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Is the X17 predicted, or discovered? It depends upon what the meaning of the word "is", is. Prediction vs. Discovery In particle physics, phrases like "the Higgs boson" or "dark matter" actually stand in for mechanisms or effects, which could be realized in many different concrete models. And even when you specify a model, it's still ambiguous because you ...

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[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) ...

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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 ...

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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 ...

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The classical Coulomb potential can be recovered in the non-relativistic limit of the tree-level Feynman diagram between two charged particles. Applying the Born approximation to QM scattering, we find that the scattering amplitude for a process with interaction potential $V(x)$ is $$\mathcal{A}(\lvert p \rangle \to \lvert p'\rangle) - 1 = 2\pi \delta(E_p -... 23 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 the particle for gravity (called graviton) does exist, it's just they haven't found it yet. ... 22 Why can't light move electrically charged object[?] ... But then why can't one move charged objects by lighting them? The polarity of the electromagnetic wave associated with visible light changes hundreds of trillions of times per second. This is fast enough that a charged macroscopic object won't accelerate noticeably before the polarity changes and it ... 20 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 ... 19 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 ... 16 I want to clear this up: Electrically charged objects emit photons (by the way, why don't we see them?), and the exchange of these photons make unlike charges attract, like charges repel. Electrically macroscopic objects belong to the classical regime. Once one introduces photons, one is in the quantum mechanical regime, and needs quantum mechanical ... 15 The properties of the graviton include: it has spin 2 it is massless (so the force it carries is long range) it couples to the stress-energy tensor, which basically means everything should be able to emit and absorb gravitons it has zero electric charge and zero color charge These are extremely distinctive properties, so it's really unlikely that we've ... 14 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 All particles that are able to interact electromagnetically (not just electrons)can emit or absorb photons. So by definition if there was a graviton all particles that interact gravitationally could emit and absorb them. Your question gives the constraint of which particles with rest mass would emit/absorb gravitons. Mass is a measure of the particles ... 10 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, "... 9 I'm afraid the account you've read is very misleading, though it's just one of many such misleading accounts. Sadly the popular science press frequently mislead in this way. If we quantise gravity using the usual quantum field theory approach then we get a particle called the graviton. Roughly speaking the graviton is the quantum of the gravitational wave i.... 8 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, ... 8 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 ... 8 There is the (non-genetal) relation between the free energy of interacting of two currents J^{a}, J^{b} and the propagator:$$ U = -\frac{1}{2} \int d^{4}xd^{4}y J^{a}(x) D_{ab}(x - y)J^{b}(y). $$It's not general, but it realizes the simple example which can help you to understand how to get the expression for force. The structure of field which causes ... 8 Just thought I'd add some interesting examples to illustrate that light can, and does, in fact, move charged objects: The Photoelectric effect. For example: UV light shining on an aluminum plate. The UV photons carry so much energy that they "kick" electrons off" from the aluminum atoms. In a general scale, this is essentially called ionization. You can ... 7 You say: Gravity depends on mass but this is not so. The source of the gravitational field is an object called the stress-energy tensor. One element of this object is the energy density, and mass contributes to this through Einstein's well known equation E = mc^2, but mass is not required to generate a gravitational field. Even massless particles like ... 7 It depends on your definition of force. Force means a change in momentum, ~dp/dt , so any change in momentum in a Feynman diagram is a force. For example this diagram for compton scattering says yes. If one is talking of gauge theories and exchanged bosons , because those are the ones that build up the three, electromagnetic, weak, strong ( maybe four ... 7 There are multiple ways to interpret Coulomb's law in quantum electrodynamics (QED). Interestingly, they don't lead to quite the same conclusion (but there is no inconsistency because they are not defined in the same way). The most commonly used way (that ACuriousMind refers to in his comment) consists in relating the notion of classical potential with that ... 7 I think that the only honest answer to this "how" question is (for the electromagnetic interaction -- the strong and weak interactions are analogous) with a mathematical formula:$$\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 ...

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Elementary particle interaction crossections and lifetimes are calculated in perturbative expansions that are set up using Feynman integrals. The wavy line represents the exchanged boson , in this case a photon which has zero mass. The wavy line in the integral is the propagator of the electromagnetic interaction in this case. In momentum space the ...

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The situation is not symmetric at all: This diagram describes a force between two fermions, but a diagram such as just doesn't exist (in the Standard Model). Fermions can in fact mediate a force between bosons, like in: Such diagrams are highly suppressed loop diagrams though, and the one above would after renormalization be seen as just one contribution ...

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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 \$\...

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