COPYRIGHT

William Collins

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This eBook first published in Great Britain by William Collins in 2016

Text © John and Mary Gribbin 2016

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John and Mary Gribbin assert the moral right to be identified as the authors of this work

Edited by Patricia Briggs

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Source ISBN: 978-0-00-814560-6

Ebook Edition © October 2016 ISBN: 9780008145613

Version: 2016-09-22

CONTENTS

COVER

TITLE PAGE

COPYRIGHT

INTRODUCTION

1 THE UPWARD THRUST OF WATER

2 MEASURING THE DIAMETER OF THE EARTH

3 THE EYE AS A PINHOLE CAMERA

4 DISSECTING THE HUMAN BODY

5 MEASURING THE MAGNETIC FIELD OF THE EARTH

6 MEASURING INERTIA

7 CIRCULATION OF THE BLOOD

8 WEIGHING THE ATMOSPHERE

9 RESISTING THE SQUEEZE

10 REVEALING THE MICROSCOPIC WORLD

11 ALL THE COLOURS OF THE RAINBOW

12 THE SPEED OF LIGHT IS FINITE

13 VITAMIN AT SEA

14 CONDUCTING THE LIGHTNING

15 THE HEAT OF ICE

16 STEAMING AHEAD

17 BREATHING PLANTS AND PURE AIR

18 OPENING UP THE SOLAR SYSTEM

19 ANIMAL HEAT, BUT NO ANIMAL MAGIC

20 TWITCHING FROGS AND ELECTRIC PILES

21 WEIGHING THE EARTH

22 BORING EXPERIMENTS ON HEAT

23 THE FIRST VACCINE

24 FEELING INVISIBLE LIGHT

25 COSMIC RUBBLE

26 FLYING HIGH WITH HYDROGEN

27 LIGHT IS A WAVE

28 DISCOVERING ATOMS

29 ELECTRIFYING SCIENCE

30 QUANTIFYING CHEMISTRY

31 THINKING ABOUT THE POWER OF FIRE

32 A RANDOM WALK

33 THE MAGNETISM OF ELECTRICITY

34 THE DEATH OF VITALISM

35 MAKING ELECTRICITY

36 AN UPLIFTING EXPERIENCE

37 BLOOD HEAT

38 TRUMPETERS ON A TRAIN

39 THE SPEED OF ICE

40 ABSORBING RADIANT HEAT

41 THE LEVIATHAN OF PARSONSTOWN

42 CONTROVERSY AND CONTROLS

43 FROM FIRE LIGHT TO STAR LIGHT

44 PREVENTION IS BETTER THAN CURE

45 PINNING DOWN THE SPEED OF LIGHT

46 DEATH TO BACTERIA

47 THE FLOWERING OF EVOLUTION THEORY

48 THE BENZENE SNAKE DANCE

49 THE MONK AND THE PEAS

50 THE IMPORTANCE OF NOTHING

51 FEELING THE SQUEEZE

52 THE SPEED OF LIGHT IS CONSTANT

53 SPARKING RADIO INTO LIGHT

54 NOBLE GASES AND A NOBLE LORD

55 THE BIRTH OF BIOCHEMISTRY

56 ENTER THE X-RAY

57 ENTER THE ELECTRON

58 RADIOACTIVITY REVEALED

59 KNOCKING ELECTRONS WITH LIGHT

60 A PAVLOVIAN RESPONSE

61 JOURNEY TO THE CENTRE OF THE EARTH

62 INSIDE THE ATOM

63 A RULER FOR THE UNIVERSE

64 THE DISCOVERY OF NUCLEIC ACIDS

65 EVOLUTION AT WORK

66 SOMETHING TO BRAG ABOUT

67 LIGHT FROM THE DARKNESS

68 ELECTRON WAVES AND QUANTUM DUALITY

69 TAKING THE ROUGH WITH THE SMOOTH

70 AN ANTIBIOTIC BREAKTHROUGH

71 SPLITTING THE ATOM

72 MAKING VITAMIN C

73 PROBING PROTEINS

74 ARTIFICIAL RADIOACTIVITY

75 THE CAT IN THE BOX

76 FISSION GETS HEAVY

77 THE FIRST NUCLEAR REACTOR

78 THE FIRST PROGRAMMABLE COMPUTER

79 DISCOVERING THE ROLE OF DNA

80 JUMPING GENES

81 THE ALPHA HELIX

82 A BLEND OF DNA

83 THE DOUBLE HELIX

84 MAKING THE MOLECULES OF LIFE

85 MASERS AND LASERS

86 MAGNETIC STRIPES AND SEA-FLOOR SPREADING

87 DETECTING THE GHOST PARTICLE

88 A VITAL VITAMIN

89 THE BREATHING PLANET

90 THE ECHO OF THE BIG BANG

91 CLOCKING ON TO RELATIVITY

92 MAKING WAVES IN THE UNIVERSE

93 THE PACEMAKER OF ICE AGES

94 THE WORLD IS NON-LOCAL

95 THE ULTIMATE QUANTUM EXPERIMENT

96 THE ACCELERATING UNIVERSE

97 MAPPING THE HUMAN GENOME

98 FIFTEEN EQUALS THREE TIMES FIVE

99 MAKING MATTER MASSIVE

100 THE COMPOSITION OF THE UNIVERSE

EXPERIMENT 101

REFERENCES

INDEX

ACKNOWLEDGEMENTS

ABOUT THE AUTHORS

ABOUT THE PUBLISHER

© NASA/Science Photo Library

Astronaut working on the Hubble Space Telescope (HST) during a routine servicing mission.

