General Question

Bill1939's avatar

Is gravity stronger or weaker at the center of the Earth?

Asked by Bill1939 (10127points) July 4th, 2016

I have always thought that gravity was strongest at the surface of a planet. Science fiction often depicts this. However, the June 25, 2016 issue of Science News (page 5) states that “In Einstein’s general theory of relativity, massive objects warp the fabric of spacetime, creating a gravitational pull and slowing time nearby. So a clock at placed at placed at Earth’s its surface will tick ever-so-slightly slower than a clock at its surface. Such time shifts are determined by the gravitation potential, a measure of the amount of work it would take to move an object from one place to another. Since climbing up from Earth’s center would be a struggle against gravity, clocks down deep would run slow relative to surface timepieces.

“Over the roughly 4.5 billion years of Earth’s history, the gradual shaving off of fractions of a second adds up to a core that’s 2.5 years younger than the planets crust, researchers estimate in the may European Journal of Physics.”

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34 Answers

kritiper's avatar

Weak to non-existent, is my guess, since it would be the very center of the field..

CWOTUS's avatar

My understanding of the topic may be off; astrophysics isn’t my thing. But several statements in your question lead me to think that you may have misunderstood the article in one important respect.

In the first place, time dilation (slowing time, if I understand it correctly) occurs as a function of velocity through space. The faster an object moves through space, the more time slows for that object. So, because the planet essentially spins at a constant rate (in human terms, even if its spin slows infinitesimally with each passing year) then everything at the surface of the planet spins faster in terms of angular momentum than everything closer to the center of the globe. Yes? In that case, clocks would run slower at the surface of the planet than they would at the center. So it’s the crust of the planet that should be “younger” than the center, I would think.

Maybe I’m wrong about that, but the concept had already been proven, because geosynchronous satellites used for GPS – which have to move much faster in space to keep up with the planet’s surface and their synchronicity – have clocks that require correction when the signals they send are received on the planet’s surface. In other words, since the whole concept of GPS tracking relies upon the changing time difference between signals from three or more satellites with an Earth receiver to precisely locate its position on the planet’s surface, getting the time stamp right on the signals is of paramount importance. And since we know from Einstein’s work that the time signals have to be corrected, and that correction is applied and accounted for in computing the time signals to make GPS work – and we can see from ordinary observation and experience that GPS systems work – then “time dilation” is a valid and proven phenomenon.

So by extension, the clocks at the center of the planet will turn slightly faster than the clocks at the planet’s surface, which turn faster than the clocks on the geosynchronous satellites. And that would make the core of the Earth “older” than the surface.

But to answer the actual question about gravity…

Here I’m on shakier ground (so to speak). I still haven’t read anything that explains WHY we have gravity. I understand that this is one of the topics that the Higgs boson and other subatomic particle research may someday help (or hope) to answer. But I don’t think there is any satisfactory (provable) explanation yet of why gravity exists at all. We understand it empirically, of course. We use it – and fight it – all the time, and we can account for its effects mathematically. The primary thing that I understand about gravity from a practical standpoint is that it varies on the surface of the planet according to the differences in mass as the planet is structured. So, for example, if I stand next to a mountain made of, say, solid lead, the gravity that the lead exerts on me would be higher than the gravity exerted by a similarly-sized mountain of feathers.

That’s not to prove that lead is more dense than feathers; we already know that. What it means is that the lead exerts more pull on my own mass than the feathers do. I’m more gravitationally influenced by the lead than by an equal volume of feathers. That’s hardly noticeable given the attraction from the planet itself, which overwhelms the tiny force differential exerted by either the lead or the feathers, but it’s not “nothing”. (I don’t know if that means that I would weigh more next to a mountain of lead than I would next to a mountain of feathers, but I think that if I were standing on top of a mountain of lead, then I would weigh more.)

At the center of the Earth – given the Earth’s presumed core of iron and nickel (and ignoring for now the temperature which would vaporize a human body instantly) – it would seem that the body in the center of that core would be acted upon by those elements more than at the surface. The question in my mind, though, is “in which direction?”

A nominal body at the exact center (center of mass, anyway) of the planet would seem to be acted upon in all directions at once. So the apparency to the owner of that body would be “weightlessness”, I expect.

Mariah's avatar

Weaker. The Earth’s mass is all around you, pulling approximately equally on all sides and cancelling out. If you’re on the surface, the mass is all to one side of you and pulling you that way.

cazzie's avatar

Air pressure increases so one feels those effects as one lowers below sea level and it has quantitative results such as water will only boil at a higher temperature. And that is only observable in mine shafts. Going further….

