The Ontology of Physics (updated)

The question at issue here is, What sort of entities are fundamental in physics?  E.g., in the late nineteenth century through early twentieth centuries,

whether atoms were real objects or only mnemonic devices for coding chemical regularities.

-- Abraham Pais, Subtle is the Lord (1982), p. 80

The idea of atoms in some sense  goes back to Ancient Greece; and today, they are taken for granted.  So it is surprising to laymen, how long opposition to a Realist take on atoms lasted among some philosophers and physicists (e.g. Ernst Mach).  An opposing view, from a leading German chemist:

Ostwald’s ‘Energetik’, according to which molecules and atoms are but mathematical fictions, and energy, in its many forms, the prime physical reality.

-- ibid, p. 83

Good bluff Rutherford, by contrast, had sucked atoms with his mother’s milk, and claimed that he “could see the little buggers as plainly as a spoon”.

Oddly, the debate on this seemingly practical laboratory matter,  had elements more characteristic of political or theological controversy, in which neither side has a prayer of of convincing the other by rational argument:

The most remarkable fact about the nineteenth century debates on atoms and molecules   is the large extent to which chemists and physicists spoke at cross purposes, when they did not actually ignore each other.

-- ibid, p. 80

~

Since that time, a host of new physical building-blocks have been proposed, and in some cases observed: from the unexpected and rather unwelcome muon (I.I. Rabi: “Who ordered that?”), to the massless chargeless neutrino (rather like Bishop Berkeley’s “ghosts of departed quantities”), through quarks, gluons, gravitons & gravitinos, selectrons & electrinos, axions, preons, virtual photons, phonons … such as might make even Rutherford gag.  But several of these are now widely accepted -- not as mere bookkeeping mechanisms, but as entities with further properties to be discovered;  so that the smart money has been on Realism, so far.

Ernest Rutherford,  swallowing an atom  but straining at a quark

Footnote:

We shouldn’t be too hard on old Ostwald  for backing the wrong horse in the Atoms Affair.   Dissident voices today would declare  as epiphenomenal not only atoms, but even the elementary particles of which those are admittedly merely bundles:  demoted to excitation-states of superstrings;  or emerging from the combinatorial-automaton structure of the world. 

Additionally, his Energetik has enjoyed a bit of a revival in some quarters:

  https://de.wikipedia.org/wiki/Energetik_(Philosophie)

More recently, information (or, solemnly, “The Information” -- Wheeler's "It from Bit") has emerged among some as a skeleton-key to everything else in the physical world.

Thus, at the bottom of everything, behind and beyond the Maya of the particle zoo, lies:

Thales:  Water.
Oswald:  Energy.
John Wheeler:  Information.

Stewardess:  Coffee tea or mi-ilk?
The Milesian:   Just water, thanks.

Or, a more recherché candidate, from philosopher Hilary Putnam:

Nothing has more physical significance than spectral measure.

(That might strike the layman as rather a … spectral candidate for the role of firmest substrate of all.)

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The word “ontology” is not one you are likely to overhear at the bus-stop;  nor indeed does a physics-major typically run across it.   But the more we see physics as science (Wissenschaft) rather than a special kind of engineering (the “shut up and calculate” ethos of the years around WWII), the more we meet questions traditionally treated under that rubric -- and indeed, contemporaneously, even under that very name. 

As:

My own position is that the issue of ontology is crucial to quantum mechanics.
-- Roger Penrose,  The Road to Reality (2004), p. 785

Let us now return to the ontology of the consistent-histories approach.  The theory operates with entities called coarse-grained histories.  … The ontological status of the insertion of such a projector set  is still not fully clear. …  A history from a maximally refined set seems to me to provide a strong candidate for what might be regarded as ontologially ‘real’.

-- Penrose,  The Road to Reality (2004), p. 788

Present-day quantum mechanics has no credible ontology … The importance of having an ontologically coherent quantum mechanics cannot be over-estimated.
-- Penrose,  The Road to Reality (2004), p. 860, 865

Re the notion of macroscopic quantum superposition being unproblematic:

This is taking a ‘pragmatic’ stance  that does not really address the ontological issues.

