Or, in other words: is there really any difference between the universe and its mirror image? I remember once reading one of Richard Feynman’s lectures in physics in which he took on this problem. True to form, Feynman found a funny, imaginative, and perfectly clear way of spelling it out:
Imagine that we were talking to a Martian, or someone very far away, by telephone. We are not allowed to send him any actual samples to inspect; for instance, if we could send light, we could send him right-hand circularly polarized light and say, “That is right-hand light—just watch the way it is going.” But we cannot give him anything, we can only talk to him. He is far away, or in some strange location, and he cannot see anything we can see. For instance, we cannot say, “Look at Ursa major; now see how those stars are arranged. What we mean by ‘right’ is …” We are only allowed to telephone him.
Now we want to tell him all about us. Of course, first we start defining numbers, and say, “Tick, tick, two, tick, tick, tick, three, …,” so that gradually he can understand a couple of words, and so on. After a while we may become very familiar with this fellow, and he says, “What do you guys look like?” We start to describe ourselves, and say, “Well, we are six feet tall.” He says, “Wait a minute, what is six feet?” Is it possible to tell him what six feet is? Certainly! We say, “You know about the diameter of hydrogen atoms—we are 17,000,000,000 hydrogen atoms high!” That is possible because physical laws are not invariant under change of scale, and therefore we can define an absolute length. And so we define the size of the body, and tell him what the general shape is—it has prongs with five bumps sticking out on the ends, and so on, and he follows us along, and we finish describing how we look on the outside, presumably without encountering any particular difficulties. He is even making a model of us as we go along. He says, “My, you are certainly very handsome fellows; now what is on the inside?” So we start to describe the various organs on the inside, and we come to the heart, and we carefully describe the shape of it, and say, “Now put the heart on the left side.” He says, “Duhhh—the left side?” Now our problem is to describe to him which side the heart goes on without his ever seeing anything that we see, and without our ever sending any sample to him of what we mean by “right”—no standard right-handed object. Can we do it? (http://www.feynmanlectures.caltech.edu/I_52.html)
Feynman thought we would have to go to some pretty extreme lengths to explain “left” and “right” to the Martian:
In short, we can tell a Martian where to put the heart: we say, “Listen, build yourself a magnet, and put the coils in, and put the current on, and then take some cobalt and lower the temperature. Arrange the experiment so the electrons go from the foot to the head, then the direction in which the current goes through the coils is the direction that goes in on what we call the right and comes out on the left.” So it is possible to define right and left, now, by doing an experiment of this kind.
It turns out we could just tell the Martian how to build one of these cool tops:
It’s interesting that Kant thought (a) that there is no intrinsic mathematical difference between left-hand and right-hand, or clockwise vs. counterclockwise, and that (b) the fact that we can distinguish the two shows that space itself is not fully described by mathematical laws, and must possess essentially some intuitive component. Meaning: spatial objects aren’t just formulae, but have to be “seen” to be grasped. I don’t think anything coming out of the physics of the natural world would challenge Kant’s claim (a), since these physical distinctions are not purely mathematical. Still, Kant would finds these results interesting, I’m sure. And Feynman would’ve have liked those tops.