Richard Feynman once said that all the mysteries of quantum mechanics are contained in the double-slit experiment. So perhaps the simplest way to explain what quantum mechanics is about is to start right there.
In elementary school, we learned that atoms are the smallest building blocks of matter that still retain its properties. For a long time, atoms were thought to be indivisible—hence the name—but eventually it was discovered that they consist of a nucleus surrounded by orbiting electrons. These electrons are tiny particles of matter.
Now, imagine an electron source—something that emits a single electron at random intervals in a random direction. Place a detection screen nearby. When an electron hits the screen, it leaves a tiny dot where it lands. Between the source and the screen, insert a barrier with a single narrow slit cut into it. If the electron hits the barrier, it bounces back. But if it happens to travel in the direction of the slit, it passes through and leaves a dot on the screen.
After many electrons have been fired, a stripe of dots builds up on the screen in line with the slit. So far, nothing surprising.
Cut a second slit next to the first one. You might expect a second stripe to appear next to the first. But that’s not what happens. Instead, the electrons form multiple bands on the screen—a pattern that resembles interference, like what you’d get with waves.
But electrons aren’t waves. Or… are they?
The experiment was later repeated using atoms and even small molecules, and the results were essentially the same. So it’s not just electrons that behave like waves, but in fact all matter. The reason we don’t notice this in everyday life is simply because the effect is only observable at very tiny scales.
To better grasp just how strange all of this is, let’s imagine ourselves as one of those tiny electrons flying out of the source. To keep things simple, we’ll only consider electrons that manage to pass through one of the slits and leave a mark on the screen.
When only one slit is open, the electron flies straight through it and hits the screen behind, leaving a single dot. Now let’s open the second slit. The electron still goes through one slit, but it somehow “notices” that the other slit is open too. Instead of continuing in a straight line, it veers off and lands elsewhere, perhaps closer to the center or the edge of the screen, helping to form an interference pattern with the other electrons.
But how does the electron know the other slit is open? Why does that change its path? And how can an electron, a tiny particle of matter, just decide to change direction on its own?
It wouldn’t be all that surprising if the electron were a wave after all; interference patterns are exactly what we expect from waves. But don’t celebrate too soon, because this picture also has its problems.
First of all, no one has ever actually seen an “electron wave.” Whenever we observe electrons, say, with a detection screen like the one we described earlier, they always appear as particles: a tiny dot at a specific location. So it’s not even entirely clear whether these waves actually exist, or if they’re simply mathematical descriptions of how an electron behaves.
If we think of the wave in that sense – as a mathematical object – then it’s a function that tells us the probability of finding the electron at a given place at a given time. Physicists call these probability waves wave functions.
The trouble with wave functions, as mentioned earlier, is that when we observe the electron, we always find it at one specific point – we never see it spread out like a wave. Physicists explain this by saying that the wave function collapses upon observation.
And this is where we dive into the deep waters of quantum mechanics: no one really knows what happens during that collapse.
Using the wave function, physicists can predict with astonishing accuracy the probability of where the electron will appear, but no one can tell exactly where that single electron will show up when we finally look for it.
This probabilistic nature and wave-like behavior are fundamental properties of elementary particles. For instance, if we place an electron in a very small box and leave it alone, there’s a certain chance that the next time we observe it, we’ll find it outside the box. It’s as if it had teleported, or, in more traditional terms, escaped through a tunnel.
This phenomenon is known as quantum tunneling. It’s also the reason why, beyond a certain point, we can’t continue shrinking the components of integrated circuits, like those in microprocessors. At extremely small scales, electrons start to “leak.” They simply escape from the wires or even jump into neighboring ones.
Thanks to quantum mechanics, we’ve seen tremendous technological progress across almost every field. And we’ll likely owe many future technologies to it as well, such as probabilistic or quantum computers, which promise exponentially greater computing power. That’s because we have incredibly powerful mathematical tools to describe quantum systems.
Using the wave function, we can calculate the probability of a particle’s position with astonishing precision. Yet we’re still in the dark about what determines where the wave function will collapse, what exactly causes that collapse, or even what the wave function really means.
There have been many attempts to answer these fundamental questions – these are known as the interpretations of quantum mechanics. There are about a dozen of them. None is clearly better or worse than the others, but they all contain at least one deeply counterintuitive idea that clashes with common sense.
One of the most accessible interpretations is the pilot-wave theory, associated with Louis de Broglie and David Bohm. According to this model, particles travel along a guiding wave that can interfere with itself, thus steering electrons toward the locations that form the interference pattern.
