[Aaron Thomas]

Aaron Thomas asked the Naked Scientists:
   
Dear Chris,

Thanks for the great show, I am always listening to your pod-casts which I think are fantastic.

I have a question/s which I hope you can find the answer/s too........

Q. 1.0. Apparently at the earth's core there is an area of zero gravity, how big would this area be?  

Q. 1.1 If this area exists then would the mass within this area of zero g be affected by the moon or the suns gravity?

Q 1.2 If the moon or the suns gravity causes this area of zero g to move then could this be how our magnetic field is generated? could this be the dynamo?

Sincerely

Aaron Thomas

What do you think?

[Bill S]

Aaron, I don't have the scientific know-how to try to answer your question. No doubt Chris will, but just a thought in the meantime.
1.0 Think of a rotating disc, at the centre is a stationary point.  How big is it?  My thought is that it must be infinitesimally small.  Would the same not apply to the zero-gravity point at the centre of the Earth? 
1.1 If this area has measurable mass, it must be accompanied by measurable gravity (?). If it appears to be gravitationally neutral, that must be because of some external influence (?). If it is infinitesimally small, how could we measure any effect it might have?
1.2 Gravity is a two-way process; if this is an area of zero gravity, would it not be un-influenced by external gravitational sources?

[maffsolo]

Actually, I believe that there is no absolute center to the earths axes.
There is a wobble that makes the inner core slash around. The core is liquid produced by the gravitational pressure on the surface presses the earths mass.
This wobble also affects the magnetic poles  continuously being relocate.

This wobble is caused by the pulling affect of the associated orbit of the moon the earth around the sun and the earth's daily revolution.

This it the same as tethering a bucket of water with a rope and orbiting it in a big circle around your head. You can feel the forces produced at the grip of your hand.

http://alansthoughts.com/resources/2-Why+The+Earth+Wobbles+with+figures.doc

http://en.wikipedia.org/wiki/Polhode

[JP]

Q. 1.0. Apparently at the earth's core there is an area of zero gravity, how big would this area be?

When people say that, they usually mean they're assuming the earth is a perfectly uniform sphere.  In that case, when you're down below the ground, the bits of earth below you are pulling you down, while the bits above you are pulling you up, and the bits to the side of you are pulling you sideways in various directions.  If you're at the point exactly in the center of the earth, the earth is pulling you equally in all directions at the same time, so that the forces all balance each other and you float there, apparently weightless.  Of course, there is still gravity from the earth, but it's just perfectly balanced!  This would only happen at a tiny point precisely at the earth's center.

Since the earth isn't perfectly spherical and uniform, this point doesn't really exist in practice.  Let's assume it does, or at least there's a small region

Q. 1.1 If this area exists then would the mass within this area of zero g be affected by the moon or the suns gravity?

Yes.  Only the earth's gravity perfectly cancels at that point.  Other sources of gravity outside the earth would effect things at that point.

Q 1.2 If the moon or the suns gravity causes this area of zero g to move then could this be how our magnetic field is generated? could this be the dynamo?

Someone who knows geophysics better can probably answer this question more fully.  I believe most of the magnetic field has to do with currents in the liquid outer core of the earth.  The outer core is liquid primarily because of the pressure of the earth's outer layers pressing down on it, the residual heat from when the earth formed, and radioactive minerals releasing heat.  The moon's gravity squeezes the earth a bit (which you can see quite readily since it's squeezing of the oceans causes tides), so that has some effect too.  I don't think it's the main effect, and it acts on the whole earth at once, not just on the very center.  The sun's gravity doesn't really squeeze the earth, so it wouldn't have too much of an effect.

[Airthumbs]

Thanks everyone for the interesting answers to my questions,

So what I think your all saying is;

That there could be an area ,or something?, of neutral, or negative?, gravity in equilibrium with gravity at the centre of the Earth?, having no mass, infinitesimally small, existing in the gravitational central axis?, effected by external gravitational influences, its effects unknown?

[SteveFish]

A quick addition. Although gravity would be neutralized in the center of the earth, the pressure from the whole mass would be enormous.

[peppercorn]

A quick addition. Although gravity would be neutralized in the center of the earth, the pressure from the whole mass would be enormous.

The physics of how to overcome this is clearly explained in the Hollywood film 'The Core'!  [] []

[imatfaal]

Aaron - basically, if you could hollow out the centre 10 metres of the earth (insert any number here), and ignoring the fact that you would burn up and that the pressure would be unbearable (and Steve, isn't The Core a terrible film?) , you could float around in there just like you see the astronauts on the international space station.  it would have zero gravitational attraction at all points.  


