Science: A History

Origins

• One potential date for the beginning of the revival of Western Europe is 1453:
  • Publication of "On the Structure of the Human Body" by Andreas Vesalius and of "On the Revolutions of the Celestial Bodies" by Copernicus", mark the start of the scientific revolution that would transform first Europe and then the world.
  • The Turks captured Constantinople marking the end of the old Roman Empire, causing many Greek-speaking scholars fled westwards to Italy with their documents, and there the Italian humanists took up these documents and the teaching found therein to -establish civilization along the lines that head existed before the Dark Ages.
  • Perhaps equally important was the depopulation of Europe by the Black Death in the 14th C, which led the survivors to question the whole basis of society, made labor expensive and encouraged the invention of technological devices to replace manpower.
  • Also, Gutenberg's development of moveable type in the mid 15th C had an obvious impact on what was to become science, and discoveries brought back to Europe by another technological development, sailing ships capable of crossing the oceans, transformed society.
• The scientific revolution did not happen in isolation, but eventually became the driving force of Western civilization over the next 450 years.
• The development of technology is more important than human genius, and it is no surprise that the start of the scientific revolution "coincides" with the development of the telescope and the microscope.
• Western science got started because the Renaissance happened. And once it got started by giving a boost to technology it ensured that it would keep on rolling, with new scientific ideas leading to improved technology, and improved technology providing the scientists with the means to test new ideas to greater and greater accuracy. Technology came first, because it is possible to make machines by trial and error without fully understanding the principles on which they operate. But once science and technology got together, progress really took off.

Renaissance Men

• Nicolaus Copernicus (1473-1543) - "On the Revolution of the Celestial Spheres" (1543)
• Andreas Vesalius (1514-1564) - "On the Structure of the Human Body" (1543)
• William Harvey - discovery of the circulation of the blood.
• Gabriele Fallopio - discover of the fallopian tubes

The Last Mystics

• Tycho Brahe (1546-1601) - De Nova Stella (1573)
  • Sees a comet in 1577. First astronomer to imagine the planets hanging unsupported in space
• Johannes Kepler (1571-1630) - The Mystery of the Universe (1597)
  • Suggested that planets were kept moving in their orbits by a force he called "the vigour" reaching out from the Sun and pushing them along.
  • "My aim is to show that the machine of the universe is not similar to a divine animated being, but similar to a clock."
  • First and second laws about planetary orbits.
  • "Astronomia Nova" (1609)
  • "Harmony of the World" (1618) - Third law about planetary orbits
  • Writes one of the first science-fiction stories, "Dream of the Moon
• John Napier (1550-1617) - Invented logarithms

The First Scientists

• William Gilbert (1544-1603)
  • Concerning Magnetism, Magnetic Bodies, and the Great Magnet Earth (1600)
  • Discovered laws of attraction and repulsion, that the Earth acts like a magnet, names the north pole and south pole
  • Nothing new was discovered about magnetism until the discovery of electromagnetism in the 1820s
  • Galileo described Gilbert as the founder of the experimental method of science.
• Galileo Galilei (1564-1642)
  • Always carried out experiments to test hypotheses, modifying or abandoning those hypotheses if the outcomes of the experiments did not match their predictions.
  • Also investigated hydrostatics, magnetism
  • Proved that a bullet fired from a gun describes a parabola
  • Built a telescope better than any other in just 24hrs
  • Discovered the four largest moons of Jupiter early in 1610, that the Milky Way is made up of many individual starts, and that the surface of the Moon is scarred by craters and has mountain ranges several km high in book The Starry Messenger (1610)
  • He was willing to test his reasoning by clearly thought-out experiments, in public, and to stand by the results.
  • Was one of the first to develop an effective compound microscope, involving two lenses each ground with a doubly convex shape.
  • Dialogue on the Two Chief World Systems (1629)
  • Discourses and Mathematical Demonstrations Concerning Two New Sciences (1638) - summed up his life’s work on mechanics, inertia, and pendulums and the strength of bodies, as well as spelling out the scientific method. The first modern scientific textbook, spelling out that the Universe is governed by laws which can be understood by the human mind and is driven by forces whose effects can be calculated using maths.
  • Realized that moving objects have a natural tendency to keep on moving, unless they are affected by friction or some other outside force.

Science Finds its Feet

• The symbols + and - were only introduced to maths in 1540 in Robert Recorde’s « The Grounde of Artes
• In another book, « Whetstone of Witte » (1557), he introduced the equals sign.
• The multiplication symbol was introduced in 1631, and the division symbol in 1659.
• The introduction of logarithms early in the 17th C enormously simplified and speeded up the laborious processes of calculations for astronomers and other scientists.
• Logarithms are a way to "undo" exponentiation. If $b^y = x$, then we say that the logarithm base $b$ of $x$ is $y$, written as $\log_b(x) = y$. In simpler terms, a logarithm answers the question: "To what power must I raise the base $b$ to get the number $x$?"

For example, since $10^2 = 100$, we know that $\log_{10}(100) = 2$. Here, 10 is the base, 100 is the number, and 2 is the logarithm (or the exponent).

Logarithms are incredibly useful for simplifying complex calculations, especially those involving very large or very small numbers. They turn multiplication into addition, division into subtraction, and exponentiation into multiplication. This made them invaluable before the age of calculators and continues to be important in various fields like science, engineering, and finance.

• Réné Descartes (1596-1650)
  • A Discourse on the Method of Rightly Conducting Reason and seeking Truth in the Sciences (1637)
  • Cartesian co-ordinates (x,y) allowed geometry to be analyzed using algebra
  • Mediationnes de Prima Philogsophia (1641)
  • Principia Philosophiae (1644)
• Blaise Pascal (1623-1662)
• Pierre Gassendi (1592-1655) - revived the concept of atomism, which dates back to Democritus in the 5th C BC
• Evangelista Torricelli (1608-1647) - invented the barometer and created a vacuum
• Christiaan Huygens (1629-1695) - from 1658 allows ordinary people to have access to accurate timepieces.
  • Uses the Aether to explain how light is refracted
  • Treatise on Light (1690)
• Royal Society is founded in London in 1645, In Paris, the Académie des Sciences in 1667
• Ole Romer (1644-1710) Calculated the speed of light as 225,000km/s, very close to the modern calculation of 299,792km/s
• Robert Boyle (1627-1691)
  • « We assent to experience, even when its information seems contrary to reason. »
  • The Spring of the Air (1660) - treats air pumpts and the problem of raising water by suction. Boyle’s la. Seen as a turning point for chemistry.
  • Came close to discovering Oxygen - essential similarities between the processes of respiration and combustion.
  • His most famous book, The Sceptical Chymist (1661)
  • Origin of Forms and Qualities (1666)
• Marcelo Malpighi (1628-1694) - Circulation of the blood
• Giovanni Borelli (1608-1679)
  • On the Movement of Animals (1680-1681) - treated the body as a system of levers acted on by the forces exerted by the muscles, and analysed geometrically how muscles in the human body acted in walking and running.
• Edward Tyson (1650-1708) - founding father of comparative anatomy
  • Anatomy of a Porpess
  • Orang-Outang - evidence that humans and chimps were built to the same body plan. The place of human beings as part of the animal kingdom is clearly established.

The Newtonian Revolution

• Robert Hooke (1635-1703):
  • Micrographia (1665) - the first subtantial book on microscopy by any major scientist. Marked the moment when microscopy came of age as a scientific discipline. Included the "cellular" structure of slices of cork - first use of the word cell
  • He realized that the orbital motion results from the tendency of the Moon to move in a straight line, plus a single force pulling it toward the Earth.
• Isaac Newton (1643-1727):
  • Calculus makes it possible to calculate accurately, from a known starting situation, things that vary as time passes, such as the position of a planet in its orbit. Provided the mathematical tools needed to study processes in which change occurs. Modern physical science simply would not exist without calculus.
  • Newton wondered whether, if the influence of the Earth's gravity could extend to the top of the tree, it might extend all the way to the Moon. He then calculated the force required to hold the Moon in its orbit and the force required to make the apple fall from the tree could both be explained by the Earth's gravity if the force fell off as one over the square of the distance from the centre of the Earth.
  • Principia Mathematica (1687) - He laid the foundations for the whole of physics, not only spelling out the implications of his inverse square law of gravity and the three laws of motion, which describe the behavior of everything in the Universe, but making it clear that the laws of physics are indeed universal laws that affect everything. The realization that the world works on essentially mechanical principles that can be understood by humand beings, and is not run in accordance with magic or whims of capricious gods.
  • Opticks (1704)

