Eli5 what is anti matter?
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To add to the other answers: Antimatter is actually inevitable if you assume just a few basic principles:
1. the existence of particles that can scatter and interact with each other
2. the existence of some charge that can be exchanged during such an interaction and that is conserved by the interaction. Meaning that total amount of charge before and after the interaction is constant.
3. the uncertainty principle, you cannot know the position and momentum of a particle at the same time
4. locality, all physical processes happen due to interactions at single points, as opposed to over a distance. If you want to act over a distance a particle has to be exchanged
5. special relativity, more specifically, we need “[relativity of simultaneity](https://en.wikipedia.org/wiki/Relativity_of_simultaneity)”, which describes the fact that 2 observers can disagree about the order of events, as long as the events happen far from each other.
Now to understand how antimatter comes about take a look at a completely hypothetical scattering event.
Incoming are two particles A,B. Assume particle A carries one of our hypthetical charges and B doesn’t (“is neutral”). They exchange an intermediary particle C which carries the charge from A to B, so that after wards A is neutral and B is charged.
There are two events to consider: first, A emits a charge-carrier C and secondly, particle B absorbs it. Due to the uncertainty principle these 2 events can be far away from each other. In fact and this is important (!): they can be so far away that by relativity of simultaneity the order of these two events is swapped for another observer.
This second observer will interpret what he sees another way: A charge carrying particle flies off from the initially neutral B and after that it arrives at the positively charged A, changing A to neutral.
In the first case (A->C->B), the charge carrier C has to have positive charge which he carries to B. Whereas in the second case, (B->C->A) the charge carrier has to carry negative charge! The second observer sees the anti-matter version of the exchange particle!
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