The Eclipse That Changed the World
The first planet discovered since antiquity was found by mistake. Its discovery resulted in the greatest triumph of Newton’s clockwork universe, which eventually led to its greatest failure at the hands of a German patent clerk. Out of the ashes of that failure a new paradigm would rise, not just for scientists, but for the world as we know it—and it was a solar eclipse that provided the proof.
On a clear night in 1781, William Herschel, a musician and self-taught astronomer born in Germany but raised in England, pointed a telescope at a relatively unremarkable piece of sky. In his eyepiece he saw a faint bluish ball. At first, he took it to be a new comet. But as other astronomers turned their telescopes toward the new object, its changing position revealed motion more like a planet around the Sun than a comet plunging toward it. They named the new planet Uranus, after the Greek god of the sky. But something about Uranus wasn’t quite right. No single orbital solution to Newton’s law of gravity fit all of its observed positions. At the Paris Observatory, Urbain-Jean-Joseph Le Verrier was given the task of reconciling these observations. Le Verrier was a brilliant young mathematical astronomer (what today we might call a theoretical astrophysicist), and in 1846 he published a paper with a startling hypothesis: Uranus was not alone.
For almost two hundred years, Newton’s law of gravity worked spectacularly well at explaining the motion of everything from a cannonball’s arc to the movement of the Moon and stars. Everything about gravity was mathematically predictable. According to Le Verrier’s calculations, the only way Uranus could move as it did was if it were influenced by an as yet undiscovered planet even farther from the Sun. To assert was one thing; to reveal, another. Le Verrier worked backward from effect to cause and on August 31, 1846, announced the precise celestial coordinates where astronomers should turn their telescopes to reveal the unseen planet. Less than a month later, Neptune was found precisely where Le Verrier said it would be.
Neptune’s discovery was the highest triumph yet of Newton and science; it was confirmation of the contemporary assertion by philosophers that if one could but know all the “forces that set nature in motion, and all positions of all items of which nature is composed,” then the future and past could be predicted with infinite precision. And such a world of mathematical precision it was. Life in the nineteenth century was accelerating rapidly, fueled by the new technology of iron and steam made possible by the physics of heat and motion. Smoking mechanical ships now crossed the seas in mere days, while locomotives raced across continents, for the first time moving people faster than any creature could walk or run. In Europe and the Americas, electricity and the telegraph made the world smaller than its physical geography and connected any two points at the speed of light (or at least the speed of a telegrapher’s typing). In this world, Le Verrier’s name, along with those of other French luminaries of science and technology, would be engraved in the steel girders of the tower that Gustave Eiffel would eventually erect over the city of Paris.
Red prominences of hydrogen gas erupt off the Sun along with two bright “Baily’s Beads,” where sunlight shines through mountain valleys along the edge of the darkened disk of the Moon. Only during a total solar eclipse is the Sun’s corona ever visible to the human eye on Earth. (Image copyright 1991, Fred Espenak, MrEclipse.com)
The Full Moon darkens as it passes into and out of the Earth’s shadow during a total lunar eclipse. It turns orange as the only light that falls upon it is filtered through our atmosphere. (Image by the author)
A portion of the 1,000-year-old Mayan Dresden Codex. The red and black bars and dots surrounding the unique half-black, half-white figures being eaten by a serpent are numbers in the Mayan base-20 system of counting. They represent 177 and 148 (6 and 5 lunar months, respectively), the number of days between eclipse seasons. (Image courtesy SLUB Dresden, Mscr.Dresd.R.310, http://digital.slub-dresden.de/id280742827, [CC-BY-SA 4.0])
An annular eclipse of the Sun by Mars’s moon Phobos as photographed from the surface of the Red Planet by the NASA rover Curiosity on August 20, 2013. (Image courtesy NASA / JPL-Caltech / Malin Space Science Systems / Texas A&M University)
A sundial sits on each NASA rover on Mars today. The “MarsDial” (left) in the lab, (middle) on Mars soon after landing, (right) covered in Martian dust after five miles of driving. (Image courtesy NASA / JPL / Cornell University)
Engraving for Francis Baily’s paper on the July 8, 1842, total solar eclipse, when he saw the corona and mysterious red “protuberances”—in actuality, solar prominences. (Image scan from the UCLA Library Collection; courtesy Royal Astronomical Society)
A false-color image of the Sun photographed in the red light of hydrogen gas, revealing a group of dark sunspots toward the lower right and numerous prominences erupting off the surface along the lower limb of the Sun. The dark tendril along the center of the Sun is a prominence erupting straight toward the camera, while the bright arc between the sunspots is a rare solar flare, an explosion of exceptional power due to the sudden release of energy within the Sun’s magnetic fields. (Image courtesy Paul B. Jones)
