# Gravity & General Relativity?

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#### tituscicero

##### Guest
A can’t seem to stop thinking about this so I would like to bounce this off some brains.

I was thinking, as usual, about artificial gravity and all the wonderful application thereof and conducting thought experiments trying to figure out how one can manipulate gravity without large mass’ or energy. One of the thought experiments that I was playing with is the hole in the ground test: if I was in a hole as big as a large house in the center of the earth (cool as a summers day) there would be no gravity. Right? Because gravity for all sides of me would be equal and therefore each force would cancel each other out and I would float around….in molten iron. Ahhhh…gives new meaning to steam baths.

I then started thinking about a hole 100 yards under the surface. According to Einstein’s Theory of General Relativity (took a class on it but am still confused) gravity is caused by mass basically disrupting space-time by its presence and creating an inward curve around the mass in question. It is the force of this inward curve of space-time “flowing” into the mass that causes gravity and planets to orbit our most beautiful star.

Here is the rub: If I was in a hole in the ground would the curve not actually be in the hole and the curve around the earth really shouldn’t affect me. Does space-time flow through mass? Since the curve of space-time in the hole would be much smaller should not then gravity be much weaker depending on the size of the hole. If it does flow through mass where does it go? Space-time is a thing that is always expanding? That thing does have friction against mass because if it didn’t we wouldn’t have gravity? So if it did go through mass and into the center of the earth (as it would else gravity would not work in the hole) then once space-time reached the center it would be stuck, right? It doesn’t make any sense. If 5 billion years of space-time was stuck in the center of the earth always expanding it would rip the center apart and, I think, time would be like very different down there?

Another thing I don’t understand about gravity and the geodesic curvatures of space-time theory is why does mass that is more densely packed create more gravity? Common sense would dictate that since the mass is smaller that it would create less gravity because the curve is smaller (though I know the universe has nothing to do with common sense.) The complete opposite happens though: if I took all the mass of the earth and squeezed into a basketball would the gravity not be exponentially greater if not creating a black hole? I mean, I would create a bigger curve than that basketball.

Does this make any sense? Am I insane and need to be committed or am I suffering from the American Public Education system?

Can someone explain where my thought processes have gone astray?

Thank you

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#### derekmcd

##### Guest
You packed a whole lot of questions into a single post. Maybe you could help us be refining your thoughts a bit more directly.

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#### DrRocket

##### Guest
tituscicero":1kph54zg said:
...

Does this make any sense?

No.

Am I insane and need to be committed or am I suffering from the American Public Education system?

Thank you

I don't know if you are insame. But you seem to be suffering badly from some flaw in education. You have completely scrambled the general theory of relativity -- scrambled it so badly that it is not possible to point out individual errors. Your concept of space-time is so wrong that it is impossible to determine where to begin to correct it.

However, you don't need Einstein's general relativity to address the general questions that you are raising. Newton's theory will do nicely. And it is quite simple. The gravitational field is dependent only on the mass inside an imaginary sphere centerd at the Earth's center with radius at your point of concern. The mass in the shell above you contributes nothing, since as you observed the effects of the mass in that shell concel one another.

This is all a matter of distribution of mass. As you not in Einstein's general relativity you also consider energy, which is just mass in disguise. There is also a pressure term in the stress-energy tensor. But the bottom line is that there is no way to "manaipulate" gravity without manipulating those terms.

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#### tituscicero

##### Guest
DrRocket":2bubrntk said:
I don't know if you are insame. But you seem to be suffering badly from some flaw in education. You have completely scrambled the general theory of relativity -- scrambled it so badly that it is not possible to point out individual errors. Your concept of space-time is so wrong that it is impossible to determine where to begin to correct it.

Damn-it Dr im a Computer Scientist not a Physicist. How is that I scramblled Theory of GR exactly? Gravity is not caused by curvatures in space time created by mass/energy. Gee wiz, how wrong I am. Okay, what is gravity caused by?

DrRocket":2bubrntk said:
However, you don't need Einstein's general relativity to address the general questions that you are raising. Newton's theory will do nicely. And it is quite simple. The gravitational field is dependent only on the mass inside an imaginary sphere centerd at the Earth's center with radius at your point of concern. The mass in the shell above you contributes nothing, since as you observed the effects of the mass in that shell concel one another.

This is all a matter of distribution of mass. As you not in Einstein's general relativity you also consider energy, which is just mass in disguise. There is also a pressure term in the stress-energy tensor. But the bottom line is that there is no way to "manaipulate" gravity without manipulating those terms.

