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On Being a Physicist

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<p>When I turned the last page of Albert Camus' The Myth of Sisyphus many years ago, I was surprised by the text's having achieved an overarching feeling of optimism. After all, a man condemned to pushing a rock up a hill with full knowledge that it will roll back down, requiring him to start pushing anew, is not the sort of story that you'd expect to have a happy ending.</p>
<p>Yet Camus found much hope in the ability of Sisyphus to exert free will, to press on against insurmountable obstacles, and to assert his choice to survive even when condemned to an absurd task within an indifferent universe. By relinquishing everything beyond immediate experience, and ceasing to search for any kind of deeper understanding or deeper meaning, Sisyphus, Camus argued, triumphs.</p><h3>The lure of physics</h3><p>I was struck by Camus' ability to discern hope where most others would see only despair. But as a teenager, and only more so in the decades since, I found that I couldn't embrace Camus' assertion that a deeper understanding of the universe would fail to make life more rich or worthwhile.</p><p>Whereas Sisyphus was Camus' hero, the greatest of scientists—Newton, Einstein, Niels Bohr, and Richard Feynman—became mine. And when I read Feynman's description of a rose—in which he explained how he could experience the fragrance and beauty of the flower as fully as anyone, but how his knowledge of physics enriched the experience enormously because he could also take in the wonder and magnificence of the underlying molecular, atomic, and subatomic processes—I was hooked for good. I wanted what Feynman described: to assess life and to experience the universe on all possible levels, not just those that happened to be accessible to our frail human senses. The search for the deepest understanding of the cosmos became my lifeblood.</p><p></p>
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<img alt="Albert Camus" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/on-being-physicist-02.width-990_uMYvlqY.jpg"/>
<figcaption>Greene was impressed by the optimism that Camus (seen here in 1959) found in the face of despair, as witnessed in <cite>The Myth of Sisyphus</cite>. But he disagreed that a search for deeper meaning was futile.
        
        <div class="credit">© Bettmann/CORBIS</div>
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<p></p><h3>A deep engagement</h3><p>As a professional physicist, I have long since realized that there was much naïveté in my high school infatuation with physics. Physicists generally do not spend their working days contemplating flowers in a state of cosmic awe. Instead, we devote much of our time to grappling with complex mathematical equations scrawled across well-scored chalkboards. Progress can be slow. Promising ideas, more often than not, lead nowhere. That's the nature of scientific research.</p><p>It took the brashness of a Newton to plant the flag of modern scientific inquiry and never turn back.</p><p>Yet, even during periods of minimal progress, I've found that the effort spent puzzling and calculating has only made me feel a closer connection to the cosmos. I've found that you can come to know the universe not only by resolving its mysteries, but also by immersing yourself within them. Answers are great. Answers confirmed by experiment are greater still. But even answers that are ultimately proven wrong represent the result of a deep engagement with the cosmos—an engagement that sheds intense illumination on the questions, and hence on the universe itself. Even when the rock associated with a particular scientific exploration happens to roll back to square one, we nevertheless learn something and our experience of the cosmos is enriched.</p>
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<h3>On the shoulders of giants</h3><p>Of course, the history of science reveals that the rock of our collective scientific inquiry—with contributions from innumerable scientists across the continents and through the centuries—does not roll down the mountain. Unlike Sisyphus, we don't begin from scratch. Each generation takes over from the previous, pays homage to its predecessors' hard work, insight, and creativity, and pushes up a little further.</p><p>New theories and more refined measurements are the mark of scientific progress, and such progress builds on what came before, almost never wiping the slate clean. Because this is the case, our task is far from absurd or pointless. In pushing the rock up the mountain, we undertake the most exquisite and noble of tasks: to unveil this place we call home, to revel in the wonders we discover, and to hand off our knowledge to those who follow.</p><p></p>
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<img alt="Isaac Newton" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/on-being-physicist-03.width-990_JYvqCIT.jpg"/>
<figcaption>"It took the brashness of a Newton to plant the flag of modern scientific inquiry and never turn back," Greene writes. Here, Sir Isaac as depicted in an engraving from the 1800s.
        
