Light Years E-Book

Brian Clegg

• Herausgeber: Icon Books
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 483

Veröffentlichungsjahr: 2015

Beschreibung

This is the story of light and the people who were determined to unlock the secrets of one of the greatest puzzles of the Universe. Acclaimed science writer Brian Clegg recounts how civilisations from the Ancient Egyptians to the Mayans understood light spiritually, and looks at the first scientific grapplings with light by the ancient Chinese sage Mo Ti, the Greek philosopher Empedocles, Arab genius Alhazen and others. Clegg also explores the contribution of artists such as Brunellschi, Leonardo de Vinci and Durer to our understanding of light and examines the great revolutionaries of light theory including Galileo, Descartes, Isaac Newton, Michael Faraday and Albert Einstein. In this new edition of one of his best-loved books, Clegg finally discusses the work of scientists such as Maxwell, Edison and Gould that led to light-driven inventions from the camera to the laser, CDs and optical computers and explains the mind-bending advances of quantum physics.

Leseprobe

Praise for the previous editions of Light Years

‘A fascinating book on a fascinating subject. It brings together all aspects of light in an unusual and compelling way.’ Sir Patrick Moore

‘Light’s properties often seem mysterious to the point of being unfathomable. Yet in this extraordinary book Brian Clegg manages to explain them through the lives of those so fixated with light that they have shaped our perception of it … Clegg’s accessible writing style manages to encapsulate the lives of light’s disciples with humorous and interesting anecdotes … [He] also provides real scientific insight into how light behaves. He explains complex theories through lucid metaphors, without resorting to the elaborate diagrams so beloved of some popular science writers … Clegg indulges in future gazing, too, the results of which are quite awesome …’ Karen Peploe, New Scientist

‘This immensely likeable work of pop science traces “man’s enduring fascination with light”, from Aristotle’s plans for a death ray (burning enemy ships with a giant array of mirrors) through to a recent experiment that seems to have sent Mozart’s 40th Symphony faster than light, and thus back through time. Clegg is very good at explaining the bizarre properties of light …’ Steven Poole, The Guardian

‘A fascinating, non-technical treatment of the concept of light … an excellent resource … makes for compelling reading.’ ScienceScope, the magazine of the US National Science Teachers’ Association

LIGHT YEARS

LIGHT YEARS

THEEXTRAORDINARYSTORY OFMANKIND’SFASCINATIONWITHLIGHT

BRIAN CLEGG

This revised edition published in the UK in 2015 by Icon Books Ltd, Omnibus Business Centre, 39–41 North Road, London N7 9DP email: [email protected]

Originally published in 2001 by Piatkus, and in a fully revised version in 2008 by Macmillan

Sold in the UK, Europe and Asia by Faber & Faber Ltd Bloomsbury House, 74–77 Great Russell Street, London WC1B 3DA or their agents

Distributed in the UK, Europe and Asia by TBS Ltd, TBS Distribution Centre, Colchester Road, Frating Green, Colchester CO7 7DW

Distributed in the USA by Consortium Book Sales & Distribution 34 13th Avenue NE, Suite 101, Minneapolis, MN 55413

Distributed in Australia and New Zealand by Allen & Unwin Pty Ltd, PO Box 8500, 83 Alexander Street, Crows Nest, NSW 2065

Distributed in South Africa by Jonathan Ball, Office B4, The District, 41 Sir Lowry Road, Woodstock 7925

Distributed in Canada by Publishers Group Canada, 76 Stafford Street, Unit 300 Toronto, Ontario M6J 2S1

ISBN: 978-184831-814-4

Text copyright © 2008, 2015 Brian Clegg

The author has asserted his moral rights.

No part of this book may be reproduced in any form, or by any means, without prior permission in writing from the publisher.

Typeset in Janson Text by Marie Doherty

Printed and bound in the UK by Clays Ltd, St Ives plc

Contents

Preface

Acknowledgements

1. At the speed of light

2. The philosophers

3. Out of the darkness

4. Engines of light

5. Seeing further

6. Light’s anatomy

7. Death of the ether

8. Fearful symmetry

9. QED

10. Tangled light

11. Tyger! Tyger!

Browsing

Further reading

Index

About the author

Science writer Brian Clegg studied physics at Cambridge University and specialises in making the strangest aspects of the universe – from infinity to time travel and quantum theory – accessible to the general reader. He is editor of www.popularscience.co.uk and a Fellow of the Royal Society of Arts. His previous books include Inflight Science, Build Your Own Time Machine, The Universe Inside You, Dice World, The Quantum Age, Science for Life and Introducing Infinity: A Graphic Guide.

www.brianclegg.net

Preface

And God said ‘Let there be light’: And there was light. GENESIS 1:3

Light is something that we take for granted. It is a fact of life, available at the press of a switch. It is the absence of darkness, the everyday gift of the Sun. It is a small part of the physics we are taught at school, a thing of ray diagrams and geometry, a natural phenomenon without substance. But light is not so easily compartmentalized. Its beguiling combination of fragility and endurance, of delicacy and power, captures the imagination just as it has fascinated scientists through the ages.

For thousands of years, uncovering the nature of light has proved an irresistible challenge. It forms a scientific quest that has endured from the conjectures of the ancient Greeks to the work of twentieth century geniuses like Albert Einstein and Richard Feynman. By combining light’s history with the latest research we can assemble a complete picture of this remarkable phenomenon and its place at the centre of creation.

What first was seen as merely the mechanism of sight has proved to be so much more. The source of all life on Earth, providing warmth, powering the weather, driving the photo-synthetic process that generates oxygen. The self-sustaining interplay of magnetism and electricity that lies behind Einstein’s special relativity. The fundamental glue that keeps all matter together. And perhaps even the key to time itself.