A petroglyph in Chaco Culture National Historical Park in northwestern New Mexico, thought to represent the total solar eclipse visible from there during the height of Chacoan culture on July 11, 1097. The curling lines may depict the looping tendrils of the corona, and possibly a solar eruption, that would have been visible during totality. (Image by the author)
Spacecraft image of the Sun, using ultraviolet light from iron atoms at a temperature of 1 million degrees looping along magnetic field lines. Sunspots are sources of high magnetic activity and so appear to be bright. On the right, the Solar Dynamic Observatory spacecraft captures a partial lunar eclipse from space. The black silhouette of the Moon reveals the jagged edge of lunar mountains, which are dwarfed by a close-up of an active solar region larger than the entire Earth. (Image courtesy NASA/SDO and the AIA, EVE, and HMI science teams)
Commemorative stamp issued by the national post office of Hungary on the occasion of the August 11, 1999, total solar eclipse across Europe. Inset shows the “diamond ring” as seen at the onset of totality during the eclipse from central Hungary. (Image and stamp courtesy of the author)
The total solar eclipse of November 3, 2013, was visible to passengers off the coast of North Africa aboard the sailing ship Star Flyer. Both ship and shadow intersected for forty seconds as they crossed the Atlantic Ocean going in opposite directions. (Image by the author)
Four commemorative travel posters created for four different solar eclipses: annular eclipse from Chaco Culture National Historical Park, 2012 (top left); total eclipse during mid-Atlantic crossing, 2013; total eclipse from high-altitude near-supersonic aircraft above the Faroe Islands, 2015; and the “All-American” total solar eclipse of 2017. (Posters created by the author)
Map of the path of totality for the August 21, 2017, total solar eclipse crossing the United States. Maximum durations of totality along the midline of the path are given at four-second intervals. (Map courtesy Michael Zeiler, www.GreatAmericanEclipse.com)
World map showing the paths of totality for future total solar eclipses over the fifty years from 2015 to 2065. See Table 7.1 for durations. (Map courtesy Michael Zeiler, www.GreatAmericanEclipse.com)
But Le Verrier’s work did not end with Neptune; at the other end of the solar system he saw that there was something wrong with Mercury. Like the planet Venus, Mercury periodically passes in front of the solar disk. But where a transit of Venus is rare, Mercury’s transits are common, happening at intervals between three and thirteen years apart. Le Verrier realized that for all of Mercury’s transits, the times that had been predicted didn’t quite match what had actually been seen. The differences were small, only a few seconds—easily explained by errors in an individual observer’s clock. The problem was that nearly every observer reported this error, and all in the same direction (every transit always starting early). What’s more, over the century that they’d been observed, the discrepancies had been growing. For the man who was Newton’s champion, the mathematical solution was clear: our solar system must contain yet another planet, this time one hidden in the glare of the Sun. Just as Mercury’s transits revealed its influence, perhaps transits of the mystery planet would reveal its existence.
Almost immediately upon announcing his conclusion, Le Verrier received word that the mystery planet had already been seen. Dr. Edmond Modeste Lescarbault, an obscure French physician living outside Paris, claimed to have seen just such a small circular spot pass across the Sun. Visiting Lescarbault incognito, Le Verrier cross-examined the doctor, “whose means of observation were certainly of the scantiest,” he later reported. “His telescope was a small one of only two or three inches aperture; as a timekeeper he had only an old watch with no second-hand, so that he was obliged to use a pendulum consisting of a bullet at the end of a string for counting seconds, and to save paper he made his calculations on boards which he planed off whenever he wished to erase an old computation and make way for a new.”
At the end of his visit with Lescarbault, Le Verrier announced to the world that his predicted planet had been seen. He named it Vulcan after the Greek god of fire. Using Lescarbault’s time and duration of transit, Le Verrier calculated that it must circle the Sun once every nineteen days and seven hours, and that during roughly half of all solar eclipses it would appear within 8 degrees of the Sun and as bright as a first-magnitude star (one of the brightest in the sky). In addition, twice each year—around April and October—it would be visible transiting across the solar disk.
The next transit would occur on or about March 22, 1860, just two and a half months later. To find such a planet, astronomers all over the world would need to monitor the Sun constantly without any gaps in their observations, lest the planet pass across the Sun unseen. By telegraph the news went out to far-flung observers. Yet no such transit was ever seen. Even worse, an astronomer in Brazil published a paper claiming to have observed the Sun at exactly the same time as Lescarbault and to have seen no transit at all. This would be a pattern repeated over and over again for the next fifty years: for every astronomer (noted and otherwise) who chanced to see a dark dot on the Sun “exactly” as Vulcan would appear, another reported fruitless hours spent looking for any such spot. Detecting Vulcan during a total solar eclipse proved no easier.
