DOC.
111
AUGUST
1915 123
and tried
all
sorts
of
possibilities, e.g., required:
The
system
must be chosen
such
that the
equations
rtr=0'*=1-4)
are
satisfied
throughout.[4]
At least
it
seemed definite to
me a
priori
that
a
transformation
group
exceed-
ing
the
Lorentz
group
must
exist,
because those observations summed
up
in
the
words
“relativity principle”
and
“equivalency principle”
point to
it.
The coordinate limitation
that
was finally
introduced deserves particular trust
because it establishes
a
link between it and the
postulate
of the event’s
complete
determination.
A
theoretical
differential
geometric
interpretation of
preferred
systems
would
be of
great
value. The weakest
point
of
the
theory
as
it
stands
today
consists
precisely
in
this,
that the
group
of
justified
transformations
are
by
no means
closely
assessable.[5]
There
is not
even
any
exact
proof
that
arbitrary
motions
can
be
transformed
to motionlessness. This is because
the
difficulties connected
with the
dissimilarity
of
elliptic
&
hyperbolic types
of differential
equations
stand
in
the
way
of
a general
observation. The
equation
3V
ç?V
dx2
dy2
0’
can
be solved
with
arbitrarily
given
boundary
values for
(p
p
given
The
equation
d2p
d2p
dx2
dy2
by
contrast, cannot.
Now,
how does
it
look for the
complicated
transformation
conditions
of
the
gen.
theory
of rel.? I
am
stuck there
like
a
bewildered
ox.
Maybe
we
could
gain
an
overview
of
the
question
if
the
geometric
interpretation
you
are
looking
for
is
found.
Cordial
greetings
and
best
wishes for
progress
in
your
efforts!
Yours,
A.
Einstein.
I
am
leaving
for Switzerland for
about
3 weeks
(26
Aug.
until
about
15 September.
Address
there
for
any
letters: Prof.
H.
Zangger, Berg
St.,
Zurich).