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- Herausgeber: Icon Books
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
- Veröffentlichungsjahr: 2019
SHORTLISTED FOR THE ROYAL SOCIETY INSIGHT INVESTMENT SCIENCE BOOK PRIZE 2019. 'An accessible primer on all things quantum' - Sunday Times Quantum physics is strange. It tells us that a particle can be in two places at once. Indeed, that particle is also a wave, and everything in the quantum world can be described entirely in terms of waves, or entirely in terms of particles, whichever you prefer. All of this was clear by the end of the 1920s. But to the great distress of many physicists, let alone ordinary mortals, nobody has ever been able to come up with a common sense explanation of what is going on. Physicists have sought 'quanta of solace' in a variety of more or less convincing interpretations. Popular science master John Gribbin takes us on a delightfully mind-bending tour through the 'big six', from the Copenhagen interpretation via the pilot wave and many worlds approaches. All of them are crazy, and some are more crazy than others, but in this world crazy does not necessarily mean wrong, and being more crazy does not necessarily mean more wrong.
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SIX IMPOSSIBLE THINGS
The ‘Quanta of Solace’ and the Mysteries of the Subatomic World
JOHN GRIBBIN
CONTENTS
LIST OF ILLUSTRATIONS
Richard Feynman
The double-slit experiment
Electron diffraction
John Bell
Niels Bohr
Erwin Schrödinger
Werner Heisenberg
Louis de Broglie
David Bohm
David Deutsch
Anthony Leggett
Lee Smolin
‘Alice laughed: “There’s no use trying,” she said; “one can’t believe impossible things.”
“I daresay you haven’t had much practice,” said the Queen. “When I was younger, I always did it for half an hour a day. Why, sometimes I’ve believed as many as six impossible things before breakfast.”’
Alice’s Adventures in Wonderland
SOLACEn. (pl. -es) comfort or consolation in a time of great distress.
PREFACE
What’s it all About, Alfie? The Need for Quantum Solace
Quantum physics is strange. At least, it is strange to us, because the rules of the quantum world, which govern the way the world works at the level of atoms and subatomic particles (the behaviour of light and matter, as Richard Feynman put it), are not the rules that we are familiar with – the rules of what we call ‘common sense’.
The quantum rules seem to be telling us that a cat can be both alive and dead at the same time, while a particle can be in two places at once. Indeed, that particle is also a wave, and everything in the quantum world can be described entirely in terms of waves, or entirely in terms of particles, whichever you prefer. Erwin Schrödinger found the equations describing the quantum world of waves, Werner Heisenberg found the equations describing the quantum world of particles, and Paul Dirac proved that the two versions of reality are exactly equivalent to one another as descriptions of that quantum world. All of this was clear by the end of the 1920s. But to the great distress of many physicists, let alone ordinary mortals, nobody (then or since) has been able to come up with a common sense explanation of what is going on.
One response to this has been to ignore the problem, in the hope that it will go away. The equations (whichever version you prefer) work if you want to do things like design a laser, explain the structure of DNA, or build a quantum computer. Generations of students have been told, in effect, to ‘shut up and calculate’ – don’t ask what the equations mean, just crunch the numbers. This is the equivalent of sticking your fingers in your ears while going ‘la-la-la, I can’t hear you’. More thoughtful physicists have sought solace in other ways. They have come up with a variety of more or less desperate remedies to ‘explain’ what is going on in the quantum world.
These remedies, the quanta of solace, are called ‘interpretations’. At the level of the equations, none of these interpretations is better than any other, although the interpreters and their followers will each tell you that their own favoured interpretation is the one true faith, and all those who follow other faiths are heretics. On the other hand, none of the interpretations is worse than any of the others, mathematically speaking. Most probably, this means that we are missing something. One day, a glorious new description of the world may be discovered that makes all the same predictions as present-day quantum theory, but also makes sense. Well, at least we can hope.
Meanwhile, I thought it might be worth offering an agnostic overview of some of the main interpretations of quantum physics. All of them are crazy, compared with common sense, and some are more crazy than others, but in this world crazy does not necessarily mean wrong, and being more crazy does not necessarily mean more wrong. I have chosen six examples, the traditional half-dozen, largely in order to justify using the quotation from Alice. I have my own views on their relative merits, which I hope I shall not reveal, leaving you to make your own choice – or, indeed, to stick your fingers in your ears while going ‘la-la-la, I can’t hear you’.
Before offering those interpretations, though, I ought to make it clear just what it is we are trying to interpret. Science often proceeds in fits and starts. In this case, though, it seems appropriate to begin, with another nod to Charles Lutwidge Dodgson, with two fits.
John Gribbin
June 2018
FIT THE FIRST
The Central Mystery
The weirdness of the quantum world is encapsulated in what is formally known as the ‘double-slit experiment’. Richard Feynman, who was awarded the Nobel Prize for his contributions to quantum physics, preferred to call it ‘the experiment with two holes’, and said that it is ‘a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery … the basic peculiarities of all quantum mechanics.’* This may come as a surprise to anyone who only remembers the experiment from school physics, where it is used to ‘prove’ that light is a form of wave.
