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7/27/2019 CHANG, Hasok (1995) the Quantum Counter-revolution; Internal Conflicts in Scientific Change
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13552198(95)00010-O
The Quantum Counter-Revolution: Internal
Conflicts in Scientific Change
Hasok Chang*
Many of the experiments that produced the empirical basis of quantum mechanics
relied on classical assumptions that contradicted quantum mechanics. Historically
this did not cause practical problems, as classical mechanics was used mostly
when it did not happen to diverge too much from quantum mechanics in the
quantitative sense. That fortunate circumstance, however, did not alleviatethe conceptual problems involved in understanding the classical experimental
reasoning in quantum-mechanical terms. In general, this type of difficulty can be
expected when a coherent scientific tradition undergoes a theoretical upheaval.
The problem may be circumvented through the use of phenomenological theory in
experimentation during the period of theoretical instability.
1. The Classical Infiltration
The transition from classical to quantum physics is widely regarded as a classicexample of a scientific revolution. That impression is deceptive, perhaps for
several reasons. One important reason is that many important experiments in
quantum physics employed reasoning based on classical mechanics. This
created a peculiar situation: the experiments were designed on the basis of
assumptions from classical mechanics, while their results were used to support
the new quantum mechanics,’ according to which classical mechanics is
presumably incorrect. Isn’t the use of a rejected theory bound to lead to errors,
or at least constitute contradictions with the accepted theory? How wasquantum physics so successful if it harboured such contradictions within it‘?
In exploring this apparently self-destructive situation, I will start with an
example that is striking not least of all because it is so commonplace. In
*Department of History and Philosophy of Science, University College London, Cower Street.
London WClE 6BT, U.K.
Received 8 April 1994; in revised form 13 April 1995.
‘Throughout the paper, I will use the term ‘quantum mechanics’ to refer to the theory and
‘quantum physics’ to refer to the whole tradition including experimentation, and similarly for
‘classical mechanics’ and ‘classical physics’. More specifically, what I mean by ‘quantum mechanics’is the theory of non-relativistic quantum mechanics which became more or less complete in the late
1920s with the works of Heisenberg, Schrodinger. Dirac. etc.
Pergamon Stud. Hist. Phil. Mod. Phys.. Vol. 26, No. 2, pp. 121-136, 1995
Copyright @,N 996 Elsevier Science Ltd
Printed in Great Britain. All rights reserved
135552198/95 $9.50+00.00
121
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Quantum Counter-Revolution 123
Now the problem is that the laws of classical mechanics are expected to fail,
when applied to microscopic particles. After all, that is one of the chief reasons
why quantum mechanics was invented in the first place. How, then, can theclassical experimental reasoning be correct? By using a rejected theory in
experimentation, were the physicists not getting into errors or, to put it more
neutrally, contradictions with the general physical theory they believed? As a
matter of historical fact experimentation and theory often do not change in step
with each other, as stressed by Peter Galison,’ but the situation here is a little
bit more perverse than just that. Quantum mechanics seems to be a revolution-
ary new theory that is sabotaged by the surviving elements of the old theory
buried in the methods of experimentation. This is the origin of the metaphor of
counter-revolution that I have used in the title of this paper.
The theoretical foundation of the magnetic deflection method was provided
by classical electromagnetism and Newtonian mechanics. This combination
had a nearly universal role in the investigation of the properties and behaviour
of charged particles. The magnetic deflection method itself was used as a matter
of course in studies of cathode rays, radioactivity, photoelectricity, cosmic rays,
and the penetration of matter by microscopic particles. There were also many
other important techniques that made use of the deflection of charged particles
in magnetic and electric fields.
