Until the End of Time: Mind, Matter, and Our Search for Meaning in an Evolving Universe

by Brian Greene

“I do mathematics because once you prove a theorem, it stands. Forever.”1

I had never really asked myself why I was so deeply attracted to mathematics and physics. Solving problems, learning how the universe is put together—that’s what had always captivated me. I now became convinced that I was drawn to these disciplines because they hovered above the impermanent nature of the everyday. However overblown my youthful sensibilities rendered my commitment, I was suddenly sure I wanted to be part of a journey toward insights so fundamental that they would never change. Let governments rise and fall, let World Series be won and lost, let legends of film, television, and stage come and go. I wanted to spend my life catching a glimpse of something transcendent.

one of the resident advisors suggested I take a look at Oswald Spengler’s Decline of the West. A German historian and philosopher, Spengler had an abiding interest in both mathematics and science, no doubt the very reason his book had been recommended. The aspects responsible for the book’s fame and scorn—predictions of political implosion, a veiled espousal of fascism—are deeply troubling and have since been used to support insidious ideologies, but I was too narrowly focused for any of this to register. Instead, I was intrigued by Spengler’s vision of an all-encompassing set of principles that would reveal hidden patterns playing out across disparate cultures, on par with the patterns articulated by calculus and Euclidean geometry that had transformed understanding in physics and mathematics.2 Spengler was talking my language. It was inspiring for a text on history to revere math and physics as a template for progress. But then came an observation that caught me thoroughly by surprise: “Man is the only being that knows death; all others become old, but with a consciousness wholly limited to the moment which must seem to them eternal,” knowledge that instills the “essentially human fear in the presence of death.” Spengler concluded that “every religion, every scientific investigation, every philosophy proceeds from it.”3

The appeal of a law of nature might be its timeless quality. But what drives us to seek the timeless, to search for qualities that may last forever? Perhaps it all comes from our singular awareness that we are anything but timeless, that our lives are anything but forever. Resonating with my newfound thinking on math, physics, and the allure of eternity, this felt on target. It was an approach to human motivation grounded in a plausible reaction to a pervasive recognition. It was an approach that didn’t make it up on the fly. As I continued to think about this conclusion, it seemed to promise something grander still. Science, as Spengler noted, is one response to the knowledge of our inescapable end. And so is religion. And so is philosophy. But, really, why stop there? According to Otto Rank, an early disciple of Freud who was fascinated by the human creative process, we surely shouldn’t. The artist, in Rank’s assessment, is someone whose “creative impulse…attempts to turn ephemeral life into personal immortality.”4 Jean-Paul Sartre went farther, noting that life itself is drained of meaning “when you have lost the illusion of being eternal.”5 The suggestion, then, threading its way through these and other thinkers who followed, is that much of human culture—from artistic exploration to scientific discovery—is driven by life reflecting on the finite nature of life.

Across cultures and through the ages, we have placed significant value on permanence. The ways we have done so are abundant: some seek absolute truth, others strive for enduring legacies, some build formidable monuments, others pursue immutable laws, and others still turn with fervor toward one or another version of the everlasting. Eternity, as these preoccupations demonstrate, has a powerful pull on the mind aware that its material duration is limited.

We emerge from laws that, as far as we can tell, are timeless, and yet we exist for the briefest moment of time. We are guided by laws that operate without concern for destination, and yet we constantly ask ourselves where we are headed. We are shaped by laws that seem not to require an underlying rationale, and yet we persistently seek meaning and purpose. In short, we will survey the universe from the beginning of time to something akin to the end, and through the journey explore the breathtaking ways in which restless and inventive minds have illuminated and responded to the fundamental transience of everything.

In the fullness of time all that lives will die. For more than three billion years, as species simple and complex found their place in earth’s hierarchy, the scythe of death has cast a persistent shadow over the flowering of life. Diversity spread as life crawled from the oceans, strode on land, and took flight in the skies. But wait long enough and the ledger of birth and death, with entries more numerous than stars in the galaxy, will balance with dispassionate precision. The unfolding of any given life is beyond prediction. The final fate of any given life is a foregone conclusion.

