Special Relativity

The contributions that Albert Einstein made to theoretical physics were numerous,
and they deeply impacted how we think about time, space, matter and energy.  In the
following paragraphs I will try to give the reader a basic understanding of one of
Einstein’s most influential theories and some of the astounding consequences of it.

The name of the theory contains the word “special” to help distinguish it from a more
broad theory of relativity proposed by Galileo Galilei.  Galileo pointed out that it is
very difficult to measure the “true” speed of an object.  He showed that observed
speed depends on the speed of one’s frame of reference.  For example, a computer
might seem to be lying still compared to its table top, yet it is actually hurtling (along
with the planet that it is on) at a tremendous speed through space.  Similarly, a
passenger inside of a boat might look at a stationary mast outside of her window
and determine that it must be the mast that is moving, and not herself, because of the
way that the mast travels by her window and thus past her frame of reference.  
Galileo promulgated that it is necessary to have an external frame of reference to
determine relative speeds.  The concept of an objective, external frame of reference
necessitates that space and time are absolute, universal constants.  Einstein used a
theory that he had about light, to help update Galileo’s assumptions, and destroy the
conventional conceptions regarding absolute space and time.

Before Einstein, scientists knew that light traveled at a constant and finite speed-
186,000 miles per second (designated by the letter “c”).  What they didn’t know is
what medium it traveled through.   Because sound waves travel through air, and
ocean waves travel through bodies of water, physicists supposed that light traveled
through a hypothetical substance that they called “ether”.  In 1887 Albert Michelson
and Edward Morley performed an experiment that influenced Einstein to question the
existence of this imaginary substance.  The Michelson- Morley experiment attempted
to measure this “ether” rushing by the earth, just like you can sense air passing by
you when riding a bike.  Before they performed the experiment they assumed that the
tremendous velocity of the earth would add to the velocity of light if light was shot in
the same direction as the earth’s movement.  Conversely they assumed that light
shot in the opposite direction would be noticeably slower, because of a concept
called “additive velocities.”

For example, if you are carrying a loaded pistol with you in a race car and are
traveling at 200mph, then you, the bullets and the car all have forward velocity of
200mph.  As your intuition might suggest to you, if you shoot the gun ahead of you on
the race track, the bullet will have its normal velocity plus the 200 mph added to it
from the speed of the car (relative to an observer standing next to the track or
course).  But if you were to shoot the gun behind you, the stationary observer would
notice that the speed of the bullet was less than normal.  The bullet would be traveling
200mph less than the normal velocity of a bullet fired by a stationary observer.  The
same thing was expected of light, but it turns out that light is not affected by additive
velocities.  The Michelson Morley experiment detected the same exact speed of light
in both directions, and the scientific community did not know what to think.  

Einstein interpreted this finding in an innovative way.  He figured that the speed of
light must be absolute, and that nothing can travel faster than it.  If the speed of light
was the speed limit of the universe, then how could you explain velocities relative to
it?  Well, Einstein reasoned that time slows down for rapidly moving objects.  In fact,
the amount that time slows down for a moving object increases with its speed.  So,
the reason that the light that was shot off, in the same direction as the velocity of the
earth, was not additively faster is because time on the Earth was actually slower than
time would be for a stationary observer.  This is kind of non-intuitive, but consider a
real life experiment…

To test this theory, scientists synchronized two atomic clocks, they left one at an
airport, and placed another in a supersonic jet.  After the jet flies around at high
speeds, and lands, scientists then compare the two clocks.  The clock that was in the
jet is slightly behind the clock that was left on the ground.  Again, this is because time
slows down for objects traveling at high speeds.  It is interesting to note that
physicists have determined that because light travels at “c” light particles (photons)
do not experience time and therefore do not age.

Another example of this phenomenon is illustrated by Einstein’s “Twins Paradox.”  
This is the story of two twins (born, of course, on the same day).  One twin goes into
space and travels in his space ship at near light speeds.  He doesn’t notice that time
passes slower for him than it does for his brother at home- but it does.  He can’t
notice this difference though because when time slows down, everything slows down:
biological aging, atomic clocks, radioactive half-life, phenomenological
consciousness… So by the time the astronaut twin comes back to the earth, he finds
that much more time has passed on the earth, than passed in his spaceship.  When
he meets his twin, he might be surprised to see that his twin looks much older than
he does.  This concept of relative time was not ignored by science fiction writers or
by real life time machine builders.  

Building a vehicle that is sufficiently fast to serve as a time machine is very difficult
and as we will see in the next essay, it becomes progressively harder to increase
your speed- especially near the speed of light.  Also, it is theoretically impossible to
achieve the speed of light – a sad truth for time machine builders.  In reality though,
we are all time travelers. Every time we go for a jog, or step in a car, train, bus or
plane, our experience of time slows (ever so slightly) compared to a stationary

To learn more
click here for information about Einstein’s “energy-mass equivalency

Before Relativity

Before writing his article “On the Electrodynamics of Moving Bodies,” now known to
the world as special relativity, Einstein published three other important papers: 1)  An
article on the “photoelectric effect” helped to further define the quantum (the
fundamental unit of light) and its dual nature as both a wave and a particle, thus
adding to our knowledge about how light propagates, and how it interacts with matter
(an area first explored in 1900 by Max Plank).  2) He explored the physics of
“brownian motion,” how air molecules travel and interact with one another.  The
equations that he derived in this paper, for example, help scientists make more
accurate predictions about how smoke dissipates with time; they also explain the
seemingly random movements of very small amounts of pollen (he explained that
pollen is seen to vibrate under a microscope because of the way that it is
bombarded by air molecules).  3) He also wrote a very influential paper examining
how molecules bond and interact.  All of these papers, along with his theory of
special relativity, were completed in 1905- quite a productive year for Einstein.
These three findings were monumental, but the development of special relativity is
probably one of Einstein’s most widely recognized achievements.