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LONDON, NEW YORK, MELBOURNE, MUNICH, AND DELHI DK LONDON PROJECT ART EDITOR Katie Cavanagh SENIOR EDITOR Georgina Palffy US EDITOR Jane Perlmutter US SENIOR EDITOR Margaret Parrish produced for DK by TALL TREE LTD. Published in the United States by DK Publishing 4th ﬂoor, 345 Hudson Street New York, New York 10014 EDITORS Rob Colson Camilla Hallinan David John 14 15 16 17 18 10 9 8 7 6 5 4 3 2 1 001–192893–July/2014 DESIGN AND ART DIRECTION Ben Ruocco MANAGING ART EDITOR Lee Grifﬁths DK DELHI MANAGING EDITOR Stephanie Farrow PROJECT EDITOR Priyaneet Singh PUBLISHING DIRECTOR Jonathan Metcalf ART DIRECTOR Phil Ormerod ASSISTANT ART EDITOR Vidit Vashisht Copyright © 2014 Dorling Kindersley Limited All rights reserved Without limiting the rights under copyright reserved above, no part of this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted, in any form, or by any means (electronic, mechanical, photocopying, recording, or otherwise), without the prior written permission of both the copyright owner and the above publisher of this book. DTP DESIGNER Jaypal Chauhan Published in Great Britain by Dorling Kindersley Limited JACKET DESIGNER Laura Brim MANAGING EDITOR Kingshuk Ghoshal A catalog record for this book is available from the Library of Congress. JACKET EDITOR Maud Whatley MANAGING ART EDITOR Govind Mittal JACKET DESIGN DEVELOPMENT MANAGER Sophia MTT PREPRODUCTION MANAGER Balwant Singh PUBLISHER Andrew Macintyre ISBN: 978-1-4654-1965-1 PREPRODUCTION PRODUCER Adam Stoneham DK books are available at special discounts when purchased in bulk for sales promotions, premiums, fund-raising, or educational use. For details, contact: DK Publishing Special Markets, 345 Hudson Street, New York, New York 10014 or SpecialSales@dk.com. PRODUCER Mandy Inness Printed and bound in China by Leo Paper Products Ltd. original styling by ILLUSTRATIONS James Graham, Peter Liddiard STUDIO 8 Discover more at www.dk.com CONTRIBUTORS ADAM HART-DAVIS, ; CONSULTANT EDITOR Adam Hart-Davis trained as a chemist at the universities of Oxford and York, and Alberta, Canada. He spent ﬁve years editing science books, and has been making television and radio programs about science, technology, mathematics, and history, as producer and host, for 30 years. He has written 30 books on science, technology, and history. JOHN FARNDON John Farndon is a science writer whose books have been short-listed for the Royal Society junior science book prize four times and for the Society of Authors Education Award. His books include The Great Scientists and The Oceans Atlas. He was a contributor to DK’s Science and Science Year by Year. DAN GREEN Dan Green is an author and science writer. He has an MA in Natural Sciences from Cambridge University and has written over 40 titles. He received two separate nominations for the Royal Society Young People’s Book Prize 2013 and his Basher Science series has sold over 2 million copies. DEREK HARVEY Derek Harvey is a naturalist with a particular interest in evolutionary biology, and a writer for titles that include DK’s Science and The Natural History Book. He studied Zoology at the University of Liverpool, taught a generation of biologists, and has led expeditions to Costa Rica and Madagascar. PENNY JOHNSON Penny Johnson started out as an aeronautical engineer, working on military aircraft for 10 years before becoming a science teacher, then a publisher producing science courses for schools. Penny has been a full-time educational writer for over 10 years. DOUGLAS PALMER Douglas Palmer, a science writer based in Cambridge, Britain, has published more than 20 books in the last 14 years—most recently an app (NHM Evolution) for the Natural History Museum, London, and DK’s WOW Dinosaur book for children. He is also a lecturer for the University of Cambridge Institute of Continuing Education. STEVE PARKER Steve Parker is a writer and editor of more than 300 information books specializing in science, particularly biology and allied life sciences. He holds a BSc in Zoology, is a Senior Scientiﬁc Fellow of the Zoological Society of London, and has authored titles for a range of ages and publishers. Steve has received numerous awards, most recently the 2013 UK School Library Association Information Book Award for Science Crazy. GILES SPARROW Giles Sparrow studied astronomy at University College London and Science Communication at Imperial College, London, and is a best-selling science and astronomy author. His books include Cosmos, Spaceﬂight, The Universe in 100 Key Discoveries, and Physics in Minutes, as well as contributions to DK books such as Universe and Space. CONTENTS 10 INTRODUCTION THE BEGINNING OF SCIENCE 600 BCE–1400 CE 20 21 22 23 SCIENTIFIC REVOLUTION 1400–1700 34 Eclipses of the Sun can be predicted Thales of Miletus At the center of everything is the Sun Nicolaus Copernicus 40 Now hear the fourfold roots of everything Empedocles The orbit of every planet is an ellipse Johannes Kepler 42 Measuring the circumference of Earth Eratosthenes A falling body accelerates uniformly Galileo Galilei 44 The human is related to the lower beings Al-Tusi The globe of the Earth is a magnet William Gilbert 45 Not by arguing, but by trying Francis Bacon 46 Touching the spring of the air Robert Boyle 50 55 Layers of rock form on top of one another Nicolas Steno Is light a particle or a wave? Christiaan Huygens 56 Microscopic observations of animalcules Antonie van Leeuwenhoek 24 A ﬂoating object displaces its own volume in liquid Archimedes 52 The ﬁrst observation of a transit of Venus Jeremiah Horrocks 58 Measuring the speed of light Ole Rømer 26 The Sun is like ﬁre, the Moon is like water Zhang Heng 53 Organisms develop in a series of steps Jan Swammerdam 60 One species never springs from the seed of another John Ray 28 Light travels in straight lines into our eyes Alhazen 54 All living things are composed of cells Robert Hooke 62 Gravity affects everything in the universe Isaac Newton EXPANDING HORIZONS 96 No vestige of a beginning and no prospect of an end James Hutton 1700–1800 102 The attraction of mountains 74 104 The mystery of nature 76 78 80 81 82 84 85 Nature does not proceed by leaps and bounds Carl Linnaeus The heat that disappears in the conversion of water into vapor is not lost Joseph Black Inﬂammable air Henry Cavendish Winds, as they come nearer the equator, become more easterly George Hadley A strong current comes out of the Gulf of Florida Benjamin Franklin Dephlogisticated air Joseph Priestley In nature, nothing is created, nothing is lost, everything changes Antoine Lavoisier The mass of a plant comes from the air Jan Ingenhousz 86 Discovering new planets William Herschel 88 The diminution of the velocity of light John Michell 90 Setting the electric ﬂuid in motion Alessandro Volta Nevil Maskelyne in the structure and fertilization of ﬂowers Christian Sprengel 105 Elements always combine the same way Joseph Proust A CENTURY OF PROGRESS 1800–1900 115 Mapping the rocks of a nation William Smith 116 She knows to what tribe the bones belong Mary Anning 118 The inheritance of acquired characteristics Jean-Baptiste Lamarck 119 Every chemical compound has two parts Jöns Jakob Berzelius 120 The electric conﬂict is not restricted to the conducting wire Hans Christian Ørsted 121 One day, sir, you may tax it Michael Faraday 110 The experiments may be repeated with great ease when the Sun shines Thomas Young 112 Ascertaining the relative weights of ultimate particles John Dalton 114 The chemical effects produced by electricity Humphry Davy 122 Heat penetrates every substance in the universe Joseph Fourier 124 The artiﬁcial production of organic substances from inorganic substances Friedrich Wöhler 126 Winds never blow in a straight line Gaspard-Gustave de Coriolis 127 On the colored light of the binary stars Christian Doppler 128 The glacier was God’s great plough Louis Agassiz 130 Nature can be represented as one great whole Alexander von Humboldt 136 Light travels more slowly 226 Particles have wavelike in water than in air Léon Foucault properties Erwin Schrödinger 138 Living force may be 234 Uncertainty is inevitable converted into heat James Joule 139 Statistical analysis of molecular movement Ludwig Boltzmann 140 Plastic is not what I meant to invent Leo Baekeland 142 I have called this principle natural selection Charles Darwin 150 Forecasting the weather Werner Heisenberg 186 Rays were coming from the tube Wilhelm Röntgen 188 Seeing into the Earth Richard Dixon Oldham 190 Radiation is an atomic property of the elements Marie Curie 196 A contagious living ﬂuid Martinus Beijerinck Robert FitzRoy 156 Omne vivum ex vivo — all life from life Louis Pasteur 160 One of the snakes grabbed its own tail August Kekulé 166 The deﬁnitely expressed average proportion of three to one Gregor Mendel 172 An evolutionary link between birds and dinosaurs Thomas Henry Huxley 174 An apparent periodicity of properties Dmitri Mendeleev 180 Light and magnetism are affectations of the same substance James Clerk Maxwell A PARADIGM SHIFT 1900–1945 202 Quanta are discrete packets of energy Max Planck 206 Now I know what the atom looks like Ernest Rutherford 214 Gravity is a distortion in the space-time continuum Albert Einstein 222 Earth’s drifting continents are giant pieces in an ever-changing jigsaw Alfred Wegener 224 Chromosomes play a role in heredity Thomas Hunt Morgan 236 The universe is big… and getting bigger Edwin Hubble 242 The radius of space began at zero Georges Lemaître 246 Every particle of matter has an antimatter counterpart Paul Dirac 248 There is an upper limit beyond which a collapsing stellar core becomes unstable Subrahmanyan Chandrasekhar 249 Life itself is a process of obtaining knowledge Konrad Lorenz 250 95 percent of the 315 Earth and all its life forms universe is missing Fritz Zwicky make up a single living organism called Gaia James Lovelock 252 A universal computing 316 A cloud is made of billows machine Alan Turing upon billows Benoît Mandelbrot 254 The nature of the 317 A quantum model chemical bond Linus Pauling of computing Yuri Manin 260 An awesome power is 318 Genes can move from locked inside the nucleus of an atom J. Robert Oppenheimer FUNDAMENTAL BUILDING BLOCKS 1945–PRESENT 270 We are made of stardust species to species Michael Syvanen 320 The soccer ball can 286 A perfect game of tic-tac-toe Donald Michie 292 The unity of fundamental forces Sheldon Glashow Fred Hoyle 271 Jumping genes 294 We are the cause of Barbara McClintock global warming Charles Keeling 272 The strange theory of 296 The butterﬂy effect light and matter Richard Feynman 274 Life is not a miracle Harold Urey and Stanley Miller 276 We wish to suggest a structure for the salt of deoxyribose nucleic acid (DNA) James Watson and Francis Crick happen happens Hugh Everett III 322 Insert genes into humans to cure disease William French Anderson 324 Designing new life forms on a computer screen Craig Venter 326 A new law of nature Ian Wilmut Edward Lorenz 298 A vacuum is not exactly nothing Peter Higgs 327 Worlds beyond the solar system Geoffrey Marcy 300 Symbiosis is everywhere Lynn Margulis 302 Quarks come in threes Murray Gell-Mann 308 A theory of everything? Gabriele Veneziano 284 Everything that can withstand a lot of pressure Harry Kroto 314 Black holes evaporate Stephen Hawking 328 DIRECTORY 340 GLOSSARY 344 INDEX 352 ACKNOWLEDGMENTS INTRODU CTION 12 INTRODUCTION S cience is an ongoing search for truth—a perpetual struggle to discover how the universe works that goes back to the earliest civilizations. Driven by human curiosity, it has relied on reasoning, observation, and experiment. The best known of the ancient Greek philosophers, Aristotle, wrote widely on scientiﬁc subjects and laid foundations for much of the work that has followed. He was a good observer of nature, but he relied entirely on thought and argument, and did no experiments. As a result, he got a number of things wrong. He asserted that big objects fall faster than little ones, for example, and that if one object had twice the weight of another, it would fall twice as fast. Although this is mistaken, no one doubted it until the Italian astronomer Galileo Galilei disproved the idea in 1590. While it may seem obvious today that a good scientist must rely on empirical evidence, this was not always apparent. The scientiﬁc method A logical system for the scientiﬁc process was ﬁrst put forward by the English philosopher Francis Bacon in the early 17th century. Building on the work of the Arab scientist Alhazen 600 years earlier, and soon to be reinforced by the French