        <div class="credit">© Georgios Kollidas/iStockphoto</div>
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<p></p><h3>An intimacy with truth</h3><p>For a species that, by cosmic time scales, has only just learned to walk upright, the challenges are staggering. Yet, over the last 300 years, as we've progressed from classical to relativistic and then to quantum reality, and have now moved on to explorations of unified reality, our minds and instruments have swept across the grand expanse of space and time, bringing us closer than ever to a world that has proved a deft master of disguise. And as we've continued to slowly unmask the cosmos, we've gained the intimacy that comes only from closing in on the clarity of truth. The explorations have far to go, but to many it feels as though our species is finally reaching childhood's end.</p><p>To be sure, our coming of age here on the outskirts of the Milky Way has been a long time in the making. In one way or another, we've been exploring our world and contemplating the cosmos for thousands of years. But for most of that time we made only brief forays into the unknown, each time returning home somewhat wiser but largely unchanged. It took the brashness of a Newton to plant the flag of modern scientific inquiry and never turn back. We've been heading higher ever since.</p>
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<p data-block-key="rmsil">Columbia&#x27;s Brian Greene explains why history&#x27;s greatest scientific thinkers, from Newton to Feynman, became his heroes.</p>

Einstein showed that time and space are not fixed entities, as they seem to us, but are actually changeable. As Brian Greene explains, it's all based on an observer's state of motion compared to that of another observer moving at a different speed.

Special Relativity in a Nutshell

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<p></p><h3>Time and space are relative</h3><p>Each of us carries our own clock, our own monitor of the passage of time. Each clock is equally precise, yet when we move relative to one another, these clocks do not agree. They fall out of synchronization; they measure different amounts of elapsed time between two chosen events. The same is true of distance. Each of us carries our own yardstick, our own monitor of distance in space. Each yardstick is equally precise, yet when we move relative to one another, these yardsticks do not agree; they measure different distances between the locations of two specified events.</p><p>If space and time did not behave this way, the speed of light would not be constant and would depend on the observer's state of motion. But it is constant; space and time do behave this way. Space and time adjust themselves in an exactly compensating manner so that observations of light's speed yield the same result, regardless of the observer's velocity.</p><p>Getting the quantitative details of precisely how the measurements of space and time differ is more involved, but requires only high school algebra. It is not the depth of mathematics that makes Einstein's special relativity challenging. It is the degree to which the ideas are foreign and apparently inconsistent with our everyday experiences. But once Einstein had the key insight—the realization that he needed to break with the more than 200-year-old Newtonian perspective on space and time—it was not hard to fill in the details. He was able to show precisely how one person's measurements of distances and durations must differ from those of another in order to ensure that each measures an identical value for the speed of light.</p><p></p>
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<img alt="man running by clocks" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/special-relativity-nutshell-02.width-990_1BplYSu.jpg"/>
<figcaption>Einstein taught us that we must consider not only motion through space but motion through time. The two are inextricably intertwined.
        
