is a catch-all phrase for both the theory of special relativity
and the theory of general relativity. Albert Einstein is the father
of both theories, even though special relativity has it's roots
in earlier work.
to popular belief, the theory of relativity does not say
that everything is relative. It does say that the speed
of light is constant. Since light is constant, special relativity
reasons, things that we once thought were constant, namely length,
mass, and time, are not constant.
do we mean by constant? Think of it this way: in the newtonian
model, no velocity was constant. For example, if you see someone
riding their bike, you might say that they are going 15 miles per
hour in relation to you. However, if you then started jogging along
side of the bike, you might say that the bikes velocity is only
5 miles per hour, because you are running 10 miles per hour. Velocities
are relative. In relativity theory, this is still for the most part
true, with the exception of light. If you are not moving and light
is riding a bicycle (for discussion's sake) toward you at the speed
of light, you would measure it's speed as the speed of light. However,
if you then started jogging along side of the bike, you would still
measure it's velocity as the speed of light, even though you are
now going 10 miles per hour. In fact, you could be going a million
miles per hour in relation to your original position, but you would
still measure the speed of light the same.
all relativity tells us. Relativity also tells us that
and that gravity is the net effect of the curvature of space-time
as a result of mass.
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is Special Relativity?
relativity is the theory published by Einstein in 1905. Specifically,
special relativity says that light is constant, and as velocity
increases length decreases, mass increases, and time slows down.
See Why Relativity Works for a detailed explanation.
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is General Relativity?
Relativity is a "generalized" and enhanced version of
special relativity. General relativity describes the same odd behavior
at high velocities as special relativity, but adds a twist. General
relativity throws in gravity.
realized that there was no difference in the force of gravity and
the force of acceleration. For example, someone in a rocket ship
without windows cannot tell whether the ship is a rest on Earth
or is accelerating through outer-space. The net effect (the person's
feet pushing against the floor of the spaceship) is identical. If
gravity has characteristics of motion, then strong gravitational
feilds must make matter behave similar to high velocities.
comes from the way the theory explains the phenomena. General relativity
can theoretically explain any scenario in space-time.
is meant by "the curvature of space-time"? To
know that, we must first learn what space-time is.
everything you can see is made out of atoms. Those atoms combine
to form molecules, etc. Air is made out of atoms, water is made
out of atoms, just about everything around us is made out of atoms.
Imagine if we could see down to the atomic level. We could see all
the atoms that make up everything. But what is in between the atoms?
is space-time. Time is an important part of it, because the theories
of relativity tell us that as you move through space you also move
through time. Einstein thought of space-time having four dimensions:
up-down, east-west, north-south, and a fourth dimension that is
time multiplied by i ( i is defined
as the square root of -1). Thus, you have all the dimensions
of space and one time-like dimension that makes up space-time. How
does it curve?
this illustration, space-time is like a rubber sheet on which
a massive ball is placed. The mass of the ball "warps"
This is a profile of the same illustration.
On the right
you can see an illustration of this concept. Imagine space-time
is the grid, and the blue sphere is something that is massive, like
a star. The mass of the star causes space-time to curve. The greater
the curve, the greater the attractive force of gravity. Notice that
space-time is more curved closer to the object.
In the lower
illustration, imagine that you are walking on the top straight line
of the grid. You can measure the strength of the force pulling you
closer to the ball by measuring the distance from the top straight
line you are standing on to the bottom curved line. As you walk
toward the ball, the distance between the line and the curve increases,
thus the strength of the force increases.
course, is not a perfect illustration, as it is a 3 dimensional
representation of a 4 dimensional concept.
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