The Purpose of Undergrad Labs

Chad Orzel asks what experimental setup he should use for an undergrad photoelectric effect lab:

…we have two different set-ups for doing a photoelectric effect experiment. One of these is a PASCO apparatus with the phototube wired to a circuit inside an actual black box. You shine light into the tube, press a button, and the output of the box rises to the stopping potential for that frequency in a more-or-less exponential manner. This gives very nice results, often within 1% of the accepted value of Planck’s Constant.

The other is an old-school lab, using a homemade monochromator and a phototube with an external voltage generator supplying the stopping potential. For each color of light, the students watch the output of the phototube on an oscilloscope, measure the output voltage for a handful of applied voltages, and extrapolate to find the stopping potential. This is much closer to the way the experiments were originally done, but it also tends to give results that differ from the accepted value by 20-30%.

Your answer would depend a lot on what you think the purpose of the lab should be. My view, like many of the commenters at Uncertain Principles, is that the purpose of labs is to let students learn how to conduct experiments. By this I don’t mean how to use the specific equipment involved (though it’s useful to do so), but how to calculate and justify experimental errors, how to explain why your data is evidence for/against a model, general principles about the points at which to take data when one is calibrating equipment versus when one is taking the actual measurements being used to test the model, etc. From this perspective, the old-school setup seems like a clear winner — it seems doable but still challenging enough, methodologically, to test the students’ experimental skills. Chad, however, says that

…the purpose of the lab is to show that experimental measurements of the photoelectric effect agree well with the Einstein model. The more complicated version doesn’t really add to that understanding, and in fact, the complication tends to obscure the physics. Students spend so much time fretting over the experimental details that they lose track of what it’s supposed to show.

You can argue that they’re learning lab skills in the process, but I’m not all that impressed. The only really useful thing they get out of it is how to use an oscilloscope, and there are other ways to teach that. There’s some fuzzy data-selection heuristic stuff going on in deciding exactly what to use as the stopping potential for any given point, but it’s hard to explain that in such a way that they don’t leave the lab thinking “it’s ok to fiddle with the data to get something closer to the target value.” That’s not only not what we’d like them to learn, but is actively harmful.

The thing is, I don’t see how doing the experiment with the PASCO black-box detectors would “show that experimental measurements of the photoelectric effect agree well with the Einstein model.” Suppose I was skeptical that Einstein’s model has been experimentally validated. Would I be convinced that it has by doing the PASCO experiment? No, because I don’t know that the apparatus accurately converts the energies it measures into stopping potentials, and that the values output by the apparatus actually are those of the stopping potentials. It’s natural to suspect that what the black box is doing isn’t what my instructor claims it’s doing, since the instructor has a vested interest in telling lies-to-children about what the box does. (This suspicion may not be justified. But we can see that inferring the model’s goodness from the black box experiment involves an extra epistemic step, so it isn’t obviously unreasonable to be less convinced by the black box version than by the old school version.)

To summarise:

1. From a teaching-the-methodology point of view, the ‘historical’ experiment wins for me.
2. From a convincing-students-the-model-is-right point of view, the historical experiment wins too, because the black box experiment isn’t any more convincing to a skeptic of the model.

Incidentally, the comments to Chad’s post highlighted to me how terrible my undergrad physics labs were. A few commenters, including Chad, say that they give the students the equipment in bits and leave them to figure out how to put them together in order to conduct the experiment. Except for introductory labs in which we did extremely simple experiments like measuring g using inclined ramps and shit, I don’t remember having to do any major assembly work for my labs. And what little assembly work we had to do would be laid out in painstaking detail in the lab manual.

Distilled Neuroticism

I was disappointed by Frost, the last Thomas Bernhard work I read. It was his first novel, and it showed. The characters were neurotic in that classic Bernhard way but their narratives didn’t really hit the mark of their neuroticism. The prose failed the characters; there weren’t those sentences that struck me dumb with the sheer precision with which they distilled the neuroticism. I didn’t enjoy most of Frost. Today I checked out Extinction from the library. It’s one of his last works, and it is much better. I’m only on page 15 and I can say I haven’t enjoyed a work of fiction this much in a long time. (Granted, I’ve not been reading much fiction recently.) Some excerpts (emphases Bernhard’s; translation by David McLintock):