© Caltech/MIT/Ligo Labs/Science Photo Library

LIGO gravitational wave detector. Aerial photograph of the Livingston detector site for the Laser Interferometer Gravitational-Wave Observatory (LIGO). LIGO compares measurements between two detector sites 3000 kilometres apart, one near Hanford, Washington, USA, and the other near Livingston, Louisiana, USA. Each site is an L-shaped ultra-high vacuum system, four kilometres long on each side. Laser interferometers are used to look for small changes caused by gravitational waves. LIGO has been operating since 2002, with an advanced upgrade (aLIGO) operating since 2015. On 11 February 2016 it was announced that gravitational waves had been detected by LIGO. The signal was detected on 14 September 2015, and was the result of two black holes colliding.

INTRODUCTION

Science is nothing without experiments. As the Nobel Prize-winning physicist Richard Feynman said: ‘In general, we look for a new law by the following process: First we guess it; then we compute the consequences of the guess to see what would be implied if this law that we guessed is right; then we compare the result of the computation to nature, with experiment or experience [observation of the world], compare it directly with observation, to see if it works. If it disagrees with experiment, it is wrong. In that simple statement is the key to science. It does not make any difference how beautiful your guess is, it does not make any difference how smart you are, who made the guess, or what his name is — if it disagrees with experiment, it is wrong.’¹

© Physics Today Collection/American Institute of Physics/Science Photo Library

American physicist Richard Feynman (1918–1988).

Those words – if it disagrees with experiment, it is wrong – provide the simplest summary of what science is all about. People sometimes wonder why it took so long for science to get started. After all, the Ancient Greeks were just as clever as us, and some of them had both the curiosity and the leisure to philosophize about the nature of the world. But, by and large, with a few exceptions, that is all they did – philosophise. We do not intend to denigrate philosophy by this remark; it has its own place in the roll of human achievements. But it is not science. For example, these philosophers debated the question of whether a light object and a heavy object dropped at the same time would hit the ground at the same time, or whether the heavier object would fall more quickly. But they did not test their ideas by dropping objects with different weights from the top of a tall tower; that experiment would not be carried until the seventeenth century (although not, as we shall explain, by Galileo; see here). Indeed, it was just at the beginning of the seventeenth century that the English physician and scientist* William Gilbert (see here) first spelled out clearly the scientific method later summed up so succinctly by Feynman. In 1600, writing in his book De Magnete, Gilbert described his work, notably concerning magnetism, as ‘a new kind of philosophizing’, and went on: ‘If any see fit not to agree with the opinions here expressed and not to accept certain of my paradoxes, still let them note the great multitude of experiments and discoveries … we have dug them up and demonstrated them with much pains and sleepless nights and great money expense. Enjoy them you, and if ye can, employ them for better purposes … Many things in our reasonings and our hypothesese will perhaps seem hard to accept, being at variance with the general opinion; but I have no doubt that hereafter they will win authoritativeness from the demonstrations themselves.’²

© National Library of Medicine/Science Photo Library

William Gilbert (1544–1603), English physician and physicist. In 1600 Gilbert published De Magnete (Concerning Magnetism), a pioneering study in magnetism, which contained the first description of the scientific method, and greatly influenced Galileo.

In other words, if it disagrees with experiment, it is wrong. The reference to ‘great money expense’ also strikes a chord in the modern age, when scientific advances seem to require the construction of expensive instruments, such as the Large Hadron Collider at CERN, probing the structure of matter on the smallest scale, or the orbiting automatic observatories that reveal the details of the Big Bang in which the Universe was born. This highlights the other key to the relatively late development of science. It required (and requires) technology. There is, in fact, a synergy between science and technology, with each feeding off the other. Around the time that Gilbert was writing, lenses developed for spectacles were adapted to make telescopes, used to study, among other things, the heavens. This encouraged the development of better lenses, which benefited, among other things, people with poor eyesight.

A more dramatic example comes from the nineteenth century. Steam engines were initially developed largely by trial and error. The existence of steam engines inspired scientists to investigate what was going on inside them, often out of curiosity rather than any deliberate intention to design a better steam engine. But as the science of thermodynamics developed, inevitably this fed back into the design of more efficient engines. However, the most striking example of the importance of technology for the advancement of science is one that is far less obvious and surprises many people at first sight. It is the vacuum pump, in its many guises down the ages. Without efficient vacuum pumps, it would have been impossible to study the behaviour of ‘cathode rays’ in evacuated glass tubes in the nineteenth century, or to discover that these ‘rays’ are actually streams of particles – electrons – broken off from the supposedly unbreakable atom. And coming right up to date, the beam pipes in the Large Hadron Collider form the biggest vacuum system in the world, within which the vacuum is more perfect than the vacuum of ‘empty’ space. Without vacuum pumps, we would not know that the Higgs particle (see here) exists; in fact, we would not have known enough about the subatomic world to even speculate that such an entity might exist.

