How is this possible if quarks are point particles?
Think: how can the solar system have a "diameter" ?
You set it as the diameter of a volume that encloses all the solar system objects right?
Same with a nucleus.
Nuclei are not just composed of quarks though - but also of the strong nuclear force.
But what it boils down to is the definition of "size" here.
Clearly a nuclei has a size in the sense of a limited extent - or else how do you tell two nuclei apart.
Clearly the size is not zero since some objects fired at nuclei hit it.
You measure the size of stuff by scattering other stuff off it and making observations about the scattering.
i.e. when you measure the size of a ball, you are scattering light off it (detected by your eyes) and comparing the scatter pattern (image) with that of some standard object (ruler).
Nuclei scatter objects, and from the pattern we can assign a size and thus a diameter.
But bear in mind that the diameter of such small objects is a statistical average rather than a well-defined boundary.
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