2.2 Spacetime Summary

summary

- General relativity
- Some of the rules for gravity were well understood by Newton, though not the mechanism.
- Based on the insight that the speed of light is always constant in empty space, Einstein theorized that gravity describes the bending of space and time by mass and energy.
- Space is not a passive entity in which we live; rather, it’s a dynamical entity that is warped by matter and energy.
- The effects of gravity are explained by objects moving on natural paths in this curved spacetime.
- Einstein’s equations describe the dynamics of spacetime and its interactions with matter.
- Einstein predicted that the warping of spacetime creates ripples that emit energy as “gravitational waves” that travel and are detectable.

- Black Holes and Quantum Gravity
- Einstein also predicted that regions of spacetime can warp into black holes with singularities of infinite gravity if matter is dense enough to trap light.
- We really need a quantum theory of gravity to fully understand black holes and the Big Bang.
- There’s a huge controversy within theoretical physics whether a quantum understanding of black holes implies firewalls at the event horizon (a wall of extremely high energy at the point of no return) as an edge of spacetime, or alternatively, black hole interiors that reflect the complexity of the matter that formed them.
- Many open questions warrant future investigation: How do quantum mechanics and gravity interact? Can different spacetimes be in superpositions? Do we even need gravity since we can basically ignore gravity at atomic scales?

- AdS/CFT Correspondence
- It took until 1997 to model quantum mechanics and gravity analytically. Juan Maldacena was a pioneer in this work and showed that three-dimensional gravitational physics can be represented by an underlying two-dimensional quantum system without gravity. This is called AdS/CFT correspondence.
- Professor Van Raamsdonk compares reality to a computer: a memory chip underlies the dynamics on the screen (the user interface), with a code translating between hardware and software. The 2-D quantum system is like the memory chip and the 3-D gravitational system is like the interface on the screen, or more aptly, like a hologram.
- Maldacena gave us the rules for 2-D quantum systems, but we still need to figure out the code to translate them into rules for 3-D gravitational systems.
- In other words, how is spacetime encoded in the quantum memory chip?