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 Hope this talk will give us opportunity to explore some things often neglected. My experience: math formalism, not philosophy or what it means. Fascinating questions, rich history of debate and controversy. Still interesting conversation, keeps philosophers employed. To do this focus on one of the whackiest interpretations: proposes infinite multiverse in which every possible result of every experiment is realized and exists in its own distinct world. Sounds like science fiction, most physicists think not serious, me included. Had occasion to look more closely and have come to appreciate some ideas not as whacky as made out. I do not expect to convince you this is the way the world works, mostly want to convince you its interesting. Might be the way the world works Contrast teaches some interesting lessons about orthodox QM, and gives us interesting alternatives to think about. Guy Blaylock - Bryn Mawr 9/21/09 Guy Blaylock - Bryn Mawr 9/21/09

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. This is common sense. Realism (tree falling in forest, scatological habits of bears in the woods) Locality is direct contact (punch in the nose), or chain of local interactions (baseball to the face), not voodoo. The only reason I mention it is because QM violates these principles. Guy Blaylock - Bryn Mawr 9/21/09

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 says we couldn’t predict even if we had all the info. QM is still predictive! It’s just not precisely predictive. It is statistical. non-determinism non-locality Guy Blaylock - Bryn Mawr 9/21/09

Guy Blaylock - Bryn Mawr 9/21/09 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. Realism says objects have independent existence and characteristics. Obviously if a theory is non-realist, it is also non-deterministic. Common sense. Only infants and philosophers would disagree. Guy Blaylock - Bryn Mawr 9/21/09

Guy Blaylock - Bryn Mawr 9/21/09 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)] Building collapse in Surat India See Sci Am Dec 2007 Everett left after graduation from Princeton to become a defense analyst, develeped generalized lagrange multipliers in operations research DeWitt Ph.D. harvard, inst. 4 advanced study, chapel hill NC, and Austin 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

Comparison with Copenhagen 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 (sort of) no collapse Complex entangled superposition When an external observer comes along, there is a very different process. Very ill-defined. Introduced as an ad-hoc piece to explain how experimenters get a single result instead of a superposition. 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

Guy Blaylock - Bryn Mawr 9/21/09 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. measurement collapse 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. Note the origin of “relative state” here Each component of the observer sees herself (and her particle) as a single entity Whatever the particle state, the experimenter must agree; that’s entanglement! DeWitt would call the separate components of the superposition separate worlds measurement entanglement Guy Blaylock - Bryn Mawr 9/21/09

Entanglement is Natural Entanglement is the natural consequence of any quantum interaction! e.g. elastic scattering: initial: + + + + … final: Entanglement is how we describe microscopic interactions in both MWI and Orthodoxy! Orthodoxy says its only different for a macroscopic ‘external’ observer. entangled state: actually a continuous superposition: Guy Blaylock - Bryn Mawr 9/21/09

Everett relative states I 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

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. Sequences have the same relative probabilties as orthodox QM 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

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. Nothing too mysterious, math formalism is the same. It’s the interpretation that’s different. In conventional qm we say we collapse to one particular result. In MWI we say we imagine following one particular branch of many possible real branches. Say each one is In this way, MWI reproduces the Born probabilities of conventional QM. Guy Blaylock - Bryn Mawr 9/21/09

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

Guy Blaylock - Bryn Mawr 9/21/09 Orthodoxy summary Orthodoxy is … non-local When 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-realist A superposition represents an undefined state. non-deterministic Collapse to a particular final state is a random process! measurement collapse or Robert M. Gingrich, Christoph Adami, “Quantum Entanglement of Moving Bodies”, Phys. Rev. Lett. 89, June 2002. Guy Blaylock - Bryn Mawr 9/21/09

Guy Blaylock - Bryn Mawr 9/21/09 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 E1 been split, two or four? Factoring shows E1 has only been split in two by her local measurement. Isn’t splitting equivalent to collapse? No, splitting is local. 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

MWI is deterministic, realist Many Worlds is … local Splitting along MWI branches is a local process. See previous. realist All 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?) deterministic The 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?) Can’t explain it for yourself in this branch. “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

Guy Blaylock - Bryn Mawr 9/21/09 The Measurement Problem I Shroedinger’s Cat Until the box is opened and examined by the researcher, the cat is in a super-position of being alive and dead. Bill the cat from Bloom County by Berk Breathed Bill the cat, known for hairballs and fleas, killed and brought back to life dozens of times, epitome of filth and moral depravity Gained respectability only after becoming lead singer for heavy metal rock band. Does the cat not know if it is alive or dead? with apologies to Berk Breathed Guy Blaylock - Bryn Mawr 9/21/09

Guy Blaylock - Bryn Mawr 9/21/09 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

Guy Blaylock - Bryn Mawr 9/21/09 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

Advantages and Disadvantages 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