well - it's time for a definition.
                
     O          i'm a big fan of Turing; and mathematically i'm
  O  O   O      a contructivist. so i'm willing to take a risk
     O          and restrict myself to computable structures.
 O              since we started all this with "people living
 O  O           inside numbers", let's stick to numbers. (no
 O      O       worries - computable numbers are isomorphic to
 O      O       computable structures. and heck - Turing's big
        OO      paper was about these numbers. so.)
     O OO       
O  O O          definition: the multiverse is the set of
O     O O       computable numbers.
OOO             
 OO   O         ...
  O OO   O      
  O  O O O      for the layman - what does this mean?
  O    O O      
O O O  O O      a computable structure is anything that is
O   O OO        produced in a step-by-step manner, following a
OO OO O         fixed set of rules, where no intelligence is
OOOO    O       required. its anything that could be produced by
O O O OOO       a machine.
O O O OOO       
O O OOOO        computable numbers are those number that are
O OOOOO O       made in the same way. pi is a good example -
O  O OO OO      there is a well known ruleset for writing out
OOO O O  O      the digits of pi. it requires a lot of work -
O  OO OOOO      but the process is simple.
O OOO O OO      
OOOO  O OO      it may not seem like it, but we don't lose
OO OOO OOO      anything by considering only numbers. the
OO OOO O O      more complicated structures (words, sets,
OO O OOO O      hierarchies, arrangements of atoms, ...) can be
OOOO OO OO      encoded as big numbers. (trust me, it's okay.)
OOOO OO OO      
OOOOOOO  O      besides - the other universe are bound to be
OOOOO OO O      extremely strange to us. maybe they don't have
 OOOO OO O      movement, only growth? we'd better convert
OOOOO OOOO      everything over to numbers so that we're all on
OOOOOOOOOO      equal footing.