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44

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 ...


40

[Edit June 2, 2016: A significantly updated version of the material below can be found in the two articles https://www.physicsforums.com/insights/misconceptions-virtual-particles/ and https://www.physicsforums.com/insights/physics-virtual-particles/ ] Let me give a second, more technical answer. Observable particles. In QFT, observable (hence real) ...


36

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 ...


22

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 gravity,...


20

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 ...


13

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 (...


13

The other answers explain that there's no paradox but they don't explain why the particular particle called photon is massless. It's massless because it is the messenger particle responsible for electromagnetism which is a long-range force. Its range is infinite so the mass has to be zero. One may view the Coulomb potential as the zero-mass limit ($m\to 0$) ...


13

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 ...


13

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 ...


12

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. ...


9

Virtual particles are not real. They come, as I've said in many answers on this site, from a naive interpretation of Feynman diagrams which should not be taken as an actual, exact description of how the physics works. The actual description of an interaction in the quantum field theory is more complicated than "photons are exchanged". In particular, "...


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 ...


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 ...


7

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, ...


7

I think that the only honest answer to this "how" question is (for the electromagnetic interaction -- the strong and weak interactions are analogous) with a mathematical formula: $$\mathcal{L}_\text{int} = e\overline{\psi} \gamma^\mu A_\mu \psi.$$ This is the term that we add to our model to describe the interaction of electrons ($\overline\psi,\psi$) with ...


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 ...


6

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 ...


6

When e.g. a neutron decays, there is no "real" W-boson inside, in the sense that it could be detected at every point. Instead, the decay of the neutron involves a "virtual" W-boson, a W-boson that only exists for a very short time. Quantum mechanics allows the energy conservation law to be violated by $\Delta E$ for a very short time $\Delta t$ as long as $\...


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 ...


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

I think this would be tricky, since any force mediator (at least from conventional thinking) must have a three-valent vertex, two of which are the charged object and one of them is the force carrier. If the force carrier is a fermion, I don't think this combination can be Lorentz invariant (spin zero combination).


5

To add to the other answer: Your intuition is right in a way. The fact that the $W$ and $Z$ bosons are so heavy is the reason for the weakness of the interaction. For example, $\pi^+$ mesons can decay over the weak interaction, the process is described by the following Feynman diagram: According to the Feynman rules, the probability amplitude of such a ...


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

There is a simple way to see that, without no much mathematics. Fundamental matter particles are spin one-half fermions (neutrinos, electrons, quarks). Each particle corresponds to several degrees of freedom, say $2$. Now, let us see these 2 degrees of freedom as a complex $2$-"vector" (in fact, it is not a Lorentz vector, it is a Weyl spinor, but this is ...


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 ...



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