Einstein's War

by Matthew Stanley

  When Planck talked, people listened. Einstein’s papers on molecular motions and the light quantum fit well into other work being done at the time, and helped solve long-standing problems in science. Their importance was clear. Less clear was what to do with “On the Electrodynamics of Moving Bodies.” There was little initial reaction to Einstein’s ideas about time dilation and length contraction. Einstein even sent copies of the paper to physicists who he thought might be interested, including G.F.C. Searle, a physicist in Cambridge. Searle couldn’t make heads or tails of it. Those who read the paper usually interpreted it as being a modest contribution to the physics of the ether and not a wholesale restructuring of our basic categories of experience.

  But Planck was intrigued by the “absolute, invariant features” of the theory—that is, the way it allowed for the laws of physics to be universal. This may seem strange, given that we call it the theory of relativity. Einstein actually never liked that name and didn’t use it until 1911. Planck wrote the first paper that dealt with relativity. He presented Einstein’s paper at his physics seminar in Berlin that winter, and set his assistant Max von Laue to studying the theory—and to find out who, exactly, this obscure patent clerk was.

  CHAPTER 2

  Science Across Nations

  “This unconstructable and unvisualizable dogmatism.”

  THE SAME WINTER that Planck was trying to excite his colleagues about relativity, Eddington had a final breakfast with Trimble in Cambridge and moved to 4 Bennett Park in the London neighborhood of Blackheath, just south of the observatory. Although he was hired for his mathematical acuity, his duties were overwhelmingly of the practical and observational variety. He set about learning all the techniques he would need and that Cambridge never taught him. The Astronomer Royal’s initial letter had praised Eddington’s skill with astronomical instruments, but in fact Eddington spent his last two weeks in Cambridge frantically learning how to use basic tools.

  The Royal Observatory’s most important project of the time was the new star catalogue, which listed the exact location of more than twelve thousand stars. His first job was to double-check all of those locations. This required long nights at the telescope making minute measurements. There was no room for error or carelessness. In all, he spent more than a thousand hours at the telescope working on the catalogue. Even more challenging were the social duties that came with the position. When he met with the observatory board, he had to wear an old-fashioned frock coat and top hat. When he was taken to dinner at the Royal Astronomical Society Club, a toast was taken to his health and the teetotaler Eddington felt obligated to drink. Adjusting to life in London was made easier by visits from his mother and sister (they visited all the usual tourist spots) and Trimble (they went to see The Taming of the Shrew and swam at the Westminster Baths).

  Not all of the measurements needed by His Majesty Edward VII could be done in Greenwich. Part of Eddington’s duties was to travel the world to make official determinations of longitude and latitude of various outposts of the British Empire. These were the official map coordinates that commercial vessels used to make their fortunes and that the Royal Navy used to enforce order. One of the most important outposts was Malta, the tiny island absolutely essential to strategic control of the Mediterranean. The two previous measurements of Malta disagreed by one second of longitude. This could conceivably cause danger for some wayward frigate; more to the point, it was unacceptable to not know the exact location of any British possession.

  Eddington nervously gathered the equipment he would need—literally a ton of very expensive gear, with which he had been practicing for months. He sailed on the Japan, a passenger and mail ship. This was his first long sea voyage, on which he read Tristram Shandy and Don Quixote. After nine days he arrived at St. George’s Bay on Malta. The telegraph station where he was to work overlooked a sandy beach perfect for swimming, but he had an enormous amount of work to do and had to stay focused. He had little opportunity to see the island and declared it an awful place.

  His first task was to build the “hut”—a temporary shelter for the equipment. This makeshift observatory contained a telescope, a clock, and elaborate electrical equipment. The process for measuring longitude was fairly straightforward. Eddington would use the telescope to measure the exact time in Malta (determined by the height of a particular star above the horizon). He would then send a signal by electric telegraph back to Greenwich, where, after accounting for transmission time of the signal, they would determine the equivalent time in London by measuring the height of the same celestial body. Because Greenwich was far to the west, it would be earlier than in Malta (the basis of time zones). The difference in time would then be precisely converted into distance.

  This was a standard technique for determining longitude anywhere in the world connected to the telegraph network. It was very accurate but required good weather at both observing stations. A snowstorm in southeast England interrupted the work, as did several cloudy nights. Eddington spent about ten hours a day crammed into the hut, assisted by a petty officer from the naval base. After three solid weeks of work he found his goal: Malta lay at 0 hours, 58 minutes, 2.595 seconds east longitude. The precision of his measurement was, as expected, about eleven inches. Eddington became known for the speed with which he could carry out the complex calculations involved in estimating error, once deriving a formula on the back of a dinner menu when someone mentioned the need.

  On the journey home Eddington stopped in Tunis, making his first visit to Africa. Tunis was still a French colonial possession, and it was an easy steamer journey to visit Marseilles as well. He deeply enjoyed travel (with the occasional exception such as Malta) and often spent his holidays on the Continent. He took his mother to Norway and his sister to Germany, and usually took advantage of these trips to meet and work with astronomers in those countries. When in Holland on vacation he visited the observatory in Leiden and conferred with the astronomer Jacobus Kapteyn in Groningen. It was Kapteyn’s work that got Eddington interested in the problem of the rotation of the Milky Way, and they connected on a personal level as well. The next year Eddington hosted Kapteyn for a visit in London.

