Quantum Enigma
Physics
Encounters Consciousness
Second
Edition
Bruce
Rosenblum and Fred Kuttner
Chapter
7 (partial)
The
Two-Slit Experiment: The Observer Problem
[The two-slit experiment] contains the only mystery. We
cannot make the mystery go away by “explaining” how it works …
In telling you how it works we will have told you about the basic
peculiarities of all quantum mechanics. —Richard Feynman
In this chapter, we go for rigor in
presenting the quantum enigma. In the rest of the book, we more
loosely ponder what it all might mean.
The two-slit experiment, the
archetypal demonstration of quantum phenomena, displays physics
encounter with consciousness. Quoting Feynman above, “We cannot
make the mystery go away….” But we’ll tell how it works.
The
two-slit experiment is, in part, an interference experiment. We
described interference for light waves in chapter 4 . Interference
has been demonstrated with photons, electrons, atoms, and large
molecules, and is being attempted with yet bigger things.
Demonstrating interference with photons is an easy classroom
demonstration. The slits can be two lines scratched on an opaque
film. Shining a laser pointer through the slits, you can display a
clear interference pattern. Electron interference is not so easy, but
you can buy a dramatic classroom demonstration apparatus for a few
thousand dollars. Demonstrating interference with atoms or molecules
is trickier and far more expensive. But it’s basically the same
idea. Since electrons or atoms would collide with air molecules,
interference with objects other than photons must be displayed in a
container from which the air is removed, but we’ll not worry about
such technical “details.”
Since the quantum mystery is the same
in every case, and since talk presents no budget problems, we will
talk in terms of atoms. Today we can see individual atoms, even pick
them up and put them down one at a time. We first briefly describe
the standard version of the two-slit experiment. We then follow it
with a completely equivalent version that contrasts nicely with the
shell game of chapter 6 .
In describing the interference of
light waves in chapter 4 , we noted that to get a well-defined
interference pattern, the light should be of a single color. That
means light of a narrow range of frequencies and wavelengths. The
same applies for atoms. The atoms should all have essentially the
same de Broglie wavelength, which just means they should all come
with the same speed.
Figure 7.1 Top: The two-slit
diaphragm. Bottom: Edge-on view of atom source, two-slit diaphragm,
and detection screen with atoms in interference pattern.
Our slits
are the two openings as shown in figure 7.1 . You send in atoms from
the left. Coming through the slits, they land on a screen to the
right, which we show in figure 7.2 . (We don’t care about any atoms
that fail to come through the slits.)
You record where on the screen
the atoms land. They land only in certain regions. The distribution
of atoms yields the pattern shown in figure 7.2 . (It’s the same as
our figure 5.7 for light waves.)
Figure 7.2 Interference pattern formed
by atoms coming through two narrow slits
The pattern, an interference
pattern, comes about, as with any waves, because each atom’s
wavefunction comes through both slits. At some places on the screen,
crests from the top slit arrive together with crests from the bottom
slit. Waves from the two slits then add to produce regions of large
waviness. Elsewhere, crests from one slit arrive with troughs from
the other to cancel, producing regions of zero waviness. The waviness
someplace is the probability of finding an atom there. You thus find
regions where many atoms have hit and regions where few atoms have
hit. In the “orthodox” Copenhagen interpretation of what’s
going on, the wavefunction of each atom collapsed at the particular
point where it hit, where it was “observed” by the macroscopic
screen.
Since each atom’s wavefunction followed a rule that
depended on the spacing of the slits, something of each atom must
have come through both slits. Quantum theory has no atom in addition
to the wavefunction of the atom. Accordingly, each atom itself must
have been a spread-out thing coming through both of the
well-separated slits.
Figure 7.3 Distribution of atoms
coming through a single narrow slit
However, you could have done this
experiment with only one slit open. Each atom’s wavefunction could
then have come through only a single one of the narrow slits. You
still find atoms landing on the screen. There could, of course, be no
interference because each atom’s wavefunction came through only a
single slit. But since each atom’s wavefunction came through a
single narrow slit, each atom is established to be a compact thing, a
particle. The atoms fall in the uniform distribution shown in figure
7.3 .
You thus could choose to demonstrate, with both slits open,
that atoms are spread-out things. Or, with only a single slit open,
you could choose to demonstrate the opposite, that atoms are compact
particles. This is, of course, the wave-particle paradox discussed
for de Broglie waves in chapter 5 . We just told the story for atoms
in terms of today’s quantum theory.
Our
Box-Pairs Version of the Two-Slit Experiment
Here’s
a completely equivalent version of the two-slit experiment in which
you can choose to show that an object, an atom, for example, was
wholly in a single box. But you could have chosen to show that that
same atom was not wholly in a single box. Telling the story with
atoms captured in boxes, you can decide at your leisure which
contradictory situation you wish to demonstrate. This way of telling
the story more dramatically displays the quantum challenge to our
commonsense intuition that an observer-independent physical reality
exists “out there.” We’ll refer to our box pairs again— and
again —in future chapters. So we tell it carefully here.
