When black holes turn white

Can bounc­ing black holes help physi­cists find the ul­ti­mate the­ory of ev­ery­thing?

All About Space - - Contents - Re­ported by Colin Stuart

Has a new de­tec­tion fi­nally re­vealed the ul­ti­mate the­ory of life, the uni­verse and ev­ery­thing?

Some­where out there in the vast­ness of space lurks a black hole smaller than the full stop at the end of this sen­tence. Mi­nus­cule but mighty, it could hold the key to un­lock­ing some of the great­est mys­ter­ies in the uni­verse.

Black holes are the ul­ti­mate cos­mic lab­o­ra­tory, a way for physi­cists to test out their the­o­ries in an en­vi­ron­ment so ex­treme that space and time are curved and warped. Even light can­not re­sist their eter­nal grasp, so we see no light re­flected from them at all. We can only spot them when their grav­ity af­fects some­thing vis­i­ble or they merge to cre­ate grav­i­ta­tional waves. Few places have such a high amount of en­ergy in such a small space.

But what hap­pens if you fall into one? The bad news is you’re un­likely to sur­vive the or­deal. The dif­fer­ence in grav­ity between your feet and your head would even­tu­ally get so ex­treme that it would over­come the forces hold­ing your atoms to­gether. You’d be torn apart into thin strips of hu­man spaghetti, which is where the process gets its whim­si­cal name: spaghet­ti­fi­ca­tion. Where do your spaghet­ti­fied atoms ul­ti­mately end up? What’s at the bot­tom of a black hole?

“With a black hole you get sucked in, but with a white hole things can only come out”

Francesca Vi­dotto

Our best an­swer cur­rently comes from our lead­ing the­ory of grav­ity: Ein­stein’s Gen­eral The­ory of Rel­a­tiv­ity. It tells us that a sin­gu­lar­ity awaits – an in­fin­itely small, in­fin­itely dense point where space and time cease to be. Hit it and you’re im­me­di­ately erased from ex­is­tence. Yet if you crush some­thing down much smaller than an atom you en­ter the arena of quan­tum physics. At the mo­ment we’re yet to take its weird and won­der­ful rules into ac­count at the bot­tom of black holes be­cause we have no way of com­bin­ing it with gen­eral rel­a­tiv­ity. The search for such a the­ory of ‘quan­tum grav­ity’ is the ul­ti­mate goal for many physi­cists. A No­bel Prize would surely be in the off­ing for any­one who finds one that ac­cu­rately de­scribes our uni­verse. It might also help us ex­plain where our cos­mos came from be­cause, ac­cord­ing to gen­eral rel­a­tiv­ity, the other place you find a sin­gu­lar­ity is at the mo­ment of cre­ation – the Big Bang – where time and space sprang into ex­is­tence.

Carlo Rovelli, di­rec­tor of the quan­tum grav­ity group at Aix-Mar­seille Univer­sity in France, doesn’t be­lieve in sin­gu­lar­i­ties. “You can­not com­press things too much,” he says. “It is a uni­ver­sal thing in na­ture.” He ar­gues we need quan­tum grav­ity to help ex­plain what hap­pens in­stead. Rovelli is a founder of one ap­proach to this thorny prob­lem of get­ting the two the­o­ries to play nicely to­gether: loop quan­tum grav­ity (LQG). Ac­cord­ing to Ein­stein, the fab­ric of space-time is smooth. How­ever, pro­po­nents of LQG sug­gest that it isn’t. “That’s not sur­pris­ing,” says Rovelli. “Other things in the uni­verse like light and the en­ergy of elec­trons come in chunks.” He sug­gests space is not smooth, but grainy – it’s also made of tiny lit­tle chunks or loops. Think of it like a piece of cloth; at first glance it may seem smooth, but look at it un­der a mi­cro­scope and you’ll see that it’s re­ally made of a se­ries of stitches.

If you ap­ply this logic to the depths of a black hole you get a re­mark­able re­sult. Oc­ca­sion­ally a black hole might ’bounce’ into its polar op­po­site: a white hole. “With a black hole you get sucked in, but with a white hole things can only come out,” says Francesca Vi­dotto from Rad­boud Univer­sity in The Nether­lands.

