Quantum physics, celebrated for its predictive success, has also become notorious for being an inscrutable mass of paradoxes.
One of the founders of the theory, Niels Bohr, stated that “those who are not shocked when they first come across quantum theory cannot possibly have understood it.” Nobel laureate Richard Feynman said, “I think I can safely say that nobody understands quantum mechanics.”
The shocking aspects of quantum theory can be summarized by three issues: uncertainty, nonlocality and the measurement problem (or the problem of “Schrödinger’s Cat”).
The first issue consists in the fact that the tiny objects described by quantum theory, such as the constituents of atoms — protons and electrons, for example — cannot be pinned down to definite locations and speeds at the same time. If one of these properties is definite, the other must be in a quantum superposition, a kind of “fuzziness” that we never see in the ordinary macroscopic world of experience.
The second issue arises in certain kinds of composite systems, such as pairs of electrons, in a so-called “entangled” state. If you send two such electrons off to the opposite ends of the galaxy, quantum physics tells us that they are still somehow in direct communication, such that the result of a measurement performed on one of them is instantly known to the other. This seems to be in conflict with another very successful theory, Einstein’s theory of relativity, which tells us that no signal can be transferred faster than the speed of light.
The third issue comes from Erwin Schrödinger’s observation that quantum physics seems to tell us that measuring instruments become “entangled” with the quantum objects they are measuring in a way that dictates that even macroscopic objects, like cats, inherit the “fuzziness” of the quantum world. In this case, the famous unfortunate cat seemingly ends up in a superposition of “alive and dead” based on the superposition of a radioactive atom in an uncertain state of “decayed and undecayed.”
It may come as a surprise to learn that there is a way to make sense of all three of these seemingly paradoxical features of quantum mechanics. However, there is, of course, a price to pay for that solution: a paradigm change as startling as the one that accompanied Einstein’s theory of relativity — which told us, despite our intuitions, that there is no such thing as absolute space or time. Quantum physics requires that we “think outside the box,” and that box turns out to be space-time itself. The message of quantum physics is that not only is there no absolute space or time, but that reality extends beyond space-time. Metaphorically speaking, space-time is just the “tip of the iceberg”: Below the surface is a vast, unseen world of possibility. And it is that vast, unseen world that is described by quantum physics.
This is not a wholly new idea: Another founder of quantum theory, Werner Heisenberg, stated that a quantum object is “something standing in the middle between the idea of an event and the actual event, a strange kind of physical reality just in the middle between possibility and reality.” Heisenberg called this “potentia,” a concept originally introduced by the ancient Greek philosopher Aristotle. It turns out that if we apply Heisenberg’s insight to an intriguing interpretation of quantum theory called the transactional interpretation (TI), we gain a unified understanding of all three paradoxical aspects of quantum theory.
Ruth E. Kastner is a quantum physicist in the Foundations of Physics Group at the University of Maryland. In her latest book is Understanding Our Unseen Reality: Solving Quantum Riddles.