And the Winner is… Many-Worlds!

This is one of sev­eral short­ened in­dices into the Quan­tum Physics Se­quence.

Macro­scopic quan­tum su­per­po­si­tions, a.k.a. the “many-wor­lds in­ter­pre­ta­tion” or MWI, was pro­posed in 1957 and brought to the gen­eral at­ten­tion of the sci­en­tific com­mu­nity in 1970. Ever since, MWI has steadily gained in pop­u­lar­ity. As of 2008, MWI may or may not be en­dorsed by a ma­jor­ity of the­o­ret­i­cal physi­cists (at­tempted opinion polls con­flict on this point). Of course, Science is not sup­posed to be an opinion poll, but any­one who tells you that MWI is “sci­ence fic­tion” is sim­ply ig­no­rant.

When a the­ory is slowly per­suad­ing sci­en­tists de­spite all aca­demic in­er­tia, and more and more grad­u­ate stu­dents grow up fa­mil­iar with it, at what point should one go ahead and de­clare a tem­po­rary win­ner pend­ing new ev­i­dence?

Read­ing through the refer­enced posts will give you a very ba­sic in­tro­duc­tion to quan­tum me­chan­ics—alge­bra is in­volved, but no calcu­lus—by which you may nonethe­less gain an un­der­stand­ing suffi­cient to see, and not just be told, that the mod­ern case for many-wor­lds has be­come over­whelming. Not just plau­si­ble, not just strong, but over­whelming. Sin­gle-world ver­sions of quan­tum me­chan­ics just don’t work, and all the leg­endary con­fus­ing­ness and mys­te­ri­ous­ness of quan­tum me­chan­ics stems from this es­sen­tial fact. But enough tel­ling—let me show you.

  • Quan­tum Ex­pla­na­tions: Quan­tum me­chan­ics doesn’t de­serve its fear­some rep­u­ta­tion. If you tell peo­ple some­thing is sup­posed to be mys­te­ri­ous, they won’t un­der­stand it. It’s hu­man in­tu­itions that are “strange” or “weird”; physics it­self is perfectly nor­mal. Talk­ing about his­tor­i­cal er­ro­neous con­cepts like “par­ti­cles” or “waves” is just ask­ing to con­fuse peo­ple; pre­sent the real, unified quan­tum physics straight out. The se­ries will take a strictly re­al­ist per­spec­tive—quan­tum equa­tions de­scribe some­thing that is real and out there.

  • Con­figu­ra­tions and Am­pli­tude: A pre­limi­nary glimpse at the stuff re­al­ity is made of. The clas­sic split-pho­ton ex­per­i­ment with half-silvered mir­rors. Alter­na­tive path­ways the pho­ton can take, can can­cel each other out. The mys­te­ri­ous mea­sur­ing tool that tells us the rel­a­tive squared mod­uli.

  • Joint Con­figu­ra­tions: The laws of physics are in­her­ently over math­e­mat­i­cal en­tities, con­figu­ra­tions, that in­volve mul­ti­ple par­ti­cles. A ba­sic, on­tolog­i­cally ex­is­tent en­tity, ac­cord­ing to our cur­rent un­der­stand­ing of quan­tum me­chan­ics, does not look like a pho­ton—it looks like a con­figu­ra­tion of the uni­verse with “A pho­ton here, a pho­ton there.” Am­pli­tude flows be­tween these con­figu­ra­tions can can­cel or add; this gives us a way to de­tect which con­figu­ra­tions are dis­tinct.

  • Distinct Con­figu­ra­tions: Since con­figu­ra­tions are over the com­bined state of all the el­e­ments in a sys­tem, adding a sen­sor that de­tects whether a par­ti­cle went one way or the other, be­comes a new el­e­ment of the sys­tem that can make con­figu­ra­tions “dis­tinct” in­stead of “iden­ti­cal”. This con­fused the liv­ing daylights out of early quan­tum ex­per­i­menters, be­cause it meant that things be­haved differ­ently when they tried to “mea­sure” them. But it’s not only mea­sur­ing in­stru­ments that do the trick—any sen­si­tive phys­i­cal el­e­ment will do—and the dis­tinct­ness of con­figu­ra­tions is a phys­i­cal fact, not a fact about our knowl­edge. There is no need to sup­pose that the uni­verse cares what we think.

