# What is the cause of gravity?

1. Jun 9, 2006

### hover

What causes gravity???

I really don't understand it . What causes gravity? Relativity says its the "bending" or "curving" of space but what causes gravity? Also anybody thats interested in observing gravity look http://www.fourmilab.net/gravitation/foobar/" [Broken].
thx

Last edited by a moderator: May 2, 2017
2. Jun 10, 2006

### Robert100

I think the real question is why we have two seemingly incompatible ways of describing gravity.

Einstein accepts that objects have inertia, and thus travel in a straight line. The only hitch Einstein introduces is that mass generates a curvature of space itself. Thus, objects attempt to travel in a straight line through a curved space, thus generating the pathway that we describe as "falling", "orbiting", "moving under a gravitational influence", etc.

But - and correct me if I am wrong, people - this picture doesn't seem to depend on the existence of any force-carrying particles, such as the graviton.

Einstein's math about GR and SR is unquestionably correct, but as far as I can tell, his interpretation (the standard geometrical interpretation) does not require the existence of gravitons. :surprised

The standard model supposes that force-carrying particles, gravitons, are what mediates the gravitational interaction.

While I am fully aware that we do not have a workable model of quantum gravity at the moment, what bothers me is the seeming total incompatibility between these two models.

If gravitons actually exist, and are necessary to transmit a gravitational force, then isn't the geometrical bending of space superfluous? (And vice-versa.) :yuck:

Could someone help me clarify my error in understanding this? I'd like to see some way that these two views are, at least in principle, compatible.

For instance: (A) are gravitons envisoned as particles that cause that distortion of space, thus creating gravity? (B) Does the distortion of space manifest itself as particles? (And if so, how?) (C) ...any other views...

Robert

3. Jun 10, 2006

### turbo

It gets more complex than that. The gravitational attraction between two objects is dependent on the mass of the objects - mass that (in popular models) is conferred by matter's interaction with a hypothetical particle - the Higgs Boson. If the graviton and Higgs Boson are real, their fields must be remarkably congruent over all the Universe, or gravitational effects would not be consistent from one place to another.

4. Jun 11, 2006

### Robert100

Can somone help clarify this debate? Curvature, particle or both?

I asked what I thought was a basic, and critically important question here yesterday, and I was disappointed that no answer was given. I thus searched the physicsforum archives to see if this issue was discussed previously. Well, it was, but nothing approaching a consensus appeared, and I cannot tell which of the two views given represent the thinking of mainstream physicists (if either.)

The basic question asked then, and now, is this: As I understand it, Einstein's theory of General Relativity (GR) seems to describe gravity as being caused by curved space-time alone. No gravitons are necessary. However, quantum field theory is said to predict the existence of a graviton (which is massless, like a photon) and it is through the interaction of gravitons that gravity is produced.

On the surface it seems as if these are two totally imcompatible world views. One has go to go. Well, in discussions here and elsewhere we always get the same result. Some people say "Yes, these two views are really imcompatible" while another groups saus "No, you're crazy, they are totally compatible."

Umm, both groups can't be correct. I would it appreciate it if people would look through the various answers to this question, and let me know what they think?

One very interesting answer is here. Thoughts?

http://www.lns.cornell.edu/spr/1999-02/msg0014781.html
Re: Gravity: Wave-Particle vs Space-Time Warpage
Subject: Re: Gravity: Wave-Particle vs Space-Time Warpage
From: baez@galaxy.ucr.edu (john baez)
Date: 14 Feb 1999 00:00:00 GMT ​

Other answers came from a discussion on this forum. Some people here answered that GR's model of a curved space causing gravity is totally at odds with the graviton model of gravity. For instance:

selfAdjoint states:
"The graviton is a theoretical particle predicted by string theory but not yet demonstrated to exist. If it exists and is the cause of gravity it will do away with spacetime curvature and produce gravity the way the photon carries electromagnetism. Then gravity would be a (quantum) force in your sense."

"At the present time however, our best theory of gravity is Einstein's 1915 General Theory of Relativity. One of the basic propositions of that theory is the Principle of Equivalence: On a sufficiantly small scale, it is impossible to tell the difference between an imposed force and a gravitational one. Notice that the shape is the shape of spacetime, not just of space. Therefore a curved geodesic goes through time as well as space, and by curving, causes those travelling along it to experience an acceleration. Anything that produces an acceleration deserves the name force, no?"

Nenad writes:
"...because if the graviton is found, then there is no curvature of space time, the reson there is gravity will be because of the graviton. You cant have both explaining the same thing."

However, some people come to exact opposite conclusion! They hold that the GR model is compatible with the graviton model of gravity. For instance:

jtolliver writes:
>> yes, if the graviton is found, General Relativity will be wrong.