The question of whether I had loved my parents and my brother was one that I at once fended off with the word naturally, but it remained fundamentally unanswered. For ages I had not had what is called a good relationship with either my parents or my brother but had one marked by tension and, in recent years, indifference. The truth is that for a long time I had wanted to know nothing about Wolfsegg or about them, and they, conversely, had wanted to know nothing about me. Hence our mutual relations were more or less confined to the exigencies of existence. Twenty years ago, I thought, your parents not only released you from Wolfsegg, after wanting to chain you there for life, but dismissed you from their feelings. During those twenty years my brother had envied me for having left Wolfsegg, for my megalomanic self-sufficiency, as he once put it, and hated me for my relentless insistence on freedom. My sisters’ distrust had always exceeded the bounds of what is acceptable among siblings, and when once I turned my back on Wolfsegg, and therefore on them, they too pursued me with their hatred. This is the truth. I picked up the suitcase. It was, as usual, too heavy. I really don’t need it, I thought, as I have everything I need at Wolfsegg. Why cumber myself with a suitcase? Having decided to travel without a suitcase, I proceeded to unpack my clothes and return them, item by item, to the closet. It’s natural to love one’s parents, and it’s equally natural to love one’s brother and sisters, I thought, standing by the window again and looking down on the deserted Piazza Minerva. We therefore fail to notice that from a certain moment onward we hate them, without wanting to, just as naturally as we previously loved them, for all kinds of reasons that we become aware of only years later, often decades later. We can’t determine precisely when we stopped loving them and started hating them, and we don’t try, basically because we are afraid to. Anyone who leaves his family, against their will and as implacably as I left mine, has to reckon with their hatred, and the greater their previous love for him, the greater their hatred when he has done what he swore to do. For decades their hatred caused me suffering, I reflected, but I haven’t suffered from it for years now; I’ve become used to it and it no longer hurts me. And their hatred of me inevitably led me to hate them, but in recent years they haven’t suffered from my hatred either. They despised me, their Roman, just as I despised them, the Wolfseggers. Basically they stopped thinking about me, just as I stopped thinking about them most of the time. They always referred to me as a charlatan, a blatherer, a parasite who battened on them and everyone else. The sole term I could apply to them was blockheads. Their death, which can only have been caused by a road accident, I told myself, in no way alters the facts. There was no danger of my yielding to sentimentality. My hands did not shake as I read the telegram, and my body did not tremble. I’ll tell Gambetti that my parents and my brother have died and that I must postpone our lessons for a few days, I thought. After all, I won’t be staying at Wolfsegg for more than a few days; a week will be enough, even allowing for unforeseen complications. For a moment I considered taking Gambetti with me, fearing the superior force of the Wolfseggers and wishing for an ally with whom I could defend myself against their onslaught, someone of my own kind who would be a partner in a desperate and possibly hopeless situation, but I immediately abandoned the idea, as I wanted to spare Gambetti a confrontation with Wolfsegg. He’d see that everything I’ve told him in recent years is actually quite tame compared with the reality, I thought. At one moment I thought of taking him with me, the next moment I thought better of it. Finally I decided against taking him. I’d spend too much time with him, and this would cause something of a stir that I’d probably find disagreeable, I thought. They wouldn’t understand a person like Gambetti at Wolfsegg, where harmless strangers are invariably greeted with hostility. They’ve always rejected anything unfamiliar, they’ve never welcomed anything or anyone unfamiliar, as I usually do.

[...]I had treated Gambetti to a highly intemperate description of my parents and told him that my brother was rather a bad character, and irremediably stupid. I had described Wolfsegg as a citadel of brainlessness and spoken of the dreadful prevailing climate, which dominated and ruthlessly destroyed all who were forced to live — or rather to exist — there. But I also told him about the glories of Wolfsegg — about the beauty of the autumn, of the winter cold and the silence in the surrounding woods and valleys, which I loved more than anything. Nature there was ruthless, I said, but utterly clear and magnificent. Yet this clear and magnificent nature was not appreciated by those who lived in the midst of it, because they were too brainless. If my family didn’t exist, but only the walls they live in, I told Gambetti, Wolfsegg would be the perfect place for me, as there’s no other so congenial to my spirit. But I can’t abolish my family just because I want to, I said. I can hear myself saying these words, and the terrible meaning they took on now that my parents and my brother were actually dead made me repeat them aloud as I stood at the window, looking down on the Piazza Minerva. But I can’t abolish my family just because I want to. Uttering these words to Gambetti, I had felt the utmost distaste for the people they referred to. I now found myself repeating them aloud in a distinctly theatrical manner. Like an actor who has to rehearse his lines because they are to be spoken before a large audience, I momentarily took the sting out of them. They suddenly ceased to be annihilating. However, these words, But I can’t abolish my family just because I want to, again forced their way to the forefront of my mind and seized possession of me. I tried to stifle them, but they would not be stifled. I no longer enunciated them clearly but gabbled them to myself several times, trying to make them seem ludicrous, but despite my attempts to stifle them and make them seem ludicrous they became all the more menacing and suddenly acquired a greater force than any words I had ever uttered. You can’t drown out these words, I told myself — you’ll have to live with them. This realization brought a sudden calm into my situation. But I can’t abolish my family just because I want to. I spoke the words once more, but this time in the tone I had used to Gambetti. They now meant what they had meant then.

