Video: Play our game: Choose your destiny in the multiverse
Every decision you make may spawn parallel universes where people are suffering because of your choice. Welcome to the quantum moral maze
Iām rich. Iām a movie star. Iām king of the world. Iām also poor. Iām homeless. Lots of me are dead.
Iām none of these. Not in this universe. But in the multiverse Iām all of them, and more.
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Iām not a megalomaniac or a fantasist, but I do have a fascination with what-ifs. In the many-worlds interpretation of quantum mechanics, every decision I take in this world creates new universes: one for each and every choice I could possibly make. Thereās a boundless collection of parallel worlds, full of innumerable near-copies of me (and you). The multiverse: an endless succession of what-ifs.
In one of those worlds, Iāve just written a paragraph which explains that more clearly.
This worries me. If many worlds is correct ā and many physicists think it is ā my actions shape the course not just of my life, but of the lives of my duplicates in other worlds.
āIn the many-worlds interpretation, when you make a choice the other choices also happen,ā says David Deutsch, a quantum physicist at the University of Oxford. āIf there is a small chance of an adverse consequence, say someone being killed, it seems on the face of it that we have to take into account the fact that in reality someone will be killed, if only in another universe.ā
Iām alive, dead, rich and poor (Image: Lucas SimƵes)
Should I feel bad about the parallel Rowans that end up suffering as a result of my actions? If I drive carelessly here, I might get a fine, but one of my other selves might crash and kill himself. Or worse, kill my parallel family. How am I supposed to live with the knowledge that I am just one of umpteen Rowans in the multiverse, and that my decisions reach farther than I can ever know?
You might think I should just ignore it. After all, the many-worlds interpretation says Iāll never meet those other versions of me. So why worry about them?
Well, most of us try to live by a moral code because we believe the things we do affect other people, even ones weāll never meet. We worry about how our shopping habits affect workers in distant countries; about as-yet-unborn generations suffering for our carbon emissions. Deutsch points out that we readily accept that attempted murder has moral implications, albeit less serious than actual murder. So why shouldnāt we afford some consideration to our other selves?
Max Tegmark, professor of physics at the Massachusetts Institute of Technology, understands my quandary. A leading advocate of the multiverse, heās thought long and hard about what it means to live in one. āI feel a strong kinship with parallel Maxes, even though I never get to meet them. They share my values, my feelings, my memories ā theyāre closer to me than brothers,ā he says.
Taking the cosmic perspective makes it difficult for Tegmark to feel sorry for himself: thereās always another Max who has it worse than him. If he has a near-miss while driving, he says he takes the experience more seriously than he did before he knew about the multiverse. āThe minimum tribute I can pay to that dead Max is to really think through what happened and learn some lessons.ā
Tegmark is obviously a multiverse believer. Once, he would have been an outsider. When many-worlds was first proposed by Hugh Everett, then a graduate student at Princeton University, it met with a scornful reception. Everett struggled to get it published, and eventually left academia in disgust. But its elegant explanations for some puzzling quantum phenomena have convinced more and more physicists over the past 50 years. āMulti-universe physics has the same kind of experimental basis as the theory that there were once dinosaurs,ā says Deutsch.
āMulti-universe physics has the same kind of experimental basis as dinosaursā
Nor can we avoid its consequences. Every time we make a decision that involves probability ā such as whether to take an umbrella in case of rain ā our decision causes the universe to branch, explains Andreas Albrecht at the University of California, Davis. In one universe, we take the umbrella and stay dry; in another, we donāt, and we get wet. The fundamental variability of the universe forces such choices upon us. āThereās no escaping them,ā says Albrecht.
Thatās a momentous realisation. Weāre living in a time akin to Copernicus realising that Earth wasnāt at the centre of the universe, or when Darwin realised that humans were not created separately from the other animals. Both of those realisations reshaped our conception of our place in the universe, our philosophy and morality. The multiverse looks like the next great humbler of humanity.
āThat these worlds are actually out there somewhere, but we cannot access them: I think thatās an amazing and remarkable thing,ā says physicist Seth Lloyd, a colleague of Tegmarkās at MIT. āItās sort of distressing really.ā Why, I ask: because it diminishes humanityās status even more?
āNo, not for that reason. Iāve always enjoyed the gradual marginalisation of humanity,ā he says. āNo, it was somehow tidier to have the universe be the cosmos. But actually now Iām liking it more, now that youāve pointed out that itās really like the ultimate step in the marginalisation of human beings. I think thatās much better. I enjoy that.ā
Enjoyable though the multiverse might be as an concept, itās tough for us humans to get our heads around its implications ā even for physicists themselves. When Tegmarkās wife was in labour with Philip, their eldest son, he found himself hoping that everything would go well. Then he admonished himself.
