Times of Our Lives
Gravity, along with dark energy, plays a key role in the timing of our cosmic appearance and sets strict limits on the span of life anywhere in the universe.
By Robert L. Jaffe
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Behind Me—dips Eternity— Before Me—Immortality— Myself—the Term between —Emily Dickinson
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ime passes. We are all swept up in its flow. Human affairs are measured in seconds, days, and years—the times of our lives. But that perception of time is as myopic as the view of the universe was centuries ago, when people looked outward or inward with only the naked eye. Most people nowadays are accustomed to the idea that nature may work in the simplest or most dramatic ways in places too small or too far away to see. Physicists understand the need for giant particle accelerators to peer deep within the atom; astronomers recognize the need for exquisitely tuned telescopes to look at distant galaxies. Should anyone be surprised that the same is true of time? And just as the insights of physicists and astronomers have widened perspectives on space, so, too, can they help all of us explore the flow of time beyond the little eddy in which we live.
Aided by extraordinary instruments, physicists have discovered that nature tends toward two extremes. At one extreme are the hectic tempos inherent in the microworld of atoms, atomic nuclei, quarks, and the fundamental forces that drive them—electromagnetism, and the strong and weak nuclear forces. The time intervals corresponding to those forces are far faster than anything that people can perceive directly. At the other extreme is the great, slow waltz of cosmology, times over which the universe itself evolves. Given nature’s preference for extremes, how is it that human beings inhabit a middle world of seconds, days, and years? After all, we are made of quarks and electrons, which swirl and vibrate at a fever pitch. The times of our lives seem arbitrary and irrelevant, compared with either fundamental or cosmological times. Where did they come from? What do they have to do with the laws of physics, or with the processes that enabled us to evolve and to observe the universe?
Last month I explained how each of nature’s fundamental forces comes with its own internal clock, and how each runs exceedingly fast [see “As Time Goes By,” by Robert L. Jaffe, October 2006]. But what about the vast timescales of cosmology? In that direction the terrain is still obscure, the crucial discovery was made just a few years ago, and the central questions are far from settled. What fixes the rhythm of cosmology? No one knows. But thanks to the recent discovery of the mysterious “dark energy” that dominates all the other forms of energy in the universe, cosmologists now know that the unit of cosmological time in the universe—the time over which the universe has changed in a fundamental way—is about 10 billion years.
To appreciate the vastness of that interval, consider that 10 billion years is about 1039 beats of the clock of the strong force—the time it takes a quark to orbit once within a proton. Why is the scale of cosmological time so vastly longer than the “heartbeat” of the fundamental forces of the universe? The question may well be the deepest mystery in modern physics.
And what of the times of our lives? Remarkably, if one looks carefully enough, the middle-size span of a human life seems to re-emerge with renewed significance. There is reason to think that the span of complex life-forms may be roughly the same throughout the universe, a consequence of a delicate balance between the fundamental forces and the force of gravity, which express themselves most dramatically in the microworld and in the stars.
The modern era in cosmology began with Einstein’s formulation of general relativity, his sweeping extension of Newton’s theory of gravity. By embedding gravity in a geometrical picture of space and time, Einstein was able to think in grand terms about the global structure of the universe. General relativity made it possible to ask, What sets the tempo of cosmological change? Einstein conjectured that the universe was static, and by inference, eternal. He had no evidence to the contrary.
Einstein also thought, correctly, that all bits of matter attract each other under gravity’s irresistible force. But that posed a problem: how could a static universe resist collapsing under gravity’s universal attraction? To avoid such a catastrophic outcome, Einstein postulated what he called the cosmological constant, which fills all space with energy and, most important, exerts a constant outward pressure that counterbalances gravity, suspending the universe in a delicate, static equilibrium.
In 1929, not long after Einstein introduced general relativity, the American astronomer Edwin P. Hubble discovered the first evidence that Einstein’s initial picture was wrong: the universe is neither static nor eternal. Instead, Hubble showed, the universe is expanding. Distant galaxies are racing away from the Milky Way and from one another like spots inked on the surface of an inflating balloon. Long ago the universe was smaller, and it was expanding faster. Since then, gravity has been acting as a brake on the expansion. Most important, from Einstein’s point of view, Hubble’s universe had no need of an outward pressure to keep it from collapsing. When Einstein learned of Hubble’s work, he discarded the cosmological constant, in later years calling it his “biggest blunder.”
