Hoboken, May 22, 2024. I was jogging along the Hudson recently, thinking about quantum mechanics, when a creepy memory popped into my head. It dates back to the late 1970s, when I was painting houses in Denver. One evening after work I found myself in a saloon on Denver’s eastern outskirts. Behind the bar was an aquarium inhabited by a snaggle-toothed, snub-nosed, milky-eyed fish. A blind piranha, the bartender informed me.

Watch this, the bartender said. He netted a few minnows from a fishbowl and dropped them into the piranha’s lair. The piranha froze for an instant, then darted this way and that, jaws snapping, as the minnows fled. The piranha kept thudding into the glass walls of its prison; that explained its deformed snout. After gobbling all the minnows, the piranha resumed hovering listlessly.

What does this creature have to do with quantum mechanics? Here’s what. Quantum principles underpin our modern scientific worldview and much of our technology, including the laptop on which I’m writing these words. And yet a century after its invention, physicists and philosophers cannot agree on what quantum mechanics means. The theory poses deep questions about “reality,” whatever that is.

Richard Feynman advised us to accept that nature makes no sense. “Do not keep saying to yourself… ‘But how can [nature] be like that?’” Feynman warns in The Character of Physical Law, “because you will get down the drain, into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that.”

Most physicists follow Feynman’s advice. Ignoring the oddness of quantum theory, they simply apply it to accomplish tasks such as building better computers.

Another deep-thinking physicist, John Bell, deplored this situation. In his 1987 essay collection Speakable and Unspeakable in Quantum Mechanics, Bell chides physicists who apply quantum theory while blithely disregarding its “fundamental obscurity”; he calls them “sleepwalkers.” But Bell acknowledges that efforts to make sense of, or “interpret,” quantum mechanics have failed. He likens interpretations such as the many-world hypothesis and pilot-wave theory to “literary fiction.”

Today, there are more interpretations than ever, but they deepen rather than dispelling the weirdness at the heart of things. The more I dwell on puzzles such as superposition, entanglement and the measurement problem, the more I identify with the piranha. I’m blindly thrashing about for insights, epiphanies, revelations. Every now and then I think I’ve grasped some slippery truth, but my satisfaction is always fleeting. Sooner or later, I end up bouncing off an invisible barrier. I don’t really know what’s going on. I’m in the dark.

The main difference between me and the piranha is that it’s inside the aquarium, and I’m on the outside, looking in. My world is bigger than the piranha’s, I know things the fish can’t know. But it’s all too easy to imagine some enlightened, super-intelligent being standing outside our world, looking at us with the same pity and smug superiority that we feel toward the piranha.

Plato presents himself as this enlightened being in his famous parable of the cave, which I make my first-year humanities classes read every semester. The parable describes people confined to a cave for their entire lives. They are prisoners, but they don’t know they are prisoners. An evil trickster behind them has built a fire, by means of which he projects shadows of everything from aardvarks to zebras onto the cave-wall in front of the prisoners. The cave-dwellers mistake these shadows for reality. Only by escaping the cave can the prisoners discover the brilliant, sunlit reality beyond it.

We are the benighted prisoners in the cave, and Plato, the enlightened philosopher, is trying to drag us into the light. But isn’t it possible that Plato and other self-appointed saviors are charlatans? Or loons? Given our profound capacity for self-deception, isn’t it likely that when you think you’ve left the cave, you’ve actually just swapped one set of illusions for another? These are questions with which I torment my students. Below I paraphrase a few of their responses:

1.     The cave is just our ignorance. We escape the cave by going to college and studying science, history, philosophy and so on. We can reduce our ignorance still further with the help of reliable news sources like The New York Times or Fox News. 

2.     Plato wasn’t really talking about worldly knowledge; he was talking about spiritual enlightenment. We can leave the cave and see the light by praying or meditating or whatever and by heeding the teachings of enlightened sages like Plato, Buddha or Jesus.

3.     Ultimate reality is unknowable, so if we escape one cave, we just end up in another cave. It might have better furnishing and lighting, but it’s still a cave. No one ever totally escapes the cave, not even Plato, Buddha or Jesus. Maybe not even God. 

4.     Who cares if we’re in a cave or not? If we’re having fun, that’s all that matters.

The suggestion that not even God can escape the cave is mine. But my students come up with the other options on their own, with minimal prodding from me. Although few of my students have the courage to voice Option #4—Who cares?—I suspect it’s what many of them think, especially the business majors.

By the time we’re done with this exercise, I start feeling guilty about rubbing the faces of the non-business majors in the world’s inscrutability. To make them feel a little better, I bring up another possibility that usually doesn’t occur to them:

If we realize we’re in the cave, isn’t that the same, sort of, as escaping from it? Better yet, if we can never escape the cave, isn’t that the same as saying that there is no cave, because this everyday world is the one and only reality? And doesn’t that imply that we should just chill out and enjoy ourselves, like the business majors said?

Maybe. On good days, I look out the window of my apartment at the shining Hudson River, crisscrossed by boats, and at the Manhattan skyline, a symbol of humanity’s ever-growing power over nature, and I think, Yes, this is reality, there is nothing else. But then I remember the quantum mist at the core of reality, which not even the smartest sages can penetrate, and to which most of us are oblivious. And I remember the piranha, bumping over and over again into the walls of its prison, blind to its own blindness.

Further Reading:

On God, Quantum Mechanics and My Agnostic Schtick

Quantum Mechanics, the Chinese Room and the Limits of Understanding

Is the Schrödinger Equation True?

Defending My Naïve Realism

Is Derealization a Delusion or Insight?

Conservation of Ignorance: A New Law of Nature

Nicaragua, Quantum Mechanics and Other “Solutions” to Habituation

Self-plagiarism alert: This column is a vastly improved version of one I wrote for Scientificamerican.com. The blind piranha also makes an appearance in my free online book My Quantum Experiment.