  • Where Philos­o­phy Meets Science: In ret­ro­spect, sup­pos­ing that quan­tum physics had any­thing to do with con­scious­ness was a big mis­take. Could philoso­phers have told the physi­cists so? But we don’t usu­ally see philoso­phers spon­sor­ing ma­jor ad­vances in physics; why not?

  • Can You Prove Two Par­ti­cles Are Iden­ti­cal?: You wouldn’t think that it would be pos­si­ble to do an ex­per­i­ment that told you that two par­ti­cles are com­pletely iden­ti­cal—not just to the limit of ex­per­i­men­tal pre­ci­sion, but perfectly. You could even give a pre­cise-sound­ing philo­soph­i­cal ar­gu­ment for why it was not pos­si­ble—but the ar­gu­ment would have a deeply buried as­sump­tion. Quan­tum physics vi­o­lates this deep as­sump­tion, mak­ing the ex­per­i­ment easy.

  • Clas­si­cal Con­figu­ra­tion Spaces: How to vi­su­al­ize the state of a sys­tem of two 1-di­men­sional par­ti­cles, as a sin­gle point in 2-di­men­sional space. A pre­limi­nary step be­fore mov­ing into...

  • The Quan­tum Arena: In­stead of a sys­tem state be­ing as­so­ci­ated with a sin­gle point in a clas­si­cal con­figu­ra­tion space, the in­stan­ta­neous real state of a quan­tum sys­tem is a com­plex am­pli­tude dis­tri­bu­tion over a quan­tum con­figu­ra­tion space. What cre­ates the illu­sion of “in­di­vi­d­ual par­ti­cles”, like an elec­tron caught in a trap, is a plaid dis­tri­bu­tion - one that hap­pens to fac­tor into the product of two parts. It is the whole dis­tri­bu­tion that evolves when a quan­tum sys­tem evolves. In­di­vi­d­ual con­figu­ra­tions don’t have physics; am­pli­tude dis­tri­bu­tions have physics. Quan­tum en­tan­gle­ment is the gen­eral case; quan­tum in­de­pen­dence is the spe­cial case.

  • Feyn­man Paths: In­stead of think­ing that a pho­ton takes a sin­gle straight path through space, we can re­gard it as tak­ing all pos­si­ble paths through space, and adding the am­pli­tudes for ev­ery pos­si­ble path. Nearly all the paths can­cel out—un­less we do clever quan­tum things, so that some paths add in­stead of can­cel­ing out. Then we can make light do funny tricks for us, like re­flect­ing off a mir­ror in such a way that the an­gle of in­ci­dence doesn’t equal the an­gle of re­flec­tion. But or­di­nar­ily, nearly all the paths ex­cept an ex­tremely nar­row band, can­cel out—this is one of the keys to re­cov­er­ing the hal­lu­ci­na­tion of clas­si­cal physics.

  • No In­di­vi­d­ual Par­ti­cles: One of the chief ways to con­fuse your­self while think­ing about quan­tum me­chan­ics, is to think as if pho­tons were lit­tle billiard balls bounc­ing around. The ap­pear­ance of lit­tle billiard balls is a spe­cial case of a deeper level on which there are only mul­ti­par­ti­cle con­figu­ra­tions and am­pli­tude flows. It is easy to set up phys­i­cal situ­a­tions in which there ex­ists no fact of the mat­ter as to which elec­tron was origi­nally which.

  • De­co­her­ence: A quan­tum sys­tem that fac­tor­izes can evolve into a sys­tem that doesn’t fac­tor­ize, de­stroy­ing the illu­sion of in­de­pen­dence. But en­tan­gling a quan­tum sys­tem with its en­vi­ron­ment, can ap­pear to de­stroy en­tan­gle­ments that are already pre­sent. En­tan­gle­ment with the en­vi­ron­ment can sep­a­rate out the pieces of an am­pli­tude dis­tri­bu­tion, pre­vent­ing them from in­ter­act­ing with each other. De­co­her­ence is fun­da­men­tally sym­met­ric in time, but ap­pears asym­met­ric be­cause of the sec­ond law of ther­mo­dy­nam­ics.

