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.