SUMMARIES

OF ETH

COURSES

627

of Lorentz's

theory.

Einstein

proceeded

by

giving a

detailed

explanation

of Fizeau's

experiment

on

the basis of Lorentz's

theory. According

to Sidler's

notes

Einstein

stated

that the

assumption

of

a

constant

speed

of

light

is the

core

of Lorentz's

theory.

After

a

brief

treatment

of

the

principle

of

relativity

in

classical

mechanics,

Einstein

noted that

according

to

Lorentz's

theory

it

apparently

does

not

apply

to

electrodynam-

ics. He next

discussed Michelson's

experiment

and

mentioned

its

explanation

by

Lorentz and

FitzGerald,

before

giving

a

detailed

exposition

on

the

significance

of

length

and

time

measurements. In the

course

of

this

exposition

he also

alluded

to

a

remark

by

Laue

on

the

reversibility

of the clock

synchronization procedure.

The

lengthy

discussion

on

the

significance

of coordinate transformations

and the

notion of

simultaneity

is

concluded

by

posing

the

problem

of

finding

transformation

equations

that would make the

principle

of

relativity compatible

with the

principle

of

the

con-

stancy

of the

speed

of

light.

This

problem

is

solved

by

deriving

the Lorentz transformations with

the

help

of

Minkowski's four-dimensional formalism.

In

the

course

of the

ensuing

discussion of

the relativistic transformation

equations,

Einstein mentioned

a

paradox equivalent

to

the twin

paradox,

but

referring

only

to

clocks

and not to

space

travelers.

He also

briefly

alluded

to

the

experimental

confirmation of

relativity. Coming

back

to

the

Fizeau

experiment,

Einstein

stated

that

it

represents

the

most

important support

for

the

theory

of

relativity.

After

a

brief

discussion

of

aberration and

the

addition of

velocities,

Einstein

focused

on

the transformation of the Maxwell-Lorentz

equations.

He discussed

the

case

of relative motion

between

a

conductor

and

a

magnet,

and

emphasized

that the

duality

of

explanations which, according

to

the

Maxwell-Lorentz

theory, depend

on

the

state

of motion of

the

observer

was one

of the

key

arguments

that motivated him

to

pursue

this

case

when he

was

working

on

the

theory

of

relativity.

The

subsequent lengthy

section of Sidler's

notes

is

devoted

to the

revision of

con-

cepts

from classical mechanics

required

by

the

theory

of

relativity.

Central

to

this

section

is

a

discussion of the

concept

of

energy

and its

relationship

to

mass.

Before

Einstein started the

systematic

treatment

of four-dimensional

electrodynamics,

he

gave

a

detailed introduction

to tensor

calculus. The discussion of covariant

electrodynamics

begins

with

the construction of

the

four-vector of

the

electrical

current

and then leads

to

the covariant formulation of Maxwell's

equations

for

empty

space.

The

next

topic

is

energy-momentum

conservation,

which

is

characterized

as one

of the

most

impor-

tant

aspects

of

relativity theory.

He

discussed the inertia of

energy

in

detail

and

showed

how

an

extended

physical system

can

be

treated

as

being equivalent

to

a mass

point

(see

the related

argument

in Einstein 1914c

[Doc. 24]).

Einstein revealed himself

as

uncertain whether

or

not

the

stress-

energy

tensor

is always symmetric.

He next

dis-

cussed the

electrodynamics

of

ponderable

matter.

The

two

concluding subjects

of

Einstein's

course are

relativistic

hydrodynamics

and

gravitation.

Einstein's discussion of

gravitation,

as

reconstructed

from

Dällenbach's

notes,

closely

follows

his

contemporary publications (see

the editorial

notes, "Einstein

on

Gravitation and

Relativity:

The Static

Field,"

pp.

122-128, and

"Einstein

on

Gravi–