        <div class="credit">© Mark Evans/iStockphoto</div>
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<h3>An extremely simple idea</h3><p>To get a fuller sense of what Einstein found, imagine that Bart has a skateboard with a maximum speed of 65 miles per hour. If he heads due north at top speed—reading, whistling, yawning, and occasionally glancing at the road—and then merges onto a highway pointing in a northeasterly direction, his speed in the northward direction will be less than 65 miles per hour. The reason is clear. Initially, all his speed was devoted to northward motion, but when he shifted direction some of that speed was diverted into eastward motion, leaving a little less for heading north.</p><p>The combined speed of any object's motion through space and its motion through time is always precisely equal to the speed of light.</p><p>This extremely simple idea actually allows us to capture the core insight of special relativity. Here's how:</p><p>We are used to the fact that objects can move through space, but there is another kind of motion that is equally important: Objects also move through time. Right now, the watch on your wrist and the clock on the wall are ticking away, showing that you and everything around you are relentlessly moving through time, relentlessly moving from one second to the next and the next. Newton thought that motion through time was totally separate from motion through space—he thought these two kinds of motion had nothing to do with each other. But Einstein found that they are intimately linked.</p><h3>Motion through time</h3><p>In fact, the revolutionary discovery of special relativity is this: When you look at something like a parked car, which from your viewpoint is stationary—not moving through space, that is—all of its motion is through time. The car, its driver, the street, you, your clothes are all moving through time in perfect synch: second followed by second, ticking away uniformly.</p><p></p>
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<img alt="speeding car" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/special-relativity-nutshell-03.width-990_SeelOym.jpg"/>
<figcaption>All the "motion" of a motionless car is through time, but as soon as it speeds away, some of that car's motion through time is diverted into motion through space.
        
        <div class="credit">© David Birkbeck/iStockphoto</div>
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<p>But if the car speeds away, some of its motion through time is diverted into motion through space. And just as Bart's speed in the northward direction slowed down when he diverted some of his northward motion into eastward motion, the speed of the car through time slows down when it diverts some of its motion through time into motion through space. This means that the car's progress through time slows down, and therefore time elapses more slowly for the moving car and its driver than it elapses for you and everything else that remains stationary.</p><p>That, in a nutshell, is special relativity.</p>
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<h3>Complementary motions</h3><p>In fact, we can be a bit more precise and take the description one step further. Bart had no choice but to limit his top speed to 65 miles per hour. This is important to the story, because if he sped up enough when he angled northeast, he could have compensated for the speed diversion and thereby maintained the same net speed toward the north. But no matter how hard he revved the skateboard's engine, his total speed—the combination of his speed toward the north and his speed toward the east—remained fixed at the maximum of 65 miles per hour. And so when he shifted his direction a bit toward the east, he necessarily caused a decreased northward speed.</p><p>Special relativity declares a similar law for all motion: The combined speed of any object's motion through space and its motion through time is always precisely equal to the speed of light. At first, you may instinctively recoil from this statement since we are all used to the idea that nothing but light can travel at light speed. But that familiar idea refers solely to motion through space. We are now talking about something related, yet richer: an object's combined motion through space and time.</p><p>The key fact, Einstein discovered, is that these two kinds of motion are always complementary. When the parked car you were looking at speeds away, what really happens is that some of its light-speed motion is diverted from motion through time into motion through space, keeping their combined total unchanged. Such diversion unassailably means that the car's motion through time slows down.</p><p></p>
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<img alt="melting clock" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/special-relativity-nutshell-04.width-990_fvxy3ae.jpg"/>
<figcaption>Because motion through both time and space must always add up to the speed of light, when an object (such as a photon) moves through space at light speed, Einstein reasoned, there's no "room" for motion through time, and time, consequently, stops.
        
        <div class="credit">© Barbara Henry/iStockphoto</div>
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<h3>Stopping time</h3><p>As an example, imagine Bart could go 500 million mph on his skateboard—that's about three-fourths the speed of light, which is about 670 million mph—and his sister Lisa, who is standing still, could see his watch as he sped past her. She would see that it was ticking about two-thirds as fast as her own. For every three hours that passed on Lisa's watch, she would see that only two had passed on Bart's. His rapid motion through space would have proved a significant drain on his speed through time.</p><p>This is not dexterous wordplay, sleight of hand, or psychological illusion. This is how the universe works.</p><p>Moreover, the maximum speed through space is reached when all light-speed motion through time is fully diverted into light-speed motion through space—one way of understanding why it is impossible to go through space at greater than light speed. Light, which always travels at light speed through space, is special in that it always achieves such total diversion.</p><p>And just as driving due east leaves no motion for traveling north, moving at light speed through space leaves no motion for traveling through time! Time stops when traveling at the speed of light through space. A watch worn by a particle of light would not tick at all. Light realizes the dreams of Ponce de León and the cosmetics industry: It doesn't age.</p><h3>The nature of nature</h3><p>As this description makes clear, the effects of special relativity are most pronounced when speeds (through space) are a significant fraction of light speed. But the unfamiliar, complementary nature of motion through space and time always applies. The lesser the speed, the smaller the deviation from prerelativity physics—from common sense, that is—but the deviation is still there, to be sure.</p><p></p>
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<img alt="streaking comet" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/special-relativity-nutshell-05.width-990_gnn2HUb.jpg"/>
<figcaption>We may not, indeed we cannot, intuitively grasp the relativity of space and time. Nevertheless, that is the way the universe works, Greene says. Here, Comet Hale-Bopp.
        
