What is space-time? Space has three dimensions and time has before and after. John sees space and time as independent of each other. Ken adds that common sense dictates that space and time are absolute things. Time, he says, contains the entire spatial manifold which moves through time as a single unit. Leibniz said this was incoherent and violated the principle of sufficient reason. The philosopher claimed that there would be no discernible difference between one position in absolute space and another. The idea of absolute space, Leibniz claimed, is an empty one. The due talk about the speed of light and its role as the measuring stick of the geometry of the universe, and try to get at what space-time really is.

John and Ken welcome guest Tim Maudlin, Professor of Philosophy at New York University and author of Philosophy of Physics: Space and Time. John asks Tim what drew him to study the philosophy of space-time rather than the physics of the matter. Tim explains that in undergraduate, he studied both philosophy and physics, and in both cases he was driven by the intellectual desire to get to the bottom of things; the conceptual side of things, however, appealed to him more. John wonders whether it makes any difference if space and time are two separate things or just a single entity; Tim explains that in everyday life it doesn’t, but if the theory of relativity holds, then the fundamental structure of things – that we all enjoy the very same moment – is shaken up. Ken asks whether each place in space has its own time. Tim explains that the measure of time is the measure of the path you are traveling through space-time; different paths have different lengths. Is there empirical information back the theory up? And if Leibniz were right, wouldn’t quantum mechanics be wrong?  

Ken wonders what philosophy has to contribute to the debate of space and time. Tim explains that philosophy’s contributions in the field of space and time come more from philosophical training than from philosophical doctrine.  Philosophy forces you to think about the very bottom of things, the fundamental, conceptual level and try to get extremely clear and precise about the fundamental concepts. That is absolutely necessary when you’re doing foundational work in physics, Tim explains! What’s not so clear is the relation between mathematical models and physical reality. And isn’t time just change? Does time have direction? John, and Ken welcome questions from the audience, and they continue the discussion.

• Roving Philosophical Reporter (Seek to 7:30): Philosophy Talk's Reporter Shuka Kalantari investigates the paradoxes of time travel in movies and shows like Back to the Future (the pre-destination paradox), The Butterfly Effect (the aptly named butterfly effect paradox), and Dr. Who (the bootstrap paradox).