Expanding Horizons

• In the century after Newton, there was a huge expansion of the realm which science attempted to explain.
• Edmond Halley (1656-1741):
  • Questioned the accepted date of the Creation, 4004 BC
  • Studied variations in atmospheric pressure and winds
  • Experimented with a diving bell to a depth of 18m
  • 1698 Voyage of the Paramore for a year to the South Atlantic
  • "A Synopsis of the Astronomy of Comets" (1705)
  • Flamseed's star catalogue containing 3,000 star positions (1725)
  • 1720 Appointed as Astronomer Royal
  • Correctly predicted the return of Halley's comet after his death in 1758-9 and transits of Venus in 1761 and 1769. These latter were used to work out the distance to the Sun at 153m km, very close to the modern measurement of 149.6m km.
• John Ray (1627-1705) - the biological equivalent of Newton:
  • Ornithology (1677)
  • History of Fishes (1686)
  • History of Plants (1686, 88, 1704) - covering 18,000 plants, classified plants in terms of their family relationships, morphology, distribution and habitats.
  • Established the species as the basic unit of taxonomy
  • History of Insects (1710)
  • Made the study of botany and zoology a scientific pursuit, bring order and logic to the investigation of the natural world
  • Invented a clear taxonomic system based on physiology, morphology, and anatomy, paving the way for the work of Linnaeus.
• Carl Linnaeus (1707-1778)
  • Plants reproduced sexually and had male and female parts
  • Could not understand anything that is not systematically ordered.
  • Systema Naturae (1735) - Introduces the binomial system of classifying every species with a two-word name. Introduced the terms Mammalia, Primates, and Homo sapiens.
  • Provided descriptions of 7700 species of plants and 4400 species of animals. Everything in the living world was arranged in a hierarchy of family relationships, from the broad classifications of their Kingdom and class down through the subdivisions Order and Genus to the Species itself. This systm preserves the last vestige of Latin in scientific work right up to the present day.
  • Paved the way from which the relationships between species and the laws of evolution would begin to become clear in the 19th C.
  • Was the first person to include man in a system of biological classification. I have yet to find any characteristics which enable man to be distinguished on scientific principles from an ape
  • Nowadays, using DNA evidence, man would be classified as a chimpanzee.
• Anders Celsius (1701-1744)
• Nicolaus Steno (1638-1686)
• Comte de Buffon (1707-1788):
  • Histoire Naturelle in 44 volumes between 1749-1804 - the first work to cover the whole of natural history
  • Speculated that the Earth had formed out of material thrown out of the Sun as the result of the impact of a comet.
  • Calculated that the Earth must be at least 75k years old
• Jean Fourier (1768-1830):
  • Fourier Analysis - mathematical techniques for dealing with time-varying phenomena
  • Developed sets of equations to describe heat flow
  • Calculated the age of the Earth as 100m years
• Georges Couvier (1769-1832)
  • Lectures in Comparative Anatomy (1800)
  • Probably the most influential biologist in the world in the 1830s
  • Compared the bodies of meat-eating and plant-eating animals
  • Arranged all animals into four major groups (vertebrates, molluscs, articulates, and radiates), which each had its own kind of anatomy.
  • Almost single-handedly invented the science of paleontology - could say which strata of fossils was older and younger
  • Discours sur la Théorie de la Terre (1825)
• Jean-Baptiste Lamarck (1744-1829) - urged naturalists to describe the natural world, without wasting time on theories purporting to explain it.

Enlightened Science I: Chemistry Catches Up

• The basic feature of the Enlightenment was a belief in the superiority of reason over superstition. This incorporated the idea that humankind was in the process of progressing socially, so that the future would be an improvement of the past; and one of those improvements was a challenge to orthodox religion with its overtones of superstition. Both the American and the French revolutions were justified intellectually, in part, on the basis of human rights, a guiding principle of Enlightenment philosophers such as Voltaire and activists such as Thomas Paine. The success of Newtonion physics in providing a mathematical description of an ordered world clearly played a big part in the flowering of this movement in the 18th century, encouraging philosophers of a rationalist persuasion, and also encouraging chemists and biologists to think that their parts of the natural world might be explained on the basis of simple laws.
• The idea of order and rationality as a way to investigate the world had taken root by the early 18th C and seemed the obvious way forward.
• The Industrial Revolution probably took place first in England (around 1740-1780 due to Britain being an « island of coal » but also because the Newtonian mechanistic world became firmly established there. The IR stimulated interest in topics like heat and thermodynamics (the connection between heat and motion) and provided new tools for scientists to use in their investigations of the world.
• Astronomy, physics, botany, and zoology could make progress with simple tools, but chemists needed, above all, a reliable and controllable source of heat to encourage chemical reactions.
• Gabriel Fahrenheit (1686-1736)- Invented the alcohol thermometer in 1709, the mercury thermometer in 1714 (along with the Fahrenheit scale).
• Anders Celsius (1701-1744) - came up with his scale in 1742.
• Thomas Newcomen (1663-1729) - Completed the first practical steam engine to pump water from mines in 1712
• From the 1740s onward, progress was rapid (if sometimes confused).
• William Cullen (1710-1790) - Invented the first refrigerator.
• Joseph Black (1728-1799):
  • Discovered carbon dioxide and showed for the first time that air is a mixture of gases and not a single substance.
  • Using his usual careful, quantitative approach, made a crucial distinction between the concepts of heat and temperature.
  • Gave the name « specific heat » to the amount of heat required to raise the temperature of a certain amount of a chosen substance by a certain amount.
• James Watt (1736-1819)
  • First person to take a set of ideas from the cutting edge of then-current research in science and apply them to make a major technological advance.
  • The scale effect - a small object loses heat more rapidly than a large object of the same shape because the small object has a larger surface area, across which heat escapes, in proportion to its volume, which stores heat.
• He improved on Newcomen’s steam engine by using two cylinders, one which was kept hot (in which the piston moved) and one which was kept cold.
  • Patented his steam engine in 1769.
• Joseph Priestley (1733-1804):
  • Wrote a history of electricity in1767.
  • When he began, only two gases (« air » and carbon dioxide (« fixed air ») were known.
  • Identified another ten gases, including ammonia, hydrogen choloride, nitrous oxide (laughing gas) and sulphur dioxide.
  • Greatest discovery was Oxygen (in 1774) - explaining it in terms of the philogiston model. This model would be doomed when people started noticing that things get heavier when they burn, not lighter.
  • Discovered carbon dioxide over a brewers vat and invented « soda water », a craze for which spread across Europe.
  • Showed how the ability of air to sustain life would somehow be « used up » in respiration and that the respirability of the air could be restored by the presence of planes - the first hints of the process of photosynthesis in which carbon dioxide is broken down and oxygen is released.
• Carl Scheele (1742-1786) - Realized that air is a mixture of two substances, one of which prevents burning, while the other promotes combustion.
• Science progresses incrementally, building on what has already been discovered and making use of the technology of the day.
• Henry Cavendish (1731-1810):
  • The Cavendish Laboratory in Cambridge, built in the 1870s is named after him.
  • Discovered Hydrogen (« inflammable air ») in 1776. He thought that the gas was released by the metals involved in the reaction (we now know that it comes from the acids), and thought that it was phlogiston.
  • Showed that water was not an element and is somehow formed from a mixture of two other substances, a key step in the transformation of alchemy into chemistry.
  • Found a previously unknown gas, argon, present in tiny traces (0.93%) in the atmosphere.
  • In « the Cavendish experiment », weighed the Earth using a torsion balance.
• Antoine Lavoisier (1743-1794):
  • Thought of as the greatest chemist of all
  • Showed that diamond is combustible.
  • Showed that sulphur gains weight when it burns, his first, independent, step to the modern understanding of combustion as a process involving oxygen from the air combining with the substance that is being burnt.
  • Gave oxygen its name (in 1779)
  • Coming towards the idea that animals keep warm by converting carbon (from their food) into carbon dioxide (which they breathe out) by combining it with oxygen from the air (which they breathe in), seeing respiration as a very slow form of combustion. This was a key step in setting human beings in their context as complicated systems obeying the same laws as falling stones or burning candles, and showed that there was no need to invoke anything outside the known world of science to produce the life-associated warmth of the human body - no need for Harvey’s « natural heat ».
  • First person to appreciate that water is a compound substance formed from a combination of « inflammable air » and oxygen in the same sort of way that « fixed air » is formed from a combination of carbon and oxygen.
  • Elements of Chemistry (1789) - which laid the foundations for chemistry as a genuinely scientific discipline and almost an equivalent to chemistry of newton’s Principia to Physics. Marks the moment when chemistry becomes recognizable as the discipline of today.
  • Proposed new names based on a logical system of nomenclature (such as oxygen, hydrogen, and sulphuric acid), introduced a logical way to name compounds, such as nitrates. By giving chemistry a logical language, he greatly eased the task of chemists trying to communicate their discoveries to one another.
  • Guillotined on 8 May 1794.

Enlightened Science II: Progress on All Fronts

• In the decade following the death of newton, the term « Physics » started to be used in plae of « natural philosophy.
• Pieter van Musschenbrook (1692-1761):
  • « Essaie de Physique » (1737 - one of the first books to use the term in its modern sense
  • Invented the Leiden Jar - a device that could store large quantities of electricity
• First steps towards an understanding of static elecricity.
• There are two kinds of electricity (positive and negative charge) and similar kinds repel one another while opposite kinds attract.
• The importance of insulating material in preventing electricity draining away from charged objects.
• Benjamin Franklin (1706-1790):
  • Developed a one fluid model of electricity where a physical transfer of the single fluid occurs when an object becomes electrically charged, leaving one surface with « negative » charge and the other with « positive » charge. ie, charge is conserved - there is always the same amount of electricity, but it can be moved around, and overall the amount of negative charge must balance the amount of positive charge.
  • Showed that electricity can magnetize and demagnetize iron needles.
• Charles Coulomb (1736-1806) - Convinced everyone that both electrical and magnetic forces obey an inverse square law - now coulomb’s law.
• Luigi Galvini (1737-1è98) - twitching of frog’s legs is caused by electricity stored or manufactured in the muscles of the frog.
• Alessandro Volta (1745-1827):
  • Created the voltaic pile, forerunner of the modern battery. Beore this, the study of electricity was essentially confined to the investigation of static electricity. After 1800, physicicists could work with electric currents, which they could turn on and off at will.
  • Current from such a pile could be used to decompose water into hydrogen and oxygen.
• Pierre-Louis de Maupertuis (1698-1759):
  • The principle of least action - nature is lazy. Eg, light always travels in straight lines.
• Leonhard Euler (1707-1783) - regarded as the most prolific mathematician of all time.
  • Introduced the use of the letters e and i
  • Described mathematically the refraction of light.
• William (1738-1822) and Caroline (1750-1848) Herschel discovered Uranus, the first planet that had not been known to the ancients.
• John Michell (1724-1793) - first person to come up with the idea of what are now known as black holes in a paper read to the Royal Society in 1783.
• Pierre Simon Laplace (1749-1827)
  • Explained variations in planetary orbits follow a cycle 929 years long (so the Solar System is stable after all.
  • Exposition du système du monde (1796) - one of the most influential books about science ever published, summing up where physics stood at the end of the 18th C. « The simplicity of nature is not to be measured by that of our conceptions. Infinitely varied in its effects, nature is simple only in its causes, and its economy consists in producing a great number of phenomena, often very complicated, by means of a small number of general laws.
  • Nebular hypothesis of the origin of the Solar System - it was practically certain that the planets had formed together from a cloud of material around the young Sun, shrinking down into a plane as the cloud, or nebula, contracted.
  • Laplace’s version of black holes.
• One reason why science did progress so rapidly in the 19th C was that by the end of the 1790s it was obvious to all but the most blinkered of the old school that the ideas of phlogiston and caloric wer both dead and buried.
• James Hutton (1726-1797):
  • Develops the idea of uniformitarianism - that the same uniform processes are at work all the time and mould the surface of the Earth continually. The idea that occasional great acts of violence are needed to explain the observed features of the Earth become known as catastrophism. Previously the received wisdom was a combination of catastrophism and Neptunism.
  • Theory of the Earth (1795). John Playfair wrote a masterly, clear summary of it in 1802 as Illustrations of the Huttonian Theory of the Earth.

