Leonard Susskind

The language of physics is mathematics, and it cannot be done honestly without mathematics. That makes it inaccessible. The language of literature is English or Chinese or whatever, and that makes it accessible. And literature is about the human condition. Physics is about the nonhuman condition. It’s not a taste that all human beings have.

People are also social creatures, and literature fits in with that. Physics is perceived as a lonesome, nerdy kind of enterprise that has very little to do with human feelings and the things that excite people day-to-day about each other. Yet physicists in their own working environment are very social creatures.


Leonard Susskind

oh wow I love this a lot

What is your favorite, deep, elegant, or beautiful explanation?

Leonard Susskind: Stanford professor of theoretical physics, one the greatest minds on the second half of the 20th century, close friend of my personal hero Richard Feynman, author and incredible story teller; answers the question “What is your favorite, deep, elegant or beautiful explanation?” which is the question of the year on edge.org

“That’s a tough question for a theoretical physicist; theoretical physics is all about deep, elegant, beautiful explanations; and there are just so many to choose from.

Personally my favorites are explanations that that get a lot for a little. In physics that means a simple equation or a very general principle. I have to admit though, that no equation or principle appeals to me more than Darwinian evolution, with the selfish gene mechanism thrown in. To me it has what the best physics explanations have: a kind of mathematical inevitability. But there are many people who can explain evolution much better than I, so I will stick to what I know

The guiding star for me, as a physicist, has always been Boltzmann’s explanation of second law of thermodynamics: the law that says that entropy never decreases. To the physicists of the late 19 th century this was a very serious paradox. Nature is full of irreversible phenomena: things that easily happen but could not possibly happen in reverse order. However, the fundamental laws of physics are completely reversible: any solution of Newton’s equations can be run backward and it is still a solution. So if entropy can increase, the laws of physics say it must be able to decrease. But experience says otherwise. For example, if you watch a movie of a nuclear explosion in reverse, you know very well that it’s fake. As a rule, things go one way and not the other. Entropy increases.

What Boltzmann realized is that the second law—entropy never decreases—is not a law in the same sense as Newton’s law of gravity, or Faraday’s law of induction. It’s a probabilistic law that has the same status as the following obvious claim; if you flip a coin a million times you will not get a million heads. It simply won’t happen. But is it possible? Yes, it is; it violates no law of physics. Is it likely? Not at all. Boltzmann’s formulation of the second law was very similar. Instead of saying entropy does not decrease, he said entropy probably doesn’t decrease. But if you wait around long enough in a closed environment, you will eventually see entropy decrease: by accident, particles and dust will come together and form a perfectly assembled bomb. How long? According to Boltzmann’s principles the answer is the exponential of the entropy created when the bomb explodes. That is a very long time, a lot longer than the time to flip a million heads in

I’ll give you a simple example to see how it is possible for things to be more probable one way than the other, despite both being possible. Imagine a high hill that comes to a narrow point—a needle point—at the top. Now imagine a bowling ball balanced at the top of the hill. A tiny breeze comes along. The ball rolls off the hill and you catch it at the bottom. Next, run it in reverse: the ball leaves your hand, rolls up the hill, and with infinite finesse, comes to the top—and stops! Is it possible? It is. Is it likely? It is not. You would have to have almost perfect precision to get the ball to the top, let alone to have it stop dead-balanced. The same is true with the bomb. If you could reverse every atom and particle with sufficient accuracy, you could make the explosion products reassemble themselves. But a tiny inaccuracy in the motion of just one single particle, and all you would get is more

Here’s another example: drop a bit of black ink into a tub of water. The ink spreads out and eventually makes the water grey. Will a tub of grey water ever clear up and produce a small drop of ink? Not impossible, but very unlikely.

Boltzmann was the first to understand the statistical foundation for the second law, but he was also the first to understand the inadequacy of his own formulation. Suppose that you came upon a tub that had been filled a zillion years ago, and had not been disturbed since. You notice the odd fact that it contains a somewhat localized cloud of ink. The first thing you might ask is what will happen next. The answer is that the ink will almost certainly spread out more. But by the same token, if you ask what most likely took place a moment before, the answer would be the same: it was probably more spread out a moment ago than it is now. The most likely explanations would be that the ink-blob is just a momentary fluctuation.

Actually I don’t think you would come to that conclusion at all. A much more reasonable explanation is that for reasons unknown, the tub started not-so-long-ago with a concentrated drop of ink, which then spread. Understanding why ink and water go one way becomes a problem of “initial conditions”. What set up the concentration of ink in the first place?

