Gravitons, why?

[rigney]

Question: What is a graviton? Why are we consumed with an idea that can only be explained mathmatically, but in no other fashion? I'm not looking for an arguement over something I know absolutely nothing about, but just wondering? Is the universe based on such antiquated ploys as entropy and gravity, or will the LHC in Cern shake our brains to their roots?

 

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

Edited November 19, 2010 by rigney

[ajb]

Really the idea of a graviton comes from the great success of quantum electrodynamics where the photon is understood as the quanta of electromagnetic radiation. You think of the photon as being the "exchange particle" responsible for all electromagnetic phenomena.

 

It was very natural for the physics community in the 1940's and 1950's to wonder if the same techniques that were applied to electrodynamics could be applied to gravity in the form of Einstein's general relativity.

 

Well it turns out that one cannot do this. The first barrier was that general relativity has a much more rich structure as far as the symmetries and interactions are concerned. The photon carries no electric charge and so does not act as a source of the electromagnetic field. The graviton (if it is realised) has energy and so would self-interact.

 

Yang and Mills in 1954 generalised electrodynamics to allow for a "self-interacting photon". Their motivation was to find some theory half-way between electrodynamics and general relativity. Study of such a theory, the community believed could help to understand how to quantise gravity.

 

Alas, this was not realised. However, Yang-Mills theory went on to be the major component of particle physics when it was realised that it could be used to describe the strong and weak forces. (It is also very important in mathematics.)

 

The other barrier is that trying to use the naive methods of quantisation that were applied to electrodynamics to general relativity produces an infinite number of infinities. The techniques of "renormalisation" to remove these that were so successful in electrodynamics (and later Yang-Mills theory) just could not cope.

 

However, from quite general arguments you know that a graviton should be massless and a spin-2 boson. In any modern theory, even if gravity was not what you were looking for any particle state that is massless and spin-2 is called the graviton. Again from quite general arguments you know that any classical theory of a massless spin-2 particle is more or less going to be general relativity.

 

In short, quantum electrodynamics works so well with the idea of a photon and thus it was expected that quantum gravity should be a theory of gravitons. This may not be the case after all...

[D H]

Question: What is a graviton? Why are we consumed with an idea that can only be explained mathmatically, but in no other fashion? I'm not looking for an arguement over something I know absolutely nothing about, but just wondering? Is the universe based on such antiquated ploys as entropy and gravity, or will the LHC in Cern shake our brains to their roots?

If the standard model is correct the LHC should find the Higgs boson. The nature of the Higgs field is one of the things the LHC is being used to investigate. The LHC almost certainly is not going to find a graviton for the simple reason that it is not looking for signs of a graviton. How can it? Physicists don't even know what to look for -- yet.

 

There are several reasons to think that the graviton, or some thing(s) like it, does(do) exist. General relativity admits singularities, something most physicists view as indicative of a shortcoming in the theory. Another reason is that while general relativity does a very good job of matching reality, physically it is not that deep. Physicists like to ask "what makes that happen?" The answer from GR is "because the math says so." Poking deeper can't occur because poking deeper runs smack dab into the axioms. Finally, there is a problem with mixing the classical nature of general relativity with the quantum nature of the rest of physics. For example, see Steve Carlip's paper on this topic, http://arxiv.org/abs/0803.3456.

 

 

 

[ajb]

It should be noted, or really I should have said it explicitly that quantum gravity is not necessarily a theory of gravitons. Via non-perturbative renormalisation group flow methods it is believed that quantum general relativity (or something similar with extra terms) is asymptotically safe.

 

In loose terms the full theory maybe well behaved outside of perturbation theory. Particles are really a notion form perturbation theory, that is based on small oscillations about some classical vacua. As quantum gravity may be fine in a non-perturbative framework it is quite possible that quantum gravity is not a theory of gravitons at all.

 

I believe other questions like the unitarity etc are still quite open.

[rigney]

Really the idea of a graviton comes from the great success of quantum electrodynamics where the photon is understood as the quanta of electromagnetic radiation. You think of the photon as being the "exchange particle" responsible for all electromagnetic phenomena.

 

It was very natural for the physics community in the 1940's and 1950's to wonder if the same techniques that were applied to electrodynamics could be applied to gravity in the form of Einstein's general relativity.

