From Copernicus to Einstein

FROM COPERNICUS TO EINSTEIN

*

From Coper KLICTULS JL

BY HANS REICHENBAGH Translated by Ralph B.

Winn

NEW YORK PHILOSOPHICAL LIBRARY, MCMXL I I

INC.

FROM

COPERNICUS

TO

EINSTEIN

COPYRIGHT MCMXLII BY

PHILOSOPHICAL 15

INC. LIBRARY, EAST 40TH STREET, NEW YORK

Composed and Printed in the United States of America By Steingould Corp., New York

TYPOGRAPHY BY SIDNEY SOLOMON

CONTENTS CHAPTER

World

1.

The Copernican View

2.

Ether

29

3.

49

4.

The The

5.

General Theory of Relativity

6.

Space and Time

of the

Special Theory of Relativity Relativity of

Motion

6201251

11

73 85

107

ILLUSTRATIONS FIGURE 1.

A Tycho

2.

Roemer s Observation

3.

The Phenomenon

4.

The Total Spectrum The Diagram of Michelson s Experiment

5. 6.

A

Brahe s Sextant

7.

A Diagram of

8.

Einstein

9.

The Curvature

s

32

Measurement

38

46

54

of the

59 63

86

Experiment"

of Light-Rays in Einstein

10.

The

11.

Major Segrave s at

Moon

the Course of a Light-Signal

"Box

Einstein

s

of Interference

of the of Speed Light

Diagram

of Jupiter

20

Tower

Full Speed

in

1,000

Potsdam

s

Box

95 100

Horsepower Auto 1

10

Chapter

1

:

THE COPERNICAN VIEW OF THE WORLD

THIS

little book purports to serve as an introduction to the great problems of space, time and motion. The in

concerned with are very old. Men have been forming ideas concerning space and time since times im memorial, and curiously enough, have been writing and quiries

it is

fighting about these things with the greatest interest, even fanaticism. This has been a strange strife, indeed, having little to

do with economic

necessities

;

it

has always dealt

with abstract things, far removed from our daily life and with no direct influence upon our daily activities. Why

do

we need

earth or

Can

know whether the sun revolves around the vice versa? What business of ours is it, anyway? to

knowledge be of any use to us? No sooner have we uttered these questions than we become aware of their foolishness. It may not be of any use to us, but we want to know something about these problems. We do not want to go blindly through the world. We desire more than a mere existence. We need this

these cosmic perspectives in order to be able to experi ence a feeling for our place in the world. The ultimate

the meaning of our actions and as to the questions as to meaning of life in general always tend to involve astro nomical problems. Here lies the mystery surrounding 11

From Copernicus To

Einstein

wonder we experience at the sight of the starry sky, the wonder growing in proportion to our understanding of immense distances of space and of the stars inner nature. Here is the source of scientific astronomy, here

as well as

lies

the

popular astronomy.

These two branches have diverged

in the course of

Astronomy, as a science, has come to primitive wonder: instead, it approaches the

their development.

forget

its

realm of

stars

with sober research and calculation. This

disenchantment with

its

subject-matter,

which

scientific

study invariably entails, has permeated astronomy to a greater degree than the layman realizes. In observing the

astronomers of today, culate,

how

lations,

one

nothing

is

little

how

they measure, take notes, cal

attention fiiey

pay

to

mysterious specu be the find wonderful struc to may surprised ture of learning so cut and dry at a close range. Yet

feeling of

more wrong and more objectionable than the a heartbreaking loss, with which some people

regard the vanishing mysticism of the skies. Although science may have destroyed a few naive fantasies, what she has put in their place can well bear the loss.

is

so

immensely greater that we

perseverance and energy, of course, to com prehend the discoveries of science; but whoever under takes the study is bound to learn many more surprising It takes

from

than a naive study of nature can disclose. Scientific astronomy has always exercised, in fact, a great

things

influence

it

upon everyday thinking and upon

conception of the universe. If 12

it is difficult

the popular

today to pro-

The Copernican View

of the

World

nounce the name of Copernicus without thinking of a turning point of history,

it is

name

not only because the

connected with a profound transformation in the science, but also because all our knowledge and thinking have been deeply affected by his discovery. The statement is

that the earth does not occupy the center of the

means more than an astronomical asserting that

man

is

fact;

we

world

interpret

as

not the center of the world, that

to us everything which appears large and mighty

reality of the smallest significance,

cosmic standards.

it

The

statement has

as a result of scientific

thousands of years, yet

is

in

when measured by been made possible

of development in the course it definitely contradicts our im

mediate experience. It takes a great deal of training in no to believe in it at all Nowadays we are thinking because we have been longer conscious of these things, since childhood in the Copernican view of

brought up

the world. However,

it

cannot be denied that the view

our senses, that every immediate the evidence shows the earth as standing still while

belies the testimony of

heavens are moving. And who among us can declare in the tremendous all seriousness that he is able to imagine

comprehend the cosmic distances The signifi earthly ways of measurement? in the fact that he lies

size of the sun or to

defying all cance of Copernicus

precisely

all broke with an old belief apparently supported by immediate sensory experiences. He could do it only be

cause he had at his disposal a considerable

accumulated

scientific

thought and

amount of

scientific data,

only 13

From Copernicus To

Einstein

because he himself had followed the road of disillusion

ment

knowledge before he glimpsed new and broader

in

perspectives.

