Quantum physics

(the physics of matter and energy at atomic and subatomic scales)

What is quantum physics?

Quantum physics describes how matter and energy behave at very small scales — atoms, electrons, and photons — where the ordinary rules of everyday mechanics stop giving accurate predictions, and a different, well-tested set of rules takes over.

Wave-particle duality: an established experimental fact

Light and matter both show behavior associated with waves (like interference patterns) and behavior associated with particles (like arriving in discrete, countable units), depending on how they're measured. This was demonstrated directly by the double-slit experiment: light passed through two narrow, closely spaced slits produces an interference pattern on a screen behind them — alternating bright and dark bands — exactly the pattern expected from overlapping waves. Yet the same light, detected at the screen, always arrives in discrete, individual units (photons), never as a smoothly spread-out wave. This dual behavior has been confirmed repeatedly, with light, electrons, and even larger particles, and is not a matter of interpretation — it is directly observed.

The photoelectric effect

Shining light on certain metals knocks electrons loose from the surface — but only if the light's frequency is above a certain threshold, no matter how bright the light is. Dim light above the threshold frequency ejects electrons immediately; extremely bright light below the threshold ejects none at all. This puzzled physicists until Albert Einstein explained it in 1905 (for which he later won the Nobel Prize): light itself is made of discrete packets of energy called photons, each with energy E = hf, where f is the light's frequency and h is a fixed constant (Planck's constant). Only a photon with enough individual energy — meaning a high enough frequency — can knock an electron loose, regardless of how many lower-energy photons (dim or bright, below the threshold) arrive.

Quantization: energy in discrete steps

Many quantities that classical physics treats as continuous — like the energy an electron can have inside an atom — turn out, at this scale, to only take on specific, discrete values, with nothing allowed in between. This is where the word "quantum" comes from: physical quantities changing in fixed, indivisible steps (quanta), rather than smoothly.

Why quantum physics matters

Quantum physics isn't an abstract curiosity — it correctly and precisely predicts the behavior of atoms, and every laser, transistor, and computer chip relies directly on quantum mechanical effects to function. It is one of the most rigorously and repeatedly tested theories in the history of science, confirmed to extraordinary precision in laboratory experiments.