āIt was going to go well, and it was going to end in tragedy, in different parallel universes. So what did it mean for me to hope that it was going to go well?ā He couldnāt even hope that the fraction of parallel universes where the birth went well was a large one, because that fraction could in principle be calculated. āSo it doesnāt make any sense to say āIām hoping something about this numberā. It is what it is.ā
Hope, it turns out, is the next casualty of the multiverse. You make a decision, and you end up on a branch of the multiverse with a āgoodā outcome, or you find yourself on a ābadā branch. You canāt wish your way on to a good one. Tegmark acknowledges this is not easy to live with. āItās tough to get your emotions to sync with what you believe,ā he says.
Too tough for me. How am I meant to live without hope?
Slippery concept
What do other non-specialists make of the multiverse? When Hugh Everett died in 1982, aged just 51, his teenage son Mark found his body. I asked him if his fatherās work had influenced him. āAlthough I consider myself an Everettian by default, itās all beyond me for the most part, having inherited none of my fatherās mathematical smarts,ā says Mark, long-time frontman of the band Eels. āHow can I grasp any of it except in small moments? Iām having a hard enough time dealing with this world lately. I only hope some of the other worlds are easier for me to figure out.ā
I know how he feels. Perhaps a philosopher can help me take a broader perspective. I turn to David Papineau of Kingās College London. āSay you put your money on a horse which you think is a very good bet,ā he says. āIt turns out that it doesnāt win, and you lose all your money. You think, āI wish I hadnāt done that.ā But you brought benefits to your cousins in other universes where the horse won. Youāve just drawn the short straw in finding yourself in the universe where it lost. You didnāt do anything wrong. Thereās no sense that the action you took earlier was a mistake.ā
Hmm. I doubt āI didnāt make a mistakeā would get much traction with my partner if I bet all our savings on a horse this afternoon and find myself on the āwrongā branch. But then, that wouldnāt be the sensible thing to do ā and one of the great attractions of Everettās interpretation, according to Papineau, is that itās not āmessyā, as long as you act rationally.
With orthodox thinking, there are two ways of evaluating risky actions, he explains. First, did you make the choice that was most in line with the odds? If we needed money, and my stake had been proportionate, it might have been. Second, did it work out well? There are any number of reasons it might not ā the horse might fall, or just defy the odds and trail in last.
It offends Papineau that these two ways of being ārightā ā choosing wisely and getting lucky ā donāt go hand-in-hand. āThe idea that the right thing to do might turn out to have been the wrong thing seems to me to be a very ugly feature of orthodox thinking,ā he says. This doesnāt arise in the many-worlds interpretation, where every choice is made and every outcome occurs. That leaves no place for hope or luck, but nor does it leave room for remorse. Itās an elegant, if cold-blooded, way to look at things.
This elegance has always been part of the multiverseās appeal. In quantum mechanics, every object in the universe is described by a mathematical entity called a wave function, which describes how the properties of subatomic particles can take several values simultaneously. The trouble is, this fuzziness vanishes as soon as we measure any of those properties. The original explanation for this ā the so-called Copenhagen interpretation ā says the wave function collapses to a single value whenever a measurement is made.
Hugh Everett called this enforced separation of the quantum world from the everyday, classical one a āmonstrosityā, and decided to find out what happened if the wave function did not collapse. The resulting mathematics showed that the universe would split every time a measurement is made ā or in human terms, whenever we make a decision with multiple possible outcomes. Thatās the many-worlds interpretation.
God of elegance
For Don Page, a theoretical physicist at the University of Alberta in Edmonton, Canada, this elegance goes far beyond human actions. Page is both a hard-core Everettian and a committed Christian. Like many modern physicists, he agrees with Everettās stance that collapsing the wave function is unnecessarily complicated. Whatās more, for Page it has a happy side effect: it explains why his God tolerates the existence of evil.
āGod has values,ā he says. āHe wants us to enjoy life, but he also wants to create an elegant universe.ā To God the importance of elegance comes before that of suffering, which, Page infers, is why bad things happen. āGod wonāt collapse the wave function to cure people of cancer, or prevent earthquakes or whatever, because that would make the universe much more inelegant.ā
āGod wants us to enjoy life. But he also wants to create an elegant universeā
For Page, that is an intellectually satisfying solution to the problem of evil. And whatās more, many worlds may even take care of free will. Page doesnāt actually believe we have free will, because he feels we live in a reality in which God determines everything, so it is impossible for humans to act independently. But in the many-worlds interpretation every possible action is actually taken. āIt doesnāt mean that itās fixed that I do one particular course of action. In the multiverse, Iām doing all of them,ā says Page.
There are limits to Pageās willingness to leave his fate to the multiverse, however. Seth Lloyd once offered him $1 million to play quantum Russian roulette, which is a good game for a multiverse aficionado: you canāt lose (see āFour aspects of the multiverseā). Page thought about it, then declined: he didnāt like the thought of his wifeās distress in the worlds where he died.
