Introduction to
Quantum Physics

For centuries, Newton's laws described the universe with elegant precision. Apples fell, planets orbited, billiard balls collided — and every result could be predicted if you knew the initial conditions. The universe was a clockwork machine.

Then, around 1900, physicists started looking at the very small. They studied atoms, electrons, and light at tiny scales. And what they found broke every rule. Particles behaved like waves. Waves behaved like particles. Things existed in two places at once until someone looked at them. The clockwork universe had a hidden basement — and down there, nothing made intuitive sense.

This lesson is your first trip into that basement. No equations required. Just open eyes and a willingness to let go of "common sense."

In 1801, Thomas Young set up one of the most famous experiments in physics. He fired light through two narrow slits and watched the pattern on a screen behind them. If light were particles — like tiny bullets — you'd expect two bright bands, one behind each slit.

Instead, he saw an interference pattern: alternating bright and dark bands, like ripples in a pond crossing each other. Light was behaving as a wave. But decades later, Einstein showed that light also comes in discrete packets — photons — that behave like particles.

Here's where it gets unsettling: when you fire single photons one at a time, the interference pattern still builds up. Each photon somehow goes through both slits simultaneously and interferes with itself. Switch to a single slit below and watch the pattern change.

You probably learned about the atom as a miniature solar system: electrons orbiting the nucleus like planets around a star. That picture, the Bohr model from 1913, was a crucial step forward — it explained why atoms emit light in specific colors. But it was still wrong in a fundamental way.

Electrons don't orbit. They don't even have definite positions. Instead, an electron exists as a probability cloud — a smeared-out region of space where the electron is likely to be found. These regions are called orbitals, and their shapes are nothing like circles. The simplest (s orbital) is a sphere. The next (p orbital) is a dumbbell. The d orbital looks like a four-leaf clover.

Each orbital represents a specific energy level. An electron can only exist at those discrete levels — it can never hover "in between." This is quantisation: the idea that at tiny scales, energy comes in specific amounts, like steps on a staircase rather than a ramp.

Imagine rolling a ball toward a hill that's too high to climb. In the classical world, the ball bounces back every time. That's obvious. Obvious — until you go quantum.

At the subatomic level, particles can pass through barriers they shouldn't have enough energy to overcome. It's called quantum tunneling, and it's not a rare curiosity — it's how the Sun generates energy. The fusion reactions inside our star happen because protons tunnel through the electric repulsion that should keep them apart.

The trick is that a quantum particle isn't just a hard ball. Its wave function extends through the barrier, and if the barrier is thin enough, there's a real probability the particle will appear on the other side. No secret passage. No trick. Just probability doing what probability does at quantum scales.

This is probably the most famous — and most misunderstood — idea in quantum physics. Schrödinger's cat is both alive and dead? Not quite. But the underlying principle is real, and it's stranger than the metaphor.

A quantum particle can exist in a superposition of states. An electron can spin "up" and "down" simultaneously. A photon can be polarised in two directions at once. These aren't two possibilities waiting to be revealed — the particle genuinely has both properties until a measurement forces it to "choose."

This is called wave function collapse. Before measurement, the mathematics says the particle is in a blend of states. After measurement, it snaps to one definite result. What triggers the collapse? That question has been argued about for a hundred years — and it still doesn't have a universally agreed answer.

In 1935, Einstein, Podolsky, and Rosen published a paper arguing that quantum mechanics must be incomplete. Their reasoning involved a phenomenon that Einstein famously called "spooky action at a distance" — what we now call quantum entanglement.

When two particles become entangled, their properties are linked. Measure one particle's spin and you instantly know the spin of the other — no matter how far apart they are. Not at the speed of light. Instantly. This isn't sending information faster than light (the no-communication theorem prevents that), but the correlation itself is immediate and has been verified in countless experiments.

In 2022, Alain Aspect, John Clauser, and Anton Zeilinger shared the Nobel Prize in Physics for their experiments with entangled photons, proving that this effect is real and cannot be explained by any local hidden variables. Einstein was right that it was spooky. He was wrong that it couldn't be real.

Quantum physics isn't just a thought experiment. It's the foundation of modern technology. Every smartphone, every computer chip, every LED screen, every MRI machine, every laser pointer in your desk drawer — all of them rely on quantum mechanics to function.

Transistors — the billions of switches inside every processor — work because of quantum tunneling and band gap physics. Lasers work because of stimulated emission, a quantum process Einstein predicted in 1917. MRI machines detect quantum spin states of hydrogen nuclei in your body to build images of your organs.

And the next wave is coming: quantum computing. Instead of classical bits (0 or 1), quantum computers use qubits that can be in superposition — effectively encoding 0 and 1 simultaneously. The Bloch sphere below shows how a qubit's state can point anywhere on a sphere, not just up or down.