Okay, as the poster above suggested I will put my question is something more easily consumed:

• First and new question: is the curve in space-time caused by energy/mass not the reason for gravity.
How it is that x amount of mass causes more gravity when packed into a smaller package if the "curve" should then be smaller? (i.e. black hole)
If, again, the curvature of space-time does cause gravity how does a curvature outside a mass effect that which is inside a mass.

PS: please don’t post just to prove how smart you are. I don’t care. I just want to know the answers to my above questions if there are answers to be had. I’m not here to engage in measuring my member with others to see which is longer and which has more girth…I grew out of that after age 15.

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#### DrRocket

##### Guest
tituscicero":1khwkgfi said:
Damn-it Dr im a Computer Scientist not a Physicist. How is that I scramblled Theory of GR exactly? Gravity is not caused by curvatures in space time created by mass/energy. Gee wiz, how wrong I am. Okay, what is gravity caused by?

DrRocket":1khwkgfi said:
However, you don't need Einstein's general relativity to address the general questions that you are raising. Newton's theory will do nicely. And it is quite simple. The gravitational field is dependent only on the mass inside an imaginary sphere centered at the Earth's center with radius at your point of concern. The mass in the shell above you contributes nothing, since as you observed the effects of the mass in that shell cancel one another.

This is all a matter of distribution of mass. As you not in Einstein's general relativity you also consider energy, which is just mass in disguise. There is also a pressure term in the stress-energy tensor. But the bottom line is that there is no way to "manipulate" gravity without manipulating those terms.

Okay, as the poster above suggested I will put my question is something more easily consumed:

• First and new question: is the curve in space-time caused by energy/mass not the reason for gravity.
How it is that x amount of mass causes more gravity when packed into a smaller package if the "curve" should then be smaller? (i.e. black hole)
If, again, the curvature of space-time does cause gravity how does a curvature outside a mass effect that which is inside a mass.

PS: please don’t post just to prove how smart you are. I don’t care. I just want to know the answers to my above questions if there are answers to be had. I’m not here to engage in measuring my member with others to see which is longer and which has more girth…I grew out of that after age 15.

If you would like your question answered you need to ask it a bit more politely. Or put your member where the sun don't shine.

A large part of the reason for answering these questions involves the benefit to lurkers as well as to you. Even if you don't "get it" someone else reading the thread may. And frankly if you want to be snotty I don't care if you get it or not. But I do care if the lurkers benefit.

The issue is not "curvatures in space" but curvature OF space-time. Gravitation is an effect of the geometry of space-time.

The curvature of space-time is expressed in terms of a tensor that is determined by the distribution of matter and energy in the universe. But space-time doesn't flow around or through anything. Space-time is not expanding or contracting either -- although there is a sense in which space-like slices of space time might be said to be expanding. Space-time itself, because it includes both space and time -- all of space and all of time -- is static. It is not like Newtonian mechanics in which time has a constant "flow". Time is just part of the whole enchilada and all of time is included. Space-time contains the entire past, present, and future. It is only when you look at the "world line" of a particle or body that you begin to see the notions that we call dynamics.

It makes no sense to talk about the "curve" of space-time going around you while you are in a hole. There is no "curve" of space -time, only a curved geometry, which is reflected in the metric. What you call gravity is the curvature of space-time. Gravity is not caused by the curvature, it is the curvature.

The effect of high mass/energy density is to create locally high curvature, at the extreme end you get a black hole. But at a distance the density is not important, and the result is primarily dependent on the amount of mass and not the density per se. So Newtonian gravity works in almost all situations. General relativity is only necessary for situations requiring extreme accuracy or where there is extreme density and hence extreme curvature.

I don't understand your question regarding how curvature outside a mass affects that which is inside a mass. Curvature is a tensor field. The curvature at a point in space determines what we call gravity at that point whether it is inside a mass or not. The gravitational field of the Earth pulls on your heat just as strongly as it pulls on your feet.

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#### bowman316

##### Guest
Imagine holding a paper towel flat in the air, and placing an object in the center. There would be slope going down the the object. This is the theory behind space time. The heavier the object, the larger the depression will be. The more surface area the object has, the more distributed the weight is, and so therefore, I think one object weighting 1 lb and having a small diamater will have a bigger depression than another object of the same weight, but a larger diamater.

Almost like how a penny normally sinks in water, unless you shape it to displace more water.

And maybe a black hole is an object so dense, that it brakes thru the "paper towel" and ruptures space time.

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