Looking back at the life and work of the extraordinary people who have uncovered light’s secrets provides both an understanding of light and a front row seat in the development of the remarkable new light-based technologies that are appearing as we enter the twenty-first century. Technologies that have the potential to transform reality itself.

When I was at university studying physics, I was overwhelmed by the power and beauty of light, yet so much that I read at the time on this remarkable subject made it seem dull. You only have to look at optical diagrams with rays and lenses and focal points to feel a yawn coming on. Coming back to light now has been wonderful, a chance to rekindle the amazement and delight I felt 30 years ago. That sense of wonder is what Light Years is all about.

Acknowledgements

Thanks to all those who have helped in the production of various editions of this book, including my former agent Peter Cox, Sara Abdulla and Duncan Heath.

Specific thanks to Professor Edward H. Adelson, for permission to reproduce his stunning optical illusion, and Professor Günter Nimtz for his considerable input and helpful comments on the manuscript. And a final thank you to the many individuals who have patiently helped with information and assistance. It would be boring to list them, but they know who they are.

Chapter One

At the speed of light

For now we see through a glass, darkly. ST PAUL

Imagine this. The dawn light is creeping into your room. You get up from your bed and open the curtains. Outside the window, the inferno of an active volcano distorts the air. A river of red-hot lava is streaming down the scarred mountain-side. A rain of ash falls near the window, yet you hear nothing, feel nothing.

Quickly, you move to the second window and pull back the curtain. Here, even though it’s morning, the sky is black, a crisper black than you have ever seen. The stars stand out, laser sharp. Before you is a rugged, near-white plain, surrounded by impossibly high, needle peaks. And then your eye is caught by something else. Standing out from the blackness is a bright circle of blues and greens with traceries of white. You are seeing the Earth from the surface of the Moon.

Nervously, half-expecting the air to rush out of the room, you open the window, to be struck by a burst of vertigo. Behind the glass is a leaden yellow-grey sky, hanging heavy over the already-bustling city streets 25 floors below. Nothing that you saw through the window glass exists. There is no volcano, no lunar landscape; there are no stars.

A magical tunnel

Close the window again and still the Earth is riding serenely in the sky. It’s as if the window’s glass were not looking out of the side of the building, but opening instead onto a magical tunnel leading straight onto the surface of the Moon. There are no video screens or electronics involved, just glass with very special properties. This is slow glass, first dreamed up by the 1970s visionary writer Bob Shaw. A special glass that light takes months or even years to pass through.

With such remarkable glass it would only take a good site in front of a beautiful view to create such stunning windows. If light takes a year to get from one side of the glass to the other, then one year after it is put in position, the first glimpse of the landscape will reach the other side. As the light takes a year to pass through, everything that has been happening in front of the glass will be seen a year later behind it. Shift the glass into a building and it carries a year’s worth of light with it. You’ve got a window on an exotic location for as long as it takes the remaining light to make its slow journey through the material.

The ultimate speed

It was only in the late 1990s that technology caught up with the imagination and made slow glass a possibility. The discoveries described in this chapter demonstrate the remarkable power of the new light technology. Later, we will plunge back 2,500 years to follow the story of humanity’s fascination with light. In that story, light’s immense speed will be a recurring theme. For slow glass it presents a particular problem.

A beam of light travels at around 300,000 kilometres each second in the vacuum of space, a speed that belies our experience of nature. A hummingbird’s wings flap 4,200 times in a minute, near invisible to the human eye. Yet in the duration of a single flap of those wings, a beam of light could have crossed the Atlantic Ocean. On 20 July 1969, Apollo 11 landed on the Moon after a journey of four days. If it had, instead, set off for the nearest star, Alpha Centauri, which light takes four years to reach, the Apollo capsule would still be travelling after a thousand millennia.

In glass, light moves a little slower than it does in space, but it would still require a window 5,000,000,000,000 kilometres thick to hold a year’s worth of light. If slow glass is to be made, there is an enormous challenge to face. There has to be a way to apply the brakes, to slow light down by a factor of a billion billion or more. Unlikely though this sounds, in the late 1990s a substance was created that can do just that.

Einstein’s strange matter

The substance with the amazing effect on light is a strange form of matter called a Bose–Einstein condensate (physicists have to have a particularly good day to come up with a snappy name like ‘photon’ or ‘quark’). We are used to matter coming in three types – solid, liquid and gas. Since the 1920s it has been known that there is a fourth form of matter, generated in the raging nuclear furnace of the Sun – plasma. This is the next stage beyond a gas, where the easily removed electrons have been broken off the atoms and the result is a soup of ions – atoms with some electrons missing – and the electrons themselves.

The four states of matter – solid, liquid, gas and plasma – have a startling parallel in a theory developed over 2,000 years ago. The Greek philosopher Empedocles thought that everything was made up of four elements – earth, water, air and fire – each equivalent to one of the modern states. Some of the ancients thought there should be a fifth element, the substance from which the heavens were constructed, called the quintessence. This handily corresponds with a then-hypothetical fifth state of matter that Einstein dreamed up. The idea also dates back to the 1920s, when a young Indian physicist called Satyendra Bose wrote to the world-famous scientist describing his ideas. Einstein would have received many letters from scientific hopefuls, but this one caught his attention. Bose had found a totally new way to describe light.

Thanks to Einstein’s theories, light had begun to be thought of as photons – tiny, insubstantial particles that shot through space like bullets from a gun. Bose experimented with describing light mathematically as if these photons were a collection of particles that was already well understood – a gas. Einstein helped Bose firm up the maths, but was also inspired to imagine a fifth state of matter. By applying intense cold or pressure to a material he believed that it would eventually reach a state where it would no longer be an ordinary substance; instead it would share some of the characteristics of light itself. Such a state of matter is a Bose–Einstein condensate, the material that could provide the key to producing slow glass.