"Not really. Any lorentz-invariant interaction that preserves causality can be described in terms of particles. GR satisfies those conditions. Gravitational waves have many properties traditionally associated with particles (If you pretend spacetime is flat and you add gravitational forces to compensate), such as energy-momentum. Using curved space-time(the technique traditionally used in GR), and using flat space-time with an additional force that makes it act exactly as though it were curved(this was the technique used in classical mechanics, except that that force didn't quite make it act exactly as curved space-time) are just different ways of looking at the same thing. The first way is much more convient mathematically, and the second way doesn't really explain why we have this additional force. The graviton's only look like particles when we assume spacetime is flat. If we assume spacetime is curved we see it is really only an effect of the curvature of spacetime, even though it looks exactly like a particle."

pmb_phy writes:

...It is not the curvature of spacetime that causes gravitational acceleration. Spacetime curvature causes two particles moving under to have a relative acceleration between them, i.e. spacetime curvature causes tidal accelerations. But you can have a gravitational force in the absense of spacetime curvature."

> The graviton is a theoretical particle predicted by string theory
> but not yet demonstrated to exist.

It doesn't come from string theory. It comes from quantum gravity/quantum field theory.

> If it exists and is the cause of gravity it will do away
> with spacetime curvature ...

That is incorrect. The graviton is responsible for producing gravitational forces, not just tidal forces. Therefore one should aslo be able to detect gravitons in a gravitational field even when the spacetime is flat. The graviton, if it exists, will have a relative existance in that sense.

> yes, if the graviton is found, General Relativity will be wrong.
> if the graviton is found, then there is no curvature of space time,
> the reson there is gravity will be because of the graviton. You cant
> have both explaining the same thing.

That is very much incorrect. You can most certainly have both. If the graviton is detected then we will have the mechanism behind GR. Gravitons are certainly not inconsistent with general relativity.

> Couldnt you say the graviton cause the curvature of spacetime...lol

Absolutely. That is 100% correct.

Gravity is not like other forces like the electric of magnetic forces. The gravitational force is an inertial force where the others aren't.

Ok, so which view is correct? (References to peer-reviewed lit, or books by respected authors, would also be appreciated.)

Thanks,

Robert

Last edited by a moderator: May 2, 2017
5. Jun 11, 2006

### pervect

Staff Emeritus
I guess I missed it because the thread started with a philosphical question - from a different poster, I see as I look back. Broad philosphical questions like the thread-starting question don't really have answers.

OK, this gets into something that I can talk about, unlike "what causes gravity".

Gravity can be understood from the point of view of "gravitons", though this is not how GR is usually formulated.

If you want to keep up with the main GR community, you need to learn the curvature formalism.

For an example of how gravity can be understood from a "particle" point of view, try

http://xxx.lanl.gov/abs/astro-ph/0006423

which I have mentioned in the past, if you refine your search you can probably find the thread. (There was some past discussion and clarification of some of the issues there, so it'll probably be worth your while).

I should add that (as you will see if you read this article) actually understanding physics from the "particle" point of view so that one can make useful calculations from it is a very difficult job. A lot of people "latch onto" this idea because they think it is simple, but actually using it for anything useful is not particularly simple. Not only is it not simple, but it can even be misleading.

A short list of common misconceptions:

1) Gravity is not carried by real "gravitons", but by virtual "gravitons", just as the columb force between charges is carried by virtual photons.

1a) "Gravitons" will not necessarily lead to artificial gravity, just as electromagnetism has not led to tractor beams. We can generate "real" photons, but to generate electrostatic attraction requires electric charges. We can generate photons, but we can't generate virtual photons to pull or push an object away except by charging it, or by using a magnetic field.

2) The columb force between two charges does not aberrate in the manner that electromagnetic radiation aberrates due to motion of the charges. Neither does gravitational attraction.

Anwyay, getting back to your question - it is possible to understand gravity from more of a "graviton" POV, there is no fundamental conflict. This leads to an "effective field theory" version of quauntum gravity. However, this "effective field theory" is still only good at low energies (note that the Tevatron is "low energy" in this context).

Also note that this approach doesn't really get rid of curvature. We construct an artifical "flat" background metric for this theory, but curvature emerges from the theory naturally when we look at what we can measure. Atoms are our rulers and clocks, and the "flat background" expression of this theory has atoms changing sizes and ticking faster/more slowly as a function of position. The "physical" expression of the theory uses standard SI units to define times and distances, and this physical version of the theory makes the rulers and clocks static, but as a result space-time becomes curved and dynamical (i.e. the curvature varies with matter density).

Both expressions represent the same theory. Essentially we use non-standard coordinates in one version with an artifical, flat background metric, then when we switch to standard SI-unit based coordianates, we wind up with a curved metric.

