Big Bang. Simon Singh

invested in the best possible light sources and mirrors for his experiment and took every conceivable precaution in assembling the apparatus. Everything was carefully aligned, levelled and polished. To increase the sensitivity of his equipment and minimise errors, he even floated the main assembly in a vast bath of mercury, thereby isolating it from external influences such as the tremors caused by distant footsteps. The whole point of this experiment was to prove the existence of the ether, and Michelson had done everything possible to maximise the chance of its detection – which is why he was so astonished by his complete and utter failure to detect any difference in the arrival times of the two perpendicular beams of light. There was no sign of the ether whatsoever. It was a shocking result.

      Desperate to find out what had gone wrong, Michelson recruited the chemist Edward Morley. Together they rebuilt the apparatus, improving each piece of equipment to make the experiment even more sensitive, and then they carried out the measurements over and over again. Eventually, in 1887, after seven years of repeating their experiment, they published their definitive results. There was still no sign of the ether. Therefore they were forced to conclude that the ether did not exist.

      Bearing in mind its ridiculous set of properties – it was supposed to be the least dense yet the most rigid substance in the universe – it should have come as no surprise that the ether was a fiction. Nevertheless, scientists discarded it with great reluctance because it had been the only conceivable way to explain how light was transmitted. Even Michelson had problems coming to terms with his own conclusion. He once nostalgically referred to the ‘beloved old ether, which is now abandoned, though I personally still cling a little to it’.

      The crisis of the non-existent ether was magnified because it was supposed to have been responsible for carrying both the electric and magnetic fields as well as light. The dire situation was nicely summarised by the science writer Banesh Hoffmann:

      First we had the luminiferous ether,

      Then we had the electromagnetic ether,

      And now we haven’t e(i)ther.

      So, by the end of the nineteenth century Michelson had proved that the ether did not exist. Ironically, he had built his career on a whole series of successful experiments relating to optics, but his greatest triumph was the result of a failed experiment. His goal all along had been to prove the existence of the ether, not its absence. Physicists now had to accept that light could somehow travel through a vacuum – through space devoid of any medium.

      Michelson’s achievement had required expensive, specialist experimental apparatus and years of dedicated effort. At roughly the same time, a lone teenager, unaware of Michelson’s experimental breakthrough, had also concluded that the ether did not exist, but on the basis of theoretical arguments alone. His name was Albert Einstein.

      

      Einstein’s Thought Experiments

      Einstein’s youthful prowess and his later full-blown genius sprang largely from his immense inquisitiveness about the world around him. Throughout his prolific, revolutionary and visionary career he never stopped wondering about the underlying laws that governed the universe. Even at the age of five, he became engrossed in the mysterious workings of a compass given to him by his father. What was the invisible force that tugged at the needle, and why did it always point to the north? The nature of magnetism became a lifelong fascination, typical of Einstein’s insatiable appetite for exploring apparently trivial phenomena.

      As Einstein told his biographer Carl Selig: ‘I have no special talents. I am only passionately curious.’ He also noted:‘The important thing is not to stop questioning. Curiosity has its own reason for existing. One cannot help but be in awe when one contemplates the mysteries of eternity, of life, of the marvellous structure of reality. It is enough if one tries to comprehend only a little of this mystery every day.’ The Nobel Laureate Isidor Isaac Rabi reinforced this point: ‘I think physicists are the Peter Pans of the human race. They never grow up and they keep their curiosity.’

      In this respect, Einstein had much in common with Galileo. Einstein once wrote:‘We are in the position of a little child entering a huge library, whose walls are covered to the ceiling with books in many different languages.’ Galileo made a similar analogy, but he condensed the entire library of nature into a single grand book and a single language, which his curiosity compelled him to decipher: ‘It is written in the language of mathematics, and its characters are triangles, circles and other geometrical figures, without which it is humanly impossible to understand a single word of it; without these one is wandering about in a dark labyrinth.’

      Also linking Galileo and Einstein was a common interest in the principle of relativity. Galileo had discovered the principle of relativity, but it was Einstein who would fully exploit it. Simply stated, Galilean relativity argues that all motion is relative, which means that it is impossible to detect whether or not you are moving without referring to an external reference frame. Galileo stated vividly what he meant by relativity in the Dialogue:

      Shut yourself up with a friend in the main cabin below deck on some large ship, and have with you there some flies, butterflies and other small flying animals. Have a large bowl of water with some fish in it; hang up a bottle that empties drop by drop into a wide vessel beneath it. With the ship standing still, observe carefully how all the little animals fly with equal speed to all sides of the cabin; how the fish swim indifferently in all directions; how the drops fall into the vessel beneath. And, in throwing something to your friend, you need to throw it no more strongly in one direction than another, the distances being equal; and jumping with your feet together, you pass equal spaces in every direction.

      When you have observed all these things carefully … have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship moves or stands still.

      In other words, as long as you are moving at constant speed in a straight line, there is nothing you can do to measure how fast you are travelling, or indeed to tell whether you are moving at all. This is because everything around you is moving at the same velocity, and all phenomena (e.g. dripping bottles, flying butterflies) happen the same regardless of whether you are moving or stationary. Also, Galileo’s scenario takes place ‘in the main cabin below deck’, so you are isolated, which removes any hope of detecting any relative motion by referring to an external frame of reference. If you isolate yourself in a similar way by sitting with your ears plugged and your eyes shut inside a train on a smooth track, then it is very difficult to tell if the train is racing along at 100 km/h or whether it is still stuck at the station, which is another demonstration of Galilean relativity.

      This was one of Galileo’s greatest discoveries, because it helped to convince sceptical astronomers that the Earth does indeed go round the Sun. Anti-Copernican critics had argued that the Earth could not go around the Sun because we would feel this motion as a constant wind or as the ground being pulled from under our feet, and clearly this does not happen. However, Galileo’s principle of relativity explained that we would not sense the Earth’s tremendous velocity through space because everything from the ground to the atmosphere is moving through space at the same speed as we are. A moving Earth is effectively the same environment as the one we would experience if the Earth were static.

      In general, Galileo’s theory of relativity stated that you can never tell if you are moving quickly or moving slowly or moving at all. This holds true whether you are isolated on the Earth, or ear-plugged and blinkered on a train, or tucked away below deck on a ship, or cut off from an external reference frame in some other way.

      Unaware that Michelson and Morley had disproved the existence of the ether, Einstein used Galileo’s principle of relativity as his bedrock for exploring whether or not the ether existed. In particular, he invoked Galilean relativity in the context of a thought experiment, also known as a gedanken experiment (from the German word for ‘thought’). This is a purely imaginary experiment that is conducted only in the physicist’s head, usually because it involves a procedure that is not in practice achievable in the real world. Although