Thought Experiments

The Wikipedia page outlines 7 types of thought experiments:

• Prefactual: Speculating on possible future outcomes given the present.
  • "What will be the outcome if event \(E\) occurs?".
• Counterfactual: Speculating on possible outcomes of a different past.
  • "What might have happened if \(A\) had happened instead of \(B\)?".
• Semifactual: Speculating on the extent to which things might have remained the same despite there being a different past.
  • "Even though \(X\) happened instead of \(E\), would \(Y\) have still occurred?"
• Predictive: Speculating on what happens when we project the present circumstances into the future.
• Hindcasting: Evaluating a forecast model after an event has happened to test its validity.
• Retrodiction: Moving backward in time step-by-step from the present to the speculated past to establish the ultimate case of a specific event. Involves going from visible effects such as symptoms and signs to their prior cases. In terms of analogies: retrodiction is to diagnosis what prediction is to prognosis.
  • Reverse engineering.
• Backcasting: Establishing the description of a very definite and specific future situation. Involves moving backward in time step-by-step from the future to the present to reveal the mechanism through which that particular specific future could be attained from the present. It is not concerned with predicting the future.
  • Working backward from a particular future to the present to determine what policy measures would be required to reach that future.

Some examples:

• (, ) - Thought experiment to determine whether gravitational waves have energy - consider 2 beads with sliding on a rod with some friction. As gravitational waves pass over the rod, the proper distance between the two beads oscillate, hence they rub against the rod, dissipating heat, which must come from the gravitational wave.
• Actually a thorough list is in the Wikipedia page linked above - see the Physics and Computer science ones.