philosopher René Descartes, Bacon’s scientiﬁc method requires scientists to make observations, form a theory to explain what is going on, and then conduct an experiment to see whether the theory works. If it seems to be true, then the results may be sent out for peer review, in which people working in the same or a similar ﬁeld are invited to pick holes in the argument, and so falsify the theory, or to repeat the experiment to make sure that the results are correct. Making a testable hypothesis or a prediction is always useful. English astronomer Edmond Halley, observing the comet of 1682, realized that it was similar to All truths are easy to understand once they are discovered; the point is to discover them. Galileo Galilei comets reported in 1531 and 1607, and suggested that all three were the same object, in orbit around the Sun. He predicted that it would return in 1758, and he was right, though only just—it was spotted on December 25. Today, the comet is known as Halley’s Comet. Since astronomers are rarely able to perform experiments, evidence can come only from observation. Experiments may test a theory, or be purely speculative. When the New Zealand-born physicist Ernest Rutherford watched his students ﬁre alpha particles at gold leaf in a search for small deﬂections, he suggested putting the detector beside the source, and to their astonishment some of the alpha particles bounced back off the paper-thin foil. Rutherford said it was as though an artillery shell had bounced back off tissue paper— and this led him to a new idea about the structure of the atom. An experiment is all the more compelling if the scientist, while proposing a new mechanism or theory, can make a prediction about the outcome. If the experiment produces the predicted result, the scientist then has supporting evidence for the theory. Even so, science can never prove that a theory is correct; as the INTRODUCTION 13 20th-century philosopher of science Karl Popper pointed out, it can only disprove things. Every experiment that gives predicted answers is supporting evidence, but one experiment that fails may bring an entire theory crashing down. Over the centuries, long-held concepts such as a geocentric universe, the four bodily humors, the ﬁre-element phlogiston, and a mysterious medium called ether have all been disproved and replaced with new theories. These in turn are only theories, and may yet be disproved, although in many cases this is unlikely given the evidence in their support. Progression of ideas Science rarely proceeds in simple, logical steps. Discoveries may be made simultaneously by scientists working independently, but almost every advance depends in some measure on previous work and theories. One reason for building the vast apparatus known as the Large Hadron Collider, or LHC, was to search for the Higgs particle, whose existence was predicted 40 years earlier, in 1964. That prediction rested on decades of theoretical work on the structure of the atom, going back to Rutherford and the work of Danish physicist Niels Bohr in the 1920s, which depended on the discovery of the electron in 1897, which in turn depended on the discovery of cathode rays in 1869. Those could not have been found without the vacuum pump and, in 1799, the invention of the battery—and so the chain goes back through decades and centuries. The great English physicist Isaac Newton famously said, “If I have seen further, it is by standing on the shoulders of giants.” He meant primarily Galileo, but he had probably also seen a copy of Alhazen’s Optics. The ﬁrst scientists The ﬁrst philosophers with a scientiﬁc outlook were active in the ancient Greek world during the 6th and 5th centuries BCE. Thales of Miletus predicted an eclipse of the Sun in 585 BCE; Pythagoras set up a mathematical school in what is now southern Italy 50 years later, and Xenophanes, after ﬁnding seashells on a mountain, reasoned that the whole Earth must at one time have been covered by sea. In Sicily in the 4th century BCE, Empedocles asserted that earth, air, ﬁre, and water are the “fourfold roots of everything.” He also took his followers up to the volcanic crater of Mt. Etna and jumped in, apparently to show he was immortal—and as a result we remember him to this day. Stargazers Meanwhile, in India, China, and the Mediterranean, people tried to make sense of the movements of the heavenly bodies. They made star maps—partly as navigational aids—and named stars and groups of stars. They also noted that a few traced irregular paths when viewed against the “ﬁxed stars.” The Greeks called these wandering stars “planets.” The Chinese spotted Halley’s comet in 240 BCE and, in 1054, a supernova that is now known as the Crab Nebula. ❯❯ If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things. René Descartes 14 INTRODUCTION House of Wisdom In the late 8th century CE, the Abbasid caliphate set up the House of Wisdom, a magniﬁcent library, in its new capital, Baghdad. This inspired rapid advances in Islamic science and technology. Many ingenious mechanical devices were invented, along with the astrolabe, a navigational device that used the positions of the stars. Alchemy ﬂourished, and techniques such as distillation appeared. Scholars at the library collected all the most important books from Greece and from India, and translated them into Arabic, which is how the West later rediscovered the works of the ancients, and learned of the “Arabic” numerals, including zero, that were imported from India. Birth of modern science As the monopoly of the Church over scientiﬁc truth began to weaken in the Western world, the year 1543 saw the publication of two groundbreaking books. Belgian anatomist Andreas Vesalius produced De Humani Corporis Fabrica, which described his dissections of human corpses with exquisite illustrations. In the same year, Polish physician Nicolaus Copernicus published De Revolutionibus Orbium Coelestium, which stated ﬁrmly that the Sun is the center of the universe, overturning the Earth-centered model ﬁgured out by Ptolemy of Alexandria a millennium earlier. In 1600, English physician William Gilbert published De Magnete in which he explained that compass needles point north because Earth itself is a magnet. He even argued that Earth’s core is made of iron. In 1623, another English physician, William Harvey, described for the ﬁrst time how the heart acts as a pump and drives blood around the body, thereby quashing forever earlier theories that dated back 1,400 years to the Greco-Roman physician Galen. In the 1660s, Anglo-Irish chemist Robert Boyle produced a string of books, including The Sceptical Chymist, in which he deﬁned a chemical element. This marked the birth of chemistry as a science, as distinct from the mystical alchemy from which it arose. Robert Hooke, who worked for a time as Boyle’s assistant, produced the ﬁrst scientiﬁc best seller, Micrographia, in 1665. His superb fold-out illustrations of subjects such as a ﬂea and the eye of a ﬂy opened up a microscopic world no one had seen before. Then in 1687 came what many view as the most important science book of all time, Isaac Newton’s Philosophiæ Naturalis Principia Mathematica, commonly known as the Principia. His laws of motion and principle of universal gravity form the basis for classical physics. Elements, atoms, evolution In the 18th century, French chemist Antoine Lavoisier discovered the role of oxygen in combustion, discrediting the old theory of phlogiston. Soon a host of new gases and their properties were being investigated. Thinking about the gases in the atmosphere led British meteorologist John Dalton to I seem to have been only like a boy playing on the seashore, and diverting myself in now and then ﬁnding a smoother pebble…whilst the great ocean of truth lay all undiscovered before me. Isaac Newton INTRODUCTION 15 suggest that each element consisted of unique atoms, and propose the idea of atomic weights. Then German chemist August Kekulé developed the basis of molecular structure, while Russian inventor Dmitri Mendeleev laid out the ﬁrst generally accepted periodic table of the elements. The invention of the electric battery by Alessandro Volta in Italy in 1799 opened up new ﬁelds of science, into which marched Danish physicist Hans Christian Ørsted and British contemporary Michael Faraday, discovering new elements and electromagnetism, which led to the invention of the electric motor. Meanwhile, the ideas of classical physics were applied to the atmosphere, the stars, the speed of light, and the nature of heat, which developed into the science of thermodynamics. Geologists studying rock strata began to reconstruct Earth’s past. Paleontology became fashionable as the remains of extinct creatures began to turn up. Mary Anning, an untutored British girl, became a world-famous assembler of fossil remains. With the dinosaurs came ideas of evolution, most famously from British naturalist Charles Darwin, and new theories on the origins and ecology of life. Uncertainty and inﬁnity At the turn of the 20th century, a young German named Albert Einstein proposed his theory of relativity, shaking classical physics and ending the idea of an absolute time and space. New models of the atom were proposed; light was shown to act as both a particle and a wave; and another German, Werner Heisenberg, demonstrated that the universe was uncertain. What has been most impressive about the last century, however, is how technical advances have enabled science to advance faster than ever before, leap-frogging ideas with increasing precision. Ever more powerful particle colliders revealed new fundamental units of matter. Stronger telescopes showed that the universe is Reality is merely an illusion, albeit a very persistent one. Albert Einstein expanding, and started with a Big Bang. The idea of black holes began to take root. Dark matter and dark energy, whatever they were, seemed to ﬁll the universe, and astronomers began to discover new worlds—planets in orbit around distant stars, some of which may even harbor life. British mathematician Alan Turing thought of the universal computing machine, and within 50 years we had personal computers, the worldwide web, and smartphones. Secrets of life In biology, chromosomes were shown to be the basis of inheritance and the chemical structure of DNA was decoded. Just 40 years later this led to the human genome project, which seemed a daunting task in prospect, and yet, aided by computing, got faster and faster as it progressed. DNA sequencing is now an almost routine laboratory operation, gene therapy has moved from a hope into reality, and the ﬁrst mammal has been cloned. As today’s scientists build on these and other achievements, the relentless search for the truth continues. It seems likely that there will always be more questions than answers, but future discoveries will surely continue to amaze. ■ THE BEG OF SCIE 600 –14O0 BCE CE INNING NCE 18 INTRODUCTION Thales of Miletus predicts the eclipse of the Sun that brings the Battle of Halys to an end. Xenophanes ﬁnds seashells on mountains, and concludes that the whole Earth was once covered with water. Aristotle writes a string of books on subjects including physics, biology, and zoology. Aristarchus of Samos suggests that the Sun, rather than Earth, is the center of the universe. 