
The young centre of the Earth (2016) - ungerik
https://arxiv.org/abs/1604.05507
======
woah
How is it possible that the core is several years younger if the core and the
surface have gone around the sun the same number of times?

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bityard
General Relativity

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wlesieutre
Parent comment is making a joke about “year” as a unit measuring how many
times you go around the sun. Less time has passed for the core per relativity,
but is it not the same number of years?

The more official definition of years defines it in terms of days, which break
down further to SI’s rigorously defined second as the base unit of time. But
it’s still weird to think about.

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bladedtoys
Unless the second is defined on a Hubble flow co-moving observer, it's ill-
defined enough to be ambiguous by at least 2 years out of 4 billion.

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wlesieutre
Yes, you wouldn't be able to measure it accurately, but it's nonetheless a
static amount of time unrelated to the Earth's orbital period.

I'm not an astrophysicist, but I'd guess the Earth's orbital period isn't
quite the same as when the planet first formed either. So using today's
defined measurement of "years", the planet's age and the number of times it's
been around the sun might not match up at the surface either. The impact that
formed the moon must have changed our velocity a bit, right? And the sun has
been losing mass (into energy via fusion) since it first formed.

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nyc640
Here is a more accessible explanation for those (like me) who are unfamiliar
with the maths: [https://www.sciencealert.com/earth-s-core-is-2-5-years-
young...](https://www.sciencealert.com/earth-s-core-is-2-5-years-younger-than-
its-crust-thanks-to-the-curvature-of-space-time)

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civilian
From that article:

> If you want an even more stark illustration of gravity’s effect on time,
> take the Sun - the team calculates that the Sun’s core is around 40,000
> years younger than its surface.

Wow!

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tomxor
> 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.

Excellent, I hope this value is also encouraged more in schools now than when
I was there.

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madengr
What's the deepest core sample; 10 km? I assume isotope dating is no where
near accurate enough to see the difference.

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danbruc
Even if it were accurate enough, I think plate tectonics moves the crust
around enough such that one would have a hard time finding a piece of rock
that has been more or less always in the same depth. And I have no idea how
static the core is, maybe even it undergoes some mixing process over long
enough periods of time.

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dredmorbius
If there _are_ any such stable deep-crust locations, they're most likely
located under a continental location rather than oceanic, if my understanding
is correct. Ocean basins reform at a higher rate (and hence: are more-
recently-exposed crust) than continental plates.

The _very_ oldest regions are two locations, presently in South Africa and
Australia, previously joined, dating back several billions of years.

The Jack Hills region in Australia has been dated to 4.4 billions of years
(the Earth itself is 4.5 billion):

[https://www.nationalgeographic.com/news/2014/2/140224-oldest...](https://www.nationalgeographic.com/news/2014/2/140224-oldest-
crust-australia-zircon-science/)

A region presently under Greenland was dated to 3.8 billion years, in 2007:

[https://www.newscientist.com/article/dn11438-oldest-chunk-
of...](https://www.newscientist.com/article/dn11438-oldest-chunk-of-earths-
crust-ever-found/)

The Nuvvuagittuq Greenstone Belt, near Hudson Bay, Canada, has been dated to
between 3.7 and 4.3 billion years old:

[https://en.wikipedia.org/wiki/Nuvvuagittuq_Greenstone_Belt](https://en.wikipedia.org/wiki/Nuvvuagittuq_Greenstone_Belt)

All of these are _surface_ rock. My thought is that drilling from these
locations might find very old subsurface structures as well. Though whether
any relativistic time dilation could be observed is hard to say.

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kstrauser
It seems like it would be the opposite:

\- If you are smack dab in the middle of the Earth, although you'd be mashed,
gravitational forces should be equal in all directions and therefore cancel
out to zero.

\- Ergo, if you are on the surface of the Earth, gravity should make your
clocks run slower than in the middle.

What am I missing that makes this untrue?

