The building blocks of scientific knowledge
Scientists use several kinds of ideas to make sense of the world. These words have special meanings in science that are different from everyday speech. In everyday language, "it’s just a theory" means a guess. In science, a theory is one of the strongest forms of knowledge we have.
| Term | What it does | Example |
|---|---|---|
| Hypothesis | Proposes a testable explanation | Plants grow faster in blue light than red light. |
| Scientific law | Describes what happens, often as an equation | Newton’s law of gravitation |
| Scientific theory | Explains why it happens, with lots of evidence | The theory of evolution by natural selection |
| Scientific model | Represents a system to help explain or predict | The Bohr model of the atom |
Hypotheses
A hypothesis is a tentative explanation that can be tested. It must be falsifiable: there must be some possible observation that would show it is wrong. "Invisible fairies make flowers bloom" can’t be tested, so it isn’t scientific.
Laws describe
A scientific law describes a pattern that always occurs under certain conditions. Laws are often written as equations.
- The law of conservation of mass: mass is neither created nor destroyed in a chemical reaction.
- Boyle’s law: at constant temperature, P₁V₁ = P₂V₂ for a gas.
- Newton’s second law: F = ma.
Laws tell us what happens, but not why.
Theories explain
A scientific theory is a broad explanation that ties together many observations, laws and tested hypotheses. It is supported by a large body of evidence and has survived many attempts to prove it wrong.
Examples:
- Cell theory: all living things are made of cells, and cells come from other cells.
- Atomic theory: matter is made of atoms.
- Plate tectonics: Earth’s outer layer is made of moving plates.
- The germ theory of disease: many diseases are caused by microorganisms.
A common myth is that theories "graduate" into laws once they are proven. They don’t. Laws and theories are different kinds of knowledge. A theory can explain a law; for example, the kinetic theory of gases explains why Boyle’s law works.
Models represent
A scientific model is a simplified representation of a system. Models can be physical (a 3-D DNA model), conceptual (a diagram of the carbon cycle), mathematical (an equation for population growth) or computer-based (a climate simulation).
Models are useful because they:
- make complex systems easier to understand
- let us make predictions
- can be tested against real data
But every model has limits. The Bohr model shows electrons moving in neat circular orbits. It explains some things, like the colours of light given off by hydrogen, but the modern quantum model replaced it because electrons don’t really follow fixed paths.
How science changes
Scientific knowledge is reliable but always open to revision. When new evidence doesn’t fit, models and theories are refined or, rarely, replaced. A major change in a field’s basic ideas is called a paradigm shift.
| Old idea | New idea | What changed it |
|---|---|---|
| Earth at the centre of the universe | Sun-centred solar system | Observations by Copernicus, Galileo and Kepler |
| Continents fixed in place | Plate tectonics | Sea-floor spreading evidence in the 1960s |
| Newton’s gravity works everywhere | Einstein’s general relativity | Mercury’s orbit; light bending near the Sun |
Newton’s laws weren’t thrown away. They still work extremely well for everyday speeds and sizes. Relativity simply covers a wider range.
Common mistakes
- Saying "just a theory." Scientific theories are strongly supported.
- Thinking theories become laws.
- Treating a model as the real thing.
Try it yourself
Classify each as a hypothesis, law, theory or model:
- Energy cannot be created or destroyed.
- A diagram showing how the heart pumps blood.
- If students sleep more, their test scores will improve.
- Living things share common ancestors and change over generations through natural selection.
(Answers: law; model; hypothesis; theory)
Quick recap
- Hypotheses are testable, falsifiable explanations.
- Laws describe what happens, often with equations.
- Theories explain why, backed by lots of evidence.
- Models simplify systems to help explain and predict, but have limits.
- Scientific knowledge is reliable yet open to change with new evidence.