Chat with a friend about an established scientific theory, and she might reply, “Well, that’s just a theory.” But a conversation about an established scientific law rarely ends with “Well, that’s just a law.” Why is that? What is the difference between a theory and a law… and is one “better”? Below, Matt Anticole shows why science needs both laws and theories to understand the whole picture.

Scientific laws and theories have different jobs to do. A scientific law predicts the results of certain initial conditions. It might predict your unborn child’s possible hair colors, or how far a baseball travels when launched at a certain angle.

In contrast, a theory tries to provide the most logical explanation about why things happen as they do. A theory might invoke dominant and recessive genes to explain how brown-haired parents ended up with a red-headed child, or use gravity to shed light on the parabolic trajectory of a baseball.

In simplest terms, a law predicts what happens while a theory proposes why. A theory will never grow up into a law, though the development of one often triggers progress on the other.

We weren’t handed a universal instruction manual. Instead, we continually propose, challenge, revise, or even replace our scientific ideas as a work in progress. Laws usually resist change since they wouldn’t have been adopted if they didn’t fit the data, though we occasionally revise laws in the face of new unexpected information. A theory’s acceptance, however, is often gladiatorial. Multiple theories may compete to supply the best explanation of a new scientific discovery. Upon further research, scientists tend to favor the theory that can explain most of the data, though there may still be gaps in our understanding.

Even incorrect theories have their value. Discredited alchemy was the birthplace of modern chemistry, and medicine made great strides long before we understood the roles of bacteria and viruses. That said, better theories often lead to exciting new discoveries that were unimaginable under the old way of thinking. Nor should we assume all of our current scientific theories will stand the test of time. A single unexpected result is enough to challenge the status quo. However, vulnerability to some potentially better explanation doesn’t weaken a current scientific theory. Instead, it shields science from becoming unchallenged dogma.

A good scientific law is a finely-tuned machine, accomplishing its task brilliantly but ignorant of why it works as well as it does. A good scientific theory is a bruised, but unbowed, fighter who risks defeat if unable to overpower or adapt to the next challenger. Though different, science needs both laws and theories to understand the whole picture. So the next time someone comments that it’s just a theory, challenge them to go nine rounds with the champ and see if they can do any better.

Watch the full TED-Ed Lesson: What’s the difference between a scientific law and theory?:

Animation by Zedem Media/TED-Ed

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