DOC.

1

PRINCIPLE OF RELATIVITY

5

"relativity theory."

As

an

aid for the theoretical deduction of the laws of

nature,

this

theory

has

already proven

itself. Its

significance

lies in the fact that it

provides

conditions which

every general

law of

nature

must

satisfy,

for the

theory

teaches that

phenomena

in nature

are

such that the laws do not

depend upon

the

state

of motion

of the observer onto whom the

phenomena

are spatially

and

temporally

related.

Of

the

major

results of the

theory

of

relativity

two

should be mentioned

here,

because

they

are

also of interest

to

the

layman.

First: the

hypothesis

of

the existence

of

a

space-filling

medium for

light propagation,

the so-called

light-ether,

has

to

be

abandoned.

Light appears according

to

this

theory no longer

as a

state of motion of

an

unknown

carrier,

but

rather

as a physical

structure with

a

physical

existence of

its

own.

Second: the

theory

establishes that the inertia of

a

body

is

not

an absolutely

unchangeable

constant,

but rather that

it

grows

with the

energy

content.

The

important

conservation theorems

of

mass

and

of

energy

melt into

a

single

theorem:

the

energy

of

a

body

also determines its

mass.

Is the

theory

of

relativity,

sketched

above,

basically

complete-or is

it

only

a

first

step

in

a

continuing development?

On this

question physicists

who

value the

theory

of

relativity

still hold

differing opinions. Nevertheless,

weighty arguments speak

for

the second alternative. We have stated above that the laws

of

nature

are

the

same

for

a

"uniformly moving"

observer

as

they

are

for

one

"at rest." This

means

it is

impossible

for

an

observer

to

find criteria that would allow him

to

decide if he is at

rest

or

in

a

state

of

uniform motion. "At rest"

and

"in uniform

motion"

are

physically

equivalent.

This raises the

question

of

whether the

principle

of

relativity

is limited

to

uniform motion. Could the laws of

nature

not be such that

they

are

the

same

for

two observers

who

are

in nonuniform

motion relative to

each

other?

In

recent

years

it has turned

out

that such

an

extension

of

relativity theory

can

be carried

out,

and

that it leads

to

a

general theory

of

gravitation

which contains the Newtonian

theory

as a

first

approximation. According

to

this

theory, light rays

suffer

a

curvature in

a

gravitational

field;

though minute,

it is

just

within the

range

of

astronomical

measurement.

The future

will

teach

us

whether

this

generalized relativity theory,

which from

an

epistemological aspect

is

very satisfying,

conforms

to

reality.