Schroedinger's Universe?
Suppose that we have a box: kind of box that might contain, for example, a cat. Suppose that no energy is flowing into or out of the box. And suppose we them ask: what is going on inside the box?
Sean Carroll will answer, in accord with all experimental evidence and of the most sophisticated quantum mechanical thinking, that nothing is going on inside the box:
The possible measurement outcomes... spinning clockwise or counterclockwise... only become “real”... when the quantum system interacts [outside] with a large number of degrees of freedom, becomes entangled with them, and decoherence occurs.... [But] what dynamic processes are occurring while the wave function isn’t changing at all? Your first guess here--nothing at all “happens” inside a wave function that doesn’t evolve with time--is completely correct....
Surprisingly, this claim--“nothing is happening if the quantum state isn’t changing with time”--manages to be controversial!
People have this idea that a time-independent quantum state has a rich inner life, with civilizations rising and falling within even though the state is literally exactly the same at every moment in time....
Believing that all sorts of things are happening inside a time-independent state creates cosmological problems, in particular the Boltzmann Brain problem, where conscious observers keep popping into existence in empty space. So we’re in the funny situation where believing the correct thing--that nothing is happening when the quantum state isn’t changing--solves a problem, and yet some people prefer to believe the incorrect thing, even though that creates problems for them...
But what if the box contains a universe? And what if we never open the box? What happened inside the box