The water and ink is an analogy for the question of why entropy increases. It increases because it is most likely that it will increase. But the equations say that it is also most likely that it increases toward the past. To understand why we have this sense of direction, one must ask the same question that Boltzmann did: Why was the entropy very small at the beginning? What created the universe in such a special low-entropy way? That’s a cosmological question that we are still very uncertain about.

I began telling you what my favorite explanation is, and I ended up telling you what my favorite unsolved problem is. I apologize for not following the instructions. But that’s the way of all good explanations. The better they are, the more questions they raise.”

Simulating a black hole

40 years ago Stephen Hawking predicted that black holes emit a special kind of radiation. Consequently black holes are theoratically able to shrink and even vanish. This radiation arises when virtual particles (pairs of particles developing because of quantum fluctuations inside the vacuum; usually they nearly instantly destroy each other) are near the event horizon. Then the virtual particle pair gets divided: one disappears in the black hole (and its quantum mechanical information) and the other one becomes real. Thus the black hole radiates but unfortunately this radiation is so low that astronomical observations are nearly impossible.
Therefore scientists have to simulate black holes to get empirical evidence. The physicist Jeff Steinhauer of the Technion, the University of Technology of Haifa in Israel exactly did this. He realized an idea of physicist Bill Unruh with an acoustical event horizon. He uses a fog made of rubidium atoms which is only slightly above the absolute zero. Because they are trapped inside an electromagnetic field these atoms become a Bose-Einstein Condensate. Inside of this condensate the acoustic velocity is only a half millimeter per second. With the help of accelerating some above this speed an artificial event horizon is created. The low temperatures lead to quantum fluctuations: pairs of phonons develop. In the simulation these pairs also get divided: one gets caught by the supersonic event horizon; the other one becomes some kind of Hawking radiation.
It is still not sure if this experiment really simulates black holes. According to Ulf Leonhardt it does not proof for sure that the two phonons are entangled. Thus it is not sure if the pairs arised out of one fluctuation. Leonhardt even doubts that the fog of atoms is a real Bose-Einstein Condensate. Leonard Susskind thinks this experiment does not reveal the mysteries of black holes: for instance it does not explain the information paradox, because acoustic black holes do not destroy information.


NEW THEORY OF GRAVITY MIGHT EXPLAIN DARK MATTER A new theory of gravity might explain the curious motions of stars in galaxies. Emergent gravity, as the new theory is called, predicts the exact same deviation of motions that is usually explained by inserting dark matter in the theory. Prof. Erik Verlinde, renowned expert in string theory at the University of Amsterdam and the Delta Institute for Theoretical Physics, published a new research paper today in which he expands his groundbreaking views on the nature of gravity. In 2010, Erik Verlinde surprised the world with a completely new theory of gravity. According to Verlinde, gravity is not a fundamental force of nature, but an emergent phenomenon. In the same way that temperature arises from the movement of microscopic particles, gravity emerges from the changes of fundamental bits of information, stored in the very structure of spacetime. Newton’s Law from Information In his 2010 article [http://link.springer.com/article/10.1007/JHEP04%282011%29029], Verlinde showed how Newton’s famous second law, which describes how apples fall from trees and satellites stay in orbit, can be derived from these underlying microscopic building blocks. Extending his previous work and work done by others, Verlinde now shows how to understand the curious behaviour of stars in galaxies without adding the puzzling dark matter. Puzzling Star Velocities The outer regions of galaxies, like our own Milky Way, rotate much faster around the centre than can be accounted for by the quantity of ordinary matter like stars, planets and interstellar gasses. Something else has to produce the required amount of gravitational force, and so dark matter entered the scene. Dark matter seems to dominate our universe: more than 80% of all matter must have a dark nature. Hitherto, the alleged dark matter particles have never been observed, despite many efforts to detect them. No Need for Dark Matter According to Erik Verlinde, there is no need to add a mysterious dark matter particle to the theory. In a new paper, which appeared today on the ArXiv preprint server, Verlinde shows how his theory of gravity accurately predicts the velocities by which the stars rotate around the center of the Milky Way, as well as the motion of stars inside other galaxies. “We have evidence that this new view of gravity actually agrees with the observations, “ says Verlinde. “At large scales, it seems, gravity just doesn’t behave the way Einstein’s theory predicts.” At first glance, Verlinde’s theory has features similar to modified theories of gravity like MOND (modified Newtonian Dynamics, Mordehai Milgrom (1983)). However, where MOND tunes the theory to match the observations, Verlinde’s theory starts from first principles. “A totally different starting point,” according to Verlinde. Adapting the Holographic Principle One of the ingredients in Verlinde’s theory is an adaptation of the holographic principle, introduced by his tutor Gerard ‘t Hooft (Nobel Prize 1999, Utrecht University) and Leonard Susskind (Stanford University). According to the holographic principle, all the information in the entire universe can be described on a giant imaginary sphere around it. Verlinde now shows that this idea is not quite correct: part of the information in our universe is contained in space itself. Information in the Bulk This extra information is required to describe that other dark component of the universe: the dark energy, which is held responsible for the accelerated expansion of the universe. Investigating the effects of this additional information on ordinary matter, Verlinde comes to a stunning conclusion. Whereas ordinary gravity can be encoded using the information on the imaginary sphere around the universe only – as he showed in his 2010 work – the result of the additional information in the bulk of space is a force that nicely matches the one so far attributed to dark matter. On the Brink of a Scientific Revolution Gravity is in dire need of new approaches like the one by Verlinde, since it doesn’t combine well with quantum physics. Both theories, the crown jewels of 20th century physics, cannot be true at the same time. The problems arise in extreme conditions: near black holes, or during the Big Bang. Verlinde: “Many theoretical physicists like me are working on a revision of the theory, and some major advancements have been made. We might be standing on the brink of a new scientific revolution that will radically change our views on the very nature of space, time and gravity.”

