The Hardest Questions in Physics | World Science Festival

Susskind's core methodology involves being drawn to situations where two seemingly correct physical principles fundamentally conflict, believing that resolving these paradoxes leads to profound breakthroughs. This approach shaped his contributions to string theory and black hole physics.

When there's something that you are pretty sure has to be right, and there's something else, which you're pretty sure has to be right, but they conflict with each other, they can't both be true. To me, that's where my mind is drawn.

Stephen Hawking proposed that information falling into a black hole is permanently lost, violating the quantum mechanical principle of unitarity (information conservation). Susskind and Gerard 't Hooft intuitively rejected this, leading to a major scientific conflict.

Stephen thought that black holes were a very specialist way. That they would be, in fact, I mean, I would have to say that he pretty much thought that his legacy in physics would be that information is not conserved.

Susskind conceived the holographic principle, suggesting that the event horizon of a black hole acts like a 2D hologram, encoding all the 3D information that falls into it. This allows information to be "stored" on the surface, reconciling information conservation with black hole physics.

It struck me at the time that the horizon of a black hole was similar to a hologram. All of the information that falls onto the black hole appears to fall toward the horizon and get stuck there and never go through it like the holographic film.

The holographic principle gained significant traction after Juan Maldacena provided a precise mathematical instance (AdS/CFT correspondence) where a quantum mechanical description of a space's interior was entirely encoded on its boundary, proving the principle's consistency.

Juan Maldacena... put together a description of a particular kind of space time, anti-de Sitter space... in which all of the activity, the encoding of information, was on the surface. In other words, it really was an instance, a precise... mathematically precise instance of the holographic principle in action.

String theory originated from an attempt to describe the internal structure of particles like protons. However, it unexpectedly predicted a particle identical to a graviton, suggesting that the same mathematics could describe gravity at extremely small scales, thus unifying quantum mechanics and gravity.

The particle was in every respect, similar to a graviton. But we weren't trying to describe gravity. We were trying to describe protons... They realized that the same mathematics that we had been using to try to describe protons and neutrons, the same mathematics, the same string ideas could apply on 19 orders of magnitude smaller distance scale could apply to the graviton.

Traditional attempts to quantize general relativity (applying quantum rules to a classical theory of gravity) consistently failed. Susskind argues that gravity and quantum mechanics are too fundamentally intertwined to be separated and then reunited; they are "almost the same thing."

I think what we've discovered over the last 20, 25 years is that it's a mistake... because gravity and quantum mechanics are too closely connected to pull them apart and then subject them to quantization and put them together again.

Modern high-energy physics experiments, like those needed to test string theory or supersymmetry, require immense resources and decades to build and operate, making direct experimental validation extremely difficult and slow, akin to building ancient cathedrals.

As technology has become more and more remote and difficult and so forth, it started taking a year to do an experiment, then it started taking five years to do an experiment. At some point, when big accelerators were necessary, it became a whole scientific lifetime to put together an accelerator.

String theory requires extra spatial dimensions curled up to a tiny size. The vast number of ways these dimensions can be curled up (e.g., 10^500) suggests a "landscape" of many possible universes, each with different physical laws, with our universe being one where conditions allow for life.

The number now, we often bandy about a number. It's somewhat meaningless, but 10 to the 500 is a number... That's an enormous number of different ways that the extra dimensions could be curled up.