Stephen M. Barr
Professor of Physics
at the University of Delaware.
Not
in any direct way. That is, it doesn’t provide an argument for the
existence of God. But it does so indirectly, by providing an
argument against the philosophy called materialism (or
“physicalism”), which is the main intellectual opponent of belief
in God in today’s world.
Materialism
is an atheistic philosophy that says that all of reality is reducible
to matter and its interactions. It has gained ground because many
people think that it’s supported by science. They think that
physics has shown the material world to be a closed system of cause
and effect, sealed off from the influence of any non-physical
realities — if any there be. Since our minds and thoughts obviously
do affect the physical world, it would follow that they are
themselves merely physical phenomena. No room for a spiritual soul or
free will: for materialists we are just “machines made of meat.”
Quantum
mechanics, however, throws a monkey wrench into this simple
mechanical view of things. No less a figure than Eugene Wigner, a
Nobel Prize winner in physics, claimed that materialism — at least
with regard to the human mind — is not “logically consistent with
present quantum mechanics.” And on the basis of quantum mechanics,
Sir Rudolf Peierls, another great 20th-century physicist, said, “the
premise that you can describe in terms of physics the whole function
of a human being … including [his] knowledge, and [his]
consciousness, is untenable. There is still something missing.”
How,
one might ask, can quantum mechanics have anything to say about the
human mind? Isn’t it about things that can be physically measured,
such as particles and forces? It is; but while minds cannot be
measured, it is ultimately minds that do the measuring. And that, as
we shall see, is a fact that cannot be ignored in trying to make
sense of quantum mechanics. If one claims that it is possible (in
principle) to give a complete physical description of what goes on
during a measurement — including the mind of the person who is
doing the measuring — one is led into severe difficulties. This was
pointed out in the 1930s by the great mathematician John von Neumann.
Though I cannot go into technicalities in an essay such as this, I
will try to sketch the argument.
It
all begins with the fact that quantum mechanics is inherently
probabilistic. Of course, even in “classical physics” (i.e. the
physics that preceded quantum mechanics and that still is adequate
for many purposes) one sometimes uses probabilities; but one wouldn’t
have to if one had enough information. Quantum mechanics is
radically different: it says that even if one had complete
information about the state of a physical system, the laws of physics
would typically only predict probabilities of future outcomes. These
probabilities are encoded in something called the “wavefunction”
of the system.
A
familiar example of this is the idea of “half-life.” Radioactive
nuclei are liable to “decay” into smaller nuclei and other
particles. If a certain type of nucleus has a half-life of, say, an
hour, it means that a nucleus of that type has a 50% chance of
decaying within 1 hour, a 75% chance within two hours, and so on. The
quantum mechanical equations do not (and cannot) tell you when a
particular nucleus will decay, only the probability of it doing so as
a function of time. This is not something peculiar to nuclei. The
principles of quantum mechanics apply to all physical systems, and
those principles are inherently and inescapably probabilistic.
This
is where the problem begins. It is a paradoxical (but entirely
logical) fact that a probability only makes sense if it is the
probability of something definite. For example, to say that Jane has
a 70% chance of passing the French exam only means something if at
some point she takes the exam and gets a definite grade. At that
point, the probability of her passing no longer remains 70%, but
suddenly jumps to 100% (if she passes) or 0% (if she fails). In other
words, probabilities of events that lie in between 0 and 100% must at
some point jump to 0 or 100% or else they meant nothing in the first
place.
This
raises a thorny issue for quantum mechanics. The master equation that
governs how wavefunctions change with time (the “Schrödinger
equation”) does not yield probabilities that suddenly jump to 0 or
100%, but rather ones that vary smoothly and that generally remain
greater than 0 and less than 100%. Radioactive nuclei are a good
example. The Schrödinger equation says that the “survival
probability” of a nucleus (i.e. the probability of its not having
decayed) starts off at 100%, and then falls continuously, reaching
50% after one half-life, 25% after two half-lives, and so on — but
never reaching zero. In other words, the Schrödinger equation only
gives probabilities of decaying, never an actual decay! (If there
were an actual decay, the survival probability should jump to 0 at
that point.)
