This post is part of a series of posts originally written for my blog at Forbes.com that I’m copying to my personal site, so I have a (more) stable (-ish) archive of them. This is just the text of the original post, from May 2016, without the images that appeared with it, some of which were lost during a Forbes platform change a couple of years ago; I may try to recover them and replace them in this post later.
Last week, I wrote a big long post about why entanglement doesn’t allow faster-than-light communication, and included a passing mention that was related somehow to the “Many-Worlds” Interpretation of quantum physics. That post was long already, so I didn’t have room to say more, but that remark probably deserves unpacking.
Now, entanglement as a general phenomenon is kind of mysterious, so saying that it can help you understand another totally mysterious quantum idea might seem less than perfectly helpful. the nice thing about entanglement, though, is that even if you don’t think about the underlying mechanism at all, it provides a very concrete set of results, verified in countless experiments, that we can use to ground our thinking about deeper issues.
A generic entanglement experiment looks like this:
Two experimental physicists, traditionally named “Alice” and “Bob” share a pair of particles emitted from a source of some sort (represented as a canonical black box, because it doesn’t matter how it works), whose states may or may not be entangled. These could be any of a huge range of systems, but for the sake of concreteness, let’s imagine that they’re electron spins. Two electrons fly out of the box, one to Alice and one to Bob, and the scientists measure what direction the magnetic moment is pointing, recording a 1 for up and a 0 for down. Their measurements are tables of numbers that look kind of like this:
Alice and Bob each record a random string of 1’s and 0’s, split 50/50; nothing about the individual lists tells you anything about entanglement. The only way to distinguish between entangled and not-entangled particles is to compare Alice’s list of measurements to Bob’s. If the particles are entangled, they’ll correlate nearly perfectly (maybe the occasional experimental error creeping in, shown in red), while if they’re not entangled, they’ll only agree about half of the time.
Of course, the universe is much bigger than just Alice and Bob and their two electrons. So let’s imagine a third person, usually called “Eve” in a communications contest, who’s mad at Alice and Bob for some reason, and wants to mess up their experiment. Eve inserts an extra element in the path between the black-box source and Bob’s detector that rotates the spin of Bob’s electron. This is easy enough to do by, say, putting an extra magnet in the path of the beam.
When this happens, Bob’s detector will give very different results than Alice’s. When Alice detects a spin-up electron and records a “1,” Bob ought to also get a spin-up electron. Instead, thanks to Eve’s meddling, he’s getting an electron whose spin has been rotated 45 degrees from the “up” direction, and thus has a 50/50 chance of being detected as spin-up or spin-down. In this scenario, they’ll find lots of cases where their spin measurements disagree, and conclude that their particles are not, in fact, entangled.
This does not mean, however, that Eve has broken the entanglement between Alice’s particle and Bob’s. On the contrary, that entanglement is still there, it’s just hidden from view. But if Alice and Bob know about Eve, Bob can adjust his detector to account for her meddling:
When Bob turns his detector to compensate for Eve’s extra rotation, Alice and Bob will recover their nearly-perfect correlation between measurements. Once entangled, the particles remain entangled even though Eve’s messing with them, provided Alice and Bob make the right set of measurements. As long as they know that Eve’s meddling, and approximately how much she’s changing things, they can correct for that outside influence.
This toy-model scenario explains why physicists studying entanglement need to work very hard to isolate their experiments from the outside world. Very few physicists have honest-to-God malicious interlopers changing their experimental parameters, but there are lots of random environmental factors that can play the role of “Eve.” A stray magnetic field in one part of the apparatus can rotate the electron spin just as effectively as an evil electromagnet, so experiments are carefully shielded, and numerous sanity-check tests are done to ensure that Bob’s making the right sort of measurements to correlate with the measurements Alice is making.
The killer for these kinds of experiments are unmeasured interactions with the external environment. If you know there’s something perturbing the experiment, you can correct for it, and see the entanglement that’s really there. You can’t correct for interactions you don’t know about– the “unknown unknowns” in Donald Rumsfeld’s immortal taxonomy of problems— and that’s what ultimately limits our ability to do entanglement experiments. It’s not that the entanglement itself is fragile– it’s actually pretty robust, nearly as hard to truly destroy as to create in the first place– but the ability to measure it depends on knowing exactly what measurement you ought to be making.
What’s this got to do with the Many-Worlds Interpretation? Well, the whole reason we have Many-Worlds and all the other interpretations of quantum physics is that when we look at the everyday world around us, we don’t see all the weird stuff quantum physics says ought to be there. We don’t see superposition states, with cats that are alive and dead at the same time, and we don’t generally notice entanglement-type correlations between distant measurements unless we work really hard to find them.
One traditional way of dealing with this is through a “collapse” type interpretation, saying that there’s some as-yet-unknown mechanism that pushes superposition states toward a single definite state, and destroys correlations between entangled particles. There are a bunch of variants of this, differing in how, exactly, they talk about the collapse process.
Many-Worlds takes a different approach, saying that superposition states and entanglement correlations continue to exist, but are hidden from us by unmeasured and random interactions with the environment. We don’t see quantum superpositions of actual cats not because the cats are really in a single state, but because of the undetected meddling of trillions of Eves. If we knew in detail exactly what the environment was doing to perturb our measurements, we could correct for it and see the quantum effects that are really there, but not only do we not know what the environment is doing, what it’s doing is changing all the time, making it impossible to compensate. The universe is really in a massive superposition of massively entangled states, but we can’t see it because of all the environmental perturbations screwing up our experiments.
It should be noted that this process of unmeasured perturbations hiding quantum effects is not unique to Many-Worlds– it gets the catch-all name “decoherence,” and shows up in basically every modern approach to quantum physics. Its role is (arguably, at least) more important in Many-Worlds type approaches than others, but it’s a real process regardless of your choice of interpretation. The process is often explained very badly– one of the things I worked hardest at when writing How to Teach Quantum Physics to Your Dog was trying to find a better way to talk about decoherence than most of the popular treatments out there– but really, it’s just a bunch of undetected Eves messing with your experimental measurements.