UNLIKE MANY of his physicist peers, Sean Carroll welcomes the interest, curiosity, and what must sometimes seem the naïve questions of laypeople. A theoretical physicist and research professor at Caltech, he is also a consummate communicator of science. He tweets and has a widely followed blog, where he has described his work as focusing “on two big themes”: first on “the foundations of quantum mechanics, especially connections to cosmology and emergent spacetime,” and second on the evolution of entropy and complexity. Of the latter he writes:

The arrow of time (the difference between the past and future) in our observable universe can be traced to low-entropy conditions near the Big Bang, and I’ve proposed models to help explain that puzzling cosmological feature. As entropy grows, complexity first appears and then eventually fades away, and I’m interested in understanding the processes by which that happens.

Part of Carroll’s appeal is that he applies his scientific acumen to subjects closer to the human condition. He continues:

I have further aspirations to insinuate my research into other areas. I’m interested in different kinds of philosophy (metaphysics, ontology, epistemology, metaethics), the behavior of complex systems, and also in the origin of life.

Hence The Big Picture.

This is Carroll’s fourth book. His first, Spacetime and Geometry, an overview of general relativity, appeared in 2003. Most recently, he tackled the Higgs boson in The Particle at the End of the Universe (2012). Even if you’re only peripherally interested in cosmology or particle physics, you will recall the excitement in July of that year when scientists at the Large Hadron Collider at CERN announced that their magnificent machine had finally coughed up the Higgs boson, the carrier of a field that endows our world’s other particles with mass. We would not be here without it, which accounts for its wince-inducing nickname of “the God particle.” Fans of science who watched the CERN press conference on TV will probably never forget the rapturous applause and the sight of 83-year-old Peter Higgs, the eponymous proposer of the field in the 1960s, dabbing his eyes with a handkerchief.

Two years earlier, Carroll published From Eternity to Here, on the arrow of time, whose flight from past to future is accompanied by ever-increasing entropy (disorder), as mandated by the second law of thermodynamics. That law may prompt existential angst in some of us, but Carroll is happy with it. Human beings are complex phenomena, having emerged over 14 billion years from the boring orderliness of a low-entropy, hot, dense infant universe into a cold, expanding, disorderly, and fruitful one. “Complex structures,” he points out in The Big Picture, “can form, not despite the growth of entropy but because entropy is growing. Living organisms can maintain their structural integrity, not despite the second law but because of it.” And yet, in the end we are doomed to a horribly named “heat death.” The universe, whose expansion is accelerating,

seems likely to reach a state of thermal equilibrium. At that point it won’t be possible for anything living to survive; life relies on increasing entropy, and in equilibrium there’s no more entropy left to generate. […] That’s us. Ephemeral patterns of complexity, riding a wave of increasing entropy from simple beginnings to a simple end.

¤

Be that as it may, there’s plenty of time left and plenty of questions to attempt to answer. Carroll lays much of this territory out, and his latest book is a good read. If you doubt that a book about the nature of physical reality and how we can best think about it could be a page-turner, you’ll be disabused of this notion by The Big Picture. Its scope is breathtaking, while its tone is friendly, tolerant, sympathetic. Carroll knows that people have a hard time contemplating an essentially pointless universe: “As we understand the world better, the idea that it has a transcendent purpose seems increasingly untenable.” His recommendation is reliance on Bayesian reasoning (updating your hypotheses as more and more evidence becomes available), as well as a relaxed stance he calls “poetic naturalism”:

Within poetic naturalism we can distinguish among three different kinds of stories we can tell about the world. There is the deepest, most fundamental description we can imagine — the whole universe, exactly described in every microscopic detail. […] Then there are “emergent” or “effective” descriptions, valid within some limited domain. That’s where we talk about ships and people, macroscopic collections of stuff that we group into individual entities as part of this higher-level vocabulary. Finally, there are values: concepts of right and wrong, purpose and duty, or beauty and ugliness. […] [T]hese are not determined by the scientific goal of fitting the data. We have other goals: we want to be good people [and] find meaning in our lives. Figuring out the best way to talk about the world is an important part of working toward these goals. […] The world exists; beauty and goodness are things that we bring to it.

Along the way, he discusses the evolution of humankind’s worldview, from Aristotle through Galileo, René Descartes, Pierre-Simon Laplace, David Hume, Charles Darwin, Albert Einstein, the awful conundrums of quantum theory and the swamps of consciousness, on up to philosophers David Chalmers and Daniel Dennett. This is, unavoidably, a march from a theological to a secular point of view, and Carroll, who was raised an Episcopalian and majored in astronomy at Villanova, a Catholic university, is a confirmed atheist (ontogeny recapitulates phylogeny). But a kindly one, not at all in the mold of the evolutionary biologist and atheist crusader Richard Dawkins. He writes:

I hope I never make the mistake of treating people who disagree with me about the fundamental nature of reality as my enemies. The important distinction is not between theists and naturalists; it’s between people who care enough about the universe to make a good-faith effort to understand it, and those who fit it into a predetermined box or simply take it for granted. The universe is much bigger than you or me, and the quest to figure it out unites people with a spectrum of substantive beliefs.

Good-faith, naturalistically inclined theists will find room to breathe here. There are those whose position on God is that He (or It — or something similarly omniscient, omnipotent, and omnipresent) is evolving. Not here at the moment — except for the notion that something that will happen has, in a sense, already happened, somewhere. (Spacetime is relative, after all.) On this wee planet alone, in the boondocks of just one of 100 billion or so observable galaxies, the only life-forms 3.5 billion years ago were unicellular and not what could be called sentient. Now just look around you. Freeways, Coca-Cola, baseball, politicians, PhD theses, large hadron colliders … Given, say, another 20 billion years, what might we see, around here or more likely over there, in another part of the universe? “God” might very well be a bottom-up rather than a top-down phenomenon — natural, rather than supernatural. (Wake up, you ultra-Darwinists!)

Carroll himself is a “fearless Everettian” — that is, he accepts the late theorist (and fellow atheist) Hugh Everett III’s idea of the world as a quantum wave function. This is known as the “Many-Worlds” interpretation of quantum mechanics, a consequence of the fact that at the deepest micro level, things are not anywhere in particular. Or they are everywhere. And each position lives in its own world, so of course there are many such universes. Quantum mechanics is existentially upsetting; Niels Bohr famously said that if you do not get schwindlig (dizzy) contemplating it, then you have not really understood it. But it works, and so we must live with it. As Carroll notes:

Many people object to Many-Worlds because they simply don’t like the idea of all those universes out there. Especially unobservable universes — the theory predicts them, but there’s no practical way of ever seeing them. This is not a very thoughtful objection. If our best theory predicts that something is true, we should place a relatively high Bayesian credence that it actually is true, until a better theory comes along. If you have some visceral or a priori bad feeling about multiple universes, then by all means work on better formulations of quantum mechanics. But a bad feeling is not a principled stance.

Carroll has renounced dizziness and existential angst. He concludes this admirable book by urging us to realize the “abiding joy in puzzling out the nature of reality” and to embrace “the fundamental task of creating meaning and of mattering for ourselves and those around us in the brief amount of time we have in the world.” It put me in mind of the concluding thoughts of another cosmologist, Henry David Thoreau, at the end of Walden:

I do not say that John or Jonathan will realize all this; but such is the character of that morrow which mere lapse of time can never make to dawn. The light which puts out our eyes is darkness to us. Only that day dawns to which we are awake. There is more day to dawn. The sun is but a morning star.

¤

Sara Lippincott, a former nonfiction editor at The New Yorker, is a freelance book editor in Los Angeles, specializing in science.