On August 7, 1869, the Sioux City Daily Times reported that a party of four astronomy enthusiasts in St. Paul County, Iowa, claimed to have seen a “star” one-sixth the size of Mercury near the totally eclipsed Sun. However, the Des Moines State Register announced that a professional astronomer who had set up his observatory in the path of totality had “searched that region thoroughly, and found nothing that would indicate the existence of planets of that kind.” The disagreement over Vulcan’s existence eventually became so absurd that, according to the New York Times, a positive opinion of Vulcan was a dangerous matter for any young astronomer. The planet Vulcan had become “an astronomical sea-serpent.” Although it might exist, the older, more established astronomers would scoff that “Professor So and So never saw it,” and then they would hint, “with sneering astronomic smiles, that too much tea sometimes plays strange pranks with the imagination, and that an astronomer who cannot tell a planet from a fly that walks across his object-glass is no sort of man from whom any discoveries of moment need be expected.”
All that appeared to change on July 29, 1878, when two independent astronomers claimed to have seen Vulcan during a total solar eclipse from two widely separated locations within minutes of one another. Reporting from Rawlins, Wyoming Territory, Professor James Craig Watson of the University of Michigan reported, “I had committed to memory the relative positions of the stars in the neighborhood of the sun, and I had placed the chart of the region conveniently before me for ready reference, whenever required. . . . The object which I had in the field shone with a ruddy light, and it certainly had a disc larger than the spurious disc of a star.” Meanwhile Lewis Swift, the famous discoverer of no fewer than thirteen comets (including Comet Swift-Tuttle, responsible for the annual Perseid meteor shower each August), reported from totality in Denver, Colorado, that in searching for Vulcan, “about one minute after totality I observed two stars, by estimation 3o S.W. of the sun. . . . [B]y careful comparison, they appeared exactly of the same magnitude, and both as red as Mars. I looked closely for twinkling, but they were as free from it as the planet Saturn. They both, at the time, seemed to my eye and mind, to have a small round disk, about like the planet Uranus.”
Upon this news the Princeton astronomer Dr. C. A. Young wrote that “one brilliant discovery will probably develop from this occasion, and hold a conspicuous place in the annals of science. The planet Vulcan, after so long eluding the hunters, showing them from time to time only uncertain trace and signs, appears at last to have been fairly run down and captured.” Given the brightness of the new planet, Young calculated Vulcan’s size could be no more than four hundred miles in diameter. This was tiny enough to have escaped detection for so long, but too small to cause the change in Mercury’s orbit in the way that was seen. Perhaps there were multiple Vulcans? In fact, it soon became clear from comparison of Watson’s and Swift’s notes that their locations didn’t match—they couldn’t possibly be reporting the same planet.
When no subsequent transits appeared, each man became adamant that what he had seen was real. The New York Times wrote, “Prof. Swift arrived in town to-night, and in an interview with a reporter stated that he had no more doubt that he discovered Vulcan than that he had been to Colorado.” In time, Swift became sure that the new photographic technology being used by eclipse-chasers like Pierre Janssen would confirm his discovery. When it didn’t, Swift was ready to explain that this was not surprising, as the photographic emulsions were more sensitive to blue light, and not the red color he’d seen Vulcan display. “Happily the time is not far distant when the problem can be settled,” wrote Swift in 1883. “The great eclipse of 1886 will afford an admirable and comparatively easy opportunity, if rightly managed, to dispel every doubt. . . . Until then let us hold the matter in abeyance. My faith in their existence was never stronger than to-day.”
But the eclipse of 1886 revealed nothing about Vulcan. As the nineteenth century gave way to the twentieth, the triumph of Newtonian physics that had led to the discovery of Neptune was now producing nothing but failure. With each eclipse that revealed no sign of the elusive planet, further excuses and refinements were required to explain away Mercury’s motion. What began with a single planet had already been modified to multiple planets, which in turn became a plethora of planetoids, a belt of debris between Mercury and Venus, and finally nothing more than a simple ring of dust. Nothing ever fit. And yet still Mercury moved.
Perhaps the fault was not in our stars but rather in our physics? The astronomer Asaph Hall, discoverer of the two moons of Mars, suggested an unsettling possibility in the summer of 1894: maybe gravity didn’t work the way they thought. Was it possible that Newton was wrong? For scientists, the defining characteristic of our field is the experiment. If an experiment’s results do not confirm our hypothesis, it is the hypothesis that must change. This is an ideal that depends on there being a simple, crucial experiment whose result is agreed upon by all involved to be the deciding factor of which idea is correct. The reality is never this simple. It is always easier to add an extra parameter to a previously successful theory than to scrap the whole thing and start from scratch: Mercury doesn’t move as expected? Simple, just keep adding ever tinier planets. But the question arises, when do you stop making excuses and look for a new hypothesis?
