What IS the

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What IS the Theory of Relativity?

A POWERPOINT BY JORDAN
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Produced by scientist Albert Einstein in the
early 1900s, the theory of relativity is
recognized as one of the greatest achievements of
the 20th century.
BUT WHAT IS IT?
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The theory of relativity is classified into two
parts Special relativity And General relativity
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The special theory of relativity was presented to
the public in 1905. For the sake of simplicity,
Einstein formulated this theory around uniform
(unchanging) motion (includes standing still),
unlike the theory of general relativity that
followed in 1915.
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But first, to make things a lot easier, I will
explain the concept of reference frames.
A reference frame is a virtual platform in which
scientists make observations. They may be moving
or at rest, accelerated or moving uniformly.
Essentially, it is your point of view.

• In his paper On the Electrodynamics of Moving
  Bodies, Einstein introduced two basic concepts
• The laws of physics are the same in all uniformly
  moving reference frames.
• 2. The speed of light is the same in all
  uniformly moving reference frames.

But what do those mean?
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The 1st concept, where the laws of physics are
the same in all uniformly moving reference
frames, had already been accepted by most
scientists, but it was the 2nd concept, the
unchanging speed of light, that hit them.
Let me explain why
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You see, with Mechanical waves (like sound,
seismic, and water waves), the speed of the wave
relative to you depends on your speed. For
exampleThe speed of sound waves is 340.29 m/s.
lets say that a jet plane is traveling at a
uniform 300 m/s. Thus, the speed of sound waves
traveling away from behind the jet would have a
speed of 640.29 m/s relative to the jet.
Likewise, the speed of sound waves traveling away
from the front of the jet would be 40.29 m/s
relative to the jet. That is why supersonic
travel is possible.
But that does not work with light waves.
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The speed of light waves is 299,792,458 m/s. Lets
say that a rocket was traveling at a speed of
1,000,000 m/s. However, the speed of light
traveling away in all directions would still be
299,792,458 m/s, which would be the same speed
measured by those at rest on the earth.
But, according to Einstein, that is not only for
light waves. Lets say that the rocket shot a
bullet in front of it at 99.99 of the speed of
light. The bullet would not go faster than light,
just because of the rockets speed. So, the theory
of relativity applies to high speeds in general,
not just light waves.
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Einstein was famous for his thought
experiments. In one of those, he imagined
himself racing a light wave. If he could get to
its speed, he would see a stationary wave.
However, the 2nd statement of special relativity
says that light travels at the same speed for all
observers, so Einstein rationalized that nothing
can reach the speed of light.
Now, lets get into the confusing part. The part
about SIMULTANIOUS EVENTS.
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Lets say that there was an experiment set up on
a train, uniformly moving, in which two light
bulbs were placed at either end of the box car an
equal distance from a sheet of paper. If the
light waves from either end of the box car hit
the film at the exact same time, a bell rings.
BELL
LIGHTBULB
LIGHTBULB
RAILWAY TRACKS
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Lets say that there is a person sitting in the
train and a person sitting by the railway tracks.
BELL
LIGHTBULB
LIGHTBULB
RAILWAY TRACKS
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The person in the train turns on the lights at
the exact same time. Because of the 2nd concept
in the theory of relativity (stating that light
travels the same speed to all observers), he sees
the beams of light travel at the same speed and
hit the film at the exact same time.
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(For the simplicity of the eye, light waves will
be shown as red arrows, red being easier to see
than yellow)
BELL
LIGHTBULB
LIGHTBULB
RAILWAY TRACKS
The bell rings.
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Now we will show it from the perspective of a
person outside the box car on an unmoving bench.
Because of the 2nd concept of special relativity,
the light waves have to travel at the same speed
in relative to both people. Thus, for the person
sitting on the bench, the bulbs would appear to
be turned on at different times. Let me show you
why
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TRAIN MOVING THIS WAY
Remember, the light must travel the same speed
regardless of the movement of the train. So, the
back lightbulb seems to travel farther than the
lightbulb in front.
BELL
LIGHTBULB
LIGHTBULB
BELL
RAILWAY TRACKS
The bell rings.
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Despite the seemingly different observations,
there is one truth. The bell rings. So who is
right? The observer in the train or the observer
on the bench? Einsteins surprising answer is
simply that they both are.
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So, what we can deduce from this experiment is
that what is simultaneous to one person is not
simultaneous to another, i. e., simultaneity is
relative.
Two other effects of relativity are length
contraction and time dilation. These effects are
closely intertwined with the relativity of
simultaneity. Heres why speed is distance
divided by time, right? So, if the speed of light
(and other fast-moving objects) is the same in
all RFs, then it needs be that distance and time
must have different values. However, because of
our very low average speeds, the effects of these
are unnoticeable in daily life.
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Now that were done with special relativity, it
is time to move on to
General Relativity.
Einstein realized that the limiting to uniform
motion was a very large hindrance to the special
theory of relativity. So he created the General
theory of relativity in 1915, being called
general because it had a broader range.
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To come up with one of the key ideas of General
Relativity, Einstein used one of his famous
thought experiments. Einstein imagined himself in
an unmoving elevator. He felt the usual downward
pull caused by gravity. Einstein begins to think
outside the box, If there are no windows, than I
cant tell whether I am in an unmoving elevator
in the earths gravitational field or in an
elevator hitched to the back of an accelerating
rocket. So Einstein theorized that there is no
difference between an accelerated reference frame
in space and a uniformly-moving reference frame
in a gravitational field, therefore a
gravitational pull in one direction equals an
acceleration in the other direction. That theory
is called the Principle of Equivalence. Let me
explain
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This is a graph of an accelerating car.
As you can see, the cars reference frames
space-time geometry is curved. So are all other
accelerated reference frames have curved
space-time geometry.
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Einstein also said that matter itself bends the
fabric of space-time. To understand matter
bending space-time, I will revert to a 2-D
model. Lets say that the fabric of space-time is
like a trampoline. Let us also say that there is
no friction at all. Just a trampoline. Now, take
a marble. The marble is attached to the
trampoline, not by gravity, but in a way that it
cant leave but still has no friction. If you
roll a marble across the trampoline, it will keep
rolling until it hits the other side. Now, if we
place a bowling ball in the center, it will make
a dip in the trampoline. Now, if you roll the
marble it will curve in its path towards the
bowling ball. In the frictionless atmosphere, a
perfect roll could send it orbiting the bowling
ball indefinitely. Now imagine looking down at it
from 10 feet in the air. All you see is a marble
orbiting a bowling ball. You cant see the dip
the bowling ball makes. You may wonder why the
marble doesnt follow the laws of inertia, and
you may conclude that some force is causing it to
break from inertia. Then Einstein looks at it
from below, then comes and tells you about the
dip the bowling ball makes. Of course, this was
just a 2-D model of it, 4-D being impossible to
comprehend.
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So, as you see, an acceleration in one direction
equals a gravitational pull in the other
direction, both dealing with curving space-time.
In General relativity, Einstein had a few
predictions. One is that clocks are affected by
gravity (for simplicity, scientists refer to
gravity instead of curved space-time). Another
was that light would curve in a gravitational
field. All of the many ideas in the theories of
relativity have been scientifically ratified, and
are still continually proven to be reliable and
correct.
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Credits

• I got all of my information, ideas, and examples
  from two amazing books
• Great Ideas in Physics By Alan Lightman
• And
• Physics for the Rest of Us by Roger S. Jones.