DOCS.
467,
468
AUGUST
1913
351
Because of
their
smallness,
none
of
these effects
is
amenable
to verification,
but
they
are
plausible
in
and of
themselves,
as
Mach has
so nicely
shown in his
critique
of
Newton's
Principia
in his
Mechanik.[17]
With cordial
greetings
to
you
and
your
family,
also from
my
wife, your
very
devoted
A.
Einstein
468. To
Erwin Freundlich
[Zurich, mid-August
1913][1]
Highly
esteemed
Colleague,
Thank
you
so
much for
your
interesting
letter. It
is
thanks
to
your
zeal that the
astronomers have
now
also
started
to
show
interest
in the
important
question
about the
bending
of
light
rays.[2]
That
the idea of
a
bending
of
light rays
was
bound
to
emerge
at
the time
of the
emission
theory is
quite
natural,
as
is
the
fact
that the numerical result
is
exactly
the
same as
that
according
to
the
equivalence
hypothesis.[3]
You
must have
received
my
new
paper
in
the
meantime. It
shows
that
an
extended
relativity
theory
is
quite compatible
with
the
equivalence
hypothesis.[4]
My
view
about the other
current
theories of
gravitation is
as
follows.
Abraham's
theory,
according to
which
light
is
deflected
as
it
is
also in
my theory,
is
inconsistent
from
the
standpoint
of the
theory
of
invariants.[5]
The other
relativity
theories of
gravitation
are
those of
Mie[6]
and
Nordström.[7]
The
first
is
fanciful
and
possesses,
in
my
opinion,
a vanishingly
small
intrinsic
probability.
But the
latter
one
is
very
reasonable and
points
to
a
contradiction-free
way
in which to succeed
without the
equivalence hypothesis.
According
to
Nordström there
exists
a
red
shift of
spectral lines,
as
in
my theory,
but
there
is
no bending
of
light rays
in
the
gravitational
field.
The
investigations
during
the
next
solar
eclipse[8]
will show which
of
the two
conceptions corresponds
to
the
facts.
Nothing
can
be
done here
by
theoretical methods.
Next
year
you
astronomers
can
provide
a
simply
invaluable
service to
theor.
physics
in this matter. We will
get
reliable information
that
will
tell
us
whether
it is
proper to generalize
the
relativity
theory
further
or
whether
we
must stop
after the
first
step.
With best
wishes,
yours,
A.
Einstein
P.S.
Our
astronomer[9]
showed
me a
short
paper
in
an
American
journal
in which
it
is proposed
that
one
should
rigidly
link
several
optical systems
to
one
another instead
of
simultaneously photographing
the
two
edges
of the
sun
with
one
instrument.
This
seems
reasonable
to
my layman's
mind.