As cultural anthropologist Ernest Becker maintained, we are under a constant existential tension, pulled toward the sky by a consciousness that can soar to the heights of Shakespeare, Beethoven, and Einstein but tethered to earth by a physical form that will decay to dust. “Man is literally split in two: he has an awareness of his own splendid uniqueness in that he sticks out of nature with a towering majesty, and yet he goes back into the ground a few feet in order blindly and dumbly to rot and disappear forever.”2 According to Becker, we are impelled by such awareness to deny death the capacity to erase us. Some soothe the existential yearning through commitment to family, a team, a movement, a religion, a nation—constructs that will outlast the individual’s allotted time on earth. Others leave behind creative expressions, artifacts that extend the duration of their presence symbolically. “We fly to Beauty,” said Emerson, “as an asylum from the terrors of finite nature.”3 Others still seek to vanquish death by winning or conquering, as if stature, power, and wealth command an immunity unavailable to the common mortal.

Planet earth, which Carl Sagan described as a “mote of dust suspended on a sunbeam,” is an evanescent bloom in an exquisite cosmos that will ultimately be barren. Motes of dust, nearby or distant, dance on sunbeams for merely a moment. Still, here on earth we have punctuated our moment with astonishing feats of insight, creativity, and ingenuity as each generation has built on the achievements of those who have gone before, seeking clarity on how it all came to be, pursuing coherence in where it is all going, and longing for an answer to why it all matters. Such is the story of this book.

A saga that ranges from quarks to consciousness is a hefty chronicle. Still, the different stories are interlaced. Don Quixote speaks to humankind’s yearning for the heroic, told through the fragile Alonso Quijano, a character created in the imagination of Miguel de Cervantes, a living, breathing, thinking, sensing, feeling collection of bone, tissue, and cells that, during his lifetime, supported organic processes of energy transformation and waste excretion, which themselves relied on atomic and molecular movements honed by billions of years of evolution on a planet forged from the detritus of supernova explosions scattered throughout a realm of space emerging from the big bang. Yet to read Don Quixote’s travails is to gain an understanding of human nature that would remain opaque if embedded in a description of the movements of the knight-errant’s molecules and atoms or conveyed through an elaboration of the neuronal processes crackling in Cervantes’s mind while writing the novel. Connected though they surely are, different stories, told with different languages and focused on different levels of reality, provide vastly different insights.

unlike the opposable thumb or upright gait—inherited physiological features tightly linked to specific adaptive behaviors—many of the brain’s inherited characteristics mold predilections rather than definitive actions. We are influenced by these predispositions but human activity emerges from a comingling of behavioral tendencies with our complex, deliberative, self-reflective minds.

The interval on the cosmic timeline in which conditions allow for the existence of self-reflective beings may well be extremely narrow. Take a cursory glance at the whole shebang, and you might miss life entirely. Nabokov’s description of a human life as a “brief crack of light between two eternities of darkness”6 may apply to the phenomenon of life itself. We mourn our transience and take comfort in a symbolic transcendence, the legacy of having participated in the journey at all. You and I won’t be here, but others will, and what you and I do, what you and I create, what you and I leave behind contributes to what will be and how future life will live. But in a universe that will ultimately be devoid of life and consciousness, even a symbolic legacy—a whisper intended for our distant descendants—will disappear into the void. Where, then, does that leave us?

Over time, my emotional engagement with these ideas has refined. Now, more often than not, contemplating the far future leaves me with a feeling of calm and connection, as if my own identity hardly matters because it has been subsumed by what I can only describe as a feeling of gratitude for the gift of experience. Since, more than likely, you don’t know me personally, let me put this in context. I’m open-minded with a sensibility that demands rigor. I come from a world in which you make your case with equations and replicable data, a world in which validity is determined by unambiguous calculations that yield predictions matching experiments digit by digit, sometimes as far as a dozen places beyond the decimal point. So the first time I had one of these moments of calm connection—I happened to be at a Starbucks in New York City—I was deeply suspicious. Perhaps my Earl Grey was tainted with some bad soy milk. Or perhaps I was losing my mind.