-- Roger Penrose,  The Road to Reality (2004), p. 812

And, full-bore:

Many contemporary thinkers seem to have supposed that, in discarding its mechanist ontology, physics had discarded its ontology:  matter had been dematerialized … The very progress of physics itself  seemed to them to call for the renunciation of mechanism and materialism  in favour of the de-ontologised view of science presented by Mach.

-- John Watkins, Science and Skepticism (1984), p. 138

 

In philosophy proper, ontology is fundamental, being prior to anything else.   In its application to or rather analogue within  physics, by contrast, it historically comes behindhand, as a setting in order of what-all several centuries of reflection and experiment have come up with:  We may think of it as a kind of cast of characters, not fully drawn-up until the play has been written:  in the course of writing it, you find out you need a ladies-maid, and so eventually she is placed upon the prefatory page of Dramatis personae, that typographically precedes the play itself -- and as a nice afterthought, you name her Lisette.  Similarly, particle physics did not begin by being defined, a priori, as (back among the Greeks) the Science of Atoms, or (later) as the Science of the Proton, the Neutron, and the Electron, or (later still-- the Barock Age) as the Menagerie-management of the Particle-zoo (with a fixed given roster of inmates), nor as the Curating of the Wiggling of Strings.  There is a thematic continuity throughout all these stages, but the staffage keeps changing.

As for the role of this Ontology, or Cast of Characters, it is not (despite the spectral example of traditional metaphysics per se) just something to admire from afar, like Mount Rushmore, but rather, as Goedel said pragmatically re which axioms we should adopt for math and logic, they should themselves possess generative potential -- by their fruits ye shall know them.  Thus, hard-headedly:

Kepler’s theoretical ontology, unlike Gilbert’s, was not organically related to his laws;  even if it could be squared with the latter, which seems doubtful, it failed to make any contribution to the testable content of his system.

-- John Watkins, Science and Skepticism (1984), p. 197

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As remarked earlier, I may well go to my grave without ever grasping the concept of an observable, much less physical ontology in general.   Still, it is helpful towards organizing my thoughts, to have an online scribble-space, so that the matter is, so to speak, officially a topic, a project under way.   For now, this is just a whiteboard on which to stow some juicy quotes.  Your own juicy contributions are more than welcome.
For a more general surview of the ontology of the various sciences, click here.

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Physics may be defined as the art of saying things about stuff (or stuff about things -- predications concerning entities, for the fastidious).  But what are these entities, whereof we predicate?  In the first place -- observables.

P.A.M. Dirac, The Principles of Quantum Mechanics (1930; 4^th edn. 1958), p. 116:

From our assumption that the energy is an observable, there are sufficient stationary states for an arbitrary state to be dependent on them.

For a layman, this is bemusing.  The assumption that it’s an observable?   Can you observe it, or can’t you?  -- Evidently there is much more to qualifying as  “an observable” than merely being … observable.
(Compare Einstein, in one of his Zen moments: "It is the theory that decides what we can observe.")

P.A.M. Dirac, The Principles of Quantum Mechanics (4^th edn. 1958), p. 458 (re certain eigenstates):

Science contains many examples of theoretical concepts which are limits of things met with in practice  and are useful for the precise formulation of laws of nature, although they are not realizable experimentally, and this is just one more of them.

Emphasis added.  “Limits” in the mathematical sense.

Note that “not realizable experimentally” does not constitute much of a disability.  What, after all, is?  “Carthage lost the Punic Wars”; “I love you”; “E8 is a 248-dimensional rotation-space”:  no, almost nothing is.

Robert Lindsay & Henry Margenau, Foundations of Physics (1936), p.402:

Quantities such as position, energy, momentum, and the like, capable of measurement… will be called observables,  although it is not intended to imply that they are observable directly.

The caveat is troubling enough;  but now this:

In quantum mechanics, the state of a system is no longer defined by means of a number of variables having an immediate intuitive appeal … In fact, it is not defined in terms of observables at all;  it is simply a function in configuration space.

