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–