 

Well it turns out that one cannot do this. The first barrier was that general relativity has a much more rich structure as far as the symmetries and interactions are concerned. The photon carries no electric charge and so does not act as a source of the electromagnetic field. The graviton (if it is realised) has energy and so would self-interact.

 

Yang and Mills in 1954 generalised electrodynamics to allow for a "self-interacting photon". Their motivation was to find some theory half-way between electrodynamics and general relativity. Study of such a theory, the community believed could help to understand how to quantise gravity.

 

Alas, this was not realised. However, Yang-Mills theory went on to be the major component of particle physics when it was realised that it could be used to describe the strong and weak forces. (It is also very important in mathematics.)

 

The other barrier is that trying to use the naive methods of quantisation that were applied to electrodynamics to general relativity produces an infinite number of infinities. The techniques of "renormalisation" to remove these that were so successful in electrodynamics (and later Yang-Mills theory) just could not cope.

 

However, from quite general arguments you know that a graviton should be massless and a spin-2 boson. In any modern theory, even if gravity was not what you were looking for any particle state that is massless and spin-2 is called the graviton. Again from quite general arguments you know that any classical theory of a massless spin-2 particle is more or less going to be general relativity.

 

In short, quantum electrodynamics works so well with the idea of a photon and thus it was expected that quantum gravity should be a theory of gravitons. This may not be the case after all...

 

Appreciate your reply and wish somehow to get a more qualified understanding of the http link so I might better discuss the topic.

 

If the standard model is correct the LHC should find the Higgs boson. The nature of the Higgs field is one of the things the LHC is being used to investigate. The LHC almost certainly is not going to find a graviton for the simple reason that it is not looking for signs of a graviton. How can it? Physicists don't even know what to look for -- yet.

 

There are several reasons to think that the graviton, or some thing(s) like it, does(do) exist. General relativity admits singularities, something most physicists view as indicative of a shortcoming in the theory. Another reason is that while general relativity does a very good job of matching reality, physically it is not that deep. Physicists like to ask "what makes that happen?" The answer from GR is "because the math says so." Poking deeper can't occur because poking deeper runs smack dab into the axioms. Finally, there is a problem with mixing the classical nature of general relativity with the quantum nature of the rest of physics. For example, see Steve Carlip's paper on this topic, http://arxiv.org/abs/0803.3456.

 

After reading my question over a few times, I realized it wasn't very well written. But the http link was what got me to thinking about gravity again at all. And since Math is not my strong suite, believe me; it's a scramble for every bit of information I can put together and rationalize. Guys like you and ajb have a good grasp on the subject and I appreciate the fact that you even answer.

[Incendia]

As I have mentioned in a different post: They could be hidden inside quarks<and leptons>...Quarks<and leptons> do have gravity...

 

Taken from wikipedia:

Additionally, it can be shown that any massless spin-2 field would be indistinguishable from gravitation, because a massless spin-2 field must couple to (interact with) the stress-energy tensor in the same way that the gravitational field does. ^[4] This result suggests that if a massless spin-2 particle is discovered, it must be the graviton, so that the only experimental verification needed for the graviton may simply be the discovery of a massless spin-2 particle

[rigney]

As I have mentioned in a different post: They could be hidden inside quarks<and leptons>...Quarks<and leptons> do have gravity...

 

Taken from wikipedia:

 

My problem is, I personally can't define gravity; and my ignorance keeps me believeing that it's a branch of magnetism. I'm sure Math would help me a lot.

[Incendia]

It isn't a branch of magnetism though...where did you hear/read that?!

Gravity is it's own force. Define it as: That force...you know...that one that everything with mass has...that force that pulls things with mass closer to each other and if it wasn't for other forces would mean that the universe would have stayed a singularity.

[rigney]

It isn't a branch of magnetism though...where did you hear/read that?!

Gravity is it's own force. Define it as: That force...you know...that one that everything with mass has...that force that pulls things with mass closer to each other and if it wasn't for other forces would mean that the universe would have stayed a singularity.

