Wigner designed the experiment to illustrate his belief that
consciousness is necessary to the quantum mechanical measurement
process. If a material device is substituted for the conscious friend,
the linearity of the wave function implies that the state of the system
is in a linear sum of possible states. It is simply a larger
indeterminate system.
Thing is, it requires not only positing a collapse mechanism that is
non-unitary, non-Lorentz-invariant, non-time-reversible, and on and on
and on, but also positing a dichotomy between things that constitute a
"material device" and some other sort of stuff that does not (but you
can bet the name for it would start with an "S" and rhyme with "hole",
and be suggested as proof of the existence of some dude whose name
rhymes with Todd).
Or we can posit that Wigner's friend is also a "material device", in
which case you realize that Wigner's friend just gets replicated into
parallel worlds, and so does Wigner, and so does everyone eventually.
Which is philosophically somewhat disturbing, and being a "material
device" perhaps even more so.
This is probably why the *obvious truth* about QM is regarded as
controversial instead of a settled matter: it flies in the face of not
only commonsense intuition (I don't *feel* like I'm being duplicated!)
but also nearly all widespread spiritual and theological beliefs (anyone
remember the phrase "God does not play dice with the universe"?) and
even our intuition about free will.
Yet, the experimental evidence says we must either accept this, or posit
a non-unitary, non-Lorentz-invariant, non-time-reversible .......
However, a conscious observer (according to his reasoning) must be in
either one state or the other, hence conscious observations are
different, hence consciousness is not material.
There's Wigner's non sequitur; if a conscious observer was in a
superposition of states, and if consciousness was *part of the brain's
function* rather than some mysterious external thing, then the observer
would have two sets of experiences and in fact two consciousnesses, each
experiencing only one of them.
What happens if you superpose a computer adding 1 and 2 and a computer
adding 3 and 4? Two additions take place, separately but simultaneously,
producing a 3 and a 7, respectively. Neither operation influences the other.
So, what happens if you superpose a computer running a self-aware
program on one set of inputs and a computer running a self-aware program
on a second set of inputs? Again, two separate self-aware computations
take place, separately but simultaneously, and neither operation
influences the other.
The implication is that Wigner cannot tell by introspection that he
*isn't* one of two (or many more) superposed Wigners, each having
received separate inputs, none influencing the others, because of that
last part.
The idea has become known as the consciousness causes collapse
interpretation.
Which I'm quite sure will eventually join a list that also contains
phlogiston, hollow Earth theory, and cold fusion.
Oh, and what *does* happen to free will if you're just a "material device"?
Why, nothing, of course. You only have problems there if you assume that
"you" are floating out there somewhere, "willing" your brain and body to
do something, and if that brain and body are deterministic all the
"willing" in the universe won't influence them. But that presupposes the
very dualism we're now presuming to be absent. So, instead, your will is
something internal; it arises from the mechanical processes of your brain.
You have the sense of being able to do anything you want to do, within
physics's constraints. This comes from the brain's labeling certain
states of the universe as reachable if certain actions are taken. All of
that is algorithmic; chess software does similar things under the hood
to see if it has a checkmate in N moves and then act to win the game if
it does.
So what is "will"? Ultimately it comes from whatever determines what you
"want" to do, and what you then decide as a way of trying to bring it
about. If what you "want" is a result of mechanical processes, and so
are those subsequent decisions, what of it? You still want things; you
can still figure out ways to try to get them and make the attempt. You
don't magically lose these capabilities, anymore than a chess program
suddenly loses the capability to win most games against human players,
just because you discover that the whole process is mechanical!
It was all along, and it never bothered you before you knew about it.