Maybe your parents told you to never talk politics or religion in polite company. It’s good advice which you should follow if you ever sit down with the Queen, the Pope, or your boss. It’s good advice in scientific circles, too, but with one addition: Never bring up interpretations of quantum mechanics. If you can imagine an assortment of cultists struggling to convert each other while playing Star Wars Trivial Pursuit, you’ll have a rough idea of the academic ambiance surrounding the topic.
Quantum mechanics tells us that atoms—and everything else on an atomic scale—behave differently than the larger objects we interact with in everyday life. And while quantum mechanics can also help predict events which happen on a tiny scale, they don’t tell us why nature behaves that way.
Recently one theory, which goes by the name of the “Many Worlds Interpretation,” has been gaining ground, moving from amusing obscurity to dark-horse frontrunner. Details vary among specific versions, but the overarching concept is that the universe splits constantly into multiple universes that are slightly different from one another, and between which all communication is cut off. Sound improbable? Maybe. Nevertheless, it’s one of the best guesses we have right now about Life, the Universe, and Everything.
The road to the Many Worlds Interpretation began in the early 20th century, when a serious problem was emerging in science. Massive amounts of evidence showed nature behaving strangely in very small scales. This didn’t make sense. Everything up to that point indicated that matter was matter, and should behave the same whether it was a planet, a tennis ball, or an atom. Why did the laws of motion simply break down at small scales?
One extreme example of the disconnect between macro and micro was the discovery of “superpositions” of states. As a conceptual example, everyday experience tells us that a coin flip can have two results: The coin can end up in the “Heads” state or “Tails” state. At a small enough scale, however, the coin would be in both states at the same time. The problem is, we can’t see the quantum superposition directly, because every time we look at the coin, it’s either “Heads” or “Tails.” It sounds a little fishy, like someone claiming to be invisible as long as no one’s watching, but without the concept of superposition, our predictions about reality come out hopelessly wrong.
The classic real-life demonstration of superposition involves photons—which are particles of light—sent flying one by one at a screen that has two holes for them to pass through. A charge-coupled device (CCD) digital camera is placed on the other side of the screen, and over time, the CCD camera collects enough photons to make an image. Cover the right hole and the image is a bright spot on the left, meaning that photons only made it through the left hole. We say that these photons were in the “Left” state. Cover the left, and the image is a bright spot on the right, so the photons were in the “Right” state. If we uncover both holes we might expect to see two spots—the right photons plus the left—but we don’t. Instead, we see rippling circles, like the surface of a pond after two stones are dropped in. The pattern comes from interference, which is what we would expect if each and every photon had gone through both holes at once. So, a possible conclusion is that instead of being in the “Right” or “Left” state, each photon was in a superposition of both states. A Many Worlds Interpretation.
Disturbingly, measuring a superposition is unpredictable. Not just in the sense that lottery drawings are unpredictable; if we knew everything about every lottery ball, and the exact way the air whirls around in the randomizing machine, the winning numbers could be predicted. But nothing in quantum mechanics or any other physical theory tells us how to predict which state we’ll see when we look at a superposition. So far, it appears that there is simply no rule that determines it.
Quantum mechanics consists of a set of rules that predict how nature works at these small scales. It does the job phenomenally well, and is unanimously agreed upon as the most powerful and accurate physical theory available. Still, it doesn’t always sit well with the scientific community. It predicts behavior accurately, but that behavior is, at a macro scale, impossible. Many scientists insist that there must be some explanation. After all, billiard balls act like billiard balls, even if atoms don’t, and we do a damn good job at predicting billiard balls using nothing but Isaac Newton’s handiwork. Somewhere between the world of particles and the world of humans, there must be a boundary where everything starts to behave normally.
Through time and debate, factions have developed to explain how this boundary works. Some say that quantum mechanics is incomplete, and that we’re ignoring some important piece of information that would let us make exact predictions. Some say it doesn’t matter how it works as long as we can build working rocket ships and computer chips (cynically referred to as the “shut up and calculate” interpretation).
The big winner for decades has been the Copenhagen Interpretation, named after the home base of Neils Bohr, who advocated the position that everything behaves quantum mechanically until it’s observed. That doesn’t have to mean human observation; it just means that the system interacts with something in the rest of the universe: A human, a frog, a water molecule or a ray of sunshine. Once a quantum system is observed, the one true reality “collapses” out of all possible ones. This isn’t entirely satisfactory, since the collapse has to be irreversible and non-deterministic, and also, travel faster than light, which are all pretty big problems for physicists. Still, it’s been the canonical understanding since the 1920s, and it’s the one that budding young physicists learn in college.
In 1957, a PhD student named Hugh Everett came up with what seems in retrospect to be a pretty obvious idea: What if there is no boundary between the human and quantum worlds? That is, what happens if we apply quantum mechanics at every level of the calculation? When he put in the equations and turned the crank, it worked out perfectly. Everything we observe is predicted exactly, except for one small problem: Every time a quantum mechanical system interacts with anything else, the universe breaks. Sometimes it splits into two, sometimes more. In each universe our quantum mechanical coin flip has a different result. That tiny change might have big consequences for the universe … or it might not. But by now there should be trillions, or trillions of trillions, or even infinitely many universes, if the Many Worlds Interpretation theory is true. How do we as humans fit into this? From the viewpoint of Many Worlds, humans are just a pile of atoms that obey known physics, not unlike a stalk of broccoli or a fence post (no offense). So we split, too, all the time, with the rest of observed reality. We don’t notice because once a world splits off, it can’t interact with “ours” any more. This would explain why we can’t predict quantum measurements. If you flip that quantum coin, you actually get both results. Or rather, one of you gets heads and one of you gets tails. The question of which result you’ll see doesn’t make any sense if you see both. This splitting continues, probably millions of times per second, and a cascading waterfall of realities is formed. Some are similar and some are very different. They might contain human beings, or not. In some universes, maybe this article is about nano-robots instead of quantum mechanics. But you’ll never read that article, because once the split occurs, the universes lose contact. Communicating between them might not turn out to be fundamentally impossible (the jury is still out), but it would at least be akin to making a window spontaneously un-break, or a bonfire un-burn, except on a universal scale.
Hugh Everett, by the way, was roundly ignored by some of the biggest names in physics when he presented his idea, and went on to make a comfortable living solving optimization problems for the Pentagon. His work was rediscovered and popularized by a handful of loyal physicists, mathematicians and computer scientists in the following decades, and by some accounts, Many Worlds is now the most supported interpretation of quantum mechanics among physicists.
The battle rages on, however. As in most emotional discussions, there is a great deal of misunderstanding among different quantum interpretation schools, and many initiates don’t fully understand their own viewpoints. There are also some legitimate concerns that have been only partially addressed by the Many Worlds Interpretation. If you want to get an idea of the terms of the debate, check out Wikipedia’s talk pages for any quantum mechanics article, or do a search for “interpretations of quantum mechanics.” A warning: I don’t know about other universes, but in this one, it’s not pretty.
For an excellent (but long) discussion on quantum mechanics and Many Worlds, check out the Quantum Physics Sequence by Eliezer Yudkowsky at lesswrong.com.
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