 

Say I put some small scraps of paper on the table, they will lie there 'til a breeze comes along and sends them flying. They stay in place because of a thing we call gravity. But, if I rub a rubber or plastic comb on my shirt a few times and move it close to the scraps of paper, they will literally jump to the comb. It's an attraction referred to as "static electricity". But why? It defies what we consider gravity, even if only momentarily. Since everything in the universe is in constant motion and being stroked by materials in space, such as; anti matter, dark matter, dark energy and who knows what else?, could this be the balance of inbalances? Our earth spins on its axis at 1,000 mph, flies around the sun at 65,000 mph and zips through our galaxy at 600,000 mph. I have no idea just how fast our galaxy is moving away from what ever? Perhaps it's a thing science might consider. I know I can't. Just wish I had a better grasp on math and sub forces. Edited November 20, 2010 by rigney

[Incendia]

Okay...Gravity is a weak force. It is separate to magnetism. Electromagnetism is a stronger force than Gravity. The four main forces: Gravity, Electromagnetism, Strong force, Weak force. It has been like that for ages. Is this 'gravity being a form of magnetism' just your way of trying to combine the four forces? I don't like it. Gravity and electromagnetism are completely different for many reasons.

[rigney]

Okay...Gravity is a weak force. It is separate to magnetism. Electromagnetism is a stronger force than Gravity. The four main forces: Gravity, Electromagnetism, Strong force, Weak force. It has been like that for ages. Is this 'gravity being a form of magnetism' just your way of trying to combine the four forces? I don't like it. Gravity and electromagnetism are completely different for many reasons.

Sorry guy, There was no intent to tweak your sensitivity or intellect, nor to think my thoughts aught to be etched in stone. And honestly I haven't the knowledge to make such an assumption, other than it being just a rough idea. Edited November 21, 2010 by rigney

[Incendia]

Okay...Chance be on your side in unifying the four forces. You may be interested in this slice of news I posted: http://www.scienceforums.net/topic/52700-physics-gravity-helps-unify-natures-forces/

[pioneer]

Part of the problem we face has to do with particle accelerator-collider data. These experiments do not occur at extreme gravity, and therefore do not reflect the idea that charge becomes close to zero at extreme gravity. The result is we might be creating sub-particle states that would not exist at extreme gravity. This will bias theory, with synthetic states of matter that would be more appropriate to a compression at low gravity, rather than an expansion at extreme gravity.

 

For example, the electron is considered an elementary particle, yet at extreme gravity if charge is zero, doesn't that mean this particle is no longer fundamental, since the charge of the electron is not there? One will not see that in particle accelerator data at low gravity. However, since the low gravity compression data shows the electron is fundamental, so we tend to fixate on matter-antimatter, which is a moot point at extreme gravity.

[rigney]

Somehow, when I try discussing gravity, I'm totally lost. Statements made by scientists leave me dazed. My thought on it is, if gravity is a spontaneous response faster than light; could it possibly be the "instantaneous teleportation" many scientists are working on? The links below is much more than I know about gravity, which isn't much. Just keep on discussing it so that I might be more informed.

 

http://www.xs4all.nl/~johanw/PhysFAQ/Relativity/GR/gravity.html

 

http://www.nytimes.com/2003/11/11/science/what-is-gravity-really.html

Edited November 21, 2010 by rigney

[Incendia]

Gravity travels at the same speed as light...not faster. Nothing travels faster than light...

Edited November 22, 2010 by ProcuratorIncendia

[lemur]

Part of the problem we face has to do with particle accelerator-collider data. These experiments do not occur at extreme gravity, and therefore do not reflect the idea that charge becomes close to zero at extreme gravity.

 

Where did you get the idea that charge decreases with gravity? I have wondered about this but I would like to know if there is some website that discusses it further. What would happen to charge as gravitation increases? Is there such a thing as conservation of force?

[rigney]

Gravity travels at the same speed as light...not faster. Nothing travels faster than light...

Can this statement be absolutely true? Even Einstein admitted that he didn't know for sure. Many hypotheticals are used in physics to describe clouded situations. Most though, if not all; have been thought out and mathematically concluded to be correct. The instantaneous teleportation thing is somehow used in quantum mechanics, which scientists readily admit the term doesn't imply faster than light. An interesting hypothetical I just came across and never heard of before is: Tachyons. They are supposidly hypothetical subatomic particles that always travel faster than the speed of light. Another is : the speed o gravity.I don't know?

 

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

 

http://www.xs4all.nl/~johanw/PhysFAQ/Relativity/GR/grav_speed.html

Edited November 22, 2010 by rigney