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Theoretical Physicist, Author, and Science Educator
Because of relativity, time moves faster the farther away you are from the center of the earth, meaning that your head ages slightly quicker than your feet.

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Big Think: How have today’s super-accurate clocks changed the way we understand time?

Michio Kaku: Believe it or not, a clock on your head beats at a different rate than a clock on your foot. We can now measure this and it has practical importance as well. First of all, Einstein said that time is a river. It's a gigantic river were all swept up in the river of time and it can speed up and it slows down. For example, on the moon; did you know, that time beats faster on the moon than it does on the earth and we can measure this? On Jupiter, time beats slower on Jupiter that it does on the Earth. So time beats at different rates. When an object moves very fast, time slows down inside that rocket that is moving very fast. If that rocket is then placed on the Moon, time beats faster. So we have to do competing effects. This has a direct implication with regards to your cell phone.

Your cell phone has GPS which allows you to locate objects on planet Earth by focusing in on satellites orbiting the planet Earth. Satellites travel at 18,000 mph if they’re in lower earth orbit; therefore time slows down for those low-lying satellites. But if satellites go farther and farther away, gravity gets less and less and less so time speeds up. So in outer space we have two competing effects. Fast satellite slowdown in time, very faraway satellites speed up inside and in fact, at one radius you could calculate time beats exactly at the same rate as it does on the planet earth. So what does it mean? Your GPS system would totally fail without Einstein's theory of special and general relativity.

This also means that the top of your head because it is farther from the center of the earth, beats at a faster rate... time beats at a faster rate then your feet. So you can actually show that even within your own body now—our instruments are so accurate—that you could show within your own body the fact that time beats at different rates within your own body.

Directed / Produced by
Jonathan Fowler & Elizabeth Rodd

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