Because we are not always able to carry out physical experiments to learn more about our world, scientists, philosophers, and others use thought experiments. This type of experiment is a way to use the imagination to investigate puzzles and new theories. Anyone can conduct thought experiments. All that is needed is to conceive of some problem or question, imagine in your mind how it might be tested or resolved, develop in your mind a method to do this, and rationalize what the results might be and how these results would affect the real world.
By no means are all thought experiments confined to scientific areas. They are used in philosophical arguments, in discussions about ethics and morals, and in other areas. Sometimes they are used to disprove preexisting theories and sometimes to advance new theories.
Thought experiments are nothing new—they were carried out by thinkers more than 2,000 years ago. A very early experiment, known as “Lucretius’ Spear,” is described in his poem De Rerum Naturam (“On the Nature of Things”). Lucretius used the technique in thinking about the nature of space. He described what might happen if one could carry a heavy spear to the edge of the universe and then hurl it forward. Only two results seem possible: Either the spear would keep going, in which case obviously there was something beyond the edge, or the spear would be stopped at the boundary. If there were indeed a boundary, then what was outside the boundary? In either result, space must be infinite.
Other Greek philosophers, such as Ptolemy, Plato, Socrates, and Aristotle, all used what we now call thought experiments. Some of the most important were created during the Renaissance, when thinkers such as Galileo Galilei, Rene Descartes, and Isaac Newton used them to develop principles still important today.
Newton, in his Newton’s Bucket experiment, argued for the existence of absolute space and absolute motion. In other words, he attempted to show that space and motion are not dependent on anything else. If all matter in the universe were removed, space and motion would still exist because they do not depend on anything outside of themselves.
Newton’s Bucket described an imaginary experiment in which a bucket is filled partway with water and then suspended by a rope from an overhead rafter. The bucket is then turned around and around until the rope is tightly wound. At this point, the water is perfectly level. However, when the bucket is released, it begins to spin. As it spins faster, just as water stirred in a cup gradually begins to rise around the edges and sink in the center, so too would the water in the bucket act.
At the beginning, while the water was level but the bucket had just begun spinning, there was a relative relationship between the water and the bucket. As the speed increases, even though the bucket and water are moving at the same speed, the level of the water changes. Newton felt this proved that the level of the water did not depend on the motion of the bucket; therefore space and motion were not in a relative relationship. Later thinking about space, time, and motion brought this idea into question.
A thought experiment by Albert Einstein dealt in a way with the questions of absolute motion or rest. Einstein described a magnet and a spiral wire. We know that a magnet next to a wire can create an electrical current. He imagined what would happen if the magnet were at absolute rest (assuming there is such a thing). The wire, because of its proximity, would develop an electric current. If the wire is stationary and the magnet moves, there would still be a current induced in the wire. Finally, if both were moving relative to each other, the induced current still would be created. These and other Einstein thought experiments all fed into his special theory of relativity, the first principle of which is that the speed of light is the same for all observers no matter what their position is in relation to the origin of the light source.
Anticipating concepts of relativity by more than 400 years was Giordano Bruno (1548-1600). In several thought experiments Bruno expounded relativistic concepts. For example, in the Argument of the Third Dialogue of his De l’infinito universo et mondi (On the Infinite Universe and Worlds), Bruno noted that looking down from a mountain on a sea on a bright night, we would see the whole sea illuminated. However, afloat on the sea at the same time, we would just see a small bright area near us.
Closely allied to these mental experiments are counterfactuals, which are used in many subject areas as well as in speculative fiction, sometimes called alternative history. Counterfactuals spring from taking a significant event in history and imagining what the world might be like had the event not happened. What if the Great Wall of China had never been built? Would the constant barbarian invasions have prevented developments in China that later influenced the West? In the United States, what if the South had won the Civil War?
Historiographers may use thought experiments to explore the development of various schools of political thought, such as Marxism. Social and public policy questions can draw on these techniques to try to shed light on moral and ethical questions such as abortion. A moral philosopher named Judith Jarvis Thompson set up the “Famous Violinist Problem.” This experiment describes a situation where an individual is kidnapped. This individual’s internal organs are linked to a world- famous violinist whose organs have failed and will die unless saved by this operation. By staying on this support for 9 months, the violinist will recover and survive. If disconnected before 9 months, the violinist will die and all the great performances that might have been will never take place. Do the perpetrators have a moral right to take such an action?
Today few scientists deny the usefulness of thought experiments. In fields such as philosophy, ethics, and linguistics, some thinkers raise questions about the validity of this way of thinking. These objections range from concerns about their imprac- ticality, overreliance on intuition, divorcement from reality, to the tendency toward oversimplification of complex issues. However, when looking at the long list of scientific advances spurred by these experiments, it is difficult to deny their importance.
See also Aristotle; Aristotle and Plato; Bruno, Giordano; Darwin, Charles; Descartes, Rene; Einstein, Albert; Galilei, Galileo; Gosse, Philip Henry; Histories, Alternative; Lucretius; Newton, Isaac; Nicholas of Cusa (Cusanus)
Cohen, M. (2005). Wittgenstein’s beetle and other classic thought experiments. Malden, MA: Blackwell.
Isaacson, W. (2007). Einstein: His life and universe. New York: Simon & Schuster.
Sorensen, R. (1992). Thought experiments. New York: Oxford University Press.