Nearly 80 years after the theory was developed, a Danish scientist has used a Bose–Einstein condensate to drag the speed of light back to a crawl. Her name is Lene Vestergaard Hau, one of the few women to take an active part in the history of light. In 1998, Hau’s team set up an experiment where two lasers were blasted through the centre of a vessel containing sodium atoms that had been cooled to form a Bose–Einstein condensate. Normally the condensate would be totally opaque, but the first laser creates a sort of ladder through the condensate that the second light beam can claw its way along – at vastly reduced speeds. Initially light was measured travelling at around 17 metres per second – 20 million times slower than normal. Within a year, Hau and her team, working at Edwin Land’s Rowland Institute for Science at Harvard University, had pushed the speed down to below a metre per second – and more was to follow, as we will discover later.

Hau’s material is not quite slow glass. There is one more problem to overcome. Imagine you had a piece of special glass one centimetre thick that took a year for light to get through. It would live up to expectations if you were looking straight through the glass. But things would be different when looking at the edges of the scene. Now the light is arriving at an angle, passing through more of the glass before it gets to you. It could easily travel through half as much glass again, and so take half as long again to get through. With an ordinary window the difference isn’t noticeable, but light that hits slow glass at an angle would take months longer to arrive than the light that arrived straight on. Views from every direction would appear at different times, combining images to produce a nightmare confusion.

To overcome this effect, a slow glass window has to do more than just let light through. It needs to capture the whole image at the surface of the window, whatever the angle the light has arrived from. That total view then must pass through the window as a piece, rather than as masses of uncoordinated rays heading in all directions. This requirement isn’t as impossible as it sounds. It is very similar to the way in which a hologram is produced, combining the rays of light from different directions to make a unified picture. In the hologram, this gives a three-dimensional view that changes as you move, built into a flat, two-dimensional photograph. It is such an image, with three dimensions compressed into two, that would have to be sent through the window. The combination of holographic techniques and a very slow material would deliver true slow glass.

While the technology required at the moment to have such control on light’s speed is formidable, the mere fact of its existence gives some hope that in the future slow glass may move from fiction to practical reality. The first lasers, after all, were heavy-duty, complex devices requiring conditions that were inconceivable outside the laboratory – yet some modern lasers can fit on a pinhead and are happy to function in the unprotected environment of a consumer product like a laser pointer.

Breaking the light barrier

If the possibilities of slow glass, bringing light to a virtual standstill, are fascinating, then taking the opposite tack, pushing light above its normal speed, has even more remarkable consequences. As we will explore in detail in Chapter 8, Einstein’s special theory of relativity showed that light was the fastest thing in existence. Nothing, he argued, could exceed that 300,000 kilometres per second. According to the special theory of relativity, any solid object approaching the speed of light would get heavier and heavier until its mass was infinite. Even the speed of an insubstantial snippet of information should never get past the 300,000 kilometres per second barrier, because the peculiarities of relativity mean that a faster than light signal would travel backwards in time. If it were possible to broadcast a message fast enough, we could use light to say hello to our ancestors.

Such technology would transform human existence. If a signal could be sent back even a tiny fraction of a second it would make it possible to build computers that worked thousands of times faster than current machines, limited as they are by the speed of internal communication. With information sent even further back, disasters could be averted by broadcasting warnings. All gambling based on prediction, from the roulette wheel to the stock market, would be destroyed. There is hardly an aspect of life that would not be fundamentally changed. Yet this is not the most dramatic implication of sending a message back in time.

The very foundations of reality would come under threat. Being able to send a message into the past would shatter the rigid connection of cause and effect. For most scientists this is enough to prove that getting a message past light speed is impossible. It’s not that they have any objection to getting a sneak preview of lottery results this way, but rather that bewildering paradoxes emerge when information is sent backwards through time.

Time COPs

It is easy to feel the impact of the paradox by considering a simple time transmitter that could send a radio message back just a few seconds. This transmitter is fitted with a radio control, so it can be switched on and off remotely. At noon precisely, the transmitter is used to send a message back in time. This message is the signal to the transmitter’s own radio control. When the message is received at five seconds before noon, it switches the transmitter off. Now, when noon arrives, the transmitter is switched off. So how could the message have been sent? But if the message wasn’t sent, the transmitter would still be switched on.

Rather than deal with such mind-bending possibilities, physicists resort to the ‘Causal Ordering Postulate’, sometimes known as the time COP. This sounds impressive, but amounts to little more than saying that effect never can come before cause. (It’s actually a little more sophisticated, allowing the effect to precede the cause if there’s no way the effect can influence the cause, but the result is the same.) It follows that anything that would endanger the relationship of cause and effect, like sending a message back in time, is impossible. Professor Raymond Chiao of the University of California, a leading exponent of superluminal physics – the science of faster than light motion – believes there is no way to send a message back through time. But Chiao’s own experiments in the late 1990s opened a loophole in the light speed barrier.

At the sub-microscopic level of photons, the minuscule particles that make up a beam of light, the everyday expectations of the world fall apart. The familiar, predictable behaviour of objects disappear, leaving only probability and uncertainty. This is the world of quantum physics, discovered by Max Planck and Albert Einstein around a hundred years ago. Thanks to the bizarre nature of reality at the quantum level, individual photons of light have a small but real chance of jumping through solid objects and appearing on the other side in a process known as tunnelling.

Quantum short cut

Tunnelling emerges from the bizarre statistical view that quantum mechanics takes. Generally speaking, quantum mechanics expects, just as we would in the normal world, that when a car drives into a wall it bounces back. Every now and then, though, quantum theory says it should pass straight through. The probability is incredibly low – far less than winning a lottery week after week after week – but it exists. In a beam of light there are many, many photons, and the chance that a single photon will cross an apparently impenetrable barrier is much higher than that of a whole car jumping through a wall. This phenomenon, tunnelling, has been widely observed. In fact, if it weren’t for tunnelling, there would be no life on Earth.

