Today I’d like to discuss a pet peeve of mine. In many physics textbooks, one reads phrases such as:
a certain physical system obeys a certain law of physics
Here’s an example taken from page 147 of Chemical Principles, by Steven S. Zumdahl, Cengage, 2009 (although I am not trying to single out this author … examples are numerous, and this just happens to be the first one that was handy):
It is important to recognize that the ideal gas law is an empirical equation—it is based on experimental measurements of the properties of gases. A gas that obeys this equation is said to behave ideally. That is, this equation defines the behavior of an ideal gas, which is a hypothetical substance. The ideal gas equation is best regarded as a limiting law—it expresses behavior that real gases approach at low pressures and high temperatures. Most gases obey this equation closely enough at pressures below 1 atm that only minimal errors result from assuming ideal behavior.
What bothers me is that such statements might be misconstrued to mean that the equation tells the gas how to behave. And then the gas goes out and obeys and behaves properly, just the way any law-abiding citizen does when he or she obeys the laws of the land.
I suppose that no scientist believes this, but I can’t be sure; in any case, if we don’t believe this, why are we writing unclearly? Clarity is courtesy.
Because the laws of science are not like the laws in our legal systems. They are descriptive, not prescriptive.
I get the feeling from reading some authors that they really believe that there is something “out there in the universe” that makes the universe behave as it does. This is not the way I see the world.
If you really believe that a scientific law tells a physical system how to behave, what happens when there is a historic change of perspective (a “scientific revolution”)? One can end up tied up in mental knots. Imagine saying in the 19th century that a physical system “obeys” Newton’s laws of motions, only to have to revise your opinion in the 20th century in light of Einstein’s theory of relativity. Do you now say that the same system “obeys” the equations of relativistic mechanics, and only approximately “obeys” Newton’s laws? Or do you say that you were wrong in your earlier statement, but now you’ve got it right; this is problematic, because how do you know you’re right? What if there is yet another revolutionary change in perspective? I find this awkward.
Consider some of what the history of science teaches. First, since science originated as the product of men and not as a revelation, it may develop further as the continuing product of men. If a scientific law is not an eternal truth but merely a generalization which, to some man or group of men, conveniently described a set of observations, then to some other man or group of men, another generalization might seem even more convenient. Once it is grasped that scientific truth is limited and not absolute, scientific truth becomes capable of further refinement. Until that is understood, scientific research has no meaning.
For me, the key word is describe. A scientific law is a convenient description of observations. The law of science does not tell the world how to be, the world just is; science is a human attempt to engage with the mysteries of the world, and to attempt to understand them.
Imagine a work of art, and a written description of the art work. Nobody would confuse the written description with the work of art, nor would we say that the description created the art. The work of art was created by someone, and the writing describes it.
In the case of the world, nobody knows whether it had a creator or not. There is no argument that is convincing, one way or the other, so let’s leave this aside.
But nevertheless, you could consider the universe as a beautiful work of art, even if it were not created by an artist. There are many ways to describe this work of art, many perspectives. One can engage with the world via poetry, literature, the arts, even sports. Science is a wonderful way to engage with the world, and brings its own special perspectives.
But if we are writing a description of a work of art, let’s not fool ourselves that our description is a blueprint or instruction manual that is obeyed in creating the work of art. The work of art is just there, and our writing is a description.
The world just is. Our equations do not tell the world what to do.
(This post first appeared at my other (now deleted) blog, and was transferred to this blog on 22 January 2021.)