Quantum Mechanics by Dr Steven Spencer
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This document provides an outline and overview of quantum mechanics and how our understanding of matter has developed over time. It discusses how classical physics no longer fully explained phenomena at the atomic and subatomic levels. Early quantum theory developed models using quanta, photons, wave-particle duality, and matrix and wave mechanics to describe the quantum realm. Later developments showed the complex and strange nature of quantum systems, including superposition, entanglement, and how observation influences outcomes. The document examines interpretations of quantum mechanics but leaves open what it ultimately means for our understanding of reality at the quantum scale.
Quantum Mechanics by Dr Steven Spencer
- 1. How We See the Ma,er That Makes All Things Steven Spencer Applied Mathema;cian Philosophers Corner 2nd October 2012 1
- 2. Outline • The world of classical physics • Things don’t quite add up! • Enter the quantum • The mechanics and the quantum • Stranger and stranger • What does it mean? • Where is the mind in all this? • Are you sure? 2
- 3. The world of classical physics • Determinis,c (Laplace 1749 -‐ 1827) • External observer and isolated experimental systems • Par,cles (Newton 1642 -‐1727) • Waves (Hooke, Huygens, Young, Maxwell) 3 • Atomic theory (Dalton)
- 4. Things don’t quite add up • Black-‐body thermal radia,on → colours • Stable atoms • Line emission spectra • Photoelectric effect 4
- 5. Enter the Quantum • The physics of atomic and sub-‐atomic scales? • Energy is emiUed in ‘bits’ (quanta) (Planck, 1900) → tricky maths? • Energy in light is in ‘grainy bits’ (photons) which are par$cles with wave proper,es → photoelectric effect (Einstein, 1905) 5
- 6. Two views of EM radia;on • Light is a wave! Light is a par,cle! 6
- 7. The Mechanics and the Quantum • Orbital atomic theory (Rutherford, 1911) • Quan,sed electron orbit atomic model (Bohr, 1913) → ‘flights and perchings’ • Par,cles are waves! (de Broglie, 1924) → orbi,ng electrons have integer wavelengths! • Matrix mechanics (Heisenberg, 1925) → unanschaulich atomic world! 7
- 8. • Wave mechanics (Schrodinger, 1926) → how the quantum state of a system evolves with ,me. Anschaulich atomic world? • Complementarity – wave-‐par,cle duality • Determinis$c descrip,on for wave func$ons + Sta$s$cal descrip,on of maUer and energy. • Uncertainty principle (Heisenberg, 1927) Momentum and posi$on cannot be simultaneously measured with unlimited precision. 8
- 9. The observer and the system • The role of the classical observer (measurement) of a quantum system becomes central and hotly contested – “We are not only observers. We are par,cipators. In some strange sense, this is a par,cipatory universe.” – John A. Wheeler 9
- 10. The Quantum World? • Wave func,on evolu,on U(objec,ve & determinis,c) + measurement R(subjec,ve) = confusion?!? 10
- 11. Electron Double Slit Experiment (Feynman) • Detector ‘D’ turned on or off by observer → affects paUern on screen by double slit. Observer affects outcome of experiment! 11
- 12. Stranger and stranger • Superposi;on of states → possible in quantum but not classical world (Schrodinger’s cat). • Quantum emtanglement → Many par,cle systems have a complicated combined wavefunc,on even at ‘large’ distances! • Einstein-‐Podolski-‐Rosen (EPR) effect – QM is either ‘non-‐local’ (faster than light influences) or is not a complete theory! • Bell inequali,es – QM wins again! 12
- 13. Schrodinger’s Cat Thought Experiment! • Quantum superposi,on affects the classical world? 13
- 14. ‘Spooky ac;on at a distance’ (Einstein) • Before detec,on the electron wave is ‘everywhere’, at detec,on the wave func,on collapses throughout the universe! 14
- 15. • Single source of two photons – ‘spin’ observa,on method at one end affects observa,on at a distant point! Do par,cles communicate with one another or are they one en,ty ?!? 15
- 16. • Standard QM violates Bell’s theorem. Separated par,cles are connected (entanglement)!?! • Result confirmed by experiment (Aspect, 1982)!! 16
- 17. What does it mean? • Star;ng point – no experiment has ever been found that concludes against QM maths! • Whatever happened to determinism? “God does not play dice” – A. Einstein vs “A physical object has an ontologically undetermined component that is not due to the epistemological limita,ons of physicists’ understanding” – A. Eddington 17
- 18. • Ontologies (interpreta;ons) – a) Copenhagen (Bohr, Heisenberg and Born, 1927) – Complementarity + uncertainty principle + measurement + correspondence principle. QM describes knowledge NOT reality! “There is no quantum world. There is only an abstract physical descrip,on.” -‐ N. Bohr “The idea of intermediate kinds of reality was just the price one had to pay” – W. Heisenberg 18
- 19. b) Many-‐worlds / rela;ve state (EvereU, 1957) – universal wavefunc,on never collapses → mul,verse c) Environmental decoherence – rapid disappearance of quantum superposi,ons by interac,on with environment (useful for many interpreta,ons). d) Ensemble (Einstein) – Minimalist, sta,s,cal – wavefunc,on for large numbers of par$cles only. e) Rela;onal – different observers see different quantum states. 19
- 20. f) Pilot-‐wave (de Broglie, Bohm) – Par,cles guided by wavefunc,on. Non-‐local, holis,c universe, hidden variables. g) Objec;ve collapse (Penrose) – physical mechanism of collapse – extended QM. h) Conciousness collapse -‐ subjec,ve reduc,on (von Neumann/Wigner) & par,cipatory anthropic principle (J.A. Wheeler) g) New theories (many!) – objec,ve R wave func,on collapse / non-‐linear U func,on. 20
- 21. Where is the mind in all this? • What is a brain? • Dense network – more than 104 cell bodies and km of wiring per cubic mm! • Mul,ple cell types: spiking Q: Is the mind neurons (1011 cells) for info (consciousness) processing, analog neurons highly organised & ‘supporter’ cells . brain activity? 21
- 22. Neuronal Behaviour ⇒ 22
- 23. Quantum Theory of Mind • Non-‐algorithmic thought → cannot be modelled by a digital (Turing) computer. • Consciousness as a quantum mechanical phenomenon (Penrose & Hameroff) – Hypothesis: Neuron microtubules within neurons support quantum superposi,ons (non-‐computable behaviour) + macroscopic quantum entanglement across brain. • Highly controversial – decoherence counter-‐argument! • Physical collapse of quantum wavefunc,on of microtubules essen,al for consciousness (Orch-‐OR). 23
- 24. Complex Dynamical Behaviour Theory of Mind • Intrinsic non-‐linear dynamics of each individual neuron + network dynamics (aUractors, bifurca,ons of behaviour, small changes in inputs lead to large changes in outputs). • ‘Simple’ non-‐linear models found for behaviour of individual spiking neurons → reproduce complex burs,ng behaviour. • Apparently non-‐algorithmic behaviour (some,mes chaos) from algorithmic (determinis,c) components. • Consciousness as an emergent phenomenon from a neural network complex dynamical system of the physical brain. 24
- 25. Are you sure? • An underlying level of reality? • Quantum state decoherence and gravitons? • Non-‐linear quantum theory • The classical-‐quantum divide -‐ looking for decoherence • To be con;nued… 25
- 26. THANK YOU!!! 26