The light of the Sun that heats the Earth and triggers the release of oxygen through photosynthesis is produced by a deceptively simple process. In the intense furnace of the core of a star (like the Sun), charged particles of the most basic element, hydrogen, combine to make helium, the next element up the chain. In this process energy is released. The reaction can only happen if hydrogen particles come into close contact, but each particle is positively charged. These charges repel each other, like magnets when the same poles are brought together. Even in the Sun’s heart, the particles can’t combine – just as well, or there would be an immense explosion, burning out the Sun in a second as all the hydrogen was converted. The repelling force forms a barrier that has to be overcome to form helium, just as we have to fight against gravity to jump over a physical barrier like a fence. It is the strange reality of quantum physics that makes this possible. Some hydrogen particles jump through the barrier to fuse together – they have tunnelled.

To give an accurate picture of what is happening, we should really drop the term ‘tunnelling’. It implies slowly grinding your way through an obstacle. What really happens is much more startling. At one moment a particle is one side of the barrier, the next it is on the other. It jumps rather than tunnels, but instead of flying over a physical barrier, it actually passes from one position to the other without moving through the points in between. This instant jump means that any photons travelling along a path that includes a barrier to tunnel through manage to get along that path at faster than the speed of light.

Chiao and his team demonstrated this strange phenomenon, measuring light travelling at 1.7 times the normal speed. If this light beam could be made to carry a signal, that message would, according to relativity, be shifted backwards in time. But Professor Chiao wasn’t worried about destroying the fabric of reality. His experiment relied on generating individual photons, and the mechanism that made this possible provided no way of controlling when a photon would emerge. Without such control, the photons could not carry a message. Equally, there was no way of deciding which photons would get through the barrier – most don’t – and so it seemed impossible to keep a signal flowing. Without the ability to send a message there would be no chance of disrupting causality.

At the time, Professor Chiao was unaware of developments in another laboratory in Cologne, Germany. The refined tones of Mozart’s 40th Symphony, clearly a message, were about to be transmitted at four times the speed of light. The stakes for reality were about to be raised.

But before exploring the nature of these faster than light experiments and how they could pose a threat to existence itself, we need to do some time travelling of our own, taking a 2500 year trip back to a time when the very existence of light seemed as close to magic as it did to science.

Chapter Two

The philosophers

The Atoms of Democritus And Newton’s Particles of light Are sands upon the Red Sea shore, Where Israel’s tents do shine so bright. WILLIAM BLAKE

In Neolithic Britain, from around 3000 BC, Stonehenge acted as both a temple of light and a marker of the important seasonal changes predicted by the movement of the Sun. By the time Stonehenge was at its peak, 1500 years later, the Egyptians linked the Sun with a god – the god, Ra, creator of the Universe, first among the deities. The Sun was thought to be Ra’s eye, the source of all life and creation. Light and warmth poured from the god, at the same time a gift and something to fear. In a papyrus dating from 1300 BC, a priest-scribe noted the thoughts of Ra himself:

I am the one who opens his eyes and there is light. When his eyes close, darkness falls.

To the Egyptian people, always poised on the balance between flood and drought, Ra was generous, but also terrible. To look directly into the eye of the god was to be blinded. Even to glance at his glory caused pain. When the ancient Egyptians dabbled with monotheism it was the Sun’s disk, the Aten, that was the focus of their worship. The temples at Akhetaten, the newly built capital dedicated to the Sun, carried a eulogy to the benefits of Aten:

You are beautiful, great, shining and high above every land, and your rays enfold the lands to the limit of all you have made … You, sole god, who no other is like.

Though not directly a part of God, light was still given great significance in the early Jewish beliefs that gave rise to modern Judaism, Christianity and Islam. In the dramatic biblical Genesis creation story, light is a part of the first day of time, along with the Earth, the heaven and the waters. Later, as the ancient Greek civilization formed the foundations of modern Western culture, light reappeared in the religion and legends of the Greeks.

Flying too close to the Sun

The picture most of us have of ancient Greek religion is a fuzzy mix of childhood stories, more like a work of fiction than a religious text. It’s easy to think of this impression as a reflection of our ignorance, flavoured by Disney cartoon characters and Hollywood epics. But that picture is surprisingly accurate. There was no written core of the Greek religion – no equivalent of the Bible or the Qur’an – instead, there was a complex web of myth: stories told to illustrate the nature of divinity, always combining entertainment with education. This flexible structure meant that even the gods themselves changed with time. Originally it was Helios who rode the Sun’s chariot across the sky, but he became absorbed into the central figure of Apollo, son of Zeus.

In these ever-changing and developing tales, the most striking human interaction with light came in the experience of Daedalus and Icarus. Daedalus was an inventor, said to have designed the labyrinth for King Minos of Crete that contained the half-man, half-bull monstrosity, the Minotaur. Architects of secret structures of the time were sometimes killed to destroy their knowledge. It might have been better for Minos if Daedalus had suffered that fate. The inventor gave the secret of the labyrinth to the king’s daughter, Ariadne, who passed it on to her lover Theseus. After Theseus managed to kill the Minotaur and escape, Daedalus was imprisoned along with his son Icarus.

Daedalus built wings of wax and feathers so the two of them could fly away to safety, but Icarus was too bold, revelling in the freedom of flight as he soared higher and higher. Forgetting his purpose, he trespassed on the territory of the Sun. The heat of the Sun’s fierce rays melted the wax on his wings, leaving Icarus to plunge to his death in the sea. This story was a pointed parable, demonstrating the dangers of the quest for knowledge and of wanting too much for the self. It has been used countless times since to provide imagery to illustrate this risk. Yet it was not long after the myth probably first emerged, around the seventh century BC, that the Greeks began a dogged pursuit of knowledge.

