50 DOCS.
72,
73
JANUARY
1908
all bodies
experience
the
same
acceleration
in
a
gravitational
field
very strongly
invites
the
assumption
that
an
accelerated coordinate
system
and
an
acceleration-free
coordinate
system
with
a
homogeneous gravitational
field
are
to
be viewed
as
totally
equivalent
things.[6]
On the basis
of
this
assumption
one
arrives
at quite
plausible
consequences.[7]
As
soon as
I receive
the
reprints
of
my
paper dealing
with this
subject,
I
will send
you a copy.
Yours
very respectfully,
A.
Einstein
73.
To
Arnold
Sommerfeld
Bern, 14 January 1908
Highly
esteemed Professor
Sommerfeld:
Your letter made
me uncommonly happy; never
before
has
a physicist
been
so
frank
with
me
and
at
the
same
time
so
kind.[1] I
therefore
cannot help
but
start this
letter
with
a
remark of
a
personal
nature.
Thanks
to
my having
hit
upon
the fortunate idea
of
introducing
the
relativity
principle
into
physics, you
(and others)
enormously
overesti-
mate
my
scientific
abilities,
to the
point
where
this makes
me
somewhat uncomfortable.
I
do not
want to
ply you
with
self-criticism;
self-criticism is
rarely
good
for
anything,
and
is
worthless
to
others
too. But
let
me assure
you
that
if
I
were
in Munich and
had the
time,
I
would sit in
on
your
lectures
in
order
to perfect
my knowledge
of mathematical
physics.-
So,
first to
the
question
of whether
I
consider
the relativistic treatment
of,
e.g.,
the
mechanics
of electrons
as
definitive.
No,
certainly
not.
It
seems
to
me
too
that
a
physical
theory
can
be
satisfactory only
when
it builds
up
its structures from
elementary
foundations. The
theory
of
relativity
is not
more
conclusively
and
absolutely
satisfactory
than,
for
example,
classical
thermodynamics
was
before Boltzmann had
interpreted
entropy
as
probability.[2]
If
the
Michelson-Morley
experiment
had
not put
us
in
the
worst predicament,
no one
would have
perceived
the
relativity
theory
as a
(half)
salvation.
Besides,
I
believe
that
we are
still
far
from
having satisfactory
elementary
foundations
for
electrical
and mechanical
processes.
I
have
come
to
this
pessimistic
view
mainly
as a
result
of
endless,
vain efforts
to
interpret
the second
universal
constant
in
Planck's radiation
law in
an
intuitive
way.[3]
I
even
seriously
doubt that
it will be
possible
to
maintain the
general
validity
of
Maxwell's
equations
for
empty space.
I
am
very
interested
in
your investigation on
the
propagation
of
signals.[4]
But
since
I
didn't
know
that
you
will
publish
this
investigation,
I
did not
consider
it
proper
to ask
you
for
further
reports
about
it when
I
was
writing my
last
letter,[5]
because it could have
cost
you
time
to
satisfy
this
request.