Aristotle
taught that to discover Nature’s laws one should start with the
simplest examples, and from them move on to greater generalizations.
Galileo accepted that injunction, but warned that we must rely on
only what is experimentally demonstrable, even if the results violate
our deepest intuitions. Considering the idealized behavior of
isolated objects, the moon, the planets, and apples, Newton
formulated his universal equation of motion. The two-slit experiment,
the simplest display of quantum phenomena, follows this path. We
carefully treat our box-pairs version with atoms.
In
the shell game of chapter 6 , the pea had equal probability for being
under each shell. Probability was not the complete description of the
physical situation. There was also an actual pea definitely under one
shell or the other. Observation did not change that physical
situation. We will put equal parts of the waviness of a single atom
in each of two boxes, so that the atom has equal probability for
being in either box. But, unlike the shell game, there is no “actual
atom” in a particular box. The wavefunction divided into both boxes
is the complete description of the physical situation. And here,
unlike the shell game, observation does change the physical
situation.
To
display the quantum enigma, it is not necessary to tell how our box
pairs are prepared. However, since we’ve already spoken of
wavefunctions, we will describe the preparation. After that, however,
we will display the quantum enigma telling only what you would
actually see. We will describe the box-pairs experiment, without
mentioning quantum theory, or wavefunctions, without even mentioning
waves.
Here’s
how the atoms were put into the box pairs. Any wave can be reflected.
A semitransparent mirror reflects part of a wave and allows the rest
to go through. A glass windowpane, for example, allows some light
through and reflects some. At the glass, the wavefunction of each
individual photon splits. Part of the photon wavefunction is
reflected and part is transmitted. We can also have a semitransparent
mirror for atoms. It splits an atom’s wavefunction into two wave
packets. One packet goes through, and another is reflected.
The arrangement of mirrors and boxes in figure 7.4 allows for the
trapping of the two parts of an atom’s wavefunction in a pair of
boxes. We send in a single atom at a known speed and close the doors
of the boxes when the wavefunction packets are inside the boxes.
After that, each part of the wavefunction bounces back and forth in
its box. In figure 7.4 we show the wavefunction and waviness at three
successive times.
Figure
7.4 Mirror and box-pair setup allowing the trapping of a wavefunction
in a pair of boxes. An atom’s wavefunction is shown at three
different times.
We
know there is one, and only one, atom in each box pair because we
observed an atom and sent one into each box pair. These days, with
the proper tools, we can see and deal with individual atoms and
molecules. With a scanning tunneling microscope, for example, we can
pick up and put down, single atoms.
Holding
an atom in a box without disturbing its wavefunction would be tricky,
but it’s certainly doable. Dividing the wavefunction of an atom
into well-separated regions is accomplished in every actual
interference experiment with atoms. Capturing the atoms in physical
boxes is actually not needed for our demonstration. A defined region
of space would be enough. We like to think of each region defined by
a box because it’s more like the shell game. We can then consider
the atom sitting there waiting for us to choose what to do with it,
rather than have the atom zip through a two-slit diaphragm on its way
to the detection screen.
From
here on, the description of our box-pairs experiment will not mention
wavefunctions, or waves at all. We just tell what you would actually
see. We describe quantum-theory-neutral observations. By doing this
we emphasize that the quantum enigma arises directly from
experimental observations. The existence of the quantum enigma does
not depend on the quantum theory !
The
“Interference Experiment”
You
are presented with large number of box pairs. (They were prepared as
we described above, but for the demonstration of the enigma, you need
not know anything of the preparation.) Position a box pair in front
of a screen on which an impacting atom would stick. Open a narrow
slit in each box, at about the same time. An atom hits the screen.
Repeat this with many identically positioned box pairs. You find that
atoms cluster in some regions of the screen, but avoid other regions.
The pattern is the same as that shown previously in figure 7.2 for a
pair of slits. Each atom followed a rule allowing it to land in
certain regions and forbidding it from landing in other regions.
Figure
7.5 Interference patterns formed by atoms coming through two narrow
slits with different slit separations
Now
repeat this procedure with a new set of box pairs. This time have a
different spacing between the boxes of each pair. You find the
regions where the atoms clustered are spaced differently. The larger
the spacing between the boxes of a pair, the smaller is the spacing
between the places where atoms land. We illustrate this with figure
7.5 . Each and every atom followed a rule that depends on the spacing
of its box pair. Each atom therefore had to “know” its box-pair
spacing.
Clearly,
the experiment we just described is an interference experiment, like
the two-slit experiment, and we’ll now call it an “interference
experiment.” But we did not use any property of waves. Something of
each atom had to come from each box because where atoms landed
depended on the box-pair spacing. This interference experiment
establishes that each atom had been a spread-out thing, in both boxes
of its pair. (Nothing done outside the boxes while the atom is still
inside has any effect at all.)