The development of philosophical thought as a legitimate activity was triggered by a change in circumstances of the Greek people. From around 650 BC, the aristocratic groups that had been in control were overthrown by tyrants. Given the connotations this word has today, it’s perhaps surprising that these tyrants were largely welcomed – the ‘tyrant’ label only meant that they had seized power unofficially, not that their actions were oppressive. Likely to be wealthy commoners, the tyrants proved popular, supporting trade and encouraging the economy. With new political and trading strength came the opportunity to take a step back, to take time to think, rather than being concerned with mere survival. This new ease of living made it possible for the Greeks, always a people inclined to structure, to develop the schools of philosophy.

Light – in fact, all of nature – came to be treated in a new way. There was still religious awe, but alongside it was room for practical curiosity and logic. The religion did not go away (though not every philosopher subscribed to religious beliefs), but now there was something more. By 500 BC, light was being considered in some detail, particularly by Empedocles. By a coincidence, this interest was echoed on the far side of the Earth. The followers of Chinese philosopher Mo-Tzu had also taken on the challenge of light.

Light from the east

Unlike the relative calm of the Greeks, China was in upheaval. Though the great Zhou dynasty that had seen the first unification of that huge country had more than 200 years still to run, Chinese society was breaking into factions. The political instability of the time seemed to demand a very practical philosophy. It saw the rise of Legalism, a school that prided itself on efficient, soulless statecraft rather than purity of thought. Yet it also produced Mo-Tzu, who has been described as China’s first true philosopher in the Western sense.

Mo-Tzu was said to be a disgruntled follower of Confucius, who became frustrated by his master’s aristocratic leanings and emphasis on ritual. Mo-Tzu’s philosophy combined pragmatism and an emphasis on universal love. Those who followed Mo-Tzu took an approach to light that was every bit as practical as Legalism’s transformation of bureaucracy into a high art. They measured and observed, noting how flat and curved mirrors produced different types of reflection. They found that by letting light shine through a tiny pinhole in a piece of wood they could project a weak, upside-down image onto a white surface. This discovery was the earliest known camera obscura, a device that remained popular into Victorian times, and eventually gave rise to all our present day photographic equipment.

The inner light

By contrast with his Chinese counterparts, Empedocles did not experiment; to do so was alien to Greek values. Instead he looked inside himself for his inspiration. Light and sight seemed inextricably entwined, so Empedocles pictured light as a beam of fire transmitted from the eyes. The two approaches, Chinese and Greek, were the total opposite of the modern stereotype that labels Eastern culture as inward looking and contemplative while the West is considered obsessed with externals, with measurement and analysis.

This tendency to ignore experiment was not quite as irrational as it seems to modern eyes. The Greeks argued that our senses were easy to fool. What we experienced was not always a helpful guide. It was more important to look within. It’s certainly true that the senses have their limits. Optical illusions like the one in Figure 2.1, produced by Professor Edward H. Adelson at MIT, demonstrate just how fallible our sense are.

Figure 2.1 Checkerboard optical illusion (courtesy of Professor H. Adelson)

In this picture, the squares labelled A and B are exactly the same shade of grey. Because of the way our brain is programmed to handle objects and shadows we are fooled into thinking that the square labelled B is much lighter, but it isn’t. (If you don’t believe this is true, see the animation at http://www.universeinsideyou.com/experiment3.html which moves square A alongside square B. They really are the same shade.) Unfortunately, the limitations of our senses don’t make our mental processes any more accurate, but the Greeks believed that the only hope of finding the truth was through pure reason.

Empedocles, born around 492 BC, had a privileged upbringing in Acragas (now Agrigento) on the Sicilian coast. His rich family was prepared to indulge the passionate enthusiasms that soon brought him to the attention of others. His followers considered him a seer, but according to historian George Sarton, contemporary critics, perhaps detecting a tendency in Empedocles to make sure that the dice rolled his way, considered him a charlatan. Certainly Empedocles had a high opinion of his own worth. In later years he flaunted many of the trappings of royalty, from a purple robe and golden girdle to a constantly present group of fawning attendants.

There is little doubt, though, that Empedocles was driven by fervent curiosity and could not have been satisfied with the easy life of home. He travelled the Greek world in a search for knowledge. He was an archetypal educated man of his culture, embodying a fascination with the nature of the world. But for all his enthusiasm and originality, Empedocles brought the cumbersome baggage of Greek philosophy to his scientific studies. He had no concept of using experiment to prove ideas – debate and the application of pure thought were the only tools he could employ.

Much of Empedocles’ time was taken up with medicine. He seems to have had real skill as a healer, and made the most of his practical ability to build up his image, passing off cures that he knew were perfectly natural as miracles. When he wasn’t peddling remedies, Empedocles shared an interest with his contemporaries in the nature of matter. How was a solid substance made up? Was light itself matter, or something different? Empedocles’ most far-reaching (if wildly wrong) contribution to this debate was to devise the four elements. Everything, he decided, could be broken down to the essential components of earth, air, fire and water. (There’s a strong parallel here with the much earlier Genesis creation story, where the first things created are the Earth, heavens, light and water.)

There is some logic to Empedocles’ theory. For instance, when a piece of wood burned it gave off fire, smoke (a form of air), and ash (a kind of earth). This simplistic picture was taken up by two of the big names to influence Western development – Aristotle and Plato – and became the accepted view for over 2,000 years. It still bizarrely crops up in some New Age and alternative philosophies today.

A poet as well as a philosopher (his songs were the surprise hit of the Olympic Games in 440 bc), Empedocles conjured up a flowery picture of the mechanism of sight. In his book On Nature he says that Aphrodite, the goddess of love

kindle[d] the fire of the eye at the primal hearth of the universe, confining it with tissues in the sphere of the eyeball.

