*A. J. Leggett*

- Published in print:
- 2006
- Published Online:
- September 2007
- ISBN:
- 9780199211241
- eISBN:
- 9780191706837
- Item type:
- chapter

- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199211241.003.0005
- Subject:
- Physics, Particle Physics / Astrophysics / Cosmology

This chapter discusses three fundamental questions which the majority of the physics community believes are not worthy of attention and a minority believes are crucial and in urgent need of ...
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This chapter discusses three fundamental questions which the majority of the physics community believes are not worthy of attention and a minority believes are crucial and in urgent need of attention. The first concerns the “anthropic principle”: to what extent is it an “explanation” of basic physical data, such as the dimensionality of space-time, the values of the fundamental constants, etc., to observe that were they appreciably different, human life and consciousness could not have evolved to the point of asking the question? The second has to do with the “arrow of time”: how is the everyday sense of the “flow” of time from past to future consistent with the invariance of the laws of physics under time reversal? The third problem is how to incorporate the occurrence of definite outcomes within the framework of quantum mechanics (the “quantum measurement problem”).Less

This chapter discusses three fundamental questions which the majority of the physics community believes are not worthy of attention and a minority believes are crucial and in urgent need of attention. The first concerns the “anthropic principle”: to what extent is it an “explanation” of basic physical data, such as the dimensionality of space-time, the values of the fundamental constants, etc., to observe that were they appreciably different, human life and consciousness could not have evolved to the point of asking the question? The second has to do with the “arrow of time”: how is the everyday sense of the “flow” of time from past to future consistent with the invariance of the laws of physics under time reversal? The third problem is how to incorporate the occurrence of definite outcomes within the framework of quantum mechanics (the “quantum measurement problem”).

*Gershon Kurizki and Goren Gordon*

- Published in print:
- 2020
- Published Online:
- July 2020
- ISBN:
- 9780198787464
- eISBN:
- 9780191829512
- Item type:
- chapter

- Publisher:
- Oxford University Press
- DOI:
- 10.1093/oso/9780198787464.003.0004
- Subject:
- Physics, Theoretical, Computational, and Statistical Physics, Particle Physics / Astrophysics / Cosmology

Chapter 4 introduces a great QM mystery: the notion of quantum measurements. Henry is in a superposition of versions localized in several places, but when Eve measures Henry’s position she (as a ...
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Chapter 4 introduces a great QM mystery: the notion of quantum measurements. Henry is in a superposition of versions localized in several places, but when Eve measures Henry’s position she (as a classical observer) either sees Henry or she does not. Physical reality is made of such measurements. Eve’s measurement projects or collapses Henry’s superposition state to a single location. The meaning of quantum-state or wavefunction “collapse” and the role of the observer have been at the heart of the historical debate concerning the interpretation of QM. Whereas Von Neumann and Wigner stressed the inseparability of the observed (measured) world from the human mind, alternative “observer-free” views were suggested, such as Everett’s many-world interpretation or Zurek’s quantum Darwinism that replaces the observer by the environment. In the appendix to this chapter the notion of probability amplitudes is elucidated, new notations for operators are introduced and projection operators are presented.Less

Chapter 4 introduces a great QM mystery: the notion of quantum measurements. Henry is in a superposition of versions localized in several places, but when Eve measures Henry’s position she (as a classical observer) either sees Henry or she does not. Physical reality is made of such measurements. Eve’s measurement projects or collapses Henry’s superposition state to a single location. The meaning of quantum-state or wavefunction “collapse” and the role of the observer have been at the heart of the historical debate concerning the interpretation of QM. Whereas Von Neumann and Wigner stressed the inseparability of the observed (measured) world from the human mind, alternative “observer-free” views were suggested, such as Everett’s many-world interpretation or Zurek’s quantum Darwinism that replaces the observer by the environment. In the appendix to this chapter the notion of probability amplitudes is elucidated, new notations for operators are introduced and projection operators are presented.

