back to article Earth's core is younger than its crust surface

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 …

  1. allthecoolshortnamesweretaken

    So Feynman was a bit off - probably because he never got around to sit down and do the math properly. Which, in the 1960ies, may have been based on a different perception of the solar system's age, so even his proper math might have been off by today's baselines.

    The amazing thing is to come up with this in the first place. Honestly, who even thinks about stuff like this? Gravitational effects, yes, but the focus there usually is black holes or other singularities* or space travel. But thinking about the effects on the very rock you're standing on - that guy was one serious thinker.

    * A plural for singularity, now that feels kinda funny.

    1. Ian Michael Gumby

      A bit off?

      Just out of curiosity, does any of the calculations take in to account the fluidity of the earth's core? So that something in the core could come up and away from the core and then recycle and flow back to the core?

      Just going out on a limb but the further away from the core, the less of a factor the time dilation effect, so that it could be that Feynman is closer to the truth?

      1. Steven Jones

        Re: A bit off?

        There's very little exchange of material from the core as it's overwhelmingly made of dense metals (80% or so of which is believed to be iron). That core is believed to have formed very early on when the Earth was still molten and the relatively small amount of material that has moved since is really not going to make a significant differences to the calculations.

        1. Anonymous Coward
          Anonymous Coward

          Re: A bit off?

          "There's very little exchange of material from the core as it's overwhelmingly made of dense metals"

          Not sure what density has to do with exchange of materials? The inner core is believed to consist primarily of a nickel-iron alloy. However, a iron-nickel alloy under core pressure is denser than the core, implying also the presence of lighter elements in the core.

          It is likely that the outer core at least has extensive exchange of material as it's liquid...

          1. Steven Jones

            Re: A bit off?

            The density matters because the mechanism for movement will be largely convection and the denser elements will tend to remain at the core. That's why the core is much more dense than the layers above, not because it's under a lot of pressure (which will be a relatively minor matter compared to the density difference between different elements).

      2. Anonymous Coward
        Anonymous Coward

        Re: A bit off?

        Material doesn't really leave the Core. As well s the Inner Core being a giant ballbearing of solid iron/nickel alloy, there's also a profound density difference across the Core Mantle boundary which means that convection can't take place.

    2. Anonymous Coward
      Anonymous Coward

      " Honestly, who even thinks about stuff like this?"

      Lots of people. GPS signals wouldn't work without accounting for it either, it's not just the big things you highlighted.

    3. DropBear
      Facepalm

      "* A plural for singularity, now that feels kinda funny."

      ...only if you never learned any maths, wherein thoroughly non-remarkable functions (like the humble "tangent(x)") can have a dozen of them before you even reach 0 on the x axis...

  2. Anonymous Coward
    Anonymous Coward

    What is time

    You realize that time, t, is just a parameter we used to define our equations-of-change. It made it easier for us to describe those equations in terms of t.

    It's phrased in terms of human perception, so the unit of seconds is a time scale that we perceive, its not nano-seconds, or centuries which would be too short or too long to perceive.

    And since we all live on the surface of earth, it is defined to be constant regardless of gravity because we perceive nothing else.

    So Einsteins fixup to time is really a difference between human perception of time and the truer underlying measure of change, but we didn't understand that when we first defined t.

    And you can ask "why can't we go back in time", but of course if we did, we would 'unperceive' change. The electronics of our brains would 'unlearn' as we did. The neuron signals would go outward away from the brain to the eyes and the senses. Rather than inwards to learn from the change.

    Asking instead "why doesn't time go 1 million times faster..." well of course it does, its only us that perceive change in terms of seconds not centuries. If our brains could perceive change at the much slower rate, time would appear to be going much slower than seconds.

    So it really does go k times faster, and even negative k, but our perception of it can only ever be of +ve values of k near the value +1.

    1. find users who cut cat tail

      Re: What is time

      Each observer has its own ‘proper time’ and for everyone their proper times clock at exactly the same speed, as far as any time-like factors in laws of physics are concerned. Beside that you can define more or less arbitrary space-time coordinates, which is done routinely in general relativity, but they are just that, arbitrary coordinates.