To
recap: (a) Probabilities in quantum mechanics must be the
probabilities of definite events. (b) When definite events happen,
some probabilities should jump to 0 or 100%. However, (c) the
mathematics that describes all physical processes (the Schrödinger
equation) does not describe such jumps. One begins to see how one
might reach the conclusion that not everything that happens is a
physical process describable by the equations of physics.
So
how do minds enter the picture? The traditional understanding is
that the “definite events” whose probabilities one calculates in
quantum mechanics are the outcomes of “measurements” or
“observations” (the words are used interchangeably). If someone
(traditionally called “the observer”) checks to see if, say, a
nucleus has decayed (perhaps using a Geiger counter), he or she must
get a definite answer: yes or no. Obviously, at that point the
probability of the nucleus having decayed (or survived) should jump
to 0 or 100%, because the observer then knows the result with
certainty. This is just common sense. The probabilities assigned to
events refer to someone’s state of knowledge: before I know the
outcome of Jane’s exam I can only say that she has a 70% chance of
passing; whereas after I know I must say either 0 or 100%.
Thus,
the traditional view is that the probabilities in quantum mechanics —
and hence the “wavefunction” that encodes them — refer to the
state of knowledge of some “observer”. (In the words of the
famous physicist Sir James Jeans, wavefunctions are “knowledge
waves.”) An observer’s knowledge — and hence the wavefunction
that encodes it — makes a discontinuous jump when he/she comes to
know the outcome of a measurement (the famous “quantum jump”,
traditionally called the “collapse of the wave function”). But
the Schrödinger equations that describe any physical process do not
give such jumps! So something must be involved when knowledge
changes besides physical processes.
An
obvious question is why one needs to talk about knowledge and minds
at all. Couldn’t an inanimate physical device (say, a Geiger
counter) carry out a “measurement”? That would run into the very
problem pointed out by von Neumann: If the “observer” were just a
purely physical entity, such as a Geiger counter, one could in
principle write down a bigger wavefunction that described not only
the thing being measured but also the observer. And, when calculated
with the Schrödinger equation, that bigger wave function would not
jump! Again: as long as only purely physical entities are involved,
they are governed by an equation that says that the probabilities
don’t jump.
That’s
why, when Peierls was asked whether a machine could be an “observer,”
he said no, explaining that “the quantum mechanical description is
in terms of knowledge, and knowledge requires somebody who knows.”
Not a purely physical thing, but a mind.
But
what if one refuses to accept this conclusion, and maintains that
only physical entities exist and that all observers and their minds
are entirely describable by the equations of physics? Then the
quantum probabilities remain in limbo, not 0 and 100% (in general)
but hovering somewhere in between. They never get resolved into
unique and definite outcomes, but somehow all possibilities remain
always in play. One would thus be forced into what is called the
“Many Worlds Interpretation” (MWI) of quantum mechanics.
In
MWI, reality is divided into many branches corresponding to all the
possible outcomes of all physical situations. If a probability was
70% before a measurement, it doesn’t jump to 0 or 100%; it stays
70% after the measurement, because in 70% of the branches there’s
one result and in 30% there’s the other result! For example, in
some branches of reality a particular nucleus has decayed — and
“you” observe that it has, while in other branches it has not
decayed — and “you” observe that it has not. (There are
versions of “you” in every branch.) In the Many Worlds picture,
you exist in a virtually infinite number of versions: in some
branches of reality you are reading this article, in others you are
asleep in bed, in others you have never been born. Even proponents of
the Many Worlds idea admit that it sounds crazy and strains
credulity.
The
upshot is this: If the mathematics of quantum mechanics is right (as
most fundamental physicists believe), and if materialism is right,
one is forced to accept the Many Worlds Interpretation of quantum
mechanics. And that is awfully heavy baggage for materialism to
carry.
If,
on the other hand, we accept the more traditional understanding of
quantum mechanics that goes back to von Neumann, one is led by its
logic (as Wigner and Peierls were) to the conclusion that not
everything is just matter in motion, and that in particular there is
something about the human mind that transcends matter and its laws.
It then becomes possible to take seriously certain questions that
materialism had ruled out of court: If the human mind transcends
matter to some extent, could there not exist minds that transcend the
physical universe altogether? And might there not even exist an
ultimate Mind?
Discussion
Summary
In
my BQO essay I sought to explain in non-technical language the main
issues that lead to different “interpretations” of quantum
mechanics, and why they present a choice between the anti-materialist
implications of the traditional Copenhagen interpretation and the
bizarre and (for most people) incredible implications of the Many
Worlds Interpretation (MWI).
