"Books are the plane, and the train, and the road. They are the destination, and the journey. They are home."
- Anna Quindlen
In space, there should be no material difference between left and right, forward and backward, up and down. However, for us there is a substantive difference between up and down because we live in the spatial vicinity of a massive object - the earth - which exerts a gravitational pull on us.
And in spacetime, there should be no material difference between past and future. But for us there is a substantive difference because we live in the temporal vicinity of a massive event - the Big Bang - which gave rise to the arrow of time.
Or so Sean Carroll argues in From Eternity to Here: The Quest for the Ultimate Theory of Time. He says that the reason we remember the past and not the future, the reason effects always follow causes and never vice versa, the reason we perceive time always moving in a particular direction, is because of entropy. For whatever reason (physicists collectively shrug their shoulders), the Big Bang was an extremely low-entropy event, and ever since entropy has been increasing and pulling the arrow of time with it. Even if the arrow were to somehow reverse and run time backwards, supposedly we would still remember the low-entropy direction as the "past", and feel we were moving toward the high-entropy "future".
Entropy itself is an interesting concept, one that most of us are taught in high school physics class is synonymous with "disorder". But that's not really what it is - entropy measures "the number of microstates corresponding to each macrostate". Basically, the more possible configurations of all the particles within a substance, the higher the entropy, and the more likely the substance is to eventually reach that state. After all, there are many more ways egg molecules can be situated in an omelet than in an unbroken egg, where all the yolk molecules must cluster together, and the albumin, and the shell and so forth. It's just statistics that any given egg is likely to end up in the high-entropy condition. (And another name for these statistics is the Second Law of Thermodynamics.)
Since everything is far more likely to be in a high-entropy condition than a low one, it remains a mystery why the universe currently has relatively low entropy, and apparently had even less in the past. Yet that seems to be the case, and so we need the Past Hypothesis: when considering the past, we ignore the statistically-preferred high-entropy condition, and assume that the closer we get in time to the Big Bang, the more the entropy decreases.
Why? Well, that's the question the book grapples with, because nobody has the faintest clue.
Carroll scoffs at popular concepts like the anthropic principle, irreversible temporal systems, and a symmetrical, entropy-decreasing Big Crunch, as irrelevant or inadequate to explain the mystery. Instead, he's fond of the idea of "baby universes", spawned spontaneously from the maximal-entropy atomic soup of impossibly ancient dead universes. The creation of the baby would have properties similar to the Big Bang - inflation, expansion, low entropy, an arrow of time, and so on. Maybe the reason we observe such crazily low levels of entropy is because our universe is just a toddler.
Admittedly, this is a cool idea, but it's not really science. It's not remotely empirical, testable, or falsifiable (which Carroll freely admits). And veering off into crazy metaphysical theories is fun and all, but I mean... we could all be brains in jars, too, or the elaborate fever dream of some minor god. Who knows?
I enjoyed this book, though. Its explanation of quantum mechanics was far more lucid and understandable than anything I'd read previously - and it uses cats, but more entertainingly (and humanely) than Schrödinger did. Its speculations on time travel and wormholes and multiple universes gave my inner sci fi geek plenty to chew on. And it introduced me to the concept of Boltzmann brains, which was worth the price of admission in and of itself.
But mostly, I just found Carroll's writing to be clear, funny, and affecting. As an example, the following paragraph, which encapsulates everything I consider beautiful and fulfilling about science:
We find ourselves, not as a central player in the life of the cosmos, but as a tiny epiphenomenon, flourishing for a brief moment as we ride a wave of increasing entropy from the Big Bang to the quiet emptiness of the future universe. Purpose and meaning are not to be found in the laws of nature, or in the plans of any external agent who made things that way; it is our job to create them. One of those purposes - among many - stems from our urge to explain the world around us the best we can. If our lives are brief and undirected, at least we can take pride in our mutual courage as we struggle to understand things much greater than ourselves.