Learn the science · 125 ideas
The science, A to Z
Every idea our stories rely on, explained in plain words, with the stories where you can see it in action.
A
Aberration of starlightBecause the Earth moves through falling starlight, stars appear shifted slightly towards the direction of travel, like rain that seems to slant when you run.AI agentAn AI model given a goal and tools (a browser, files, email) and allowed to act on its own in a loop: look, plan, act, check.Air pressureThe push of the air's weight on every surface, about 1013 hectopascals at sea level; differences in it make air flow.AnaesthesiaDrugs that remove sensation; general anaesthesia makes a patient unconscious and free of pain during surgery.AntibioticsDrugs, many first made by moulds and soil bacteria, that kill bacteria or stop them growing without harming our own cells.Atomic time and leap secondsAn extra second occasionally added to clocks so that atomic time stays in step with Earth's slightly irregular spin.AuroraCurtains of light in the night sky made when charged particles from the Sun strike oxygen and nitrogen in the upper atmosphere.Axial tilt and the seasonsEarth's spin axis leans about 23.4 degrees, which gives us seasons, solstices, the midnight sun and the polar night.
B
Beer–Lambert lawLight or X-rays passing through a material fade exponentially with the distance travelled and how strongly the material absorbs.Bioelectricity and nerve signalsLiving cells use electrical signals: nerves and muscles fire by letting charged atoms rush across their membranes.BioluminescenceLight made by living things through a chemical reaction, in which an enzyme called luciferase makes luciferin react with oxygen.Boyle's lawAt a fixed temperature, squeeze a gas into half the space and its pressure doubles; release the pressure and it expands.BuoyancyThe upward push of a fluid on anything immersed in it, equal to the weight of the fluid it pushes aside.
C
CalderaA huge basin left when a volcano empties its magma chamber in a great eruption and the ground above collapses into the space.Centre of massThe balance point of an object, where its weight effectively acts; the object topples once that point moves beyond its base.Confirmation biasThe tendency to look for, favour and remember evidence that fits what we already believe, and to overlook evidence that does not.Controlled trialTesting a treatment by comparing groups that differ in one thing only, ideally with neither patients nor researchers knowing who got what.Coronal mass ejectionA huge cloud of magnetised plasma blasted from the Sun's outer atmosphere; when one hits Earth it can set off a geomagnetic storm.Cosmic microwave backgroundThe faint glow of microwaves filling the whole sky, left over from the hot early universe, at under 3 degrees above absolute zero.Cosmic raysHigh-energy particles, mostly protons, that stream in from space and smash into the atmosphere, showering secondary particles to the ground.
D
Deficiency diseaseIllnesses caused not by germs but by missing nutrients, such as scurvy from too little vitamin C and pellagra from too little niacin.Delay-line memoryEarly computer memory that kept bits alive as pulses travelling through a tube, caught at the far end and sent round again.Demographic transitionThe shift from high birth and death rates to low ones: deaths fall first, the population booms, then families get smaller and growth slows.DendrochronologyDating wood and reading past climate from the yearly growth rings of trees, matched across many samples.Diffraction and interferenceWaves bend around edges and spread through gaps, then overlap to make patterns of strong and weak, bright and dark.Dissociative amnesiaMemory loss caused by severe stress or trauma rather than an injury or illness of the brain; its rare form with sudden travel is called a fugue.Doppler effectThe change in pitch or frequency you notice when a source of waves moves towards or away from you.Dyes and pigmentsColoured substances absorb some wavelengths of light and send back the rest; the colour we see is what is left over.
E
Echolocation and sonarFinding things by sending out a sound and timing the echo, as bats, dolphins, orcas and ships' sonar all do.Electromagnetic inductionA changing magnetic field drives an electric current in a nearby wire: the principle behind generators, transformers and solar-storm surges.Electromagnetic spectrumThe full family of light-like waves, from radio waves to gamma rays; visible light is only a narrow slice in the middle.EnzymesProteins that speed up chemical reactions in living things, each acting on particular molecules without being used up.Escalation of commitmentSticking with, and investing more in, a failing decision because we have already backed it, especially when we are personally responsible for it.Exponential growthGrowth by a fixed percentage each period, so a population keeps doubling in the same time and soon becomes enormous.
F
Finding longitudeLongitude is a matter of time: the Earth turns 15 degrees an hour, so the difference between two local clocks gives the distance east or west.Frequency and pitchFrequency is how many times a wave vibrates each second; pitch is how high or low we hear a sound.FrictionThe force that resists surfaces sliding over each other; slippery materials such as PTFE have very low friction.
G
Geomagnetic stormA disturbance of Earth's magnetic field caused by a blast of solar plasma, bringing aurorae far from the poles and surges in long wires.Germ theoryThe idea, now proven, that many diseases are caused by microscopic organisms spreading between people, not by bad air.Giant-impact hypothesisThe leading idea that the Moon formed from debris thrown out when a Mars-sized body called Theia struck the young Earth.Gravitational perturbationSmall tugs from other bodies that pull a planet or comet slightly off its simple orbit; measuring them can reveal unseen masses.Gravity anomalyA small difference in the pull of gravity from place to place, caused by denser or lighter rock underground. Surveys map them to see hidden structures.Greenhouse effectGases such as carbon dioxide, water vapour and methane trap some of the heat Earth gives off, keeping the planet warm.