Carl Hempel, “Problems and Changes in the Empiricist Criterion of Meaning” (1950):

Green, soft, liquid, longer than  designate observable characteristics, while bivalent, radioactive, better electric conductor, and introvert do not.

This odd assertion, by a well-known philosopher of science, seems more psychological than scientific.  It is reminiscent of Locke’s distinction between simple and composite ideas.

Eugen Merzbacher, Quantum Mechanics (1961, 2^nd edn. 1970), p. 153:

Following Dirac, we call observable any Hermitian operator which possesses a complete set of eigenfunctions.

This might sound opaque to some, but for a math guy it’s the clearest statement yet, by far.  Of course, what it amounts to physically, intuitively, is something else…

Gerald Holton, The scientific imagination (1978), p. 202:

The idea of making quantitative indicators of anything at all  fascinates some persons, and repels others as dangerous or absurd.  This difference is caused largely by thematically incompatible -- and therefore often unresolvable -- personal views concerning the ability of quantifiables to lead to … the deepest reality.

Note the silly dichotomy -- as though failing to lead to "the deepest reality" (a deeply suspect term) meant that they couldn't be "indicators of anything at all".

~

I had some fun above, playing with a rumpled old word like stuff, shoving it before the microphone of science.  Here a gifted popularizer  makes similar play  with pronouns:

[In its] Einsteinian reframing … is spacetime a something?

-- Brian Greene, The Fabric of the Cosmos (2004), p. 39

In that historical context, the question concerned the ontological status of (the novelty) ‘spacetime’, as opposed to the traditional notions of the independent entities, space, and time.

(More recently, spacetime has been demoted in some theories -- not returning to a Cartesian product of space and time, but being derived as an epiphenomenon of more fundamental items.  Thus, twistor theory, among others.)

If there is no aether to provide the standard of rest, what is the what  with respect to which this speed is to be interpreted?

-- Brian Greene, The Fabric of the Cosmos (2004), p. 45

If an individual electron is also a wave, what is it that is waving?

-- Brian Greene, The Fabric of the Cosmos (2004), p. 88

(Here the wordplay inheres not in the pronoun what, but in the verb.  He could more conventionally have written, “What is the medium for the wave?”, but the startling verbal formulation ‘makes it strange’, confronting us with something more fundamental.)

~

Stephen Hawking, A Brief History of Time (1988; 2^nd edn. 1996) p. 75:

The fact that confinement prevents one from observing an isolated quark or gluon  might seem to make the whole notion of quarks and gluons as particles   somewhat metaphysical.  However, there is another property of the strong nuclear force, called asymptotic freedom.  The concept of these entities  was already well-defined, or not, as the case may be:  certainly well-defined as bookkeeping conventions, if nothing more.   Asymptotic freedom -- “at high energies, the strong force becomes much weaker, and the quarks and gluons behave almost like free particles” -- simply adds a further mode of observing their effects:  and in this case, their effects when they are relatively ineffectual -- quarks on holiday.

Failure to be observable in isolation certainly doesn't make a thing "metaphysical" (in the colloquial bad sense intended here).  You cannot observe a "brother" in isolation:  dissect him down to his last tissues, nothing will reveal his brotherhood but the historical context.  Nor, perhaps, can you observe Coulomb attraction in a single isolated particle -- it takes two to tangle.  (I might be wrong on this -- the photon cloud and all that.  But how does the cloud tell you whether you've got an attraction or a repulsion?)

Steven Weinberg, Dreams of a Final Theory (1992),  p. 181:

The positivist concentration on observables like particle positions and momenta  has stood in the way of a “realist” interpretation of quantum mechanics, in which the wave function is the representation of physical reality.

Wiki, "Quantum field theory" (excellent article, btw):

In quantum field theory, unlike in quantum mechanics, position is not an observable.

From the point of view of quantum field theory, particles are identical if and only if they are excitations of the same underlying quantum field.  Thus, the question ‘Why are all electrons identical?” arises from mistakenly regarding individual electrons as fundamental objects, when in fact it is only the electron field that is fundamental.

The global phase of the wave function  is arbitrary, and does not represent something physical.

