Quantum Mechanics I: Uncertainty, Observation, and the Quantum Retcon

This article on quantum mechanics is part of the Science in Sci-fi, Fact in Fantasy blog series. Each week, we tackle one of the scientific or technological concepts pervasive in sci-fi (space travel, genetic engineering, artificial intelligence, etc.) with input from an expert. Please join the mailing list to be notified every time new content is posted.

The Expert: Casey Berger

Casey E. Berger earned a PhD in computational physics from UNC Chapel Hill and has bachelor’s degrees in philosophy and film production from Boston University and in physics from the Ohio State University. Since school is obviously her natural habitat, she now works as a professor of physics, where she studies the complicated interactions of subatomic particles and encourages the next generation of brilliant scientists. She is also the author of the forthcoming Resonance Saga (Aethon Books 2021), a space opera trilogy set in the distant future of the Milky Way galaxy. Check out her website, or follow her on Twitter (@CEBwrites) or Instagram (@caseybergerbooks)

Uncertainty, Observation, and the Quantum Retcon

When you go into a coffee shop to pick up your morning espresso drink, you may choose your order in the moment, depending on your mood. Or you may be the kind of person who has a “usual,” and the baristas already have it started when they see you come in the door. But whether you are a creature of routine or someone who mixes up their morning coffee order, in the end the drink you order is either a latte or a cappuccino. Either a chai or a cortado. Many possibilities may exist in your head when you step up to the counter, but when you speak your order aloud, those possibilities coalesce into one.

In the world of classical physics—the world we can see and hear and smell and touch—if you turn your back as the baristas make your drink, what they make won’t be a mixture of latte-cappuccino-chai-americano-cortado whose identity only solidifies when you take a sip and discover it is in fact a cappuccino. That cappuccino will have been a cappuccino-in-the-making the entire time.

Not so in the world of quantum physics. The classical world is a world of cause and effect. You order a cappuccino, and so the baristas extract the espresso and measure out the appropriate amount of milk for a cappuccino. At no point is there any chance of your drink being a latte except perhaps due to human error. But the quantum world is probabilistic. Things have a certain chance of being in any of many states, and until you actually measure the thing and interact with it, it is not definitively in any of those states.

Quantum mechanics describes the behavior of very small objects: particles smaller than the eye can see, like atoms or electrons. These particles make up the ordinary, classical objects that we interact with every day. The coffee cup you’re holding as you exit the shop is made up of more quantum particles than there are stars in the universe, and while each individual particle is governed by the bizarre laws of quantum mechanics, they work together to create the predictable nature of a cup of coffee that never ceases to be a cup of coffee (at least, until you’ve consumed it and your body is converting into other compounds).

This is historically kind of hard to swallow in the physics realm. It’s a reminder of how little we truly understand about the universe we inhabit. In fiction, quantum mechanics is often used as a substitute for magic. Have a plot device with no scientific explanation? It’s quantum. Did a thing that looks more like sorcery but you’re writing science fiction? It’s quantum.

The rules of the quantum world may seem very strange to us classical objects, but they are rules, which means there are patterns and expectations for their behavior that scientists can study and predict and test. Those rules often get lost when quantum mechanics appears in pop culture.

Make it so

We writers think things into existence every day. Our minds invent worlds, cultures, technologies, characters, historical events, and then we make them real inside our fictional creations. It’s no surprise, then, that the concept of uncertainty in quantum mechanics would be appealing to speculative fiction writers.

This use of quantum mechanics often appears in speculative works as character believes something so strongly that the entire fabric of reality changes to make it so. If we’re working with fantasy, magic is involved, but if it’s science fiction… well, that’s quantum mechanics, of course! In an early episode of Buffy the Vampire Slayer—a show that combines fantasy, science fiction, and the paranormal into one snarky, speculative blend—Giles calls this out exactly:

Giles:  Of course! I’ve been investigating the mystical causes of invisibility when I should have looked at the quantum mechanical! Physics.

