Shadows

If you know me, you'll know I am positively obsessed with quantum physics and the whole Many Worlds Interpretation business.  No day goes by without me thinking about it, and my overriding obsession is to try to find a way to explain it to the less obsessed.  And, that's not easy - as my many mad ramblings at friends in the pub have proved to me. You think you have it simmered down to a perfect elevator pitch, and then, an opportunity! a potential convert! and the words get all jumbled.  Because it is only then that the mathematical knowledge assumed becomes apparent.

About a year ago I read The Fabric Of Reality, and I realized that, thankfully, there exists a far, far smarter person with the same obsession as me.  Chapter 2 - Shadows - consists of the most perfect explanation - derivation even - of the reality of parallel worlds.  I wish I'd thought of it.

The author David Deutsch starts off with the standard double slit experiment.  Then, like Feynman in QED, he points out the experimental fact that you can reduce the intensity of the light until the flashes of light on the screen caused by individual photons are distinguishable.  Opening a 2nd hole actually reduces to zero the probability of a flash in some locations on the screen.  This, he argues, implies that something must be going through both holes each time a flash appears, and not just one hole.  Then he tries to derive the nature of these things.  Since only one flash appears, only one of them is a standard "photon" that interacts strongly with objects such as the screen.  The others are not standard photons, but on the other hand, they are blocked and reflected by the same things that block and reflect photons.  This leads him to invent the name "shadow photons" and he shows that although they do not interact with the screen, they do interact with the photon that does, though weakly through interference effects.  He then goes on to show that there must be an enormous number of these "shadow photons" for each real "photon", so many that they would destroy any mirror or wall they hit if they did interact with it.  But since they do not destroy real mirrors, or real walls, yet are deflected and absorbed by something when a real mirror or real wall is present, there must be shadow mirror atoms and shadow wall atoms whenever there are real mirror atoms and real wall atoms present.  He goes on to show that shadow walls and shadow mirrors interact only with one shadow photon each, i.e. that the shadow walls, mirrors, and photons are partitioned into shadow universes of which we experience only one directly.

The real piece de la resistance is when Deutsch does away with the walls and mirrors and replaces them with a frog sitting in a dark place, staring at a light source from which a photon emanates just once a day.  Frogs have marvellous retinas that can detect single photons (unlike ours which need about 10 to activate).  A chain of events triggered by a photon landing on the retina of the frog causes this particular frog to jump.  But, like with the double slit experiment there are shadow photons which cause flashes only in shadow retinas, causing shadow frogs to jump in some shadow worlds, and not in others.




I have to admit to finding this blogging business quite exciting!  I wonder whether it will wear off, or will I still be posting this time next week?