        <div class="credit">© tpuerzer/iStockphoto</div>
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<p>Truly. This is not dexterous wordplay, sleight of hand, or psychological illusion. This is how the universe works.</p><p>In 1971, Joseph Hafele and Richard Keating flew state-of-the-art cesium-beam atomic clocks around the world on a commercial Pan Am jet. When they compared the clocks flown on the plane with identical clocks left stationary on the ground, they found that less time had elapsed on the moving clocks. The difference was tiny—a few hundred billionths of a second—but it was precisely in accord with Einstein's discoveries. You can't get much more nuts-and-bolts than that.</p>
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<p data-block-key="jm9nw">Brian Greene explains Einstein&#x27;s notion of the mutability of space and time in a way you can readily understand it.</p>

In 1947, eight years before his death, Einstein wrote to a friend that he could not seriously believe in quantum mechanics because "physics should represent a reality in time and space, free from spooky actions at a distance." He was referring to quantum entanglement, one of the quantum world's most bizarre attributes.

Spooky Action at a Distance

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<p>With Newton and his precise, deterministic mathematical formulation of motion, the description changed again. The universe was likened to an enormous, grand clockwork: After being wound and set into its initial state, the clockwork universe ticks from one moment to the next with complete regularity and predictability.</p><p>Einstein's special and general relativity pointed out important subtleties of the clockwork metaphor: There is no single, preferred, universal clock; there is no consensus on what constitutes a moment, what constitutes a <i>now</i>. Even so, you can still tell a clockworklike story about the evolving universe. The clock is your clock. The story is your story. But the universe unfolds with the same regularity and predictability as in the Newtonian framework. If by some means you know the state of the universe right now—if you know where <i>every</i> particle is and how fast and in what direction each is moving—then, Newton and Einstein agree, you can, in principle, use the laws of physics to predict everything about the universe arbitrarily far into the future or to figure out what it was like arbitrarily far into the past.</p>
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<h3>Enter quantum weirdness</h3><p>Quantum mechanics breaks with this tradition. We <i>can't</i> ever know the exact location and exact velocity of even a single particle. We <i>can't</i> predict with total certainty the outcome of even the simplest of experiments, let alone the evolution of the entire cosmos. Quantum mechanics shows that the best we can ever do is predict the <i>probability</i> that an experiment will turn out this way or that. And as quantum mechanics has been verified through decades of fantastically accurate experiments, the Newtonian cosmic clock, even with its Einsteinian updating, is an untenable metaphor; it is demonstrably <i>not</i> how the world works.</p><p>Something that happens over here can be entwined with something that happens over there.</p><p>But the break with the past is yet more complete. Even though Newton's and Einstein's theories differ sharply on the nature of space and time, they do agree on certain basic facts, certain truths that appear to be self-evident.</p><p></p>
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<figcaption>Space, whatever else it is, serves as the medium that separates two distinct things, like two birds, Greene says. But quantum entanglement belies that apparent truth.
        
