Perhaps
there isn't enough written by Niels Bohr to secure a faithful
interpretation of his philosophy of quantum physics. That's despite
the fact that Bohr did say a lot more than many other physicists on philosophical issues.
In
more precise terms, Bohr never published a paper on the philosophy
of quantum mechanics. (Though, clearly, he published much on physics
itself - sometimes with philosophical interjections.)That's not a
surprise: Bohr was a physicist, not a philosopher. So, again, because
of that lack of technical philosophical detail, and philosophical
argument, it's no wonder that philosophers argue about what exactly
Bohr's philosophical position was. It's also worth noting that many
of his philosophical remarks on quantum mechanics came after he'd
done his important and relevant work in physics. (His published philosophical positions came largely after the
1920s - and sometimes a lot later than that.)
Thus
it may also seem odd that there are books on “Bohr's philosophy”.
And we also have books actually of (rather than about)
Bohr's philosophy - such as The
Philosophical Writings of Niels Bohr (in
three volumes). Nonetheless, the problems I've just
highlighted are summed up by one reviewer of this book. He
writes:
“Niels
Bohr's view of the world is always something to keep in mind. But
this book's title is misleading: if you're trying to understand how
the (back then) new discoveries and theories in physics may have
affected and /or influenced Niels Bohr from a non-theoretical,
philosophical point of view, you won't find it here.”
To sum up Bohr's philosophical position. I would say that
“subjectivist” is a far better term than “idealist” to sum up
Bohr's philosophy of quantum physics. And, in turn, “anti-realist”
is far better than “subjectivist”. The term “idealism”, for
one, comes with far too much philosophical and historical baggage to
affix it to Bohr's
positions. In addition, it's clear that Bohr fused subjectivism
with intersubjectivism. (The latter position is something that the
logical positivist Rudolf Carnap had to take on board in order to
escape from the possible “solipsism” of his 1928
Aufbau position.) Indeed, as a physicist, Bohr could
hardly not have fused subjectivism and intersubjectivism.
***************************************
i)
Niels Bohr's Ant-Realism?
Although
I'm shoehorning terms from late-20th-century analytic philosophy into physics here, it
can be said that Niels Bohr perfectly expressed an
anti-realist position on physics (without ever actually using the
term “anti-realism”) in the following:
“Physics
is not about how the world is, it is about what we can say about the
world.”
And
in his The
Unity of Human Knowledge (1958-1962), Bohr offers us a
slightly more detailed anti-realist account of physics:
“Physics
is to be regarded not so much as the study of something a
priori
given, but rather as the development of methods of ordering and
surveying human experience.”
Of
course Bohr wasn't alone. For example, the
Swiss-American theoretical physicist, Wolfgang Pauli, went further
when he rejected the opposition (i.e., between reality itself and
what we can can know about reality) entirely when he
stated the following
(as
quoted by N. David Mermin):
“One should no
more rack one's brain about the problem of whether something one
cannot know anything about exists all the same, than about the
ancient question of how many angels are able to sit on the point of a
needle.”
In
other words, “how Nature is” amounts to no more than a metaphysician's dream. All we
have is “what we can say about Nature”. And, at the
quantum-mechanical level, what we can say is what we can say with
mathematics. Consequently, just about everything else is analogical
and/or imagistic. Indeed the analogical stuff can (or does) often
mislead us. And perhaps that's also partly the source of quantum
mechanic's "weirdness".
Despite
all that, the quotes above don't in any way suggest that (as the
philosopher Michael J. Loux puts it about idealism and anti-realism) physicists will “we
make it all up”.
(Or, as some say about postmodernism, that “anything
goes”.)
And neither does it commit physicists to idealism. What it does tell us is that
we don't have something “a
priori given”:
what is given is mediated through “human experience”. How can it
be otherwise? Nonetheless, not all of this need to be entirely
“subjective” either - it is often intersubjective.
As Bohr himself puts
it:
“In
this respect our task must be to account for such experience in a
manner independent of individual subjective judgement and therefore
objective in the sense that it can be unambiguously communicated in
ordinary human language.”
