# On tunneling to the center of the earth

1. Oct 25, 2009

### ernestpworrel

Would it be possible to tunnel directly to the center of the earth using currently available means, or would the centrifugal effect cause you to unwittingly continue on toward the nearest point on the surface so that you emerge near the point where you began without ever having gone beyond a certain depth?

2. Oct 25, 2009

### Staff: Mentor

Technologically, it isn't hard to make a straight tunnel in whatever direction you want. But depth is a problem.

3. Oct 25, 2009

### arildno

There might be some tiny problems once you get into the magma layer, though..

4. Oct 25, 2009

### hamster143

It's not possible to tunnel to the center of the Earth, but for a different reason. Once you get past 20 to 50 miles of crust, Earth is liquid...

5. Oct 25, 2009

### ernestpworrel

Whoa there people. I'm aware of the reality of the situation. :) Let's just assume that the earth is made of the same, solid material all the way through. But it is a rotating body, therefore wouldn't there be a centrifugal effect which prevents one from tunneling through to the center?

6. Oct 25, 2009

### D H

Staff Emeritus
The mantle is, for the most part, solid. You have to go down about 1800 miles to get to something that is truly liquid.

Why would you think that? We can drill straight down on the surface, and that is where centrifugal force (which isn't quite real) is strongest.

7. Oct 25, 2009

### Staff: Mentor

When you used the phrase "currently available means" I assumed that meant you were asking a question about reality.... ok, then....
No. Centrifugal force acts directly outward. You're probably thinking of the coriolis force. But I can't see why it would be a problem: drill bits can be steered.

8. Oct 25, 2009

### ernestpworrel

I'm not sure if it would be the coriolis force, because that directs objects along a curved path. I'm referring to the force that directs an object that is sitting on a disc along a straight vector and outwards from the center when the disc is rotated at a certain speed. I'm aware that current drilling technology allows us to drill several miles below the earth's crust, but that may be due to the fact that the drill is situated on the surface. I'm also actually aware that centrifugal force is strongest near the edge of a rotating disc. I'm thinking that once you reach a certain depth, gravitational forces may be overcome by centrifugal forces.

My reason is this. The gravitational acceleration that we feel is caused by the curvature of spacetime due to the mass of the earth, but inside the earth there is no curvature because there is not enough of a difference in density. Therefore if gravity continues to an arbitrary point in the center, as GR says it does, then it would have to be a constant which neither increases nor decreases. Centrifugal force, however, increases linearly with distance from the center. So there should be a region in which gravitational acceleration is overpowered by the centrifugal effect.

Now there's probably something that prevents this centrifugal force from extending all the way to the surface, allowing us to penetrate several miles. One possibility is that gravitational acceleration is reduced but still not constant until that region is passed.

9. Oct 25, 2009

### ernestpworrel

Thanks, but I'm actually talking about a tunneling-machine rather than a horrendously long drill.

10. Oct 25, 2009

### Phrak

Those who have had the interesting job of tunneling deep into the kimberly pipes in search of diamonds in Africa. They've experienced rocks under pressure--or releasing pressure, as the case may be.

At times, the rocks explode out of the bottom of the cut.

11. Oct 25, 2009

### arithmetix

wow!

12. Oct 25, 2009

### Staff: Mentor

I don't understand what you mean. Why would there be any problems with that?

13. Oct 25, 2009

### TurtleMeister

I believe he thinks that the drilling machine will reach a point where it's own weight will be so low as to make drilling difficult.

14. Oct 25, 2009

### ernestpworrel

That's pretty close to what I'm saying.

15. Oct 25, 2009

### DrZoidberg

At the surface the centrifugal force is much weaker then gravity.
As you go deeper gravity will get weaker but so will the centrifugal force.
At the center both will be zero.
There is no point at wich the centrifugal force is stronger then gravity.

16. Oct 26, 2009

### ernestpworrel

So the gravitational force is not reduced to a constant inside the earth but actually decreases with depth?

17. Oct 26, 2009

### DrZoidberg

Why should it be a constant in the earth?

18. Oct 26, 2009

### D H

Staff Emeritus
If the Earth had a uniform density the gravitational force inside the Earth would decrease linearly with depth, hitting zero at the center of the Earth. The Earth does not have a uniform density. The Earth's core represents only 16.3% of the Earth's volume but 32.5% of its mass. Rather than falling linearly with depth, gravity inside the Earth falls slightly and then starts rising, reaching a maximum at the mantle/core boundary. Only then does it start dropping precipitously toward zero at the center of the Earth.

19. Oct 26, 2009

### ernestpworrel

So you're saying that inside the earth's mantle, where the density is roughly uniform, gravity still increases with depth. Why would it increase? What is causing the curvature?

20. Oct 26, 2009

### Phrak

Three inflection points? Really? I didn't know.