einstein
einstein
einstein
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Suppose that at the exact instant (from the viewpoint of the person on the embankment) when lightning strikes at points A and B, there is a<br />
passenger at the midpoint of the train, M t , just passing the observer who is at the midpoint alongside the tracks, M. If the train was motionless<br />
relative to the embankment, the passenger inside would see the lightning flashes simultaneously, just as the observer on the embankment would.<br />
But if the train is moving to the right relative to the embankment, the observer inside will be rushing closer toward place B while the light signals<br />
are traveling. Thus he will be positioned slightly to the right by the time the light arrives; as a result, he will see the light from the strike at place B<br />
before he will see the light from the strike at place A. So he will assert that lightning hit at B before it did so at A, and the strikes were not<br />
simultaneous.<br />
“We thus arrive at the important result: Events that are simultaneous with reference to the embankment are not simultaneous with respect to the<br />
train,” said Einstein. The principle of relativity says that there is no way to decree that the embankment is “at rest” and the train “in motion.” We can<br />
say only that they are in motion relative to each other. So there is no “real” or “right” answer. There is no way to say that any two events are<br />
“absolutely” or “really” simultaneous. 43<br />
This is a simple insight, but also a radical one. It means that there is no absolute time. Instead, all moving reference frames have their own<br />
relative time. Although Einstein refrained from saying that this leap was as truly “revolutionary” as the one he made about light quanta, it did in fact<br />
transform science. “This was a change in the very foundation of physics, an unexpected and very radical change that required all the courage of a<br />
young and revolutionary genius,” noted Werner Heisenberg, who later contributed to a similar feat with his principle of quantum uncertainty. 44<br />
In his 1905 paper, Einstein used a vivid image, which we can imagine him conceiving as he watched the trains moving into the Bern station past<br />
the rows of clocks that were synchronized with the one atop the town’s famed tower. “Our judgments in which time plays a part are always<br />
judgments of simultaneous events,” he wrote. “If, for instance, I say, ‘That train arrives here at 7 o’clock,’ I mean something like this: ‘The pointing of<br />
the small hand of my watch to 7 and the arrival of the train are simultaneous events.’ ” Once again, however, observers who are moving rapidly<br />
relative to one another will have a different view on whether two distant events are simultaneous.<br />
The concept of absolute time—meaning a time that exists in “reality” and tick-tocks along independent of any observations of it—had been a<br />
mainstay of physics ever since Newton had made it a premise of his Principia 216 years earlier. The same was true for absolute space and<br />
distance.“Absolute, true, and mathematical time, of itself and from its own nature, flows equably without relation to anything external,” he famously<br />
wrote in Book 1 of the Principia. “Absolute space, in its own nature, without relation to anything external, remains always similar and immovable.”<br />
But even Newton seemed discomforted by the fact that these concepts could not be directly observed. “Absolute time is not an object of<br />
perception,” he admitted. He resorted to relying on the presence of God to get him out of the dilemma. “The Deity endures forever and is<br />
everywhere present, and by existing always and everywhere, He constitutes duration and space.” 45<br />
Ernst Mach, whose books had influenced Einstein and his fellow members of the Olympia Academy, lambasted Newton’s notion of absolute time<br />
as a “useless metaphysical concept” that “cannot be produced in experience.” Newton, he charged, “acted contrary to his expressed intention only<br />
to investigate actual facts.” 46<br />
Henri Poincaré also pointed out the weakness of Newton’s concept of absolute time in his book Science and Hypothesis, another favorite of the<br />
Olympia Academy. “Not only do we have no direct intuition of the equality of two times, we do not even have one of the simultaneity of two events<br />
occurring in different places,” he wrote. 47<br />
Both Mach and Poincaré were, it thus seems, useful in providing a foundation for Einstein’s great breakthrough. But he owed even more, he later<br />
said, to the skepticism he learned from the Scottish philosopher David Hume regarding mental constructs that were divorced from purely factual<br />
observations.<br />
Given the number of times in his papers that he uses thought experiments involving moving trains and distant clocks, it is also logical to surmise<br />
that he was helped in visualizing and articulating his thoughts by the trains that moved past Bern’s clock tower and the rows of synchronized clocks<br />
on the station platform. Indeed, there is a tale that involves him discussing his new theory with friends by pointing to (or at least referring to) the<br />
synchronized clocks of Bern and the unsynchronized steeple clock visible in the neighboring village of Muni. 48<br />
Peter Galison provides a thought-provoking study of the technological ethos in his book Einstein’s Clocks, Poincaré’s Maps. Clock coordination<br />
was in the air at the time. Bern had inaugurated an urban time network of electrically synchronized clocks in 1890, and a decade later, by the time<br />
Einstein had arrived, finding ways to make them more accurate and coordinate them with clocks in other cities became a Swiss passion.<br />
In addition, Einstein’s chief duty at the patent office, in partnership with Besso, was evaluating electromechanical devices. This included a flood<br />
of applications for ways to synchronize clocks by using electric signals. From 1901 to 1904, Galison notes, there were twenty-eight such patents<br />
issued in Bern.<br />
One of them, for example, was called “Installation with Central Clock for Indicating the Time Simultaneously in Several Places Separated from<br />
One Another.” A similar application arrived on April 25, just three weeks before Einstein had his breakthrough conversation with Besso; it involved<br />
a clock with an electromagnetically controlled pendulum that could be coordinated with another such clock through an electric signal. What these<br />
applications had in common was that they used signals that traveled at the speed of light. 49<br />
We should be careful not to overemphasize the role played by the technological backdrop of the patent office. Although clocks are part of<br />
Einstein’s description of his theory, his point is about the difficulties that observers in relative motion have in using light signals to synchronize<br />
them, something that was not an issue for the patent applicants. 50<br />
Nevertheless, it is interesting to note that almost the entire first two sections of his relativity paper deal directly and in vivid practical detail (in a<br />
manner so different from the writings of, say, Lorentz and Maxwell) with the two real-world technological phenomena he knew best. He writes about<br />
the generation of “electric currents of the same magnitude” due to the “equality of relative motion” of coils and magnets, and the use of “a light<br />
signal” to make sure that “two clocks are synchronous.”<br />
As Einstein himself stated, his time in the patent office “stimulated me to see the physical ramifications of theoretical concepts.” 51 And Alexander<br />
Moszkowski, who compiled a book in 1921 based on conversations with Einstein, noted that Einstein believed there was “a definite connection<br />
between the knowledge acquired at the patent office and the theoretical results.” 52<br />
“On the Electrodynamics of Moving Bodies”<br />
Now let’s look at how Einstein articulated all of this in the famous paper that the Annalen der Physik received on June 30, 1905. For all its<br />
momentous import, it may be one of the most spunky and enjoyable papers in all of science. Most of its insights are conveyed in words and vivid<br />
thought experiments, rather than in complex equations. There is some math involved, but it is mainly what a good high school senior could