Six Impossible Things

The 'Quanta of Solace' and the Mysteries of the Subatomic World

John Gribbin

• Publisher: Icon Books
• Year: 2019
• Print ISBN:
  9781785784996
• eBook ISBN:
  9781785785009
• Topic: Physical Sciences
• Subtopic: Quantum Theory

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.

SOLACE
1

The Not So Wonderful Copenhagen Interpretation

The interpretation of quantum physics that became the standard way of looking at things for decades is based on the idea of waves – and on largely forgetting the caveat ‘as if’. In the 1920s, physicists already knew that the quantum world could be described in either of two mathematical ways. One involved waves, summed up in the Schrödinger equation. The other involved pure numbers, in the form of arrays called matrices, developed from the work of Werner Heisenberg and Paul Dirac. They gave the same answers, so it was a matter of choice which one to work with; and since most physicists already had some familiarity with wave equations, that was what they chose. In any quantum calculations, however, what you calculate is the relationship between two states of a system, where the system may be an electron, the experiment with two holes, or (in principle) the entire Universe – or anything in between the electron and the Universe. If you have a set of parameters describing the system in state A, you can calculate the probability that it will be in state B after a certain time. But there is nothing which tells you what is going on in between.

Niels Bohr
Getty Images

The archetypal example is an electron in an atom. Electrons can, for some calculations, be thought of as if (that caveat) they are in orbits which correspond to different amounts of energy. When an atom emits energy in the form of light, an electron disappears from one orbit and appears in another orbit closer to the nucleus of the atom. When an atom absorbs light, an electron disappears from one orbit and appears in one further out from the nucleus of the atom. But it does not move from one orbit to the other. First it is here, then it is there. This is known as a quantum jump (or a quantum leap*). Schrödinger intended his wave mechanics to explain what happens during the leap, but it didn’t, and he said: ‘If all this damned quantum jumping were really here to stay, I should be sorry I ever got involved with quantum theory.’ Alas for Schrödinger, it was, and is, here to stay. The matrix approach is more honest, since it does not pretend to try to tell us what is happening between state A and state B, but it provides less solace than the Schrödinger equation.

What was for decades the standard way of looking at the quantum world became known as the Copenhagen Interpretation, because it was vigorously promoted by Niels Bohr, a forceful personality who was based in that city. This name (actually given to the package of ideas by Werner Heisenberg) caused considerable irritation to Max Born, who was not a member of Bohr’s team, and did not work in Copenhagen, but whose ideas about probability were an integral part of the interpretation. Bohr so dominated any discussions about quantum physics at the end of the 1920s that as well as getting his home town recognised in this way he dissed an alternative, completely viable interpretation of quantum mechanics so thoroughly that it was neglected for two decades. I shall present it as Solace 2.

Bohr was essentially a pragmatist who was happy to stick together different bits and pieces of ideas to make a working package without worrying too much about what it all meant. As a result, there is no straightforward, definitive statement of what the Copenhagen Interpretation is, although Bohr came close to such a revelation in a talk he gave at Como, in Italy, in 1927 – long before the interpretation got its name. The conference at which that talk was given was a landmark moment in physics, because it marked the point where physicists were presented with the tools they would require in order to ‘shut up and calculate’, applying quantum mechanics to the solutions of practical problems involving atoms and molecules (for example, chemistry, lasers, and molecular biology) without having to think about the fundamentals of what it all meant.

Bohr’s pragmatic approach extended to his interpretation. He said that we do not know anything except for the outcomes of experiments. These outcomes depend on what the experiments are designed to measure – on the questions we choose to ask of the quantum world (of nature). These questions are coloured by our everyday experiences of the world, on a scale much larger than atoms and other quantum entities. So we may guess that electrons are particles, and build an experiment designed to test this in an obvious way by measuring the momentum of an electron, thinking of the electron as a tiny pool ball. When we do so, lo and behold, the experiment measures the momentum of the electron, confirming our notion that electrons are particles. But a friend of ours has a different idea. She thinks that electrons are waves, and designs an experiment to measure the wavelength of an electron. Lo and behold, her experiment gives a measurement of the wavelength, confirming her notion that electrons are waves. So what, says Bohr. Just because the electron behaves as if it were a particle when you are looking for particles, or as if it were a wave when you are looking for waves, doesn’t mean that it is either, let alone both. What you see is what you get, and what you see depends on what you chose to look for. It is meaningless, according to the Copenhagen Interpretation, to ask what quantum entities such as electrons and atoms are, or what they are doing, when nobody is measuring them – looking at them, if you like.

So far, so pragmatic, and nothing really too alarming. But Bohr quickly takes us into muddy waters. This is where probability comes in. When Schrödinger came up with his wave equation, he thought of it as being a literal description of an electron (or other quantum entity; electrons are the simplest example to use for illustration). To him, an electron was a wave. But Bohr took Schrödinger’s ball and ran off with it, combining it with Born’s ideas on the role of probability to produce a bizarre and troubling concoction which worked (and still works), as far as quantum calculating was concerned, but makes your head hurt when you stop to think about it. The equation that Schrödinger gave us is, on this new picture, to be thought of as a ‘probability wave’, and the chance of finding an electron at any location is determined by ‘the square of the wave function’, essentially by multiplying the equation that describes the wave by itself, at any point. When we make a measurement, or observe a quantum entity, the wave function ‘collapses’ to a point, determined by the probabilities. But although some locations are more likely than others, in principle the electron could appear anywhere that the wave function has spread to. A very simple example highlights the oddity of this behaviour.

