# What are gravitons exactly and how will they help us prove string

I recently watched the documentary, "The Elegant Universe," by Brian Greene. During the show, gravitons were mentioned. What are gravitons exactly and how will they help us prove string theory? Was he trying to explain that gravity is a lot stronger than we perceive it to be? Thank you in advance for your responses.

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mathman

Gravitons are theoretical particles which carry the gravitational force. String theory predicts gravitons with the right properties, so it is a hint for the validity of string theory. However, there has been no explicit experimental observation of gravitons.

PAllen
2019 Award

Gravitons are theoretical particles which carry the gravitational force. String theory predicts gravitons with the right properties, so it is a hint for the validity of string theory. However, there has been no explicit experimental observation of gravitons.
To amplify this a little, gravitons had been proposed long before string theory, simply because a quantum theory requires a force carrying particle, and if gravity is viewed as force you would expect them. Further, their required properties were worked out, such that they match predictions of GR. The really distinguishing achievement of the string research program is that without assuming gravity or gravitons, they came out as prediction. So far as I know, this is unique to string theory among all quantum gravity research programs. For example, any program that starts from quantizing GR is assuming gravity at the start.

Unfortunately, it is likely that (unlike photons), they will never be directly detected, even if a theory involving them is ultimately established. See:

http://arxiv.org/abs/gr-qc/0601043

So how does gravitons play a part in string theory when talking about branes?

So how does gravitons play a part in string theory when talking about branes?
The graviton is one of the fundamental particles that string theory defines that unifies the four forces of nature. The graviton, like the photon and other particles, are just another vibration mode of the string. It is theorized the gravitons pass through multiple dimensions (other than our 4 that we live in(3d plus time)) and that the gravity we feel is actually the interaction of the gravitons from other dimensions influencing our space time that we live in.

The graviton is one of the fundamental particles that string theory defines that unifies the four forces of nature.
How will the graviton unify the four forces of nature?
Also, how will this allow us to contact other branes?

jtbell
Mentor

How will the graviton unify the four forces of nature?
The graviton doesn't do the unification. It's a product of string theory which is what does the unification. (If string theory actually works, that is.)

The graviton doesn't do the unification.
So what will unify the forces then?
Is it string theory that will unify them?

string theory unifies but without any experimental or observational evidence.
it is only a mathematical theory. an ultra high level mathematics.

string theory unifies but without any experimental or observational evidence.
it is only a mathematical theory. an ultra high level mathematics.

Yes. String theory is a mathematical solution that unifies the four forces of nature (electromagentism, gravity, weak and strong force). Gravity uses gravitons, electromagentism uses electrons and/or photons, the weak force which is similar to uranium decay has its particle that I can't recall (I want to say Boson but I am not sure) and the strong force that holds the everyday things we know and use everyday (Higgs Boson would be that particle, I believe but I am not sure, I have trouble keeping track of all these theoretical particles). All these particles are different vibrations of the string. By the way, nobody has defined what a string is other than it would be the fundamental particle, where the strings come from, what it is made of, where does it get its energy to vibrate and why does it vibrate in various modes to give various particle characteristics. And nobody has found a graviton particle yet either.

I read somewhere on this forum that it would take a particle accelerator the diameter of the milky way galaxy to identify a string. That won't happen in our lifetime.

There are experiments being devised that may be able to prove string theory, indirectly, by inference: if this is true, then that is true. But I don't recall what those experiments may consist of and what the successful outcomes might be. I think if they find the Higgs Boson at CERN that may be a step forward in the puzzle but so far, no luck.

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PAllen
2019 Award

Yes. String theory is a mathematical solution that unifies the four forces of nature (electromagentism, gravity, weak and strong force). Gravity uses gravitons, electromagentism uses electrons and/or photons, the weak force which is similar to uranium decay has its particle that I can't recall (I want to say Boson but I am not sure) and the strong force that holds the everyday things we know and use everyday (Higgs Boson would be that particle, I believe but I am not sure, I have trouble keeping track of all these theoretical particles). All these particles are different vibrations of the string. By the way, nobody has defined what a string is other than it would be the fundamental particle, where the strings come from, what it is made of, where does it get its energy to vibrate and why does it vibrate in various modes to give various particle characteristics. And nobody has found a graviton particle yet either.