Another
way to frame the argument is in terms of the “ontological status”
of wavefunctions. The most obvious thing is to think of a
wavefunction as simply a straightforward description of “the world
as it is”. But that is equivalent to the MWI, because generally
speaking the wavefunction of a system contains a large number of
branches in which the system behaves in different ways. The
alternative view (adopted in the Copenhagen interpretation) is that a
wavefunction is not an account of the world as it is, but of some
observer’s state of knowledge of the world. That interpretation
brings knowledge (and therefore mind) into the discussion as
something that is as fundamental as matter, because wavefunctions
themselves are fundamental to our understanding of the world.
These
seem to be the only viable choices if one accepts the present
formalism of quantum mechanics. The third possibility, then, is to
say that this formalism needs to be changed somehow. Several
commentators on my article either mentioned or proposed approaches to
modifying quantum mechanics. Two well-known approaches that they
mentioned were “hidden variables theories” and “objective
collapse theories”. Certainly, science can never be absolutely
certain that it has arrived at a final and complete description of
the physical world; so it will always remain a possibility that
present quantum mechanics is incomplete and that a modification of
its formalism will resolve all its puzzles, paradoxes, and
conundrums. However, as I explained in my responses, the present
formalism of quantum mechanics has been spectacularly successful
since the 1920’s in describing with astonishing precision a vast
range of phenomena. It seems less likely than it once did that it
will have to be modified. (Many, including one commentator on my
essay, have seen the difficulty of combining present quantum
mechanics with Einstein’s theory of gravity as evidence that the
former will have to be modified. Superstring theory shows, however,
that Einstein’s theory can be consistently “quantized”.)
Not
only does the traditional interpretation of quantum mechanics have
anti-materialist implications, but, as noted by more than one
commentator, it is compatible with philosophical idealism (which
makes mind the only fundamental reality) and even with solipsism
(which makes one’s own mind the only reality). If we reject such
extremes and take the view that both matter and mind are fundamental
and equally real, then a question arises: how are they related to
each other?
Connected
with this question is a common objection made to the Copenhagen
interpretation, which is that it seems to say that the minds of
“observers” have a spooky influence on the physical world. In
particular, the idea that “wavefunction collapse” is caused by
“observations” seems to suggest that human knowers actually cause
events to happen by knowing them. As one commentator pointed out,
this would be a radical reversal of traditional ideas about the
relation of knower and known. Indeed, the traditional interpretation
of quantum mechanics strikes many people as “subjectivist”. It
is questionable whether the Copenhagen Interpretation really has
these “anti-realist” implications. At the very least, however, it
makes problematic the question of what is “really going on when no
one is looking”. In other words, if the wavefunction of a system
is not a straightforward description of “the world as it is,” but
just of some observer’s knowledge of it — then what WOULD a
description of the world as it is look like? This is, indeed, an
awkward question for the traditional interpretation to deal with —
though various attempts have been made to answer it. A second
problem connected with the relation between mind and matter, raised
by another commentator to my essay, is how the relation between mind
and brain is to be thought of. This is quite mysterious at present.
A third question is what kinds of minds qualify as “observers” in
the Copenhagen interpretation of quantum mechanics. One commentator
asked why it would have to be only human minds. The answer is that
it wouldn’t, but it seems it would have to be a mind capable of
knowing the results of measurements and one not entirely describable
by physics. Purely physical devices or inanimate things such as atoms
and bacteria would not seem to qualify, whereas humans certainly do.
Somewhere in between there is a line, but where it should be drawn is
debatable.
It
has long been suggested that the special role of mind in quantum
mechanics as traditionally interpreted, together with the
indeterminacy of the theory, may provide an opening for free will to
operate in the world. Though I did not discuss this in my essay,
several commentators did. Again, this is a murky and much debated
subject.
Altogether,
it is clear that quantum mechanics raises more philosophical
questions that it solves.
Here
are two “Big Questions” that it raises.
Two
New Big Questions:
-
Does quantum indeterminacy provide an opening for free will?
-
In the context of the traditional Copenhagen interpretation of quantum mechanics, what would a complete description of the world through time, apart from what any “observer” knows about it, look like?