# What is the reason for gravity?

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I have learned from physics articles that the speed of gravity is equal to the speed of light. I'm just curious.., could light be the reason for gravity? If not, what is the reason for gravity? Just asking...

If my lack of knowledge is causing me to ask or say something stupid, I apologize in advanced.

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So, General Relativity (GR) is the best description of classical gravity we have. According to GR it is energy-momentum that acts as the source of gravity.

As light has energy-momentum, light can act as a source of gravity.

The statement that gravity travels at the speed of light refers to gravitational waves. These satisfy the relativistic wave equation for massless fields. Thus they travel at the speed of light. More physically, you should think of small changes in an object creates small fluctuations in the gravitational field that travel away from the object at the speed of light.

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Gravity is a property or consequence of the interaction between Spacetime and Matter. Spacetime tells Matter how to move and Matter tells Spacetime how to bend.

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OK, I think the best explanation I have read as to how gravity works is from Einstein's Universe by Nigel Caulder (pp. 82-84). It is based on Einstein's wild and crazy idea that time and space are not rigid but flexible, and change or warp in the presence of mass/energy. (This has been verified again and again by numerous experiments and observations.) This is my version of Caulder's explanation. Here goes.

Why does an apple fall from a tree? According to general relativity, the Earth's mass/energy does two things: 1) Space warp - space is stretched more and more as it gets closer to the Earth (in the radial direction), and 2) Time warp - time slows down more and more as it gets closer to the Earth. (All this as seen from an observer far away.)

Warping of Space - An apple. once freed from hanging on the tree moves toward the greater stretching of space, or towards the Earth. So the warping of space by the Earth gives the apple its direction; towards the ground. Just below the apple where gravity is a tiny bit stronger (closer to the Earth), space is a tiny bit longer. And just below that point, gravity is a little bit stronger still, thus space is a tiny bit longer still, and so on. The apple moves towards this continuously increased stretching of space; in other words downward towards the Earth.

Wapring of Time - OK, so the apple heads towards the Earth because of the warping of space, but why it does continually gain speed (accelerate) as it falls? Time gets slower and slower as the apple approaches the Earth because the Earth's mass/energy slows down time. And the closer yit is to the Earth, the more time is slowed down. So as the apple moves closer and closer to the Earth's surface, the atoms and molecules and the particles within them (quarks and electrons) vibrate more slowly. This reduced vibration means the apple has less and less internal energy of motion (kinetic energy) as it falls.

So? Well, per Conservations of Mass/Energy, the apple's total energy must be maintained. So the apple speeds up its overall motion so that its total energy remains the same! In other words, because the Earth's mass/energy slows down time more and more as the apple approaches the ground, the apple has to speed up more and more (accelerate) to maintain a constant overall energy.

In summary, 1) the apple heads toward the ground due to the continuous stretching of space below it, and 2) the apple continually increases its speed (some 32 feet per second every second) so that its overall kinetic energy increase makes up for its internal loss of kinetic energy due to the continual slowing of time. This is, albeit, a bit simplified, but I think it gives some feel for the dynamics of Einstein's theory of gravity.

I hope this helps.

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I'll just add that in general conservation of energy in General Relativity is a very tricky and subtle issue. Really, it requires lots of symmetry.

The case of the test particle i.e. the apple in the Earth's gravitational field the test particle's radial motion can be described by motion in an effective potential with terms of order $\frac{1}{r}, \:\frac{1}{r^{2}}, \: \frac{1}{r^{3}}$.

You can then employ everything you learnt in mechanics 101 to describe the motion.

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So, General Relativity (GR) is the best description of classical gravity we have. According to GR it is energy-momentum that acts as the source of gravity.

As light has energy-momentum, light can act as a source of gravity.

The statement that gravity travels at the speed of light refers to gravitational waves. These satisfy the relativistic wave equation for massless fields. Thus they travel at the speed of light. More physically, you should think of small changes in an object creates small fluctuations in the gravitational field that travel away from the object at the speed of light.

There is in fact no way to define a global energy-momentum vector in a general curved spacetime.

Another way of saying this is if two different objects of different mass approached a mass of a larger value, then each of the two would have its own view of GR spacetime without any way to associate the two. Yet, each proceed predictably from any frame.

Yet, the solar system for example does follow a "locally global" plan.

So, what exactly is the source of gravity?

I am really curious.

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Thank you for your replies, ajb. If I am understanding you correctly, using general relativity you can show that gravitational waves propagate at the speed of light. There has to be a motion, such as an interaction between two or more gravitating objects, to produce gravity waves, right? You said it requires a lot of symmetry. My question is, would a lone object in an empty universe produce gravity? Or would an interaction between it and another object be a necessary requirement? It would seem to me that gravity would still hold the individual atoms in a lone star together, even if there were no other object to "detect" the gravity?

