ThroughT the worm­hole

COULD WE TRAVEL THROUGH A BLACK HOLE TO TAKE A SHORT­CUT INTO AN­OTHER GAL­AXY?

Focus-Science and Technology - - CONTENTS - WORDS: PROF ROBERT MATTHEWS

We find out if we could take a short­cut into an­other gal­axy by plung­ing through a black hole.

Ever since a trip through a worm­hole was first por­trayed in 2001: A Space Odyssey 50 years ago, the idea of them has cap­tured the pub­lic imag­i­na­tion. And small won­der: they’re the ul­ti­mate form of cos­mic travel: a way of zip­ping across gal­ax­ies in an in­stant.

But while worm­holes have be­come a sta­ple of science fic­tion, among sci­en­tists they’ve been a source of end­less frus­tra- tion. Not be­cause the idea is ridicu­lous, but be­cause it isn’t. The as­ton­ish­ing fact is that worm­holes are a nat­u­ral con­se­quence of cur­rent the­o­ries of grav­ity, and were in­ves­ti­gated by Ein­stein him­self over 80 years ago. Ever since, re­searchers have been try­ing to find out if such a bizarre the­o­ret­i­cal pos­si­bil­ity could be a re­al­ity.

And now they have made a ma­jor break­through – one which ex­ploits deep con­nec­tions be­tween the na­ture of space and time and the laws of the sub­atomic world. The re­sult is a new un­der­stand­ing of ex­actly what’s re­quired to make a re­al­life worm­hole.

Ein­stein first in­ves­ti­gated the prop­er­ties of worm­holes with his col­league Nathan Rosen in 1935, us­ing his the­ory of grav­ity known as Gen­eral Rel­a­tiv­ity. They found that what we now call a black hole could be con­nected to an­other via a tube-like ‘throat’. Now called the Ein­stein-Rosen bridge, this seemed to open the way to tak­ing short­cuts through space and time,

en­ter­ing a black hole in one part of the Uni­verse and emerg­ing from an­other per­haps mil­lions of light-years away, but with­out tak­ing mil­lions of years to do so – thus ef­fec­tively trav­el­ling faster than the speed of light.

It was a stun­ning idea, but in the early 1960s it was dealt a se­vere blow by John Wheeler, the bril­liant US physi­cist who first coined the terms ‘black hole’ and ‘ worm­hole’. To­gether with fel­low the­o­rist Robert Fuller, he showed that the Ein­stein-Rosen bridge would col­lapse al­most as soon as it formed. As Dr Daniel Jaf­feris, as­so­ciate pro­fes­sor of physics at Har­vard Univer­sity ex­plains: “We could jump in from op­po­site sides and meet in the con­nected in­te­rior, but then we would both be doomed.”

Jaf­feris is one of an elite group of the­o­rists around the world search­ing for ways to dodge this prob­lem. For years, the most promis­ing idea has been to sup­port the bridge us­ing a type of ‘ex­otic mat­ter’ with

“They’re the ul­ti­mate form of cos­mic travel: a way of zip­ping across gal­ax­ies”

neg­a­tive en­ergy. As its name sug­gests, this is pretty weird stuff – so weird it’s ca­pa­ble of bend­ing the nor­mal rules of grav­ity. While or­di­nary mat­ter al­ways gen­er­ates a grav­i­ta­tional pull, the neg­a­tive en­ergy pro­duced by this ex­otic mat­ter gen­er­ates an anti­grav­i­ta­tional re­pul­sion. Amaz­ingly, such en­ergy is known to ex­ist. In the 1990s, as­tronomers dis­cov­ered that the whole Uni­verse is ex­pand­ing un­der the anti­grav­i­ta­tional ef­fect of so- called ‘dark en­ergy’. There’s just one prob­lem - the ex­act ori­gins of dark en­ergy are as yet un­known. The same goes for the ex­otic mat­ter – no one has any idea how to cre­ate the stuff, let alone use it keep a worm­hole open long enough to fly through.

THE WORM HAS TURNED

But now the de­bate over such so- called tra­vers­a­ble worm­holes has taken a rad­i­cal new turn. It fol­lows the discovery of a new way of keep­ing the bridge in­tact based on a sur­pris­ing link be­tween worm­holes and quan­tum the­ory (the laws of the sub­atomic world). It emerged dur­ing at­tempts to solve a prob­lem that has ob­sessed some of the great­est the­o­rists of our time, in­clud­ing the late Stephen Hawk­ing: what hap­pens to ob­jects that fall into a black hole?

