Introduction to Quantum Physics

A different set of rules

Everything you have learned in physics so far — forces, motion, energy, gravity — belongs to what we call classical physics. It describes the everyday world brilliantly: balls, cars, planets, bridges. But around the year 1900, physicists discovered that when you zoom right down to the scale of atoms, electrons and particles of light, nature stops following these familiar rules. Down there, a strange and beautiful new set of laws takes over. We call it quantum physics (or quantum mechanics).

Quantum physics is famous for being weird — particles that act like waves, things that seem to be in two places at once, and a fundamental limit on what we can know. Richard Feynman, one of its greatest contributors, joked that "nobody really understands quantum mechanics." And yet it is the most precisely tested theory in the history of science, and it powers much of modern technology. To explore it, we need to start with the building blocks of matter — see introduction to atoms.

The quantum idea: energy comes in packets

The word quantum means "a fixed amount" (its plural is quanta). The founding idea of quantum physics is that energy is not smooth and continuous — it comes in tiny, indivisible packets.

Think of money. You might imagine you could pay any amount at all, but in fact the smallest unit is one penny; you cannot pay half a penny. In the same way, energy at the atomic scale comes in smallest "coins." In 1900, Max Planck found he could only explain the light glowing from hot objects if energy was emitted in discrete chunks. A few years later, Albert Einstein showed that light itself comes in packets, now called photons — each photon carrying a fixed amount of energy depending on its colour (frequency). Blue light photons carry more energy than red ones. This explained the photoelectric effect, where light knocks electrons out of metal, and it won Einstein his Nobel Prize.

This "graininess" is invisible in everyday life because the packets are unimaginably small, just as a beach looks like a smooth surface until you get close enough to see individual grains of sand.

Electrons in atoms only sit on certain "steps"

The quantum idea immediately explains something about atoms. Electrons around a nucleus cannot have just any energy — they can only occupy certain fixed energy levels, like steps on a staircase rather than points on a ramp. An electron can jump from one step to another, but never rest in between.

When an electron drops from a higher step to a lower one, it releases the energy difference as a single photon of light of a specific colour. This is why each chemical element glows with its own unique set of colours when heated — the basis of the bright lines you see when you split light through a prism, and how astronomers identify what distant stars are made of. The colours are a direct fingerprint of the quantum energy levels inside atoms.

Wave-particle duality

Here is where things get genuinely strange. Classically, a wave (like ripples on water) and a particle (like a tiny ball) are completely different things. But in the quantum world, the same object can behave like both.

• Light had long been understood as a wave — it ripples, bends and interferes like the patterns covered in waves and vibrations. Yet the photon experiments show it also arrives in particle-like packets.
• Matter, surprisingly, works the other way too. In 1924, Louis de Broglie proposed that particles such as electrons also have a wavelength and can behave like waves. Experiments soon proved him right — electrons can be made to diffract and interfere, just like light. This is the basis of the electron microscope.

This dual nature is called wave-particle duality. A quantum object is not "really" a wave that sometimes looks like a particle, nor the reverse. It is something genuinely new, and which face it shows depends on how you choose to look at it.

The double-slit experiment: nature's deepest mystery

No experiment captures quantum strangeness better than the double-slit experiment, which Feynman called the heart of quantum mechanics.

Fire a beam of light or electrons at a barrier with two narrow slits, with a screen behind. If they were simple particles, you would expect two bright bands behind the two slits. Instead you get an interference pattern — many alternating bright and dark bands — exactly what happens when waves pass through both slits and overlap, reinforcing in some places and cancelling in others. So far, so wave-like.

Now the twist. Fire the electrons one at a time, so only a single electron is ever in the apparatus. Each one lands as a single dot — clearly a particle. But as thousands of dots build up, they slowly form... the interference pattern again. Each lone electron seems to pass through both slits at once, interfering with itself, before landing as a single dot.

Stranger still: if you place a detector to watch which slit each electron goes through, the interference pattern vanishes and you get two simple bands. The mere act of measuring which path was taken changes the outcome. The electron behaves as a wave when unobserved and as a particle when its path is measured.

Superposition and probability

To describe this, quantum physics says that before measurement, a particle exists in a superposition — a blend of all its possible states at once. The unmeasured electron is, in a real sense, "going through both slits." Only when it is measured does it settle into one definite outcome.

Crucially, quantum physics can only predict the probability of each outcome, not the exact result of a single event. This troubled even Einstein, who protested that "God does not play dice." But every experiment since has confirmed that nature really is, at its core, probabilistic. We can predict the pattern of millions of electrons with stunning precision, but never exactly where the next one will land.

The uncertainty principle

In 1927, Werner Heisenberg discovered another deep limit. His uncertainty principle states that there are pairs of properties — most famously a particle's position and its momentum (mass × velocity) — that cannot both be known precisely at the same time. The more accurately you pin down where a particle is, the more uncertain its momentum becomes, and vice versa.

This is not a failure of our instruments — it is a fundamental feature of reality. A quantum particle simply does not possess a perfectly definite position and momentum simultaneously. It is one reason electrons are pictured as fuzzy "clouds" of probability around a nucleus rather than tiny planets on neat orbits.

The same fuzziness has a remarkable consequence: empty space is never truly empty. Particle pairs constantly flicker in and out of existence — the very effect behind Hawking radiation, discussed in black holes explained.

Why quantum physics matters

For all its strangeness, quantum physics is profoundly practical. It explains why materials are solid, why the periodic table has the shape it does, and how chemistry works. It made possible:

• Lasers and LEDs, which rely on electrons jumping between energy levels.
• Computer chips and transistors, the heart of every phone and computer.
• MRI scanners, solar panels, digital camera sensors and atomic clocks (which keep GPS accurate).
• And, at the frontier, quantum computers, which use superposition to tackle problems impossible for ordinary machines.

Roughly a third of the modern economy rests on technology that works only because the quantum rules are true.

Try it yourself! 🧪

You cannot run a true double-slit experiment at home easily, but you can see wave interference with your own eyes and understand the principle behind it.

Demo 1 — interference with light. In a dark room, shine a small bright light (a laser pointer is ideal but be careful never to aim it at eyes) at a wall. Hold up a piece of fine fabric, a feather, or two pencils held almost touching to make a tiny gap, in front of the beam. You will see the light spread into a pattern of bright and dark stripes rather than a single dot. Those stripes are an interference pattern — direct evidence that light travels as a wave, exactly the wave behaviour that makes the double-slit result so puzzling when single particles produce the same pattern.

Demo 2 — interference with water. Fill a shallow tray with water and let it settle. Dip two fingertips in and out together, side by side, making two sets of ripples. Watch where the ripples meet: in some places they add up into bigger waves, and in others they cancel out into calm — the same overlapping pattern that quantum particles somehow produce on their own. This is the clearest everyday picture of what "interference" means, the phenomenon at the very heart of the quantum world.

These demos won't show the quantum weirdness itself — for that you'd need single particles and detectors — but they let you see the wave behaviour that makes that weirdness so astonishing.