© Science Source/Science Photo Library

Robert Hooke’s hand-crafted microscope.

But we know that atoms and even subatomic particles exist, in a much more fundamental way than the Ancient Greek philosophers who speculated about such things, because we have been able (and, equally significantly, we have been willing) to carry out experiments to test our ideas. The ‘guesses’ that Feynman refers to are more properly referred to as hypotheses. Scientists look at the world around them, and make hypotheses (guesses) about what is going on. For example, they hypothesise that a heavy object and a lighter object dropped at the same time will hit the ground at different times. Then they drop objects from a high tower, and find that the hypothesis is wrong. There is an alternative hypothesis: that heavy and light objects fall at the same rate. Experiment proves that this is correct, so this hypothesis gets elevated to the status of a theory. A theory is a hypothesis that has been tested by experiment and passed those tests. Human nature being what it is, of course, it is not always so straightforward and clear cut. Adherents to the failed hypothesis may try desperately to find a way to shore it up and explain things without accepting the experimental evidence. But in the long run, the truth will out – if only because the die hards really do die.

Non-scientists sometimes get confused by this distinction between a hypothesis and a theory, not least because many scientists are guilty of sloppy use of the terminology. In everyday language, if I have a ‘theory’ about something (such as the reason why some people like Marmite and others don’t) this is really just a guess, or a hypothesis; this is not what the word ‘theory’ means in science. Critics of Darwin’s theory who do not understand science sometimes say that it is ‘only a theory’, with the implication ‘my guess is as good as his’. But Darwin’s theory of natural selection starts from the observed fact of evolution, and explains how evolution occurs. In spite of what those critics might think, it is more than a hypothesis – not just a guess – because it has been tested by experiment, and has passed those tests. Darwin’s theory of evolution by natural selection is ‘only’ a theory in the same way that Newton’s theory of gravity is ‘only’ a theory. Newton started from the observed facts of the ways things fall or orbit around the Earth and the Sun, and developed an idea of how gravity works – gravity involving an inverse square law of attraction. Experiments (and further observations, which throughout this book we include under the heading ‘experiments’) confirmed this.

© Paul D. Stewart/Science Photo Library

Charles Darwin’s illustration, from his book Fertilisation of Orchids, of Cypripedium (slipper orchid, Paphiopedilum), beneath a photograph of an early variety of Sandford orchid cultivar.

Gravity provides another example of how science works. Newton’s theory passed every test at first, but as observations improved it turned out that the theory could not explain certain subtleties in the orbit of Mercury, the closest planet to the Sun, which orbits where gravity is strong – that is, where there is a strong gravitational field. In the twentieth century, Albert Einstein came up with an idea, which became known as the general theory of relativity, that explained everything that Newton’s theory explained, but which also explained the orbit of Mercury and correctly predicted the way light gets bent as it passes near the Sun (see here). Einstein’s theory is still the best theory of gravity we have, in the sense that it is the most complete. But that does not mean that Newton’s theory has to be discarded. It still works perfectly within certain limits, such as in describing how things move under the influence of gravity in less extreme circumstances, in the so-called ‘weak field approximation’, and is fine for calculating the orbit of the Earth around the Sun, or for calculating the trajectory of a spaceprobe sent to rendezvous with a comet.

Contrary to what is sometimes taught, science does not proceed by revolutions, except on very rare occasions. It is incremental, building on what has gone before. Einstein’s theory builds on, but does not replace, Newton’s theory. The idea of atoms as little hard balls bouncing off one another works fine if you want to calculate the pressure of a gas inside a box, but has to be modified if you want to calculate how electrons jumping about within atoms produce the coloured lines of a spectrum of light. No experiment will ever prove the theories of Einstein or Darwin ‘wrong’ in the sense that they have to be thrown away or require us to start again, but they may be shown to be incomplete, in the way Newton’s theory was shown to be incomplete. Better theories of gravity or evolution would need to explain all the things that the present theories explain, and more besides.

Don’t just take our word for it. In his book Quantum Theory, Paul Dirac, possibly the greatest genius of the quantum pioneers, wrote: ‘When one looks back over the development of physics, one sees that it can be pictured as a rather steady development with many small steps and superposed on that a number of big jumps. These big jumps usually consist in overcoming a prejudice … And then a physicist has to replace this prejudice by something more precise, and leading to some entirely new conception of nature.’³

All of this should be clear from the selection of experiments that we have chosen in order to mark the historical growth of science, starting with a couple of those rare pre-1600 exceptions that did amount to more than mere philosophising, and coming up to date with the discovery of what the Universe at large is made of. This choice is necessarily a personal one, and limited by the constraint of choosing exactly 100 experiments. There is so much more that we could have included. But one obvious feature of the story, which we realized as we were researching this book, is not a matter of personal choice, but another example of the way science works. Some of the experiments reported here come in clusters, with several in a similar area of science in a short span of time – for example, in the development of atomic/quantum physics. This is what happens when scientists succeed in ‘overcoming a prejudice’. When a breakthrough is made, it leads to new ideas (new ‘guesses’, as Feynman would have said, but, crucially, informed guesses) and new experiments, which tumble out almost on top of each other until that seam is exhausted.

