Einstein: The Man and The Genius

# Thanks to Time Dilation, Earth's Core Is Younger Than Its Surface

Time may seem like one of the most predictable, dependable measures we've got. But despite the reliable tick, tick, tick of your wristwatch, time doesn't always flow at the same rate. Einstein's theories say that time varies depending on things like gravity and acceleration, so that time moves more slowly the further into an object's gravity you go. That means that the center of the Earth must be younger than its surface. According to a 2016 study, it is — by more than we ever believed.

## Check Your Work

In a lecture at Caltech in the 1960s, legendary physicist Richard Feynman famously said that because of time dilation — that result of Einstein's theory of relativity that makes gravity distort time — "the center of the Earth should be a day or two younger than the surface." Since it was Feynman, a science rockstar by all accounts, other scientists accepted this at face value.

But in 2016, a team of Danish physicists decided they should probably check the math. Nothing against Feynman, of course. "... just because someone has become famous, this person is evidently not necessarily right on all matters," the authors write in their study, which was published in the European Journal of Physics. "Feynman himself would most likely have agreed with this and he would probably not have fallen for his own miscalculation."

The physicists performed a back-of-the-envelope calculation and found that, due to the effects of physics (and not, say, geological processes), the Earth's core is in fact 2.5 years younger than its surface.

## Day-to-Day Dilation

This isn't just a mathematical abstraction. The effects of time dilation actually affect things you use every day. Take GPS maps, for example. GPS satellites orbit the Earth at 12,500 miles up (20,117 km), which is considerably further out in Earth's gravity well than everything on its surface is. That means that time moves faster for GPS satellites than it does for clocks here on Earth.

The one snag in this factoid is that satellites are also moving at 8,670 miles per hour (13,953 kph), and acceleration slows time down. That means time isn't as fast for satellites as it would be if they were stationary. But these two effects don't cancel each other out entirely — gravity has a bigger effect on the satellites than speed does. As a result, clocks on GPS satellites run a few nanoseconds slower than clocks here on Earth.

The effect can also be observed in humans. Astronaut Scott Kelly came back to Earth younger than his twin. He spent 11 months aboard the International Space Station, shaving 13 milliseconds off his Earth age in the process. How's that for sibling rivalry?

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To get a taste of Richard Feynman's famous lectures, check out "Six Easy Pieces: Essentials of Physics Explained by Its Most Brilliant Teacher." It's free with a 30-day trial of Audible. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

Ashley Hamer

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