Our hopes of finding a theory of everything depend on upsetting a balance that Einstein cherished
Coincidence is not generally something scientists have much truck with. If two things are genuinely unrelated, there is little further of interest to be said. If the coincidence keeps turning up, however, there must be some deeper underlying link. Then it is the job of science to tease out what it is and so explain why there was no coincidence in the first place.
That makes it rather odd that a large chunk of modern physics is precariously balanced on a whopping coincidence.
This coincidence is essential to the way we view and define mass. It is so fundamental to the world's workings that most of us encounter its consequences every day without giving them another thought. Yet it has vexed some of the best minds in physics for centuries. Galileo and Newton grappled with it, and ended up just accepting it, rather than understanding it. Einstein went one better: he declared it a principle of nature. He went on to use this "equivalence principle" as the fundament of his general theory of relativity, still our best stab at explaining the mysterious force of gravity.
But there is a problem. If we want to find some bigger, better theory that can unify gravity with the other forces that dictate the world's workings, the equivalence principle cannot stay. We must either unmask this coincidence - or radically rethink how physics can progress from here.
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