What ex­actly trig­gers the change? Ac­cord­ing to Vi­dotto it is sim­ple chance. Quan­tum physics is de­fined by prob­a­bil­ity. You can never say ex­actly where an ob­ject is or what state it is in, only where it is more likely to be when you make a mea­sure­ment. But the smaller an ob­ject, the more likely it is for un­usual things to hap­pen. Vi­dotto says an ob­ject has a timescale over which it can dis­play these weird quan­tum prop­er­ties. “For large ob­jects, like a per­son or a cat, this time is much larger than the age of the uni­verse,” she says. “For a planet-sized black hole it is about the age of the uni­verse.” But for a black hole just half a mil­lime­tre across you’d ex­pect it to have hap­pened fairly of­ten al­ready across the cos­mos. We nor­mally think of black holes as much big­ger than that – formed by the deaths of the most mas­sive stars. How­ever, as­tronomers also imag­ine there may be pri­mor­dial black holes out there. Tiny ones formed in the early uni­verse shortly after the Big Bang. Some of those could now be mak­ing this odd tran­si­tion into a white hole.

If that’s true we should be able to see ev­i­dence of it hap­pen­ing with our tele­scopes. “You would ex­pect an ex­plo­sion,” says Vi­dotto. Such a det­o­na­tion would trig­ger the rapid re­lease of huge

amounts of en­ergy. How en­er­getic this ra­di­a­tion is depends on the size of the black hole. For black holes the size of your hand or smaller you’d ex­pect it to be the ra­dio part of the spec­trum. And over the last decade as­tronomers have found a hand­ful of un­ex­plained events that might just fit the bill: fast ra­dio bursts (FRBs).

The first was spot­ted in 2007 and, while there are still many mys­ter­ies sur­round­ing them, it is clear they are com­ing from be­yond our galaxy. The near­est em­anated from over a bil­lion light years away. Some as­tronomers have even sug­gested they might be at­tempts by aliens to get in con­tact. Far more likely is that they have some as­tro­nom­i­cal ori­gin, but what ex­actly? Per­haps they are gen­er­ated by col­lid­ing black holes or neu­tron stars. How­ever, there is a way we might be able to prove once and for all that they re­ally are com­ing from black holes bounc­ing into white holes.

Ac­cord­ing to cal­cu­la­tions by Rovelli and Vi­dotto, more dis­tant bursts should have more en­ergy than those nearby. That’s be­cause black holes are thought to evap­o­rate over time by re­leas­ing Hawk­ing ra­di­a­tion, named after the late physi­cist Stephen Hawk­ing. Younger black holes in the dis­tant uni­verse should there­fore be big­ger and re­lease more en­ergy than older black holes closer to us that have had more time to evap­o­rate.

This is in di­rect con­trast to the way things nor­mally work in as­tron­omy. As the uni­verse ex­pands it di­lutes the amount of en­ergy in a given amount of space. There’s more space between us and a dis­tant ob­ject to stretch, so far-away ob­jects have their en­ergy wa­tered down more than those close to us. With bounc­ing black holes you’d ex­pect the two ef­fects to can­cel each other out, mean­ing these ex­plo­sive events would have a sim­i­lar en­ergy across a wide range of cos­mic dis­tances. Ac­cord­ing to Vi­dotto, ob­serv­ing this be­hav­iour “would be a smok­ing gun for our the­ory”.

There are some po­ten­tial snags, how­ever. The FRBs dis­cov­ered so far are not of the ex­act en­ergy you would ex­pect from a black hole to white hole bounce. That may not be the end of the world ac­cord­ing to Hal Hag­gard from Bard Col­lege in

New York. “Given how im­pre­cise the cal­cu­la­tions are it’s not sur­pris­ing,” he says. “It’s in the right ball park.” More con­cern­ing is that as­tronomers have iden­ti­fied a re­peat­ing fast ra­dio burst called FRB 121102. First dis­cov­ered in 2012, more than 15 dis­tinct pulses are as­so­ci­ated with the same source. “There’s noth­ing in the white hole the­ory that calls for that,” says Hag­gard. “If more and more of these re­peat­ing bursts are found then that goes against this pro­posal.”

He be­lieves the white hole in­ter­pre­ta­tion is ex­tremely spec­u­la­tive, but the pay-off is po­ten­tially huge. “It’s ex­cit­ing be­cause there are so few ways

“It’s ex­cit­ing be­cause there are so few ways to test quan­tum grav­ity cur­rently”

Hal Hag­gard

to test quan­tum grav­ity cur­rently on the ta­ble.” But con­firm­ing a black to white hole tran­si­tion wouldn’t im­me­di­ately crown loop quan­tum grav­ity the vic­tor. Hag­gard says the ap­proach taken so far is “a generic model that doesn’t lever­age any­thing spe­cific about the the­ory of quan­tum grav­ity you’re us­ing”. How­ever, fur­ther de­tailed ob­ser­va­tions of how the ex­plo­sions played out could do the trick. “De­tailed anal­y­sis of the sig­nals would be able to dis­tin­guish between the­o­ries, and that’s why this is so ex­cit­ing,” says Hag­gard.