What
explains a whole atom appearing on the screen while some of it had to
come from each box of its pair? Might it not make sense to say that
part of each atom was in each box? In that case, part of the atom
emerged from each box of its pair, and then congealed to the spot on
the screen where you found it. That reasonable-sounding idea doesn’t
work. Here’s why:
The “Which Box?” Experiment
Instead
of doing the experiment by opening the slits in the boxes of a pair
at the same time, choose a different experiment: Open a slit in one
box, and then later , open a slit in the other box. Opening one box,
you sometimes find a whole atom impacts the screen. If so, when you
open the other box of that pair, nothing comes out. If you open a box
and nothing appears on the screen, then, for sure, an atom will
appear on the screen when you open the second box. Repeatedly opening
boxes of a set of box pairs one box at a time, you determine which
box the whole atom was in. You demonstrate that there had been a
whole atom in one box, and that the other box of that pair contained
nothing . With atoms wholly in a single box, box spacing would not be
relevant. Indeed, you find a uniform distribution of atoms hitting
the screen, as previously seen in figure 7.3 for the case of a single
slit being open.
There’s
a more direct way to establish that each atom was wholly in a single
box. Simply look in a box to see which box held the atom. It doesn’t
matter how you look. You can, for example, shine an appropriate light
beam into the box and see a glint from the atom. About half the time
you will find a whole atom in the looked-in box; about half the time
you find the box empty. If there is no atom in the box you first look
in, it will always be in the other. If you find an atom in one box,
the other box of its pair will be totally empty . No observation, of
any kind, would find anything at all in that empty box. This
“which-box?” experiment, or “look-in-the-box” experiment,
establishes that each atom was concentrated in a single box of its
pair, that it was not spread out over both boxes.
But
before you looked, you could have done an interference experiment
establishing that something of each atom had been in both boxes. You
therefore could choose to prove either that each atom had been wholly
in a single box, or you could choose to prove that each atom had not
been wholly in a single box. You can choose to prove either of two
contradictory situations.
The
ability to prove either of two contradictory results is puzzling.
Wanting to explore further, some have asked: “What if you do both
experiments with the same atoms? What if you open the box pairs at
the same time, in order to get an interference pattern, but also look
to see which box each atom came out of.” Such looking is
essentially a which-box experiment. Absolutely anything you do that
allows you to know which box the atom was in defeats the atom’s
ability to obey
the rule giving an interference pattern.
Seeking
a loophole, a logician might note that the interference experiment
relies on circumstantial evidence. It uses one fact, the interference
pattern, to establish another fact, that each atom came out of both
boxes. This is true of any interference experiment. Finding no other
reasonable explanation, physics universally accepts interference as
establishing spread-out waviness. As in our legal system,
circumstantial evidence can establish a conclusion beyond a
reasonable doubt.
A
theory leading to a logical contradiction is necessarily an incorrect
theory. Does the ability to demonstrate either of two contradictory
things about atoms (and other objects) invalidate quantum theory? No.
You did not demonstrate the contradiction with exactly the same
things. You did the two experiments with different atoms.
The
Quantum Enigma
Here’s
a logically conceivable explanation of your ability to prove either
of two contradictory things: Those box pairs for which you chose an
interference experiment actually contained objects spread out over
both boxes, not wholly in a single box. And those box pairs for which
you chose a which-box experiment actually contained compact objects
wholly in a single box. How could you establish otherwise?
You
reject this explanation. You reject it because you know that, given a
set of box pairs, you could have made either choice. You freely chose
which experiment to do. You have free will. At least your choices
were not predetermined by a physical situation external to your body,
by what was supposedly “actually” in the box pairs.
Did
your free choice determine the external physical situation? Or did
the external physical situation predetermine your choice? Either way,
it doesn’t make sense. It’s the unresolved quantum enigma.
Important
point: We experience an enigma because we believe that we could have
done other than what we actually did. A denial of this freedom of
choice requires our behavior to be programmed to correlate with the
world external to our bodies. The quantum enigma arises from our
conscious perception of free will. This mystery connecting
consciousness with the physical world displays physics’ encounter
with consciousness.
History
Creation
To
a certain extent at least, our present actions obviously determine
the future. But obviously, our present actions cannot determine the
past . The past is the “unchangeable truth of history.” Or is it?
Finding
an atom in a single box means the whole atom came to that box on a
particular single path after its earlier encounter with the
semi-transparent mirror. Choosing an interference experiment would
establish a different history: that aspects of the atom came on two
paths to both boxes after its earlier encounter with the
semi-transparent mirror.
The
creation of past history is even more counterintuitive than the
creation of a present situation. Nevertheless, that’s what the
box-pairs experiment, or any version of the two-slit experiment,
implies. Quantum theory has any observation creating its relevant
history.
