Physics at large assumes that all matter, even at the subatomic level, interacts gravitationally. Prevailing theory is that even though two lone protons have an incredibly strong electrostatic repulsion, they also at the same time have an incredibly weak gravitational attraction. Whether this has been experimentally verified with certainty or is just taken for granted, I don’t know. It kind of makes me wonder: how much about particle interaction has been taken for granted?

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