J. J. Thomson used combined electric and magnetic deflection to measure the
charge-to-mass ratio of electrons. 3 Ernest Rutherford used a similar technique
to study the properties of alpha particles4 and then used the alpha particles in
the investigation of atomic structure, with the assumption that they would be
classically deflected by charged elements within the atoms. In his Nobel
prize-winning work on isotopes F. W. Aston employed the mass spectrograph,
an ingenious instrument of his own invention, in which a combination of
electric and magnetic deflections separated particles out according to their massregardless of their velocities. The mechanism of the mass spectrograph is
schematically represented in Fig. 2. This instrument was invented in 1919, and
constant improvements were made by many researchers until the precision of
one in a million was reached in 1940.5
‘P. Galison, ‘History, Philosophy, and the Central Metaphor’, Science i n Cont ext Z(1) (1988).
197-212; P. Galison, Image and Logic (Chicago: University of Chicago Press, forthcoming), Chap. 9.
‘J. J. Thomson, ‘Cathode Rays’, Phi lo sophical M agazine 44 (1897), 293-316.
4 For instance, E. Rutherford, ‘The Mass and Velocity of the a-Particles Expelled from Radium
and Actinium’, Phi lo sophical M agazine 12 (1906), 348-371, and E. Rutherford and H. Robinson,
‘The Mass and Velocities of the a-Particles from Radioactive Substances’, Phil osophicul M agazine
28 (1914), 552-572.
‘See F. W. Aston, Mass Spect ra and I sotopes, 2nd edn (London: Edward Arnold and Co., 1942).
Figure 2 is taken from p. 40 of this book. For the history of increasing precision, see pp. 1022116.
In this type of arrangement, precision can be increased by effecting an increase in the distance
between the region where the deflection takes place and the screen on which the particles are finally
detected. since that would allow a more precise measurement of the angle at which the particles
continued overle@’
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124 Studi es in Hi stor y and Phil osophy of M odern Physics
Fig. 2. The mechanism of the mass spectrograph. S, and S, are slits through which the purticles me
initially collimated. The particles undergo electrostutic defection while they pass between phtes P,
and Pr, and then magnetic deflection when they pass through the circular region centred at 0. Particles
of the same mass and different velocities will disperse as they go through the electric ield, but they will
be focused’ again by the magneticheld, to point F.
Not only deflection but acceleration and retardation by electric fields also
played an important role in experiments. It is a common practice to this day to
prepare particles of a certain energy by accelerating them across a certain
amount of electrostatic potential. This was the technique used in the Franck-
Hertz experiment of 1914, which provided important early confirmation for
Niels Bohr’s atomic theory.6 In his experiments on the photoelectric effect
published in 1916, Robert A. Millikan measured the kinetic energy of photo-
electrons by noting how much electrostatic potential was sufficient to bring
them to a stop. Millikan’s experimental arrangement is shown in Fig. 3.7 His
measurements vindicated Einstein’s ‘corpuscular’ theory of the photoelectric
Note S-continued
emerge out of the deflecting region. A similar point is made by Heisenberg, with regard to a set-up
that just involves a magnetic field intended for momentum measurement; see W. Heisenberg, The
Physical Principles of the Quantum Theory, trans. Carl Eckart and F. C. Hoyt (New York: Dover,
1949) pp. 28-30.
6_i. Franck and G. Hertz, ‘cber Zusammenstiibe zwischen Elektronen und den Molekiilen
des Quecksilberdampfes und die Ionisierungsspannung desselben’, Verhandlungen der Deutschen
Physikalischen Gesellschufi 16 (1914), 16&166. The interesting historical issues surrounding the
interpretation of the Franck-Hertz experiment are discussed in the following: G. L. Trigg, Crucial
Experiments in Modern Physics (New York: Van Nostrand Reinhold, 1971), Chap. 6; G. Hon,
‘Franck and Hertz versus Townsend: A Study of Two Types of Experimental Error’, Historical
Studies in the Physical and Biol ogical Sciences 20(l) (1989). 79-106, see pp. 81-92.
‘R. A. Millikan, ‘A Direct Photoelectric Determination of Planck’s -“h” ‘, Physical Review 7
(1916), 355-388. Figure 3 is from p. 362. Millikan’s exuerimental arrangement was in fact a eood
deal more complicated than the-brief description in the text would suggest; for details: see
H. Chang, Measurement and the Disunity of Quantum Physics, Ph.D. Dissertation, Stanford
University, 1993, Chap. 2.
