Atoms and light: lasers and LEDs
An electron in an atom can only have certain energies, called energy levels. When it falls from a higher to a lower level, the atom gives out a packet of light called a photon. Its energy is E = hf, where f is the frequency of the light and h is Planck's constant.
LED bulbs, lasers in barcode scanners and fibre-optic internet all use this idea. In a laser the atoms are made to give out photons together, so the light is one colour and travels in a thin straight beam.
Chips: tiny switches
A semiconductor like silicon conducts a little, and we can switch its conduction on and off. A transistor is a tiny switch made this way. A chip has billions of them. Today the smallest parts are only a few nanometres wide (a nanometre is a billionth of a metre).
If you make each switch half as wide, four times as many fit in the same area. That is why phones keep getting more powerful. Very small sizes need quantum physics to understand, because electrons start to behave like waves.
Quantum computers
A normal bit is either 0 or 1. A qubit can be in a mix of 0 and 1 at the same time. Picture an arrow on a ball. Up means 0, down means 1, and a tilted arrow is a mix. When we read the qubit, we get 0 or 1, with a chance set by the tilt: P(1) = sin²(θ/2).
A group of qubits can work on many possibilities together. Quantum computers are not faster at everything, but they may help with problems like designing new medicines and materials. They are still experimental and need very cold, very quiet machines.
Superconductors
A superconductor is a material that, below a certain temperature (its critical temperature), has exactly zero electrical resistance. Current can flow in a loop for a very long time with no loss. A superconductor also pushes magnets away, so a magnet can float above it.
Some ceramic materials turn superconducting at about 92 K (−181 °C), which can be reached with cheap liquid nitrogen at 77 K. Superconducting magnets are used in MRI machines and in particle accelerators. A big goal is a superconductor that works at room temperature, which would save huge amounts of electricity.
Fusion energy
In nuclear fusion two light nuclei (such as two forms of hydrogen) join to make a heavier nucleus (helium) and release energy. The Sun is a giant fusion machine. Nuclei repel each other because both are positive, so to push them together we need extreme heat, about 100 million °C in an Earth reactor, and a way to hold the hot gas, such as strong magnetic fields.
Fusion makes no carbon dioxide and its fuel is plentiful. Many countries share the big experimental reactor ITER in France. Making fusion produce more energy than we put in, steadily and cheaply, is still a challenge.
Ripples in space and the next questions
When two very heavy objects such as black holes circle each other, they send out ripples in space called gravitational waves. They were first detected in 2015 by the LIGO detectors, and they let us "hear" events in the universe that give out no light. Other open questions include what dark matter is and how the universe began.
Try it: in the 3D, visit each station and move its slider. Predict what will happen before you move it.
Key formulas and definitions
- Photon energy: E = h f
- Photon wavelength: λ (nm) ≈ 1240 ÷ E (eV)
- Chip switches on an N × N grid: N²; halving the size gives 4× as many
- Qubit chance of reading 1: P(1) = sin²(θ/2)
- Superconductor: R = 0 below its critical temperature
Worked examples
1. A chip has 1 million switches in a square. If each switch is made half as wide, how many fit in the same area?
Halving the width means 2 times as many along each side, so 2 × 2 = 4 times in the area. 4 × 1 million = 4 million switches.
2. A qubit arrow is tilted by θ = 90° from the "0" direction. What is the chance of reading 1?
P(1) = sin²(θ/2) = sin²(45°) = (0.707)² = 0.5. So the chance is 50%, as likely 1 as 0.
3. An electron falls and gives out a photon of energy 2.0 eV. What is the wavelength? Which colour is it?
λ ≈ 1240 ÷ 2.0 = 620 nm. That is orange-red light.
Common mistakes
- Thinking a qubit is "both 0 and 1" so the answer is both. When read, you always get one result, 0 or 1.
- Saying fusion and fission are the same. Fission splits a heavy nucleus; fusion joins light nuclei.
- Thinking a superconductor works at any temperature. It only works below its critical temperature.
- Thinking faster chips come only from higher power. Mostly they come from smaller, more numerous switches.