Sidelights on Relativity

Sidelights on Relativity

The Project Gutenberg EBook of Sidelights on Relativity, by Albert
Einstein #2 in our series by Albert Einstein
Copyright laws are changing all over the world. Be sure to check the
copyright laws for your country before downloading or redistributing
this or any other Project Gutenberg eBook.
This header should be the first thing seen when viewing this Project
Gutenberg file. Please do not remove it. Do not change or edit the
header without written permission.
Please read the "legal small print," and other information about the
eBook and Project Gutenberg at the bottom of this file. Included is
important information about your specific rights and restrictions in how
the file may be used. You can also find out about how to make a
donation to Project Gutenberg, and how to get involved.
**Welcome To The World of Free Plain Vanilla Electronic Texts**
**eBooks Readable By Both Humans and By Computers, Since
1971**
*****These eBooks Were Prepared By Thousands of
Volunteers!*****

Title: Sidelights on Relativity
Author: Albert Einstein
Release Date: January, 2005 [EBook #7333] [Yes, we are more than
one year ahead of schedule] [This file was first posted on April 15,
2003]
Edition: 10
Language: English
Character set encoding: ASCII
*** START OF THE PROJECT GUTENBERG EBOOK
SIDELIGHTS ON RELATIVITY ***

Produced by David Starner, William Fishburne and the Online
Distributed Proofreading Team.

SIDELIGHTS ON RELATIVITY
By Albert Einstein
Contents
ETHER AND THE THEORY OF RELATIVITY
An Address delivered on May 5th, 1920, in the University of Leyden
GEOMETRY AND EXPERIENCE
An expanded form of an Address to the Prussian Academy of Sciences
in Berlin on January 27th, 1921.

ETHER AND THE THEORY OF RELATIVITY
An Address delivered on May 5th, 1920, in the University of Leyden

How does it come about that alongside of the idea of ponderable matter,
which is derived by abstraction from everyday life, the physicists set
the idea of the existence of another kind of matter, the ether? The
explanation is probably to be sought in those phenomena which have

given rise to the theory of action at a distance, and in the properties of
light which have led to the undulatory theory. Let us devote a little
while to the consideration of these two subjects.
Outside of physics we know nothing of action at a distance. When we
try to connect cause and effect in the experiences which natural objects
afford us, it seems at first as if there were no other mutual actions than
those of immediate contact, e.g. the communication of motion by
impact, push and pull, heating or inducing combustion by means of a
flame, etc. It is true that even in everyday experience weight, which is
in a sense action at a distance, plays a very important part. But since in
daily experience the weight of bodies meets us as something constant,
something not linked to any cause which is variable in time or place,
we do not in everyday life speculate as to the cause of gravity, and
therefore do not become conscious of its character as action at a
distance. It was Newton's theory of gravitation that first assigned a
cause for gravity by interpreting it as action at a distance, proceeding
from masses. Newton's theory is probably the greatest stride ever made
in the effort towards the causal nexus of natural phenomena. And yet
this theory evoked a lively sense of discomfort among Newton's
contemporaries, because it seemed to be in conflict with the principle
springing from the rest of experience, that there can be reciprocal
action only through contact, and not through immediate action at a
distance. It is only with reluctance that man's desire for knowledge
endures a dualism of this kind. How was unity to be preserved in his
comprehension of the forces of nature? Either by trying to look upon
contact forces as being themselves distant forces which admittedly are
observable only at a very small distance--and this was the road which
Newton's followers, who were entirely under the spell of his doctrine,
mostly preferred to take; or by assuming that the Newtonian action at a
distance is only apparently immediate action at a distance, but in truth
is conveyed by a medium permeating space, whether by movements or
by elastic deformation of this medium. Thus the endeavour toward a
unified view of the nature of forces leads to the hypothesis of an ether.
This hypothesis, to be sure, did not at first bring with it any advance in
the theory of gravitation or in physics generally, so that it became
customary to treat Newton's law of force as an axiom not further
reducible. But the ether hypothesis was bound always to play some part

in physical science, even if at first only a latent part.
When