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So it is said that particles are excitations of fields. We are given examples like photons for electromagnetic field, gravitons for gravity, etc. We are also told that normal matter is an excitation. However the former are all virtual particles that can never be detected. So how are particles like protons that can be detected on the same footing?

Qmechanic
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user43470
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    why do you say that photons are virtual and cannot be detected? I detect them with my bare eyes (in fact they are the only thing that I detect with my eyes) –  Jan 27 '20 at 16:03
  • I said that because when the positive end of a magnet attracts the negative end of another you don't see the photons. – user43470 Jan 27 '20 at 16:07
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    Maybe I don't understand what you mean by "see". Do you mean see in your bare eyes? because then indeed the photons emitted by the end of the magnet are not visible, but that is because they are not in a wave-length that your eye can react with. There is a very simple experimental apparatus that detects them - the magnet itself! the fact that it is attracted is a direct observation of the photons. there are other ways to "see" them, of course –  Jan 27 '20 at 16:11
  • Omg! I was under the impression that nothing could detect it! So my question is quite baseless considering what you've said. However one thing. What is the field that the matter particles are an excitation of? – user43470 Jan 27 '20 at 16:13
  • see replies here on detecting individual photons: https://physics.stackexchange.com/questions/102313/how-does-one-detect-a-single-photon regarding the field - the matter is an excitation of relevant fermionic fields https://en.wikipedia.org/wiki/Fermionic_field –  Jan 27 '20 at 16:20
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    This might be helpful: https://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/ –  Jan 27 '20 at 16:33
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    Every particle, massive or not, is associated with a quantum field. Electrons are modes of the electron field, etc... – Cosmas Zachos Jan 27 '20 at 17:14
  • Alright. But I'm still confused. When we talk about fields we think of things that cause force. What force does the electron field cause? The only thing is seems to do is cause disturbance in the electromagnetic field. Does that mean that electron field and EM field is the same? – user43470 Jan 27 '20 at 17:35
  • No, they're different fields. A photon is an excitation of the electromagnetic field, an electron (or positron) is an excitation of the electron field. – PM 2Ring Jan 27 '20 at 18:08

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You are confused because of the difference between static EM fields and EM radiation.

An electromagnetic field (also EM field) is a classical (i.e. non-quantum) field produced by moving electric charges.[1] It is the field described by classical electrodynamics and is the classical counterpart to the quantized electromagnetic field tensor in quantum electrodynamics.

https://en.wikipedia.org/wiki/Electromagnetic_field

Now in your example, the magnets' static EM fields are what are represented by these virtual photons. These virtual photons are not real, they are just a mathematical description of a phenomenon called the static EM field. In reality we do not really know how these static fields affect each other (or other particles), we just see that they do in experiments. We describe this with virtual particles.

In physics, electromagnetic radiation (EM radiation or EMR) refers to the waves (or their quanta, photons) of the electromagnetic field, propagating (radiating) through space, carrying electromagnetic radiant energy.[1] It includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.[2]

https://en.wikipedia.org/wiki/Electromagnetic_radiation

Now EM waves are what are made up (QFT) of real photons, the very quanta of light. These photons can be detected in experiments.

Árpád Szendrei
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  • Yes but why are the fields different? Why does the EM field produce force while the electron field does not? – user43470 Jan 28 '20 at 01:47
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    @user43470 what you are calling force, is the static field (mediated by virtual particles), the static field of the magnet in your case is what represents the force. The electron does have a static field. You are confusing the word field. In QFT, fields are everywhere, and all particles are just excitation of the fields. These fields are what we call far fields. What I am talking about, the static field is a near field. https://en.wikipedia.org/wiki/Near_and_far_field – Árpád Szendrei Jan 28 '20 at 04:53
  • okay so are there forces mediated by virtual particles in the electron field? Because when we talk about the EM field we have interactions between objects with different charges and all. I haven't heard of any interactions happening with electrons intrinsically to the electron field. Am I missing something? – user43470 Jan 28 '20 at 09:16
  • @user43470 when you have two electrons, they do have their own near field (static), and these charges repel, this repulsion is caused by the static field, and this is what we mathematically model with virtual photons. – Árpád Szendrei Jan 28 '20 at 16:05
  • Sorry, didn't you say that that field between two electrons was the near field of the electromagnetic field? If not what would you call the near field of the EM field? – user43470 Jan 28 '20 at 16:10
  • @user43470 when you are talking about interactions, like the repulsion of two electrons, that we model with virtual photons, that is the near field of the EM field. Because photons are excitation of the EM field. When you say that the electron itself is an excitation, then we say that the electron is the excitation of the electron field. The electron field and the EM field can interact too, for example when the electron emits a real photon, and the energy gets transferred to the EM field, and then another electron absorbs it, the energy returns to the electron field. – Árpád Szendrei Jan 28 '20 at 16:32
  • Alright. I've understood so far. However I still have a doubt. What is the near field of the electron field? We have interactions between electrons which comes under the EM field. So what are considered as interactions in the electron field? And what is the corresponding virtual particle associated with it? Or are you saying that the interaction between two electrons is an interaction between the electron field and the EM field? – user43470 Jan 28 '20 at 16:47
  • @user43470 The interaction between two electrons is mediated by what we model with virtual photons. Photons are the excitation of the EM field. This interaction is mediated by virtual photons. The interaction means that the energy is transferred through the EM field (photons). – Árpád Szendrei Jan 28 '20 at 21:34