Wiki, "Implicate and explicate order" (of interest only to those who are already devotees of guru-physicist David Bohm):

 Bohm’s paradigm is inherently antithetical to reductionism … and can be regarded as a form of ontological holism.

Wiki, “Introduction to Gauge Theory”:

The electric field and the magnetic field are observable, while the more fundamental electromagnetic potentials V and A  are not.

~

In this ontological context, it is far from clear how the phrase ‘more like’ is to be applied.  Comparison of historical theories gives no sense that their ontologies are approaching a limit:  in some fundamental ways, Einstein’s general relativity resembles Aristotle’s physics more than Newton’s.

-- Thomas Kuhn, in I. Lakatos & A. Musgrave, eds., Criticism and the Growth of Knowledge (1970), p. 265

Cf. too Dirac’s remarks (1951) that the aether concept was ripe for resuscitation.

[Update 8 May 2012] And now this:

http://www.nature.com/news/a-boost-for-quantum-reality-1.10602

The philosophical status of the wavefunction — the entity that determines the probability of different outcomes of measurements on quantum-mechanical particles — would seem to be an unlikely subject for emotional debate. Yet online discussion of a paper claiming to show mathematically that the wavefunction is real has ranged from ardently star-struck to downright vitriolic since the article was first released as a preprint in November 2011.

The paper, thought by some to be one of the most important in quantum foundations in decades, was finally published last week in Nature Physics

They say that the mathematics leaves no doubt that the wavefunction is not just a statistical tool, but rather, a real, objective state of a quantum system.

I told you so...

Physicists reify space-time. They elevate it from a four-dimensional diagram used to record their experience into the kind of “real essence” that Bohr warned us not to seek.
-- David Mermin (March 2014), at:

http://www.nature.com/news/physics-qbism-puts-the-scientist-back-into-science-1.14912?WT.ec_id=NATURE-20140327

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Not the same as the question of the building-blocks (ontological bricks) of physics, but related to it, is that of the Boundaries of Disciplines:  between physics and neighboring fields (chemistry, mathematics, …) and within physics itself (mechanics, astronomy, electromagnetism, condensed-matter, nucleonics, quantum theory, …).   In one sense, the question is idle -- you are working on whatever project you are working on, with methods appropriate thereto, however outsiders might classify them.  But it also has practical consequences, e.g. in the writing of textbooks.  As:

The traditional teaching of thermodynamics and statistical mechanics  as distinct subjects,  has often left students with their knowledge  compartmentalized, and has left them ill-prepared to accept newer ideas such as spin temperature or negative temperature  as legitimate and natural.

-- F. Reif, Fundamentals of statistical and thermal physics (1965), p. viii

That, from the textbook we used in stat mech at Harvard -- in the physics department, though previously I had only met notions of enthalpy, temperature, free energy, and entropy, in a chemistry course.

Similarly, Lindsay & Margenau remark, in their historical overview Foundations of Physics (1936), that they are moving away from treating optics and electrodynamics as distinct disciplines, “the former being, since Mawell’s time, really a branch of the latter.”

~

God’s-truth vs Hocus-pocus:

It is tempting to dismiss these quantum waves  as mathematical contrivances … but in the laboratory these “probability waves” can be manipulated with mirrors …

-- George Johnson,  A Shortcut Through Time (2003), p. 38

~

The prototypical example of an ontological ‘bit’ of chemistry and physics, is the atom (the ‘indivisible’ in its Greek etymology).  But later perspectives can get quite unprototypical:

A neutron star … is basically a giant atomic nucleus, stabilized by gravity.

-- J. Richard Gott, The Cosmic Web (2016), p. 29

.

Zen Physics

A wristwatch worn

by a particle of light

would  not  tick     a-tall.

(-- after Brian Greene; slightly modified, to please the Muse)

Dr. Greene delivers himself of this epigram, in the course of explaining that we

 each of us, all of us,

 even Achilles and the tortoise,

move equally at the speed of light.

To be sure, in a Pickwickian sense (dividing our motion between space and time).

~

Clocks on Earth    tick

a tiny bit

s l o w l y

relative to those in interstellar space.

[ -- Richard Gott, The Cosmic Web (2016), p. 16 ]

The Big Bang itself   is at the bottom

where all the worldlines  converge.

