The relevance of conscious observers

In $3.5 we proved that an instrument governed by the laws of
quantum theory is not capable of making a proper measurement,
that is, it cannot cause the wavefunction of a system to change to
a state corresponding to a particular value of the quantity to be
measured. As an example we saw that, in the potential barrier
experiment, even after the attempted measurement of transmission
or reflection, the wavefunction still contained pieces corresponding
to both possibilities.
We must not, of course, conclude from this that true
measurements are impossible. We know that they occur. We can
observe which of the two detectors flashes and hence deduce
whether or not a particle has passed through the barrier. Our brain
certainly does not permit both possibilities. Thus, although a
simple, microscopic, instrument, obeying the laws of quantum
theory, does not reduce wavefunctions, they are certainly reduced
by the time the information reaches our brain.
What, then, is responsible for the reduction and what are the
characteristics of ‘instruments’ that are able to cause it? We do not
know the answers to these questions. It could be that, with increasing
complexity and size, correction terms in the equations of quantum
mechanics become more significant, so that any macroscopic
apparatus can do the reduction. On the other hand, it could be that
something totally new is required and that some things possess it whereas others do not. In either case it is an obvious question to
ask whether there are other features of wavefunction-reducing
systems that distinguish them from simpler systems. One obvious
possibility that arises here is to go to the extreme end of the chain
of observation and consider the possibility that the reduction does
not occur until we know that it must, i.e. that it only occurs when
conscious observers are involved.
Such a wild suggestion tends to horrify the austere minds of
most physicists. We fear that it takes our subject, beloved for its
high standards of objectivity, rigour, precision and experimental
support, into a realm where nothing can be properly defined, where
feelings and personality replace detached measurement, even, perhaps,
to put it on a par with astrology and the reading of tea leaves!
From another point of view, however, it should perhaps be seen as
an exciting new development. Maybe it allows the possibility that
the enormously successful methodology of physics might enter a
totally new field of investigation. This would be a revolution that
would, in its significance, dwarf those to which we referred in 4 1.1.
Although it is probably fair to say that such a revolution is
unlikely, we should, before dismissing it entirely, remember that
J C Maxwell, the creator of the theory of electromagnetism and
undoubtedly one of the greatest physicists of all time, once
expressed the view that the study of atoms would be forever outside
the scope of physics! Such a precedent will guard us from making
similar rash statements about consciousness.
If we are to consider seriously the relevance of consciousness in
the collapse of wavefunctions we must ask, and at least try to
answer, the question of what it is. To this topic we turn in our next
section.