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Chat with a friend about
an established scientific theory

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

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But a conversation about
an established scientific law

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

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Why is that?

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What is the difference 
between a theory and a law,

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and is one better?

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Scientific laws and theories
have different jobs to do.

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A scientific law predicts
the results of certain initial conditions.

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

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

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

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about why things happen as they do.

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A theory might invoke
dominant and recessive genes

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

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

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In simplest terms,

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a law predicts what happens
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
often triggers progress on the other.

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

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to explain the nature of planetary orbits.

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He developed three brilliant laws
of planetary motion

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while he was studying decades 
of precise astronomical data

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in an effort to find support 
for his theory.

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While his three laws 
are still in use today,

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gravity replaced his theory of harmonics
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 
a universal instruction manual.

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Instead, we continually propose,
challenge, revise, or even replace

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our scientific ideas 
as a work in progress.

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Laws usually resist change

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

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

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A theory's acceptance, however,
is often gladiatorial.

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Multiple theories may compete
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 
that can explain most of the data,

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though there may still 
be gaps in our understanding.

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Scientists also like 
when a new theory successfully predicts

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previously unobserved phenomena,

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like when Dmitri Mendeleev's theory
about the periodic table

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predicted several undiscovered elements.

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The term scientific theory
covers a broad swath.

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Some theories are new ideas 
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, 
evolution, and climate change,

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have endured years 
of experimental confirmation

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before earning acceptance by the majority 
of the scientific community.

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You would need to learn more about
a specific explanation

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before you'd know how well 
scientists perceive it.

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The word theory
alone doesn't tell you.

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In full disclosure,

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the scientific community has bet
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 
discarded in favor of better ones.

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But even incorrect theories 
have their value.

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Discredited alchemy was the birthplace
of modern chemistry,

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and medicine made great strides

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long before we understood the roles
of bacteria and viruses.

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That said, better theories often lead
to exciting new discoveries

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that were unimaginable 
under the old way of thinking.

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Nor should we assume 
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
to challenge the status quo.

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However, vulnerability to some potentially
better explanation

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doesn't weaken 
a current scientific theory.

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Instead, it shields science from becoming
unchallenged dogma.

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A good scientific law 
is a finely-tuned machine,

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accomplishing its task brilliantly

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but ignorant of why it works
as well as it does.

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A good scientific theory is a bruised,
but unbowed, fighter

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

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Though different,

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

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

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challenge them to go nine rounds
with the champ

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and see if they can do any better.