Erwin Schrödinger
Getty Images

Think of a single electron trapped in a box. The probability wave spreads out to fill up the box evenly, meaning that there is an equal chance of finding the electron at any location inside the box. Now drop a partition down the middle of the box. Common sense tells us that the electron must now be trapped in one half of the box. But the Copenhagen Interpretation (CI) says that the probability wave still fills each half of the box and the electron might with equal probability be found on either side of the partition. Now divide the box in two downthe centre of the partition. Keep one half-box in your laboratory, and put the other one on a rocket which takes it to Mars. Still, according to Bohr, there is a 50:50 chance of the electron popping up in the box in the lab or the one on Mars. Now open the box in your lab. Either you find an electron, or you don’t. But either way, the wave function has collapsed. If your box is empty, the electron is on Mars; if you have the electron, the other box is empty. This is not the same as saying that the electron ‘always was’ in one half of the box or the other; the CI insists that the collapse only happens when the contents of the box in the lab are examined. This is the kernel of the idea behind the EPR ‘paradox’, and Schrödinger’s famous puzzle involving a dead-and-alive cat. But before going into that story, I want to look at how the Copenhagen Interpretation ‘explains’ the experiment with two holes.

According to the CI, which I was taught as a student, and which too many students are still taught today, as ‘the’ way to ‘understand’ quantum mechanics, an electron is emitted from a source – an electron gun – on one side of the experiment as a particle. It immediately dissolves into a ‘probability wave’ which spreads through the experiment and heads towards the detector screen on the other side. This wave passes through however many holes are open, interfering with itself or not as appropriate, and arrives at the detector as a pattern of probabilities, higher in some places and lower in others, spread across the screen. At that instant, the wave ‘collapses’ and turns back into a particle, whose position on the screen is chosen at random, but in accordance with the probabilities. This is called ‘the collapse of the wave function’. The electron travels as a wave but arrives as a particle.

The wave, however, carries more than just probabilities. If the quantum entity has a choice of states it can be in, such as an electron which may be spin up or spin down, both states are somehow included in the wave function, the situation called a ‘superposition of states’, and the state the entity settles into at the point of detection, or interaction with another entity, is also determined at the moment the wave function collapses. In a lecture at the University of St Andrews in 1955, Werner Heisenberg said ‘the transition from the “possible” to the “actual” takes place during the act of observation’.

This works as a method of calculating quantum behaviour, as if things like electrons really did behave like this. But it also poses many puzzles. One of the most puzzling is a so-called ‘delayed choice’ experiment, dreamed up by the physicist John Wheeler. He started from the fact that when photons are fired one at a time through the experiment with two holes they still build up an interference pattern on the detector screen. But according to the CI, if a device is placed between the two holes and the detector screen to monitor which hole the photon goes through, the interference pattern will vanish, showing that each photon really did go through just one of the holes. The ‘delayed choice’ comes in because we can decide whether or not to monitor the photons after they have passed the screen with two holes. Of course, human reactions are not fast enough to do this. But experiments have been carried out with automatic monitoring devices to do exactly this, switching the monitors on or off after the photons have passed the holes. They show that the interference pattern does indeed disappear when the photons are monitored, meaning that each photon (or the probability wave) only goes through one hole – even though the decision to monitor the photon was made only after it had passed the holes.

Werner Heisenberg
Getty Images

Wheeler pointed out that you can imagine a similar experiment on a literally cosmic scale. In a phenomenon known as gravitational lensing, light from a distant object, such as a quasar, is focused by the gravity of an intervening object, such as a galaxy, so that it follows two (or more) paths around the gravitational lens. This makes two images of the object in detectors here on Earth. In principle, instead of making those two images it would be possible to merge the light coming different ways around the gravitational lens to make an interference pattern, caused by waves going both ways round the lens. A cosmic version of the experiment with two holes. But then we could monitor the photons before they get a chance to make the interference pattern to see which way round the lens they have come. In that case, according to the results of the laboratory-scale experiments, the interference pattern would disappear. The quasar might be 10 billion light years away, the galaxy acting as a gravitational lens might be 5 billion light years away. But according to everything we know from experiment, what the photons were doing billions of years ago and billions of light years away is affected by what we choose to measure here and now. What is going on? As Wheeler himself put it, ‘the Copenhagen Interpretation commands us not to ask such things’.† Not so wonderful, then.

In essence, the Copenhagen Interpretation says that a quantum entity does not have a certain property – any property – until it is measured. Which raises all kinds of questions about what constitutes a measurement. Does human intelligence ha...

Table of contents

• Cover
• Title Page
• Contents
• List of Illustrations
• Epigraph
• PREFACE: What’s it all About, Alfie? The Need for Quantum Solace
• FIT THE FIRST: The Central Mystery
• FIT THE SECOND: The Tangled Web
• SOLACE 1: The Not So Wonderful Copenhagen Interpretation
• SOLACE 2: The Not So Impossible Pilot Wave Interpretation
• SOLACE 3: The Excess Baggage Many Worlds Interpretation
• SOLACE 4: The Incoherent Decoherence Interpretation
• SOLACE 5: The Ensemble Non-Interpretation
• SOLACE 6: The Timeless Transactional Interpretation
• CONCLUSION: There Ain’t No Sanity Clause
• Further Reading
• About the Author
• Also by John Gribbin
• Copyright