“If a future experiment should demonstrate that antiparticles have a negative gravitational mass, it will deliver a mortal blow to the entire relativistic theory of gravity by disproving the principle of equivalence. An antiapple might fall up in a true gravitational field, but it could hardly do so in Einstein’s accelerated spaceship. If it did, an outside observer would see it moving at twice the acceleration of the ship, with no force at all acting on it. The discovery of antigravity would thus force upon us a choice between Newton’s law of inertia and Einstein’s equivalence principle. The author earnestly hopes that this will not come to pass.”
—Scientific American, March 1961
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