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comprehend. “The whole paper is a testament to the power of simple language to convey deep and powerfully disturbing ideas,” says the science<br />
writer Dennis Overbye. 53<br />
The paper starts with the “asymmetry” that a magnet and wire loop induce an electric current based only on their relative motion to one another,<br />
but since the days of Faraday there had been two different theoretical explanations for the current produced depending on whether it was the<br />
magnet or the loop that was in motion. 54 “The observable phenomenon here depends only on the relative motion of the conductor and the magnet,”<br />
Einstein writes, “whereas the customary view draws a sharp distinction between the two cases in which either the one or the other of these bodies<br />
is in motion.” 55<br />
The distinction between the two cases was based on the belief, which most scientists still held, that there was such a thing as a state of “rest”<br />
with respect to the ether. But the magnet-and-coil example, along with every observation made on light, “suggest that the phenomena of<br />
electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest.” This prompts Einstein to raise “to the<br />
status of a postulate” the principle of relativity, which holds that the laws of mechanics and electrodynamics are the same in all reference systems<br />
moving at constant velocity relative to one another.<br />
Einstein goes on to propound the other postulate upon which his theory was premised: the constancy of the speed of light “independent of the<br />
state of motion of the emitting body.” Then, with the casual stroke of a pen, and the marvelously insouciant word “superfluous,” the rebellious patent<br />
examiner dismissed two generations’ worth of accrued scientific dogma: “The introduction of a ‘light ether’ will prove to be superfluous, inasmuch<br />
as the view to be developed here will not require a ‘space at absolute rest.’ ”<br />
Using these two postulates, Einstein explained the great conceptual step he had taken during his talk with Besso. “Two events which, viewed<br />
from a system of coordinates, are simultaneous, can no longer be looked upon as simultaneous events when envisaged from a system which is in<br />
motion relative to that system.” In other words, there is no such thing as absolute simultaneity.<br />
In phrases so simple as to be seductive, Einstein pointed out that time itself can be defined only by referring to simultaneous events, such as the<br />
small hand of a watch pointing to 7 as a train arrives. The obvious yet still astonishing conclusion: with no such thing as absolute simultaneity, there<br />
is no such thing as “real” or absolute time. As he later put it, “There is no audible tick-tock everywhere in the world that can be considered as<br />
time.” 56<br />
Moreover, this realization also meant overturning the other assumption that Newton made at the beginning of his Principia. Einstein showed that<br />
if time is relative, so too are space and distance: “If the man in the carriage covers the distance w in a unit of time—measured from the train—then<br />
this distance—as measured from the embankment—is not necessarily also equal to w.” 57<br />
Einstein explained this by asking us to picture a rod that has a certain length when it is measured while it is stationary relative to the observer.<br />
Now imagine that the rod is moving. How long is the rod?<br />
One way to determine this is by moving alongside the rod, at the same speed, and superimposing a measuring stick on it. But how long would<br />
the rod be if measured by someone not in motion with it? In that case, a way to measure the moving rod would be to determine, based on<br />
synchronized stationary clocks, the precise location of each end of the rod at a specific moment, and then use a stationary ruler to measure the<br />
distance between these two points. Einstein shows that these methods will produce different results.<br />
Why? Because the two stationary clocks have been synchronized by a stationary observer. But what happens if an observer who is moving as<br />
fast as the rod tries to synchronize those clocks? She would synchronize them differently, because she would have a different perception of<br />
simultaneity. As Einstein put it, “Observers moving with the moving rod would thus find that the two clocks were not synchronous, while observers in<br />
the stationary system would declare the clocks to be synchronous.”<br />
Another consequence of special relativity is that a person standing on the platform will observe that time goes more slowly on a train speeding<br />
past. Imagine that on the train there is a “clock” made up of a mirror on the floor and one on the ceiling and a beam of light that bounces up and<br />
down between them. From the perspective of a woman on the train, the light goes straight up and then straight down. But from the perspective of a<br />
man standing on the platform, it appears that the light is starting at the bottom but moving on a diagonal to get to the ceiling mirror, which has<br />
zipped ahead a tiny bit, then bouncing down on a diagonal back to the mirror on the floor, which has in turn zipped ahead a tiny bit. For both<br />
observers, the speed of the light is the same (that is Einstein’s great given). The man on the track observes the distance the light has to travel as<br />
being longer than the woman on the train observes it to be. Thus, from the perspective of the man on the track, time is going by more slowly inside<br />
the speeding train. 58<br />
Another way to picture this is to use Galileo’s ship. Imagine a light beam being shot down from the top of the mast to the deck. To an observer on<br />
the ship, the light beam will travel the exact length of the mast. To an observer on land, however, the light beam will travel a diagonal formed by the<br />
length of the mast plus the distance (it’s a fast ship) that the ship has traveled forward during the time it took the light to get from the top to the<br />
bottom of the mast. To both observers, the speed of light is the same. To the observer on land, it traveled farther before it reached the deck. In other<br />
words, the exact same event (a light beam sent from the top of the mast hitting the deck) took longer when viewed by a person on land than by a<br />
person on the ship. 59<br />
This phenomenon, called time dilation, leads to what is known as the twin paradox. If a man stays on the platform while his twin sister takes off in<br />
a spaceship that travels long distances at nearly the speed of light, when she returns she would be younger than he is. But because motion is<br />
relative, this seems to present a paradox. The sister on the spaceship might think it’s her brother on earth who is doing the fast traveling, and when<br />
they are rejoined she would expect to observe that it was he who did not age much.<br />
Could they each come back younger than the other one? Of course not. The phenomenon does not work in both directions. Because the<br />
spaceship does not travel at a constant velocity, but instead must turn around, it’s the twin on the spaceship, not the one on earth, who would age<br />
more slowly.<br />
The phenomenon of time dilation has been experimentally confirmed, even by using test clocks on commercial planes. But in our normal life, it<br />
has no real impact, because our motion relative to any other observer is never anything near the speed of light. In fact, if you spent almost your<br />
entire life on an airplane, you would have aged merely 0.00005 seconds or so less than your twin on earth when you returned, an effect that would<br />
likely be counteracted by a lifetime spent eating airline food. 60<br />
Special relativity has many other curious manifestations. Think again about that light clock on the train. What happens as the train approaches<br />
the speed of light relative to an observer on the platform? It would take almost forever for a light beam in the train to bounce from the floor to the<br />
moving ceiling and back to the moving floor. Thus time on the train would almost stand still from the perspective of an observer on the platform.<br />
As an object approaches the speed of light, its apparent mass also increases. Newton’s law that force equals mass times acceleration still<br />
holds, but as the apparent mass increases, more and more force will produce less and less acceleration. There is no way to apply enough force to<br />
push even a pebble faster than the speed of light. That’s the ultimate speed limit of the universe, and no particle or piece of information can go<br />
faster than that, according to Einstein’s theory.