Le 19-nov.-05, à 22:56, Russell Standish a écrit :
> On Sat, Nov 19, 2005 at 04:22:58PM +0100, Bruno Marchal wrote:
>> Now observation and knowledge are defined in the logics of
>> self-reference, i.e. by transformation of G and G*, and so are each
>> multiplied by two. Actually and amazingly for the knower (the first
>> person) G and G* give the same logic, like if the first person
>> conflates truth and provability. But for the notion of observation, G
>> and G* give again different logics, so that the observer can
>> distinguish communicable observations ("physical facts") and non
>> communicable observations (sensations, I would argue).
>
> Are you now saying that your operators
>
> Pp = Bp & -B-p
>
> and
>
> Op = Bp & p & -B-p
>
> correspond to "to observe" (Op being "to validly observe" I
> suppose)?. Previously, you would say that Pp is "to bet on p", and Op
> "to correctly bet on p", which never really made sense to me. What's
> the French word you would use for this - I may know it, or perhaps I
> can figure the relevant English term from a dictionary.
Let me first explain in few words a plausible logician conception of a
"multiverse". I borrow the term "multiverse" from FOR, but I think we
should be neutral about what is really a universe, or a world, or a
state, or an observer-moment: the only thing which matter is that we
have many of them, and that they are related by a relation of
accessibility. So a multiverse is just a set W (of elements called
"worlds") and a binary relation R defined on it. Let us use the letter
a, b, c, d, ... for the worlds. So aRb just means that the world b is
accessible from the world a. You can travel from a to b.
Note that I am not pretending that the "real multiverse" (perhaps the
quantum one) is of that type, but it is good to begin with that
familiar sort of Kripke multiverse, and then to correct it.
Now we assume that all the worlds obey classical logic: if p is true
in world a, and if q is true in world a, then propositional formula
like (p & q), (p -> q) etc. are true at a, and ~p is false at a, etc.
In particular, all classical tautologies are true in all worlds of all
multiverse independently of the assignment of truth value to the
sentence letter p, q, r, etc.
The main idea of Kripke has consisted in saying that the modal formula
Bp (also written []p) is true at world a, if p is true in all the
worlds you can access from a. p is relatively necessary at a.
For example, if the world are countries and if you have to pay taxes in
all countries that you can access from where you are, then taxes are
necessary (relatively to a).
That is, p is "necessary" at world a if p is true for all worlds b such
that aRb. It is intuitively normal: a proposition is necessary for you
if it is true in all world you can access.
Then a proposition is possible at world a if it is not necessary that
~a. So "possible p", written Dp, or <>p, can be seen as an abbreviation
of ~B~p. Note that if Dp is true at a, it means there is an accessible
world (where p is true) from a. In particular, given that the constant
true t is true in all worlds, Dt really means I can access to some
world (I am alive, if you want).
Now, there are relation between the structure of the multiverse, i.e.
the nature of its accessibility relation, and the formula which are
true in each world. It should be easy to guess that if the multiverse
is reflexive (i.e. all worlds are accessible from themselves) then the
formula Bp -> p is true in all the worlds, independently of the truth
value of the sentence letters. Slightly less easy: the reverse is true:
if Bp -> p is true in all worlds, independently of the assignment of
true/false to the sentence letters, then the multiverse is reflexive.
We say that the reflexive multiverse characterizes the formula Bp -> p.
It means the formula remains invariant when we travel in that
multiverse.
It can be shown that the symmetrical multiverse, that is those where
the accessibility relation is symmetric, characterizes the formula p ->
BDp. The transitive multiverse characterizes Bp -> BBp. etc.
Of special interest in this thread are the dead-end world, or
cul-de-sac observer-moment (we have use many name for them). A world a,
in a multiverse W, is said to be a dead end or a cul-de-sac world if,
when you are in a, there is no more world in which you can acceded. So,
in such world no proposition are possible, so whatever proposition p
is, ~Dp is always false. By classical logic B~p is always true. This is
true whatever p is, in particular this is true for its negation ~p. So
in a dead end world, all proposition are necessary and none is
possible. Not a funny place!
Now, when B represents the Godel-Lob provability predicate, i.e. when B
represents provability in or by a "sufficiently rich" formal
system/machine, it can be shown that the "humble multiverse", that is
those where all worlds have access to a dead end world, characterizes
B. In that case Dp = ~B~p = "~p is not provable" = "p is consistent"
(because if you cannot prove ~p, you will not get a contradiction by
adding ~p as axiom, that is you will not prove ~p, that is p is
consistent (with your formal theory or for your machine). So "humble
multiverse" characterizes the formula Dp -> ~BDp, which is the second
incompleteness theorem of Godel: consistent p -> not provable
consistent p. So the humble multiverse characterize the machine's
discourses.
This prevents us of defining "probability(p) = 1" in world a by Bp is
true at a, because if a is a dead end then Bp is true (D~p is false)
although the probabilities are senseless.
Now, as you know, I limit the interview of machine to the correct and
consistent one. This is just a mathematical trick. For those machine Bp
-> Dp is true, but not provable by the machine. So we can define
"probability(p) = 1" by "Bp & Dp". It means that we define the
probability one of a proposition p, by p is true in all accessible
worlds and there is (at least one) accessible world. The incompleteness
forces us to put explicitly the consistency as a requirement. It
corresponds to the correct bets, or to the observation of laws
(invariant truth of the multiverse).
Actually, adding that Dp to Bp is so much constraining that we loose
the Kripke multiverse structure in the process, but we get instead a
more interesting proximity relation, and ultimately we get (translating
the comp hyp itself in the language of the machine) an orthogonality
structure on the worlds of the multiverse, making it looking like the
quantum multiverse inferred by the observing physicists.
Observation is implicitly defined here by measurement capable of
selecting alternatives on which we are able to bet (or to gamble ?).
The french word is "parier".
I will not explain the nuance between Bp & Dp and Bp & Dp & p. It just
happen that they are not equivalent in the humble multiverse, that is
they are not equivalent in the discourse of the correct machine, and
this provide nuances. Arithmetical quantum logics appear for all first
person nuances put on the provability predicate (with the comp hyp),
giving three arithmetical interpretation of some quantum logic. Could
explain this latter but I'm afraid it is a bit more technical.
Hoping this could help (to make logician's and physicist's talk closer
perhaps). 'course, Kripke uses himself the term "frame" instead of
multiverse, and "model" when each of the proposition letters (p, q, r,
...) are assigned to true or false (1 or 0, t or f) in each world.
Bruno
http://iridia.ulb.ac.be/~marchal/
Received on Mon Nov 21 2005 - 09:48:56 PST