In this view, the electron remains a particle, but it’s accompanied by a wave that “guides” its motion.
The catch is that the guiding wave is non-local, meaning an interaction with one particle can instantaneously affect another particle, even one located on the opposite side of the universe. The effect is immediate.
It’s as if everything in the universe were somehow connected. For the guiding wave, space and distance seem not to exist. And to fully describe the state of just one particle, we’d theoretically need to know the state of all the other particles in the universe, since they all exert an influence. That’s… pretty bizarre.
This kind of non-locality is a key property in quantum mechanics, and to test whether it’s real, three physicists came up with a remarkably clever experiment. Their work was so groundbreaking that it earned them the 2022 Nobel Prize in Physics.
The core of the experiment involves creating a pair of entangled particles. In this context, entangled means the particles are linked in such a way that they complement each other in some measurable property.
For electrons, one such property is called spin. It’s often described as a kind of angular momentum, but in reality, electrons don’t actually “spin” like tiny tops. So it’s better to think of spin as an intrinsic property, like mass or electric charge.
Spin can be either “up” or “down,” and in an entangled pair, if one electron is spin-up, the other must be spin-down. In fact, an electron’s state can be more complex, something like up–down–up, and then its entangled partner would be down–up–down.
Spin is also described by a wave function, which, according to quantum mechanics, collapses the moment it is measured. Until that moment, the electron doesn’t have a definite spin value. This can be tested experimentally.
If spin were predetermined, then after repeating the same measurement many times, we would get a certain statistical distribution of results. But if quantum mechanics is correct, we get a different statistical pattern—one that cannot be explained by predefined values.
Imagine rolling a die. We can’t predict the result of any single roll, but after 100 rolls, we expect roughly equal counts of each number. If the results are skewed, with six showing up far more often, we might suspect the die is loaded. This is exactly how electron spin behaves: as if nature is using a loaded die. But the strange part is that how the die is loaded depends on how the measurement device is configured.
And since the measurement device only needs to be configured right before the measurement itself, it appears that the electron doesn’t decide its spin until that very moment. But once we measure the spin of one electron, the spin of the other electron – its entangled partner – must instantly fall into the opposite state.
This happens even if the two measurement devices are separated by thousands of light-years. That’s the kind of non-locality that’s built into quantum mechanics, and what makes it so hard to accept.
The experiment was originally based on a thought experiment by Albert Einstein, designed to disprove quantum mechanics’ non-local nature, which directly contradicts relativity. Einstein believed quantum mechanics was incomplete, and that there must be hidden variables, unknown but fixed values that determine things like spin before any measurement.
He assumed, for instance, that spin had a definite value all along, even if we could only find it out by measuring. Unfortunately, Einstein didn’t live to see the real-world experiment, which didn’t disprove quantum mechanics. On the contrary, it confirmed that non-locality is real.
So the spin really does acquire its final value at the moment of measurement, and in doing so, instantly changes the state of its partner particle, even one light-year away.
There have been attempts to eliminate this non-locality from quantum mechanics, but the alternative interpretations are no less bizarre. Take, for example, Everett’s “Many-Worlds Interpretation.” In this view, the wave function is real and never collapses. Instead, at every measurement, the universe splits into countless parallel universes—one for every possible outcome. In each universe, the electron ends up with a different spin.
I don’t know about you, but to me, the idea of an infinite number of universes sounds pretty wild.
And yet, when Richard Feynman wrote down the equations of quantum electrodynamics, he did so based on a very similar idea. He proposed that during the double-slit experiment, an electron doesn’t follow just one path, it explores all possible paths. If we add up all these possible paths, weighted by their probability, we get the final result: the interference pattern.
The only problem is that an electron can reach the screen in infinitely many ways. So to calculate the final result, you would have to sum up an infinite number of paths.
Most people don’t like dealing with infinities in math, but Feynman wasn’t most people. He figured out how to cancel out the infinities in a clever way, and using this method, he achieved incredibly precise results.
Quantum electrodynamics, thanks to Feynman’s approach, gave us a powerful understanding of how matter works. It helped explain chemical reactions and many other phenomena, and earned Feynman the Nobel Prize in Physics.
Still, in a later lecture, Feynman remarked that the mathematics he used was “absurd,” and admitted that he didn’t truly understand what he was describing. In fact, he claimed, no onein the world really understands quantum mechanics.
There are people who think they do—but, according to him,they’re mistaken.
