The Turing machine operates by shuffling a long strip of paper back and forth
And, at the end of Chapter Six, Feynman comes to the following amazing conclusion: "It seems that the laws of physics present no barrier to reducing the size of computers until bits are the size of atoms, and quantum behavior holds dominant sway."
Feynman had come to his conclusion: quantum computers were possible.
However, in his book, Feynman starts by introducing the principles which lie behind the conventional classical computer. "Classical" physics means the state of physics as it existed before 1900, which means it pre-dates the discovery of quantum mechanics and relativity. It is the physics of Newton — not the physics of Einstein and Bohr. Classical physics is also the physics of the great Scottish 19th century physicist James Clerk Maxwell who viewed light as a wave rather than a stream of particles. The computers we use today are all classical computers, based on classical physics.
The template for the classical computer is the Turing machine. The Turing machine was invented by Alan Turing in 1936, and it is the simplest possible general computing device. The Turing machine operates by shuffling a long strip of paper back and forth. The tape has a series of zeroes and ones written on it, representing input data and output data. The Turing machine has a probe pointed at the paper which can read, write, or erase the zeroes and ones on the strip: