Isaac Newton had a problem with the concept of action-at-a-distance. On one hand, like other 17th-century mechanist philosophers, he was deeply suspicious of the idea. As he wrote to the theologian Richard Bentley in 1693:
[T]hat one body may act upon another at a distance through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to the other, is to me so great an absurdity, that I believe no man who has in philosophical matters a competent faculty of thinking, can ever fall into it.
On the other hand, there’s his own theory of gravity, published in his Principia several years earlier. It says that one body can exert a force on another, at arbitrary distance, without the need for any intermediary. What was a poor genius to do?
How Newton dealt with this dilemma in his own mind is still a matter for debate. Privately, his letter to Bentley continues: ‘Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial, I have left open to the consideration of my readers.’ In public, he seems to express disdain for the question: ‘I have not as yet been able to deduce from phenomena the reason for these properties of gravity, and I do not feign hypotheses.’
Two centuries later, Albert Einstein got Newton off the hook – though not before he’d made the problem even worse. Einstein’s 1905 theory of special relativity raised a new difficulty for Newton’s theory of gravity. Instantaneous action-at-a-distance requires that the distant effect is simultaneous with the local cause. According to special relativity, however, simultaneity is relative to the observer. Different observers disagree about which pairs of events are simultaneous, and there’s simply no fact of the matter about who is right.
Without simultaneity at a distance, the notion of instantaneous action-at-a-distance doesn’t make sense. By making Newton’s theory of gravity even more problematic, special relativity gave Einstein an extra motivation for developing his own theory. He succeeded in his theory of general relativity (GR) 10 years later. GR explains gravity in terms of the curvature of spacetime, and abandons the idea that it acts instantaneously. In GR, gravitational effects propagate at the speed of light. If the Sun suddenly vanished, it would be eight minutes before the Earth reacted.
Unfortunately for Einstein, the physics of action-at-a-distance turned out to be Whack-a-Mole. He had knocked it on the head in one place, but it popped up in another – and he deserves some of the credit for that, too. Another of Einstein’s great discoveries in 1905 – the one he thought most important – was that light can behave like individual particles, now called photons. This became one of the foundations of the new theory of quantum mechanics, developed by Erwin Schrödinger, Werner Heisenberg and others in the 1920s.
Einstein was never happy with quantum mechanics. As he complained later to Max Born, another quantum pioneer: ‘The theory cannot be reconciled with the idea that physics should represent reality in time and space, free from spooky actions at a distance.’
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