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
Iâm rich. Iâm a movie star. Iâm king of the world. Iâm also poor. Iâm homeless. Lots of me are dead.
Iâm none of these. Not in this universe. But in the multiverse Iâm all of them, and more.
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Iâm 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. Thereâs 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, Iâve just written a paragraph which explains that more clearly.
This worries me. If many worlds is correct â and many physicists think it is â my actions shape the course not just of my life, but of the lives of my duplicates in other worlds.
âIn the many-worlds interpretation, when you make a choice the other choices also happen,â says David Deutsch, a quantum physicist at the University of Oxford. âIf 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.â
Iâm alive, dead, rich and poor (Image: Lucas SimĂ”es)
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 Iâll 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 weâll 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 shouldnât 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, heâs thought long and hard about what it means to live in one. âI feel a strong kinship with parallel Maxes, even though I never get to meet them. They share my values, my feelings, my memories â theyâre closer to me than brothers,â he says.
Taking the cosmic perspective makes it difficult for Tegmark to feel sorry for himself: thereâs 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. âThe minimum tribute I can pay to that dead Max is to really think through what happened and learn some lessons.â
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. âMulti-universe physics has the same kind of experimental basis as the theory that there were once dinosaurs,â says Deutsch.
âMulti-universe physics has the same kind of experimental basis as dinosaursâ
Nor can we avoid its consequences. Every time we make a decision that involves probability â such as whether to take an umbrella in case of rain â 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 donât, and we get wet. The fundamental variability of the universe forces such choices upon us. âThereâs no escaping them,â says Albrecht.
Thatâs a momentous realisation. Weâre living in a time akin to Copernicus realising that Earth wasnât 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.
âThat these worlds are actually out there somewhere, but we cannot access them: I think thatâs an amazing and remarkable thing,â says physicist Seth Lloyd, a colleague of Tegmarkâs at MIT. âItâs sort of distressing really.â Why, I ask: because it diminishes humanityâs status even more?
âNo, not for that reason. Iâve always enjoyed the gradual marginalisation of humanity,â he says. âNo, it was somehow tidier to have the universe be the cosmos. But actually now Iâm liking it more, now that youâve pointed out that itâs really like the ultimate step in the marginalisation of human beings. I think thatâs much better. I enjoy that.â
Enjoyable though the multiverse might be as an concept, itâs tough for us humans to get our heads around its implications â even for physicists themselves. When Tegmarkâs wife was in labour with Philip, their eldest son, he found himself hoping that everything would go well. Then he admonished himself.
âIt 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?â He couldnât 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. âSo it doesnât make any sense to say âIâm hoping something about this numberâ. It is what it is.â
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 âgoodâ outcome, or you find yourself on a âbadâ branch. You canât wish your way on to a good one. Tegmark acknowledges this is not easy to live with. âItâs tough to get your emotions to sync with what you believe,â 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 fatherâs work had influenced him. âAlthough I consider myself an Everettian by default, itâs all beyond me for the most part, having inherited none of my fatherâs mathematical smarts,â says Mark, long-time frontman of the band Eels. âHow can I grasp any of it except in small moments? Iâm 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.â
I know how he feels. Perhaps a philosopher can help me take a broader perspective. I turn to David Papineau of Kingâs College London. âSay you put your money on a horse which you think is a very good bet,â he says. âIt turns out that it doesnât win, and you lose all your money. You think, âI wish I hadnât done that.â But you brought benefits to your cousins in other universes where the horse won. Youâve just drawn the short straw in finding yourself in the universe where it lost. You didnât do anything wrong. Thereâs no sense that the action you took earlier was a mistake.â
Hmm. I doubt âI didnât make a mistakeâ would get much traction with my partner if I bet all our savings on a horse this afternoon and find myself on the âwrongâ branch. But then, that wouldnât be the sensible thing to do â and one of the great attractions of Everettâs interpretation, according to Papineau, is that itâs not âmessyâ, 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 â the horse might fall, or just defy the odds and trail in last.
