# How do gravitons mediate the force of gravitation in universe ?

• Redi
In summary, the force of gravitation in the universe is mediated by hypothetical particles called gravitons. These particles are the quantum representation of gravity waves, which are responsible for the non-wave-like curvature of spacetime. This concept is based on the particle-wave duality and is still a hypothesis in the realm of general relativity. However, it has proven to be a useful tool in understanding the force of gravity and its relationship to other fundamental forces, such as electromagnetism. Unlike other gauge theories, the force of gravity does not require the exchange of virtual particles to explain its interaction, but can be described through a potential.
Redi
How do gravitons mediate the force of gravitation in universe ?

Gravitons are hypothetical.
And "how" is not something physics can answer on a fundamental level - and that question is very fundamental.

Redi said:
How do gravitons mediate the force of gravitation in universe ?

Gravity is caused by the spacetime curvature. There are also waves of the spacetime curvature, the gravity waves. These are two forms of spacetime curvature. They are quite different.

The "gravity" (force-causing) curvature is non-wave-like, may be static and does not carry kinetic energy.
The "wave" curvature is never static, is wave-like (satisfies some wave equation) and does carry kinetic energy.

From the QM standpoint the gravity waves are some elementary particles. We call them gravitons. This is due to the particle-wave duality. Every particle is a wave, every wave is a particle. In case of gravitons this is of course a hypothesis, since we had to extrapolate QM to the GR realm and we don't know if QM still holds at that energy level.

Now, to your question. Mathematically, you can express non-wave-like curvature as a weighted sum of different gravity waves, at least to the first order. This is also true the other way - you can express a gravity wave as a sum of non-wave-like spacetime deformations, to the first order. This is a purely mathematical trick. But it QM it's a basis of some important theorems. We can deduce much about the gravitational force knowing it can be rewritten as a sum of gravitons. It doesn't mean that there are some actual gravitons flying here and there. It's just a part of QM formalism.

We don't know if the above construction is sound. Physicists invented it as an analogy to the quantum electromagnetism. This approach turned to be very useful. The force between charged particles, mediated by the electromagnetic field was expressed as sum of photons, the electromagnetic field quanta (waves). Maybe this is also the case with massive particles and the gravity field (the spacetime curvature).

For ordinary gauge theories like QED and QCD there are so-called physical gauges (e.g. Coulomb gauge) where gauge fixing = solving the Gauß constraint introduces a "potential". That means that to zeroth order no virtual particle exchange is required to explain the interaction. E.g. the el.-mag. force is described via a 1/r potential; photons are required for corrections only.

Yes, and in the original (1926) formulation of the quantum theory of radiation by Dirac, the electromagnetic field was separated into a radiation field and a static Coulomb interaction. The radiation field was then subjected to the usual quantum procedure, while the Coulomb interaction was treated as an unquantized classical interaction potential.

And this can be made exact, at least for abelian gauge theories; for non-abelian gauge theories the "potential" turns out to be a non-local, gauge-field dependent operator; anyway - it is NOT something like the exchange of perturbative virtual particles.

## 1. What are gravitons?

Gravitons are hypothetical particles that are believed to mediate the force of gravitation in the universe. They are predicted by the theory of quantum gravity, which attempts to reconcile the principles of quantum mechanics with those of general relativity.

## 2. How do gravitons mediate the force of gravitation?

Gravitons are thought to be constantly exchanged between particles with mass, creating a gravitational force between them. This exchange of gravitons is similar to how photons mediate the electromagnetic force between charged particles.

## 3. Are gravitons proven to exist?

No, gravitons have not yet been directly observed or detected. They are a theoretical concept that has not yet been experimentally confirmed. However, many scientists believe in their existence based on their importance in the theory of quantum gravity.

## 4. Can gravitons travel faster than the speed of light?

No, according to the theory of relativity, nothing can travel faster than the speed of light. This includes gravitons, which are predicted to have no mass and therefore must travel at the speed of light.

## 5. How do gravitons fit into our current understanding of the universe?

Gravitons are a key component in the theory of quantum gravity, which is still being developed and tested. They help explain the force of gravitation on a microscopic level and could potentially lead to a more complete understanding of the universe and its fundamental laws.

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