Although this language is poetic, Empedocles was being literal. He envisaged actual fire, passing through special channels to separate it from the waters of the eye and flowing out in a blazing stream to the objects that were seen. Such a dramatic picture has to be understood in the context of his four elements – light had to be composed of fire, as it could hardly be earth, air or water.

This fiery light from the eye, the accepted reality for over a thousand years, seems hopelessly flawed to a modern mind. If light originates in the eyes, why can we not still see when the Sun goes down? Empedocles had not missed the contribution of the Sun. In fact he even suggested that the Earth caused the darkness of night by blocking the Sun’s rays, a concept that was well ahead of his time. Yet he was able to separate two quite independent kinds of light in his mind. The sunlight he regarded as a facilitator that enabled the eye’s light to make sight possible. Imagine opening a box to let light into it. The action of moving the box lid doesn’t generate light, it just makes it possible for light to get in. Similarly, Empedocles believed that the Sun only made it possible for the light from the eye to function correctly.

Empedocles’ theory was influenced by more than the practicalities of vision. To Greek thinking, the very nature of what was seen, or at least how it was described, was coloured by this inner view. Homer, writing perhaps 400 years before Empedocles, described the sea as wine-dark – yet no one would now consider the colour of the sea to resemble wine. In fact, both blue and green seem to have represented very different concepts from their present day meanings. The closest word there was in ancient Greek to blue is kyanos, which from the context in which it was used suggests darkness rather than a colour.

A similar confusion exists over chloros, the word that is most similar to green; it was applied to both blood and honey. It seems that chloros was not really a colour, but rather a state of freshness, of new, growing life. A facile explanation for this attitude to colour would be that the ancient Greeks were more susceptible to colour blindness than we are, but there is no evidence to support this. Instead it seems that the feelings attached to an object were given more significance that any observed colouring. The principal light was the inner light not the outer.

Seeing in the dark

Although Empedocles’ theories would not be discarded for a millennium and more, they weren’t the only attempt to describe how light worked. The most significant competition came from the atomists. This faction was a spin-off from the school that Pythagoras set up before even Empedocles was at work. Two fourth century BC philosophers, Leucippus and Democritus, devised the almost prescient concept that everything was made up of tiny indivisible particles – atoms (literally ‘uncuttable’ in the Greek original). As they believed that all creation was constructed in this way, they also thought that light must consist of such tiny particles, flowing in a stream like a spray of fine powder from source to observer.

The ideas of the atomists were not forgotten, but always remained on the fringe of acceptability. Even when Newton attended Cambridge in the 1600s, the atomist view was not considered particularly significant, but it appealed very much to Newton himself, and he was to construct a whole theory of light that was driven by the atomists’ ideals (see Chapter 5). For the moment, though, it was Empedocles’ theory that remained the accepted truth, reinforced by the contribution of Plato, the highly influential philosopher born in Athens around 428 bc.

Plato (probably a nickname meaning ‘broad shouldered’ – he may actually have been called Aristocles) was the youngest son of an extremely wealthy family. He dabbled in politics, but the upheavals following the final Peloponnesian war between Athens and Sparta made this a dangerous pursuit. The execution of his philosophical master, Socrates, in 399 BC brought this message home with terrible force. Socrates was technically charged with heresy – neglecting the gods and introducing his own deities – but in reality, his crime was more likely to have been his active criticism of those in power. Socrates’ fate brought Plato to think that the study of mathematics, science and philosophy was a safer option.

Although famed as philosopher, Plato’s doctrines are not the easiest to pin down as they appear as a series of dialogues – almost as fiction – rather than clear expositions of fact. But some of his scientific views, and specifically that of the mechanics of sight, are more clearly recorded.

Plato was conscious of the problems that the inability to see in the dark presented. He expanded the part of Empedocles’ theory that dealt with sight as a special interaction between the light of the eye and the light of the outer world. Plato thought that the two merged into a single link, tying together the object being looked at and the inner person. The conjoining of the two types of light produced, Plato thought, an optical highway to channel information on what was being seen to the soul.

Despite his attempt at rationalization, Plato’s view remained purely philosophical. It lacked the mathematical reasoning we would now think of as scientific. But less than a hundred years later, another great Greek name was putting a different spin on the nature of vision. Euclid, working around 300 BC, was two generations on from Plato – in fact he was probably educated by Plato’s pupils. That is, if he existed at all.

Euclid’s rays

That there can be doubt about the existence of such a well-known historical figure may seem bizarre, but there is insufficient evidence to be sure if the works of Euclid are attributable to a single man, a teacher and his pupils, or even a group of philosophers operating under a fictional name (this has occurred since, when a team of mathematicians published a series of works under the constructed name Bourbaki). This uncertainty makes any biographical information about Euclid at best speculative.

Whatever the reality of his existence, Euclid was obsessed with geometry. He applied the unwavering measure of spatial mathematics to the behaviour of sight. Yet despite this logical approach, Euclid managed to further refine the light-from-the-eyes theory rather than dismiss it entirely.

As Einstein would, more than 2,000 years later, when realizing the unique nature of light’s speed, Euclid used a thought experiment, acting out a hypothetical situation in his mind to test his deductions. He imagined looking for a needle that had been dropped on the ground. As he searched, even though he was looking in the right general direction, he didn’t see the needle. Then all of a sudden it sprang into view. Euclid reasoned that light from the Sun must always be hitting the needle and reaching the eye, so if that were the only light, we ought to be able to see the needle immediately. Sight, though, he argued, was dependent on the sunlight’s interaction with a ray that shone from the eye, and that ray needed a conscious focus on the object.

This sounds very similar to Plato’s theory, but Euclid’s big step forward was the idea that this ray from the eye travelled in a straight line, bounced off the needle (or whatever was being looked at) and was reflected back into the eye. The specifics might have been faulty, but he had painted a picture of light that would make a true scientific view possible. Suddenly light had been transformed from a diffuse vaporous phenomenon to something that travelled in straight lines, its behaviour predictable by the new-fangled mathematics of geometry. That light travelled in straight lines would be a fundamental assumption all the way up to the twentieth century, when the distorting mirror of Einstein’s genius would throw even this basic premise into question.