This
means that even if a multitude of different minds come to state
various things about x, they're still (at least roughly)
saying similar things about x. So, yes, minds matter. But that doesn't mean that anything goes or that physicists make it all
up. After all, if a multitude of minds agree that the earth isn't
flat, then surely that doesn't also mean that all those minds make
that fact up.
It's true that Niels Bohr did seem to argue that our
experimental results didn't reflect a reality which existed
independently of our measurements. Yet surely if there were
a different reality, then there would be different results. This
isn't of course to say that there's a perfect mirroring of that
reality with our results. Though there is a mind-independent reality
which somehow determines or causes our experimental results. This, of
course, isn't the claim that our theories or terms perfectly mirror
(or mirror at all) that mind-independent reality. The claim is
very simple and somewhat obvious: the mind-independent world can't
be factored out.
So
Bohr was wrong to say that nothing exists until it is measured.
However, in a strong sense, it may as well not do. That is, we
can say very little about that x as it is before it's measured
or experimented upon. In that sense, it serves almost no purpose. Yet
it still has causal effects on what we say. It's still an x
which has specific results on our experiments and measurements. Thus
if that x were substituted with y, then we'd have
different experimental results and different measurements. In that
sense, the noumenon (to use a Kantian term) that is x
is obviously of vital importance.
In
the same vain. Simply because our measurements and experiments
produce changes in what's been measured, that doesn't also mean that
what's been measured is somehow factored out. And neither does it
mean that there's no relation between what's being measured and what
we say about what's being measured. Again, there will be no
perfect mirroring because such a notion hardly makes sense.
However, there may be very strong relations between what we
say and what we measure. Those relations may be symmetrical,
isomorphic, etc. in nature. There may also be various correspondences
(though not mirrorings) or modelings of various kinds.
Bohr
himself went further and into detail on this.
For
example, he claimed that the spin of an electron or the momentum of
an atom aren't things which reflect what is the case
mind-independently. It's true that there's (obviously) some level of
contingency in our descriptions and experimental results. However,
the fact that physicists have stuck with words like
“spin” and “momentum” (as well as the precise measurements of
spin and momentum) must mean that they're getting at least something
right.
Bohr
also gave an explicitly anti-essentialist position when he
said that
“our
description of nature the purpose is not to disclose the real essence
of the phenomena but only to track down, as far as possible,
relations between the manifold aspects of our experience”.
Nonetheless,
Bohr does (again) seem to needlessly play down the world and,
correspondingly, play up mind or “our experience”.
For
one, we can deny “real essence” yet also
play down the idealist,
subjectivist or phenomenalist implications of Bohr's words. After
all, when it comes to elementary particles, the distinction between essential and contingent properties is hard to maintain in the sense
that besides spatial, temporal and relational properties, properties such as spin, mass and energy are the only properties particles have.
So, for example, when you give the spin or mass of an electron, you're not
factoring out a whole host of contingent properties of that electron in
one's descriptions.
The
subjectivist import of Bohr's words is also apparent when he said
that we
“only
to track down, as far as possible, relations between the manifold
aspects of our experience".
Why
not say, instead, that we should track down the relations between the
“manifold aspects of experience” and what (causally) gives rise
to that experience? It's true that this is a difficult ontological
and epistemological nut to crack. However, it's a nut which
exists. Thus if it's literally all about the manifold
aspects of our experiences, then why talk about “experiments”,
“observations” or “electrons” at all? Why not give a purely
phenomenological account of the specific experiences of physicists at
particular times? Indeed why single out what physicists say in the
first place – why not ask sociologists or fishermen?
Subjectivism
vs. Intersubjectivism
Despite
all the above, Bohr himself notes the problem with over-stressing what
he calls the “subjective element”. Or at least he does so when it
came to Albert Einstein's relativity theory. Here is an
example:
“Today
we know that 'simultaneity' contains a subjective element, inasmuch
as two events that appear simultaneous to an observer at rest are not
necessarily simultaneous to an observer in motion.”
The
above is the “subjective element”. What follows is what we can
call the intersubjective element of exactly the same
situation:
“However,
the relativistic description is also objective inasmuch as every
observer can deduce by calculation what the other observer will
perceive or has perceived.”
