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
I鈥檓 rich. I鈥檓 a movie star. I鈥檓 king of the world. I鈥檓 also poor. I鈥檓 homeless. Lots of me are dead.
I鈥檓 none of these. Not in this universe. But in the multiverse I鈥檓 all of them, and more.
Advertisement
I鈥檓 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. There鈥檚 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, I鈥檝e just written a paragraph which explains that more clearly.
This worries me. If many worlds is correct 鈥 and many physicists think it is 鈥 my actions shape the course not just of my life, but of the lives of my duplicates in other worlds.
鈥淚n the many-worlds interpretation, when you make a choice the other choices also happen,鈥 says David Deutsch, a quantum physicist at the University of Oxford. 鈥淚f 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.鈥
I鈥檓 alive, dead, rich and poor (Image: Lucas Sim玫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 I鈥檒l 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 we鈥檒l 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 shouldn鈥檛 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, he鈥檚 thought long and hard about what it means to live in one. 鈥淚 feel a strong kinship with parallel Maxes, even though I never get to meet them. They share my values, my feelings, my memories 鈥 they鈥檙e closer to me than brothers,鈥 he says.
Taking the cosmic perspective makes it difficult for Tegmark to feel sorry for himself: there鈥檚 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. 鈥淭he minimum tribute I can pay to that dead Max is to really think through what happened and learn some lessons.鈥
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. 鈥淢ulti-universe physics has the same kind of experimental basis as the theory that there were once dinosaurs,鈥 says Deutsch.
鈥淢ulti-universe physics has the same kind of experimental basis as dinosaurs鈥
Nor can we avoid its consequences. Every time we make a decision that involves probability 鈥 such as whether to take an umbrella in case of rain 鈥 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 don鈥檛, and we get wet. The fundamental variability of the universe forces such choices upon us. 鈥淭here鈥檚 no escaping them,鈥 says Albrecht.
That鈥檚 a momentous realisation. We鈥檙e living in a time akin to Copernicus realising that Earth wasn鈥檛 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.
鈥淭hat these worlds are actually out there somewhere, but we cannot access them: I think that鈥檚 an amazing and remarkable thing,鈥 says physicist Seth Lloyd, a colleague of Tegmark鈥檚 at MIT. 鈥淚t鈥檚 sort of distressing really.鈥 Why, I ask: because it diminishes humanity鈥檚 status even more?
鈥淣o, not for that reason. I鈥檝e always enjoyed the gradual marginalisation of humanity,鈥 he says. 鈥淣o, it was somehow tidier to have the universe be the cosmos. But actually now I鈥檓 liking it more, now that you鈥檝e pointed out that it鈥檚 really like the ultimate step in the marginalisation of human beings. I think that鈥檚 much better. I enjoy that.鈥
Enjoyable though the multiverse might be as an concept, it鈥檚 tough for us humans to get our heads around its implications 鈥 even for physicists themselves. When Tegmark鈥檚 wife was in labour with Philip, their eldest son, he found himself hoping that everything would go well. Then he admonished himself.
鈥淚t 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?鈥 He couldn鈥檛 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. 鈥淪o it doesn鈥檛 make any sense to say 鈥業鈥檓 hoping something about this number鈥. It is what it is.鈥
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 鈥済ood鈥 outcome, or you find yourself on a 鈥渂ad鈥 branch. You can鈥檛 wish your way on to a good one. Tegmark acknowledges this is not easy to live with. 鈥淚t鈥檚 tough to get your emotions to sync with what you believe,鈥 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 father鈥檚 work had influenced him. 鈥淎lthough I consider myself an Everettian by default, it鈥檚 all beyond me for the most part, having inherited none of my father鈥檚 mathematical smarts,鈥 says Mark, long-time frontman of the band Eels. 鈥淗ow can I grasp any of it except in small moments? I鈥檓 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.鈥
I know how he feels. Perhaps a philosopher can help me take a broader perspective. I turn to David Papineau of King鈥檚 College London. 鈥淪ay you put your money on a horse which you think is a very good bet,鈥 he says. 鈥淚t turns out that it doesn鈥檛 win, and you lose all your money. You think, 鈥業 wish I hadn鈥檛 done that.鈥 But you brought benefits to your cousins in other universes where the horse won. You鈥檝e just drawn the short straw in finding yourself in the universe where it lost. You didn鈥檛 do anything wrong. There鈥檚 no sense that the action you took earlier was a mistake.鈥
Hmm. I doubt 鈥淚 didn鈥檛 make a mistake鈥 would get much traction with my partner if I bet all our savings on a horse this afternoon and find myself on the 鈥渨rong鈥 branch. But then, that wouldn鈥檛 be the sensible thing to do 鈥 and one of the great attractions of Everett鈥檚 interpretation, according to Papineau, is that it鈥檚 not 鈥渕essy鈥, 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 鈥 the horse might fall, or just defy the odds and trail in last.
