Turning Points in Physics. A Series of Lectures Given at Oxford University in Trinity Term 1958

195 Pages·1959·3.602 MB·English

by  R.J. Blin-Stoyle, D. ter Haar, K. Mendelssohn, J. De Boer, H. Brinkman and H. B. G. Casimir (Auth.)

SERIES IN PHYSICS
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TURNING POINTS
IN PHYSICS
A SERIES OF LECTURES
GIVEN AT OXFORD UNIVERSITY
IN TRINITY TERM 1958
BY
R.J.BLIN-STOYLE
D.TER HAAR
K. MENDELSSOHN
G. TEMPLE
F. WAISMANN
D.H.WILKINSON
WITH AN INTRODUCTION
BY A. C. CROMBIE
1959
NORTH-HOLLAND PUBLISHING COMPANY,
AMSTERDAM
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INTRODUCTION
by
A. C. CROMBIE
Lecturer in the History of Science, Oxford University
In his intelligent and attractive essay, Faraday as a Dis
coverer (new ed., London, 1877, pp. 66-7), John Tyndall in
troduced his account of Faraday's investigation of the laws
of electro-chemical decomposition as follows:
"In our conceptions and reasonings regarding the forces
of nature, we perpetually make use of symbols which,
when they possess a high representative value, we dig
nify with the name of theories. Thus, prompted by cer
tain analogies we ascribe electrical phenomena to the
action of a peculiar fluid, sometimes flowing, sometimes
at rest. Such conceptions have their advantages and their
disadvantages; they afford peaceful lodging to the intel
lect for a time, but they also circumscribe it, and by and
by, when the mind has grown too large for its lodging,
it often finds difficulty in breaking down the walls of
what has become its prison instead of its home."
Tyndall had first used these words in a Friday evening lec
ture at the Royal Institution, and he copied them here, he
said, "because they remind me of Faraday's voice, re
sponding to the utterance by an emphatic 'hear ! hear !' " He
went on to point out how sensitive Faraday himself was to
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INTRODUCTION
the tyranny and temptations of symbols and analogies, es
pecially of words and concepts transferred from their
original context to another. As Faraday wrote in his Ex
perimental Researches into Electricity (London, 1849, vol.
1, p. 515):
"The word current is so expressive in common language,
that when applied in the consideration of electrical phe
nomena we can hardly divest it sufficiently of its mean
ing, or prevent our minds from being prejudiced by it."
Similarly he objected to "pole", because it suggested the
idea of attraction, to which he did not want to be prema
turely committed. So, with the help of William Whewell,
he divised the neutral terminology of electrode, electrolyte,
anion, cation,... with meanings defined primarily within
the context of electro-chemistry.
This glimpse into Faraday's mind provided by Tyndall
serves to introduce the purpose of the essays comprising
this volume. It has become the practice in recent years in
Oxford to arrange in the Trinity Term, as part of the in
struction in the history and philosophy of science given in
the University, lectures designed to introduce critical dis
cussions of the history of some major aspect of scientific
thought in the nineteenth and twentieth centuries. The
theme for the course in 1958 was "Turning-points in phys
ical theory". The chapters that follow are based on the
papers read on selected aspects of this vast theme. All of
them are by colleagues available in Oxford.
There are many reasons for fundamental changes in
scientific theory. At the empirical end of the scale, one
obvious reason is the discovery of disagreement between
observations and theoretical expectations, brought about
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INTRODUCTION
perhaps by the invention of new techniques which greatly
extend the range of possible observations. An elementary
example from the early history of modern science is the
part played in the reform of astronomical theory, from
Tycho Brahe and Kepler to Newton, by the extension given
to the range and accuracy of observation by the invention
of new instruments, including the telescope. But many of
the deepest changes in scientific theory are initiated not
directly by new observations but by re-thinking, by having
new ideas and asking new questions from a different point
of view. The classic example, again from the seventeenth
century, is the formulation of the conception of inertia.
The initial change was theoretical; a new range of possible
observations followed from the new theory rather than
brought it about. The new theory of motion also gener
ated fresh problems and thus created the need for further
new theories, for example for the theory of gravitation to
explain the now unexplained planetary orbits, and so the
process continued once the first theoretical innovation had
been made. But much more was involved in the change
than simply the replacement of one theoretical conception
of motion by another; to do this at all meant replacing the
accepted Aristotelian assumptions about the nature of
physics and criteria of satisfactory explanation by the as
sumptions and criteria found in classical mechanics. The
reasons for making so radical a change went far beyond the
mere data of observation, even though their control and
justification was sought and found in the test of observation.
Among the various reasons that go to bring about a
major turning-point in scientific theory, one of the com
monest seems to be some kind of mental uneasiness with
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INTRODUCTION
accepted criteria of satisfactory explanation. This is clear,
for example, in Faraday's growing wariness over the ac
cepted assumptions of the mechanistic physics inherited
from the seventeenth century, and especially over the
question of action at a distance. It was, to use TyndalTs
phrase, the need he felt to break down walls of this kind and
to find a new home for theory, rather than questions
strictly testable by experiment, that sent him off in a new