H
Half-lifeThe time it takes for half of a radioactive substance, or anything decaying at a steady rate, to disappear.Harmonic analysisBreaking a complicated repeating signal into a sum of simple sine waves, used to predict tides and to build sounds.HeliopauseThe boundary where the Sun's outflowing wind of particles is stopped by the gas between the stars: the edge of the Sun's bubble.Hill sphereThe region around a planet or other body where its own gravity, rather than the Sun's, controls what orbits it.How fibre filters catch particlesMasks and air filters are not sieves: their fibres catch particles by impaction, interception and diffusion as the air swerves past.Hydrostatic pressureThe pressure of a liquid's own weight, rising steadily with depth: about one extra atmosphere for every 10 metres of sea.Hyperbolic growthGrowth that speeds up as the quantity gets bigger, so fast that the curve reaches infinity at a definite moment, unlike exponential growth.Hypervelocity impactWhen an asteroid or meteorite hits at many kilometres per second, the shock melts rock instantly and forges minerals found almost nowhere else.
I
Inertial navigation and gimbal lockKnowing where you point and how you move from spinning gyroscopes and accelerometers alone, with no outside signal; three nested gimbals can jam.InfrasoundSound too low-pitched for human ears, below about 20 hertz, which can travel thousands of kilometres through the air.Invasive speciesA species moved by people to a place where it does not naturally live, which then spreads and causes harm.Inverse-square lawLight, sound, gravity and radiation spreading from a point weaken with the square of distance: twice as far, a quarter as strong.Ionising radiationRadiation energetic enough to knock electrons off atoms and damage living cells; its effect on the body is measured in sieverts.Iron meteoriteMeteorites of nickel-iron metal from the cores of shattered asteroids, recognisable by criss-cross crystal patterns grown over millions of years.IsotopesVersions of an element with the same number of protons but different numbers of neutrons; their ratios act as natural fingerprints.
K
KarstLandscape where rainwater slowly dissolves limestone, carving caves, underground rivers and sinkholes, so water flows below ground rather than in rivers.Kepler's lawsJohannes Kepler's three rules: planets move on ellipses, sweep equal areas in equal times, and take longer the farther out they are.
L
Lightning and lightning conductorsA giant spark that discharges electric charge built up in storm clouds; a grounded metal rod gives it a safe path to earth.Limnic eruptionA rare disaster in which carbon dioxide dissolved in a deep lake suddenly bursts out, smothering the surrounding land in heavy gas.Logistic growthGrowth that starts out fast and nearly exponential, then slows and levels off at a ceiling: the S-shaped curve first described by Verhulst in 1838.Lung float testAn old forensic test: lungs that have breathed air float in water, while lungs that never breathed are dense and sink.
M
MagnetosphereThe bubble of space controlled by Earth's magnetic field. It deflects the solar wind, and solar storms can squeeze it hard.Mass psychogenic illnessReal symptoms that spread through a group by anxiety and suggestion rather than by any germ or poison.Meteor entry and fusion crustA space rock entering the atmosphere is heated so fiercely that its surface melts and streams away, leaving a thin glassy crust.Meteor showerWhen Earth crosses the dust trail of a comet, grains burn up in the air as meteors that seem to stream from one point: the radiant.Misinformation effectInformation heard after an event, such as the wording of a question, can change what people remember about the event itself.MonocultureGrowing one crop, often one genetically identical variety, over large areas: efficient, but a single disease can spread through all of it.MutualismA partnership between two species in which both benefit, from bees and flowers to birds that lead people to honey.
N
Natural selectionIndividuals with inherited traits that help them survive and breed leave more offspring, so those traits become commoner over generations.Nebular hypothesisThe Sun and planets formed from a collapsing, spinning cloud of gas and dust that flattened into a disc in which the planets grew.Negative feedbackA control loop that counteracts change, pushing a system back towards its set point; it keeps body temperature and blood sugar steady.
O
Ocean currents and gyresWind and Earth's spin drive great circling currents in each ocean basin, with fast currents such as the Gulf Stream on their western sides.Operant conditioningLearning by consequences: behaviour that is rewarded becomes more frequent, and behaviour that is punished becomes less so.Orbital decayThe slow sinking of a satellite's orbit as thin air at the edge of space drags on it, until it falls back to Earth.
