The definition of "theory" depends on who is using the word.
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Defining the word theory is tricky. Scientists use it one way, the average Joe another.
In casual parlance, a theory is basically an idea or thought. It probably has no carefully collected data to back it up, let alone any rigorous hypothesis testing or experiments. In the world of science, however, a theory is a broad explanation of a phenomenon or phenomena that is testable, falsifiable and has multiple lines of evidence.
“Genuinely successful theories interconnect information from previously disparate areas of experience,” said Adolf Grünbaum, the Andrew Mellon Professor of Philosophy at the University of Pittsburgh. For example, fossil records, DNA evidence and biogeography are connected under the theory of evolution.
A theory differs from a hypothesis in its scope. For example, I can have a hypothesis that if I throw a penny off the Empire State Building it will fall to the ground. But the theory of gravity goes vastly beyond throwing objects off a building. The theory of gravity also explains the motion of planetary bodies and orbiting satellites. So, a hypothesis is like a mini-theory. It attempts to explain an event, and is testable and falsifiable like a theory is, but on a more narrow scale.
Hypotheses and theories are part of the scientific method — the process of asking and answering questions by experimentation. The method exists to ensure that science is as accurate as possible in explaining the world and that arguments rely on observable evidence. The main components of the scientific method are that experiments should be repeatable, so other scientists can verify or nullify the results; data should be collected by observation and experimentation; and experiments should be documented and shared.
Theories are formed after numerous hypotheses are vetted using the scientific method. Hypotheses are tested, data is collected, and the results are documented, shared and retested. Then a theory that explains the data and predicts the outcomes of future experiments is formed. Typically, researchers in different fields of study tend to rely on different methods — ones deemed the best suited for their subjects or objects of study.
One component of a theory that can frustrate scientists and non-scientists alike is that a theory is never proven and can always be revised. Experiments testing a theory either corroborate it or falsify it. Even Isaac Newton’s laws of gravity were revised after 200 years, when Albert Einstein found flaws and devised his theory of relativity.
In general, a law is said to be without exception, such as the second law of thermodynamics, which says isolated systems that are not in equilibrium move from more ordered to less ordered states (or less energy available to do work). Very few theories become laws. Michael Weisberg, associate professor of philosophy at the University of Pennsylvania, said in the future even fewer theories are likely to become laws, since theories and laws are so frequently revised.
“We can accumulate incredible amounts of evidence and say this is unlikely to be untrue, but we can never give a proof,” Weisberg said.
On the other hand though, scientists do not look fondly upon those who pooh-pooh scientific theories such as evolution as being just a theory. “It is skullduggery,” said Grünbaum. Yes, evolution is a theory, but a theory supported by an enormous body of evidence.
“Ultimately, to say something is a theory in science is an honorific,” Weisberg said, meaning it commands esteem or respect based on the many tests it has withstood.