direction. This was perfectly compatible with his being the
prince of experimenters. Physical theory after Faraday saw
a succession of similar re-appraisals of the assumptions that
could be considered acceptable, of special importance being
those associated with the introduction of probability into
physics and with the atomic theory, radioactivity, quan
tum theory, and relativity. In all these cases the presup
positions being made by physicists about the world and
about the scope of physics served, as with Faraday, on the
one hand to regulate the form it was held that an explana
tion must take, and on the other to direct attention and
stimulate the scientific imagination in particular directions.
The pressure forcing change began sometimes with new
theoretical presuppositions, sometimes with new experi
mental discoveries. The resulting interplay between as
sumptions, theories, and experimental discoveries is the
main subject of this volume.
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CHAPTER I
THE END OF MECHANISTIC PHILOSOPHY
AND THE RISE OF FIELD PHYSICS
by
R. J. BLIN-STOYLE
Senior Research Officer in Theoretical Physics, Oxford University
l. Physical Theory
In the physical world one is confronted by a series of ob
servational data gleaned from experiment and more or less
accurately known as the case may be. It is the objective of
the physicist and in particular the theoretical physicist at
any time to correlate and explain these data within the
framework of a theory. A successful theory must be con
sistent with all the available data and in the case of possible
alternatives simplicity is generally used as a weighting
factor. Such a theory can then be used to predict the results
of, as yet, unperformed experiments, and the success of the
theory is judged by whether such predictions are in fact
borne out by further experimental enquiry. If this is not
found to be so it is then necessary to modify or possibly
radically change the theory in the light of the new facts.
Thus one has a continuous evolution of physical theory and
so far the end or ideal limit of this evolutionary chain, even
supposing that it exists, has never been in sight. However,
the belief that there is a simple underlying theory or an ob
jective truth which manifests itself in the phenomena of the
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THE END OF MECHANISTIC PHILOSOPHY [Ch. I, § 2
physical world is fundamental to all physical thought and
the stimulus for all research.
It is the aim of this first lecture to describe a radical modi
fication to the form of physical theories which had been
prevalent up to the middle of the nineteenth century. This
modification led to a considerable simplification in the
nature of physical theory and also led to a very satisfying
unification of two separate branches of physics. This change
in outlook can justly be termed a "turning point in physics".
To appreciate the nature of this revolution in physical
theory it is necessary first of all to consider the philosophy
that had set the pattern for all physical theory for the last
two centuries prior to the revolution.
2. The Mechanistic Philosophy
Perhaps one can trace the first cause of the mechanistic
viewpoint to Copernicus who in 1543 proposed the now
currently accepted relationship between the earth and the
sun and to Kepler (1571-1630) who laid stress on the im
portance of setting up a mathematical scheme for a proper
description of nature. It was the teaching of Kepler that
provided the chief inspiration for René Descartes (1596-
1650) who can properly be called the originator of the
mechanistic philosophy. One of the problems of that time
was to account in a satisfactory manner for the actions
transmitted between bodies not in contact such as, for ex
ample, the effect of the moon on tides and the mutual at
traction or repulsion of magnetic bodies. In his rejection of
all "qualities", Descartes was unable to accept an explana
tion in terms of "action at a distance". His philosophy had
led him to identify substance-material with extension and to
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Ch. I, § 2] THE END OF MECHANISTIC PHILOSOPHY
the conclusion that space was a plenum occupied by an
aether with mechanical properties. The interaction between
two spatially separated bodies was then accounted for in
terms of forces transmitted by the aether. In this mechanis
tic manner he wished to account for all physical pheno
mena manifested by the inanimate world.
If the concept of force between material bodies is to be
used we must look a little more closely at its implications
and in particular at the inter-relation between force and
motion. Motion had been a fundamental problem for thou
sands of years. Intuitively the idea exists that force is the
cause of motion; as Aristotle wrote in the Mechanics : "The
moving body comes to a standstill when the force which
pushes it along can no longer act so as to push it." Aristotle's
great authority in Europe unfortunately led to the reten
tion of this fallacious idea for a long time and it was not
until Galileo (i 564-1642) came on to the scene that the
correct interpretation of motion was given and accepted.
It is well-known as Newton's (1642-1727) first law of mo
tion: "Every body perseveres in its state of rest, or of uni
form motion in a right line, unless it is compelled to change
that state by forces impressed thereon." Galileo saw quite
clearly that the effect of a force on motion is not to main
tain the motion but to change it. Commonplace examples
of this are now, of course, well-known and need not be
quoted. Newton stated the idea in his Principia in the
following way:
"An impressed force is an action exerted upon a body in
order to change its state, either of rest, or of moving uni
formly forward in a right line."
If we extend the above considerations to motion other
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