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Albert Einstein was famous for many things, but his greatest
brainchild is the theory of relativity. It forever changed our understanding of
space and time.
What is relativity? Succinctly put, it is the notion that
the laws
of physics are the same everywhere. We here on Earth obey the same laws of
light and gravity as someone in a far off corner of the universe.
The universality of physics means that history is provincial.
Different viewers will see the timing and spacing of events differently. What
for us is a million years may just be a blink of an eye for someone flying in a
high speed rocket or falling into a black hole.
It's all relative.
Special relativity
Einstein's theory is divided into special and general
relativity.
Special relativity came first and is based on the speed of
light being constant for everyone. That may seem simple enough, but it has
far-reaching consequences.
Einstein
came to this conclusion in 1905 after experimental
evidence showed that the speed of light didn't change as the Earth swung around
the Sun.
This result was surprising to physicists because the speed
of most other things does depend on what direction the observer is moving. If
you drive your car alongside a railroad track, a train coming at you will seem
to be moving much faster than if you turned around and followed it in the same
direction.
Einstein said that all observers will measure the speed of
light to be 186,000 miles per second, no matter how fast and what direction
they are moving.
This maxim prompted the comedian Stephen Wright to ask:
"If you are in a spaceship that is traveling at the speed of light, and
you turn on the headlights, does anything happen?"
The answer is the headlights turn on normally, but only from
the perspective of someone inside the spaceship. For someone standing outside
watching the ship fly by, the headlights do not appear to turn on: light comes
out but it takes an eternity for the beams to get ahead of the spaceship.
These contradictory versions arise because rulers and
clocks—the things that mark time and space—are not the same for different
observers. If the speed of light is to be held constant as Einstein said, then
time and space cannot be absolute; they must be subjective.
For instance, a 100-foot-long spaceship traveling at 99.99
percent the speed of light will appear one foot long to a stationary observer,
but it will remain its normal length for those onboard.
Perhaps even weirder, time passes slower the faster one
goes. If a twin rides in the speeding spaceship to some distant star and then
comes back, she will be younger than her sister who stayed on Earth.
Mass, too, depends on speed.
The faster an object moves, the more massive it becomes. In fact, no
spaceship can ever reach 100 percent of the speed of light because its mass
would grow to infinity.
This relationship between mass and speed is often expressed
as a relationship between mass and energy:
E=mc^2, where E is energy, m is mass and c is the speed of light.
General relativity
Einstein wasn't done upsetting our understanding of time and
space. He went on to generalize his theory by including acceleration and found
that this distorted the shape of time and space.
To stick with the above example: imagine the spaceship
speeds up by firing its thrusters. Those onboard will stick to the ground just
as if they were on Earth. Einstein claimed that the force we call gravity is
indistinguishable from being in an accelerating ship.
This by itself was not so revolutionary, but when Einstein
worked out the complex math (it took him 10 years), he discovered that space
and time are curved near a massive object, and this curvature is what we
experience as the force of gravity.
It is difficult to picture the curved geometry of general
relativity, but if one thinks of space-time as a kind of fabric, then a massive
object stretches the surrounding fabric such that anything passing nearby
no longer follows a straight line.
The equations of general relativity predict a number of
phenomena, many of which have been confirmed:
The warping
of space-time around a black hole is more intense than anywhere else. If
the space-faring twin fell
into a black hole, she would be stretched out like spaghetti.
Luckily for her it would all be over in a few seconds. But
her sister on Earth would never see it end—watching her poor sister inching
incrementally toward the black hole over the age of the universe.
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