Prove It!

Standard introductory science exam question: "What is the difference between a law and a theory?"

Standard student answer: "A law has been proven and a theory has not."

Wrong! Wrong! Wrong!

Ok wise guy, what's right then?

By the time you have completed this material you should be able to

First let's get this straight.  Proof, as we mean it when we say "prove me wrong", has nothing to do with science.  While we might use the word "proof" in science, it is not a scientific idea.  Proving is an exercise in logic.  The other meaning of the word "proof", as it is used when we refer to whiskey – "this is 90 proof" – actually has its origin in the meaning of the word as we use it in science, but that's a whole different story!

Here's a definition of what it means to prove something: "Proof is arriving at a logical conclusion, based on the available evidence."  Notice that this has absolutely nothing to do with being right or wrong.  It also has nothing to do with science either, since you can have logical conclusions in Social Studies, English, or any other subject.  The word proof is used a lot in law, and the idea isn't a lot different if we use it in science.

In science we collect empirical evidence through the process of experimentation.  If we collect enough evidence, we will probably notice patterns or regularities in the evidence, and then we will develop generalizations that describe what we have observed.  These generalized descriptions of observed events are called scientific laws.

Scientific law: a generalized description, usually expressed in mathematical terms, which describes the empirical behavior of matter.

Scientific laws describe things.  They do not explain them.

Example: drop a pencil, drop a book, drop a ball, drop a ruler, in fact drop anything that is heavier than air, and it will fall.  The generalized description is the law of gravity ("objects that are heavier than air fall when released").  Notice that this doesn't tell us what gravity is, but it does describe its behavior.  Of course, if we were to really do this "scientifically", we'd make measurements about the speed of the fall, and then we'd probably state the law as Isaac Newton did mathematically.

Remember this very important idea:  Isaac Newton knew what gravity did (he could describe its behavior), but he could not explain why gravity did it.  Even today the topic of what gravity really is, is an active topic for scientific discussion.

In fact, we can call scientific observations, "facts".  At least we can do this if we define a fact as an observed event.  That is pretty much what it means in a court of law.  The concept of fact in law and in science are very similar.   A fact in law is an observation.  The judge doesn't care what your opinion is.   The only information most witnesses are allowed to give in court is what they have observed.

Of course, our observations are not perfect, as they are limited by experimental errors, both systematic and random.  That doesn't mean our observations are bad.   It just means that they have limitations, of which we must be aware.  These limitations mean that our conclusions are also not perfect.  Scientists will hardly ever give you "the right answer".  Instead, they'll say things like "its likely that", or "it's probable that".  They know that observation has limits.

In a legal trial, both the defense and the prosecution present what they believe to be the facts in the case.  Then it is up to the judge and/or jury to determine whether the facts support the defense's case (and the charged person is found not guilty), or whether the facts better support the prosecution's case (and the charged person is found guilty).   In other words, the lawyers try to prove their case.  They try to present convincing arguments that show that their interpretation of the facts is the correct explanation for what happened.

In science, a theory or hypothesis is the explanation for the events that have been observed.  Scientists create hypotheses to try and explain the observed behavior of the world.  They do so by creating an explanation that can be tested (an hypothesis).   If a lot of evidence is collected to support the hypothesis, then they'll probably accept the hypothesis as a good explanation, and it may, given even more testing, become an accepted theory.   This is pretty much what is meant by proving something in court.  If there is enough evidence, then you will be able to convince your peers that your explanation is good. 

Proof and truth: proving something does not make it true.  It just means that you have convinced other people that the evidence supports your conclusion.  There are many examples in law of people who have been "proven guilty beyond a reasonable doubt" who were not guilty at all.  And most of us probably suspect that there are guilty people who get off.

Proof does not equal truth.

Just as in court, proof in science does not equal truth.  No scientist will ever claim that  a theory is true.  What they will do is state that the evidence agrees with the theory.  Of course, sometimes new evidence shows up that we didn't have before.  Then in science, as in law, we will reach a new conclusion.

Be careful though.  You'd be really wrong if you thought scientists didn't have confidence in their ideas.  Knowing that something may change, and believing strongly that right now there is no better answer than the accepted one, are not mutually exclusive points of view.

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David Milgaard was convicted of the 1969 rape and murder of Saskatoon nursing assistant Gail Miller. He was set free in 1992 after 23 years in jail, and exonerated in 1997 following DNA tests.  In 1999, Larry Fisher was convicted of the murder.

So now, let's go back to the question: "What is the difference between a law and a theory?"  Here is an acceptable answer.  "Scientific laws are the evidence used to support a conclusion.  Scientific hypotheses and theories are our best attempts at explaining the behavior of the world, in ways that can be tested by further experiment.  The facts (the scientific laws) must convince us that our theory is a good explanation for what happened."

Notice that we didn't even use the word proof once in the last paragraph?  That's most likely a good idea.  When talking about scientific laws and theories, you probably should never use the word.  Even if you understand it's correct meaning, someone you are speaking to will not.  Maybe you know that scientific theories are proven to be supported by observations, but most people won't.  So don't use the word proof when talking about science.  It's just too confusing.  Use words like "convince", or "support", or "agrees with" and you won't run into trouble.

We don't prove theories (and hypotheses) true.  We just use the observations to convince ourselves (and others) that we have a good idea.  Scientists have a lot of confidence in scientific theories, because they know there is a lot of evidence to back them up.

Actually of course, every single theory there is in science has been proven.  If there was no evidence – no proof – to back up an idea, scientists would not accept it.  The evidence convinces us that the theory is a good explanation of what is happening.  If theories don't agree with the evidence, and sometimes we find that they don't, then scientists will be trying to create a better explanation that does.  But they won't abandon the current one, even if they know it is flawed, until they have managed to think up a better explanation that agrees with the evidence.

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Last Updated:April 26, 2009
Copyright 2000 David Dice All rights reserved