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The Many-Weirdnesses Interpretation of Quantum Mechanics

The Many-Weirdnesses Interpretation of Quantum Mechanics. Weirdness in orthodox quantum mechanics Weirdness in the ‘Many-Worlds’ interpretation A comparison of weirdnesses MWI may be less weird than what you already believe. Guy Blaylock - Bryn Mawr 9/21/09.

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The Many-Weirdnesses Interpretation of Quantum Mechanics

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  1. The Many-Weirdnesses Interpretation ofQuantum Mechanics • Weirdness in orthodox quantum mechanics • Weirdness in the ‘Many-Worlds’ interpretation • A comparison of weirdnesses • MWI may be less weird than what you already believe Guy Blaylock - Bryn Mawr 9/21/09 Guy Blaylock - Bryn Mawr 9/21/09

  2. Characteristics of a Garden Variety Classical Scientific Theory (scientific) realism – characteristics or qualities of a system exist and are well defined, independent of any outside influence or observation. determinism – complete knowledge of the current state of a physical system is sufficient to determine the future state of the system. locality – actions at one location do not immediately have any effect at a separate location. Guy Blaylock - Bryn Mawr 9/21/09

  3. Two Characteristics of (orthodox) Quantum Mechanics The outcome of certain measurements can never be precisely predicted no matter how well you know the initial conditions. Roll the dice. What happens in one part of the universe can instantaneously affect the behavior of a distant part of the universe. The effects of these actions are not localized to one region, but rather, they permeate all space. non-determinism non-locality Guy Blaylock - Bryn Mawr 9/21/09

  4. Determinism & Realism In the orthodox interpretation of QM, the idea of non-determinism is embodied within the more extreme concept of non-realism. non-realism Non-realism implies that when an object is out of sight and isolated from its surroundings, its location becomes not only unknown, but undefined. In order for it to acquire a well-defined location, somebody must see it, or it must interact in some other way with the environment around it. Guy Blaylock - Bryn Mawr 9/21/09

  5. History of the Worlds 1957 Hugh Everett writes a thesis on the “relative state” interpretation of QM [Hugh Everett III, “Relative State’ Formulation of Quantum Mechanics”, Rev. Mod. Phys. 29, 454-462 (1957)] Bryce DeWitt popularizes, embellishes and somewhat misrepresents the concept in the “many worlds” interpretation [Bryce S. DeWitt , “Quantum mechanics and Reality”, Physics Today 23, 30-35. (1970)] The essence of Everett’s many worlds interpretation is the same as orthodox QM except that collapse does not happen. Superpositions persist. “…every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth in myriads of copies of itself.” Guy Blaylock - Bryn Mawr 9/21/09

  6. Orthodoxy Many Worlds Comparison with Copenhagen • wave function evolves via a linear deterministic wave equation • superposition of states • amplitude squared gives probability à la Born • random collapse to a single answer • ditto • ditto • ditto (sort of) • no collapse Process 1 - deterministic continuous change of wave function according to wave equation Process 2 - discontinuous change brought about by ‘observation’ MW Orthodoxy Guy Blaylock - Bryn Mawr 9/21/09

  7. measurement collapse measurement entanglement Consider the measurement of a spin 1/2 particle… The Difference Orthodoxy says before you make the measurement, the state may exist in a superposition. After a non-deterministic collapse, the system (experimenter & particle) is in one of two definite states. or MWI says after you make the measurement, the state still exists in a superposition, along with the experimenter, who is herself described by a more inclusive, entangled superposition. Guy Blaylock - Bryn Mawr 9/21/09

  8. actually a continuous superposition: Entanglement is Natural Entanglement is the natural consequence of any quantum interaction! e.g. elastic scattering: initial: + + + + … final: entangled state: Guy Blaylock - Bryn Mawr 9/21/09

  9. Everett says: even without collapse, experience of MWI observer agrees with that of the orthodox ‘external observer’. Everett relative states I Suppose an experimenter measures a spin. Two possibilities result. Moreover, repeated measurements of the same spin will yield identical results. It looks as if the particle spin has ‘collapsed’. Guy Blaylock - Bryn Mawr 9/21/09

