Ultimately Untestable?

Contemporary physics is facing a problem—some physicists would even describe it as a crisis. And as is true for most crises, controversy erupts. Factions appear and engage in debate. Emotions bubble up. Yes, even staid physicists can become emotional, and a few of them even become impassioned.

The cause of this particular crisis is the shortcomings of what has come to be known as the “standard model” of physics. It describes the fundamental behavior of matter—which is essentially the core of physics. But the standard model, although it has been very successful in describing most of the behavior of aggregate matter, does not explain the behavior of elementary particles. The model does very well at the macro level, but is quite useless at the micro level.

Beginning about 100 years ago, a few physicists developed a new micro-matter theory that's come to be called quantum mechanics. It does a very nice job of describing the behavior of subatomic particles—such as electrons, protons, and neutrons. The problem—the crisis—lies in physicists being unable to reconcile the two theories.

What's worse, while the standard model has been verified by countless experiments, that's not true for several aspects of quantum theory. While many predictions of quantum mechanics have been verified by tests, there are some aspects of the theory that seem to be beyond experimental investigation. And that really bothers some physicists. The time-tested scientific principle is that scientific theories must be proven by experiment. So what do you do, when you can't put your theory to the test?

A rough parallel is the theory that was put forth by the Ptolemaic model for the universe—the nearly 2,000-year-old idea that placed the Earth at the center of the universe. Ptolemy's model was very cumbersome and complex, but it explained various phenomena for centuries. It was finally replaced by the far simpler Copernican (sun-centered) model in the 17^th century by Kepler, but his model's proof had to wait until science had the tools to do the experiment and irrefutably demonstrate that the new model was correct. The crucial tool that did the job: the telescope.

Quantum physics today is in a rather similar place. It could be that we may someday have the tools to run the experiments and confirm the correct theory (or correct theories). Yet some physicists are convinced we may never be able to do the experiments. If so, do we abandon the venerable rule that all theories must be testable, or just abandon those theories that can't be tested? The debate continues. Emotions are roiling.