# Question: Does and/or Can Gravity exist indepent of objects?

1. Jun 5, 2010

### linda McHenry

Question: Does and/or Can Gravity exist indepent of objects?

2. Jun 5, 2010

### Staff: Mentor

Re: Gravity

Welcome to PF.

Gravity results from mass.

3. Jun 5, 2010

### linda McHenry

Re: Gravity

Okay, so it is dependent of objects.

So how does mass create gravity?

4. Jun 5, 2010

### Staff: Mentor

Re: Gravity

The exact mechanism is unknown. Ultimately, that is the way it is for fundamental forces like gravity and magnetism.

5. Jun 5, 2010

### arildno

Re: Gravity

We know the general manner of how gravity will affect masses, and that is all we need to make effective predictions and suchlike.

But why masses interdepend through the force of gravity in the first place, this is, as russ have said, a big unknown.

6. Jun 5, 2010

### linda McHenry

Re: Gravity

If gravity is the product of objects, then, does gravity itself contain mass?

7. Jun 5, 2010

Re: Gravity

8. Jun 5, 2010

### linda McHenry

Re: Gravity

Maybe my question wasn't a logical flow of thought. So here is what I'm trying to understand. Since you brought up the subject of Magnetism earlier, maybe this will help.
I can vaguely comprehend how magnetism force works and how it may not contain mass. I understand that two magnetic and electrically charged objects are attracted to each other. But, by comparison, can you elaborate a bit further how gravity works? How one object relates to another one by way of a gravitational force? Please understand I am not a physicist, so give me the layman's short version, please.

9. Jun 5, 2010

### Staff: Mentor

Re: Gravity

Could you explain it to me? I haven't got a clue!
Not really, no. I'm afraid the answers you seek do not exist.

10. Jun 5, 2010

### linda McHenry

Re: Gravity

Thank You for trying to answer my question.

11. Jun 5, 2010

### Glenlan

Re: Gravity

Has anyone ever considered whether gravity is a consequence of the expansion of the universe?

The thought experiment goes like this: If the universe is expanding, then matter must remain the same size, or we would not even notice the expansion; so is gravity matter's way of resisting expansion?

12. Jun 5, 2010

### vandegg

Re: Gravity

One way that gravity can be explained is as the curvature of space. Mass curves space around it the way a bowling ball would curve a bed spring, and other masses fall into the crevice created. This is a slightly more detailed explanation of how it works but fundamentally the answer is still "we don't know why."

13. Jun 6, 2010

### undidly

Re: Gravity

""curve a bed spring,""
I think you mean a bed mattress.

Imagine a mattress in the shape of a hollow sphere.
Make it spin.
Centrifugal force will make any masses inside press into the mattress.
These are the analogy with the curvature of space.
The masses roll or slide into each others dips in the mattress.
They appear to attract each other.
This is gravity.
Our universe is very large ,is 4 dimensional and may be spinning.
We are inside the universe.

14. Jun 6, 2010

### Naty1

Re: Gravity

Linda:

While in everyday life, gravity results from mass, in relativity, ala Einstein, gravity results from mass, energy and pressure.

In everyday, Newtonian, physics gravity is teated [approximately, but very accurately as a force. But Einstein discovered a more accurate description of gravity as the bending of space, and oddly, time rather than as a conventional "force".

As John Wheeler said
"Mass tell space how to curve, space tells mass how to move." (or something close to that)

which is a simplified version using "space" instead of "spacetime"...

Newtonian, or force based gravity, works well for the movement of planets and spaceships and the trajectory of a baseball for example, but fails horribly for GPS navigation systems and around black holes where relativistic effects must be taken into account.

We now know that space, time and gravity are interwoven, that is, related to each other and affect each other, but why that is remains a mystery.

15. Jun 6, 2010

### vandegg

Re: Gravity

I meant bed spring. I dunno why you would differentiate between the two or think i confused the two. In the example i gave nothing is spinning. Everything is in free fall and gravity curves space around massive objects causing other objects to fall into them. If the universe were spinning it would need to be spinning relative to something else.

16. Jun 6, 2010

### Naty1

Re: Gravity

The cosmological constant has no effect on mass since particle forces totally over whlem the cosmological force which is evident only on intergalactic distances.....
so while you idea is potentially interesting, it does not seem to predict nor explain much.

