Earth's core is younger than its crust
Thanks to Einstein's theory of general relativity, we know gravity warps time. Because gravity is stronger at the center of the Earth than at its surface, time actually moves slightly slower there. Physicists calculated that the Earth's solid iron inner core is about two and a half years younger than the planet's crust, despite forming at the same time.
The Curvature of Spacetime and Time
In classical Newtonian physics, time was considered an absolute, universal constant that flowed at the exact same rate everywhere in the cosmos. Albert Einstein overturned this view with his general theory of relativity, demonstrating that space and time are inextricably bound together into a four-dimensional fabric known as spacetime. Massive objects do not simply exert an invisible pull across empty space; rather, their mass warps and curves the surrounding spacetime geometry. This curvature alters not only how objects move through spatial dimensions, but also how they move through time.
One direct consequence of this warping is gravitational time dilation: clocks located in regions of stronger gravitational potential tick more slowly relative to clocks situated in regions of weaker gravitational potential. The closer an object is to the center of a mass concentration, the deeper it sits within the gravitational well, and the more slowly its local time elapses compared to an observer located farther away in empty space.
Potential Versus Force: The Core Paradox
A common point of confusion arises when comparing gravitational force with gravitational potential. At the exact center of the Earth, a person would experience weightlessness because the surrounding mass pulls equally in all directions, causing the net gravitational acceleration to cancel out to zero. However, gravitational time dilation is governed by gravitational potential—the total amount of work required to move an object out of the gravitational field to an infinite distance—rather than the local gravitational force.
Even though the net gravitational force at Earth's center is zero, the gravitational potential reaches its deepest point there. Every kilogram of rock and metal above the center contributes to the depth of the planet's gravitational well. Consequently, an observer at the center of the Earth is in the deepest gravitational potential on the planet, meaning that local clocks at the core run slower than clocks anywhere on the surface.