John Gribbin - Six Impossible Things: The Mystery of the Quantum World

John Gribbin

The MIT Press

Cambridge, Massachusetts

London, England

2019 John Gribbin

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Library of Congress Cataloging-in-Publication Data

Names: Gribbin, John, 1946- author.

Title: Six impossible things : the mystery of the quantum world / John Gribbin.

Description: Cambridge, Massachusetts ; London, England : The MIT Press, [2019] | Includes bibliographical references.

Identifiers: LCCN 2019005642| ISBN 9780262043236 (hardcover ; alk. paper) | ISBN 0262043238 (hardcover ; alk. paper)

Subjects: LCSH: Quantum theory.

Classification: LCC QC174.12 .G748 2019 | DDC 530.12--dc23 LC record available at https://lccn.loc.gov/2019005642

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Alice laughed: Theres no use trying, she said; one cant believe impossible things.

I daresay you havent had much practice, said the Queen. When I was younger, I always did it for half an hour a day. Why, sometimes Ive believed as many as six impossible things before breakfast.

Alices Adventures in Wonderland

solace

noun: solace; plural noun: solaces comfort or consolation in a time of great distress

d_r0

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 behavior of light and matter, as Richard Feynman put it), are not the rules that we are familiar withthe 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 Schrdinger 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 calculatedont 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 cant 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 favored 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 choiceor, indeed, to stick your fingers in your ears while going la-la-la, I cant 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

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.

Figure 1.1

Physicist and mathematician Richard Feynman leans on a sculpture while speaking at Cal Tech University, Pasadena, California. (Photo by Joe Munroe/Hulton Archive/Getty Images)

The school version of the experiment involves a darkened room in which light is shone on to a simple screena sheet of card or paperin 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 traveling 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.

Figure 1.2

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.