Occam’s Razor is the supposition that the simplest explanation that fits all the data is usually the correct one. Unfortunately, there is rarely any agreement on what explanation is “simplest,” and “usually correct” is not the same as “always correct.” In his book The Structure of Scientific Revolutions, the philosopher Thomas Kuhn explains that science does not progress by a constant stream of crucial experiments, with scientists constantly reevaluating all of their assumptions and successes. Rather, scientists use the results of previous experiments to build a framework, or paradigm, upon which to hang all of their new experimental results, gradually constructing a picture of the universe. Based on this evolving picture at any given moment, scientists think of new experiments to perform and decide how to interpret their results. The majority of the time, we are simply filling in the missing pieces of a picture we have inherited from those who came before.
When experiments don’t provide the results we expect (e.g., when the planet Vulcan fails to appear), we look for reasons that allow us to keep as much of the framework as we can, even if the details of the picture become more complicated than we would like (e.g., rings of small asteroids especially oriented to affect the planet Mercury and no other). Eventually, someone comes along who suggests a completely different framework that creates an entirely new picture—a new paradigm—by which to interpret our previous results. Whether this new paradigm is “simpler” than what came before, thus satisfying Occam’s Razor, is rarely agreed upon by the scientists of the time. According to Kuhn, scientists during these scientific revolutions rarely have a rational reason for choosing one framework over another.
For instance, Copernicus proposed a new paradigm in which the Earth was only one planet among many in orbit around the Sun. When Galileo’s telescope revealed moons orbiting Jupiter, and previously unknown features of our Moon, the Sun, and the other planets of the solar system, it refuted a tenet of the old paradigm, which had said that all motion must center on the Earth, while confirming a central aspect of the new paradigm: that the heavenly spheres were physical places just like the Earth. Copernicus’s model of planets orbiting the Sun in perfect circles did not, however, predict the positions of the planets as well as Ptolemy’s complex systems of celestial spheres within spheres did. Understandably, a reasonable person could choose the old complexity over the new simplicity when simplicity didn’t work. But new paradigms suggest new hypotheses—with new experiments that might make no sense under the old framework.
The parallax motion of nearby stars over the course of a year is a phenomenon that makes no sense in a universe where the Earth doesn’t move. Likewise, a universe where the Earth is simply another planet requires that the Moon, planets, and Sun also exhibit features and turn on their axes just like the Earth. In time, all of these phenomena were revealed through the ever-increasing magnification of telescopes. Meanwhile, the physical laws of Kepler and Newton revealed that the same force that causes an apple to drop here on Earth makes the planets orbit the Sun in elliptical, not circular, paths, a discovery that produced even better agreement with observations than Ptolemy’s crystalline spheres. Whatever reason individual scientists have for accepting one framework over another, eventually no serious scientist is left to propose new additions to the old paradigm, and the scientific revolution is complete. From that moment onward, it becomes the task of new scientists to understand the implications of new laws within the new framework.
At the dawn of the twentieth century, astronomers believed they understood the framework of Copernicus, Galileo, and Newton so completely that the well-respected astronomer A. A. Michelson could write:
The more important fundamental laws and facts of physical science have all been discovered, and these are so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote. Nevertheless, it has been found that there are apparent exceptions to most of these laws, and this is particularly true when the observations are pushed to a limit, i.e., whenever the circumstances of experiment are such that extreme cases can be examined. Such examination almost surely leads, not to the overthrow of the law, but to the discovery of other facts and laws whose action produces the apparent exceptions.
By this reasoning, since the famous astronomers of the nineteenth century were doing nothing more than filling in details of well-established laws, then perhaps it is no coincidence that today almost no one remembers their names. But the nineteenth century was also the greatest time of discovery for forces that had seemingly little to do with planets or gravity: electricity, magnetism, and optics. In 1865, while astronomers were busy looking for Vulcan, the Scottish physicist James Clerk Maxwell unified these phenomena into a set of four equations that revealed light to be a wave of changing electric and magnetic fields.aa We tend not to think of electricity and magnetism outside of the electronics that power our daily lives, yet four of the five senses with which we perceive the world—taste, touch, sound, and smell—are just the interactions of atoms and molecules via their electric fields (while sight is the direct detection of light).
In 1895, at the age of sixteen, Albert Einstein thought very deeply about light and the ramifications of the laws that governed it. For instance, he wondered, what would a person see if he could ride a beam of light? A person on a beach sees waves crashing one after another on the shore, the water waving in and out. But a person surfing on a wave, traveling at its speed, rides a constant crest that no longer appears to “wave.” Maxwell’s equations provided no solution for this possibility with light. They required that for light to be “light,” it must move precisely at the speed of light. In fact, Michelson and other astronomers had verified this experimentally: no matter where you looked or how you moved, light always moved past you with the same speed.