I read somewhere on this forum that it would take a particle accelerator the diameter of the milky way galaxy to identify a string. That won't happen in our lifetime.

There are experiments being devised that may be able to prove string theory, indirectly, by inference: if this is true, then that is true. But I don't recall what those experiments may consist of and what the successful outcomes might be. I think if they find the Higgs Boson at CERN that may be a step forward in the puzzle but so far, no luck.
Some corrections: weak force is mediated by the W and Z vector bosons; strong force by gluons. Higgs particle is proposed to break electroweak symmetry giving observed masses to particles.

I would describe string/M-theory as a research program rather than a theory, analagous to Einstein's heuristic, partial results between 1908 and 1916. Given assumptions about the theory he was aiming at, he could make partial conclusions and tentative predictions. String theory is in that state (no actual theory that can be written down in any form), with lot's of theoretical partial results (e.g. the microstructure of near-extremal black holes, that supersymmetry is real, that supergravity is subsumed), but no clear testable predictions unique to string/M theory, yet. An open issue is whether it will ever be able to make general testable predictions short of anthropic arguments.

There is no specific string/M prediction for LHC, that I know of. For example, if LHC discovers supersymmetric particles (I hope), that would be supportive, but supersymmetry was proposed well before and independently of string/M theory.

marcus
Gold Member
Dearly Missed

Nice patient balanced objective short comment, effectively without bias in either direction.
Remind me to strive for that more often It's easy to lose patience.

Thanks PAllen for the corrections. I am but a lowly mechanical engineer who enjoys reading about theoretical physics and learning from a lot of the threads that are discussed here in this forum.

No argument from me that String Theory is a research program. That is exactly what it is.

Thank you for your help. I'm understanding gravitons and string theory better than before.

Your welcome. Remember, the game here is that every force, to be a force, has to have a particle associated with it. That is fundamental to string theory as I understand it. The vibrating string unites the four forces.

I find that I understand things better when I try to explain in simple terms in writing. Although the subject we are discussing is very complex some of it can be explained with words. For me, being an engineer in aerospace, you have to have a physical feel, intuition of things to determine what the solution might be, almost approaching an art. This comes with years of experience and crunching the equations. In pure research which is being discussed here you really have to think more out of the box in order to make the quantum leaps (no pun intended) of understanding of what the true reality is and have the mathematical prowess to put that understanding into math that describes that understanding. As an engineer, I would take the knowledge gained from the pure research here and make for example, a quantum teleportation device to replace the Post Office and Cars and all forms of transportation, and do this as a research and development project.

The other question that would be very complex and would require math to reinforce would be why does the string need 11 dimensions of existence. These strings exist in all 11 dimensions. We only live in 4 of the 11 dimensions, as far as we can tell. We may never know. We are not hardwired to perceive the other dimensions, if they exist.

An interesting look on trying to infer the existence of gravitons by experiment:

http://arxiv.org/pdf/1002.2962

This is sort of analogous to the Lamb shift in QED. Interesting how this might be able to provide some insight into the existence of gravitons in the same way we can 'know' that photons are real particles.

PAllen
2019 Award

An interesting look on trying to infer the existence of gravitons by experiment:

http://arxiv.org/pdf/1002.2962

This is sort of analogous to the Lamb shift in QED. Interesting how this might be able to provide some insight into the existence of gravitons in the same way we can 'know' that photons are real particles.
Interesting complement to the paper I quoted. The paper I quoted could be interpreted as saying "you're never going to get an analog of CCD devices or photoelectric effect with gravitons, where you directly detect graviton". This paper is saying, well you may be able to detect gravitons indirectly by their collective contribution to the state of a sensitive system.

It was your link in fact that sparked the discussion on the Lamb shift, almost conclusively say some, showing that the photon really does exist, and then applying this to gravity. So thanks for the link!

But I'd like to know the reasoning that's going on through this paper. It's a bit thick for me as a student in astrophysics that hasn't really gotten into enough QM, so a bit of explanation would be very valuable to me (and others I'd imagine).