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So I was thinking about how a positive and a negative charge (or positive/positive, negative/negative) interact. I have read previously about how photons carry the electromagnetic force. However, how does this occur exactly?

How does a photon compel a particle to move towards/away from another? At the very least, if we say that a positive charge emits photons and a negative one absorbs them (seems wrong, but I guess that's what happens?), then wouldn't the negative one be repelled away instead?

Similarly, for gravitation, I understand that it's the curvature of spacetime that results in the "force". However, what compels a particle to move down the curvature of spacetime and not just stay where it is? In a regular rubber sheet situation, it is gravity that pulls an object down a curve. However, in spacetime there's no such equivalent.

Any help would be greatly appreciated!

Qmechanic
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ashiswin
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  • https://chat.stackexchange.com/transcript/message/50969499#50969499 refer this from beginning.I hope this helps you a bit. – P-S-S Jul 21 '19 at 03:36
  • Possible duplicate of How are fundamental forces transmitted? – BioPhysicist Jul 21 '19 at 04:28
  • The idea that particles move “down” the curvature of spacetime is wrong. Freely-falling particles (i.e., those with no non-gravitational forces acting on them) simply move “straight ahead” through curved spacetime. It’s like “Newton’s First Law” but in curved spacetime. – G. Smith Jul 21 '19 at 05:33
  • The idea that photons are only emitted by positive charges and only absorbed by negative ones is wrong. Repulsion of like-sign charges is also mediated by photons. Both positive and negative charges can emit and absorb photons. – G. Smith Jul 21 '19 at 05:34
  • How does a photon compel a particle to move towards/away from another? By transporting four-momentum between the two charged particles. – G. Smith Jul 21 '19 at 05:39
  • @G.Smith, so the photon itself would not possess the momentum? Like does it carry four momentum on top of its own momentum? Otherwise wouldn't it cause the particle it impacts to move away from the original particle? – ashiswin Jul 21 '19 at 05:49
  • No, I’m talking about the photon’s own four-momentum. The four-momentum of virtual particles can be weird. (Virtual particles are “off-mass-shell”.) So you can’t use your classical intuition about how the photon’s momentum relates to its direction of motion. You can’t even say whether the photon went from particle 1 to particle 2 or vice versa. But four-momentum is conserved in the Feynman diagram for the interaction. – G. Smith Jul 21 '19 at 05:54

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To properly understand electromagnetic interactions, you need to study quantum electrodynamics (QED). In that case, the easiest way to think about it is in terms of Feynman diagrams. It is not a physical photon that mediates the force, but rather a virtual photon. The difference between physical and virtual photon can only really be appreciated by studying the full mathematical formalism of QED. The idea of macroscopic particles being attracted or repelled from each other is a difficult concept to explain. This is because the transition from the quantum field theory description to the macroscopic world is poorly understood. For an especially easy and intuitive explanation of why charged particles are attracted or repelled from each other see the textbook "Student Friendly Quantum Field Theory" by Robert D. Klauber. It contains a section addressing this very question. Here is a short extract from that section:

Feel My Black Hole
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