DOC.
173
DECEMBER
1915 167
173. To Paul Ehrenfest
[Berlin,]
26
December
[1915][1]
Dear
Ehrenfest,
I just
received
your
postcard
with
the
kind invitation.
As
a
matter of
fact,
every
muscle
in
my body
is
itching
to set
off.
But
my
mother
is
visiting[2]
and
I
cannot
act
on
it.
I
am even
booked for
Easter,
as
then
I
am
going
to
my
Albert.
So
since
I
am
already
tied
up
and cannot drive
out
to
meet
you,
at least
I
shall
prove
in
writing
my
desire to visit
you
with unusual talkativeness. Einstein has it
easy.
Every year
he retracts what
he wrote
in the
preceding year;
now
the
sorry
business falls
to
me
of
justifying my
latest retraction.
In
§12
of
my
paper
of last
year, everything
is correct
(in
the
first
3
paragraphs)
except
for
what
is printed
at
the
end of
the third
paragraph
in
spaced
type.[3]
A
contradiction
to
the
uniqueness
of
the
event does not
follow
at all from
the
fact
that both
systems
G(x)
and
G'(x),
related to
the
same
frame of
reference, satisfy
the
conditions of
the
grav.
field.
The
seemingly compelling
part
of this reflection
founders
immediately
when
you
consider
that
1)
the
reference
system
has
no
real
meaning
2)
that the
(simultaneous)
materialization of
two
different
g
systems
(more
aptly
put, two
different
grav.
fields)
within the
same area
of
the continuum
is,
according
to
the
nature of
the
theory, impossible.
In
place
of
§12
the
following
consideration must
appear.
Whatever is
phys-
ically
real
in
events
in
the
universe
(as opposed
to
that
which
is
dependent
on
the
choice of
a
reference
system)
consist in
spatio-temporal
coincidences
and
in
nothing
else![4]
For
ex.,
the
intersection
points of
two world
lines
are
real,
or
the
statement
that
they
do not intersect each other.
Therefore,
those statements
relating
to
the
physically
real do not
lose
validity
from
the
absence of
a
(unique)
coordinate transformation. When
two
systems
in
the
guv’s
(or gen.,
the
variables
used
to
describe
the
world) are
constituted
in such
a
way
that the
second
can
be
obtained
from
the
first
by
mere
space-time
transformation,
then
they are
entirely
equivalent.
This is because
they
have in
common
all
the
spatio-temporal point
coincidences,
that
is,
all
the
observables.
This consideration shows
simultaneously
how
natural the
requirement
of
gen-
eral covariance is.
Now
to
the
second
point.
The main
equations
Edta/dx