      You can observe that proper times of distant objects go slower or faster (i.e. physical processes appear to be slower or faster) -- which is exactly the difference between the crust and core times here.

      Humans simply experience their proper times because that is the physical time determining the evolution of everything. In principle you can imagine a time difference for instance between your head and limbs, but for any non-negligible difference the corresponding tidal forces would be enormous.

      1. Dave 126 Silver badge

        Re: What is time

        >In principle you can imagine a time difference for instance between your head and limbs

        That's actually been shown experimentally! Seriously, physicists as NIST have observed difference in reading between two nuclear clocks, one 0.33 m higher than the other. The difference is as theory predicted, and it is equivilenet to 90 billionths of second over the course of a 79 year lifetime.

        http://www.nist.gov/public_affairs/releases/aluminum-atomic-clock_092310.cfm

        1. find users who cut cat tail

          Re: What is time

          > That's actually been shown experimentally!

          Sure, it has been *measured* thanks to the insane precision of nuclear clocks. I was talking about direct sensory input, ‘feeling’ that time goes differently in different pats of your body. Which should be a funny feeling, except for the terrible tidal forces killing you instantly.

      2. Rich 11 Silver badge

        Re: What is time

        You can observe that proper times of distant objects go slower or faster (i.e. physical processes appear to be slower or faster) -- which is exactly the difference between the crust and core times here.

        It's not a matter of distance per se. If you are deeper in a gravity well then space-time is being distorted more. To an observer higher up in the well, your time appears to them to be passing more slowly than does theirs.

        1. JeffyPoooh
          Pint

          Re: What is time

          R11: "If you are deeper in a gravity well then space-time is being distorted more."

          Which leads to the following thought experiment...

          You're inside a box (like Einstein's elevator). The box is either floating 'weightless' alone in the interstellar void, or it's hovering 'weightless' in the hollow exact-center of a very huge mass (deep in a huge gravity well). What experiment can you perform inside your sealed box to distinguish these two?

          Conclusion: We're in a conceptually similar box called The Universe. This is where some of the fundamental constants (e.g. 'c') come from.

          Maybe. Maybe not.

          1. Ian Michael Gumby
            Coffee/keyboard

            @ Jeffy Poooh ... Re: What is time

            You didn't happen to take your cat along with you in to the box, now did you?

            You didn't happen to catch his name? (Schrodinger was it? )

      3. Anonymous Coward
        Anonymous Coward

        Re: What is time

        See my comment about dog years below (when it gets posted, post-time is not in sync with real time).

        My point is, that to be useful as a measure of change, delta-t at the earths surface has to be equal to delta-t at the core. Because the earths core and earths crust regularly interact, and its silly to have each layer of the earth in a different time scales.

        @"proper times because that is the physical time determining the evolution of everything"

        Proper? Well no, we could define change in all manner of parameters, so for example if I throw a ball its path is parabolic, i.e. y = ax + bx^2, a parabolic curve defined as a function of x. That equation is not actually true, it treats gravity as fixed and constant, if we'd defined it properly, we'd have a much longer equation.

        Then when we parameterize that equation for a 'change' value the correctly deals with gravity, (which we'll call tReal ), we then have a definition which we could then use BOTH at the crust, AND at the center of the earth and everywhere inbetween. There's nothing special about our perception of time that makes it the proper perception. But all that physics *we* defined in terms of t, where t is time defined in terms of our incorrect perception of it.

        Why is it incorrect?

        Put it this way, the dog years thing:

        I was there at the birth of my dog. My dog is older than me in dog-years. This is because dog-years is not a real measure of change, its an arbitrary scaling of my dogs years as a measure of how old it is in human age terms. Which leads to impossible faults like me being at its birth and it being older than me. Or like the earths core being younger than the earth. The fault, (which Jake below notices but doesn't understand the implications) is such a major flaw in a parameter intended to be the measure of change.

        Hence using t, a human perception of time, is an incorrect measure of change.

