Fundamentals Page 6

  The design of ever more precise and accurate clocks is a challenging, wonderfully creative branch of modern physics. A recent example is close to my heart: It may be possible to orchestrate large numbers of atoms, cooperating within a new state of matter that I predicted and that was subsequently observed—a “time crystal”—to improve on the accuracy of single-atom atomic clocks.

  Resolving Short Times

  Just as we discussed earlier for space, when we come to extremely short periods of time, we must measure it in different, less direct ways. In the spatial case, we saw that x-ray diffraction and scattering in the style of Geiger and Marsden gave information that could be converted into maps (that is, images) of the atomic and subatomic world. Those techniques involve observing how targets—namely, the objects we want to image—change the motion of incident x-rays and of incident particles, respectively.

  To resolve the structure of rapid events, we employ methods of a similar kind, but focus on changes in energy rather than changes in direction of motion. The world of rapid events is full of wonders and surprises. Let me spotlight a couple of highlights—briefly, as befits the subject matter.

  Using high-powered lasers, it is possible to resolve the sequence of events that occur in many chemical and biochemical processes. Femtochemistry constructs those timelines, in steps as small as 10−15 seconds (one femtosecond). With understanding, increasingly, comes control. Lasik eye surgery exploits femtosecond laser pulses to remodel patients’ corneas.

  It is possible to resolve even shorter times by using high-energy accelerators. We’ll explore examples of this more deeply later. The Higgs particle, whose discovery was a major triumph for twenty-first-century physics, is highly unstable. It lives for only about 10−22 second. Thus, in order to discern evidence for its existence, physicists had to reconstruct events on that time scale.

  THE FUTURE OF TIME

  Engineering Physical Time

  Einstein’s theory of general relativity has gone from triumph to triumph as our theory of gravity. It teaches us that space-time can bend and distort. That fact helps to fuel dreams of time travel, portals, wormholes, and warp drives. Might those fantasies and desires become engineering realities?

  I see little hope that we’ll be able to manipulate physical time in the foreseeable future. Ironically, the Laser Interferometer Gravitational-Wave Observatory’s (LIGO) observation of gravitational waves, which is the most recent major confirmation of general relativity, and perhaps the purest, also demonstrates the problem starkly.

  LIGO is an exquisite instrument, designed to detect tiny distortions in space-time. It is sensitive to changes in the relative positions of mirrors, separated by four kilometers, that are a thousand times smaller than the size of an atomic nucleus. Yet even with that kind of sensitivity, LIGO was barely able to detect distortions produced by the violent merger of two black holes, each several times the mass of the Sun. The message is simple: Space-time can be distorted, but it’s very hard work.

  Engineering Psychological Time: Hopping and Cycling

  Physical time is very stiff. For practical purposes it flows steadily and in one direction, the same for every entity in the physical universe. Psychological time is quite different. It can meander, branch, and jump around quite nimbly. We can revisit the past, consulting memory. In doing this, we can move through it quickly, or slowly, or in jumps. Or we can change it, by imagining how things might have been. We routinely imagine alternative futures and plan actions to realize desirable ones. That may be the central task of our frontal lobes—those massive, convoluted outcroppings of brain that uniquely distinguish humans among animals.

  Computers are essentially ageless, and they can revisit previous states precisely, and they can pursue several programs in parallel. An artificial intelligence rooted in those platforms will be able to engineer its psychological time with great precision and flexibility. Notably, it could set up states that lead to pleasure, and relive them repeatedly, while experiencing each as fresh.

  Engineering Psychological Time: Speed

  There’s a big gap between the human speed of thought—which we estimated at a few tens per second—and the existing speed of electron-motion-powered thought, as embodied in computer clock rates. It’s about a factor of a billion, as we’ve discussed. Basic femtosecond atomic processes are even faster, by an additional factor of many thousands. Thus, there’s room to pack a lot more life into each moment.

  Artfully evolved humans, cyborgs, or completely artificial intelligences have plenty of room to transcend the (presently) standard speed of thought. Barring catastrophic nuclear war or climate change, that will soon come to pass—I’d guess within a few decades.

  More fancifully, we can imagine forms of intelligence based on subatomic processes, which can be even quicker. Robert Forward’s delightful hard sci-fi novel Dragon’s Egg plays on that theme. He imagines an intelligent form of life, the cheela, evolving on the surface of a neutron star. There, nuclear chemistry rather than atomic chemistry would rule. Nuclear chemistry involves much larger exchanges of energy than atomic chemistry, and therefore operates faster. Epochs of cheela history pass in the blink of a human eye. The human astronauts who come upon a savage, scientifically backward form of life discover, half an hour later, that the cheela, given access to their libraries, have far outstripped them.