        <div class="credit">© Flavio Vallenari/iStockphoto</div>
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<h3>How we think of space</h3><p>If there is space between two objects—if there are two birds in the sky and one is way off to your right and the other is way off to your left—we can and do consider the two objects to be independent. We regard them as separate and distinct entities. Space, whatever it is fundamentally, provides the medium that separates and distinguishes one object from another. That is what space does. Things occupying different locations in space are different things.</p><p>Moreover, in order for one object to influence another, it must in some way negotiate the space that separates them. One bird can fly to the other, traversing the space between them, and then peck or nudge its companion. One person can influence another by shooting a slingshot, causing a pebble to traverse the space between them, or by yelling, causing a domino effect of bouncing air molecules, one jostling the next until some bang into the recipient's eardrum.</p><p>Being yet more sophisticated, one can exert influence on another by firing a laser, causing an electromagnetic wave—a beam of light—to traverse the intervening space. Or, being more ambitious, one can hypothetically shake or move a massive body (like the moon), sending a gravitational disturbance speeding from one location to another.</p><p>To be sure, if we are over here we can influence someone over there, but no matter how we do it, the procedure always involves someone or something traveling from here to there, and only when the someone or something gets there can the influence be exerted.</p><p></p>
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<img alt="voodoo totem" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/spooky-action-distance-03.width-990_zwuXiuh.jpg"/>
<figcaption>Quantum entanglement brings to mind voodoo (here, a voodoo idol from Benin, West Africa). But the scientific evidence that it exists is overwhelming, Greene says.
        
        <div class="credit">© Peeter Viisimaa/iStockphoto</div>
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<h3>Voodoo reality</h3><p>Physicists call this feature of the universe <i>locality</i>, emphasizing the point that you can directly affect only things that are next to you, that are local. Voodoo contravenes locality, since it involves doing something over here and affecting something over there without the need for anything to travel from here to there, but common experience leads us to think that verifiable, repeatable experiments would confirm locality. And most do.</p><p>But a class of experiments performed during the last couple of decades has shown that something we do over here (such as measuring certain properties of a particle) <i>can</i> be subtly entwined with something that happens over there (such as the outcome of measuring certain properties of another distant particle), <i>without</i> anything being sent from here to there.</p><p>Intervening space does not ensure that two objects are separate.</p><p>While intuitively baffling, this phenomenon fully conforms to the laws of quantum mechanics, and was predicted using quantum mechanics long before the technology existed to do the experiment and observe, remarkably, that the prediction is correct. This sounds like voodoo; Einstein, who was among the first physicists to recognize—and sharply criticize—this possible feature of quantum mechanics, called it "spooky." But the long-distance links these experiments confirm are extremely delicate and are, in a precise sense, fundamentally beyond our ability to control.</p><p></p>
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<img alt="star cluster" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/spooky-action-distance-04.width-990_Px55h3v.jpg"/>
<figcaption>Quantum connections between two particles can persist even if the two particles are on opposite sides of the universe. (Here, a star cluster within our Milky Way galaxy.)
        
        <div class="credit">NASA/JPL-Caltech, D. Figer (Space Telescope Science Institute/Rochester Institute of Technology), E. Churchwell (University of Wisconsin, Madison) and the GLIMPSE Legacy Team</div>
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<h3>The twain shall meet</h3><p>Nevertheless, these results, coming from both theoretical and experimental considerations, strongly support the conclusion that the universe admits interconnections that are not local. Something that happens over here can be entwined with something that happens over there even if nothing travels from here to there—and even if there isn't enough time for anything, even light, to travel between the events.</p><p>This means that space cannot be thought of as it once was: Intervening space, <i>regardless of how much there is</i>, does not ensure that two objects are separate, since quantum mechanics allows an entanglement, a kind of connection, to exist between them. A particle, like one of the countless number that make up you or me, can run but it can't hide.</p><p>According to quantum theory and the many experiments that bear out its predictions, the quantum connection between two particles can persist <i>even if they are on opposite sides of the universe</i>. From the standpoint of their entanglement, notwithstanding the many trillions of miles of space between them, it's as if they are right on top of each other.</p><p>Numerous assaults on our conception of reality are emerging from modern physics. But of those that have been experimentally verified, I find none more mind-boggling than this recent realization that our universe is not local.</p>
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<p>That&#x27;s what Einstein called it, dismissively, but ultra-strange quantum entanglement does exist, Brian Greene writes.</p>

Inflation explains how everything from galaxies to dust may have come about.

