The Problem of Relativity
When Copernicus proposed that the Earth went around the
Sun, it was a great challenge to physicists because they had to
deal with the problem of
What does it mean to say something is standing still when it
is actually on the surface of a planet moving at great speed
Is there any such thing as ABSOLUTE REST? Can we ever
say we are REALLY not moving, or can we only say, we’re
not moving RELATIVE to the Earth, though we are moving
RELATIVE to the Moon, the Sun and the stars.
Beginning with Galileo and ending with Newton it was realized that
certain kinds of motion, especially INERTIAL MOTION, meaning
constant speed motion in a straight line, are equivalent to being at
rest. If you are moving with constant velocity there is no experiment
you which will tell you that you are REALLY moving.
Of course you can always look at the window.
Also, if you go around a corner then you can feel a mysterious
centrifugal force pushing on you.
For this reason Newton insisted that there was an ABSOLUTE
SPACE which defined a state of being ABSOLUTELY AT REST.
Therefore as we travel around the Sun, we think we are rest, but we
are REALLY moving, with respect to invisible space.
Measuring the Speed of Light.
Early measurements of the speed of light were hampered by the
lack of a sufficiently precise clock. The use of the telescope in
astronomy permitted the first reasonable estimate of the speed, by
timing the motions of the Moons of Jupiter at different times of
the year. When the Earth is farther from the Sun it take light
longer to reach us from Jupiter, so the Moons appear to be “late”
in their motions. It is not that they are any later, but that the
messenger is delayed as a result of the fact that we, the observers,
In the 19
Century much more accurate measurements of the
speed of light became possible when people realized they could
take advantage of the wave nature of light. Light is itself a kind
of clock, because it has a very regular period or frequency. One
can use interference to compare the time it takes two different
light rays to travel two different paths. If you measure the path
lengths very carefully then the light itself provides the clock by
which to measure the time take, which gives you the speed.
This is the basis of the INTERFEROMETER, invented by the
great American physicist Albert Michelson.
Absolute Rest and the Luminiferous Ether
By the end of the 19
Century physicists though they had a way
of proving that the Earth was REALLY in motion, with respect
to Newton’s Absolute Space.
If light is a wave it must have a medium. 19
called it the Luminiferous (“light
bearing”) Ether, because the
Ether was the “fifth element” of the ancient Greeks which made
If the Earth is moving through the Ether, then there ought to be
an “Ether Wind” which would cause light to move more slowly
when it is directed “into the wind.” It ought to be possible to
measure a small difference in the speed of light when it is
moving in different directions (about the size of v/c, where v is
the speed of the Earth’s motion through the ether).
Relative Motion and Light
Suppose Light was made up of particles, as Newton thought.
If you shine a laser light from your spaceship towards
another spaceship, and if your spaceship is moving
towards the other spaceship at speed v, what speed does
the pilot of the other spaceship think the light is moving
at? What speed do you (in your spaceship) think it is
YOU: c OTHER PILOT: c
v OTHER PILOT: c + v
YOU: c OTHER PILOT: c + v
v OTHER PILOT: c
Correct Answer: C
So in this case you think the light is
moving at the same speed it always does. You can’t do an
experiment on your own laser light to discover that you are
REALLY moving. Only the guy on the other spaceship can
do that, but he only discovers how fast you are moving
RELATIVE to him.
What if Light is Wave?
Imagine you are on ship and you drop a rock into the water.
The ship is moving through the water with velocity v.
The water is moving in a current with velocity w (relative
to the land). The waves move through the water with
velocity c. What speed do you see the wave moving at?
What speed does a landlubber on the shore see the wave
YOU: c LANDLUBBER: c
v LANDLUBBER: c
YOU: c + w LANDLUBBER: c + v
YOU: c LANDLUBBER: c + w
v LANDLUBBER: c + w
Notice that your answer depends on the velocity at which you
are moving through the water. If you measure the speed of the
waves, then you will know how fast you are moving through the
water, even if you are out of sight of land. The landlubber also
can measure how fast he is “moving” with respect to the water,
since he measures the speed to be c + w.
So if light is a wave, and the luminiferous ether is the sea, then
we ought to be able to measure how fast the Earth is moving
through this sea just by measuring the Ether Wind. Then we
could assume that the Ether is Absolutely at Rest and we could
prove that the Earth (and maybe the whole solar system) is
REALLY in motion.
No Such Luck
Unfortunately despite the best efforts of Michelson and others no
one was able to measure any difference in the speed of light in any
direction on Earth.
