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.
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.
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Before an experiment: “Bean plants grow taller with more hours of light each day”
Elementary
The hypothesis “Bean plants grow taller with more hours of light each day” is not a theory, because no body of evidence supports it yet.
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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 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: Mathematics and science both have words for statements that are proposed, tested, or established, and the words are often confused. 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.