Weapons of light

Shortly after Euclid’s time, another great philosopher took the ideas of straight line optics and came close to using them to build a death ray. Born in 287 BC, Archimedes lived practically his entire life in Syracuse in Sicily, though he probably spent some time in Alexandria, as he often exchanged personal letters with mathematicians based there. He is now remembered for his mechanical concepts and for carrying on Euclid’s work. Archimedes certainly had an obsessive enthusiasm with geometry. Plutarch, writing 350 years later, wryly observed that Archimedes’ servants had to drag him from his work to get him to the baths to wash him, and when he was there, Archimedes would still be drawing diagrams using the embers of the fires, and even marking out lines on his naked body as he was being washed and anointed.

Like Euclid, Archimedes was fascinated by light and particularly by mirrors. He wrote a book on optics, now lost along with all the detail of his optical theories. Archimedes lived in an unsettled time for Greece. The Romans, whom the Greeks had contemptuously dismissed as insignificant barbarians, were sweeping across Greek territories. The once great Hellenic civilization was on the verge of collapse. And Archimedes, for all his genius, ended up in the wrong place at the wrong time. He had designed engines of war that were used to bombard invading ships, but despite these, the Romans seemed unstoppable.

It was 212 BC. With the enemy closing in on Syracuse, Archimedes had the inspiration of using light itself as a weapon. He knew that small, curved mirrors could concentrate the rays of the Sun enough to set kindling alight. This ability to focus energy at a distance seemed an ideal way to attack the Roman’s vulnerably flammable wooden ships before they were even in range of his projectile weapons.

Archimedes drew up plans for great curved metal sheets to be fixed in frames on the harbour. These dazzling constructions were intended to capture the Sun’s rays, focusing them to a point until the undiluted heat of the day became a miniature furnace. But the mirrors were never made. Perhaps the craftsmen, more used to blacksmithing than precision engineering, found them too much of a challenge. Perhaps the stricken city had lost so much to the war effort that it could not find time and money to construct the mirrors. Perhaps even the great Archimedes was laughed at when he claimed it was possible to destroy the Roman enemies without even touching them.

It may have been the mirrors that Archimedes was still working on in his last minutes. He was said to be drawing and re-drawing diagrams when one of the invading Roman soldiers found him. Without looking up, Archimedes cursed the interruption. ‘Do not disturb my diagrams.’ They were his last words. The soldier who found the 75-year-old man was in no mood to tolerate such disrespect from a member of a defeated race. Archimedes was slaughtered without compassion.

On the brink

The Romans did not entirely eliminate Greek culture. There remained one last flowering of scientific philosophy before Western civilization took the fall into darkness. The man responsible was Ptolemy, living in the Greek city of Alexandria on the edge of Egypt in the second century ad. It was a time of painful transition. Ptolemy was not Greek in the classical sense. His name alone suggests it. In fact he is sometimes mistakenly called an Egyptian.

More properly he was Claudius Ptolemaeus, the first name showing his Roman citizenry, the second that he lived in Egypt. Nonetheless he was born in Greece and followed the Greek tradition of scholarship. Hardly anything more is known about Ptolemy as a man, apart from the astoundingly precise dates of his first and last recorded observations at Alexandria on 26 March 127 and 2 February 141. Ptolemy made his name with a major study of astronomy – his system, built on Aristotle’s view that the Sun and the planets revolved around the Earth on crystal spheres, would remain the absolute standard for another 1,400 years. But his optical work was equally lasting.

His most significant observation of light was, like his model of the solar system, both influential and wrong. He studied the way a beam of light bent as it passed into water, the process known as refraction. By noticing that a coin seems to move if it is placed at the bottom of an empty cup and then water is poured in, and by adding in Euclid’s ideas of straight line optics, he was able to say quite correctly that light bends inwards towards a straight line into the water or glass when it enters a denser material from air. The reverse happens on the way out.

So far, so good. And Ptolemy also listed many measurements that he made in establishing just how much the light was bent – an approach that was totally contrary to the traditional Greek tactic of untested theorising and much closer to the modern scientific method. Unfortunately, his deductions from his data were not correct. He thought that there was a fixed proportion between the angle at which the light hit the material and the angle it bent to when it got inside. While this is almost true for small angles, it gets further and further from reality as the angles get bigger. The whole business of refraction took hundreds of years to untangle.

Ptolemy was not without later detractors. The sixteenth century astronomer Tycho Brahe, who constructed the best maps of the stars to be made before telescopes were available, thought that Ptolemy had passed off measurements of star positions made by the earlier Hipparchus as his own. The data produced by Hipparchus, working around 150 BC, is lost, making it impossible to compare the two. But Ptolemy’s observations had a consistent error that suggested he might have copied earlier data, then tried to adjust it for the passage of time. Ptolemy made it clear how much he depended on the work of Hipparchus, leaving himself open to posthumous attack. The ever-volatile Isaac Newton attacked Ptolemy vehemently, accusing him of:

A crime committed against fellow scientists and scholars, a betrayal of the ethics and integrity of his profession that has forever deprived mankind of fundamental information about an important area of astronomy and history.

Certain that Ptolemy had invented data to fit his theories, Newton went on to say:

Instead of abandoning the theories, he deliberately fabricated observations from the theories so that he could claim that the observations prove the validity of his theories. In every scientific or scholarly setting known, this practice is called fraud, and it is a crime against science and scholarship.

