QED: The Strange Theory of Light and Matter: Summary & Key Insights

In the old days, we believed light was a wave, a continuous undulation through a mysterious medium called the ether. But the more we looked, the more that idea began to fail. Experiments—starting with Einstein’s explanation of the photoelectric effect—revealed that light behaves as if it’s made up of particles, distinct packets of energy we now call photons. Each of these photons carries a specific amount of energy determined by the light’s color or frequency.

When you shine light upon a mirror, each photon bounces off the surface at just the right angle to make reflection seem smooth and perfect. Yet, microcosmically, it’s not orderly at all. Each photon could bounce from any atom on the mirror, could take any possible path, and might not even reflect at all! Still, when you average the collective behavior of trillions of photons, what you perceive is that neat angle of reflection—the angle of incidence equaling the angle of reflection—that seems so reassuringly classical.

The same story holds for refraction: light bends in water or glass because the photons traveling through the new medium interact with the electrons of the atoms, slowing their effective propagation. But again, this is no deterministic process. It’s the combined sum of all the possible paths, weighted by their probabilities, that constructs the smooth beam of refracted light your eyes detect.

Understanding light as made up of particles doesn’t destroy the wave picture—it completes it. Light sometimes behaves like a wave, sometimes like a particle, but deep down, it’s neither. It’s something subtler: a combination of potentialities, each contributing its possibility to the final observable event. That’s why I say that photons don’t decide whether to act as a wave or a particle—they just do whatever quantum mechanics allows them to do, and both options emerge naturally depending on how we look at them.

Once you accept that light is composed of photons, the next step is stranger: these photons don’t have fixed paths or deterministic outcomes. They operate under probabilities—but not the ordinary kind. In the quantum world, we use what I call probability amplitudes. You can think of each amplitude as a little arrow with a direction and a length. The direction represents the phase or the timing of the event, while the length represents how likely that event is to occur.

When two or more quantum events might give the same observable result—say, a photon reflecting off two different layers of a glass sheet—we add their arrows together, tail to tip. The total probability amplitude is the sum of these arrows, and only after adding them do we calculate the probability, which is the square of the amplitude’s length.

Sometimes these arrows point in similar directions, reinforcing each other and making the event more probable. Sometimes they point oppositely, canceling each other and making the event less likely or even impossible. This simple geometric picture allows us to grasp interference, the heart of quantum mechanics. What seems like mysterious self-cancellation of waves is simply the addition of these unseen arrows, each representing a possible path.

In our everyday life, the effects of interference are usually washed out because so many random factors scramble the phases. But in controlled experiments—like shining laser light through two slits—you can see interference vividly. Those bright and dark fringes on the screen are direct manifestations of the arrows adding up differently at each point. It’s the first window into the inherently probabilistic nature of reality.