I. INTRODUCTION

In order to appreciate
the fundamental importance
of Relativity,
it is necessary to know
how it arose.

Whenever a "revolution" takes place,
in any domain,
it is always preceded by
some maladjustment producing a tension,
which ultimately causes a break,
followed by a greater stability --
at least for the time being.

What was the maladjustment in Physics
in the latter part of the 19th century,
which led to the creation of
the "revolutionary" Relativity Theory?

Let us summarize it briefly:

It has been assumed that
all space is filled with ether,
through which radio waves and light waves
are transmitted --
any modern child talks quite glibly
about "wave-lengths"
in connection with the radio.

Now, if there is an ether,
does it surround the earth
and travel with it,
or does it remain stationary
while the earth travels through it?

Various known facts indicate that
the ether does NOT travel with the earth.
If, then, the earth is moving THROUGH the ether,
there must be an "ether wind,"
just as a person riding on a bicycle
through still air,
feels an air wind blowing in his face.

And so an experiment was performed
by Michelson and Morley
in 1887,
to detect this ether wind;
and much to the surprise of everyone,
no ether wind was observed.

This unexpected result was explained by
a Dutch physicist, Lorentz, in 1895,
in a way which will be described
in Chapter II.
The search for the ether wind
was then resumed
by means of other experiments.

But, again and again,
to the consternation of physicists,
no ether wind could be detected,
until it seemed that
nature was in a "conspiracy"
to prevent our finding this effect!

At this point
Einstein took up this problem,
and decided that
a natural "conspiracy"
must be a natural LAW operating.
And to answer this question
as to what is this law,
he proposed his Theory of Relativity,
published in two papers,
one in 1905 and the other in 1915.

He first found it necessary to
re-examine the fundamental ideas
upon which classical physics was based,
and proposed certain vital changes in them.
He then made
A VERY LIMITED NUMBER OF
MOST REASONABLE ASSUMPTIONS
from which he deduced his theory.
So fruitful did his analysis prove to be
that by means of it he succeeded in:

(1) Cleaning up the fundamental ideas.
(2) Explaining the Michelson-Morley experiment
in a much more rational way than
had previously done.
(3) Doing away with
other outstanding difficulties
in physics.

(4) Deriving a
NEW LAW OF GRAVITATION
much more adequate than the
Newtonian one
(See Part II.: The General Theory)
and which led to several
important predictions
which could be verified by experiment;
and which have been so verified
since then.
(5) Explaining
QUITE COINCIDENTALLY
a famous discrepancy in astronomy
which had worried the astronomers
for many years
(This is also discussed in
The General Theory).

Thus; the Theory of Relativity had
a profound philosophical bearing
on ALL of physics,
as well as explaining
many SPECIFIC outstanding difficulties
that had seemed to be entirely
UNRELATED,
and of further increasing our knowledge
of the physical world
by suggesting a number of
NEW experiments which have led to
NEW discoveries.

No other physical theory
has proven so powerful
though based on so FEW assumptions.

As we shall see.