585 BCE C.500 BCE C.325 BCE C.250 BCE C.530 BCE C.450 BCE C.300 BCE C.240 BCE Pythagoras founds a mathematical school at Croton in what is now southern Italy. Empedocles suggests that everything on Earth is made from combinations of earth, air, ﬁre, and water. Theophrastus writes Enquiry into plants and The causes of plants, founding the discipline of botany. Archimedes discovers that a king’s crown is not pure gold by measuring the upthrust of displaced water. T he scientiﬁc study of the world has its roots in Mesopotamia. Following the invention of agriculture and writing, people had the time to devote to study and the means to pass the results of those studies on to the next generation. Early science was inspired by the wonder of the night sky. From the fourth millennium BCE, Sumerian priests studied the stars, recording their results on clay tablets. They did not leave records of their methods, but a tablet dating from 1800 BCE shows knowledge of the properties of right-angled triangles. Ancient Greece The ancient Greeks did not see science as a separate subject from philosophy, but the ﬁrst ﬁgure whose work is recognizably scientiﬁc is probably Thales of Miletus, of whom Plato said that he spent so much time dreaming and looking at the stars that he once fell into a well. Possibly using data from earlier Babylonians, in 585 BCE, Thales predicted a solar eclipse, demonstrating the power of a scientiﬁc approach. Ancient Greece was not a single country, but rather a loose collection of city states. Miletus (now in Turkey) was the birthplace of several noted philosophers. Many other early Greek philosophers studied in Athens. Here, Aristotle was an astute observer, but he did not conduct experiments; he believed that, if he could bring together enough intelligent men, the truth would emerge. The engineer Archimedes, who lived at Syracuse on the island of Sicily, explored the properties of ﬂuids. A new center of learning developed at Alexandria, founded at the mouth of the Nile by Alexander the Great in 331 BCE. Here Eratosthenes measured the size of Earth, Ctesibius made accurate clocks, and Hero invented the steam engine. Meanwhile, the librarians in Alexandria collected the best books they could ﬁnd to build the best library in the world, which was burned down when Romans and Christians took over the city. Science in Asia Science ﬂourished independently in China. The Chinese invented gunpowder—and with it ﬁreworks, rockets, and guns—and made bellows for working metal. They invented the ﬁrst seismograph and the ﬁrst compass. In 1054 CE, THE BEGINNING OF SCIENCE 19 Eratosthenes, a friend of Archimedes, calculates the circumference of Earth from the shadows of the Sun at midday on midsummer day. Hipparchus discovers the precession of Earth’s orbit and compiles the Western world’s ﬁrst star catalogue. Claudius Ptolemy’s Almagest becomes the authoritative text on astronomy in the West, even though it contains many errors. Persian astronomer, Abd al-Rahman al-Suﬁ updates the Almagest, and gives many stars the Arabic names used today. C.240 BCE C.130 BCE C.150 CE 964 C.230 BCE Ctesibius builds clepsydras—water clocks—that remain for centuries the most accurate timepieces in the world. Chinese astronomers observed a supernova, which was identiﬁed as the Crab Nebula in 1731. Some of the most advanced technology in the ﬁrst millennium CE, including the spinning wheel, was developed in India, and Chinese missions were sent to study Indian farming techniques. Indian mathematicians developed what we now call the “Arabic” number system, including negative numbers and zero, and gave deﬁnitions of the trigonometric functions sine and cosine. The Golden Age of Islam In the middle of the 8th century, the Islamic Abbasid Caliphate moved the capital of its empire from Damascus to Baghdad. Guided by the Quranic slogan “The ink of a scholar is more holy than the blood C.120 CE In China, Zhang Heng discusses the nature of eclipses, and compiles a catalogue of 2,500 stars. 628 1021 Indian mathematician Brahmagupta outlines the ﬁrst rules to use the number zero. Alhazen, one of the ﬁrst experimental scientists, conducts original research on vision and optics. of a martyr,” Caliph Harun al-Rashid founded the House of Wisdom in his new capital, intending it to be a library and center for research. Scholars collected books from the old Greek city states and India and translated them into Arabic. This is how many of the ancient texts would eventually reach the West, where they were largely unknown in the Middle Ages. By the middle of the 9th century, the library in Baghdad had grown to become a ﬁne successor to the library at Alexandria. Among those who were inspired by the House of Wisdom were several astronomers, notably al-Suﬁ, who built on the work of Hipparchus and Ptolemy. Astronomy was of practical use to Arab nomads for navigation, since they steered their camels across the desert at night. Alhazen, born in Basra and educated in Baghdad, was one of the ﬁrst experimental scientists, and his book on optics has been likened in importance to the work of Isaac Newton. Arab alchemists devised distillation and other new techniques, and coined words such as alkali, aldehyde, and alcohol. Physician al-Razi introduced soap, distinguished for the ﬁrst time between smallpox and measles, and wrote in one of his many books “The doctor’s aim is to do good, even to our enemies.” Al-Khwarizmi and other mathematicians invented algebra and algorithms; and engineer al-Jazari invented the crank-connecting rod system, which is still used in bicycles and cars. It would take several centuries for European scientists to catch up with these developments. ■ 20 ECLIPSES OF THE SUN CAN BE PREDICTED THALES OF MILETUS (624–546 BCE) IN CONTEXT BRANCH Astronomy BEFORE c.2000 BCE European monuments such as Stonehenge may have been used to calculate eclipses. c.1800 BCE In ancient Babylon, astronomers produce the ﬁrst recorded mathematical description of the movement of heavenly bodies. 2nd millennium BCE Babylonian astronomers develop methods for predicting eclipses, but these are based on observations of the Moon, not mathematical cycles. AFTER c.140 BCE Greek astronomer Hipparchus develops a system to predict eclipses using the Saros cycle of movements of the Sun and Moon. B orn in a Greek colony in Asia Minor, Thales of Miletus is often viewed as the founder of Western philosophy, but he was also a key ﬁgure in the early development of science. He was recognized in his lifetime for his thinking on mathematics, physics, and astronomy. Perhaps Thales’s most famous achievement is also his most controversial. According to the Greek historian Herodotus, writing more than a century after the event, Thales is said to have predicted a …day became night, and this change of the day Thales the Milesian had foretold… Herodotus solar eclipse, now dated to May 28, 585 BCE, which famously brought a battle between the warring Lydians and Medes to a halt. Contested history Thales’s achievement was not to be repeated for several centuries, and historians of science have long argued about how, and even if, he achieved it. Some argue that Herodotus’s account is inaccurate and vague, but Thales’s feat seems to have been widely known and was taken as fact by later writers, who knew to treat Herodotus’s word with caution. Assuming it is true, it is likely that Thales had discovered an 18-year cycle in the movements of the Sun and Moon, known as the Saros cycle, which was used by later Greek astronomers to predict eclipses. Whatever method Thales used, his prediction had a dramatic effect on the battle at the river Halys, in modern-day Turkey. The eclipse ended not only the battle, but also a 15-year war between the Medes and the Lydians. ■ See also: Zhang Heng 26–27 ■ Nicolaus Copernicus 34–39 Johannes Kepler 40–41 ■ Jeremiah Horrocks 52 ■ THE BEGINNING OF SCIENCE 21 NOW HEAR THE FOURFOLD ROOTS OF EVERYTHING EMPEDOCLES (490–430 BCE) IN CONTEXT BRANCH Chemistry BEFORE c.585 BCE Thales suggests the whole world is made of water. c.535 BCE Anaximenes thinks that everything is made from air, from which water and then stones are made. AFTER c.400 BCE The Greek thinker Democritus proposes that the world is ultimately made of tiny indivisible particles—atoms. 1661 In his work Sceptical Chymist, Robert Boyle provides a deﬁnition of elements. 1808 John Dalton’s atomic theory states that each element has atoms of different masses. 1869 Dmitri Mendeleev proposes a periodic table, arranging the elements in groups according to their shared properties. T he nature of matter concerned many ancient Greek thinkers. Having seen liquid water, solid ice, and gaseous mist, Thales of Miletus believed that everything must be made of water. Aristotle suggested that “nourishment of all things is moist and even the hot is created from the wet and lives by it.” Writing two generations after Thales, Anaximenes suggested that the world is made of air, reasoning that when air condenses it produces mist, and then rain, and eventually stones. Born at Agrigentum on the island of Sicily, the physician and poet Empedocles devised a more complex theory: that everything is made of four roots—he did not use the word elements—namely earth, air, ﬁre, and water. Combining these roots would produce qualities such as heat and wetness to make earth, stone, and all plants and animals. Originally, the four roots formed a perfect sphere, held together by love, the centripetal force. But gradually strife, the See also: Robert Boyle 46–49 ■ Empedocles saw the four roots of matter as two pairs of opposites: ﬁre/water and air/earth, which combine to produce everything we see. Fire Hot Dry Air Earth Cold Wet Water centrifugal force, began to pull them apart. For Empedocles, love and strife are the two forces that shape the universe. In this world, strife tends to predominate, which is why life is so difﬁcult. This relatively simple theory dominated European thought— which referred to the “four humors”—with little reﬁnement until the development of modern chemistry in the 17th century. ■ John Dalton 112–13 ■ Dmitri Mendeleev 174–79 22 MEASURING THE CIRCUMFERENCE OF EARTH ERATOSTHENES (276–194 ) BCE IN CONTEXT BRANCH Geography BEFORE 6th century BCE Greek mathematician Pythagoras suggests Earth may be spherical, not ﬂat. 3rd century BCE Aristarchus of Samos is the ﬁrst to place the Sun at the center of the known universe and uses a trigonometric method to estimate the relative sizes of the Sun and the Moon and their distances from Earth. Late 3rd century BCE Eratosthenes introduces the concepts of parallels and meridians to his maps (equivalent to modern longitude and latitude). AFTER 18th century The true circumference and shape of Earth is found through enormous efforts by French and Spanish scientists. T he Greek astronomer and mathematician Eratosthenes is best remembered as the ﬁrst person to measure the size of Earth, but he is also regarded as the founder of geography—not only coining the word, but also establishing many of the basic principles used to measure locations on our planet. Born at Cyrene (in modern-day Libya), Eratosthenes traveled widely in the Greek world, studying in Athens and Alexandria, and eventually becoming the librarian of Alexandria’s Great Library. It was in Alexandria that Eratosthenes heard a report that at the town of Swenet, south of Alexandria, the Sun passed directly overhead on the summer solstice (the longest day of the year, when the Sun rises highest in the sky). Assuming the Sun was so distant that its rays were almost parallel to each other when they hit Earth, he used a vertical rod, or “gnomon,” to project the Sun’s shadow at the same moment in Alexandria. Here, he determined, the Sun was See also: Nicolaus Copernicus 34–39 ■ 7.2° south of the zenith—which is 1/50th of the circumference of a circle. Therefore, he reasoned, the separation of the two cities along a north–south meridian must be 1/50th of Earth’s circumference. This allowed him to ﬁgure out the size of our planet at 230,000 stadia, or 24,662 miles (39,690 km)—an error of less than 2 percent. ■ Sunlight reached Swenet at right angles, but cast a shadow at Alexandria. The angle of the shadow cast by the gnomon allowed Eratosthenes to calculate Earth’s circumference. 7.2˚ Alexandria 7.2˚ Gnomon Swenet Earth Sunrays Johannes Kepler 40–41 THE BEGINNING OF SCIENCE 23 THE HUMAN IS RELATED TO THE LOWER BEINGS AL-TUSI (1201–1274) IN CONTEXT BRANCH Biology BEFORE c.550 BCE Anaximander of Miletus proposes that animal life began in the water, and evolved from there. c.340 BCE Plato’s theory of forms argues that species are unchangeable. c.300 BCE Epicurus says that many other species have been created in the past, but only the most successful survive to have offspring. AFTER 1377 Ibn Khaldun writes in Muqaddimah that humans developed from monkeys. 1809 Jean-Baptiste Lamarck proposes a theory of evolution of species. 