Some­thing’s wrong with grav­ity

A new the­ory could re­write the laws of physics as we know them, and ƂPCNN[ ex­plain what dark mat­ter is

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

What if the force that holds the Uni­verse to­gether doesn’t ex­ist?

Sci­en­tific rid­dles don’t come much more baf­fling than this: en­tire gal­ax­ies seem to be in the grip of some­thing that af­fects their be­hav­iour, but no one knows what this ‘some­thing’ is. If it’s a form of mat­ter, then it must be the most abun­dant mat­ter in the cos­mos, yet all at­tempts to get a sam­ple of it have failed. Not even the Large Hadron Col­lider has seen a glimpse of it. It re­mains as enig­matic as its name: dark mat­ter.

Now, one the­o­rist has pro­voked con­tro­versy with a dev­as­tat­ingly sim­ple ex­pla­na­tion for why dark mat­ter still hasn’t been found: it doesn’t ex­ist.

But that’s not the only rea­son Prof Erik Ver­linde of the Uni­ver­sity of Am­s­ter­dam is at­tract­ing so much at­ten­tion. Af­ter all, oth­ers have pre­vi­ously sug­gested dark mat­ter may be some kind of il­lu­sion.

What sets Ver­linde apart is his ex­pla­na­tion for the source of the il­lu­sion. He be­lieves it’s the re­sult of noth­ing less than a fun­da­men­tal mis­con­cep­tion about the most fa­mil­iar force in the Uni­verse: grav­ity.

It’s a claim that brings Ver­linde up against the work of some of the great­est minds in science – in­clud­ing Al­bert Ein­stein, whose cel­e­brated the­ory of grav­ity is one of the cor­ner­stones of mod­ern physics. Known as Gen­eral Rel­a­tiv­ity, it has led to a host of tri­umphs, in­clud­ing the de­tec­tion in 2015 of grav­i­ta­tional waves – rip­ples in the fab­ric of space-time caused by the col­li­sion of two black holes.

THE TRUTH ABOUT GRAV­ITY

Ver­linde has spent years piec­ing to­gether clues from the­ory and ob­ser­va­tion to cre­ate a whole new vi­sion of the force we call grav­ity. Now his ideas are be­ing put to the test, with in­trigu­ing re­sults. And at the cen­tre of them all is the mys­tery of dark mat­ter.

Ver­linde has been hailed as the in­tel­lec­tual suc­ces­sor to Ein­stein in the me­dia, yet he sees his goal in more down to earth terms. “I’m just try­ing to ex­plain where grav­ity comes from,” he says.

That might seem a bizarre state­ment, com­ing a cen­tury af­ter Ein­stein showed that grav­ity is the re­sult of mat­ter warp­ing space and time around it. Yet ac­cord­ing to Ver­linde, this over­looks the fact that Gen­eral Rel­a­tiv­ity re­mains just a de­scrip­tion of the force we call grav­ity. It leaves unan­swered the key ques­tion of ex­actly how mat­ter af­fects space and time.

To carry out his re­search, Ver­linde has had to grap­ple with some of the deep­est prob­lems in science, in­clud­ing the quest for the so- called The­ory of

Ev­ery­thing – a the­ory that unites grav­ity with quan­tum me­chan­ics that has been con­sid­ered the holy grail of physics for decades.

The­o­rists have long known that Gen­eral Rel­a­tiv­ity can­not be the last word about grav­ity. That’s be­cause it fails to in­cor­po­rate the other cornerstone of mod­ern physics, quan­tum the­ory. As well as de­scrib­ing the sub­atomic world with as­ton­ish­ing pre­ci­sion, quan­tum the­ory has been able to ac­count for all the fun­da­men­tal forces of na­ture apart from one: grav­ity. Since the 1950s, the­o­rists have tried to marry the two views of na­ture to pro­duce one over­ar­ch­ing the­ory.

The prob­lem, says Ver­linde, is that they are based on such rad­i­cally dif­fer­ent views of re­al­ity. For ex­am­ple, Gen­eral Rel­a­tiv­ity pre­sumes that it’s pos­si­ble to pin down pre­cisely where par­ti­cles are and how they’re mov­ing, while quan­tum the­ory shows that’s im­pos­si­ble. “So tak­ing grav­ity into ac­count gives us a bit of a prob­lem”, ex­plains Ver­linde.

For years, he worked on su­per­string the­ory, which many be­lieve to be the most promis­ing way of over­com­ing these prob­lems. Yet de­spite decades of ef­fort and a host of mind-bog­gling ideas, there is still no hard ev­i­dence that it works.

This has led Ver­linde down a dif­fer­ent path in search for the truth about grav­ity. The ori­gins of this truth lie in a se­ries of sur­pris­ing con­nec­tions be­tween grav­ity and an ap­par­ently un­re­lated part of science: ther­mo­dy­nam­ics, the physics of heat.