[...]The present situation is one I’ve never envisaged, I thought. I had considered every other possible situation time and again, but never this. I had often dreamt of dying and leaving them behind, of leaving them alone without me, of freeing them from me by my own death, but never of being left behind by them. The fact that they were now dead and I was alive was not only utterly unforeseen, but quite sensational. It was this sensational element, this overwhelming sensation, that I found shattering, not the simple fact that they were dead, irrevocably dead. Though my parents had been pathetic in every way, I had always regarded them as demons, and now suddenly, overnight, they had shrunk to the ridiculous, grotesque photo that I had in front of me and was studying with the most shameless intensity. The same was true of the photo of my brother. All your life you feared these people more than any others, I thought, and this fear cast a monstrous blight on your life. All your life you repeatedly tried to escape from them, but you always failed. You went to Vienna to escape from them, to London, to Paris, to Ankara, to Istanbul, and finally to Rome — all to no avail. They had to have a fatal accident and shrink to this ridiculous scrap of paper called a photograph before they could cease to harm you. The persecution mania’s over, I thought. They’re dead. You’re free. Looking at the photograph of my brother on his sailboat at Sankt Wolfgang, I felt sorry for him for the first time. In the photo he now seemed far more comic than when I had first looked at it. I was alarmed by my ruthless honesty. My parents too looked comic in the photo taken at Victoria Station. All three of them, lying on the desk in front of me, not four inches in height, fashionably dressed and in grotesque physical attitudes that betrayed mental attitudes no less grotesque, were even more comic than when I had looked at them before.

Incidentally, the cover photo for the edition of Extinction I’m reading is August Sander’s Farm Girls. An excellent fit for the excerpts above.

Another time reversal puzzle

Wolfgang has a nice stat mech puzzle up at his extremely under-read physics-ish blog. As I write in the comments there, I think there’s a major flaw with his reasoning, but I’m not entirely comfortable with the implications of what I propose, either.

Wheeler on “basic quantities of nature”

After many months of letting it sit around, I took a glance at the Chandrasekhar-autographed (-owned?) relativity conference volume I’d bought for almost nothing at a library book sale. Wheeler has a paper in there on “superspace”, which for him meant the configuration space of general relativity. He speaks of a theory in which the universe undergoes cycles of collapse and re-expansion, with properties like its number of dimensions, its coupling constants, and its particle masses coming out different with each re-expansion. Then he explains why, in this theory, a particle mass is not a “basic quantity of nature”:

On this view a particle mass is not a basic quantity of nature. It has as little claim to that title as does the mass of the water droplet that hangs from the ceiling of the shower. Ask why it has its mass, and find oneself asking why one takes a shower where the value of g happens to be 980cm/sec². Ask why the particle has its mass, and end up asking why we happen to be living in this particular cycle of the universe. One cycle, one set of masses. Another cycle, another set of masses. That is the picture.

The paragraph itself raises interesting questions, of course. It seems that Wheeler’s idea of a ‘basic quantity of nature’ is a quantity that isn’t explained (or caused?) by what I roughly think of as ‘contingent’ physical configurations of the universe. But the notions of ‘contingency’ and explanation (or cause) aren’t fleshed out.

What interested me more, though, was that Wheeler has a reference in this paragraph to Leibniz’s Théodicée, Leibniz’s correspondence with Samuel Clarke, Landau and Lifshitz’s Statistical Physics, and a few physics papers from the 1960s. It’s been nearly two years since I read the Leibniz-Clarke correspondence, but I can’t recall anything in there that is obviously relevant to Wheeler’s point about basic quantities of nature. The Principle of Sufficient Reason? If we take PSR seriously, though, we’d have to say there aren’t any ‘unexplained’ (again, lacking a firm notion of explanation here) physical quantities, which is a quite different thing from saying that if a physical quantity is explained by some contingent physical configuration, then it is not a basic quantity of nature. That is, PSR would seem to rule out unexplained physical quantities rather than serving as a justification for classifying them as ‘basic’ quantities.

And — what do Landau and Lifshitz say that is remotely related to what Leibniz said, or to Wheeler’s point?