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?
Read the full explanation
Understanding General Relativity and Gravitational Time Dilation Near Massive Bodies
Imagine stretching a rubber sheet taut. Now place a heavy bowling ball in the center—it creates a deep dimple. That’s what a massive body does to the fabric of the universe, which is the combined structure of space and time. In this picture, Earth’s mass makes a dent in time as well as space. The dent means that time itself flows more slowly deep inside the dent, where gravity is stronger. Think of it like this: a clock at the bottom of a well ticks a tiny bit slower than one at the top. The same effect applies to you: your head ages a hair faster than your feet! This effect is called gravitational time dilation, and it is a real, measured consequence of Einstein’s theory of general relativity.
A deeper explanation
General relativity tells us that gravity is not an invisible force pulling objects, but rather the curvature of spacetime caused by mass and energy. This curvature guides how objects move and sets the rate at which time flows. The more massive the body—and the closer you are to it—the deeper the curvature, and the slower time passes relative to a distant observer. Why? Because just as a heavier ball makes a deeper dent in a rubber sheet, a denser concentration of mass creates a steeper ‘time well.’ This is not a mechanical toll, but a fundamental change in the nature of spacetime itself. Near the event horizon of a black hole, this effect becomes extreme: time appears to stop as seen from the outside. This principle is not just theoretical—it is essential for the GPS navigation system in your phone, which must correct for time moving slightly faster in orbit, several millionths of a second each day, or your location would be off by kilometers.