## Tuesday, January 28, 2014

### Dyson against wave-function collapse

Famous mathematical physicist Freeman Dyson answers 2014 : WHAT SCIENTIFIC IDEA IS READY FOR RETIREMENT?:
The Collapse Of The Wave-Function

Fourscore and seven years ago, Erwin SchrÃ¶dinger invented wave-functions as a way to describe the behavior of atoms and other small objects. According to the rules of quantum mechanics, the motions of objects are unpredictable. The wave-function tells us only the probabilities of the possible motions. When an object is observed, the observer sees where it is, and the uncertainty of the motion disappears. Knowledge removes uncertainty. There is no mystery here.

Unfortunately, people writing about quantum mechanics often use the phrase "collapse of the wave-function" to describe what happens when an object is observed. This phrase gives a misleading idea that the wave-function itself is a physical object. A physical object can collapse when it bumps into an obstacle. But a wave-function cannot be a physical object. A wave-function is a description of a probability, and a probability is a statement of ignorance. Ignorance is not a physical object, and neither is a wave-function. When new knowledge displaces ignorance, the wave-function does not collapse; it merely becomes irrelevant.
Dyson was a genius, but this is nonsense. Observing an electron does not just remove uncertainty; it alters the electron.

The wave-function may not be a physical object, but it still collapses. Dyson says that it does not collapse, but becomes irrelevant and is replaced with a new wave-function. That is what collapse means -- the old wave-function is projected to a subspace based on the observation.

I thought that he was going to advocate the many worlds interpretation (MWI), as those are the main one who argue against collapse of the wave-function. They argue that the collapsing part of the wave-function is really escaping to a parallel universe. The argument is based on a belief that wave-function uncertainty should be some sort of conserved quantity like energy, so they postulate a vast collection of unobservable alternate universes.

Dyson is botching up an explanation of conventional quantum mechanics. I guess that is better than advocating some completely unscientific multiverse idea.

David Deutsch likes the MWI, and hence dislikes wave-function collapse, but his answer objects to quantum jumps:
The term "quantum jump has entered everyday language as a metaphor for a large, discontinuous change. It has also become widespread in the vast but sadly repetitive landscape of pseudo-science and mysticism. ...

OK, maybe some physicists still subscribe to an exception to that, namely the so-called "collapse of the wave function" when an object is observed by a conscious observer. But that nonsense is not the nonsense I am referring to here. ...

Quantum jumps are an instance of what used to be called "action at a distance": something at one location having an effect, not mediated by anything physical, at another location. Newton called this "so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it". And the error has analogues in fields quite distant from classical and quantum physics. For example in political philosophy the "quantum jump" is called revolution, and the absurd error is that progress can be made by violently sweeping away existing political institutions and starting from scratch. In the philosophy of science it is Thomas Kuhn's idea that science proceeds via revolutions—i.e. victories of one faction over another, both of which are unable to alter their respective "paradigms" rationally. In biology the "quantum jump" is called saltation: the appearance of a new adaptation from one generation to the next, and the absurd error is called saltationism.

A new article explains that reality of the wave function is a continuing debate:
It is not exaggerated to claim that one of the major divides in the foundations of non-relativistic quantum mechanics derives from the way physicists and philosophers understand the status of the wave function. On the instrumentalist side of the camp, the wave function is regarded as a mere instrument to calculate probabilities that have been established by previous measurement outcomes.1 On the other “realistic” camp, the wave function is regarded as a new physical entity or a physical field of some sort.
That's right, those are the two main views. Both are tenable, I guess, but you should be suspicious of anyone who makes strong claims based on the reality of the wave function, without recognizing the other view.