1858 Alfred Russel Wallace and Charles Darwin suggest a theory of evolution by means of natural selection. A Persian scholar born in Baghdad in 1201, during the Golden Age of Islam, Nazir al-Din al-Tusi was a poet, philosopher, mathematician, and astronomer, and one of the ﬁrst to propose a system of evolution. He suggested that the universe had once comprised identical elements that had gradually drifted apart, with some becoming minerals and others, changing more quickly, developing into plants and animals. In Akhlaq-i-Nasri, al-Tusi’s work on ethics, he set out a hierarchy of life forms, in which animals were higher than plants and humans were higher than other animals. He regarded the conscious will of animals as a step toward the consciousness of humans. Animals are able to move consciously to search for food, and can learn new things. In this ability to learn, al-Tusi saw an ability to reason: “The trained horse or hunting falcon is at a higher point of development in the animal world,” he said, adding, “The ﬁrst steps of human perfection begin from here.” The organisms that can gain the new features faster are more variable. As a result, they gain advantages over other creatures. al-Tusi Al-Tusi believed that organisms changed over time, seeing in that change a progression toward perfection. He thought of humans as being on a “middle step of the evolutionary stairway,” potentially able by means of their will to reach a higher developmental level. He was the ﬁrst to suggest that not only do organisms change over time, but that the whole range of life has evolved from a time when there was no life at all. ■ See also: Carl Linnaeus 74–75 ■ Jean-Baptiste Lamarck 118 Charles Darwin 142–49 ■ Barbara McClintock 271 ■ 24 A FLOATING OBJECT DISPLACES ITS OWN VOLUME IN LIQUID ARCHIMEDES (287–212 ) BCE IN CONTEXT BRANCH Physics BEFORE 3rd millennium BCE Metalworkers discover that melting metals and mixing them together produces an alloy that is stronger than either of the original metals. 600 BCE In ancient Greece, coins are made from an alloy of gold and silver called electrum. AFTER 1687 In his Principia Mathematica, Isaac Newton outlines his theory of gravity, explaining how there is a force that pulls everything toward the center of Earth—and vice versa. 1738 Swiss mathematician Daniel Bernoulli develops his kinetic theory of ﬂuids, explaining how ﬂuids exert pressure on objects by the random movement of molecules in the ﬂuid. T he Roman author Vitruvius, writing in the 1st century BCE, recounts the possibly apocryphal story of an incident that happened two centuries earlier. Hieron II, the King of Sicily, had ordered a new gold crown. When the crown was delivered, Hieron suspected that the crown maker had substituted silver for some of the gold, melting the silver with the remaining gold so that the color looked the same as pure gold. The king asked his chief scientist, Archimedes, to investigate. Archimedes puzzled over the problem. The new crown was precious, and must not be damaged Silver is less dense than gold, so a lump of silver will have a greater volume than a lump of gold of the same weight. A crown made partly of silver will have greater volume and displace more water than a lump of pure gold of the same weight as the crown. The difference in upthrust between the two is small, but it can be detected if you hang them on a balance in water. The displaced water causes an upthrust. The partly silver crown experiences a greater upthrust than the gold. Eureka! THE BEGINNING OF SCIENCE 25 See also: Nicolaus Copernicus 34–39 ■ in any way. He went to the public baths in Syracuse to ponder the problem. The bath was full to the brim, and when he climbed in, he noticed two things: the water level rose, making some water slop over the side, and he felt weightless. He shouted “Eureka!” (I have found the answer!) and ran home stark naked. Measuring volume Archimedes had realized that if he lowered the crown into a bucket ﬁlled to the brim with water, it would displace some water— exactly the same amount as its own volume—and he could measure how much water spilled out. This would tell him the volume of the crown. Silver is less dense than gold, so a silver crown of the same weight would be bigger than a gold crown, and would displace more water. Therefore, an adulterated crown would displace more water than a pure gold crown—and more than a lump of gold of the same weight. In practice, the effect would have been small and difﬁcult to measure. But Archimedes had also Archimedes Isaac Newton 62–69 realized that any object immersed in a liquid experiences an upthrust (upward force) equal to the weight of the liquid it has displaced. Archimedes probably solved the puzzle by hanging the crown and an equal weight of pure gold on opposite ends of a stick, which he then suspended by its center so that the two weights balanced. Then he lowered the whole thing into a bath of water. If the crown was pure gold, it and the lump of gold would experience an equal upthrust, and the stick would stay horizontal. If the crown contained some silver, however, the volume of the crown would be greater than the volume of the lump of gold—the crown would displace more water, and the stick would tilt sharply. Archimedes’ idea became known as Archimedes’ principle, which states that the upthrust on an object in a ﬂuid is equal to the weight of the ﬂuid the object displaces. This principle explains how objects made of dense material can still ﬂoat on water. A steel ship that weighs one ton will sink until Archimedes was possibly the greatest mathematician in the ancient world. Born around 287 BCE, he was killed by a soldier when his home town Syracuse was taken by the Romans in 212 BCE. He had devised several fearsome weapons to keep at bay the Roman warships that attacked Syracuse—a catapult, a crane to lift the bows of a ship out of the water, and a death array of mirrors to focus the Sun’s rays and set ﬁre to a ship. He probably invented the Archimedes screw, still used today for irrigation, during a stay in Egypt. A solid heavier than a ﬂuid will, if placed in it, descend to the bottom of the ﬂuid, and the solid will, when weighed in the ﬂuid, be lighter than its true weight by the weight of the ﬂuid displaced. Archimedes it has displaced one ton of water, but then will sink no further. Its deep, hollow hull has a greater volume and displaces more water than a lump of steel of the same weight, and is therefore buoyed up by a greater upthrust. Vitruvius tells us that Hieron’s crown was indeed found to contain some silver, and that the crown maker was duly punished. ■ Archimedes also calculated an approximation for pi (the ratio of a circle’s circumference to its diameter), and wrote down the laws of levers and pulleys. The achievement Archimedes was most proud of was a mathematical proof that the smallest cylinder that any given sphere can ﬁt into has exactly 1.5 times the sphere’s volume. A sphere and a cylinder are carved into Archimedes’ tombstone. Key work c.250 BCE On Floating Bodies 26 THE SUN IS LIKE FIRE, THE MOON IS LIKE WATER ZHANG HENG (78–139 CE) IN CONTEXT BRANCH Physics BEFORE 140 BCE Hipparchus ﬁgures out how to predict eclipses. 150 CE Ptolemy improves on Hipparchus’s work, and produces practical tables for calculating the future positions of the celestial bodies. AFTER 11th century Shen Kuo writes the Dream Pool Essays, in which he uses the waxing and waning of the Moon to demonstrate that all heavenly bodies (though not Earth) are spherical. During the day Earth is bright, with shadows, because of sunlight. The Moon is sometimes bright, with shadows. The Moon must be bright because of sunlight. 1543 Nicolaus Copernicus publishes On the Revolutions of the Celestial Spheres, in which he describes a heliocentric system. 1609 Johannes Kepler explains the movements of the planets as free-ﬂoating bodies describing ellipses. Therefore the Sun is like ﬁre, the Moon like water. I n about 140 BCE, the Greek astronomer Hipparchus, probably the ﬁnest astronomer of the ancient world, compiled a catalogue of some 850 stars. He also explained how to predict the movements of the Sun and Moon and the dates of eclipses. In his work Almagest of about 150 CE, Ptolemy of Alexandria listed 1,000 stars and 48 constellations. Most of this work was effectively an updated version of what Hipparchus had written, but in a more practical form. In the West, the Almagest became the standard astronomy text throughout the Middle Ages. Its tables included all the information needed to calculate the future positions of the Sun and Moon, the planets and the major stars, and also eclipses of the Sun and Moon. In 120 CE, the Chinese polymath Zhang Heng produced a work entitled Ling Xian, or The Spiritual Constitution of the Universe. In it, he wrote that “the sky is like a hen’s egg, and is as round as a crossbow pellet, and Earth is like the yolk of the egg, lying alone at the center. The sky is large and the Earth small.” This was, following Hipparchus and Ptolemy, a universe THE BEGINNING OF SCIENCE 27 See also: Nicolaus Copernicus 34–39 Isaac Newton 62–69 ■ Johannes Kepler 40–41 ■ Eclipses of the Moon and planets Sun, and the Moon’s darkness is due to the light of the Sun being obstructed. The side that faces the Sun is fully lit, and the side that is away from it is dark.” Zhang also described a lunar eclipse, where the Sun’s light cannot reach the Moon because Earth is in the way. He recognized that the planets were also “like water,” reﬂecting light, and so were also subject to eclipses: “When [a similar effect] happens with a planet, we call it an occultation; when the Moon passes across the Sun’s path then there is a solar eclipse.” In the 11th century, another Chinese astronomer, Shen Kuo, expanded on Zhang’s work in one signiﬁcant respect. He showed that observations of the waxing and waning of the Moon proved that the celestial bodies were spherical. ■ Zhang was fascinated by eclipses. He wrote, “The Sun is like ﬁre and the Moon like water. The ﬁre gives out light and the water reﬂects it. Thus the Moon’s brightness is produced from the radiance of the The crescent outline of Venus is about to be occulted by the Moon. Zhang’s observations led him to conclude that, like the Moon, the planets did not produce their own light. The Moon and the planets are Yin; they have shape but no light. Jing Fang with Earth at its center. Zhang catalogued 2,500 “brightly shining” stars and 124 constellations, and added that “of the very small stars there are 11,520.” Zhang Heng Zhang Heng was born in 78 CE in the town of Xi’e, in what is now Henan Province, in Han Dynasty China. At 17, he left home to study literature and train to be a writer. By his late 20s, Zhang had become a skilled mathematician and was called to the court of Emperor An-ti, who, in 115 CE, made him Chief Astrologer. Zhang lived at a time of rapid advances in science. In addition to his astronomical work, he devised a waterpowered armillary sphere (a model of the celestial objects) and invented the world’s ﬁrst seismometer, which was ridiculed until, in 138 CE, it successfully recorded an earthquake 250 miles (400 km) away. He also invented the ﬁrst odometer to measure distances traveled in vehicles, and a nonmagnetic, southpointing compass in the form of a chariot. Zhang was a distinguished poet, whose works give us vivid insights into the cultural life of his day. Key works c.120 CE The Spiritual Constitution of the Universe c.120 CE The Map of the Ling Xian 28 LIGHT TRAVELS IN STRAIGHT LINES INTO OUR EYES ALHAZEN (c.965–1040) IN CONTEXT BRANCH Physics The light of the Sun bounces off objects. The light bounces off in straight lines. Light travels in straight lines into our eyes. To see, we need to do nothing but open our eyes. BEFORE 350 BCE Aristotle argues that vision derives from physical forms entering the eye from an object. 300 BCE Euclid argues that the eye sends out beams that are bounced back to the eye. 980s Ibn Sahl investigates refraction of light and deduces the laws of refraction. AFTER 1240 English bishop Robert Grosseteste uses geometry in his experiments with optics and accurately describes the nature of color. 