Quantum: A Guide For The Perplexed Read online

  Contents

  Cover

  Title page

  Dedication

  Introduction

  1 Nature’s Conjuring Trick

  Buckyballs and the Dual-Slit Experiment Markus Arndt & Anton Zeilinger

  2 Origins

  3 Probability and Chance

  Radioactive Decay Ron Johnson

  4 Spooky Connections

  Quantum Chaology Michael Berry

  5 The Watchers and the Watched

  6 The Great Debate

  Quantum Reality, According to de Broglie and Bohm Chris Dewdney

  7 The Subatomic World

  Elementary Constituents Frank Close

  8 The Search for the Ultimate Theory

  Accentuating the Negative Paul Davies

  9 Putting the Quantum to Work

  Bose-Einstein Condensates Ed Hinds

  Quantum Mechanics and Biology Johnjoe McFadden

  10 Into the New Millennium

  Quantum Computing Andrew Steane

  Further Reading

  Picture Credits

  Acknowledgements

  Index

  Author Biography

  Copyright

  This book is dedicated to my father, to whom I am indebted for, among many things, first telling me about some strange theory called quantum mechanics.

  Introduction

  During my teens, I was an avid reader of a magazine called The Unexplained that was full of accounts of UFO sightings, Bermuda Triangle stories, and assorted other paranormal phenomena. I remember the tingle of excitement I would feel as I opened each issue to be reassured that the world was full of weird and wonderful occurrences that no one understood. Best of all were the fascinating photographs that seemed to have been taken with a cheap camera, a shaky hand, in thick fog on a dark night. They would purport to provide evidence for flying saucers, ghostly apparitions, and Loch Ness monsters. I particularly remember one morbid picture showing the charred remains of an old lady’s dismembered feet, still inside their cosy slippers and lying next to a pile of ash in a living room; all that remained of the old dear after an incident of ‘spontaneous human combustion’.

  I have no idea whether that magazine is still going – I certainly have not come across it recently – but the public’s fascination with all manner of paranormal phenomena that appear to have avoided being neatly labelled, classified and packaged by science continues unabated. It seems many people take comfort in the knowledge that there are still corners of our world that are holding out against science’s inexorable advance, where magic, mystery, and otherworldliness still survive and thrive.

  This is quite a shame; I find it frustrating that all science’s victories in explaining and rationalizing the multitude of phenomena in our Universe are sometimes regarded as mundane or lacking in wonder. One physicist who let this get under his skin was Richard Feynman, who won the Nobel Prize in 1965 for his contribution to our understanding of the nature of light. He wrote:

  ‘Poets say science takes away from the beauty of stars – mere globs of gas atoms. Nothing is “mere”. I too see the stars on a desert night, and feel them. But do I see less or more? … What is the pattern, or the meaning, or the why? It does not do harm to the mystery to know a little more about it. For far more marvellous is the truth than any artists of the past imagined it. Why do the poets of the present not speak of it?’

  These days, with so many popularizations of science that the public has access to in books, magazines, television documentaries, and the Internet, I believe attitudes are changing. But there remains one area of science that cannot be entirely rationalized using everyday language, or explained in simple, easily digestible concepts and sound bites. I refer not to any speculative, half-baked idea based on some pseudo-scientific arguments such as ESP or, worse still, astrology. On the contrary, this subject is very much mainstream science. In fact, it is a field of study that is so pervasive, so fundamental to our understanding of nature, that it underpins a large fraction of all physical sciences. It is described by a theory the discovery of which was without doubt the single most important scientific advance of the twentieth century. By some curious coincidence, it is also the subject of this book.

  Quantum mechanics is remarkable for two seemingly contradictory reasons. On the one hand, it is so fundamental to our understanding of the workings of our world that it lies at the very heart of most of the technological advances made in the past half a century. On the other hand, no one seems to know exactly what it means!

  When it comes to the world of the quantum we really are crossing into a quite extraordinary domain. A domain where it seems we are free to choose any one of a number of explanations for what is observed, each of which is in its way so astonishingly strange that it even makes tales of alien abductions sound perfectly reasonable.

  If only people knew how frustratingly and yet wonderfully un-mundane the quantum world really is, how our familiar and solid reality ultimately rests so tenuously on an unfathomable ghostly reality beneath. No need any longer for tales of the Bermuda Triangle or poltergeist activities; quantum phenomena are much stranger. And while just about every recorded paranormal incident can be explained away with no more than a pinch of common sense, quantum theory has been tested, prodded, and probed in every imaginable way for nearly a hundred years. It is a pity none of the predictions of quantum mechanics made it, as far as I am aware, into an issue of The Unexplained.

