The Philosophy of Physics: From Newton's World to Quantum Resurgence

What is Philosophy of Physics?

• Definition: Systematic study of our best theories of physics, encompassing Newtonian mechanics, relativity, quantum mechanics, the Standard Model, and cosmology.
  • Involves asking questions about the conceptual and mathematical architecture of theories, their implications for reality, and their connections to philosophical areas like time, causality, and identity.
• Relationship to Physics 'Proper': Continuous with physics, differing primarily in emphasis.
  • Philosophers focus on understanding the structure and implications of existing theories.
  • Physicists engage in building and experimentally testing new theories.
• Disciplinary Connections:
  • History: Understanding a theory's development and the tools used to create it.
  • Mathematics: Shoring up subtle mathematical details for a full understanding.
  • Philosophy 'Proper': Appreciating philosophical scrutiny of notions like time, causation, and identity.

Historical Perspective: From Newton to the 20th Century

• Isaac Newton (1643-1727): Laid groundwork for modern science and using mathematics to solve physics problems.
  • His Principia presaged Einstein's work on general relativity and contained philosophical reflections on space, time, and motion.
• Leibniz-Clarke Correspondence: Explored philosophical questions arising from Newton's mechanics.
  • Addressed the difference between relative and absolute motion, determining global properties of the Universe, and understanding identity in physics.
• 19th Century Developments:
  • Non-Euclidean Geometries: Work by János Bolyai, Nikolai Lobachevsky, and Bernhard Riemann led to the "Problem of Space," questioning the necessity of Euclidean space.
  • Ernst Mach's Principle: Challenged Newton's assumption of inert space and time, proposing that spacetime cannot exist without matter.

The Shifting Attitudes and a Third Golden Age

• Decline in Interaction: After Einstein and Bohr's debates on quantum theory, some physicists became hostile to philosophy.
  • Figures like Steven Weinberg ("Against Philosophy" in Dreams of a Final Theory) and Stephen Hawking ("philosophy is dead") expressed this sentiment.
• Reasons for the Shift:
  • Bohr's Obscure Philosophy: Suggested reality is ineffable, discouraging the pursuit of a complete picture of the world.
  • Particle Physics' Pragmatism: Collaborative and pragmatic approach focused on developing tools without deep conceptual understanding.
• Resurgence of Interaction:
  • Quantum Information Theory and Quantum Computing: Grappling with concepts like decoherence, which relates to interpretations of quantum mechanics.
  • Alternative Theories of Gravitation: Taxonomizing theories based on Einstein's "equivalence principle," requiring philosophical understanding.
  • Cosmology: Concerns about modern cosmology being "permanently underdetermined" by evidence, prompting dialogue with philosophers.
• Current State: Philosophy of physics is thriving with experts in physics, philosophy, mathematics, and history.
  • Examples include the Radboud Centre for Natural Philosophy and Sean Carroll's position as Homewood Professor of Natural Philosophy at Johns Hopkins University.