HISTORY OF PHYSICS

Electricity and magnetism: 5th century BC

Two natural phenomena, central to the study of , are observed and speculated upon by Greek natural scientists – probably in the 5th century BC, though Aristotle gives credit for the first observation of each to the shadowy figure of Thales.

One such phenomenon is the strange property of amber. If rubbed with fur it will attract feathers or bits of straw. Modern science, in its terms for the forces involved, acknowledges this Greek experiment with amber (electron in Greek). The behaviour of the amber is caused by what we call electricity, resulting from the transfer of what are now known as electrons.

The other natural phenomenon also derives its scientific name from Greek experiments. It is lodestone, a naturally occurring mineral (formed of iron oxide), which will surprisingly attract small pieces of iron.

The Greeks find this mineral in a region of Thessaly called Magnesia. They call it lithos magnetis, the ‘stone of Magnesia’. Thus the magnet is identified and named, though like rubbed amber it will only be a source of interest and amusement for the next 1000 years and more – until a practical purpose is found for it in the form of the compass.

Democritus and the atom: c.420 BC

In the late 5th century BC Democritus sets out an interesting theory of elemental physics. Notions of a similar kind have been hinted at by other Greek thinkers, but never so fully elaborated.

He states that all matter is composed of eternal, indivisible, indestructible and infinitely small substances which cling together in different combinations to form the objects perceptible to us. The Greek word for indivisible is atomos. This theory gives birth to the atom.

Democritus describes an extraordinary beginning to the universe. He explains that originally all atoms were whirling about in a chaotic manner, until collisions brought them together to form ever larger units – including eventually the world and all that is in it.

His theory will find few followers over the centuries. But his imagination provides an astonishing first glimpse of the Big Bang.

Science’s siesta: 8th – 15th century AD

In the profoundly Christian centuries of the European Middle Ages the prevailing mood is not conducive to scientific enquiry. God knows best, and so He should – since He created everything. Where practical knowledge is required, there are ancient authorities whose conclusions are accepted without question – Ptolemy in the field of astronomy, Galen on matters anatomical.

A few untypical scholars show an interest in scientific research. The 13th-century Franciscan friar Roger Bacon is the most often quoted example, but his studies include alchemy and astrology as well as optics and astronomy. The practical scepticism required for science must await the Renaissance.

Gilbert and the amber force: AD 1600

The year 1600 is a good one for William Gilbert. He is appointed court physician to Queen Elizabeth, and the summary of his life-long research into magnetism is published as De magnete, magneticisque corporibus, et de magno magnete tellure (Of the magnet, of magnetic bodies, and of the earth as a great magnet).

As the title states, Gilbert’s work has led him to the grand conclusion that compasses behave as they do because the earth itself is a vast magnet. He introduces the term ‘magnetic pole’, and states that the magnetic poles lie near the geographic poles.

Gilbert describes useful practical experiments, revealing how iron can be magnetized for use in compasses without relying on rare and expensive lodestone. Hammering the metal will do the trick, if the iron is correctly aligned with the earth’s magnetic field.

Gilbert’s researches also involve him in the mysterious property of amber, recognized 2000 years previously by Greek scientists. He identifies this as a force and coins a term for it from elektron, the Greek for amber. He calls it, in an invented Latin phrase, vis electrica – the ‘amber force’. Electricity has found its name.

and the Discorsi: AD 1634-1638

In December 1633 Galileo is place under house arrest, on the pope’s orders, because of his work on astronomy. Finding himself confined to his small estate at Arcetri near Florence, his response is typically positive. He settles down to explain and prove his early and less controversial discoveries in the mechanical sciences.

Two are particularly well known. The first he is said to have observed as a student in Pisa, when he watches a lamp swinging in the cathedral, times it by his own pulse, and discovers that each swing takes the same amount of time regardless of how far the lamp travels. At Arcetri he demonstrates this principle of the pendulum experimentally, and suggests its possible use in relation to clocks.

His other most famous discovery in physics, proved theoretically in about 1604 when he is professor of mathematics in Padua, is that bodies falling in a vacuum do so at the same speed and at a uniform rate of acceleration. (There is as yet no vacuum in which to demonstrate this law, but Boyle is able to do so later in the century.) While at Padua Galileo also works out the laws of ballistics, or the dynamics of objects moving through the air in a curve rather than falling directly to earth.

Written up and proved mathematically during 1634, these theorems are published in Leiden in 1638 as the Discorsi e dimostrazioni matematichè intorno à due nuove scienze attenenti alla mecanica et i movementi locali.

Galileo’s title claims to introduce two new sciences, mechanics and ‘local movements’, and his book stands at the start of mathematical physics. He is the first to use mathematics to understand and explain physical phenomena, and he is the first to make rigorous use of experiment to check results provided by theory. The attractive notion of his dropping weights from the leaning tower of Pisa, to check on the behaviour of falling bodies, is only a legend. But he certainly, if more mundanely, rolls balls down inclined planes for the same purpose.

Galileo provides the foundation on which (born in the year of Galileo’s death) soon builds.

