The wave function exists in the map, not in the territory.
That is not an uncontroversial fact. For instance, Roger Penrose, from the Emperor’s New Mind
OBJECTIVITY AND MEASURABILITY OF QUANTUM STATES
Despite the fact that we are normally only provided with probabilities for the outcome of an experiment, there seems to
be something objective about a quantum-mechanical state. It is often asserted that the state-vector is merely a
convenient description of ‘our knowledge’ concerning a physical system or, perhaps, that the state-vector does not really
describe a single system but merely provides probability information about an ‘ensemble’ of a large number of similarly
prepared systems. Such sentiments strike me as unreasonably timid concerning what quantum mechanics has to tell us
about the actuality of the physical world.
Some of this caution, or doubt, concerning the ‘physical reality’ of state-vectors appears to spring from the fact that
what is physically measurable is strictly limited, according to theory. Let us consider an electron’s state of spin, as
described above. Suppose that the spin-state happens to be |a), but we do not know this; that is, we do not know the
direction a in which the electron is supposed to be spinning. Can we determine this direction by measurement? No, we
cannot. The best that we can do is extract ‘one bit’ of information that is, the answer to a single yes no question. We
may select some direction P in space and measure the electron’s spin in that direction. We get either the answer YES or
NO, but thereafter, we have lost the information about the original direction of spin. With a YES answer we know that
the state is now proportional to |p), and with a NO answer we know that the state is now in the direction opposite to p.
In neither case does this tell us the direction a of the state before measurement, but merely gives some probability
information about a.
On the other hand, there would seem to be something completely objective about the direction a itself, in which the
electron ‘happened to be spinning’ before the measurement was made For we might have chosen to measure the
electron’s spin in the direction a -and the electron has to be prepared to give the answer YES, with certainty, if we
happened to have guessed right in this way! Somehow, the ‘information’ that the electron must actually give this
answer is stored in the electron’s state of spin.
It seems to me that we must make a distinction between what is ‘objective’ and what is ‘measurable’ in discussing the
question of physical reality, according to quantum mechanics. The state- vector of a system is, indeed, not measurable,
in the sense that one cannot ascertain, by experiments performed on the system, This objectivity is a feature of our
taking the standard quantum-mechanical formalism seriously. In a non-standard viewpoint, the system might actually
‘know’, ahead of time, the result that it would give to any measurement. This could leave us with a different, apparently
objective, picture of physical reality.
precisely (up to proportionality) what that state is; but the state vector does seem to be (again up to proportionality) a
completely objective property of the system, being completely characterized by the results that it must give to
experiments that one might perform.
In the case of a single spin-one-half panicle, such as an electron, this objectivity is not unreasonable because it merely
asserts that there is some direction in which the electron’s spin is precisely defined, even though we may not know what
that direction is.
(However, we shall be seeing later that this ‘objective’ picture is much stranger with more complicated systems- even
for a system which consists merely of a pair of spin-one-half particles. ) But need the electron’s spin have any
physically defined state at all before it is measured? In many cases it will not have, because it cannot be considered as
a quantum system on its own; instead, the quantum state roust generally be taken as describing an electron inextricably
entangled with a large number of other particles.
In particular circumstances, however, the electron (at least as regards its spin) can be considered on its own. In such
circumstances, such as when its spin has actually previously been measured in some (perhaps unknown) direction and
then the electron has remained undisturbed for a while, the electron does have a perfectly objectively defined direction
of spin, according to standard quantum theory.
resolved. The possible relevance of quantum effects to brain
function will be considered in the final two chapters.
That is not an uncontroversial fact. For instance, Roger Penrose, from the Emperor’s New Mind
OBJECTIVITY AND MEASURABILITY OF QUANTUM STATES Despite the fact that we are normally only provided with probabilities for the outcome of an experiment, there seems to be something objective about a quantum-mechanical state. It is often asserted that the state-vector is merely a convenient description of ‘our knowledge’ concerning a physical system or, perhaps, that the state-vector does not really describe a single system but merely provides probability information about an ‘ensemble’ of a large number of similarly prepared systems. Such sentiments strike me as unreasonably timid concerning what quantum mechanics has to tell us about the actuality of the physical world. Some of this caution, or doubt, concerning the ‘physical reality’ of state-vectors appears to spring from the fact that what is physically measurable is strictly limited, according to theory. Let us consider an electron’s state of spin, as described above. Suppose that the spin-state happens to be |a), but we do not know this; that is, we do not know the direction a in which the electron is supposed to be spinning. Can we determine this direction by measurement? No, we cannot. The best that we can do is extract ‘one bit’ of information that is, the answer to a single yes no question. We may select some direction P in space and measure the electron’s spin in that direction. We get either the answer YES or NO, but thereafter, we have lost the information about the original direction of spin. With a YES answer we know that the state is now proportional to |p), and with a NO answer we know that the state is now in the direction opposite to p. In neither case does this tell us the direction a of the state before measurement, but merely gives some probability information about a. On the other hand, there would seem to be something completely objective about the direction a itself, in which the electron ‘happened to be spinning’ before the measurement was made For we might have chosen to measure the electron’s spin in the direction a -and the electron has to be prepared to give the answer YES, with certainty, if we happened to have guessed right in this way! Somehow, the ‘information’ that the electron must actually give this answer is stored in the electron’s state of spin. It seems to me that we must make a distinction between what is ‘objective’ and what is ‘measurable’ in discussing the question of physical reality, according to quantum mechanics. The state- vector of a system is, indeed, not measurable, in the sense that one cannot ascertain, by experiments performed on the system, This objectivity is a feature of our taking the standard quantum-mechanical formalism seriously. In a non-standard viewpoint, the system might actually ‘know’, ahead of time, the result that it would give to any measurement. This could leave us with a different, apparently objective, picture of physical reality. precisely (up to proportionality) what that state is; but the state vector does seem to be (again up to proportionality) a completely objective property of the system, being completely characterized by the results that it must give to experiments that one might perform. In the case of a single spin-one-half panicle, such as an electron, this objectivity is not unreasonable because it merely asserts that there is some direction in which the electron’s spin is precisely defined, even though we may not know what that direction is. (However, we shall be seeing later that this ‘objective’ picture is much stranger with more complicated systems- even for a system which consists merely of a pair of spin-one-half particles. ) But need the electron’s spin have any physically defined state at all before it is measured? In many cases it will not have, because it cannot be considered as a quantum system on its own; instead, the quantum state roust generally be taken as describing an electron inextricably entangled with a large number of other particles. In particular circumstances, however, the electron (at least as regards its spin) can be considered on its own. In such circumstances, such as when its spin has actually previously been measured in some (perhaps unknown) direction and then the electron has remained undisturbed for a while, the electron does have a perfectly objectively defined direction of spin, according to standard quantum theory. resolved. The possible relevance of quantum effects to brain function will be considered in the final two chapters.