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Quantum Coun ter-Revolu t i on
Fig. 3. The set -up of Millikan’s phot oel ectri c experi ment . There i s an elect rostut ic pot ent i al d$ference.V, betw een t he phot osensit i ve met al and t he el ectr ode recei vi ng t he phot oel ectr ons. bot h enclosed i n a
vacuum chamber (roughly t he ri ght hal f of t he$gure). Li ght ent eri ng t hrough the hol e l ubel ied 0 it rt
t he ri ght-hand edge of the chumber) fal l s upon t he nearby met al cyli nder (one of the three ut t ached
to the \ vheel W ut t he cent re of t he chamber). The elect rode i s a Faraday cyli nder made qf coppe r ,
ly i ng betw een 0 und t he phot osensit iv e met al sampl e, and conn er tcd o an elect rometer (not shown).
M il l i kan determined the value of Vat w hich the photocurrent became zero. and culculat ed the ini ti al
ki neti c energy of t he phot oelectr ons as e V.
effect* and, as a useful by-product, gave a precise measurement of the value of
Planck’s constant. Millikan estimated the precision of this value at OS’%, which
made it the most precise value to that time, and one of the most precise for
many years to come.
Here I have provided only an illustrative list of quantum-physical exper-
iments that made use of classical reasoning. The list could be extended quite
easily. It seems that experimenters relied on classical reasoning in almost all
experiments investigating or utilizing the ‘particle-like’ aspects of microscopic
objects: location, velocity, acceleration, mass, electric charge, etc. The infiltra-tion of quantum-physical experimentation by classical reasoning was deep and
widespread.
‘It is interesting to note, however, that Millikan himself objected strongly to Einstein’s concept
of the photon for many years. He thought that his experimental results confirmed the ‘Einstein
equation’ beyond any doubt, but still did not agree with the ‘Einstein conception’, which blatantly
ignored the wave nature of light. Op. cit ., pp. 3.55 and 383.
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Quantum Coun ter-Revolu t i on 121
This answer points to a well-known doctrine, usually traced back to Bohr,
that the use of classical theory in experiments is justified because all measure-
ment situations have to be described in classical terms. One version of this ideastates that our measuring instruments are macroscopic things, so it is fine to
describe them with classical mechanics, which continues to apply quite well to
everyday-sized things. That is irrelevant to the problem under consideration.
which is about the use of classical theory in describing microscopic particles. In
the passage just cited, Bohr seems to be arguing that even microscopic particles
must be described in classical terms when they are in a measurement situation.
Although Heisenberg records no explicit objections, it is difficult to see how one
can retain classical mechanics just in case of measurement interactions, while
rejecting it in all other situations.
More useful for the present discussion is something suggested by another
part of Bohr’s philosophy, namely the correspondence principle. Bohr noted
that in the realm of ‘high quantum numbers’ there was an asymptotic
convergence between the predictions of his atomic theory with its quantum
jumps, and the standard classical theory based on the conception that radiation
was produced by the oscillation of electrons in atoms. Bohr raised this
observation of convergence to a general principle, which served as a useful tool
in theory-building. What Bohr’s observation suggests to me is that there are
various circumstances under which the predictions of classical and quantum
mechanics converge. Bohr’s rule about the region of high quantum numbers
specifies just one such circumstance, and there are some other well-known
cases. Ehrenfest’s theorem states that the laws of classical dynamics hold for the
average values of corresponding quantum-mechanical quantities. The classical
laws of energy and momentum conservation are satisfied to a high accuracy in
quantum physics. There is also the result that in a harmonic-oscillator potential
the ground-state wave packet does not spread out, which misled ErwinSchrodinger into thinking that wave packets in general could represent classical
particles in a straightforward manner.”
The use of classical mechanics in experiments would only generate minor
contradictions, if just those parts of classical mechanics that converge with
quantum mechanics were used. I believe that most of the experiments I have
discussed satisfied that condition .I4 One important factor in general was the
practice of averaging, which would have worked to smooth out errors resulting
from the classical-quantum discrepancy, according to Ehrenfest’s theorem.“For a concise description of Schriidinger’s original interpretation of the wave function, see M.
