## Lex Fridman Podcast – Andrew Strominger: Black Holes, Quantum Gravity, and Theoretical Physics

Andrew Strominger is a theoretical physicist at Harvard.

## Quantum gravity

Quantum gravity is an attempt to reconcile general relativity and quantum mechanics into a consistent theory that can describe phenomena at the smallest scales of space-time. Some approaches and frameworks that have been proposed to tackle quantum gravity are string theory, loop quantum gravity, and asymptotic safety.

## String theory

String theory is one of the leading candidates for a theory of quantum gravity. The theory posits that the fundamental constituents of matter and energy are not point-like particles **but tiny vibrating strings. **

String theory can incorporate all the known forces and particles of nature as well as predict new ones. Some of the implications and predictions of string theory are** extra dimensions, supersymmetry, dualities, and branes.**

## Holographic principle

The holographic principle is a conjecture that states that information in some volume of space-time can be stored on the boundary of that region. The holographic principle can be used to understand the behavior of black holes and quantum gravity.

Examples and applications of the holographic principle: dS/CFT correspondence, de Sitter space, and soft hair on black holes.

## de Sitter space

de Sitter space is a type of space-time that has a positive cosmological constant and a constant positive curvature. de Sitter space is relevant for cosmology and quantum gravity as it can model an accelerating or inflating universe.

A few challenges and open questions arise from studying de Sitter space: its stability, entropy, holography, and observability.

## Black holes and the speed of light

The speed of light is a fundamental constant in physics that sets a limit on how fast information can travel in space-time. The speed of light is invariant under Lorentz transformations and relates to causality and time dilation.

**The black hole information paradox:** The paradox is a contradiction between the unitarity principle of quantum mechanics, which states that information is conserved in any physical process, and the classical description of black holes, which states that information is lost when it falls into a black hole.

Some possible resolutions or implications of the paradox are firewalls, complementarity, remnant scenarios, or quantum hair on black holes.

Einstein was a very courageous person. He was willing to go where his thoughts led him, even if it meant contradicting himself or contradicting what he had previously believed.

Quantum gravity is an attempt to reconcile two of the greatest intellectual achievements of the 20th century: general relativity and quantum mechanics.

The holographic principle is a very deep and profound idea that says that the information in some region of space-time can be encoded on the boundary of that region.