Hubble’s concept of an expanding universe is the foundation of modern cosmology, according to which the universe was born in a great explosion, the big bang, some 13 or 14 billion years ago. Until quite recently, cosmologists generally thought the force of the big bang and the retarding effects of gravity were perfectly balanced, so that the expansion would exhaust itself only in the infinite future. Hubble’s universe is almost as unchanging as Einstein’s. After its violent birth, its uniform expansion continues without cosmological incident forever. Its present age has no significance except that it happens to be the moment that human beings have come along to make observations and debate cosmological questions.
All that changed in the 1990s, when astronomers tried to verify one of the central predictions of Hubble’s cosmology, that the expansion of the universe should be slowing down. But how could astronomers measure such a universal deceleration? As the dots in the balloon analogy suggest, the more distant a galaxy, the faster it is receding. Yet when astronomers observe a distant galaxy, they are looking deep into the past, to the moment the light they observe was actually emitted. In that early epoch, the universe was smaller and, according to Hubble’s standard cosmology, expanding more rapidly. So if the universe is decelerating, distant galaxies should be receding slightly faster than the present rate of expansion of the universe would suggest.
To almost everyone’s surprise, the results of precise studies showed that distant galaxies are receding slightly slower, not faster, than expected from the present expansion rate. The universal expansion is no longer decelerating at all; in the past few billion years it has begun to accelerate. Nevertheless, all the other features of Hubble’s standard cosmology appear, so far, to be correct. What to do? Although the answer is not certain, a consensus has emerged that Einstein’s discarded cosmological constant fills the bill: space is, in fact, imbued with an energy density and an outward pressure that augments the expansion of the universe.
Remarkably, investigators pulling on another thread of the fabric of cosmology were reaching the same conclusion at just about the same time. They were auditing the relative contributions of various forms of matter and energy to the total energy of the universe. Cosmologists now know that visible matter and all the forms of light (“radiation”) together account for only about 4 percent of the energy in the universe.
Another 22 percent or so is a mysterious, nonluminous, and ghostly stuff known as “dark matter,” which does not interact appreciably with proton-neutron-electron stuff like us. Dark matter has never been observed directly and no one knows what it is—except that cosmologists are rather certain that like other matter, it has mass, carries momentum when it moves, and “feels” the force of gravity.
The rest of the universe, a whopping 74 percent, is “dark energy.” Neither matter nor radiation, dark energy has energy, exerts pressure, and affects gravity, but unlike ordinary matter, it does not carry momentum. It is like nothing encountered before, but it is exactly like the substance Einstein postulated as the cosmological constant. And, to the delight of cosmologists, the pressure generated by dark energy is close to the value necessary to fuel the newly discovered acceleration of the universal expansion.
As the universe expands, new space is created, and with it, a minute amount of new dark energy. Like a sales tax, the dark energy is a fixed percentage of the newly created volume of space. Although ordinary matter and dark matter dwarf dark energy in familiar places such as the Milky Way, matter is concentrated only here and there. Beyond such clumps, dark energy is everywhere. When it is all added up, dark energy dominates.
The discovery of dark energy has finally brought time into cosmology. Long ago, the universe was small. It included the same amount of matter and even more radiant energy than it does now. But because the universe was small, it included very little dark energy. By contrast, in the distant future, after the universe has expanded to many times its current size, matter and radiation will be further diluted and dark energy will dominate overwhelmingly.
Our epoch is special [see illustration below]. Now is when matter and radiation, on the one hand, and dark energy, on the other, are comparable fractions of the stuff of the universe. When the universe was about 70 percent of its present age, the fractions were equal. (Cosmologists measure time in orders of magnitude, and so the distinction between the present age of the universe and 70 percent of the present age is hardly significant.) But now the balance is shifting with what cosmologists consider breakneck speed. In the cosmologically recent past, when the universe was one-tenth its present age, matter accounted for more than 98 percent of the stuff in the universe. In the cosmologically not-too-distant future, when the universe is only five times its present age, matter will account for a mere fifteen parts per million of the stuff in the universe!