  Engineering Psychological Time: Persistence

  In Gulliver’s Travels, Jonathan Swift introduced a race of immortals, the Struldbruggs. Though immortal, the Struldbruggs grow old. They become frail, miserable creatures that are a burden to society. The misery or evil of immortality is a common theme in myth and literature. The intended lesson: When it comes to longevity, be careful what you wish for.

  Frankly, I think this is sour grapes. The destruction of memory and learning by death is horrifying and wasteful. Extending the healthful human lifespan should be one of the main goals of science.

  3

  THERE ARE VERY FEW INGREDIENTS

  As children, we learn to deal with many sorts of things: other people, animals, plants, water, soil, stones, wind, the Sun and the Moon, stars, clouds, books, smartphones, and many others. We develop different models for how to identify each of those things, how they might affect us, and how we can affect them. The idea that all of those things are made from a handful of basic building blocks, each occurring in great numbers, is not an important part of those models. It is, however, a central lesson of science.

  ATOMS AND BEYOND

  If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms.

  —Richard Feynman

  The word “atom” derives from a Greek root meaning “without parts.” For a long time, scientists thought that the smallest objects that can be exchanged in chemical reactions were the ultimate, indivisible units of matter. Those basic chemical building blocks got to be called “atoms,” and that name has stuck.

  But when people studied matter in more extreme conditions than are commonly encountered in chemistry, they discovered that chemical “atoms” can be broken into smaller units. Thus, the “atoms” of chemistry, which are the objects that go under that name in most of the scientific literature, are not “atoms” in the sense of being our ultimate building blocks.

  The traditional atom of chemistry consists of electrons surrounding an atomic nucleus. The nucleus can be further analyzed into protons and neutrons. That’s not the end of the story. Our best world-models today build up atoms from electrons, photons, quarks, and gluons. As we’ll see, there are good reasons to think that this really is the last word.

  These discoveries, which form part of our fundamentals, continue the spirit of th
e atomic hypothesis. They suggest that we should rephrase (and maybe rename) it, though. Instead of “all things are made of atoms,” we should say that “all things are made of elementary particles.” But whichever way you state it, the central message is clear: It pays to analyze matter into the smallest units you can. After doing that correctly, you can build back up, conceptually, and construct the physical world.

  The modern scientific construction of physical reality from a few simple ingredients requires that we reimagine both what we mean by “simple ingredients” and how we do “construction.” Our everyday experiences do not prepare us well for the modern versions of those concepts.

  PRINCIPLES: REALITY AND ITS RIVALS

  The most basic ingredients of physical reality are a few principles and properties. Those principles and properties are expressed through things we call elementary particles. But the “elementary particles” differ in important ways from any objects of common experience, and to understand them properly we must start with the principles and properties.

  Four (Deceptively) Easy Principles

  Four simple yet profound general principles govern how the world works. I’ll first state them all at once, telegraphically, and then spell them out in more depth.

  The basic laws describe change. It is useful to separate the description of the world into two parts: states and laws. States describe “what there is,” while laws describe “how things change.”

  The basic laws are universal. That is, the basic laws hold everywhere, and for all times.

  The basic laws are local. That is, the behavior of an object in the immediate future depends only on current conditions in its immediate vicinity. The standard scientific jargon for this principle is locality.

  The basic laws are precise. The laws are precise, and they admit no exceptions. Thus, they can be formulated as mathematical equations.

  The simplicity of those general principles is deceptive. They are far from self-evident. They may not even be completely true. Their strength derives not from any logical necessity, but from their proven success. They have pointed us to an impressively successful description of how the physical world actually works, as this book aims to document.

  Over the bulk of human history, people have held many different views about how the physical world works. Ideas that contradict one or more of our principles have been recorded in folklore, in history, and—until recently—in the works of learned academics, philosophers, and theologians. Some, such as astrology, telepathy, clairvoyance, and witchcraft, bring in forces that act powerfully across big separations in space and time. Others, such as extrasensory perception, telekinesis, prayer-induced miracles, and magical thinking, assign prominent roles in shaping the course of physical events to mind and will. Most of those ideas are “reasonable” extensions of the world-models we build up as children, in which our mind is disembodied and our will controls our body. Historically, most people’s world-models have accepted many or all of them.

  Only a tiny percentage of people over the course of human history have aspired to make precise predictions about what happens next under carefully controlled conditions, or even imagined that such a thing might be possible. Yet that possibility is the central message of our principles. Our general principles were first clearly formulated in the seventeenth century. They are the core lessons of the Scientific Revolution.

  The message of the first principle is simply that “What happens next?” is a more approachable question, and proves to be a much more fruitful question, than “Why are things the way they are?” “What happens next?” is an approachable question because, thanks to our second and third principles, we can do experiments to answer it. That is, we can make an accurate copy of the situation we’re interested in—set up the same state—and observe what happens in the copy.

