Where Does Time Run Slowest?

Hello, curious humans. I'm Orbi! Your head is a tiny bit older than your feet, and physicists have measured it. So where in the universe does time run slowest? We climb a ladder of real places, each with a real clock: a mountain, the center of the Earth, the space station, GPS satellites, the Moon, Mars, Jupiter, the surface of the Sun, a white dwarf, a neutron star and the edge of a black hole. And at the end, the place where time runs fastest of all. Along the way: why you'd float at the center of the Earth and yet that's where Earth's time is slowest (it's how deep you sit in gravity, not how hard it pulls); why astronaut Scott Kelly came home a few milliseconds younger; why every GPS clock is built to tick slow on purpose; the 1971 experiment where atomic clocks flew around the world with their own airline seats ("Mr. Clock"); why clocks on the Moon and Mars run fast, and why NASA is setting up a time standard for the Moon; how the Sun's own light proves its clocks are slow; and the 1925 "confirmation" of Einstein that was off by four times. Jupiter's number is our own calculation (general relativity with standard planetary data); every other number comes from the sources below. Earth's interior in the animation is drawn from the PREM Earth model. Chapters 0:00 Older than your feet 0:57 Higher is faster 1:51 Weightless, and slowest 2:50 Speed fights back 4:13 Mr. Clock 5:09 The Moon and Mars run fast 6:10 The first slower places 6:57 Extreme clocks 8:21 The fastest clocks Sources Chou, Hume, Rosenband & Wineland 2010, "Optical Clocks and Relativity", Science 329, 1630 Pound & Rebka 1960, "Apparent Weight of Photons", Physical Review Letters 4, 337 Uggerhøj, Mikkelsen & Faye 2016, "The young centre of the Earth", European Journal of Physics 37, 035602 Dziewonski & Anderson 1981, "Preliminary reference Earth model" (PREM), Physics of the Earth and Planetary Interiors 25, 297 Ashby 2003, "Relativity in the Global Positioning System", Living Reviews in Relativity 6, 1 Hafele & Keating 1972, "Around-the-World Atomic Clocks: Predicted / Observed Relativistic Time Gains", Science 177, 166–170 Space.com (M. Wall), 13 July 2016, on Scott and Mark Kelly; NASA Twins Study Ashby & Patla 2024, Astronomical Journal (time on the Moon); NIST, "What Time Is It on the Moon?" (2024); NASA, Coordinated Lunar Time (2024) Ashby & Patla 2025, "A Comparative Study of Time on Mars with Lunar and Terrestrial Clocks", Astronomical Journal, doi:10.3847/1538-3881/ae0c16 González Hernández et al. 2020, "The solar gravitational redshift from HARPS-LFC Moon spectra", Astronomy & Astrophysics 643, A146 Joyce et al. 2018, "The gravitational redshift of Sirius B", MNRAS; Holberg 2010, Journal for the History of Astronomy 41, 41 Riley et al. 2019 and Miller et al. 2019, Astrophysical Journal Letters (NICER neutron-star measurements) Thorne 2014, The Science of Interstellar (W. W. Norton) Piffl et al. 2014, "The RAVE survey: the Galactic escape speed and the mass of the Milky Way", A&A 562, A91; Planck 2018 results (CMB dipole) Image credits Aluminum-ion clock, NIST 2010: J. Burrus/NIST Mount Everest from the International Space Station: NASA (ISS066-E-141209) International Space Station: NASA (ISS072-E-316172) Scott and Mark Kelly: NASA/Bill Ingalls GPS III satellite (illustration): GPS.gov Earth from the ISS (video): NASA Johnson Space Center Full Moon: NASA (Apollo 11, AS11-44-6667) Mars, Murray Buttes: NASA/JPL-Caltech/MSSS (Curiosity rover) Jupiter: NASA/JPL/Space Science Institute (Cassini) The Sun: NASA/GSFC/Solar Dynamics Observatory (image and video) Sirius A and B in X-rays: NASA/SAO/CXC (Chandra X-ray Observatory) All other visuals (the clocks, the ladder, the cut-away Earth, the orbits, the wells, the neutron star, the black hole) are our own illustrations and animations, computed from the sources above; they are not photographs. Orbi and the library illustrations: Orbi Observatory. Music and sound: original score made for Orbi Observatory.