The Darwinian Revolution

• Arguably the most important idea in the whole of science. This is the nub of the theory of natural selection:
  • Offspring resemble their parents, but in each generation there are slight differences between individuals.
  • Only the individuals best suited to the environemnt survive to reproduce, so the slight differences which make them successful are selectively passed on to the next generation and become the norm.
  • When conditions change, or when species colonize new territory, species change to match the new conditions and new species arise as a result.
  • Natural selection explains how, given enought time, evolution could produce an antelope adapted to a grazing lifestyle, the grass itself, a lion adapted to eat antelope, a bird that depends on a certain kind of seed for its food, or any other species on Earth today, including humankind, from a single, simple common ancestor.
• Charles Lyell (1797-1875):
  • Principles of Geology (1830) - Third volume in 1833, 12th and final in 1875
  • The first person to make his living as a science writer.
  • Elements of Geology (1838)
  • The leading geologist of his time
  • In 1841 went on a year-long visit to the US by steamship, and was surprised how the railways had already spread over what was until recently unknown territory.
• Evolutionary ideas can be traced back to the ancient Greeks, and there were discussions about species change by Francis Bacon in 1620 and Leibnitz, while Buffon also discussed the migration and evolution of species. The difference with Darwin and Russel was that they came up with a sound scientific theory.
• Erasmus Darwin (1731-1802):
  • Zoonomia (1794, 1796) in two volumes sets out his ideas on evolution.
  • God still exists for Erasmus, but only as the first cause who set the processes of life on Earth working. There is no place here for a God who intervenes to create new species from time to time, but a clear sense that whatever the origins of life itself, once life existed it evolved and adapted in accordance with natural laws, with no outside intervention.
• Jean-Baptiste Pierre Antoine de Monet de Lamarck (1744-1829):
  • Flore française (1778)
  • Classified insects and worms into « invertebrates ».
  • Histoire naturelle des animaux sans vertèbres (7 volumes) (1815-1822):
  • His four laws - which become progressively more wrong:
    • First law - By virtue of life’s own powers there is a constant tendency for the volume of all organic bodies to increase and for the dimensions of their parts to extend up to a limit determined by life itself. (true)
    • The production of new organs in animals results from newly experienced needs which persist, and from new movements which the needs give rise to and maintain (not wrong but Lamarck means, wrongly, that the new organs develop within individuals, not by tiny changes from one generation to the next)
    • Third Law: The development of organs and their faculties bears a constant relationship to the use of the organs in question (definitely wrong - the idea that the giraffe’s neck gets long because it is stretching for leaves)
    • Fourth Law: Everything which has been acquired or changed in the organization of an individual during its lifetime is preserved in the reproductive process and is transmitted to the next generation by those who experienced the alterations (definitely wrong)
• Charles Darwin (1809-1882)
  • Learned about the argument between the Neptunists, who thought that the Earth’s features had been shaped by water, and the Vulcanists, who saw heat as the driving force - he preferred the latter explanation.
  • 1831 - start of a 5 year voyage on the Beagle
  • The Voyage of the Beagle (1839)
  • Calls the great age of the Earth, « the gift of time »
  • On the Origin of Species by Means of Natural Selection (1859) - prompted by Lyell’s advances
• Thomas Malthus (1766-1834):
  • Britain’s first professor of political economy.
  • Essay on the Principle of Population (1798) - populations have the power to grow geometrically, but are held in check by pestilence, predators, and especially by the limited amount of food available (as well as by war, in the case of humans). Most offspring die without reproducing, if nature takes its course.
• Alfred Russel Wallace (1823-1913)
  • 1847 - self-funded a four year , two-man expedition to South America, exploring and collecting in the jungles of Brazil.
  • The great problem of the origin of species was already distinctly formulated in my mind… I firmly believed that a full and careful study of the facts of nature would ultimately lead to a solution of the mystery
  • 1854 - goes to the Far East because he decided that the best way to pursue his interest in the species problem would be to visit a region of the globe which had not already been explored by other naturalists.
  • Stays there for eight years, publishing 40 scientific papers, and establishing the geographical ranges of different species
  • Wallace developed the idea of evolution as like the branching of a huge tree, with different branches growing from a single trunk, and continually dividing and splitting down to the little twigs, still growing, which represent the diversity of living species (all derived from a common stock) in the world today. He presented these ideas in 1855 without, at that point, offering an explanation for how or why speciation occurred.
  • Breakthrough in 1858, remembering Malthus - in every generation the inferior would inevitably be killed off and the superior would remain.
  • On the Tendency of Varieties to Depart Indefinitely from the Original Type (1858)
• Thomas Henry Huxley (1825-1895)
  • Darwin’s bulldog
  • Helped to establish science as a profession that people were paid to do.
• Darwin was the first scientist listed here who was born after 1800. Wallace was the first who dies after 1900.