A problem for the non-specialist is that the information on which those guesses are based is itself based on the whole edifice of science, a series of experiments going back for centuries. The vacuum in the Large Hadron Collider has its origins in the work of Evangelista Torricelli in the seventeenth century (see here). But Torricelli could never have imagined the existence of the Higgs particle, let alone an experiment to detect it. The first steps in such a series are relatively easy to understand, even for non-scientists, not least thanks to the successes of science over the years. It is now ‘obvious’ to us that objects with different weight will fall at the same rate, just as it was ‘obvious’ to the ancients that they would not. But when it gets to the Higgs particle and the composition of the Universe, unless you have a degree (or two) in physics, it may be far from obvious that the story makes sense. At some level, things have to be taken on trust. But the key to that trust is that everything in the scientific world view is based on experiment, by which term we include observations of phenomena predicted by theories and hypotheses, such as the bending of light as it goes past the Sun (see here). If you find that some of the concepts described here fly in the face of common sense, remember what Gilbert said. They may ‘seem hard to accept, being at variance with the general opinion’; but they ‘win authoritativeness from the demonstrations [experiments] themselves’. And above all, if it disagrees with experiment, it is wrong.

* The term ‘scientist’ was not coined until much later, but we shall use it for convenience to describe all the thinkers or ‘natural philosophers’ of centuries past.

No. 1
THE UPWARD THRUST OF WATER

One of the first, and most famous, scientific experiments was carried out by Archimedes, who lived in the third century BC. Not much is known about Archimedes’ personal life, but it seems that he was a relative of King Hieron II of Syracuse, in Sicily, and, after extensive travels, he settled down as the King’s astronomer and mathematician. According to legend, King Hieron had a new crown, probably in the form of a laurel wreath, made for him from a bar of gold he supplied to the jeweller, to give as an offering to the gods in a temple. He suspected that the jeweller had kept some of the gold and mixed in cheaper silver instead to make up the same weight for the crown. This would be a doubly serious matter; not only would the king be cheated, but the gods might be offended at being given an inferior offering. So Hieron ordered Archimedes to find out if the crown was made of pure gold – without, of course, damaging it in any way. Archimedes had no idea how to do this, and worried about the problem for days. Then, when stepping in to a bath filled to the brim, he noticed how the water slopped over the side as it was displaced by his body. The story has come down to us from Vitruvius, a Roman architect, in a book written two centuries after Archimedes had died. We do not know where he got it from, but this is where we get the image of Archimedes immediately realizing how to test the crown, and becoming so excited that he ran out into the street, naked and wet, shouting ‘Eureka!’ (‘I have found it!’).

© Science Photo Library

An imaginative portrayal of the Greek mathematician and physicist Archimedes (287–212 BC) in his bath. Archimedes showed that an object immersed in a fluid is supported with a force equal to the displaced fluid’s weight (Archimedes’ principle).

What Archimedes had realized was that the volume of water displaced from the bath was equal to the volume of his body immersed in the water. As silver is less dense than gold, if the crown were made of a mixture of silver and gold it would have to be bigger than a crown made of pure gold in order to have the same weight. And he could measure the volume of the crown, without damaging it, by immersing it in water and seeing how much water was displaced.

Nobody knows exactly how Archimedes carried out the experiment. But the most likely method is based on an observation he described in his book, On Floating Bodies. There, Archimedes explained that the upward force (buoyancy) exerted on an object placed in water (or any other fluid) is equal to the weight of fluid that is displaced. This is now known as Archimedes’ Principle. And, of course, the weight of water displaced will be proportional to the volume of water displaced.

The obvious way to use this to test the purity of the crown, as Archimedes must have realized, would be to balance the crown against exactly the same weight of pure gold on a beam balance above a tank of water. Then, the balance is lowered until the crown and the pure gold sample are immersed in the water, while the balance arm stays above it. If both objects are made of pure gold, they will each displace the same volume (and therefore the same weight) of water, experience the same buoyancy force, and stay in balance. But if the crown is less dense than gold it will have a bigger volume, displace more water, and be more buoyant than the pure gold, so the balance will tip down on the side of the gold. The beauty of this experiment is that you don’t actually have to measure the volume of the crown, or the volume of water that it displaces; you just watch to see if the balance tilts.

That, it seems, is exactly what happened. Archimedes did the experiment (or something very similar) and found that the jeweller had indeed cheated the king. About five centuries after Vitruvius, the story was re-told in a Latin poem ‘Carmen de ponderibus et mensuris’ which described the use of such a hydrostatic balance, and in the twelfth century a manuscript called ‘Mappae clavicula’ gave detailed instructions on how to make weighings in this way to calculate the proportion of silver in the adulterated crown.