The
Quantum Theory Description
Now
that we have established the experimental basis of the enigma, we
offer quantum theory’s explanation. Since we can choose to observe
an atom to be in either of two contradictory situations, how does
quantum theory describe the state of the atom before we observe it?
The theory describes the world in mathematical terms. In those terms,
when an atom can be observed in either of two contradictory
situations, or “states,” the wavefunction of the total physical
situation is written as the sum of the wavefunctions of those two
states separately. Expressing this mathematics in words, the
wavefunction of one of the states is “the-atom-is-wholly-in-the-top
-box.” The wavefunction of the other state is
“the-atom-is-wholly-in-the-bot tom-box.” The wavefunction of the
unobserved atom is “the-atom-is-wholly-in-the-top -box” plus
“the-atom-is-wholly-in-the-bot tom-box.” The atom is said to be
in a “superposition” of these two states. It is simultaneously in
both states. On looking in a box, this sum, or superposition, collapses randomly to one or the other term of the superposition. But before
we look, the atom is simultaneously in both boxes. The atom is in two
places at once.
Observation
collapses the waviness, the probability, to a specific actuality. But
what constitutes an “observation”? Observation is ultimately not
explained within quantum theory. What constitutes observation is
controversial. The pragmatic Copenhagen interpretation of quantum
mechanics, physics’ “orthodox” position (more fully discussed
in chapter 10 ) defines any recording of a microscopic event by a
macroscopic measuring instrument as an observation. Or, more
strictly, any interaction of a microscopic system with a macroscopic
system constitutes an observation if it would make a demonstration of
interference essentially impossible. Not all physicists accept this
for-all-practical-purposes interpretation of observation. We leave
the issue, for now.
Chapter
16 (partial)
The
Mystery of Consciousness
What
is meant by consciousness we need not discuss; it is beyond all
doubt. —Sigmund Freud
Consciousness
poses the most baffling problems in the science of the mind. There is
nothing that we know more intimately than conscious experience, but
there is nothing that is harder to explain. —David Chalmers
Does
consciousness collapse wavefunctions? That question, raised at the
beginning of quantum theory, cannot be answered. It can’t even be
well posed. Consciousness itself is a mystery.
When
we described the experimentally demonstrated quantum facts and the
quantum theory explaining those facts (as distinct from the theory’s
several contending interpretations), we presented the undisputed
consensus of the physics community. We cannot describe such a
consensus in our discussion of consciousness. There is none. There
is, of course, a large amount of undisputed experimental data, but
diametrically opposed explanations of that data are strongly held. We
have our own take, but, you may notice, we waver.
Until
the 1960s, behaviorist-dominated psychology avoided the term
“consciousness” in any discussion that presumed to be scientific.
There has since been an explosion of interest in consciousness. Some
attribute this to the striking developments in brain imaging
technology that allow seeing which parts of the brain become active
with particular stimuli. But according to an editor of the Journal of
Consciousness Studies :
It
is more likely that the re-emergence of consciousness studies
occurred for sociological reasons: The students of the 1960s, who
enjoyed a rich extra-curricular approach to “consciousness studies”
(even if some of them didn’t inhale), are now running the science
departments.
Interest
in the foundations of quantum mechanics grows at the same time as
does interest in consciousness. And connections are seriously
proposed. There’s something in the air.
What
Is Consciousness?
We’ve
talked about consciousness but never clearly defined it. Dictionary
definitions of “consciousness” are little better than those for
“physics.” We’ve been using “consciousness” as roughly
equivalent to “awareness.” For us, “consciousness” most
definitely includes the perception of free choice by the
experimenter. This use of “consciousness” is that quite standard
in the treatment of the quantum measurement problem. Ultimately, a
definition is manifest by the use of the term. (As Humpty Dumpty told
Alice: “When I use a word … it means just what I choose it to
mean,” and the philosopher Wittgenstein, who taught that a word is
defined by its use, would more or less agree.)
One
can know of the existence of consciousness in no other way than
through our first-person feeling of awareness, or the second-person
reports of others. (In our following chapter we suggest an apparent
quantum challenge to this limitation.)
We
do not discuss many of the things found in treatments of
consciousness from a psychological point of view. We do not, for
example, talk about optical illusions, mental disturbance,
self-consciousness, or Freud's seat of hidden emotions, the
unconscious.
Our
concern is with that “consciousness” related to the observer's
free choice of experiment, the consciousness that physics encounters.
Our
frequent example of physics’ encounter with consciousness is the
decision to observe an object in a single box causing it to be wholly
there. We say “causing” only because the observer presumably
could have chosen to do an interference observation establishing a
contradictory situation—that the object was not wholly in a single
box. The observer could have, we assume, chosen to establish that the
object was a wave simultaneously in two boxes.
Does
such a demonstration necessarily require a conscious observer?