Jammer, The Conceptual Development of Quantum M echanics. 2nd edn (Tomash Publishers and
American Institute of Physics, 1989), pp. 300-301.
“‘It is beyond the scope of this paper to discuss each case in detail, and no general arguments
could be expected. Elsewhere (see note 7) I have made a detailed quantitative study of one case,
namely Millikan’s photoelectric experiment, That is a case in which it might well be expected that
the classicalquantum discrepancy should create a visible effect, but the result of my investigation
is far from conclusive in that direction.
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128 Studi es in History and Phil osophy of M odern Physics
Averaging came in two forms. First, there was the long-standing custom of
making many runs of an experiment and averaging the results, as a way of
dealing with uncontrolled inaccuracies that were assumed to be random. Thispractice was not originally designed for handling the quantum-mechanical
deviations from classical predictions, but it ended up serving the purpose quite
well since those deviations are random. Second, when cloud chambers, Geiger
counters and other powerful amplification techniques were not available, the
detection of microscopic particles was possible only through collecting a large
number of them. This means, in effect, that a statistical distribution was
obtained, and the value taken in the observation was the peak value, which is
the average value for symmetric distributions.
Even if there had been remaining inaccuracies owing to the classical-
quantum discrepancy, their consequences would not always have been so
significant. In many experiments, inaccuracies owing to other factors were so
great as to swamp any additional inaccuracy resulting from the classical-
quantum gap. It should also be remembered that many important experiments
did not have to be very precise in order to serve their function. For instance, in
demonstrating that cathode rays were streams of charged particles, it was
important to show that they could be deflected by electric and magnetic
fields in the expected directions, but not so important to find out how great
the deflections were. Even in more quantitative experiments, high precision
was sometimes not an overriding concern. For example, it was important to
get rough measures of the energies of alpha and beta rays, but their precise
values had few consequences for the rest of physics until later, when energy
conservation and nuclear structure became important topics.
Experimental physics sailed smoothly through the conceptual storm out of
which quantum mechanics emerged, because it used mostly those parts of
classical mechanics that happened to converge with quantum mechanics closelyenough for the particular purposes it served. Here I have found one way in
which experiments can aford not to change in step with theory, even as they
retain a close relationship with theory. The practices of the early twentieth-
century quantum physicists seem to constitute an eminently sensible strategy of
maintaining a stable tradition of theory-based experimentation, even when the
stability in the realm of theory itself is quite low. However, as a more detailed
examination of the experiments will show, this effective strategy was not
consciously devised, but owed its origin to serendipity.A very nice illustration of this serendipity is provided by the case of
Rutherford’s work on atomic structure. Rutherford and his co-workers were
triumphant in their achievements obtained through the technique of alpha-
particle scattering, starting with the demonstration of the existence of the
atomic nucleus. Rutherford was deeply troubled, therefore, when he was told
later that the new quantum mechanics invalidated the classical assumptions that
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Quantum Count er-Revolu t ion 129
formed the basis of his scattering experiments. And he was puzzled, as there
seemed to be no serious disagreements between the results of his experiments
and the predictions of the new theory. Norman Feather, recounting this story,states that the puzzle was solved by 1929, after Wentzel and Mott showed that
‘uniquely for scattering in an inverse-square-law field, the final result--the
expression for the differential cross-section for scattering-is precisely the same
whether classical mechanics or wave mechanics is used in the calculation’.Js
When Rutherford used classical mechanics in designing his experiments, he
did not do so on the basis of any awareness that those pieces of classical
mechanics used by him would closely approximate the results of quantum
mechanics. After all, it is absurd to demand such awareness; it is quite
impossible to know in advance which parts of current theory will turn out to be
convergent with some unknown future theory, especially when the shape of that
future theory might depend on the results of the very experiments that one is
trying to design. So it would seem that scientists at a time of theory change are
forced to choose between blindness and paralysis. This apparent dilemma will
be addressed in Section 4.
