Vol 448|5 July 2007 COMMENTARY Many lives in many worlds Accepting quantum physics to be universally true, argues Max Tegmark, means that you should also believe in parallel universes.

lmost all of my colleagues have an opin- ion about it, but almost none of them Ahave read it. The first draft of Hugh Everett’s PhD thesis, the shortened official version of which celebrates its 50th birthday this year, is buried in the out-of-print book The Many-Worlds Interpretation of Quantum ENTERPRISES/RGA MIRAGE Mechanics. I remember my excitement on finding it in a small Berkeley book store back in grad school, and still view it as one of the most brilliant texts I’ve ever read. By the time Everett started his graduate work with John Archibald Wheeler at Prin- ceton University in New Jersey quantum mechanics had chalked up stunning suc- cesses in explaining the atomic realm, yet debate raged on as to what its mathematical formalism really meant. I was fortunate to get to discuss quantum mechanics with Wheeler during my postdoctorate years in Princeton, but never had the chance to meet Everett. Quantum mechanics specifies the state of the Universe not in classical terms, such as the positions and velocities of all particles, but in terms of a mathematical object called a Is it only in fiction that we can experience parallel lives? If atoms can be in two places at once, so can you. wavefunction. According to the Schrödinger equation, this wavefunction evolves over time at all times; in other words, that the wavefunc- Everett’s no-collapse idea is not yet at stage in a deterministic fashion that mathematicians tion of the Universe never collapses. That’s it three, but after being widely dismissed as too term ‘unitary’. Although quantum mechanics — no mention of parallel universes or splitting crazy during the 1970s and 1980s, it has gradu- is often described as inherently random and worlds, which are implications of the theory ally gained more acceptance. At an informal uncertain, there is nothing random or uncer- rather than postulates. His brilliant insight poll taken at a conference on the foundations tain about the way the wavefunction evolves. was that this collapse-free quantum theory is, of quantum theory in 1999, physicists rated the The sticky part is how to connect this wave- in fact, consistent with observation. Although idea more highly than the alternatives, although function with what we observe. it predicts that a wavefunction many more physicists were still ‘undecided’2. I Many legitimate wavefunctions “If we dismiss describing one classical reality believe the upward trend is clear. correspond to counterintuitive theories because gradually evolves into a wave- Why the change? I think there are several situations, such as Schrödinger’s they seem weird, function describing a super- reasons. Predictions of other types of paral- cat being dead and alive at position of many such realities lel universes from cosmological inflation the same time in a ‘superpo- we risk missing true — the many worlds — observ- and string theory have increased tolerance sition’ of states. In the 1920s, breakthroughs.” ers subjectively experience this for weird-sounding ideas. New experiments physicists explained away this splitting merely as a slight ran- have demonstrated quantum weirdness in weirdness by postulating that the wavefunction domness (see ‘Not so random’, overleaf), with ever larger systems. Finally, the discovery of a ‘collapsed’ into some random but definite probabilities consistent with those calculated process known as decoherence has answered classical outcome whenever someone made using the wavefunction-collapse recipe. crucial questions that Everett’s work had left an observation. This add-on had the virtue dangling. of explaining observations, but rendered Gaining acceptance For example, if these parallel universes exist, the theory incomplete, because there was no It is often said that important scientific dis- why don’t we perceive them? Quantum super- mathematics specifying what constituted an coveries go though three phases: first they are positions cannot be confined — as most quan- observation — that is, when the wavefunction completely ignored, then they are violently tum experiments are — to the micro world. was supposed to collapse. attacked, and finally they are brushed aside Because you are made of atoms, then if atoms Everett’s theory is simple to state but has as well known. Everett’s discovery was no can be in two places at once in super position, complex consequences, including parallel exception: it took more than a decade before so can you. universes. The theory can be summed up by it started getting noticed. But it was too late for The breakthrough came in 1970 with a saying that the Schrödinger equation applies Everett, who left academia disillusioned1. seminal paper by H. Dieter Zeh, who showed

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where mathematical simplicity is sacrificed to to sacrificed is simplicity mathematical where demonstrated in systems with many atoms, atoms, many with systems in demonstrated on page 15. page on

words’ interpretation of quantum mechanics, mechanics, quantum of interpretation words’ been have superpositions collapse-free See also Editorial on page 1 and News Feature Feature News and 1 page on Editorial also See

the former, you would probably prefer a ‘many ‘many a prefer probably would you former, the Indeed, collapsing. wavefunction the observe quant-ph/0101077 (2001). quant-ph/0101077

2. Tegmark, M. & Wheeler, J. A. http://arxiv.org/pdf/ A. J. Wheeler, & M. Tegmark, 2.

guage or mathematical language? If you opt for for opt you If language? mathematical or guage a system they probe, it says, they will not not will they says, it probe, they system a

http://space.mit.edu/home/tegmark/everett/ (2003). http://space.mit.edu/home/tegmark/everett/

words, what is more basic to you: human lan- human you: to basic more is what words, future lab experiments: no matter how large large how matter no experiments: lab future

III Everett, Hugh of Sketch Biographical E. Shikhovtsev, 1.

