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
-- 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 imporance of having an ontologically coherent quantum mechanics cannot be over-estimated.
-- Penrose, The Road to Reality (2004), p. 860, 865
-- 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
~ ~ ~ ~ ~
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.
~ ~ ~ ~ ~
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; 4th 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.")
(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 (4th 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, 2nd 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; 2nd 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:
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:
-- David Mermin (March 2014), at:
.
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