Atoms and Molecules

• During the 19th C, science shifts from being a gentlemanly hobby to a well-populated profession
• Humphry Davy (1778-1829):
  • Learned French and red Lavoisier’s Traité Elémentaire in the original French
  • Carried out experiments with nitrous oxide (laughing gas) and discovered that it could be used during surgical operations
  • Saw a significant relationship between chemistry and electricity.
  • Royal Instituation established by Count Rumford in 1799 and he became its director in 1802
  • One of the last great amateur scientists and also one of the first professional ones.
  • 1806 - Isolated two previously unknown metals and called them potassium and sodium.
  • 1810 - isolated and named chlorine. Defined an element as a substance that cannot be decomposed by any chemical process, showed that chlorine is an element, and established that the key component of all acids is hydrogen, not oxygen.
  • Appointed Michael Faraday (his eventual successor) as an assistant in the RI
  • Designed the famous miners’ safety lamp which bears his name.
  • In 1820, elected President of the Royal Society
• John Dalton (1766-1844)
  • When he was born, there were perhaps 300 scientists in the world, when he started work there were a thousand, by his death 10k, and by 1900 100k. The number of scientists doubled every 15 years (though generally populations were doubling over longer periods)
  • Recorded daily meteorological observations daily from 1787 until his death.
  • Discussed the nature of water vapour and its relationship to air, describing it in terms of particles which exist between the particles of air, so that the equal and opposite pressures of the surrounding air particles on a particule of vapour cannot bring it nearer to another particle of vapour, without which no condensation can take place - a precursor to his atomic theory.
  • Was color-blind and did a detailed analysis of the condition, which became known as Daltonism
  • In 1801, came up with the law of partial pressures, which says that the total pressure exerted by a mixture of gases in a container is the sum of the pressures each gas would exert on its own under the same conditions.
  • In the early 1800s became convinced that each element was made up of a different kind of atom, what made one element different from another was the weight of its atoms, and that elementary atoms could be neither created nor destroyed (though they could combine to form « compound atoms » (molecules).
  • The main flaw in the model is that he did not realize that elements such as hydrogen are composed of molecules, not individual atoms and so he got some combinations wrong.
  • A New System of Chemical Philosophy (1808)
• It took almost half a century for the Daltonian atom to become really fixed as a feature of chemistry
• Jöns Berzelius (1779-1848):
  • One of the first people to formulate the idea that compounds are composed of electrically positive and negative parts
  • Studied 2000 different compounds over 6 years
  • Invented the modern alphabetical system of nomenclature for the elements.
  • Isolated and identified several elements, including selenium, thorium, lithium and vanadium
  • Chemists were beginning to group elements into families with similar properties. He gave the name « halogens » (meaning salt-formers) to the group including chlorine, bromine, and iodine.
  • Coined the terms « organic chemistry », « catalysis », and « protein »
  • Textbook of Chemistry (1803)
  • Experimenters had long been aware that everything in the material world falls into one of two varieties of chemical substances. Some can be heated and seem superficially to change their character (glowing red hot, melting, evaporating, etc), but when cooled, revert back to the same chemical state they started from. Others, such as sugar or wood, are completely altered by the action of the heat, so that it is very difficult to « unburn » a piece of wood. In 1807, he formalized the distinction. The first, associated with non-living systems, he called « inorganic », and the second, associated with living systems, « organic ». It became clear that organic materials are made up of much more complex compounds, but there was also belief in a vague « life force ».
• Joseph Louis Gay-Lussac (1778-1850)
  • In 1809 published that gases combine in simple proportions by volume, and that the volume of the products of the reaction (if they are also gaseous) is related in a simple way to the volumes of the reacting gases.
• Amadeo Avogadro (1776-1856):
  • In 1811, gave the hypothesis that at a given temperature, the same volume of any gas contains the same number of particles.
  • Realized that oxygen and other elements could exist in polyatomic molecular form. Two volumes of hydrogen contain twice as many molecules as one volume of oxygen, and when they combine, each oxygen molecule provides one atom to each pair of hydrogen molecules, making the same number of molecules as there were in the original volume of Hydrogen.
• William Prout (1785-1850):
  • Suggested that the atomic weights of all elements are exact multiples of the atomic weight of hydrogen, implying that in some way heavier elements might be built up from hydrogen.
  • In the 20th C, with the discovery of isotopes (atoms of the same element with slightly different atomic weights, but each isotype having an atomic weight a precise multiples of the weight of one hydrogen atom) the puzzle was resolved (since chemically-determined atomic weights are an average of those of all the isotopes of an element present, and Prout’s hypothesis was confirmed.
• Friedrich Wöhler (1800-1882):
  • Accidentally discovered that organic materials could be manufactured from substances that had never been associated with life and the definition of organic changed. There was no mysterious life force.
  • Organic compounds are often complex, containing elements of different elements, and they all contain carbon, which is the reason for their complexity.
• Now we say that an organic molecule is any molecule containing carbon and organic chemistry is the chemistry of carbon and its compounds. Life is seen as a product of carbon chemistry, obeying the same chemical rules that operate throughout the world of atoms and molecules. Natural selection tells us that we are part of the animal kingdom with no evidence of a uniquely human soul. Chemistry tells us that animals and plants are part of the physical world, with no evidence of a special « life force ».
• Edward Frankland (1825-1899)
  • Analyzed valency, a measure of the ability of one element to combine with another or as soon became clear, the ability of atoms of a particular element to combine with other atoms.
• Archibald Couper (1831-1892):
  • Introduced the concept of « bonds », simplifying the representation of valency. Hydrogen is now said to have a valency of 1, meaning that it can form one bond with another atom. Oxygen has a valency of 2, meaning that it can form two bonds. Nitrogen has a valency of 3 and carbon has a valency of 4, so it can form four separate bonds with four separate atoms, including other atoms of carbon at the same time.
  • He saw that organic chemistry might consist of a chain of carbon atoms holding hands with other atoms attached to the spare bonds at the sides of the chain.
• Friedrich August Kekulé (1829-1896):
  • Saw that carbon atoms could link up in rings (often six atoms in a hexagon) with bonds sticking out from the ring to link up with other atoms or other rings of atoms.
• Stanislao Cannizzaro (1826-1910)
  • Drew the essential distinction between atoms and molecules
  • Showed how the observed behavior of gases together with Avogadro’s hypothesis could be used to calculate atomic and molecular weights relative to the weight of one hydrogen atom, and drew up a table of atomic and molecular weights.
• John Newlands (1837-1898):
  • Realized that if the elements are arranged in order of their atomic weight, there is a repeating pattern in which elements at regular intervals, with atomic eights separated by amounts that are multiples of eight times that of hydrogen, have similar properties.
• Dmitri Mendeleev (1834-1907):
  • Principles of Chemistry (1868, 1870)
  • On the Relation of the Properties to the Atomic Weights of Elements
  • Rearranges the elements slightly in order to make them fit the pattern he had discovered and leaves gaps in the periodic table for elements which had not yet been discovered.
  • It turns out that the chemical properties of an element depend on the number of protons in the nucleus of each atom (the atomic number), while its atomic weight depends on the total number of protons plus neutrons in the nucleus
  • By 1871 he had refined his table to incorporate all of the 63 known elements, with three gaps, which would be filled over the next 15 years with just the properties predicted by him - gallium (1875), scandium (1879) and germanium (1886)
  • From a mass of data, Mendeleyev found a pattern and made a prediction that could be tested by experiment, and found a deep truth about the nature of the chemical world.
• Themodynamics frew out of the industrial revolution and fed back into it. At the beginning of the 19th C, there was no consensus about the nature of heat, the term was coined in 1849 by William Thomson, by the 1860s the basis laws and principles had been worked out, and 40 years later would be used in a definitive proof of the reality of atoms.
• Sadi Carnot (1796-1832)
  • Réflexions sur la puissance motive du feu (1824) - analyzed the efficiency of engines converting heat into work, provided a scientific definition of work, showed that work is done as heat passes from a higher temperature to a lower temperature, and even suggested the possibility of the internal combustion engine.
  • First person to appreciate that heat and work are interchangeable and worked out how much work a given amount of heat can do.

Julius Robert von Mayer (1814-1878):

• • Knew Lavoisier’s work which showed that warm-blooded animals are kept warm by the slow combustion of food, which acts as fuel, with oxygen in the body. He knew that bright red blood, rich in oxygen, is carried around the body from the lungs in arteries, while dark purple blood, deficient in oxygen, is carried back to the lungs by veins.
  • Realized that the reason why the venous blood was rich in oxygen was that in the heat of the tropics the body had to burn less fuel (and therefore consume less oxygen) to keep warm. He saw that this implied that all forms of heat and energy are interchangeable - heat from muscular exertion, the heat of the Sun, heat from burning coal, or whatever - and that heat, or energy, could never be created but only changed from one form to another.
• James Joule (1818-1889):
  • Gave two lectures in Manchester in 1847 setting out the law of conservation of energy and its importance to the physical world.
  • The Joule-Thomson effect - the way in which gases cool as they expand, the principle on which a refrigerator operates.
• William Thomson/Lord Kelvin (1824-1907):
  • Responsible for the success of the first working transatlantic telegraph cable
  • Established thermodynamics as a scientific discipline in the middle of the 19th C.
  • Established the absolute scale of temperature, which is based on the idea that heat is equivalent to work, and that a certain change in temperature corresponds to a certain amount of work. There is a minimum possible temperature (-273C) at which no more work can be done because no heat can be extracted from a system.
• Laws of thermodynamics:
  • First law - heat is work
  • Second law - Heat cannot, of its own volition, move from a colder object to a hotter object. Things wear out - everything wears out, including the Universe itself
  • Writes in 1852: Within a finite period of past time the earth must have been, and within a finite period of time to come the earth must again be unfit for the habitation of man as at present constituted, unless operations have been or are to be performed which are impossible under the laws to which the known operations going on at present in the material world are subject.
  • This was the first real scientific recognition that the Earth (and, by implication, the Universe had a definite beginning. He worked out the age of the sun as at least a few tens of millions of years based on the most efficient processes for generating heat known at the time (until the later discovery of radiation).
• Rudolf Clausius (1822-1888)
  • Defined « entropy ». it always increases. It is only possible for order to be preserved or to increase in local regions, such as the Earth, where there is a flow of energy from outside (ie the Sun) to feed off. But it is a law of nature that the decrease in entropy produced by life on earth feeding off the Sun is smaller than the increase in entropy associated with the processes that keep the Sun shining, whatever they may be. This cannot go on for ever - the supply of energy from the Sun is not inexhaustible.
  • Introduced the idea of a mean free path - the average distance that a molecule travels between collisions, and it is tiny. Every molecule experiences more than 8bn collisions per second. It is the shortness of the mean free path and the frequency of these collisions that gives the illusion that a gas is a smooth, continuous fluid, when it is really made up of a vast number of tiny particles in constant motion with nothing at all in the gaps between the particles. This led to a full understanding of the relationship between heat and motion - the temperature of an object is a measure of the mean speed with which the atoms and molecules that makeup the object are moving - and the final abandonment of the caloric
• James Clerk Maxwell (1831-1879):
  • Applied Clausius’s ideas to explain many of the observed properties of gases, such as their viscosity and the way they cool when they expand (because atoms and molecules in a gas attract one another slightly, so work has to be done to overcome this attraction when the gas expands, slowing the particles down and therefore making the gas cooler)
  • Maxwell-Boltzmann distribution - statistical rule describing the distribution of the velocities (or kinetic energies) of the molecules in a gas around their mean.
• Albert Einstein (1879-1955):
  • Obsessed with the idea of proving that atoms are real.
  • Used the second law or the tendency of all differences in the Universe to average out. A system in which there is a clear pattern (or even a vague pattern) has more order, and therefore less entropy, than a system in which there is no pattern. Nature abhors differences.
  • Osmetic pressure depends on the number of molecules of the solute (in this case sugar) in the solution. The more concentrated the solution, the greater the pressure.
  • Einstein explained Brownian motion, the jerky, zig-zag fashion of grains in water. The motion might be caused by the impact of molecules with the grains. A better way might be in statistical terms. The process is now known as a « random walk » and the statistics behind it turn out to be important, for example, in describing the decay of radioactive elements.
  • The blue color of the sky is caused by the way light is scattered by the molecules of gas in the air itself.