  International travel became a standard part of his professional life. In 1909 the British Association for the Advancement of Science (known colloquially as “the BA”) held its annual meeting in Canada. It was fairly common for British scientific organizations to make this sort of trip to help build imperial solidarity. Eddington gladly went along, departing from Liverpool on the Empress of Ireland. The six-day ocean trip gave him many opportunities to get to know other scientists. He didn’t care much for Winnipeg in the summer (he declared the plains “not much to look at”), but the Rocky Mountains and Niagara Falls made a lasting impression. On his way home he passed through Boston to meet with the Harvard astronomer Edward Charles Pickering. Scientific journeys would eventually take him to every continent save Antarctica.

  The other focus of his travel was the Friends’ Guild of Teachers, a Quaker organization dedicated to experiments in education and applying their religious values to the problems of modern society. Their annual meetings were held in various locations across the United Kingdom, and Eddington attended nearly all of them (many with his sister as well). He was strongly committed to education as a powerful tool for improving the world. He remained an active participant in the guild his entire life and served as president for several years.

  Eddington often planned travel to those annual meetings to coincide with a holiday with Trimble. They would spend a week or two together hiking. Many stories came out of these trips: being drenched by rain for six days straight; going to the movies but enjoying watching the locals’ behavior more than the actual film; Trimble insisting on stopping to investigate every interesting bit of geology. Eddington loved glissading (sliding down hillsides), and on one adventure in Wales tore the seat of his trousers. As they walked to the nearest town, Trimble walked close
behind to conceal the opening. Village children gave “shrill cries” at the sight and Eddington remarked, “It was perhaps a mercy that we knew no Welsh.” When they reached their destination, they borrowed needle and thread and Eddington insisted that Trimble sew up his pants without taking them off. Trimble said he accomplished it without injury but added, “I believe I sewed him to his shirt.”

  Even beyond these trips, Eddington made regular trips to Cambridge to see Trimble. Bicycling was a favorite pastime on those visits. Eddington, always an obsessive counter, kept a fantastically detailed journal of his cycling adventures. When mere tallies were not enough (in 1905 he rode 2,669 miles), he began recording his progress with his infamous n-number: the number of times n he had cycled more than n miles (that is, when n was 37, he had taken 37 trips of 37 miles or more). Letters to friends would often include an update to n. After Trimble finished his degree, he went to London and stayed with Eddington on and off for nearly two years.

  When his duties of measurement and observation allowed him time, Eddington continued his theoretical investigations of the Milky Way. At this time, it was not at all clear what size or shape our galaxy was, or even whether it was unique or just one of many similar bodies in the universe. Eddington and Kapteyn’s work gave some of the first insight into the structure and movement of nearby stars. Eddington was able to use the measurements he was making at Greenwich to solidify their calculations. The mathematics involved was challenging, though, and he also contacted the German astronomer Karl Schwarzschild for advice. Eddington worked up a full paper on stellar motions for the examination to become a fellow at Cambridge, which also became his first communication to the Royal Astronomical Society. This paper won him the prestigious Smith’s Prize. He even received an offer of a professorship of physics at his old college in Manchester. He declined, having decided that he preferred the life of an astronomer. The stars had captured him.

  * * *

  EINSTEIN WAS STILL working at the patent office. While his miracle year had attracted attention, his civil-service job paid more than the open academic positions. Not much changed for his daily life in Bern, save being promoted to Technical Expert II Class. His star within the physics community was certainly rising, though, despite his dismal networking skills (he didn’t even attend a physics conference until 1909). The diligent experimentalist Jean Baptiste Perrin in Paris had confirmed his predictions about molecular motions, essentially proving the existence of atoms. The Nobel Prize winners Philipp Lenard and Wilhelm Röntgen wrote to him about his light quantum theory.

  Most important to Einstein, though, was the correspondence he began with Hendrik Lorentz. Einstein had admired the elder physicist for many years and even felt that Lorentz had in many ways anticipated his own work. Many of the equations used in special relativity were in fact written down by Lorentz long before 1905, though the physical meaning attributed to them was quite different. The Dutch physicist was unsure exactly what the implications of relativity were, but he nonetheless saw a kindred spirit and took Einstein under his wing. Their relationship quickly became parental, and Einstein wrote, “I admire that man more than anyone else, I might say I love him.”

  Only a handful of scientists took notice of relativity, though. Planck continued to evangelize for it. Hermann Minkowski, Einstein’s old mathematics teacher, was amazed at what his lazy student had accomplished and asked for an offprint. Arnold Sommerfeld, the dean of Munich physics, mixed admiration with some casual anti-Semitism:

  Works of genius though they are, this unconstructable and unvisualizable dogmatism seems to me to contain something almost unhealthy. An Englishman would scarcely have produced this theory; perhaps it reflects . . . the abstract-conceptual character of the Semite.