Though governed by elegant mathematical laws that allow for all manner of wondrous physical processes, the universe will play host to life and mind only temporarily. If you take that in fully, envisioning a future bereft of stars and planets and things that think, your regard for our era can appreciate toward reverence. And that is the feeling I had experienced at Starbucks. The calm and connection marked a shift from grasping for a receding future to the feeling of inhabiting a breathtaking if transient present. It was a shift, for me, compelled by a cosmological counterpart to the guidance offered through the ages by poets and philosophers, writers and artists, spiritual sages and mindfulness teachers, among countless others who tell us the simple but surprisingly subtle truth that life is in the here and now. It’s a mind-set that is hard to maintain but one that has infused the thinking of many. We see it in Emily Dickinson’s “Forever—is composed of Nows”8 and Thoreau’s “eternity in each moment.”9 It is a perspective, I’ve found, that becomes all the more palpable when we immerse ourselves in the full expanse of time—beginning to end—a cosmological backdrop that provides unmatched clarity on how singular and fleeting the here and now actually is. The purpose of this book is to provide that clarity.

The deep-seated affinity for something permanent, for what Franz Kafka identified as our need for “something indestructible,”10 will then propel our continued march toward the distant future, allowing us to assess the prospects for everything we hold dear, everything constituting reality as we know it, from planets and stars, galaxies and black holes, to life and mind. Across it all, the human spirit of discovery will shine through. We are ambitious explorers seeking to grasp a vast reality. Centuries of effort have illuminated dark terrains of matter, mind, and the cosmos. During millennia to come, the spheres of illumination will grow larger and brighter. The journey so far has already made evident that reality is governed by mathematical laws that are indifferent to codes of conduct, standards of beauty, needs for companionship, longings for understanding, and quests for purpose. Yet, through language and story, art and myth, religion and science, we have harnessed our small part of the dispassionate, relentless, mechanical unfolding of the cosmos to give voice to our pervasive need for coherence and value and meaning. It is an exquisite but temporary contribution. As our trek across time will make clear, life is likely transient, and all understanding that arose with its emergence will almost certainly dissolve with its conclusion. Nothing is permanent. Nothing is absolute. And so, in the search for value and purpose, the only insights of relevance, the only answers of...

One line of thought that informed Russell’s position is relevant to our exploration here. “So far as scientific evidence goes,” Russell noted, “the universe has crawled by slow stages to a somewhat pitiful result on this earth and is going to crawl by still more pitiful stages to a condition of universal death.” With such a bleak outlook, Russell concluded, “if this is to be taken as evidence of purpose, I can only say that the purpose is one that does not appeal to me. I see no reason, therefore, to believe in any sort of God.”2 The theological thread will be stitched into later chapters. Here, I want to focus on Russell’s reference to scientific evidence for a “universal death.” It comes from a nineteenth-century discovery with roots as humble as its conclusions are profound.

Roughly 95 percent of the heat generated by burning wood or coal was lost to the environment as waste. This inspired a handful of scientists to think deeply about the physical principles governing steam engines, seeking ways to burn less and get more. Over the course of many decades their research gradually led to an iconic result that has become justly famous: the second law of thermodynamics. In (highly) colloquial terms, the law declares that the production of waste is unavoidable. And what makes the second law vitally important is that while steam engines were the catalyst, the law is universally applicable. The second law describes a fundamental characteristic inherent in all matter and energy, regardless of structure or form, whether animate or inanimate. The law reveals (loosely, again) that everything in the universe has an overwhelming tendency to run down, to degrade, to wither.

we are saved by a change in perspective. Large collections can sometimes yield their own powerful simplifications. It is surely difficult, impossible really, to predict exactly when you will next sneeze. However, if we broaden our view to the larger collection of all humans on earth, we can predict that in the next second there’ll be roughly eighty thousand sneezes worldwide.6 The point is that by shifting to a statistical perspective, earth’s large population becomes the key—not the obstacle—to predictive power. Large groups often display statistical regularities absent at the level of the individual.

