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

[linda McHenry]

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

 

[russ_watters]

Welcome to PF.

Gravity results from mass.

 

[linda McHenry]

Welcome to PF.

Gravity results from mass.

Okay, so it is dependent of objects.

So how does mass create gravity?

 

[russ_watters]

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

 

[arildno]

Okay, so it is dependent of objects.

So how does mass create 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.

 

[linda McHenry]

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

 

[russ_watters]

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

No. That doesn't follow logically.

 

[linda McHenry]

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

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.

 

[russ_watters]

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.

Could you explain it to me? I haven't got a clue!

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?

Not really, no. I'm afraid the answers you seek do not exist.

 

[linda McHenry]

Thank You for trying to answer my question.

 

[Glenlan]

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?

 

[vandegg]

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."

 

[undidly]

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."

""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.

 

[Naty1]

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.

 

[vandegg]

""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.

I meant bed spring. I don't know 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.

 

[Naty1]

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?

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.

 

[linda McHenry]

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.

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?

 

[vandegg]

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?

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.

 

[Dav333]

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?

 

[D H]

Just curious why there is no gravity or very little outside Earth's orbit ...

What makes you think this?

 

[K^2]

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

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.

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?

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 let's 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.

 

[Dav333]

What makes you think this?

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?

 

[vandegg]

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?

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

 

[cragar]

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,

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+

 

[K^2]

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.

 

[cragar]

so are you saying gravitons interact with other gravitons , you have an advisor you must be in grad school .

 

[K^2]

They should, by all means. The "straight" line to a massive body is going to depend on the curvature of space-time. How is that really different from gravitons from that body interacting with other gravitons along the way?

This is starting to be a little over my head, though. I have a pretty clear idea on how gravitons from linearized gravity should behave after second quantization. I have some basic understanding of curved space-times, and how to work with them classically. But when the two come together, it's starting to give me a headache. I know there are ways of writing, say, Dirac's Eqns in curved space-time, but has anybody even figured out how to deal with quantum object's causing gravity? I mean, can you imagine a space-time curvature in superposition? I'm having difficulties with it.

Yeah, I'm a Ph.D. Candidate in particle theory. My advisor does seem to have some interest in quantum gravity, though, so I'll bug him with some questions.

Edit: I wonder if anybody tried to second-quantize the metric tensor field. That seems like the way to go here.

 

[Dav333]

But what about with the ship that's 100's of thousands of km from earth? or if an its on its way to mars? The astronauts will still float around right?

 

[cragar]

On my above question , if an electron and positron collided an produced a photon it seems that the field strength between the photon and the pair production between them would haft to be the same or conservation of energy would not hold . can gravitons be emitted from a B field . If gravitons interact with other gravitons then a graviton could not escape a black hole , does the graviton mediate the force or is it the field .

 

[cragar]

Is it possible for two photons to collide and produce a graviton , but then photons don't interact directly . If a graviton could turn into 2 photons would this imply that photons are interacting ,

 

[K^2]

If gravitons interact with other gravitons then a graviton could not escape a black hole , does the graviton mediate the force or is it the field .

When you second-quantize a field, you end up with gauge bosons that mediate the force in that field. These are quasi-particles, really. Convenient way of working with field theory. Think about phonons in the solids. They aren't "real", but they are extremely convenient to work with, and things do end up working as if there are particles of "sound" bouncing around. Same with electromagnetic field and photons. (Note that in QFT all particles have associated fields. It's more of the particle-wave duality.)

No, gravitons cannot escape black hole. Neither can gravity itself. Nothing from bellow event horizon can interact in any way with anything above. Otherwise, there is transfer of energy, and the Cosmic Censor is violated. That'd be bad.

Black hole's gravity all comes from mass that's still above event horizon. Things that are still falling in. Of course, to an outside observer, falling in takes an infinite amount of time. So anything that has fallen into black hole is still falling. Still above event horizon. That even includes the star that initially collapsed to produce the black hole. It's still there, just outside event horizon, getting closer and closer but not quite getting there. It's almost infinitely red-shifted by now, of course, so there won't be much in terms of EM radiation escaping, but it can still interact gravitationally and electromagnetically with the stuff outside.

Is it possible for two photons to collide and produce a graviton , but then photons don't interact directly . If a graviton could turn into 2 photons would this imply that photons are interacting

You can think of ep annihilation the same way. If electron and positron can come together to produce a pair of photons, then a pair of photons can come together to produce an ep pair. That's an interaction.

Particle production does add non-linear behavior to EM field. Which means that photons really can interact. I think the only condition is that photons traveling in the same direction cannot be interacting. That has to do with them traveling at the speed of light and light not "aging". Though I don't entirely understand why.

Edit: Talked to my advisor. He reminded me that it's the total angular momentum that is conserved. So a graviton can actually couple to anything. He also recommended Feynman's lecture on Gravitation, which I'm now going to read to see if I can understand it a little better.

 

[cragar]

K^2 it is an honor to read your post's , it is interesting to read about what you wrote about gravity not be able to escape a black hole .

 

[Antiphon]

You can have gravity with nothing in the space but gravity. That's why general relativity is a nonlinear theory.

I'm probably going to regret this, but here are all the sources of gravity as I understand them:

1) gravity
2) rest mass
3) energy
3.a kinetic
3.b potential
4) stress (force/unit area)

You have to be careful with 3.b and 4. I can't come up with an example of potential energy where there isn't stress so they may be the same case.

 

[Antiphon]

No, they are different. Stress is a separate source of gravity as far as I can tell.

 

[K^2]

Technically, there is only one source of gravity. Stress-energy tensor. But it's going to get contributions from all of the above, plus how fast these contributing sources are going.

And stress, in this case, just contributes as a form of potential energy. The name stress-energy tensor is derived from its similarity to stress tensor from classical statics. Other than the name, it has nothing actually to do with mechanical stress.