If

we

endeavor

to trace, in the

following pages, the

development of the problems of space and time, begin ning with the discovery of Copernicus and closing with the still less accessible theory of the Copernicus of our day, we have no other alternative than to apply hard scientific

to

thought

every step of the way.

that the discoveries of

modern

s

doctrines are by no

add

made new scientific materials.

science have been

possible only by the abundance of

Einstein

We must

means an outgrowth

of

astronomical reflections alone; they are grounded in the facts of the theory of electricity

able to

quainted with

all of their sources.

several sources tivity.

and light

comprehend them only insofar

While

is

the

as well. as

we

We are get ac

This derivation from

characteristic of the theory of rela

modern source gave

rise to the special

theory of relativity, the older sources provided the terial for the construction of the tivity,

in

ma

general theory of rela

which the old and new knowledge became

blended in a magnificent unity. In this chapter we shall deal with old material in the next two chapters we shall present the special theory of ;

and its origin; and the last three chapters will be devoted to the blending of the material and, therefore, relativity

to the general theory of relativity.

The

world-picture found by Copernicus goes back to the ancient Greeks. It was systematized about 140 A.D. 14

The Copernican View

of the

World

by Ptolemy Claudius of Alexandria and outlined in his famous work Almagest. The most important feature of the Ptolemaic scheme of the universe is the principle that the earth

is

the center of the world.

The heavenly

globe revolves around it; and Ptolemy knew full well that it has the same spherical shape below the horizon,

which

knew

it

assumes above the horizon. In

fact,

Ptolemy

a sphere. His proofs to this knowledge of astronomy. He shows, the existence of curvature from north to south.

even that the earth

is

effect reveal a great first

As

of

all,

the Polar Star stands higher in the north and lower

in the south, the surface of the earth

The. proof

ingly curved.

from west

to east reveals

must be correspond

of the existence of curvature

even better observation.

When

the clocks are set by the sun in two places located west and east, and when an eclipse of the moon is thus ob served,

it

eclipse

is

will be seen at different times. However, the a single objective event and should be seen

everywhere

same time. Hence we conclude that the two places are not in accord. This can

at the

the clocks at

*

be accounted for by the curvature of the earth in the west-east direction the sun passes the line of the meridian :

at different

moments

in different places.

In spite of the recognition of the spherical shape of the earth, Ptolemy was far from admitting its movement

He contended,

on the contrary, that

it

was impossible for

the earth to be

moving at all, either in a rotating or in a progressive manner. As far as the former is concerned, he admitted the

possibility of such

an opinion, as long 15

From

as the

movement

Copernicus

of the stars

To

Einstein

was considered. However,

when we

take into consideration everything that happens around us and in the air, this view so he argues be

comes obviously absurd. For the earth, during its rota in the tion, would have to leave the air behind. Objects not being able to follow the rotation, would have to be also left behind. pro in is equally impossible for, gressive motion of the earth

atmosphere, such

as flying birds,

A

that case, the earth

sphere, and

would leave the center

we would

see

of the heavenly

by night a smaller part of the

sphere) and

by day a larger one. One can see from these arguments that the great

astronomer has devoted

much

serious

thought

to

the of

problem. In the light of his rather limited knowledge mechanics and of the heavenly spaces, his reasoning must

have seemed quite conclusive. As far as his last objection was concerned, he could .not have suspected that the interstellar distances shift of the earth

were

so great as to

make

the lateral

completely unnoticeable.

The planets are characterized, according to Ptolemy, in the by common movements. Their path, as observed circular orbits. As sky, is determined by superimposed One must a result, there arise the so-called "epicycles."

admit that Ptolemy has deeply understood the nature of with the planetary movements. When one gets acquainted

Copernican conception, one discovers the facts revealed behind Ptolemy s epicycles: the loop of the planets course mirrors their double motion as regards the earth.

In the 16

first

place, they

move

in a circle

around the sun,

The Copernican View

and

in the second place, this

the earth which, in

its

of the

World

movement

turn, revolves

is

observed from

around the sun.

The Ptolemaic

conception of the universe dominated the learned people s minds for more than one thousand years.

The man who undermined

this firm tradition

of required great independence an thought as well as great scientific knowledge, for only nature could give insight into the ultimate relations of

Nicholas Copernicus

him

the ability to discern

new approaches

to truth.

Frauenburg was long known as a learned astronomer before his new ideas were presented; he had studied in Italy all branches of science, he had

The canon

of

acted as doctor and church administrator in his home so well recog town, and his astronomic knowledge was nized that in 1514 he was asked by the Lateran Coun for his opinion on questions of calendar reform. His new ideas concerning the system of the universe were cil

he formed, in their essence, at the age of 33, However, the did not promulgate them at that time, but devoted and demon following years to a thorough elaboration

Only excerpts of his doctrine were published during his lifetime. His main work en stration of his theories.

titled

"Of

the Rotation of Celestial

Bodies"

appeared

read the proofs only on

only after his death in 1546. He his death-bed and thus failed to notice that his friend Osiander supplied the work with a foreword which con tained a cautious compromise with the opinions of the

Church. If

we examine

the proofs given by Copernicus of his 17

From

new theory, we

find

Copernicus

To

them quite

Einstein

of view of present-day knowledge. to cite as a distinct

of his system.

He

from the point was able, in fact,

insufficient

He

advantage only the greater simplicity

regards

improbable that the stars their large orbits and finds it

it

move with

as

great speed in mo)re likely that the earth rotates

on,