A Theory of Everything?

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<p>Although it is by no means obvious, this simple replacement of point-particle material constituents with strings resolves the incompatibility between quantum mechanics and general relativity (which, as currently formulated, <i>cannot both be right</i>). String theory thereby unravels the central Gordian knot of contemporary theoretical physics. This is a tremendous achievement, but it is only part of the reason string theory has generated such excitement.</p><h3>Field of dreams</h3><p>In Einstein's day, the strong and weak forces had not yet been discovered, but he found the existence of even two distinct forces—gravity and electromagnetism—deeply troubling. Einstein did not accept that nature is founded on such an extravagant design. This launched his 30-year voyage in search of the so-called <i>unified field theory</i> that he hoped would show that these two forces are really manifestations of one grand underlying principle. This quixotic quest isolated Einstein from the mainstream of physics, which, understandably, was far more excited about delving into the newly emerging framework of quantum mechanics. He wrote to a friend in the early 1940s, "I have become a lonely old chap who is mainly known because he doesn't wear socks and who is exhibited as a curiosity on special occasions."</p><p></p>
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<figcaption>"I have become a lonely old chap," Einstein told a friend in the early 1940s. This photo was taken in 1940, when Einstein was deep into his ultimately futile search for a unified field theory.
        
        <div class="credit">© Bettmann/Corbis</div>
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<p>Einstein was simply ahead of his time. More than half a century later, his dream of a unified theory has become the Holy Grail of modern physics. And a sizeable part of the physics and mathematics community is becoming increasingly convinced that string theory may provide the answer. From one principle—that everything at its most microscopic level consists of combinations of vibrating strands—string theory provides a single explanatory framework capable of encompassing all forces and all matter.</p><p>String theory is sometimes described as possibly being the "theory of everything."</p><p>String theory proclaims, for instance, that the observed particle properties—that is, the different masses and other properties of both the fundamental particles and the force particles associated with the four forces of nature (the strong and weak nuclear forces, electromagnetism, and gravity)—are a reflection of the various ways in which a string can vibrate. Just as the strings on a violin or on a piano have resonant frequencies at which they prefer to vibrate—patterns that our ears sense as various musical notes and their higher harmonics—the same holds true for the loops of string theory. But rather than producing musical notes, each of the preferred mass and force charges are determined by the string's oscillatory pattern. The electron is a string vibrating one way, the up-quark is a string vibrating another way, and so on.</p><p></p>
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<figcaption>The way a string-theory string vibrates may confer the properties of various fundamental particles, just as the way a violin's string vibrates confers the resonant frequencies we recognize as different musical notes.
        
        <div class="credit">© Bojan Fatur/iStockphoto</div>
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<p>Far from being a collection of chaotic experimental facts, particle properties in string theory are the manifestation of one and the same physical feature: the resonant patterns of vibration—the music, so to speak—of fundamental loops of string. The same idea applies to the forces of nature as well. Force particles are also associated with particular patterns of string vibration and hence everything, all matter and all forces, is unified under the same rubric of microscopic string oscillations—the "notes" that strings can play.</p>
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<h3>A theory to end theories</h3><p>For the first time in the history of physics we therefore have a framework with the capacity to explain every fundamental feature upon which the universe is constructed. For this reason string theory is sometimes described as possibly being the "theory of everything" (T.O.E.) or the "ultimate" or "final" theory. These grandiose descriptive terms are meant to signify the deepest possible theory of physics—a theory that underlies all others, one that does not require or even allow for a deeper explanatory base.</p><p>In practice, many string theorists take a more down-to-earth approach and think of a T.O.E. in the more limited sense of a theory that can explain the properties of the fundamental particles and the properties of the forces by which they interact and influence one another. A staunch reductionist would claim that this is no limitation at all, and that in principle absolutely everything, from the Big Bang to daydreams, can be described in terms of underlying microscopic physical processes involving the fundamental constituents of matter. If you understand everything about the ingredients, the reductionist argues, you understand everything.</p><p></p>
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<figcaption>According to reductionist string theorists, everything, from the Big Bang and the formation of the first stars (depicted here) to the daydreams we humans have 13 billion years later, can be described in terms of microscopic physical processes involving strings.
        