Let There Be Light

• Francesco Grimaldi (1618-1663)
  • Diffraction - direct evidence that light travels as a wave
• Euler published his model of light in 1746 and specifically made the analogy between light waves and sound waves and described the Sun as « a bell ringing out light »
• Thomas Young (1773-1829)
  • Explained the focusing mechanism of the eye (the way muscles change the shape of the lens of the eye)
  • Explained astigmatism as an uneven curvature in the cornea.
  • First person to appreciate that color vision is produced by a combination of three primary colors (red, green, and blue) which affect different receptors in the eye and explained color blindness as a failure of one or more sets of these receptors
  • Played a leading role in deciphering the Rosetta Stone.
  • « Outlines of Experiments and Enquiries Respecting Sound and Light » (1800) - proposing that different colors of light correspond to different wavelengths.
  • The idea of interference in light waves (like interfering ripples on a pond).
  • Calculated the wavelength of red and violet light
  • Young’s double slit experiment - where there can be dark where the peaks and troughs of two light waves cancel each other out.
• Augustin Fresnel (1788-1827):
  • Developed an efficient lens made of concentric annular rings of glass, each with a slightly different curvature (the Fresnel lens)
• William Wollaston (1766-1828):
  • Discovered the elements rhodium and palladium
• Josef von Frauenhofer (1787-1826):
  • Frauenhofer lines
  • Diffraction grating
  • Counted 576 dark lines between the red and violet ends of the spectrum and proved that they were a property of the light itself
• Robert Bunsen (1811-1899) and Gustav Kirchoff (1824-1887):
  • Each element, when hot, produces a characteristic pattern of bright lines in the spectrum, like the pair of yellow lines associated with sodium. Each pattern is as distinctive as a fingerprint or a barcode.
• Norman Lockyer (1836-1920):
  • Identified Helium based on the fact that it had an unknown fingerprint.
• Michael Faraday (1791-1867):
  • Oersted had discovered in 1820 that there is a magnetic effect associated with an electric current and when a magnetic compass needle is held over a wire carrying an electric current it is deflected to point across the wire at right angles.
  • Faraday realized that the wire should be forced to move in a circle around a fixed magnet - « The effort of the wire is always to pass off at a right angle from the pole of the magnet, indeed to go in a circle around it.
  • This is the basis of the electric motor - sixty years later, electric trains were running in Germany and the USA.
  • Liquefies chlorine in 1823.
  • Discovers benzene in 1825, which had the archetypal ring structure of importance to the molecules of life.
  • If an electric current can induce a magnetic force, can a magnet induce an electric current.
  • An electric current passing through a wire wound in a helix/coil would make it act like a bar magnet with poles, and if it is wound round an iron rod, the rod becomes a magnet when the current is turned on.
  • Invented the electric generator or dynamo, which uses the relative motion of coils of wire and magnets to generate electric currents.
  • Introduced terms « electrolyte », « electrode », « anode », « cathode », and « ion ».
  • Introduced concept of « lines of force » in 1831
  • In 1832, suggested that magnetic forces take time to travel across space, proposing that a wave motion was involved and making a tenuous connection with light
  • Magnetic, electric, and gravitational lines of force filled the Universe, according to Faraday, and were the reality with which the seemingly material entities that make up the world are interconnected. The material world, from atoms to the Sun and Earth and beyond, was simply a result of knots in the various fields.
• James Clerk Maxwell (1831-1879):
  • Definitively explained light as an electromagnetic phenomenon
  • His work on color vision was the foundation for color photography, for color TV and computer monitors.
  • Published a set of four papers On Physical Lines of Force (1861-2) - « light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena. »
  • « A Dynamical Theory of the Electromagnetic Field » (1864) - summed up all of classical electricity and magnetism in four equations (Maxwell’s equations). This unified electricity and magnetism in the electromagnetic field.
  • Placed alongside Newton in the pantheon of great scientists - Newton’s laws and Maxwell’s equations explained everything known to physics at the end of the 1860s.
  • Introduced the constant c, which is the speed of light (299,792,458 m/s) - Light (including radiant heat and other radiations, if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws;
  • Predicted radio waves.
  • « Treatise on Electricity and Magnetism » (1873)
  • Set up and led the Cavendish Laboratory from 1874.
• Heinrich Hertz (1857-1894)
  • Confirmed the existence of radio waves, showing that they travel at the speed of light and can be reflected, refracted and diffracted.

The Last Hurrah of Classical Science

• Radioactivity is discovered at the end of the 19th C, turning geology into geophysics, providing a way to keep the Earth warm inside for long enough (and stars shining long enough) to support the increasingly long estimates of the age of the Earth (and the Universe).
• Radioactivity howed that the Earth's interior was not, in fact, cooling dramatically at all
• Permanentism held that continents were always continents and oceans always oceans, while contractionism was a variant of catastrophism
• Eduard Suess (1831-1914):
  • Proposed that Australia, India and Africa were fragments of Gondwanaland, an ancient landmass.
• Alfred Wegener (1880-1930):
  • Father of the theory of continental drift
  • Greenland expedition in 1912-3
  • "The Origin of Continents and Oceans" (1915)
  • Saw that the continents and ocean floors are fundamentally different
  • Proposed that there was once a single supercontinent, Pangea, which had broken up to create today's continents, but had no reason for its breakup.
  • Died on second Greenland expedition in 1930
• We now know that the Atlantic is widening at a rate of a few centimeters a year
• Arthur Holmes (1890-1965)
  • More than any other, "the man who measured the age of the Earth".
  • "Principals of Physical Geology" (1944) - a nod to Lyell
  • Apart from the radioactive heating, the key component was time - "solid" rock, warmed from beneath, could stretch and flow, like very thick treacle, but only very slowly. Convection currents inside the Earth, coming from heat generated by radioactive decay broke up Pangea. These currents would move continents at a rate of about 5cm/yr, so the Atlantic could be made from an initial crack over a period of about 100m years.
• In the early 20th C can now date sample of rock from the proportions of lead and uranium isotopes they contain, and these rocks ere much older than would be allowed by the idea of heat being released from the Sun only as a result of its gravitational collapse.
• Walter Elsasser (1904-1991):
  • The Earth's magnetism is generated by a natural internal dynamo
• Edward Bullard (1907-1980):
  • The Earth's magnetic field is the product of circulating conducting fluids in the hot fluid core (basically convection and rotation in molten iron)
  • The "Bullard fit" of the continents, published in 1965, shows them all stuck together in a single landmass.
• Rocks are magnetised when they are laid down, as molten material flowing from volcanoes or cracks in the Earth's crust, and once set they preserve the pattern of the magnetic field in which they formed, becoming like bar magnets, but realized that not all rocks are magnetized in the same direction and there is evidence that the geomagnetic field had flipped at some time in the past. Maybe this is due to the core's dynamo dying out and then building again in the opposite sense. The Sun also has magnetic reversal every 11 years or so, linked to the sun spot cycle.
• Much more knowledge of the sea bed, which is two-thirds of the Earth's surface.
• In the 1960s, see the ocean basins as the sites of action in continental drift with the continents being just carried along for the ride.
• The Atlantic is getting wider and the Pacific is narrowing. Eventually, America and Asia will collide, while the Red Sea is a new region of upwelling activity, which will splinter Africa away from Arabia to the east.
• The sea floor spreading is like a slow conveyor belt, endlessly looping round and round, but over the surface of the planet, everything cancels out and the planet stays the same size.
• Tuzo Wilson (1908-1993) - Coined the term "plate" for the rigid portions of the Earth's crust.
• Plate tectonics - seismically quiet regions of the globe are quiet because they form rigid plates (six large ones and about twelve small ones, covering the entire surface of the globe):
  • An individual plate may be just oceanic crust, just continental crust, or both.
  • Most of the interesting geological activity on Earth is happening at the boundaries between plates (plate margins).
  • Constructive margins are where new oceanic crust is being made at ocean ridges and spreading out on either side.
  • Destructive margins are where one plate is being pushed under the edge of another, going down at an angle of 45 degrees and melting back into the magma below
  • Conservative margins are where plates are merely rubbing sideways against each other as they rotate (eg San Andreas Fault)
  • Mountain ranges and former sea beds in the hearts of continents today, show that this activity has been going on since long before the breakup of Pangea, and that supercontinents have repeatedly broken up and rebuilt in different patterns.
• Louis Agassiz (1807-1873):
  • Coined the term "Ice Age"
  • "Studies on Glaciers" (1840)
  • Compared to a Great Flood, glaciation was uniformitarian.
• Joseph Adhémar (1797-1862):
  • "Revolutions of the Sea"
  • On 4 July each year, the Earth is furthest from the Sun and is moving at its slowest, and on 3 January it is at its closest, but the difference is only about 3% of the average 150km distance.
  • The Earth wobbles like a spinning top over the course of each 22k years, suggesting an alternating cycle of ice ages in the northern and southern hemispheres, but this last point turns out to be wrong
• Climate is determined by the distance of a planet from the Sun, the latitude of interest, and the angle at which the Sun’s rays strike the surface at that latitude. The key to Ice Ages is cool summers, not extra-cold winters. The last Ice Ages was intense about 80,000 years ago and ended about 10-15k years ago.
• The natural state of the Earth throughout most of its long history has been completely ice-free. But occasionally, at intervals separated by hundreds of millions of years, one or other hemisphere is plunged into a period of cold lasting for several million years, an Ice Epoch. There was an Ice Epoch lasting for about 20m years in the Permian and ending about 250m years ago
• After the Permian Ice Epoch ended, the world was warm for 200m years, while the dinosaurs flourished, but it started cooling again 55m years ago and by 10m years ago, glaciers returned.
• For the last 5m years, the Earth has been in what may be a unique state, with ice caps over both poles. In this period we have seen a succession of full Ice Ages about 100k years long, separated by warmer Interglacials like the present day, lasting about 10k years. The present Interglacial could be coming to an end naturally in a couple of thousand years — less time than the span of recorded history.
• It may be this unusual sequence of Ice Ages and Interglacials caused by continental drift that provided suitable selection pressures for us to break away from our hominid cousins and ratchet up our intelligence and adaptability as the key requirements for survival on the fringes of forests.

