Antimatter and Virtual Photons
"What is antimatter?"
Antimatter is nothing extraordinary. The name has a historical reason. For ages, people were calling stuff around them "matter". Then in the thirties and forties of the 20 century, they realize, that in our world, a different type of matter exists, which is very very similar to the usual one, but not exactly the same. Scientist discovered, that to all known particles, there exists a partner, which has certain properties exactly the same as the original particle ( mass, spin, ... ) but some of the properties are different ( charge, isospin ... ) Since all of the particles have this partner, in principle there could be another world made from them. They started to call the partners ANTIPARTICLES or ANTIMATTER. To answer what an antiparticle is, one needs to go to the fundamental ideas of the theory, which describes the world of particles. This theory is called Quantum Field Theory (QFT). It defines in its basic, what we mean by a particle. ( Particle are represented as certain "quantized fields". For example a photon is a quantum of the electromagnetic field. Do not worry if you do not recall what a field or quantization is, it is not necessary for what I want to say. The bottom line is that QFT gives us the definition of particles and describes their properties.) When you introduce particles, you realize, that for every particle you automatically introduce its partner, the antiparticle. And this fact leads us to your next question:
"Are there other properties/quantum numbers besides the charge that are different?"
The answer to your question is , YES, there are other properties that are different. As an example I will mention the charge, the isospin projection to a given axis, lepton number, baryon number, strangeness, internal parity ( in case of the spin 1/2 particles called fermions) and many others. And of course, there are many, that stay the same, mass, spin, isospin, lifetime ... .
"Why is the charge the key parameter?"
As you see, the charge is not the key parameter, it is just the most common and most known quantity.
"Why do particle-antiparticle annihilate when they meet?"
Now we are ready to answer this question as well. I mentioned, that the Quantum Field Theory defines what a particle is. There are many QFTs. However, our world seems to be described by one, which is called The Standard Model. This theory is the most successful and most precise theory that we have ever had. It gives as certain predictions which are studied and verified in High-energy laboratories, such as Fermilab. This theory tells us everything about the interactions of elementary particles. One of its results is, that a free neutron decays into a proton electron and an electron-antineutrino. Another result is the calculation of the magnetic moment of the electron with a precision of 12 decimal places. It also describes, what kind of processes (reactions) occur, when a particle meets its antipartner. In some cases this particle antiparticle pair annihilates ( creates a photon pair) , in other cases they create a Z^0 boson, which then decays according to the rules of the Standard Model. So, the bottom line is, that we can answer the question, what will happen if a particle meets its antipartner, but a question WHY they annihilate is not answerable, or the answer would be, because Nature dictates it :-)
OK. Enough of antimatter, let us talk about virtual photons.
"Why is that we cannot see the photons that mediate the electromagnetic force?"
The answer to this questions turns out to be simpler than one would actually expect. We cannot see the virtual photons because THEY ARE VIRTUAL and not real!!! They cannot get to our eyes. We cannot measure them. They are internal properties of quantum systems. They are not necessarily massless, they do not have to obey conservation of energy for a short time period ( related to the Heisenberg relation). They exist for very very short time. In the concept of Feynman diagrams, only particles which are represented by external legs are measurable, visible or detectable. It is because it is the external leg which interacts with our apparatus, eyes ... . For instance, let us take a scattering of an electron on a positron (antielectron). It is described by the Feynman diagram shown on the picture.
The solid lines are the electrons, the curled line is the exchanged virtual photon. This diagram has 2 incoming particles e+e-( the incoming legs), and 2 outgoing particles e+e- ( the outgoing legs) All of them could be ( at least in principle involved in future/past processes, because they have free "leg ends" which can interact. However the internal, virtual photon has both of its ends used up on the electron system, therefore that is it for our photon. It cannot interact with anything else, not with our eyes, not with our detectors, ... . That is why we cannot see them. Hopefully my answers helped you to view your questions from a different side. Please read physics and enjoy thinking. If we can help you with something else, please let us know.
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