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ajkjk
It's not the gravitational force that determines the relative passage of the
time but the gravitational potential, which can be much higher despite not
experiencing any force.

~~~
kstrauser
Oh, interesting. Thanks for the clarification!

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p1esk
What about the center of a black hole? Is it younger than the surface?

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Filligree
The question is slightly ill-formed. I understand why you'd ask it, but there
are a couple of possible answers, none of which work directly.

\- No, because a black hole has nothing underneath the event horizon. (This is
true from the perspective of anyone outside it, at least to a first
approximation. Gravity is affected by curvature, and whatever might be in the
centre _cannot be the cause of the event horizon_ , except historically, but
the event horizon is stable in itself. It's a self-sustaining cascade of
space-time.)

\- Mu, because once you pass the event horizon the extreme warping of space-
time turns "inwards" into "future", making the centre of the hole a point in
time, not space. This makes it difficult to determine what the question is
asking.

\- Maybe, because singularities probably don't exist. There should be
_something_ in the centre, which might evolve over time. It might just be a
faster-than-light cascade of space-time, though. We still can't answer your
question, because we don't know how to accurately model the 'singularity'. (As
anything other than a singularity, and that's essentially just the math giving
up.)

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redblacktree
> self-sustaining cascade of space-time

I am in awe of these words and fail to understand them in equal parts.

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qubex
The way I usually explain it is that gravity is a form of energy, and that
energy has mass, and that at the event horizon the energy of the gravity is
exactly sufficient to maintain the gravity itself, basically becoming a self-
powering loop of gravitational energy.

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gumby
This only works because the core is denser than the surface.

(hint: imagine the limit case where the earth were a hollow sphere)

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pdonis
_> imagine the limit case where the earth were a hollow sphere_

If it were a hollow sphere with the same total mass as our actual Earth, as
measured from the outside, then clocks everywhere inside the hollow sphere
would run slow relative to clocks just on the outer surface of the sphere, by
roughly the same amount as clocks at the center of the actual Earth. The
"clock rate" in the interior of the sphere's material would decrease a lot
faster, per unit distance down from the outer surface, than it does in our
actual Earth, because the material of the hollow sphere would have to be a lot
denser than the material of our actual Earth, in order to have the same total
mass.

~~~
gumby
What is the force of gravity at any point in such a sphere? So what does GR
have to say?

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lalaithion
The force of gravity is 0. But the potential of gravity is higher than on the
outside, so clocks run slower.

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pdonis
_> the potential of gravity is higher than on the outside, so clocks run
slower_

Correction: the potential of gravity is _lower_ inside the sphere than
outside, so clocks run slower.

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spuz
Does the same apply to the Sun? How old is the centre of the Sun compared to
its surface?

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lisper
There's a little bit of smoke-and-mirrors going on here. Yes, the same effect
happens on/in the sun (in fact it's even more extreme because the sun's
gravitational field is much stronger than earth's). But no, it's not quite
correct to say that the center of the sun -- or of the earth for that matter
-- is "older" than the surface because the material in both bodies moves
around. _If_ you put a clock at the center and another synchronized clock at
the surface and left them running for a few billion years and then compared
them, you'd see a difference. But of course there are no such clocks. The only
"clock" is the material that exists in both locations, and in all cases this
material moves around over time. On earth there is some stability because
heavier material sinks towards the center and more or less stays there, while
lighter material floats to the surface and again more or less stays there. But
the sun is made almost entirely of hydrogen and helium and that material is
constantly being moved around due to convection.

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Kutta
There is no convection in the core of the sun. It is plausible that material
has mostly stayed there since the formation of the sun.

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dredmorbius
What form(s) of motion are there, if any, according to theory?

(I'll presume ... direct observations ... are limited.)

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hirundo
> Comments: Published version, apart from minor editing (e.g. corrections of
> 'center' to 'centre')

> Journal reference: Eur.J.Phys. vol. 37, 035602 (2016)

If Brexit occurs, Britain will no longer be a European nation, and therefore
the European Journal of Physics will no longer be obliged to accept "centre"
as the standard spelling of "center", right?

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tasssko
We will most certainly still be a European nation. Just not a member of the
European Union. We are still located on the European continent. Its spelled
centre.

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hirundo
“Some years ago I opined that London was not really an English city any more.
Since then, virtually all my friends from abroad have confirmed my observation
So there must be some truth in it.” -- John Cleese

In that sense, perhaps London remains a European city while still the capital
of a not really European nation.

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bregma
It's all a wibbly-wobbly, timey-wimey big BIG ball of string.

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dashesyan
There's a "yo momma" joke just waiting to be made here...

~~~
keanebean86
According to General relativity your mom is actually more attractive compared
to other mothers...