I don't understand the second part of your follow up:

having no mass, infinitesimally small, existing in the gravitational central axis?, effected by external gravitational influences, its effects unknown?

Rephrase it and we will try to answer

[JP]

A quick addition. Although gravity would be neutralized in the center of the earth, the pressure from the whole mass would be enormous.

The physics of how to overcome this is clearly explained in the Hollywood film 'The Core'!  [] []

Well, yes.  With your unobtanium suit, you'd be just fine!

[peppercorn]

A quick addition. Although gravity would be neutralized in the center of the earth, the pressure from the whole mass would be enormous.

The physics of how to overcome this is clearly explained in the Hollywood film 'The Core'!  [] []

Well, yes.  With your unobtanium suit, you'd be just fine!

Damn! It's at the dry cleaners! []

[Airthumbs]

I think what I am trying to get at is the size of the area of zero "g" and what effect it might have if any.  In previous comments someone suggested that this area would be very small.   Could it really be big enough to float in?  Also once I can find out how big this area is, then I will then be able to possibly visualise it, moving around within the core being effected by the Earths wobble and other gravitational influences, in relation to its position. If the earth is not perfectly spherical and thus causes the center of mass to change then would this point not be moving? I also think that an area which must be as as hot as the surface of the sun at the very center of the earth having no gravity is quite interesting. What also interests me now is how big this area would be in the Sun or even the biggest star VY Canis Majoris? Would this area interact with material in the inner core, meaning what does a substance composed primarily of a nickel-iron alloy at a pressure of 330 to 360 gigapascals and at 5,430 degree centigrade do when in zero "g" would its chemical properties change?

[imatfaal]

Aaron - as most of the replies have stated, the shell theorem relates to a hollow sphere.  The gravity in a solid sphere is related by 1/r; that is to say that in a mineshaft reaching to the centre of the earth (ignoring impossible heat and pressure) a miner would feel less attraction to the centre of mass as he descended. 

you seem to be conflating mathematical ideas with real world problems - we cannot make a hollow room at the centre of the earth, let alone the sun; but we can predict the gravitational attraction if we did. 

Theoretically, no matter how big the celestial object and what percentage was hollowed out, as long as it is spherically symmetrical there is no net gravitational attraction within the shell.  Practically, on an engineering side there are real experts in these forums and geographically bass, jb and their ilk on the geology board seem to be massively knowledgeable.  But you have to ask the right question!

[SteveFish]

Sheesh, I don't know what all the fuss is about. Just take your tinfoil suit, your shades, and a couple of beers and it could be great!

Aaron, the actual point of zero gravity would be infinitesimally small, but the region of microgravity (e.g. in close earth orbit) where you couldn't personally tell the difference, would be quite large, perhaps on the order of tens of miles. Steve

[Bored chemist]

If you hollowed out nearly the whole of the earth and just left the outermost mile (or whatever)  as a thin spherical shell, the gravity anywhere inside it would be zero.

It wouldn't just be a point of zero g in the middle; the whole of the region would have zero g.

[SteveFish]

Bored chemist, I don't understand what you say at all. Please explain. It seems to me that because of the inverse square law you would feel a much greater attraction of gravity from the side of the hollow sphere that is very close than from the other side that is 7,000 miles away. Steve

[Geezer]

Bored chemist, I don't understand what you say at all. Please explain. It seems to me that because of the inverse square law you would feel a much greater attraction of gravity from the side of the hollow sphere that is very close than from the other side that is 7,000 miles away. Steve

Steve, you need to check out Shell Theorem. I keep getting it wrong myself  []

[Bored chemist]

Bored chemist, I don't understand what you say at all. Please explain. It seems to me that because of the inverse square law you would feel a much greater attraction of gravity from the side of the hollow sphere that is very close than from the other side that is 7,000 miles away. Steve

If, for example, you are very near the bottom of the shell, it's true that the bottom bit is nearer, but most of the shell is above you. More stuff further away balances out less stuff nearer.
Or, as Geezer says, check out the theorem.
http://en.wikipedia.org/wiki/Shell_theorem#Inside_a_shell

[SteveFish]

Thanks guys, you made my day.

[Bill S]

If, for example, you are very near the bottom of the shell

Which bit is the bottom??? []

[Bored chemist]

If, for example, you are very near the bottom of the shell

Which bit is the bottom??? []

The other end,from the top.
Of course, it doesn't really matter but it makes it easier to refer to the stuff above you and below you rather than trying to keep it abstract.