Is It Even Possible to Understand Quantum Mechanics?

A big problem with physics is that it’s invented by humans. We see stuff around us and use our observations to build a mental model of the world. Children do this, and of course physicists keep right on doing it. In many cases, this is awesome. It's how we got Newton's laws of motion. But it goes sideways when we try to picture the behavior of tiny, invisible things like subatomic particles.This is the “quantum realm,” as they call it in Ant-Man. It's not really a place you can visit in a special suit; you're already in it, and it's in you. But quantum phenomena are so alien that this deep layer of reality may as well be an alternate universe. The physicist Richard Feynman once famously said, “Nobody really understands quantum mechanics.”Feynman didn't mean we can't explain quantum behavior. Today we have models that predict quantum outcomes with incredible accuracy. What he meant is that it's impossible to ever wrap our macro-scale arms around it. The stuff I'm about to tell you is true, and it makes no sense whatsoever. You just have to accept the absurdity and proceed from there.To see this in action, let's revisit one of the most famous experiments in science: the double-slit experiment. Thomas Young first used this setup in 1801 to show that light travels in waves. Then, in 1961, Claus Jönsson built a minuscule version to show that subatomic particles with mass also behave like waves.Think about that. In our world of big things, if you toss a marble into a pond, it causes ripples, yeah? Well, in the quantum realm, it's like physical matter is both a marble and a ripple. And I'm afraid it only gets more preposterous from there.The Double-Slit ExperimentLet's start in the world of big things. Imagine a wall with two narrow, vertical windows, and we have a machine that shoots tennis balls. Some hit the wall, some pass through on the left, some pass through on the right. In back there's a second wall covered with Velcro, so when a ball hits this back wall, it sticks. What does that look like?Photograph: Courtesy of Rhett AllainAs you’d expect, the balls end up in two clumps, corresponding to the two inlets. But what if we run the same experiment with teeny-tiny slits and a machine that shoots electrons? Well, we tend to picture electrons as little balls orbiting an atomic nucleus, so by analogy you might expect a similar result on the screen. Nope. Instead of two clumps of electron hits, we get multiple bands:Photograph: Courtesy of Rhett AllainThis is very similar to what you see when light passes through two slits, as Thomas Young discovered. Since light is a wave, it does two things: First, there is diffraction. As the light passes through each opening, it spreads out like an ocean wave passing through a gap in a sea wall. Second, the waves from the two slits overlap and create an interference pattern.Photograph: Science History Images/AlamyWhere the waves are in phase, you get a bright spot. Where they're out of phase, you get a dark spot. Here's what that looks like in real life with a red laser as the light source:Photograph: Courtesy of Rhett AllainSo, if electrons produce an effect similar to that of light, can we model these tiny particles of matter as waves? Yes, and this is the idea behind Schrödinger's equation, which is the foundation of quantum mechanics. It tells us how a quantum system changes across time and space.SuperpositionThat's useful, but it's not entirely satisfying. We still want to know what's happening here. Are electrons truly like waves that pass through both slits and interfere, or does any given electron pass through one slit like a tennis ball? The answer is … yes.We can turn our electron gun down so that it shoots just one electron at a time. And guess what? Even when the electrons go through one at a time, so that they can’t interfere with one another, the same interference pattern of multiple bands emerges over time. They are somehow interfering with themselves.So physicists don't say the electron goes through one or the other slit. Instead, we say the particle is in a superposition state—it’s a combination of going through the left slit and going through the right slit. And that is not the same as going through both slits.Wait! Couldn't we actually determine which slit an electron went through? Absolutely. We can place a tiny light source near each slit, and if it reflects off the electron, we'll know which one it went through.But guess what? If you use a detector like this, you no longer get the interference pattern. You get a result that looks like it's an electron going through one slit (the tennis ball pattern). The double-slit pattern occurs only if you don't measure it! We say that measurement “collapses the wave function” of a system, so that the electron is no longer in superposition.That's just silly, right? It's crazy that the act of measurement changes the outcome. Does this mean that physics experiments can change reality? Maybe.What About That Zombie Cat?I mentioned Erwin Schrödinger above. He was one of the pioneers of quantum theory, but he couldn't bring himself to accept this idea of superposition. Using his macro-world common sense, he insisted there must be a flaw in the model, and that a complete theory of quantum mechanics would eventually be deterministic like classical physics.When scientists don't like something, they make up absurd scenarios to invite mockery of its proponents. So Schrödinger created a famous thought experiment you may have heard of. It's called Schrödinger's Cat, and it goes like this:There is a cat in a box. This box also contains a radioactive atom, a detector, and a vial of poison. If the atom decays and produces radiation, the detector will open the vial and the cat will die. But if the box is closed, the atom is in a superposition state, being both decayed and not decayed—meaning the cat is both dead and alive. Only when you open the box and observe the cat will its wave function collapse into either dead or alive.Nice try, but Schrödinger's scenario was easily dismissed. First, no one was saying superposition translated to big objects like cats. Second, there was already a measurement before the box was opened: The device that releases the poison was monitoring the state of the atom.In fact, Schrödinger was wrong—as was Einstein, who agreed with him. Since then, numerous experiments have proved that superposition is real and that uncertainty is fundamental to the universe. And a good thing it is, too, even if it makes no sense. Superposition is what gives quantum computers their immense parallel processing power.Heads and TailsIt's important to understand that superposition of states isn't the same as saying we don't know. If you flip a coin and cover it up when it lands, the coin is already either heads or tails—you just don't yet know which. In the double-slit experiment, we know: It goes through only one slit and it goes through both slits.Quantum systems have other properties that will scramble your brain if you try to grok them—like quantum tunneling—or entanglement, where measuring the state of one particle determines the state of another, no matter how far away it is. This happens instantly, with no time for a signal to travel between them.But by now these are well-established phenomena, so is it fair to call them bizarre? Are we only exposing the limitations of our human minds? Clearly, the level of reality we inhabit is only part of the story, so we should be skeptical of our intuition. If nothing else, quantum mechanics teaches humility. We keep doing science—and the farther we go, it seems, the greater the mystery.