If, at the core, time theoretically travels more slowly, then the effects of gravity are increased. Also, we know from the pressure in the centre of suns and gas giants that gravity is exactly centered at the core. There is absolutely no weightlessness. Only the most pressure that can be felt due to the mass of the body surrounding you.

Zaku's avatar

Gravity would be zero at the center, because it would be pulling equally in all directions, offsetting itself.

In the visual metaphor where gravity is shown to create curved depressions, the bottom of the curve is flat, for zero acceleration.

CWOTUS's avatar

No, I can see where it would be strongest at the center, because it’s not a one-way force. It’s not as though an object at the center of the mass (the planet) would be pulled “outward” only by each point on the mass that surrounds it. It would be pulled nearly identically in all directions, meaning that it would be pulled inward, too, and in that way compressed.

Rarebear's avatar

At the exact center of mass of the Earth you would be weightless.

LostInParadise's avatar

I understand that the net gravitational force is zero, but what would if feel like? If I had equal forces pulling at my head and feet, wouldn’t I notice it? Imagine someone having two people holding the arms and pushing with equal force in opposite directions. The net force is zero but you would definitely feel both of them.

flutherother's avatar

While gravity cancels out at the centre of the Earth the gravitational potential is greater than at the surface. For example a beam of light shining up from the centre of the Earth through an imaginary tunnel will be red shifted at the surface. Gravitational time dilation will therefore be greater at the centre of the Earth than at the surface despite feeling no gravitational force there.

Rarebear's avatar

@LostInParadise No, you wouldn’t feel the differences. Gravity is very weak and although there may be a tiny amount of difference between one side of your body and the other, it wouldn’t be noticeable. If the Earth were a neutron star, where a tablespoon would weigh a billion tons or something and was much denser, then yes, you’d feel a difference.

Zaku's avatar

@CWOTUS Except not. Equal and opposite forces counter each other out. It’s not like squishing an object between two forces, because gravity isn’t a physical object pushing from the outside. Gravity attracts every piece of matter without regard to whatever is in between, and it doesn’t cause stress, it just negates the gravitational force from the opposite directions.

@LostInParadise You wouldn’t feel any gravitational force yourself (you never feel gravitational force, anyway, because it’s always evenly distributed over your body – you only feel things in pulls you into, or things it pulls against you), but at the center of the Earth, you’d still be superheated immediately, melted and crushed by the all the molten metal weighted down by the entire Earth being pulled towards the center by gravity. It’d all be so ridiculously powerful that you’d die before getting to sense any of it though, even if a bubble could be opened and you teleported into it somehow. After all, look at what happens when a relatively small crack merely opens in the crust and we get a volcanic eruption, tsunami or earthquake.

SecondHandStoke's avatar

To put it very simply gravity is negated at Earth’s center.

Nothing I’ve read suggests the “age difference” idea is incorrect.

But consider this:

Earth’s center is still in motion in relation to the universe, or perhaps more significantly, our galaxy and even more significantly, local bodies.

SmartAZ's avatar

Isaac Newton published “Shell Theorem” to show that gravity is zero inside a hollow sphere. Inside a solid sphere, gravity gets weaker as you dig deeper because the attraction from all sides cancels, and there is less matter beneath your feet pulling down. When you reach the center, gravity cancels in all directions. You can look that up at by typing “shell theorem” in the search box.

All gravity depends on the universal gravitation constant, and that has never been measured to any reliable number. It varies in the third decimal.

cazzie's avatar

Anyone here know a Dr of Physics we can ask? (tee hee)

Response moderated (Spam)
Bill1939's avatar

@cazzie, the site you provided answers my question. Gravity at the center of the Earth is weaker. However, the articles conclusion that time would be slower there, relative to the surface, seemed to contradict this. Following a Wikipedia link seemed to be saying that time would be slowed at the center.

I then went to the link that @Rarebear provided. While the mathematics is far beyond my ability to follow, relative velocity time dilatation resolved what appeared to be the conflict; “The sign of the effect is that clocks tick slower when they’re deeper in the potential [gravity] well. That is, a clock at the Earth’s surface ticks 1.0000000003 times faster than one at the center.”

If I understand this information correctly, because gravity at the center is effectively zero the effect of relative velocity time dilation slows time.

Thanks to everyone who attempted to resolve my confusion.

cazzie's avatar

So, no gravity but lots of pressure?

RealEyesRealizeRealLies's avatar

Wow what a great question. Never considered it. Glad you asked. Enjoying the replies.


cazzie's avatar

Good program from BBC. Forces of Nature with Brian Cox.

