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In a German interview some physicists were asked, what they would ask Einstein, if he were alive today. One of them wanted to know how Einstein came up with the idea of the equivalence principle, that inertial mass is the same as gravitational mass. Correct me if I'm wrong, but I always believed that was known way before Einstein. And I have an experiment in mind where one famous guy standing on a leaning tower and letting two things fall with different mass to test if gravity acts on every object the same.

My question is, does somebody know the name of this famous person, who conducted the experiment? I can't remember it.

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    $\begingroup$ Galileo Galilei, see en.wikipedia.org/wiki/Galileo_Galilei $\endgroup$ – hyportnex Apr 6 at 16:31
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    $\begingroup$ "And I have an experiment in mind where one famous guy standing on a church and letting two things fall with different mass to test if gravity acts on every object the same." I think you mean Galileo Galilei. $\endgroup$ – Gert Apr 6 at 16:32
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    $\begingroup$ I'm voting to close this question as off-topic because it's about the history of physics, not physics primarily. $\endgroup$ – Gert Apr 6 at 16:32
  • $\begingroup$ It's also notable that he never did this experiment. He experimented with sloped planes, but the two body drop was his thought experiment: what if we connect the bodies with a piece of chain and then reduce and reduce its length until there is no chain? $\endgroup$ – kkm Apr 8 at 2:29
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There are lots of different ways of stating the equivalence principle, and they're not all logically ... er ... equivalent to each other.

Names that come up in this connection, from before Einstein, are Galileo and Eotvos. Experiments that test the equality of gravitational and inertial mass are called Eotvos experiments.

Einstein's version of the equivalence principle is more radical than anything that Galileo or Newton envisioned. It entails throwing away the whole concept of a universal reference frame and instead defining inertial frames as local, free-falling frames. Previously, people had imagined that there was a frame of the "fixed stars," which was as good an approximation to an inertial frame as we would ever need. But according to relativity, observers in New York and Beijing can't even have the same frame of reference.

Taking the equivalence principle seriously also leads to the prediction of gravitational time dilation, which is something that nobody before Einstein had imagined.

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