Barometer and atmospheric pressure: AD 1643-1646

Like many significant discoveries, the principle of the barometer is observed by accident. Evangelista Torricelli, assistant to Galileo at the end of his life, is interested in why it is more difficult to pump water from a well in which the water lies far below ground level. He suspects that the reason may be the weight of the extra column of air above the water, and he devises a way of testing this theory.

He fills a glass tube, open at only one end, with mercury. Submerging the open end in a bath of mercury, and raising the tube to a vertical position, he finds that the mercury slips a little way down the tube. He reasons that the weight of air on the mercury in the bath is supporting the weight of the column of mercury in the tube.

If this is true, then the space in the glass tube above the mercury column must be a vacuum. This plunges him into instant controversy with traditionalists, wedded to the ancient theory – going as far back as Aristotle – that ‘nature abhors a vacuum’. But it also encourages von Guericke, in the next decade, to develop the vacuum pump.

The concept of variable atmospheric pressure occurs to Torricelli when he notices, in 1643, that the height of his column of mercury sometimes varies slightly from its normal level, which is 760 mm above the mercury level in the bath. Observation suggests that these variations relate closely to changes in the weather. The barometer is born.

With the weight of air thus established, Torricelli is able to predict that there must be less atmospheric pressure at higher altitudes. It is not hard to imagine an experiment which would test this, but the fame for proving the point in 1646 attaches to Blaise Pascal – though it is not even he who carries out the research.

Having a weak constitution, Pascal persuades his more robust brother-in-law to carry a barometer to different levels of the 4000-foot Puy de Dôme, near Clermont, and to take readings. The brother-in-law descends from the mountain with the welcome news that the readings were indeed different. Atmospheric pressure varies with altitude.

Von Guericke and the vacuum: AD 1654-1657

Spectators in the town square of Regensburg, on 8 May 1654, are treated to perhaps the most dramatic demonstration in the of science. Otto von Guericke, burgomaster of Magdeburg and part-time experimenter in physics, is about to demonstrate the reality of a vacuum.

Aristotle declared that there can be no such thing as empty space, but von Guericke has spent several years perfecting an air pump which can achieve just that. He now produces two hollow metal hemispheres and places them loosely together. There is no locking device. Von Guericke works for a while at his pump, attached by a tube to one of the hemispheres. He then signals that he is ready.

Sixteen horses are harnessed in two teams of eight. Each team is attached to one of the hemispheres. Whipped in opposite directions, the horses fail to pull the sphere apart. Yet when von Guericke undoes a nozzle of some kind, the two halves separate easily.

A mysterious point has been very forcefully made. Von Guericke’s experiments are first described in a book of 1657 (Mechanica Hydraulica-Pneumatica by Kaspar Schott). The vacuum thus becomes available to the scientific community as an experimental medium. Von Guericke himself uses it to demonstrate that a bell is muffled in a vacuum and a flame extinguished. , too, soon borrows the device.

Robert Boyle: AD 1661-1666

The experimental methods of modern science are considerably advanced by the work of Robert Boyle during the 1660s. He is skilful at devising experiments to test theories, though an early success is merely a matter of using von Guericke’s air pump to create a vacuum in which he can observe the behaviour of falling bodies. He is able to demonstrate the truth of Galileo’s proposition that all objects will fall at the same speed in a vacuum.

But Boyle also uses the air pump to make significant discoveries of his own – most notably that reduction in pressure reduces the boiling temperature of a liquid (water boils at 100° at normal air pressure, but at only 46°C if the pressure is reduced to one tenth).

Boyle’s best-known experiment involves a U-shaped glass tube open at one end. Air is trapped in the closed end by a column of mercury. Boyle can show that if the weight of mercury is doubled, the volume of air is halved. The conclusion is the principle known still in Britain and the USA as Boyle’s Law – that pressure and volume are inversely proportional for a fixed mass of gas at a constant temperature.

Boyle’s most famous work has a title perfectly expressing a correct scientific attitude. The Sceptical Chymist appears in 1661. Boyle is properly sceptical about contemporary theories on the nature of matter, which still derive mainly from the Greek theory of four elements.

His own notions are much closer to the truth. Indeed it is he who introduces the concept of the element in its modern sense, suggesting that such entities are ‘primitive and simple, or perfectly unmingled bodies’. Elements, as he imagines them, are ‘corpuscles’ of different sorts and sizes which arrange themselves into compounds – the chemical substances familiar to our senses. Compounds, he argues, can be broken down into their constituent elements. Boyle’s ideas in this field are further developed in his Origin of Forms and Qualities (1666).

Chemistry is Boyle’s prime interest, but he also makes intelligent contributions in the field of pure physics.

In an important work of 1663, Experiments and Considerations Touching Colours, Boyle argues that colours have no intrinsic identity but are modifications in light reflected from different surfaces. (This is demonstrated within a few years by Newton in his work on the spectrum.)

As a man of his time, Boyle is as much interested in theology as science. It comes as a shock to read his requirements for the annual Boyle lecture which he founds in his will. Instead of discussing science, the lecturers are to prove the truth of Christianity against ‘notorious infidels, viz., atheists, theists, pagans, Jews and Mahommedans’. The rules specifically forbid any mention of disagreement among Christian sects.

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