  A crucial point of the second principle, which helps make that “obvious” suggestion—to perform experiments—a practical one, is that we can do the experiments anywhere and at any time. According to the second principle, universality, we will always find the same fundamental laws.

  The third principle, locality, allows another crucial simplification. It tells us that in formulating the laws, we do not need to take into account the whole universe, or all of history. It tells us, more precisely, that we can aspire to control all the relevant conditions by taking appropriate precautions here and now.

  Finally, the fourth principle, precision, is an invitation to ambition. It says that if we describe the laws using appropriate concepts, we can get a description that is brief yet complete and fully accurate. It is also a challenge: We should not be satisfied with less.

  In short, these principles assure us that by doing experiments, we can discover precise, universal laws that govern how things change. Science pursues that goal systematically, and relentlessly.

  Principles one through four, working together, give us a strategy to make discoveries. We start by studying what happens in precisely defined, simple situations that we can set up repeatedly. Having mastered those, we can try to deduce what will happen in more complicated situations.

  Babies—even animal babies—use that same experimental strategy to get in tune with physical reality. We humans learn, for example, how to throw a ball, how to bring food to our mouths, and hundreds of other practical procedures to make changes in the physical world by weaving together experiences at different times and places, under different conditions. Scientists, and people who open themselves to science, are born-again explorers. But we “babies” get to benefit in our explorations from logical minds, sense-enhancing instruments, and the work of explorers who came before us.

  Newton and Locality

  Newton was extremely unhappy with one of his most glorious discoveries. According to Newton’s law, the gravitational force that one body (call it body B) exerts on another one (call it body A) acts immediately, with no delay in time, however far the two bodies are separated. This implies that you cannot predict the motion of A based solely on conditions in the immediate neighborhood of A—specifically, you have to know where B is. Newton was deeply dissatisfied with that feature of his own law, as he expressed in a letter to his friend Richard Bentley:

  That one body may act upon another at a distance through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it.

  Newton realized that his law of gravity is not local—in other words, that it fails to embody our third principle—and he did not like it.

  This perceived flaw was, for Newton and for several generations of scientists who followed him, purely theoretical. Newton’s law of gravity worked spectacularly well in practice. You might say its shortcomings were aesthetic, or even, for Newton himself, theological. It seemed to represent a lapse in God’s usually excellent taste.

  Newton’s faith in our third principle—the principle of local action—proved amazingly prescient. Many decades after his death, starting in the mid-nineteenth century, physicists filled the passive “vacuum”—a nothingness, or Void—that Newton complained about with force-transmitting materials, which we call fields. Fields, rather than particles, are the fundamental building blocks of matter in modern physics.*

  A Case Study: Atomic Clocks

  Atomic clocks are a superb example of our fundamental principles at work.

  Vibrating atoms supply the heartbeat of atomic clocks. Their physical state determines how they change—in this case, how fast they vibrate (fulfilling the first principle). Importantly, experimenters have measured rates of atomic vibrations at different times and places, and always found consistent answers (fulfilling the second principle)—once they take a few laboratory precautions (exploiting, and fulfilling, the third principle). And, as we discus
sed previously, atomic rates of vibration have been measured with exquisite precision, with consistent results (fulfilling the fourth principle).

  The trickiest part, both in this case and in most experiments, is taking “necessary precautions.” To get consistent results, experimenters need to make sure that the complicated, finely tuned apparatus they use to trap atoms and observe their behavior—including lasers, fancy cooling equipment, vacuum chambers, and a lot of complicated electronics—is stable. You must shield it against the effects of ground-shifting tremors set in motion by passing trucks, and the seismic rumblings of Earth itself. You mustn’t let playful children or heedless students wander through the lab, touching things. The point of the third principle—locality—is that these precautions, and other humdrum corrections for temperature, humidity, and so forth, all relate to local conditions. (The truck might be far away, but what counts are tremors at the lab itself.) Thankfully, you don’t have to worry about the distant universe, what happened in the past, or what will happen in the future.

  The heart of the matter is the atoms. What vagaries do we need to control for before we get to the reproducible, exquisitely precise results that atomic clocks are famous for? Basically, just four things. We need to keep the atoms of interest separated from other atoms. That’s what the cooling apparatus and vacuum chambers are for. And we have to keep track of electric, magnetic, and gravitational conditions where the atom is—the value of the electric, magnetic, and gravitational fields, as we say. Those fields can be measured locally, by monitoring how charged particles move and how fast bodies fall. Once you’ve made appropriate corrections for that small list of local conditions, you’ve done enough. At that point, you will always observe a consistent rate of atomic vibration, with extremely high precision—or else you will have made a great discovery, which has eluded all previous experimenters!