On 31 Dec 2013, at 21:19, meekerdb wrote:

## Advertising

On 12/31/2013 1:37 AM, Bruno Marchal wrote:On 30 Dec 2013, at 20:00, meekerdb wrote:On 12/30/2013 3:09 AM, Bruno Marchal wrote:But that's essentially everything, since everything is(presumably) quantum. But notice the limitation of quantumcomputers, if it has N qubits it takes 2^Ncomplex numbers to specify its state, BUT you can only retrieveN bits of information from it (c.f. Holevo's theorem). So itdoesn't really act like 2^N parallel computers.OK, but nobody pretended the contrary. You can still extract Nbits depending on the 2^N results, by doing some Fouriertransfrom on all results obtained in "parallel universes". Thismeans that the 2^N computations have to occur in *some* sense.But they pretend that the number 2^N is so large that it cannotexist in whole universe, much less in that little quantum computerand therefore there must be other worlds which contain theseenormous number of bits. What Holevo's theorem shows is the onecan regard all those interference terms as mere calculationfictions in going from N bit inputs to N bit outputs. It isconceptually no different than doing a calculation in ordinaryprobability theory: I start with some initial conditions and Iintroduce a probability distribution and compute a probability forsome event. In that intermediate step I introduced a continuousprobability distribution which implies an *infinite* number ofbits. Nobody thinks this requires an infinite number of worlds.Then you need to add some selection principle to QM. If QC worksthrough QM, the "parallel" computation are done in our quasi-classical world as in any other branch, and this is tested by doinga Fourier Transform which required the computation do be done insome non fictitious way (or you are adding some non linear magic inQM at some place).I don't understand your comment. It is my point that thecomputations are done in our world via the interference of wavefunctions - which have to be in the same world in order to interfere.

`Not sure that this makes sense to me. The wave is the"wave of`

`universe", it evolves in a Hilbert Space, not in any world or branch.`

`The term "world" is confusing here.`

I take "worlds" to mean quasi-classical worlds and a quasi-classicalworld may be supported by many different quantum "worlds". But inDeutsch's famous proposed experiment, a quantum AI computer afterfactoring some prime by Shor's algorithm may not be able to tell usanything more than the factors - because those factors comes out thesame in almost all branches and hence correspond to the same quasi-classical world.

`Because we manage to get the result in our quasi-classical world. Yet,`

`we get an indirect information about what has been computed in the`

`other branch. We can compute a predicate, which outputs 1 or 0, and`

`decide, in our branch, to test if all output were identical or not.`

`Also with Deutsch quantum brain, the observer can know that he has`

`seen a definite result in each branch, but can't remember which one.`

`So he can test the presence of the alternate realities, but not much`

`more (but still more) than with the usual two slits.`

`Then in arithmetic, the parallel realities existence is an elementary`

`theorem of RA. So we expect the MW, and are happy that some physicists`

`already confirm this when abandoning the collapse, ...which is all`

`absurd by itself, note.`

Bruno http://iridia.ulb.ac.be/~marchal/ -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.