Physics
General Relativity and Gravitational Time Dilation Near Massive Bodies
Quick fact
The stronger the gravity, the slower time runs. This has been measured: a clock on the ground loses about 30 microseconds per year compared to one in a GPS satellite 20,000 km above Earth, and GPS systems must correct for this to stay accurate.
Why this is interesting
Your phone’s GPS works because time moves at a different speed on the ground than it does in orbit. But why would time—something we think is constant—change based on where you are?