The Double Slit Experiment: When Reality Gets Weird

If you want to understand why physicists lose sleep at night, look no further than the double slit experiment. This famous experiment, involving subatomic particles such as electrons and photons, is a deceptively simple setup that has been bending minds for over two centuries, and it remains one of the most profound demonstrations of quantum mechanics’ strange nature.

Introduction to Quantum Physics

Quantum physics is the science that explores how the universe behaves at its smallest scales—think atoms, electrons, and photons. Unlike the predictable world of classical physics, quantum physics is full of surprises, where particles can act like waves and waves can act like particles. This is known as wave-particle duality, and it’s at the heart of some of the most famous experiments in science.

One of the best ways to see quantum physics in action is through the double slit experiment. In this classic slit experiment, a beam of particles—whether electrons, photons, or even atoms—passes through two parallel slits. Instead of forming two simple bands on the screen behind, the particles create an interference pattern, a series of light and dark stripes. This pattern is the result of constructive and destructive interference, where the waves from each slit combine to amplify or cancel each other out. The double slit experiment shows that even things we think of as solid particles can display wave like behavior, challenging our everyday understanding of how the world works.

The double slit experiment has been performed with all sorts of particles, and every time, it reveals the same strange truth: at the quantum level, reality doesn’t play by the rules we expect. It’s a powerful demonstration of wave particle duality and a cornerstone of quantum physics.

Thomas Young’s Contribution

Back in 1801, British scientist Thomas Young performed what would become the original double slit experiment. Using a beam of light passing through two parallel slits, Young observed an interference pattern of alternating light and dark bands on a screen behind the slits. This was a groundbreaking discovery—light, which many thought behaved only as particles, was clearly showing wave like behavior.

Young’s experiment provided strong evidence for the wave theory of light, challenging the prevailing particle theory of his time. By demonstrating that light could interfere with itself, creating patterns only possible with waves, Young laid the foundation for future explorations into the nature of light and matter. His work with parallel slits and the resulting interference pattern became a key stepping stone toward the development of quantum mechanics, forever changing our understanding of the universe.

What is the double slit experment?

The Setup

Imagine you have a barrier with two parallel slits cut into it—these two slits are crucial for the experiment—and behind that, a screen to detect whatever passes through. Now fire something—let’s start with particles like tiny balls—at the barrier. Some will go through one slit, some through the other, and you’ll see two distinct bands as the pattern on the screen behind them. Exactly what you’d expect.

Now try it with waves, like ripples in water—water waves are a helpful analogy here. The waves pass through both slits and interfere with each other, producing interference patterns—peaks meeting peaks create bigger waves through constructive interference, while peaks meeting troughs cancel out. When two waves overlap, their superposition leads to these effects. On your screen, you’ll see multiple bands forming a diffraction pattern, which is characteristic of light waves as well. This is classic wave behavior, and it makes perfect sense: as light passes through the two slits, it is diffracted, and the resulting light diffracted produces the observed pattern on the screen.

In contrast, if you use a single slit instead of two, the pattern on the screen changes, showing a different diffraction pattern that further highlights the wave nature of light.

Enter the Electrons in the Double Slit

Here’s where things get interesting. Fire electrons (tiny particles of matter) at the slits one at a time—each as an individual electron. You might expect them to behave like little bullets, creating two bands. Instead, you get a double slit interference pattern—the signature of waves—demonstrated at the atomic scale. Even though you’re sending them through one at a time, each electron somehow “knows” about both slits and creates a wave-like pattern, showing that it behaves as both a particle and a wave in this quantum system.

It’s as if each electron is interfering with itself, passing through both slits simultaneously. This behavior is described by the electron’s wave function, which governs the probability of where the electron will be detected. When a measurement is made to determine which slit the electron passes through, its particle like behavior is revealed, and the interference pattern disappears. Similar results are observed when single photons are used—each photon passes through the slits and forms an interference pattern, highlighting the quantum states of light. Other particles, such as atoms and even molecules, have also been used in these experiments, further demonstrating quantum behavior at the atomic scale.