1604 Johannes Kepler’s theory of the retinal image is based directly on Alhazen’s work. 1620s Alhazen’s ideas inﬂuence Francis Bacon, who advocates a scientiﬁc method based on experiment. T he Arab astronomer and mathematician Alhazen, who lived in Baghdad, in present-day Iraq, during the Golden Age of Islamic civilization, was arguably the world’s ﬁrst experimental scientist. While earlier Greek and Persian thinkers had explained the natural world in various ways, they had arrived at their conclusions through abstract reasoning, not through physical experiments. Alhazen, working in a thriving Islamic culture of curiosity and inquiry, was the ﬁrst to use what we now call the scientiﬁc method: setting up hypotheses and methodically testing them with experiments. As he observed: “The seeker after truth is not one who studies the writings of the ancients and…puts his trust in them, but rather the one who suspects his faith in them and questions what he gathers from them, the one who submits to argument and demonstration.” Understanding vision Alhazen is remembered today as a founder of the science of optics. His most important works were studies of the structure of the eye and the process of vision. The THE BEGINNING OF SCIENCE 29 See also: Johannes Kepler 40–41 ■ Francis Bacon 45 Object ■ Christiaan Huygens 50–51 Image is upside down and back to front Pinhole ■ Isaac Newton 62–69 and is focused by a lens onto a sensitive surface (the retina) at the back of the eye. However, even though he recognized the eye as a lens, he did not explain how the eye or the brain forms an image. Experiments with light Light rays travel from the object Greek scholars Euclid and, later, Ptolemy believed that vision derived from “rays” that beamed out of the eye and bounced back from whatever a person was looking at. Alhazen showed, through the observation of shadows and reﬂection, that light bounces off objects and travels in straight lines into our eyes. Vision was a passive, rather than an active, phenomenon, at least until it reached the retina. Alhazen provided the ﬁrst scientiﬁc description of a camera obscura, an optical device that projects an upside-down image on a screen. He noted that, “from each point of every colored body, illuminated by any light, issue light and color along every straight line that can be drawn from that point.” In order to see things, we have only to open our eyes to let in the light. There is no need for the eye to send out rays, even if it could. Alhazen also found, through his experiments with bulls’ eyes, that light enters a small hole (the pupil) Alhazen The duty of the man who investigates the writings of scientists, if learning the truth is his goal, is to make himself an enemy of all that he reads. Alhazen Abu Ali al-Hassan ibn alHaytham (known in the West as Alhazen) was born in Basra, in present-day Iraq, and educated in Baghdad. As a young man he was given a government job in Basra, but soon became bored. One story has it that, on hearing about the problems resulting from the annual ﬂooding of the Nile in Egypt, he wrote to Caliph al-Hakim offering to build a dam to regulate the deluge, and was received with honor in Cairo. However, when he Alhazen’s monumental, sevenvolume Book of Optics set out his theory of light and his theory of vision. It remained the main authority on the subject until Newton’s Principia was published 650 years later. The book explores the interaction of light with lenses, and describes the phenomenon of refraction (change in the direction) of light—700 years before Dutch scientist Willebrord van Roijen Snell’s law of refraction. It also examines the refraction of light by the atmosphere, and describes shadows, rainbows, and eclipses. Optics greatly inﬂuenced later Western scientists, including Francis Bacon, one of the scientists responsible for reviving Alhazen’s scientiﬁc method during the Renaissance in Europe. ■ traveled south of the city, and saw the sheer size of the river— which is almost 1 mile (1.6 km) wide at Aswan—he realized the task was impossible with the technology then available. To avoid the caliph’s retribution he feigned insanity and remained under house arrest for 12 years. In that time he did his most important work. Key works 1011–21 Book of Optics c.1030 A Discourse on Light c.1030 On the Light of the Moon SCIENTI REVOLU 1400 –1700 FIC TION 32 INTRODUCTION Nicolaus Copernicus publishes De Revolutionibus Orbium Coelestium, outlining a heliocentric universe. 1543 Johannes Kepler suggests that Mars has an elliptical orbit. Francis Bacon publishes Novum Organum Scientarum and The New Atlantis, outlining the scientiﬁc method. Evangelista Torricelli invents the barometer. 1609 1620S 1643 1600 1610 1639 Astronomer William Gilbert publishes De Magnete, a treatise on magnetism, and suggests that Earth is a magnet. Galileo observes the moons of Jupiter and experiments with balls rolling down slopes. Jeremiah Horrocks observes the transit of Venus. T he Islamic Golden Age was a great ﬂowering of the sciences and arts that began in the capital of the Abbasid Caliphate, Baghdad, in the mid-8th century and lasted for about 500 years. It laid the foundations for experimentation and the modern scientiﬁc method. In the same period in Europe, however, several hundred years were to pass before scientiﬁc thought was to overcome the restrictions of religious dogma. Dangerous thinking For centuries, the Catholic Church’s view of the universe was based on Aristotle’s idea that Earth was at the orbital center of all celestial bodies. Then, in about 1532, after years of struggling with its complex mathematics, Polish physician Nicolaus Copernicus completed his heretical model of the universe that had the Sun at its center. Aware of the heresy, he was careful to state that it was only a mathematical model, and he waited until he was on the point of death before publishing, but the Copernican model quickly won many advocates. German astrologer Johannes Kepler reﬁned Copernicus’s theory using observations by his Danish mentor Tycho Brahe, and calculated that the orbits of Mars and, by inference, the other planets were ellipses. Improved telescopes allowed Italian polymath Galileo Galilei to identify four moons of Jupiter in 1610. The new cosmology’s explanatory power was becoming undeniable. Galileo also demonstrated the power of scientiﬁc experiment, investigating the physics of falling 1660S Robert Boyle publishes New Experiments Physico-Mechanical: Touching the Spring of the Air, and its Effects, investigating air pressure. objects and devising the pendulum as an effective timekeeper, which Dutchman Christiaan Huygens used to build the ﬁrst pendulum clock in 1657. English philosopher Francis Bacon wrote two books laying out his ideas for a scientiﬁc method, and the theoretical groundwork for modern science, based on experiment, observation, and measurement, was developed. New discoveries followed thick and fast. Robert Boyle used an air pump to investigate the properties of air, while Huygens and English physicist Isaac Newton came up with opposing theories of how light travels, establishing the science of optics. Danish astronomer Ole Rømer noted discrepancies in the timetable of eclipses of the moons of Jupiter, and used these to calculate an approximate value SCIENTIFIC REVOLUTION 33 In Micrographia, Robert Hooke introduces the world to the anatomy of ﬂeas, bees, and cork. 1665 Jan Swammerdam describes how insects develop in stages in Historia Insectorum Generalis. Ole Rømer uses the moons of Jupiter to show that light has a ﬁnite speed. John Ray publishes Historia Plantarum, an encyclopedia of the plant kingdom. 1669 1676 1686 1669 Nicolas Steno writes about solids (fossils and crystals) contained within solids. for the speed of light. Rømer’s compatriot, Bishop Nicolas Steno, was sceptical of much ancient wisdom, and developed his own ideas in both anatomy and geology. He laid down the principles of stratigraphy (the study of rock layers), establishing a new scientiﬁc basis for geology. Microworlds Throughout the 17th century, developments in technology drove scientiﬁc discovery at the smallest scale. In the early 1600s, Dutch eyeglasses-makers developed the ﬁrst microscopes, and, later that century, Robert Hooke built his own and made beautiful drawings of his ﬁndings, revealing the intricate structure of tiny bugs such as ﬂeas for the ﬁrst time. Dutch fabric-store owner 1670S Antonie van Leeuwenhoek observes single-celled organisms, sperm, and even bacteria with simple microscopes. 1678 1687 Christiaan Huygens ﬁrst announces his wave theory of light, which will later contrast with Isaac Newton’s idea of light as corpuscular. Isaac Newton outlines his laws of motion in Philosophiae Naturalis Principia Mathematica. Antonie van Leeuwenhoek, perhaps inspired by Hooke’s drawings, made hundreds of his own microscopes and found tiny life forms in places where no one had thought of looking before, such as water. Leeuwenhoek had discovered single-celled life forms such as protists and bacteria, which he called “animalcules.” When he reported his ﬁndings to the British Royal Society, they sent three priests to certify that he had really seen such things. Dutch microscopist Jan Swammerdam showed that egg, larva, pupa, and adult are all stages in the development of an insect, and not separate animals created by God. Old ideas dating back to Aristotle were swept away by these new discoveries. Meanwhile, English biologist John Ray compiled an enormous encyclopedia of plants, which marked the ﬁrst serious attempt at systematic classiﬁcation. Mathematical analysis Heralding the Enlightenment, these discoveries laid the groundwork for the modern scientiﬁc disciplines of astronomy, chemistry, geology, physics, and biology. The century’s crowning achievement came with Newton’s treatise Philosophiæ Naturalis Principia Mathematica, which laid out his laws of motion and gravity. Newtonian physics was to remain the best description of the physical world for more than two centuries, and together with the analytical techniques of calculus developed independently by Newton and Gottfried Wilhelm Leibniz, it would provide a powerful tool for future scientiﬁc study. ■ AT THE CENTER SUN OF EVERYTHING IS THE NICOLAUS COPERNICUS (1473 –1543) 36 NICOLAUS COPERNICUS IN CONTEXT BRANCH Astronomy BEFORE 3rd century BCE In a work called The Sand Reckoner, Archimedes reports the ideas of Aristarchus of Samos, who proposed that the universe was much larger than commonly believed, and that the Sun was at its center. 150 CE Ptolemy of Alexandria uses mathematics to describe a geocentric (Earth-centered) model of the universe. AFTER 1609 Johannes Kepler resolves the outstanding conﬂicts in the heliocentric (Sun-centered) model of the solar system by proposing elliptical orbits. 1610 After observing the moons of Jupiter, Galileo becomes convinced that Copernicus was right. T hroughout its early history, Western thought was shaped by an idea of the universe that placed Earth at the center of everything. This “geocentric model” seemed at ﬁrst to be rooted in everyday observations and common sense— we do not feel any motion of the ground on which we stand, and superﬁcially there seems to be no observational evidence that our planet is in motion either. Surely the simplest explanation was that the Sun, Moon, planets and stars were all spinning around Earth at different rates? This system appears to have been widely accepted in the ancient world, and became entrenched in classical philosophy through the works of Plato and Aristotle in the 4th century BCE. However, when the ancient Greeks measured the movements of the planets, it became clear that the geocentric system had problems. The orbits of the known planets—ﬁve wandering lights in the sky—followed complex paths. Mercury and Venus were always seen in the morning and evening skies, describing tight loops around Earth appears to be stationary, with the Sun, Moon, planets, and stars orbiting it. However, a model of the universe with Earth at its center cannot describe the movement of the planets without using a very complicated system. If the Lord Almighty had consulted me before embarking on creation thus, I should have recommended something simpler. Alfonso X King of Castile the Sun. Mars, Jupiter, and Saturn, meanwhile, took 780 days, 12 years, and 30 years respectively to circle against the background stars, their motion complicated by “retrograde” loops in which they slowed and temporarily reversed the general direction of their motion. Ptolemaic system To explain these complications, Greek astronomers introduced the idea of epicycles—“sub-orbits” around which the planets circled as the central “pivot” points of the Placing the Sun at the center produces a far more elegant model, with Earth and the planets orbiting the Sun, and the stars a huge distance away. At the center of everything is the Sun. SCIENTIFIC REVOLUTION 37 See also: Zhang Heng 26–27 Edwin Hubble 236–41 ■ Johannes Kepler 40–41 sub-orbits were carried around the Sun. This system was best reﬁned by the great Greco-Roman astronomer and geographer Ptolemy of Alexandria in the 2nd century CE. Even in the classical world, however, there were differences of opinion—the Greek thinker Aristarchus of Samos, for instance, used ingenious trigonometric measurements to calculate the relative distances of the Sun and Moon in the 3rd century BCE. He found that the Sun was huge, and this inspired him to suggest that the Sun was a more likely pivot point for the motion of the cosmos. However, the Ptolemaic system ultimately won out over rival theories, with far-reaching implications. While the Roman ■ Galileo Galilei 42–43 ■ Empire dwindled in subsequent centuries, the Christian Church inherited many of its assumptions. The idea that Earth was the center of everything, and that man was the pinnacle of God’s creation, with dominion over Earth, became a central tenet of Christianity and held sway in Europe until the 16th