Robert DiSalle: Gravity, Geometry, Philosophy: 100 Years in Einstein's Universe

The speaker, Carl Heifer, opens the event commemorating the 100th anniversary of Einstein's general theory of relativity. He introduces the Rotman Institute of Philosophy's series of events for 2015 and welcomes Professor Robert Desal, an expert in philosophy and physical theories. Desal is expected to discuss Einstein's work and its philosophical underpinnings, emphasizing Einstein's role as a philosopher-scientist. The introduction also highlights the importance of philosophy in the development of scientific theories, particularly in Einstein's case.

Professor Desal delves into the questions that define our understanding of the universe as Einsteinian, despite the potential for future theories to supersede Einstein's. He discusses the conceptual revolution brought about by Einstein, which includes questions about the universe's origin, the curvature of space-time, and the nature of black holes. Desal also explores various historical conceptions of geometry and how Einstein's theory of relativity dynamically connects geometry with the physical world, marking a significant departure from traditional views.

Desal highlights the practical applications of Einstein's theories, such as GPS navigation, which relies on corrections for time dilation due to both special and general relativity. He then transitions to the conceptual impact of Einstein's work, like the prediction of black holes and gravitational lensing, which have expanded our understanding of the universe's large-scale structure. Desal emphasizes that even if Einstein's theory is not the final word on gravitation, it has provided invaluable insights into the cosmos.

The speaker outlines Einstein's philosophical journey that led to the general theory of relativity. Starting with a simple question about the Earth's rotation, Einstein explored the philosophical implications of Newtonian mechanics and the nature of physical laws. He questioned the privileged status of certain reference frames and the unobservable causes in Newtonian physics, advocating for a theory based on observable facts. This philosophical inquiry laid the groundwork for a new understanding of gravity and its relation to the fabric of space-time.

Einstein's principle of equivalence, which states that there is no experiment that can distinguish between gravity and acceleration, is discussed in detail. The speaker explains how this principle led to a deeper understanding of inertial frames and the equivalence of gravitational and inertial effects. This concept is central to the development of general relativity and challenges the traditional Newtonian view of gravity.

Desal introduces the geodesic principle, which posits that the path of a falling body is physically equivalent to an inertial path in the absence of forces. This concept is key to understanding how Einstein reinterpreted the motion of bodies in a gravitational field as the natural motion in curved space-time. The speaker illustrates this with the analogy of the Earth's surface and how straight lines of longitude indicate its curvature, drawing a parallel to the behavior of inertial trajectories in space-time indicating space-time curvature.

The speaker discusses the radical idea that geometry, as understood in general relativity, is not static but dynamic, varying with the distribution of mass-energy in the universe. This dynamical geometry is a fundamental aspect of Einstein's theory, where the curvature of space-time is directly related to the presence of mass, thus geometrizing the concept of gravity and making geometry a physical field influenced by mass-energy distribution.

Desal reflects on the enduring legacy of Einstein's general theory of relativity, considering its potential survival beyond the development of a quantum theory of gravity. He suggests that while the theory may not be the final word on gravity, it could continue to be useful for predictions and practical applications, serve as a limiting case for future theories, and provide unique insights into the large-scale structure of the universe. The speaker concludes by celebrating Einstein's work as a product of critical philosophical analysis and a testament to his identity as a philosopher-scientist.