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# Theory of the earth's gravity.

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Would it make sense that the closer you get to the center of the earth, the less you weigh? I mean, If you got to the center of the earth somehow, you wouldn't just fall to the other side. You would just float there. So my theory is: The closer you get to the center of the earth, the less gravity effects you.

Do you guys agree with that?

Edited by realmclovin

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The closer you get to the center of gravity the MORE it affects you.

This is why planets closer to the center of the sun move faster than those farther out. This is why if you were to fall to the center of the earth you might go a little ways past it due to momentum, but would swing back around and draw closer and closer until you held constant at the center.

Try this. Click the lines box too to show the paths.

Rown

P.S. Forewarning... this will probably get locked for simplicity. Sorry man.

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Not at all. To be honest, you are simply going into a sub-theory of Gravity, so you're not helping your cause. The theory of gravity seems plausible, but not provable by any means, just like Einstein's Theory of Relativity.

Most of the sciences they teach in middle school are only theories, remember that buddy.

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The closer you get to the center of gravity the MORE it affects you.

This is why planets closer to the center of the sun move faster than those farther out. This is why if you were to fall to the center of the earth you might go a little ways past it due to momentum, but would swing back around and draw closer and closer until you held constant at the center.

How would gravity affect you more if you are closer to the center of the earth? You would gain momentum if you fell straight there, but if you were just outside the center, wouldn't it be equivelent to being on a small planet?

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Sorry rown, but my computer sucks, I can't download anything onto it. D:

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Well that's a shame, try to limit double posts though. Use the edit button. It's an etiquette thing.

Also it's late. And realistically I shouldn't be talking physics... ever.

That said, let's see if I can talk some more physics.

If you were at the center of a sphere having equal mass on all sides, you would feel no gravity. However I still feel the momentum of traveling there should count for something. The shape of the human body though means there's already a variable that could effect that perfect zero. There would be a slight tug on your extremities from the surrounding mass.

/info from five minutes of web surfing the subject

Rown

P.S. K^2 came online and the though occurred to me that he might not make stops in here because he's tired of the unending non-science. That damned Vulcan made me sad. :[

Edited by Rown

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Tug? Don't you mean squeeze?

Just so you know, I am in the 8th grade. Physics facinates me, when I am a Freshman next year, I'm taking geometry.

Edited by realmclovin

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Forgive my naive grasp of the notion here, but I think you're right - if, let's say, there were an indestructible, pressurized room in the very center of the Earth, you'd have .5G acting on you in every possible direction.

But we should not forget that the deepest we've ever gone is only something like 4 miles, and even at such a shallow depth we're talking about 300 degree temperatures.

So before you ever got anywhere close, you'd be utterly destroyed by the temperature and immense pressure. Fun times.

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If we assume the earth to be hollow and completely uniform then the gravity any where inside would all be the dame and it really wouldn't matter that the human body was irregular shaped. The physics behind it are not that bad, I remember this being discussed in the long-long-ago of college physics, but I couldn't show the math without studying up. Either way, that's what Google is for.

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Not at all. To be honest, you are simply going into a sub-theory of Gravity, so you're not helping your cause. The theory of gravity seems plausible, but not provable by any means, just like Einstein's Theory of Relativity.

Most of the sciences they teach in middle school are only theories, remember that buddy.

Theories predict what should happen. When evidences support the predictions, theories are accepted until there are contrary evidences. The stuff you learn in school are well-accepted and well-established (scientific consensus). The theories you learn can be considered "true" or "fact" for simplicity.

Edited by spaceeinstein

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I thought the current model for understanding gravity is related to the idea that Space isn't flat.

The easiest way I know how to explain it, is grab a a bed sheet. Hold the four corners tight and then place a ball ( basketball, soccer ) on the sheet. Then place a smaller ball ( golf ball ) and watch as it rolls towards the bigger ball. That curvature is the current model in explaining how gravity works. It isn't so much a force pushing down on the Earth, but the way the Earth curves Space around it.

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Actually, wasn't that the same theory for Relativity (Space-time)? I mean, relativity states that larger objects curve the fabric to make smaller objects curve around them, while gravity states all objects pull on each other. It's just a matter of how strong and how large it's gravitational pull can be.

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Actually, wasn't that the same theory for Relativity (Space-time)? I mean, relativity states that larger objects curve the fabric to make smaller objects curve around them, while gravity states all objects pull on each other. It's just a matter of how strong and how large it's gravitational pull can be.

