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eclipse prediction should have settled the argument once and for all. Yet, as we have already seen in the case of the Sun-centred versus Earth-centred debate, factors beyond pure logic and reason sometimes influence the scientific consensus. Cassini was senior to Römer and also outlived him, so by political clout and simply by being alive he was able to sway opinion against Römer’s argument that light had a finite speed. A few decades later, however, Cassini and his colleagues gave way to a new generation of scientists who would take an unbiased look at Römer’s conclusion, test it for themselves and accept it.

      Once scientists had established that the speed of light was finite, they set about trying to solve yet another mystery concerning its propagation: what was the medium responsible for carrying light? Scientists knew that sound could travel in a variety of media –talkative humans send sound waves through the medium of gaseous air, whales sing to each other through the medium of liquid water, and we can hear the chattering of our teeth through the medium of the solid bones between teeth and ears. Light can also travel through gases, liquids and solids, such as air, water and glass, but there was a fundamental difference between light and sound, as demonstrated by Otto von Guericke, the Burgomeister of Magdeburg, Germany, who conducted a whole series of famous experiments in 1657.

      Von Guericke had invented the first vacuum pump and was keen to explore the strange properties of the vacuum. In one experiment he placed two large brass hemispheres face to face and evacuated the air from inside them so that they behaved like two exceedingly powerful suction cups. Then, in a marvellous display of scientific showmanship, he demonstrated that it was impossible for two teams of eight horses to pull the hemispheres apart.

      Although this equine tug-of-war showed the power of the vacuum, it said nothing about the nature of light. This question was addressed in a somewhat daintier experiment, which required von Guericke to evacuate a glass jar containing a ringing bell. As the air was sucked out of the jar, the audience could no longer hear the ringing, but they could still see the clapper hitting the bell. It was clear, therefore, that sound could not travel through a vacuum. At the same time, the experiment showed that light could travel through a vacuum because the bell did not vanish and the jar did not darken. Bizarrely, if light could travel through a vacuum, then something could travel through nothing.

      Confronted with this apparent paradox, scientists began to wonder if a vacuum was really empty. The jar had been evacuated of air, but perhaps there was something remaining inside, something that provided the medium for conveying light. By the nineteenth century, physicists had proposed that the entire universe was permeated by a substance they termed the luminiferous ether, which somehow acted as a medium for carrying light. This hypothetical substance had to possess some remarkable properties, as pointed out by the great Victorian scientist Lord Kelvin:

      Now what is the luminiferous ether? It is matter prodigiously less dense than air – millions and millions and millions of times less dense than air. We can form some sort of idea of its limitations. We believe it is a real thing, with great rigidity in comparison with its density: it may be made to vibrate 400 million million times per second; and yet be of such density as not to produce the slightest resistance to any body going through it.

      In other words, the ether was incredibly strong, yet strangely insubstantial. It was also transparent, frictionless and chemically inert. It was all around us, yet it was clearly hard to identify because nobody had ever seen it, grabbed it or bumped into it. Nevertheless, Albert Michelson, America’s first Nobel Laureate in physics, believed that he could prove its existence.

      Michelson’s Jewish parents had fled persecution in Prussia in 1854, when he was just two years old. He grew up and studied in San Francisco before going on to join the US Naval Academy, where he graduated a lowly twenty-fifth in seamanship, but top in optics. This prompted the Academy’s superintendent to remark: ‘If in the future you’d give less attention to those scientific things and more to your naval gunnery, there might come a time when you would know enough to be of some service to your country.’ Michelson sensibly moved into full-time optics research, and in 1878, aged just twenty-five, he determined the speed of light to be 299,910 ± 50 km/s, which was twenty times more accurate than any previous estimation.

      Then, in 1880, Michelson devised the experiment that he hoped would prove the existence of the light-bearing ether. His equipment split a single light beam into two separate perpendicular beams. One beam travelled in the same direction as the Earth’s movement through space, while the other beam moved in a direction at a right angle to the first beam. Both beams travelled an equal distance, were reflected off mirrors, and then returned to combine into a single beam. Upon combining they underwent a process known as interference, which allowed Michelson to compare the two beams and identify any discrepancy in travel times.

      Michelson knew that the Earth travels at roughly 100,000 km/h around the Sun, which presumably meant that it also passed through the ether at this speed. Since the ether was supposed to be a steady medium that permeated the universe, the Earth’s passage through the universe would create a sort of ether wind. This would be similar to the sort of pseudo-wind you would feel if you were speeding along in an open-top car on a still day – there is no actual wind, but there seems to be one due to your own motion. Therefore, if light is carried in and by the ether, its speed should be affected by the ether wind. More specifically, in Michelson’s experiment one light beam would be travelling into and against the ether wind and should thus have its speed significantly affected, while the other beam would be travelling across the ether wind and its speed should be less affected. If the travel times for the two beams were different, then Michelson would be able to use this discrepancy as strong evidence in favour of the ether’s existence.

      This experiment to detect the ether wind was complicated, so Michelson explained the underlying premise in terms of a puzzle:

      Suppose we have a river of width 100 feet, and two swimmers who both swim at the same speed, say 5 feet per second. The river flows at a steady rate of 3 feet per second. The swimmers race in the following way: they both start at the same point on one bank. One swims directly across the river to the closest point on the opposite bank, then turns around and swims back. The other stays on one side of the river, swimming downstream a distance (measured along the bank) exactly equal to the width of the river, then swims back to the start. Who wins? [See Figure 20 for the solution.]

      Figure 20 Albert Michelson used this swimming puzzle to explain his ether experiment. The two swimmers play the same role as the two beams of light heading in perpendicular directions, then both returning to the same starting point. One swims first with and then against the current, while the other swims across the current – just as one light beam travels with and against the ether wind, and the other across it. The puzzle is to work out the winner of a race over a distance of 200 feet between two swimmers who both can swim at 5 feet per second in still water. Swimmer A goes downstream 100 feet and back upstream 100 feet, whereas swimmer B goes across the river and back, also covering two legs of 100 feet. The river has a 3 ft/s current.

      The time of swimmer A, going downstream and then upstream, is easy to analyse. With the current, the swimmer has an overall speed of 8 ft/s (5 + 3 ft/s), so the 100 feet takes just 12.5 seconds. Coming back against the current means that he is swimming at only 2 ft/s (5 - 3 ft/s), so swimming this 100 feet takes him 50 seconds. Therefore his total time is 62.5 seconds to swim 200 feet.

      Swimmer B, going across the river, has to swim at an angle in order to compensate for the current. Pythagoras’ theorem tells us that if he swims at 5 ft/s at the correct angle, he will have an upstream component of 3 ft/s, which cancels the effect of the current, and a cross-stream component of 4 ft/s. Therefore he swims the first width of 100 feet in just 25 seconds, and then takes another 25 seconds to return, giving a total time of 50 seconds to swim 200 feet. Although both swimmers would swim at the same speed in still water, the swimmer crossing the current wins the race against the swimmer who goes with and against the current. Hence, Michelson suspected that a light beam travelling across the ether wind would have a shorter travel time than a beam travelling with and then against the ether wind. He designed an experiment to see if this was really

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