Science expressed only in terms of what happens is getting in the way of progress.
BY CHIARA MARLETTO
If you could soar high in the sky, as red kites often do in search of prey, and look down at the domain of all things known and yet to be known, you would see something very curious: a vast class of things that science has so far almost entirely neglected. These things are central to our understanding of physical reality, both at the everyday level and at the level of the most fundamental phenomena in physics—yet they have traditionally been regarded as impossible to incorporate into fundamental scientific explanations. They are facts not about what is—“the actual”—but about what could or could not be. In order to distinguish them from the actual, they are called counterfactuals.
Suppose that some future space mission visited a remote planet in another solar system, and that they left a stainless-steel box there, containing among other things the critical edition of, say, William Blake’s poems. That the poetry book is subsequently sitting somewhere on that planet is a factual property of it. That the words in it could be read is a counterfactual property, which is true regardless of whether those words will ever be read by anyone. The box may be never found; and yet that those words could be read would still be true—and laden with significance. It would signify, for instance, that a civilization visited the planet, and much about its degree of sophistication.
To further grasp the importance of counterfactual properties, and their difference from actual properties, imagine a computer programmed to produce on its display a string of zeroes. That is a factual property of the computer, to do with its actual state—with what is. The fact that it could be reprogrammed to output other strings is a counterfactual property of the computer. The computer may never be so programmed; but the fact that it could is an essential fact about it, without which it would not qualify as a computer.
The counterfactuals that matter to science and physics, and that have so far been neglected, are facts about what could or could not be made to happen to physical systems; about what is possible or impossible. They are fundamental because they express essential features of the laws of physics—the rules that govern every system in the universe. For instance, a counterfactual property imposed by the laws of physics is that it is impossible to build a perpetual motion machine. A perpetual motion machine is not simply an object that moves forever once set into motion: It must also generate some useful sort of motion. If this device could exist, it would produce energy out of no energy. It could be harnessed to make your car run forever without using fuel of any sort. Any sequence of transformations turning something without energy into something with energy, without depleting any energy supply, is impossible in our universe: It could not be made to happen, because of a fundamental law that physicists call the principle of conservation of energy.
We cannot bring about transformations that laws of physics declare to be impossible.
Another significant counterfactual property of physical systems, central to thermodynamics, is that a steam engine is possible. A steam engine is a device that transforms energy of one sort into energy of a different sort, and it can perform useful tasks, such as moving a piston, without ever violating that principle of conservation of energy. Actual steam engines (those that have been built so far) are factual properties of our universe. The possibility of building a steam engine, which existed long before the first one was actually built, is a counterfactual.
So the fundamental types of counterfactuals that occur in physics are of two kinds: One is the impossibility of performing a transformation (e.g., building a perpetual motion machine); the other is the possibility of performing a transformation (e.g., building a steam engine). Both are cardinal properties of the laws of physics; and, among other things, they have crucial implications for our endeavors: No matter how hard we try, or how ingeniously we think, we cannot bring about transformations that the laws of physics declare to be impossible. But by thinking hard enough, we can come up with more and better ways of performing a possible transformation—for instance, that of constructing a steam engine—which can then improve over time…