It offends Papineau that these two ways of being 鈥渞ight鈥 鈥 choosing wisely and getting lucky 鈥 don鈥檛 go hand-in-hand. 鈥淭he 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,鈥 he says. This doesn鈥檛 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. It鈥檚 an elegant, if cold-blooded, way to look at things.
This elegance has always been part of the multiverse鈥檚 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 鈥 the so-called Copenhagen interpretation 鈥 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 鈥渕onstrosity鈥, 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 鈥 or in human terms, whenever we make a decision with multiple possible outcomes. That鈥檚 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 Everett鈥檚 stance that collapsing the wave function is unnecessarily complicated. What鈥檚 more, for Page it has a happy side effect: it explains why his God tolerates the existence of evil.
鈥淕od has values,鈥 he says. 鈥淗e wants us to enjoy life, but he also wants to create an elegant universe.鈥 To God the importance of elegance comes before that of suffering, which, Page infers, is why bad things happen. 鈥淕od won鈥檛 collapse the wave function to cure people of cancer, or prevent earthquakes or whatever, because that would make the universe much more inelegant.鈥
鈥淕od wants us to enjoy life. But he also wants to create an elegant universe鈥
For Page, that is an intellectually satisfying solution to the problem of evil. And what鈥檚 more, many worlds may even take care of free will. Page doesn鈥檛 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. 鈥淚t doesn鈥檛 mean that it鈥檚 fixed that I do one particular course of action. In the multiverse, I鈥檓 doing all of them,鈥 says Page.
There are limits to Page鈥檚 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 can鈥檛 lose (see 鈥Four aspects of the multiverse鈥). Page thought about it, then declined: he didn鈥檛 like the thought of his wife鈥檚 distress in the worlds where he died.
Like Tegmark, Page seems to values the multiverse for the perspective it offers. 鈥淥ne of my teenage children wants to get a motorcycle, which my wife and I think is pretty dangerous,鈥 Page says. 鈥淏ut if I say: 鈥極kay, maybe most of the time you鈥檇 survive, but there鈥檚 going to be some part of you, some branch, in which you get seriously maimed in a motorcycle accident鈥欌 Maybe I鈥檒l try it.鈥
Double Deutsch
I鈥檓 somewhat relieved to find that even many-worlds experts ultimately behave in much the same way as people who know nothing of it. But I鈥檝e also realised that it shapes the way they think about their decisions. Perhaps it鈥檚 more natural for us to think about how our actions affect our 鈥渙ther selves鈥 than about the arid probabilities of risk and reward.
If anyone鈥檚 going to buck this trend, it鈥檚 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.
鈥淒ecision 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,鈥 he explains. 鈥淎nd 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.鈥 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, Deutsch鈥檚 approach could be wrong, a possibility he accepts, though he is adamant the multiverse exists. But if he鈥檚 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, it鈥檚 not big at all 鈥 and he just does it. 鈥淭he multiverse has definitely made me a happier person,鈥 he says. 鈥淚t鈥檚 given me courage to take chances to be bold in life.鈥
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: 鈥Life in the multiverse means endless possibilities鈥
Read more: 鈥Hugh Everett: The man who gave us the multiverse鈥
Four aspects of the multiverse
1 The wave function
This mathematical entity describes the properties of any quantum system. Such properties 鈥 an atom鈥檚 direction of spin, say 鈥 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 鈥 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 鈥渃ollapse鈥 when the measurement is taken, but it isn鈥檛 clear how this happens. (Schr枚dinger鈥檚 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 atom鈥檚 spin is up; in another universe, it鈥檚 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 you鈥檒l 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 鈥渃entral mystery鈥 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 鈥渃hoose鈥. One way to explain the interference pattern through many worlds, by contrast, is that each photon only ever goes through only one slit 鈥 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 鈥渜ubits鈥 which can take many more states than the binary states available to the 鈥渂its鈥 used by classical computers. In the multiverse interpretation, it鈥檚 because it conducts the necessary calculations in many universes at once.
4 Quantum Russian roulette
This amounts to playing the role of Schr枚dinger鈥檚 cat. You鈥檒l need a gun whose firing is controlled by a quantum property, such as an atom鈥檚 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 鈥減lay鈥, the less likely you are to survive.
If the multiverse is real, on the other hand, there always will be a universe in which 鈥測ou鈥 are alive, no matter how long you play. What鈥檚 more, you might always end up in it, thanks to the exalted status of the 鈥渙bserver鈥 in quantum mechanics. You would just hear a series of clicks as the gun failed to fire every time 鈥 and realise you鈥檙e 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 鈥淟ife in the multiverse鈥
