Friday Science: Hawking 9 (The Arrow of Time)

Friday reviews of Stephen Hawking's A Brief History of Time so far.
Chapter 1: Heliocentric
Chapter 2: Spacetime
Chapter 3: Expansion of the Universe
Chapter 4: Uncertainty Principle
Chapter 5: Elementary Particles and the Forces of Nature
Chapter 6: Black Holes
Chapter 7: Black Holes Ain't So Black
Chapter 8: The Origin and Fate of the Universe

Chapter 9: The Arrow of Time
Getting close to the end. The problem we are dealing with in this chapter is the fact that, on the quantum level, nothing prevents a forward or backward movement in time. In the macro-universe, time only can move in one direction. In the micro-universe, this simply is not the case.

The first reason for this is what Hawking calls the "thermodynamic" arrow of time. We easily identify with a cup shattering on the floor. We do not identify with a cup unfalling and unshattering.

Another arrow is the "cosmological" arrow. The universe is expanding. My sense is that Hawking, writing this book in the late 80s, hoped that eventually this expansion would stop and recontract, making possible an oscillating big bang of sorts. That view has largely been eliminated in the last twenty years

A third arrow he mentions is the "psychological" arrow. This one I am less convinced of. It seems to be related to the anthropic principle. Basically, he argues that our brains are just wired to see time moving in only one direction.

Short chapter. I'm sure I don't fully know the depth of some of what he is saying. But I think I know enough to know that subsequent developments have trashed some of what he said.

The universe has a prevailing arrow of time, based on the second law of thermodynamics and its expansion. On the micro-level, this may not always be the case.