Concepts Explained: Theory

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The video below explains six famous thought experiments, from the ancient Greeks to Albert Einstein, that have changed the way we see the world. Can a cat be both alive and dead? Can a computer think? How does a tortoise beat Achilles in a race? Transcripts are below the video.


  1. Achilles and the Tortoise  Ancient mathematical trickery proves that a mighty hero cannot overtake a tortoise.
  2. The Grandfather Paradox  A well known story that questions the logic of time travel.
  3. The Chinese Room  An argument against computers ever being truly intelligent.
  4. Hilbert’s Infinite Hotel  A never-ending hotel, always full of guests, helps to explain the nature of infinity.
  5. The Twin Paradox  An identical twin in space illustrates Einstein’s special theory of relativity.
  6. Schrödinger’s Cat  This famous experiment tackling quantum theory involves a cat in a potentially lethal box.

Adventures in thought (Duration, 6:40)

Source: Open University (2012). 60 Second Adventures in Thought.

^ Achilles and the Tortoise
How could a humble tortoise beat the legendary Greek hero Achilles in a race? The Greek philosopher Zeno liked a challenge and came up with this paradox. First, the tortoise is given a slight head start. Anyone fancying a flutter would still rush to put their money on Achilles. But Zeno pointed out that, to overtake him, Achilles would first have to cover the distance to the point where the tortoise began. In that time the tortoise would have moved – so Achilles would have to cover that distance, giving the tortoise time to amble forwards a bit more. Logically this would carry on forever. However small the gap between them, the tortoise would still be able to move forwards while Achilles was catching up. Meaning that Achilles could never overtake. Taken to an extreme, this bizarre paradox suggests that all movement is impossible. But it did lead to the realisation that something finite can be divided an infinite number of times. This concept of ‘an Infinite Series’ is used in finance, to work out mortgage payments… Which is why they take an infinite amount of time to pay off!

^ The Grandfather Paradox
Will Time Travel ever be possible? Rene Barjavel was a French journalist and science fiction writer who spent a lot of his time thinking about time travel. In 1943, Barjavel asked what would happen if a man went back in time, to a date before his parents were born, and killed his own grandfather? With no grandfather, one of the man’s parents would never have been born – and therefore the man himself would never have existed. So there would be nobody to go back in time and kill the grandfather in the first place. Or the last place – depending on how you look at it. The Grandfather Paradox has been a mainstay of philosophy, physics and the entire Back to the Future Trilogy. Some people have tried to defend time travel, with arguments like the Parallel Universe Resolution – in which the changes made by the traveler create a new, separate history, branching off from the existing one. But the Grandfather Paradox prevails. Although the paradox only suggests that traveling backwards in time is impossible – it does not say anything about going the other way…


^ The Chinese Room
Can a machine ever be truly called intelligent? American philosopher and Rhodes scholar John Searle certainly can. In 1980 he proposed The Chinese Room thought experiment in order to challenge the concept of strong ‘artificial intelligence’ – and not because of some Eighties design fad. He imagines himself in a room with boxes of Chinese characters he cannot understand and a book of instructions, which he can. If a Chinese speaker outside the room passes him messages under the door, Searle can follow instructions from the book to select an appropriate response. The person on the other side would think they’re chatting with a Chinese speaker – just one who does not get out much. But really it is a confused philosopher. Now, according to Alan Turing, the father of computer science, if a computer programme can convince a human they are communicating with another human, then it could be said to think. The Chinese Room suggests that, however well you program a computer it does not understand Chinese – it only simulates that knowledge – which isn’t really intelligence. But then sometimes humans aren’t that intelligent either.

^ Hilbert’s Infinite Hotel
A Grand Hotel with an infinite number of rooms – and an infinite number of guests in those rooms. That was the idea of German mathematician David Hilbert, friend of Albert Einstein and enemy of chambermaids the world over. To challenge our ideas about infinity he asked what happens if someone new comes along looking for a place to stay? Hilbert’s answer is to make each guest shift along one room – the guest in room one moves to room two and so on – so the new guest would have a space in room one. And the guest book would have an infinite number of complaints. But what about when a coach containing an infinite number of new guests pulls up – surely he cannot accommodate all of them? Hilbert frees up an infinite number of rooms by asking the guests to move to the room number, which is double their current one, leaving the infinitely many odd numbers free. Easy for the guest in room one. Not so easy for the man in room eight million, six hundred thousand, five hundred and ninetyseven. Hilbert’s paradox has fascinated mathematicians, physicists and philosophers – even theologians. And they all agree you should get down early for breakfast.

^ The Twin Paradox
Albert Einstein didn’t have a twin brother – but he had some funny ideas of what you could do with one. He imagined two identical twins – let’s call them Al and Bert. Now, Al is a couch potato, but Bert likes to travel, so he hops into a spaceship and zooms off at close to the speed of light. That’s when Einstein’s special theory of relativity kicks in. It says that the faster you travel through space, the slower you move through time. So from Al’s point of view Bert’s time would be moving slower than his own. To put it another way, time might fly when you’re having fun – but when clocks fly, they run more slowly in relativity. After a while Bert decides to head back, still at close to the speed of light, and returns to his brother with his holiday snaps. But when Bert arrives home, Al will now be older than his twin. Which makes their double dates a lot more awkward. Although it seems implausible, Einstein just followed his theory to its logical conclusion and it turns out he was right. This concept of ‘time dilation’ provides the basis for our Global Positioning System – which is how your Sat Nav knows you need to ‘turn left in two hundred yards.’

^ Schrödinger’s Cat
Erwin Schrödinger was a physicist, a theoretical biologist and probably more of a dog person. In the nineteen-twenties, scientists discovered Quantum Mechanics, which said that some particles are so tiny; you cannot even measure them without changing them. But the theory only worked if, before you measure them, the particle is in a ‘super-position’ of every possible state all at the same time. To tackle that, Schrodinger imagined a cat in a box with a radioactive particle and a Geiger counter attached to a vial of poison. If the particle decays, it triggers the Geiger counter, releases the poison and bye bye Tiddles. But if the particle is in two states – both decayed and not decayed – the cat is also in two states – both dead and not dead. Until someone looks in the box. In practice, it is impossible to put a cat into a superposition. You’d have the animal rights lobby up in arms. But you can isolate atoms, and they do seem to be in two states at once. Quantum mechanics challenges our whole perception of reality. So maybe it is understandable that Schrödinger himself decided he didn’t like it – and was sorry he ever started on about cats.




01. Beginnings
02. Who am I?
03. Being human
04. Knowledge
05. Beauty
06. Love
07. Freedom
08. Society
09. The good life
10. Right from wrong
11. Justice
12. Technology


01. Astronomy
02. Microgravity
03. Thoughts on theory
04. Theology explained
05. History of the English language
06. History of Money
07. Economics explained
08. The European Union explained