EINSTEIN

ON THE THEORY OF RELATIVITY

I

Einstein

was

the first

physicist

to formulate

clearly

the

new

kinematical

foundation

for

all

of

physics

inherent in

Lorentz's

electron

theory.

This kinematics

emerged

in 1905 from

his critical examination

of

the

physical significance

of

the

concepts

of

spatial

and

temporal

intervals. The examination, based

on a

careful definition

of

the

simultaneity

of

distant

events,

showed that the

concept

of

a

universal

or

absolute time,

on

which

Newtonian

kinematics

is based,

has to be

abandoned;

and that the Galilean

transformations between

the coordinates

of

two inertial frames

of

reference has to be

replaced by a

set

of

spatial

and

temporal

transformations that

agree formally

with

a

set that Lorentz had

introduced

earlier,

with

a

quite

different

interpretation. Through

its

interpretation

of

these transfor-

mations

as

elements

of

a space-time symmetry

group

corresponding

to the

new

kinematics,

the

special theory

of

relativity,

as

it later

came

to be called,

provided physicists

with

a

powerful guide

in the search for

new dynamical

theories

of

fields and

particles,

and

grad-

ually

led to

a deeper appreciation

of

the role

of

symmetry

criteria in

physics.

The

special

theory

of

relativity

also

provided philosophers

with abundant material for reflection

on

the

new

views

of

space

and time. The

special theory,

like Newtonian

mechanics,

still

assigns

a privileged

status

to the class

of

inertial frames

of

reference. The

attempt

to generalize

the

theory

to include

gravitation

led Einstein to formulate the

equivalence

principle

in

1907. This

was

the

first

step

in his search for

a new theory

of

gravitation

denying a privi-

leged

role to inertial

frames, a theory

that

is

now

known

as

the

general

theory

of

relativity.

Einstein

presented

the

special theory

in

Einstein

1905r

(Doc.

23),

a paper

which

is

a

landmark

in

the

development

of

modern

physics.

In the first

part

of

this

paper

Einstein

presented

the

new

kinematics,

basing

it

on

two

postulates,

the

relativity

principle

and the

principle

of

the

constancy

of

the

velocity

of

light.

In the second

part,

he

applied

his kine-

matical results to the

solution

of

a

number

of

problems

in the

optics

and

electrodynamics

of

moving

bodies. This volume includes

a

number

of

other

papers on

relativity.

Three

of

these,

Einstein 1905s

(Doc. 24),

1906e

(Doc. 35),

and 1907h

(Doc.

45),

present

argu-

ments

for

one

of

the

most

important consequences

of

the

theory,

the

equivalence

of

mass

and

energy.

Two

papers,

Einstein

1906g

(Doc. 36)

and 1907e

(Doc.

41),

suggest new

experimental

tests

of

the

theory.

In his

reply

to

a

paper

by

Ehrenfest,

Einstein

1907g

(Doc.

44), Einstein clarified the kinematical nature

of

the

special theory.

Einstein

1907j (Doc.

47) is

the first

major

review

of

the foundations

of

the

theory, as

well

as

of

its

applications

to date

(corrections

appeared

in Einstein 1908b

[Doc. 49]).

The review also

contains

dis-

cussions

of

several

topics

Einstein had not

previously

treated.

Particularly

notable

are

Einstein's

comments

on

the

equivalence

principle

and its

relationship

to the

problem

of

gravitation.

A

brief

discussion

comment,

Einstein

et

al.

1909b

(Doc. 59),

concerns an

objection

to the

theory.

Two

papers on

the relativistic

electrodynamics

of

moving

media

were

written in collaboration with Jakob

Laub,

Einstein

and

Laub 1908a

(Doc.

51),

1908b

(Doc. 52).

These

papers

and corrections

in

Einstein

and

Laub

1908c,

1909

(Docs.

53 and