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Many of the great advances in science are marked by the discovery that an aspect of nature we thought was fundamental is actually an illusion, due to the coarseness of our sensory perceptions. Thus, air and water appear to us to be continuous fluids, but we discover on deeper experiment that they are made of atoms. The Earth appears to us motionless, but a deeper understanding teaches us that it moves relative to the sun and the galaxy.

One persistent illusion is that physical objects only interact with other objects they are close to. This is called the principle of locality. We can express this more precisely by the law that the strengths of forces between any two objects falls off quickly—at least by some power of the distance between them. This can be explained by positing that the bodies do not interact directly, but only through the mediation of a field, such as an electromagnetic field, which propagates from one body to the other. Fields spread out as they propagate, with the field lines covering a constantly greater area—providing a natural explanation for the laws that say the forces between charges and masses fall off like the square of the distance between them.

Locality is an aspect of an even more compelling illusion: that we exist within an absolute space, with respect to which we mark our positions as we move “through” it. Thus, Newton opined that motion is ultimately defined as change of position with respect to absolute space. If this seems obscure—because no measurement can establish a relation of a physical object to this imagined absolute space—Newton assured us that absolute space is seen by God, making your location relative to it an aspect of the divinity of the world. We humans must make do with relative positions and motions—which are defined relative to physical objects we can see.

Leibniz broke the mystification by declaring that all that exists is relative positions and motions. He proposed as a matter of principle that any acceptable science of motion must be formulated in terms of relative motions alone. And this, after two centuries of waiting, is what Einstein delivered to us in his general theory of relativity. In this glorious construction, space is subsumed into spacetime, which is explicable as a dynamically evolving network of relationships.

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