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The young centre of the Earth (2016)

arxiv.org

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Re: The young centre of the Earth (2016)

#51

Post by pdonis »

gumby wrote: post #11

This only works because the core is denser than the surface. (hint: imagine the limit case where the earth were a hollow sphere)

> 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.

Re: The young centre of the Earth (2016)

#52

Post by pdonis »

lisper wrote: post #23
spuz wrote: post #17

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

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…

> 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

Correction: "younger", not "older".

Re: The young centre of the Earth (2016)

#53

Post by gumby »

pdonis wrote: post #51
gumby wrote: post #11

This only works because the core is denser than the surface. (hint: imagine the limit case where the earth were a hollow sphere)

> 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 wo…

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

Re: The young centre of the Earth (2016)

#54

Post by lisper »

Kutta wrote: post #46
lisper wrote: post #23

Earlier quoted context omitted.

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…

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.

Heh, you're right. TIL.

Re: The young centre of the Earth (2016)

#55

Post by mechatrocity »

hirundo wrote: post #31

> 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?

Centre is the standard for all English countries except the US, no? We use centre in Canada

Re: The young centre of the Earth (2016)

#56

Post by mehrdadn »

colechristensen wrote: post #28
woah wrote: post #5

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?

Time isn't universal, it doesn't tick for everybody at the same rate, in fact every particle has its own unique clock. One thing that changes how your clock flows is being in a gravity well. We have clocks which can measure it over just a few feet of height difference. Gravity isn't so much a "force" but the bending of space and time . Clocks moving differently is the result.

> We have clocks which can measure it over just a few feet of height difference

Wait, seriously? Do you have a link??

Re: The young centre of the Earth (2016)

#57

Post by lalaithion »

gumby wrote: post #53
pdonis wrote: post #51

Earlier quoted context omitted.

> 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 wo…

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

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

Re: The young centre of the Earth (2016)

#58

Post by civilian »

nyc640 wrote: post #13

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...

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!

Re: The young centre of the Earth (2016)

#59

Post by colechristensen »

mehrdadn wrote: post #56
colechristensen wrote: post #28

Earlier quoted context omitted.

Time isn't universal, it doesn't tick for everybody at the same rate, in fact every particle has its own unique clock. One thing that changes how your clock flows is being in a gravity well. We have clocks which can measure it over just a few feet of height difference. Gravity isn't so much a "force" but the bending of space and time . Clocks moving differently is the result.

> We have clocks which can measure it over just a few feet of height difference Wait, seriously? Do you have a link??

Measuring the height of a mountain with a portable atomic clock (1000m accuracy) https://www.nature.com/articles/s41567-017-0042-3

NIST measuring speed differences https://science.sciencemag.org/content/329/5999/1630.long

Re: The young centre of the Earth (2016)

#60

Post by mehrdadn »

colechristensen wrote: post #59
mehrdadn wrote: post #56

Earlier quoted context omitted.

> We have clocks which can measure it over just a few feet of height difference Wait, seriously? Do you have a link??

Measuring the height of a mountain with a portable atomic clock (1000m accuracy) https://www.nature.com/articles/s41567-017-0042-3 NIST measuring speed differences https://science.sciencemag.org/content/329/5999/1630.long

Amazing. They observe fractional frequency changes of around 4.1E-17. Thanks for sharing the links!
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