P
ParallaxThe apparent shift of a nearby object against a distant background when seen from two places; the size of the shift gives its distance.Plate tectonicsEarth's outer shell is broken into plates that slowly move, pulling apart at ridges and rifts and colliding elsewhere.Pneumonic plaguePlague of the lungs: the one form of plague that spreads directly from person to person, in droplets breathed out by the sick.PollinationMoving pollen from a flower's male parts to the female stigma so seeds and fruit can form; insects, wind, birds or human hands can do it.Population bottleneckA sharp crash in a population's numbers that leaves the survivors with only a fraction of the original genetic variety.Positive feedbackA loop in which a change strengthens the very process that caused it, so it grows faster and faster until something runs out or breaks.Putrefaction and decomposition gasAfter death, bacteria that live without oxygen break down tissue and release gases that make a body swell, and sometimes burst.Pyroclastic flowA ground-hugging avalanche of hot volcanic gas, ash and rock that can race downhill at hundreds of kilometres per hour.
Q
R
Radio wavesThe longest-wavelength electromagnetic waves, travelling at the speed of light and used to carry sound and signals without wires.Radioactive decayUnstable atomic nuclei break down by themselves, giving off radiation as alpha particles, beta particles or gamma rays.Radiocarbon datingDating once-living material by measuring how much radioactive carbon-14 is left; half of it decays every 5,730 years.Radioisotope thermoelectric generatorA power source with no moving parts that turns the heat of decaying radioactive material into electricity, for spacecraft too far from the Sun for solar panels.Rayleigh scatteringAir molecules scatter blue light far more than red, which is why the sky is blue and the setting Sun looks red.Red-teamingAttacking your own system on purpose, in a controlled test, to find its weaknesses before a real attacker or a real failure does.RefractionWaves change direction when their speed changes, bending light through air and glass, and sound and swell through the sea.Reproduction numberHow many people, on average, each infected person goes on to infect; above 1 an outbreak grows, below 1 it dies out.ResonanceWhen small pushes arrive in step with an object's natural rhythm, its vibrations build up into a large swing.Roche limitThe closest a moon held together by its own gravity can orbit a planet before tides tear it apart, about two and a half planet radii.
S
Salt domeA huge plug of rock salt that has slowly flowed upwards through heavier rock, sometimes reaching close to the surface; many hold oil, gas or mines.Saros cycleAn eclipse cycle of about 18 years and 11 days, after which the Sun, Earth and Moon return to nearly the same arrangement.Seismic wavesVibrations that travel through the Earth from earthquakes and explosions; seismographs record them and reveal where they came from.Shock waveA pressure front that moves faster than sound, with a sudden jump in pressure, temperature and density; explosions make them.Shocked quartzQuartz crossed by fine parallel lines, formed only by extreme, sudden pressure such as a meteorite impact. It is a fingerprint of impact craters.Soil liquefactionWhen shaking makes waterlogged sand lose its strength and behave like a liquid, so buildings tilt and sink.Spark-gap transmitterThe first kind of radio transmitter: a spark across a gap sets a circuit ringing, sending out bursts of radio waves that spread over a wide band of frequencies.SpectroscopySplitting light into its colours to read a chemical fingerprint: each element emits and absorbs light at its own wavelengths.Speed of lightLight travels at 299,792 kilometres per second, so we always see distant things as they were when the light left them.Speed of soundHow fast sound travels: about 343 metres per second in air, and more than four times faster in water.Standard candleA star or other object whose true brightness is known, so how dim it looks reveals how far away it is.Stored-program computerA computer that keeps its instructions in the same memory as its data, so a new job only needs new instructions to be loaded.Structural colourColour made by the shape of a material rather than by pigment: tiny repeating layers or ridges that reflect one wavelength much more strongly than others.SupersaturationWhen air or water holds more vapour or gas than it normally can; the excess condenses or bubbles out as soon as it has somewhere to start.
T
Temperature inversionA layer of warm air sitting on top of cooler air near the ground. It acts like a lid, trapping smoke, fog and pollution low down.TidesThe twice-daily rise and fall of the sea, driven by the Moon and Sun; it is largest at spring tides and smallest at neap tides.Time dilationEinstein's relativity predicts that moving clocks run slow and clocks deeper in gravity run slow, effects measured with atomic clocks.TriangulationFixing the position of a distant point by measuring angles to it from the two ends of a baseline of known length.Trophic cascadeWhen removing or adding a predator ripples down a food chain, changing the numbers of its prey and of what the prey eats.Tropical yearThe time for the seasons to repeat, about 365.2422 days; calendars need leap years to keep up with it.TsunamiA series of long, powerful waves made when a huge volume of water is suddenly displaced by an earthquake, landslide or eruption.Tuned circuitA coil and a capacitor joined together: electricity sloshes between them at one natural frequency, so the circuit picks out signals near that frequency.
U
Ultraviolet visionThe ability of many birds, insects and other animals to see light just beyond violet, which human eyes block and cannot detect.Universal gravitationNewton's law that every mass attracts every other, with a force that grows with their masses and falls with the square of their distance.
V
VaccinationTraining the immune system with a harmless or weakened version of a germ so the body is ready if the real one arrives.Vector-borne diseaseDiseases passed between people or animals by living carriers, usually blood-feeding insects such as mosquitoes, fleas and ticks.Volcanic aerosol coolingBig eruptions loft sulfur gases that become a haze of tiny droplets high in the stratosphere, reflecting sunlight and cooling the planet.