        The earths core really isn't younger than the crust, its just the incorrect way we defined t.

    2. jzl

      Re: What is time

      "So Einsteins fixup to time is really a difference between human perception of time and the truer underlying measure of change"

      Nope.

      Stick to commenting on politics, where actual facts don't matter. Leave the physics alone.

    3. Lars Silver badge
      Happy

      Re: What is time

      Indeed a timeless question.

      As a kid I spent a lot of time in a library during the winter. I had my fair share of Biggles, Tarzan and such but I soon found out adults have more interesting book to read like science, and stuff one had to hide from parents at home. Anyway, one book I still remember after 60 years, Cosmos (1 and 2) was written by some guy (not Sagan) who left Russia for the USA and started to write about science in a "popular" way.

      Writing about Einstein and why it took other "boffins" so many years to understand him he claimed one reason is that we understand things like a kg a meter and speed as an physical experience but we are rubbish at understanding time.

      To explain his point he has this simple and straightforward example.

      There is a river and a boat goes upriver downriver from A to B and then from B to A with no current in the river. And then the same trip but with a stable current, say 2 knots while the ships speed is 20 through the water in both cases. No hidden agenda here just pure mathematics/logic.

      Now his question is this - does it take more, less or equal time with the current.

      Take your pic but then prove it on paper and then prove your answer with one spoken sentence (pure logic) and you will understand why I still remember it.

      1. myhandler

        Re: What is time

        Lars.. can you expand on this..

        It takes the same time surely - what am I missing?

        Something to do with derivative of speed against time and distance?

        Too old for this.

        1. Lars Silver badge
          Happy

          Re: What is time

          @ myhandler

          Try paper and pen and you get it. Time.

        2. jzl

          Re: What is time

          If you go upriver twice as slow, you have to come back twice as fast to take the same time, but the speed downstream can't be the inverse of the speed upstream. Basically, (22/20) <> (20/18)

          1. myhandler

            Re: What is time

            Thanks.. ok.. but no light bulbs gone off.. why is this meant to be revelatory?

            There I was pondering the extra 2 miles an hour increasing the mass as it approaches the speed of light, and vice versa, but that seemed contrived

            1. jzl

              Re: What is time

              It's not revelatory. Lars is pretty pleased with himself for understanding some GCSE arithmetic :)

              1. Lars Silver badge
                Happy

                Re: What is time

                @ myhandler and jzl

                Thanks for responding to my time rant.

                The reason I still remember that "time example" is that while I knew the answer as I read the book, I still had to check it on paper because I went for the equal time answer and I just did not get it.

                The whole idea with this example is to prove that we are rubbish when it comes to understanding time.

                But then I got it, and yes jzl, I was very very proud about my self, I was eleven and I felt a genius. Hasn't happened much often since then.

                The solution in one sentence is that the time against the disadvantage (against the current) is always longer than the time during the advantage (down stream). The stronger the current the greater the difference. But what we idiots do is to add 18 to 22 and then we divide by 2 and we get 20 and we will say the time is equal. We suddenly forget that speed is the relation between distance and time. We don't get the time at all.

                The funny thing is that some guys are so totally convinced the time must be equal that they even refuse to do the calculation. For them I have invented a modern version that you can solve in your head. Goes like this. You have a trip to do of 200km in your car. Then you decide you will use 2h driving 100km/h and everything is fine. But then after having made that first 100km you realize that your average speed was in fact 80km/h. Cleaver as you are you then decide you drive the rest of the trip, 100km at 120km/h to make it in 2h. But when your 2h are up you are still 10km short of the 200km. That you can work out in your head.

                Now, I am not obsessed with this, but during my "barfly" -period, long ago, often short of beer, I found that one can win a beer or two with that silly question. Only once did I have to avoid a punch in the nose.

                Regards Lars

  3. jake Silver badge

    Lack of critical thinking, methinks ...

    The Earth was molten for how long?

    The heavy elements (mostly iron) became the core while the Earth was molten, right?

    The core developed before the crust developed, by definition, right?

