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What does quantum physics teach us about the concept of physical reality?
http://www.physics.ox.ac.uk/users/polkinghorne2010/pdf/healey.pdf
When the layman says "reality" he usually thinks that he is speaking of something which is
self-evidently known; while to me it appears to be specifically the most important and
extremely difficult task of our time to work on the elaboration of a new idea of reality.
(Wolfgang Pauli, in a letter to Markus Fierz dated 12th August 1948)
2.2 Bohr on physical reality
Bohr thought quantum theory made Einstein’s characterization of physical reality obsolete.
Already in the transcript of the Como lecture in which he introduced the idea of
complementarity he wrote “the quantum postulate implies that any observation of atomic
phenomena will involve an interaction with the agency of observation not to be neglected.
Accordingly, an independent reality in the ordinary physical sense can neither be ascribed to
the phenomena nor to the agencies of observation.” (“The quantum postulate and the recent
development of atomic theory” (Nature 121 (1928), p.580) So he took the quantum postulate
to imply that any attempt by physics to “grasp reality as it is thought independently of its
being observed” is doomed to failure, thus rendering Einstein’s sense of ‘physical reality’
vacuous.
But I think an equally important reason why Einstein and (later) Bell were
unable to make much of Bohr’s doctrine of complementarity was that their view of the
relation of physics to reality was so different from Bohr’s. This may be brought home by
Bohr’s striking remark “There is no quantum world. There is only an abstract quantum description.
It is wrong to think that the task of physics is to find out how nature is. Physics concerns what
we can say about nature.”
2.3 Pauli on physical reality
Bohr presented his doctrine of complementarity after discussions with Pauli, who should
therefore be considered to have contributed significantly to its formulation. While Bohr’s
reply to EPR did not effectively address the grounds of Einstein’s divergent view of physical
reality, Pauli’s letter to Bohr of February 15th, 1955 shows that Pauli, at least, understood
Einstein’s viewpoint reasonably well, while continuing to disagree with it.
“it seems to me quite appropriate to call the conceptual description of nature in classical
physics, which Einstein so emphatically wishes to retain, “the ideal of the detached observer”.
To put it drastically the observer has according to this ideal to disappear entirely in a
discrete manner as hidden spectator, never as actor, nature being left alone in a
predetermined course of events, independent of the way in which phenomena are observed.
‘Like the moon has a definite position’ Einstein said to me last winter, ‘whether or not we look
at the moon, the same must hold for the atomic objects, as there is no sharp distinction
possible between these and macroscopic objects. Observation cannot create an element of
reality like position, there must be something contained in the complete description of
physical reality which corresponds to the possibility of observing a position, already before
the observation has actually been made’. I hope, that I quoted Einstein correctly; it is always
difficult to quote somebody out of memory with whom one does not agree. It is precisely this
kind of postulate which I call the ideal of the detached observer.
In quantum mechanics, on the contrary, an observation hic et nunc changes in general
the ‘state’ of the observed system in such a way not contained in the mathematically
formulated laws, which only apply to the automatical time-dependence of the state of a closed
system. I think here on the passage to a new phenomenon by observation which is technically
taken into account by the so-called ‘reduction of the wavepackets’. As it is allowed to consider
the instruments of observation as a kind of prolongation of the sense organs of the observer, I
consider the unpredictable change of the state by a single observation—in spite of the
objective character of the results of every observation and notwithstanding the statistical
laws for the frequencies of repeated observation under equal conditions—to be an
abandonment of the idea of the isolation (detachment) of the observer from the course of
physical events outside himself.”
In his essay “Probability and physics”, Pauli quotes Einstein as saying “There is such a
thing as the real state of a physical system, which exists objectively, independently of any
observation or measurement, and can in principle be described by the modes of expression
used in physics”.
Pauli continues
“However, these formulations of Einstein’s are only a paraphrase of the ideal of a special
form of physics, namely the “classical” form. This ideal, so pertinently characterised by
Einstein, I would call that of the detached observer. In point of fact ‘existent’ and ‘non-
existent’, or ‘real’ and ‘unreal’, are not unique characterisations of complementary qualities,
which can be checked only by statistical sequences of experiments using different
arrangements, freely chosen, which may in some cases be mutually exclusive. The new theory
on the contrary generalizes these classical ideals and postulates. Under the pressure of the
physical facts summed up under the heading “finiteness of the quantum of action”, this logical
generalisation has emerged into a higher synthesis as a finally satisfactory solution of earlier
contradictions: The mathematical inclusion, in quantum mechanics, of the possibilities of
natural events has turned out to be a sufficiently wide framework to embrace the irrational
actuality of the single event as well. It may also, as comprehending the rational and
irrational aspects of an essentially paradoxical reality, be designated as a theory of
becoming.”
(From “Probability and Physics”, Dialectica 8 (1954) reprinted in Wolfgang Pauli:
Writings on Physics and Philosophy, pp.47-8)
Pauli frequently uses the word ‘irrational’ when referring to physical reality, and here he
even dubs reality “paradoxical”, right after maintaining that quantum theory provides “a
finally satisfactory solution of earlier contradictions.” This at least suggests that in Pauli’s
usage ‘rational’ means ‘in conformity to physical law’ while ‘irrational’ does not mean
‘contradictory’ or ‘beyond human understanding’ but rather ‘outside the scope of physical
law; the random outcome of an observer’s action’. (In a 1954 letter to Born, Pauli says “The
appearance of a definite position during the subsequent observation...is then regarded as a
‘creation’ existing outside the laws of nature.”) But this leaves open what Pauli meant by
calling reality “essentially paradoxical”. Though he never succeeded in formulating a clear
view of physical reality in the light of quantum theory, Pauli’s letters and publications
continue to provide a rich source of insights for someone trying to do so.
