DOC.
43
JANUARY
1915 53
Since it
seems
that
general
covariance in the
physics equations
is out
of
the
question
for
arbitrary
transformations,
you
will
also
naturally
admit the
existence
of
preferred
coordinate
systems.
Your
adapted
systems
are
just
such
ones,
and
the
point
of
departure
for
your theory
of
gravitation
was
precisely
that the
form
of
the
equations
changes
in
a specific way
upon
substitution
of
an
accelerated
system
against
the
original
one.
I
do not have to
say
any
more
about
that,
and
I
just
want to
add
the
following
comment.
Supposing
we
could conduct
our
experiments
in
a
portion of
space
that
is
very
far
away
from all celestial
bodies,
and
we
had learned
that with
a
suitable choice
of
coordinate
system a mass-point approaching
others not too
closely moves along
a
straight
course
at
a
constant
velocity;
we
also find
that
Maxwell’s
equations
are
valid in
this coordinate
system.
The older
theory
of
relativity
would
then
show
us
that
a
whole
group
of coordinate
systems
exists for which all this
equally applies,
and
we
must hence
acknowledge
all of
them
as
on a par
with
one
another.
Having
arrived
thus
far
we
would
certainly prefer
these coordinate
systems
over
all
others,
since
we
would not
want
to
spoil
the
simpler description
through
the introduction
of
a
coordinate
system
that
does not
belong
with the others mentioned. If
we
did
so,
“gravitation
terms” would
then
appear
in
the
equations
and
we
would
not
be
too pleased
with
them,
because
we
would not
be
able to
indicate
any
bodies whose
“influence”
or
“effect”
could be
attributed
to such terms.
The
case
would be similar to
just
now,
when
the
meaning
of
the
terms
2wdy/dt,w2x2, etc.,
in
equations
(2)
was
the
subject
of discussion.
Now
that
I
am
bothering
you
with such
a
long
letter,
you
will
surely
allow
me
to continue
on
for
a
while. You
say, p.
1031,
“We
are
looking
in vain for
a
sufficient
reason
for
why
one
of
these
systems ought
to
be
more
suitable than
another
to
serve as a
reference
system
in
the
formulation of
the natural
laws;
on
the
contrary,
we
feel
compelled
to postulate
the
equality
of status
of
both
systems.”
Are
you
not
going
a
bit
too
far here
by presenting
a
personal
view
as
self-evident?
As
a
matter
of
fact, prior physicists
have
thought
it
possible
to
find
the
“sufficient reason”
you speak
of
in
that both
systems
move
in
a
different
way
in reference
to
the
ether.
You
are
correct in
your
observation
only
because
you
are
not at
all
interested
in the ether. This
conception may
ultimately
be
preferable
to
the
former,
but it
is,
of
course,
not
the
only possible
one.
In
your
article in
Kultur der
Gegenwart
I find in the discussion
of
the “contrac-
tion
hypothesis” (Michelson’s experiment)
the
remark: “This
manner
of
thinking
up
ad
hoc
hypotheses
to
cope
with
experiments yielding negative
results
is
very
unsatisfactory.”[9]
Poincaré
has also said
this,[10]
and
I
myself
also
agreed
with
that; I felt
the
need for
a more
general
theory,
as
I
later
attempted to
develop[11]