When Einstein conducts one of his thought experiments, he is modelling in his mind the universe as he conceives it. He imagines that he is travelling on a beam of light. He is holding up a mirror twelve inches from his face. What happens? he asks himself. There is nothing to be seen in the mirror. At 186,000 miles per second, nothing can overtake me; nothing travels faster. So, if my image cannot travel faster than the speed of light, then it can never arrive at the surface of the mirror. That’s the thought experiment, based upon the Einsteinian model. And if I now create a phrase to encapsulate it and say ‘travelling at the speed of light is an endless stare into an empty mirror’ I have created my metaphor. One thing – the speed of light – is another – a stare into an empty mirror; each belongs to a different intellectual genus.
When Heisenberg first began to formulate what came to be known as the Principle of Uncertainty he was prompted by his profound unhappiness with the phrase ‘the path of the electron’. Others thought this was a straightforward description of the truth; he reckoned it was a pernicious and misleading metaphor. The problem here was like the problem Wittgenstein identified in Augustine’s thinking about time. A metaphor is so pervasive that it is not seen as a metaphor at all; it is assumed to be merely a transparent filament connecting us with the observable world. But ‘path’ here, as Heisenberg realised, is a Newtonian notion, with an attendant repertoire of expectations: a specified object makes its way across a designated region which can be mapped.