On Sat, Jan 30, 2021 at 2:29 PM smitra <[email protected]> wrote: > On 30-01-2021 01:41, Bruce Kellett wrote: > > On Sat, Jan 30, 2021 at 11:20 AM smitra <[email protected]> wrote: > >> > >> This argument is wrong for two reasons. First, your definition of > >> irreversibility is wrong, it has nothing to do with the practical > >> impossibility to reverse the evolution of the state. Time evolution is > >> said to be reversible if two different initial state will evolve to two > >> different final state, which is true for unitary time evolution. > > > > You are making exactly the same mistake as was made earlier with > > Deutsch's definition of 'world'. You are using a technical definition > > that does not always relate to the usual meaning of the term. > > 'Reversible' means that the situation can be reversed. In this > > context, it means that coherence can be restored. If you want to mean > > something different, then you should use a different term, and your > > objection collapses. > > The usual meaning is wrong, the technical definition is what it is for a > good reason.
The usual meaning is what we require in these circumstances. That no physical process exists to get the initial state > back e.g. because time reversibility is not possible due to CP violation > can be the case, but that does not capture the aspect of reversibility > that one needs. Also in the example you raise with photons escaping and > whether or not you can then get interference, as that's also not a > relevant issue in the MWI when we focus on a the state of a local > observer. > The argument that I have presented does not depend on MWI. What you do in MWI is not really relevant because MWI still does not allow for the reversibility of processes involving the escape of IR photons to outer space. I am considering a particular case in order to show that universal unitary evolution does not capture an essential element of the physics. If you start by insisting that evolution is necessarily always unitary, then you have begged the question. For these reasons MWI does not get you off the hook here. > > >> The second mistake that leads to the wrong conclusion that a pure > >> state > >> evolves to a mixed state is that this requires entanglement with an > >> infinite number of physical degrees of freedom when, precisely due > >> to > >> locality (finite c), only a finite number of degrees of freedom get > >> entangled at any given time. > > > > This is technically incorrect. There is no requirement for an infinite > > number of degrees of freedom. Escape of just one IR photon to outer > > space is sufficient to destroy reversibility. Then, in order to > > reflect this irreversibility, the off-diagonal elements of the density > > matrix should be exactly zero (reducing the pure state to a mixture). > > Unitary evolution cannot give this, so unitary evolution, by itself, > > is unable to capture that whole reality about the physical state. > > You are then replacing the density matrix by the reduced density matrix > and then claiming that the reduced density matrix describes a mixed > state. That's true but irrelevant if you want to capture the whole > reality of the physical state. > What on earth is "the whole reality of the physical state"? We are looking at the physics of a particular situation in which the physical state is such that reversibility is impossible in principle, because reversibility would violate the known laws of physics. The point is that unitary evolution does not capture the reality of this situation -- the probability of reversing is known to be exactly zero, so the off-diagonal elements of the density matrix must vanish. This is a non-unitary requirement. Therefore, unitary evolution does not capture the physics of this situation. MWI does not help you here. >> What this shows is that the notion of a World is only approximate, and > >> therefore cannot play any role in defining what observation is, because > >> we obviously do observe things and that must then have a mathematically > >> exact formulation, not an approximate one, no matter how accurate > >> that approximation is. > > > > The definition of 'world' in the context of QM is made exact precisely > > because of this irreversibility. Worlds are well-defined and distinct > > precisely because they can no longer interact or recohere. The laws of > > physics ensure this. > > > > Your argument is based on replacing the exact physical state by the > reduced density matrix, so you are smuggling in the Copenhagen > interpetation in by hand, you are not really considering the MWI. > No. I agree that this is inconsistent with MWI, since MWI insists on universal unitary evolution. If such unitary evolution cannot capture essential elements of the physical situation, then it must be wrong. You simply beg the question if you insist that MWI must be correct. It is under test, so you cannot logically assume it is true from the start. However, I have not made any appeal to Copenhagen or any other particular interpretation. I am simply pointing to a physical result that must be explainable by whatever interpretation or theory you adopt. >> A definition of observation should involve defining the algorithm that > >> defines the observer and the content of the observation in terms of > >> the relevant local physical degrees of freedom. > > > > Bullshit. > > > > You make arbitrary appeals to algorithms that do not exist. Besides, > > nothing that I have said is unique to the process of conscious > > observation. It is true for any interaction whatsoever in the quantum > > domain. > > If you disagree, you could define observers and observations in the > context of the MWI in a different way and discuss that, but what you are > doing is setting up an argument that is incompatible with the MWI by > invoking collapse. > I am pointing out that some collapse is a necessary consequence of the physics -- I am not prejudging the issue as you seem to be doing. >> There is no need to define a > >> "World" which is a meaningless concept, observer's are in principle > >> only aware of their own physical state. > > > > And that physical state is part of a unique world. The word 'world' is > > useful, and has clear operational content. > > > >> That state can contain information > >> about the environment, but what matters is not the environment but > >> the computational state of the algorithm that defines the observer. > > > > Forget algorithms. We are talking about physics, here, not > > computationalism. > > > >> This can be done rigorously in the MWI by invoking entanglement to > >> get to correlations between slightly different inputs and outputs of the > >> algorithm such that the spread in the inputs and outputs is below > >> the resolution the observer can detect. > > > > So what? > > To get to a meaningful definition of observers and observations. If your > model cannot distinguish Alice from making a measurement or Bob from > making measurement, it's no good for the sort of discussions in this > email list. How do you make out that physics does not make a distinction between Alice making a measurement and Bob making a measurement? If one can only discuss things on the list by assuming from the start that computationalism and MWI is necessarily correct, then the email list is pretty useless. I thought the idea was to allow free and open discussion of the strengths and weaknesses of this type of theory. Begging the question is not a characteristic of free and open discussion of the merits of any theory. Bruce Of course, you need to simplify things, but one can then > work in a simplified model where Alice and Bob are similar programs that > have a different memory content while they can do the same sort of > measurements and store the information from those measurements in their > memories. The state Bob finds himself in is then always some bitstring > that is then in some entangled superposition with the environment, with > the terms in that superposition referring to slightly different physical > states that fall within Bob's detection resolution. So, Bob's subjective > states are then coarse grained versions of the bit string, which allows > for small counterfactual differences to physically exist (counterfactual > w.r.t. a hypothetical superobserver that can observe the exact > bitstring). These counterfactual inputs and outputs then define the > algorithm. The lack of existence of counterfactuals in a classical > determinsitc setting makes the movie graph argument possible with leads > to a paradox for computationalism. > -- 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 view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAFxXSLTTQXJZnTnrR0uynxxSeTHm-Lc-YN%2BcOXn4TJDue1zvZA%40mail.gmail.com.