Archimedes’ Principle also explains why a ship made of steel can float. A solid lump of steel displaces a relatively small amount of water, much less than its own weight, and sinks. But if the same amount of steel is spread out in the shape of a boat, or even a simple bowl (like a coracle), a larger volume of water is displaced, weighing more than the weight of the steel, resulting in a large enough upwards force to make the boat float.

No. 2
MEASURING THE DIAMETER OF THE EARTH

The first scientific attempt to measure the size of the Earth was made by a Greek polymath, Eratosthenes of Cyrene (276–194 BC), who was in charge of the Library of Alexandria in the third century BC. He was a contemporary and friend of Archimedes. His experiment involved some observations of his own, made in Alexandria, but combined with evidence from a far away place, the city then known as Syene (now Aswan), which he had never visited.

Eratosthenes learned that each year on the day of the summer solstice, when the Sun is at its highest in the sky, it was exactly overhead at Syene, south of Alexandria. Travellers told how the reflection of the Sun could be seen at the bottom of a deep well in Syene on that day. Even at the summer solstice, the Sun is not directly overhead at Alexandria, because, as Eratosthenes appreciated, the Earth is round. So he made careful measurements of the difference between the angle made by the Sun and the vertical at the time of the solstice, working out that this corresponded to one-fiftieth of a circle, or 7º 12ʹ of arc. Simple geometry told him that this meant that the distance from Alexandria to Syene was one-fiftieth of the circumference of the Earth, assuming (which is not quite true) that Syene lies due south of Alexandria.

The distance from Syene to Alexandria was well known even in Eratosthenes’ day (it is about 800 kilometres in modern units). Egyptian records gave the distance as 5,000 stades, and Eratosthenes checked this by asking camel train drivers how long it took them to make the journey (some sources say he hired a man to pace out the distance; but this may be apocryphal). This gave him a figure of 694 stades per degree, which he rounded off to 700. Multiplying by 360 gave him the circumference of the Earth – 252,000 stades (he could have just multiplied 5,000 by 50 to get the ‘answer’ 250,000, but apparently he did it the hard way).

So what is this in modern units? Unfortunately for us, the Greeks and Egyptians used slightly different stades, but the likelihood is that Eratosthenes, being Greek, used the Greek measurement, where one stade corresponds to 185 metres, which gives a circumference of 46,620 kilometres, only 16.3 per cent too big. In the unlikely event that he used the Egyptian measurement, with one stade corresponding to 157.5 metres, he would have come up with a figure of 39,690 kilometres, just a bit too small (less than 2 per cent smaller than the actual distance, 40,008 kilometres). Either way, it is impressive.

© Sheila Terry/Science Photo Library

Eratosthenes (c. 276–194 BC).

That was by no means the only impressive achievement of Eratosthenes. He used the information he found in the books in the Library of Alexandria to produce a three-volume book of his own in which he mapped and described the entire known world. He used grids of overlapping lines, like modern lines of latitude and lines of longitude, to locate places, and invented many of the terms still used by geographers today. More than four hundred cities were named and located in the book. Unfortunately the book itself, called Geographika, was lost, but parts of it have been reconstructed from references to it in other works. Book Two of Geographika included Eratosthenes’ estimate of the size of the Earth. According to Ptolemy, Eratosthenes measured the tilt of the Earth’s axis, which is related to the measurement of the circumference, very accurately, getting a value of ¹¹/₈₃ of 180º, which is 23º 51ʹ 15ʺ. He also worked out a calendar that included leap years, and he tried to establish a chronology of literary and political events going back to the siege of Troy.

© Collection Abecasis/Science Photo Library

The World by Eratosthenes. 1886 replica of a map of the known world according to the Ancient Greek geographer, mathematician and astronomer Eratosthenes.

Eratosthenes was very much an all-rounder, so much so that he had the nickname ‘Beta’, because he was second best at everything, according to his contemporaries. The Greek geographer Strabo, who lived from about 64 BC to AD 24, described Eratosthenes as the best mathematician among the geographers, and the best geographer among the mathematicians. In mathematics, he is known for a technique called ‘the sieve of Eratosthenes’, used to find prime numbers. This simple method, which he invented, involves making a list (or grid) of all the numbers up to the biggest one you are interested in (for example, 1 to 1000). Then, you cross off from the list all the multiples of 2, the first prime number (4, 6, 8 and so on, but not 2 itself), and check that the next lowest number not crossed off is prime (if it isn’t, you have made a mistake!). If it is, cross off all the multiples of that number (but not the number itself), and so on. Once you get to the end of the list, the numbers that have not been crossed off form the list of primes.

No. 3
THE EYE AS A PINHOLE CAMERA

After the decline of classical civilization and before the European Renaissance, scientific knowledge was preserved and improved in the Arabic world. Greek texts were translated into Arabic and later from Arabic into Latin, which is how they became known to Europeans. But the Arabs also carried out original scientific work. The greatest scientist of the Middle Ages, the ‘Arabic Newton’, was Abu Ali al-Hassan ibn al-Haytham, known for short as Alhazen, who lived from about 965 to 1040 and carried out experiments in optics on either side of the year 1000. His influential book was published in Europe in Latin as Opticae Thesaurus (The Treasury of Optics) in 1572, five centuries after his death. It was a major influence on the ‘natural philosophers’ who started the scientific revolution in Europe.