The school version of the experiment involves a darkened room in which light is shone on to a simple screen – a sheet of card or paper – in which there are two pinholes, or in some versions two narrow parallel slits. Beyond this screen there is a second screen, without any holes. Light from the two holes in the first screen travels across to the second screen, where it makes a pattern of light and shade. The way light spreads out from the two holes is called diffraction, and the pattern is called an interference pattern, because it is the result of two beams of light, one from each of the two holes, spreading out and interfering with each other. And it exactly matches the pattern you would expect if light is travelling as a form of wave. In some places, the waves add together and make a bright patch on the second screen; in other places the peak of one wave coincides with the trough of the other wave, so they cancel each other out to leave a dark patch. You can see exactly the same kind of interference pattern in the ripples produced on a still pond if you drop two pebbles into it at the same time. One of the distinctive features of this kind of interference is that the brightest patch of light on the second screen is not directly behind either of the two holes, but exactly halfway between those points, just where, if light was actually a stream of particles, you would expect the second screen to be completely dark. If light was made of a stream of particles, you would expect to see a bright patch behind each hole, and darkness in between those patches of light.
Richard FeynmanGetty Images
When light passes through two slits in a screen, waves spread out from each slit to make an interference pattern, like ripples on a pond.
So far, so good. This proves that light travels as a wave, as Thomas Young realised at the beginning of the nineteenth century. Unfortunately, at the beginning of the twentieth century another kind of experiment showed light behaving as a stream of particles. These experiments involved electrons being knocked out of a metal surface by a beam of light – the photoelectric effect. When the energy of the ejected electrons was measured, it turned out that for any given colour of light the energy of each electron was always the same. For a bright light there are more electrons ejected, but they still all have the same energy as each other, and this is the same as the energy of each of the smaller number of electrons ejected when the light is dimmed. It was Albert Einstein who explained this in terms of particles of light, what we now call photons – or in his language, quanta of light. The amount of energy carried by a photon depends on the colour of the light, but for any colour all photons have the same energy. As Einstein put it, ‘the simplest conception is that a light quantum transfers its entire energy to a single electron’. Turning up the light just provides more photons (light quanta), each with the same energy to give to the electrons. It was for this work, not his theories of relativity, that Einstein was awarded the Nobel Prize. After a hundred years of thinking of light as a wave, physicists had to start thinking of it as a particle – but how could that explain the experiment with two holes?
It got worse. After seeing the wave nature of light cast into doubt by the photoelectric effect experiments, in the 1920s physicists were discomfited by evidence that electrons, the archetypal particles of the subatomic world, could behave as waves. The experiments involved beams of electrons being fired through thin sheets of gold foil, between one ten-thousandth and one hundred-thousandth of a millimetre thick, and studied on the other side. The studies showed that the electron beams had been diffracted as they passed through the gaps between the array of atoms in the metal, just like light being diffracted as it passed through the experiment with two holes. George Thomson, who carried out those experiments, received a Nobel Prize for proving that electrons are waves. His father, J.J. Thomson, had received a Nobel Prize for proving that electrons are particles (and was still around to see George get his prize). Both awards were justified. Nothing demonstrates more clearly the weirdness of the quantum world. But this still isn’t the whole story.
The puzzle of wave-particle duality, as it became known, lay at the heart of theorising about the meaning of quantum mechanics from the 1920s onward. Much of this theorising about the foundations of quantum mechanics provided the solace for physicists that I discuss later. But the puzzle was brought forth in all its glory in a series of beautiful experiments beginning in the 1970s, so for now I shall skip half a century of solace-seeking to give you the up-to-date facts about the central mystery. If you find what follows hard to accept, remember that as Mark Twain put it, ‘truth is stranger than fiction, but it is because Fiction is obliged to stick to possibilities; Truth isn’t.’
In 1974, three Italian physicists, Pier Giorgio Merli, Gian Franco Missiroli, and Giulio Pozzi, developed a technique to monitor the equivalent of the experiment with two holes for electrons. Instead of a beam of light, they used a beam of electrons, boiled off from a hot wire, which travelled through a device called an electron biprism. The electrons go into the biprism through a single entrance, but encounter an electric field which splits the beam in two, with half the electrons emerging from one exit, and half emerging from another exit. Then they arrive at a detector screen, like a computer screen, where each electron makes a white spot as it arrives. The spots persist, so as more and more electrons pass through the experiment a pattern builds up on the screen. When a single electron is fired through the biprism, there is a 50:50 chance of it going one way or the other, and it makes a single spot on the screen. When a beam of many electrons is fired through the experiment, they make many overlapping spots on the screen, and these spots combine to make a pattern – the interference pattern expected for waves.
In itself, this is not too alarming. Even if the electrons are particles, there are a lot of them in the beam, and they could be interacting with each other on their way through the experiment to make the interference pattern. After all, water waves make interference patterns, and water is made up of molecules, which can be regarded as particles. But there is more.
The Italian experiment was so precise that individual electrons could be fired through it one at a time, and sent on their way like airliners departing from a busy airport. Like those aircraft, the electrons were widely spaced. The distance from the electron source (actually a bit more sophisticated than a hot wire) to the detector screen was 10 metres, and each electron in the stream did not leave the source until its predecessor had already arrived at its destination. You can (I hope) guess what happened when thousands of electrons were fired one after the other through the experiment to build up a pattern on the detector screen. They made an interference pattern. If the individual particles were acting together to make a pattern in the same sort of way that water molecules interact to make a pattern, then the interaction was taking place across both time and space. This kind of experiment became known as ‘single-electron double-slit diffraction’.
When electrons are fired one at a time through the equivalent of the double-slit experiment for light, each electron makes a blob of light on the detector screen. But the blobs build up over time to make an interference pattern, as if they were waves (see image overleaf).
Adapted from A. Tonomura et al., Am. J. Phys. (1989)
Although the Italian team published these startling results in 1976, they failed to make waves of their own in the world of physics. At that time, few physicists worried about how
