DOC. 26 TIME,
SPACE, AND GRAVITATION 213
EINSTEIN
ON
HIS
THEORY.
TIME,
SPACE. AND
GRAVITATION.
THE NEWTONIAN
SYSTEM.
By
Dr. Albert Einstein.
I respond
with
pleasure to your
Corre-
spondent's
request
that
I
should write
some-
thing
for
The Times
on
the
Theory of
Relativity.
After the lamentable breach in the former
international
relations
existing
among men
of
science,
it
is with
joy
and
gratefulness
that
I
accept this
opportunity
of
communication
with
English astronomers and
physicists.
It
was
in
accordance with the
high
and
proud
tradition of
English science that
English
scientific
men
should have
given
their time and labour,
and
that
English
institutions should have
provided
the material
means,
to
test
a
theory
that
had
been
completed
and
published
in the
country
of their
enemies in the midst of
war.
Although
investigation
of the influence
of
the solar
gravi-
tational
field
on rays
of
light
is
a
purely
objec-
tive
matter,
I
am none
the less
very
glad to
express
my
personal
thanks
to my English
col-
leagues
in this branch
of
science
;
for
without
their aid I
should not have
obtained
proof
of
the
most
vital deduction from
my theory.
There
are
several kinds of
theory
in
Physics.
Most
of them
are
constructive.
These attempt
to
build
a
picture
of
complex phenomena
out of
some
relatively
simple
proposition.
The kinetic
theory
of
gases,
for instance, attempts to
refer
to molecular
movement
the mechanical, thermal,
and diffusional
properties of gases.
When
we
say
that
we
understand
a
group
of
natural
phenomena,
we mean
that
we
have found
a
constructive
theory
which embraces them.
THEORIES OF PRINCIPLE.
But
in
addition
to
this
most weighty group
of
theories,
there is another
group consisting
of
what
I
call theories
of
principle.
These
employ
the
analytic,
not
the
synthetic method.
Their
starting-point
and foundation
are
not hypo-
thetical
constituents,
but
empirically
observed
general
properties
of
phenomena, principles
from which mathematical formulae
are
deduced
of such
a
kind that
they apply
to
every
case
which
presents
itself.
Thermodynamics,
for in-
stance,
starting from the fact
that
perpetual
motion
never occurs
in
ordinary experience,
attempts
to
deduce from
this, by
analytic
pro-
cesses, a
theory
which will
apply
in
every case.
The
merit
of constructive theories is their
com-
prehensiveness,
adaptability, and
clarity,
that
of the theories of
principle,
their logical
per-
fection, and the
security
of
their
foundation.
The
theory
of
relativity is
a
theory
of prin-
ciple.
To
understand it,
the
principles on
which
it
rests must be
grasped.
But before stating
these it is
necessary
to
point
out
that the theory
of
relativity
is
like
a
house with
two
separate
stories,
the
special relativity
theory
and the
general
theory
of
relativity.
Since the time
of
the ancient Greeks it has
been well
known that in
describing
the motion
of
a
body
we
must refer
to
another
body.
The
motion
of
a
railway
train is described with
reference
to
the
ground,
of
a
planet with
reference
to
the total
assemblage of
visible fixed
stars. In
physics
the bodies
to
which motions
are
spatially referred
are
termed
systems of
co–
ordinates. The laws
of
mechanics
of
Galileo
and Newton
can
be formulated
only by using
a
system
of co-ordinates.
The state of motion of
a
system
of
co–
ordinates cannot
be
chosen arbitrarily if
the
laws of mechanics
are
to
hold
good
(it must
be free from
twisting and from
acceleration).
The
system
of co-ordinates
employed
in
mechanics
is
called
an inertia-system.
The state
of motion
of
an
inertia-system,
so
far
as
mechanics
are
concerned,
is not restricted
by
nature to
one
condition. The condition
in the
following proposition
suffices
:
a
system
of
co-ordinates
moving
in the
same
direction and
at the
same
rate
as a
system of
inertia
is itself
a
system
of inertia. The
special
relativity
theory
is therefore the
application
of the
following
pro-
position to any
natural
process
:-"Every
law
of nature
which holds good with
respect
to
a
co-ordinate
system
K must
also hold
good
for
any
other
system
K'.
provided
that K and
K'
are
in uniform movement of translation.
The second
principle
on
which the
special
relativity
theory rests
is that of the
constancy
of
the
velocity
of
light
in
a vacuum.
Light
in
a
vacuum
has
a
definite and constant
velocity,
independent of
the
velocity
of its
source.
Physicists
owe
their confidence in this
proposition to
the Maxwell-Lorentz
theory
of
electro-dynamics.
The
two
principles
which I have
mentioned
have received
strong experimental
confirmation,
but do
not
seem
to
be
logically
compatible.
The
special
relativity
theory
achieved their
logical
reconciliation
by making
a
change
in
kinematics,
that is
to
say,
in
the doctrine of the
physical
laws
of space
and time. It became evident
that
a
statement of the coincidence of two events
could have
a
meaning only
in
connexion with
a
system
of
co-ordinates,
that the
mass
of
bodies
and the
rate
of movement of clocks must
depend
on
their state of motion with
regard to
the
co-ordinates.
THE OLDER
PHYSICS.
But the older
physics, including
the
laws of
motion
of
Galileo
and
Newton,
clashed with the
relativistic kinematics
that I have
indicated.
The latter
gave
origin to certain
generalized
mathematical
conditions
with which the laws of
nature
would have
to
confirm if the two
funda-
mental
principles were compatible. Physics
had
to
be
modified. The most
notable
change
was
a new
law of motion for
(very rapidly)
moving
mass-points,
and
this
soon came
to
be
verified
in the
case
of electrically-laden
particles.
The
most
important result
of
the
special relativity
system
concerned
the
inert
mass
of
a
material
system.
It became
evident
that
the
inertia of
such
a
system
must depend
on
its
energy-con-
tent,
so
that
we
were
driven
to
the
concep-
tion that inert
mass
was
nothing
else
than
latent
energy.
The doctrine of the conservation
of
mass
lost its
independence
and became
merged
in the doctrine of conservation of
energy.