ideas built upon string theory. This is one of the most well respected ‘theories of everything’ (Stephen Hawking is a fan) and it posits that one-dimensional vibrating objects known as ‘strings’ are the elementary particles of the universe.

Maldacena has welcomed the new research by Hyakutake and his team, telling the journal Nature that the findings are “an interesting way to test many ideas in quantum gravity and string theory.”

Leonard Susskind, a theoretical physicist regarded as one of the fathers of string theory, added that the work by the Japanese team “numerically confirmed, perhaps for the first time, something we were fairly sure had to be true, but was still a conjecture.”

Here is the original press release from Nature:

A team of physicists has provided some of the clearest evidence yet that our Universe could be just one big projection.

In 1997, theoretical physicist Juan Maldacena proposed1 that an audacious model of the Universe in which gravity arises from infinitesimally thin, vibrating strings could be reinterpreted in terms of well-established physics. The mathematically intricate world of strings, which exist in nine dimensions of space plus one of time, would be merely a hologram: the real action would play out in a simpler, flatter cosmos where there is no gravity.

Maldacena’s idea thrilled physicists because it offered a way to put the popular but still unproven theory of strings on solid footing — and because it solved apparent inconsistencies between quantum physics and Einstein’s theory of gravity. It provided physicists with a mathematical Rosetta stone, a ‘duality’, that allowed them to translate back and forth between the two languages, and solve problems in one model that seemed intractable in the other and vice versa. But although the validity of Maldacena’s ideas has pretty much been taken for granted ever since, a rigorous proof has been elusive.

In two papers posted on the arXiv repository, Yoshifumi Hyakutake of Ibaraki University in Japan and his colleagues now provide, if not an actual proof, at least compelling evidence that Maldacena’s conjecture is true.

In one paper2, Hyakutake computes the internal energy of a black hole, the position of its event horizon (the boundary between the black hole and the rest of the Universe), its entropy and other properties based on the predictions of string theory as well as the effects of so-called virtual particles that continuously pop into and out of existence. In the other3, he and his collaborators calculate the internal energy of the corresponding lower-dimensional cosmos with no gravity. The two computer calculations match.

“It seems to be a correct computation,” says Maldacena, who is now at the Institute for Advanced Study in Princeton, New Jersey and who did not contribute to the team’s work.

The findings “are an interesting way to test many ideas in quantum gravity and string theory”, Maldacena adds. The two papers, he notes, are the culmination of a series of articles contributed by the Japanese team over the past few years. “The whole sequence of papers is very nice because it tests the dual [nature of the universes] in regimes where there are no analytic tests.”

“They have numerically confirmed, perhaps for the first time, something we were fairly sure had to be true, but was still a conjecture — namely that the thermodynamics of certain black holes can be reproduced from a lower-dimensional universe,” says Leonard Susskind, a theoretical physicist at Stanford University in California who was among the first theoreticians to explore the idea of holographic universes.

Neither of the model universes explored by the Japanese team resembles our own, Maldacena notes. The cosmos with a black hole has ten dimensions, with eight of them forming an eight-dimensional sphere. The lower-dimensional, gravity-free one has but a single dimension, and its menagerie of quantum particles resembles a group of idealized springs, or harmonic oscillators, attached to one another.

Nevertheless, says Maldacena, the numerical proof that these two seemingly disparate worlds are actually identical gives hope that the gravitational properties of our Universe can one day be explained by a simpler cosmos purely in terms of quantum theory.