Equivalence principle: 1 in 10^15
The equality of inertial mass and gravitational mass — Einstein's central clue for general relativity — has been confirmed experimentally to 1 part in 10^15.
Snapshot · Dwarkesh Podcast
Adam Brown – A deep but accessible introduction to general relativity
Explore episode
Jul 10, 2026
Where this was said
The coincidence that led Einstein to general relativity
At 14:30 · chapter starts 0:00
Brown opens his mini-lecture with Newton's second law (F = ma), his first law (zero force means straight-line motion), and then his gravitational force law: F = GMm/r². He flags the immediate problem — Newton's law implies that jiggling the Sun would instantaneously change the gravitational force felt at Earth, violating the finiteness of the speed of light. He draws a historical parallel to the electrostatic force law, which faced the same problem and was resolved by Maxwell's full theory of electromagnetism. That success story sets up the hope — and the difficulty — of doing something analogous for gravity.
Einstein's decade-long pursuit
Einstein took roughly 10 years of dogged pursuit from his 'happiest thought' in 1907 to writing down the final form of general relativity in 1915.
Why You Can't Do the Same Trick for Gravity as for Electromagnetism
Electrostatics was upgraded into a relativistically consistent Maxwell theory, and you'd think you could do the same for gravity. But there's a sign flip: like charges repel, like masses attract. Do the same math and you get a theory where gravity repels. That forced Einstein down a completely different path.
Newton's Embarrassing Coincidence
In Newtonian physics it's just a coincidence that inertial mass and gravitational mass are identical. But we've tested it to 1 part in 10^15. Einstein refused to accept a coincidence that precise. That refusal became the core of general relativity.
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