It so happened that Maxwell’s equations had other problems with moving observers: charged particles in motion produced magnetic fields, while stationary ones did not. Depending on whether I am standing next to a proton or moving by one at a constant rate, I will see it produce different fields. This may not seem like a profound problem, but it puzzled Einstein and would eventually lead him to solve the mystery of Mercury—and in the process overturn how we understand the nature of time and space. The reason this is so puzzling is that the Relativity Principle, which has its origins in Galileo’s efforts to prove that the Earth can move without us feeling it, states that there is no experiment that can reveal whether a person is at rest or in constant, uniform motion. This principle must be true, since even when we are at rest in a laboratory, any experiment we do there is really flying at tremendous speeds as the Earth hurtles through space. Everything is always in motion relative to something. But Maxwell’s laws do not satisfy this principle.
Let’s say I hold two protons while standing inside a rocket moving at a constant speed. I open my hand and, because we are at rest with respect to one another, all I see is a repulsive electric force between them (their like charges repel). They fly apart and take exactly one second to hit the surrounding wall. But a friend standing outside the rocket will see me and the two protons in motion. There is now an additional magnetic force of attraction between the protons that causes them to move apart more slowly. By her watch, the same protons now take two seconds to hit the wall. But by the Relativity Principle, we can’t both be correct; otherwise, measuring the time it took the protons to fly apart would be a simple test to determine who was in motion and who was not. Einstein found that both observers could be correct, but only if time passed differently for people at different velocities. Moving clocks, he said, must run more slowly than those at rest; the closer a clock approached the speed of light, the slower time would run. The answer to Einstein’s original question was that a person moving at the speed of light would see a light wave stop waving, because time would stop altogether.
Einstein was not the first to propose a solution where space and time were relative. Henri Poincaré, a French mathematician and philosopher, had done so in his book La Science et l’hypothese (Science and Hypothesis), which was widely read among the intelligentsia of turn-of-the-century Europe. Einstein was influenced by Poincaré (as was, apparently, the artist Pablo Picasso). Einstein, however, was the one to take these radical ideas to their natural mathematical conclusions and have the courage to declare that this was how reality worked—no matter how much it might disagree with common sense (common sense, according to Einstein, being just the collection of prejudices someone acquired by the time they were eighteen).
In 1905, Einstein published his Special Theory of Relativity specifically looking at how motion at constant velocity (a “special” case of motion) solved the problems with Maxwell’s equations. The only way for the laws of electricity, magnetism, and light to work for any observer traveling at any constant speed was if the following were true:
1. Moving clocks run slowly compared with clocks at rest.
2. Moving meter-sticks are shorter compared with meter-sticks at rest.
3. Events that are simultaneous for observers at rest need not be simultaneous to observers moving at a constant speed. Simultaneity is relative.
As revolutionary as these ideas were, Einstein realized his Relativity theory was incomplete. What about any motion, including those where an observer was accelerating? It was this question that brought Einstein into the realm of Newton: he realized that an extension of the Relativity Principle to acceleration required that any physical phenomenon that happened while accelerating must also happen in a sufficiently large gravitational field.
We call this the Equivalence Principle, and there is a good chance you have experienced it for yourself. If you have ever been to Disneyland, you may have seen a ride where you sit in a room and feel as if you were zooming through space in a rocket. When the view out the front “window” shows the rocket blast off, hidden hydraulics tip the room backward, letting the Earth’s gravity pull you back into your seat. The pull of gravity is indistinguishable from the feeling when you accelerate, and it is the reason the ride works. The same principle is at work in movies like 2001: A Space Odyssey or Interstellar, where a spinning space station produces an acceleration which the characters experience as “artificial” gravity.
Einstein spent another decade working through the mathematics of this more “general” theory of Relativity. By the time he completed his equations in 1915, he had come up with two other strange conclusions:
1. Gravity is a curvature of the fabric of a four-dimensional “spacetime” (three dimensions of space and one dimension of time).
2. Clocks close to a massive object run more slowly compared with those farther away.
Gravity was no longer a force between objects with mass, as Newton proposed, but a warping of the fabric of both space and time that reproduced all the planetary motion predicted by Newton’s and Kepler’s laws. If you’ve ever seen a coin roll into one of those giant plastic funnels at a science museum, you will have seen it sweep in and out of the “gravity well” at the bottom in a way that reproduces the elliptical orbits and changing velocities of planets around the Sun. Get very close to a massive object like the Sun, and the warpage in spacetime causes planetary orbits to curve a little more than the amount Newton’s and Kepler’s laws predicted. The added curvature causes a planet to overshoot the point where its orbit began, and each new orbit begins a little farther along than the one before. As a result, its orbital axis shifts a tiny amount each trip around and traces out a shape like a flower, with each orbit a petal (rather than a single constant ellipse). A person on a planet farther from the Sun, like the Earth, sees a planet closer to the Sun, like Mercury, cross the Sun earlier than Newton’s and Kepler’s laws would have otherwise predicted, and over time, the discrepancy grows.
