big even to be written down in standard notation 

 with digits of ordinary size on paper that could be 

 fitted into the part of the universe we can observe. 

 But the important point is that the number is finite. 



Not only is the number of possible states of an O- 

 region finite; the number of its possible histories is 

 finite as well. A history is described by a sequence of 

 states at successive moments of time. Which histories 

 are possible in quantum physics differ immensely 

 from the ones possible m the classical world. In the 

 quantum world the same initial state can lead to a 

 multitude of outcomes to which only probabilities 

 can be assigned. Consequently, the range of possible 

 histories is greatly enlarged. 



Once again, though, the fuzziness imposed by 

 quantum uncertainty makes it impossible to distin- 

 guish histories that are too close to each other. The 

 histories of various O-regions can be distinguished 

 only if the successive moments in the histories are 

 separated by sufficiently large (though still tiny) inter- 

 vals of time. Giving such a coarse-grained history is 

 much like recording a movie in a digital format: you 

 simply specify the cell "addresses" for all the particles 

 in the O-region at each moment in the finite se- 

 quence ot intervals. Any finite number of states, each 

 specified for a finite number of moments, yields only 

 a finite number of distinct histories of the system. 



Garriga and I did a quick, back-ot-the-envelope 

 estimate of the number of possible histories that can 

 unfold in an O-region between the big bang and the 

 present. As one might expect, we got yet another 

 "googolplexic" number: 10 to the power 10 (a 

 googolplex is 10 to the power of a googol). The ac- 

 tual numbers of the quantum states and of possible 

 histories in an O-region are not particularly impor- 

 tant, but the fact that those numbers are finite has 

 far-reaching implications, indeed. 



It may be helpful to take stock of the cosmic story 

 so far. It follows from the theory of inflation that 

 island universes are internally infinite, so that each 

 of them comprises an infinite number of O-regions. 

 And it follows from quantum uncertainty that only 

 a finite number of histories can unfold in any O- 

 region. Putting those two statements together, it 

 follows that every single possible history should be 

 repeated an infinite number of times within any of 

 the island universes — including, of course, the one 

 we inhabit. In quantum mechanics, anything not 

 strictly forbidden by the conservation laws of 

 physics must happen — and has happened — in an in- 

 finite number of O-regions. 



Among the infinitely replayed scripts are some 

 very bizarre histories. For example, a planet similar 

 to our Earth can suddenly collapse to form a black 



hole. Such an event is extremely unlikely, but all that 

 means is that, before encountering it, one would, in 

 all probability, have to survey an enormous number 

 of O-regions within our island universe. 



A striking consequence of the new picture of the 

 world is that among the infinite number of O-re- 

 gions is an infinite subset of those regions with his- 

 tories absolutely identical to ours. Yes, dear reader, 

 scores of your duplicates are now reading copies of 

 this article. They live on planets exactly like Earth, 

 with all its mountains, cities, trees, and butterflies. 



How far away are these earths populated by our 

 clones? Matter in our O-region can assume about 



With humankind reduced 

 to absolute cosmic insignificance, 

 our descent from the center 

 of the world is now complete. 



10 to the 10 90 distinct states. A box large enough 

 to hold, say, a googolplex O-regions should exhaust 

 all those possibilities by a large margin. Such a box 

 would be roughly a googolplex light-years across. At 

 greater distances, O-regions, including ours, should 

 recur. There should also be regions whose histories 

 differ from ours, with all possible variations. They all 

 belong to the same spacetime, and given enough 

 time, our descendants might, in principle, be able to 

 visit the inhabitants of another O-region. 



But we will never be able to reach another one of 

 the island universes (each with its own infinite com- 

 plement of O-regions), or even send a message 

 there. The reason is that no one will be able to keep 

 up with the boundaries of the island universe, 

 which are expanding faster than the speed of light. 

 Thus, we will never be able to reach the shores of 

 the inflating sea and bask in the light of the new 

 suns that will be born there. We cannot even pro- 

 claim our own existence to the future civilizations 

 that will thrive around these suns. 



Regardless of their accessibility, other universes 

 and other O-regions will surely change our larger 

 sense of place. In the worldview that has emerged 

 trom the ideas of eternal inflation, our civilization 

 is anything but unique. Instead, countless identical 

 civilizations are scattered across the cosmos. With 

 humankind reduced to absolute cosmic insignifi- 

 cance, our descent from the center of the world, a 

 process begun by Copernicus (in this world and on 

 countless others) is now complete. □ 



This article was adapted from Many Worlds in One: I he Search for 

 Other Universes, by . Ilex I ilenkin, which is being published in July 

 by Hill and Wang, 



July/August 2006 NATURAL HISTORY 



47 



