Time Moves Faster Up A Mountain – And That’s Why Earth’s Core Is 2.5 Years Younger Than Its Surface

Time is relative. The ticking of the clock, even of the most precise clocks we can make, does not exist in an absolute. It depends on the gravity you are experiencing and on your speed. Being on the surface of a sizable planet moving at high speed through space, spinning around a yellow star and on its axis, you can imagine produces a very particular timeframe.

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We have recently seen that Mars’s time is different from Earth’s own, and so is the Moon's. We need to go further, though. Our clocks are now so accurate that even a difference of a few meters in altitude might shift them. Researchers want to test this with a never-before-seen experiment. They are using an optical atomic clock, the most precise clock humanity has ever built, on top of Mount Blue Sky, a 4,348-meter-tall (14,266-foot-tall) mountain in Colorado.

Time dilation and the age of Earth's core

We often think of time dilation when we think of speed, usually with the thought experiment known as the twin paradox. One twin gets on a spaceship going near the speed of light, and the other one stays on Earth. The faster you move, the slower the clock will tick for you, so once the twin is back on Earth, they are younger than their earthbound sibling.

Being in a gravitational field also changes the flow of time. The deeper you are in a gravitational field, the slower your clocks will go. A few years ago, researchers estimated with high precision how the different layers of our planet would affect the ticking of clocks within the Earth. They found, in the more accurate model, that the crust is about 2.49 years older than the core.

For decades, conventional wisdom placed the age of the core to a few days younger than the crust, but that value was very off. In a simplistic model, researchers calculated that the core is at least 1.5 years younger than the surface, which assumes the Earth to be a perfect uniform sphere. The more realistic one uses the Preliminary Reference Earth Model, a one-dimensional model used to work out where the different layers are located and how they affect the gravitational field.

Optical clocks on mountains are just the beginning

Optical atomic clocks are the next step in precision timekeeping, being over 100 times more accurate than regular atomic clocks, with researchers trying to increase the precision tenfold still. The best measurement has reached an uncertainty in time measurements of about eight parts per tenth of a billionth of a billionth. That would be like a clock that loses a second once every 39.15 billion years, to be exact, which is slightly less than three times the age of the universe.

These clocks use clouds of atoms at very cold temperatures and lasers, which excite the electrons of those atoms at a specific resonant frequency. The ticking of the clock is given by the electrons going back to their original place, releasing light, and the oscillation of that light can be measured with outstanding precision.

It is so precise that two clocks can be used to measure the gravitational differences of the planet. As we know, it is not at all a perfect sphere. Taking a clock such as this up a mountain is a very interesting idea. Mount Blue Sky has an astronomical observatory, so it can accommodate another scientific experiment.

“This is unprecedented,” Professor Scott Diddams, from the University of Colorado Boulder (CU Boulder), said in a statement. “When we built the first optical clocks 25 years ago, we never would have dreamed such a combination of performance and remote operation would be possible.”

This campaign is a promising step forward for this technology. Making these clocks portable and smaller could allow for their use in a variety of applications. Scientists could use them to track changes in the elevation of land masses due to glaciers disappearing or the internal motion of the Earth. Optical atomic clocks can also be used to probe the limits of our physics, both in general relativity and quantum mechanics.

The team will use laser communication and fiber-optic cables to connect the optical clock on top of Mount Blue Sky with one in the lab at CU Boulder. The times of the two clocks will be compared, so that the subtle time dilation can be quantified. Mount Blue Sky is not the limit; it’s the very beginning of a new way to test these clocks.