Since the 1970s, black holes have emerged as a central area for theorists. Here are five points why.
- Black holes are unique physics; no classical, semi-classical, or quantum descriptions can describe them completely. It provides a way to understand quantum gravity. Understanding horizons, singularities, information through it, and many more make black holes perfect for learning new physics.
- Black holes have unique entropy. Bekenstein, in 1971 gave (with arbitrary constants only perfected by Hawking later in 1973) a formula for entropy $S=A/4$ in Planck units. Bekenstein also provided a generalized second law $S = S_{BH} + S_{out}$, which conjectured that generalized second law can never be violated, this went through many tests. Bekenstein also proposed a unique bound on the entropy of falling objects, now known as the Bekenstein bound. (Which on the other hand has been explored a lot by many setting up holographic bounds, covariant bounds, and so on.)
- Black holes evaporate. Hawking, in 1973, did the field theory calculations and showed that the positive flux of radiation goes to the future infinity, and the negative flux goes inside the horizon. This is radiation. With negative flux, black hole mass is reduced, and thus it is called evaporated. But this is not fitting since the black hole starts with a pure state, and in this scenario, it will end in the mixed state as Hawking radiation. This is called the Information problem.
- The last mentioned point is why most ponder over black holes. Information preservation is a must, so physicists devised many roundabouts and alternatives to information loss proposed by Hawking. This includes Fuzzballs, Firewalls, Complimentarity, baby universes, etc. None of them exactly solves it; some create further paradoxes. It is unsettling that information could be lost.
- There are beautiful ideas regarding wormholes through the double-sided eternal AdS black holes. ER=EPR is quite a line that joins the geometry and entanglement. Quantum teleportation, in theory, is also possible due to information theory and the web- traversable and non-traversable,