With irreversibility being so central to how things evolve, you would think we could easily identify its mathematical origin within the laws of physics. Surely, we should be able to point to something specific in the equations that ensures that although things can transform from this to that, the math forbids them from transforming from that to this. But for hundreds of years the equations we’ve developed have failed to offer us anything of the sort. Instead, as the laws of physics have been continually refined, passing through the hands of Newton (classical mechanics), Maxwell (electromagnetism), Einstein (relativistic physics), and the dozens of scientists responsible for quantum physics, one feature has remained stable: the laws have steadfastly adhered to a complete insensitivity to what we humans call future and what we call past.

In the real world we don’t see Olympic divers popping out of pools feetfirst and landing calmly on springboards. We don’t see shards of stained glass jumping up from the floor and reassembling into a Tiffany lamp. Clips from films run in reverse are amusing for the very reason that what we see projected differs so thoroughly from anything we experience. And yet, according to the math, the events depicted in reverse-run clips are fully in keeping with the laws of physics. Why then is our experience so lopsided? Why do we only ever see events unfold in one temporal orientation and never the reverse? A key part of the answer is revealed by the notion of entropy, a concept that will be essential to our understanding of the cosmic unfolding.

we have now illustrated the essential concept of entropy. The entropy of a given configuration of the pennies is the size of its group—the number of fellow configurations that pretty much look like the given configuration.9 If there are many such look-alikes, the given configuration has high entropy. If there are few such look-alikes, the given configuration has low entropy. All else being equal, a random shake is more likely to belong to a group with higher entropy since such groups have more members. This formulation also connects with the colloquial uses of entropy I referenced at the outset of this section. Intuitively, messy configurations (think of a chaotic desktop piled high with scattered documents, pens, and paper clips) have high entropy because a great many rearrangements of the constituents all pretty much look the same; randomly rearrange a messy configuration and it still looks messy. Orderly configurations (think of a pristine desktop with all documents, pens, and paper clips neatly placed in their designated positions) have low entropy because very few rearrangements of the constituents look the same. As with the pennies, high entropy beckons because messy arrangements far outnumber orderly ones.

The organization of particles into groups of look-alikes provides an extraordinarily powerful schema. Just as randomly tossed pennies are more likely to belong to a group with greater membership (with higher entropy), so too for randomly bouncing particles. The realization is as straightforward as its implications are far-reaching: Whether the bouncing particles are in a steam engine, in your room, or anywhere else, by understanding the typical features of the most commonplace configurations (those that belong to the groupings with the greatest membership), we can make predictions about the system’s macroscopic qualities—the very qualities we care about. These are statistical predictions, to be sure, but ones with a fantastically high probability of being accurate. And the kicker is that we achieve all this while avoiding the insurmountable complexity of analyzing the trajectories of an absurdly large number of particles.

To summarize: Having fewer molecules, or having lower temperature, or filling a smaller volume results in lower entropy. Having more molecules, or having higher temperature, or filling a larger volume results in higher entropy. From this brief survey, let me underscore one way of thinking about entropy, lacking in precision but providing a useful rule of thumb. You should expect to encounter high-entropy states. Because such states can be realized by a great many different arrangements of the constituent particles, they’re typical, pedestrian, easily configured, a dime a dozen. By contrast, if you encounter a low-entropy state it should command your attention. Low entropy means there are far fewer ways the given macrostate can be realized by its microscopic ingredients, and so such configurations are hard to come by, they’re unusual, they’re carefully arranged, they’re rare. Step out of a long hot shower and find the steam uniformly spread throughout your bathroom: high entropy and totally unsurprising. Step out of a long hot shower and find the steam all clustered in a perfect little cube hovering in front of the mirror: low entropy and extraordinarily unusual. So unusual, in fact, that were you to encounter such a configuration you should be extremely skeptical of the explanation that you’ve simply come upon one of those unlikely things that on occasion happen. That could be the explanation. But I’d bet my life it isn’t. Just as you’d suspect there’s a reason beyond mere...

low-entropy configurations should be viewed as a diagnostic, a clue that powerful organizing influences may be responsible for the order we’ve encountered. In the late 1800s, armed with these ideas, many of his own devising, Austrian physicist Ludwig Boltzmann believed he could address the question that launched this section of our discussion: What distinguishes the future from the past? His answer relied on a quality of entropy articulated by the second law of thermodynamics.

