chang, hasok (1995) the quantum counter-revolution; internal conflicts in scientific change

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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

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121



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@’



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.



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.



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.



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



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.



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



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



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



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.



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.

chang, hasok (1995) the quantum counter-revolution; internal conflicts in scientific change

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