century. However, this does not mean that astronomy stagnated for a millennium and a half after Ptolemy. The ability to accurately predict the movements of the planets was not only a scientiﬁc and philosophical puzzle, but also had supposed practical purposes thanks to the superstitions of astrology. Stargazers of all persuasions had good reason Sun Mars Mercury William Herschel 86–87 ■ to attempt ever more accurate measurements of the motions of the planets. Arabic scholarship The later centuries of the ﬁrst millennium corresponded with the ﬁrst great ﬂowering of Arabic science. The rapid spread of Islam across the Middle East and North Africa from the 7th century brought Arab thinkers into contact with classical texts, including the astronomical writings of Ptolemy and others. The practice of “positional astronomy”—calculating the positions of heavenly bodies— reached its apogee in Spain, which had become a dynamic melting pot of Islamic, Jewish, and Christian thought. In the late 13th century, King Alfonso X of Castile sponsored the compilation of the Alfonsine Tables, which combined new observations with centuries of Islamic records to bring new precision to the Ptolemaic system and provide the data that would be used to calculate planetary positions until the early 17th century. Venus Questioning Ptolemy Earth Moon Jupiter Saturn Ptolemy’s model of the universe has Earth unmoving at the center, with the Sun, Moon, and the ﬁve known planets following circular orbits around it. To make their orbits agree with observations, Ptolemy added smaller epicycles to each planet’s movement. However, by this point the Ptolemaic model was becoming absurdly complicated, with yet more epicycles added to keep prediction in line with observation. In 1377, French philosopher Nicole Oresme, Bishop of Lisieux, addressed this problem head-on in the work Livre du Ciel et du Monde (Book of the Heavens and the Earth). He demonstrated the lack of observational proof that Earth was static, and argued that there was no reason to suppose that it ❯❯ 38 NICOLAUS COPERNICUS was not in motion. Yet, despite his demolition of the evidence for the Ptolemaic system, Oresme concluded that he did not himself believe in a moving Earth. By the beginning of the 16th century, the situation had become very different. The twin forces of the Renaissance and the Protestant Reformation saw many old religious dogmas opened up to question. It was in this context that Nicolaus Copernicus, a Polish Catholic canon from the province of Warmia, put forward the ﬁrst modern heliocentric theory, shifting the center of the universe from Earth to the Sun. Copernicus ﬁrst published his ideas in a short pamphlet known as the Commentariolus, circulated among friends from around 1514. His theory was similar in essence to the system proposed by Aristarchus, and while it overcame many of the earlier model’s failings, it remained deeply attached to certain pillars of Ptolemaic thought—most signiﬁcantly the idea that the orbits of celestial objects were mounted on crystalline spheres that rotated in perfect circular motion. As a result, Copernicus had to introduce “epicycles” of his own in order to regulate the speed of planetary Since the Sun remains stationary, whatever appears as a motion of the Sun is due to the motion of the Earth. Nicolaus Copernicus motions on certain parts of their orbits. One important implication of his model was that it vastly increased the size of the universe. If Earth was moving around the Sun, then this should give itself away through parallax effects caused by our changing point of view: the stars should appear to shift back and forth across the sky throughout the year. Because they do not do so, they must be very far away indeed. The Copernican model soon proved itself far more accurate than any reﬁnement of the old Ptolemaic system, and word spread among intellectual circles across Europe. Notice even reached Rome, where, contrary to popular belief, the model was at ﬁrst welcomed in some Catholic circles. The new model caused enough of a stir for German mathematician Georg Joachim Rheticus to travel to Warmia and become Copernicus’s pupil and assistant from 1539. This 17th-century illustration of the Copernican system shows the planets in circular orbits around the Sun. Copernicus believed that the planets were attached to heavenly spheres. It was Rheticus who published the ﬁrst widely circulated account of the Copernican system, known as the Narratio Prima, in 1540. Rheticus urged the aging priest to publish his own work in full— something that Copernicus had contemplated for many years, but only conceded to in 1543 as he lay on his deathbed. Mathematical tool Published posthumously, De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres) was not initially greeted with outrage, even though any suggestion that Earth was in motion directly contradicted several passages of Scripture and was SCIENTIFIC REVOLUTION 39 therefore regarded as heretical by both Catholic and Protestant theologians. To sidestep the issue, a preface had been inserted that explained the heliocentric model as purely a mathematical tool for prediction, not a description of the physical universe. In his life, however, Copernicus himself had shown no such reservations. Despite its heretical implications, the Copernican model was used for the calculations involved in the great calendar reform introduced by Pope Gregory XIII in 1582. However, new problems with the model’s predictive accuracy soon began to emerge, thanks to the meticulous observations of the Danish astronomer Tycho Brahe (1546–1601), which showed that the Copernican model did not adequately describe planetary motions. Brahe attempted to resolve these contradictions with a model of his own in which the planets went around the Sun but the Sun and Moon remained in orbit around Earth. The real solution—that of elliptical orbits— would only be found by his pupil Johannes Kepler. It would be six decades before Copernicanism became truly emblematic of the split caused in Europe by the Reformation of the Earth in January Sun As though seated on a royal throne, the Sun governs the family of planets revolving around it. Nicolaus Copernicus Nicolaus Copernicus Church, thanks largely to the controversy surrounding Italian scientist Galileo Galilei. Galileo’s 1610 observations of the phases displayed by Venus and the presence of moons orbiting Jupiter convinced him that the heliocentric theory was correct, and his ardent support for it, from the heart of Catholic Italy, was ultimately expressed in his Dialogue Concerning the Two Chief World Systems (1632). This led Galileo into conﬂict with the papacy, one result of which was the retrospective censorship of controversial passages in De Revolutionibus in 1616. This prohibition would not be lifted for more than two centuries. ■ As Earth moves around the Sun, the apparent position of stars at different distances changes due to an effect called parallax. Since the stars are so far away, the effect is small and can only be detected using telescopes. Near Key works star 1514 Commentariolus 1543 De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres) Apparent position Earth in July Born in the Polish city of Torun in 1473, Nicolaus Copernicus was the youngest of four children of a wealthy merchant. His father died when Nicolaus was 10. An uncle took him under his wing and oversaw his education at the University of Krakow. He spent several years in Italy studying medicine and law, returning in 1503 to Poland, where he joined the canonry under his uncle, who was now Prince-Bishop of Warmia. Copernicus was a master of both languages and mathematics, translating several important works and developing ideas about economics, as well as working on his astronomical theories. The theory he outlined in De Revolutionibus was daunting in its mathematical complexity, so while many recognized its signiﬁcance, it was not widely adopted by astronomers for practical everyday use. Distant stars 40 THE ORBIT OF EVERY PLANET IS AN ELLIPSE JOHANNES KEPLER (1571–1630) IN CONTEXT BRANCH Astronomy BEFORE 150 CE Ptolemy of Alexandria publishes the Algamest, a model of the universe built on the assumption that Earth lies at its center and the Sun, Moon, planets and stars revolve around it in circular orbits on ﬁxed celestial spheres. 16th century The idea of a Sun-centered cosmology begins to gain followers through the ideas of Nicolaus Copernicus. AFTER 1639 Jeremiah Horrocks uses Kepler’s ideas to predict and view a transit of Venus across the face of the Sun. 1687 Isaac Newton’s laws of motion and gravitation reveal the physical principles that give rise to Kepler’s laws. W hile the work of Nicolaus Copernicus on celestial orbits, published in 1543, made a convincing case for a heliocentric (Sun-centered) model of the universe, his system suffered from signiﬁcant problems. Unable to break free from ancient ideas that heavenly bodies were mounted on crystal spheres, Copernicus had stated that the planets orbited the Sun on perfect circular paths, and was forced to introduce a variety of complications to his model to account for their irregularities. Supernova and comets In the latter half of the 16th century, Danish nobleman Tycho Brahe (1546–1601) made observations that The birth of a new star in a constellation shows that the heavens beyond the planets are not unchanging. Observations of comets show that they move among the planets, crossing their orbits. If the planets are not ﬁxed onto spheres, an elliptical orbit around the Sun best explains their observed motion. This suggests that heavenly bodies are not attached to ﬁxed celestial spheres. The orbit of every planet is an ellipse. SCIENTIFIC REVOLUTION 41 See also: Nicolaus Copernicus 34–39 Isaac Newton 62–69 ■ would prove vital to resolving the problems. A bright supernova explosion seen in the constellation of Cassiopeia in 1572 undermined the Copernican idea that the universe beyond the planets was unchanging. In 1577, Brahe plotted the motion of a comet. Comets had been thought of as local phenomena, closer than the Moon, but Brahe’s observations showed that the comet must lie well beyond the Moon, and was in fact moving among the planets. In one stroke, this evidence demolished the idea of “heavenly spheres.” However, Brahe remained wedded to the idea of circular orbits in his geocentric (Earth-centered) model. In 1597, Brahe was invited to Prague, where he spent his last years as Imperial Mathematician to Emperor Rudolph II. Here he was joined by German astrologer Johannes Kepler, who continued Brahe’s work after his death. Breaking with circles Kepler had already begun to calculate a new orbit for Mars from Brahe’s observations, and around this time concluded that its orbit must be ovoid (egg-shaped) rather Kepler’s laws state that planets follow elliptical orbits with the Sun as one of the two foci of the ellipse. In any given time, t, a line joining the planets to the Sun sweeps across equal areas (A) in the ellipse. Focus Jeremiah Horrocks 52 ■ than truly circular. Kepler formulated a heliocentric model with ovoid orbits, but this still did not match the observational data. In 1605, he concluded that Mars must instead orbit the Sun in an ellipse—a “stretched circle” with the Sun as one of two focus points. In his Astronomia Nova (New Astronomy) of 1609, he outlined two laws of planetary motion. The ﬁrst law stated that the orbit of every planet is an ellipse. The second law stated that a line joining a planet to the Sun sweeps across equal areas during equal periods of time. This means that the speed of the planets increases the closer they are to the Sun. A third law, in 1619, described the relationship of a planet’s year to its distance from the Sun: the square of a planet’s orbital period (year) is proportional to the cube of its distance from the Sun. So a planet that is twice the distance from the Sun than another planet will have a year that is almost three times as long. The nature of the force keeping the planets in orbit was unknown. Kepler believed it was magnetic, but it would be 1687 before Newton showed that it was gravity. ■ t Focus A t A A Sun Planet Johannes Kepler Born in the city of Weil der Stadt near Stuttgart, southern Germany, in 1571, Johannes Kepler witnessed the Great Comet of 1577 as a small child, marking the start of his fascination with the heavens. While studying at the University of Tübingen, he developed a reputation as a brilliant mathematician and astrologer. He corresponded with various leading astronomers of the time, including Tycho Brahe, ultimately moving to Prague in 1600 to become Brahe’s student and academic heir. Following Brahe’s death in 1601, Kepler took on the post of Imperial Mathematician, with a royal commission to complete Brahe’s work on the so-called Rudolphine Tables for predicting the movements of the planets. He completed this work in Linz, Austria, where he worked from 1612 until his death in 1630. Key works t 1596 The Cosmic Mystery 1609 Astronomia Nova (New Astronomy) 1619 The Harmony of the World 1627 Rudolphine Tables 42 A FALLING BODY ACCELERATES UNIFORMLY GALILEO GALILEI (1564–1642) IN CONTEXT BRANCH Physics BEFORE 4th century BCE Aristotle develops ideas about forces and motion, but does not test them experimentally. 