LETTERS 



Infinite Multiverse 



Alex Vilenkin's article, 

 "Beyond the Big Bang" 

 [7—8/06], is stimulating and 

 fantastic at the same time. 

 The mind boggles at the 

 thought that there are 

 replica worlds in an infinite 

 number of universes creat- 

 ed by "eternal inflation." 



How would the author's 

 theory change if universe 

 creation (each big bang) 

 were a stochastic rather 

 than a deterministic event? 

 That is, what if the laws of 

 physics in even some small 

 way were random in those 

 first trillionths of a second 

 of creation? 

 Walter Ulrich 

 Pearland, Texas 



Alex Vilenkin's assertion 

 that the number of uni- 

 verses is infinite relies on 

 an analysis that shows the 

 rate of increase of false- 

 vacuum regions to be larg- 

 er than their decay rate. 

 Can that conclusion be 

 tested experimentally? 

 Fred Haag 



Burnt Hills, New York 



Alex Vilenkin's article looks 

 like another example of a 

 theory that cannot be tested. 

 Mr. Vilenkin is careful to 

 establish that the size of an 

 "island universe" would be 

 so large that communicating 

 with another universe or 

 testing the existence of an- 

 other would be impossible. 

 To me, as a mere mathe- 

 matician, that circumstance 

 seems to imply there is no 

 way for us to determine 

 whether Mr. Vilenkin's the- 

 ory is true or false. 



Another problem I have 

 is Mr. Vilenkin's assertion 

 that since there is only a fi- 



nite number of possible 

 kinds of "island universes," 

 and since there is an infinite 

 number of them scattered 

 about the universe, there 

 must be an infinite number 

 of island universes just like 

 ours. But Mr. Vilenkin's 

 conclusion does not follow; 

 all that follows is that there 

 must be at least one island 

 universe that occurs an infi- 

 nite number of times. 

 Nolan Wallach 

 University of California, 

 San Diego 



There is indeed an infinite 

 number of universes in 

 which Alex Vilenkin reach- 

 es his conclusion: "With 

 humankind reduced to ab- 

 solute cosmic insignificance, 

 our descent from the center 

 of the world ... is now 

 complete." There is also an 

 infinite number of universes 

 in which Mr. Vilenkin con- 

 cludes the exact opposite. I 

 quote: "With humankind 

 restored to absolute cosmic 

 significance, our ascent to 

 the center of the world . . . 

 is now complete." 



How does he (or rather 

 they) reach this conclusion? 

 Check out a copy of the al- 

 ternative Natural History] 

 And which conclusion is 



superior? One suspects this 

 matter must be settled 

 among themselves by the 

 alternative Alexes. 

 Robert P. Largess 

 Boston, Massachusetts 



Alex Vilenkin replies: 

 Walter Ulrich asks what 

 would happen if different is- 

 land universes are character- 

 ized by different values of 

 the constants of nature (and 

 thus by different laws of 

 physics). That possibility is 

 being actively discussed by 

 cosmologists. The "choice" 

 of the constants is stochastic, 

 and the theory of eternal 

 inflation is used to predict 

 the most probable values 

 that we (typical observers in 

 the multiverse) are most 

 likely to observe. One such 

 prediction (for the so-called 

 cosmological constant) has 

 already been confirmed — 

 perhaps the first evidence 

 that there is indeed a huge 

 multiverse out there. 



Fred Haag wonders 

 whether a certain property 

 of the false vacuum can be 

 verified experimentally. But 

 a false vacuum will not be 

 generated in the laboratory 

 any time soon, because of 

 the enormous energy re- 

 quired to do so. Still, there is 



a good theoretical reason to 

 think that the rate of false- 

 vacuum expansion is greater 

 than the rate of its decay If 

 it were not, the universe 

 would not expand by much 

 before the false vacuum de- 

 cayed, and inflation would 

 not be able to account for 

 the huge expanding universe 

 that we observe. 



The points raised by 

 Nolan Wallach are discussed 

 in detail in my book Many 

 Worlds in One. Briefly, other 

 island universes are not di- 

 rectly observable, but their 

 existence follows from the 

 theory of inflation. The 

 theory is supported by data 

 in the observable part of the 

 universe, and that gives us 

 reason to believe its conclu- 

 sions about the parts we 

 cannot observe. 



It is true that infinite 

 space does not, by itself, 

 guarantee that all possibili- 

 ties are realized. Random 

 quantum fluctuations, how- 

 ever, supply an additional 

 ingredient. All possible is- 

 land universes that do not 

 violate the laws of physics 

 have nonzero quantum me- 

 chanical probabilities, and 

 so they must occur some- 

 where in the multiverse. 



Collateral Damage 



David Barraclough's arti- 

 cle, "Bushels of Bots" 

 [6/06], noted that every 

 large animal species (in- 

 cluding the rhinoceros) 

 hosts several endemic spe- 

 cies of parasites and com- 

 mensals, such as flukes, 

 protists, roundworms, and 

 tapeworms. When a host 

 species goes extinct, it car- 

 ries several other species 

 "down" with it. 



It is hard for most people 



"/ said I wanted a wild beast in bed, not a wildebeest." 



10 



NATURAL history September 2006 



