In­for­ma­tion en­ter­ing one of these high-grav­ity ob­jects might not be de­stroyed but ooz­ing into an­other cos­mos en­tirely

All About Space - - Contents - Writ­ten by Colin Stu­art

In­vis­i­ble, enig­matic and in­fu­ri­at­ing, black holes are as­tound­ing. Formed from the ex­plo­sive deaths of the most mas­sive stars, they push our very un­der­stand­ing of space and time to its limit. They are re­gions of such con­cen­trated grav­ity that es­cap­ing from their clutches is im­pos­si­ble for those ven­tur­ing too close. Once you've crossed the event hori­zon, you'd have to travel faster than the speed of light to es­cape but noth­ing can travel faster than the speed of light. Breach the event hori­zon and you're doomed to obliv­ion. What's more, you can­not hail any­one for help.

These mon­sters are so vex­ing be­cause at var­i­ous times they are both big and small. They start as the size of a star, where Ein­stein's gen­eral the­ory of rel­a­tiv­ity rules the roost. But, as the core of the dead star col­lapses to form the black hole, mat­ter is con­cen­trated down into an ever-smaller space. Even­tu­ally it moves into a realm dom­i­nated by the rules of the su­per-small – the weird and won­der­ful world of quan­tum physics.

Both of these the­o­ries have rightly been lauded for their in­di­vid­ual ex­plana­tory power. Ein­stein pub­lished his revo­lu­tion­ary the­ory in 1915 and so far it has passed ev­ery test thrown at it with fly­ing colours. The re­cent dis­cov­ery of the grav­i­ta­tional waves it pre­dicted was a real tri­umph. Equally, our modern tech­no­log­i­cal age is built on a thor­ough un­der­stand­ing of quan­tum physics. Yet physi­cists can­not get the two the­o­ries to play to­gether nicely. There is no cur­rently ac­cepted the­ory of “quan­tum grav­ity” that com­bines the two neatly on the same scale. Black holes in par­tic­u­lar em­bar­rass us by con­fronting us with the re­al­ity of this dilemma.

One of the most fa­mous at­tempts to rec­on­cile the two the­o­ries with the physics of black holes was pro­vided by Stephen Hawk­ing in 1974. In a well-stud­ied quan­tum phe­nom­e­non, a pair of sub­atomic par­ti­cles can si­mul­ta­ne­ously pop into ex­is­tence as long as they dis­ap­pear again very quickly. Hawk­ing imag­ined this hap­pen­ing right on the event hori­zon of a black hole. One par­ti­cle is doomed, the other is free to es­cape. They can never be re­united, mean­ing a black hole must slowly lose en­ergy to its im­me­di­ate en­vi­ron­ment. Ac­cord­ing to Hawk­ing, black holes evap­o­rate over time in this way through the emis­sion of one half of these par­ti­cle pairs – an ef­fect known as Hawk­ing ra­di­a­tion.

How­ever, that idea im­me­di­ately threw up a prob­lem be­cause his cal­cu­la­tions showed that the na­ture of Hawk­ing ra­di­a­tion de­pends solely on the mass of the black hole. Ya­sunori No­mura, a re­searcher at the Berke­ley Center for The­o­ret­i­cal Physics, likes to imag­ine throw­ing two books into the void. “One is Shake­speare, the other is Pent­house,” he says. While both books con­tain dif­fer­ent words, they both have ex­actly the same mass. As it only de­pends on the mass of the black hole, No­mura says the re­sult­ing Hawk­ing ra­di­a­tion is iden­ti­cal in both cases. “It looks like the in­for­ma­tion about whether it was Shake­speare or Pent­house is com­pletely lost,” he says. Quan­tum

Stephen Hawk­ing was one of the first to suc­cess­fully ap­ply quan­tum physics to black holes

“A black hole can never com­pletely evap­o­rate away. In­stead, a mi­nus­cule husk would al­ways re­main”

physics says that in­for­ma­tion can­not be cre­ated or de­stroyed. So where does the in­for­ma­tion go? This prob­lem has be­come known as the ‘Black Hole In­for­ma­tion Para­dox’.

Many physi­cists have wres­tled with how to solve this thorny is­sue. In 2015, Hawk­ing him­self de­tailed a new idea, re-ex­plor­ing the no­tion he'd had 40 years ear­lier. His rad­i­cal so­lu­tion to the in­for­ma­tion para­dox is that the in­for­ma­tion con­tained within the two books never ac­tu­ally makes it into the black hole. “I pro­pose that the in­for­ma­tion is stored not in the in­te­rior of the black hole as one might ex­pect, but on its bound­ary, the event hori­zon,” he said at a con­fer­ence in Swe­den on Hawk­ing ra­di­a­tion held that year. Ac­cord­ing to Hawk­ing, in­for­ma­tion about three-di­men­sional ob­jects fall­ing in ends up en­coded as a two-di­men­sional holo­gram on the event hori­zon. Later, out­go­ing Hawk­ing ra­di­a­tion re-de­liv­ers this in­for­ma­tion back into the uni­verse. Given enough time, some­one would, in prin­ci­ple, be able to re­cover the in­for­ma­tion con­tained within the books. Hawk­ing would go on to tell the con­fer­ence that black holes are not the eter­nal pris­ons they were once thought to be.