The second law of thermodynamics focuses on entropy. Unlike the first law, the second is not a law of conservation. It is a law of growth. The second law declares that over time there is an overwhelming tendency of entropy to increase. In colloquial terms, special configurations tend to evolve toward ordinary ones (your carefully pressed shirt becomes creased and wrinkled) or order tends to descend into disorder (your organized garage degenerates into a haphazard mess of tools, storage boxes, and sporting equipment). While this depiction provides fine intuitive imagery, Boltzmann’s statistical formulation of entropy allows us to describe the second law with precision and, just as important, gain a clear understanding of why it’s true.

if a physical system is not already in the highest-entropy state available, it is overwhelmingly likely that it will evolve toward it. The explanation, illustrated well by the bread’s aroma, rests on the most basic reasoning: because the number of configurations with more entropy is enormously greater than those with less entropy (by the very definition of entropy), the odds are enormously larger that random jostling—the relentless bumping and vibrating of atoms and molecules—will drive the system toward higher entropy, not lower. The progression will continue until we reach a configuration with the highest entropy available. From that point onward the jostling will tend to drive the constituents to migrate among the (typically) gargantuan number of configurations of the highest-entropy states.14 That’s the second law of thermodynamics. And that’s why it is true.

Consider, as an example, a stick of dynamite. Because all the energy stored in the dynamite is contained in a tight, compact, orderly chemical package, the energy is easy to harness. Place the dynamite where you want its energy deposited and light the fuse. That’s it. Post-explosion, all of the dynamite’s energy still exists. That’s the first law in action. But because the dynamite’s energy has been transformed into the rapid and chaotic motion of widely dispersed particles, harnessing the energy is now extremely difficult. So, although the total amount of energy doesn’t change, the character of the energy does. Before the explosion, we say that the dynamite’s energy is high quality: it’s concentrated and easy to access. After the explosion, we say that the energy is low quality: it’s spread out and difficult to utilize. And since the exploding dynamite fully abides by the second law, going from order to disorder—from low entropy to high entropy—we associate low entropy with high-quality energy and high entropy with low-quality energy. Yes, I know. It’s a lot of highs and lows to keep track of. But the conclusion is pithy: whereas the first law of thermodynamics declares that the quantity of energy is conserved over time, the second law declares that the quality of that energy deteriorates over time. Why then is the future different from the past? The answer, apparent from what we’ve now developed, is that the energy powering the future is of lower quality than that poweri...

While spectacularly unlikely, the laws of physics do allow entropy to go down.

All the entropically increasing processes you’ve experienced day in and day out during your entire life—from the mundane of a shattering glass to the profound of bodily aging—can happen in reverse. Entropy can decrease. It’s just ridiculously unlikely.

A highly ordered, exceedingly low entropy starting point at the big bang is why today’s universe is not entropically maxed out, allowing for an eventful future that differs from the past.

Grab hold of a sauté pan’s hot handle and it feels like heat is flowing to your hand. But does anything actually flow? There was a time long ago when scientists thought the answer was yes. They envisioned a fluidlike substance, called “caloric,” which would flow from hotter locations to cooler ones much like a river flows from upstream to downstream. In time, the more refined understanding of matter’s ingredients provided a different description. When you grasp the pan’s handle, its faster-moving molecules collide with the slower-moving molecules in your hand, on average causing the speed of those in your hand to go up and those in the handle to go down. You sense the increased speed of the molecules in your hand as warmth; the temperature of your hand has increased. Correspondingly, the slower speed of the molecules in the handle means its temperature has decreased. What flows, then, is not a substance. The molecules in the handle stay in the handle, and those in your hand stay in your hand. Instead, much as information flows from one person to the next in a game of telephone, molecular agitation flows from molecules in the handle to those in your hand when you grab it. And so, whereas matter itself does not flow from handle to hand, a quality of matter—average molecular speed—does. That is what we mean by the flow of heat. The same description applies to entropy.

Since your hand interacts with the pan’s handle, you can’t apply the second law to the handle on its own. You must include both the handle and your hand (and, to be more precise, the entire pan, the stove, the surrounding air, and so on). And a careful accounting shows that the increase in the entropy of your hand outstrips the decrease in the entropy of the handle, ensuring that the total entropy does indeed go up.