  I must make it clear from the outset that it is not the theory of quantum mechanics that is weird or illogical. On the contrary, it is a beautifully accurate and logical mathematical construction that describes Nature superbly well. In fact, without quantum mechanics we would not be able to understand the basics of modern chemistry, or electronics, or material science. Without quantum mechanics we would not have invented the silicon chip or the laser; there would be no television sets, computers, microwaves, CD and DVD players, mobile phones, and so much more that we take for granted in our technological age.

  Quantum mechanics accurately predicts and explains the behaviour of the very building blocks of matter – not just the atoms, but the particles that make up the atoms – with incredible accuracy. It has led us to a very precise and almost complete understanding of how subatomic particles interact with each other and connect up to form the world we see around us, and of which we are of course a part.

  Thus, we seem to be faced with a bit of a contradiction. How can a scientific theory be so successful in explaining so many ‘how’s and ‘why’s, and yet still be so obscure?

  Most practising physicists who use the rules and mathematical formulae of quantum mechanics on a daily basis will say that they do not have a problem with it. After all, they know that it works. It has helped us to understand a vast array of phenomena in nature, its mathematical framework and formulation is precise and well understood and, despite the numerous attempts of many who have doubted it, has survived with flying colours every conceivable experimental test thrown at it. Indeed, it is not uncommon for physicists to become irritated by those of their colleagues who still feel unable to come to terms with the counter-intuitive and bizarre nature of the subatomic world forced upon us by the theory. After all, what right do we have to expect nature at the unimaginably tiny scale of atoms to behave in a way familiar to us from our everyday experiences on the scale of humans, cars, trees, and buildings? It is not that the theory of quantum mechanics is a strange description of Nature but that Nature herself behaves in a surprising and counter-intuitive way. And if quantum mechanics provides us with the theoretical tools to understand everything we observe then we have no right to blame Nature –
or the theory – for our intellectual shortcomings.

  Many physicists become impatient with those seeking a more intuitive interpretation of quantum mechanics, and take what is in my view a rather unscientific stance. They will say ‘Why don’t you just shut up and simply use the quantum tools to make predictions about results of experiments? It is a futile waste of time to seek complete enlightenment about anything that cannot be checked experimentally.’

  In fact, the standard interpretation of quantum mechanics – the one that is in general taught to all physics students – has built into it strict rules and conditions that physicists must adhere to regarding the sort of information they are able to extract from Nature given a particular experimental set-up. I know this must sound unnecessarily obscure to appear so early on in the book but you must appreciate from the outset that quantum mechanics is like no other intellectual human endeavour, either before it or since.

  Like most physicists I have spent many years thinking about quantum mechanics, both from a professional point of view as a practising researcher and as someone interested in its deeper meaning – a field known as the foundations of quantum mechanics. Maybe the twenty years or so I have been grappling with quantum mechanics is not enough time for me to have ‘come to terms’ with it yet. But I feel I have heard enough sides of the debate (and believe me it is still ongoing despite the optimistic and in some ways dishonest claims to the contrary by those adhering to a particular interpretation) for me to at least take a step back from the fray. Most of what I cover in this book will, I hope, not be controversial, and where I do touch on issues relating to ‘what is going on’ I hope to adopt a neutral and objective position. I am not a supporter of any particular interpretation of quantum mechanics, but I do have clear views on the issue. You are of course free to disagree with these but I am sure I can win you over – unless you are one of the ‘shut up and calculate’ brigade, in which case you should not be reading this book but doing something more useful instead!

  All I will say for the time being is that my favoured version is called the ‘shut up while you calculate’ interpretation. This way I am free to worry about quantum mechanics when I am not busy using it.

  But this book is not just about the meaning of quantum mechanics. It is also about its successes, both in explaining so many phenomena, and in its many past, present, and future applications in our everyday lives. I will thus take you on a journey through philosophy, subatomic physics, and theories of higher dimensions to today’s high-tech world of lasers and microchips and tomorrow’s remarkable world of quantum magic.

  But while I hope this all sounds fascinating, it is natural for the complete novice to the field to first ask what all the fuss is about. There are many ways of highlighting the weird nature of quantum mechanics, some from everyday examples we are familiar with and which we take for granted, others by employing ‘thought experiments’: idealized situations that do not need to be realized in the laboratory to be appreciated. Indeed, nothing brings home so ruthlessly and beautifully the mystery of quantum mechanics as the experiment with the double slit. So, that is where I will begin.

  Chapter 1

  Nature’s Conjuring Trick

  Before I start throwing around too much science this early on in the book I will describe a simple experiment. It will, I predict, sound like magic. Indeed you may well wish not to believe a word of it; that is up to you. Like any magician worth his salt, I will not, at this stage, reveal to you exactly how or why it works. Unlike a conjuring trick, however, you will slowly begin to appreciate as the story unfolds that there is no sleight of hand, no hidden mirrors or secret compartments. In fact, you should be left concluding that there is no rational explanation of how things could possibly be the way I outline them.