There is no time

before    that …

-- ibid, p. 81

Metaphysical Infrastructure

The widest postulate of rationality is that the world is rationally intelligible throughout… The whole war of the philosophies is over that point of faith.

-- William James,  The Principles of Psychology (1890)

… unverifiable, unfalsifiable, and yet not arbitrary conceptions or hypotheses -- a class to which I have referred as thematic presuppositions … necessary for scientific work

-- Gerald Holton, The scientific imagination (1978), p. 99

[Postmodernism uses] a technique of metatheory (theory about theories), by which scholars analyze  not so much the subject-matter of the scientific discipline  as the cultural and psychological reasons  particular scientists  think the way they do.  The analyst places emphasis on “root metaphors”, those ruling images in the thinker’s mind  by which he designs theory and experiement.

-- Edward O. Wilson, Consilience (1998), p. 42

There is no such thing as philosophy-free science;  there is only science whose philosophical baggage is taken on board without examination.

-- Daniel Dennett, Darwin’s Dangerous Idea (1995)

… Whewell’s views about latent axioms, things which  at first  are not even credible, but which settle down into first principles.

-- Augustus de Morgan

Metaphysics (what we think reality is fundamentally like) can affect our ethics.

-- Edward Craig,  Philosophy: A Very Short Introduction (2002), p.  43

Awhile back, in a ground-breaking article, I proved the existence of coffee-cups.  But before the International Philosophic Society officially crowns this achievement with the name  of Justice’s Theorem (or even Justice’s Great Theorem, in contradistinction to Justice’s Little Theorem, to appear), we must confess that the proof was made possible only by the use of certain major metaphysical assumptions, up to and including the existence of God.  (Indeed, in a slightly strengthened version, known as  “Boshaft-ist-Er-Nicht” -- which still falls well short of God as conceived by Christianity.  So, you can still believe in coffee-cups without being Christian, though it helps.)  Given these assumptions, the actual deduction could have simply been “left as an exercise for the reader.”  So perhaps it should be demoted to, say, Justice’s Corollary, or even David’s Afterthought.

That was, of course, the whole point of the exercise:  to argue that many things we take for granted, and ought to take for granted, are not in fact straightforwardly empirically given, but must be derived with the aid of fundamental metaphysical principles,  which we seldom explicitly acknowledge in daily life, and which it has become the fashion  to deny.

Thus, Paul Davies, a physicist and a Christian, in The Goldilocks Enigma (2006), p. 262:

If there is no coherent scheme of things, then the success of the scientific enterprise to date  is rendered totally enigmatic, and science can be pursued only with a completely unjustified faith  that the methods used hitherto  will continue to uncover reasonlessly existing order  beneath the surface appearance of things.

A “coherent scheme of things” is basically a metaphysical notion.

He goes on to characterize the adaptationist fantasy (which we satirized here), that

brains have evolved to recognize patterns, and that -- again for no reason -- the deep patterns of physics and cosmology  resemble the patterns of the everyday world on our planet (which in fact they mostly don’t).

This all seems rock-solid.  In the satire, I wrenched it up a notch, noting the even greater implausibility of far-flung mathematicians from many different times and cultures  unanimously agreeing on the truth of such things as the Urysohn Metrization Theorem, which are farther from our experience even than quasars or quarks. 

On the other hand, where Davies, a believer, nonetheless has recourse to the phrase “unjustified faith” to belittle a credulous embrace of the scientific endeavor, Cohen & Nagel (secularists) use the same word “faith” (in the sense of naïve faith) to belittle a belief in something very like a “coherent scheme of things”:

The reader may perhaps believe that the inference from the observed conjunction of factors  to an invariable conjunction  is legitimate in virtue of the “uniformity of nature”.  We shall not disturb the reader’s faith in this familiar doctrine  at this point.  But, as we shall see presently, such a faith has no evidential value  in demonstrating the existence of invariable connections.

-- Morris Cohen & Ernest Nagel,  An Introduction to Logic and Scientific Method (1934), p. 254

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Philosophers by profession attempt to dig behind the appearance of things, and to uncover unsuspected incoherencies and hidden assumptions.  People just mowing their lawn, do not.   Midway between these  lie the scientists.

