340

DOC.

17

THE THEORY OF RELATIVITY

Doc.

17

The

Theory

of

Relativity1

by

A.

Einstein

[Naturforschende

Gesellschaft

in Zürich.

Vierteljahrsschrift

56

(1911): 1-14]

[1]

The

one

basic

pillar upon

which

the

theory designated as

the

"theory

of

relativity"

rests

is

the so-called

principle

of

relativity.

First

I will

try

to make

clear

what

is

understood

by

the

principle

of

relativity.

Picture

to

yourself

two

physicists.

Let both

physicists

be

equipped

with

every physical

instrument

imaginable;

let

each

of them

have

a laboratory.

Suppose

that the

laboratory

of

one

of the

physicists

is arranged

somewhere

in

an

open

field,

and that of the second

in

a

railroad

car traveling

at constant

velocity

in

a

given

direction. The

principle

of

relativity

states

the

following:

if,

using

all

their

equipment,

these

two

physicists were

to

study

all

the

laws

of

nature,

one

in his

stationary laboratory

and the other

in his

laboratory

on

the

train, they

would discover

exactly

the

same

laws

of

nature,

provided

that the train

is not

shaking

and

is

traveling

in

uniform motion.

Somewhat

more abstractly, we can

say:

according

to

the

principle

of

relativity,

the

laws

of

nature

are

independent

of the translational motion of the reference

system.

Let

us

consider the role that

this

principle

of

relativity plays

in classical mechanics.

Classical mechanics

is

based

first

and foremost

on

Galileo's

principle, according

to which

a body

not

subjected

to

the

influences

of other bodies

finds

itself

in

uniform,

rectilinear

motion. If

this

principle

holds for

one

of

the

laboratories mentioned

above,

then

it holds

for the other

one as

well. This

we can

deduce

directly

from

intuition;

however, we can

also

deduce

this

from the

equations

of Newtonian

mechanics if

we

transform these

equations

to

a

reference

system

that

moves uniformly

relative to

the

original

reference

system.

All I have

been

talking

about

is

laboratories.

However,

in

mathematical

physics,

it

is

customary

to

relate

things

to

coordinate

systems

and

not to

a

specific

laboratory.

What

is

essential

in this

relating-to-something is

the

following:

when

we

state

anything

whatsoever about the

location

of

a

point,

we always

indicate

the

coincidence

of

this

point

with

some point

of

a specific

other

physical system.

If,

for

example,

I

choose

myself as

this

material

point,

and

say,

"I

am

at this

location

in this

hall,"

then

I have

brought

myself

into

spatial

coincidence with

a

certain

point

of

this

hall,

or

rather,

I have

asserted

this coincidence. This

is

done

in

mathematical

physics by using

three

numbers,

the

so–

called

coordinates,

to indicate with which

points

of

the

rigid system,

called

the coordinate

system,

the

point

whose

location

is

to

be described

coincides.

1

Lecture

given

at

the

meeting

of the Zurich

Naturforschende Gesellschaft on

16 January

1911.