Theoretical physics: Complexity on the horizon

A concept developed for computer science could have a key role in fundamental physics — and point the way to a new understanding of space and time.

When physicist Leonard Susskind gives talks these days, he often wears a black T-shirt proclaiming “I ♥ Complexity”. In place of the heart is a Mandelbrot set, a fractal pattern widely recognized as a symbol for complexity at its most beautiful.

That pretty much sums up his message. The 74-year-old Susskind, a theorist at Stanford University in California, has long been a leader in efforts to unify quantum mechanics with the general theory of relativity — Albert Einstein’s framework for gravity. The quest for the elusive unified theory has led him to advocate counter-intuitive ideas, such as superstring theory or the concept that our three-dimensional Universe is actually a two-dimensional hologram. But now he is part of a small group of researchers arguing for a new and equally odd idea: that the key to this mysterious theory of everything is to be found in the branch of computer science known as computational complexity.

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There is a philosophy that says that if something is unobservable – unobservable in principle – it is not part of science. If there is no way to falsify or confirm a hypothesis, it belongs to the realm of metaphysical speculation, together with astrology and spiritualism. By that standard, most of the universe has no scientific reality – it’s just a figment of our imaginations.
—  Leonard Susskind

The universe is vastly bigger than the portion that we can see; and, on a very large scale it is as varied as possible. In other words, rather than being a homogeneous, mono-colored blanket, it is a crazy-quilt patchwork of different environments. This is not an idle speculation. There is a growing body of empirical evidence confirming the inflationary theory of cosmology, which underlies the hugeness and hypothetical diversity of the universe.

Meanwhile string theorists, much to the regret of many of them, are discovering that the number of possible environments described by their equations is far beyond millions or billions. This enormous space of possibilities, whose multiplicity may exceed ten to the 500 power, is called the Landscape. If these things prove to be true, then some features of the laws of physics (maybe most) will be local environmental facts rather than written-in-stone laws: laws that could not be otherwise. The explanation of some numerical coincidences will necessarily be that most of the multiverse is uninhabitable, but in some very tiny fraction conditions are fine-tuned enough for intelligent life to form.

—  Leonard Susskind

PHYSICS OF THE DAY: An Introduction To Black Holes, Information And The String Theory Revolution by Leonard Susskind

A unique exposition of the foundations of the quantum theory of black holes including the impact of string theory, the idea of black hole complementarily and the holographic principle<BR>bull; Aims to educate the physicist or student of physics who is not an expert on string theory, on the revolution that has grown out of black hole physics and string theory

Particles are particles, waves are waves. How can a particle be a wave?
a wave in an ocean, that’s not a particle. The ocean is made out of particles but the ocean are not particles. And rocks are not waves, rocks are rocks. So a rock is an example of a particle, and the ocean is an example of a wave. And now somebodies trying to tell you, a rock is the ocean. What?
—  leonard susskind on quantum mechanics.


It gets interesting around 34 min, if you want to skip ahead