Time is a concept so central to our daily lives. Yet, the closer scientists look at it, the more it seems to fall apart. Time ticks by differently at sea level than it does on a mountaintop. The universe's expansion slows time's passage. "And some scientists think time might not even be 'real' — or at least not fundamental," says NPR science correspondent Geoff Brumfiel. In this encore episode, Geoff joins Short Wave Scientist in Residence Regina G. Barber to bend our brains with his learnings about the true nature of time. Along the way, we visit the atomic clocks at the National Institute of Standards and Technology, consider distant exploding stars and parse the remains of subatomic collisions. Want to know more about fundamental physics? Email shortwave@npr.org.