Mr. Turing's Computer

Computers in the past could only do one kind of thing at a time. One computer could add some numbers together, but nothing else. Another could find the smallest of some numbers, but nothing else. You could give them different numbers to work with, but the computer would always do the same kind of thing with them.

To make the computer do something else, you had to open it up and put all its pieces back in a different way. This was very hard and slow!

So a man named Mr. Babbage thought: what if some of the numbers you gave the computer were what told it what to do? That way you could have just one computer, and you could quickly make it be a number-adding computer, or a smallest-number-finding computer, or any kind of computer you wanted, just by giving it different numbers. But although Mr. Babbage and his friend Ms. Lovelace tried very hard to make a computer like that, they could not do it.

But later a man named Mr. Turing thought up a way to make that computer. He imagined a long piece of paper with numbers written on it, and imagined a computer moving left and right that paper and reading the numbers on it, and sometimes changing the numbers. This computer could only see one number on the paper at a time, and also only remember one thing at a time, but that was enough for the computer to know what to do next. Everyone was amazed that such a simple computer could do anything that any other computer then could do; all you had to do was put the right numbers on the paper first, and then the computer could do something different! Mr. Turing's idea was enough to let people build computers that finally acted like Mr. Babbage's and Ms. Lovelace's dream computer.

Even though Mr. Turing's computer sounds way too simple when you think about our computers today, our computers can't do anything that Mr. Turing's imagined computer can't. Our computers can look at many many numbers and remember many many things at the same time, but this only makes them faster than Mr. Turing's computer, not actually different in any important way. (Though of course being fast is very important if you want to have any fun or do any real work on a computer!)