Given the high stakes, for­tu­nately there are other ways a black hole to white hole bounce could show it­self. Ac­cord­ing to Vi­dotto the ex­plo­sive event should also gen­er­ate gamma rays – the high­est en­ergy part of the elec­tro­mag­netic spec­trum. Al­though we do al­ready have gam­maray tele­scopes in space peer­ing into the uni­verse, Vi­dotto says “they are not yet op­ti­mised to see in

“What is re­mark­able is that no new physics is needed. No strings, no new forces and no new par­ti­cles”

Carlo Rovelli

such high-en­ergy gamma rays”. Fu­ture gamma-ray ob­ser­va­to­ries may well be up to the task, how­ever. In the mean­time there’s a third way in: syn­chro­tron emis­sion. Par­ti­cles like elec­trons would be ac­cel­er­ated through strong mag­netic fields dur­ing the high-en­ergy ex­plo­sion, emit­ting ra­di­a­tion as they do so. “The chal­lenge is how can we dis­tin­guish these cos­mic rays from all the other sources in the sky,” says Vi­dotto.

If any one of these en­deav­ours is ul­ti­mately suc­cess­ful, con­firm­ing a black hole to white hole tran­si­tion won’t just help with the mys­tery of quan­tum grav­ity. It could also tackle an equally per­plex­ing puz­zle cur­rently frus­trat­ing as­tronomers: dark mat­ter. When we look at gal­ax­ies and clus­ters of gal­ax­ies there ap­pears to be far more grav­ity than can be ac­counted for us­ing vis­i­ble ma­te­rial like stars and gas alone. In­stead as­tronomers have sug­gested there is some hid­den ma­te­rial skulk­ing in the shad­ows which acts like a ga­lac­tic glue, help­ing bind gal­ax­ies to­gether with its own grav­i­ta­tional pull. The most fash­ion­able con­tender for this dark mat­ter has been su­per­sym­me­try – the idea that along­side the fa­mil­iar sub-atomic par­ti­cles like elec­trons and pro­tons there are big­ger par­ti­cles that are their mir­ror im­ages. The light­est of these su­per­sym­met­ric par­ti­cles has been the go-to ex­pla­na­tion for dark mat­ter for well over a decade. De­spite a lot of search­ing, no one has ever found a su­per­sym­met­ric par­ti­cle.

That’s caus­ing some physi­cists to look else­where for an ex­pla­na­tion. Rovelli be­lieves the rem­nants left be­hind as a black hole transitions into a white hole could go some way to pro­vid­ing the miss­ing grav­ity. Be­ing so small, they would be hard to de­tect other than by their col­lec­tive grav­i­ta­tional pull. “What is re­mark­able is that no new physics is needed. No strings, no new forces and no new par­ti­cles,” Rovelli says, re­fer­ring to string the­ory – an al­ter­na­tive way to at­tack the prob­lem of quan­tum grav­ity. Hag­gard agrees it’s pos­si­ble that “they could make up a sub­stan­tial frac­tion of dark mat­ter”. He also says that “dark mat­ter may not be one thing – it may be a mix­ture of par­ti­cles we haven’t dis­cov­ered and some­thing else”. That some­thing else could be black holes turn­ing white.

For now as­tronomers are left in a tan­ta­lis­ing po­si­tion. Through fast ra­dio bursts we might not only have the first clues that black holes can morph into their polar op­po­sites, but also a way to tackle the ul­ti­mate ques­tions about the na­ture of space and time it­self. Then again, we may not. Only more ob­ser­va­tions with more tele­scopes from one end of the elec­tro­mag­netic spec­trum to other will tell us whether to call the No­bel com­mit­tee or re­turn to the draw­ing board. The stakes couldn’t be higher.

What is a white hole?Black holes are places where you can go in and you can never es­cape, while a white hole is a place where you can leave but cannever go back.

Dif­fer­ences in the strength of grav­ity across an ob­ject stretches it as it ap­proaches a black hole

Al­though in­vis­i­ble, a black hole can of­ten be de­tected by its ef­fect on its sur­round­ings

Ein­stein said that space and time are wo­ven to­gether into a smooth, con­tin­u­ous fab­ric called space-time

Could dark mat­ter be made up of black holes?

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