Yeah, it is The Theory of Relativity. Like I said, my understand is that the Space Time curvature is a more accurate description of how Gravity works. It isn't so much a force pushing down but the external pressure of Space on the body in Space.

So, the bigger the object the bigger the curvature the bigger the external pressure pushing down on it.

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ADVISORY: I am not a sciencematician.

The strength of any body or object's gravitational field is determined using the object's mass, and the distance from said object. So the closer you get to the earth, the greater the gravity.

That said, if you went below the surface of the Earth, gravity would begin to decrease as you got closer to the center of the earth, and would be zero (assuming the earth as a perfect sphere) at the center.

First, the entire mass of the Earth would no longer be on the one side of you once you got below the surface so less pull from that side, and second there would be mass on the other side of you pulling in the opposite direction.

However, zero gravity at the center of (an again "perfectly spherical) Earth it isn't because "gravity effects you less." Gravity is effecting you just the same. It's just that instead of all of the Earth's mass pulling you down towards all the Earth's mass, all the Earth's mass would be pulling you outward equally in every direction, which would result in you experiencing zero gravity.

Though it's kind of moot (as Otter said) seeing as you'd likely be killed by pressure, heat, etc. long before you got anywhere near the center.

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That, and Earth isn't a sphere.

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Yes, it's actually closer to an oblate spheroid, but that wouldn't effect gravity much in terms of being inside the Earth or under the surface, since on a global scale the Earth is still symmetrical.

So even though the diameter of the Earth is something like 25 miles longer along the equator than it is from pole to pole, at the center of the Earth the differences would still be on either side of you.

Other things to take into account would be the specific gravity of the various parts of the earth. As in water, dirt, various elements, (which would also mean their temperature and the pressure they're under.) Also the differences in topography, which again are minor when speaking on a global scale.

So while mathematically there would be zero gravity at the center of a perfectly spherical Earth or even a perfect oblate spheroid, the actual "center" of the Earth might be a bit off "center" based on all those other factors.

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Not if the lower cores of the Earth are dense enough in - density to negate the gravity of the Crust. Remember the Earth's Crust is a very small part of the actual planet, and most of the core is heavy metal and molten Iron, so it's not like a few million tons of Rockface and soil is going to throw off the core considering how dense we think it is.

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You know, somebody should just PM me when you guys are stuck like this.

For the most part, Sweets is correct. If Earth was a perfect sphere of equal density, you could break it up into many, many thin shells. A spherical shell produces gravity on the outside equal to gravity of a point mass at the center with the same mass. On the inside, a spherical shell produces no gravity. So as you go down, only the shells still bellow you have any affect, and there are fewer and fewer of these as you go down, so gravity decreases.

However, Earth is not of equal density. Core is denser than the crust, so for the first few hundred kilometers or so the gravity is actually going to increase. At the center, it still goes to zero.

All of this assumes classical gravity, of course, but the relativistic corrections to Earth's gravity are tiny, and are only significant for GPS systems.

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The closer you get to the center of gravity the MORE it affects you.

This is why planets closer to the center of the sun move faster than those farther out. This is why if you were to fall to the center of the earth you might go a little ways past it due to momentum, but would swing back around and draw closer and closer until you held constant at the center.

How would gravity affect you more if you are closer to the center of the earth? You would gain momentum if you fell straight there, but if you were just outside the center, wouldn't it be equivelent to being on a small planet?

Momentum is affected by many more things than the force of gravity itself- terminal velocity is the point at which air resistance overpowers the force of gravity and prevents a body from accelerating any further. Though the force of gravity will be increasing the closer you get to the centre of the earth, so will the density of air (which is also affected by gravity), so you won't get any faster. Which means it's not really feasible to test gravity through acceleration unless it's done in a vacuum and therefore air resistance is irrelevant.

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gravity is a theory? im pretty sure it's fact, you know the laws of gravity? i think gravity makes perfect sense. all matter has its own gravitational pull, which is caused by the object warping the fabric of space/time, and it puts an indentation in it, where the objects with greater mass/density have a larger indentation and objects are effected by the gravitational pull. don't really see why gravity needs to be debated. seems to make sense.

