Living in Many Worlds

Some com­menters have re­cently ex­pressed dis­tur­bance at the thought of con­stantly split­ting into zillions of other peo­ple, as is the straight­for­ward and un­avoid­able pre­dic­tion of quan­tum me­chan­ics.

Others have con­fessed them­selves un­clear as to the im­pli­ca­tions of many-wor­lds for plan­ning: If you de­cide to buckle your seat belt in this world, does that in­crease the chance of an­other self un­buck­ling their seat belt? Are you be­ing self­ish at their ex­pense?

Just re­mem­ber Egan’s Law: It all adds up to nor­mal­ity.

(After Greg Egan, in Quaran­tine.[1])

Frank Sul­loway said [2]:

Iron­i­cally, psy­cho­anal­y­sis has it over Dar­winism pre­cisely be­cause its pre­dic­tions are so out­landish and its ex­pla­na­tions are so coun­ter­in­tu­itive that we think, Is that re­ally true? How rad­i­cal! Freud’s ideas are so in­trigu­ing that peo­ple are will­ing to pay for them, while one of the great dis­ad­van­tages of Dar­winism is that we feel we know it already, be­cause, in a sense, we do.

When Ein­stein over­threw the New­to­nian ver­sion of grav­ity, ap­ples didn’t stop fal­ling, planets didn’t swerve into the Sun. Every new the­ory of physics must cap­ture the suc­cess­ful pre­dic­tions of the old the­ory it dis­placed; it should pre­dict that the sky will be blue, rather than green.

So don’t think that many-wor­lds is there to make strange, rad­i­cal, ex­cit­ing pre­dic­tions. It all adds up to nor­mal­ity.

Then why should any­one care?

Be­cause there was once asked the ques­tion, fas­ci­nat­ing unto a ra­tio­nal­ist: What all adds up to nor­mal­ity?

And the an­swer to this ques­tion turns out to be: quan­tum me­chan­ics. It is quan­tum me­chan­ics that adds up to nor­mal­ity.

If there were some­thing else there in­stead of quan­tum me­chan­ics, then the world would look strange and un­usual.

Bear this in mind, when you are won­der­ing how to live in the strange new uni­verse of many wor­lds: You have always been there.

Reli­gions, an­thro­pol­o­gists tell us, usu­ally ex­hibit a prop­erty called min­i­mal coun­ter­in­tu­itive­ness; they are startling enough to be mem­o­rable, but not so bizarre as to be difficult to mem­o­rize. Anu­bis has the head of a dog, which makes him mem­o­rable, but the rest of him is the body of a man. Spirits can see through walls; but they still be­come hun­gry.

But physics is not a re­li­gion, set to sur­prise you just ex­actly enough to be mem­o­rable. The un­der­ly­ing phe­nom­ena are so coun­ter­in­tu­itive that it takes long study for hu­mans to come to grips with them. But the sur­face phe­nom­ena are en­tirely or­di­nary. You will never catch a glimpse of an­other world out of the cor­ner of your eye. You will never hear the voice of some other self. That is un­am­bigu­ously pro­hibited out­right by the laws. Sorry, you’re just schizophrenic.

The act of mak­ing de­ci­sions has no spe­cial in­ter­ac­tion with the pro­cess that branches wor­lds. In your mind, in your imag­i­na­tion, a de­ci­sion seems like a branch­ing point where the world could go two differ­ent ways. But you would feel just the same un­cer­tainty, vi­su­al­ize just the same al­ter­na­tives, if there were only one world. That’s what peo­ple thought for cen­turies be­fore quan­tum me­chan­ics, and they still vi­su­al­ized al­ter­na­tive out­comes that could re­sult from their de­ci­sions.

De­ci­sion and de­co­her­ence are en­tirely or­thog­o­nal con­cepts. If your brain never be­came de­co­her­ent, then that sin­gle cog­ni­tive pro­cess would still have to imag­ine differ­ent choices and their differ­ent out­comes. And a rock, which makes no de­ci­sions, obeys the same laws of quan­tum me­chan­ics as any­thing else, and splits fran­ti­cally as it lies in one place.

You don’t split when you come to a de­ci­sion in par­tic­u­lar, any more than you par­tic­u­larly split when you take a breath. You’re just split­ting all the time as the re­sult of de­co­her­ence, which has noth­ing to do with choices.

There is a pop­u­la­tion of wor­lds, and in each world, it all adds up to nor­mal­ity: ap­ples don’t stop fal­ling. In each world, peo­ple choose the course that seems best to them. Maybe they hap­pen on a differ­ent line of think­ing, and see new im­pli­ca­tions or miss oth­ers, and come to a differ­ent choice. But it’s not that one world chooses each choice. It’s not that one ver­sion of you chooses what seems best, and an­other ver­sion chooses what seems worst. In each world, ap­ples go on fal­ling and peo­ple go on do­ing what seems like a good idea.

Yes, you can nit­pick ex­cep­tions to this rule, but they’re nor­mal ex­cep­tions. It all adds up to nor­mal­ity, in all the wor­lds.

You can­not “choose which world to end up in.” In all the wor­lds, peo­ple’s choices de­ter­mine out­comes in the same way they would in just one sin­gle world.

The choice you make here does not have some strange bal­anc­ing in­fluence on some world el­se­where. There is no causal com­mu­ni­ca­tion be­tween de­co­her­ent wor­lds. In each world, peo­ple’s choices con­trol the fu­ture of that world, not some other world.

