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 March 2018, without the images that appeared with it (which were mostly fairly generic photos), though I may add back the couple of images that were original to me.

My post the other day about how classical rules emerge from quantum ones was spinning off from an NPR post by Adam Becker on quantum reality. The idea of the post is also based on something I’d been thinking about recently, in the process of revising my forthcoming book on quantum physics (due out in December)– writing about quantum physics of everyday life forced me to deal with that a bunch.

I don’t want that post to leave the impression that quantum physics isn’t weird, though, because it very much is. “Weird” here meaning “behaving in ways that run counter to our expectations from classical physics.” The fact that the rules of classical physics have quantum roots doesn’t change the fact that they’re very different from the underlying quantum rules.

Of course, that doesn’t mean there can’t be quibbles and subtleties about what, exactly, counts as “weird” in quantum physics. In the NPR piece, Becker spends a fair bit of space talking about the shift from particles with definite positions that can be specified with only three numbers to a quantum wavefunction consisting of “an infinity of numbers, scattered across all of space.” This is mostly to set up a discussion of quantum measurement, but I think it mis-states things in a couple of significant ways.

For one thing, he’s short-changed the ideal classical picture by a factor of two– to really specify everything you need to know about an electron, you need not just three numbers for its position, but three more numbers for the components of its velocity. The quantum wavefunction contains that information as well as information about the position, because the momentum of a particle is related to its wavelength.

More importantly, I think the weirdness of the wavefunction giving you a probability is over-sold a bit. You can also describe classical particles in terms of a probability distribution, and in fact I would argue that a truly responsible treatment of classical physics demands that you describe the motion of particles in terms of probability. Any measurement of a particle’s initial position and velocity will necessarily be limited in precision, and that will lead to some range in the possible outcomes of any projection into the future. If you’re trying to describe, say, the flight of a golf ball, you can have a pretty good idea of where it’s going to land, but it’d take an amazing amount of effort to narrow the landing zone to better than several square meters.

Both quantum and classical physics, then, should describe objects of interest in terms of an infinity of numbers, in the form of a probability distribution covering all of space. The thing that’s weird about quantum wavefunctions isn’t their scope, but the fact that they’re subtly different than classical probability distributions– to describe the motion of a classical particle like a golf ball, you work with probability distributions directly, but the quantum wavefunction is more like the square root of the probability distribution. This is what makes it possible to see wave phenomena like interference and diffraction. If a classical particle has two possible paths from its initial position to its final position, the final probability distribution is just the sum of the distributions from each path, and will generally have two distinct lumps. A quantum particle, on the other hand, will produce a probability distribution with lots more wiggles in it, reflecting the wave nature of the particles.

So, the weirdness of quantum probability is more subtle than just the notion of probability itself. Quantum measurement, the process that gets you from the probability distribution to the single measured outcome, is more strange, but there are ways to make that less weird, too. That’s what interpretations like the Many-Worlds Interpretation are for– it’s a simple explanation of how the world can consist of objects in superpositions of multiple states without us ever noticing that. You can also remove a lot of the weirdness of quantum measurement by taking a more epistemic approach to the theory, and saying that the wavefunction is really describing our state of knowledge about the system, rather than a real, physical thing. (There are some significant issues with the latter approach, but they’re very technical and not without controversy, so it remains an approach with many backers.)

There’s one bit of quantum physics, though, that I think is inescapably weird, and that’s quantum entanglement. This is a topic I’ve written about a lot– see, for example, this analogy to a sudoku-like game, or this one about how you make entangled particles, or even this one about how to use quantum entanglement to better understand the Many-Worlds Interpretation. There are links in those posts to lots of other posts, as well.

In quantum physics, “entanglement” refers to a correlation between the states of two particles that leaves the state of the individual particles indeterminate, but lets you know the collective state of the two with absolute certainty. That is, you can’t say for certain whether the state of particle A will be 0 or 1 until you measure it, but if you get a 1 for particle A you know with 100% confidence that the state of particle B will also be 1.

The idea of an indeterminate-but-correlated state is weird, but might not seem all that weird, but it gets stranger: the result of these measurements does not depend on the distance between the particles or the time between the measurements. You could put one particle in New York and the other in San Francisco, and measure them a nanosecond apart, and the correlation would still be absolute. Quantum physics is “non-local” in this way, meaning that particles can be correlated with each other in ways that don’t seem to respect distance in space or the speed of light.

As bits of physics go, that’s as weird as it comes. It’s also absolutely real, confirmed by experiment. Explaining how that works, and how it’s not actually a tool for sending information faster than the speed of light has been one of the most active and fruitful research areas in quantum phhysics over the last couple of decades. (I’ll put in a plug here for George Musser’s Spooky Action at a Distance, which is a good general-audience overview of the field, and Dance of the Photons by Anton Zeilinger, who’s one of the biggest names in the field of entanglement experiments.)

These are, of course, just my opinions about what’s weird and what’s not, and you’ll get some argument about some aspects of this. If you pin down a bunch of people in the field, though, and ask them to name the single weirdest aspect of quantum physics, odds are they’ll tell you it’s quantum entanglement.