The conversation turns to time travel, where Rizzi takes a notably conservative position. He argues that time is fundamentally the measure of motion rather than a spatial dimension, making backward time travel logically impossible since the past no longer exists. Forward time travel, he explains, amounts to decoupling oneself from the motions of the universe, something already achieved in primitive form through frozen embryos and the time dilation experienced by orbiting astronauts.
In a surprising philosophical turn, Rizzi builds a careful argument for the existence of the human soul and its immortality. He reasons that ideas lack the defining property of material things, having parts outside each other, and therefore cannot be physically destroyed. This non-material aspect of human nature, he contends, necessarily persists after bodily death, though separated from the senses it would be unable to acquire new knowledge on its own.
Key Moments
String theory: half its predicted particles haven't been found: Asked about string theory, Rizzi calls it the most promising mathematical handle on physical reality but warns it has no supporting data and predicts a whole class of supersymmetric particles, none of which have been observed. He gently mocks the supporters who say 'we found half of them.'
Why angular momentum is so hard in Einstein's universe: Rizzi explains that Newtonian momentum is conserved cleanly, but in general relativity gravity itself travels as waves, leaving spacetime perpetually rippled. Trying to define angular momentum is like trying to find a place to stand on a frothing sea - which is exactly the problem he solved in 1997.
Backward time travel is impossible - the past does not exist: Rizzi tells Art the mathematics of relativity treat time as a line that can bend into a circle, but argues this is a category error: time is not a line, the past does not exist anymore, and therefore backward time travel is intrinsically impossible.
Time is the measure of motion - before the Big Bang, no time: Drawing on Aristotle, Rizzi tells Art that time is the measure of motion. Where there is no motion there is no time - meaning before the Big Bang, with nothing to move, there was literally no time.
Forward time travel is just decoupling from the universe's motion: Rizzi reframes forward time travel as anything that decouples a person from the changes happening around them - frozen embryos, suspended animation, or relativistic travel like the twin paradox. The traveling twin returns younger because less has happened to him.