# Tag Info

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You're quite right that the other fundamental forces of Nature possess mediator particles, e.g. the photon for the electromagnetic force. For gravity, a graviton particle has been postulated, and is included in the five standard string theories which are candidates for quantum gravity. From a quantum field theory perspective, the graviton arises as an ...

25

Let me give a second, more technical answer. Observable particles. In QFT, observable (hence real) particles of mass $m$ are conventionally defined as being associated with poles of the S-matrix at energy $E=mc^2$ in the rest frame of the system (Peskin/Schroeder, An introduction to QFT, p.236). If the pole is at a real energy, the mass is real and the ...

25

The short answer for why gravity is unique is that it is the theory of a massless, spin-2 field. To contrast with the other forces, the strong, weak and electromagnetic forces are all theories of spin-1 particles. Although it's not immediately obvious, this property alone basically fixes all of the essential features of gravity. To begin with, the fact ...

20

Since you don't fully understand the answer of JamalS, I'll try to explain it shorter and easier for you. If all other forces of nature have some particles associated with them why should gravity be an exception? No, it isn't an exception. Physicists believe that they do appear, it's just they haven't found it yet. Standard Model doesn't have ...

19

Brief answer: Read only the bold part (and ignore grammar then). The answer you already mentioned lies in Quantum Field Theory (QFT). But to fully understand it, you must give up a particle as a point-like thing that is well-localized. There is one Quantum Field per sort of particle, e.g. the electron field for all electrons, and the photon field for all ...

11

Gravitons are the particles you get from quantizing General Relativity. Since we don't know yet how to correctly quantize GR (or whether trying to quantize it is actually the right way to go forward; for all we know it might just be an effective theory where the more fundamental theory has to be quantized instead), we cannot know for sure whether the ...

11

I wonder whether graviton is indeed hypothetical or does its existence directly follow from modern physics? At the moment we believe that at the micro level the underlying framework of nature is quantum mechanical and from that level the classical mechanics, thermodynamics and electromagnetism emerge. "Believe" means physicists have gathered an ...

10

I don't think the other answers have clearly called out that we do not know. Yes, we do have the (rather wonderful) theory of general relativity (GR), which does an excellent job of explaining the effect of gravity. It does this by relating the presence of mass (strictly "stress-energy") to the structure of space-time. It also states how that effect ...

10

That's an interesting question, even though it might be biased by the definition of forces, and on what particles they apply. For instance, if you want to describe the force that exists between photon (even though direct photon-photon scattering has not been observed yet), it is mainly due to electron loops, so in that case the `force' is fermionic. On a ...

8

The simplest Feynman diagram for an interaction between two particles looks like a letter "H". The cross-bar is a force-carrier being exchanged. At each vertex, you have a particle either emitting or absorbing a force-carrier. If the force-carrier has a half-integer spin, then you can't emit or absorb it without violating conservation of angular momentum. ...

8

For example, how do two charged particles know that they are to move apart from each other? Do they communicate with each other somehow through some means? Yes, specifically the electromagnetic field. To give a simplistic view, a charged particle produces an electric field to indicate its presence and a magnetic field to indicate its motion. Any ...

8

When you ask "Why is gravity such a unique force?" then you should know that in the framework of General Relativity gravity is not a force at all. In General Relativity energy (for example the mass of an object) cause curvature. The movement of other objects is influenced by this curvature - they travel along the path of shortest distance between two points ...

8

Gravitons are hypothetical, but they're far less hypothetical than most of the other particles which theorists speculate about (such as axions, magnetic monopoles, strings, sterile neutrinos, and the like). That probably sounds a little strange. Let me explain. We don't have a complete theory of quantum gravity. But we do actually have an extremely ...

7

An addendum to the answers of Daniel Grumiller and sb1: The major difference of the gravitational field and other fields is that according to general relativity the gravitational field defines space and time and therefore defines the relation of events. It is true that it is possible to do an "arbitrary" split of a certain linear approximation of the ...

6

Photons are force carrying bosons and come in both virtual and real varieties. There is nothing wrong with that. Virtual means off-shell, and real means on-shell. Even on-shell weak bosons decay very quickly, however, because there are plenty of modes with the right quantum numbers and much lower total mass (and thus lots of phase space). I want to ...

6

All observed particles are real particles in the sense that, unlike virtual particles, their properties are verifiable by experiment. In particular, W and Z bosons are real but unstable particles at energies above the energy equivalent of their rest mass. They also arise as unobservable virtual particles in scattering processing exchanging a W or Z boson, ...

6

A graviton is a theorised particle (specifically a boson). It is said to be its own antiparticle. It has a special role in general relativity (being that the science is all about gravitation and the warping of space/time). In relation to your mention of string theory, it is said to be a closed string with a very low-frequency vibration. Closed strings are ...

5

In quantum field theories, interactions are exerted by the exchange of a force carrier particle. For the electromagnetic force, this is the photon, for the strong force, the gluon and for the weak force, the W and Z bosons. All these force carrying particles have been observed. Now it is imagined, that if we are able to find a quantized version of gravity, ...

5

The short answer in quantum field theories is "by the exchange of virtual particles". Look up "virtual particles" on Wikipedia to get a sense of it. The ancillary questions you asked about the energy budget, and the derivation of Coulomb's law are very much the right ones. They should all be answered there. For example, energy is conserved except for ...

4

You say: Gravity depends on mass but this is not so. The source of the gravitational field is an object called the stress-energy tensor. One element of this object is the energy density, and mass contributes to this through Einstein's well known equation $E = mc^2$, but mass is not required to generate a gravitational field. Even massless particles ...

4

These are just my thoughts as someone who studied the subject for a while: The concept of virtual photons that mediate interaction should not be seen as "what really happens". A virtual photon is not a real object (hence the name "virtual"), but an artifact of perturbation theory. If we knew an effective way (or even "a" way) to do the calculations without ...

4

The difference between the Higgs boson and the bosons of the three/four fundamental (depending whether you include gravity as a quantized theory or not) actions is that the latter are associated with gauge symmetries, while the Higgs plays a role in spontaneous symmetry breaking. Photons, W- and Z-bosons, gluons and gravitons arise from the requirement that ...

3

The other forces are also just the result of "spacetime bending", just in a different way. There is no fundamental difference in the description of the other forces through gauge theories and gravity through relativity.1 The reason why it is often said that it is different is that our usual methods of quantizing a theory fail when applied to gravity. But to ...

3

That's a very nice answer by ACuriousMind. I would like to add something, though. GR is actually not like other gauge theories in some of its aspects (apart from having lots of similarities). For starters, it is background-independent and highly non-linear. In ordinary QFT we usually deal with perturbative expansions, which make sence only for weak-coupled ...

3

It depends on your definition of force. Force means a change in momentum, ~dp/dt , so any change in momentum in a Feynman diagram is a force. For example this diagram for compton scattering says yes. If one is talking of gauge theories and exchanged bosons , because those are the ones that build up the three, electromagnetic, weak, strong ( maybe ...

3

Let's look theoretically to your question. Let's introduce linearized GR and then let's derive the wave equations. It is exactly the second Bianchi equation for the Weyl tensor. We want to associate some particle to the gravity wave (only for linearized gravity limit). For associating we must do at least two things: 1) Show that equation for hypothetical ...

3

Gravitons are hypothesised to be the mediating particle for the gravitational force. Even if Gravity appears to be the curvature of spacetime rather than an actual force, a "graviton" is still needed as means of communication of matter/energy to "tell" spacetime to curve. Gravitons afaik are hypothesised rather than predicted (might be wrong here) due to ...

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