Doesn’t everyone want to remain young at heart? This might just be true in the case of Earth! Calculations have shown that the Earth’s core is actually 2.5 years younger than its surface. Like most things nowadays, the reason for this unbelievably true phenomenon can be understood through the General Theory of Relativity.
Albert Einstein might have been the one to introduce us to the concept of relativity, but its consequences are more profound than scientists previously thought. As you might know, light moves slowly near massive objects like stars, planets or black holes. For most everyday objects with a small gravitational field around them, this effect can be completely overlooked. According to Einstein’s general relativity, the more massive an object, the more the fabric of space-time is ‘warped’ by its existence, which creates a gravitational pull.
Any object with a gravitational field massive enough can actually bend time around it. This means that time actually moves faster around the objects. This effect can be observed in geostationary satellites that orbit the Earth and are responsible for all our navigational and GPS systems! The infinitely precise atomic clocks aboard these satellites must be corrected regularly to account for ‘time-loss’, without which we would observe errors of more than 10km/day in the satellite’s location analysis.
Interestingly, even the mass of Earth’s core is great enough to exert a gravitational force on the planet’s surface. Due to this force, time moves faster at the surface than the core. It’s something theoretical physicist Richard Feynman predicted back in the 1960’s, reportedly stating in a series of lectures that the curvature of space-time should, in theory, put an age gap between Earth’s center and surface of a day or two.
Scientists to the Rescue!
Enter physicist Ulrik Uggerhøj from Aarhus University in Denmark, who himself admits to citing Feynman’s famous speculation in previous papers before coming to the realization that no study had gone to the effort of confirming the numbers. So, Uggerhøj recruited a team, and they got to calculating what’s known as the gravitational potential of the surface and the core.
Earth: “I’m Not That Old!”
The Earth came into existence about 4.5 billion years ago and its core has been exerting a gravitational force on the surface ever since. The team assumed the Earth to be a uniformly dense sphere in order to simplify calculations. Although this isn’t true, they had to do it in order to get a rough idea of how to eventually proceed (hopefully) with more dexterity and accuracy. They observed that for every second at the surface, a time lag of about 0.0000000003 of a second is observed at the core, according to the relativity calculations. This might seem negligible, but over the course of 4.5 billion years, that time lag has compounded to 1.5 years.
However, the core is much denser than the layers above it. To produce a more precise estimate for the crust-core age difference, the team used a more realistic density distribution. The Earth becomes denser towards the center, but not linearly, so geologists have different models to describe the interior of our planet. In their paper, the team used the Preliminary Reference Earth Model, a one-dimensional model used to work out where the different layers separate. A realistic model needs a more complex mathematical calculation, but the answer was still easily obtained. Using a more realistic model of Earth’s density, the team found the difference in age to actually be around 2.5 years!
Now, before you say anything, we still assume that Earth’s core probably formed earlier than its surface, but due to the influence of gravity on time, it’s been aging more slowly than its outward-facing counterpart.
Sun: “You Ain’t Got Nuthin On Me!”
If you want an even more stark illustration of gravity’s effect on time, consider the Sun. The team calculates that the Sun’s core is around 40,000 years younger than its surface. This estimate does not even take into account the fact that the Sun’s core is a factory that keeps on replacing itself with new materials, technically making it even younger than the Sun’s surface if we don’t bring relativity into the conversation. This means that relativity basically acts like an anti-aging method on a cosmic scale, which is very good news when you’re as heavy as the Sun!
Of course, relativity is just a theory, albeit a strong one. There is no way of actually determining the age of a planet’s core unless we drill down a hole and get a sample for ourselves. What we do know is that the findings by the team not only serve as an example of the influence of gravity over time, but the problems that can arise when scientists take the words of famous colleagues at face value, but don’t bother to check the math themselves!