Introduction: The Return of the Aether.- 1 Quantum Theory and the Special Theory of Relativity.- 1.1 Compatibility of Nonlocal Correlations with the Principle of Relativity.- 1.1.1 Introduction.- 1.1.2 Consequences from the Principle of Relativity.- 1.1.3 Assuming the Existence of an Aether, Born’s Rule and Nonlocal Correlations Follow from the Principle of Relativity.- 1.2 The de Broglie-Bohm Causal Interpretation of Quantum Theory.- 1.2.1 Quantum Potential and Guiding Wave.- 1.2.2 Applications: Wave Packets and the “Particle in the Box”.- 1.2.3 Many-Body Systems and Relativistic Formulations.- 2 Quantum Cybernetics.- 2.1 The Idea.- 2.1.1 Circular Causality between Waves and “Particles”.- 2.1.2 Quantum Systems as Self-Referential, “Autonomous” Units.- 2.2 The Formalism.- 2.2.1 Against Rash Abstractions.- 2.2.2 Relativistic Quantum Cybernetics.- 2.2.3 de Broglie’s Thermodynamic Arguments.- 2.2.4 Feynman’s Clocks and Arrows: A Lesson in Organizational Coherence.- 2.2.5 Quantum Feedback and Open Questions.- 3 Experiments.- 3.1 Quantum Postselection Experiments and the Delayed Choice Quantum Eraser.- 3.2 Late-Choice Experiments.- 3.3 Realistic Solution of the “Relativistic EPR-Dilemma”.- 3.4 Superluminal Signaling and “Causal Paradoxes”.- 4 Gravity as a Pure Quantum Phenomenon: Mach’s Principle Revisited.- 4.1 Introduction.- 4.2 A Generalized Principle of Relativity.- 4.3 Derivation of Einstein’s Equations for the Gravitational Field.- 4.4 Conclusion: Quantum Cybernetics and Mach’s Principle.- 5 Implications of Circular Causality at the Quantum Level.- 5.1 The Historical Context.- 5.2 Future Perspectives.- Coda: On the Meaning of Nonlocality.- References.