  10. Everett relative states II Suppose the experimenter measures many identically prepared spins. Along any branch, the number of ups tends to equal the number of downs (6 branches out of 16 with 2 up and 2 down). As more measurements are done, the branches tend more and more towards equal up and down. The odds for a measurement sequence along any one branch are the same as predicted by conventional QM. Say each one is Guy Blaylock - Bryn Mawr 9/21/09

  11. Everett relative states III Suppose the amplitudes for up and down are not equal. The odds for going down any branch are given by the amplitude of that component of the superposition, just like the odds of collapsing to that particular result are given by the same amplitude in conventional QM. Say each one is In this way, MWI reproduces the Born probabilities of conventional QM. Guy Blaylock - Bryn Mawr 9/21/09

  12. Advantage of no collapse • MWI restores: • locality • realism • determinism • a sensible measurement process Guy Blaylock - Bryn Mawr 9/21/09

  13. measurement collapse Orthodoxy summary Orthodoxy is … • non-localWhen an entangled state is collapsed by interacting with one of the two entangled partners, the other partner is collapsed via a non-local process (see EPR). • e.g. • non-realistA superposition represents an undefined state. • non-deterministicCollapse to a particular final state is a random process! or Guy Blaylock - Bryn Mawr 9/21/09

  14. MWI is local Many Worlds is local!In the absence of collapse, the remaining measurement process is entanglement (or ‘entangled splitting’) and is purely local. Imagine an experiment in which one spin is measured in the  basis and the other spin is measured in the  basis. Into how many pieces has E1been split, two or four? Factoring shows E1 has only been split in two by her local measurement. When the two experimenters communicate their results to each other, each experimenter is split again, but this occurs only via a chain of local interactions at sublight speed. Guy Blaylock - Bryn Mawr 9/21/09

  15. MWI is deterministic, realist Many Worlds is … • localSplitting along MWI branches is a local process. See previous. • realistAll possibilities do in fact exist in one branch or another. Instead of one reality in an ill-defined state, there are multiple definite realities. (a little too much realism?) • deterministicThe wave functions evolve according to a deterministic wave equation and every possible results of a measurement is realized in its own world. Although an experimenter may still end up wondering how she ended up with a particular measurement result. (not usefully predictive?) “it is quite likely that at some future time we may get an improved quantum mechanics in which there will be a return to determinism” - P.A.M. Dirac Guy Blaylock - Bryn Mawr 9/21/09

  16. Shroedinger’s Cat The Measurement Problem I Until the box is opened and examined by the researcher, the cat is in a super-position of being alive and dead. with apologies to Berk Breathed Guy Blaylock - Bryn Mawr 9/21/09

  17. Wigner’s Friend The Measurement Problem II When and how does collapse occur? Wigner’s press agent Wigner’s friend Wigner Guy Blaylock - Bryn Mawr 9/21/09

  18. Quantum Suicide Tests of MWI A daring proponent of MWI presses an almost fully loaded gun to his head and pulls the trigger. If MWI is correct, he will have the experience of always surviving the suicide attempt. His consciousness continues only in those worlds where he lives. [Max Tegmark, “The Interpretation of Quantum Mechanics: Many Worlds or Many Words?”, Fortsch. Phys. 46, 855-862 (1998) and arXiv:quant-ph/9709032] Quantum Immortality See also: There’s always some branch that avoids death (debatable). We should all expect to live forever. …and on a grand scale… Observation within a model of the universe that predicts low probability of life could be evidence of MWI. [Don N. Page, “Observational Consequences of Many-Worlds Quantum Theory”, arXiv:quant-ph/9904004] Guy Blaylock - Bryn Mawr 9/21/09

  19. multiple cats • Advantages and Disadvantages • restores realism, determinism, locality • offers an answer for the measurement problem • science fiction terminology (though Fred Hoyle would approve) • risky testing • popular among cosmologists • too many cats Guy Blaylock - Bryn Mawr 9/21/09

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