Tim May wrote:

>I've known of the Everything list, and of course the 
>Everett/DeWitt/Niven/Wheeler/Egan/Tegmark ideas for a while...I 
>remember reading Larry Niven's "All the Myriad Ways" around 1970 or 
>so. We used to sit around in the early 70s debating the Everett 
>model, which a couple of science fiction writers were making much 
>of, and which had gained new popularity after Bryce DeWitt dusted 
>off the idea and began publishing a lot on it. (DeWitt assigned much 
>credit to his student, RN Graham, and even called the MWI the 
>"Everett-Wheeler-Graham" theory. Way too charitable to Graham, I 
>think.)I learned of your list a while back, but wasn't 
>super-interested in what I thought (and partially still think) is a 
>fanciful idea, akin to the late David Lewis' "plurality of worlds" 
>philosophy (that everything we can imagine must have reality). Saul 
>Kripke's "possible worlds" work was more interesting, as it is 
>closely linked to linguistics and AI and predictions about the 
>future ("If Oracle were to announce bad earnings tomorrow, then this 
>is what would probably happen," a possible worlds "story" which is 
>of course very close to some discussions of alternate _presents_. In 
>fact, no different, except more practical.)



Mmh... Most people here have a good understanding of the "many"-idea, by which
I mean have realised that the idea of a unique universe is far more speculative
that O universe or many universe. I will not insist.
Perhaps you could read the very interesting paper by Louis Crane, 
which is quite
convincing on the importance of category theory in frames of quantum gravity+
observers, and which use cleverly Everett relative states. He even 
concludes that
his proposal can be seen as an attempt to fuse the many worlds with general
relativity. It is a paper by Crane entitled "categorical physics". He gives
the TEX source somewhere on the net. (If you don't find it I can eventually
send you a photocopy). One day I will talk you about the work of Yetter on
models for non commutative linear logic. Yetter is at a relevent crossroad of
logic and physics imo. (Then I guess you heard about Crane, Kauffman Yetter
papers?)
About linguistic use of Kripke, most of them, imo, use technical approach
of language to explain problems ... away. Beware sophisticated tricks for
putting interesting (but hard) problems under the rug.




>Personally, I'm not (yet) "taking seriously" either the David Lewis 
>"plurality of worlds" or Max Tegmark "everything" or Greg Egan "all 
>topologies model" ideas. At least not yet. I need to learn a lot 
>more of the language first.


I am more problem driven, and even "mind-body" problem driven. I gave an
argument in this list (argument on which my phd thesis is based) that 
IF we are machine then physics is utimately reducible to machine's 
psychology. The laws
of physics emerge from some collection of sharable "dreams" by machines, where
a dream is basically a computation seen from first person point of view.
And first person notion are captured by 1) memory, 2) S4 modal logic, 
3) toposes
(but only in my technical notes for the 3).
I hope you will continue to read my posts now that I confess that I am not
sure physics is the *fundamental* way to get a TOE. Still, my favorite theory
is quantum mechanics, so much that I want to provide serious foundation for it.
Looking at nature, and other empirical evidence is not enough serious for me!
I may be wrong, or the computationalist hypothesis may be wrong, don't depress
to quickly ;) Thesis and links in my URL below.




>I saw a streaming video talk given by the topologist Michael 
>Freedman at MSRI. URL for his talk "Anyons in Mathematics, Computer 
>Science, and Physics" is:
http://www.msri.org/publications/ln/msri/2000/subfactors/freedman/1/
>Some interesting stuff on quantum computation and the braid 
>category, but inasmuch as I know even less about knots than about 
>category theory, I can't say much about his work.



Thanks, I will look at it.




>  >Have you read Everett?
>
>Hugh Everett, I assume you mean. Yes, indeed. I have the book edited 
>by Bryce DeWitt and Neill Graham, "The Many-Worlds Interpretation of 
>Quantum Mechanics," 1973. I think this is how many in the physics 
>community encountered MWI, through DeWitt's late 60s, early 70s 
>re-analysis.


Yes. Note that Everett just abandons the wave collapse postulate. The "many-
world" expression came latter (through DeWitt I think, indeed). I mean what
Everett proposed is a new theory, not a new interpretation of a theory.


>Ironically, my general relativity instructor at UC Santa Barbara was 
>Jim Hartle, known then (1973) for his work on photon black holes and 
>such, and later famous for collaborations with Gell-Mann on 
>"consistent histories" and 'wave function of the universe" and with 
>Hawking.


Even before, Hartle makes the same derivation as Graham, about the same time.
Consistent histories are good ways to tackle QM basic problem seriously. I have
appreciated the work of Savvidou, a student of Isham. Savvidou's 
thesis is available
on Los Alamos archive. You have been lucky having Hartle as intructors :)




>  >Quite important. He just embeds the physicist in the
>>physical world. My own work is a (radical) generalisation of that idea
>>in the sense that I embed the "arithmetician" in the arithmetical world,
>>making it a first order citizen.
>
>Sounds intriguing. I'm currently less-focused on the role of human 
>(or machine) observers.


Ah, but if you search for a TOE, you will never get rid of the human, 
or machine or
whatever sort of observers ....
And honestly TOPOSES are objective approach toward the subject and its horizon.
I disagree with Smolin use of topos for dismissing the "other" worlds. That is
just a modern reinstantiation of the solipsistic move. What exist is 
what I feel?
I am much more platonist than that! ('course Smolin book is very 
nice, but I don't
follow him on that point).



>Isham makes an excellent point about time-varying sets, echoed by 
>Smolin. In a nutshell, while the logic of a quantum universe (or 
>cosmological universe, perhaps) may follow a Heyting logic where 
>"the cat is neither alive nor dead," once _any_ observation or 
>measurement, whether a machine or a written note or a memory or 
>whatever, then the logic is Boolean, as we "are used to."

>Now obviously we're all familiar with this as the basic "measurement 
>collapses the wave function" model, so there is at first glance 
>nothing new here (you skeptics out there are right to be skeptical). 
>However, the topos-theoretical point of view, in which topos logic 
>(Heyting) is used instead of Boolean logic, seems to me to make the 
>"interpretation" problem (Copenhagen vs. MWI vs. Cramer vs. ...) 
>largely go away.


Everett clearly proposed a new *formulation* of QM. Just SWE (+ math decor).
The collapse of the wave packet has never been succesfully explained. 
It introduces
a cut subject/object which has never been succesfully defined. To use 
TOPOS against
Everett formulation is like using an electronical microscope to 
sharpen a flint.



>I just wish mathematicians would do more of what John Baez in his 
>papers: show the reader the motivations.


Gosh...I wish too. But for some mathematicians, transparent 
motivations are just
forbidden, alas!


Bruno

PS
    1) Another nice book on toposes for logically minded reader is the book
by J.L. BELL "Toposes and Local Set Theories" (Oxford Science Publications,
Clarendon Press 1988). It is the same Bell who wrote an important paper
on "a new approach of quantum logic" which plays some role in my work 
(ref in the
thesis). It is not the J.S. BELL of BELL's inequality.
    2) The notion of categoricalness for a model in logic is not 
related to category theory.
-- 
http://iridia.ulb.ac.be/~marchal/

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