274 THEORY OF RELATIVITY
point.[140]
It follows from this
equality
that there
is
no
observable difference between
the
behavior of
a
mechanical
system
in
a gravitation-free uniformly accelerating
reference
frame
and
the
behavior of
the
same system
in
an
inertial frame in which there
is a
uniform
gravitational
field.
Generalizing
this mechanical
equivalence,
Einstein
postulated
"the
complete
physical
equivalence
of
a gravitational
field and
a
corresponding
acceleration
of
the reference
system"
("die
völlige
physikalische Gleichwertigkeit
von
Gravitationsfeld
und
entsprechender
Beschleunigung
des
Bezugssystems"),
which allows
one
"to
replace
a homogeneous gravitational
field
by
a uniformly
accelerated reference
system"
("ein
homogenes
Gravitationsfeld durch ein
gleichförmig beschleunigtes
Bezugssystem
zu
ersetzen").[141]
This
equivalence
enabled him to draw conclusions about the effects
of
a
uniform
gravitational
field
on physical processes
from the
analysis
of
such
processes
in
a
uniformly
accelerated reference
system.
He thus deduced that
spectral
lines from the
sun
show
a
shift toward
longer wave lengths
when
compared
to the
corresponding
terrestrial
lines
(an
effect
now
called the
gravitational
red
shift)
and that
a light ray
passing through
a gravitational
field
is
deflected from
a straight-line path;
but he noted
that
the
effect
is
too
small to be observed in the
earth's
gravitational
field.
Although
his
correspondence during
the
intervening years
contains several references to the
problem
of
gravitation,
Einstein
did not
publish
again on
this
topic
until
1911
(see
Volume
4,
the editorial note,
"Einstein
on
Gravitation
and
Relativity").[142]
[140]
In
Einstein
1934
(first
published
in
En-
glish
in
Einstein
1933),
Einstein stated
that he
was
convinced
of
this
equality even though
not
yet aware
of
the
experiments
of
Eötvös
(see
Eöt-
vös
1890).
[141]
Einstein
1907j
(Doc.
47), p.
454.
[142]
See
Einstein
1911g.