Quantum mechanics provides the most accurate description of our physical universe so far. It reigns supreme in its scope, from the smallest known particles that are the quarks to the largest conglomeration of objects that is the cosmos. It accounts for all the observed properties of the elements of the periodic table, and the molecules that form from them, so that life as we know it has quantum mechanics at its foundation. Every experiment to date to test quantum mechanics to exacting standards has only increased our confidence that it paints a correct picture of the universe. Yet quantum mechanics is weird to the core.
The confirmed predictions of quantum mechanics run smack against our sane expectations and corporeal experiences. Your pet can only be either dead or alive, but never dead and alive at once, a seemingly irrefutable fact established by our everyday experiences of “reality”. Yet quantum mechanics says otherwise. Your pet cannot only be either dead or alive but it can also be both dead and alive at once—not a paralyzed half-dead-half-alive pet but a whole pet simultaneously dead and alive. This is clearly a travesty of logic to the highest order and heretical at best. But your pet in some vague state of being dead and alive is an established fact by real experiments that can be replicated anytime, anywhere. Physical reality according to quantum mechanics is not the physical reality that we know of. In some ways our experiences are mere reductions of the real reality according to quantum mechanics.
The culprit behind quantum weirdness is the principle known as the principle of quantum superposition. Given two mutually exclusive alternatives, say dead or alive, the superposition principle says that if there is nothing in principle to distinguish the two alternatives then the alternatives are simultaneously true. So if you have a living cat and isolate it from the rest of the universe, say by placing it inside a sufficiently isolated box, then quantum mechanics says that the cat will eventually be in a morbid state of being dead and alive at once. That is so because isolating the cat from the rest of the universe removes all possibilities of distinguishing the two mutually exclusive states of the cat. By the superposition principle, the two alternatives must be simultaneously true so that the cat is dead and alive. But how do we know that that indeed is the correct description of the cat when we are not looking in the first place? Without the superposition principle, there would not have been any atom.
The principle of superposition is weird. But it spawns to a much weirder phenomenon—quantum entanglement. Entanglement arises when two initially isolated objects come into contact with each other, say when two electrons collide. The interaction makes the separate objects into a single, inseparable quantum object. The individual identities of the initial objects have been lost and one can only ascertain the state of the entangled objects as a whole but not the separate states of the constituent objects. Equivalently for entangled objects, we cannot infer the state of the whole by looking at its parts. This is contrary to our expectations. Given a dismantled chair, you can infer from the parts by mere inspection that they came from a chair. But not when the parts are quantum objects that have entangled. In our everyday experience, the information on the whole is contained entirely on its parts. For entangled objects, it is possible that no information on the whole is ever encoded on its parts, so that no amount of inspection on the parts may yield information on their combined system.
Quantum entanglement between two objects also arises when the objects have been produced at the same time from a common source, such as when the objects are fragments of a disintegrating body. This happens, for example, when a photon, the grain of light, disintegrates into an electron and a positron, a process known as pair creation. The electron and the positron are entangled in this case and are inextricably linked to one another. They are not separate objects but form a unit entity, even when they fly apart to the “opposite ends” of the universe, separated by millions of light years. As such anything that happens to the electron has an instantaneous effect on the positron however the two are far apart. The link between them provided by quantum entanglement seems to act as a hyper-highway allowing an infinitely fast transmission of information from the electron to the positron, that is, at a speed infinitely faster than the speed of light. Einstein called it “spooky action at a distance”, which he could not accept as reflecting the true description of physical reality and thus rejected it. However, entanglement prevailed over Einstein.
Quantum mechanics is weird. And its weirdness does not stop at quantum entanglement. The quantum superposition principle has flung open the door to the odd reality that has been hidden from us. If there is anything we have learned from recent developments, it is this: The room that superposition principle has led us into is vast and much remains to be explored. Perhaps farther down the room we may be able to reconcile our everyday version of reality with the version of quantum mechanics. However, it is more likely that our reality is a mere mirage of the weird quantum reality. And the best that we can do is to embrace weirdness and conquer the future with it.