What hap­pens in par­al­lel uni­verses?

The ‘many-worlds’ isn't the only type of mul­ti­ple cos­mos con­sid­ered by physi­cists

All About Space - - Black Holes Are Leaking! -

Dan­ish physi­cist Niels Bohr was in­stru­men­tal in de­vel­op­ing the Copen­hagen in­ter­pre­ta­tion of quan­tum the­ory

Ai­dan Chatwin-Davies, from Cal­tech in Cal­i­for­nia, is an­other the­o­ret­i­cal physi­cist not fond of fire­walls. He has re­cently found an al­ter­na­tive way to aban­don a blaz­ing event hori­zon. He says all we have to do is think of black holes in terms of the many worlds in­ter­pre­ta­tion of quan­tum physics - an idea first de­vised by physi­cist Hugh Everett in the 1950s as an al­ter­na­tive way of think­ing about the weird sub-atomic world.

Quan­tum physics fa­mously says that a par­ti­cle can be in two places at once, or in two dif­fer­ent states si­mul­ta­ne­ously. The orig­i­nal in­ter­pre­ta­tion of this idea, favoured by Niels Bohr and de­vised in Den­mark, is known as the Copen­hagen in­ter­pre­ta­tion. It ar­gues that only once the par­ti­cle is mea­sured does it ‘de­cide’ which state to ap­pear in. How­ever, fel­low physi­cist Er­win Schrödinger de­vised his fa­mous Schrödinger's Cat thought ex­per­i­ment to show up holes in this ar­gu­ment.

The epony­mous fe­line is trapped in a sealed box with a ham­mer and a vial of poi­son. Whether or not the ham­mer falls to crack the vial de­pends on the out­come of a mea­sure­ment on a quan­tum par­ti­cle. The Copen­hagen in­ter­pre­ta­tion says that the par­ti­cle is si­mul­ta­ne­ously in both states at once un­til the mea­sure­ment is made. That means the ham­mer falls and doesn't fall and the cat is alive and dead un­til the par­ti­cle is mea­sured. But why does the act of mea­sur­ing force na­ture to choose? Everett's al­ter­na­tive ‘many-worlds‘ pic­ture was to sug­gest that it doesn't – both out­comes oc­cur.

The uni­verse splits into two dis­tinct ver­sions (or branches) – one where the cat lives and an­other where it per­ishes.

“If you are try­ing to de­scribe the for­ma­tion and evap­o­ra­tion of a black hole truly quan­tum grav­i­ta­tion­ally then you would ex­pect mul­ti­ple ver­sions of the black hole,” says Chatwin-Davies, just like there are two ver­sions of the cat. The im­pli­ca­tions for the in­for­ma­tion para­dox are pro­found. “If you're sit­ting around mon­i­tor­ing the Hawk­ing ra­di­a­tion com­ing out of a black hole you should ex­pect to see a loss of in­for­ma­tion,” he says. That's be­cause you're lim­ited to one of the many branches the black hole now ex­ists in. The in­for­ma­tion about an in­falling ob­ject isn't de­stroyed, it is sim­ply shared out across the many branches of re­al­ity. Throw Ham­let into a black hole and Act I may emerge in this uni­verse's Hawk­ing ra­di­a­tion, but Act II in an­other.

No­mura agrees. “Fo­cus on one world and clearly you can­not re­cover all the ini­tial in­for­ma­tion,” he says. What ef­fect does this have on the fire­wall? “The state­ment that you have to go smoothly into a black hole ap­plies only to each branch of the many worlds,” says No­mura. “Whereas the rules about quan­tum in­for­ma­tion ap­ply to the whole set of worlds.” Ac­cord­ing to No­mura, the Fire­wall Para­dox re­sults from con­fus­ing these dif­fer­ences. Chatwin-Davies is on the same page. Com­par­ing the two “is like com­par­ing ap­ples and or­anges,” he says.

So, as with many times in the his­tory of physics, an­swer­ing one ques­tion has thrown up sev­eral oth­ers. In­for­ma­tion fall­ing into a black hole may be im­printed as a holo­gram on the event hori­zon and car­ried back into space by Hawk­ing ra­di­a­tion. It could be that sev­er­ing the link be­tween the quan­tum par­ti­cles re­spon­si­ble for Hawk­ing ra­di­a­tion in­cin­er­ates you to a crisp as you en­ter, or that in­for­ma­tion could be hid­den in the Hawk­ing ra­di­a­tion af­ter all. Fi­nally, it could even be pos­si­ble that in­for­ma­tion fall­ing into a black hole is shared out among the many ver­sions of re­al­ity that splin­ter off as a black hole evolves. Un­til we crack the elu­sive code of quan­tum grav­ity, it is hard to know who is right.

“If you're mon­i­tor­ing the Hawk­ing ra­di­a­tion from a black hole you should ex­pect to see a loss of in­for­ma­tion”

ai­dan chatwin-Davies

Level 3 Where your fu­ture self ex­ists One ap­proach says that the uni­verse splin­ters into mul­ti­ple copies ev­ery time a quan­tum event takes place. This could make you im­mor­tal. Imag­ine hook­ing a gun to a ma­chine that fires upon a pos­i­tive re­sult of a 50:50 quan­tum mea­sure­ment. Ev­ery time a mea­sure­ment is made your uni­verse would splin­ter. As you're only able to per­ceive a uni­verse in which you didn't die, you'd be­lieve you'd sur­vived ev­ery mea­sure­ment. Level 1 Where an iden­ti­cal Earth ex­ists There is a limit to how far we can see into space. We can only see places from which light has had chance to reach us since the Big Bang. If you could ven­ture be­yond this cos­mic hori­zon you might end up reach­ing an­other part of the uni­verse where atoms are ar­ranged in pre­cisely the same fash­ion as they are here – an­other Earth and an­other you. Level 2 The ex­pand­ing uni­verse we can’t reachString the­ory – the idea that ev­ery­thing around us is made up of tiny vi­brat­ing strings – was the­o­rised to in at­tempt to com­bine the gen­eral the­ory of rel­a­tiv­ity and quan­tum the­ory. String the­o­rists need there to be seven ad­di­tional di­men­sions to the three of space and one of time that we ex­pe­ri­ence. Level 4 The uni­verse next door Cos­mol­o­gists in­tro­duced a mod­i­fi­ca­tion to the Big Bang the­ory in the 1980s to ad­dress some of its fail­ures. This patch is known as in­fla­tion, yet when they looked at what could have caused this to hap­pen they found that they couldn't get it to hap­pen just once. In­stead, eter­nal in­fla­tion is con­stantly cre­at­ing neigh­bour­ing uni­verses.Level 1Level 2Level 3Level 4

The ESA's Planck space tele­scope ob­served the CMB (in­set) for nearly 4.5 years

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