It offends Papineau that these two ways of being ârightâ â choosing wisely and getting lucky â donât go hand-in-hand. âThe 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,â he says. This doesnât 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. Itâs an elegant, if cold-blooded, way to look at things.
This elegance has always been part of the multiverseâs 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 â the so-called Copenhagen interpretation â 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 âmonstrosityâ, 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 â or in human terms, whenever we make a decision with multiple possible outcomes. Thatâs 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 Everettâs stance that collapsing the wave function is unnecessarily complicated. Whatâs more, for Page it has a happy side effect: it explains why his God tolerates the existence of evil.
âGod has values,â he says. âHe wants us to enjoy life, but he also wants to create an elegant universe.â To God the importance of elegance comes before that of suffering, which, Page infers, is why bad things happen. âGod wonât collapse the wave function to cure people of cancer, or prevent earthquakes or whatever, because that would make the universe much more inelegant.â
âGod wants us to enjoy life. But he also wants to create an elegant universeâ
For Page, that is an intellectually satisfying solution to the problem of evil. And whatâs more, many worlds may even take care of free will. Page doesnât 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. âIt doesnât mean that itâs fixed that I do one particular course of action. In the multiverse, Iâm doing all of them,â says Page.
There are limits to Pageâs 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 canât lose (see âFour aspects of the multiverseâ). Page thought about it, then declined: he didnât like the thought of his wifeâs distress in the worlds where he died.
Like Tegmark, Page seems to values the multiverse for the perspective it offers. âOne of my teenage children wants to get a motorcycle, which my wife and I think is pretty dangerous,â Page says. âBut if I say: âOkay, maybe most of the time youâd survive, but thereâs going to be some part of you, some branch, in which you get seriously maimed in a motorcycle accidentâ⊠Maybe Iâll try it.â
Double Deutsch
Iâm somewhat relieved to find that even many-worlds experts ultimately behave in much the same way as people who know nothing of it. But Iâve also realised that it shapes the way they think about their decisions. Perhaps itâs more natural for us to think about how our actions affect our âother selvesâ than about the arid probabilities of risk and reward.
If anyoneâs going to buck this trend, itâs 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.
âDecision 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,â he explains. âAnd 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.â 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, Deutschâs approach could be wrong, a possibility he accepts, though he is adamant the multiverse exists. But if heâs 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, itâs not big at all â and he just does it. âThe multiverse has definitely made me a happier person,â he says. âItâs given me courage to take chances to be bold in life.â
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: âLife in the multiverse means endless possibilitiesâ
Read more: âHugh Everett: The man who gave us the multiverseâ
Four aspects of the multiverse
1 The wave function
This mathematical entity describes the properties of any quantum system. Such properties â an atomâs direction of spin, say â 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 â 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 âcollapseâ when the measurement is taken, but it isnât clear how this happens. (Schrödingerâs 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 atomâs spin is up; in another universe, itâs 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 youâll 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 âcentral mysteryâ 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 âchooseâ. One way to explain the interference pattern through many worlds, by contrast, is that each photon only ever goes through only one slit â 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 âqubitsâ which can take many more states than the binary states available to the âbitsâ used by classical computers. In the multiverse interpretation, itâs because it conducts the necessary calculations in many universes at once.
4 Quantum Russian roulette
This amounts to playing the role of Schrödingerâs cat. Youâll need a gun whose firing is controlled by a quantum property, such as an atomâs 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 âplayâ, the less likely you are to survive.
If the multiverse is real, on the other hand, there always will be a universe in which âyouâ are alive, no matter how long you play. Whatâs more, you might always end up in it, thanks to the exalted status of the âobserverâ in quantum mechanics. You would just hear a series of clicks as the gun failed to fire every time â and realise youâre 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 âLife in the multiverseâ