Couldn’t a not-conscious mechanical robot, or even a Geiger
counter, do the observing? It depends on what you mean by
“observing.” For now, just recall that if that robot or Geiger
counter were isolated from the rest of the world, and was governed by
quantum theory, it would merely become entangled as part of a total
superposition state, as did Schrödinger’s cat. In that sense, it
would not observe .
The
quantum enigma arises from the assumption that experimenters can
freely choose between two experiments, two experiments that yield
contradictory results. We assume that the experimenters had the “free
will” to make that choice. However, we can’t evade the quantum
enigma by denying the free will of the experimenters, that is, merely
by having their choices somehow determined by the electrochemistry of
their brains. To evade the quantum enigma, the required denial of
free will must go much further. It must include the denial of
counterfactual definiteness. That denial must include the assumption
of a “conspiratorial” world. (In our example, the experimenter’s
“choices” would have to match the physical situation in the box
pairs.)
Today’s
discussions of free will in psychology or neurophysiology usually
focus more narrowly on whether the choices we make are somehow
predetermined by the electrochemistry of our brain. This free will
issue is therefore peripheral to the quantum enigma. But “free
will” constantly comes up in connection with the quantum enigma.
Free
Will
Problems
with free will arise in several contexts. Here’s an old one: Since
God is omnipotent, it might seem unfair that we be held responsible
for anything we do. God, after all, had control. Medieval theologians
resolved this issue by deciding that every train of events starts
with a “remote efficient cause” and ends with a “final cause,”
both in God’s hands. Causes in between come about through our free
choices, for which we will be held accountable on judgment day.
This
medieval concern is not completely remote from that of today’s
philosophers of morality. Similarly, criminal defense lawyers can
make the concern practical by arguing that the defendant’s actions
were determined by genetics and environment rather than by free will.
We, however, will deal with a more straightforward free-will issue.
Classical
physics, Newtonian physics, is completely deterministic. An
“all-seeing eye,” viewing the situation of the universe at one
time, can know its entire future. If classical physics applied to
everything , there would be no place for free will.
However,
free will can happily coexist with classical physics. In chapter 3 on
the Newtonian worldview, we told how physics, in days gone by, could
stop at the boundary of the human body, or certainly at the then
completely mysterious brain. Scientists could dismiss free will as
not their concern and leave it to the philosophers and theologians.
That
dismissal does not come so easily today as scientists study the
operation of the brain, its electrochemistry, and its response to
stimuli. They deal with the brain as a physical object whose behavior
is governed by physical laws. Free will does not fit readily into
that picture. It lurks as a specter off in a corner.
Most
neurophysiologists and psychologists tacitly ignore that corner. Some
though, taking a physical model to apply broadly, deny that free will
exists and claim that our perception of free will is an illusion. The
controversy this creates will be right up front when we soon discuss
the “hard problem” of consciousness.
How
could you demonstrate the existence of free will? Perhaps all we have
is our own feeling of free will and the claim of free will that
others make. If no demonstration is at all possible, perhaps the
existence of free will is meaningless. Here’s a counter to that
argument: Though you can’t demonstrate your feeling of pain to
someone else, you know it exists, and it’s certainly not
meaningless.
A
famous free-will experiment has generated fierce argument. In the
early 1980s, Benjamin Libet had his subjects flex their wrist at a
time of their choice, but without forethought. He determined the
order of three critical times: the time of the “readiness
potential,” a voltage that can be detected with electrodes on the
scalp almost a second before any voluntary action actually occurs;
the time of the wrist flexing; and the time the subjects reported
that they had made their decision to flex (by watching a fast-moving
clock).
One
might expect the order to be (1) decision, (2) readiness potential,
(3) action. In fact, the readiness potential preceded the reported
decision time. Does this show that some deterministic function in the
brain brought about the supposedly free decision? Some, not
necessarily Libet, do argue this way. But the times involved are
fractions of a second, and the meaning of the reported decision time
is hard to evaluate. Moreover, since the wrist action is supposed to
be initiated without any “preplanning,” the experimental result
seems, at best, ambiguous evidence against conscious free will.
In
2008, John-Dylan Haynes went beyond fractions of a second. He and his
colleagues monitored neural activity with functional magnetic
resonance imaging (fMRI). As letters appeared on a screen in front of
them, subjects were asked to push the button in their right hand or
the one in their left whenever they felt like it, or randomly. They
then reported the letter they saw when they decided which button to
push. From the fMRI signal, the researchers could predict seventy
percent of the time (guessing works fifty percent of the time) the
button that would be pushed, as much as ten seconds before the
reported decision time. Haynes commented: “This doesn’t rule out
free will, but it does make it implausible.”
Does
it really? Presumably, if a subject were told during that ten-second
interval, “You are going to push the left-hand button,” they
could still freely choose to push the right-hand button. Being able
to roughly predict someone’s behavior from an fMRI does not
seriously challenge their free will. Predicting behavior from facial
expression also works quite well.
