Gravitons are supposed to mediate the force of gravity, but wouldn't that require Earth, Sun and basically everything else is constantly sending out gravitons to everything else?
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6$\begingroup$ The idea of particle interactions as "two particles exchanging virtual particles" should not be interpreted literally. $\endgroup$– CharlieCommented Aug 28, 2021 at 10:03
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$\begingroup$ The electric field due to a point charge can be Fourier decomposed into many photons so surely gravity should be similar. There are examples where the particle exchange analogy is unhelpful, but I don't think this is one of them. $\endgroup$– Connor BehanCommented Aug 28, 2021 at 11:28
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$\begingroup$ @Charlie But if the exchange of particles isn't real what causes gravity? I am a layman so sorry for my confusion. $\endgroup$– user311753Commented Aug 28, 2021 at 13:42
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1$\begingroup$ We don't know what "causes gravity" because different theories of gravity can involve vastly different mathematical structure. The "virtual particle exchange" is akin to a mnemonic in QFT for writing large cumbersome mathematical expressions, however in general relativity there is no suggestion of virtual particles. So even though both QFT and GR are very predictive models, they have very different internal structure and this internal structure doesn't really have a physical interpretation (usually). $\endgroup$– CharlieCommented Aug 28, 2021 at 13:59
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$\begingroup$ You may find this answer about virtual photons helpful: physics.stackexchange.com/a/83415/123208 But also see physics.stackexchange.com/a/275099/123208 $\endgroup$– PM 2RingCommented Aug 28, 2021 at 16:37
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As Charlie said, interactions are not an exchange of real particles, and neither even an exchange of virtual particles, since the latter are in fact mathematical artifacts of quantum field theory. You can see gravitons as elementary perturbations/excitations of the metric field when it is treated perturbatively: \begin{equation} \underbrace{g_{\mu \nu}}_{\text{general metric}}=\underbrace{\eta_{\mu \nu}}_{\text{Minkowski metric}}+\underbrace{h_{\mu \nu}}_{\text{perturbation}}. \end{equation} Since gravitons are linked to the concept of metric, they are also linked to the concept of curvature of space-time. This is why one often read or hear that gravitons are quanta of the curvature of space-time. An interaction in QFT and QFT in curved space-time is nothing but a way the gauge fields are curved, leading the matter field perturbations to move accordingly. This is the same with gravitation, but the "field is curved" part is to be understood literally.
answered Aug 28, 2021 at 11:20
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$\begingroup$ So the basic particle interactions are nothing more than mathematical artifacts/models, and their physical interpretation is an exchange of virtual particles? Is this the reason why we cannot detect virtual particles directly in the first place? $\endgroup$– TachyonCommented Aug 28, 2021 at 11:28
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1$\begingroup$ @Tachyon This is an interesting point, you are right to ask this question! What we call "virtual particles" are the internal lines of Feynman diagrams, so these are really just mathematics because Feynman diagrams represent the amplitude matrix, which comes from the $S$ matrix. But what we call virtual particles when we say we can not detect them, is just a consequence of the uncertainty principle. The wikipedia entry is quite good at explaining this. "Virtual particles" is a loosely defined term and is in fact plural. $\endgroup$ Commented Aug 28, 2021 at 12:50
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$\begingroup$ "we say we can not detect them, is just a consequence of the uncertainty principle." - You mean that we CAN detect them but only their complementary properties such as position or momentum? That is the uncertainty principle, right? Then they would be detectable? However, I have heard that experimentalists cannot detect virtual particles. Then why would you say that it is a consequence of the uncertainty principle if they are undetectable? $\endgroup$– TachyonCommented Aug 28, 2021 at 16:49
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1$\begingroup$ I mean that these virtual particles can be thanks to the uncertainty principle $\Delta E\Delta t \geq \frac{1}{2} \hbar$: you cannot say you are in presence of a perfect vacuum (in the sense of perfectly 0 energy, 0 particles) if you observe the system within a sufficiently small time frame. This is the usual explanation of virtual particles in the sense of something real but undetectable. You can interpret them as fluctuations of the quantum fields. $\endgroup$ Commented Aug 28, 2021 at 18:08
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$\begingroup$ So, Earth and Sun are not exchanging gravitons but they curve Gravitational field and this creates gravity? $\endgroup$– user311753Commented Aug 29, 2021 at 9:54