On Thu, Nov 16, 2017 at 10:20:45AM +1100, Bruce Kellett wrote: > On 16/11/2017 9:14 am, Russell Standish wrote: > > But not all measurements are measurements of the position of > > something. What about measuring the voltage of a circuit using an A->D > > converter? > > A surrogate measurement of the position of a pointer on a voltmeter. >
There is no d'Arsenval style meter here with a pointer, just an A->D converter + whatever recording device you want to attach to it. Where is the position measurement in that case? > > Or the measurement of the momentum of a charged particle in > > an electron multier? > > Do you mean "electron multiplier"? Would than not be an energy measurement? The direction component of the momentum can be determined by the orientation of the multiplier tube. At least that is the sort of setup I seem to remember using during my honours thesis - of course at 30+ years ago, my memory of the details could be a bit unreliable. > Momentum of a charged particle is often measured by tracking the path of the > charged particle in a magnetic field. > > > > > In MWI, we normally assume that there are > > > > two branches of the universe with different choices made by the > > > > experimenter. > > > That is really an oversimplification. It is done because it is simpler to > > > work with two-state systems, and position measurements are of a continuous > > > variable, so are not neatly two-valued. > > > > > The choice between circularly polarised filter and linear polarised > > filters is binary. Obviously, there follows the choice of orientation, > > which is continuous... > > Whatever the choice of orientation, the outcome is binary, pass or not pass. > Yes - but I was referring to what is the preferred basis, not the experimental outcome. > > > > > Unless there is some sort of superdeterminism in play, > > > > where the experimenter does not have the freedom to choose. But > > > > superdeterminism is certainly not a popular idea. > > > No, superdeterminism does not have many advocates. > > > > > > > > Observers have nothing to do with it. In Zurek's account, it is the > > > > > fact > > > > > that the results of interactions, be they measurements or not, are > > > > > recorded > > > > > multiple times in the environment via decoherence, that is the mark > > > > > of an > > > > > irreversible quantum event. > > > > If you put a system in contact with a completely symmetric heat bath, > > > > there will be no preferred basis selected by einselection. > > > The environment of a measurement or an interaction is not generally a > > > symmetric heat bath. > > If there is no experimenter, just an environment, then we must > > consider all possible environments in superposition. That will have > > maximal symmetry. > > "All possible environments in superposition"? That is seriously > under-defined. Besides, that would only have a pseudo-symmetry. > What is pseudosymmetry in this context? The only meaning that term seems to have on the internet relates to when two crystals intergrow (or "twin") giving rise to an apparent crystal symmetry. That doesn't seem applicable here. > > > > If you measure a spin component (space quantization) > > > you get one of two spots on a screen downstream of the S-G magnet. These > > > are > > > not symmetric wrt the rest of the environment. > > That is because we're considering an SG experiment, with an SG > > experimenter. That breaks the symmetry. > > The environment breaks the symmetry. The environment may contain an > experimenter, but need not. A camera would do the job. The camera merely decoheres the system, which remains in a superposition of the two possible outcomes of the SG experiment. To break that symmetry requires an observer looking at the photo plate, or downloading the image from the camera's CCD and observing it on a screen. > > In the first place, it is unlikely that all possible outcomes of an > experiment are equally likely. But I think you are confusing symmetry > breaking with the observer self-locating in one of the possible outcome > worlds. Where is the confusion? Prior to self-location, the observer is in all possible worlds, afterwards in just one. > That breaks the symmetry for him, perhaps. But he is not important > for that, because any future developments in the separate worlds will break > any residual symmetry. The observer is nothing more than just one > possibility for such a future development. > Einselection imposes no more symmetry breaking than the asymmetry present in the environment. For that assymetry to get into the environment requires conscious self-location (to use your term). > > > The many worlds > > > in QM are not symmetric anyway. > > > > > Not completely, but far more symmetric that the world we inhabit. > > Only in that the many worlds of QM contain more possibilities. Observers are > not necessary for breaking this symmetry. The bottom line is that > observation is not necessary for the existence of the world. Observation may > tell us about this particular world, but that is secondary. > I still don't see that. See my response to your Stern-Gerlach camera example above. > Bruce > > -- > 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 [email protected]. > To post to this group, send email to [email protected]. > Visit this group at https://groups.google.com/group/everything-list. > For more options, visit https://groups.google.com/d/optout. -- ---------------------------------------------------------------------------- Dr Russell Standish Phone 0425 253119 (mobile) Principal, High Performance Coders Visiting Senior Research Fellow [email protected] Economics, Kingston University http://www.hpcoders.com.au ---------------------------------------------------------------------------- -- 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 [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