To absorb heat is to absorb energy that is carried by random molecular motion. That energy, in turn, drives the receiving molecules to move more quickly or spread more widely, thus contributing to an increase in entropy. The conclusion, then, is that to shift entropy from here to there, heat needs to flow from here to there. And when heat flows from here to there, entropy shifts from here to there. In short, entropy rides the wave of flowing heat.

entropy is overwhelmingly likely to increase toward the future, making reverse-run sequences (in which entropy would decrease) fantastically improbable.

over time, physical systems will, with fantastic likelihood, evolve from configurations of lower entropy toward configurations of higher entropy. If a system, like a steam engine, seeks to maintain its structural integrity, it must stave off the natural drive toward increased entropy by transferring the entropy it builds up to its surroundings. To do so, the engine must release waste heat to the environment.

You Are a Steam Engine With the importance of resetting the entropy each time a steam engine goes through a cycle, you might wonder what would happen if the entropy reset were to fail. That’s tantamount to the steam engine not expelling adequate waste heat, and so with each cycle the engine would get hotter until it would overheat and break down. If a steam engine were to suffer such a fate it might prove inconvenient but, assuming there were no injuries, would likely not drive anyone into an existential crisis. Yet the very same physics is central to whether life and mind can persist indefinitely far into the future. The reason is that what holds for the steam engine holds for you.

We are all waging a relentless battle to resist the persistent accumulation of waste, the unstoppable rise of entropy. For us to survive, the environment must absorb and carry away all the waste, all the entropy, we generate. Which raises the question, Does the environment—by which we now mean the observable universe—provide a bottomless pit for absorbing such waste? Can life dance the entropic two-step indefinitely? Or might there come a time when the universe is, in effect, stuffed and so is unable to absorb the waste heat generated by the very activities that define us, bringing an end to life and mind? In the lachrymose phrasing of Russell, is it true that “all the labors of the ages, all the devotion, all the inspiration, all the noonday brightness of human genius, are destined to extinction in the vast death of the solar system, and that the whole temple of Man’s achievement must inevitably be buried beneath the debris of a universe in ruins”?17

Discovered in 1927 by German physicist Werner Heisenberg, the uncertainty principle demonstrated that there are features of the world—like the position and the speed of a particle—that a classical physicist in the mold of Isaac Newton would adamantly claim can be specified with complete certainty but that a quantum physicist realizes are burdened by a quantum fuzziness that makes them uncertain. It’s as if the classical tradition viewed the world through pristine, polished spectacles that brought all physical features into perfectly sharp focus, while the spectacles donned by the quantum perspective are inherently foggy. In the large, everyday world of common experience, the quantum fog is too thin to impact our vision, so the classical and quantum perspectives are barely distinguishable. But the smaller you probe, the foggier the quantum lenses become and the fuzzier the view. The metaphor might suggest that all we need do is clean the quantum lenses. But the uncertainty principle established that no matter how fastidious we are and regardless of the advanced equipment we use, there will always be a minimal amount of fogginess that cannot be wiped away. In fact, my phrasing betrays the bias of human experience. It is only by comparison with the demonstrably incorrect classical view—the view we humans discovered first because it’s both simpler and extraordinarily accurate on the scales accessible to human senses—that quantum reality seems hazy. It is actually the classical...

Notwithstanding centuries of scientific progress, we are no closer to answering the question raised by Gottfried Leibniz—“Why is there something rather than nothing?”—than we were when the German philosopher first expressed this lean distillation of the mystery of existence. Not that people haven’t proposed creative ideas and provocative theories. But in asking a question of ultimate origin, we are seeking an answer that requires no antecedent, an answer that does not shift the question one step further back, an answer that is immune to the follow-on questions “Why were things this way instead of that?” or “Why these laws instead of those?” No explanation yet proposed has achieved this or even come close.