Ev­ery­one knows there’s no es­cap­ing a black hole once in­side it: the pull of grav­ity is too strong even for light to evade its clutches. Yet Hawk­ing fa­mously showed that a black hole doesn’t last for­ever, but even­tu­ally ex­plodes in a burst of in­tense

“The good news is that tra­vers­a­ble worm­holes re­ally can ex­ist”

ra­di­a­tion, leav­ing no trace of what­ever fell into it. The trou­ble is, this con­tra­dicts one of the key prin­ci­ples of quan­tum the­ory, which states that in­for­ma­tion can never be de­stroyed. Black holes, how­ever, seem quite ca­pa­ble of ut­terly de­stroy­ing in­for­ma­tion about what they’ve con­sumed. This is the no­to­ri­ous ‘black hole in­for­ma­tion para­dox’, and it hints at a big gap in our un­der­stand­ing of how the Uni­verse works.

For decades, Hawk­ing and many oth­ers tried to re­solve the para­dox with­out suc­cess. But now there’s grow­ing ex­cite­ment that the an­swer has been found. And it lies in the abil­ity of worm­holes to pro­vide a way out of black holes. Put sim­ply, the­o­rists think the sup­pos­edly in­escapable bound­ary of a black hole – the so-called event hori­zon – is rid­dled with

tiny worm­holes that al­low in­for­ma­tion to seep out, along with the ra­di­a­tion which Hawk­ing showed de­stroys black holes. This, in turn, has led to new in­sights into the na­ture of worm­holes, and whether they can be tra­versed.

Un­til now, the only known way to tra­verse a worm­hole was to stop the Ein­stein-Rosen bridge col­laps­ing us­ing the neg­a­tive en­ergy of ex­otic mat­ter. “Quan­tum ef­fects al­low some neg­a­tive en­ergy,” ex­plains Jaf­feris. “But it was long sus­pected that what is re­quired for a tra­vers­a­ble worm­hole is phys­i­cally im­pos­si­ble.”

Now, Jaf­feris and his col­leagues Dr Ping Gao and Dr Aron Wall think they’ve dis­cov­ered an­other source. “What we found is that a di­rect in­ter­ac­tion be­tween the [black holes at the] two ends of a non-tra­vers­a­ble worm­hole can lead to neg­a­tive en­ergy,” says Jaf­feris. The re­sult­ing anti­grav­i­ta­tional ef­fect then stops the Ein­stein-Rosen bridge from col­laps­ing, there­fore mak­ing the worm­hole tra­vers­a­ble.

When Jaf­feris and his col­leagues say “di­rect in­ter­ac­tion”, they mean that the two black holes form­ing the mouths of the worm­hole are af­fect­ing each other across real, or­di­nary space. “Bi­nary black hole sys­tems con­sum­ing each other’s Hawk­ing ra­di­a­tion is a good ex­am­ple,” says Jaf­feris. “The con­sum­ing of the ra­di­a­tion is the di­rect con­nec­tion.”

IN A TAN­GLE

So, the good news is that tra­vers­a­ble worm­holes re­ally can ex­ist. Bet­ter still, ac­cord­ing to Jaf­feris there’s no prob­lem send­ing a hu­man through one of them, at least in prin­ci­ple. But, per­haps un­sur­pris­ingly, there are some ma­jor prob­lems to over­come. First, the black holes can’t just be the stan­dard type formed from the col­lapsed rem­nants of huge stars; they have to be ‘max­i­mally en­tan­gled’. This refers to a strange quan­tum con­nec­tion that can ex­ist be­tween two ob­jects, so that any­thing done to one af­fects the other in­stantly – no mat­ter how far apart they are.

Like neg­a­tive en­ergy, the bizarre phe­nom­e­non of quan­tum en­tan­gle­ment re­ally ex­ists. It was first de­tected in lab ex­per­i­ments nearly 40 years ago, and it’s now be­ing in­ves­ti­gated by com­pa­nies like Google for cre­at­ing ul­tra-fast quan­tum com­put­ers. Yet while sub­atomic par­ti­cles can be en­tan­gled rel­a­tively eas­ily in the lab, no one has any idea how to do the same with black holes. “We can’t even make un­en­tan­gled black holes, let

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alone pre­cisely quan­tum en­tan­gled ones,” ex­plains Jaf­feris.