Like Tegmark, Page seems to values the multiverse for the perspective it offers. āOne of my teenage children wants to get a motorcycle, which my wife and I think is pretty dangerous,ā Page says. āBut if I say: āOkay, maybe most of the time youād survive, but thereās going to be some part of you, some branch, in which you get seriously maimed in a motorcycle accidentāā¦ Maybe Iāll try it.ā
Double Deutsch
Iām somewhat relieved to find that even many-worlds experts ultimately behave in much the same way as people who know nothing of it. But Iāve also realised that it shapes the way they think about their decisions. Perhaps itās more natural for us to think about how our actions affect our āother selvesā than about the arid probabilities of risk and reward.
If anyoneās going to buck this trend, itās surely David Deutsch, probably the most hard-core of Everettians. Surely he can give me the last word on what it means to live in the multiverse. He does, but it is by no means the answer I was expecting.
āDecision theory in the multiverse tells us that we should value things that happen in more universes more, and things that happen in fewer universes less,ā he explains. āAnd it tells us that the amount by which we should value them more or less is, barring exotic circumstances, exactly such that we should behave as if we were valuing the risks according to probabilities in a classical universe.ā So the right thing to do remains the right thing to do.
So has my quest been for nothing? Not at all. For one thing, Deutschās approach could be wrong, a possibility he accepts, though he is adamant the multiverse exists. But if heās right, his conclusion only reinforces what his peers have been telling me: the best way to live in the multiverse is to think carefully about how you live your life in this one.
Thinking of what-ifs as having some kind of reality can help us to do that. Tegmark says many worlds has made him think differently about life. He sometimes fears doing something because it feels too big a deal. But then he realises that in the grander context of the multiverse, itās not big at all ā and he just does it. āThe multiverse has definitely made me a happier person,ā he says. āItās given me courage to take chances to be bold in life.ā
I hope it will do the same for me. We might not stop feeling hope or remorse, but the multiverse can help put those feelings in perspective. And while the multiverse may not require a change in our morality, it can help us think harder about our choices and actions. The cosmos reaches far further than we ever appreciated. But so, it seems, do we.
Leader: āLife in the multiverse means endless possibilitiesā
Read more: āHugh Everett: The man who gave us the multiverseā
Four aspects of the multiverse
1 The wave function
This mathematical entity describes the properties of any quantum system. Such properties ā an atomās direction of spin, say ā can take several values at once, in what is known as quantum superposition. But when we measure such a property we only get a single value ā in the case of spin, it is either up or down.
In the traditional Copenhagen interpretation of quantum mechanics, the wave function is said to ācollapseā when the measurement is taken, but it isnāt clear how this happens. (Schrƶdingerās famous cat, neither alive nor dead until someone looks inside its box, illustrates this.) In the multiverse, the wave function never collapses: rather, it describes the property across multiple universes. In this universe, the atomās spin is up; in another universe, itās down.
2 Wave-particle duality
In the landmark experiment, photons are sent one at a time towards a pair of slits, with a phosphorescent screen behind them. Take a measurement at either slit, and youāll register individual photons passing particle-like through one or the other. But leave the apparatus alone, and an interference pattern will build up on the screen, as if each photon had passed through both slits simultaneously and diffracted at each, like a classical wave.
This dual character has been described as the ācentral mysteryā of quantum mechanics. In the Copenhagen interpretation, it is down to wave function collapse. Left to its own devices, each photon would pass through both slits simultaneously: the measurement at the slit forces it to āchooseā. One way to explain the interference pattern through many worlds, by contrast, is that each photon only ever goes through only one slit ā the pattern comes about when a photon interacts with its clone passing through the other slit in a parallel universe.
3 Quantum computing
Though quantum computers are in their infancy, they are in theory incredibly powerful, capable of solving complex problems far faster than any ordinary computer. In the Copenhagen interpretation, this is because the computer is working with entangled āqubitsā which can take many more states than the binary states available to the ābitsā used by classical computers. In the multiverse interpretation, itās because it conducts the necessary calculations in many universes at once.
4 Quantum Russian roulette
This amounts to playing the role of Schrƶdingerās cat. Youāll need a gun whose firing is controlled by a quantum property, such as an atomās spin, which has two possible states when measured. If the Copenhagen interpretation is right, you have the familiar 50-50 odds of survival. The more times you āplayā, the less likely you are to survive.
If the multiverse is real, on the other hand, there always will be a universe in which āyouā are alive, no matter how long you play. Whatās more, you might always end up in it, thanks to the exalted status of the āobserverā in quantum mechanics. You would just hear a series of clicks as the gun failed to fire every time ā and realise youāre immortal. But be warned: even if you can get hold of a quantum gun, physicists have long argued about how this most decisive of experiments would actually work out.
This article appeared in print under the headline āLife in the multiverseā
