Quantum Theory Cannot Hurt You

by Marcus Chown

  QUANTUM THEORY

  CANNOT HURT

  YOU

  A Guide to the Universe

  Marcus Chown

  To Patrick, who, when I’m down and wondering why everyone seems to be against me, consoles me by saying: “That’s because you’re a complete bastard, Marcus!”

  CONTENTS

  Foreword

  Acknowledgements

  PART ONE: SMALL THINGS

  1 Breathing in Einstein

  2 Why God Plays Dice with the Universe

  3 The Schizophrenic Atom

  4 Uncertainty and the Limits of Knowledge

  5 The Telepathic Universe

  6 Identicalness and the Roots of Diversity

  PART TWO: BIG THINGS

  7 The Death of Space and Time

  8 E =mc2 and the Weight of Sunshine

  9 The Force of Gravity Does Not Exist

  10 The Ultimate Rabbit Out of a Hat

  Glossary

  Further Reading

  Index

  FOREWORD

  One of the following is true:

  Every breath you take contains an atom breathed out by Marilyn Monroe.

  There is a liquid that can run uphill.

  You age faster at the top of a building than at the bottom.

  An atom can be in many different places at once, the equivalent of you being in New York and London at the same time.

  The entire human race would fit in the volume of a sugar cube.

  One per cent of the static on a television tuned between stations is the relic of the Big Bang.

  Time travel is not forbidden by the laws of physics.

  A cup of coffee weighs more when it is hot than when it is cold.

  The faster you travel, the slimmer you get.

  No, I’m joking. They are all true!

  As a science writer I am constantly amazed by how much stranger science is than science fiction, how much more incredible the Universe is than anything we could possibly have invented. Despite this, however, very few of the extraordinary discoveries of the past century seem to have trickled through into the public consciousness.

  The two towering achievements of the past 100 years are quantum theory, our picture of atoms and their constituents, and Einstein’s general theory of relativity, our picture of space, time, and gravity. Between them the two explain virtually everything about the world and about us. In fact, it can be argued that quantum theory has actually created the modern world, not only explaining why the ground beneath our feet is solid and why the Sun shines but also making possible computers and lasers and nuclear reactors. Relativity may not be as ubiquitous in the everyday world. Nevertheless, it has taught us that there are things called black holes from which nothing, not even light, can escape; that the Universe has not existed forever but was born in a titanic explosion called the Big Bang; and that time machines—remarkably—may be possible.

  Although I have read many popular accounts of these topics, the explanations have often left me baffled, even with my science background. I can only guess what it must be like for nonscientists.

  Einstein said: “Most of the fundamental ideas of science are essentially simple and may, as a rule, be expressed in a language comprehensible to everyone.” All my experience tells me he was right. My idea in writing this book was to try to help ordinary people understand the principal ideas of 21st-century physics. All I had to do was identify the key ideas behind quantum theory and relativity, which turn out to be deceptively simple, and then show how absolutely everything else follows from them logically and unavoidably.

  Easier said than done. Quantum theory in particular is a patchwork of fragments, accrued over the past 80 years, that nobody seems to have sewn together into a seamless garment. What’s more, crucial pieces of the theory, such as “decoherence”—which explains why atoms but not people can be in two places at once—seem to be beyond the power of physicists to communicate in any intelligible way. After corresponding with many “experts,” and beginning to think that decoherence should be renamed “incoherence,” it dawned on me that maybe the experts didn’t completely understand it themselves. In a way this was liberating. Since a coherent picture seemed not to exist, I realised that I had to piece together my own from insights gleaned from different people. Because of this, many of the explanations given here you will not find anywhere else. I hope they help lift some of the fog that surrounds the key ideas of modern physics and that you can begin to appreciate what a breathtakingly amazing Universe we find ourselves in.

  PART ONE

  SMALL THINGS

  1

  BREATHING IN EINSTEIN

  HOW WE DISCOVERED THAT EVERYTHING IS MADE OF ATOMS AND THAT ATOMS ARE MOSTLY EMPTY SPACE

  A hydrogen atom in a cell at the end of my nose was once part of an elephant’s trunk.

  Jostein Gaarder

  We never had any intention of using the weapon. But they were such a terribly troublesome race. They insisted on seeing us as the “enemy” despite all our efforts at reassurance. When they fired their entire nuclear stockpile at our ship, orbiting high above their blue planet, our patience simply ran out.

  The weapon was simple but effective. It squeezed out all the empty space from matter.

  As the commander of our Sirian expedition examined the shimmering metallic cube, barely 1 centimetre across, he shook his primary head despairingly. Hard to believe that this was all that was left of the “human race”!

  If the idea of the entire human race fitting into the volume of a sugar cube sounds like science fiction, think again. It is a remarkable fact that 99.9999999999999 per cent of the volume of ordinary matter is empty space. If there were some way to squeeze all the empty space out of the atoms in our bodies, humanity would indeed fit into the space occupied by a sugar cube.

  The appalling emptiness of atoms is only one of the extraordinary characteristics of the building blocks of matter. Another, of course, is their size. It would take 10 million atoms laid end to end to span the width of a single full stop on this page, which raises the question, how did we ever discover that everything is made of atoms in the first place?