Still,
Bohr ends on a non-classical note when
he says that
“[f]or
all that, we have come a long way from the classical ideal of
objective descriptions”.
And
Bohr makes roughly the same point elsewhere when he says that
“every
physical process may be said to have objective and subjective
features”.
He
continued:
“Admittedly,
even in our future encounters with reality we shall have to
distinguish between the objective and the subjective side, to make a
division between the two. But the location of the separation may
depend on the way things are looked at; to a certain extent it can be
chosen at will.”
To
put all that in more basic terms: it's clear that Bohr's position (or
positions) could never be deemed to be entirely subjectivist.
Indeed isn't that obvious? Though, as we can see in the quotes above, there
is indeed an element of subjectivism (or experientialism) in Bohr's
statements.
******************************
ii)
The Case of Waves & Particles
As
Bohr
put it, the words “wave” and “particle” are “classical
terms”:
“However
far the phenomena transcend the scope of classical physical
explanation, the account of all evidence must be expressed in
classical terms.”
Bohr
extends what he says about classical terms and refers to “what
we have done” in our “experiments”. He
wrote:
“The
argument is that simply by the word 'experiment' we refer
to a situation where we can tell others what we have done and what we
have learned and that, therefore, the account of the experimental
arrangement and of the results of the observations must be expressed
in unambiguous language with suitable application of the terminology
of classical physics.”
In
a certain sense, we have nothing more than classical terms because
even when we attempt to describe the “strange” goings-on
of the quantum world, we still use classical terms. Why? Because
there's nothing else we can use other than the mathematics.
Mathematics
has just been mentioned. Bohr makes it clear that only mathematics
gives us a true of picture of the quantum realm. He
wrote:
"We
must be clear that when it comes to atoms, language can be used only
as in poetry. The poet, too, is not nearly so concerned with
describing facts as with creating images and establishing mental
connections.”
This,
again, is a roundabout of saying that only the mathematics is
adequate when it comes to describing sub-atomic phenomena. Thus, by
this definition, the words “particle” and “wave” simply can't
do the full job. They can do part of the job; though not the
full job. Indeed it's not just the word “particle”
that's problematic, thinking of particles as things is too.
Bohr wote:
"Isolated
material particles are abstractions, their properties being definable
and observable only through their interaction with other systems."
In
other words, it can't possibly be about the world “as it is in
itself”. Every statement we make about the world comes with a lot
of contingent baggage. There is of course a causal set of relations
to any x – though those very same relations can be described in an
indefinite number of ways.
So
when
Louis de Broglie argued
(in 1924)
that
that every moving particle (yes, particle)
can be equally described as either a wave or a particle, that may
well be because this x
is neither
a wave or a particle. Nonetheless, describing x
as a wave or a particle still helps both physicists and ourselves.
It's
also the case that the words “particle” and “wave” carry far
too much baggage. After all, the original wave-particle experiments
had water
waves
in mind. Is that a good thing when it comes to talk about things
happening at the subatomic level? Yes, it is if it helps us get of
grip of things. The same goes for the word “particle”. Thus there
are a host of good reasons as to why x should
be seen as a particle. Yet there are also a host of equally good
reasons as to why we shouldn't
see it as a particle.
For
instance, is a photon a particle? Well, they don't have mass for a
start. And that's partly why the physicist Willis Lamb said:
“At
the first of the 1960's Rochester Coherence Conferences, I suggested
that a license be required for use of the word photon, and offered to
give such license to properly qualified people.”
Lamb
then went on to say that “[t]here are very good substitute words
for 'photon' (e.g., 'radiation' or 'light')”.
Bohr
himself argued that the reality behind our measurements and
experiments is that there is neither
a
particle nor a wave. Indeed linguistic practicalities led me to want
to say: The
reality behind the particle/wave measurements and experiments is that
there is neither a particle nor a wave.
But we must still talk about something.
However,
that
something
isn't necessarily a thing
as such (this is grammar speaking here). And we give that x
(which
doesn't need to be a thing,
only a something)
the
name “wave” or “particle”.
