Naruto: From an Uchiha Native to the God of Heaven
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Not all gauge bosons (mediator particles) were stipulated by relativity to have zero rest mass. Among these fundamental forces, the weak interaction was rather unusual, but it was too complicated to have any real bearing on the current question.
Setting aside how the other party had managed to confine light, massless matter alone was already highly counterintuitive.
It simply could not be observed.
So no one knew what a real photon's rest mass actually was, but based on existing theory, they could only define it as zero.
Why could massless matter not be observed...?
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