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. 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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