Does Time Flow Differently in Space?

“Byeoli, how does this figure out our location so precisely?”

“Thanks to signals sent by GPS satellites orbiting Earth. But here’s something interesting, the clocks inside these satellites don’t actually run at the exact same speed as clocks on Earth.”

“What do you mean? Time runs differently?”

“Yes. Measured very precisely, a satellite’s clock runs a tiny bit faster each day than a clock on Earth. There are two reasons, one is that the satellite sits where gravity is weaker than on Earth, and the other is that the satellite is moving at very high speed.”

Hyeongseop looked thoroughly confused.

“Why would weaker gravity make time go faster?”

“A scientist named Einstein figured this out. The stronger the gravity somewhere, the more slowly time flows there. Earth’s surface has relatively strong gravity, while up in space it’s weaker, so time inside the satellite flows just slightly faster than on Earth.”

“But does moving fast have something to do with time too?”

“Yes, and it works the opposite way, moving very fast actually makes time flow more slowly. Satellites move at tens of thousands of kilometers per hour, so this effect slows their time down a little. Add both effects together, and overall, the satellite’s clock ends up running just slightly faster than Earth’s.”

“Why does such a tiny difference even matter? It seems incredibly small.”

“Small as it looks, it really matters. Without accounting for this tiny difference, position errors could grow by several kilometers in just a single day. That’s why GPS satellites have built-in corrections that calculate this time difference in advance.”

“So every time we use a map to find directions, Einstein’s theory is actually working behind the scenes?”

“Exactly. Every time you use a map app to find your way, somewhere invisible, the universe’s laws of time are quietly being calculated.”

“So could astronauts end up slightly different ages when they come back to Earth?”

“Extremely slightly, yes. An astronaut who spent a long time on the International Space Station would have aged just a tiny fraction less than someone who stayed on Earth the whole time. Of course, it’s nowhere near a difference a person could ever actually feel, but with a precise enough clock, that difference really does show up.”

Hyeongseop looked quietly down at his wristwatch. Knowing that, at this very moment, time was flowing just slightly faster somewhere high above in space made him realize that time wasn’t some fixed, unchanging thing after all, it was something strange that shifted depending on where in the universe you happened to be.