Bruno Marchal wrote:
> 
> On 05 Mar 2009, at 12:43, Stathis Papaioannou wrote:
> 
>> 2009/3/5 Bruno Marchal <marc...@ulb.ac.be>:
>>
>>> Sure. But note that "a lot of things happens", including the white
>>> rabbits and aberrant histories. Quantum intefrence and decoherence
>>> explains why those aberrant histories are relatively rare.
>> Could it be that some things which seem physically possible, like the
>> matter in my keyboard spontaneously rearranging itself into a
>> miniature fire-breathing dragon, are actually impossible under MWI,
>> i.e. don't occur in any branch of the multiverse?
> 
> 
> If we take seriously *classical* quantum mechanics into account, or  
> even *special relativistic quantum mechanics* into account, I don't  
> see how we could prevent such happening (your keyboard becoming a  
> dragon) in the multiverse. It just follows from the math. Of course  
> the probability that your keyboard become a firing dragon in your  
> branch is much little than winning the big lottery every nanosecond  
> during 100^100 millennia. The main reason is that in such theories  
> position and momentum are described by continuous variables, and the  
> quantum splitting or observers differentiation operate on the  
> continuum. They are even a continuum of variant among your possible  
> dragons, but this remains relatively rare.
> 
> Of course we have good reason to dismiss both classical quantum  
> mechanics and special relativistic mechanics as the "real theory",  
> given that they "forget" the unavoidable problem of quantization of  
> gravitation, and thus of space-time.
> 
> If we take into account gravitation, we have a choice of theories on  
> which physicists are still debating a lot. I would say that with the  
> "superstring" sort of theories, the multiverse generates still a  
> continuum of differentiation of stories, and that keyboard-dragon  
> transformation will still happen in many branches (but will still be  
> very rare, for the same reason as above). If we take the Loop-Gravity  
> kind of theories, then gravitation (which curves space-time) is  
> properly quantized, and we get eventually a discrete space-time. In  
> that case, if we add the assumption that the physical universe is  
> sufficiently little, it may be that the keyboard-dragon transformation  
> does not occur, in the resulting finite or enumerable multiverse. 

This is what I've suggested before.  There may be a smallest non-zero 
probability, so quantum evolution is not strictly unitary and after sufficient 
decoherence the off diagonal terms of the reduced density matrix become 
strictly 
zero.

Brent

>Now,  
> *this* would be a problem for comp, because comp implies indeed that  
> everything consistent happens somewhere indeed (unless Günther is  
> right and that some comp super-selection rule applies, but I don't see  
> where such super-selection could come from).
> 
> Of course keyboard-dragon types of transformations are utterly NOT  
> verifiable, even in the ironical first person way of quantum or comp  
> suicide. If you decide to kill yourself until your keyboard transforms  
> itself into a firing dragon, a "simple" evaluation of the  
> probabilities will show that you have 99,9999... % of chance of  
> surviving only with a brain making you believing that such a  
> transformation has occurred, when it has not. It is the general  
> practical weakness of comp or quantum suicide: if you ask for  
> something *near-impossible",  suicide will send you in dreamland (1  
> person view), and probably in a asylum (3 person view).
> 
> 
> Bruno
> 
> 
> 
> http://iridia.ulb.ac.be/~marchal/
> 
> 
> 
> 
> > 
> 


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