  The idea that everything is made of atoms was actually first suggested by the Greek philosopher Democritus in about 440 BC.1 Picking up a rock—or it may have been a branch or a clay pot—he asked himself the question: “If I cut this in half, then in half again, can I go on cutting it in half forever?” His answer was an emphatic no. It was inconceivable to him that matter could be subdivided forever. Sooner or later, he reasoned, a tiny grain of matter would be reached that could be cut no smaller. Since the Greek for “uncuttable” was “a-tomos,” Democritus called the hypothetical building blocks of all matter “atoms.”

  Since atoms were too small to be seen with the senses, finding evidence for them was always going to be difficult. Nevertheless, a way was found by the 18th-century Swiss mathematician Daniel Bernoulli. Bernoulli realised that, although atoms were impossible to observe directly, it might still be possible to observe them indirectly. In particular, he reasoned that if a large enough number of atoms acted together, they might have a big enough effect to be obvious in the everyday world. All he needed was to find a place in nature where this happened. He found one—in a “gas.”

  Bernoulli imagined a gas like air or steam as a collection of billions upon billions of atoms in perpetual frenzied motion like a swarm of angry bees. This vivid picture immediately suggested an explanation for the “pressure” of a gas, which kept a balloon inflated or pushed against the piston of a steam engine. When confined in any container, the atoms of a gas would drum relentlessly on the walls like hailstones on a tin roof. Their combined effect wou
ld be to create a jittery force that, to our coarse senses, would seem like a constant force pushing back the walls.

  But Bernoulli’s microscopic explanation of pressure provided more than a convenient mental picture of what was going on in a gas. Crucially, it led to a specific prediction. If a gas were squeezed into half its original volume, the gas atoms would need to fly only half as far between collisions with the container walls. They would therefore collide twice as frequently with those walls, doubling the pressure. And if the gas were squeezed into a third of its volume, the atoms would collide three times as frequently, trebling the pressure. And so on.

  Exactly this behaviour was observed by the English scientist Robert Boyle in 1660. It confirmed Bernoulli’s picture of a gas. And since Bernoulli’s picture was of tiny grainlike atoms flying hither and thither through empty space, it bolstered the case for the existence of atoms. Despite this success, however, definitive evidence for the existence of atoms did not come until the beginning of the 20th century. It was buried in an obscure phenomenon called Brownian motion.

  Brownian motion is named after Robert Brown, a botanist who sailed to Australia on the Flinders expedition of 1801. During his time down under, he classified 4,000 species of antipodean plants; in the process, he discovered the nucleus of living cells. But he is best remembered for his observation in 1827 of pollen grains suspended in water. To Brown, squinting through a magnifying lens, it seemed as if the grains were undergoing a curious jittery motion, zigzagging their way through the liquid like drunkards lurching home from a pub.

  Brown never solved the mystery of the wayward pollen grains. That breakthrough had to wait for Albert Einstein, aged 26 and in the midst of the greatest explosion of creativity in the history of science. In his “miraculous year” of 1905, not only did Einstein overthrow Newton, supplanting Newtonian ideas about motion with his special theory of relativity, but he finally penetrated the 80-year-old mystery of Brownian motion.

  The reason for the crazy dance of pollen grains, according to Einstein, was that they were under continual machine-gun bombardment by tiny water molecules. Imagine a giant inflatable rubber ball, taller than a person, being pushed about a field by a large number of people. If each person pushes in their own particular direction, without any regard for the others, at any instant there will be slightly more people on one side than another. This imbalance is enough to cause the ball to move erratically about the field. Similarly, the erratic motion of a pollen grain can be caused by slightly more water molecules bombarding it from one side than from another.

  Einstein devised a mathematical theory to describe Brownian motion. It predicted how far and how fast the average pollen grain should travel in response to the relentless battering it was receiving from the water molecules all around. Everything hinged on the size of the water molecules, since the bigger they were the bigger would be the imbalance of forces on the pollen grain and the more exaggerated its consequent Brownian motion.

  The French physicist Jean Baptiste Perrin compared his observations of water-suspended “gamboge” particles, a yellow gum resin from a Cambodian tree, with the predictions of Einstein’s theory. He was able to deduce the size of water molecules and hence the atoms out of which they were built. He concluded that atoms were only about one 10-billionth of a metre across—so small that it would take 10 million, laid end to end, to span the width of a full stop.

  Atoms were so small, in fact, that if the billions upon billions of them in a single breath were spread evenly throughout Earth’s atmosphere, every breath-sized volume of the atmosphere would end up containing several of those atoms. Put another way, every breath you take contains at least one atom breathed out by Albert Einstein—or Julius Caesar or Marilyn Monroe or even the last Tyrannosaurus Rex to walk on Earth!

  What is more, the atoms of Earth’s “biosphere” are constantly recycled. When an organism dies, it decays and its constituent atoms are returned to the soil and the atmosphere to be incorporated into plants that are later eaten by animals and humans. “A carbon atom in my cardiac muscle was once in the tail of a dinosaur,” writes Norwegian novelist Jostein Gaarder in Sophie’s World.

