Theories of Everything: Ideas in Profile

THEORIES OF EVERYTHING

FRANK CLOSE is the author of the award winning Half-Life, a biography of Bruno Pontecorvo, Antimatter, The Infinity Puzzle, Very Short Introductions to Nothing and Particle Physics. He is Professor of Physics at Oxford University and a former Head of Communications and Public Education at CERN. In 2014 he was awarded the Michael Faraday Award of the Royal Society for science communication, and is the only scientist to have won an Association of British Science Writers’ Prize on three occasions.

ALSO BY FRANK CLOSE

An Introduction to Quarks and Partons

End: Cosmic Catastrophe and the Fate of the Universe

Too Hot to Handle: The Story of the Race for Cold Fusion

Lucifer’s Legacy: The Meaning of Asymmetry

Particle Physics: A Very Short Introduction

Nuclear Physics: A Very Short Introduction

The Particle Odyssey: A Journey to the Heart of the Matter

The New Cosmic Onion: Quarks and the Nature of the Universe

The Void

Neutrino

Antimatter

The Infinity Puzzle

Half Life: The Divided Life of Bruno Pontecorvo, Physicist or Spy

Theories of Everything

FRANK CLOSE

First published in Great Britain in 2017 by

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eISBN 978 1 78283 309 3

Index by Bill Johncocks

Contents

1LORD KELVIN’S HUBRIS

2WHAT IS A THEORY OF EVERYTHING, AND WHAT IS ‘EVERYTHING’?

3NEWTON’S THEORY OF INANIMATE TORTOISES

4QUANTUM THEORIES OF SMALL THINGS

5WEIGHTY MATTERS

6THE CLOUDS OF QUANTUM GRAVITY

7BACK TO THE FUTURE

Notes and references

Further reading

Acknowledgements

Index

1

Lord Kelvin’s Hubris

In 1980, Stephen Hawking speculated that the end might be in sight for theoretical physics and that the arrival of a Theory of Everything might be imminent. Was he unconsciously echoing the assertion by the American scientist Albert Michelson in 1894 that ‘the grand underlying principles have been firmly established. Further truths of physics are to be looked for in the sixth place of decimals’,¹ or by Lord Kelvin in 1900 that ‘there is nothing new to be discovered in physics now. All that remains is more and more precise measurement’²? Perhaps not.

Nature alone knows what extends beyond the horizons of our present vision, and it repeatedly reveals the limits of our imagination. Within a few years of Lord Kelvin’s remarks, the discovery of the nuclear atom, and the rise of quantum mechanics and relativity, made the exuberance of those titans of nineteenth-century science appear naive. The truth, however, is more nuanced, and in consequence the implications are rather different. The words of Lord Kelvin (certainly) and Albert Michelson (to a degree) have been taken out of context and often misquoted. When carefully interpreted, what they actually said has a more profound message for seekers of the theory of everything.

Lord Kelvin’s enduring and strongly held belief that the main role of physics was to measure known quantities to great precision had in fact inspired Michelson’s remarks. Lord Kelvin had been impressed by Maxwell’s theory of electromagnetic radiation as well as by thermodynamics, a description of heat based on mechanics, of which Kelvin himself was a prime architect. It might be possible, he felt, to understand the concept of energy in terms of the motion of particles, as the broad underlying principles seemed to be at hand.

On Friday 27 April, 1900, Kelvin gave a speech about his vision at the Royal Institution in London, the place where Michael Faraday had made the discoveries in the fields of electricity and magnetism that underpinned the new physics. Instead of making an uncritical claim that the synthesis of light, heat and mechanics meant that the end of physics was imminent, Kelvin began his speech thus: ‘The beauty and clearness of the dynamical theory, which asserts heat and light to be modes of motion, is at present obscured by two clouds.’³ This became known as the ‘two clouds’ speech.

In contrast to the folklore that Lord Kelvin was arrogantly announcing the end of physics, he was actually drawing attention to two outstanding puzzles. If he was wrong, it was in the hope that the ‘two clouds’ were mere puffs in an otherwise clear blue sky. In reality, they were the heralds of storms. Their removal would require the construction of two great pillars of twentieth-century physics: Einstein’s theory of relativity, and the quantum theory.

So Lord Kelvin was wrong in detail, certainly, but he was nonetheless well aware of the limitations of late-nineteenth-century physics. Indeed, when he made those remarks, clues to the impending revolutions in twentieth-century physics were, with hindsight, already in plain view. This will be worth bearing in mind when we come to assess modern claims that the end of physics is once more in sight.

2

What is a theory of everything, and what is ‘everything’?

Theories of everything can be roughly described as theories which draw on work in all relevant branches of current knowledge – physics, astronomy, mathematics, and so on – which seek to explain everything about the universe that is currently known. From this, it is easy to see that a theory of everything is a moving target. An explanatory account of the known universe may reign supreme for decades, even centuries. During that time it may be the basis for numerous scientific and technological advances. Then, perhaps as a direct or an indirect result of these advances, a new discovery is made, adding to the ‘everything’ which is known and which cannot be explained by the accepted theory in terms consistent with itself. A new theory of the new ‘everything’ is then required. And so the cycle continues.

