Thought Experiments

A thought experiment is a mental exercise in which the thinker constructs a scenario that perhaps does not or could not actually exist in the physical world but which helps one to visualize some problem. For example, you might be asked to imagine what the world would be like for someone who lacked the sense of sight and hearing. Or, what would things look like to a person travelling faster than the speed of light?

Philosophers and scientists use thought experiments all the time as they're very useful heuristic tools.

New Scientist has an article in which they discuss seven of the most famous thought experiments in science and philosophy. Here's their lede and three of their examples. I've added the videos:

If you imagined that thought experiments were mere mental gymnastics meant to bamboozle the uninitiated, think again. Take Schrödinger’s cat, perhaps the most famous example, which involves a cat that is simultaneously alive and dead. It seems bizarre – and that’s the point. It was designed as a slap on the wrist for quantum theorists, to show that a theory that predicts such nonsense must be missing something. Current thinking is that perhaps nothing is missing, and quantum theory really is as weird as it seems.

But other thought experiments have forced us to reformulate the laws that describe nature. Take Maxwell’s demon, which appears to break the laws of thermodynamics. It showed us that thermodynamics really was missing something (see “Matter, energy… knowledge: How to harness physics’ demonic power“).

Newton’s cannon

Take one gigantic cannon, put it on top of a mountain so high it reaches above the atmosphere, and fire horizontally. Irresponsible, perhaps, but instructive. If the cannonball is fired at a low speed, gravity will soon drag it down to the ground along a tightly curving arc. If you add more gunpowder, the ball will go faster and its arc will be more gradual, taking it further around the curve of the Earth. Fire it fast enough and the cannonball’s path will not meet the ground at all – it will fly all the way around and hit you in the back of the head.

This thought experiment helped Newton show that gravity is a universal force: the force we see pulling cannonballs and apples to Earth can also explain the orbit of the moon around Earth, and Earth around the sun.

We are used to the idea of universal forces now. We know that nuclear reactions fuel the distant stars, and that exoplanets can be magnetic. But before Newton there was no expectation that the celestial realm should have the same rules as Earth. His cannonball blew a big hole in such heavenly pretentions.

Achilles and the tortoise

Two-and-a-half millennia ago, the Greek philosopher Zeno of Elea apparently proved that motion is an illusion. One of his paradoxes sets fleet-of-foot Achilles to chase a tortoise that has a small head start. Achilles can never catch the tortoise, argued Zeno, because first he must reach the point where the tortoise started, but by then the tortoise has moved on to a new position. So then Achilles must run there – by which time the tortoise has moved on again. The “dichotomy paradox” is more general: to cover any distance, you must first cover half that distance, then half of what’s left, then half of what’s left, and so on for ever. It seems that you can never get there, no matter what the original distance or how fast you move.

Since then, mathematicians have pointed out that although these arguments take an infinite amount of time to pan out, real motion doesn’t have to. We know for instance that that an infinite series of terms can add up to something finite. If you add an infinite series of fractions starting with ½ and halving in value with each new term (½ + ¼ +1/8…), the infinite sum is equal to 1. You can use maths like this to represent the distance traveled or the time taken in Zeno’s paradox, so — phew – motion is possible after all. That said, Zeno’s paradox may manifest itself for real in the quantum world.

The Chinese room

Can a computer be conscious? In an attempt to disprove this idea of “strong artificial intelligence”, John Searle, a philosopher at the University of California, Berkeley, imagined himself inside a room of dictionaries and rule books that hold instructions for translating Chinese to English and vice versa. Someone posts a question through the door written in Chinese, and using his rule books Searle works out an appropriate answer. To the questioner it would seem there is a mind in the room that understands Chinese, even though there isn’t. Searle claims that a hypothetical rule-bound computer designed to speak Chinese would be the same — a mere machine with no understanding.

There are many objections to this thought experiment. Some argue that although Searle does not understand Chinese, he is part of a larger system, including the rule books, that does. You might balk at the idea that a mind could be made from a person plus some books, but it’s only a very dim mind, taking perhaps years or millennia to respond to one question.

Another interpretation is that Searle’s idea merely highlights the mystery of “other minds”: that you can’t know whether a computer, a penguin or the person next door is conscious in the same way as you are. If the Chinese room doesn’t disprove strong AI, thinking about it could help us to find out what’s missing from our understanding of consciousness.

Read the others. They're fascinating.