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The implications for cosmology are fundamental. A benchmark timescale, independent of human observers, has finally emerged. The unit of cosmological time is the age of the universe at which the balance between matter and dark energy shifted in favor of dark energy: once again, about 10 billion years.
That number introduces a strange coincidence: the lifetime of a hospitable star like our Sun is also about 10 billion years. Why is the match so close? It is a mystery. Astrophysicists can calculate the lifetimes of stars from the laws of gravity, electromagnetism, and the weak and the strong forces. Massive stars burn fast and die young, but bright, stable, medium-size stars like our Sun live billions of years. Long ago the universe was hot, structureless, and uninhabited; stars had yet to form. Far in the future the universe will be cold, dark, and—once more—uninhabited; stars will no longer shine. Today is the epoch of stars, and thus the epoch of life.
Our era is also the epoch of transition from a matter-dominated universe, whose expansion was, until fairly recently, decelerating, to a universe dominated by dark energy that will eventually cause everything to fly apart. This coincidence haunts modern cosmology. Is it an accident or is there some underlying connection between the dark energy, which no one understands, and the complex balance of fundamental forces that lead stars to shine for billions of years? Or is the only connection, as some speculate, that if the two timescales did not coincide, we would not be here to discuss it?
If the density of dark energy throughout space were too large, the universe would have blown apart long ago, before galaxies, stars, and observers like us could have appeared. If the pressure of dark energy pulled inward rather than pushing outward, which seems entirely possible, then the universe would have slammed shut in a “big crunch” long before stars had time to form. Perhaps only a universe in which the laws of physics enable the lifetimes of hospitable stars to match the clock of cosmological time would evolve observers who look at the night sky and ponder the mysteries of cosmology.
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That “cosmic coincidence” has given new urgency to a question that has long lingered in the shadows between physics and metaphysics: could the laws of physics in our universe be determined, not by some wonderful unified theory, but instead, at least in part, because had they been otherwise, no observer could have evolved? Could there be a multitude of universes, each a different throw of the cosmic dice, each born with a different cosmological clock and a different set of physical laws? If so, most of them might be wasting away unobserved, and only a few, perhaps a very few, would turn out to be lucky enough to be noticed, to give rise to beings that could observe them. In this strange and speculative picture, we happen to live in a particularly hospitable universe, one blessed with a long, lazy, and calm cosmological tempo, compared to the helter-skelter time scales of fundamental processes.
What do we see if we try to tune the frame speed of the mind’s eye to the tempo of cosmological time? Is our universe merely one event, one tick of the cosmological clock, part of a larger drama in which universes wink in and out of existence? “Tick”: a universe with too much dark energy, which rips apart before stars can form; “tock”: another universe, where the dark matter attracts rather than repels, so this universe collapses in on itself before any structure can form; “tick”: a universe like ours, delicately balanced and long lived; “tock”: something entirely different.
Trying to comprehend the universe by itself may be like trying to understand a single person alone, without ancestors or offspring or a society to put the person in some context. Perhaps the full story is a sequence or ensemble of universes—a “multiverse,” as cosmologists are calling it—and the properties of our universe have to be understood in that larger context. Perhaps we need to grasp scales of time even longer and more alien than we attempt today. Perhaps we ought to be thinking about a theory of universes. If it seems ludicrous to regard the universe as so ephemeral, perhaps it is because our limited, human perspective on time has prevented us from perceiving the important rhythms of cosmology.
We seem lost in time. What set the pace of life on Earth? How did the day, on the order of 105 seconds, and the year, 107 seconds, emerge as the times of our lives? Are they characteristic times for life throughout the universe? If the search for extraterrestrial intelligence ever succeeds, will we humans find that the aliens live for a few billion seconds, as we do, or will their lifetimes be measured in milliseconds or aeons?
No one knows for sure, but modern physics paired with natural selection suggests an answer. Instead of arising directly out of the laws of physics or cosmology, the times of our lives seem to emerge from a fascinating and subtle interplay of the great forces that control the universe. And there is reason to believe that the time of our lives may well be the timescale of all life in the universe.
