What is space-time made of?

This was a really informative lecture. Fay goes from the Laws of Thermodynamics to Hawking's Laws of Black Hole Dynamics and explains the fundamental nature of space-time through the logic that follows. There are a lot of examples of unification and she presents the information so that most people can understand it. The Q&A is also excellent.

She explains the underlying concepts first, so it doesn't get really good until after 30m in.

The main thing that I took away from this was that the key part of the black hole is surface area of the event horizon and not actually the black hole. And that this event horizon is really only empty space-time.

This leaves us on the threshold of an understanding of quantum gravity. She also goes on to marry the granularity of space-time causality and how it illustrates causal horizons including black hole horizons which are only one type.

Fay proposes that entropy counts the number of simplest causal relations that straddle the horizon.

Around 50m in she goes into the expanding universe & the cosmological constant (λ) or the energy density of empty space-time. Einstein originally postulated the cosmological constant and after Hubble's observations, Einstein thought his constant was his greatest error. But we now know that Einstein was wrong about being wrong. Rafael Sorkin predicted that the cosmological constant would be non-zero. Some unknown mechanic of the universe pushes this towards zero but it cannot be non-zero because of Werner Heisenberg's Uncertainty Principle. She restates this principle in the context of space-time and energy density by saying that the smaller the fluctuations in space-time volume, the larger the fluctuations in λ‎.

She suggests that we can prove the atomicity of space-time by looking at how Brownian motion gave us a clue that lead to a greater understanding as to the existence of atoms.

Richard Feynman said, "All things are made of atoms." Fay suggests that this includes space-time. I think it only makes sense, space-time must be quantized by a relationship to the Planck unit. Perhaps gravity is the Brownian motion clue that proves the atomicity of space-time.

Sorkin believes that the successful solution of quantum gravity will involve both a reevaluation of gravity in terms of a discrete structure underlying continuous spacetime, and also a reformulation of quantum mechanics. He also hypothesises that the phenomena of topology change and the thermodynamics of the black hole structure provide important clues to the formation of the final synthesis. In this framework he has examined the quantum properties of topological geons (particles created directly from the spacetime topology). His findings include that the topological geons can exhibit remarkable statistical properties. He also discovered evidence that topology change is a required feature of any consistent quantum gravity theory. He has hunted the origin of a black hole's entropy to discover more about how it relates to the synthesis of quantum mechanics and the theory of general relativity.

A lot of great lectures on the Perimeter Institute Recorded Seminar Archive (PIRSA), including Fay Dowker and Rafael Sorkin.

An Invitation to Causal Sets - Lecture 1
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