INTRODUCTION TO
VOLUME
2
xxix
on
the basis
of
these
theories
by
means
of
both
principle
and
constructive theories.
At
first,
he seemed
to
suggest
that
some
version
of
a
generalized
mechanical
sys-
tem,
involving only a
finite number
of
degrees
of
freedom,
might provide a new
basis for all
of
physics,
including
radiation.[73]
But
by
the end
of
the decade,
as
we
have
seen,
he
hoped
to derive the
granular
structure
of
radiation and
electricity
from
some
form
of
nonlinear field
theory.
Einstein's
efforts to
incorporate gravitation
into the
theory
of
relativity
led
him
in 1907 to formulate
a new
formal
principle,
later named the
principle
of
equiva-
lence.[74]
He stressed
that,
when
gravitational
effects
are
taken into account, it is
impossible
to maintain the
privileged
role that inertial frames
of
reference still
have in the
original relativity theory.
He concluded
that,
if
gravitation
is
to be
included,
it is
necessary
to
extend the
relativity principle.
The search for
a group
of
transformations,
wider than the Lorentz
group,
under which the laws
of
physics
remain invariant when
gravitation
is
included,
lasted from 1907 until the end
of
1915,
leading
finally
to what Einstein considered his
greatest achievement,
the
general theory
of
relativity.
[73]
See Einstein 1905i
(Doc. 14),
pp.
132-
133,
and Einstein 1907h
(Doc.
45),
p.
372.
[74]
See the editorial note,
"Einstein
on
the
Theory
of
Relativity,"
pp.
273-274.
The
similarity
that he
saw
between
relativity
and
thermodyamics,
discussed
above,
may
have
pre-
pared
him to
expect
that, like the second law
of
thermodynamics, the
original principle
of
rela-
tivity
would have its limits.
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