Background boosters for elementary teachers

Q: How does a scientific theory become a scientific law? By Bill Robertson

A theory doesn’t become A: a law. End of story, end of this issue of Science 101. Just kidding—it’s all about the how and why, and that hasn’t been answered. I’d like to step back a bit and ad- dress a common misconception that unfortunately permeates sci- ence education. See if this sounds familiar: Scientists begin with a hypothesis, which is sort of a guess of what might happen. When the sci- entists investigate the hypothesis, they follow a line of reasoning and Brian Diskin eventually formulate a theory. Once a theory has been tested thoroughly and is accepted, it becomes a scien- tific law. Nice progression, and not Look, Kid, I don’t know why. Ask him to explain it! what happens. To understand how scientists proceed in their investi- chemistry says that the total mass volume are related, but we have no gations, it will help to understand of the things that react together is explanation for why one quantity each term individually. What’s a equal to the total mass of the things increases or decreases based on the hypothesis, what’s a theory, and that are produced in the reaction. changes in the other quantity. We what’s a law? I’ll deal with these in Newton’s second law states that only know that they increase and reverse order. the net external force applied to an decrease according to the law. When object is equal to the mass of the we measure the forces applied to an The Law of Laws object times the acceleration of the object, the mass of the object, and To understand what a law is, you object. These things are called laws the acceleration of the object, the just have to look at various laws because every time people have quantities obey Newton’s second in science. Boyle’s law states that tested them, they hold true*. law. That the quantities obey New- when the temperature of a gas is But let’s examine what laws do ton’s second law does not explain constant, the pressure of the gas not tell us. Laws do not explain why the quantities obey Newton’s times the volume of the gas is al- why things behave the way they do. second law. They just do. ways a constant, regardless of how There is no mechanism involved So, scientific laws simply explain the pressure and volume change. in the laws. In Boyle’s law, for ex- how things behave. They do not The law of conservation of mass in ample, we know how pressure and result from theories but are rather *To be accurate, I have to explain that each law I have mentioned fails to hold true under certain circumstances. Within defined constraints, though, they do hold true. As long as we keep those constraints in mind, we can consider these things to be laws.

56 Science and Children descriptions of how the physical explains a law. The kinetic theory scientists write down every hypoth- world does what it does. Maybe of gases explains Boyle’s law, but eses they have and report the results it’s the term law that makes people the theory does not evolve into the as part of a final report of research? think scientific laws have a special law. The highest award for a theory Probably not, because hypotheses place, sort of the ultimate in sci- is that it is a good theory, not that it are simply steps that are a normal entific thought. Not so. Laws are becomes a law. part of developing understanding descriptions of what happens and of a problem. The truth about how nothing more. That doesn’t mean An Educated Guess? scientists investigate things is that laws are trivial, though. Laws help But what is the role of the hypoth- they mess around a whole lot, often us develop technologies and de- esis? For all the space spent in el- without benefit of the statement of a velop theories based on the laws. ementary science books discussing formal hypothesis, and then narrow Newton’s laws help us figure out hypotheses, you would think this their focus. Ask someone who cur- how to get a few people to the was a really important thing. Hy- rently is doing research in any field Moon, even if we don’t know why potheses are important, but maybe of science what his or her hypothesis Newton’s laws work. a bit overrated. Many books define is, and I’m betting he or she will say, a hypothesis as an “educated guess” “...... Ummm ... Huh?” Ask that Theories Explained that’s based on an understanding of same scientist, “What problem are And with that, let’s move on to the situation. That’s not complete, you working on?” and you’d better theories. Theories generally pro- though. A hypothesis is an educated sit down and get ready for a lengthy vide mechanisms that explain guess coupled with an explanation discussion. Finally, do hypotheses the things we observe. There is a for why that guess should come become theories? Only in the sense theory known as the kinetic theory true. For example, two students that they contribute to a scientist’s of gases that provides a mechanism might guess that a puddle of water understanding of a problem. They for understanding Boyle’s law. outside will disappear overnight. are part of the formation of theories, In the kinetic theory of gases, we One student might say that this will but seldom are they the central ker- make various assumptions about happen because of evaporation, nel from which theories grow. how the molecules in a gas act. In and the other might say this will To summarize, laws in science the most basic form, we assume happen because the water will soak are a formal way of stating how the that gas molecules run into one into the concrete. A true test of these world behaves. Laws do not gener- another without sticking and that hypotheses should include a test of ally provide any kind of mechanism they move about randomly. We the explanations as well as what they that explains why the law is true. assume that the molecules have an observe. Now, scientists are always Theories generally provide mecha- average kinetic energy based on the guessing what will happen in a given nisms that explain laws as well as temperature. Those basic assump- experiment, expecting certain re- explain other observations. While tions lead to Boyle’s law holding sults and explaining to themselves one might use various laws in for- true. Because the kinetic theory of why they expect those results. mulating a theory, that’s the extent gases provides a mechanism rather Scientists have expectations any of the connection. Theories can than just a description of results, time they investigate the real world. never become laws, because laws it qualifies as a theory. The kinetic If those scientists guess wrong on form the body of evidence upon theory of gases will never become a either what they will observe or the which we base theories. Laws can law, because that’s not what theories explanation for it, they revise their help with formulating theories, but become. If a theory is any good, it thinking or redo a procedure. But do theories do not develop into laws.

January 2009 57 Finally, hypotheses, while a natural The phrase “just a theory” might a fact, often in retort to the claim part of the scientific process, do not have significance if a theory were that it is just a theory. Evolution generally evolve into theories. They the precursor to something better, isn’t a fact. Rather than claiming are an important component of like maybe a law. Not so, though. so, I think scientists would be better developing theories, but not neces- The atomic theory and the theory served to agree that evolution is a sarily the central component. of general relativity, to name two, theory and then proceed to explain I’ll end with an application of are among the most solid things what a theory is—a coherent ex- some of these ideas to a contentious we know in science. Of course I’m planation that undergoes constant issue, which is the ongoing battle not putting the theory of evolution testing and often revision over a between teaching evolution and on the same level as the theory of period of time. teaching intelligent design. One of general relativity. Too many unex- the arguments from the intelligent plained things in the former. Still, Bill Robertson (wrobert9@ design crowd is that evolution is evolution is a pretty good working ix.netcom.com) is the author of “just a theory.” But a well-devel- theory. And one more thing I just the NSTA Press book series, Stop oped theory is pretty much the have to add. I have heard too many Faking It! Finally Understanding pinnacle of scientific achievement. scientists claim that evolution is Science So You Can Teach It.

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