Is the term "gravity waves" the same as "gravity" itself?

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Yet, the solar system for example does follow a "locally global" plan.

So, what exactly is the source of gravity?

I am really curious.

If I understand your question right, when describing the solar system the planets are assumed to be test particles. That is they couple to gravity but do not create their own gravitational field. This is a reasonable approximation as the Sun is much more massive than the planets.

Technically, as the Field Equations are highly non-linear we cannot simply add solutions to get another.

If I am understanding you correctly, using general relativity you can show that gravitational waves propagate at the speed of light.

That is correct. You can linearise the Field Equations (then gauge fix) to produce a relativistic wave equation.

There has to be a motion, such as an interaction between two or more gravitating objects, to produce gravity waves, right?

I am not sure in total generality.

You said it requires a lot of symmetry.

You need symmetries for conservation laws. In a sense, if the space-time has enough symmetry then our ideas about conservations laws in Minkwoski space-time carry over to more general curved space-time. In general we do not have equality between local descriptions and global description of conserved objects.

My question is, would a lone object in an empty universe produce gravity? Or would an interaction between it and another object be a necessary requirement? It would seem to me that gravity would still hold the individual atoms in a lone star together, even if there were no other object to "detect" the gravity?

This would be a very idealised situation, but one that physics thinks about. For example, the Schwarzschild solution describes the gravitational field created by an isolated spherically symmetric object.

Is the term "gravity waves" the same as "gravity" itself?

In a way yes. The local geometry of space-time is the gravitational field. A gravitational wave is a very particular configuration of the local geometry.

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If I understand your question right, when describing the solar system the planets are assumed to be test particles. That is they couple to gravity but do not create their own gravitational field. This is a reasonable approximation as the Sun is much more massive than the planets.

I am OK with this.

But, what is the source of gravity?

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I am OK with this.

But, what is the source of gravity?

See post #2.

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Magnets attract by photon exchange,

Masses attract by graviton exchange

(This happens at the cost of energy stored in atoms. Gradually energy is consumed to produce these forces)

(graviton is yet to be detected in lab)

Edited by zacinfinite
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See post #2.

That is a description and not a cause.

In other words, GR will say how it works, but not why it works that way.

Of course, let's leave off dark energy and dark matter.

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That is a description and not a cause.

In other words, GR will say how it works, but not why it works that way.

Why should GR be held to a higher standard than any other part of physics?

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Why should GR be held to a higher standard than any other part of physics?

OK, is the strong nuclear force under the standard model implemented by say gluons?

See, that is a why in a way.

Why does the strong nuclear force exist. Because of gluons.

Edited by vuquta
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OK, is the strong nuclear force under the standard model implemented by say gluons?

See, that is a why in a way.

Why does the strong nuclear force exist. Because of gluons.

Gluons are the mechanism. It doesn't answer the question of why the force exists. But if that's what you want, the proposed particle for gravitation is the graviton.

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!

Moderator Note

The continuation of the gluon discussion has been moved to a thread in the quantum theory subforum... Please continue it there...

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the classical explanation for gravity is that active gravitational mass creates a gravitational field that acts on passive gravitational mass causing it to accelerate toward the active gravitational mass.

the general relativity explanation for gravity is that inertial mass causes massive bodies to move in straight lines (or rather geodesics) through 4D spacetime which is itself distorted by the same massive bodies thus explaining the fact that all objects fall at the same rate in any given gravitational field. (this is known as the equivalence principle and strictly speaking means that passive gravitational mass is always proportional to inertial mass.)

http://en.wikipedia.org/wiki/Equivalence_principle

if that doesnt answer your question then I dont know what more you want.

Edited by granpa
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I have learned from physics articles that the speed of gravity is equal to the speed of light. I'm just curious.., could light be the reason for gravity? If not, what is the reason for gravity? Just asking...

Maybe "gravitons" are anti-photons. This is, of course, speculative but couldn't it be the case that as smaller particles fuse to form heavier ones, the energy released as radiation also generates a proportional amount of anti-radiation that remains in the form of mass/gravitation? Just as particles always emit a certain amount of low-level radiation, they may also consistently emit a certain level of gravitation, although I wonder why this level wouldn't fluctuate the way radiation-levels do, as a response to absorption and re-emission. Still, I wonder if gravitation could build up in particles in such a way that causes extremely large particles to decay faster. Maybe gravity-generation is very similar to energy-generation except mass has less volatility and dynamism in comparison with processes that store and release energy.

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Maybe "gravitons" are anti-photons.
Unless you consider "maybe a house cat is a horse" an interesting statement to explore they aren't.
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This is, of course, speculative

This isn't the place for speculation.

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Unless you consider "maybe a house cat is a horse" an interesting statement to explore they aren't.

