Dear Freinds, I just found the following discussion that, IMHO, bears on the ideas in the thread on "Computational irreducibility and the simulability of worlds".

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Kindest regards, Stephen On Thu, 15 Apr 2004 15:14:38 +0000 (UTC), in sci.physics.research [EMAIL PROTECTED] (James Baugh) wrote: >"p.valletta" <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>... >> The Wave-function of the Universe is based on QM principles of probabilty >> and thus of observertional quantities. Quantum Tunneling is a good >> experimental method of 'Wave-function' analysis. >> >> Is it true that there is a corresponding 'Particle-function', based on the >> same variables? and could we use this to devise an experiment whereby a >> 'particle' tunnels outside of the Universe revealing the >> 'Universe-wavefunction'? > >Pardon Me, >I began with a short note and it turned into a sermon, >not all of which may apply to your inquiry. >******** > >[First] >Before using such phrases as "The wave function of the Universe" >first keep in mind what a "wave-funtion" means. >In QM a wave function is a distribution of probability >amplitudes over a range of possible obserables. >(Usually but not always position observations, e.g. >we can have a wave function over momentum space.) > >In other words it describes for example how likely we are to see >a particle pass through a pin-hole. The arguments (x,y,z,t) >describe where the pin-hole i.e. measuring device "is" not where >the particle "is". > >In (other) other words a wave function describes >*What We Know* about the system w.r.t. >the probability of future observations. >Before you write down a wave function >of "the universe" or any other posited >system you first need to define what >"the universe" means in terms of what >measurements you can make. > >[Second] >Given we exist within "the universe" our knowledge of the universe >cannot be complete (in the quantum sense of maximal). A measuring >device cannot measure itself. (Thermodynamics is an integral part >of the measuring process, you must have a heat sink to cool the >amplifier which amplifies the measurement signal. Measurement >is by definition gain > 0 dB.) >So at best we should only define a Density Operator for the universe. > >[Third] >Having said this let me point out that "outside the universe" >also has no operational meaning. If we can reach it then it >can reach us in the sense of exchanging interactions and thus >it is part of "The Universe" where one defines "The Universe" as >a limiting case of the largest system possible. > >Quantum theory does not posit extra dimensions or multiple universes. >(Unless you need a plot device for a Science Fiction script.) >It simply gives us a more "stable" description of what we may >observe and how one act of observation can alter the outcome >of future observations. > >Too many people have become confuse by reading >"Many Worlds" re-interpretations of quatum theory >which essentially tries to re-embed quantum theory >within a larger classical framework. >The quantum language is by definition >a more general language than classical. >It simple relaxes assumptions about actions you may >effect on a system (i.e. that all acts of >observation commute and thus have no effect on one another.) > >The same sort of confusion arises when you play with relativized >space-time but try to stick it back into a context of absolute time. >In the example of the twin paradox the question of >"Which twin is *really* older?" is just such a case. >Once you relativize you mustn't frame questions in >terms of the old absolutes. > >Quantum theory relativizes the concept of physical state. >Holding on to the same absolute assumptions that one has >relativized again leads to a paradox. In >this case the EPR paradox in the form of assumptions >about classical probabilities intrensically vested >with a particle when the probabilities of QM are >intrensically non-local correlations between system >and observer. > >"Collapsing wave functions" are exactly analogous to >"collapsing average value of a Lotto ticket" when the drawing >is held. > >[Fourth] >I would qualify that the above doesn't preclude >extra "hyperspace" dimensions. However assuming >there are we must ask if our "universe" is a "thin membrane" or >if rather it extends into those extra dimensions completely. >It is one thing to posit extra dimensions >and quite another to assume physical objects are already >localized (i.e. thin) with respect to those dimensions. >This is the point of string theory. (although it too has >serious conceptual problems.) > >Our three dimensional >spatial picture may be just a cross section of a broader >picture where an electron is a rope looping around >4 dimensional hyper-space (ignoring time). But again >these are just mental pictures. The physics is in how >things behave and that is described mathematically >where one can add as many dimensions as one wants. >Don't confuse the mathematical model for the physics. > >Too many people do including >quite a few PhD's in Physics. > >Finally with regard to "Tunneling". Tunneling >is a phenomenon where classical theory predicts >something is impossible while quantum theory >predicts it is improbable but not impossible. >Think of tunneling this way. I design an >escape-proof prison. Suppose that I can prove >mathmatically that . . . > >***provided the guards follow protocol*** > >it is impossible for you to escape. >But in the "real world" the guards >are only human and mistakes >have a certain probability of occuring. >Given this then I can only prove in the >real world that the probability of you >escaping is very very small. > >The classical potential describing >say the binding of an electron within >an atom is just an idealized average description >of the quantum electromagnetic field. >The actual electromagnetic field you >could view as "noisy" but its better just >to say >****The e-m potential only has meaning in QM >as defined by the probabilistic behavior of a charged particle*** > >There is only a mystery when you try to >impose a classical picture on the behavior. >Anytime you see the word "potential" remember >it is an artifact used to describe how something >else behaves. > >Carry this back to descriptions of "The Universe" >"The Universe" is not a four dimensional space-time >manifold with fields affixed to it. That is just a model. >The universe is a bunch of interacting processes. >The structure of their interaction we break down into >pieces which classically we describe as localized particles >or continuous waves. Both are imperfect approximations and >the "quantum wave functions" are just a bit better. >The best description is simply to look at how different >actions correlate. "Look for a click here after seeing a click there" >(where what's clicking we classically describe as a particle detector.) >This is usually done with the operator algebras or Lie groups. >(And usually done badly with infinite dimensional versions, hence >the problems with divergences when quantizing gravitation.) > > >Regards >J. Baugh