What does quantum physics teach us about the concept of physical reality?
http://www.physics.ox.ac.uk/users/polkinghorne2010/pdf/healey.pdf
When the layman says "reality" he usually thinks that he is speaking of something which is
self-evidently known; while to me it appears to be specifically the most important and
extremely difficult task of our time to work on the elaboration of a new idea of reality.
(Wolfgang Pauli, in a letter to Markus Fierz dated 12th August 1948)
2.2 Bohr on physical reality
Bohr thought quantum theory made Einstein’s characterization of physical reality obsolete.
Already in the transcript of the Como lecture in which he introduced the idea of
complementarity he wrote “the quantum postulate implies that any observation of atomic
phenomena will involve an interaction with the agency of observation not to be neglected.
Accordingly, an independent reality in the ordinary physical sense can neither be ascribed to
the phenomena nor to the agencies of observation.” (“The quantum postulate and the recent
development of atomic theory” (Nature 121 (1928), p.580) So he took the quantum postulate
to imply that any attempt by physics to “grasp reality as it is thought independently of its
being observed” is doomed to failure, thus rendering Einstein’s sense of ‘physical reality’
vacuous.
But I think an equally important reason why Einstein and (later) Bell were
unable to make much of Bohr’s doctrine of complementarity was that their view of the
relation of physics to reality was so different from Bohr’s. This may be brought home by
Bohr’s striking remark “There is no quantum world. There is only an abstract quantum description.
It is wrong to think that the task of physics is to find out how nature is. Physics concerns what
we can say about nature.”
2.3 Pauli on physical reality
Bohr presented his doctrine of complementarity after discussions with Pauli, who should
therefore be considered to have contributed significantly to its formulation. While Bohr’s
reply to EPR did not effectively address the grounds of Einstein’s divergent view of physical
reality, Pauli’s letter to Bohr of February 15th, 1955 shows that Pauli, at least, understood
Einstein’s viewpoint reasonably well, while continuing to disagree with it.
“it seems to me quite appropriate to call the conceptual description of nature in classical
physics, which Einstein so emphatically wishes to retain, “the ideal of the detached observer”.
To put it drastically the observer has according to this ideal to disappear entirely in a
discrete manner as hidden spectator, never as actor, nature being left alone in a
predetermined course of events, independent of the way in which phenomena are observed.
‘Like the moon has a definite position’ Einstein said to me last winter, ‘whether or not we look
at the moon, the same must hold for the atomic objects, as there is no sharp distinction
possible between these and macroscopic objects. Observation cannot create an element of
reality like position, there must be something contained in the complete description of
physical reality which corresponds to the possibility of observing a position, already before
the observation has actually been made’. I hope, that I quoted Einstein correctly; it is always
difficult to quote somebody out of memory with whom one does not agree. It is precisely this
kind of postulate which I call the ideal of the detached observer.
In quantum mechanics, on the contrary, an observation hic et nunc changes in general
the ‘state’ of the observed system in such a way not contained in the mathematically
formulated laws, which only apply to the automatical time-dependence of the state of a closed
system. I think here on the passage to a new phenomenon by observation which is technically
taken into account by the so-called ‘reduction of the wavepackets’. As it is allowed to consider
the instruments of observation as a kind of prolongation of the sense organs of the observer, I
consider the unpredictable change of the state by a single observation—in spite of the
objective character of the results of every observation and notwithstanding the statistical
laws for the frequencies of repeated observation under equal conditions—to be an
abandonment of the idea of the isolation (detachment) of the observer from the course of
physical events outside himself.”
In his essay “Probability and physics”, Pauli quotes Einstein as saying “There is such a
thing as the real state of a physical system, which exists objectively, independently of any
observation or measurement, and can in principle be described by the modes of expression
used in physics”.
Pauli continues
“However, these formulations of Einstein’s are only a paraphrase of the ideal of a special
form of physics, namely the “classical” form. This ideal, so pertinently characterised by
Einstein, I would call that of the detached observer. In point of fact ‘existent’ and ‘non-
existent’, or ‘real’ and ‘unreal’, are not unique characterisations of complementary qualities,
which can be checked only by statistical sequences of experiments using different
arrangements, freely chosen, which may in some cases be mutually exclusive. The new theory
on the contrary generalizes these classical ideals and postulates. Under the pressure of the
physical facts summed up under the heading “finiteness of the quantum of action”, this logical
generalisation has emerged into a higher synthesis as a finally satisfactory solution of earlier
contradictions: The mathematical inclusion, in quantum mechanics, of the possibilities of
natural events has turned out to be a sufficiently wide framework to embrace the irrational
actuality of the single event as well. It may also, as comprehending the rational and
irrational aspects of an essentially paradoxical reality, be designated as a theory of
becoming.”
(From “Probability and Physics”, Dialectica 8 (1954) reprinted in Wolfgang Pauli:
Writings on Physics and Philosophy, pp.47-8)
Pauli frequently uses the word ‘irrational’ when referring to physical reality, and here he
even dubs reality “paradoxical”, right after maintaining that quantum theory provides “a
finally satisfactory solution of earlier contradictions.” This at least suggests that in Pauli’s
usage ‘rational’ means ‘in conformity to physical law’ while ‘irrational’ does not mean
‘contradictory’ or ‘beyond human understanding’ but rather ‘outside the scope of physical
law; the random outcome of an observer’s action’. (In a 1954 letter to Born, Pauli says “The
appearance of a definite position during the subsequent observation...is then regarded as a
‘creation’ existing outside the laws of nature.”) But this leaves open what Pauli meant by
calling reality “essentially paradoxical”. Though he never succeeded in formulating a clear
view of physical reality in the light of quantum theory, Pauli’s letters and publications
continue to provide a rich source of insights for someone trying to do so.