Announcing his new General Theory of Relativity to the Prussian Academy of Sciences on November 18, 1915, Einstein stated, “In this work, I found an important confirmation of this radical Relativity theory; it exhibits itself namely in the secular turning of Mercury in the course of its orbital motion, as was discovered by Le Verrier. Namely, the approximately 45 arcseconds per century amount is qualitatively and quantitatively explained without the special hypotheses that he had to assume.”
“Furthermore,” continued Einstein, “it shows that this theory has a stronger (doubly strong) light bending effect in consequence through the gravitational field” than what could be explained by Newton’s gravity alone. In other words, just like with Mercury, light traveling close to a massive object has its path deflected by the curvature of spacetime more than could be accounted for by Newton’s laws alone.
The mark of a successful scientific theory is that it ties together a wide range of physical phenomena, explaining accurately what is already seen and predicting results for experiments yet to be performed. General Relativity did exactly that. It tied together space, time, motion, light, electricity, magnetism, matter, and gravity. It explained what more than fifty years of transit and solar eclipse observations had failed to verify, and it suggested a result for a new test: the bending of starlight passing near a mass like the Sun. When could such a phenomenon be tested?
The answer was: during a total solar eclipse. The change in light would be subtle. Starlight passing through the Sun’s warped gravitational field would cause the light to reach Earth from a slightly different direction than if the Sun were not present. Stars near the Sun on the sky would all appear slightly shifted away from its disk when compared to their known positions. A photograph of the totally eclipsed Sun would record the faint light of all the stars that were momentarily visible. Compare these positions with a photo of the same star field without the Sun (perhaps taken as little as a couple of months after the eclipse), and the change in the stars’ positions should reveal any gravitational influence of the Sun.
FIGURE 5.1. Starlight passing close by the Sun appears to arrive at the Earth from a different location due to the curvature of spacetime. (Image by the author)
The first attempt to measure this deflection of starlight during a solar eclipse, in 1912, failed because of bad weather.cc The next opportunity occurred on August 21, 1914, from southern Russia. Unfortunately, the assassination of Archduke Ferdinand in July of that year led to the German invasion of Russia on the first of August and the start of World War I. Einstein’s colleague Erwin Freundlich, who had traveled there specifically to measure the deflection of starlight, found himself instantly converted from visiting astronomer to enemy alien. He had all of his instruments confiscated and was instantly arrested by the Russians. As the United States remained officially neutral, a group of astronomers from California’s Lick Observatory was allowed to remain, but they ran out of luck when the weather turned bad, and Russian officials impounded their instruments for the rest of the war.
World War I hindered the proof of Relativity in more ways. While today we may forget that Einstein was German, scientists of the day did not. A British zoologist included in his paper of 1918 a note saying, “No quotations from German authors published since August, 1914, are included. ‘Hostes humani generis’ [enemies of the human race].” The director of the Observatory of Turin in Italy gave lectures on the nature of “German science and Latin science” exhorting his audience to not look to German scientists “for bold initiative, for flashes of genius, for fruitful ideas, for results expressed in a brilliant, clear, comprehensive, and simple formula, as are all the laws of Nature. . . .
FIGURE 5.2. Engraving from The Graphic magazine, 1914, illustrating the two shadows descending upon Europe—the total solar eclipse of August 21, 1914, and the near-simultaneous outbreak of war. (Image courtesy Michael Zeiler, private collection)
Even where hatred of Germany wasn’t evident, many of the Allied scientists who heard of Einstein’s theory of gravitation were sure it must be wrong. The American astronomer Heber Curtis was the leader of Lick’s efforts to search for both Vulcan and any deflection of starlight. The photographic plates he acquired during the 1918 total solar eclipse in Washington State produced nothing but ambiguous results (in part because of thin clouds, but mostly thanks to cobbled together equipment replacing the pieces still impounded in Russia). Yet still he thought them worth publishing, so that, “When the Einstein theory goes into the discard, as I prophesy it will go within ten years, these negative or indecisive results will be more highly regarded than at present.”
Ironically, by war’s end it was a British astronomer, Sir Arthur Eddington, who was the unofficial spokesman for the German theory of Relativity. Thanks to a colleague in Holland, Eddington had the only copy of Einstein’s paper to reach England during the war, and he was the primary person spreading word that the mystery of Mercury’s orbit had been solved. As much as Allied scientists may have been loath to accept the broader implications of Einstein’s theory, the solution to the problem of Mercury could not be ignored. As a devout Quaker, Eddington was a pacifist like Einstein during a time when pacifism was not a popular position on either side of the war. In England, those who refused to serve on the basis of conscientious objection faced possible imprisonment. According to one of Eddington’s students, Subrahmanyan Chandrasekhar, it was a social disgrace to even associate with a conscientious objector; the view among the older faculty members at Cambridge University where Eddington taught was that it would bring disgrace upon the institution to even have one in their midst.
