This volume tries to continue a tradition of reviews of the contemporary research on the foundations of modern physics begun by the volume on the Einstein­ Podolsky-Rosen paradox that appeared a few years ago. (I) Its publication coin­ cides with the hundredth anniversary of de Broglie's birth (1892), a very welcome superposition, given the lasting influence of the Einstein-de Broglie conception of wave-particle duality. The present book, however, contains papers based on a broad spectrum of basic ideas, some even opposite to those that Einstein and de Broglie would have liked. The order of the contributions in this book is alphabetical by first author's name. It is important here to stress the presence of three reviews of fundamental experimental data, by Hasselbach (electron interferometry), Rauch (neutron interferometry), and Tonomura (Aharonov-Bohm effect). Hasselbach reviews several interesting experiments performed in 1Ubingen with the electron biprism interferometer. Wave-particle duality is brought out in striking ways, e. g. , in the buildup of an interference pattern out of single events. The Sagnac effect for electrons is also discussed. The chapter by Rauch presents interesting results on wave-particle duality for neutrons. Of particular interest are the differences between stochastic and deterministic absorption in the neutron interferometer, and the concrete evidence for the quantum-mechanical 41T-symmetry of spinors. In the short chapter by Tonomura, conclusive evidence for the reality of the Aharonov­ Bohm effect is reviewed, collected in experiments based on advanced technologies of electron holography and microlithography.

1 Probability, Pseudoprobability, Mean Values.- 1. Introduction.- 2. Pairs of Operators.- 3. Wigner's Pseudodensity.- 4. Probability Spaces.- 5. Quantum Mechanics and Fluid Dynamics.- 6. Marcinkiewicz Space and Stationary Functions.- 7. Conclusion.- 2 Local Vacua.- 1. From Bell's Theorem to Where?.- 2. Some Basic Facts about Nonlocality.- 3. How to Get Rid of the Wave Function Collapse.- 4. The Physical Nature of the Local Vacuum.- 5. Dirac's Covariant Vacuum as an Example.- 6. Conclusion and Outlook.- 3 Causal Quantum Theory: Why a Nonstarter?.- 1. Introduction.- 2. Formalism and Interpretation: An Example.- 3. The "Copenhagen" Interpretation.- 4. A Logically Possible, Empirically Viable Alternative: Causal Interpretations.- 5. A Modern "Double-Slit" Experiment.- 6. The Value of an Alternative Interpretation.- 7. Oposing Commitments, Opposing Schools.- 8. Forging the "Copenhagen" Interpretation.- 9. An Alternative Historical Scenario?.- 10. Internal versus External Explanations.- 4 Duality of Fluctuations, Fields, and More.- 1. Introduction.- 2. Duality of Fluctuations.- 3. Duality of Fields.- 4. Duality of the Radiation Process.- 5. The Unobservable Wave.- 6. Discussion: Measurement and Duality.- 5 Dualism within Dualism: Open Questions.- 6 Recent Contributions of Electron Interferometry to Wave-Particle Duality.- 1. Introduction.- 2. The Novel Electron-Optical Biprism Interferometer.- 3. Electron Interferometric Verifications of Wave-Particle Duality.- 7 The Aharonov-Bohm Effect From the Point of View of Local Realism.- 1. Introduction.- 2. Theory of the AB Effect.- 3. Looking Beyond the Mathematical Formalism.- 4. Local Realist Interpretation of the AB Effect.- 8 Are Two-Beam Self-Interferences Mass-Independent? Not Thoroughly Known (?) Role of theMass.- 1. Introduction and Historical Backgrounds.- 2. Propositions of New Test Experiments.- 3. Is the Dirac Assumption Approximate or Not?.- 4. Hypothetical Model: The In-flight Transition before Arrival at Detector.- 5. Theoretical Perspectives.- 6. Epistemological Remarks.- 9 Wave Mechanics and Relativity.- 1. Can We Reach Wave Mechanics Starting from Classical Mechanics?.- 2. Minimal Conditions of Wave-Particle Dualism: They Are Incompatible with Classical Mechanics and Require Relativity.- 3. A Note about a Reasoning of Hamilton.- 10 Unsharp Particle-Wave Duality in Double-Slit Experiments.- 1. Introduction.- 2. The Photon Split-Beam Experiment.- 3. The Unsharp Measuring Process.- 4. Information Theoretical Considerations.- 5. Realization of the Unsharp Joint Measurement of P(A) and P(B).- 6. Neutron Interference Experiments.- 11 Some Arguments against the Existence of de Broglie Waves.- 1. Distinguishing Schrödinger's from de Broglie's Waves.- 2. De Broglie's Initial Concept of de Broglie Waves.- 3. Discussion of Models.- 4. Conclusion.- 12 On the "Completeness" of Quantum Mechanics.- 1. Background.- 2. Covering Theories: An Example from Electromagnetism.- 3. A Simple Covering Theory of Quantum Mechanics.- 4. Alternative "Necessitations" of the Covering Theory, Equation (2).- 13 Neutron Interferometric Tests of Quantum Mechanics.- 1. Introduction.- 2. Stochastic versus Deterministic Absorption.- 3. Wave Particle Measure.- 4. 4?-Symmetry of Spinors.- 5. Spin State Interferometry.- 6. Double Coil Experiments.- 7. Macroscopic Quantities in Uncertainty Relations.- 8. Experiments in Progress or in Preparation.- 9. Discussion.- 14 Gedanken Experiments on Duality.- 1. Approach.- 2. Wave-Particle Duality.- 3. Nonlocality.- 4. Combining Experiments onNonlocality and Wave-Particle Duality.- 5. Testing the Proposed Approach.- 6. Another Possible Experiment.- 7. Comparison with Concrete Situations.- 8. Nonlocality and Relativity.- 9. Extending the Approach to the Case of Momentum Correlated Photons.- 15 Wind Effect of Empty Quantum Waves in a Pfleegor-Mandel-Type Experiment for Electrons.- 1. Experiments Showing the Interference of Independent Photon Beams.- 2. Further Development of the Idea of Empty Waves.- 3. The Normal Double-Slit Experiment.- 4. Double-Slit Experiment with Two Incoherent Sources.- 5. Three Calculations of Particle Density Compared.- 6. Numerical Results Obtained with the "Guidance Formula".- 16 Two-Photon Interference and the Question of Empty Waves.- 1. Introduction.- 2. Detection-Probability Model.- 3. Two-Photon Detection.- 4. Violation of "Strong" Bell-Type Inequalities.- 5. Two-Photon Interference on a Screen.- 6. Two-Photon Interference beyond a Beam Splitter.- 7. The Wang-Zou-Mandel Experiment.- 17 Experiments on the Aharonov-Bohm Effect.- 1. Introduction.- 2. The Aharonov-Bohm Effect.- 3. Formation Process of Electron Interference Pattern-Demonstration of Wave-Particle Duality.- 4. Confirmation Experiments on the AB Effect.- 5. Conclusions.