by Marcus Chown
But Maxwell’s theory of electromagnetism marks not only a profound change in our view of ultimate reality; it also contains the seeds of several scientific revolutions. The fact that a furnace should in theory contain an infinite number of electromagnetic vibrations – an utterly nonsensical conclusion – caused German physicist Max Planck to propose in 1900 that there must be an energy cut-off and that electromagnetic energy comes only in discrete chunks or ‘quanta’, the most energetic of which are too costly to be made in a furnace. This marked the birth of ‘quantum theory’, the modern description of atoms and their constituents.
Maxwell’s theory also contains the seed of relativity. The fact that the speed of light appears in the theory as an absolute, with no reference to the motion of its source or of anyone observing it, led Albert Einstein to propose in 1905 that the speed of light is the rock on which the universe is built, while space and time are but shifting sand. In fact, the ‘special’ theory of relativity reveals that space and time are aspects of the same thing: the seamless entity of ‘space–time’.
Special relativity provides further insight into Maxwell’s theory by resolving a paradox that is at its heart. A magnetic field arises whenever an electric charge such as an electron is moving, thus changing its electric field.§ But what if you were able to shrink yourself down to the size of such a mote of matter and catch up with it? Since you would now be stationary with respect to the particle, you would see an electric field, but no magnetic field. How can a magnetic field exist for one person but not for another? There is only way out of the paradox, according to Einstein: by recognising that electric and magnetic fields, like space and time, are not fundamental things; the fundamental things are the electromagnetic field and space–time. How much electric field, magnetic field, space and time you see individually depends on how fast you are moving.
Arguably the most important aspect of Maxwell’s theory of electromagnetism, however, is the concept of the field – invented by Faraday but given mathematical expression by Maxwell.
Faraday recognised that the electromagnetic field was a new kind of entity which differed from matter and could transmit effects from place to place. He intuited that electricity and magnetism are best understood via the field rather than via charged bodies and currents. When a current flows in a wire, the most important aspect of the phenomenon is not the current itself but the fields of electric and magnetic force that extend through space in its vicinity. This elevation of the field to a position of pre-eminence was Faraday’s greatest and most prescient achievement. He had anticipated the future of physics.
In the modern view, it is the fields – not just the electromagnetic field but the electron field, the up-quark field, the Higgs field, and so on – that are the ultimate entities from which the universe is constructed. In this picture, the fundamental forces, including the electromagnetic force, exist for no other reason than to enforce something known as ‘local gauge invariance’, which I will explain in more detail in Chapter 8. It is remarkable that the electrical and magnetic phenomena discovered during centuries of experimenting and theorising turn out to be nothing more than a consequence of this incredibly simple and universal principle.
Maxwell’s uniting of nature’s electric and magnetic forces in his theory of electromagnetism has been called the second great unification in physics, after Newton’s unification of the laws of heaven and Earth in his universal theory of gravity. Maxwell is now widely considered to be the most important physicist between the time of Newton and Einstein. Those who taunted him at school in Edinburgh, and whose own names nobody now remembers, came to learn their mistake during their lifetimes.
Maxwell did not live to see the prediction of this theory of electromagnetism fulfilled in the creation of Hertzian, or radio, waves. But although he died tragically young, he did not die as young as Hertz, who contracted a rare disease called Wegener’s granulomatosis, in which the immune system attacks the blood vessels, mainly in the ears, nose, sinuses, kidneys and lungs.13 Despite several operations, he died of septicaemia on 1 January 1894. He was just thirty-six.
In his last letter, sent to his parents on 9 December 1893, Hertz wrote, ‘If anything should really befall me, you are not to mourn; rather you must be proud a little and consider that I am among the especially elect destined to live for only a short while and yet to live enough.’14 He had no inkling that his discovery would change the world and make possible the twentieth and twenty-first centuries. Asked about the ramifications of his discovery, he said, ‘Nothing, I guess.’ And, when pressed, ‘It’s of no use whatsoever. This is just an experiment that proves Maestro Maxwell was right.’15
But despite the Hertz’s failure to recognise the importance of his discovery, it changed the world irrevocably. Radio, TV, Wi-Fi, microwave ovens, radar … the list of technologies it spawned is endless. Our ultra-connected world, which we take for granted and in which the air around us is criss-crossed by the invisible chatter of a billion voices, was born in Karlsruhe on 12 December 1887.
