Thought is so powerful that we can use it to reveal aspects of reality that cannot be reached any other way
Sometimes an experiment is impossible. But that doesn't necessarily stop us from doing it – in our heads. Such thought experiments are one of the most impressive demonstrations of the power and scope of human thought.
The ancient Greeks knew about thought experiments in mathematics. Today they are most common in physics. Galileo described the first, which dealt with the speed at which stones of different sizes would fall when dropped.
The most famous is Schrödinger's cat, which demonstrates the implausibility of a certain interpretation of quantum mechanics using a classic reductio ad absurdum. Erwin Schrödinger was later proved right in a real-world version of the experiment.
Einstein performed another famous one at age 16, when he imagined himself running alongside a beam of light. This flight of fancy, he later said, sowed the seed for special relativity.
A vivid imagination was also important to Kary Mullis, who shared a Nobel prize in chemistry for inventing a way of copying DNA. He did it, he said, by imagining himself "down there with the molecules".
Thought experiments can also help us explore moral issues. The trolley problem, for instance, asks whether would you act to avert an accident that is about to kill 10 people if your deliberate intervention would save the 10, but intentionally kill 1 other person.
Thought isn't always a reliable guide to reality, however. In 1935 Einstein, Nathan Rosen and Boris Podolsky imagined the properties of two "entangled" particles. They used the absurdity of the outcome to claim that quantum theory must be incomplete. However, we've since developed the technology to do the experiment for real, and in this case reality turns out to be truly absurd.
Sometimes thought leads nowhere, as in considerations of what happens to information absorbed by a black hole. In 2004 Stephen Hawking conceded a bet in the face of "proof" that information is not destroyed by the black hole. It turns out he gave in too soon: the question is still wide open.
Nonetheless, "thought experiments are incredibly useful for distilling the essential elements of a situation", says Dave Wineland of the National Institute for Standards and Technology (NIST) in Boulder, Colorado. Wineland won the Nobel prize in physics last year for his experimental work in quantum physics. Despite our impressive array of experimental abilities, thought experiments can still challenge and improve our understanding of the world, he says.
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