    How many hundreds of millions of years was it between "molten" and "crust development? It certainly wasn't a couple of years. In fact, it's still going on as the Earth cools ...

    1. Anonymous Coward
      Anonymous Coward

      Re: Lack of critical thinking, methinks ...

      "I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."

      1. Dave 126 Silver badge

        Re: Lack of critical thinking, methinks ...

        >"I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."

        The Reg made an error in its reporting, and introduced the word 'crust' when it wasn't present in the source material. Just like using Wikipedia, it's important to check the references yourself.

        Welcome to the internet.

        1. Dave 126 Silver badge

          Re: Lack of critical thinking, methinks ...

          Abstract

          We treat, as an illustrative example of gravitational time dilation in relativity, the observation that the centre of the Earth is younger than the surface by an appreciable amount. Richard Feynman first made this insightful point and presented an estimate of the size of the effect in a talk; a transcription was later published in which the time difference is quoted as 'one or two days'. However, a back-of-the-envelope calculation shows that the result is in fact a few years. In this paper we present this estimate alongside a more elaborate analysis yielding a difference of two and a half years. The aim is to provide a fairly complete solution to the relativity of the 'aging' of an object due to differences in the gravitational potential. This solution—accessible at the undergraduate level—can be used for educational purposes, as an example in the classroom. Finally, we also briefly discuss why exchanging 'years' for 'days'—which in retrospect is a quite simple, but significant, mistake—has been repeated seemingly uncritically, albeit in a few cases only. 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.

      2. jake Silver badge

        Re: Lack of critical thinking, methinks ...

        ""I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.""

        I think the word you were looking for was "comprehend".

        1. John Gamble

          Re: Lack of critical thinking, methinks ...

          Having now read your correction to a famous quote in the history of computer science, I look forward to your edits of Defoe, Darwin, Newton, Ruskin, and Shakespeare.

        2. TitterYeNot
          Coat

          Re: Lack of critical thinking, methinks ...

          "I think the word you were looking for was 'comprehend'."

          To aprehend - to grasp or understand a concept.

          A most appropriate verb to use when so many comments here demonstrate a complete failure to grasp one of the basic concepts of general relativity i.e. time is relative and not constant.

          Though it begs the question, if a higher gravitational potential at the Earth's surface causes relative time dilation, what implications does this have for the turtle at the bottom?

          I know, I know, I'm being silly, there cannot be a turtle at the bottom. It's turtles all the way down...

    2. Anonymous Coward
      Anonymous Coward

      Re: Lack of critical thinking, methinks ...

      Even if it was a few hundred million years that's a single digit percentage against the age of the Earth. Take a single digit percent off 2 1/2 years and you get ... maybe 2 1/3 years. Not enough to worry about, especially since this was never going to be an exact calculation to begin with.

      That doesn't even get into the question of when you start the clock for the Earth's age. Is it when mass starts to coalesce in the orbit we occupy? When it finishes that process and is a sphere about the size of today's Earth? When the surface begins to solidify into a crust?

    3. This post has been deleted by its author

    4. Dave 126 Silver badge

      In partial defence of jake:

      This Reg article has used the word 'crust' in error. The authors of the paper did not use the word 'crust', only 'surface'. Jake is correct - the crust took tens of millions of years to form in the first place, and has been recycled through tectonic activity many times since.

      That said, jake should know better than to trust the Reg at face value - it's important to check the source material directly.

      The purpose of the paper, as the authors intended it, was as a teaching aid for undergraduate students to challenge established views. Feynman would have approved.

    5. Anonymous Coward
      Anonymous Coward

      Re: Lack of critical thinking, methinks ...

      This is the "time dilation effect" i.e. gravity affects time, since gravity inside the center of a sphere is lower and effectively zero at the center, time is faster.... wait.... shouldn't that make it older from our perspective?

      Oh no matter, basically we have a measure of change that has an error in it, since it doesn't account for gravity. That error means you can't compare two times at two different gravity values. Even though they clearly interact regularly, at t = 0, {t= 1, tcore=1.1}, {t=2, tcore=1.2}... the time definition for each is messed up.