  • The So-Called Heisen­berg Uncer­tainty Prin­ci­ple: Un­like clas­si­cal physics, in quan­tum physics it is not pos­si­ble to sep­a­rate out a par­ti­cle’s “po­si­tion” from its “mo­men­tum”. The evolu­tion of the am­pli­tude dis­tri­bu­tion over time, in­volves things like tak­ing the sec­ond deriva­tive in space and mul­ti­ply­ing by i to get the first deriva­tive in time. The end re­sult of this time evolu­tion rule is that blobs of par­ti­cle-pres­ence ap­pear to race around in phys­i­cal space. The no­tion of “an ex­act par­tic­u­lar mo­men­tum” or “an ex­act par­tic­u­lar po­si­tion” is not some­thing that can phys­i­cally hap­pen, it is a tool for an­a­lyz­ing am­pli­tude dis­tri­bu­tions by tak­ing them apart into a sum of sim­pler waves. This uses the as­sump­tion and fact of lin­ear­ity: the evolu­tion of the whole wave­func­tion seems to always be the ad­di­tive sum of the evolu­tion of its pieces. Us­ing this tool, we can see that if you take apart the same dis­tri­bu­tion into a sum of po­si­tions and a sum of mo­menta, they can­not both be sharply con­cen­trated at the same time. When you “ob­serve” a par­ti­cle’s po­si­tion, that is, de­co­here its po­si­tional dis­tri­bu­tion by mak­ing it in­ter­act with a sen­sor, you take its wave packet apart into two pieces; then the two pieces evolve differ­ently. The Heisen­berg Prin­ci­ple definitely does not say that know­ing about the par­ti­cle, or con­sciously see­ing it, will make the uni­verse be­have differ­ently.

  • Belief in the Im­plied In­visi­ble: If a space­ship goes over the cos­molog­i­cal hori­zon rel­a­tive to us, so that it can no longer com­mu­ni­cate with us, should we be­lieve that the space­ship in­stantly ceases to ex­ist?

  • Where Physics Meets Ex­pe­rience: Meet the Eb­bo­ri­ans, who re­pro­duce by fis­sion. The Eb­bo­rian brain is like a thick sheet of pa­per that splits down its thick­ness. They fre­quently ex­pe­rience di­vid­ing into two minds, and can talk to their other selves. It seems that their unified the­ory of physics is al­most finished, and can an­swer ev­ery ques­tion, when one Eb­bo­rian asks: When ex­actly does one Eb­bo­rian be­come two peo­ple?

  • Where Ex­pe­rience Con­fuses Physi­cists: It then turns out that the en­tire planet of Eb­bore is split­ting along a fourth-di­men­sional thick­ness, du­pli­cat­ing all the peo­ple within it. But why does the ap­par­ent chance of “end­ing up” in one of those wor­lds, equal the square of the fourth-di­men­sional thick­ness? Many mys­te­ri­ous an­swers are pro­posed to this ques­tion, and one non-mys­te­ri­ous one.

  • On Be­ing De­co­her­ent: When a sen­sor mea­sures a par­ti­cle whose am­pli­tude dis­tri­bu­tion stretches over space—per­haps see­ing if the par­ti­cle is to the left or right of some di­vid­ing line—then the stan­dard laws of quan­tum me­chan­ics call for the sen­sor+par­ti­cle sys­tem to evolve into a state of (par­ti­cle left, sen­sor mea­sures LEFT) + (par­ti­cle right, sen­sor mea­sures RIGHT). But when we hu­mans look at the sen­sor, it only seems to say “LEFT” or “RIGHT”, never a mix­ture like “LIGFT”. This, of course, is be­cause we our­selves are made of par­ti­cles, and sub­ject to the stan­dard quan­tum laws that im­ply de­co­her­ence. Un­der stan­dard quan­tum laws, the fi­nal state is (par­ti­cle left, sen­sor mea­sures LEFT, hu­man sees “LEFT”) + (par­ti­cle right, sen­sor mea­sures RIGHT, hu­man sees “RIGHT”).

  • The Con­scious Sorites Para­dox: De­co­her­ence is im­plicit in quan­tum physics, not an ex­tra law on top of it. Ask­ing ex­actly when “one world” splits into “two wor­lds” may be like ask­ing when, if you keep re­mov­ing grains of sand from a pile, it stops be­ing a “heap”. Even if you’re in­side the world, there may not be a definite an­swer. This puz­zle does not arise only in quan­tum physics; the Eb­bo­ri­ans could face it in a clas­si­cal uni­verse, or we could build sen­tient flat com­put­ers that split down their thick­ness. Is this re­ally a physi­cist’s prob­lem?

