For millennia scientists have been struggling to answer the question of whether objects need some kind of contact with each other to know how to move through space. This notion is called the principle of locality.
As an example, I can know about all the objects in an utterly dark room by reaching out with my hands (and by bumping into things). Likewise a magnet can know where another magnet is by physical contact of the north and south magnetic poles, or equally well by contact with the magnetic field repelling the two north (or two south) poles.
Now then, if objects are not bumping into each other, or bumping into each other’s magnetic (or other) fields? Can such two objects ever somehow know instantly of each other’s existence? The ancient Greeks thought not, arguing that instant knowledge of other objects is supernatural and therefore not scientific.
About 2000 years later Newton presented the first formal description of gravity. Newton’s universal law of gravity can quantify the gravitational pull that we feel towards the center of the Earth, as well as the pull of our own gigantic Milky Way galaxy as it is pulled towards its far away counterpart called the Andromeda galaxy two million light years away.
One problem is that Newton could not explain how objects separated by any distance do find out about each other’s existence. Is there a telephone cable, a cell phone electromagnetic field, or nothing at all? If communication is truly instant, then it would break the principle of locality.
This spooky ‘action at a distance’ which allows information suddenly to jump across empty space bothered Newton. It annoyed Einstein enough for him to found a way past this scary theory by introducing gravitational fields which fill up all the space between objects. Now it is possible for information to be transmitted between objects via the fields.
As an analogy, one can imagine suspending a thin rubber sheet above the ground. If one puts a heavy lead ball in the middle it will cause an indentation in the sheet. Now if one starts another lead ball rolling along the edge of the sheet it will start rolling towards the center in response to the indentation.
On another front, Einstein also helped to establish the new field of quantum mechanics. Here again Einstein found examples of spooky actions at a distance. In the next installment, we will look at a thought experiment of Einstein’s that is still under debate today.