      It's like measuring time in dog years, we know its wrong to scale time by the average human life over the average dog life, its arbitrary and clearly broken, but well my dog is "older" than I am (in dog years), yet I was there are his birth. That fact clearly demonstrates the error in the time unit I'm using. And in a similar way, a younger core that was created before the outer core illustrates the problem with our time unit.

  4. frank ly

    Gravity

    I'd have thought that the gravitational field at the centre of the earth is zero, assuming a spherical earth. You can only treat a planet as a point source of gravity if you're outside it, or just on the surface. Under the surface, you have to do an integration involving slices of the planet 'above' and 'below' you. At the centre, everything cancels out and you have what is effectively a one body Lagrangian point.

    1. Julian Bradfield

      Re: Gravity

      The time dilation between two points depends on the difference of the gravitational potential between them, not on the field strength at either point. The centre of the earth has a zero field, but a low potential relative to the surface (if you drop something down a tube to the centre of the earth, all that kinetic energy has to come from somewhere!).

      1. This post has been deleted by its author

    2. Francis Boyle

      Have an upvote

      for asking a good question which elicited an excellent answer.

  5. Novex

    Pitching for down votes... ;-)

    So, the age of the earth is dependent on where you measure it from, because it's all relative. Measured from the surface, the centre is the same age as the surface. Measured from the centre, the surface is the same age as the centre. The difference between the two, which would have a single start point about 4.5 billion years ago, is down to the way atoms and particles and everything else in the centre are slowed down by gravitational dilation while at the surface they move just a little faster because there's less gravitational effect.

    1. Anonymous Coward
      Anonymous Coward

      Re: Pitching for down votes... ;-)

      Pitching AGAINST the down votes:

      No, its Physics as an unchallengable religion, that can't face up to a few thought experiments that point to the problems.

      As if down-voting make the problem go away.

  6. Blofeld's Cat
    Coat

    So ...

    Dateline 22 October 4004 BC, about 3:30 pm: Place the planet Magrathea:

    Two figures stand on a construction gantry surrounded by partly assembled dinosaur skeletons and surplus firmament.

    "Right that's the crust finished, shall we call it a day?"

    "Nah, we'll pour the core in before we go. It'll only take a couple of hours or so to heat it up."

    "OK, but I'm taking tomorrow off for a rest."

  7. Anonymous Coward
    Unhappy

    My brain hurts, can we have an article about kittens please.

    1. Kane

      "My brain hurts, can we have an article about kittens please."

      Here you go.

    2. GrumpenKraut
      Coat

      > My brain hurts, can we have an article about kittens please.

      OK, fine. Do you want kittens in low or in high gravity?

      Mine is the one with the 13 volumes of "Annoying questions that will make you hugely unpopular" in the pockets.

  8. james 68

    Quite a large pachyderm in the room

    Seems to me like nobody considered a little thing called plate subduction, wherein the earths tectonic plates constantly move, when forced against each other one plate will be forced under the other, as that plate is pushed further into the underlying magma it in turn "melts" once again becoming magma itself. At areas where the plates pull apart, new land is formed by upwelling magma (Hawaiian island chain is a good example of this).

    Long story short - the earths crust is much much younger than the core as it is constantly being "recycled". This scientific hypothesis which states the opposite is only relevant in a static earth model where there is no subduction or upwelling to recycle the crust.

    Currently Mars has no plate tectonics, but the hypothesis doesn't even work there, as Mars did have plate tectonics in the past meaning that even its crust is in fact younger than its core.

    I guess this is a problem specific to when physicists try to work out geology.

    1. Christoph

      Re: Quite a large pachyderm in the room

      No. The rocks themselves may be younger, but the material they are made of is not. Those same atoms have been cycled around through various different rocks. Just because they have been melted and re-solidified does not make them physically younger.

