A trio of researchers in Denmark has calculated the relative ages of the surface of the Earth versus its core and has found that the core is 2.5 years younger than the crust. In a paper published in the *European Journal of Physics*, U I Uggerhøj and R E Mikkelsen with Aarhus University and J Faye with the University of Copenhagen, describe the math involved in their effort and their results.

During one of his famous lectures at Caltech in the 1960's, Richard Feynman remarked that due to time dilation, the Earth's core is actually younger than its crust—a difference he suggested that was likely a "day or two." Since that time, physicists have accepted both the notion that the core is younger than the surface, and the amount of time given by Feynman, without checking the math.

General relativity suggests that really big objects, like planets and stars, actually warp the fabric of spacetime, which results in a gravitational pull capable of slowing down time. Thus, an object closer to Earth's center would feel a stronger pull—a clock set near the core would run slower than one placed at the surface, which means that the material that makes up the core is actually younger than the material that makes up the crust. This seems counterintuitive to our sense of reason. Such oddities have long been taken for granted in physics, as has the degree of time difference offered by Feynman during his lecture. In this new effort, the research trio ran the math to discover the actual number involved. They found that over the course of our planet's 4.5 billion year history, the pull of gravity causes the core to be approximately 2.5 years younger than the crust—ignoring geological processes, of course.

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, simply because of their prestige. No one should be above review, of course, which, as many who knew Feynman can attest, was one of his mantras.

**Explore further:**
Famous Feynman lectures put online with free access

**More information:**
U I Uggerhøj et al. The young centre of the Earth, *European Journal of Physics* (2016). DOI: 10.1088/0143-0807/37/3/035602

## Kedas

I don't get what they are saying here, do they want to talk about the speed of time or a time difference?

## Noumenal

## NiteSkyGerl

## pdavisgenoa

In four and a half billion years 2.5 years younger is NOT "much" younger. If anything the headline should have said "slightly" younger.

Please restrain yourself from hyperbole in the future.

## Steelwolf

## antigoracle

## mauriceg

## Guy_Underbridge

## antigoracle

Or, we can compare midgets and basketball players of the same age.

## Whydening Gyre

Can anyone say - Naptime...?

## kevin_s

## kevin_s

## kevin_s

## Da Schneib

The calculations are interesting, I suppose.

## ogg_ogg

BTW, a look at their paper's DOI or Wikipedia's article on Earth's Gravity (near bottom) have graphs displaying gravitational acceleration below Earth's surface (estimates).

## kevin_s

## nilbud

You should learn to read. Then you might stand a chance of not making a fool of yourself again.

## Da Schneib

I had a look and all they did was account for density; they didn't bother with heat or stress.

[contd]

## Da Schneib

They did have a look at time dilation on the surface due to the Earth's rotation but it was on the close order of 10^-5 years and they comment that's "negligible" in the paper. I expect the stress and heat terms would be of similar or even smaller order. It's the mass that does most of the gravitating; energy, whether heat or stress, is of lesser gravitational influence by a factor of c², as we all know from the famous equation E = mc².

The big factor seems to be the density gradient.

Also worthy of note, the amount of time dilation here is microscopic; it's on the order of 10^-9 at 1G on the Earth's surface. It takes really, really strong gravity to make much difference.

## billpress11

## lengould100

## Tenstats

## Tenstats

## Tenstats

## thisisminesothere

They said "much younger than THOUGHT". We THOUGHT it was a day or two. It turns out its 2.5 years. Which in comparison to a day or two is MUCH younger.

## MrData

## torbjorn_b_g_larsson

So it isn't about geology, but FWIW it is believed from state observations of iron and thermal models of the core that it has is now solid in the center.

They use the Core Theory field approximation for GR. I'll cite the paper "The gravitational potential influences the rate at which time passes." That potential decrease towards the center with a factor 3/2 in the simplest estimate. They then look at gravitational redshift of photons, frequency at center decrease with potential difference to space which is positive, reversely passed time dilates at center. [Eq. 4]

[tbctd ]

## torbjorn_b_g_larsson

It is also not (directly) about the behavior of masses at the center. But yes, by the classical field approximation a mass at the center of a massive sphere is weightless. By hydrostatic behavior of Earth the small deviation from a sphere would not affect that much. But there would be some residual tidal effects pulling at the mass, so it can't be expected to stay at the center.

Unless the core is solid of course, but in reality the tidal effects would move around the whole core some.

[I better add that the gravitational potential is negative, of course. That may else be confusing.]

## gkam

## EnsignFlandry

The 2.5 years is obviously a typo of some sort.