**Wigner's Friend**: Eugene Wigner published in 1961 a widely reprinted
article \[Wigner 1967/1983\] entitled "Remarks on the Mind-Body Problem"
in which he stresses the basic role played by consciousness in quantum
theory. But if consciousness is basic then the question immediately
arises: Whose consciousness? To explore this issue Wigner considers a
situation in which his "friend", rather than he himself, is observing
the effects of an atomic process, the radiation of a visible photon.

.

In order to formulate the problem Wigner first explains the entry of
consciousness into physical theory:

> When the province of physical theory was extended to encompass
> microscopic phenomena, through the creation of quantum mechanics, the
> concept of consciousness came to the fore again: it was not possible
> to formulate the laws of quantum mechanics without reference to the
> consciousness.^3^ All that quantum mechanics purports to describe are
> probability connections between subsequent impressions (also called
> 'apperceptions') of consciousness, and even though the dividing line
> between the observer, whose consciousness is being affected, and the
> observed physical object can be shifted towards one or the other to a
> considerable degree,^4^ it cannot be eliminated.

His reference 4 \[von Neumann 1932, Ch. VI\] is to von Neumann's work
\[→ orthodox interpretation\] on the shifting of the boundary between
those aspects of nature that are described in the mathematical language
of quantum theory, and those that are described in the psychological
language by means of which we describe our actual and possible conscious
experiences. The job of quantum theory is to make predictions about
connections between such experiences. His reference 3 \[Heisenberg
1958\] was to Heisenberg's famous pronouncement:

The conception of objective reality ... evaporated into the ...
mathematics that represents no longer the behavior of elementary
particles but rather our knowledge of this behavior.

The concept of "our knowledge" is reasonably clear insofar as "we are
able to communicate to others what we have done and what we have
learnt." \[Bohr, 1962, p.3\].

But in practice different people often know different things.

The thought experiment considered by Wigner involves, essentially, an
atomic state that emits a visible photon into an optical system that
directs the rays emitted from the atom in certain directions into the
retina of the eye of Wigner's friend, and directs the rays emitted in
other direction to some other place. The wave function of the atom plus
the photon will be a → superposition of components corresponding to
different directions of the photon emission. If the interaction of the
photon with the retina, and of the retina with the brain of the
friend\-\--who is presumed to be attending to what she is seeing\-\--is
now included in the physical description, then the state of his friend's
brain generated by the purely physical laws of motion would include a
part that corresponds to her observing the flash and another part
corresponding to her not observing the flash. When Wigner asks his
friend whether she saw the flash, then, upon his registering of her
response, the wave function (quantum state) that represents his
knowledge of her brain and body will suddenly jumps to one state or the
other. Yet before he learned about her reaction his representation of
her state was in a combination of the "I observed a flash" and "I
observed no flash" alternatives.

Wigner is willing to admit that, if the purely physically described laws
entail it, then an unobserved inanimate measuring device could exist in
a state that represents a combination of two macroscopically different
states. However, although solipsism may be a *logical* possibility,
"everyone believes that the phenomena of sensation is widely shared by
organisms that we consider to be living". And, accordingly, his friend
will surely report that she did \[or did not\] experience the flash \[as
the case may be\] *before* she reported that fact to him. Wigner
concludes from these considerations that his friend was "not in a state
of suspended animation" before *he* learned about her state: he
concludes that her quantum state became one or the other of these two
alternatives when *she* became conscious of the flash, not when *he*
came to know what she reported.

Wigner asserts that "The preceding argument for the difference in the
roles of inanimate tools of observation and observers with
consciousness\-\--hence for a violation of physical laws where
consciousness plays a role\-\--is entirely cogent so long as one accepts
the tenets of orthodox quantum theory and all their consequences."

Wigner proposes, then, that "the being with a consciousness must have a
different role in quantum mechanics than the inanimate measuring
device." He proposes, in essence, that the occurrence of a conscious
experience is an objective reality that is correlated to a change in an
objective wave function. "Our knowledge" can then be interpreted to be
the aggregate of the conscious knowledge of *all* systems that possess
consciousness. This allows quantum theory to be regarded as an objective
theory that describes the interaction between an objective physical
aspect that is described in terms of the mathematical language of
quantum theory, and an objective mental aspect that is described in
terms of the concepts of thoughts, ideas, and feelings\-\--i.e., in
terms of the concepts of psychology. This move allows what had
originally been a fundamentally anthropocentric, pragmatic, subjective
theory to be elevated into a non-anthropocentric objective theory of an
objective reality having physically described aspects and
psychologically described aspects related in the specific way specified
by the → orthodox interpretation quantum theory spelled out by John von
Neumann (1932).

Bohr, N. (1962): *Essays 1958/1962 on atomic physics and human
knowledge*, New York, Wiley.

:

Heisenberg, W. (1958): The representation of nature in contemporary
physics, *Daedalus,*

(Summer), 95-108.

Von Neumann, J, (1932): *Mathematische Grundlagen der Quantnmechanik,*
Springer, Berlin. (Translated as *Mathematical Foundations of* *Quantum
Mechanics,*

Princeton University Press, 1955)

Wigner, E. (1961): "Remarks on the Mind-Body Problem", in *The Scientist
Speculates*, I. J. Good, ed. pp. 284-302, Heinemann, London (1961);
Basic Books, New York (1962). Reprinted in Wigner 1967/1983.

Wigner, E. (1967/1983): *Symmetries and Reflections* , Indiana
University Press, Bloomington, Indiana (1967) pp.171-184 and in Quantum
Theory and Measurement,

J.A. Wheeler and W. H. Zurek eds. pp.168-181 Princeton University Press
(1983).

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