Why is syncing atomic clocks still one of the hardest problems in physics and engineering?
In this episode, we speak with Judah Levine—legendary NIST physicist and one of the key architects of modern timekeeping—about the invisible systems that hold the digital world together. Levine explains why synchronizing atomic clocks across the planet is far more complex than the clocks themselves, and why seemingly simple ideas like “round-trip delay” break down in real-world media such as fiber optics and the internet.
We explore how UTC is built from hundreds of atomic clocks, the difference between keeping time and *transferring* time, and the surprising challenges introduced by asymmetric delays, chromatic dispersion, and environmental noise. Levine walks us through the evolution of cesium clocks, the rise of optical clocks, and the technologies that make GPS, finance, power grids, and global communication possible.
Along the way, we discuss the history of time synchronization, from railroad schedules to radio frequencies to modern satellite systems; the ongoing debate over leap seconds; and why the future of precision timing depends not just on better clocks, but on better *engineering* to deliver those clocks’ performance to the real world.
Whether you’re curious about atomic clocks, relativity, fiber optics, GPS, the structure of time itself, or the hidden physics behind everyday technology, this conversation offers a rare look at how science, engineering, and careful statistical thinking keep modern civilization in sync—down to the nanosecond.
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Xinghui Yin: https://x.com/XinghuiYin
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Timestamps:
00:00 – Intro
01:03 – What is UTC?
05:50 – Timekeeping for Satellites
07:08 – How Radio Created Better Clocks
18:32 – From Astronomy to Atoms
25:25 – Why are there 24 Hours in a Day?
29:55 – Why Synchronizing Clocks is so Hard
47:09 – How did Judah get into Clocks?
53:29 – Is UTC Vulnerable to Hackers?
1:06:41 – Cesium vs Optical Atomic Clocks
1:11:23 – How Cesium Clocks Work
1:23:35 – Why Cesium Clocks are Imperfect
1:26:17 – Judah’s 3 Year Experiment
1:29:30 – Statistics with Clocks
1:33:40 – Is Time Real?
1:36:29 – Is the Universe Slowing Down?
1:40:29 – Atomic Time and General Relativity
1:42:17 – What’s Left for Atomic Clocks?
1:54:34 – What would Judah do with Unlimited Funding?
1:58:57 – Judah's Past in Programming
2:02:55 – Advice for Young Scientists