Inner Space

• Heinrich Geissler (1814-1879)
  • Invented the vacuum tube, technology that led to the discovery of electrons ("cathode rays") and X-rays.
• William Crookes (1832-1919)
  • Developed an improved vacuum tube known as the Crookes tube, with an even better (harder) vacuum
• JJ Thomson (1856-1940)
  • Found that cathode rays move much more slowly than light.
  • 1897 is often regarded as the year of the "discovery" of the electron - the atom was definitely not indivisible
• William Röntgen (1845-1923):
  • Discovered X-rays, which are not deflected by electric or magnetic fields, though they are a form of electromagnetic wave with wavelengths much shorter than visible light (or even ultraviolet light)
• Henri Becquerel (1852-1908
  • Phosphorescent salts can produce energy, it seems, out of nothing at all.
• Marie Curie (1867-1934)
  • In 1898 discovered that pitchblende is more radioactive than uranium, and must contain another, highly radioactive, element.
  • Discovered polonium and radium
  • discovered that radium outputs enough energy to heat gram after gram of water to boiling point seemingly endlessly.
• Ernest Rutherford (1871-1937)
  • Found that the radiation discovered by Becquerel is made up of two components, alpha radiation (with a short range) and beta radiation (with a much longer range and more penetrating power).
  • Discovered that during radioactive decay, an atom is converted into an atom of a different element
  • Pointed out that this storehouse of energy gave the Earth a possible lifetime of at least hundreds of millions of years
  • Alpha particles are the same as helium atoms which have lost two units of negative electric charge (they have lost two electrons)
  • Most of the mass and charge of an atom is concentrated in a tiny central nucleus, surrounded by a cloud of electrons.
  • An individual alpha particle weights 8000 times as much as an individual electron. The nucleus occupies only about one hundred-thousandth of the diameter of an atom, which is mostly empty space, filled with a web of electromagnetic forces linking the positive and negative charges.
  • Everything we think of as solid matter is mostly empty space.
  • Nitrogen atoms bombarded with alpha particles were converted into a form of oxygen, with the ejection of a hydrogen nucleus (a proton). This was the first artificial transmutation of an element, and marked the beginning of nuclear physics.
  • By 1920, experiments involving just a few atoms were becoming routine.
  • Beryllium exposed to alpha particles produced a new form of radiation explained in terms of gamma rays. The alpha radiation was knocking neutral particles out of the beryllium nuclei, and these neutral particles were in turn knocing protons (hydrogen nuclei) out of the paraffin
• Max Planck (1858-1947)
  • h is Planck's constant
• Albert Einstein (1879-1955)
  • Found that electromagnetic radiation behaves as if it consisted of mutually independent energy quanta
  • Light could be seen behaving either as a wave (the double-slit experiment) or as a stream of particles (the photoelectric effect). How could this be?
• Niels Bohr (1885-1962)
  • The reason why atoms are stable is entirely thanks to quantum physics.
  • Said that electrons had to stay in their orbits around the nucleus because they are not physically capable of continuously emitting radiation, as they would be if classical laws applied. An electron can only emit quanta of energy, one at a time, and this would correspond to it jumping down from one orbit to another.
  • Each allowed orbit had room for only a certain number of electrons, and that electrons further out from the nucleus cannot jump inwards if the inner orbits are already full.
• Louis de Broglie (1892-1987):
  • Just as electromagnetic waves could be described in terms of particles, all material particles, such as electrons, could be described in terms of waves.
  • The momentum of a particle multiplied by its wavelength is equal to Planck's constant.
  • Everything has dual wave-particle character, but the wave aspect scarcely matters at all above the molecular level (though it cannot entirely be ignored for whole atoms)
• Schrôdinger, Heisenberg, and Dirac:
  • The uncertainty principle - we cannot know, as a matter of principle, the present in all its details. The size of the smallest electron orbit in an atom is as small as it can be without violating the uncertainty principle.
  • Existence of a previously unknown particle, with the same mass as an electron but positive charge - the positron or antimatter.
  • The strong nuclear force (or strong force) is 100x stronger than the electric force, which is why there are about a hundred protons in the largest stable nuclei; any more, and electric repulsion overcomes the strong force and blows the nucleus apart. But the strong force, unlike electric, magnetic, and gravitational forces, does not obey an inverse square rule. It is very strong indeed over an extremely limited range, and cannot essentially be felt at all beyond that range.
• Wolfgang Pauli and Enrico Fermi
  • The weak nuclear force and the neutrino.
• Atoms are made of protons, neutrons, and electrons. The nucleus contains protons and neutrons, held together by the strong force, in which beta decay can take place as an effect of the weak force. The electrons are in a cloud outside the nucleus, held in place by electromagnetic forces but only allowed to occupy certain energy states by the rules of quantum physics. On large scales, gravity is important in holding bigger lumps of matter together.
• This gives us four particles (proton, neutron, electron, and neutrino plus their associated antiparticles) and four forces (electromagnetism, the strong and weak nuclear forces, and gravity. That is sufficient to explain everything that is detectable to our senses
• when two charged particles, such as two electrons, or an electron and proton, interact, they can be thought of as doing so by the exchange of photons. Two electrons, say, may move towards one another, exchange photons and be deflected on to new paths. It is this exchange of photons which produces the repulsion which shows up as an inverse square law, a law which emerges naturally from QED
• Protons and neutrons can be though of as composed on entities called quarks, held together by the exchange of entities analogous to photons, and that the strong nuclear force is just an outward manifestation of this deeper force at work.