Proxy it or ask me.

gondwanalon's avatar

Sir Isaac Newton might conclude that there is no gravity in the center of the Earth. But Albert Einstein’s theory of general relativity is in conflict with that.

According to Einstein the gravitation observed on Earth is the result of the Earth’s mass creating a distortion of the space time continuum. This distortion is what holds the Earth together. The center of the Earth does not escape that distortion. In fact the center of the Earth is the focal point of that distortion. Therefore the gravitational pull is greatest at the center of the Earth. That is why the center of the Earth is made of a very dense (mostly iron) core material.

cazzie's avatar

That’s what I thought to, @gondwanalon ..

Rarebear's avatar

@cazzie No, there is plenty of gravity. It’s just equal on all sides so it cancels out (assuming the Earth is perfectly symmetrical, which, of course, it isn’t.)

flutherother's avatar

There is a lot of pressure at the centre of the Earth but that is not the same as gravity.

gondwanalon's avatar

@flutherother The pressure at the center of the Earth is the result of gravity.

flutherother's avatar

Yes but the pressure is transmitted by the material and doesn’t prove there is gravity at the Earth’s centre.

gondwanalon's avatar

I’m not a physicist and I can’t prove anything about gravity. All I’m saying is that a strong gravitational force at the center of the Earth is consistent with Einstein’s general theory of relativity.

SmartAZ's avatar

In Einstein’s theory the gravitational field, manifest in the curvature of spacetime, is coupled to its sources by the field equations, the sources being described by an appropriate energy-momentum tensor, and so the Principle of Superposition does not apply. This means that one cannot simply pile up masses in any given spacetime because the field equations must be solved for each and every configuration of matter proposed.
(This paragraph was taken from Yahoo News but they deleted it from the article. Apparently somebody couldn’t stand the implication of what it said.)
Second source

gondwanalon's avatar

I didn’t study Einstein’ mathematical equations. I saw illustrations in a magazine and in a film that explains his theory. Check out this you tube video and try to comprehend how there could possibly be no gravity at the point where all matter is forced to go.

cazzie's avatar

I know someone who IS a professor of physics and I’ll ask him. We usually have better things gs to talk about, like his upcoming vist. ;)

msh's avatar

I am not trying to stir the pot again, however the recent arrival of Juno into orbit around Jupiter held me fast as it occurred. It was phenomenal to watch the process and success of it’s placement to coincide with Jupiter’s exposure to the Sun. I have questioned what we will gain via the various data that Juno sends back, if indeed Einstein’s Space-Time Continuum would hold with this new field of data from Jupiter. If, as I read and watched the suggested info from above, does this mean that the same properties and conjectures would fit the same examples using Earth as a basis of comparison, to those of Jupiter and it’s planetary differences? Putting Mass in the equation? Or is that one of the awaited data to be sent as Juno dives in towards the Planet- and it’s doom?
I do not usually ponder such ideas unless nudged. As here. (Excellent Question!)
Now I am mixing in alternative variances and if it might interact with the turbulance of Jupiter’s surface- and incredible gravitational pull. Or would these ideas not be viable in this particular mission? I am working with conjecture using known data from earth and joining it together.
Too far off base? Anyone?
This was my pre-Juno nudge I guess:

(I love it when Stephen Colbert brings this guest on to ‘do’ Science: food for the brain’s what if’s. Catch their other Space clarification projects if this appeals.)
Apples and oranges perhaps to the gravitational pull from earth’s center vs outer crust in addition to these waves/distortions with Einstein’s theory…. but yet does the info being spoken of in the clip on gravitational waves effect things enough to change the time variance question and ideas about the Earth from above also? Jupiter’s incredible gravitational pull would change from the variance of the waves also? No?
Or am I thinking of the movement of the Earth as shown, with the reaction to the ‘trampoline’ when putting Jupiter into the mix? Real data to be sent back by Juno vs conjectures on theories will eventually happen with returned data?
Be nice. Astronomy class back in the Dark Ages only touched briefly on Einstein’s various theories about Space. (After my beloved Apollo Era and in the birthing stages of Space Shuttles.)

SmartAZ's avatar

Everybody carries on about Einstein. He said “If you can’t explain what you know so a bar maid understands it, you don’t know it.” Many or most physicists today can not meet that standard. They can’t blind people with brilliance, so they fall back to baffling them with baloney.

Rarebear's avatar

@SmartAZ Yup. I had kind of given up on the thread and threw up my hands when I read the babble, but I’m glad you went there.

msh's avatar

Oh well. C’est la vie. C’est le guerre.
Drinks anyone? Belly on up…

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