        <div class="credit">NASA/JPL-Caltech/R. Hurt (SSC)</div>
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<p>The reductionist philosophy easily ignites heated debate. Many find it fatuous and downright repugnant to claim that the wonders of life and the universe are mere reflections of microscopic particles engaged in a pointless dance fully choreographed by the laws of physics. Is it really the case that feelings of joy, sorrow, or boredom are nothing but chemical reactions in the brain—reactions between molecules and atoms that, even more microscopically, are reactions between some of the fundamental particles, which are really just vibrating strings?</p><p>In response to this line of criticism, Nobel laureate Steven Weinberg cautions in Dreams of a Final Theory:</p><p>At the other end of the spectrum are the opponents of reductionism who are appalled by what they feel to be the bleakness of modern science. To whatever extent they and their world can be reduced to a matter of particles or fields and their interactions, they feel diminished by that knowledge....I would not try to answer these critics with a pep talk about the beauties of modern science. The reductionist worldview is chilling and impersonal. It has to be accepted as it is, not because we like it, but because that is the way the world works.</p><p>Some agree with this stark view, some don't.</p><p>Others have tried to argue that developments such as chaos theory tell us that new kinds of laws come into play when the level of complexity of a system increases. Understanding the behavior of an electron or quark is one thing; using this knowledge to understand the behavior of a tornado is quite another. On this point, most agree. But opinions diverge on whether the diverse and often unexpected phenomena that can occur in systems more complex than individual particles truly represent new physical principles at work, or whether the principles involved are derivative, relying, albeit in a terribly complicated way, on the physical principles governing the enormously large number of elementary constituents.</p><p>The discovery of the final theory would not spell the end of science. Quite the contrary.</p><p>My own feeling is that they do not represent new and independent laws of physics. Although it would be hard to explain the properties of a tornado in terms of the physics of electrons and quarks, I see this as a matter of calculational impasse, not an indicator of the need for new physical laws. But again, there are some who disagree with this view.</p><p></p>
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<figcaption>Greene sees our inability to explain the properties of a tornado in terms of the physics of its constituent particles as a matter of "calculational impasse" rather than an indication that we need new physical laws.
        
        <div class="credit">© Clint Spencer/iStockphoto</div>
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<h3>A fresh start for science</h3><p>What is largely beyond question, and is of primary importance to the journey described in my book The Elegant Universe, is that even if one accepts the debatable reasoning of the staunch reductionist, principle is one thing and practice quite another. Almost everyone agrees that finding the T.O.E. would in no way mean that psychology, biology, geology, chemistry, or even physics had been solved or in some sense subsumed. The universe is such a wonderfully rich and complex place that the discovery of the final theory, in the sense we are describing here, would not spell the end of science.</p><p>Quite the contrary: The discovery of the T.O.E.—the ultimate explanation of the universe at its most microscopic level, a theory that does not rely on any deeper explanation—would provide the firmest foundation on which to build our understanding of the world. Its discovery would mark a beginning, not an end. The ultimate theory would provide an unshakable pillar of coherence forever assuring us that the universe is a comprehensible place.</p>
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<p>Physicist-author Brian Greene explains why string theory might hold the key to unifying the four forces of nature.</p>

Brian Greene

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On Being a Physicist

Special Relativity in a Nutshell

Spooky Action at a Distance

A Theory of Everything?

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