Belief
in our free will arises from our conscious perception that we make
choices between possible alternatives. If free will is just an
illusion, and we’re all just sophisticated robots controlled by our
neurochemistry with perhaps a bit of thermal randomness, is our
consciousness then also an illusion? (If so, what is it that is
having that illusion?)
Though
it is hard to fit free will into our usual scientific worldview, we
cannot, ourselves, with any seriousness, doubt it. J. A. Hobson’s
comment seems apt to us: “Those of us with common sense are amazed
at the resistance put up by psychologists, physiologists, and
philosophers to the obvious reality of free will.”
If
you’re going to deny free will, stopping at the electrochemistry of
the brain is arbitrary. After all, the motivation for suggesting such
denial is the Newtonian determinism of classical physics. Being
logically consistent, and thus accepting that reasoning all the way,
we come to the completely deterministic world where the “all-seeing
eye” can know the entire future of everything, including our
experimenters’ supposedly-free choice leading to the quantum
enigma.
Unlike
arbitrarily stopping at the electrochemistry of the brain, accepting
complete determinism does evade the quantum enigma. For most of us,
being “robots” in a completely deterministic world is too much to
swallow. However, accepting both free will and the undisputed quantum
experiments, we come to the quantum enigma. And to quantum theory for
an explanation.
And
quantum theory, unlike classical physics, is not a theory of the
physical world independent of the experimenters’ freely made
decisions, their free will. According to John Bell:
It has turned out that quantum mechanics cannot be “completed”
into a locally causal theory, at least as long as one allows …
freely operating experimenters.
Before
Bell’s theorem, “free will”–or an explicit assumption of
“freely operating experimenters”–was not something seen in a
book about physics. It was certainly not seen in a serious physics
journal . That’s of course changing. In December 2010, for example,
the prestigious journal Physical Review Letters published a
calculation of precisely how much free will would have to be given up
to account for the correlations observed by freely operating
experimenters performing twin-state photon experiments. It’s 14%.
What that means in human terms is not clear.
Let’s explore observation by Bell’s “freely operating
experimenters.” Recall Pascual Jordan’s defining statement of the
Copenhagen interpretation, the working physicist’s interpretation:
“Observations not only disturb what is to be measured, they produce
it.” “Observation” here is an open-ended term, but the creation
of physical reality by any kind of observation is hard to accept.
However, it’s not a new notion.
From
Berkeley to Behaviorism
The
idea of physical reality being created by its observation goes back
thousands of years to Vedic philosophy, but we skip ahead to the
eighteenth century. In the wake of Newton’s mechanics, the
materialist view that all that exists is matter governed by
mechanical forces gained wide acceptance. Not everybody was happy
with it.
The
idealist philosopher George Berkeley saw Newtonian thinking as
demeaning our status as freely choosing moral beings. Classical
physics seemed to leave little room for God, and that appalled him.
He was, after all, a bishop. (It was common in those days for English
academics to be ordained as Anglican priests, though the celibacy of
Newton’s day was no longer required. Berkeley married.)
Berkeley rejected materialism with the motto esse est percipi , “to
be is to be perceived,” meaning all that exists is created by its
observation. To the old question, “If a tree falls in the forest
with no one around to hear it fall, is there any sound?” Berkeley’s
answer would presumably be that there wasn’t even a tree were it
not observed.
Though
Berkeley’s almost solipsistic stance may seem a bit batty, many
idealist philosophers of his day were enthusiastic about it. Not so
Samuel Johnson, who supposedly responded by kicking a stone, stubbing
his toe, and declaring, “I refute him thus!” Stone kicking made
little impression on those partial to Berkeley’s thinking, which
is, of course, impossible to disprove.
God
may be omnipotent but he is not omniscient. If God’s observation
collapses the wavefunctions of large things to reality, quantum
experiments indicate that He is not observing the small.
The
idea that the world around us was being created by its observation
never took hold. Most practical people, surely most scientists of the
eighteenth century, considered the world to be made up of solid
little particles, which some called “atoms.” These were presumed
to obey mechanical laws much as did those larger particles, the
planets. While physical scientists might speculate about the mind,
and some used hydraulic pictures for it instead of today’s computer
models, for the most part they ignored it.
In
the nineteenth and much of the twentieth century, scientific thinking
was generally equated with materialist thinking. Even in psychology
departments, consciousness did not warrant serious study. Behaviorism
became the dominant view. People were to be studied as “black
boxes” that received stimuli as input and provided behaviors as
output. Correlating the behaviors with the stimuli was all that
science needed to say about what goes on inside. If you knew the
behavior corresponding to every stimulus, you would know all there is
to know about the mind.
The
behaviorist approach had success in revealing how people respond and,
in some sense, why they act as they do. But it did not even address
the internal state, the feeling of conscious awareness and the making
of apparently free choices. According to behaviorism’s leading
spokesman, B. F. Skinner, the assumption of a conscious free will was
unscientific. But with the rise of humanistic psychology in the
latter part of the twentieth century, behaviorist ideas seemed
sterile.