Lord Kelvin’s two clouds heralded paradigm shifts in our understanding of space and time, and of the microscopic structure of matter. Given that nuclear physics and quantum physics are so rich and far-reaching, and that Albert Einstein’s relativity theory absorbed Isaac Newton’s great works on mechanics and gravitation, one might wonder how nineteenth-century science could have been blind to them. The explanation of how these fundamental pillars of wisdom remained hidden for so long, while Isaac Newton, James Clerk Maxwell and Lord Kelvin created theories of everything then known, touches on profound properties of our universe, and arguably on our ability to successfully decode its laws.

A theory of everything (or TOE, as it is sometimes abbreviated) would have to describe nature across all distances, times and energies. Our experience is limited to a mere fraction of these vast ranges, though over the centuries it has grown. In practice, nature does not cover the spectrum homogenously, so we can build theories of subsets of phenomena where ignorance in one area need not prevent progress in others.

That we have been able to advance our understanding without having a theory of truly everything is a consequence of the way natural phenomena can be grouped into discrete regimes: they form what I have referred to as a ‘cosmic onion’, whose component layers are linked together but whose contents are, to an excellent approximation, independent of one another. A theory of everything-at-one-layer succeeds because nature effectively consigns manifestations of other layers to quarantine. Suitably isolated, they play no effective role in the description of phenomena at the layer of interest.

In this book I shall illustrate this compartmentalisation for the material universe in discrete scales of size, and quantify the different scales of energy, temperature or spatial resolution we need to study to expose their dynamics. For example, before the twentieth century, physics was limited to phenomena below the temperature of blast furnaces: the millions of degrees where nuclear physics takes over were out of range, let alone the thousands of trillions at which the Higgs boson bubbles into view.

Thus we can build a theory of everything, where ‘everything’ means ‘within a specific limited range of energy’. That is how science has grown historically. It took centuries to reach the conditions revealed by the Large Hadron Collider at CERN, but along the way scientists developed a sequence of theories that were applicable to different ranges of energy.

For example, at the human scale such a theory already exists. Mathematical relationships accounting for everything bigger than the atomic nucleus have been with us since the work of Austrian physicist Erwin Schrödinger, the German physicist Werner Heisenberg and the Cambridge mathematician Paul Dirac, ninety years ago. The equations of this theory, which describe the behaviour of electrons and atoms, are taught to students. Their simplicity, however, is highly misleading as they are difficult to manipulate and impossible to solve except for a few simple cases. It is only with the development in recent years of powerful computers that the range of such solvable problems has grown. No one has deduced from these equations the properties of simple amino acids, let alone the workings of DNA, though this has hardly held back the astonishing development of modern biology. Similarly, starting from Isaac Newton’s ‘theory of everything-large-that-moves’, we can predict solar or lunar eclipses with certainty, but not the weather.

Thus when Dirac’s theory of everything is applied to the behaviour of electrons in the periphery of atoms, the complexities of the atomic nucleus can be isolated, and ignored. The theory of everything-for-sequencing-the-genetic-code may flow from the symbols A, C, G and T, which represent adenine, cytosine, guanine and thymine – the linked units of nucleic acids of a DNA strand. Dirac’s more fundamental theory of atomic physics and chemistry, which underpins the existence and structure of complex molecules, may be consigned to quarantine if your primary interest is the manipulation of those chains of amino acids encoded in A, C, G and T.

Even today, some energy domains have no theory at all, and the modern quest for theories of truly everything involves finding theories to cover the whole energy scale. The progress of science has not been restricted by the lack of an all-embracing theory of everything, nor by our inability to solve the equations of those ‘theories of something’ that have been formulated. One of the themes of this book is to consider whether the quest for a ‘theory of truly everything’ is a realistic goal, and to illustrate how practical science is largely independent of it.

The book’s structure will illustrate how the gift of nature that enables science to quarantine areas of ‘everything’ has seeded the advance of theoretical physics down the centuries. Chapters 3 and 4 review this history up to the present day, starting with Newton’s mechanics in the seventeenth century and its application to thermodynamics in the nineteenth. Electricity, magnetism and light were described by Maxwell’s theory in the nineteenth century, but new data led to the birth of quantum theory and special relativity theory. The marriage of relativity, quantum theory and mechanics led to Dirac’s fundamental theory, which underpins chemistry and the structure of DNA, and inspires the current standard model or core theory of particles and forces, with the recently discovered Higgs boson as its capstone. Chapter 5 describes theories of gravity, and their flowering in general relativity theory, while the challenge of finding a viable quantum theory of gravity is the theme of Chapter 6. In the concluding pages these ideas are brought together in assessment of the likely direction to a final theory of everything.

But first, the title of the book itself suggests two questions: what is a theory, and what is ‘everything’? ‘Life, the universe and everything’ runs the mantra. In this book, ‘life’ and, to a large extent, ‘the universe’ will be in quarantine: ‘everything’ refers to the rest, namely the inanimate contents of the universe. The ultimate challenge for theoretical physics is