Reviews Physics World  June 2021

Light at the end of the tunnel

David Appell reviews Lightspeed: the Ghostly Aether and the Race to Measure the Speed of Light by John Spence

Observational clues The transit of Venus allows a geometrical measurement of the Earth–Sun distance, and a measurement of the speed of light. (Courtesy: iStock/milehightraveler)

It is arguably the most important constant in physics, it is easily visualized by anyone and its symbol is widely known among the public: c, the speed of light in vacuum. If your neighbour knows an equation, it’s probably Albert Einstein’s E = mc2, and even schoolchildren learn that light takes about eight minutes to travel from the Sun to the Earth. Light and its speed have shocked physicists time and again – in 2001 it was even brought to a complete standstill in a cloud of supercooled atoms – and perhaps it is not finished yet.

In his highly informative and entertaining book Lightspeed: the Ghostly Aether and the Race to Measure the Speed of Light, John Spence, who is Richard Snell Professor of Physics at Arizona State University, recounts the history of humanity’s attempts to understand light and measure its speed. This may seem like a narrow focus, but light is so fundamental to the nature of the universe that this history encompasses most of the essential developments in physics, and features all of the giants – from Galileo to Einstein to today’s quantum computer scientists.

Light is so fundamental to the nature of the universe that this history encompasses most of the essential developments in physics

Spence begins with the story of Ole Rømer, a Danish astronomer who in 1676 established that the speed of light was finite. Showing ingenuity typical of the long string of methods that would follow over the centuries, Rømer used previous eclipse observations to predict a 10-minute delay in an upcoming eclipse of Jupiter’s moon Io as seen from Earth. This prediction was confirmed, bringing him instant recognition and suggesting that, according to the astronomical numbers of the day, light took about 11 minutes to travel from the Sun to the Earth. Today we know it to be 8 minutes 19 seconds on average.

This chapter is characteristic of the delights of the whole book; it not only thoroughly explores the science, but is also full of interesting diversions, making for a rich and engaging narrative. The biographical information, historical insights and personal stories combine into a hearty stew of a book, spiced with knowledge from Spence’s own long, luminous career as an optical physicist.

The anecdotes sprinkled throughout bring the famous scientists to life in their interactions with one another. One humorous story describes how, in his elder years, the eminent physicist William Thompson, Lord Kelvin, became highly opinionated and not much of a listener. He lived until 1907, when physics was quite a mess (though Einstein was beginning to straighten some of it out). Spence recounts how J J Thomson, discoverer of the electron, quipped that “[Kelvin] was a counter-example to the idea that a good emitter is a good absorber.”

I’ve always been a sucker for stories like this; in part they were what drew me into physics. But then I’m a romantic, and I think Spence is too. He skilfully covers the great Michael Faraday and James Clerk Maxwell of the 19th century, one an intuitive experimentalist and the other a magical theorist, whose work fit together like hand and glove.

Spence includes a story about how Maxwell derived his formula for the speed, v, of oscillating electromagnetic waves in a vacuum during a summer stay at his Glenlair residence in Scotland. Maxwell came up with the formula v = 1/√ε0μ0, but didn’t have the numerical values of the constants ε0 and μ0 to hand – they were in documents at his apartment in London. So he had to wait the rest of the summer until a 400-mile return on a steam train to evaluate the result, after which he found that he had indeed predicted the speed of light from his theory of electricity and magnetism.

Spence shows how our knowledge of light was filled in piece by piece as astronomers and physicists worked through observational clues like the transit of Venus, where geometry allows an estimate of the Earth–Sun distance to be made. (Hilarity ensues as British and French ships fan across the world to measure – or attempt to measure – the 1761 transit.) By the late 19th century physicists had worked themselves into a befuddled lather over the “aether”, the substance that they believed must fill the universe to allow the transmission of light. The towering figures of that era – Lord Rayleigh, Kelvin, Henri Poincaré – and nearly everyone else, simply could not conceive of light’s transmission through vacuous space.

Albert Michelson and Edward Morley’s eponymous experiment in 1887 – “the most famous negative result in physics” – cast serious doubt on the aether’s existence, confounding everyone. It took the singular genius of Einstein to cut through the imbroglio by assuming, correctly, that the velocity of light is independent of the velocity of its source, at once rewriting the human understanding of time and space.

Even in the telling of this profound shift, we are treated to another humanizing story. At a dinner at Caltech in 1931, Einstein asked Michelson why he had spent so many years doing tedious measurements with interferometers like the famous one he and Morley had used in the 1877 experiment. “Because,” Michelson simply replied, “I think it is fun.”

Spence ends his book with a chapter on faster-than-light schemes and nonlocal influences arising from the Einstein–Podolsky–Rosen paradox and the now well known “spooky action at a distance”. He also touches on discussions of the nature of quantum reality as deduced from the violation of Bell’s theorem. This last chapter feels rushed, taking on too many mind-blowing topics while not explaining anything in enough detail to grasp.

It doesn’t change my overall opinion of the book, though. This volume will work for everyone from a high-school student (just ignore the few equations, although they aren’t that difficult anyway) to anyone more well versed in physics who’s interested in an entertaining history of how we came to our current understanding. On finishing the book, I can’t help wondering how close to complete this understanding will turn out to be. Is there light at the end of the tunnel? Who knows?