Galileo

Galileo knew that most people would predict that a heavy body (H) would fall faster than a lighter body (L). 

But, his eyes were fixed on our solar system, and these bodies were not falling, so he conducted a thought experiment to prove another possible prediction.  Science and art are wonderfully connected by experimental thinking. Here is Galileo’s well-known thought experiment:

1. Suppose we connect the two bodies by a string, thereby making the compound object H+L.
2. One could predict that H+L should fall faster than H by itself because of the compound weight: H+L > H.
3. However, it’s also possible to use the same logic to claim that the compound body should fall slower than H because of L’s drag so that H+L < H.
4. This yields a contradiction. It means that logical consequence is absurd or reductio ad absurdum because H = L = H+L.

On the Moon, Neil Armstrong showed the whole world that Galileo was right a half-century later. He let go of a hammer and a feather in the absence of atmospheric friction while standing on the Moon, and, sure enough, they hit the Moon’s surface at the same time. This is the predictive power of thought experiments.