Last edited: Jun 11, 2006
6. Jun 15, 2006

### Robert100

Marcus has recently led me to a provocative article on the nature of gravity, an article by Freeman J. Dyson entitled "The World on a String".

It is a review of "The Fabric of the Cosmos: Space, Time, and the Texture of Reality" by Brian Greene. It was published in The New York Review of Books, Volume 51, Number 8 May 13, 2004

The complete text of this review is here.
http://www.jefallbright.net/node/2538 [Broken]

Dyson hypothesizes that the search for quantum gravity may simply be wrong, that gravitons may simply not exist. I would be very interested to know what others think of his proposal. Strengths? Weaknesses?

Robert

Last edited by a moderator: May 2, 2017
7. Jun 16, 2006

### yogi

The notion of discrete particles traveling between masses to produce attractive forces is one more fanciful attempt to explain all phenomena in terms of particle theory. While Dyson's critique is incorrect in some aspects, I would agree that time spent looking for other explanations will be more rewarding in the long run. Einstein's theories are based upon the notion that the forces produced between physically separated objects are due to a modification (conditioning) of space, and not to traveling entities. The appeal of the graviton is probably due in a large part to the success of QED; virtual photons are conjured to explain the electic force between two charged entities - and the existence of these photons is touted as real because of the high correlation between experimental measurements and theory - but the calculations do not really depend upon the existence of virtual photons - QED is simply an alternative way to arrive at the predictions of perturbation theory - and alternatively the coefficients of the alpha expansion - which Dyson himself helped develop.

8. Jun 16, 2006

### pervect

Staff Emeritus
I think that any test of quantum gravity will probably be indirect, like the ones proposed in the article - rather than looking directly for quantized gravitatioanl radiation as Dyson proposes.

http://physicsweb.org/articles/news/2/6/17/1

As far as gravity goes, we are currently free to use either a quantum "effective field theory" formulation or the traditional geometrical explanation. Both of them work at the sorts of energy levels that we can access. We expect that we will need a better theory to explain higher-energy events, but it may be very difficult to get the needed data to experimentally support such a theory. We're doing the best we can (via cosmological observations, and building bigger particle accelerators).

My overall reaction to Dyson's theory is it needs to be fleshed out more. I don't see what sort of predictions it makes, other than negative ones that will be exremely difficult to test.

I'd also like to add that as a practical matter, I think the current classical approach to GR is the easiest way to calculate a lot of predicted results, such as the binary neutron star inspiral, and (hopefully, in the near future) gravitational wave signatures.

Last edited by a moderator: May 2, 2017
9. Jun 16, 2006

### Leo Foucault II

Hello everyone
I am enjoying reading the posts on gravity as I have been toying with it for a while.
I did want to know if anyone, anywhere in the world has said yes we do have proof of String Theory.
I may be wrong, but I think it is not real, but just simply a desperate move to create something that we can give to the students and keep them in school.
Yes, it is a wonderful path of math, and a nice idea, but please show us something.
Einstein did as Brian Greene says (Paint a picture of gravity) but this is all he did just a picture.
If you know what gravity is, then you can use math or simple physics and gravity will do work.
Question them when they say it is so small that it can never be used. It is indeed so large that it can spin the earth.

10. Jun 16, 2006

### pervect

Staff Emeritus
I've started another thread that expands a bit on the point I was trying to make earlier. This has the original quote from Einstein that I was trying to think of, and also a minor correction in terminology.

https://www.physicsforums.com/showpost.php?p=1014241&postcount=1

The short version is that a flat marble table can be made "non-flat" by changing the rulers with which one measures it. The quote from Einstein gives an analogy that allows us to see how space-time can be flat with t he distorted rulers postulated in

http://xxx.lanl.gov/abs/astro-ph/0006423

but yet appears to be non-flat when we use "physical" rulers, made out of platinum bars (or, to use a more modern method, by counting interference fringes of a known wavelength of light).

11. Jun 24, 2006

### CarlB

Yogi, I think that this is a cool concept. I haven't seen it in the literature, can you give me references? I'm not sure how to search for it. The name of a few researchers would be appreciated.

Carl

12. Jun 24, 2006

### pmb_phy

General Relativity is often mis-stated as being the relativistic explanation of gravity. It is not. It is a relativistic description of gravity. As Eddington put it (Nature, March 14, 1918, p. 36)
Pete

13. Jun 28, 2006

### Thrice

Relativity will be compatible with any theory that surpasses it. The new theory will probably reduce to relativity in some sort of limit.

14. Jun 28, 2006

### Farsight

That strikes a chord with me, Yogi. Do you know if the same can be said of Gluons?

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