Yet di­rect in­ter­ac­tion be­tween two black holes comes with a catch: it for­bids any amaz­ing time travel trick­ery. But could it still al­low faster-than-light travel? That’s a tricky ques­tion, says Jaf­feris. Grav­ity, space and time are all in­ti­mately linked, and that messes with the very no­tion of speed. Ac­cord­ing to Jaf­feris, cal­cu­la­tions based on the worm­hole types stud­ied so far sug­gest that us­ing them would ac­tu­ally be slower than sim­ply trav­el­ling di­rectly through space. He ad­mits, though, that the de­tails have yet to be fully worked out. So, it seems that science fact is still run­ning a lit­tle be­hind science fic­tion. The laws of na­ture seem to in­sist that worm­holes can ei­ther per­form amaz­ing feats but col­lapse in an in­stant, or be tra­vers­a­ble but use­less.

Yet time and again, na­ture has sprung big sur­prises on the­o­rists. The mere pos­si­bil­ity of black holes was dis­puted for decades, and Ein­stein him­self re­fused to be­lieve in quan­tum en­tan­gle­ment. Could it be that some­where in the Uni­verse lie nat­u­ral worm­holes per­form­ing their mir­a­cles?

SCIENCE OR SCI-FI?

The pos­si­bil­ity of ob­serv­ing a real-life worm­hole is now the fo­cus of re­search by the­o­rists us­ing a mix of math­e­mat­ics and com­puter mod­els. The chal­lenge is spot­ting the dif­fer­ence be­tween nor­mal

“The mere pos­si­bil­ity of black holes was dis­puted for decades, and Ein­stein him­self re­fused to be­lieve in quan­tum en­tan­gle­ment”

black holes and those that are the por­tals of worm­holes. Ac­cord­ing to Ra­jibul Shaikh, a grav­ity the­o­rist at the Tata In­sti­tute of Fun­da­men­tal Re­search in Mum­bai, In­dia, the an­swer may lie in sub­tle dif­fer­ences in the way they af­fect their sur­round­ings – and in par­tic­u­lar the be­hav­iour of light. “As pre­dicted by Ein­stein’s Gen­eral Rel­a­tiv­ity, pho­tons un­dergo bend­ing in a grav­i­ta­tional field,” he ex­plains.

The in­tense grav­ity of black holes cre­ates in­cred­i­bly hot, bright ac­cre­tion discs around them, formed of mat­ter spi­ralling down to its doom. The oth­er­wise in­vis­i­ble hosts of these discs then re­veal their pres­ence as a pitch-black shadow cast on them. It’s the shape of this shadow that could re­veal when a black hole is ac­tu­ally some­thing even more bizarre. Ac­cord­ing to Shaikh, the tell­tale signs of a worm­hole come from the grav­i­ta­tional ef­fect of its throat on the re­sult­ing shadow.

“What I found is that the shape of the shadow of a slowly ro­tat­ing worm­hole would be very sim­i­lar to the al­most per­fectly disc-like shadow cast by a slowly ro­tat­ing black hole,” he ex­plains. “But a faster spin­ning worm­hole would cast a shadow which is more dis­torted than that of a black hole with the same spin.”

He stresses that re­search is still in progress, and the re­sults so far are based on spe­cific types of black holes and worm­holes. “There’s no guar­an­tee the type of ro­tat­ing worm­holes I con­sid­ered are the most com­mon.”

But Shaikh points out that as­tronomers al­ready have the means to de­tect the ef­fects pre­dicted to ex­ist around worm­holes. Known as the Event Hori­zon Tele­scope (EHT), it con­sists of a global net­work of ra­dio an­ten­nas able to make stud­ies of black holes and worm­holes. “And it has al­ready started tak­ing data,” says Shaikh.

It could just be that, half a cen­tury af­ter it made its de­but on movie screens, the space-time worm­hole is about to be­come more than just science fic­tion.

Black holes will de­stroy any­thing that is drawn into them, like this star. Yet worm­holes could pro­vide a way out

BE­LOW: Artist’s im­pres­sion of the event hori­zon – the point of no re­turn – of the black hole at the cen­tre of our Gal­axy

ABOVE: Dark en­ergy, as vi­su­alised here, is re­spon­si­ble for the ex­pan­sion of the Uni­verse

ABOVE: In fu­ture, could we travel to black holes to cap­ture sam­ples of Hawk­ing ra­di­a­tion to help im­prove our un­der­stand­ing of worm­holes?

RIGHT: Sci­en­tists can study the shadow that a black hole casts on its hot, bright ac­cre­tion disc. Cer­tain shapes of shadow may re­veal that the black hole is, in fact, a worm­hole

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