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A difference of 2.5 years between the core and surface is not directly related to geology, but to general relativity, which shows how gravity alters the passage of time within the planet
The Earth’s core is about 2.5 years younger than the surface: time passes more slowly in the deep regions of the planet’s gravitational well.
The difference does not occur because the center formed later, but because clocks in different gravitational potentials do not age at the same rate. The explanation shifts the topic to physics.
The Earth’s crust continuously changes, with volcanoes, erosion, and tectonic plates creating new parts while others are destroyed. However, the case of the planet’s center involves time, gravity, and space-time.
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The Earth formed about 4.5 billion years ago, from dust and rocks orbiting the young Sun. Gravity gathered these materials, which collided and formed the planet.
Earth’s core ages more slowly
Einstein’s general relativity overturned the idea of universal time. There is no cosmic metronome that ticks at the same rate everywhere. Time depends on an object’s position and movement.
A clock subjected to stronger gravitational attraction runs more slowly. In general relativity, gravity is not a simple force pulling objects but an alteration in the geometry of space-time.
Mass curves space-time, and this alters the passage of time. Therefore, clocks located in different gravitational potentials do not agree with each other, within the same planet.
At the exact center of a perfectly spherical Earth, gravity would pull equally in all directions. An object there would seem light, but this does not eliminate the effect on time.
The central point is in the potential. A clock placed at the center is at the bottom of the Earth’s gravitational well. To reach the surface and space, it would need to escape this well.
The force may disappear in the middle, but the potential remains lower there. This is why the deepest clock advances more slowly. The greater the mass, the greater the effect.
Calculations changed famous estimate
The idea that the Earth’s center is younger was popularized by physicist Richard Feynman, Nobel laureate. In his lectures, he showed that general relativity applies not only to black holes and galaxies.
Feynman used the example to bring this physics closer to the planet beneath our feet. He estimated that the Earth’s center should be one or two days younger than the surface.
The idea was correct, but the value was far from the later result. A 2016 paper revisited the issue and redid the calculations with more precise data.
A perfectly uniform Earth would already make its center approximately 1.58 years younger than the surface. However, the Earth is not uniform. Its mass is concentrated inward.
The interior, including the outer core, is much denser than the crust and mantle. This central concentration deepens the gravitational potential and increases time dilation.
Using the Preliminary Reference Earth Model, the PREM, researchers calculated a difference between the core and the surface: about 2.5 years.
Still, the phrase gained traction by being associated with Feynman. The same mathematics applies to estimates on other planets in the Solar System and even the Sun.
Physics also affects GPS
The difference of 2.5 years on a planetary scale may seem like a curiosity, but the same physics has a practical effect on GPS. The system works with satellites and atomic clocks.
These satellites orbit above the Earth, where gravity is weaker than on the ground. Just as the surface ages faster than the center, it ages more slowly than the satellites.
However, there is another effect in the opposite direction: the orbital movement makes the satellites’ clocks run slower. The net result is that they gain about 38 microseconds per day.
This difference occurs in relation to those on Earth. The correction needs to be included in the system for positioning to work.
A microsecond is equivalent to one-millionth of a second, but this scale is decisive. Light travels about 300 meters in a microsecond. Without correction, a fixed position would quickly become useless.
The same physics also becomes a tool to study the Earth. Optical atomic clocks are so precise that the effect of gravity on time can reveal height differences of centimeters.
Newer clocks advance below this scale. The NIST described the best current atomic clocks as sensitive enough to detect height differences smaller than a centimeter.
Geology and relativity do not cancel each other out
The relativistic effect should not be confused with geology. The Earth’s core is stable, while the crust is constantly changing. The core formed early when dense molten iron sank inward.
This process occurred during planetary differentiation. Much of the crust is younger because it has been recycled by plate tectonics and volcanism. Relativistically, however, the core has experienced less time.
The two statements can coexist because they address different aspects of planetary history. Even so, 2.5 years are insignificant when compared to billions of years.
The idea shows that a planet does not have a single age in a physical sense. It has many proper times, in layers, like its rocks and metals, not a single physical age for the entire planet.
In the end, the story of the Earth’s core serves as a reminder that time does not pass in a unique and universal way. It passes somewhere, under certain conditions, and gravity directly participates in this rhythm.
With information from zmescience.
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