Would you ‘kill’ one person to save five? Famous thought experiments

Philosophers and scientists (mainly theoretical physicists) use thought experiments, experiments carried out in the imagination, to probe a variety of very dense ideas, often when conducting physical experiments is extremely difficult or impossible. These thought experiments are fascinating to ponder and today I will describe four famous examples that illuminate/explain certain philosophical, scientific and philosophical ideas.

The Trolley Problem

A madman has strapped five innocent people to a railway line along which a driverless-locomotive hurtles towards them. You are a lone spectator surveying the scene. At your hand is a lever that would allow you to divert the runaway locomotive on to another track, thereby sparing five lives. However, the madman has strapped a single person to the other track, who will die if you divert the locomotive. Should you pull the lever?

Utilitarian philosophy holds that the most moral action is the one that creates the greatest good for the greatest number of people and therefore you should pull the lever. However, if you pull the lever you are partially responsible for the death of the person on the other track – you are compliant in an immoral act. But if you do nothing, five people die, which is also immoral. Most people would pull the lever, as would I.

In another version of this experiment you stand on a bridge above the track watching the locomotive career towards the five people strapped to the railway line. A huge man stands in front of you on the bridge. You can save the five people below by pushing this man on to the track where his bulk will stop the train, but he will die. Most people would not push the man though the outcome is the same as pulling the lever in the previous version.

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This thought experiment illustrates that there is no completely moral action.

Einstein’s Train and Light Experiment

Einstein imagines sitting on a railway platform watching a steadily moving train carriage pass by. His friend sits in the middle of the train carriage. A light bulb is switched on in the middle of the train carriage as it passes Einstein. The friend sees the light hit both end walls of the carriage simultaneously. But Einstein sees the light hit the rear wall of the carriage before it hits the front wall because the rear wall is moving forward to meet the light, which travels a shorter distance.

This thought experiment showed that time passes differently for someone standing still than for someone moving, bolstering Einstein’s belief that space and time are relative, a cornerstone of his Special Theory of Relativity.

Galileo’s Falling Bodies Experiment

Aristotle (384-322 BC) erroneously taught that heavier objects fall to Earth faster than lighter objects. A simple experiment would have settled the matter, but the Greeks didn’t do experiments. Aristotle’s reputation was so great that nobody questioned his teaching for a long time.

But, while teaching at a Pisa (1589-1592), Galileo did a thought experiment that showed all bodies fall independently of their weight. He considered dropping three identical weights simultaneously from the same height – all three weights hit the ground at the same time. Now, he mentally combined two of the weights using a weightless glue.

This produces no physical change in these objects and when dropped they again strike the ground simultaneously. But, the second situation can be interpreted as dropping only two weights, one double the weight of the other. Galileo inferred that all bodies fall at the same rate independently of their weight.

There is no record that Galileo dropped balls from the Leaning Tower of Pisa to demonstrate this principle. But he almost certainly would have checked out his thought experiment with a practical demonstration.

The Ship of Theseus

One of the oldest thought experiments comes from the writings of Plutarch (AD 45-120) as a paradox known as the Ship of Theseus. This ship was kept seaworthy for very many years through constant repair and replacement of parts. When a plank failed it was replaced and eventually no part of the original ship remained. Is this end-product ship still the same Ship of Theseus or a completely different ship? If it is a different ship, when did it cease being The Ship of Theseus?

Philosophers use this story to help explain the nature of identity. Are objects more than the sum of their parts? Is identity solely contained in physical objects?

William Reville is an emeritus professor of biochemistry at UCC