INTRODUCTION TO VOLUME
4
xv
added
much
to
the
understanding
of
the
quantum hypothesis.
His
collaboration
with Otto
Stern
on
the
application
of
the
quantum theory
to
rotating
molecules,
on
the other
hand, at first
seemed
to provide
an
explanation
of Arnold
Eucken's
measurements
of
the
specific
heat of
hydrogen.
But
soon
afterwards the
expla-
nation lost
its
foundation when Einstein
lost confidence
in
the
most
important
feature of
the
paper, namely
the introduction
of
a
term
representing zero-point
energy.
III
Einstein's
first
attempt
at
generalizing
his
relativity theory
of
1905 to
include
accelerated frames of reference
dates
from
1907.
His
hypothesis,
formulated
in
a
review
paper
that
year,[3]
concerning
the
equality
of inertial and
gravita-
tional
mass
and
the
equivalence
of
a
homogeneous gravitational
field
with
a
uniformly
accelerated frame of reference became
a
cornerstone
for
his sub-
sequent
attempts
to
generalize special relativity.
Thus
the
separate problem
of
a
generalization
of
the
relativity theory
of
1905 to
accelerated frames
of
ref-
erence
became linked
to
the
development
of
a new theory
of
gravitation.
As
heuristically
fruitful
as
this
"principle
of
equivalence"
turned
out to be
for
predicting
such
effects
as
the
bending
of
light
rays
passing through a grav-
itational
field
and the
gravitational
redshift of
spectral lines,
Einstein
initially
hesitated to
build
a theory on
it. He
continued
to stress his
efforts
to
deal with
the
quantum
riddle.
It
was
only
when
his
hope
of
solving
this
riddle waned
that
he
began
to concentrate
on
the
gravitation problem.
Einstein also decided
to
make his results
public even
when
he
had
the
feeling
that
they were
not
yet
definitive.
Cautiously proceeding step
by step,
he
attacked
first the
case
of
a
static
gravitational
field.
At the end of March
1912 he
was
content
with
his
preliminary
results:
"Lately,
I
have worked
furiously
on
the
problem
of
grav-
itation.
I
am now finally
finished with the
statics. I
don't know
anything yet
about
the
dynamic field,
that
is
to
follow
next....
Every
step
is
devilishly
difficult,
and what
I
have derived
so
far
is
certainly
only
the
easiest
part."[4]
Although
difficulties with
the
principle
that action
equals
reaction forced
him
to
limit the
validity
of
the
principle
of
equivalence,
Einstein did
not give
it
up
when
he
decided
to
move on
to
the
dynamic case.
Another
guide
Einstein
[3]Einstein
1907j (Vol. 2,
Doc.
47).
[4]"In
der letzten Zeit arbeitete ich rasend
am
Gravitationsproblem.
Nun ist
es
so
weit,
dass
ich mit der Statik
fertig
bin.
Von dem
dynamischen
Feld weiss ich noch
gar
nichts,
das
soll
erst
jetzt
folgen.... Jeder Schritt ist verteufelt
schwierig,
und das bis
jetzt abgeleitete gewiss
noch das einfachste."
See
Einstein
to
Michele
Besso,
26 March
1912
(Vol. 5,
Doc.
377).