      1. james 68

        Re: Quite a large pachyderm in the room

        Some plates manage to get deep enough to interface with the magma-core boundary and dredge up core material, the materials from the crust mix with the cores due to these interactions at the the magma-core boundary which is why there are heavy elements for us to mine in the crust. Which would make crust and core the same age. So again screwing the hypothesis, and how about the new material from space which is constantly added to the crust (160+ tonnes per year according to NASA)? That adds up to a metric fuck-ton over geological timescales.

        If you really want to stick to atomic age and not material age then answer me this- How old is the earth? Because if you answer 4.543 billion years then you would be wrong (by your reasoning) as that is the "material age" the atoms were formed long long before that in the hearts of long dead stars from atoms which were created long long before that yadda yadda etc.

        I say it again, the hypothesis only works for a static earth model.

        1. Doctor Syntax Silver badge

          Re: Quite a large pachyderm in the room

          "Some plates...the materials from the crust mix with the cores due to these interactions at the the magma-core boundary which is why there are heavy elements for us to mine in the crust."

          And overall, just what proportion does this amount to for the core & for the crust?

          1. james 68

            Re: Quite a large pachyderm in the room

            Over the period of time earth has existed? quite a lot I'd imagine. You are I notice studiously ignoring the entirely new material added to the crust on a continuous basis, this is not an insignificant mass of material even discounting the heavy bombardment periods over the current lifespan of our planet makes an enormous difference in relative age of crust and core materials.

        2. Anonymous Coward
          Anonymous Coward

          Re: Quite a large pachyderm in the room

          'the materials from the crust mix with the cores due to these interactions at the the magma-core boundary which is why there are heavy elements for us to mine in the crust.'

          Not the case. Heavy elements such as uranium and thorium become concentrated in the Crust because they are incompatible with mineral structures in the Mantle (they're called lithophilic in older books) which means they are concentrated in melts when the Mantle undergoes partial melting . Low density melts rise up towards the surface, undergo fractional crystallisation which further concentrates heavy elements in the continental crust. Some elements, such as gold and the other platinum metals are described as siderophilic because they are relatively soluble in molten iron which meant they were preferentially drawn to the Core during its separation from the silicate bulk of the Earth.

          There's conflicting evidence of whether subducting plates get to the Core Mantle boundary - most become invisible around 600-700km in a region where there's a detachment in the Mantle. The Farallon Plate does seem to go very deep, albeit very diffuse as it merges with the surrounding Mantle. There's pretty much no evidence in magmas derived from deep-seated Mantle plumes of any Core material coming back up, a huge density difference between the lowermost Mantle and Outer Core prevents transfer of material.

    2. Doctor Syntax Silver badge

      Re: Quite a large pachyderm in the room

      Irrespective of how many times some particular stuff has actually been at the surface it's still the same stuff. You're really only looking at the difference between the time dilation effect at the surface vs just below the surface and the time spent at various depths. In terms of the overall size of the Earth it's not a great deal. What's more, the paper points out that as the Earth isn't homogeneous the effect varies depending on depth and even ignoring vertical movements, is negligible for several hundred kilometres (fig 3).

    3. Chemist

      Re: Quite a large pachyderm in the room

      "I guess this is a problem specific to when physicists try to work out geology."

      I think you are all (most) missing the point totally. This has nothing to do with fluid flows, tectonics etc.

      All these guys are saying is if you have an object the size and mass of the Earth time will run differently at the surface and in the core. It's a General Relativity thing. It's no more suprising than a GPS satellite having a different flow of time to an Earth based receiver

      1. Dave 126 Silver badge

        Re: Quite a large pachyderm in the room

        >"I guess this is a problem specific to when physicists try to work out geology."

        Not at all. The physicists in question did not use the word 'crust', they only mention the surface. The word crust was introduced by this Reg article in error.

        That is all.

    4. Lars Silver badge
      Pint

      Re: Quite a large pachyderm in the room

      @ james 68

      "the earths tectonic plates constantly move, when forced against each other one plate will be forced under the other"

      Tectonic plates, pushing or pulling. According to this fairly recent program it's more about pulling, and recycling is about replacing than producing anything new. And it will become "interesting" if nine plates becomes ten.

      https://www.youtube.com/watch?v=h1KuEFUtuJY

  9. Christopher Reeve's Horse

    the other stuff?