One scenario that cosmologists have considered imagines that the early universe was a frenzied and chaotic environment, and as a result the value of the inflaton field across space would have fluctuated wildly, somewhat like the surface of boiling water. To generate repulsive gravity and set off the bang, we need a small region of space in which the inflaton’s value was uniform (or very nearly so, taking into account quantum jitters). But finding such a uniform region amid the chaotic undulations would be like boiling a vat of water and finding a region on its agitated surface that had suddenly flattened. You have never seen that happen. Not because it’s impossible but because it’s extraordinarily unlikely. For a region of the vat’s randomly bubbling water to pass through the same height at the same moment yielding a flat, orderly, uniform, low-entropy configuration would require an astounding coincidence. Similarly, for the wildly undulating inflaton field to have acquired a uniform value within a small region of space, thus igniting inflationary expansion, would have required an astounding coincidence too. And without an explanation for how this special, orderly, low-entropy, uniform configuration came to be, physicists are deeply uneasy.8

This is not the only proposal for how inflationary expansion may have gotten off the ground. Andrei Linde, one of inflationary cosmology’s pioneers, has quipped that for every three researchers there are at least nine opinions on the matter.9 So we must leave to future research, theoretical and observational, a more definitive answer for how a small region of space became uniformly filled with an inflaton field, thus setting off a burst of spatial expansion. For now, we will simply assume that one way or another, the early universe transitioned into this low-entropy, highly ordered configuration, sparking the bang and allowing us to declare that the rest is history.

Gravity is the weakest of nature’s forces, a fact made evident by the simplest demonstration. Pick up a coin. The muscles in your arm beat out the gravitational pull of the entire earth. Whether you consider yourself soft or strapping, victory over the gravitational pull of a planet highlights gravity’s intrinsic weakness. The only reason we’re even aware of gravity is that it’s a cumulative force: every bit of the earth pulls on every bit of a coin, and on every bit of this book, and on every bit of you, and since there’s a whole lot of earth, these pulls add up to downward forces we can feel. But the gravitational attraction between two smaller things, like two electrons, is a million billion billion billion billion times weaker than their electromagnetic repulsion.

You’ve never clutched hold of your morning coffee and found it hotter than when you poured it. That’s because heat flows only from higher temperature to lower temperature, and so your hot coffee transfers some of its heat to the cooler environment, causing the coffee’s temperature to decrease.12 For our large cloud of gas, heat also flows from the hot central core to the cooler surrounding shell. Now, I can’t fault you for thinking that this flow of heat will cool the core and warm the shell, bringing their temperatures closer together, much as heat transferred from your coffee to the air brings your hot mug closer to room temperature. But—and this is remarkable and remarkably important—when gravity is directing the show, the conclusion is reversed. As heat flows out from the core, the core gets hotter and the shell gets cooler. This is surely counterintuitive, but understanding it is just a matter of connecting dots we’ve already marked. As the surrounding shell absorbs heat from the core, the additional energy drives the cloud to swell even further. The outward moving molecules, once again, strain against the inward pull of gravity, and hence are slowed even further.13 The net effect is that the temperature of the expanding shell goes down, not up. Conversely, as the core relinquishes heat, the decrease in energy causes it to contract even further. The inward-moving molecules, flowing in the same direction as the inward pull of gravity, pick up speed as they fall, and so...

When a collection of atoms gets sufficiently hot and dense, they slam together with such force that they can meld more deeply than they do in chemical processes like the burning of natural gas. Whereas chemical burning is a reaction that involves the electrons that surround atoms, nuclear fusion is a reaction that joins nuclei at the center of atoms. Through such deep melding, nuclear fusion generates copious quantities of energy manifested as rapidly moving particles. And it is such rapid thermal motion that generates an outward pressure capable of balancing the inward force of gravity. Nuclear fusion in the core thus halts the contraction. The result is a concentrated, stable, and sustained source of heat and light. A star is born.