Since the scientific investigation of nature ipso facto strives to get behind the phenomena, its practitioners -- even if not otherwise philosophically inclined -- may be led to confront such assumptions.   Thus, going way back, and down almost to our day (until atomism, Mendelism, and quantum theory put something of a kink in it), is the Principle of Continuity.   Thus Wikipedia (I quote from the German, since the English in this case is a bit lame):

„Natura non facit saltus“ (lateinisch für „Die Natur macht keine Sprünge“) ist eine Grundannahme der antiken Philosophie und Naturwissenschaft seit Aristoteles (bzw. schon seit den Eleaten: altgr. Ἡ φύσις οὐδὲν ποιεῖ ἅλματα.). In dieser Form stammt das Axiom von Carl von Linné (1707-1778) aus seinem Werk Philosophia Botanica (Stockholm, 1751). Der Gedanke wurde später im biologischen und geologischen Gradualismus aufgegriffen.

Mit dem Satz wird ausgedrückt, dass sich Prozesse bzw. Veränderungen in der Natur nicht sprunghaft und plötzlich - diskontinuierlich - vollziehen, sondern prinzipiell kontinuierlich bzw. stetig.

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A magnificent expression of this view was the Great Chain of Being, stretching up from the paramecia, through the Humble Woodchuck, mankind, and the mighty penguin, up to cherubim and archangels and beyond.

This notion is closely related to the Principle of Plenitude, likewise of ancient date:  and likewise leading to startling modern updatings, in the Many-Worlds fantasy of quantum theory, or the Multiverse (currently all the rage.)  Edward Harrison (Cosmology, 2000, p. 268) characterizes it thus:

The principle of plenitude:  everything that is not forbidden is compulsory.  Whatever is possible, must exist.  In science we see that nature is plenitudinous rather than parsimonious.

A stark contrast, then, between the (plenitudinous) objects of inquiry, and our (parsimonious) ideal of explanation.  (More on parsimony here.)

 

Or again,  “Nature abhors a vacuum”.   In the universe of our forefathers, what would otherwise have been the vacuum  was filled up with the ether;  now it is clogged with quantum foam.

Or:  Occam's razor (" lex parsimoniae"), the implement of choice for those seeking Unification.

And, 

According to the rationalistic doctrine, the causal principle is a necessity of thought (Denknotwendigkeit), an a priori regulative principle, hence a presupposition rather than a result of science.  This is the opinion of the Leibnizians, for whom the causal principle is nothing but a form of the principle of sufficient reason.

Mario Bunge, Causality and Modern Science (1959, 1963)

Nonlinear systems do not 'obey' the 'principle' (theorem) of superposition (of forces, displacements, and so forth) ... The 'principle' of superposition may be regarded as the specific form taken in physics  by the hypothesis of the independence of causes.

Mario Bunge, Causality and Modern Science (1959, 1963)

*

Note, though:  metaphysical infrastructure, not metaphysical foundations.   Metaphorically, these principles are like the skeleton in the body, which, though fundamental, is not more important than the blood or the flesh.

It is difficult, indeed, to come up with metaphysical foundations for science out of metaphysics itself.  Thus, consider Kant.  After thinking long and hard, abstractly (none of us could have done any better, within those limits), he managed to convince himself that having extension was of the very essence of physical objects, thus “All bodies are extended” is analytic.   Musing further, he concluded that having weight lay outside of what it meant to be a body, hence “All bodies have weight” is synthetic.

Presumably this last hunch was not a prescient anticipation of massless particles like photons and (perhaps) neutrinos.   In any event, the same arena furnishes examples of bodies that do indeed have rest-mass, but (if indeed they are point-particles) no extension:  electrons.

Reductionism, when its status is not that merely of a convenient method, but of a tenet or article of faith, might be deemed a metaphysical principle.

Edward Wilson, in Consilience (1998), rather stands the matter on its head, since he refers to science itself as metaphysical: “Science offers the boldest metaphysics of the age.”   He would seem to be using the word in some different sense. -- In any event he is not consistent in such use, as he later goes on to warn, more conventionally, against "the pitfalls of metaphysics".

