The Fabric of Reality: The Science of Parallel Universes--and Its Implications

by David Deutsch

The key fact is that a real, tangible photon behaves differently according to what paths are open, elsewhere in the apparatus, for something to travel along and eventually intercept the tangible photon. Something does travel along those paths, and to refuse to call it ‘real’ is merely to play with words. ‘The possible’ cannot interact with the real: non-existent entities cannot deflect real ones from their paths. If a photon is deflected, it must have been deflected by something, and I have called that thing a ‘shadow photon’. Giving it a name does not make it real, but it cannot be true that an actual event, such as the arrival and detection of a tangible photon, is caused by an imaginary event such as what that photon ‘could have done’ but did not do. It is only what really happens that can cause other things really to happen. If the complex motions of the shadow photons in an interference experiment were mere possibilities that did not in fact take place, then the interference phenomena we see would not, in fact, take place.

This is where the explanation becomes “hard to vary.” Not playing with words. The effects are real. If they aren’t, then the outcome of the experiment is not observed. Change the definition of the core explanation—tangible photons interacting with shadow photons—and the explanation falls apart.