*Jeffrey Bub*

- Published in print:
- 2016
- Published Online:
- March 2016
- ISBN:
- 9780198718536
- eISBN:
- 9780191819643
- Item type:
- chapter

- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780198718536.003.0010
- Subject:
- Physics, Particle Physics / Astrophysics / Cosmology

The interaction between Schrodinger’s cat and a radioactive atom is the sort of thing that happens in a quantum measurement. The “measurement problem” of quantum mechanics is to explain how the cat ...
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The interaction between Schrodinger’s cat and a radioactive atom is the sort of thing that happens in a quantum measurement. The “measurement problem” of quantum mechanics is to explain how the cat can be considered to end up either definitely alive or definitely dead if the final state is an entangled state of the radioactive atom and the cat. This chapter discusses Bohm’s theory and the Everett interpretation as solutions to the measurement problem. The Everett interpretation is fundamentally a proposal to solve what Pitowsky calls the “big” measurement problem: how a quantum measurement can produce a definite outcome. On the information-theoretic interpretation proposed here, the “big” measurement problem is a pseudo-problem. The chapter concludes with some remarks on the “small” measurement problem: how a classical probability distribution over macroscopic measurement outcomes emerges in a measurement process – how probabilities of “what you’ll obtain if you measure” become transformed to probabilities of “what’s there.”Less

The interaction between Schrodinger’s cat and a radioactive atom is the sort of thing that happens in a quantum measurement. The “measurement problem” of quantum mechanics is to explain how the cat can be considered to end up either definitely alive or definitely dead if the final state is an entangled state of the radioactive atom and the cat. This chapter discusses Bohm’s theory and the Everett interpretation as solutions to the measurement problem. The Everett interpretation is fundamentally a proposal to solve what Pitowsky calls the “big” measurement problem: how a quantum measurement can produce a definite outcome. On the information-theoretic interpretation proposed here, the “big” measurement problem is a pseudo-problem. The chapter concludes with some remarks on the “small” measurement problem: how a classical probability distribution over macroscopic measurement outcomes emerges in a measurement process – how probabilities of “what you’ll obtain if you measure” become transformed to probabilities of “what’s there.”

*William J. Mullin*

- Published in print:
- 2017
- Published Online:
- March 2017
- ISBN:
- 9780198795131
- eISBN:
- 9780191836480
- Item type:
- chapter

- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780198795131.003.0008
- Subject:
- Physics, Particle Physics / Astrophysics / Cosmology

The wave function gives probabilities, but is it a real physical quantity, as in the ontic interpretation of quantum mechanics, or just a measure of human belief about a physical system, as in an ...
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The wave function gives probabilities, but is it a real physical quantity, as in the ontic interpretation of quantum mechanics, or just a measure of human belief about a physical system, as in an epistemic interpretation? These questions are introduced by treating Schrödinger’s cat and Wigner’s friend. An important question is whether quantum mechanics applies to macroscopic objects, for example, cats. SQUID experiments have been performed that see quantum interference in very large objects, whereas with most macroscopic objects quantum interference disappears because of decoherence. Various views of the meaning of quantum mechanics are discussed: the Copenhagen interpretation, continuous spontaneous localization, Everett’s many-worlds interpretation, Bohm’s hidden-variable model, and quantum Bayesianism. The latter holds that the wave function represents one’s best belief about the state of a quantum system, rather than an actual physical property. This leads to discussions of recent theories and experiments explicitly designed to test wave function reality.Less

The wave function gives probabilities, but is it a real physical quantity, as in the ontic interpretation of quantum mechanics, or just a measure of human belief about a physical system, as in an epistemic interpretation? These questions are introduced by treating Schrödinger’s cat and Wigner’s friend. An important question is whether quantum mechanics applies to macroscopic objects, for example, cats. SQUID experiments have been performed that see quantum interference in very large objects, whereas with most macroscopic objects quantum interference disappears because of decoherence. Various views of the meaning of quantum mechanics are discussed: the Copenhagen interpretation, continuous spontaneous localization, Everett’s many-worlds interpretation, Bohm’s hidden-variable model, and quantum Bayesianism. The latter holds that the wave function represents one’s best belief about the state of a quantum system, rather than an actual physical property. This leads to discussions of recent theories and experiments explicitly designed to test wave function reality.