  • De­co­herece is Pointless: There is no ex­act point at which de­co­her­ence sud­denly hap­pens. All of quan­tum me­chan­ics is con­tin­u­ous and differ­en­tiable, and de­co­her­ent pro­cesses are no ex­cep­tion to this.

  • De­co­her­ent Essences: De­co­her­ence is im­plicit within physics, not an ex­tra law on top of it. You can choose rep­re­sen­ta­tions that make de­co­her­ence harder to see, just like you can choose rep­re­sen­ta­tions that make ap­ples harder to see, but ex­actly the same phys­i­cal pro­cess still goes on; the ap­ple doesn’t dis­ap­pear and nei­ther does de­co­her­ence. If you could make de­co­her­ence mag­i­cally go away by choos­ing the right rep­re­sen­ta­tion, we wouldn’t need to shield quan­tum com­put­ers from the en­vi­ron­ment.

  • The Born Prob­a­bil­ities: The last se­ri­ous mys­te­ri­ous ques­tion left in quan­tum physics: When a quan­tum world splits in two, why do we seem to have a greater prob­a­bil­ity of end­ing up in the larger blob, ex­actly pro­por­tional to the in­te­gral of the squared mod­u­lus? It’s an open prob­lem, but non-mys­te­ri­ous an­swers have been pro­posed. Try not to go funny in the head about it.

  • De­co­her­ence as Pro­jec­tion: Since quan­tum evolu­tion is lin­ear and uni­tary, de­co­her­ence can be seen as pro­ject­ing a wave­func­tion onto or­thog­o­nal sub­spaces. This can be neatly illus­trated us­ing po­larized pho­tons and the an­gle of the po­larized sheet that will ab­sorb or trans­mit them.

  • En­tan­gled Pho­tons: Us­ing our newly ac­quired un­der­stand­ing of pho­ton po­lariza­tions, we see how to con­struct a quan­tum state of two pho­tons in which, when you mea­sure one of them, the per­son in the same world as you, will always find that the op­po­site pho­ton has op­po­site quan­tum state. This is not be­cause any in­fluence is trans­mit­ted; it is just de­co­her­ence that takes place in a very sym­met­ri­cal way, as can read­ily be ob­served in our calcu­la­tions.

  • Bell’s The­o­rem: No EPR “Real­ity”: (Note: This post was de­signed to be read as a stand-alone, if de­sired.) Origi­nally, the dis­cov­er­ers of quan­tum physics thought they had dis­cov­ered an in­com­plete de­scrip­tion of re­al­ity—that there was some deeper phys­i­cal pro­cess they were miss­ing, and this was why they couldn’t pre­dict ex­actly the re­sults of quan­tum ex­per­i­ments. The math of Bell’s The­o­rem is sur­pris­ingly sim­ple, and we walk through it. Bell’s The­o­rem rules out be­ing able to lo­cally pre­dict a sin­gle, unique out­come of mea­sure­ments—rul­ing out a way that Ein­stein, Podolsky, and Rosen once defined “re­al­ity”. This shows how deep im­plicit philo­soph­i­cal as­sump­tions can go. If wor­lds can split, so that there is no sin­gle unique out­come, then Bell’s The­o­rem is no prob­lem. Bell’s The­o­rem does, how­ever, rule out the idea that quan­tum physics de­scribes our par­tial knowl­edge of a deeper phys­i­cal state that could lo­cally pro­duce sin­gle out­comes—any such de­scrip­tion will be in­con­sis­tent.

  • Spooky Ac­tion at a Dis­tance: The No-Com­mu­ni­ca­tion The­o­rem: As Ein­stein ar­gued long ago, the quan­tum physics of his era—that is, the sin­gle-global-world in­ter­pre­ta­tion of quan­tum physics, in which ex­per­i­ments have sin­gle unique ran­dom re­sults—vi­o­lates Spe­cial Rel­a­tivity; it im­poses a preferred space of si­mul­tane­ity and re­quires a mys­te­ri­ous in­fluence to be trans­mit­ted faster than light; which mys­te­ri­ous in­fluence can never be used to trans­mit any use­ful in­for­ma­tion. Get­ting rid of the sin­gle global world dis­pels this mys­tery and puts ev­ery­thing back to nor­mal again.

