arXiv:0707.2593v1 [quant-ph] 18 Jul 2007

Man y liv es in man y worlds

Max T egmark

(In this universe:) Dept. of Physics & MIT Kavli Institute,

Massachuse tts Institute of T e chnolo gy, Cambridge, M A 02139 , USA

(Dated: Published in Natur e , 448 , 23, July 200 7)

I argue that accepting quantum me chanics to b e universa lly tru e means that yo u should also

b eliev e in parallel u niverse s. I giv e my assessmen t of Everett’s th eory as it celebrates its 50th

anniversa ry .

Almost all of my co lleagues hav e an opinion a bo ut it,

but almost none of them have read it. The ﬁr st draft o f

Hugh Ev er ett’s PhD thesis, the shortened oﬃcial v er sion

of whic h celebra tes its 5 0th birthday this year, is buried

in the out-of-pr int bo ok The Many-Worlds Interpr etation

of Quant um Me chanics [1]. I remember m y excitemen t

on ﬁnding it in a small Be r keley b o ok stor e bac k in g rad

school, and still view it as one of the most brillia n t texts

I’ve ev er read.

By the time E verett started his gra duate w or k with

John Archibald Wheeler at Princeton Universit y , q ua n-

tum mechanics had chalk ed up stunning successes in ex-

plaining the atomic realm, yet a debate rag ed on as to

what its mathematical fo rmalism r eally mea nt . I was for-

tunate to get to discuss q ua nt um mechanics with Wheeler

during my p ostdo ctorate years in P rinceton, but never

had the chance to meet E verett.

Quantum mechanics spec iﬁes the state of the univ er s e

not in classica l terms, such as the p ositions and velo ci-

ties of all particles, but in terms of a mathematical ob-

ject ca lled a w avefun ctio n. According to the Sc hr¨ odinger

equation, this wav efunction evolves over time in a de-

FIG. 1: Is it only in ﬁction that we can exp erience parallel

liv es? If atoms can b e in tw o places at once, so can you.

terministic fashion that mathematicians term “unitary ” .

Although quantum mechanics is often described as inher -

ent ly random and uncertain, there is nothing r andom or

uncertain ab o ut the wa y the wa vefunction ev olves.

The sticky pa rt is ho w to co nnect this wa vefunc-

tion w ith what w e observe. Man y legitimate wav efunc-

tions cor resp ond to counterin tuitiv e situations, such as

Sch r ¨ odinger’s cat b eing dead-a nd-alive at the same time

in a “sup erp o s ition” of states. In the 19 20s, ph ysicists

explained aw ay this weirdness b y p os tulating that the

wa vefunction “collapsed” into some random but deﬁnite

classical outcome whenever s o meone made an obs e r v a-

tion. This add-on had the virtue of explaining observ a-

tions, but r endered the theor y incomplete, be c a use there

was no mathematics sp ecifying what constituted an ob-

serv ation – that is, when the wa vefunction w as supp osed

to colla pse.

Everett’s theory is simple to state but has co mplicated

implications, including par a llel universes. The theory ca n

be s ummed up by saying that the Schr¨ odinger eq uation

applies at all times; in other words, that the wa vefunction

never collapses. Th a t’s it – no menti o n of para llel uni-

verses or s plitting worlds, which are implications of the

theory r ather than po s tulates. His brilliant insight was

that this collapse- fr ee quantum theory is, in fact, consis-

ten t with observ ation. Although it predicts that a wav e-

function describing one clas sical r eality gra dually ev o lv es

in to a w avefu nction des c ribing a sup erp osition of ma ny

such realities – the many worlds – o bservers sub jectiv ely

exp e rience this splitting mer ely as a slig ht randomness

(see Figure 2), w ith probabilities consistent with those

calculated using the wa vefunction-collapse recipe.

Gaining acceptanc e

It is often said that impor tant s cient iﬁc discoveries go

though three phase s: ﬁrst they are completely ig nored,

then they ar e vio lent ly attack ed, a nd ﬁnally they are

brushed aside as well-known. Everett’s discovery was no

exception: it to ok over a decade un til it star ted getting

noticed. But it w a s to o late for Everett who left academia

disillusioned [2].

Everett’s no-co llapse idea is not yet at stage three, but

after b eing widely dismissed as too cr azy during the 197 0s

and 198 0s, it ha s g radually gained more acceptance. At

an informal p oll taken at a conference on the foundations

of quan tum theory in 19 99 ph ysicists rated the idea more

highly than the alter natives, although there w er e still