Theory (in science)
High school
A scientific theory is a well-tested explanation of a broad range of observations, supported by a large body of evidence and used to make predictions.
Examples
Examples
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The theory of evolution by natural selection
High school
Evolution by natural selection is a theory, because it explains a broad range of observations (fossils, genetics, the distribution of species) and is supported by a large body of evidence. It is not a law because it explains why species change rather than describing one pattern. It is not a hypothesis because it has be demonstrated via experiments.
Non-examples
Things that may be mistaken for examples.
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“I have a theory about who ate the cake”
Elementary
“A theory about who ate the cake” is not a scientific theory, because it is a guess with little evidence. In scientific terms, it is closer to a hypothesis.
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Before an experiment: “Bean plants grow taller with more hours of light each day”
Elementary
“Bean plants grow taller with more hours of light” is a hypothesis, because it is a prediction that an experiment can test and could show to be false. It is not a theory or a law, because no body of evidence supports it yet, and it is not a conjecture, because experiments, not proofs, decide it.
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Newton’s law of universal gravitation, $F = G m_1 m_2 / r^2$
High school
Newton’s law of gravitation is a law because it is an equation that describes how strongly masses attract. It is not a theory, because it does not explain why masses attract; general relativity, a theory, does.
Properties
- A theory is not a hypothesis that is waiting to be proved. Instead, it is an explanation for existing data and observations. Theories may be revised as new evidence is collected.
Notes
In everyday speech, “theory” often means a guess (“I have a theory about who ate the cake”), which is closer to a scientific hypothesis.
In mathematics, a “theory” is a body of results about one subject, such as group theory.
Distinctions involving this term
- Conjecture vs. hypothesis vs. law vs. theory: A conjecture is a statement believed true but not yet proved, and it belongs to mathematics, where statements are settled by proof. In science, where statements are tested by evidence, a hypothesis is a testable proposal, and a theory is a well-supported explanation of many observations. A law describes a pattern that holds in every tested case, often as an equation such as $F = G m_1 m_2 / r^2$, without explaining why the pattern holds; explaining it is the job of a theory. In everyday speech, “theory” means a guess, but in science a theory is an a principle or body of principles that explains observed data and phenomenon.