Anyhow:

The prototype of a body is a coffee-cup, with extention, mass, hardness, etc. etc.   Kant’s musings really added nothing to our understanding of these.   Subsequent scientific results did add something:  an object can lack mass, or lack extension, or both, or neither.  Tiens, tiens.   To what extent we assimilate such items to our prototype of a "body", is more a matter of metaphor than of science or philosophy.  How is an electron like a coffee-cup?  like the old riddles of childhood and thereafter (“How is a blonde like a bottle?” -- “They’re both empty from the neck up.”)

*

 

I'm not particularly wedded to this term "metaphysical", b.t.w.; mostly it is just nicely provocative.  What I'm talking about here is related to what Gerald Holton discussed using his favorite terminology of themata.  (I once visited Holton in his office one summer;  he grabbed my lapel and launched into the topic of themata, like the Ancient Mariner.)  Thus ( The scientific imagination (1978), p.20):

This prototype for explanation (classical causal sequences  account for observed accident or disorder) is a thematic commitment.  It is not an experimental or logical necessity.

*

Now for some more recent methodological/metaphysical assumptions of physics.

Edward Harrison, Cosmology (2^nd edn. 2000), p. 454:

We are unable to obtain a model of the universe without some specifically cosmological assumptions that are completely unverifiable. (George Ellis, “Cosmology and Verifiability”, 1975).  The problem is that we observe isotropy, which we cannot explain,  and we assume homogeneity, which we cannot verify.

As you see, working scientists are sometimes acutely aware of such epistemologically awkward states of affairs.   But here’s another one I’ve noticed, and am not too sure that it is widely recognized as a tacit working assumption:

~ Do I contradict myself?  Very well then, I contradict myself.

It’ll all work out in the end. ~

Gerald Holton, in his detailed investigations of the rival Ehrenfest/Millikan experiments in verifying and quantifying the hypothized granularity of electricity ("Subelectrons and Presuppositions", reprinted in The scientific imagination, 1978), refers to the "suspension of disbelief" on the part of an experimenter, faced with the blooming buzzing confusion of his contradictory daily results.  But this epistemological grace period was envisioned as lasting over the course of a series of experiments, not for decades or centuries.

Thus, classical physics predicted that orbiting electrons would continuously radiate away energy -- and thus collapse into the nucleus.  Hence, atoms could not exist.  Nor was there a glimmer of a solution on the horizon.  Physicists just lived with it.  When a solution of sorts did come, with quantum mechanics, it was of an utterly strange and unforeseen sort.

The history of science -- of physics in particular -- is riddled with such intellectually scandalous states of affairs.   And yet the trustful, What-me-worry stance of the everyday scientist  is wise in its way -- it’s that or quit physics (or discover quantum mechanics).    Perhaps, without explicitly saying so, they inwardly believe that the universe is an orderly and rational place, and that our fumbling steps are somehow guided, and that the Plan unfolds in time.   Perhaps they believe  (in their heart of hearts) … in Him.  For if not -- whence the assurance of (A) ?

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Brian Greene, The Fabric of the Cosmos (2004), p. 224:

Physicists have elevated  symmetry principles  to the highest rung on the explanatory ladder. …. Physicists believe these theories are on the right track because, in some hard-to-describe way, they feel right, and ideas of symmetry are essential to this feeling.   … Symmetries are the foundations  from which  laws  spring.”

The manly admission of the role of (most unmanly) “feeling” here, is well in point.  But it is not really subjectivism -- the personal equation -- that is in play here.  Rather, a level of explanation  deeper than that of laws,  which we cannot grasp  with our intellect.

He goes on (p. 243):

Symmetry is the essential consideration  allowing us to comprehend space and time when applied to the unuiverse as a whole.  Without invoking the power of symmetry, we’d be stuck at square one.