Modern scholars are less critical, accepting that Ptolemy added valuable observations to knowledge that was already in the public domain. As for Ptolemy’s book on optics, no one in the modern world has ever seen it, so it may seem odd that we can be sure that he listed detailed experimental data. Most of the copies of his book were destroyed as the remainder of Greek and then Roman civilization fell to the waves of barbarian invaders. Some would certainly have been lost in the destruction of the vast library in Ptolemy’s home city of Alexandria, which according to legend was devastated no less than four times.

The library was built at the instigation of an earlier Ptolemy, Ptolemy I, King of Egypt, towards the end of the fourth century BC. It became a unique centre of learning, with over half a million scrolls of information stored in its huge halls. But in 47 BC, Julius Caesar was holed up in Alexandria during the civil war with Pompey. A fire, started to destroy the Egyptian fleet, accidentally spread to the library and burned it down. That time many of the scrolls were saved, but it was then made an intentional target of destruction, twice attacked by Emperors of the collapsing Roman world and finally obliterated by the Caliph Umar, the second of the Muslim rulers to succeed Muhammad.

Around AD 630, the Caliph is said to have ordered that all books in the library that weren’t in agreement with the Qur’an should be destroyed. For good measure, any books that did agree with the Qur’an were also to be destroyed, as they merely provided unnecessary repetition. The scrolls were piled into the furnaces that powered the battered remains of the Roman heating systems and baths. Ibn al-Kifti, a later Arabic writer, notes in his Chronicle of Wise Men that ‘the number of baths was well known, but I have forgotten it’, (contemporary records suggest around four thousand). According to al-Kifti it took these thousands of furnaces, ‘six months to burn all that mass of material’.

Luckily there were other libraries, and some books survived. Later Caliphs were less intolerant of Greek learning; many of the Greek books that we now have, including the remaining fragments of Ptolemy’s Optics, come to us in translation from Arabic copies made after the fall of Rome. A new force had taken over the burden of knowledge from the ancient civilizations. It took an Arab philosopher who had studied the Greek texts to bring light out from the shadow of the Dark Ages. But first he had to avoid painful death at the hands of the Egyptian king.

Chapter Three

Out of the darkness

There are two ways of spreading light; to be the candle or the mirror that reflects it. EDITH NEWBOLD JONES WHARTON

The Dark Ages were dark indeed. Those intent on survival had little time for scientific enquiry. When there was peace, the Greeks’ preference for pure thought was joined by a Christian distrust of the pursuit of knowledge. A few islands of advancement remained. Islamic culture, emerging in the seventh century ad, required every Muslim to pursue knowledge (ilm) in order to further justice. After an initial 150 years of bloody conquest, Islam reached a more peaceful period, where this pursuit of knowledge brought light to the fore. In the West, a handful of the inward-looking academics were prepared to risk everything in a stubborn desire for knowledge. The candles of scientific discovery were few, but they burned brightly.

The safety of madness

The new Islamic intellectual hub was Baghdad. Here scholars uncovered the remains of Greek natural philosophy and added to it with their own researches. In the story of light one name stands out – Abu Ali al-Hasan ibn al-Haytham, usually called Alhazen in Western texts. He was born in Al Basrah (now Basra in Iraq) in 965 and was one of a handful of men who would transform the subjective classical theories to devise the geometrical view of optics that we take for granted today.

While young and amenable, Alhazen tried to understand the world from the purely religious viewpoint of his teachers, but he could not resist the more practical approach of dealing with the ‘how’ rather than the ‘why’ of the world’s workings. His practicality was almost his undoing. Soon famous in the Muslim world as a prodigy who could solve any problem, he was invited to Cairo by the king, al-Hakim. This was the sort of invitation it was not wise to decline. The king had a problem – the Nile. Throughout the history of Egypt, the great river had proved both a blessing and a curse. Depending on its flow, the Nile could ensure plentiful crops, flooding or drought. Al-Hakim commanded Alhazen to devise a means of controlling the Nile, of taking the power of devastation away from nature.

The young Alhazen jumped at the challenge, but soon found that he had bitten off more than he could chew. The Nile did not respond to his bidding. Alhazen was devastated. The king was not interested in failure. If Alhazen admitted his inability to control the great river he would lose more than his job. Fearing for his life, Alhazen cast around for a way out. He considered running away to Syria, but though al-Hakim’s kingdom was puny in comparison with dynastic imperial Egypt, he was still a power to be reckoned with. His arm was long, particularly when he was angry. With time running out for his potentially fatal audience with the king, Alhazen made a desperate decision. Tearing at his clothes, a wild light in his eyes, he imitated the madmen he had seen raving in the city streets. In brief moments of clarity he let it be known that the challenge of taming the Nile had driven him insane. Al-Hakim could not take his revenge on a madman. Alhazen was forced to keep up this pretence for years until the king died.

It’s easy to imagine Alhazen, confined in his pretended madness, staring out from a small, barred window at the way light moved and changed in response to the procession of the clouds and the Sun. With little more to do than watch, it became more and more obvious that light came not from the eye, but from the Sun. He noticed that after-images of brightly-lit objects remained floating in front of him when he turned away and looked back into the darkness of his cell. This surely was something external acting on his eye, rather than a response to light that originated in his own eyeball. Similarly the pain he felt when glancing at the Sun could not be a result of light that came from the eye itself. Alhazen convinced himself that light was independent of the eye.

Again, it is easy to imagine that the sight of a busy square outside his window, flooded with sunlight, was the source of Alhazen’s other great contribution to the understanding of optics. The cheerful bustle of activity must have seemed like paradise from his silent confinement. Perhaps there were children, scurrying around, yelling and throwing balls to each other. Now that he understood that light didn’t require the eye, Alhazen could bring in Euclid’s elegant straight line geometry, imagining the light flowing along lines from the Sun, beaming out in all directions. Some of those beams were hitting the square, filling it with brightness – but just one, the one that passed in a straight line into the eye enabled sight. He would have seen the glitter of reflection off mirrors and weapons and metalware in the square. When this happened, he imagined a