The “Hard Problem” of Consciousness
Behaviorism
had waned when, in the early 1990s, David Chalmers, a young
Australian philosopher, shook up the study of consciousness by
identifying the “hard problem” of consciousness. In a nutshell,
the hard problem is that of explaining how the biological brain
generates the subjective, inner world of
experience
. Chalmers’s “easy problems” include such things as the
reaction to stimuli and the reportability of mental states, and all
the rest of consciousness studies. Chalmers does not imply that his
easy problems are easy in any absolute sense. They are easy only
relative to the hard problem. Our present interest in the hard
problem of consciousness, or awareness, or experience, arises from
its apparent similarity (and connection?) to the hard problem of
quantum mechanics, the problem of observation.
Before
going on about the hard problem and the heated arguments it continues
to generate, a bit about David Chalmers: As an undergraduate student,
he studied physics and mathematics and did graduate work in
mathematics before switching to philosophy. Though it is not central
to his argument, Chalmers considers quantum mechanics likely relevant
to consciousness. The last chapter of his landmark book, The
Conscious Mind , is titled “The Interpretation of Quantum
Mechanics.” David Chalmers was a faculty colleague at the
University of California, Santa Cruz, in the philosophy department,
before he (to our regret) moved to the University of Arizona to
become a director of the Center for Consciousness Studies. He is, at
the time of this writing, back in his native Australia as the
director of the Centre for Consciousness at Australia National
University.
Chalmers’s easy problems often involve the correlation of neural
activity with physical aspects of consciousness, the “neural
correlates of consciousness.” Brain-imaging technology today allows
the detailed visualization of metabolic activity inside the thinking,
feeling brain and has stimulated fascinating studies of thought
processes.
Exploration
of what goes on inside the brain is not new. Neurosurgeons have long
correlated electrical activity and electrical stimulation with
reports of conscious perception by placing electrodes directly on the
exposed brain. This is done largely for therapeutic purposes, of
course, and scientific experimentation is limited.
Electroencephalography (EEG), the detection of electrical potentials
on the scalp, is even older. EEG can rapidly detect neuronal activity
but can’t tell much about where in the brain the activity is taking
place.
Positron
emission tomography (PET) is better at finding out just where in the
brain neurons are firing. Here, radioactive atoms, of oxygen for
example, are injected into the blood stream. Radiation detectors and
computer analysis can determine where there is an increase in
metabolic activity, and can correlate this call for more oxygen with
reports of conscious perceptions.
The
most spectacular brain imaging technology is functional magnetic
resonance imaging (fMRI). It is better than PET at localizing
activity and involves no radiation. (The examined head must, however,
be held still in a large, usually noisy, magnet.) MRI is the medical
imaging technology we described in chapter 9 as one of the practical
applications of quantum mechanics. fMRI can identify the part of the
brain that is using more oxygen during a particular brain function
responding to an external stimulus.
fMRI can correlate a brain region with the neural process involved
in, say, memory, speech, vision, or reported awareness. The
computer-generated, false-color brain images produced can display
just which regions in a brain require more blood when someone thinks,
say, of food or feels pain. Like any technique based on metabolic
activity, fMRI is not fast.
Is
the physical brain that these techniques observe, presumably all
there is to the brain, also all there is to the mind? While the work
today relating neural electrochemistry to consciousness may be
rudimentary, just suppose that improved fMRI, or some future
technology, could completely identify particular brain activations
with certain conscious experiences. This would correlate all
(reported) conscious feelings with metabolic activity, and perhaps
even with the underlying electrochemical phenomena. Such a complete
set of the neural correlates of consciousness is the ultimate goal of
much of today’s consciousness research involving the brain.
Were
this goal actually achieved, some say we would have accomplished all
that can be accomplished. Consciousness, they claim, would be
completely explained because there is nothing to it beyond the neural
activity we correlate with the experiences we call “consciousness.”
If we take apart an old pendulum clock and see how the swinging
weight driven by a spring moves the gears, we can learn all there is
to know about the workings of the clock. The claim here is that
consciousness will be similarly explained by our learning all about
the neurons making up the brain.
Francis
Crick, physicist co-discoverer of the DNA double helix, who turned
brain scientist, looked for the “awareness neuron.” For him, our
subjective experience, our consciousness, is nothing but the activity
of such neurons. His book The Astonishing Hypothesis identifies that
hypothesis:
“You,”
your joys and sorrows, your memories and your ambitions, your sense
of personal identity and free will, are in fact no more than the
behavior of a vast assembly of nerve cells and their associated
molecules.
If
so, the intuition that our consciousness and free will are
experiences beyond the mere functioning of electrons and molecules in
our brain is an illusion. Consciousness should therefore ultimately
have a reductionist explanation. It should, in principle at least, be
completely describable in terms of simpler entities, the neural
correlates of consciousness. Subjective feelings thus supposedly
“emerge” from the electrochemistry of neurons. This is akin to
the readily accepted idea that the surface tension or “wetness”
of water emerges from the interaction of hydrogen and oxygen atoms
forming contiguous molecules of water.
