The Earth, sitting on a distorted grid that represents the gravitational field.

Earth’s mass bends space-time.

Image: NASA

It’s a science fiction horror story – a young astronaut takes a year-long mission closely orbiting a black hole. When he returns home, thousands of years have passed and everyone he has ever known has been dead for centuries. Is there a hint of truth to this terrifying tale?

This imaginary astronaut’s story is a prediction from Einstein’s theories of relativity. Einstein proposed that the speed of all light is the same, no matter where you are standing, and no matter how fast you’re moving. That means space can shrink and stretch, and the steady tick of time isn’t so steady after all.

Time stretches greatly near a black hole due to immense gravity, but Einstein also predicted some effect all the time, everywhere. Experiments have proven it – time passes differently on GPS satellites than on the ground. Without correcting GPS clocks, the system wouldn’t be able to tell us where we are!

If Earth’s gravity affects us and our satellites, it also must affect the rocks beneath out feet. A famous physicist named Richard Feynman once said that Earth’s core is a day or two younger than the surface, because it’s more affected by gravity. Spurred on, a team of experts looked to Einstein’s formulas and set to calculating.

A quick calculation and a few revisions later, and the crew had an answer. Time moves slower at the centre of Earth, but only just. The difference is around the blink of an eye every ten years. But Earth formed 4.5 billion years ago, and all those blinks really add up. Right now, the surface of Earth is about 2.5 years older than its centre!

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