Notes
1 The Feynman Lectures on Physics, Volume II by Richard Feynman, Robert Leighton and Matthew Sands (Addison-Wesley, Boston, 1989).
2 ‘Tartan rosette: an animated recreation of James Clerk Maxwell’s “Tartan Ribbon” photograph, the world’s first colour photograph’ by Ron Pethig and David Peacock (James Clerk Maxwell Foundation: https://vimeo.com/130333096).
3 Davy’s invention of the safety lamp created a priority dispute, since the engineer George Stephenson came up with a similar design in the same year.
4 Ernest Rutherford had a similar experience. Always coming second in exams, he lost out on a coveted 1851 Great Exhibition scholarship to Britain to a fellow New Zealander. But at the eleventh hour, the other man, who had recently married, decided to take up a steady, well-paid government post in Auckland. Rutherford was digging potatoes on the family farm near Nelson when he got the good news. In later life, as he stood at the pinnacle of world science as Lord Rutherford, the greatest experimental physicist of the twentieth century, the thought of how nearly his life could have gone another way could bring him to tears.
5 In Frankenstein: Or the New Prometheus, Mary Shelley uses Humphry Davy as the model for the character of Professor Waldman. Victor Frankenstein studies with Waldman at the University of Ingolstadt in Bavaria. His lecture, which is very similar to Davy’s ‘A Discourse, Introductory to a Course of Lectures on Chemistry’ in 1802, inspires Frankenstein to search for the secret of life.
6 Volta himself gave Faraday a battery when he and Davy met the great man, then aged sixty-nine, in Italy in June 1814.
7 ‘Faraday’s Notebooks: Electromagnetic Rotations’ (Royal Institution of Great Britain: http://www.rigb.org/docs/faraday_notebooks__rotations_0.pdf).
8 It was James Clerk Maxwell who called Ampère ‘the Newton of Electricity’, mainly because the Frenchman formulated a law which expressed the electrical force between ‘current elements’ in much the same way that Newton formulated a law that expressed the gravitational force between masses.
9 Letter to Christian Schönbein (13 November 1845), The Letters of Faraday and Schoenbein, 1836–1862 (1899, p. 148).
10 The South Kensington Museum was renamed the Victoria and Albert Museum in 1899.
11 What Mad Pursuit: A Personal View of Scientific Discovery by Francis Crick (Basic Books, New York, 1990).
12 Now renamed 16 Palace Gardens Terrace.
13 Granulomatosis with polyangiitis (Wegener’s granulomatosis): https://www.nhs.uk/conditions/granulomatosis-with-polyangiitis/.
14 The Heinrich Hertz Wireless Experiments at Karlsruhe in the View of Modern Communication by D. Cichon and W. Wiesbeck (University of Karlsruhe, October 1995).
15 Dynamic Fields and Waves by Andrew Norton (CRC Press, Boca Raton, 2000, p. 83).
* Surprisingly, Jacob’s Well Mews, close to Marylebone High Street, has no plaque to mark the site of the forge, despite its importance in Faraday’s li
fe.
† A dielectric material consists of molecules which have a net positive charge on one side and a net negative charge on the other. In the presence of an electric force field – which, by convention, points in the direction in which positive charges move – the molecules line up along the direction of the field. The electric field of such ‘polarised’ molecules always acts to oppose and reduce the applied electric field. Maxwell christened the brief current that flows as electric charge is polarised, or ‘displaced’, a ‘displacement current’.