Celestial mechanics is a piece of the puzzle. The day, the month, the year—those are the rhythms of light and dark, of the tides, of summer and winter, of the solar system. Our lives, it seems, are tuned to the music of the spheres. But it is not so simple: we are made of biological stuff, governed by laws of physics in which the day and the year do not appear. How did the chemistry of life become locked onto the rhythms of celestial mechanics? The answer seems to lie in natural selection, a law of nature different in kind from the physical laws I have invoked so far, but no less powerful.
Life arose on Earth, a planet circled by the Moon and circling the Sun. From the very beginning, the ooze that was to become us was cooked in a crucible by a flame that rose and fell with daily, monthly, and yearly rhythms. Through countless cycles of reproduction and predation, complex organisms adapted to the rhythms of the solar system.
To be sure, some organisms have evolved lifetimes as short as a few days or as long as a few millennia. But even those exceptions stray from celestial timescales by only a couple of orders of magnitude—not very significant on a timescale that ranges from 10–24 second to 1017 seconds. Could environmental pressure or scientific progress enable an organism to extend or compress its life span by factors of a million or a billion? Although nothing is impossible, it seems highly unlikely.
So suppose human life spans are linked to celestial clocks by natural selection; still, that does not explain why the day and the year are so long compared to the timescales of fundamental physics. Why does it take so long for the Earth to circle the Sun? If that seems a foolish question, recall that the timescales of the strong interactions that fuel the Sun and the electromagnetic forces that make it shine are tiny fractions of a second. How do they manage to produce a solar system that moves to such a leisurely rhythm? The answer comes from a surprising direction: the weakness of gravity compared to the strength of the strong force. Simply stated, the year—and therefore our lives—are so long because gravity is so weak!
The evolution of life appears to require a warm, stable environment: not so hot that complex structures are destroyed, nor so cold that chemistry grinds to a halt. It also must be stable enough to preserve delicate structure while evolution works its slow and steady magic. The neighborhoods of quiescent, long-lived stars seem the preferred real estate for such processes to unfold. Stars are made when atoms are squeezed and heated so much that their nuclei begin to fuse and emit the energy that makes them shine. Gravity is what squeezes matter together and heats it up.
So why doesn’t a baseball or our Earth become a star? Because gravity is too weak. If gravity were much stronger, it would pull all the atoms in a baseball hard enough, create enough pressure, and generate enough heat to initiate fusion, leading to miniature, short-lived stars orbited by planets smaller still. Back to reality—with gravity as weak as it is, it takes about 1057 hydrogen atoms packed by gravity into a sphere to ignite fusion at the core of the sphere.
Not surprisingly, 1057 atoms is roughly the mass of our Sun. Astrophysicists estimate that stars a tenth the mass of the Sun do not ignite, whereas giants ten times its mass burn too quickly for life to evolve in their vicinity. The orbital period of a planet placed at a comfortable distance from a long-lived, stable star ranges between about a tenth of and ten times our year: quite a narrow band [see illustration below]. If gravity were stronger, stars and planetary orbits would be smaller, orbital periods shorter, and life everywhere in the universe, tuned to those periods by natural selection, briefer.
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So the time of our lives may well be the universal tempo for complex life, give or take a factor of ten. It is not carved directly into the laws of fundamental physics or written into the script of cosmology. It emerges instead in an interplay of physics and natural selection: the weakness of gravity compared to the strong force is what makes stars large and planetary periods long, when measured by the rhythm of the strong interactions. Then natural selection binds the time of our lives to celestial mechanics. There is every reason to think that those forces are active everywhere in the universe, and that all life marches to the same beat.
Since the advent of modern science, the universe has seemed a mostly cold and empty place. People have known for centuries that our place in it is nothing special. Now, it seems, the flow of time as we perceive it is far removed from the times of fundamental physics and cosmology that make the universe tick. And yet the times of our lives are likely the times of all life in the universe. The heartbeats we call seconds, the hours that pass as winter shadows move across yesterday’s snow, the years gone by since Emily Dickinson wrestled with time and eternity, these may well be universal experiences in any universe filled with life.
 Robert L. Jaffe is a professor of physics at M.I.T., where he has also served as chair of the M.I.T. faculty and director of the Center for Theoretical Physics. His research has focused on properties of quarks, the way they bind together to form particles such as protons and neutrons, and the role of quarks in the structure of the universe. This article is the second of a two-part essay. |
Copyright © Natural History Magazine, Inc., 2006