# What is the theory of gravitons

Gravitons are particles believed to be the cause of gravity but can anyone tell me their principle and how they work

Drakkith
Staff Emeritus
The graviton would be force carrier particle of gravitation, much like how the photon is the force carrier particle for the electromagnetic force (the other 2 forces have their own force carriers). These force carriers are the result of a particular type of theory known as Quantum Field Theory (QFT), which is what the current standard model of particle physics is based on. The details are well above even my head though.

Unfortunately QFT does not accurately describe gravitation. The graviton is a hypothetical force carrier that is believed to exist if gravitation is quantized like the other forces are. As it is, we're still using General Relativity, which isn't a quantum theory (meaning that it doesn't describe gravity as being quantized).

https://en.wikipedia.org/wiki/Graviton
https://en.wikipedia.org/wiki/Force_carrier

There have been attempts to quantize gravity, you may find this wiki article interesting.
https://en.wikipedia.org/wiki/Quantum_gravity
Nobody seems to have nailed it yet though.
Gravitons would exist in such a theory if somebody did, but until then they are hypothetical.

As gravitons are currently hypothetical, how might they be discovered?

And if they were, what could this discovery potentially mean to the wider physics community and The Standard Model? Which major theories would in come in direct contradiction with?

Also if someone could elaborate on how extra dimensions come into play concerning gravity then it would be appreciated.

PS. I am a high school student studying Physics.

• hsdrop
Also if someone could elaborate on how extra dimensions come into play concerning gravity then it would be appreciated.
PS. I am a high school student studying Physics.
I can take a shot at answering this one at least.
With the view of gravity as a "force", when compared to the other 'fundamental forces' (Such as Electromagnetic and Weak, also the "Strong" nuclear Force) is incredibly weak by comparison. An example is in the ability for a magnet on a string hanging downwards by the gravity of the entire earth may be deflected by a small other magnet, whilst the gravity of a 10 tonne weight wont even make the slightest visible difference.

Physicists have attempted to understand why there's such a huge difference and called it "the hierachy problem".

As such, there have been a number of theories and ideas attempting to explain and solve this "hierarchy problem" - a popular one being linked with "Loop Quantum Gravity" - based on a string theory, which in essence, basically suggests that the fundamental force carrying particles (i.e. photons for light, W and Z for Weak force, pions for strong force) are made of tiny fundamental strings that vibrate in certain ways.
According to the theory, Gravitons are also such strings, but are closed loops - the ends of the strings can join together. Whereas other strings have at least one 'end' always attached to our familiar 3+1 dimensional spacetime - these loops, are no longer so grounded and are then free to 'float away' in a higher dimensional space.
Therefore (according to the theory) despite gravitons actually possessing more similar energies as their counterparts - so many are lost to this higher dimensional space, that the resulting forces experienced in our 3+1d spacetime appear so much weaker.

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A lot of the description and language used is non-scientific and potentially misleading. I apologise for this. The attempt was to give a visual idea, rather than a technical explanation.

Chronos
Gold Member
The notion of gravity as a force is where the problem arises. According to Einstein, gravity is not a force. It is merely the curvature of spacetime in the presence of mass. The fact all bodies fall at the same rate in a gravitational field illustrates this point. Were gravity a force, heavy objects would fall proportionately faster than light objects in a gravitational field. There is no need for a carrier particle [graviton] to convey a force where none exists.

As gravitons are currently hypothetical, how might they be discovered?.
We may be able to detect quantized gravity using gravity wave telescopes. As two huge objects get closer and closer to each other, general relatively predicts a smooth transition, no energy levels. If gravity is quantized, then there should be discrepancies at the last instant of a black hole merger. They should only be able to radiate gravitational energy away in packets, so the transition would not be smooth.

The notion of gravity as a force is where the problem arises. According to Einstein, gravity is not a force. It is merely the curvature of spacetime in the presence of mass. The fact all bodies fall at the same rate in a gravitational field illustrates this point. Were gravity a force, heavy objects would fall proportionately faster than light objects in a gravitational field.

Wrong.

The strength of gravitational attraction in a quantum gravity theory would depend on strength of coupling of all other fields to the graviton field. As you correctly noted, we know from experiments that it is proportional to mass (more correctly, stress-energy tensor), and GR's only equation says precisely that.

Thus, any sensible quantum gravity theory which wants to match observations posits that this coupling is also proportional to stress-energy tensor of these "other fields", and it all works just fine.

The difficulties are elsewhere: renormalization procedure does not work for gravitons.

There is no need for a carrier particle [graviton] to convey a force where none exists.

There is. "Classical" (i.e. non-quantum) theories are mathematically incompatible with quantum ones. GR is "classical". Standard model is quantum.

haushofer