Population growth and physics?

by Stephanie Chasteen on September 19, 2009

Do physics teachers have a role to play in teaching about population growth?  One could argue that the study of physics is separate from the world of human concerns — it’s concerned with the physical laws governing how the world works.  Our role is to educate students about these abstract laws.  The rest is for philosophers.

One could also argue that, as critical thinkers, experts, and citizens of the world, our role is to help educate our students about issues critical to our world and our country.  And as physicists, we have access to information that is relevant to the world — such as world energy demands, or the mathematics of population growth.

One of the physics teachers’ listservs I subscribe to recently had a spirited debate on this topic.  After a rousing exchange, one physicist shared:

I was indoctrinated into the isolation view in my undergraduate and master’s programs.  I was told by the structure of the physics courses and by my fellow students that physics is just information and special ways of thinking about the world, as such it is morally neutral, and that the question of “right vs. wrong” applies only to what others do with it.  I heard many people say “I just do/teach physics – what other people do with it is not my responsibility”.    This separation from moral responsibility is partly a response to Oppenheimer’s claim that “physicists have known sin” – the willing development by good-hearted physicists of nuclear weapons during WWII and afterwards (“Oppenheimer: The Tragic Intellect” by Charles Thorpe p.190).  [Andy Johnson]

He changed his tack, however, after many years in the field.  What he teaches, he says, has moral implications.  He can teach his students, for example, to rely upon the authoritative teachings of physics, or he can teach them to be independent thinkers.  He can encourage or discourage questioning.

With regard to what I teach,  I cannot be a moral person if I act as if physics has nothing to contribute to the burning issues of the day such as climate change, resource depletion, population growth, and economic troubles – all of which have been shown to be symptoms of the fundamental long-term unsustainability of the capital-industrial system (e.g. see “The Limits to Growth” by Meadows, et. al).   Physics has been a key enabler in allowing this system to lurch to where it is now, and it would be worse than folly to pretend that physics doesn’t provide critically useful perspectives in helping people understand the problems and begin addressing them.

On the other hand, cautioned another teacher, we have powerful positions as experts in a highly-regarded field.  We don’t want to use our position of power and authority to “bully-pulpit,” especially about an issue so sensitive as population growth.  Would we advocate that our students refrain from having children?  Of course not, that’s the socioeconomic, political, and psychological ramifications of the scientific content.  And, are physics teachers the best people to teach about such questions?  If it includes physics — well, yes.  We’re experts in that arena.  We can teach about the energetics or chemistry of climate change, but the politics of it are beyond us.

The problem, of course, is that these lines blur.  What we choose to teach is inherently political.  And if we teach about exponential growth and make a connection between that mathematics and population growth, then we are engaging in mathematical modeling of a problem that is much too complex for the average physicist’s expertise to address.

What’s my own opinion?  I think we should teach students some of the powerful scientific ideas that underlie the important issues of our time — exponential growth, conservation of energy, the greenhouse effect, and so forth.  We should make the connection between the abstract physics and the real-world application.  We should do our best to help our students understand the issue, from a scientific point of view.  What we choose to teach shows our biases and preferences — we can’t escape that, and we’re fools if we think that we can.  But students’ decisions, or those of policymakers, aren’t our arena.

Resources for teaching about population growth:

Rice on a chessboard story

Bacteria growing in a bottle (when is the bottle half full?)

Bacteria in a large bottle double in number every minute, through cell division.
At 11:00 there is one bacterium in the bottle. At 12:00 the bottle is completely full.
At what time is the bottle half full?
How long after 12:00 would it take to fill two bottles (assuming no bacteria are lost)?

Exponential Folding (one of my podcasts):  How many times can you fold a piece of paper in half?

Use a large newspaper sheet and have folks fold it in two, repeatedly. It gets to be difficult pretty fast. So you can ask them to fold it as many times as they possibly can (like a game; who can fold more than 6x)  You can ask them to predict how thick the folded 0.1mm sheet will be once you fold it 6 times (compare to 5x or 4x).  Have your participants ‘guesstimate’ how thick the sheet would be if you could -hypothetically- fold it 50 times.  Turns out that:  (10^-4m) (2^50) is roughly 10^12 which is roughly the distance between the Sun & the Earth.

The other side of growth:  Exponential decay using M&M’s.

A few problem sets from Ed Redish’s collection:  1, 2, 3

And apparently Population Connection has a lot of free, good activities for a wide range of learners.  They also do workshops for free!  They are certainly liberal, but reportedly agenda-free.

And here is a link to how real-world population modeling is done (which doesn’t really follow an exponential curve).

And below a video from Al Bartlett, a physicist who has been giving a talk on population growth, exponential functions, and energy demands for the past 20 years.  He gave me shivers when I heard him speak.  He’s not snappy, but his points, and the math, is quite compelling.

{ 2 comments… read them below or add one }

Fran September 19, 2009 at 1:30 am

I love Al Bartlett’s talk! I have heard it at least three times, the first time in 1988! I also once had the pleasure of sitting next to him in a fluid dynamics workshop at an AAPT meeting. He is very interesting to talk with!

Also, it is nice that you compiled all these resources onto one page. I have gone ahead and deleted all those listserv e-mails because now I can just link here if I need to.

Captain Skellett September 19, 2009 at 9:11 am

I loved the rice on a chessboard story when I first heard it back in high school, the imagery has really stayed with me.

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