Action-at-a-distance is the creation of an effect without physically touching. An example is magnetism: a magnet pulls on iron filings without the necessity of touching them. This is a problematic concept to physicists. How can the iron filings “feel” at a distance that they’re near a magnet?
Gravity as action-at-a-distance. Despite this preference for local action, action at a distance has long been accepted by physicists. Newton’s theory of gravity, developed in the 1600’s and accepted for over 200 years, depends on action at a distance: a piece of matter attracts all other pieces of matter in the universe regardless of their distances from each other. In Newton’s view, this attraction is by virtue of an inherent property of matter, that is, its mass. Thus, the Earth holds the moon in an orbit and also draws an apple to the ground, both without contact.
This aspect of Newton’s theory troubled him as well as many of his contemporaries. Newton wrote:
“That one body may act upon another at a distance through a vacuum without mediation of anything else, by and through which their action and force may be conveyed from one another, is to me so great an absurdity that, I believe, no man who has in philosophic matters a competent faculty of thinking could ever fall into it.”1
As physicist, Frank Wilczek, comments on Newton’s statement, “Nevertheless, he left his equations to speak for themselves.”2
Newton’s law of gravity allowed physicists to accurately calculate the orbits of the planets, the speed at which an apple falls, and much more. So, they put aside reservations and accepted that by some unknown means, an object can, even at great distances, feel the pull of the mass of another object. It wasn’t until 1915, when Einstein’s Theory of General Relativity replaced Newton’s law of gravity, that gravity as action-at-a-distance was abandoned.
Gravity as action-at-a-distance would act instantaneously. As a note, Newton’s conception that gravity is due to mass of matter has an important implication. Should the matter instantaneously disappear, so would its mass and so would its gravity. This implies instantaneous transmission of changes in gravitational pull at indefinitely large distances.
For example, let’s say, heaven forbid, that the Earth instantaneously evaporated. What would happen to the moon? According to Newton’s Law of Gravitation, the moon would, at the same moment, no longer be pulled by Earth’s gravity.
This outcome would conflict with Einstein’s Theory of Special Relativity, developed two centuries after Newton. According to Special Relativity, information cannot travel faster than the speed of light. So, the quickest that the information that the Earth had evaporated could travel to the moon would be at the speed of light. That would take 1.3 seconds.
So, according to Newton’s Law of Gravitation, the moon would fly out of orbit instantly upon the mass of the Earth evaporating. But, according to Einstein’s Special Relativity, the poor moon would fly off 1.3 seconds later. The matter was settled experimentally in 2002, and Einstein won. In 2002, the speed of gravity was measured for the first time—it traveled at the speed of light.3
Field theory replaces action-at-a-distance. In the late 1800’s, Michael Faraday created the concept of “fields” to replace action-at-a-distance. These days, physicists speak of magnetic and electrical fields through which forces are communicated to the objects and energies they affect. For example, a magnet creates a magnetic field around it. The iron filings sit in the field and experience its force.
People also sometimes speak of a gravitational field. However, this is a throw-back to earlier ways of looking at gravity. The idea of a gravitational field has been replaced by curved spacetime in Einstein’s Theory of General Relativity.
Fields provide a physical mechanism through which force travels. Not only is this conceptualization more satisfactory to many physicists, the field approach also simplifies calculations.
Action-at-a-distance is reborn in quantum entanglement. According to quantum physics theory, the behavior of particles can instantaneously correlate with each other even if they are located at great distances from each other, possibly even when they’re on opposite sides of the universe. Such instantaneous correlation raises the possibility of information traveling faster than the speed of light, which would violate Einstein’s Special Theory of Relativity. Einstein was uncomfortable with this aspect of quantum physics (among others) and famously called it “spooky action at a distance.”
However, in 1972, the instantaneous correlation of the behavior of quantum particles at a distance was found to occur in lab experiments by John Clauser and, in 1982 confirmed by the French physicist, Alain Aspect. This effect is called “non-local causation” or “non-locality.” Physicists have developed a number of interpretations of quantum physics to explain the physical mechanism that might account for non-locality. No consensus as to an interpretation has been reached to date (2016).
1Isaac Newton, as quoted by Frank Wilczek, The Lightness of Being; Basic Books, 2008, New York; p. 77.
3 Hazel Muir, “First Speed of Gravity Measurement Revealed,” New Scientist; Jan. 7, 2003. https://www.newscientist.com/article/dn3232-first-speed-of-gravity-measurement-revealed.