© Science Source/Science Photo Library

Abu Ali al-Hassan ibn al-Haytham (known as Alhacen, or Alhazen) (965–1040).

Alhazen’s key insight was that sight is not the result of some influence reaching out from the eyes and sensing the world outside, but is caused by light entering the eye from outside. In his own words, ‘from each point of every coloured body, illuminated by any light, issue light and colour along every straight line that can be drawn from that point’. This was not an entirely original idea. Philosophers had discussed whether vision was caused by an outward influence (emission) or an inward influence (intromission) since the time of Euclid and Aristotle. But Alhazen put together a complete, coherent package of ideas which he then proved correct by experiments based on the idea of a ‘camera obscura’ (literally, a ‘dark room’; the Latin term is the source of our modern word camera). In a dark room with a heavily curtained window, if a tiny hole is made in the curtain on a sunny day an image of the outside world will be projected, upside down, on the wall opposite the window. The phenomenon had been known to the ancients, but Alhazen was the first person to describe it clearly and explain what is going on.

Alhazen realized that light travels in straight lines. Light from the top of a tree in the garden outside the window of the camera obscura will go through the hole in the curtain to the bottom of the wall opposite. Light from the base of the tree will go through the hole and up to the top of the wall. Straight lines from other points on the tree, and from other objects outside the window, go through the hole in straight lines to corresponding places on the wall to make the image.

He might have stopped there. Before Alhazen, those philosophers who thought about such things at all, such as Euclid and Aristotle, usually stopped at this stage, without actually doing experiments to test their ideas. They tried to persuade people that they were right by logic and reason, without getting their hands dirty (Archimedes, of course, was a notable, but rare, exception). What made Alhazen a real scientist was that he went a stage further. It was one thing to show how a camera obscura worked, but something else to prove that the eye works in the same way. A thousand years ago, many people would have assumed that living things were not subject to the same rules as inanimate objects. To test whether this was so, he took an eyeball from a bull, and carefully scraped away at the back of it, thinning it down until he could see on the back of the eyeball an image of what was in front of the eye, almost exactly like a tiny camera obscura. He had proved that light travels in straight lines, shown how a camera obscura works, and established that no mysterious life force is needed to explain vision, just the same physical laws that apply to non-living things. And he had done so using what became known (eventually) as the scientific method – thinking up ideas (hypotheses) about how the world works based on observation, then testing those ideas by experiment. Today, an idea that passes the experimental test is upgraded to the status of a theory, while those that fail the experimental test are discarded. As the twentieth-century physicist Richard Feynman pithily put it, ‘if it disagrees with experiment then it is wrong’. Because he understood this and put it in to practice, Alhazen was arguably the first modern scientist.

© Science Museum/Science & Society Picture Library

Alhazen’s representation of the eye as a ‘camera’.

Alhazen did much more than this. He wrote on a variety of scientific and mathematical topics, and his optical work alone filled seven books. He realized that light does not travel at infinite speed, although it is very fast, and he explained the illusion that a straight stick looks bent when one end is placed in water because light travels at different speeds in water and in air. He studied lenses and curved mirrors, working out how the curvature makes them focus light. But his place in history has been secured by how he worked as much as by what he studied. It was the true beginning of experimental science.

No. 4
DISSECTING THE HUMAN BODY

The scientific Renaissance began in the middle part of the sixteenth century, and a significant marker is the year 1543, when Copernicus published his famous book De Revolutionibus Orbium Coelestium (On the Revolution of Celestial Bodies), displacing the Earth from its supposed special position in the Universe, and Andreas Vesalius published De Humani Corporis Fabrica (On the Structure of the Human Body), going some way towards displacing humankind from a supposed special position in the animal world. Copernicus’s story is well known, and he did not, strictly speaking, carry out experiments. But Vesalius is less well known, and deserves more attention than he often gets. He did carry out experiments – on human bodies.

Vesalius was born in Brussels in 1514, but carried out his important work at the University of Padua (where he was Professor of Anatomy) in the late 1530s and early 1540s. Before that time, when human dissections were carried out (which was not very often), the actual cutting was done by barber-surgeons, who were little more (arguably less) than butchers. The professor would stand at a safe distance (literally without getting dirty) and lecture to students about what was being uncovered, using imagination as well as actual evidence. Vesalius changed all that. He carried out the dissections himself, showing as well as telling the students what was going on, and developing a much better understanding of the human body. He was helped by the civil authorities in Padua – in particular, the judge Marcantonio Contarini, who not only supplied him with the bodies of executed criminals but would time the executions to fit in with Vesalius’s need for a fresh cadaver for a lecture. This was in marked contrast to his time as a student in Paris, where Vesalius (like his fellow medical students) had been reduced to grave-robbing to get specimens for his studies.

Before Vesalius, the accepted understanding of human anatomy had been handed down since ancient times, and was based on the work of the Romano-Greek physician Claudius Galenus (known as Galen). In the Middle Ages in Europe, it was thought that the ancients had been much wiser than contemporary people, and that they had superior knowledge which could not be emulated, much less exceeded. But this was wrong. Galen was an enthusiastic dissector, but most of his work was carried out on dogs, pigs, and monkeys, because human dissection was infra dig in the second century AD. So his description of the human body was often wildly inaccurate.