3. The Persistence of Conceptual Contradictions
I have argued that the problems arising from the employment of classical
reasoning in quantum-physical experiments were ameliorated by the existence
of pockets of convergence between classical and quantum mechanics. That
addresses only half of the problem, as there is also a conceptual dimension to
be considered. Classical and quantum mechanics have such different conceptual
structures that statements made in one cannot be straightforwardly translated
into the terms of the other; this case is about as close as anything comes to
Kuhnian incommensurability. Hence, if we try to incorporate the classical
experimental reasoning into quantum mechanics, we are bound to run into
conceptual contradictions. The helpful convergence between the two theories
that I have discussed so far is only quantitative. It is instructive to note what
Bohr said in this regard about the correspondence principle:
In the limiting region of large quantum numbers there is in no wise a question of a
gradual diminution of the difference between the description by the quantum theory
of the phenomena of radiation and the ideas of classical electrodynamics, but only anasymptotic agreement of the statistical results.16
Even when the quantitative contradiction happens to be minor, the conceptual
contradiction persists.
‘sN. Feather, ‘Some Episodes of the Alpha-Particle Story, 1903-1977’, in M. Bunge and W. R.
Shea (eds), Rut herfor d and Physics at the Turn qf t he Cent ug, (New York: Science History
Publishers, 1979). pp. 7488, see p. 80.
“Quoted in D. Murdoch, N iels Bohr’s Phi losophy of Physics (Cambridge: Cambridge University
Press, 1987), p. 40. The original source is N. Bohr,“On the Application of the Quantum Theory to
Atomic Structure’, Collected Works, Vol. 111. p. 480.
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130 Studi es in History and Phil osophy of M odern Physics
To illustrate what I mean by ‘conceptual contradiction’ more clearly, I will
return to the example of the magnetic deflection method of momentum-energy
measurement. Classical theory says that a particle with a certain energy enteringthe magnetic deflection instrument here in t h is direction will come out here. The
quantitative question is: where does it come out according to quantum
mechanics? Strictly speaking, however, quantum mechanics says nothing at all
about that. If we represent quantum-mechanically a particle with a certain
energy going in a certain direction, we get something that is spread out all over
space, namely a momentum eigenstate, which is a plane wave. In order for the
magnetic deflection to make any sense, we need to be able to say where the
particle enters and exits the magnetic field. But this spread-out state has no
particular position in space at all, and therefore cannot enter or exit the
instrument at any definite places. The classical reasoning in magnetic deflection
is not incorrect but rather nonsensical, when put in quantum-mechanical terms.
If we want to understand magnetic deflection quantum-mechanically, we
cannot just say that the particle of a certain energy will come out here, rather
than there as predicted by classical mechanics. We have to do something much
more radical, because a particle that has a perfectly definite momentum cannot
come through a small hole as required in this kind of experimental set-up, and
a particle that can do so does not possess a definite value of momentum.
A common response here is that the classical reasoning can be translated into
quantum-mechanical terms approximately. More specifically: the particle can
be conceptualized as an entity with a fairly definite position and a fairly definite
momentum, the values being indefinite enough to satisfy the uncertainty
principle but sharp enough for practical purposes. That is to say, the particle
can be represented as a wave packet, even a minimum-uncertainty wave packet.
Then we can ask the classically phrased question, namely where the particle
(wave packet) should exit the magnetic field if it enters the field at a certainplace. According to Ehrenfest’s theorem, the centre of a symmetric wave packet
moves around just like a classical particle. So the classical way of talking might
make perfect sense, if we are willing to allow our particles to be a little bit
‘fuzzy’.