No­bel prize-win­ning physi­cist Ger­ard ’t Hooft has an­other idea. An ob­ject cross­ing the event hori­zon will be­gin to feel dra­matic changes in its grav­i­ta­tional field. Hawk­ing ra­di­a­tion will be af­fected by these grav­i­ta­tional changes and so carry out with it in­for­ma­tion about what the in­com­ing ob­ject was. How­ever, both Hawk­ing and ’t Hooft's ideas have a sig­nif­i­cant snag: quan­tum physics not only for­bids in­for­ma­tion from be­ing de­stroyed, it also out­laws it be­ing du­pli­cated. The ob­ject fall­ing in will carry one copy of its in­for­ma­tion, while an­other ei­ther sits as a holo­gram on the event hori­zon or is car­ried out­wards by Hawk­ing ra­di­a­tion. The mystery is far from solved.

Other re­searchers found a less dras­tic ray of hope when they dis­cov­ered a way that Hawk­ing ra­di­a­tion might pre­serve the in­for­ma­tion con­tained within ob­jects added to the black hole with­out the need for holo­grams or du­pli­cates. How­ever, they could only get this to hap­pen by dra­mat­i­cally sev­er­ing the quan­tum link be­tween the two par­ti­cles that ini­tially cre­ated the Hawk­ing ra­di­a­tion. Cut­ting the cord would lead to a sud­den burst of en­ergy. With this process hap­pen­ing all along the event hori­zon, cross­ing over it would be like en­ter­ing hell. You'd soon be in­cin­er­ated by what physi­cists have a dubbed a ‘fire­wall’. This cre­ates a new para­dox. Ein­stein's gen­eral the­ory of rel­a­tiv­ity for­bids any­thing spe­cial hap­pen­ing when you cross over the event hori­zon. Like the Earth's equa­tor, it is a purely math­e­mat­i­cal line. Why should you be set alight just be­cause you pass from the equiv­a­lent of

“The in­for­ma­tion is stored not in the in­te­rior of the black hole, but on its bound­ary, the event hori­zon”

stephen hawk­ing

one hemi­sphere into an­other? Physi­cists call this ‘The Fire­wall Para­dox’. Ap­ply­ing quan­tum physics to black holes sug­gests the ex­is­tence of Hawk­ing ra­di­a­tion. At first that im­plied in­for­ma­tion can be de­stroyed

– The In­for­ma­tion Para­dox – un­less cross­ing the event hori­zon singes you into a ball of smoke – The Fire­wall Para­dox.

“I'm just not com­fort­able with this idea,” says Ana Alonso-Ser­rano at the Max Planck In­sti­tute for Grav­i­ta­tional Physics in Ger­many. She's been look­ing for an al­ter­na­tive way out and now be­lieves she may have found one. “You don't need a fire­wall,” she says. To come to this con­clu­sion, Alon­soSer­rano looked at some of the cur­rent mod­els for how quan­tum grav­ity might work. She specif­i­cally in­ves­ti­gated some­thing called the Gen­er­alised Un­cer­tainty Prin­ci­ple (GUP), which says the more you know about a black hole's size the less you know about its en­ergy. Her work shows that more and more Hawk­ing ra­di­a­tion would be given off as the black hole evap­o­rates, chang­ing the amount of in­for­ma­tion it car­ries away. “In­for­ma­tion isn't lost

– it is hid­den in the Hawk­ing ra­di­a­tion,” she says. Alonso-Ser­rano ad­mits that her so­lu­tion “is not a com­plete res­o­lu­tion” to the prob­lem, but it has the po­ten­tial to elim­i­nate the pesky fire­wall. Her work also shows that a black hole can never com­pletely evap­o­rate away. In­stead, a mi­nus­cule husk would al­ways re­main.

Ac­cord­ing to Hawk­ing, a black hole should gently glow in Hawk­ing ra­di­a­tion

Rather than sim­ply swal­low­ing you up, could fall­ing into a black hole send you to a par­al­lel uni­verse?

Ger­ard 't Hooft thinks the grav­ity of in­falling ob­jects im­prints a black hole's Hawk­ing

ra­di­a­tion The dis­cov­ery of grav­i­ta­tional waves in 2015 fi­nally con­firmed a ma­jor pre­dic­tion of Ein­stein's gen­eral the­ory of rel­a­tiv­ity

It's pos­si­ble that ev­ery quan­tum event frac­tures the uni­verse into copies

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