It's only natural to think that light travels from point A to point B in a straight line. After all, it explains the idea behind line of sight: place an object between a light source and your eyes, and the light stops reaching you — as if it had marched in a perfectly straight beam. The reality is that this picture, while useful, is an approximation of something far stranger.

The double-slit experiment

The simplest way to see what light is actually up to is the double-slit experiment.[1] A beam of laser light is directed at a barrier with two narrow slits. If photons — the fundamental quanta of light — travelled in straight lines, we'd expect exactly two bright bands on the detector screen behind the barrier, one per slit. Instead, we see many alternating bright and dark bands spread across the screen: an interference pattern.

A schematic of the double-slit experiment. Light fans through both slits simultaneously. The overlapping probability waves produce alternating bright and dark bands — not the two bands a classical particle would leave.

Here's the genuinely remarkable part: this experiment has been run with the source dimmed so low that only one photon at a time is in flight. Each photon produces a single dot on the screen. And yet, after thousands of such dots accumulate, the same multi-band interference pattern emerges.[2] There's no second photon for each photon to interact with. Each photon interferes with itself.

This is wave–particle duality: every photon exists as a probability wave that simultaneously explores both slit paths until it encounters the screen and deposits its energy at a single point. The single-point detection is the particle behaviour, and the simultaneous existence of individual photons across space and time is the wave behaviour as evidenced by the interference pattern it builds over time.

What "passing through a slit" really means

It's tempting to say each photon has a 50–50 chance of having gone through one slit or the other. But that framing, while intuitive, is subtly wrong. Without a measurement that determines which slit the photon used, it doesn't take one path or the other — it takes both, in quantum superposition. The moment you add a detector at the slits to find out which one it went through, the interference pattern vanishes entirely. What you get instead are the two bands classical intuition predicted. That's not a limitation of experimental technique. Gaining path information and preserving interference are mutually exclusive — a fundamental consequence of the uncertainty principle, confirmed to remarkable precision by a 2025 experiment at MIT that stripped the apparatus down to its quantum essentials.[3]

What exactly is oscillating

We've mentioned the probability wave of a photon several times so far without providing a rigorous definition. So what exactly oscillates like a wave?

Photons do not move up and down like a sinusoidal wave as they are often depicted in textbooks. What actually oscillates are the electromagnetic field vectors. Electric and magnetic fields are functions that take a position in space as a three dimensional vector and return another 3 dimensional vector. This is analogous to how wind operates: wind has a speed and a direction at each point in space, and it varies through time. The difference between wind field and electric and magnetic fields is the object they act upon: wind acts on mass, while electromagnetic fields act on charged particles. The relationship between electric and magnetic fields are fully described by Maxwell's equations, the foundation of classical electromagnetism.

The probability of detecting a photon in a given region of space can be approximated (some handwaving here) by computing the electromagnetic energy density at each point — which involves squaring the magnitude of the field vectors — and integrating over that region. This rests on the Born rule of quantum mechanics: detection probability is proportional to energy density. Intuitively, wherever the field carries more energy, the photon is more likely to be found.

Quantum mechanics or classical electromagnetism?

Photons and probability waves are objects of quantum mechanics. For the next two articles we will focus on classical electromagnetism — the framework Maxwell completed in the 1860s, in which there are only continuous electric and magnetic fields evolving deterministically through space and time. Even though classical EM is more than 150 years old, it remains highly relevant today: it accurately describes how light behaves at the macroscopic scale, and it is the foundation upon which much of modern technology and quantum electrodynamics is built. Quantum electrodynamics provides the precise description for how light behaves at the quantum scale by unifying the theories of classical EM, quantum mechanics, and special relativity.

References

  1. Wikipedia contributors. Double-slit experiment. Wikipedia, The Free Encyclopedia. en.wikipedia.org/wiki/Double-slit_experiment
  2. Halder, M. et al. "Time-resolved double-slit interference pattern measurement with entangled photons." Scientific Reports 4, 4685 (2014). nature.com/articles/srep04685
  3. MIT News. "Famous double-slit experiment holds up when stripped to quantum essentials." July 2025. news.mit.edu