Quantum Theory: What is it about?

Transcription

1 Quantum Theory: What is it about? Duino 2007 Quantum Theory is about? 1.? = Something out there What is out there = Ontology = What IS = Quantum Theory is about something going on IS can be particles, fields, strings, membranes, flashes, something which is in physical space and which can make up things we experience, like tables and chairs and pointers...

2 2.? = Not Ontology = Quantum Theory is not about something going on BUT? = Statistics of outcomes of measurements = PROBLEM!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! apparatus measurement apparatus is made out of?? Which theory is about Mechanics??? if not Quantum = 80 years debate about Quantum Mechanics

3 1.Bohr/Heisenberg lesson of QM: physics MUST NOT BE about ontology physics must not talk about?? physics is about how an observer must speak = Wittgenstein quantised 2.Working physicists lesson of QM: Schrödinger wave function says it all = all =?? = Schrödinger s cat = PROBLEM!!!!!!!!!!!!!!! Who collapses the wavefunction? 3.Mathematical Physicists lesson of QM: physics is about observables = PROBLEM!!!!!!!!!!!!!!! Who observes?

4 1.),2.),3) need observer = What is an observer? The Lesson from QM: Mysteries!!!!!! But I rather share the view that such debates, and accompanying experiments such as those of Gröblacher et al.(nature 2007), allow us to look deeper into the great mysteries of quantum mechanics A. Aspect in Nature 2007 great mysteries of quantum mechanics Johannes Kepler (1600): Mysterium Cosmographicum (the mystery of the universe) Galileo Galilei: The book of nature is written in the language of mathematics

5 What are the mysteries? wave-particle duality: The development of quantum mechanics early in the twentieth century obliged physicists to change radically the concepts they used to describe the world. The main ingredient of the first quantum revolution, waveparticle duality.. A. Aspect in Nature 2007 irreducible randomness and reality: Hopes of keeping quantum mechanics real have been dashed by new measurements of neutrons quantum behaviour. Despite what our classical sensibilities require, the world is indeed fundamentally random. Gregor Weihs in Nature 2007 entanglement: Thanks to ideas developed by Albert Einstein and John S. Bell, another essential quantum ingredient, entanglement, is now leading us through the conceptual beginnings of a second quantum revolution this time based on quantum information... A. Aspect in Nature 2007

6 Mystery wave-particle duality? trivial resolution as paticle and wave in de Broglie-Bohm (Bohmian Mechanics): Mystery irreducible randomness and reality? Pauli in a letter to Born, 1954 about Einstein, the realist :...I was unable to recognize Einstein whenever you talked about him in either your letter or your manuscript. It seemed to me as if you had erected some dummy Einstein for yourself, which you then knocked down with great pomp. In particular Einstein does not consider the concept of determinism to be as fundamental as it is frequently held to be (as he told me emphatically many times)... he disputes that he uses as a criterion for the admissibility of a theory the question Is it rigorously deterministic?... he was not at all annoyed with you, but only said that you were a person who will not listen...

7 Mystery entanglement? Schrödinger s wavefunction is entangled: Schrödinger s cat Mystery:Einstein and Bell = entanglement? Einstein (EPR) and J.S. Bell: Entanglement creates superluminal action between entangled physical systems. EPR+Bell = nature is nonlocal Mystery of Quantum Mechanics? NO!!!!! Fact of nature

8 The dummy Realism knocked down with great pomp Simon Gröblacher,... Anton Zeilinger Nature 2007: Most working scientists hold fast to the concept of realism a viewpoint according to which an external reality exists independent of observation. But quantum physics has shattered some of our cornerstone beliefs. According to Bells theorem, any theory that is based on the joint assumption of realism and locality (meaning that local events cannot be affected by actions in space-like separated regions) is at variance with certain quantum predictions. Experiments with entangled pairs of particles have amply confirmed these quantum predictions, thus rendering local realistic theories untenable. Maintaining realism as a fundamental concept would therefore necessitate the introduction of spooky actions that defy locality. Here we show by both theory and experiment that a broad and rather reasonable class of such nonlocal realistic theories is incompatible with experimentally observable quantum correlations.

9 If no one observes? Is the moon still there if nobody looks? Einstein Does this mean that my observations become real only when I observe an observer observing something as it happens? This is a horrible viewpoint. Do you seriously entertain the thought that without observer there is no reality? Which observer? Any observer? Is a fly an observer? Is a star an observer? Was there no reality before 109 B.C. before life began? Or are you the observer? Then there is no reality to the world after you are dead? I know a number of otherwise respectable physicists who have bought life insurance. By what philosophy will the universe without man be understood? Richard Feynman: Lecture Notes on Graviation What if everything is an illusion and nothing exists? In that case, I definitely overpaid for my carpet. Woody Allen

10 What is observation? Irrational (unspeakable) quantum mechanics It would seem that the theory [quantum mechanics] is exclusively concerned about results of measurement, and has nothing to say about anything else. What exactly qualifies some physical systems to play the role of measurer? Was the wavefunction of the world waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer, for some better qualified system... with a Ph.D.? If the theory is to apply to anything but highly idealized laboratory operations, are we not obliged to admit that more or less measurement-like processes are going on more or less all the time, more or less everywhere. J.S. Bell, Against measurement

11 Quantum Theory is about something all the time, everywhere The world (including apparatuses) is made out of something = All mysteries disappear unromantic view of quantum mechanics (J.S.Bell) Quantum Theories without observers Two possibilities: Bohmian Mechanics GRW Collapse not part of theory (effective collapse) Bohmian Mechanics (David Bohm 1952 (de Broglie 1927)) Collapse as part of the theory (real collapse) GRW: Ghirardi-Rimini-Weber GRW-P (Pearle)- CSL Theory Continuous Spontaneous Collapse

12 Bohmian Mechanics What is Bohmian Mechanics about? particles and wavefunction What is real in Bohmian Mechanics? particles (positions) and wavefunction Q, ψ i h t ψ = Hψ ψ(t) Q(t) Determinism versus Irreducible Randomness? or Determinism yields Irreducible Randomness? Bohmian mechanics is deterministic.

13 BUT Theorem: In a typical Bohmian universe holds Quantum Equilibrium (QE) QE: If a systems wave function is ψ then its particles coordinates are distributed according to ψ 2. The proof is a Boltzmannian statistical analysis of Bohmian mechanics Bohmian Mechanics Quantum Formalism Bohmian Mechanics Limits of Knowledge. In QE is not possible to know the configuration Q(t) of a system more accurately than the

14 ψ 2 distribution allows, where ψ(q) is the system s wave function. Thus there are limits of knowledge: no matter what technology an observer uses, there are in-principle, insuperable limits to what she or he can find out about the world without paying the price of drastically changing the system s wave function, and thus the future trajectories. Analogoy: BM QM = NewtonianMechanics Epicycle T heory

15 IS is and what it is says the theory Bohmian mechanics (and also GRW) are rational (speakable) theories about the universe. They are about ontology. The ontologies are different. Which ontology is right, i.e. which theory is right? We can t say at the moment. We need to think harder, we need to understand relativity and non-locality. The true theory lies in our heads. When we spell it out, we say what IS. To deny that physics can spell out what IS, is to deny physics. The pysical world is real....[that] statement appears to me, however, to be, in itself, meaningless, as if one said: The physical world is cock-a-doodle-do. It appears to me that the real is an intrinsically empty, meaningless category (pigeon hole)... Who said that? Einstein the realist