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The Man Behind the Curtain

Physics is not always the seamless subject that it pretends to be

Tony Rothman

Probable Cause

2011-05MacroRothmanFB.jpgClick to Enlarge ImageQuantum text authors, perhaps because of the perversity of their subject, are particularly adept at sweeping conceptual difficulties under the rug. Nowhere is this more apparent than in the celebrated “two-slit” experiment, which is universally invoked to illustrate the wave-particle duality of light and which brings you face to face with the bedrock inscrutability of Nature. The experiment is simple: Shine a light beam through a pair of narrow slits in a screen and observe the results. For our purposes, the great paradoxes illustrated by the two-slit experiment, that light can act like a wave or a particle but not both at the same time, are not central. What is central is that explanations of the experiment’s results invoke both classical lights waves, on the one hand, and photons—quantum light particles—on the other.

Also central is that in analyzing this experiment textbook authors essentially throw up their hands and surrender. Recollecting that light is an electromagnetic wave, authors invariably begin by talking about the intensity of the incident light, which is a measure of the strength of the electric and magnetic fields. Then in a complete non sequitur, they shift the conversation to photons, as if the quantum-mechanical beastlets have electric and magnetic fields like classical light waves. They don’t. In fact, an accurate description of the famous experiment requires a more subtle quantum-mechanical entity known as a coherent state, which is the closest thing to a classical light wave.

What’s more, by resorting to a classical optics analogy of the experiment, authors are forgoing any explanation whatsoever. “Explanation” in physics generally means to find a causal mechanism for something to happen, a mechanism involving forces, but textbook optics affords no such explanation of slit experiments. Rather than describing how the light interacts with the slits, thus explaining why it behaves as it does, we merely demand that the light wave meet certain conditions at the slit edge and forget about the actual forces involved. The results agree well with observation, but the most widely used of such methods not only avoids the guts of the problem but is mathematically inconsistent. Not to mention that the measurement problem remains in full force.

Such examples abound throughout physics. Rather than pretending that they don’t exist, physics educators would do well to acknowledge when they invoke the Wizard working the levers from behind the curtain. Even towards the end of the twentieth century, physics was regarded as received Truth, a revelation of the face of God. Some physicists may still believe that, but I prefer to think of physics as a collection of models, models that map the territory, but are never the territory itself. That may smack of defeatism to many, but ultimate answers are not to be grasped by mortals. Physicists have indeed gone further than other scientists in describing the natural world; they should not confuse description with understanding.

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