Moreover, Everett’s theory is falsifiable by by falsifiable is theory Everett’s Moreover, perspective or the bird perspective? In other other In perspective? bird the or perspective

What is more fundamental — the frog frog the — fundamental more is What universes. parallel including predictions, Cambridge, Massachusetts, USA. Massachusetts, Cambridge,

made scientists take more seriously its other other its seriously more take scientists made randomness. at the Massachusetts Institute of Technology, Technology, of Institute Massachusetts the at

the splitting of classical storylines as quantum quantum as storylines classical of splitting the have mechanics quantum unitary by dictions In this Universe, Max Tegmark is a physicist physicist a is Tegmark Max Universe, this In

fraction of this full reality, and they perceive perceive they and reality, full this of fraction pre- successful Analogously, holes. black of ture its strangeness to be part of its charm. charm. its of part be to strangeness its ■

frog perspective, observers perceive only a tiny tiny a only perceive observers perspective, frog struc- internal the as such cannot, we things for to the weird ways of our cosmos, and even find find even and cosmos, our of ways weird the to

observe, we also take seriously its predictions predictions its seriously take also we observe, another 50 years, I believe we will be more used used more be will we believe I years, 50 another that lack a classical description. From their their From description. classical a lack that

as well as a number of quantum phenomena phenomena quantum of number a as well as can we things many predicted successfully has only mistake was to be born ahead of his time. In In time. his of ahead born be to was mistake only

(worlds), continuously splitting and merging, merging, and splitting continuously (worlds), relativity of theory general Einstein’s Because ogy than it was 50 years ago. I think Everett’s Everett’s think I ago. years 50 was it than ogy

it a vast number of classical parallel storylines storylines parallel classical of number vast a it do. will one — accept after the recent breakthroughs in cosmol- in breakthroughs recent the after accept

by this evolving wavefunction contains within within contains wavefunction evolving this by predictions its all test and observe not need we a humble suggestion that is probably easier to to easier probably is that suggestion humble a

lelism. The abstract quantum world described described world quantum abstract The lelism. falsifiable, be to theory a For theories. certain physical world is grander than we had imagined, imagined, had we than grander is world physical

over time without any kind of splitting or paral- or splitting of kind any without time over of prediction a but themselves, in theory a not Everett asked us to acknowledge that our our that acknowledge to us asked Everett

and it evolves smoothly and deterministically deterministically and smoothly evolves it and are universes parallel that is point key The phy? positively quaint. quaint. positively

universes by the standards of the time) sounds sounds time) the of standards the by universes wavefunction, one only is There simple. is verse philoso- mere or science it is question: pressing

there was really a multitude of galaxies (parallel (parallel galaxies of multitude a really was there multi- Everett’s perspective, bird the From another face we but observations, quantum

Shapley–Curtis debate of 1920 about whether whether about 1920 of debate Shapley–Curtis with consistent still is interpretation Everett’s the bird. the

by the equations, like a frog being watched by by watched being frog a like equations, the by that fact the celebrate can we years 50 After expand our horizons. To modern ears the the ears modern To horizons. our expand

of an observer living in the world described described world the in living observer an of one. ern perpetuating our instinctive reluctance to to reluctance instinctive our perpetuating

the index, I recommend buying a more mod- more a buying recommend I index, the weird, we risk missing true breakthroughs, breakthroughs, true missing risk we weird, view inside the and above, high from landscape

does not mention Everett and decoherence in in decoherence and Everett mention not does can’t observe everything or because they seem seem they because or everything observe can’t a surveying bird a like equations, ematical

you are considering a quantum textbook that that textbook quantum a considering are you dismiss theories such as Everett’s because we we because Everett’s as such theories dismiss math- its studying physicist a of view outside

tion so as to avoid any subsequent confusion. If If confusion. subsequent any avoid to as so tion The choice is yours. But I worry that if we we if that worry I But yours. is choice The the theory: physical a viewing of ways two

mental process violating the Schrödinger equa- Schrödinger the violating process mental ries of flying rocks. flying of ries between distinguish to useful it find I reality,

should be told that it is probably not a funda- a not probably is it that told be should for our distant ancestors, such as the trajecto- the as such ancestors, distant our for of nature ultimate the about thinking When it?

has utility as a calculational recipe, but students students but recipe, calculational a as utility has those aspects of physics that had survival value value survival had that physics of aspects those believe really you should But observation. with

quantum mechanics. The idea of collapse still still collapse of idea The mechanics. quantum evolution endowed us with intuition only for for only intuition with us endowed evolution agrees far so and testable is theory Everett’s So

tion collapse as a fundamental postulate of of postulate fundamental a as collapse tion mechanics to feel counterintuitive, because because counterintuitive, feel to mechanics The bird perspective bird The

quantum textbooks that introduce wavefunc- introduce that textbooks quantum Moreover, we should expect quantum quantum expect should we Moreover,