  Nonetheless, he called relativity an “inspired conceptual skeleton” and expressed hope that Lorentz could give it “real physical life.”

  In September 1907, Johannes Stark, a noted physicist in Hanover, asked Einstein for a summary of relativity theory for a science yearbook he was putting together. This provided Einstein his first opportunity to reflect on the state of his theory and think about how he wanted to extend it further. One issue arose from the problem of how to connect special relativity with the crown jewel of classical physics: Newton’s theory of gravity. It had generally been assumed that the force of gravity was instantaneous—if you destroyed the sun, Earth’s orbit would be immediately affected. But special relativity set the speed of light as the upper limit for any sort of physical effect, so Earth would not feel the sun’s destruction for about eight minutes. This difference was hardly insurmountable. Einstein just had to tweak Newton’s equations to match those of relativity.

  Unfortunately Einstein was not a fan of tweaking. He distinguished between “constructive” theories (assembled from many small details and facts) and “theories of principle” (guided by universal statements or ideas that applied in all possible cases). The former could be effective and useful, but only the latter could be logically secure and philosophically profound. Slightly changing Newton’s law of gravity would yield only a constructive theory. His efforts on the problem so far, even where they looked promising, struck him as “highly suspicious.” He needed some new foundational principles on which he could base an entirely original theory of gravity.

  Newtonian gravity had inaugurated science as it had been understood and taught for centuries. Any change to it would have enormous consequences. But there were special problems in wedding Newton and relativity. In Einstein’s theory, two observers in motion can disagree about the mass of a given object. And since gravitational force is proportional to mass, they will feel different amounts of gravity depending on their motion. This was a profound possibility: could the force of gravity, the very glue holding together the universe, be relative and malleable? Einstein decided that a close consideration of gravity was essential for his theory to grow.

  Working with Mach’s framework, Einstein’s first question was still one about direct sensory experience. How do we experience gravity? How do you know that gravity is pulling on you right now? What does it feel like to have gravity acting on you? Years later Einstein recalled the key moment in this inquiry, what he called “the happiest thought of my life.”

  I was sitting in my chair in the patent office in Bern when all of a sudden a thought occurred to me: “If a person falls freely he will not feel his own weight.” I was startled. This simple thought made a deep impression on me. It impelled me towards a theory of gravitation.

  The detail that he was sitting in his chair is actually important. Right now you are probably sitting. Think carefully for a moment about where you feel the effect of gravity on you. You don’t feel the downward pull; you feel your chair pushing upward, keeping you in place. Einstein’s realization was that it is actually this stabilizing force that makes you aware of gravity. If your inconsiderate roommate were to suddenly yank your chair out from under you, you would lose the sensation of that force. There would be a brief moment (before you hit the floor) when you would not be aware that gravity was pulling on you. Your perception of gravity would vanish (the same situation astronauts in orbit experience). Perhaps it was the case that gravity could be relative—it was not an absolute truth, but depended on your circumstances and surroundings just as time depended on your movement.

  Einstein carefully considered how we perceive gravity and pondered situations where that perception could be changed. Like the light clock, this is a thought experiment. Imagine a physicist who, after a lively night out, awakes groggily in a sealed, windowless room. No scientist would be caught without basic experimental equipment, so she begins studying the chamber. She holds a weight out and releases it, noticing that it moves quickly toward the floor. One explanation for this is that the room is sitting on the surface of a planet like the Earth and the planet’s gravity pulls the weight down toward the floor. But she realizes that there could be another explanati
on. What if the room is in deep space, far from any gravitational sources, but has a rocket strapped to the bottom? The rocket continually fires and accelerates the room in one direction. This time, when she releases the weight, it still moves toward the floor—but because the floor is accelerating upward toward it. From the scientist’s point of view, though, the two situations are identical. There is no experiment she can perform from inside the room that will let her distinguish these two situations.

  Einstein said that this thought experiment shows that gravity and acceleration are equivalent. This is again something only a positivist would say. One might object that the two situations are, in fact, physically different. But the positivist says you can’t tell, so they might as well be the same. You feel your chair pushing on your gluteus maximus right now. That might be gravity, or there might be a rocket strapped to the bottom of the chair. If you can’t tell which it is, Einstein said, it doesn’t matter which it is. To someone doing physics they are the same, or at least “of the exact same nature.” He comes to call this the equivalence principle: gravity and acceleration are indistinguishable.

  In addition to Einstein’s own concerns, this principle would help make sense of an odd feature of physics. An m for mass appears both in Newton’s equations for motion (called inertial mass, it is a measure of how hard it is to accelerate something) and his equation for gravity (called gravitational mass, it is a measure of how much gravitational force is created by it). They were generally considered to have the same value (and had actually been measured to be so in the laboratory), even though the equations did not demand it. The mathematics allowed a universe where the two kinds of mass were different, so why did we live in one where they were the same? Einstein finally had an answer: they’re the same because you can’t tell them apart.