Carroll begins by saying that Time is three things: a coordinate, a measurement and a medium. This would make you believe that it will be a philosophical essay, but instead Carroll will only deal with the Physics of Time.

Carroll begins with Relativity. His introduction to Einstein's theories is also used to debunk "time travel": it is in practice impossible to create a viable universe that contains closed timelike curves.

After General Relativity, it's the turn of Thermodynamics. Carroll introduces Boltzmann's definition of entropy (as a logarithm of the number of microstates that can implement a macrostate) and explanation of the second law (statistically, a system tends to evolve towards the macrostates with high entropy, i.e. macrostates that correspond to large numbers of microstates, basically from rare configurations towards more likely configurations) The implicit assumption is that every microstate is equally probable. The other implicit assumption of the second law is that the universe started in a state of low entropy. That creates the fundamental asymmetry that we recognize as the arrow of time: entropy tends to increase because it is a lot easier to increase than decrease, and that's because the beginning of the story was at low entropy. We assume a low-entropy past when we trust our memories of it: our memories (including photographs, videos, diaries) could have been created in a myriad of ways, but the low-entropy explanation is that they reflect what really happened, and that's all there is to it. Carroll briefly dwells on the relationship with information, and on the paradox of recurrence, but the focus now is on explaining the arrow of time.

Then he's off to Quantum Mechanics. The measurement that collapses the wave function creates an irreversible arrow of time. Carroll hints that this might be related to the law of entropy.

The final chapters are a desperate and highly technical attempt at making sense of the universe as it is, with its low entropy and all. Carroll is a believer in the multiverse, or that our universe is just one of many universes, and its laws are just one of the many possible sets, and we exist because this universe has that set rather than others (that would make life impossible).

At the very end he mentions that Time could even be just an emergent phenomenon, not a fundamental element of reality, but that's for another book.

Surprisingly this book doesn't talk at all about antimatter. Maybe it has become an old boring subject.

Carroll never doubts that the laws of nature are reversible at the microscopic level (despite Quantum Theory's evidence to the contrary), and never wonders what it could mean that gravitation (unlike the other forces) only works in one direction (it only attracts). My feeling is that the explanation for the arrow of time is irreversibility at a much more fundamental level. Carroll never doubts that Time is a continuum. I would start by assuming that Time (and spacetime in general) is discrete (as after all, Quantum Theory implies with the quantum vacuum).

TM, ®, Copyright © 2011 Piero Scaruffi