In my opinion, it is time to update the many many the update to time is it opinion, my In — and not just for quantum physics. quantum for just not and — worlds. parallel

like a collapse. a like everything we can observe and nothing else else nothing and observe can we everything Everett’s in computations parallel performing

an effect that looks like a collapse and smells smells and collapse a like looks that effect an formulate a mathematical theory that predicts predicts that theory mathematical a formulate effectively computers, classical than faster tially

generally agreed that decoherence produces produces decoherence that agreed generally in general, it has proved extremely difficult to to difficult extremely proved has it general, in ful, will be able to factor numbers exponen- numbers factor to able be will ful,

cally never collapses in the Everett view, it is is it view, Everett the in collapses never cally struggle with this you are in good company: company: good in are you this with struggle success- if which, computers quantum build

gone. Even though the wavefunction techni- wavefunction the though Even gone. with the many-worlds interpretation. If you you If interpretation. many-worlds the with to effort global a also is There scale. scopic

thing strange, much of this motivation was was motivation this of much strange, thing matical theory, then you — like me — are stuck stuck are — me like — you then theory, matical macro- human our to close tantalizingly more,

found to explain why we never perceive any- perceive never we why explain to found But if you prefer a simple and purely mathe- purely and simple a prefer you if But or atoms 10 involving objects of positions

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‘Not so random’) and decoherence had been been had decoherence and random’) so ‘Not lel universes. lel super- quantum create to attempting now are

collapse the wavefunction and eliminate paral- eliminate and wavefunction the collapse groups Several molecules. carbon-60 as such (see anyway random appear would things that

scopic superpositions. After Everett had shown shown had Everett After superpositions. scopic

perceive probabilities and not strange macro- strange not and probabilities perceive

M.T. put it, “God does not play dice”. dice”. play not does “God it, put quantum physics had been to explain why we we why explain to been had physics quantum

underlying physics is not random and, as Einstein Einstein as and, random not is physics underlying

One parallel world parallel One

Q Q into collapse wavefunction random of notion

Q Q the laws of probability apply even though the the though even apply probability of laws the

Q Q the introducing for motivation main The

gambler in each of the worlds will conclude that that conclude will worlds the of each in gambler

Q Q

Q Q Q Q Q Q philosophy? or Science

game is continued many more times, almost every every almost times, more many continued is game

Q Q Q Q Q Q

If the the If Q Q Q Q Q Q up. way same the land cards four all will

Q decoherence. to robust most the are Q Q Q Q Q

Q Q Q Q Q Q worlds two in Only probability. 50% about with

bling classical physics have special status: they they status: special have physics classical bling

Q Q Q Q Q Q

Q Q Q randomly, occur queens that seem will it worlds, Q Q Q Q Q Q Q resem- states why explains also Decoherence

Q these of most In worlds. parallel or outcomes, Q Q Q Q Q Q Q Q Q

the superposition and making it unobservable. unobservable. it making and superposition the

100 % 100 25 % 25 % 50 % 75 0 % 0 in a row, there will be 16 (2 x 2 x 2 x 2) possible possible 2) x 2 x 2 x (2 16 be will there row, a in

left or to the right, destroying the coherence of of coherence the destroying right, the to or left

Fraction of queens landing face up face landing queens of Fraction If the gambler repeats this game four times times four game this repeats gambler the If

thereby find out whether it has fallen to the the to fallen has it whether out find thereby

card fell randomly. fell card Sixteen parallel worlds parallel Sixteen

into air molecules, photons and so on, which which on, so and photons molecules, air into

unaware of the other outcome and feels as if the the if as feels and outcome other the of unaware µ 4 µ

(see ‘Not so random’) is constantly bumping bumping constantly is random’) so ‘Not (see In either of these parallel worlds, the gambler is is gambler the worlds, parallel these of either In

the world. The quantum card in our example example our in card quantum The world. the

possible outcomes — winning or losing the bet. the losing or winning — outcomes possible

long as they were kept secret from the rest of of rest the from secret kept were they as long state changes to become a superposition of two two of superposition a become to changes state

tions were found to remain observable only as as only observable remain to found were tions on the queen landing face up, the gambler’s own own gambler’s the up, face landing queen the on

in two places at once. If a gambler bets money money bets gambler a If once. at places two in the following decades. Quantum superposi- Quantum decades. following the

known as a ‘superposition’ — the card really is is really card the — ‘superposition’ a as known detail by Wojciech Zurek, Zeh and others over over others and Zeh Zurek, Wojciech by detail

balanced on its edge will fall down in what is is what in down fall will edge its on balanced

as ‘decoherence’, and was worked out in great great in out worked was and ‘decoherence’, as

Two parallel worlds parallel Two

According to quantum theory, a card perfectly perfectly card a theory, quantum to According a type of censorship. This effect became known known became effect This censorship. of type a

Not so random so Not that the Schrödinger equation itself gives rise to to rise gives itself equation Schrödinger the that

COMMENTARY 5 July 2007 July 5 448 Vol NATURE | |