Thought Experiments

Hello. My name is Roger Jones. I am coming to you from steaming downtown Pensacola, Florida.

A thought, or gedanken, experiment is the creation of a simplified, often fanciful, world for the purpose of understanding the implications of a theory. One of the first was Plato’s allegory of the cave, in which Plato postulates that people are only able to sense a part of the universe about them. The allegory explores what that means and implies. The real value of thought experiments is to generate hypotheses that can be tested in the real world.

As an example, let’s talk about the thought experiment, Newton’s Cannonball. Isaac Newton came up with this thought experiment in order to demonstrate that the mechanism that causes bodies to fall on Earth is the same mechanism that cause the planets to orbit the Sun. He called this mechanism “gravity.” This seems obvious to us now, after the fact, but it was certainly not obvious at the time. Galileo, who died on the day that Newton was born, thought that the two mechanisms were entirely different.


The thought experiment goes like this. Imagine a very tall mountain. In fact, the mountain is very much taller than anything found on Earth. Now imagine that a cannon is placed at the top of the mountain. Now imagine that we place a small amount of gunpowder in the cannon along with a standard cannonball. We fire the cannon, and the ball drops to the Earth at point A some distance from the mountain. If we put more gunpowder in the cannon, then the cannonball drops farther from the mountain at point B. As we increase the amount of gunpowder, the cannonball falls farther and farther from the mountain and lands on the backside of the Earth. If we put a sufficient amount of gunpowder in the cannon, and the cannon does not explode, we can drive the cannonball completely around the Earth such that it never lands. This is precisely what the moon does. It travels around the Earth. This suggests that the mechanism that controls orbiting objects is the same as the mechanism that controls terrestrial cannonballs.

From this simple thought experiment Newton conjectured that the gravity that controlled the motion of a cannonball was the same force that controlled the motion of the moon and the planets. Of course, he turned out to be correct.

This story illustrates the power of the thought experiment. Newton took some observed measurements, the trajectories of cannonballs, and then created an imaginary world that contained the cannonball trajectories. He then asked, “What would happen if …?” The conclusions for what happens next in the simple imaginary world led to awareness of connections in the real world.  This is the thought experiment.

This very powerful tool has great potential, in my opinion, to solve real-world everyday problems. There will be more on this later.

Read more in Confronting Complexity by Casti, Jones, and Pennock

Thought experiment in quantum physics

Thought experiments in ethics and philosophy

Various thought experiments

Mind/Body thought experiment

Allegory of Plato’s cave

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