There are serious difficulties with this view. First of all, wave packets are not
only fuzzy, but they have phases and exhibit interference. Secondly, a wave
packet of the desired type will have a ‘tail’-a ‘little bit’ that extends far away
from the centre, in principle to infinity. The existence of a little bit far awayfrom the centre is quite contrary to the classical notion of a particle. The third
and perhaps the most unmanageable difficulty is the fact that the quantum wave
packets spread out as they move along. To make things worse, the sharper they
are initially (in space), the faster they tend to spread; a delta-function, which
might be the best analogue of a point-like particle, will spread out to all of space
in no time. It may make sense to represent a microscopic particle as an entity
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Quan tum Counter-Revolut ion 131
having a finite spatial extension, but it would not make sense to represent it as
an entity that assumes a wider and wider spatial extension as it moves along.
Perhaps a better way of translating the classical reasoning is to follow, afterall, the orthodox view on the quantum wave function, usually attributed to
Max Born, which takes it as a distribution of probabilities of detecting a
particle rather than the material smearing of a particle. According to this view,
particles can be as sharply localized as one’s metaphysics allows; in Born’s
words, ‘matter can always be visualized as consisting of point masses’.17 What
the spreading of the wave packet represents, in this view, is indeterminism: if we
start with a reasonably localized wave packet and it ends up in a much wider
spread, that only means that the final location of the particle can be anywhere
within a rather wide region of space. In the context of classically reasoned
experiments this indicates the possibility of random inaccuracies, but that is
only a quantitative problem.
Even with this interpretation, however, the conceptual contradictions do not
all disappear, as there are some basic assumptions contained in classical
descriptions that are fundamentally at odds with quantum mechanics. Another
way of describing this situation is that many classical statements have no
unambiguous quantum-mechanical translations. For a simple example, take the
classical statement that a particle has position x and momentum p at a given
time. If we tried to write down a quantum-mechanical wave function with
definite values of both x and p, we would obtain a self-contradictory state-
ascription. The only reasonable quantum-mechanical interpretation of the
statement involves discarding the classical assumption that those quantities are
sharply defined, and taking x and p as indefinite values. This would result in
ambiguities, however, because nothing in the original classical statement
specifies the statistical distribution that each quantity should have, not to
mention the phase.This is a relatively obvious case, but it is not always so easy to detect the
assumptions contained in classical descriptions that contradict quantum
mechanics. For example, consider the innocent-looking classical statement that
a certain particle has velocity v. Even that contradicts quantum mechanics,
since what we mean classically by velocity is the time-derivative of position,
which does not exist unless the particle has a sharp and continuous trajectory;
that is precisely what quantum mechanics forbids. Or take the specification of
a potential energy function in quantum mechanics. The concept of potentialenergy is inherited from classical mechanics, where it is obtained by integrating
the force function over distance. Not only is there no place for force in the
formalism of quantum mechanics, but the classical notion of force also implies
that it will produce a determinate amount of acceleration when applied to a
“M. Born, Physics in My Generafion (London and New York: Pergamon Press, 1956A p. 9
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132 Studi es i n Hi st ory and Phil osophy of Modern Physics
particle of a certain mass, which is again contradictory to quantum mechanics.
There are similar problems involved in the notions of electric charge and mass,
which are also originally classical. It is only by ignoring these fundamentalconceptual differences that we can even come to make quantitative comparisons
between the two theories.
4. Provisional Phenomenalism
Was the infiltration of experiments by old theory something peculiar to
quantum physics? On the contrary, I want to suggest that such infiltration
would occur as a matter of course in a certain type of scientific change. Imagine
a scientific tradition that is more or less unified, in which the accepted theory
forms a secure basis for experimental reasoning. Suppose that experiments
began to produce anomalous results that did not fit the theory, and the
theoreticians responded by making significant changes in the theory. In such
cases it would be almost inevitable that the old theory, which had served as the
basis of the experiments, should conflict with the new theory, and it would not
be a trivial task to readjust all the elements so that the whole system regains the
coherence that was initially present.
My assessment, given in Sections 2 and 3, was that this task was performed
rather poorly in the case of quantum physics: the conceptual contradictions
remained; the quantitative contradictions also went unaddressed, although they
were ameliorated to a large extent by fortunate coincidences. Now I want to
suggest a more viable strategy of development, applying retrospectively to the
classical-quantum transition, and also generally to similar future occasions for
scientific change. Such unabashed normativism as I am about to engage in has
increasingly been shunned in recent history and philosophy of science, perhaps
for good reasons. I only want to be condi t ional ly normative. In other words,I want to talk about whether a given strategy of scientific development is
conducive to the satisfaction of given goals; this will become clearer in
the concluding section. I believe that this conditional normativism is
entirely compatible both with historical detachment and with epistemological
naturalism.