17. Jun 6, 2010

### linda McHenry

Re: Gravity

Okay, I have another question. If space, time and gravity are related to each other, is it possible to have one, or two but not the third? In other words, can space exist without gravity? Time without space? Gravity without Time? etc.

I'm wondering, also, does a black hole contain such a thing as space? I'm thinking that anything that got close to a black hole would be crushed beneath the weight of gravity, am I right? Crushed as in, nothing left but pure matter and no space?

18. Jun 6, 2010

### vandegg

Re: Gravity

No one really knows what happens inside black holes but they do take up space. A black hole is a sphere bounded by its event horizon, which is the point at which nothing inside can escape it. This radius of the sphere is directly proportional to the mass of the black hole, and so the sphere's volume increases as more mass is added.

19. Jun 15, 2010

### Dav333

Re: Gravity

Just curious why there is no gravity or very little outside earth's orbit, but yet every planet is bound by the suns gravity? Is the sun's gravity very weak on small objects & only a planet sized object feels its pull?

20. Jun 15, 2010

### D H

Staff Emeritus
Re: Gravity

What makes you think this?

21. Jun 15, 2010

### K^2

Re: Gravity

Yes. Just like electric and magnetic fields, gravitational field has energy associated with it. First order approximation can be derived really easily following E&M recipe on linearized gravity equations. I'm sure a more general GR way does exist as well. You also do get gravitational waves and gravitons. These have to have energy.

And of course, anything that has energy has gravitational mass. So gravity itself has mass. Or more precisely, curvature associated with gravity has a mass of its own producing more curvature.

But I do not think it can be self-sustaining, like glueballs. I'm pretty sure, Einstein Field Eqns with zero stress tensor produce only trivial solutions.

There is space within a black hole, but strange things happen, because the coordinates underneath event horizon cannot be projected onto the same $R^4$ space as the coordinates above the event horizon in a continuous way. There is a mathematical trick, however, that lets you project these onto a $C^4$ space. Basically, keep using Schwarzschild Solution, and just treat any roots of negative numbers as imaginary quantities. (Not nice in strictly mathematical sense, but it works as a shortcut to working it out properly in this case.) So time dilation and space contraction due to gravity will be imaginary. That is canceled out by the fact that any particle that got past event horizon is moving effectively faster than light, again, just relative to our "broken" external coordinate system. That also produces imaginary dilations and contractions, which together with gravity give you some real numbers to work with. So if you want to figure out how long it took a particle to reach singularity, you can still do it using Schwarzschild metric. Of course, that's time relative to the particle. Relative to any outside observer, the particle never even reaches event horizon.

Last edited: Jun 15, 2010
22. Jun 15, 2010

### Dav333

Re: Gravity

Well on the Apollo mission they were floating in their ship when between earth & moon. Thats why I asked if the suns gravity would only pull on large objects like earth?

23. Jun 15, 2010

### vandegg

Re: Gravity

The floating around in orbit is the result of being in free fall. Objects in orbit around the earth are constantly falling toward the earth due to the earth's gravity, which is not much lower in orbit than it is on the surface. Acceleration due to gravity in an orbit of 20km is about 9.7 m/s^2 where as on the surface of the earth it is about 9.8 m/s^2

24. Jun 16, 2010

### cragar

Re: Gravity

Interesting , so can the energy from a gravitational field turn into mass , like a photon turning into an electron and positron , Is the graviton an excitation of the field ,
so then if the graviton exists could it turn into mass . When you say that it is not self sustaining are you saying that a gravitational field cannot create another G field , like an EM field cannot create another EM field . When an electron an positron collide and produce EM radiation is the gravitational field the same strength for the photon as the 2 initial e- and e+

25. Jun 16, 2010

### K^2

Re: Gravity

No, that's the thing, it does create a G field. I'm saying, you can't have a space with nothing but gravitational fields in it. As far as I know. Though, if somebody claimed that everything around is just gravitational fields creating more gravitational fields, I don't think I'd be able to find a way to disprove it.

As for graviton splitting into a particle-anti particle pair, sure. It can couple to anything with energy, so it can certainly split into particle-anti-particle, but I'm not sure about quantum number conservations. Graviton, apparently has spin ±2. I suppose, that means it has to split into something with integer spin. So a pair of photons should work. These can split into electrons and positrons, if you really want to see something with rest mass.

I'll have to look into it a bit more. Hmm... How does a virtual gauge graviton get absorbed by, say, an electron with its ±1/2 spin? I'll probably bug my advisor about that.