einstein

comprehend. “The whole paper is a testament to the power of simple language to convey deep and powerfully disturbing ideas,” says the science
writer Dennis Overbye. 53
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,
but since the days of Faraday there had been two different theoretical explanations for the current produced depending on whether it was the
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,”
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
is in motion.” 55
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”
with respect to the ether. But the magnet-and-coil example, along with every observation made on light, “suggest that the phenomena of
electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest.” This prompts Einstein to raise “to the
status of a postulate” the principle of relativity, which holds that the laws of mechanics and electrodynamics are the same in all reference systems
moving at constant velocity relative to one another.
Einstein goes on to propound the other postulate upon which his theory was premised: the constancy of the speed of light “independent of the
state of motion of the emitting body.” Then, with the casual stroke of a pen, and the marvelously insouciant word “superfluous,” the rebellious patent
examiner dismissed two generations’ worth of accrued scientific dogma: “The introduction of a ‘light ether’ will prove to be superfluous, inasmuch
as the view to be developed here will not require a ‘space at absolute rest.’ ”
Using these two postulates, Einstein explained the great conceptual step he had taken during his talk with Besso. “Two events which, viewed
from a system of coordinates, are simultaneous, can no longer be looked upon as simultaneous events when envisaged from a system which is in
motion relative to that system.” In other words, there is no such thing as absolute simultaneity.
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
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
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
time.” 56
Moreover, this realization also meant overturning the other assumption that Newton made at the beginning of his Principia. Einstein showed that
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
this distance—as measured from the embankment—is not necessarily also equal to w.” 57
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.
Now imagine that the rod is moving. How long is the rod?
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
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
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
distance between these two points. Einstein shows that these methods will produce different results.
Why? Because the two stationary clocks have been synchronized by a stationary observer. But what happens if an observer who is moving as
fast as the rod tries to synchronize those clocks? She would synchronize them differently, because she would have a different perception of
simultaneity. As Einstein put it, “Observers moving with the moving rod would thus find that the two clocks were not synchronous, while observers in
the stationary system would declare the clocks to be synchronous.”
Another consequence of special relativity is that a person standing on the platform will observe that time goes more slowly on a train speeding
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
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
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
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
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
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
the speeding train. 58
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
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
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
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
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
person on the ship. 59
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
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
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
they are rejoined she would expect to observe that it was he who did not age much.
Could they each come back younger than the other one? Of course not. The phenomenon does not work in both directions. Because the
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
more slowly.
The phenomenon of time dilation has been experimentally confirmed, even by using test clocks on commercial planes. But in our normal life, it
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
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
likely be counteracted by a lifetime spent eating airline food. 60
Special relativity has many other curious manifestations. Think again about that light clock on the train. What happens as the train approaches
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
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.
As an object approaches the speed of light, its apparent mass also increases. Newton’s law that force equals mass times acceleration still
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
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
faster than that, according to Einstein’s theory.