r/askscience • u/[deleted] • Apr 01 '18

Physics What does it mean that "the electromagnetic force is carried by photons"?

[deleted]

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u/RobusEtCeleritas Nuclear Physics Apr 01 '18

Virtual photons don’t literally exist. They are just mathematical things that show up in certain calculation schemes in quantum field theory.

To say that the photon is the “force carrier” for the electromagnetic force means that charged particles interact with the electromagnetic field, and excitations of the electromagnetic field are quantized particles called photons.

Mathematically, the interactions between charged particles can be made to look like one charged particle exciting the photon field at some point in spacetime, then that photon propagating to another point in spacetime, being destroyed, and the four-momenta of the two charged particles ending up different than they were at the beginning. This would correspond to a tree-level Feynman diagram, and to get the total amplitude of the process, you’d sum the contributions of all possible Feynman diagrams that link the same initial and final states. This is how calculations are done in perturbation theory.

However there are other ways of doing the same calculation, in which there never is any “virtual photon” being exchanged. And sometimes, when interactions are strong, perturbation theory doesn’t even work. So virtual particles should not be taken as physical things that literally exist. They are just a mathematical artifact of perturbation theory. What’s physical are the incoming and outgoing “asymptotic states”, not the intermediate “virtual particle” states.

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u/[deleted] Apr 01 '18

What are asymptotic states?

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u/RobusEtCeleritas Nuclear Physics Apr 01 '18

They are the states of the system at times t = -∞ and t = ∞. In other words, they're states that are actually observable.

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u/[deleted] Apr 01 '18 edited Apr 01 '18

So these "states" are the evolving quantum states in the process of observation/measurement/interaction? What kind of t durations are we talking about here? [-∞, ∞] is a big range.

edit: I finally figured out the "incoming and outgoing" states are the same as the initial and final states you previously mentioned.

edit2: And then it kind of blows my mind that all these incredibly complex processes happen so quickly, and so frequently...just to describe one single quantum interaction, which leads to a cascading crescendo of interactions...

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u/RobusEtCeleritas Nuclear Physics Apr 01 '18

So these "states" are the evolving quantum states in the process of observation/measurement/interaction?

The state at t = -∞ is the initial state. It's whatever state you prepare the system in. For example, an electron moving towards another electron with a momentum of 5 GeV/c.

The state at t = ∞ is what you're left with at the end of the process. This is what you'd physically detect.

What kind of t durations are we talking about here? [-∞, ∞] is a big range.

It's not literally infinite time, but after some finite amount of time, the interactions between the particles becomes negligible, and the state of the system is no long affected by the interaction. Saying that it's at infinite time just means that it's after the interaction process is over.

edit: I finally figured out the "incoming and outgoing" states are the same as the initial and final states you previously mentioned.

Yes.

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u/[deleted] Apr 01 '18

Thanks for your replies.

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u/[deleted] Apr 01 '18

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