    I get the whole gravitational time dilation difference thing, but what interests me is how that affects processes that link the different time domains.

    In the case of the Sun, does that mean there's 39,000 years of energy production 'missing' at the surface that the core simply hasn't had [the] time to produce?

    1. Chemist

      Re: the other stuff?

      "does that mean there's 39,000 years of energy production 'missing' at the surface that the core simply hasn't had [the] time to produce?"

      Not going to even consider that when estimates show it may take a photon a million years just to reach the sun's surface from the core

      image.gsfc.nasa.gov/poetry/ask/a11354.html (0/10 for background to web-page)

    2. jzl

      Re: the other stuff?

      "In the case of the Sun, does that mean there's 39,000 years of energy production 'missing' at the surface that the core simply hasn't had [the] time to produce?"

      No. It means that the Sun is very, very, very, very, very, very, very fractionally less radiant than it would otherwise be.

  10. Ralph B

    A Simple Fix

    Obviously this 2.5 year slippage is unacceptable. I believe that the only way we can hope to keep the observers at the surface and at the core properly synchronised in future will be to construct a vacuum-filled hole through the Earth and then drop an atomic clock down it. Every 84.5 minutes both parties can set their watches to the correct time as the clock visits. (Of course, the observers at the core will have to be pretty quick at it since the clock will be passing by at 17,700mph.)

  11. mistergrantham

    goddamnit Uggerhoe &c., you're ruining my plans!

  12. Richard Scratcher
    Paris Hilton

    Duh! Not news to me.

    This theory was proven years ago by Doug McClure when he encountered that subterranean world of stone-age people.

  13. G R Goslin

    Gravitational potential?

    As was pointed out earlier. The force of gravity, at the centre of the Earth is zero. Although not exactly at the centre, since the Earth and the Moon are a mutually orbiting pair. So the gravitational centre rotates about the true centre at every rotation of the Moon in its orbit. There's a similar effect due to the Sun. However, the gravitational potential is near zero. Does this not invalidate the hypothesis? Or even reverse it?

    1. Rich 11 Silver badge

      Re: Gravitational potential?

      The sun and the moon (and let's not forget Jupiter and all the other planets) will lessen the effect very, very slightly, except for the times when they increase it very, very slightly.

  14. Palpy

    Appears to me that --

    -- the "core [centre, really] is younger than the surface" statement is -- (again!) -- a neat pedagogical tool for demonstrating the value of verifying intuition (Feynman's comment) with careful physics (the authors' calculations).

    IMHO, it's not about rocks, subduction, (or any geological effects); nor about gravitational effects of the Moon or Jupiter or the Sun. Perhaps the article would have been best presented as: "If the Earth were homogeneous in composition, geologically static, and gravitationally isolated, then the centre of its gravity well would, assuming the whole structure formed all at once 4.5 billion years ago, 'now' be 2.5 years younger than a spherical surface located at 6,371 km from that centre."

    It doesn't make a snappy lead paragraph, though. Rather dryish.

    And note that any sensible concept of "now" is relative and depends on the observer's relative position in any particular gravity field and his relative velocity -- both measured with respect to any particular point used as a time reference. I think that's approximately right, but surely someone will correct me.

    Selection of viewpoint appears to be arbitrary; it appears that there is no universal "now". It reminds me a bit of John McPhee's observation of early research on plate tectonics -- British geologists drew up maps showing continents wandering all over the globe "with respect to a fixed and undriftable England". That's kind of how I view my personal wristwatch-time: fixed and undriftable. Of course that's balderdash.

  15. Anonymous Coward
    Anonymous Coward

    Heretics, the world is flat and the top is older than the bottom.

  16. Big_Ted
    Coat

    Please; they are overcomplicating it.....

    When they switch on the LHC it created a micro black hole as was feared by many of us and sank to the centre of the earth.

    Only those of us with tin foil hats are save from these ageing rays flying round.

  17. Anonymous Coward
    Anonymous Coward

    For our US cousins

    Hint: the earth is older than 6000 years.....