The Realm of Life

• We are the most complicated things that we know about in the entire universe. This is because, on the cosmic scale of things, we are middle-sized. Small objects, like atoms, are composed of a few simple entities obeying a few simple rules. The universe is so big that the subtleties of even objects as large as stars can be ignored, and the whole cosmos can be treated as a single object made up of a reasonably smooth distribution of mass-energy, again obeying a few very simple laws. But on the scales where atoms are able to join together to make molecules, the number of compounds possible - the number of different ways in which atoms can join together to make molecules - is so great that a huge variety of different things with complicated structures can exist and interact with one another in subtile ways.
• Life as we know it is a manifestation of this ability for atoms to form a complex variety of large molecules. This complexity starts on the next scale up from atoms, with simple molecules such as water and carbon dioxide; it ends where molecules begin to be crushed out of existence by gravity.
• Imagine starting out with a set of objects made up of 10 atoms, 100 atoms, 1000 atoms, and so on, with each lump containing ten times more atoms than the one before:
  • The 24th object would be as big as a sugar cube
  • The 27th - about the size of a large mammal
  • The 39th - the size of a rock about a kilometer in diameter
  • The 54th - the size of the planet Jupiter
  • The 57th - about the size of the Sun, where even atoms are destroyed by gravity, leaving a mixture of nuclei and free electrons called a plasma.
• On this logarithmic scale, people are almost exactly halfway in size between atoms and stars. The realm of life forms like u,s investigated by Charles Darwin and his successors, is between the sizes of sugar lumps and large rocks
• The role of cells as the fundamental component of living things became clear at the end of the 1850s due to improving microscopic instruments and techniques.
• Matthias Shleiden (1804-1881), Theodor Schwann (1810-1882), John Goodsir (1814-1867), and Rudolf Virchow (1821-1902) developed the ideas of:
  • All plant tissues are made of cells.
  • All living things are made of cells.
  • Cells arise only from other cells, by division.
  • Every cell is derived from a preexisting cell and disease is no more than the response of a cell (or cells) to abnormal conditions, including tumors, which are derived from pre-existing cells in the the body;
• The microscopic techniques available at the time were more than adequate to show the structure of the cell as a bag of watery jelly with a central concentration of material, the nucleus.
• Walther Flemming (1843-1915) - discovered in 1879 that the nucleus contains thread-like structures which readily absorb colored dyes used by microscopists to stain cells and highlight their structure. These became known as chromosomes.
• August Weismann (1834-1914):
  • Saw chromosomes as the carriers of hereditary information - « heredity is brought about by the transmission from one generation to another of a substance with a definite chemical and, above all, molecular constitution, »chromatin ».
  • Spelled out two kinds of cell division:
    • For growth and development, all the chromosomes in a cell are duplicated before the cell divides, so each daughter cell obtains a copy of the original set of chromosomes
    • For production of egg or sperm cells, the amount of chromatin is halved, so that a full set of chromosomes is only restored when two such cells fused to create the potential for the development of a new individual.
  • Showed that the cells responsible for reproduction are not involved with other processes going on in the body, and the cells that make up the rest of the body are not involved with the manufacture of reproductive cells, so that Darwin’s pangenesis and Lamarck’s ideas are both definitely wrong.
• In 1909, Wilhelm Johannsen, uses term « gene »
• William Bateson (1861-1926) - coined the term « genetics »
• Gregor Mendel (1822-1884) - Father of genetics
  • In 1856, began an intensive study of the way heredity works in peas, working with 28, 000 plants, each pollinated by hand
  • There is something in a plant that determines the properties of its overall form. These genes come in pairs, which can be the same, RR or SS, or different, RS. Only one of the genes in a pair, though, is expressed in the individual plant (the phenotype). If the R is ignored and the S expressed, then the S is said to be dominant and the R recessive.
  • With this, Mendel showed that inheritance works not by blending characteristics, but by taking individual characteristics from each parent.
• Thomas Hunt Morgan (1866-1945)
  • Wanted to disprove (or limit) Mendel’s findings
  • Worked with Drosophila, fruit flies, where there is a new generation every two weeks, with each female laying hundreds of eggs at a time.
  • Drosophila have only four pairs of chromosomes and one pair has a particular significance in all sexually reproducing species.
  • Sex chromosomes are in one of two shapes, X and Y. Females always carry the XX pair, while males always have an XY combination.
  • Different varieties of a particular gene are called alleles.
  • Mendelian heredity and genetics came of age in 195, when he and his colleagues published « The Mechanism of Mendelian Heredity ».
  • « The Theory of the Gene » (1926)
• The constant reshuffling of the genetic possibilities provided by the process of reproduction encourages diversity, which explains why it is so easy for sexually reproducing species to adapt to changing environmental conditions. Asexual species do evolve, but only much more slowly.
• In human beings, there are about 30k genes that determine the phenotype. Just over 93% are homozygous - the same on each chromosome of the relevant pair, in all human beings. Just under 7% are heterozygous, which means that there is a chance that there are different alleles for that particular gene on the paired chromosomes of an individual person chosen at random. With some 2000 pairs of genes which come in at least two varieties, no two people on Earth are genetically identical (except for twin who share the same genotype because they come from the same fertilized egg)
• Friedrich Miescher (1844-1895):
  • Found that the composition of the nucleus was significantly different from that of protein. This substance, the nuclein, contains a lot of carbon, hydrogen, oxygen, and nitrogen, like other organic molecules, but he also found a significant amount of phosphorous, unlike any protein
  • The sperm cell is almost all nucleus, with only a trace of cytoplasm.
• The building block which gives its name to DNA is ribose, a sugar whose central structure consistas of four carbon atoms linked with an oxygen atom in a pentagonal ring, with other atoms (notably hydrogen-oxygen pairs, OH) attached at the corners. These attachments can be replaced by other molecules, linking the ribose units to them
  • The second building block, which attaches in this way, is a molecular group containing phosphorous, and is known as a phosphate group, acting as a link between ribose pentagons in an alternating chain.
  • The third building block come in five varieties, called bases, known as guanine, adenine, cytosine, thymine, and uracil (G, A, C, T, and U); One base is attached to each of the sugar rings in the chain, sticking out at the side.
  • The ribose pentagons gives the overall molecule its name, ribonucleic acid, or RNA
  • DNA, Deoxyribonucleic acide is almost identical, with one less oxygen atom.
  • RNA contains G, A, C, and U, while DNA contains G, A, C, and T.
  • Covalent bonds - Carbon has six protons in its nucleus (and six neutrons, as it happens), plus six electrons in its cloud. Four bonds is the maximum any atom can make, and bonds are stronger for shells closer to the central nucleus
  • Ionic bonds
  • There is no arbitrariness in the arrangement of electrons in atoms and atoms in molecules - the arrangements which are most stable in the atoms and molecules are always the arrangements with the least energy.
• X-rays are a form of electromagnetic wave, like light but with shorter wavelengths
• In a substance like sodium chloride there are no individual molecules (NaCl), but an array of sodium ions and chlorine ions arranged in a geometric pattern.
• The structure of biomolecules, such as haemoglobin, insulin, and the muscle protein, myoglobin
• Hydrogen bonds - unlike all other chemically reactive atoms (helium is not chemically reactive), hydrogen has no other electrons in inner shells to help conceal the positive charge on its proton, so some of the positive charge is « visible » to any nearby atoms or molecules. This will attract any nearby atom which has a preponderance of negative charge - such as an oxygen atom in a water molecule, which has gained extra negative charge from its two hydrogen atoms.
• Astbury showed that globular protein molecules (such as haemoglobin and myoglobin) are made up of long-chain proteins (polypeptide chains) that are folded up to make balls.
• Pauling laid out in detail the chemical structure of hair, feathers, muscles, silk, horn, and other protein, as well as the alpha-helix structure, as it became known, of the fibres themselves.
• James Watson (1928-) and Francis Crick (1916-) where influenced by Schrôdinger’s « What is Life? » (1944), which noted that in a code similar to the Morse code but with three symbols, not just dot and dash, used in groups of ten, you could form 88,572 different « letters ».
• Griffith worked out from the shapes of the molecules that adenine and thymine could fit together, linking up through a pair of hydrogen bonds, while guanine and cytosine could also fit together, linking up through a set of three hydrogen bonds, but that the four bases could not pair up in any other way.
• The structure of DNA must involve pairs of long-chain molecules, linked together by AG and CT bridges
• The genetic code is actually written in triplets, with sets of three bases, such as CTA or GGC, representing each of the twenty or so individual amino acids used in the proteins that build and run the body. When proteins are being manufactured by the cell, the relevant part of the DNA helix containing the appropriate gene uncoils, and a string of three-letter codons is copied into a strand of RNA (which of RNA and DNA was first?); this messenger RNA, whose only essential difference to DNA is that it has uracil everywhere DNA has thymine, is then used as a template to assemble a string of amino acids corresponding to the codons, which are linked together to make the required protein. It keeps doing this until no more of that particular protein is required. The DNA has long since coiled up again, and after enough protein has been manufactured the RNA is disassembled and its components reused. Just how the cell knows when and where to do all this remains to be explained, but the principles of the process were clear by the mid 1960s
• During all the copying of DNA that goes on when cells divide, there must occasionally be mistakes. Bits of DNA get copied twice, or bits get left out, or one base (one letter in the genetic code) gets accidentally replaced by another. None of this matters much in the kind of cell division that produces growth, since all that happens is that a bit of DNA in a single cell (probably not even a bit of DNA that that particular cell uses) has been changed. But when reproductive cells are produced by the special process of division that halves the amount of DNA in the daughter cells, not only is there more scope for mistakes to occur.
• By the late 1990s, it had been established that human being share 98.4% of their genetic material with the chimpanzee and the gorilla making us, in popular terminology, only « 1% human »
• The human, chimp, and gorilla lines split from a common stock just 4m ya.
• Human beings have only about 30k genes, which are capable of making at least 250k proteins. This is only twice as many genes as the fruit fly, and just 4k more than a garden weed called thale cress. A few key genes are different in us, compared with our closest relatives, and these are affecting the way the other genes operate.

