Ontologische Herausforderungen der Quantentheorie

1. Ontologische Herausforderungen der Quantentheorie Daniel von Wachter http://daniel.von-wachter.de

2. Hinweise • „Bild der Wissenschaft“ 8/2004 • Hausarbeiten

3. Übersicht • Interpretations of QT • Hidden variable theories • Collapse theories • Many worlds interpretation • Decoherence interpretation (Roland Omnès) • Does QT support • indeterministic causation? • field ontology (no discoverable conditions of diachronic identity) (quantum field theory) • Why EPR correlations? • Non-locality? • Causation faster than Einstein allows? • Common cause? • What makes laws of QT true? How do they differ from laws like the law of gravitation? Observations are effects

4. Wiederholung

5. Wiederholung

6. Wiederholung: EPR • Maudlin (483): „Since we are certain that x-spin measurements on both particles are certain to give opposite results, once we have mesured particle 1 we can be certain that if the x-spin of particle 2 is measured, particle 2 will go down. That is, after the measurement performed on particle 1, the physical dispositions of particle 2 have changed. [?] And, indeed, the measurement performed on particle 1 will change the quantum state assigned to particle 2 via the collapse of the wavefunction. Furthermore, this holds no matter how far apart the two particles are: measuring one will collapse the wavefunction, and the collapse will change the quantum state ascribed to the other particle. This is what Einstain memorably called spooky action-at-a-distance.“

7. nicht-Einsteinsche Wirkung? • Eßfeld: „Es ist nicht möglich, diese Korrelationen zu nutzen, um Signale mitÜberlichtgeschwindigkeit zu senden“ • Verschränkung durch gemeinsame Ursache?

8. Feldontologie

9. Naturgesetze • Ein Naturgesetz besagt ... • Messungen sind Wirkungen. Was sollten wir daher erwarten?

10. Wave function • If only a single particle is under consideration, the wavefunction can be visualized as a field in physical space, and the Schödinger equation then specifies how that field changes with time. • Example: a polarized filter with a detector placed behind it. A vertically polarized photon passes through and makes the detector fire. A horizontally pol. photon will not pass. • Send diagonally polarized photons. The wf has a non-zero value for the detector firing as well as for not firing. Superposition. • The wf can be used for predictions assigning probabilities to the two outcomes. (Born‘s rule) • But hasn‘t the detector either fired or not? (cf. Schrödinger‘s cat) • Bell (1955): Either the wavefunction, as given by the Schrödinger equation, is not everything, or it is not right.

11. Hidden Variable Theories • HVTs say that the wf are incomplete descriptions. • Bohm: Particles always have exact locations. Additional variables for particle locations.

12. Collapse Theories • A wf is a complete description. • But sometimes things behave differently than described by the SE. • von Neumann (1955): Most of the time systems evolve in accordance with the SE, but sometimes not. The collapse of the wavefunction. • The collapse occurs when a measurement is made, and • the state collapses to a state in which the measured quantity has a definite value (eigenstate).

13. Many Worlds Interpretation • The detector both fired and did not. • The world splits into two non-interacting parts, with different outcomes in each, and we are only aware of one world, the world we now inhabit.

14. Schrödingergleichung • Beschreibt die SG • vollständig einen Zustand? • der deterministisch zum beobachteten Ergebnis führt? (haltbar?) • der indeterministisch zum beobachteten Ergebnis führt? • unvollständig einen Zustand • der deterministisch zum beobachteten Ergebnis führt? • der indeterministisch zum beobachteten Ergebnis führt?

15. Determinism • Weak vs stront d. • Bohm‘s hidden variable theory • Most collapse theories take the c. to be indet. • But: d. and indet. can be combined with most approaches to interpretation.

16. Determinateness • Exp: look for a particle with a screen • Did the particle have a position before the flash? • According to the wave function the particle was spread out. One did not ‚find‘ the particle. • Failure of determinateness is not the same as failure of dterminism. • Once the wf has spread out, the most one can say of the particle is that it is in a state which has the propensity, in varying degrees, to cause flashes on screeens at various locations.

17. The Role of the Observer • Does the quantum formalism represent anything at all as happening in the world? • What can we learn from our knowledge about measurement interactions about the systems independent from measurements? • Before the collapse of a wave function, is there a mixed state? If so what causes the collapse? If not, ... • Consciousness • Maudlin (476): „The belief that consciousness should play a central role in an interpretation of QT must ultimately rest on views about c. which are imported into the physics rather than being derived from it.“ • What does the double slit experiment show? • Can Bohm‘s HVT take the DSE into account?

18. Uncertainty and Complementarity • Does the Uncertainty Principle state a limitation on our knowledge or a more fundamental limitation on the world itself? • If the wf is complete, then one is not ignorant of anything. • If the wf is incomplete, then ...