teaching theoretical physics in primary school

what does a bunch of toddlers breaking up play-doh into ever smaller pieces, a seven year-old dropping a tennis ball and a ruler, and a primary school classroom separating white beans from borlotti beans have to do with theoretical physics?

a lot, actually! let me explain…

a couple of weeks ago my nephew’s school in Rome was celebrating the late astrophysicist Margherita Hack’s legacy by inviting parents to go to school for some enrichment activities related to their jobs. my sister invited me to go. impressing my nephews and sharing my passion and maybe inspire a whole bunch of kids? how¹ could i refuse! there was a problem however…

how can i teach a bunch of kids about my work? the problems and tools of theoretical physics are quite delicate and abstract. the main goal could not be teaching them some specific facts or ideas. instead, i decided i’d have them go through experiences that might impart some intuition about some of the underlying physical principles.

so i thought: let’s do gravity and time! two subjects central to my research, but that also have universal and obvious impact on our lives. the activities suggest themselves.

gravity

most people have seen $F=ma$ and $F=mg$ in highschool, but i think very few people know in their bones that this implies $a=g$, that all things dropped at the same time will fall at exactly the same pace, independently of how heavy they are, their shape, what they are made of, etc… Galileo, Newton, and Einstein all recognised how important and mysterious this fact is, a fact that now stands at the core of our understanding of gravity.

so my sister and I collected a bunch of objects around her house and put them in a bag. we had kids pick two objects, weight them, and then we’d ask the class which one they thought would fall to the floor first. on the first test, the whole class bet that the heavier object would fall faster, and they were all surprised when they landed at the same time. as more and more object dropped, the class slowly started to expect that any pair would fall at the same rate. success!

actually, if you haven’t done this, i suggest you pick some objects now and drop them at the same time. actually do the experiment! it’s really neat and satisfying to see a pillow and a coin fall at the same exact rate. (or you can look at the video of my nephew Adriano doing the experience)

of course we all know that some things fall at different rates: feathers and leaves take forever to fall; maybe the intuition that lighter things fall slower comes from this. (you’d be surprised how much the shape matters in that case, actually) that’s why i also showed them clips from Brian Cox’s BBC show, and the footage from the fourth moon landing where a feather and a heavy object fall at the same exact rate in the absence of air resistance (it also allowed me to make the point that also to moon gravitates, and in fact everything gravitates).

time

i’m not gonna do the usual spiel about how the laws of physics are time-reversal invariant and that is in stark contrast with the manifestly time-oriented nature of our experience, how generations of physicists have been puzzled about it, that the firmest thing we know about it is that time-orientation is deeply related with disorder/entropy, and how we are still debating whether we need time-oriented laws or if we can recover the phenomena from special boundary conditions. but that’s kind of what i wanted to pass on to the kids.

so i wanted to impart two things: 1) some phenomena are time-oriented and some aren’t and 2) all phenomena that are time oriented have to do with entropy increasing.

i first showed them a bunch of videos of breaking glasses, and candles burning, but also pendulums swinging, balls flying in parabolas, Newton’s cradles, soap-bubbles floating. i edited the videos so that thet would play both forwards and backwards in time, and i asked the kids to guess which was the correct direction. it was quite fun to see their delight at icecream unmelting, but also their puzzlement at not being able to tell if a ball flying in the air was time-reverse or not!

then i told them that whenever things happen only one way, it’s about disorder, so i gave them a plate with white beans on one side and borlotti beans on the other side. i wanted them to experience how easy it was to mix them up, and how it was harder—but not impossible—to unmix tghem. it went like this:

• me: what will happens if you put your hands in the plate and shake the beans?
• class: they’ll mix!
• me: and will they ever unmix if you keep going?
• class: NO!
• me: but does it mean you can never unmix them?
• class: NO!

we had them race each other in teams on who could unmix them faster. they loved it i told them this is like coffee and milk (or cocoa), just easier. things move spontaneously towards disorder, and when that happens it’s just much harder to undo, but not necessarily impossible. i think² some kids actually got it.

to make the analogy more concrete, and to underscore how much harder it is to undo the mixing of coffee and milk, we did the Democritus’ thought experiment of dividing a piece of matter (play-doh) over and over again and ask ourselves whether it would be possible to keep going forever. this one was the least successful, as only a fraction (pun intended) of the kids in every class actually tried to divide the dough in as many pieces as possible, most of them just let their creativity run free and started making all sorts of stuff.

of course, the elementary-school kids could appreciated a bit more what was happening. i catered the explanations and the amount of abstract information to the age of the class. the kindergarten kids were more just happy to drop things, touch things, and look at funny videos in school, while the second elementary class was eager to dicuss whether you could feel gravity on the moon, how the ball-dropping experiment relates to microgravity in the international space station, and were even interested in the estimate of how many atoms are in a piece of plah-doh

the last time i dealt with an entire class of schoolchildren was when i was teaching English as a foreign language in a school in a rural village near Pokhara, Nepal. this was similarly chaotic and satisfying, although it was particularly rewarding to share something i am genuinely excited about with the kids.

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1. nevermind that she told them i was going to do it before i could make my mind up! :P↩︎

2. funnily enough, a lot of kids thought it was easier to unmix them than mix them. apparently kids under a certain age cannot tell the difference between things being calmer and more easy.↩︎

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