1020 Persian scholar Ibn Sina (Avicenna) writes that moving objects have innate “impetus,” slowed only by external factors such as air resistance. 1586 Flemish engineer Simon Stevin drops two lead balls of unequal weight from a church tower in Delft to show that they fall at the same speed. AFTER 1687 Isaac Newton’s Principia formulates his laws of motion. 1971 US astronaut Dave Scott demonstrates Galileo’s ideas about falling bodies by showing that a hammer and a feather fall at the same rate on the Moon, which has almost no atmosphere to cause drag. F or 2,000 years, few people challenged Aristotle’s assertion that an external force keeps things moving and that heavy objects fall faster than lighter ones. Only in the 17th century did the Italian astronomer and mathematician Galileo Galilei insist that the ideas had to be tested. He devised experiments to test how and why objects move and stop moving, and was the ﬁrst to ﬁgure out the principle of inertia—that objects resist a change in motion and need a force to start moving, speed up, or slow down. By timing objects falling, Galileo showed that the rate of fall is the same for all objects, and came to realize the part played by friction in slowing them down. A With the equipment available during the 1630s, Galileo could not directly measure the speed or acceleration of freely falling objects. By rolling balls down one ramp and up another, he showed that the speed of a ball at the bottom of the ramp depended on its starting height, not on the steepness of the ramp, and that a ball would always roll up to the same height it had started from, no matter how steep or shallow the inclines were. Galileo carried out his remaining experiments with a ramp 16 ft (5 m) long, lined with a smooth material to reduce friction. For timing, he used a large container of water with a small pipe in the bottom. He collected the water during the interval he was measuring, and weighed the water Galileo demonstrated that the speed a ball reaches at the bottom of a ramp depends only on its starting height, not the steepness of the ramp. Here, balls dropped from points A and B will reach the bottom of the ramp at the same speed. B SCIENTIFIC REVOLUTION 43 See also: Nicolaus Copernicus 34–39 ■ Count what is countable, measure what is measurable, and what is not measurable, make it measurable. Galileo Galilei collected. By letting the ball go at different points on the ramp, he showed that the distance traveled depended on the square of the time taken—in other words, the ball accelerated down the ramp. The law of falling bodies Galileo’s conclusion was that bodies all fall at the same speed in a vacuum, an idea later developed further by Isaac Newton. There is a greater force from gravity on a larger mass, but the larger mass also Galileo Galilei Isaac Newton 62–69 needs a bigger force to make it accelerate. The two effects cancel each other out, so in the absence of any other forces, all falling objects will accelerate at the same rate. We see things falling at different rates in everyday life because of the effect of air resistance, which slows objects down at different rates depending on their size and shape. A beach ball and a bowling ball of the same size will initially accelerate at the same rate. Once they are moving, the same amount of air resistance will act on them, but the size of this force will be a much greater proportion of the downward force on the beach ball than the bowling ball, and so the beach ball will slow down more. Galileo’s insistence on testing theories with careful observation and measurable experiments marks him, like Alhazen, as one of the founders of modern science. His ideas on forces and motion paved the way for Newton’s laws of motion 50 years later and underpin our understanding of movement in the universe, from atoms to galaxies. ■ Galileo was born in Pisa, but later moved with his family to Florence. In 1581, he enrolled in the University of Pisa to study medicine, then switched to mathematics and natural philosophy. He investigated many areas of science, and is perhaps most famous for his discovery of the four largest moons of Jupiter (still called the Galilean moons). Galileo’s observations led him to support the Sun-centered model of the solar system, which at the time was in opposition to the teachings of the Roman Catholic Church. In 1633, he was tried and Objects of different masses appear to fall at different rates. All moving objects are affected by air resistance. Without air resistance, all objects would fall at the same rate. A falling body accelerates uniformly. made to recant this and other ideas. He was sentenced to house arrest, which lasted the rest of his life. During his conﬁnement, he wrote a book summarizing his work on kinematics (the science of movement). Key works 1623 The Assayer 1632 Dialogue Concerning the Two Chief World Systems 1638 Discourses and Mathematical Demonstrations Relating to Two New Sciences 44 THE GLOBE OF THE EARTH IS A MAGNET WILLIAM GILBERT (1544–1603) IN CONTEXT BRANCH Geology BEFORE 6th century BCE The Greek thinker Thales of Miletus notes magnetic rocks, or lodestones. 1st century CE Chinese diviners make primitive compasses with iron ladles that swivel to point south. 1269 French scholar Pierre de Maricourt sets out the basic laws of magnetic attraction, repulsion, and poles. AFTER 1824 French mathematician Siméon Poisson models the forces in a magnetic ﬁeld. 1940s American physicist Walter Maurice Elsasser attributes Earth’s magnetic ﬁeld to iron swirling in its outer core as the planet rotates. 1958 Explorer 1 space mission shows Earth’s magnetic ﬁeld extending far out into space. B y the late 1500s, ships’ captains already relied on magnetic compasses to maintain their course across the oceans. Yet no one knew how they worked. Some thought the compass needle was attracted to the North Star, others that it was drawn to magnetic mountains in the Arctic. It was English physician William Gilbert who discovered that Earth itself is magnetic. Stronger reasons are obtained from sure experiments and demonstrated arguments than from probable conjectures and the opinions of philosophical speculators. William Gilbert Gilbert’s breakthrough came not from a ﬂash of inspiration, but from 17 years of meticulous experiment. He learned all he could from ships’ captains and compass makers, and then he made a model globe, or “terrella,” out of the magnetic rock lodestone and tested compass needles against it. The needles reacted around the terrella just as ships’ compasses did on a larger scale—showing the same patterns of declination (pointing slightly away from true north at the geographic pole, which differs from magnetic north) and inclination (tilting down from the horizontal toward the globe). Gilbert concluded, rightly, that the entire planet is a magnet and has a core of iron. He published his ideas in the book De Magnete (On the Magnet) in 1600, causing a sensation. Johannes Kepler and Galileo, in particular, were inspired by his suggestion that Earth is not ﬁxed to rotating celestial spheres, as most people still thought, but is made to spin by the invisible force of its own magnetism. ■ See also: Thales of Miletus 20 ■ Johannes Kepler 40–41 ■ Galileo Galilei 42–43 Hans Christian Ørsted 120 ■ James Clerk Maxwell 180–85 ■ SCIENTIFIC REVOLUTION 45 NOT BY ARGUING, BUT BY TRYING FRANCIS BACON (1561–1626) IN CONTEXT BRANCH Experimental science BEFORE 4th century BCE Aristotle deduces, argues, and writes, but does not test with experiments—his methods persist for the next millennium. c.750–1250 CE Arab scientists conduct experiments during the Golden Age of Islam. AFTER 1630s Galileo experiments with falling bodies. 1637 French philosopher René Descartes insists on rigorous scepticism and inquiry in his Discourse on Method. 1665 Isaac Newton uses a prism to investigate light. 1963 In Conjectures and Refutations, the Austrian philosopher Karl Popper insists that a theory may be tested and proved false, but cannot conclusively be proved correct. T he English philosopher, statesman, and scientist Francis Bacon was not the ﬁrst to conduct experiments— Alhazen and other Arab scientists conducted them 600 years earlier— but he was the ﬁrst to explain the methods of inductive reasoning and set out the scientiﬁc method. He also saw science as a “spring of a progeny of inventions, which shall overcome, to some extent, and subdue our needs and miseries.” Whether or no anything can be known, can be settled not by arguing, but by trying. Francis Bacon Evidence from experiment According to the Greek philosopher Plato, truth was found by authority and argument—if enough intelligent men discussed something for long enough, the truth would result. His student, Aristotle, saw no need for experiments. Bacon parodied such “authorities” as spiders, spinning webs from their own substance. He insisted on evidence from the real world, particularly from experiment. Two key works by Bacon laid out the future of scientiﬁc inquiry. In Novum Organum (1620), he sets out his three fundamentals for the scientiﬁc method: observation, deduction to formulate a theory that might explain what has been observed, and experiment to test whether the theory is correct. In The New Atlantis (1623), Bacon describes a ﬁctitious island and its House of Salomon—a research institution where scholars conduct pure research centered on experiment and make inventions. Sharing those goals, the Royal Society was founded in 1660 in London, with Robert Hooke as its ﬁrst Curator of Experiments. ■ See also: Alhazen 28–29 ■ Galileo Galilei 42–43 Robert Hooke 54 ■ Isaac Newton 62–69 ■ William Gilbert 44 ■ 46 TOUCHING THE SPRING OF THE AIR ROBERT BOYLE (1627–1691) IN CONTEXT BRANCH Physics BEFORE 1643 Evangelista Torricelli invents the barometer using a tube of mercury. 1648 Blaise Pascal and his brother-in-law demonstrate that air pressure decreases with altitude. 1650 Otto von Guericke performs experiments on air and vacuums, ﬁrst published in 1657. AFTER 1738 Swiss physicist Daniel Bernoulli publishes Hydrodynamica, describing a kinetic theory of gases. 