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My colleague finds a mistake in Feynman

October 11, 2012

Since I first saw it on Minds of Our Own, I loved the seed and a log question, where Harvard and MIT graduates are given a seed and a log, and asked how the seed becomes a log. Students wax scientific about photosynthesis, but then when pushed to explain where all that mass comes from, is it the air, the ground, or water, almost all of them default to thinking it’s the dirt that makes up the mass of the tree. Minds of our own goes on to show how this shows students are missing both a very basic scientific idea—that most of the mass of the tree comes from Carbon Dioxide, a gas, and this is because they don’t have a real understanding of the idea that gases has mass.

Recently, I stumbled upon a video of Richard Feynman giving his own answer to this question, via the excellent blog, It’s Okay to be Smart.

I forwarded this video to my colleague, who shares a similar love for this question (we were both indoctrinated in its value at the Klingenstein Summer Institute (Have you been teaching in an independent school for less that 5 years? If so, apply now).

Not to long after I sent the email, he wrote me back.

Sorry to say this about such a great speaker and great scientist, but Feynman gets things badly wrong here.

He suggests that the oxygen generated during photosynthesis is the product of a splitting apart of carbon and oxygen in carbon dioxide. (See min. 3:39)
In fact, all the oxygen produced by photosynthesis is generated by the splitting of water to make H+ e- and \textrm{O}_2. The role of carbon dioxide in photosynthesis is to get reduced, to have hydrogen atoms added to it, not to have oxygen removed. This is easy to demonstrate experimentally by using radioisotopically labeled \textrm{CO}_2 and \textrm{H}_2\textrm{O}. Only when the label is in the \textrm{H}_2\textrm{O} is the \textrm{O}_2 produced radioactive.

I’d like to think I would have caught this error if I’d paid closer attention to the video as I was watching it and trying to decide whether to forward it, but honestly, my recollection of the AP chemistry class I took 20 years ago is now rather faded, and redox reactions are definitely one of the gaping holes in my understanding.

But I see this as a really exciting interdisciplinary moment—here’s a great physicist, stepping pretty far outside his area of expertise to talk about a topic in biology, photosynthesis, only to be called out on incorrect chemistry by a biologist. It’s all the more exciting that Feynman made this mistake, since it shows we all make mistakes, which is a great lesson for my students to see.

This is exactly the type of interdisciplinary learning I’d like to be setting my students up to do. However, I’m not sure I am setting them up to understand ideas like this, since they see most topics in year long courses completely isolated from one another. Could a biology student hear an explanation from a physicist about photosynthesis and bring up his understanding of redox reactions from the previous year’s study of chemistry to check Feynman’s work? I’m skeptical. And I’m sure than none of them would be able to push to the extra layer of thinking about how we might be able to know this experimentally by using radioisotopes. So how do we teach science students to think in this interdisciplinary manner?

If you want even more information about this, my colleague wrote this excellent article in The American Biology Teacher: Dust Thou Art Not & unto Dust Thouh Shan’t Return: Common Mistakes in Teaching Biogeochemical Cycles.

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5 Comments leave one →
  1. October 11, 2012 7:31 am

    The radioisotope experiment is absolutely needed to trace where the O2 originates, in this case, since there are multiple sources of O2. But even if one thought they only had one source for an element, they would be wise to confirm the source.

    The example you share is a good one to share with your students because it demonstrates how the various fields of science often overlap and are reliant on each other’s expertise to make breakthroughs. The moral of the story: collaboration and understanding of other sciences is important.

    But there is another moral of the story. The ideas the students are learning in class (or if they watch a science show or read a science article) are based on experimentation – manipulation, observation and measurement of matter and energy in the real world. The ideas did not just pop into somebody’s head — not just dreamy thoughts — though that plays a role in making sense of available information.

    Another great lesson in this is how your friend discovered the paper(s)/journal articles which prove the source of O2. Or, how your friend even thought to look for the evidence in the first place. These are two additional great lessons that spring from you example. As for identifying journal articles, I’d had no practice with this until I was an undergrad. I felt swamped by the amount of information available. I was very uncertain how to “pick” the “right” articles for my projects. I had no idea where to start and felt a little intimidated by the language. Only with practice did I become more comfortable. I wish I’d been exposed to the process and offered some kind of guidance when I was in High School. The exercise in identifying “real” journal articles and interpreting them would be informative, I think, even for students who decide not to pursue the sciences. Then they will then have some sense of how/where these answers in science are generated, how they are communicated and why they are open to the rest of the scientific community to confirm or dispute.

  2. October 11, 2012 7:55 pm

    Great post. Here’s an experiment that I’d like to see: single teacher for all subjects in High School. Maybe then we could really hit at integration of subjects?

    • October 13, 2012 1:54 am

      Michael, good luck with the “single teacher for all subjects” idea. It is hard enough finding enough teachers competent in one subject, much less in all. The single teacher for elementary school grades has already done a lot of damage to math education—I’d rather not see the failed experiment extended to higher grades.

  3. October 20, 2012 5:44 pm

    Since most science teachers specialize in whatever subject it is that they teach, it makes sense that a specialist in one area would call out a specialist in another area on specific information that pertains to their expertise. Like the earlier commenter suggested, having a single teacher for all subjects would hurt the overall quality of education that students receive now. It seems that teachers would teach with a “jack of all trades, master of none” mentality. In order for a system like this to work, the quality of teachers would have to be dramatically improved. Effectively teaching material to students requires more than a passing knowledge of a subject. I think that the current system is our best option right now, with a focus towards specialization.

  4. October 28, 2012 1:17 am

    Forgive my humble opinion, I am a future teacher in Edm 310 right now and am probably woefully optimistic, but I don’t think one teacher is possible or beneficial to students. I think that if you were to open the walls of our classrooms, so to speak, so that some of other subjects can bleed into our classes, this will allow students to see the connections between subjects. I think that you could take a lesson in physics through the bio-teachers point of view, maybe even having that teacher teach the lesson, and vice versa. I could also see this as only confusing to students as they would be trying to absorb information from two different viewpoints. I don’t know, it would definitely a planning nightmare to do it this way.

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