# Earth's core is younger than its crust surface

## Time is 'bigger on the inside'

Back in the early 1960s, physicist Richard Feynman remarked that the centre of the Earth had to be a little younger than the surface, since it would experience gravitational time dilation.

Now, boffins from two Danish universities have put a value to that difference, and while they agree with his hypothesis, they've corrected his estimate of the difference: while Feynman thought it would be “a couple of days”, a full calculation suggests it's two-and-a-half years.

The paper is here at Arxiv and has been accepted by the European Journal of Physics.

The authors – Ulrik Uggerhøj and Rune Mikkelsen at Aarhus University and Jan Faye of the University of Copenhagen – don't expect their “back of the envelope” calculation to change the world, but say it should be useful for teaching physics students.

The paper actually runs two calculations for Earth, with a “realistic” model of earth yielding the 2.5-year difference in age. Even if Earth's mass was homogeneously distributed, which it's not, the difference in gravitational potential from centre to surface would cause a 1.5-year difference.

The same mathematics works just fine for the sun as well, except that because of its size, mass, and age, the core-to-surface time dilation is much bigger.

In a non-homogeneous model of the Sun, the core is around 39,000 years younger than the surface.

As the paper says: “The pedagogical value of this discussion is to show students that any number or observation, no matter who brought it forward, must be critically examined” – especially since nobody had thought to question Feynman's remark until now. ®