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Many people consider Albert Einstein’s formulation of the theory of relativity to be the starting point of
modern physics. You may have heard about the theory of relativity several times, but you may not have
known what it was exactly. In fact, Einstein formulated two parts of the theory, special relativity and
general relativity. If you are not too familiar with them, the theories may seem very strange at first.
Special Relativity
For example, say you are traveling in a car going one direction at 25 m/s, and another car is heading the
opposite direction at 15 m/s. From your vantage point, the other car would seem to be coming towards
you at 40 m/s (simple addition: 25 + 15). In other words, the other car is moving 40 m/s relative to you.
Let us move this analogy to higher speeds. You are in a spaceship moving 2.0 x 108 m/s. Another
spaceship is moving the opposite direction at 1.5 x 108 m/s. Do you say that the other spaceship is
moving 3.5 x 108 m/s relative to you? However, light moves at 3.0 x 108 m/s. Does that mean that the
other spaceship is moving faster than the speed of light relative to you? In fact, Einstein says that the
other spaceship does not move at 3.5 x 108 m/s; it would be impossible for it to do so. Einstein
calculates that the other spaceship will appear to travel about 2.6 x 108 m/s towards you. The simple
addition does not work anymore. This is Einstein’s theory of special relativity.
Special relativity says basically one thing: the speed of light in all reference frames is constant. No
matter how fast you travel, the speed of light in all directions is 3.0 x 108 m/s. This fact accounts for all
the strange things that happen when you start moving at extremely high speeds. If you are moving at 2.0
x 108 m/s and another person is stationary, both of you measure the speed of light to be 3.0 x 108. You
are probably thinking why this is so.
The only solution to this apparent paradox is that time slows down for you. If the stationary person looks
into your moving spaceship, he would see you moving much more slowly. In addition, Einstein predicts
that you will appear to be flattened as you approach the speed of light. However, since you are in the
spaceship, you do not notice these effects because your brain also slows down and everything around
you flattens as well. Thus, space and time can be unified into a single entity—space-time.
Another consequence of Einstein’s theory is that your mass increases as you approach the speed of light.
This additional mass must come from somewhere. It turns out that this mass comes from the energy of
your moving ship. Einstein found that matter and energy are unified with his most famous equation, E =
mc2. Since the speed of light squared (c2) is an extremely large number, it means that a small amount
of matter can release a vast amount of energy. This is the basis for weapons such as the hydrogen
bomb.
General Relativity
Special relativity handled situations with constant velocities quite nicely, but Einstein wondered what
would happen in an accelerating frame of reference. His conclusion was the theory of general relativity,
which state that the laws of nature in an accelerating frame are equivalent to the laws in a gravitational
field.
He used this example to describe general relativity: if you are in an elevator and suddenly the rope
breaks, you become weightless since you and the elevator are accelerating in the earth’s gravitational
field. If you had no idea what was happening, you might think that gravity was turned off somehow.
Similarly, someone in an accelerating rocket will feel a force pushing him into his seat, as though by
gravity. Thus, accelerating frames are the same as frames in a gravitational field. This is Einstein’s
principle of equivalence, showing that gravitational mass is equal to inertial mass.
Taking this one step further, if you shine a light beam into an accelerating rocket, the light beam would
bend downward since the rocket accelerates upward during the time it takes the light beam to move
across. Therefore, Einstein reasoned that a gravitational field would bend light as well. Since light beams
will take the path requiring the least amount of time between two points, it was once thought that their
paths were straight. However, in a gravitational field, the path with the least time between two points is
a curved line. Ultimately, Einstein concluded that space itself was curved.
This revelation had a serious impact on the way we view the universe. It meant that mass and energy
curves space-time. We do not perceive this curvature because light is bent along with it. However, we do
perceive a force because the curvature of space-time. For the first time, we had a geometric explanation
for the mysterious action-at-a-distance forces such as gravity. Forces like gravity are the result of the
warping of space-time in the fourth spatial dimension (the word spatial is used so it will not be confused
with time, which is a temporal dimension).
Source: ThinkQuest
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