I have three questions about Sakurai's discussion of the time-evolution operator. First question: In equation 2.12, Sakurai requires the composition property of the time-evolution operator: $$U(t_2,t_0)=U(t_2,t_1)U(t_1,t_0)$$ Why is this required? Second question: In equation 2.15, Sakurai asse...

So I was reading this wonderful article about blackhole thermodynamics. https://hapax.github.io/physics/teaching/bh-thermo/ Can someone recommend a book which covers all 4 laws of blackhole thermodynamics? One which hopefully a postgraduate can follow?

I'm watching a video about the LISA experiment, which will be used to detect gravitational waves. In there 3 space stations will be launched and will follow the Earth in its revolution around the Sun. To measure gravitational waves, the distance between two cubes of gold and platinum must be cons...