‡ When, eventually, Maxwell discovered that light was an electromagnetic wave, with an electric field oscillating in strength at right angles to an oscillating magnetic field and with both at right angles to the wave’s direction of motion, it become clear what ‘polarisation’ was. The electric (and hence the magnetic) field was free to vary in any plane whatsoever. This was the case in normal light, which turned out to be a mix of waves in which the electric field varied in all possible planes. By contrast, polarised light consisted of waves in which the electric field oscillated in a single plane – the ‘plane of polarisation’.
§ The electron, the fundamental grain of electricity, was discovered in ‘cathode rays’ by the British physicist J. J. Thomson in 1895. Such rays represent electricity in its naked state, travelling through the empty space of a ‘vacuum tube’ rather than concealed within a conducting wire. Electrons, which orbit the ‘nuclei’ of atoms, explain not only the phenomenon of electricity but all of chemistry, which is nothing more than a game of musical chairs in which electrons rearrange themselves within atoms.
3
Mirror, mirror on the wall
I think that the discovery of antimatter was perhaps the biggest jump of all the big jumps in physics in our century.
WERNER HEISENBERG1
‘How did you find the Dirac equation, Professor Dirac?’
‘I found it beautiful.’
PAUL DIRAC2
Pasadena, California, 2 August 1932
It was a window onto a new world, a new universe. An oddball of a man six thousand miles away in England would have realised that instantly, but the young man holding up the photograph – the physicist whose persistence and hard work had obtained the image – knew only that it was extraordinary. Carl Anderson, sitting at a desk on the third floor of Caltech’s Guggenheim Aeronautical Laboratory, put down the photograph and began writing the paper that would not only make his name but also make him one of the youngest people ever to win the Nobel Prize in Physics.
It had all started with Robert Millikan, the charismatic physicist whose relentless drive had transformed Pasadena’s Throop College of Technology into the world-renowned California Institute of Technology, more commonly known as Caltech. Millikan had become intrigued by the mysterious ‘radiation’ that the balloon-borne experiments of the Austrian physicist Victor Hess had revealed became stronger with altitude, indicating that its source was not the Earth but something in space.
At the time of Hess’s discovery in 1912, the only radiation known was that which emerged from unstable, or ‘radioactive’, elements, such as uranium, thorium and radium. The cores, or ‘nuclei’, of their atoms spat out subatomic bullets, in the form of alpha particles (helium nuclei), beta particles (electrons) and gamma rays (high-energy photons). As all three types of radiation rocketed through the air, they smashed apart atoms, whose ricocheting electrons could be detected when they charged up an ‘electroscope’ or triggered the rattlesnake clicking of a ‘Geiger counter’. Hess’s ‘cosmic rays’ – a name coined by Millikan in 1925 – mimicked this ‘ionising’ effect.
At the end of 1929, with Anderson nearing the end of his PhD, Millikan asked whether he would be interested in investigating cosmic rays. It was a no-brainer for the young student; he was in awe of the Caltech president, who had won the 1923 Nobel Prize in Physics for measuring the charge on an electron.3
Millikan thought cosmic rays were gamma rays with enormously higher energy than any found on Earth and that electrons they collided with would ricochet like billiard balls hit by a cue ball. By measuring the energy of such ‘Compton-scattered electrons’, it would be possible to estimate the energy of the gamma rays.* Millikan suggested that Anderson use a cloud chamber for the task, a remarkable device invented by Charles Wilson in Cambridge in 1911 which could reveal the tracks of subatomic particles. Its principle was simple and had been copied directly from nature. When moist air rises, it cools and condenses into droplets, forming clouds. Wilson mimicked this process by filling a glass chamber with moist air and suddenly cooling it. Air naturally cools when it expands, so he was able to achieve this by pulling out a piston to increase the volume of the air.
A water droplet will form only if there is a ‘seed’ such as a grain of dust around which it can condense; if the water vapour is so pure that it contains no such impurities, the condensation seeds may be provided by tiny charged ‘ions’ created when electrons are stripped from atoms by ionising radiation.
