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Chat with a friend about[br]an established scientific theory

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and she might reply,[br]"Well, that's just a theory."

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But a conversation about[br]an established scientific law

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rarely ends with,[br]"Well, that's just a law."

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Why is that?

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What is the difference [br]between a theory and a law,

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and is one better?

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Scientific laws and theories[br]have different jobs to do.

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A scientific law predicts[br]the results of certain initial conditions.

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It might predict your unborn [br]child's possible hair colors,

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or how far a baseball travels[br]when launched at a certain angle.

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In contrast, a theory tries to provide[br]the most logical explanation

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about why things happen as they do.

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A theory might invoke[br]dominant and recessive genes

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to explain how brown-haired parents[br]ended up with a red-headed child,

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or use gravity to shed light [br]on the parabolic trajectory of a baseball.

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In simplest terms,

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a law predicts what happens[br]while a theory proposes why.

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A theory will never grow up into a law,

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though the development of one[br]often triggers progress on the other.

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In the 17th century, Johannes Kepler[br]theorized cosmic musical harmonies

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to explain the nature of planetary orbits.

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He developed three brilliant laws[br]of planetary motion

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while he was studying decades [br]of precise astronomical data

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in an effort to find support [br]for his theory.

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While his three laws [br]are still in use today,

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gravity replaced his theory of harmonics[br]to explain the planets' motions.

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How did Kepler get part of it wrong?

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Well, we weren't handed [br]a universal instruction manual.

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Instead, we continually propose,[br]challenge, revise, or even replace

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our scientific ideas [br]as a work in progress.

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Laws usually resist change

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since they wouldn't have been adopted[br]if they didn't fit the data,

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though we occasionally revise laws [br]in the face of new unexpected information.

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A theory's acceptance, however,[br]is often gladiatorial.

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Multiple theories may compete[br]to supply the best explanation

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of a new scientific discovery.

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Upon further research,

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scientists tend to favor the theory [br]that can explain most of the data,

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though there may still [br]be gaps in our understanding.

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Scientists also like [br]when a new theory successfully predicts

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previously unobserved phenomena,

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like when Dmitri Mendeleev's theory[br]about the periodic table

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predicted several undiscovered elements.

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The term scientific theory[br]covers a broad swath.

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Some theories are new ideas [br]with little experimental evidence

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that scientists eye with suspicion,

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or even ridicule.

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Other theories,

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like those involving the Big Bang, [br]evolution, and climate change,

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have endured years [br]of experimental confirmation

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before earning acceptance by the majority [br]of the scientific community.

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You would need to learn more about[br]a specific explanation

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before you'd know how well [br]scientists perceive it.

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The word theory[br]alone doesn't tell you.

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In full disclosure,

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the scientific community has bet[br]on the wrong horse before:

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alchemy,

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the geocentric model,

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spontaneous generation,

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and the interstellar aether

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are just a few of many theories [br]discarded in favor of better ones.

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But even incorrect theories [br]have their value.

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Discredited alchemy was the birthplace[br]of modern chemistry,

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and medicine made great strides

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long before we understood the roles[br]of bacteria and viruses.

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That said, better theories often lead[br]to exciting new discoveries

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that were unimaginable [br]under the old way of thinking.

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Nor should we assume [br]all of our current scientific theories

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will stand the test of time.

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A single unexpected result is enough[br]to challenge the status quo.

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However, vulnerability to some potentially[br]better explanation

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doesn't weaken [br]a current scientific theory.

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Instead, it shields science from becoming[br]unchallenged dogma.

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A good scientific law [br]is a finely-tuned machine,

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accomplishing its task brilliantly

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but ignorant of why it works[br]as well as it does.

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A good scientific theory is a bruised,[br]but unbowed, fighter

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who risks defeat if unable to overpower[br]or adapt to the next challenger.

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Though different,

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science needs both laws and theories [br]to understand the whole picture.

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So next time someone comments that[br]it's just a theory,

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challenge them to go nine rounds[br]with the champ

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and see if they can do any better.