Does physics actually describe reality or is it just a useful way of talking about how things happen?
This question often comes up when I speak about my recent book. People want to know if these curious properties of the universe proposed by theoretical physics - whether the experimentally-verified ones such as quarks and antimatter or the speculative ones such as strings and wormholes - are physical objects or just mathematical abstractions that happen to work out.
Unfortunately, the answer is unsatisfying to me as well as those who ask the question:
No one knows for sure.
Or, to put it more precisely, in many cases the physicists really don't care. The mathematical models which make up physics are such that physical objects such as particles are described as a mathematical entity. The mathematics make certain predictions, which is consistent with the existence of the particle, and those predictions are then tested.
In a sense, to theoretical physicists, the quark is the same thing as the mathematical model that represents the quark and to talk about whether or not the quark is "real" is kind of missing the point - that the model which uses quarks works to describe the physical system in question each and every time. (This is somewhat akin to the model-dependent realism discussed in Stephen Hawking's latest book.)
Physics is able to state what will happen in pretty much any situation that you come across in day-to-day life with an astounding degree of accuracy. (A point well illuminated a few months back by Sean Carroll at Cosmic Variance.) But in the realm of theoretical physics, things get much murkier. Predictions are made that are consistent with the known properties of the universe, but which often cannot actually be experimentally verified in any meaningful way.
Oddly, as I was contemplating these very issues, John Horgan tackled this subject over at the Scientific American blog with an intriguing comparison between anthropology and theoretical physics. He brings into question whether the work being done in theoretical physics is more akin to literary criticism (or even literature) than to traditional "hard" science.
Even those who embrace theoretical physics call the reality of some physical theories into question. In his 2007 book The Trouble with Physics, theoretical physicist Lee Smolin discusses two different types of theories:
- Effective theories are useful at describing behaviors in reality, but do not describe the actual physical reality itself.
- Fundamental theories not only describe the behaviors of objects, but also describe the actual physical reality of the situation.
In his book, Smolin uses these two descriptions to argue that string theory may be an effective theory, but that physicists should look elsewhere for a fundamental theory of nature.
One major problem with this tactic is that it's incredibly hard to determine whether a theory is effective or fundamental. About the only way to know for sure is to rule out its fundamental nature by showing that it's flawed. If it's flawed, then it can, by definition, just be a useful model of reality and not describe the real thing ... and therefore must only be an effective theory.
But I don't think it's wrong for science to embrace a theory which may only be effective, because really it's the efficacy of a theory that we care about. What does the theory tell us? What does it predict? What tangible components of reality does it allow us to understand or manipulate? This is really the point where science is most compelling, rather than just some abstract musings about how the universe "really" works.
Is physics real, then? I'll let that question be answered, in part, by the father of quantum mechanics, Niels Bohr:
It is wrong to think that the task of physics is to find out how Nature is. Physics concerns what we say about Nature.
- Niels Bohr Quotes
- Physics Book Review: The Grand Design by Stephen Hawking & Leonard Mlodinow
- Introduction to the Scientific Method