1827 Scottish botanist Robert Brown explains the motion of pollen in water as a result of collisions with water molecules moving in random directions. I n the 17th century, several scientists across Europe investigated the properties of air, and their work was to lead Anglo-Irish scientist Robert Boyle to produce his mathematical laws describing pressure in a gas. This work was tied in to a wider debate about the nature of the space between stars and planets. The “atomists” held that there was empty space between celestial bodies, whereas the Cartesians (followers of the French philosopher René Descartes) held that the space between particles was ﬁlled with an unknown substance called the ether, and that it was impossible to produce a vacuum. SCIENTIFIC REVOLUTION 47 See also: Isaac Newton 62–69 ■ John Dalton 112–13 ■ Robert FitzRoy 150–55 Torricellian vaccum We live submerged at the bottom of an ocean of the element air, that by unquestioned experiments is known to have weight. Evangelista Torricelli Mercury Scale Pressure of mercury column Tube The barometer invented by Evangelista Torricelli used a column of mercury to measure air pressure. Torricelli correctly reasoned that it was the air pressing down on the mercury in the cistern that balanced the column of mercury in the tube. Pressure of atmosphere Barometers In Italy, the mathematician Gasparo Berti performed experiments designed to ﬁgure out why a suction pump could not raise water more than 33 ft (10 m) high. Berti took a long tube, sealed it at one end and ﬁlled it with water. He then inverted the tube with its mouth in a tub of water. The level of water in the tube fell until the column was about 30 ft (10 m) high. In 1642, fellow Italian Evangelista Torricelli, hearing of Berti’s work, constructed a similar apparatus but used mercury instead of water. Mercury is more than 13 times denser than water, so his column of liquid was only about 30 in (76 cm) high. Torricelli’s explanation for this was that the weight of the air above the mercury in the dish was pressing down on it, and that this balanced the weight of the mercury inside the column. Blaise Pascal’s experiments with barometers showed how air pressure varied with altitude. In addition to physics, Pascal also made signiﬁcant contributions to mathematics. Cistern (dish) He said that the space in the tube above the mercury was a vacuum. This is explained today in terms of pressure (force on a certain area), but the basic idea is the same. Torricelli had invented the ﬁrst mercury barometer. French scientist Blaise Pascal heard of Torricelli’s barometer in 1646, prompting him to start some experiments of his own. One of these, performed by his brother-in-law Florin Périer, was to demonstrate that air pressure changed depending on altitude. One barometer was set up on the grounds of a monastery in Clermont, and observed by a monk during the day. Périer carried the other to the top of Puy de Dôme, about 3,200 ft (1,000 m) above the town. The column of mercury was more than 3 in (8 cm) shorter at the top of the mountain than in the monastery garden. Since there is less air above a mountain than there is above the valley below it, this showed that it was indeed the weight of the air that held the liquid in the tubes of mercury or water. For this, and other work, the modern unit of pressure is named after Pascal. Air pumps The next important breakthrough was made by Prussian scientist Otto von Guericke, who made a pump that was capable of pumping some of the air out of a container. He performed his most famous ❯❯ 48 ROBERT BOYLE Men are so accustomed to judge of things by their senses that, because the air is indivisible, they ascribe but little to it, and think it but one remove from nothing. Robert Boyle demonstration in 1654, when he put two metal hemispheres together with an airtight seal between them and pumped the air out of them— two teams of horses were unable to pull the hemispheres apart. Before the air was pumped out, the air pressure inside the sealed hemispheres was the same as the air pressure outside. Without the air inside, pressure from the outside air held the hemispheres together. Robert Boyle learned of von Guericke’s experiments when they were published in 1657. To do Robert Boyle experiments of his own, Boyle commissioned Robert Hooke (p.54) to design and build an air pump. Hooke’s air pump consisted of a glass “receiver” (container) whose diameter was nearly 16 in (40 cm), a cylinder with a piston below it, and an arrangement of plugs and valves between them. Successive movements of the piston drew more and more air out of the receiver. Due to slow leaks in the seals of the equipment, the near-vacuum inside the receiver could only be maintained for a short time. Nevertheless, the machine was a great improvement on anything made previously, an example of the importance of technology to the furthering of scientiﬁc investigation. he was intent on pointing out that the results described are all from experiments, since at the time even such noted experimentalists as Galileo often also reported the results of “thought experiments.” Many of Boyle’s experiments were directly connected to air pressure. The receiver could be modiﬁed to hold a Torricelli barometer, with the tube sticking Experimental results Boyle performed a number of different experiments with the air pump, which he described in his 1660 book New Experiments Physico-Mechanical. In the book, Otto von Guericke built the ﬁrst air pump. His experiments with the pump provided evidence against Aristotle’s idea that “Nature abhors a vacuum.” Robert Boyle was born in Ireland, the 14th child of the Earl of Cork. He was tutored at home before attending Eton College in England and then touring Europe. His father died in 1643, leaving him enough money to indulge his interest in science full time. Boyle moved back to Ireland for a couple of years, but lived in Oxford from 1654 to 1668 so that he could do his work more easily, and then moved to London. Boyle was part of a group of men studying scientiﬁc subjects called the “Invisible College,” who met in London and Oxford to discuss their ideas. This group became the Royal Society in 1663, and Boyle was one of the ﬁrst council members. In addition to his interests in science, Boyle performed experiments in alchemy and wrote about theology and the origin of different human races. Key works 1660 New Experiments Physico-Mechanical: Touching the Spring of the Air and their Effects 1661 The Sceptical Chymist SCIENTIFIC REVOLUTION 49 out of the top of the receiver and sealed in place with cement. As the pressure in the receiver was reduced, the level of the mercury fell. He also performed the opposite experiment, and found that raising the pressure inside the receiver made the level of the mercury rise. This conﬁrmed the previous ﬁndings of Torricelli and Pascal. Boyle noted that it became harder and harder to pump air out of the receiver as the amount of air left decreased, and also showed that a half-inﬂated bladder in the receiver increased in volume as the air surrounding it was removed. A similar effect on the bladder could be achieved by holding it in front of a ﬁre. He gave two possible explanations for the “spring” of the air that caused these effects: each particle of the air was compressible like a spring and the whole mass of air resembled ﬂeece, or the air consisted of particles moving randomly. This was similar to the view of the Cartesians, although Boyle did not agree with the idea of the ether, but suggested that the “corpuscles” were moving in empty space. His explanation is The height of mercury in a barometer falls if you take the barometer up a mountain. The level of mercury falls as air is pumped out of the receiver in a barometer. This is because there is less air above you pressing down on the mercury. This means that the smaller the amount of air in the receiver, the lower its pressure. The “spring of the air” decreases as the mass of the air decreases. remarkably similar to the modern kinetic theory, which describes the properties of matter in terms of moving particles. Some of Boyle’s experiments were physiological, investigating the effects on birds and mice of reducing the pressure of the air, and speculating on how air is moved in and out of lungs. Boyle’s law If the height of the mercury column is less on the top of a mountain than at the foot of it, it follows that the weight of the air must be the sole cause of the phenomenon. Blaise Pascal Boyle’s law states that the pressure of a gas multiplied by its volume is a constant, as long as the amount of gas and the temperature are kept the same. In other words, if you decrease the volume of a gas, its pressure increases. It is this increased pressure that produces the spring of the air. You can feel this effect using a bicycle pump by covering the end with a ﬁnger and pushing the handle in. Although it bears his name, this law was ﬁrst proposed not by Boyle, but by English scientists Richard Towneley and Henry Power, who performed a series of experiments with a Torricelli barometer and published their results in 1663. Boyle saw an early draft of the book and discussed the results with Towneley. He conﬁrmed them by experiment and published “Mr Towneley’s hypothesis” in 1662 as part of a response to criticism of his original experiments. Boyle’s work on gases was particularly signiﬁcant because of his careful experimental technique, and also his full reporting of all his experiments and their possible sources of error, whether or not they gave the expected results. This led many to seek to extend his work. Today, Boyle’s law has been combined with laws ﬁgured out by other scientists to form the “idealgas law,” which approximates to the behavior of real gases under changes of temperature, pressure, or volume. His ideas would also eventually lead to the development of the kinetic theory. ■ 50 IS LIGHT A PARTICLE OR A WAVE? CHRISTIAAN HUYGENS (1629–1695) IN CONTEXT BRANCH Physics Huygens thought that… space is ﬁlled with an ether. Newton thought that… a source of light emits large numbers of tiny “corpuscles.” Light is disturbances in the ether spreading out as waves. The corpuscles are weightless and travel in straight lines. BEFORE 11th century Alhazen shows that light travels in straight lines. 1630 René Descartes proposes a wave description of light. 1660 Robert Hooke states that light is a vibration of the medium through which it propagates. AFTER 1803 Thomas Young describes experiments that demonstrate how light behaves as a wave. 1864 James Clerk Maxwell predicts the speed of light and concludes that light is a form of electromagnetic wave. 1900s Albert Einstein and Max Planck show that light is both a particle and a wave. The quanta of electromagnetic radiation they recognize become known as “photons.” Is light a particle or a wave? I n the 17th century, Isaac Newton and the Dutch astronomer Christiaan Huygens both pondered the true nature of light, and reached very different conclusions. The problem they faced was that any theory about the nature of light had to explain reﬂection, refraction, diffraction, and color. Refraction is the bending of light as it passes from one substance to another, and is the reason that lenses can focus light. Diffraction is the spreading out of light when it passes through a very narrow gap. Before Newton’s experiments, it was widely accepted that light gained its quality of color by interacting with matter—that SCIENTIFIC REVOLUTION 51 See also: Alhazen 28–29 ■ Robert Hooke 54 ■ Isaac Newton 62–69 James Clerk Maxwell 180–85 ■ Albert Einstein 214–21 the “rainbow” effect seen when light passes through a prism is produced because the prism has somehow stained the light. Newton demonstrated that the “white” light that we see is actually a mixture of different colors of light, and these are split up by a prism because they are all refracted by slightly different amounts. As with many natural philosophers of the time, Newton held that light was made up of a stream of particles, or “corpuscles.” This idea explained how light traveled in straight lines and “bounced” off reﬂective surfaces. It also explained refraction in terms of forces at the boundaries between different materials. Partial reﬂection However, Newton’s theory could not explain how, when light hits many surfaces, some is reﬂected and some is refracted. In 1678, Huygens argued that space was ﬁlled with weightless particles (the ether), and that light caused disturbances in the ether that Christiaan Huygens ■ Thomas Young 110–11 ■ When white light passes through a prism, it is refracted into its component parts. Huygens explained that this is due to light waves traveling at different speeds through different materials. spread out in spherical waves. Refraction was thus explained if different materials (be they ether, water, or glass) caused light waves to travel at different speeds. Huygens’ theory could explain why both reﬂection and refraction can occur at a surface. It could also explain diffraction. Huygens’ ideas made little impact at the time. This was in part due to Newton’s already giant stature as a scientist. However, a Dutch mathematician and astronomer Christiaan Huygens was born in The Hague in 1629. He studied law and mathematics at his university, then devoted some time to his own research, initially in mathematics but then also in optics, working on telescopes and grinding his own lenses. Huygens visited England several times, and met Isaac Newton in 1689. In addition to his work on light, Huygens had studied forces and motion, but he did not accept Newton’s idea of “action at a distance” to describe century later, in 1803, Thomas Young showed that light does indeed behave as a wave, and experiments in the 20th century have shown that it behaves both like a wave and a particle, although there are big differences between Huygens’ “spherical waves” and our modern models of light. Huygens said that light waves were longitudinal as they passed through a substance—the ether. Sound waves are also longitudinal waves, in which the particles of the substance the wave is passing through vibrate in the same direction as the wave is traveling. Our modern view of light waves is that they are transverse waves that behave more like waves of water. They do not need matter to propagate (transmit), while particles vibrate at right angles