    1. Lars Silver badge
      Happy

      Re: For our US cousins

      "the earth is older than 6000 years.....". So very true, I am sure I was informed about it about 37, perhaps 38 years ago. Time flies.

      PS. How should I end this comment, elegantly, to reveal that the great, great, majority of our US cousins know better. Ah damn it, know better than older than. I give up.

    2. Mark 85

      Re: For our US cousins

      It's only a subset of the cousins. Most of us understand and accept it. Those that don't understand it's more 6000 years old (certain CongressCritters and a number of fundamentalist types) view the rest of us as satan worshippers, the embodiment of evil, etc. and while they despise Sharia Law, they would implement the punishments to the rest of us in a heartbeat.

  18. stewwy
    Facepalm

    It's a Physics Question FFS

    Std assumptions apply - First posit, a spherical Cow in a weightless vacuum

    1. Swarthy

      I thought it was a sperical cow in a friction-less vacuum

      1. David Pollard

        You'll also need one of those massless pullies and some inextensible string to get it back to the surface. These are not so easy to obtain these days.

  19. Mark 85

    The article, the references, and the comments make my brain hurt.

    Is it beer o'clock yet?

  20. AustinTX
    Coffee/keyboard

    Unscientific Thought Experiment

    One little idea I like to muse about from time to time is that perhaps black holes don't form under the collapse of stars, but rather form inside a star earlier in it's life, possibly even having been a seed which drew gasses together to form that star. Either way, it starts out and remains so small for so long, that the core's mass completely hides it. It may affect the sun's "engine" at some point, but the change is gradual. A tiny black hole has too little surface area to pull in very much matter and thus will grow very slowly - at first. An embedded black hole would slowly drain the star's mass, eventually crossing a threshold and suddenly heating it up quite a bit, causing it to puff up and expel material.

    The mass of a star, starting with 1.4x the size of our own sun, is tied to the likelihood that a hole makes its appearance after supernova. A star that is too small simply never seems to produce a black hole, though it might reveal a neutron star, or just shrink and cool off. Imagine that it isn't simply pressure, but rather some other sort of stress on space-time which creates black holes.

    The article talks about a tens-of-thousands of years difference between the age of the sun's core and it's outer layers. This obviously doesn't mean that we could travel back in time by burrowing into the sun's core somehow, but I wonder if, in cases like this, that there isn't some accumulated "frame drag" that goes on. A star that is too small just can't create enough strain to 'tear' space, but larger ones inevitably do. The mass of a star seems to be related to [the difference in age of the core of a star and it's outer layers] and [the likelihood it will collapse under enough pressure to form a black hole], for what we take to be unrelated reasons.

    I propose that perhaps a black hole is actually produced by a kind of frame drag between the core and outside of massive objects like stars of sufficient size. The greater the mass, the quicker the frame drag accumulates. All else that we know about black holes is just what we observe once they become exposed, and behave like independent objects.

    1. Anonymous Coward
      Anonymous Coward

      Re: Unscientific Thought Experiment

      Because of the density, mass wouldn't be able to orbit. It'd all fall straight in.It wouldn't last a week.

  21. John F***ing Stepp

    Oh, well; down vote bait.

    The exact center of the Earth would not be subject to a gravitational force (because the mass around that point would be balanced). Full force of gravity at the surface, less the deeper you go.

    I am fairly certain that even Jules Verne knew this.

    Sometimes things go wrong 'under pressure'.

    1. Chemist

      "The exact center of the Earth would not be subject to a gravitational force (because the mass around that point would be balanced). Full force of gravity at the surface, less the deeper you go."

      Indeed so but the effect on time is due to the gravitational potential not the field and that gets stronger all the way to the centre. As others have explained if you fell down an evacuated tube connecting one side of the earth with the other you would reach max velocity at the centre and then almost pop out the other side, then oscillate around the minimum ( the centre). Better than any fairground ride but a bit same-ee after the first billion years. (Presumably there would be some loss of energy from the system to slowly damp the oscillation eventually)

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