Isaac Newton was never entirely happy with his law of universal gravitation. For decades after publishing it in 1687, he sought to understand how, exactly, two objects were able to pull on each other from afar. He and others came up with several mechanical models, in which gravity was not a pull, but a push. For example, space might be filled with unseen particles that bombard the objects on all…

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At the turn of the 20th century, the renowned mathematician David Hilbert had a grand ambition to bring a more rigorous, mathematical way of thinking into the world of physics. At the time, physicists were still plagued by debates about basic definitions — what is heat? how are molecules structured? — and Hilbert hoped that the formal logic of mathematics could provide guidance.

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