© British Library/Science Photo Library

Andreas Vesalius (1514–1564).

The big contribution Vesalius made was not just to improve the understanding of human anatomy, but to stress the importance of using the evidence in front of you and your own experiments to find things out, instead of relying on the supposedly superior wisdom of the ancients. This, of course, echoed what was happening in astronomy at the same time. Vesalius, who once wrote, ‘I am not accustomed to saying anything with certainty after only one or two observations’, used to carry out ‘parallel dissections’ in which an animal body and a human body are dissected side by side, to highlight the anatomical differences between them, explicitly correcting Galen’s errors.

Vesalius also used highly skilled artists to prepare large diagrams to use in his lectures (a sixteenth-century equivalent of PowerPoint), and six of these were published as Tabulae Anatomica Sex (Six Anatomical Pictures) in 1538. He drew three of the illustrations himself, but the other three were made by John Stephen of Kalkar (Jan Stephen van Calcar), a pupil of Titian. Stephen is also thought to have been the main illustrator for Vesalius’s masterwork, the Fabrica, which appeared in seven volumes in 1643. But Vesalius’ pioneering activity did not stop there. The Fabrica was a book for experts – other professors and doctors. In order to make his work accessible to students, and even to educated laymen, he produced alongside it, and published in the same year, a summary officially titled De Humani Corporis Fabrica Librorum Epitome (Abridgement of the Structure of the Human Body) but known as the Epitome. And all before he was 30. He then gave up teaching, and spent the rest of his career practising medicine as a Court Physician, first to the Holy Roman Emperor Charles V and then to Charles’s son, Philip II of Spain (who later sent the Spanish Armada against England).

© British Library/Science Photo Library

Sixteenth-century frontispiece from De Humani Corporis Fabrica, showing a public dissection.

This change of career may have been caused by the opposition to his ideas from some of his peers, even in Padua. Jacobus Sylvius, a physician of the old school based in Paris, said that Vesalius was mad and that any advance of anatomical knowledge beyond Galen was impossible. It was more likely, he said, not that Galen was wrong, but that the human body had changed since his time. In 1543, science still had a long way to go.

No. 5
MEASURING THE MAGNETIC FIELD OF THE EARTH

If you were looking for a key date to mark the transition from a superstitious and mystical view of the world to the scientific study of our surroundings, you could do worse than pick 1600, the year in which the first book based solely on scientific experimentation was published. The book was called De Magnete Magneticisque Corporibus, et de Magno Magnete Tellure (Concerning Magnetism, Magnetic Bodies, and the Great Magnet Earth), usually shortened to De Magnete, and it was the work of an Elizabethan physician/scientist who had spent years studying magnetic phenomena.

William Gilbert had been born in 1544, studied at Cambridge and eventually became a Court Physician, first to Elizabeth I then to James I of England and VI of Scotland. As a wealthy gentleman, he was able to indulge his passion for science as an amateur, but reportedly spent £5,000 of his own money on this ‘hobby’. He died, probably of bubonic plague, in 1603.

© Science Photo Library

Title page of the second edition of William Gilbert’s De Magnete, published in 1628.

Some of the most important experiments carried out by Gilbert concerned the magnetism of the Earth. At the time, seafarers were opening up the exploration of the world, and the magnetic compass was an invaluable tool, although nobody understood how it worked. Gilbert discussed the behaviour of compass needles with ships’ captains and navigators, and disproved by experiment many superstitions, such as the idea that a magnetic compass could be desensitized by rubbing it with garlic, or even by garlic breath. He then worked with naturally occurring magnetic rocks called lodestones, shaped into magnetized spheres that he called terrellae (meaning little Earths). He studied the magnetism of these spheres with magnetized needles which could be moved around the spheres. Gilbert showed that these behaved like compass needles at different places on Earth, and concluded that the Earth had an iron core which behaved like a bar magnet, with a North Pole and a South Pole. Before he carried out these experiments, philosophers had argued that compass needles pointed north because they were attracted to the Pole Star, or, alternatively, that there was a large magnetic island at the north geographical pole.

All of this represented a dramatic scientific leap forward, which is so obvious to modern eyes that it is hard to appreciate its revolutionary nature at the time. Gilbert regarded his terrellae as models of the real Earth, and accepted that the results he obtained – for example, in the way the angle of dip of a magnetized needle depends on its place on the magnetized sphere – could be scaled up to tell us what the Earth itself is like. He was extrapolating from models to the world at large, a key feature of science in subsequent centuries.

© Science Photo Library

William Gilbert’s illustration of the angle of dip of a magnetic field surrounding the Earth; the line AB is the equator, C is the North Pole, and D is the South Pole.