Outer Space

• Units:
  • au (Astronomical Unit) - approx 150m km - for distances within the Solar System - average distance between the Earth and the Sun
    • Earth is 1 au from the Sun (149.6m km).
    • Mars is 1.5 au from the Sun
    • Jupiter is 5.2 au from the Sun
    • Neptune is 30 au from the Sun
    • Sirius is 550 au, or 2.67 pc, away.
  • ly (Light Year) - approx 9460b km or 63,240 au - the distance light travels in a vacuum in a year
    • Speed of light - 300,000km/s
    • The Milky Way is 75,000 ly (34kpc) in diameter
    • The Andromeda Galaxy is more than 2.5m ly (780 kpc) from Earth (closest galaxy to us)
    • Proxima Centauri is 4.22 ly (1.3 pc) from Earth (closest star to the Sun)
    • Sirius is 2.67 pc from us, and is brighter than the Sun
    • 61 Cygni is 11.2 ly or 3.4 pc away.
    • Orion Nebula is 1501 ly from Earth
  • Parsec (pc) - 3.26 ly (206,265 au, 30.9t km). Also kiloparsec (kpc), megaparsec (Mpc), gigaparsec (Gpc)
    • The center of the Milky Way is more than 8kpc from the Earth
    • The nearest large galaxy cluster, the Virgo Cluster is about 16.5Mpc from the Earth
    • The galaxy RXJ1242-11, with a supermassive black hole core similar to the Milky Way’s is about 200Mpc from the Earth
    • The particle horizon (the boundary of the observable universe has a radius of approx 14Gpc
• Our understanding of the universe at large rests upon two foundations - being able to measure the distances to the stars, and being able to measure the compositions of the stars.
• The Moon, our nearest neighbor, is just 384,400km away
• The Sun is 149.6m km away.
• By the end of the 18th C, astronomers had a very good idea of the scale of the Solar System.
• Parallax effect - hold a finger out at arm’s length and close each of your eyes in turn, the position of the finger seems to move against the background of more distant objects.
• Astronomers define one parallax second of arc, or parsec, as the distance to a star which would show a displacement of one second of are on the sky from opposite ends of a baseline 1 AU (150m km) long.
• The brightness of an object is inversely proportional to the square of its distance.
• The nearest star to the Sun is 7000 times further away than Pluto
• By 1950, the distances to some 10,000 stars had been determined, and by 2000, some 120,000 stars had been measured.
• Modern astronomy, astrophysics, only began at the beginning of the 20th C because of the application of photographic techniques to preserve images of the stars
• The masses of stars
• The Doppler effect - produces a redshift, with the size of the shift indicating the relative speed of he object.
• Keppler’s laws
• The Hertzsprung- Russell diagram (HR or color-magnitude diagram) is as important to astronomy as the table of elements is to chemistry:
  • Developed by Ejnar Hertzsprung (1873-) and Henry Norris Russell (1877-).
  • The temperature of a star is closely related to its color.
  • Blue and white stars are always intrinsically bright, while some orange and red stars are bright and some are faint.
  • The color, precisely defined, can tell you the temperature of the surface emitting the light.
  • The intrisic brightness of a star (its absolute magnitude), tells you how much energy the star is radiating overall, regardless of its temperature.
  • Most stars lie on a band running diagonally across the diagram, with hot, massive stars about the same size (diameter) as the Sun at one end of the band, and cool, dim stars with less mass than the Sun at the other end.
  • The Sun itself is an average star, roughly in the middle of this so-called main sequence.
  • Red giants - large, cool but bright stars, lie above the main sequence
  • White dwarfs - small but hot stars, lie below the main sequence
• Arthur Eddington (1882-):
  • The first astrophysicist, popularized Einstein’s theories of relativity in English - « the man who proved Einstein was right »
  • With the data from the HR diagram, showed that the brighter stars are the most massive. A main sequence star 25x the mass of the Sun, for example, is 4000x as bright as the Sun. A star holds itself up by the pressure it generates in its interior, counteracting the inward pull of gravity. The more massive it is, the more weight there is pressing inwards and the more pressure it has to generate. It can only do this by burning its fuel mor quickly, thereby generating more heat, which eventually escapes from the surface of the star as more light for us to see.
  • The temperature at the heart of a star can be calculated from observations of its brightness, mass and size
  • Eddington discovered that all main sequence stars have roughly the same central temperature, even though they cover a range in masses from tens of times the mass of the Sun down to a tenth the mass of the Sun. It seems as if stars have an inbuilt thermostat; as a ball of gas shrinks under its own weight and gets hotter inside as gravitational energy is converted into heat, nothing happens to halt this process until a critical temperature is reached, when the thermostat switches on an almost inexhaustible supply of energy.
  • It was clear that « subatomic » energy must hold the key to the longevity of the Sun and stars: « If 5% of a star’s mass consists initially of hydrogen atoms, which are gradually being combined to form more complex elements, the total heat liberated will more than suffice for our demands, and we need look no further for the source of a star’s energy.
• The Mily Way is nothing special in the Universe.
• One class of pulsating stars, the Cepheids, all show a characteristic pattern of repeated brightening and dimming, but some have periods as short as a day or so, while others have periods of more than a hundred days.
• The Large and Small Magellanic Clouds are now known to be small satellite galaxies associated with the Milky Way. Hertzsprung’s calibration implied a distance to the Small Magellanic Clouds (SMC) of 30,000 ly (approx 10 kpc) - now we see these, taking account of reddening and extinction effects, as 170,000 ly (52 kpc)
• George Ellery Hale (1868-1938)
  • Organized the construction of the 100-inch Hooker telescope, completed on Mount Wilson in 1918 and still in use today. It was the largest telescope of Earth for 30 years
• Edwin Hubble (1889-1953) and Milton Humason (1891-1972)
• The band of light on the night sky known as the Milky Way is a flattened, disc-shapes system containing vast numbers of stars, and the Sun is just one star among this multitude.** The Sun is not at the center of the Milky Way
• Exploding stars, supernovae, all have roughly the same absolute maximum brightness.
• There are hundreds of billions of galaxies in the visible Universe, which extends for billions of light years in all directions.
• Our galaxy is just average in size.
• There is a relationship between the distance to a galaxy and the redshift in the redshift in the spectrum of light from it.
• Einstein’s great insight was to appreciate that there is no distinction between acceleration and gravity.
• Bernhard Reimann (1826-1866) developed the mathematical tools to describe non-Euclidean geometry of curved surfaces, following on from the work of Karl Friedrich Gauss (1777-1855).
  • In non-Euclidean geometry, parallel lines can cross one another
  • Found a general mathematical treatment which was the footing for the whole of geometry, allowing a range of different mathematical descriptions of a range of different geometries, which are all equally valid and with the familiar Euclidean geometry of everyday life as just one example
  • The best way to describe the Universe at large is in terms of curved space.
  • Concentrations of matter, such as the Sun, are now seen as making little dimples in the spacetime of an otherwise flat Universe.
• The general theory of relativity describes the relationship between spacetime and matter, with gravity as the interaction that links the two. The presence of matter bends spacetime, and the way material objects (or even light) follow the bends in spacetime is what shows up to us as gravity: «  Matter tells spacetime how to bend; spacetime tells matter how to move ».
• The equations that Einstein found had one bizarre and unexpected feature. In their original form, they did not allow for the possibility of a static universe. The equations insisted that space itself must either be stretching as time passed, or shrinking, but could not stand still. So he added another term, using the Greek letter lambda (λ) - the cosmological constant.
• You can choose different values for lambda, some of which would make the universe expand faster, at least one of which would hold it still, and some of which would make it shrink.
• The cosmological redshift is not caused by galaxies moving through space, and is not, therefore, a Doppler effect. It is cause by the space between the galaxies stretching as time passes, exactly in the way that Einstein’s equations described, but Einstein refused to believe, in 1917. If space stretches while light is en route to us from another galaxy, then the light itself will be stretched to longer wavelengths, which, for visible light, means moving it towards the red end of the spectrum. The existence of the observed redshift-distance relation (Hubble’s law) implies that the Universe was smaller in the past, not in the sense that galaxies were crammed together in a lump in a sea of empty space, but because there was not space either between the galaxies or “outside” them - there was no outside. This in turn implies a beginning to the Universe - the Big Bang model.
• The steady state model sees the Universe as eternal, always expanding, but always looking much the same as it does today because new matter, in the form of atoms of hydrogen, is continuously being created in the gaps left behind as galaxies move apart , at just the right rate to make new galaxies to fill the gaps. This was a sensible and viable alternative to the Big Band model right through the 1950s and into the 1960s - it is, after all, no more surprising that matter should be created steadily, one atom at a time, than it is to suggest that all the atoms in the Universe were created in one event, the Big Bang. But improving observations, including the new techniques of radio astronomy developed in the 2nd half of the 20th C, showed that galaxies far away across the Universe, which we see by light (or radio waves) which left them long ago, are different from nearby galaxies, proving that the Universe is changing as time passes and galaxies age.
• By the end of the 20th C the age of the Universe had been determined reasonably accurately as somewhere between 13bn and 16bn years. This is calculated from the general theory of relativity and deals with the laws of physics on the very large scale. The ages of stars are essentially calculated from the laws of quantum mechanics, physics on the very small scale. The ages of the Universe comes out to be just enough older than the ages of the oldest stars to allow the time required for the first stars to form after the Big Bang. Thus are agree physics on both the largest and smallest scales.
• The chemical elements we, and the rest of the visible Universe are made of have to come from somewhere, and Gamow guessed that the raw material for their manufacture was a hot fireball of neutrons.
• Although the raw material was assumed to be neutrons, neutrons themselves decay in this way to produce electrons and protons, which together make the first element, hydrogen. Adding a neutron to a hydrogen nucleus gives a nucleus of deuterium (heavy hydrogen), adding a further proton makes helium-3, and adding another neutron as well makes helium-4, which can also be made by the fusion of two helium-3 nuclei and the ejection of two protons, and so on. Nearly all the deuterium and helium-3 is converted into helium-4, one way or another. Alpher and Gamow looked at all of the available neutron-capture date for different elements and found that the nuclei formed most easily in this way turned out to be those of the most common elements, while nuclei that did not form readily in this way corresponded to rare elements.
• 1 April 1948 marks the beginning of Big Band cosmology as a quantitative science with the publication of the Alpher, Bethe, Gamow (alpha-beta-gamma) paper
• The cosmic microwave background radiation is the most powerful single piece of evidence that there really was a Big Bang, that the visible Universe experienced an extremely hot, dense phase about 13bn ya
• If you can only make hydrogen and helium (and tiny traces of lithium-7 and deuterium) in the Big Bang, then all the other elements must have been manufactured somewhere else. The somewhere is the inside of stars. In the 1920s and 1930s, we began to realize that the Sun and stars are not made of the same mixture of elements as the Earth.
• The composition of stellar atmospheres is dominated by hydrogen. There are a million times more hydrogen atoms present in the atmosphere of stars than there are atoms of everything else put together. Heavy elements are rare in stars and hydrogen and helium make up 99% of star stuff.
• Hans Bethe and Carl von Weizsäcker identified two processes:
  • The proton-proton chain
  • The carbon cycle, operates in a loop and requires the presence of a few nuclei of carbon, involvin protons tunnelling into these nuclei. Because it is a loop, these heavy nuclei emerge at the end of the cycle unchanged, effectively acting as catalysts. It outputs an alpha particle (a nucleus of helium-4 - the net effect is that four protons have been convered into a single nucleus of helium, with a couple of positrons and a lot of energy ejected along the way.
• Once you have carbon nuclei to work with, you can make heavier elements still by adding more alpha particles (going from carbon-12 to oxygen-16 to neon-20 and so on.
• The definitive account of how the elements are built up in this way inside stars is given in a paper in 1957 by BBFH, and this understanding of nuclear fusion processes inside stars explained how all the elements up to iron can be manufactured from hydrogen and helium. Even better, the proportions of the elements predicted to be produced match the proportions seen in the Universe at large.
• But it cannot explain the existence of elements heavier than iron, because iron nuclei represent the most stable form of everyday matter with the least energy. To make nuclei of even heavier elements - such as gold, uranium, or lead - energy has to be put in to force the nuclei to fuse together. This happens when stars rather more massive than the Sun reach the end of their lives and run out of nuclear fuel which can generate heat to hold them up. When their fuel runs out, such stars collapse dramatically in upon themselves and as they do so, enormous amounts of gravitational energy are released and converted into heat. One effect of this is to make the single star shine, for a few weeks, as brightly as a whole galaxy of ordinary stars, as it becomes a supernova; another is to provide the energy which fuses nuclei together to make the heaviest elements. And a third effect is to power a huge explosion in which most of the material of the star, including those heavy elements, is scattered through interstellar space, to form part of the raw material of new stars, planets and possibly people.
• In 1987, a supernova was seen to explode in our near neighbor, the Large Magellanic Cloud
• Apart from helium, which is an inert gas that does not take part in chemical reactions, the four most common elements in the Universe are hydrogen, carbon, oxygen and nitrogen, collectively known as CHON
• There is no evidence of a special life force, and all of life on Earth, including us, is based on chemical processes. And the four most common elements involved in the chemistry of life are hydrogen, carbon, oxygen and nitrogen. We are made out of exactly the raw materials which are most easily available in the Universe. Earth is not a special place, and life forms based on CHON are likely to be found across the Universe, not just in our Galaxy but in others.
• Four and a half centuries after the publication of De Revolutionibus, we are in the situation a that small child who has just learned the rules of the game. We are just beginning to make our first attempts to play the game, with developments such as genetic engineering and artificial intelligence.