Such
emergence forms Crick’s “astonishing hypothesis.” Is it really
so astonishing? We suspect that, to most physicists at least, it
would seem a most natural guess.
Crick’s
long-time younger collaborator, Christof Koch, takes a more nuanced
approach: Given the centrality of subjective feelings to everyday
life, it would require extra ordinary factual evidence before
concluding that qualia and feelings are illusory. The provisional
approach I take is to consider first-person experiences as brute
facts of life and seek to explain them.
In
a slightly different context, Koch further balances different views:
While
I cannot rule out that explaining consciousness may require
fundamentally new laws, I currently see no pressing need for such a
step.
…
[But] [t]he characters of brain states and of phenomenal states [
experienced states] appear too different to be completely reducible
to each other. I suspect that their relationship is more complex than
traditionally envisioned.
David
Chalmers, a principal spokesperson for a point of view diametrically
opposite to Crick’s, sees explaining consciousness purely in terms
of its neural correlates to be impossible . At best, Chalmers
maintains, such theories tell us something about the physical role
consciousness may play, but those physical theories don’t tell us
how consciousness arises:
For
any physical process we specify there will be an unanswered question:
Why should this process give rise to [conscious] experience? Given
any such process, it is conceptually coherent that it could cccccc
[exist] in the absence of experience. It follows that no mere account
of physical process will tell us why experience arises. The emergence
of experience goes beyond what can be derived from physical theory.
While
atomic theory might reductively explain the wetness of water and why
it clings to your finger, that’s a far cry from explaining your
feeling of its wetness. Chalmers, denying the possibility of any
reductive explanation of consciousness, suggests that a theory of
consciousness should take experience as a primary entity alongside
mass, charge, and space-time. He suggests that this new fundamental
property would entail new fundamental laws, which he calls
“psychophysical principles.”
Chalmers
goes on to speculate on these principles. The one he considers basic,
and the one most interesting to us, leads to a “natural hypothesis:
that information (at least some information) has two basic aspects, a
physical aspect and a phenomenal aspect.” This postulate of a
dualism recalls the situation in quantum mechanics, where the
wavefunction also has two aspects: On the one hand, it is the total
physical reality of an object, while on the other hand, that reality,
some have conjectured, is purely “information” (whatever that
means).
To
argue that conscious experience goes beyond intellectual knowing,
some tell the story of Mary. Mary is a scientist of the future who
knows everything there is to know about the perception of color. But
Mary has never been outside a room where everything is black or
white. One day she is shown something red. For the first time, Mary
experiences red. Her experience of red is something beyond her
complete knowledge of red. Or is it? You can no doubt generate for
yourself the pro and con arguments the Mary story provokes.
Philosopher
Daniel Dennett in his widely quoted book Consciousness Explained ,
describes the brain’s dealing with information as a process where
“multiple drafts” undergo constant editing, coalescing at times
to produce experience. Dennett denies the existence of a “hard
problem,” considering it a form of mind–brain dualism. He claims
to refute it by arguing:
No
physical energy or mass is associated with them [the signals from the
mind to the brain]. How then do they make a difference to what
happens in the brain cells they must affect, if the mind is to have
any influence over the body? … This confrontation between quite
standard physics and dualism is widely regarded as the inescapable
and fatal flaw of dualism.
Since Chalmers argues that consciousness obeys principles beyond
standard physics, it is not clear that an argument based on “quite
standard physics” can be a refutation of Chalmers. Moreover,
there’s a quantum loophole in Dennett’s argument: No mass or
energy is necessarily required to determine to which of the set of
possible states a wavefunction will collapse upon observation.
Our
own concern with the hard problem of consciousness arises, of course,
because physics has encountered consciousness in the quantum enigma,
which physicists call the “measurement problem.” Here, aspects of
physical observation come close to those of conscious experience. In
both cases, something beyond the normal treatment, of physics, or of
psychology, appears to be needed for a solution.
The
essential nature of the measurement problem in quantum mechanics has
been in dispute since the inception of the quantum theory. Similarly,
ever since consciousness has become scientifically discussed in
psychology and philosophy, its essential nature has been in dispute.
An example of the rather extreme divergence of opinion appeared in
2005 in the New York Times , where some leading scientists were asked
to state their beliefs. According to cognitive scientist Donald
Hoffman:
I
believe that consciousness and its contents are all that exists.
Space-time, matter and fields never were the fundamental denizens of
the universe but have always been, from their beginning, among the
humbler contents of consciousness, dependent on it for their very
being.
Psychologist
Nicholas Humphrey sees it differently:
I
believe that human consciousness is a conjuring trick, designed to
fool us into thinking we are in the presence of an inexplicable
mystery.