Wilson filled a glass chamber with ultra-pure water vapour and cooled it below the temperature at which droplets would normally form. In this ‘supercooled’ state, the water vapour was desperate to form droplets around any ions, and would do so the instant Wilson expanded and cooled his cloud chamber.
Operating the device proved to be more of an art than a science, but by illuminating the chamber with a bright light it was possible to photograph the tiny trails of bead-like water droplets left in the wake of a passing subatomic particle. Given how mind-bogglingly tiny subatomic particles are – a million million times smaller than the smallest speck visible to the naked eye – revealing their tracks was a stunning achievement. It earned Wilson a share of the 1927 Nobel Prize in Physics.
Millikan knew that if the water droplets were spread out, leaving a thin track, the particle responsible carried a relatively small electric charge; whereas, if the water droplets were crowded together, making a thick track, the particle had a relatively large charge. The charge would help to identify a particle created by a cosmic gamma ray but would not be enough to pin down its identity definitively. Millikan therefore suggested that Anderson place his cloud chamber in a magnetic field; this would bend the path of subatomic particles, with those of low momentum being more curved than those with high momentum. (Momentum, which is the product of a body’s mass and velocity, reflects the fact that a slow-moving, massive body is as hard to deflect as a fast-moving, light body.)
However, the cosmic rays and their subatomic debris were extremely penetrating – they were capable of smashing their way through a thick sheet of dense lead – which indicated that they had enormous energy and were travelling extremely fast. Such a fast-moving particle would spend very little time traversing the cloud chamber, which meant the magnetic field would have little opportunity to bend its trajectory. The only way to create a measurable deflection was to use the strongest possible magnetic field.
The experiment was a major challenge and it took a full year to assemble the apparatus in the optical shop of the Robinson Laboratory of Astrophysics, which had been set up to build a world-beating five-metre telescope for Mount Palomar Observatory near San Diego.4 The Great Depression, which had been triggered by the Wall Street Crash of 29 October 1929, was in full swing: money was tight, and Anderson had to salvage material for his experiment from local scrapyards. Fortunately, he had a long history of improvising with discarded equipment, having powered electrical experiments with used automobile batteries he had cadged from garages while a high-school student in Los Angeles.5
Anderson’s cloud chamber was the shape of a very shallow biscuit tin, three centimetres deep and seventeen centimetres in diameter. It was arranged with its long dimension vertical, and embedded in the coils of a solenoid – a tight coil of copper wire carrying an electric current. The bigger the current, the stronger the magnetic field, and the biggest current available at Caltech was produced by the 425-kilowatt generator that powered the wind tunnel in
the Guggenheim lab. This was why Anderson had installed his apparatus in the aeronautics building.
Large electric currents generate large amounts of heat, which was another major problem for Anderson. In order to stop his apparatus melting, he had to pump cooling water through pipes which spiralled around his solenoid. Although the cloud chamber at the heart of the experiment had a diameter barely greater than a tea plate, the entire apparatus ended up weighing close to two tonnes.
When operating, it was a fearsome sight. Tap water was pumped through at a rate of forty gallons a minute. Heated to almost boiling point by the current surging through the solenoid, it was piped out of the lab, down the outside wall of the Guggenheim lab and to a drain on the far side of neighbouring California Street (now California Boulevard). Anderson had no choice but to work at night because the Guggenheim’s generator was needed to power the wind tunnel during the day. Local Pasadena residents were alarmed at the sight of steam billowing upwards in the dark between the spindly palm trees on Arden Road, and it required all Millikan’s charm and powers of diplomacy to reassure them that their lives were not at risk.
Even more dramatic were the supernova-bright flashes of light that came intermittently from the third-floor window of the Guggenheim lab.6 Anderson used a powerful arc lamp to illuminate particle tracks and a camera to record them. For those strolling down Olive Walk after a pleasant dinner at Caltech’s new Athenaeum club, it seemed as if Frankenstein’s monster was being brought to life. Had anyone climbed the stairs to the third floor, the sight of Anderson, in white coat and welding glasses, crouched beside the magnetic coils of his apparatus, would have done little to put their mind at rest.