The Observer Effect

But wait—it gets weirder. Decide to measure which slit each electron actually goes through. Set up a detector to watch. The moment you do this, the interference pattern vanishes—here, the interference pattern disappears, and the interference fringes are no longer visible. Now you get two bands, just like particles, similar to the result when only one slit is open. The pattern on the screen changes from a series of interference fringes to two distinct bands, indicating that the electrons are now taking only one path through the apparatus. If you compare the cases of one or two slits being open, you see that opening the second slit is essential for the interference pattern to appear; closing it removes the possibility of interference. The electrons start behaving like particles simply because you’re watching them, and the measurement determines which path—one path—the electron takes. When the path difference between the two slits is half a wavelength, destructive interference occurs, resulting in dark bands on the screen. Importantly, according to standard quantum physics, the presence or absence of a conscious observer does not affect whether the interference pattern disappears; it is the act of measurement itself that matters.

The act of observation fundamentally changes the outcome.

Quantum Eraser

The quantum eraser phenomenon takes the weirdness of the double slit experiment to a whole new level. Imagine you set up the experiment so you can tell which slit a particle—like a photon or electron—passes through. Normally, this destroys the interference pattern, and you see two bands instead of the familiar wave like interference pattern. But what if you could somehow erase that which-path information after the particle has already gone through the slits?

That’s exactly what the quantum eraser does. By cleverly designing the experiment, scientists can “erase” the information about which path the particle took, even after it has passed the slits. When this information is erased, the interference pattern reappears, as if the particle never revealed its secret. Experiments like the delayed choice and quantum eraser experiments have shown that the very act of knowing—or not knowing—which slit a particle passed through can change the outcome, even retroactively. The quantum eraser phenomenon is a striking reminder that in the quantum world, reality is deeply tied to what we can and cannot know.

What Does the Interference Pattern Mean?

This experiment reveals something unsettling about reality at the quantum scale. Before measurement, particles exist in what’s called a superposition—they’re not in one place or another, but in a sort of probabilistic haze of possibilities. When a measurement is made, it affects the quantum system, causing the superposition to collapse into a definite state. The electron doesn’t “choose” which slit to go through until something measures it.

Physicist Richard Feynman called this “the only mystery” of quantum mechanics, arguing that all other quantum strangeness stems from this fundamental weirdness. The experiment is a cornerstone of quantum theory and demonstrates wave-particle duality: quantum objects aren’t purely waves or purely particles, but exhibit both wave nature and particle properties depending on how we observe them.

Conclusion

Why it matters?

The double slit experiment isn’t just a curiosity—it’s the foundation of quantum mechanics, which underlies all modern electronics, from smartphones to solar panels. This famous experiment, first performed by Thomas Young and others, is a landmark experiment performed to reveal the wave-particle duality of light and matter. It challenges our classical intuitions about reality and raises profound questions about the nature of observation, measurement, and existence itself.

In the original experiment, known as Young’s double slit experiment, visible light was passed through two closely spaced double slits in free space, producing a striking double slit interference pattern on a screen. The same experiment has since been repeated with electrons, atoms, and even molecules, showing that not only light waves but also particles exhibit interference. Modern versions often use laser light of a single wavelength to create clear and precise interference fringes, further confirming how light behaves as both a wave and a particle. In contrast, the single slit experiment produces a different diffraction pattern, highlighting the unique role of double slits in demonstrating wave behavior.

More than 200 years after Thomas Young first performed a version of this experiment with light, we’re still grappling with its implications. The way light behaved like a wave in the original experiment, and how light behaves in modern setups, continues to fascinate physicists. The passed light through the slits creates patterns that can only be explained by considering light as a light wave, not just as particles. Double slit interference remains central to our understanding of quantum mechanics, and the experiment itself is often used as a thought experiment to inspire debate and research.

The next time someone tells you they understand quantum mechanics, remember: if the double slit experiment doesn’t unsettle you at least a little, you might not be thinking about it hard enough.