Thought experiments

How do you deal with experiments that you would loooove to run in your course but just can’t?

We are currently working on a guide to instructing lab experiments. A colleague is writing on a piece on thought experiments, one of which I found super interesting.

Thought experiments are quite common in philosophy. Think, for example, of the one where a person has to decide whether or not to pull the switch that sends an out-of-control trolley on that branch of its tracks where it will kill one person, rather than not interfering and letting it drive straight ahead, killing five people. Or another example: the prisoners’ dilemma where confessing to a crime is in each prisoner’s best interest, but remaining silent would be the optimal solution for all prisoners taken together. While those are interesting in themselves, thought experiments can be directly relevant in science teaching, too.

The example my colleague talks about, for example, is a free fall experiment. Two metal balls, a large one and a small one, are connected with a chain, and dropped. Conventional wisdom would dictate that the heavier ball drops faster than the lighter one. However, since the two balls are connected, the lighter ball should slow down the heavier one a little bit, making the whole system fall slower than the heavy ball, yet faster than the lighter one. On the other hand, since the two balls are connected, this system is heavier than either of the balls individually, and should hence fall even faster than either of the balls. The only way to solve this contradiction is to have both balls fall at the same rate – which is what happens in reality (when friction is neglected at least).

Now the really interesting question is this one: How can we use this in our teaching? One could imagine using a similar thought experiment before running an actual experiment. Or to use thought experiments in cases where running “real” experiments is too expensive or not feasible for other reasons.

Einstein’s elevator, for example, discusses how you cannot distinguish whether an object you drop falls down due to gravity, or whether the floor accelerates towards the object if you happened to be in a space ship. There are ways to experimentally show that the effect is the same in both cases, but conducting this experiment as a thought experiment requires a lot less resources.

Another example is that of the scientist who works on colors, but has only ever seen black and white, due to some weird glasses that she has to wear day and night. She knows everything there is to know about color vision from an academic point of view: which wavelength of light has which name, what happens when the light hits our eyes, how the information is transmitted to the brain and how it is then processed. But what would happen if that scientist ever took off her glasses and, for the first time ever, saw color? This is a really nice thought experiment on how we cannot know what we don’t know.

How would you use thought experiments in your teaching?

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