Invisible wrote:
> Suppose that every atom in the known universe had a unique serial 
> number. Suppose I picked a serial number at random, and then asked you 
> to go find out where that particular atom is. (Let us assume that there 
> is no particular correspondence between serial numbers and position in 
> space.)

I don't want to spoil your gedanken experiment, but in modern physics 
you can not distinguish one (e.g.) hydrogen atom from another. It is not 
that they are two different objects that are not marked so you can not 
distinguish them, but much more fundamental. If you exchange two protons 
nothing happens. There is no way to figure that out, because there is 
absolutely no difference. You probably better say that this one proton 
is at all these places at the same time.

Perhaps you know the interpretation that a positron is just an electron 
flying backwards in time. It has been suggested that if the universe 
ends in a big crunch (probably not) then you only need one electron 
weaving forwards and backward in time. i.e. if an electron and a 
positron annihilate, what actually happens is that the electron changes 
direction again. So probably not true, but a funny image anyway.

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