INTRODUCTION TO VOLUME
3
xxix
inertial
frame.
He
postulated
that
a
frame of
reference
free
of
gravitational
fields
but
uniformly
accelerated with
respect
to
an
inertial
frame
is
completely
equivalent
to
an
inertial frame with
a
uniform
gravitational
field.
This
princi-
ple
of
equivalence
was
suggested by
the fact
that
all
bodies
are
accelerated
equally
in such
a
uniform
field;
inertial and
gravitational
masses are
always
proportional to
one
another.[69]
In
that
same
article
Einstein
was
able
to
draw
two
important
conclusions from the
equivalence
principle.
The first concerned
the
effect
of the
gravitational
field
on
time:
any physical process proceeds
faster the
greater
the
gravitational potential
at
the
position
where the
process
takes
place.
Consequently
a
spectral
line emitted
at
the
surface
of
the
sun
will
have
a
wavelength slightly longer
than the
corresponding spectral
line
pro-
duced
on
the
earth,
an
effect
known
as
the
gravitational
redshift. Einstein's
second conclusion
was
that
a
light ray proceeding
in
a
direction other than
that
of the
gravitational
field will
be bent
through
an
angle proportional
to
the
strength
of
the field.
He
pointed
out, however,
that
the
effect
of
the
earth's
gravity
is too
small for
one even
to
think
about
observing
this
effect.
When he returned
to
these
issues
in
1911,
Einstein
proceeded
to revise
and
improve
his
earlier
presentation, making
the
principle
of
equivalence
the
cen-
tral feature of
his
treatment.
Einstein
now
included
an
elegant proof,
based
on
a cyclic
process
reminiscent of
thermodynamics,
that
the
gravitational
mass
of
a
body,
as
well
as
its
inertial
mass,
is increased
by
the
amount
(E/c2)
when
the
body
absorbs
energy
E.
His
1911
paper was
specifically
prompted by
his
new
realization
that
it
should
be
possible
to
observe the
gravitational bending
of
light
after
all,
if
one
relied
on
the sun's
field
rather
than the earth's. One had
to
observe
a
star
whose
light
would travel
close
by
the
sun on
its
way
to
the
observer. This could
be
done
during
a
total
eclipse
of the
sun,
when such
a
star
would
appear
to
be
displaced
from its normal
position by
an
angle
that
Einstein
calculated to
be
0.87
seconds of
arc.[70]
Einstein
took
the initiative
in
consulting experimental colleagues
about
the
possibilities
for
checking
these results. In
August 1911
he
began
corre-
sponding
with
W. H.
Julius of Utrecht
about
the
redshift,
among
other
matters.[71]
At
about
the
same
time
he raised with Erwin Freundlich
at
Berlin
the
question
of
observing
the
deflecting
of
starlight by
the
gravitational field
[69]For
further discussion of
the
principle
of
equivalence, see
Norton
1989.
[70]This is
just
half
the
value Einstein calculated
a
few
years
later from his
new general theory
of
relativity.
See
the
notes to
Einstein
1911h
(Doc.
23).
[71]Einstein to W. H. Julius, 24
August
1911.
Julius tried
hard,
but
unsuccessfully,
to
per-
suade Einstein
to
accept an
offer
of
a
professorship
at
Utrecht.
See
their
correspondence
in
Vol.
5.
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