Sean Carroll is a distinguished cosmologist cosmologist and well-known for his contributions to science communication. “From Eternity to Here” offers an expertly thorough exploration of the nature of time, covering a wide range of concepts in theoretical physics in exquisite detail. The major focus is on time’s relation to thermodynamics and entropy, but Carroll goes above and beyond the basic considerations and takes the reader on a journey touching upon a plethora of mysteries in modern physics. The book can be rather dense and technical at points, but it’s necessary for the ambitious aim of teaching a comprehensive understanding of the physics behind time’s arrow.
Some of the most interesting ideas in the book include:
- The Past Hypothesis: Why does time go forwards and never backwards? Why do we remember the past but not the future? Why don’t scrambled eggs ever reform into eggs? These questions strike right at the heart of the mystery of time’s arrow. A “solution” can be found by assuming “The Past Hypothesis” which asserts that our universe evolved from an extremely low entropy state. Are we justified in making this assumption? Does the necessity of postulating this hypothesis point towards a deeper truth?
- Boltzmann Brains: Boltzmann brains are a disturbing and troublesome problem that plague many cosmological models. One of the major issues in cosmology is to explain why our universe originated in such a low entropy state. One approach is to consider the idea that our universe is an improbable, but not impossible, gargantuan entropy fluctuation. In a universe with infinite time in its future, at some point we can expect this to occur. Maybe our universe is just a massive entropy fluctuation? The problem with this suggestion is that the probability of the entire universe forming as an entropy fluctuation is far far smaller than the probability of a smaller system, say just your brain, forming spontaneously with all your present memories and experiences. On many proposed theories of cosmology, it’s far more likely that you are an isolated brain of this kind – a Boltzmann Brain – than an actual human being in a universe of stars and galaxies. Disturbing!
- Eternal Inflation: Maybe there’s more to reality than just our visible universe. Perhaps we’re just one little bubble in a constantly evolving and inflating multiverse. Certainly some of the mysteries behind the nature of time lead Carroll quite naturally to the consideration of an eternally inflating multiverse.
- Thermodynamic vs Gravitational Entropy: Entropy is loosely understood as a measure of disorder in a system, but what exactly counts as a disordered system depends on what forces are involved! In thermodynamics a disordered system is one that has reached thermal equilibrium. Imagine a gas of particles all at the same temperature and spread out fairly evenly in the box. This is a high entropy, disordered configuration. Alternatively, if we consider a system under the influence of gravity, spread-out-ness is actually a very low entropy, ordered state. In contrast to thermodynamics, it’s when particles are clumped and crushed into small spaces like stars, planets and black holes that their entropy is large. The relation between entropy and gravity is like looking through a key-hole into theories of quantum gravity: a rich area to explore.
- The Holographic Principle: Holograms display what appear to be three-dimensional images, but do so by scattering light off of a two-dimensional surface. In other words, the 3D image contains no more information than the 2D slice it’s created from, so it can all be compressed down into just two dimensions. The behaviour of the entropy of a black hole seems to suggest a similar thing about our universe! The entropy of a black hole depends on its surface area, not its volume – this suggests that spacetime as we know it is a construction and all the information in the universe is compressible to a lower dimension!
In terms of readability, Carroll is a naturally gifted communicator and his prose is accessible to a wide readership. It is impressive how he’s able to explain subtle concepts like entropy comprehensively using a menagerie of metaphors and analogies. Despite the quality of the writing however, this is not a book you can skim over quickly (unless you’re a physics graduate perhaps). To get the best out of this book you need to be patient and willing to re-read sections to make sure you’ve understood, there are a lot of very sophisticated ideas tackled. There’s plenty going on between the pages, but if you can keep track of the arguments and stay determined, then the rewards are plentiful.
The content of the book is excellent and even as a physics graduate there were some refreshingly concise expositions of some very sophisticated ideas, such as the Maldecena correspondence. I would recommend it to anyone with a keen interest in physics, wanting to gain a solid conceptual understanding of some of the puzzles cosmology faces. This book would be best for someone who is already fairly well-read in popular science, or an undergraduate physicist wanting a precise overview of frontiers in cosmology. I think anyone picking this up as their first popular-physics book may well struggle! Nevertheless, if you’re determined, “From Here to Eternity” is an extremely rewarding read.