Buffy:  I think I speak for everyone here when I say, huh?

Giles:  It’s a rudimentary concept that, that reality is shaped, even, even… created by our perception.

Buffy:  And with the Hellmouth below us sending out mystical energy…

Giles:  People perceived Marcie as invisible, and she became so.

Is it magic? Is it quantum mechanics? Why not both?

Uncertainty and observation

The physics behind this idea actually arises from two concepts. One is known as Heisenberg’s Uncertainty Principle (or often just “the uncertainty principle”) and as you might guess, it’s quite a bit more subtle than “wish it so; make it so.”

The uncertainty principle tells us something about the way physical properties in the universe are related to each other. Certain things (position and momentum or energy and time, for example) are linked in such a way that if you measured one with perfect precision, you could not measure the other with perfect precision as well. It doesn’t matter how good your measurement tools are—there is a fundamental limit. If you can say exactly where a particle is located when you measure it, then your measurement of that particle’s momentum cannot be exact. The best you can do is say a range it is in.

Doesn’t sound very much like something that could make a very lonely, very ignored high school girl turn invisible, does it?

The Observer Effect

The other concept that often gets wrapped up with the uncertainty principle is the observer effect, which is the idea that measurement in quantum systems is not neutral: the act of observing a quantum system actually alters the system. This is because quantum objects are not in a definite state (e.g. latte, cappuccino, cortado) until you measure them. They are in what’s known as a superposition of states—there exists any number of possible states, each with some probability, and the object’s full state is a combination of all those possibilities. This is known as superposition.

We can force a quantum system to choose one of many probable states by measuring it or interacting with it, but before that measurement occurs, it is spread across them all. Perhaps the most famous example of this is the double-slit experiment, in which light passing through two slits behaves as a wave and displays an interference pattern on the far wall. This all changes when you begin to measure the photons passing through each slit. Once the measurement determines the location of each photon, the interference pattern disappears, and instead there are two spots on the far wall, one for each slit. The photons were measured in one or the other slit, and therefore they were in that slit.

This combination of uncertainty and the ability of an observer to alter reality starts to sound quite powerful, and is unsurprisingly at the basis of a lot of quantum mechanics in the media. The cult classic Quantum Leap uses this concept often, leaning heavily on uncertainty and the observer effect to retcon the show on a regular basis. If reality is fundamentally quantum, and quantum systems are inherently uncertain and can be changed by simply observing them, then why can’t everything work that way? (With the correct amount of science applied, of course).

Applying uncertainty and observation

As demonstrated effectively by Douglas Adams, a great way to get away with a lot of handwaving in science fiction is to use humor. He uses the probabilistic nature of quantum mechanics to get around the universe’s speed limit (nothing can travel faster than light) in The Hitchhiker’s Guide to the Galaxy. Importantly, he throws in just enough real science with the humor to show that he knows the rules well enough to break them: 

The Infinite Improbability Drive is a wonderful new method of crossing interstellar distances in a few seconds; without all that tedious mucking about in hyperspace. As the Improbability Drive reaches infinite improbability, it passes through every conceivable point in every conceivable universe almost simultaneously. In other words, you’re never sure where you’ll end up or even what species you’ll be when you get there. It’s therefore important to dress accordingly.

If you’re not writing a humorous novel, there are other ways (demonstrated just as beautifully in this quote). One way to make your use of the uncertainty principle and the observer effect more true to the physics is to lean into it.

The entire universe is governed by quantum mechanics (at a small enough scale), so effects of randomness, probability, and superposition shouldn’t be confined to just one incident or one individual. Think about what broader effects might be in the world you’re building, and find other ways to incorporate this concept so that it’s embedded in the worldbuilding and not just a convenient excuse to do something that looks more like magic than science.

And most importantly, don’t just throw the word “quantum” in front of something if you can’t explain it.

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