If you’re uncomfortable with the idea of infinities, here’s another option: retrocausality.
Retrocausality means that an effect can come before its cause. But let’s not beat around the bush; this is essentially time travel. If we allow certain influences to travel backward in time, we can potentially avoid both non-locality and those pesky infinities.
In this interpretation, when we measure the spin of an electron, that act sends an influence backward through time, affecting its entangled partner. As a result, the spin is already “set” in just the right way at the moment of creation, so that when we later perform the measurement, we see the expected statistical pattern.
Put more simply: the electron “knows in advance” how the measurement device will be set up, and its spin adjusts accordingly before the measurement even takes place.
If retrocausality still feels too strange, there’s another life raft: superdeterminism.
In the experiment that confirmed quantum non-locality, there’s an unspoken assumption: that the scientists set their measurement devices randomly, just like rolling a die. Superdeterminism challenges that assumption. It claims that there is no such thing as randomness. The universe is completely deterministic, meaning everything is exactly predictable, and everything was already decided at the very beginning, at the moment of the Big Bang.
If that’s true, then there’s no need for signals traveling backward in time, infinities, or non-locality. The spin of the electron, the settings on the measuring device, and even the seemingly “weird” statistical results predicted by quantum mechanics, all of it was already baked into the universe from the start.
The math works out beautifully. You don’t need probabilities. You don’t need time travel. But the trade-off is enormous: the universe would have to be fine-tuned in a way that’s extremely hard to believe.
Think about it: if we repeat experiment 1,000 times, we’ll get the results quantum mechanics predicts all 1,000 times. According to superdeterminism, this happens because the universe decided at the very beginning, including how your brain would move your hand to set the measuring device just right to generate those exact results.
It’s like rolling a die and getting a six far more often than expected, but insisting the die isn’t rigged. “That’s just how the universe works,” you’d say. “Every time I roll it, I just happen to get a six.” Why? Because the outcome of every roll and the motion of your hand was already decided when the universe was born.
Sure, that’s possible… but it’s hard to believe.
And here’s the cherry on top: if we accept this kind of predetermination, then we have to give up on free will. Every choice we make was already written at the beginning of time. Our fate is carved in stone.
If we want to preserve free will and exempt it from the rigid rules of physical law, we end up back at the so-called “traditional” Copenhagen interpretation of quantum mechanics.
According to this view, the wave function collapses at the moment of measurement.
But there’s a problem: the measuring device itself operates according to the laws of quantum mechanics. So it can’t be the cause of the collapse. What, then, does cause the collapse? The conscious observer.
This interpretation was championed by Nobel laureate Eugene Wigner, who went so far as to suggest in his writings that we might need to return to a kind of dualism, the idea that consciousness is a fundamental entity, existing outside the material world governed by quantum laws.
Wigner wasn’t alone. Erwin Schrödinger also pondered the nature of consciousness extensively, often referencing Eastern philosophical traditions in his writings.
The philosophical questions surrounding consciousness remain unanswered to this day. And once we invite them into physics, we’re opening Pandora’s box.
We’re then forced to ask questions like: At what point does a living being become conscious? And more fundamentally: What is consciousness, anyway?
If none of the interpretations above seem convincing, don’t worry, there are more. You can find them on Wikipedia. But make no mistake: those are just as bizarre and mind-bending as the ones we’ve discussed.
Quantum mechanics is a tough nut to crack. It’s radically different from anything we experience in everyday life. It simply doesn’t fit the simplified mental models we’ve developed to make sense of the world.
When Einstein introduced the theory of relativity, he shook up our basic notions of space and time, but he gave us something in return: a four-dimensional spacetime fabric, like an invisible ether, which could be curved by massive objects and within which waves and particles behaved in predictable ways. That’s something we can imagine.
But quantum mechanics? It shatters our understanding of space and time entirely. Particles separated by light-years can affect one another instantly. Some effects might even travel backward in time.
And what do we have to replace these lost concepts? Only the mathematics.
If we accept the fundamentally probabilistic nature of reality, then everything works just fine. The behavior of quantum systems can be predicted with incredible accuracy, and our entire modern technological world is built on those predictions.
Maybe this is the best model we’ll ever have: a vast system running an unfathomable number of calculations. In a way, the universe is like a gigantic computer, and our consciousness is somehow plugged into it – an interface shaped by our senses.
And while we may never fully comprehend the true nature of reality, that’s not a failure, it’s a feature of being human. What we can take comfort in is this: Even if the models in our minds aren’t perfect, they still come remarkably close.
