6. THE PARTICIPANT
Niels Bohr, the principal architect of quantum philosophy,
wrote that we cannot forget "that in the drama of existence
we ourselves are both actors and spectators." [Essays
1958/62 on Atomic Physics and Human knowledge, p.15]
This comment succinctly captures a key point of quantum
theory: the human observers are no longer passive
witnesses to a flow of physical events that they cannot
influence. They are essential players in the action: their
"free" choices can influence the flow of physical events.
Quantum theory, in spite of its idealistic content, is
formulated in a completely realistic and practical way. It is
structured around the activities of human agents, who can
probe nature in any of many possible ways. Bohr
emphasized this freedom of the experimenters in passages
such as:
"The freedom of experimentation, presupposed in
classical physics, is of course retained and corresponds
to the free choice of experimental arrangement for which
the mathematical structure of the quantum mechanical
formalism offers the appropriate latitude."
The point here is that quantum theory, in its original form, is
set up in terms of an interaction between conscious human
agents and an invisible quantum system. As already
discussed, that system is represented in the theory by a
vector in Hilbert space. This vector usually evolves
according to the quantum law of motion, the Schroedinger
equation. But to get information about that system the
experimenter must ask specific questions by setting up
corresponding probes. For example, the experimenter can
orient the crystal in the photon experiment described earlier
in any way he chooses. Different choices correspond to
different choices of the two perpendicular directions A1 and
A2. Moreover, the human agent can choose to do or not do
the experiment, or to do it sooner or later. These choices
are, according to our basic physical theory, quantum theory,
not fixed by any yet-known laws of nature. Hence these
choices are, in that specific sense, "free choices".
We have now arrived at the crux of the matter! Quantum
theory, in its orthodox formulation, involves the human
observer not merely as a passive recipient of data, but also
as an active agent that enters into natural process in ways
that are not controlled by the known physical laws. His
specific role, as it appears in the world of experience, is to
select which experiment is to be performed: i.e., to choose
which aspect of nature will be probed. This role, as it
appears in the mathematics, is to select some one single
projection operator P. This P is the projection operator such
that the initial vector V will become PV if nature delivers the
affirmative answer 'yes' to the posed question, but will
become the vector P'V if nature delivers the negative answer
'no'. Here P' is the projection operator that keeps the vectors
that P eliminates, and hence satisfies the condition
PV+P`V=V for all V.
[Sometimes a complex question involving a combination of
several projection operators can be considered. But these
can be regarded as a sequence of individual P's, and I shall
adopt this simpler way of speaking.]
The essential point here is that, according to orthodox ideas,
nature's process of generating the experiences that appear
in our streams of consciousness cannot proceed without
particular questions being posed. But then the key questions
become:
If the known physical laws do not determine the choices that
need to be made by the human agents in order for nature's
process of generating human experiences to proceed, then
what sort of considerations do influence or determine these
choices?
and
What effect do these "free" choices have on the course of
physical events?
The evolution of the unseen system involves a sequence of
questions with Yes or No answers. Hence the interaction
between nature and agent is like a game of twenty-
questions: the agent is free to choose a sequence of Yes-No
questions, and nature delivers an answer, Yes or No, to
each, in the form of an experience that answers the posed
question. The choice of question is represented within the
mathematical description as the selection of one projection
operator P from among an infinite continuum of possibilities.
Each possible P corresponds to particular way of orienting a
set of N mutually perpendicular vectors, and, then, a
particular choice of which of these N vectors will be
eliminated to form PV. The process of making these
particular choices of P's is not specified by the Schroedinger
equation, which controls the continuous evolution of the
physical state between observations, but neither the agent's
choices of the questions nor nature's choices of the answer.
In particular, a complete account of the dynamics needs to
explain an as-yet-unexplained process that picks a
sequence of particular P's from a continuum of possibilities.
This selection process is associated, according to orthodox
quantum theory, with consciously made free choices on the
part of human beings. And, according to the quantum rules
themselves, these "free choices" can influence the course of
physical events, as we shall presently see.