  Brownian motion was the most powerful evidence for the existence of atoms. Nobody who peered down a microscope and saw the crazy dance of pollen grains under relentless bombardment could doubt that the world was ultimately made from tiny, bulletlike particles. But watching jittery pollen grains—the effect of atoms—was not the same as actually seeing atoms. This had to wait until 1980 and the invention of a remarkable device called the scanning tunnelling microscope (STM).

  The idea behind the STM, as it became known, was very simple. A blind person can “see” someone’s face simply by running a finger over it and building up a picture in their mind. The STM works in a similar way. The difference is that the “finger” is a finger of metal, a tiny stylus reminiscent of an old-fashioned gramophone needle. By dragging the needle across the surface of a material and feeding its up-and-down motion into a computer, it is possible to build up a detailed picture of the undulations of the atomic terrain.2 Of course, there is a bit more to it than that. Although the principle of the invention was simple, there were formidable practical difficulties in its realisation. For instance, a needle had to be found that was fine enough to “feel” atoms. The Nobel Prize committee certainly recognised the difficulties. It awarded Gerd Binnig and Heinrich Rohrer, the IBM researchers behind the STM, the 1986 Nobel Prize for Physics.

  Binnig and Rohrer were the first people in history to actually “see” an atom. Their STM images were some of the most remarkable in the history of science, ranking alongside that of Earth rising above the gray desolation of the Moon or the sweeping spiral staircase of DNA. Atoms looked like tiny footballs. They looked like oranges, stacked in boxes, row on row. But most of all they looked like the tiny hard grains of matter that Democritus had seen so clearly in his mind’s eye, 2,400 years before. No one else has ever made a prediction that far in advance of experimental confirmation.

  But only one side of the atom was revealed by the STM. As Democritus himself had realised, atoms were a lot more than simply tiny grains in ceaseless motion.

  NATURE’S LEGO BRICKS

  Atoms are nature’s Lego bricks. They come in a variety of different shapes and sizes, and by joining them together in any number of different ways, it is possible to make a rose, a bar of gold, or a human being. Everything is in the combinations.

  The American Nobel Prize winner Richard Feynman said: “If in some cataclysm all of scientific knowledge were destroyed and only one sentence passed on to succeeding generations, what statement would convey the most information in the fewest words?” He was in no doubt: “Everything is made of atoms.”

  The key step in proving that atoms are nature’s Lego bricks was identifying the different kinds of atoms. However, the fact that atoms were far too small to be perceived directly by the senses made the task every bit as formidable as proving that atoms were tiny grains of matter in ceaseless motion. The only way to identify different types of atoms was to find substances that were made exclusively out of atoms of a single kind.

  In 1789 the French aristocrat Antoine Lavoisier compiled a list of substances that he believed could not, by any means, be broken down into simpler substances. There were 23 “elements” in Lavoisier’s list. Though some later turned out not to be elements, many—including gold, silver, iron, and mercury—were indeed elemental. Within 40 years of Lavoisier’s death at the guillotine in 1794, the list of elements had grown to include close to 50. Nowadays, we know of 92 naturally occurring elements, from hydrogen, the lightest, to uranium, the heaviest.

  But what makes one atom different from another? For instance, how does a hydrogen atom differ from a uranium atom? The answer would come only by probing their internal structures. But atoms are so fantastically small. It seemed impossible that anyone would ever find a way to look inside one. But one man did—a New Zealander named Ernest Ruthe
rford. His ingenious idea was to use atoms to look inside other atoms.

  THE MOTH IN THE CATHEDRAL

  The phenomenon that laid bare the structure of atoms was radioactivity, discovered by the French chemist Henri Becquerel in 1896. Between 1901 and 1903, Rutherford and the English chemist Frederick Soddy found strong evidence that a radioactive atom is simply a heavy atom that is seething with excess energy. Inevitably, after a second or a year or a million years, it sheds this surplus energy by expelling some kind of particle at high speed. Physicists say it disintegrates, or “decays,” into an atom of a slightly lighter element.

  One such decay particle was the alpha particle, which Rutherford and the young German physicist Hans Geiger demonstrated was simply an atom of helium, the second lightest element after hydrogen.

  In 1903, Rutherford had measured the speed of alpha particles expelled from atoms of radioactive radium. It was an astonishing 25,000 kilometres per second—100,000 times faster than a present-day passenger jet. Here, Rutherford realised, was a perfect bullet to smash into atoms and reveal what was deep inside.

  The idea was simple. Fire alpha particles into an atom. If they encountered anything hard blocking their way, they would be deflected from their path. By firing thousands upon thousands of alpha particles and observing how they were deflected, it would be possible to build up a detailed picture of the interior of an atom.

  Rutherford’s experiment was carried out in 1909 by Geiger and a young New Zealand physicist called Ernest Marsden. Their “alphascattering” experiment used a small sample of radium, which fired off alpha particles like microscopic fireworks. The sample was placed behind a lead screen containing a narrow slit, so a thread-thin stream of alpha particles emerged on the far side. It was the world’s smallest machine gun, rattling out microscopic bullets.