In nature, waves interact with each other differently than solid objects do. Imagine throwing two rocks into a pond—the waves they create merge and influence each other in a smooth, interconnected way. Now picture two lines of billiard balls colliding. Instead of merging, they crash and scatter as individual objects. One behaves like a wave; the other like distinct particles.

In physics, the double-slit experiment reveals that tiny particles—like photons and even atoms—can behave as both waves and particles. That alone is fascinating and mysterious. But even stranger, when not observed, they behave like waves, much like ripples on water. The moment we observe them, they act like particles, smashing together like billiard balls. This bizarre shift challenges our understanding of reality and remains one of the deepest unresolved questions in theoretical physics.

Personally, I like to frame the double-slit experiment as an ongoing debate about free will in physics. It is one of the key experiments that suggests the existence of true randomness in the universe. If true randomness exists, then free will may be real. If not, then we may only think we have free will—but don’t.

Let me explain. First, we’ll explore wave-particle duality in physics—then we’ll get philosophical.

The Experiment

In the modern double-slit experiment, physicists send single atoms toward a barrier with two narrow slits. But instead of behaving like a tiny bullet and hitting exactly where we aim, each atom follows a path that appears both random and predictable at the same time.

Think about what could happen to the atom:

• It could pass through the first or second slit and land on a screen behind the barrier.
• It could miss both slits entirely and bounce back.
• It might clip the edge of a slit, altering its trajectory.

All of that makes perfect sense in a world where objects follow predictable paths.

But here’s where things get weird. Physicists observe what appears to be true randomness. The path of each individual atom is not certain. Over time, as more and more atoms are sent through the slits, their landing spots form a pattern—a pattern that looks just like a wave.

Before the double-slit experiment, scientists believed that light, like sound, required a medium to travel through. We now know that’s not true—light doesn’t need a medium. Sound, on the other hand, cannot travel in a vacuum—a fact that ruins (or at least diminishes) most sci-fi movies for physicists.

The Wave-Particle Duality

The double-slit experiment demonstrates that everything has a wave-like nature—including particles, atoms, molecules, and, in principle, even you and me. However, in our universe, we can only observe this effect clearly for objects up to about 10,000 atoms in size. Beyond that, the wave properties become negligible. Still, the fact that everything has a wave nature—especially at very small scales—remains one of the great mysteries of physics.

This phenomenon is known as the wave-particle duality of matter. It was first observed in experiments with light, then later confirmed for electrons, atoms, and even complex molecules.

In everyday life, we instinctively understand the difference between waves and particles. For example, noise-canceling headphones work because sound is a wave—by generating opposite sound waves, they cancel out external noise. But if someone fires a machine gun at you, there’s no magical “anti-bullet wave” that can cancel out the incoming bullets. However, in the quantum world, particles, atoms, and even small molecules can, in a sense, interfere and cancel each other out—strong evidence that, at tiny scales, they behave like waves.

From Light to Particles

For centuries, scientists have explored the nature of waves and particles, starting with light. We’ve known since ancient times that sound requires a medium to travel through. Aristotle and others speculated about this relationship, and by 1801, Thomas Young’s double-slit experiment demonstrated that light behaves like a wave, seemingly traveling through a medium.

At the time, scientists assumed space contained a mysterious medium called “ether” that allowed light waves to propagate. This idea held up until the famous Michelson-Morley experiment in 1887, which proved that space is essentially a vacuum. With no ether to act as a medium, scientists were forced to reconsider the nature of light, leading many to conclude that it must be a particle—a photon.

Then, in 1905, Einstein’s work on the photoelectric effect showed that light behaves as discrete photon particles when it interacts with matter. By the 1920s, quantum mechanics had expanded on this, demonstrating that photons—and later, electrons and atoms—exhibit both wave and particle behaviors. This was deeply puzzling and led to competing interpretations of what reality actually is.

The Mystery Deepens

Since then, experiments have confirmed that all quantum objects—photons, electrons, atoms, and even molecules—move both as particles and waves. Some physicists interpret this as evidence of true randomness in the universe. Others argue that it points to a multiverse—where all possible paths exist in parallel, and what seems random in one universe is simply part of a larger, wave-like reality spanning many worlds.

But here’s where things get even stranger. In the double-slit experiment, if we try to observe which slit an electron goes through, the interference pattern disappears. If we only look at where the atom lands on the screen, we see a wave-like pattern. But the moment we try to watch it pass through the slit in real-time, the pattern collapses into something completely different.

And to this day, no one can fully explain why.

Philosophical Viewpoint

Science focuses on observations and predictions—measuring what happens and forecasting what will happen next. Philosophy, on the other hand, steps back and asks: What does it all mean?

We know the universe is made up of just 17 or 18 fundamental particles and four fundamental forces. But do these things exist in a fixed state, simply flowing deterministically like a river? Are we being carried along by an inevitable tide? Or can we make decisions and “surf” the flow, exerting some degree of control?

Science allows us to predict many things with astonishing accuracy, but can it predict everything with absolute certainty? Or are there some aspects of reality that are inherently unpredictable? This is where physicists are divided. Some argue that true randomness exists in the universe, while others suggest that what looks random may have hidden, deterministic causes.

This question ties directly to free will. If true randomness exists, then perhaps we can make independent choices. If everything is deterministic—if all events are simply the result of past conditions—then maybe our sense of free will is an illusion.

Does the Multiverse Solve the Mystery?

Some physicists take the randomness we observe—such as the unpredictable movement of particles, atoms, and molecules in quantum experiments—as evidence that free will exists within our single known universe. Others take an entirely different approach, arguing that what appears random to us is actually the result of the multiverse.

In a multiverse, all possible paths exist simultaneously across an infinite number of universes. From the perspective of any one universe, the path of a single atom appears random. But if you step back and consider all universes together, these paths form an interconnected, wave-like structure. In this view, randomness is an illusion—our reality is simply one thread in a much larger tapestry of quantum possibilities.

Some physicists suggest that quantum entanglement—where particles remain mysteriously connected across vast distances—is tied to the multiverse or at least to hidden dimensions within our own universe. You might recognize this part of physics from Einstein’s famous phrases: “spooky action at a distance” and “God does not play dice with the universe.” These ideas became part of the public consciousness, reflecting the deep paradoxes of quantum mechanics that remain unresolved.

The Deep Paradox of Reality

At its core, the double-slit experiment reveals a fundamental duality in nature. The quantum world exhibits both deterministic (wave-like) and probabilistic (particle-like) behaviors. This duality is at the heart of quantum mechanics and challenges the classical, Newtonian idea of a purely deterministic universe.

Whether this means free will is real—or that we are simply riding the wave of an inevitable cosmic script—is one of the biggest unanswered questions in both science and philosophy.