If you can imag­ine de­ci­sion­mak­ing in one world, you can imag­ine de­ci­sion-mak­ing in many wor­lds: just have the world con­stantly split­ting while oth­er­wise obey­ing all the same rules.

In no world does two plus two equal five. In no world can space­ships travel faster than light. All the quan­tum wor­lds obey our laws of physics; their ex­is­tence is as­serted in the first place by our laws of physics. Since the be­gin­ning, not one un­usual thing has ever hap­pened, in this or any other world. They are all lawful.

Are there hor­rible wor­lds out there, which are ut­terly be­yond your abil­ity to af­fect? Sure. And hor­rible things hap­pened dur­ing the twelfth cen­tury, which are also be­yond your abil­ity to af­fect. But the twelfth cen­tury is not your re­spon­si­bil­ity, be­cause it has, as the quaint phrase goes, “already hap­pened.” I would sug­gest that you con­sider ev­ery world that is not in your fu­ture to be part of the “gen­er­al­ized past.”

Live in your own world. Be­fore you knew about quan­tum physics, you would not have been tempted to try liv­ing in a world that did not seem to ex­ist. Your de­ci­sions should add up to this same nor­mal­ity: you shouldn’t try to live in a quan­tum world you can’t com­mu­ni­cate with.

Your de­ci­sion the­ory should (al­most always) be the same, whether you sup­pose that there is a 90% prob­a­bil­ity of some­thing hap­pen­ing, or if it will hap­pen in 9 out of 10 wor­lds. Now, be­cause peo­ple have trou­ble han­dling prob­a­bil­ities, it may be helpful to vi­su­al­ize some­thing hap­pen­ing in 9 out of 10 wor­lds. But this just helps you use nor­mal de­ci­sion the­ory.

Now is a good time to be­gin learn­ing how to shut up and mul­ti­ply. As I note in Lot­ter­ies: A Waste of Hope:

The hu­man brain doesn’t do 64-bit float­ing-point ar­ith­metic, and it can’t de­value the emo­tional force of a pleas­ant an­ti­ci­pa­tion by a fac­tor of 0.00000001 with­out drop­ping the line of rea­son­ing en­tirely.

And in New Im­proved Lot­tery:

Between zero chance of be­com­ing wealthy, and ep­silon chance, there is an or­der-of-ep­silon differ­ence. If you doubt this, let ep­silon equal one over googol­plex.

If you’re think­ing about a world that could arise in a lawful way, but whose prob­a­bil­ity is a quadrillion to one, and some­thing very pleas­ant or very awful is hap­pen­ing in this world . . . well, it does prob­a­bly ex­ist, if it is lawful. But you should try to re­lease one quadrillionth as many neu­ro­trans­mit­ters, in your re­ward cen­ters or your aver­sive cen­ters, so that you can weigh that world ap­pro­pri­ately in your de­ci­sions. If you don’t think you can do that . . . don’t bother think­ing about it.

Other­wise you might as well go out and buy a lot­tery ticket us­ing a quan­tum ran­dom num­ber, a strat­egy that is guaran­teed to re­sult in a very tiny mega-win.

Or here’s an­other way of think­ing about it: Are you con­sid­er­ing ex­pend­ing some men­tal en­ergy on a world whose fre­quency in your fu­ture is less than a trillionth? Then go get a 10-sided die from your lo­cal gam­ing store, and, be­fore you be­gin think­ing about that strange world, start rol­ling the die. If the die comes up 9 twelve times in a row, then you can think about that world. Other­wise don’t waste your time; thought-time is a re­source to be ex­pended wisely.

You can roll the dice as many times as you like, but you can’t think about the world un­til 9 comes up twelve times in a row. Then you can think about it for a minute. After that you have to start rol­ling the die again.

This may help you to ap­pre­ci­ate the con­cept of “trillion to one” on a more visceral level.

If at any point you catch your­self think­ing that quan­tum physics might have some kind of strange, ab­nor­mal im­pli­ca­tion for ev­ery­day life—then you should prob­a­bly stop right there.

Oh, there are a few im­pli­ca­tions of many-wor­lds for ethics. Aver­age util­i­tar­i­anism sud­denly looks a lot more at­trac­tive—you don’t need to worry about cre­at­ing as many peo­ple as pos­si­ble, be­cause there are already plenty of peo­ple ex­plor­ing per­son-space. You just want the av­er­age qual­ity of life to be as high as pos­si­ble, in the fu­ture wor­lds that are your re­spon­si­bil­ity.

And you should always take joy in dis­cov­ery, as long as you per­son­ally don’t know a thing. It is mean­ingless to talk of be­ing the “first” or the “only” per­son to know a thing, when ev­ery­thing know­able is known within wor­lds that are in nei­ther your past nor your fu­ture, and are nei­ther be­fore or af­ter you.

But, by and large, it all adds up to nor­mal­ity. If your un­der­stand­ing of many-wor­lds is the tiniest bit shaky, and you are con­tem­plat­ing whether to be­lieve some strange propo­si­tion, or feel some strange emo­tion, or plan some strange strat­egy, then I can give you very sim­ple ad­vice: Don’t.

The quan­tum uni­verse is not a strange place into which you have been thrust. It is the way things have always been.


1. Greg Egan, Quaran­tine (Lon­don: Le­gend Press, 1992).

2. Robert S. Boyn­ton, “The Birth of an Idea: A Pro­file of Frank Sul­loway,” The New Yorker (Oc­to­ber 1999).