The Bizarre Reason Why the Earth’s Core Is Younger than the Crust

This is not a story of geology but a story of physics.

The Earth’s crust is constantly rewritten by volcanoes, erosion, and plate tectonics. New crust is being created while old one is being destroyed all the time. In that sense, the center of Earth is not nearly as old as the ground beneath your feet.

But that’s not what this story is about.

This story is about time and gravity. It’s about how the center of the Earth ages at a different rate than the surface of the planet, and why this is written in the very laws of physics. In fact, this isn’t a story about geology at all. The planet’s center isn’t younger because it formed later. It is younger because time itself has moved slower there.

According to recent calculations, the center of the Earth is around 2.5 years younger than clocks at the surface would put it.

Blame Relativity for This One

Our planet formed some 4.5 billion years ago, from particles of dust and rock orbiting the young Sun. Gravity pulled the materials together, forming particles that collided and merged to gradually create our planet.

That’s about where the geology in this story ends. From here on, we have more to do with Einstein.

Einstein’s general relativity killed the old idea of universal time. Time is not the same everywhere; there’s no central, cosmic, metronome that beats the same rhythm everywhere. Time depends on where you are and how you move.

A clock that’s experiencing severe gravitational attraction will tick more slowly. Gravity, in general relativity, isn’t a force that pulls objects together. Rather, it changes the geometry of space and time. Space and time are connected (in what we un-creatively call spacetime), and gravity curves this spacetime.

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So, mass curves spacetime, and this causes time to pass slower. Or, to put it differently, clocks in different gravitational potentials won’t agree with each other.

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That word, potential, matters. At the exact center of a perfectly spherical Earth, gravity pulls equally in every direction. An object there would feel weightless, at least in the idealized sense. But that does not mean the center is free of gravity’s effect on time.

A clock at the center sits at the bottom of Earth’s gravitational well. To move from the center to the surface, and then far away into space, requires climbing out of that well. The force may vanish at the middle, but the potential is lowest there. That is why the deepest clock is the slowest one.

The higher the mass, the bigger the effect. Near a black hole, time seems to completely stop (for an external observer).

Clocks in different gravitational potentials do not agree.

They Did the Math

This idea that the Earth’s center is younger was popularized by Nobel-winning physicist Richard Feynman.

In his iconic physics lectures, Feynman used to show how general relativity isn’t just about black holes and galaxies, and how it can reach the very ground beneath our feet. Feynman guesstimated that the Earth’s center should be a “day or two” younger than the surface.

Feynman’s idea was on point, but his math was way off.

A 2016 paper revisited this idea and did the math.

A perfectly uniform Earth would already make its center younger by about 1.58 years. But Earth is not uniform.

Its mass is concentrated inward. The inner and outer core are far denser than the crust and mantle. That central concentration deepens the gravitational potential well, which increases the time dilation. Using the Preliminary Reference Earth Model, or PREM, the standard seismic model of Earth’s layered interior, researchers calculated a more accurate core-surface difference: about 2.5 years. Not days, but years.

Maybe Feynman made a quick order-of-magnitude slip, or maybe the transcript garbled “years” into “days.” Either way, the number stuck because it was attached to Feynman, who was a leading authority. Using the same math, we can calculate the same thing for other planets in our solar system, or even the Sun.

This Matters a lot for Practical Reasons

You might think this is just a physics gimmick with no real significance. But it does matter in one important regard: GPS.

The Global Positioning System works because satellites carry atomic clocks. Those satellites orbit high above Earth, where gravity is weaker than on the ground. In the same way that the center of the Earth ages slower than the surface of the Earth, the surface of the Earth ages slower than the satellites.

But there’s another effect: their orbital motion produces the opposite effect, making then run slower. The net result is that GPS satellite clocks gain about 38 microseconds per day relative to clocks on Earth, a correction that must be built into the system.

A microsecond is a millionth of a second, but that’s the scale at which these systems are working. Light travels about 300 meters in a microsecond. Let the relativistic errors accumulate, and a position fix would rapidly become useless.

The same physics is now becoming a tool for studying Earth itself. Modern optical atomic clocks are so precise that gravity’s effect on time can reveal height differences of centimeters, and the newest clocks are pushing below that scale. NIST (the National Institute for Standards and Technology) has described today’s best atomic clocks as sensitive enough, in principle, to detect height differences of less than a centimeter.

This approach is called chronometric geodesy. Instead of measuring gravity by watching a mass fall or a spring stretch, scientists compare time.

Geology and Physics

It’s important not to confuse this relativistic effect with geology. As we mentioned in the start, the Earth’s core is stable, while the crust changes all the time. The core formed early, as dense molten iron sank inward during planetary differentiation. Much of the crust is younger because it has been recycled by plate tectonics and volcanism.

Relativistically, though, the core has experienced less time. Both ideas can be true at the same time, and that’s one of the joys of physics. Also, 2.5 years is completely negligible for all practical purposes when we’re talking about billions of years.

So, when we talk about planetary time, a planet doesn’t have one single age, at least not in a physical sense. It has many proper times, layered like its rocks and metals.

Ultimately, this is a reminder that time doesn’t pass. It passes somewhere.