  • De­co­her­ence is Sim­ple and

  • De­co­her­ence is Falsifi­able and Testable: An epis­tle to the physi­cists. To prob­a­bil­ity the­o­rists, words like “sim­ple”, “falsifi­able”, and “testable” have ex­act math­e­mat­i­cal mean­ings, which are there for very strong rea­sons. The (minor­ity?) fac­tion of physi­cists who say that many-wor­lds is “not falsifi­able” or that it “vi­o­lates Oc­cam’s Ra­zor” or that it is “untestable”, are com­mit­ting the same kind of math­e­mat­i­cal crime as non-physi­cists who in­vent their own the­o­ries of grav­ity that go as in­verse-cube. This is one of the rea­sons why I, a non-physi­cist, dared to talk about physics—be­cause I saw (some!) physi­cists us­ing prob­a­bil­ity the­ory in a way that was sim­ply wrong. Not just crit­i­ciz­able, but out­right math­e­mat­i­cally wrong: 2 + 2 = 3.

  • Quan­tum Non-Real­ism: “Shut up and calcu­late” is the best ap­proach you can take when none of your the­o­ries are very good. But that is not the same as claiming that “Shut up!” ac­tu­ally is a the­ory of physics. Say­ing “I don’t know what these equa­tions mean, but they seem to work” is a very differ­ent mat­ter from say­ing: “Th­ese equa­tions definitely don’t mean any­thing, they just work!”

  • Col­lapse Pos­tu­lates: Early physi­cists sim­ply didn’t think of the pos­si­bil­ity of more than one world—it just didn’t oc­cur to them, even though it’s the straight­for­ward re­sult of ap­ply­ing the quan­tum laws at all lev­els. So they ac­ci­den­tally in­vented a com­pletely and strictly un­nec­es­sary part of quan­tum the­ory to en­sure there was only one world—a law of physics that says that parts of the wave­func­tion mys­te­ri­ously and spon­ta­neously dis­ap­pear when de­co­her­ence pre­vents us from see­ing them any more. If such a law re­ally ex­isted, it would be the only non-lin­ear, non-uni­tary, non-differ­en­tiable, non-lo­cal, non-CPT-sym­met­ric, acausal, faster-than-light phe­nomenon in all of physics.

  • If Many-Wor­lds Had Come First: If early physi­cists had never made the mis­take, and thought im­me­di­ately to ap­ply the quan­tum laws at all lev­els to pro­duce macro­scopic de­co­her­ence, then “col­lapse pos­tu­lates” would to­day seem like a com­pletely crack­pot the­ory. In ad­di­tion to their other prob­lems, like FTL, the col­lapse pos­tu­late would be the only phys­i­cal law that was in­for­mally speci­fied—of­ten in du­al­is­tic (men­tal­is­tic) terms—be­cause it was the only fun­da­men­tal law adopted with­out pre­cise ev­i­dence to nail it down. Here, we get a glimpse at that al­ter­nate Earth.

  • Many Wor­lds, One Best Guess: Sum­ma­rizes the ar­gu­ments that nail down macro­scopic de­co­her­ence, aka the “many-wor­lds in­ter­pre­ta­tion”. Con­cludes that many-wor­lds wins out­right given the cur­rent state of ev­i­dence. The ar­gu­ment should have been over fifty years ago. New phys­i­cal ev­i­dence could re­open it, but we have no par­tic­u­lar rea­son to ex­pect this.

  • Liv­ing in Many Wor­lds: The many wor­lds of quan­tum me­chan­ics are not some strange, alien uni­verse into which you have been thrust. They are where you have always lived. Egan’s Law: “It all adds up to nor­mal­ity.” Then why care about quan­tum physics at all? Be­cause there’s still the ques­tion of what adds up to nor­mal­ity, and the an­swer to this ques­tion turns out to be, “Quan­tum physics.” If you’re think­ing of build­ing any strange philoso­phies around many-wor­lds, you prob­a­bly shouldn’t - that’s not what it’s for.