310
DOCS.
409,
410 JUNE
1912
for
justification
of
the
now
modified
assumption
of
the
constancy
of
c,[5]
none
can
be
advanced. An
explanation is
out
of the
question
here.
All
one can
bring up
in
support
of the
hypothesis
of the
independence
of
the
velocity
of
light
from
the
state
of motion
of
the
light source
is
its
simplicity
and
practicability.
Once
one gives
up
this
hypothesis,
then
even
the
explanation
of
shadow
formation
requires
that
one
introduce the
ugly
assumption
that the
light
emitted
by a
resonator
depends
on
the
kind
of
excitation
(excitation by "moving"
radiation
or
excitation of
another
kind).
Still,
I
think
it
is
necessary
to
look for the
above
second-order
effect,
which
should
be
easier
to
demonstrate than that of
Michelson,[6]
because
moving light sources
of several
1000 km
are
available. I
am
very surprised
that
the
relativity
theory experts
misunderstood that
most
important assumption.
I
still do
not
understand
the
theorem
on
gravitational
fields
that
you
are
telling me
about.[7]
You
must
explain
it
to
me more
clearly.
To
make this
easier for
you,
I
am
sending you my
papers
on
gravitation,
the latest
of which
you
do
not have.[8]
According
to
it,
it
appears
that the
equivalence principle
can
be
valid
only for
infinitely
small
fields,[9]
and
that,
therefore,
Born's accelerated
finite
system
cannot
be
considered
a
static
gravitational field, i.e., cannot
be
generated
by
masses
at
rest.[10]
A
rotating ring
does
not
generate
a
static
field in this
sense,
even
though
it
is
a
temporally
invariant
field.
The
reversibility
of
light
paths
will not hold in such
a
field.
My
case
corresponds
to
the
electrostatic
field in
the
theory
of
electricity,
whereas the
more
general
static
case
would
also include
the
analogue
of
the
static
magnetic
field.
I
haven't
got as
far
as
that
yet.
The
equations
I
found
apply only
to
the static
case
of
masses
at rest.
Born's
field
of
finite
extension
does
not
fall
into
this
category.
It
has not
yet
become clear to
me why
the
equivalence principle
fails
for
finite
fields
(Born).
Please
tell
me
again exactly
what it
is
that
you
have
proved.
As I said
earlier, I
don't
even
understand the
statement.
With
kindest
regards,
also from
my
wife and little
Albert,
and
also to
your
wife and
your
little
ones,
I
remain
your nosy
A.
Einstein
410. To
Anton
Lampa?
Prague,
29
June
1912
Einstein
turns
over
his duties in the Bohemian
Physical Society as
he
prepares to
take his vacation.
I have
not
yet
answered the letter
from
the
chemical
society. Also,
I
am
not
sure
whether notice
has
already
been
taken of the other
two
communications enclosed.