In the early 1970s, the­o­rists study­ing black holes – no­to­ri­ous for the in­ten­sity of their grav­ity – dis­cov­ered they must also be packed with some­thing called en­tropy. Widely used to un­der­stand the be­hav­iour of hot ob­jects, en­tropy re­flects the num­ber of ways of re­ar­rang­ing the con­stituents of ob­jects with­out chang­ing their ap­pear­ance. Cal­cu­la­tions showed that black holes con­tain the high­est pos­si­ble en­tropy that can be crammed into a given vol­ume of space. But they also

re­vealed some­thing else. Com­mon sense sug­gests that as it de­pends on the con­stituents of ob­jects, the en­tropy of a black hole should de­pend on its vol­ume. Yet the­o­rists found it de­pends only on the hole’s sur­face area. Stranger still, the cal­cu­la­tions sug­gest the black hole’s sur­face is made up of a vast patch­work of so- called Planck ar­eas. Named af­ter the epony­mous Ger­man pi­o­neer of quan­tum the­ory, Planck ar­eas are far smaller even than a sub­atomic par­ti­cle, and ap­pear to be the build­ing blocks of space-time it­self.

Pon­der­ing these mind-bend­ing con­nec­tions be­tween the physics of heat and space-time, Ver­linde be­gan to won­der if they were hints of a rad­i­cal new way of think­ing about grav­ity. Heat was once thought to be a fun­da­men­tal prop­erty of mat­ter that ex­ists in and of it­self, like elec­tric charge, for ex­am­ple, but it’s now known to ul­ti­mately be the re­sult of col­li­sions be­tween the mil­lions of atoms and mol­e­cules that make up a gas, liq­uid or solid. The faster the atoms and mol­e­cules that make up a ma­te­rial move, the more en­ergy they have and the hot­ter the ma­te­rial

ap­pears. Thus heat is ac­tu­ally an ‘emer­gent’ prop­erty. So could the sup­pos­edly fun­da­men­tal force of grav­ity also be emer­gent, its real ori­gins be­ing linked to en­tropy and those in­cred­i­bly tiny Planck ar­eas of space-time?

NEW­TON AND EIN­STEIN

In 2010, Ver­linde cre­ated a stir among the­o­rists when he pub­lished a pa­per show­ing how his the­ory could be used to ac­cu­rately de­rive both New­ton’s and Ein­stein’s laws of grav­i­ta­tion. “The sim­i­lar­i­ties with other known emer­gent phe­nom­ena such as ther­mo­dy­nam­ics have been mostly re­garded as just sug­ges­tive analo­gies,” de­clared Ver­linde. “It is time we not only no­tice the anal­ogy, and talk about the sim­i­lar­ity, but fi­nally do away with grav­ity as a fun­da­men­tal force.”

While in­trigu­ing, many the­o­rists re­mained un­con­vinced the find­ing was any­thing more than a quirk of physics. Ver­linde needed to come up with some­thing that didn’t merely re­pro­duce ex­ist­ing the­o­ries, but pre­dicted some­thing new – and testable. He now be­lieves he’s found it with the enigma of dark mat­ter.

While hints of its ex­is­tence emerged over 80 years ago in stud­ies of clus­ters of gal­ax­ies, it was a dis­cov­ery of a cu­ri­ous ef­fect in­side gal­ax­ies that first con­vinced as­tronomers to take dark mat­ter se­ri­ously.

Ac­cord­ing to New­ton’s law of grav­ity, stars fur­ther from the cen­tre of a galaxy should or­bit more slowly than those closer in. But dur­ing the 1970s, stud­ies of stars within spi­ral gal­ax­ies showed that be­yond a cer­tain dis­tance from the cen­tre, this ef­fect sim­ply van­ished. The most ob­vi­ous ex­pla­na­tion was that the stars were be­ing af­fected by the grav­ity of an in­vis­i­ble cloud of mat­ter sur­round­ing the gal­ax­ies. It soon be­came clear that what­ever this stuff was, it couldn’t be made from the stan­dard build­ing blocks of mat­ter. That sparked a global ef­fort to de­tect a vi­able al­ter­na­tive, which con­tin­ues to this day – with no suc­cess.

This has led to grow­ing sus­pi­cions that the most ob­vi­ous ex­pla­na­tion is

sim­ply wrong. In 1983, physi­cist Prof Morde­hai Mil­grom of the Weiz­mann In­sti­tute in Is­rael, pointed out a cu­ri­ous fact about the galac­tic ev­i­dence for dark mat­ter: it can also be ex­plained if New­ton’s law fails to ac­cu­rately ex­plain the mo­tions of stars in the outer reaches of gal­ax­ies feel­ing an ac­cel­er­a­tion due to grav­ity at a rate less than a cer­tain crit­i­cal value: around 100-bil­lionth that gen­er­ated by the Earth.

