April 17, 2011

WE'RE GONNA NEED A BIGGER RUG:

The Man Behind the Curtain: Physics is not always the seamless subject that it pretends to be (Tony Rothman, American Scientist)

“I want to get down to the basics. I want to learn the fundamentals. I want to understand the laws that govern the behavior of the universe.” Thousands of admissions officers and physics department chairs have smiled over such words set down by aspiring physicists in their college-application essays, and that is hardly surprising, for every future physicist writes that essay, articulating the sentiments of all of us who choose physics as a career: to touch the fundamentals, to learn how the universe operates.

It is also the view the field holds of itself and the way physics is taught: Physics is the most fundamental of the natural sciences; it explains Nature at its deepest level; the edifice it strives to construct is all-encompassing, free of internal contradictions, conceptually compelling and—above all—beautiful. The range of phenomena physics has explained is more than impressive; it underlies the whole of modern civilization. Nevertheless, as a physicist travels along his (in this case) career, the hairline cracks in the edifice become more apparent, as does the dirt swept under the rug, the fudges and the wholesale swindles, with the disconcerting result that the totality occasionally appears more like Bruegel’s Tower of Babel as dreamt by a modern slumlord, a ramshackle structure of compartmentalized models soldered together into a skewed heap of explanations as the whole jury-rigged monstrosity tumbles skyward.

Of course many grand issues remain unresolved at the frontiers of physics: What is the origin of inertia? Are there extra dimensions? Can a Theory of Everything exist? But even at the undergraduate level, far back from the front lines, deep holes exist; yet the subject is presented as one of completeness while the holes—let us say abysses—are planked over in order to camouflage the danger. It seems to me that such an approach is both intellectually dishonest and fails to stimulate the habits of inquiry and skepticism that science is meant to engender. [...]

Quantum 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.

Posted by Orrin Judd at April 17, 2011 7:01 AM
  
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