So is the Earth really Absolutely at Rest, as the ancients believed?
Modern Physicists didn’t believe this could be true.
So maybe light really doesn’t behave like a wave in this respect?
Maybe if light is emitted like a particle then it has a speed equal to
its usual speed, c, plus the speed of the object which emitted it.
Einstein proposed a different idea, which at first sight seems to
make no sense. EVERYONE who measures the speed of light, no
matter how they are moving, gets the same answer, c.
Einstein proposed two axioms which lay at the heart of his
Special Theory of Relativity
You cannot do any experiment to distinguish between
INERTIAL MOTION and REST
Every observer who measures the velocity of light finds it
to be c
So if we go back to the previous problem, in which you
shone a laser light at an oncoming spaceship, what does
Einstein’s theory say you and the other pilot should
measure the speed of light as?
YOU: c OTHER PILOT: c
YOU: c + v OTHER PILOT: c + v
YOU: c OTHER PILOT: c + v
YOU: c + v OTHER PILOT: c
Correct Answer: A
So you both think that the laser light moves with speed c. How can
this be? Suppose the other spaceship flies by incredibly fast, at a
speed of 9/10 c. Shouldn’t it see the light moving at a speed of 1/10
Why can’t the two of us agree to use similar clocks and measuring
sticks to measure the speed of the light. Won’t we agree that it
covered a given distance in a given time and that since the other
spacecraft covered the same distance in slightly less time their
relative speeds are less than c?
Einstein realized that the answer to this paradox lies in the fact that
you and the other pilot actually won’t agree on how fast his clock his
running, or even how long his meter stick is!
Relativity of Simultaneity
Einstein asked himself, what do we mean by time? Which
of the following is true, as Einstein would have it?
Time is a thing which is measured by clocks (if time didn’t
exist, we wouldn’t have clocks)
Time is a thing which is produced by clocks (if clocks
didn’t exist, we wouldn’t have time)
To Einstein time is simply the order in which events
happen. A clock is simply a device which produces
events. When we say that “Dan started the lecture at
12:30pm” we mean that Dan started talking when the
little hand on the clock pointed towards 12 and the big
hand pointed towards 6. When we measure time what
we are actually talking about is taking note of a series
of “coincidences.” When we don’t have manmade
clocks we use naturally occurring events such as
sunrise and sunset.
So time is measured by observing Simultaneous
What Einstein realized is that observers who are in
motion relative to each other will disagree about which
events are actually Simultaneous.
Suppose I drop a glass, which breaks just as the clock,
which is 10m away, strikes 1 o’clock. Meanwhile, a pilot
is flying by in a spaceship moving at 1/10 c. What will
the pilot see?
The glass gets broken just before 1 o’clock
The glass gets broken at 1 o’clock
The glass gets broken just after 1 o’clock
If the pilot had been standing still, the light from the clock
would reach him first (the clock is closer to him), but he
would know enough to allow for the time required to reach
him, and could adjust to get the time the light left from the
clock and the glass. He would decide the two events
happened simultaneously. But since he is moving, he first
receives the light from the clock, and then moves a
measurable distance towards the glass before that light
reaches him. Since that light moves at c regardless of how
he moves, it takes a little less time to reach him then it
would have had he been standing still. The result that when
he takes his clock measurement he thinks the glass broke
just before 1 o’clock because the light arrived a little earlier
then he expected.
So in Einstein’s Relativity Theory IF nothing can travel
faster then light, two people can disagree about the order in
which events can happen. Why did no one ever notice this
Because the speed of light is so fast, we have always
assumed that we can compare two events which happen in
very different places and see which happened first. We can
do this, but ONLY if the time between the two events is
greater then the time it takes light to travel between them.
As long as this is true then everyone agrees on the order in
which the events happen. Since light takes tiny fractions of
a second to cross the Earth it was only when we got
extremely accurate clocks that we could begin to notice
knowing when things happen depends on getting
information about them from wherever they took place.
Wierdness of Time and Space
Einstein’s two simple axioms lead to the conclusions that
how you are moving affects how you measure distances and
times. If you try to measure a distance as you are moving
across it, it actually seems shorter then it did before you
started traveling! It will take less time to go there then you
thought. On the other hand someone left behind will still see
the distance as being the larger value, but they will be
convinced that your clocks are running slow. But everything
on your spaceship is a kind of clock, including your body.
You will actually age more slowly than the person you left
behind, as you go on your trip. But you will only notice the
difference if you travel at an appreciable fraction of the
speed of light.