With that caveat in mind, I want to address the following question: what can
we do if we want to avoid contradictions in the period of transition between two
mutually incompatible theories? The bottom line is that we have to get theexperimental methods away from the old theory, if we want to believe in the
new one as a general physical theory. In the classical-quantum case, this means
getting the experimental methods away from classical mechanics in the
appropriate way. There are serious difficulties, discussed in Section 3, with the
obvious option of reinterpreting the classically reasoned experiments in
quantum-mechanical terms. Another option is to create a brand-new
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Quantum Count er-Revolu t ion 133
experimental tradition, but that would be quite difficult, and may not be
feasible at all.
Instead, I propose that we can make the experimental reasoning unclassicalby taking it away from the level of fundamental theory altogether. Recall the
case of magnetic deflection. The law used in the measurement, relating the
places of the particle’s entry and exit, is normally derived from the Lorentz
force law and Newton’s second law. But it does not have to rest on that
derivation. We could take it as a phenomenological regularity holding just for
the motion of charged particles travelling macroscopic distances in uniform
magnetic fields, with none of the generality of Lorentz’s or Newton’s law. As
Nancy Cartwright has pointed out, ‘8 the empirical truth or reliability of such a
phenomenological law does not rest on its derivability from fundamental
laws. The concepts occurring in the phenomenological law can be understood
operationally for the time being, not relying on precise theoretical definitions
found in classical mechanics. That way the experiment can be interpreted
to be largely independent of classical mechanics, both quantitatively and
conceptually.
In generalized terms, my proposal is that experimenters can make a
temporary retreat to a more phenomenological level of description and
prediction in times of theoretical instability. We can pick secure and well-
understood phenomenological regularities and use them as tools for construct-
ing experiments that explore less familiar phenomena. The phenomenological
regularities would not give us any ‘deep’ theoretical understanding, but provide
a basis for making practical operations. This would be a prudent thing to do
after all if our confidence in the theory is shaky. I am by no means implying that
experiments can be done without relying on any theory at all; I am only
suggesting that they might be done without relying on high-level theory, by
which I mean theory that is fundamental and applicable to a wide range of
phenomena. If the high-level theory becomes stable once again, we can attempt
to reach a deeper theoretical understanding of the experimental set-ups. What
I am advocating can be seen as a sort of ‘phenomenalism’, but only a
provisional one.
Provisionalphenomenalism is in fact not so far away from the strategy that
the quantum physicists actually used. After all, what the experimental practices
relied on was the phenomenological regularities, rather than the fundamental
laws from which they might have been derived. In Section 2, I have noted thatexperimenters often happened to use those parts of classical mechanics that
were quantitatively not too divergent from quantum mechanics. Now I want to
re-conceptualize that observation slightly, to say that the experimenters used
phenomenological regularities whose reliability in fact did not depend on
‘*N. Cartwright, HOW he Law s ofPhysi c s i e (Oxford: Ckendon Press, 1983). esp. Essay 6.
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136 Studi es in Hi stor y and Phil osophy of M odern Physics
like to thank numerous people, especially the following, for discussion and comments on earlier
drafts: Peter Gahson, Nancy Cartwright, John Dupre, Patrick Suppes, Conevery Bolton, Thomas
Kuhn, Jim Woodward, Jerry Handspicker, Jim Antal, Gerald Holton, Cathryn Carson, Michael
Friedman, Maila Walter, Edward Jurkowitz, Jordi Cat, and Sam Schweber. Different versions ofthis paper were given as talks at the following places, where the audiences gave helpful reactions:
Department of Philosophy, Stanford University; Department of the History of Science, Harvard
University; Department of Philosophy, the University of Illinois at Chicago.