  Since I can only use adjectives such as ‘weird’, ‘strange’ and ‘mysterious’ just so many times, I will waste no further time with this fanfare and get on with it. What I will describe is a real experiment and you will have to trust me that what is seen is not just theoretical speculation. The experiment is simple to do given the right apparatus and has been performed many times in many different ways. It is also important to point out that I shall describe the experiment, not using the benefit of an understanding of quantum physics, but from the point of view of the reader who does not yet know what to expect or how to come to terms with the astonishing results. I will assume that you will be trying to rationalize the results logically as we go along according to what you might regard as common sense, which is quite different to the way a quantum physicist would explain things. That will come later.

  I should first say that the trick, if I may still refer to it as a trick for now, could be performed simply by shining light on a special screen; and indeed this is often the way it is described in many texts. However, it turns out that the nature of light is itself very strange, and this reduces the dramatic effect. We learn at school that light behaves as a wave; it can be made up of different wavelengths (which give the different colours of the spectrum we see in a rainbow). It exhibits all the properties we expect of waves, such as interference (when two waves mix), diffraction (the spreading out of waves when they are squeezed through a narrow gap), and refraction (the bending of a wave as it travels through different transparent media). These phenomena are to do with the way waves behave when they encounter a barrier or when two waves meet. The reason I mention that light is strange is because this wave-like behaviour is not the whole story. In fact, Einstein won his Nobel prize for showing that light can sometimes exhibit some very unwave-like behaviour; but more of that in the next chapter. For the purposes of the two-slit trick, we can assume that light is a wave, since this doesn’t ruin the really good part.

  First, a beam of light is shone on a screen with two narrow slits in it that allow some of the light to pass through to a second screen where an interference pattern is seen. This is a sequence of light and dark bands that are due to the way the separate light waves emerging from the two slits spread out, overlap and merge before hitting the back screen. Where two wave crests (or troughs) meet they combine together to form a higher crest (or lower trough) that corresponds to more intense light and hence a bright band on the screen. But where a crest of one wave corresponds with a trough of the other they cancel out resulting in a dark patch. In between these two extremes some light survives and there is a gradual blending in of the pattern on the screen. It is therefore only because light behaves as a wave washing through both slits simultaneously that the interference pattern appears. No problem so far I hope.

  Light shone through two narrow slits will form a pattern of fringes on the screen due to interference between the light waves emerging from the slits. This will of course only happen if the light source is ‘monochromatic’ (consisting of light of a single wavelength).

  Next, a similar experiment is carried out using sand. This time the second screen is placed below the one with the slits and gravity does the work. As the sand falls onto the first screen, separate piles gradually build up on the lower one beneath the two slits. This is not surprising since each individual grain of sand must pass through one or other of the two slits; we are not dealing with waves now and there is no interference. The two piles of sand will be of the same height provided the two slits are of the same size and the sand is poured from a position above their mid-point.

  Now for the interesting part: repeating the trick with atoms. A special apparatus – let us call it an atomic gun for want of a better name – fires a beam of atoms at a screen with two appropriately narrow slits.1 On the other side, the second screen is treated with a coating that shows up a tiny bright spot wherever a single atom hits it.

  Grains of sand do not behave as waves of course and form two piles beneath the slits.

  Of course I don’t need to tell you that atoms are incredibly tiny entities and so should clearly behave in a manner similar to the sand, as opposed to spread-out waves, capable of overlapping both slits at once.
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  First, we run the experiment with just one slit open. Not surprisingly, we get a spread of light spots on the back screen behind the open slit. This slight spreading of the spots might be worrying if you already know something about wave behaviour since that is what happens to a wave passing through a narrow slit (diffraction). However, we can quickly reassure ourselves that we needn’t be too concerned just yet as some of the atoms may just be bumping off the edges of the slit rather than going cleanly through and this might account for the spread.

  Next, we open the second slit and wait for the spots to appear on the screen. If I asked you to now predict the distribution of the bright spots that builds up you would naturally guess that it would look like the two piles of sand. Namely, that a cluster of spots builds up behind each slit, giving two distinct patches of light that are brightest in their centre and gradually fade away as we move out and the ‘hits’ become rarer. The mid-point between the two bright patches will be dark, corresponding as it does to a region of the screen that is equally hard to reach for the atoms whichever slit they manage to get through.

  Now repeat the trick with atoms. When one of the slits is closed the atoms only pass through the open slit. The distribution of spots indicates where the atoms have landed. While this slight spreading is actually due to a wave property called diffraction, we can still argue that the atoms are behaving as particles and the result is no different to one of the sand piles.