How does the interaction between two particles happen?

Imagine a physical theory that suddenly appears to be contradicted by facts, measurements, experimental results or observations. Does it necessarily have to be abandoned? No, because its true implications may have simply been misunderstood. When this is the case, radical reconsiderations of what previously seemed crystalline become possible: a new perspective opens up, condemning a part of the old world to death. For example, a few years ago, suddenly, due to equations disturbing him from some kind of backward world, the old notion of mass, apparently so familiar, so clear, so well understood, lost its obvious hints.

In our mind, the notions of mass and matter are linked. They are even entangled with each other: we cannot conceive of a material thing that would be massless and, symmetrically, we struggle to imagine a mass that would not be embodied in material things, whether small or large. The mass thus appears as an intrinsic property of material objects: in short, whoever says matter says mass and whoever says mass says matter.

What could you possibly find fault with in all this? Apparently, nothing. But on examination, many things. Because what particle physicists now know is that instead of being a primitive property of so-called “elementary” particles, a property they would “by themselves” possess; simply because they are made of matter, mass appears to be only a secondary property of elementary particles, an indirect property resulting from the interaction of these particles with … The vacuum! Void, which, in fact, is not empty, but inhabited: it contains in particular what generates the mass of elementary particles.

What quantum physics tells us

It is that every interaction is a barter: two particles interact only if they exchange “something”, and this something is a characteristic particle of the interaction involved. In other words, an interaction does not take place between two particles only for the exchange of a third particle (…).

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