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The gravity is debated for several reasons. First of all, the standard model, General Relativity, is inherently non-linear. Which means it is incompatible with Quantum Mechanics. Either General Relativity or Quantum Mechanics is wrong. They cannot both be right. That's a hell of a problem, considering that both of these are the best tested theories. Some of the measurements we can make in both give about 12 decimal places of precision. Everything else we are using goes to 2-3 decimal places. And at least one of these two fundamental theories is completely wrong.

There is a very good chance that in a long run we'll have to drop quantum mechanics, but at the moment, the only real problem with quantum theory that we can see is disagreement in vacuum zero point energy. Granted, the disagreement is bad. No, that's understatement. It's by far the worst prediction of anything ever. You know these crazies with "End is near" sandwich boards? They are on the money by comparison. But there have been some major improvements in the area. Recently, a model has been suggested that excludes quark-condensate from zero-point energy. That's going to knock many orders of magnitude from error. If something like this can be done for EM field contribution, the quantum theory prediction might actually reach some plausible levels. And other than that, QM works.

So on to gravity. Why people don't trust General Relativity? Well, first of all, when we take a look at Solar System, by far, most of its mass is in the Sun. If we extrapolate this, and even account for supermassive black holes, most of the matter within our galaxy should still be in stars, and therefore, visible. We then look at other galaxies, and notice that amount of matter we estimate from the glow of the galaxy disagrees greatly with amount of mass required to make the the stars move the way they do. If we go with General Relativity, by far most of the mass in the Galaxy is "dark". Hence addition of "Dark Matter" to the standard model to balance it. Well, the problem is that addition of all that Dark Matter makes certain predictions on overall behavior of our Universe, showing that it should be expanding at highly decelerated rate, and will eventually collapse.

But, wait, the measurements suggest not only that the universe isn't decelerating its expansion, it is accelerating. What? Ok, we substitute this back into General Relativity, and see that the only way this can happen is through the "pressure" terms in the average Stress-Energy Tensor of our universe. Something has to be causing thins, and all of the known matter and Dark Matter have to be causing just the opposite effect. This is something that hangs around between galaxies pushing everything apart. Damn, now we also have "Dark Energy". And none of these things have been observed in any way, shape, or form other than as artifacts of General Relativity. So we either assume that we know absolutely nothing about what's going on in our own "backyard", or Einstein missed something in his equations.

On a historical note, before Einstein published General Relativity, the entire astronomic community were searching for a planet Vulcan, said to be located between the Sun and Mercury and explaining Mercury's odd orbit around the Sun. General Relativity explained the orbit by fixing equations of motion for orbiting body. Now everyone is searching for Dark Matter, and the more data is found showing that we need more and more dark matter and even more dark energy to balance it out, all at the same time finding absolutely no other evidence of either. So people start wondering if maybe there is something wonky going on with gravity itself.

There are two major directions in trying to replace standard model. First is Quantum Gravity. Assumption of QG is that universe is fundamentally linear, and non-linearities of General Relativity are an artifact of sub-structure of cosmos that we have not yet found. String Theory attempted to go that way. It is more and more considered to be a failed theory. Loop Quantum Gravity is the newest attempt. It shows some promise, but has many of the same problems as String Theory.

Second direction is to find a non-linear theory that includes General Relativity, but also replaces Relativistic Quantum Field Theory. Electro-gravity, explaining results of Quantum Electrodynamics in context of General Relativity would be the first step. Such a theory is likely to make more corrections to our understanding of electrodynamics than gravity, and would probably explain "dark matter" and "dark energy" effects through some new forms of long-range electromagnetic interaction. Very little is being done here, because honestly, people have no idea where to start. Every observation we've made in electrodynamics is explained by linear theory. Trying to find a non-linear model that fits is looking for needle in a haystack.

So there you have it. Problems with gravity in a nutshell. Keep in mind that none of these things would cancel the things we understand about gravity. It's merely a correction. Just like General Relativity did not change the fact that massive objects attract each other in proportion to their mass, neither will the new theory. It will merely make corrections that might explain effects on the scale of our Universe.

Edited by K^2

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Keep in mind that none of these things would cancel the things we understand about gravity. It's merely a correction.

That makes sense, but you have some sentences that read awkwardly to me.

Either General Relativity or Quantum Mechanics is wrong. They cannot both be right.

Or they could be incomplete, like Newton's laws were.

There are two major directions in trying to replace standard model.

Replace? I thought it's to improve on it.

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