The chain of events, highly idealized and simplified to be sure, shows how a star—a pocket of low entropy, a pocket of order—can be produced spontaneously even though no engineer directs the action and even though the second law of thermodynamics, with its dictum that total entropy increases, remains in full force. Compared with a steam engine, the cosmic setting is more exotic, but what we’ve found is another instance of the entropic two-step. Much as a steam engine and its surrounding environment engage in a thermodynamic dance—the steam engine releases waste heat, causing its entropy to decrease, while the environment soaks up the heat, causing its entropy to increase—a gas cloud that’s large enough for gravity to matter engages in an analogous pas de deux. As the core of such a gas cloud contracts under the pull of gravity, its entropy decreases, but in the process it releases heat that causes the entropy of the surroundings to increase. A local region of order is created within an environment that undergoes a more than compensating surge in disorder. The new feature of the gravitational version of the entropic two-step is that it’s self-sustaining. As the gas cloud contracts and emits heat, its temperature rises, causing yet more heat to flow outward and driving the two-step to continue stepping. By contrast, when the steam engine performs work and emits heat, its temperature drops. Without burning more fuel to heat the steam back up, the engine peters out. That’s why...

the nuclear force dances the entropic two-step too. When atomic nuclei fuse—as in the sun, where hydrogen nuclei fuse into helium billions and billions of times each second—the result is a more complex, more intricately organized, lower-entropy atomic cluster. In the process, some of the mass of the original nuclei is converted into energy (as prescribed by E = mc2), mostly in the form of a burst of photons that heats the star’s interior and powers the release of light from the star’s surface. And it is through such fiery starlight, which is itself a torrent of outward streaming photons, that the star transfers copious quantities of entropy to the environment. Indeed, much as we found with the steam engine, and with the contracting gas cloud, the increase in environmental entropy more than compensates for the decrease in entropy from fusing nuclei, ensuring that the net entropy goes up and the integrity of the second law is once again secured.

The upshot, anthropomorphized, is that the universe cleverly leverages the gravitational and nuclear forces to wrest a cache of untapped entropy that’s locked up inside of its material constituents. Without gravity, particles that are uniformly dispersed, like an aroma that has filled your house, have attained the highest entropy available. But with gravity, particles that are squeezed into massive and dense balls supported by nuclear fusion drive the entropy tally yet higher. Catalyzed by gravity and executed by the nuclear force, this version of the entropic two-step is danced by matter clear across the universe. It’s a process that has dominated the cosmic choreography since shortly after the big bang, resulting in vast numbers of stars—orderly astronomical structures whose heat and light, in at least one instance, enabled the emergence of life. That development, as we will explore in the next chapter, involves a counterpart to entropy—evolution—that is capable of shaping the most exquisitely complex structures in the universe.

Concentrate on a single molecule of water, an atom of hydrogen, or an individual electron, and you will find that none bear any mark delineating whether they are a constituent of something living or dead, of something animate or inanimate. Life is recognizable from the collective behavior, the large-scale organization, the overarching coordination of an enormous number of particulate constituents—even a single cell contains more than a trillion atoms. Seeking insight into life by homing in on fundamental particles is akin to experiencing a Beethoven symphony instrument by instrument, note by single note. Schrödinger himself emphasized a version of this very point in his first lecture. If a body or a brain could be impaired by the errant movement of a single atom or a handful of atoms, the survival prospects of that body or brain would be dim. To avoid such sensitivity, Schrödinger pointed out, bodies and brains are made of large collections of atoms that can maintain their overall highly coordinated functioning even as the individual atoms randomly jitter about. So Schrödinger’s goal was not to reveal life hovering within a single atom but to build upon the understanding of atoms to construct a physicist’s explanation of how a large collection might assemble into something that lives. In his view, this was an expansive quest that would likely require science to broaden its base of conceptual structures. Indeed, in an epilogue to What Is Life? touching on consciousness, Sch...

Divide and conquer has long been the rallying call of physics, a strategy that has resulted in rousing triumphs.

Wilson developed a mathematical procedure for analyzing physical systems over a range of different distances—from scales far smaller, say, than those probed by the Large Hadron Collider to the far larger atomic distances that have been accessible for well over a century—and then systematically connecting the stories, clarifying how each hands off the narrative burden to the next as the scale migrates beyond its particular domain. The method, called the renormalization group, lies at the core of modern physics. It shows how the language, conceptual framework, and equations used to analyze physics on one distance scale need to shift as we change focus to a different scale. By using it to develop a nested collection of distinct descriptions and delineating how each informs those it borders, physicists have extracted detailed predictions that have been confirmed through a great many experiments and observations.