A thought experiment is a scientific approach of running experiments inside your mind. Like an actual experiment, it has a starting problem, hypothesis, experimentation, analysis, interpretation, and a conclusion. All within the mind.
Because thought experiments happen inside the mind, one can do them anywhere and almost anytime.
This article will take through the step-by-step example of a thought experiment beginning with a problem, all the way to the conclusion.
As a case study, we will use Isaac Newton, one of the most influential physicists of all time. His theory of gravity takes inspiration from a thought experiment that he had while sitting under an apple tree.
Let’s revisit that incident …
The problem: strange movements
In Newton’s time, nobody could explain why planets and the moon move the way they do.
Astronauts had already established that planets and moons weren’t stationary. Planets were observed to revolve around the Sun and moons revolved around planets.
It was strange enough that these bodies seemed to be hanging in space, but revolving around other bodies! That was bizarre.
So, the problem is this: What causes planets and the moon to move the way they do?
Make Hypothesis: The thought experiment begins
This is where the thought experiment begins.
One day Newton was sitting under a tree when an apple fell and bonked him on the head. Rather than dismissing the incident, Newton began developing a clever hypothesis.
According to William Stukeley, Newton’s friend, it was this moment that inspired Newton to start developing a hypothesis about the motions of moons and planets.
In Newton’s own words, “I began to think of gravity extending to the orbit of the Moon…”
This was Newton’s hypothesis: That gravity from Earth is responsible for the motion of the moon.
So the hypothesis is this: the force of gravity extends all the way to the orbit of the Moon.
Experimentation: Testing the hypothesis
These steps, experimentation, and analysis usually go hand-in-hand. The bulk of the work happens here.
Experimentation – This involves running different versions of the thought experiment in your mind and asking questions – all inside your mind. You may need to go back and modify your hypothesis a little.
For example, in Newton’s case, different variations of the thought experiment could be:
- Does the force of gravity pull the moon with the same strength as everything on Earth?
- If the answer is yes. Why is the force of gravity independent of distance?
- If the answer is no. Why and how does the force vary with distance?
- How does gravity act without making contact? How does it go from the Earth to the Moon with nothing connecting them?
- Why isn’t the Moon not falling like the apple?
All these questions are potential ‘experiments’ to test the hypothesis. From one of these questions, Isaac Newton devised the famous canon ball thought experiment to explain why the Moon doesn’t fall like an apple.
Newton likely wrapped up this step with a considerable modification of his hypothesis: gravity guides the motions of the moon and the planets.
You can read this short story about Isaac Newton to follow through the thrilling step-by-step thought process leading to the canon ball thought experiment.
Analysis and interpretation
This goes in hand with experimentation and usually means leaving your mind and getting a little ‘physical’.
By ‘physical’ I mean subjecting your experiment to a rigorous logical, mathematical, and speculative analysis. You go grab a pen, paper, a calculator, and whatever tools you’ll need to make sense of your ‘experiments’.
Newton used the greatest tool he had at his disposal: mathematics.
In Newton’s own words, “I began to think of gravity extending to the orb of the Moon . . . and having thereby compared the force requisite to keep the Moon in her orb with the force of gravity at the surface of the Earth, and found them to answer pretty nearly.”
Conclusion: why thought experiment?
This is where you compile your results and state your conclusion.
For Isaac Newton, this came in the form of Philosophiæ Naturalis Principia Mathematica or simply, Principia.
Thought experiments can be a powerful way to peek the curtain behind reality and may result in truly profound insights. They are relatively cheap and aren’t bound to time or location. Anyone can take them.
However, they are not easy.
As the saying goes, “everyone saw an apple fall, but only Newton asked why”.
It takes practice, persistence, sheer curiosity, and perhaps a sense of ingenuity to perfect this approach. If you would like to sharpen this skill, there are plenty of useful resources depending on your interests.
As a simple practice, indulge me as I delve into this thought experiment about a bug in a bus.