To avoid such a scandal, and to protect Eddington from the draft, “they therefore tried through the Home Office to have Eddington deferred on the grounds that he was a most distinguished scientist and that it was not in the long-range interests of Britain to have him serve in the army,” Chandrasekhar later wrote. “Eddington was deferred with the express stipulation that if the war should end by 1919, then he should lead one of the two expeditions that were being planned for the purpose of verifying Einstein’s prediction with regard to the gravitational deflection of light.”
The eclipse on May 29, 1919, would be the best opportunity yet to test the deflection of starlight, as the Sun would be passing before the Hyades star cluster, resulting in numerous bright stars visible beside the Sun. Totality would cross the Atlantic Ocean, touching land in northern Brazil and again on the island of Principe, off the coast of western Africa. By 1919, the war had ended, but astronomers from nearly every nation that would normally mount such an expedition were out of money or simply too busy recovering from the devastation. England alone found itself prepared to set sail to test this new theory of gravity. Eddington and an assistant from Cambridge would head to Principe, while the Astronomer Royal, Frank Dyson, sent two of his assistants from the Greenwich Observatory to observe the eclipse from Brazil. Each team would take along two instruments for photographing the eclipse. Their task was to capture photos of the eclipse, as well as comparison plates of the same region of sky without the Sun present. Any change in the positions of the background stars would reveal the effects of the Sun’s gravity on light.
The observations and their analysis would be difficult. The deflection Eddington was looking for was tiny: only about an arcsecond in size (an arcsecond is 1/1,800 of the solar diameter as seen from the Earth). Any temperature, focus, or mechanical changes in the telescope could skew where the individual stars fell on a photographic plate by at least that amount. Correcting for these possibilities required taking even more comparison photos. For the team in Brazil, the eclipse would occur soon after sunrise. If they remained there an additional two months, they could photograph the same stars before dawn, with the telescope in the same position, but the Sun no longer present.
In Africa, where the eclipse would occur near noon, it would take nearly half a year for the same stars to be photographed in the same position by night. As a compromise, comparison plates were taken back in England before the expeditions set sail. This was not an ideal arrangement, but both groups were already making do with makeshift equipment pieced together from parts available in the aftermath of war. The end result was that a difficult experiment would be performed under difficult conditions where the final results, though simple in principle—a deflection of 1.75 arcseconds would confirm Einstein, a deflection of half that would support Newton—would be anything but simple in practice.
To prepare the public for the results of an experiment testing the implications of a theory that even professionals couldn’t claim to understand fully, Eddington, Dyson, and their assistants implemented a public relations program aimed at both the general populace and the scientific community. It was an effort without clear precedent. For the public, they published explanatory newspaper articles about the experiments in The Times of London and gave public talks. For the scientific community, they wrote scholarly articles in Observatory, a leading professional journal, of which Eddington was the editor. These articles continued through the launch of the expedition and up to the day of the eclipse itself, with breathless reporting from the astronomers on station as the eclipse commenced.
The Times of London reported, on June 4: “Astronomer Royal (Sir Frank Dyson) informs us that he received yesterday a cablegram from Professor A. S. Eddington and Mr. Cottingham from Princes Island [sic], West Africa, stating that the eclipse of the sun was observed there through clouds, but they are hopeful of obtaining good results.” A day later, The Times informed readers that “further telegrams from the British astronomers who observed the total eclipse of the sun last week report that the photographs taken at Sobral, Brazil, were quite successful, and the negatives already developed show all the stars that were expected to be recorded.” Upon their return, other articles highlighted their work over the glass plates to tease out the results that would determine which great mind, Newton or Einstein, would prevail.
During this time the members of the general public weren’t the only ones who needed to be properly prepared. Einstein’s mathematics and the implications of his work were so complex that even Dyson could later write, “The result was contrary to my expectations, but since we obtained it I have tried to understand the Relativity business, & it is certainly very comprehensive, though elusive and difficult.” Prior to making any official announcement of their results, however, Eddington first quietly presented the findings to a leading group of British mathematical physicists. Only with their favorable reception were Eddington and Dyson confident enough to announce what they’d found. They presented the findings to a joint meeting of the Royal Astronomical Society and the Royal Society two weeks later.
