A law describes a pattern, a theory explains why the pattern occurs, and a principle is a broad idea applied across many situations. In science, a theory is a tested explanation, not a guess.
In everyday speech, law, theory and principle are used loosely. In science each one names a different kind of statement. The way to tell them apart is to ask what the statement does.
Three different jobs. A law describes a pattern. A theory explains the pattern. A principle is a broad idea that is applied in many different situations.
A law is a statement of a pattern that is observed every time the same conditions occur. It is often written as a mathematical relationship.
Newton’s first law states that an object keeps moving at the same speed in the same direction unless a force acts on it. When a bus brakes suddenly, the brakes apply a force to the bus. No equal force acts on your upper body at first, so it keeps moving forward. You feel thrown forward.
A law states the pattern. It does not say why the pattern exists.
A theory is an explanation of why a set of observations and laws hold. It is built from evidence and has been tested many times, by many people, against new observations.
The law of constant proportions states that water always contains hydrogen and oxygen in the mass ratio 1 : 8. That is a pattern. Atomic theory explains it: water is made of molecules, each with two hydrogen atoms and one oxygen atom, and each kind of atom has a fixed mass. A fixed recipe of atoms gives a fixed ratio of masses.
Someone dismisses an idea by saying “that is just a theory”. What is wrong with this, using the scientific meaning of the word?
In science, a theory is not a guess. It is an explanation that has survived repeated testing against evidence.
An untested idea that could explain something is called a hypothesis. A hypothesis becomes part of an accepted theory only after its predictions have been checked many times. So “just a theory” mistakes the everyday meaning (a hunch) for the scientific one (a well-tested explanation).
One word, two meanings. In everyday use, “theory” means a hunch. In science, a hunch is called a hypothesis, and a theory is an explanation that has passed many tests.
A principle is a general statement that applies across many different situations. The principle of conservation of energy states that energy is not created or destroyed. It only changes from one form to another, and the total stays the same.
When you climb stairs, chemical energy stored in your body decreases. Part of it becomes gravitational potential energy, because you are higher. Most of it becomes heat in your muscles. Add the two up and the total equals the chemical energy used. The same accounting works for a pendulum, a falling ball and a braking train.
Scientists do not always use these three words consistently. Conservation of energy is called a principle in some books and a law in others. Many labels were fixed by history, when an idea was first named.
What matters is the job a statement does: describing a pattern, explaining it, or applying a broad idea. Learning which label is attached to which item matters less.
Every law, theory and principle can be changed if new evidence contradicts it. This has happened. Newton’s laws of motion give wrong answers for objects moving close to the speed of light. In 1905 Einstein’s theory of relativity replaced them for those cases. For everyday speeds, the two give answers that agree to better than one part in a billion, so Newton’s laws are still used.
Revision is how science corrects its errors. An idea that could never be changed by evidence could never be corrected.
If scientific ideas can change, why should you rely on them?
Because they change only when evidence requires it, not because of opinion.
An idea that has been tested for a century and has passed every test is very likely to keep working in the conditions where it was tested. When Newton’s laws were revised, they were not thrown away. Their range of accuracy was identified: they hold for speeds much smaller than the speed of light.
1Sort each description by the kind of statement it is.
2In science, the word “theory” means…
3“An object keeps moving at constant speed in a straight line unless a force acts on it.” This statement is a…
4Atomic theory explains why water always has hydrogen and oxygen in the mass ratio 1 : 8. What makes atomic theory a theory rather than a law?
5You climb stairs. The chemical energy used equals the gain in potential energy plus the heat produced. Which statement are you applying?
6Newton’s laws give wrong answers near the speed of light, so relativity is used there. What does this show?
7A prediction does not match a careful observation. What do scientists do?
8An untested idea that might explain an observation is called a ____.
Next: how to turn a claim into a prediction that can be tested.
Noescio