help conceiving as anything except a flat surface. With every improvement in his telescopes, Herschel pushed his observations of stars to greater and greater depths in the sky or distances from Earth or—since the speed of light coming from the stars is finite, since it does take time to reach our eyes—farther and farther into the past. “I have looked further into space than ever human being did before me,” Herschel marveled in 1813, in his old age. “I have observed stars of which the light, it can be proved, must take two million years to reach the earth.”
Even that distance, however, would seem nearby if the speculations of some astronomers at the turn of the twentieth century turned out to be true. If certain smudges at the farthest reaches of the mightiest telescopes turned out to be systems of stars outside our own—other “island universes” altogether equal in size and magnitude to our own Milky Way—then when we looked at the starlight reaching us from them we might be seeing not Herschel’s previously unfathomable two million years into the past but two hundred million years or even two thousand million years. And so they would go, these meditations on the meaning of light, ever and ever outward, further and further pastward, if not necessarily ad infinitum, then at least, quite possibly, ad absurdum.
Now Einstein reversed that trajectory. Instead of considering the implications of looking farther and farther across the universe and thereby deeper and deeper into the past, he thought about the meaning of looking nearer and nearer—or, by the same reasoning, closer and closer to the present. Look near enough, he realized, and you’ll be seeing very close indeed to the present. But only one place can you claim to be the present—and then only your present.
It was this insight that allowed Einstein to endow the idea of time with an unprecedented immediacy, in both the positional and the temporal senses of the word: here and now: the arrival of a train and the hands of a watch. Because the train and the hands of the watch occupy the same location, they also occupy the same time. For an observer standing immediately adjacent to the train, that time is, by definition, the present: seven o’clock. But someone in a different location observing the arrival of that same train—that is, someone at some distance away receiving the image of the train, which has traveled by means of electromagnetic waves from the surface of the locomotive to the eyes of this second observer at the speed of light, an almost unimaginably high yet nonetheless finite velocity—wouldn’t be able to consider the arrival of the train simultaneous with its arrival for the first observer. If light did propagate instantaneously—if the speed of light were in fact infinite—then the two observers would be seeing the arrival of the train simultaneously. And indeed, it might very well seem to them as if they were, especially if (using the modern value for the speed of light as 186,282 miles, or 299,792 kilometers, per second) the other observer happens to be standing on a street corner that’s about two-millionths of a light-second (the distance that light travels in two-millionths of a second, or slightly less than 2000 feet) away rather than, less ambiguously, near a star that’s two million light-years (or slightly less than 12 quintillion miles) away. And yes, if you were the observer on the street corner in the same town, gazing down a hill at a slowing locomotive pulling into the station, the arrival of the train for all practical purposes might as well be happening at the same moment as its arrival for the observer on the platform.
But what it is, is in your past.
Einstein was not in fact alone in recognizing the role that the velocity of light plays in the conception of time. Other physicists and philosophers had begun to note a paradox at the heart of the concept of simultaneity—that for two observers, the difference in distances has to translate into a difference in time. But where Einstein diverged from even the most radical of his contemporaries was in accepting as potentially decisive what the velocity of light is.
It was there in the math. In 1821, the British physicist Michael Faraday had decided to investigate reports from the Continent concerning electricity and magnetism by placing a magnet on a table in his basement workshop and sending an electrical impluse through a wire dangling over it. The wire began twirling, as if the electricity were sparking downward and the magnetism were influencing upward. This was, in effect, the first dynamo, the invention that would drive the industrial revolution for the rest of the century, and the product that Einstein’s own father and uncle would manufacture as the family business. But not until the 1860s did the Scottish physicist James Clerk Maxwell manage to capture Faraday’s accomplishment in mathematical form, a series of equations with an unforeseen implication. Electromagnetic waves travel at the same speed as light (and therefore, Maxwell predicted, are light): 186,282 miles, or 299,792 kilometers, per second in a vacuum. Meaning … what? That it would be more than 186,282 miles per second if you were moving away from the source of light, or less than 186,282 miles per second if you were moving toward the source? Yes—according to Newton’s mechanics. Yet it never seemed to vary.
On a planet that was spinning; a spinning planet that was orbiting the sun; a spinning planet orbiting a sun that itself was moving in relation to other stars that were moving in relation to one another—in this setting that, as Copernicus and Galileo and Newton and Herschel and so many other astronomers and mathematicians and physicists and philosophers had so persuasively established, was never at rest and therefore wouldn’t be at rest in relation to a source of light outside itself, always the answer to the question of what was the speed of light seemed to come up exactly the same. Just as Aristotelian philosophers considering the descent of an onboard stone would have overlooked the motion of the ship, so maybe several generations of Galilean physicists had been overlooking properties of electromagnetism. Maybe what you needed to consider was the motion of the stone, the motion of the ship, and the motion of the medium by which we perceive both, and together those three elements would constitute a single system in motion.
A few years earlier, his friend Besso had given Einstein a copy of the Austrian physicist Ernst Mach’s Die Mechanik in ihrer Entwicklung (The Science of Mechanics). This work, Einstein later recalled, “exercised a profound influence upon me” because it questioned “mechanics as the final basis of all physical thinking.” The issue for Mach wasn’t whether mechanics had worked well over the past two centuries in describing the motions of matter; clearly, it had. The issue wasn’t even whether mechanics could answer all questions about the physical universe, as the Kelvins of the world were constantly trying to prove. Rather, the issue for Mach—the root of his objection to Newtonian mechanics—was that it raised some questions it couldn’t answer.
For instance, absolute space, the existence of which is necessary to measure absolute motions: On close reading, Newton’s definition of it turned out to be every bit as circular as the reigning definition of the ether. “Absolute motion,” Newton had written, “is the translation of a body from one absolute place into another.” And what is place? “Place is a part of space which a body takes up, and is according to the space, either absolute or relative.” So what, then, is absolute space? “Absolute space, in its own nature, without relation to anything external, remains always similar and immovable.” Newton anticipated some criticism: “It is indeed a matter of great difficulty to discover, and effectually to distinguish, the true motions of particular bodies from the apparent; because the parts of that immovable space, in which those motions are performed, do by no means come under the observation of our senses. Yet the thing is not altogether desperate,” he reassured the reader, “for we have some arguments to guide us, partly from the apparent motions, which are the differences of the true motions; partly from the forces, which are the causes and effects of the true motions.” And what are these true, or absolute, motions? See above.
“We join with the eminent physicist Thomson [later Lord Kelvin] in our reverence and admiration of Newton,” Mach wrote in 1883. “But we can only comprehend with difficulty his opinion that the Newtonian doctrines still remain the best and most philosophical foundation that can be given.” Not that Mach was proposing an alternative to Newtonian mechanics; not that he was even suggesting physics was in need of an alternative. Rather, he was trying to remind his fellow physicists that just because