On November 6, 1919, in front of over a hundred members of the scientific community (as well as one reporter from The Times), Dyson and Eddington announced that Einstein’s theory had been confirmed. Upon hearing their analysis, the president of the Royal Society declared,
If the results obtained had been only that light was affected by gravitation, it would have been of the greatest importance. . . . But this result is not an isolated one; it is part of a whole continent of scientific ideas affecting the most fundamental concepts of physics. . . . The difference between the laws of gravitation of Einstein and Newton come only in special cases. The real interest of Einstein’s theory lies not so much in his results as in the method by which he gets them. If this theory is right, it makes us take an entirely new view of gravitation. If it is sustained that Einstein’s reasoning holds good—and it has survived two very severe tests in connection with the perihelion of Mercury and the present eclipse—then it is the result of one of the highest achievements of human thought.
The next day the headlines blared out from The Times, “REVOLUTION IN SCIENCE.—New Theory of the universe.—Newton’s Ideas Overthrown.” The groundwork Eddington laid for Relativity’s favorable—even ecstatic—embrace in London caught the attention of the American media, which had not been subjected to Eddington’s public relations onslaught of the preceding year. The headline in the next day’s New York Times read: “LIGHTS ALL ASKEW IN THE HEAVENS—Men of Science More or Less Agog Over Results of Eclipse Observations—EINSTEIN THEORY TRIUMPHS—A Book for 12 Wise Men: No more in All the World Could Comprehend it, Said Einstein, When His Daring Publishers Accepted It.”
Eddington made Einstein a genius, but the American press made Einstein a star. Due to the press coverage of the 1919 total solar eclipse, we now live in a world where Einstein’s name is universally known and synonymous with brilliance, where everyone knows that black holes “suck in” light, that science-fiction starships fly through space using their “warp drive,” and that “everything is relative.” No scientist or philosopher predicts any longer that everything that can be known is known, or even that what is known now will always be known to be true. Even our language has changed: scientists are now careful to talk about scientific “theories” instead of “laws,” even when, like the General Theory of Relativity, they have withstood nearly a century of repeated experimentation.
That repeated questioning and testing of Relativity has been vital to its success. In the years since 1919, there have been those who have claimed that given Eddington’s public (and not so public) support for Relativity, he cooked the books on his “decisive” experiment, and that he threw out data that didn’t match the answer he wanted. It’s true the observations were not all that anyone could have wanted in a decisive experiment. Errors introduced in the stellar positions due to focus, tracking, temperature, and travel were not easily, or obviously, removed by all the comparison plates, and Eddington did throw out data he felt were not trustworthy.
But in a 1979 reanalysis of the plates using modern automated measurement software, the Greenwich Observatory confirmed the earlier results of 1919. More importantly, however, even after the results of 1919, the scientific community as a whole (including Eddington and Dyson) recognized the need to confirm the results during subsequent eclipses. At the next optimal eclipse in 1922, the skeptical astronomers at Lick Observatory successfully acquired results that confirmed those of Eddington. But the testing and retesting didn’t end there. Such is the tenacity of science that the last professional eclipse expedition to measure the deflection of starlight was in 1973, led by a University of Texas team looking to test Einstein’s theory against an even newer alternative.
Since then, radio astronomers have been able to measure the same deflection of light around the Sun using quasars—distant supermassive black holes—confirming Einstein’s theory to a precision far greater than Eddington could have ever dreamed possible. Astronomers continue to subject the details of the theory to experimentation, including the detection in 2015 of gravitational waves of spacetime caused by two black holes colliding. In a fraction of a second they released ten times as much energy as the combined luminosity of every star and galaxy in the observable universe. Such waves had been predicted by Einstein in 1916, and subsequently deduced in the 1970s from the motions of two neutron stars in orbit around one another (earning two astronomers a Nobel Prize in Physics in 1993). With this new window on our universe, astronomers continue to probe the strengths and limitations of Relativity. Any weaknesses discovered in Einstein’s theory in the future will raise new questions, spurring new experiments, and in time lead to even deeper understanding of the strange and magnificent universe in which we live. The story of the orbit of Mercury demonstrates the same principle: the beauty of science is that its discoveries are never the end of a story, merely the first step in a new and different one.
a These laws are: (1) moving charges and changing electric fields create a magnetic field; (2) changing magnetic fields create currents and electric fields; (3) electric fields are caused by charges; and (4) there is no such thing as a magnetic monopole (all magnets come with two poles).
b Almost no one remembers the hypothesized planet today. In 1962, the American television writer Gene Roddenberry was drafting a story for a new science fiction TV show and his main alien crew member was labeled a “Martian.” Roddenberry later changed this to something more exotic. What caused him to make Star Trek’s Mr. Spock a Vulcan, he never said.
c The astronomer C. D. Perrine, director of the Astronomical Observatory of Córdoba in Argentina, reported rain before, during, and after the eclipse: “We suffered a total eclipse instead of observing one.”