Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!burl!ulysses!gamma!epsilon!zeta!sabre!petrus!bellcore!decvax!tektronix!hplabs!sri-unix!mikes@AMES-NAS.ARPA From: mikes@AMES-NAS.ARPA Newsgroups: net.physics Subject: none Message-ID: <520@sri-arpa.ARPA> Date: Thu, 29-Aug-85 20:05:17 EDT Article-I.D.: sri-arpa.520 Posted: Thu Aug 29 20:05:17 1985 Date-Received: Mon, 2-Sep-85 03:58:55 EDT Lines: 83 From: mikes@AMES-NAS.ARPA (Peter Mikes) Subject: QM, Many-Worlds, and holes in the argument response to: rimey@ucbmiro.ARPA (Ken Rimey) Article-I.D.: <117@ucbmiro.ARPA> Ken: You people don't know the Many Worlds Interpretation of Quantum Mechanics from a hole in the ground. Consider the apparatus used for this two-outcome experiment, and consider the experimenter. The two, together with the rest of the universe, constitute a single closed system that, according to quantum mechanics, evolves completely deterministically. For simplicity, imagine that the universe consists solely of the apparatus and the experimenter. Furthermore, imagine that the apparatus has only three possible states, A, B, and P, where P will be its state before the experiment; and imagine that the experimenter has only three possible states: p - where he sees the apparatus in state P a - where he continuously thinks "Aha! The outcome was A" and b - where he continuously thinks "Aha! The outcome was B" The universe starts out in state Pp, and eventually evolves into (Aa+Bb)/1.4.. ..probability ideas are involved only in the interpretation of the mathematical object that represents the state of the universe. They don't clutter up the theory of how to calculate that object. In particular, in the Many-Worlds view, wave functions don't "collapse". Comment: Actually, in QM THEORY the wave functions do not "collapse". (The solutions of Schrodinger eq. are not only continuous but even smooth..) The collapse is part of the interpretation, which is a necessary step in applying the theory to real world. In Copenhagen interpretation we discard the part of the solution which does not fit our new image of the universe(=collapse). In Many-World we delegate it to a 'land of shadows' (= an alternative not chosen for us) In both cases we violate the Schrodinger equation, which does not allow ANY deletion of 'unneeded states'. ( By 'new' we mean here 'post measurement' - when P evolved into either A or B ( or (A+B)/1.4.. ??)). [ The real issue is the evolution semigroup: The theory itself pictures the evolution as (loosely) a rotation (in the Hilbert space). That (I think) is what 'deterministic evolution' term used above means. Once you admit the existence of branches, selective collapsing etc - you change that group into a semigroup (such as involved in the difussion process.) In this sense the Many-World is a nice visualisation of the basic problem of the orthodox interpretation: The theory dictates a group - but when you learn something ( when YOU - the observer get information ) - is that described by such a 'rotation' - or is it 'irreversible'?. Considering how old this issue is, there is a surprising amount of ill-defined fuzzy concepts involved..) ] The connection with information (and so with measurement) is a necessary consequence of interpreting the wave function as (related to) a probability. [ Probability implies lack of information. It actually implies the 'state' (what QM would call a classical state) which is not fully known]. The ortho- dox interpretation is using these concepts and definitions ( you actually SOMETIME talk about WHERE the particle landed) but obfuscates everything by defining the wave function as a 'state' of the system and than denying that the original (classical) state exists. Would it not simplify discussion if we would decouple the concept of probability/information from the concept of the wave_function/state_of_the_system? (i.e. if we would discard the Copenhagen interpretation and it's Many_World dramatisation??). Ken: On the other hand, it is often suggested that quantum mechanics is incomplete. The problem is that the rules for how a system changes state when it is measured seem to be central features of quantum mechanics, and yet these rules make explicit reference to measurement, as if the observer played a distinguished role in the universe. This difficulty motivates much crackpot physics. If you believe the Many-Worlds idea, then this is not a problem, and rather than waste time on crackpot physics, you can get back to working on faster integrated circuits for SDI. Comment: This is a cheap shot: While the role of an (conscious) observer does open the door for many crackpot ideas - the conclusion that spending time thinking about unresolved problems in the the founda- tions of the QM is a waste of time does not follow! (At least such conclusion uses "guilt by association" reasoning). This looks like classical example of the 'practical' vs 'theoretical' approach to physical research: As usual - the brokers lead the heard astray - so that Zarathustras can work in uncrowded mountain resorts. The right solution is not to slam the door but to examine the ideas coming in to see if they relate to crack in pot or in the foundations.