Comparing cats and horses could involve different characteristics of each. What basis do you have to compare photons to gravitons, since gravitons are purely theoretical, as far as I am aware?

This isn't the place for speculation.

I did not bring up "gravitons" first in this thread. I was just playing off another post that mentioned them and considering a further theoretical possibility that they could be related to photons at the fundamental level of atomic construction/design. If particles always emit some level of radiation, why shouldn't they also be constantly emitting some level of gravitation? I'm not really trying to speculate as much as just considering avenues for how to explore possible relationships between gravity and light.

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Comparing cats and horses could involve different characteristics of each. What basis do you have to compare photons to gravitons, since gravitons are purely theoretical, as far as I am aware?

- the different spacetime structure of the related fields; rank-1 vs. rank-2 tensor.

- the different coupling to matter properties.

- anti-photons are just photons.

"purely theoretical" in this case means that no experimental evidence exists (it's a bit worse in fact). It does not mean it is an undefined term for everyone to project their ideas on.

What I meant with the example is that you can always say "maybe X is just a form of Y" as soon as you either ignore the common meaning of X or Y, or as soon as you pull the "but maybe everything is different than we think"-statement (which is quite pointless).

EDIT: Sorry if above is a bit rude. Lately, I'm slightly annoyed by people talking about modern physics without even knowing the basics. That's of course neither your fault nor a justification for being rude, so no offense meant. Content-wise, my statements still stand, of course.

Edited by timo
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I did not bring up "gravitons" first in this thread. I was just playing off another post that mentioned them and considering a further theoretical possibility that they could be related to photons at the fundamental level of atomic construction/design. If particles always emit some level of radiation, why shouldn't they also be constantly emitting some level of gravitation? I'm not really trying to speculate as much as just considering avenues for how to explore possible relationships between gravity and light.

!

Moderator Note

If one wanted to ask a question about why gravitons can't be photons, or if gravitons and photons are the same thing, either is a perfectly fine physics question to ask. As an answer to the OP, though, that's different — we have a speculations forum, and a post on why one thinks gravitons might be photons belongs there.

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- the different spacetime structure of the related fields; rank-1 vs. rank-2 tensor.

- the different coupling to matter properties.

- anti-photons are just photons.

"purely theoretical" in this case means that no experimental evidence exists (it's a bit worse in fact). It does not mean it is an undefined term for everyone to project their ideas on.

What I meant with the example is that you can always say "maybe X is just a form of Y" as soon as you either ignore the common meaning of X or Y, or as soon as you pull the "but maybe everything is different than we think"-statement (which is quite pointless).

EDIT: Sorry if above is a bit rude. Lately, I'm slightly annoyed by people talking about modern physics without even knowing the basics. That's of course neither your fault nor a justification for being rude, so no offense meant. Content-wise, my statements still stand, of course.

!

Moderator Note

If one wanted to ask a question about why gravitons can't be photons, or if gravitons and photons are the same thing, either is a perfectly fine physics question to ask. As an answer to the OP, though, that's different — we have a speculations forum, and a post on why one thinks gravitons might be photons belongs there.

Ok, first I didn't know that there was such a definite discourse about gravitons in progress, so my apologies. I thought they were just speculative projections of particle-fetishizing people. I should have asked what the theoretical claims are about gravitons and critically engaged those instead of assuming there was nothing serious to engage. Second, thanks for apologizing and not wanting to offend. I can understand your annoyance but since my interest in physics is amateur, I have gaps in what I know about various lines of theory. Also, to be honest I think one of the good things about an empirically-oriented science like physics is that one does not have to adhere to certain discourses because they are institutionally validated. What reason is there to speculate according to established professionally-developed theories but not to discuss the logic of why a graviton could or couldn't be an anti-photon?

Imo, the interesting thing is to have discussions that bring out the contours of reasoning about these things and not to simply dismiss thoughts and questions because they conflict with established knowledge. How can learning occur from just saying, "no, gravitons can't be anti-photons because that's not what they are." That's not scientific reasoning - it's insistence on established knowledge because that knowledge is grounded - instead of explaining the actual grounds themselves. Isn't that like telling someone, "because the bible says so," without even bothering to explain what the bible says and why?

Edited by lemur
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What reason is there to speculate according to established professionally-developed theories but not to discuss the logic of why a graviton could or couldn't be an anti-photon?

We want to make sure any amateur reading the thread gets legitimate physics answers instead of speculative ones. If someone reads an answer and gets the idea that gravitons might be photons, that's undesirable.

We know that gravitons have to have certain properties; timo has explained some of the issues: Because the stress-energy tensor is rank-2, the graviton must be spin-2, and photons are spin-1. And we already have photons interacting with matter via the electromagnetic force, and the gravitational force is different. The interaction must be different.

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