Vlad Gheorghiu’s Reviews > Black Holes: The Key to Understanding the Universe > Status Update

Chapter 3 introduces 1+1 Penrose diagrams - a map that stretches spacetime to infinity. Cool facts from this chapter:
1. Relativistic Doppler effect
2. Changing Penrose diagrams between two POVs appears for each POV to see the other moving slower.
3. A round trip at near light speed to Andromeda will seem as 18 years, but someone on Earth will have to wait 5 million years.

Chapter 2 introduces the spacetime interval - a constant of Nature that measures the space and time between two events. Outside observers will always measure time and space differently, but subtracting the time interval squared with the space interval squared will always give you the same spacetime interval no matter the POV. The chapter also explains light cones, to see what events can physically influence others.

Starts with a brief history of black holes (first theorised in 18th century, predicted by general relativity, skepticism of their existence, then their acceptance in the 70s, ending with the discovery of Hawking radiation). Chapter lists a good amount of quotes from other books. It also explains how not even whizzing in the electrons and neutrons can withstand the mass pressure of a star, thus becoming a blackhole.