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.

Digital or Analog?

Is the fundamental nature of the universe continuous or discrete? Does matter come in lumps or waves? Is the material universe intrinsically a digital or an analog thing?

It's an ancient debate that has relentlessly been waged for over two millennia in the Western world. (I can't speak for the Chinese; they may have been arguing about it for 5,000 years or so.) The pendulum periodically swings toward the discrete side, as scientists view the natural world as made up of particles: discrete elements like atoms. Then it will swing the other way for a period of time, as those who view nature as continuous will gain the upper hand. The dispute carries on today.

The ancient Greeks were among the first to draw opposing sides on the issue. Aristotle and his camp saw the world as continuous, while the atomists saw it as discrete. Long before technology advanced to the point that the existence of atoms could be demonstrated, their actuality was presciently posited by Democritus and his followers.

Look at a river. The flowing water looks continuous. No matter how much you subdivide it—down to a trickle or even a drop—it appears to the naked eye to be the same thing: a clear, unbroken liquid. So Aristotle and his buddies reasoned. And yet we moderns know that water is composed of discrete molecules of H₂O; tiny discontinuous entities. Modern chemistry shows us that the water molecule is further composed of atoms of hydrogen and oxygen—which are themselves made up of protons and electrons...just finer, discrete lumps of matter. 

But wait: we don't need to stop at these particles. Protons and electrons are made up of quarks and leptons...still finer particles. But wait again: modern particle physics knows that another way of looking at these most fundamental building blocks is that they are apparently composed of continuous energy fields. Some physicists call the newly-discovered Higgs boson a particle, some call it the Higgs field. So these most fundamental particles can also appear to be like “ripples” in continuous energy fields. 

Now we seem to be back to the analog. And so it goes: Is the universe continuous or discrete? Analog or digital? It seems to depend on how fine you look at it and your preference. The debate goes on.

Another conundrum of this type can be encountered when one considers light. Ever since Newton's time (18^th century) one scientific camp has considered light to be made up of particles (photons) and another camp views light as waves. Quantum mechanics entered the fray in the early 20^th century and added its weight to both sides of the argument. Is light particles or waves? Discrete or continuous? Well, the discomfiting thing about quantum mechanics is that the answer is never definitive. The answer always seems to be: depends; could be both. Depends on how you look at it. That's no help!

So here we are in the 21^st century—still not sure if nature, at its heart, is analog or digital. The debate rages on. Will we ever find an answer? Maybe a more pertinent query is: Are we even asking the right question?