As a result of these experiments, Gilbert was the first person to appreciate that, because magnetic opposites attract, the end of a magnet that points northwards (towards the north magnetic pole of the Earth) ought to be called the south pole. (In modern language, scientists sometimes refer to the ‘north-seeking’ pole and the ‘south-seeking’ pole of a magnet to avoid this confusion.) Gilbert said: ‘All who hitherto have written about the poles of the loadstone, all instrument-makers, and navigators, are egregiously mistaken in taking for the north pole of the loadstone the part of the stone that inclines to the north, and for the south pole the part that looks to the south: this we will hereafter prove to be an error. So ill-cultivated is the whole philosophy of the magnet still, even as regards its elementary principles.’

Indeed, it was Gilbert who introduced terms such as ‘magnetic pole’ and ‘electric force’ into the language. He was the first person to realize that magnetism and electricity (a word he invented) are separate phenomena, and his work on magnetism was not improved upon for two centuries, until the work of Michael Faraday.

Gilbert’s book caused a sensation in its day, and was highly influential. Galileo was one of its readers, and commented favourably on it in a letter to a friend. Indeed, Galileo described Gilbert as the founder of the scientific method. In Gilbert’s own words: ‘In the discovery of secret things, and in the investigation of hidden causes, stronger reasons are obtained from sure experiments and demonstrated arguments than from probable conjectures and the opinions of philosophical speculators.’ That is science in a nutshell, and in his book Gilbert was careful to spell out every detail of his experiments, so that other people could carry them out and see the results for themselves. But he cautions whoever does this ‘to handle the bodies carefully, skilfully and deftly, not heedlessly and bunglingly; when an experiment fails, let him not in his ignorance condemn our discoveries, for there is naught in these Books that has not been investigated and again and again done and repeated under our eyes’.

No. 6
MEASURING INERTIA

Galileo Galilei is famous for an experiment he did not carry out – but it was a real experiment, inspired by his work. He was Professor of Mathematics in Padua from 1592 until 1610, and during that time he worked in mechanics and astronomy as well as mathematics.

To put Galileo’s achievements in perspective, at the time he was working there were many people – educated people – who thought that a bullet fired horizontally from a gun, or a ball fired from a cannon, would fly a certain distance in a straight line, then stop and drop vertically to the ground. It was Galileo who first appreciated that the trajectory followed when a bullet is fired from a gun, or when an object such as a ball is thrown up in the air, is a parabola – and he carried out tests to prove this.

© New York Public Library/Science Photo Library

Nineteenth century illustration showing a glamourised version of Galileo’s experiment rolling balls down inclined planes. Although the scene depicted is fictional, Galileo really did carry out such experiments.

Among the many experiments he carried out in the years around the turn of the century there was a series of studies in which he rolled balls with different weights down inclined planes. He timed how quickly the balls moved using his pulse, and reached two important conclusions. The first was that the ‘natural’ state of a ball rolling off the slope was to continue horizontally (literally ‘towards the horizon’), unless it was stopped by friction. Without friction, he reasoned, the ball would roll on forever. This was an early insight into what Isaac Newton, following Robert Hooke, developed as his ‘First Law’ of mechanics, that an object stays at rest or moves in a straight line at a steady speed unless it is acted upon by an outside force.

Galileo’s second discovery was that the speed with which the balls rolled down the slope did not depend on their weight. For any particular slope, it took the same amount of time for any of the balls to get from the top to the bottom. This applied no matter how steep he made the slope. So he concluded – without actually dropping things vertically – that, apart from the effects of wind resistance, all falling objects would accelerate downwards at the same rate.

This infuriated some of his colleagues, philosophers of the old school who believed that Aristotle, who said that heavy objects fall faster than light objects, could not be wrong. So, in 1612, two years after Galileo moved from Padua to Pisa, one of them really did drop two weights from the leaning tower in a public demonstration intended to prove that Aristotle was right. The balls hit the ground very nearly at the same time, but not exactly. The Aristotelians said that this proved Galileo was wrong. But Galileo had an answer: ‘Aristotle says that a hundred-pound ball falling from a height of one hundred cubits hits the ground before a one-pound ball has fallen one cubit. I say they arrive at the same time. You find, on making the test, that the larger ball beats the smaller one by two inches. Now, behind those two inches you want to hide Aristotle’s ninety-nine cubits and, speaking only of my tiny error, remain silent about his enormous mistake.’⁴

Among other things, this true story highlights the power of the experimental method. If you carry out an experiment honestly, it will tell you the truth, regardless of what you want it to tell you. The Aristotelians wanted to prove Galileo was wrong, but the experiment proved he was right – within, as we would now say, the limits of possible experimental error.

By 1612 Galileo was nearly 50, and his days as an experimental physicist were essentially over. His famous clash with the Church authorities in Rome did not take place until the 1630s, and led to his spending his final years, from 1634, under house arrest at his own home (a relatively lenient sentence considering that he had been forced to confess to heresy). There, he summed up his life’s work on mechanics and promoted the scientific method pioneered by Gilbert in a great book, Discourses and Mathematical Demonstrations Concerning Two New Sciences, usually known as Two New Sciences, published in 1638 in Holland. The book was enormously influential, the first real scientific textbook, and an inspiration to scientists across Europe – except, of course, in Catholic Italy, where it was banned. As a direct result, from being a leading light in the scientific renaissance, Italy became a backwater, while the real progress was made elsewhere.

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