Well I recall, many years before, a lecture by Steven Weinberg, in which he mentioned Galileo’s sticking to circular planetary orbit, despite Kepler’s having previously demonstrated their elliptic shape.   The occasion, in most settings, of indignant Whiggish sniffs; but Weinberg wisely -- like a bolt it was -- demurred:

Galileo was perfectly within his rights to privilege such symmetry, it lies at the heart of all subsequent physics.  His error, merely, lay in mistaking the planets, those -- not godlings, as for the pagans, but -- dull lumps of earth, as lying near the center of things.

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The principles lying perdu behind our practice, need not be recondite, but they are metaphysical (as opposed to empirical) nonetheless.  Thus the charming simplicity of the complicated Quine:

The so-called scientific method is a matter of being guided by sensory stimuli, a taste for simplicity in some sense, and a taste for old things.

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I do not intend to suggest a disjunctive dichotomy between “metaphysical” assumptions on the one hand, and empirical matters on the other.  Rather, there is a sort of hierarchy (not necessarily linear) of relatively-specific facts or principles, and relatively more general.  The former will be at the front of consciousness, in practical work;  the point of the present essay is to point to the latter, lest they be forgot.  But often, both levels will clearly lie within (say) physics.

Thus:

The only reason for introducing probabilities in the study of gases  is to obviate the insuperable mathematical difficulties which a direct application of the mechanical laws would involve.  The mechanical laws, however, are utilized indirectly, for it is by their means (and also by accepting the ergodic hypothesis) that the probabilities are computed.

-- A. D’Abro, The Rise of the New Physics (1939), vol. II, p. 945

And, concerning the alleged correlation of disorder and frequency of macrostates, Georgescu-Roegen comments (in The Entropy Law... 1971):

The proposition comes very close to being a metaphysical principle. ... It is this metaphysical essence which accounts for the insuperable difficulties encountered in justifying the new Entropy Law,  either experimentally,  or analytically by derivation from other basic laws.

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Edward O. Wilson, Consilience (1998), p. 198, lists the four metascientific desiderata for any scientific theory:  parsimony; generality; consilience; predictiveness.

It is in point of consilience that he finds Creationism wanting.  “God may exist, He may be delighted with what we are up to on this minor planet, but His fine hand  is not needed to explain the biosphere.”

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Unlike considerations of symmetry or simplicity, ideas of causality do not, I believe, generally guide research as (examined or unexamined) metaphysical assumptions behind the scenes.   Rather, research proceeds on its own terms, and periodically is discovered to have implications for our notions of causality.  So, I will not here comment on that.

In the public and legal arena, by contrast, very practical decisions are influenced by our hazy ideas of causality.  A stab at clarifying a couple of these: here  and  here.

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Miscellaneous quotations on the matter of metaphysical underpinnings (with attendant vocabulary):

Unity of science” … “physical forces”  were not merely directing ideas or hypothesis of scientific endeavor:  they became almost objects of worship.  They were more than methods of research -- they became a Weltanschauung.

-- Ernest Jones, Freud: the Formative Years (1953), p. 43

Gauge invariance is not a physical principle, but a logical principle.

-- Russell Standish, Theory of Nothing (2006; 2nd edn. 2011), p. 121

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The tacit ideational underpinnings need not be strictly metaphysical;  it can be straightforwardly practical:

The real issue in this connection is not the one commonly debated, “What are the criteria for the selection of patients?”, but the underlying, usually unstated assumption, “Criteria for selection are good”.  There are strong theoretical and practical reasons … for saying that  “Selection of patients is not good” … The best policy is to pick patients at random … to increase the heterogeneity of the group.

-- Eric Berne, Group Treatment (1966)

“Tacit principle” cf. “pious assumption/lip-service”:

At present, the only statement that can be made is that it is helpful for a group therapist to have had personal experience as a patient in a therapy group.  But even that   is only a pious assumption, since  in fact  it has never been critically evaluated, and so still remains part of the lore of institutional group therapy.

-- Eric Berne, Group Treatment (1966)

[Bonus quotes]

Fundamental to knowledge are certain “analytical hypotheses” that go beyond the evidence.

-- Noam Chomsky, “Quine’s Empirical Assumptions”,  in:  D. Davidson & J. Hintikka, eds. Words and Objections (1969, 1975), p. 60

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