TEST­ING, TEST­ING

While in­trigu­ing, what Mil­grom called Mod­i­fied New­to­nian Dy­nam­ics (MOND) sim­ply re­placed one mys­tery with an­other: where did this ‘crit­i­cal ac­cel­er­a­tion’ come from? That’s what Ver­linde de­cided to find out us­ing his ideas of emer­gent grav­ity. “I quickly found a back-of-the- en­ve­lope cal­cu­la­tion that might ex­plain it, but I had to work for a num­ber of years to make this more pre­cise,” he says. And now be­lieves he has suc­ceeded.

The key lies in the ef­fect of the en­tire Uni­verse on the vi­tal in­gre­di­ent needed for the ex­is­tence of grav­ity: en­tropy. Ac­cord­ing to both New­ton and Ein­stein’s the­o­ries, the en­tropy of ob­jects like black holes in­creases with their area. But Ver­linde has shown things change on the scale of the whole Uni­verse, be­cause of dark en­ergy. First iden­ti­fied in the 1990s, dark en­ergy is a kind of anti-grav­i­ta­tional force that is pro­pel­ling the ex­pan­sion of the Uni­verse. Its ori­gins re­main mys­te­ri­ous, but cal­cu­la­tions by Ver­linde show that dark en­ergy leads to en­tropy in­creas­ing with vol­ume, not just area. That changes the be­hav­iour of grav­ity at cos­mic scales – and, says Ver­linde, the re­sult is an ac­cel­er­a­tion ef­fect cre­at­ing the il­lu­sion that dark mat­ter ex­ists.

“In an ex­pand­ing Uni­verse, the grav­i­ta­tional laws have to be ad­justed at the ac­cel­er­a­tion scale in­di­cated by MOND,” he says. Un­like MOND, how­ever, he has been able to cal­cu­late the ef­fect us­ing ba­sic physics.

Ver­linde’s the­ory does more than ex­plain why dark mat­ter has never been found. As­tronomers have long been puz­zled by a ‘law’ link­ing the bright­ness of spi­ral gal­ax­ies to their spin rate. Known as the Tully-Fisher re­la­tion, it makes no sense us­ing con­ven­tional the­o­ries of grav­ity, but Ver­linde has shown that it’s a nat­u­ral con­se­quence of the link be­tween grav­ity and en­tropy.

Fur­ther ev­i­dence back­ing Ver­linde’s the­ory comes from re­cent stud­ies of the light from dis­tant gal­ax­ies. Ac­cord­ing to Ein­stein, the grav­ity field of gal­ax­ies can bend the path of light rays. This is known as the ‘grav­i­ta­tional lens’ ef­fect. An in­ter­na­tional team of as­tronomers has found that this ef­fect is con­sis­tent with the pre­dic­tions of Ver­linde’s the­ory, with­out the need for dark mat­ter.

Now the search is on for ev­i­dence that Ver­linde’s the­ory does not just ex­plain MOND, but out­per­forms it. And here some prob­lems have emerged. As­tronomer Dr Fred­erico Lelli and his col­leagues at the Euro­pean South­ern Ob­ser­va­tory have been study­ing the or­bits of stars in gal­ax­ies, and they’re not be­hav­ing as ex­pected. “Ver­linde’s the­ory pre­dicts a stronger grav­i­ta­tional pull than MOND in the in­ner re­gions,” ex­plains Lelli. But this ef­fect doesn’t seem to

"WE'RE IN A PE­RIOD WHEN IT IS NEC­ES­SARY TO EX­PLORE MANY NEW IDEAS, AND IT TAKES A LONG TIME FOR SUCH THINGS TO SET­TLE OUT"

ex­ist: “This seems to be a se­ri­ous is­sue,” he says.

The big­gest prob­lem fac­ing Ver­linde, how­ever, is ex­plain­ing a cos­mic ‘co­in­ci­dence’. Why does the amount of dark mat­ter needed to ex­plain galaxy ro­ta­tion curves match the amount needed to ex­plain ob­ser­va­tions of the early Uni­verse? “The ob­ser­va­tional ev­i­dence for dark mat­ter from a va­ri­ety of meth­ods is all amaz­ingly con­sis­tent,” says as­tro­physi­cist Prof Neta Bah­call of Prince­ton Uni­ver­sity.

The sim­plest ex­pla­na­tion is that dark mat­ter re­ally does ex­ist, but just hasn’t been found yet. But Ver­linde points out that his work on the na­ture of grav­ity is far from com­plete. “To ex­plain these ef­fects one has to de­velop the the­ory to the point where one can de­scribe the cos­mo­log­i­cal evo­lu­tion of the Uni­verse,” he says. “I am cur­rently work­ing on these ideas, but it will take some time.”

Given the huge pay- off if he’s right, many sci­en­tists are will­ing to cut Ver­linde some slack. “We’re in a pe­riod when it is nec­es­sary to ex­plore many new ideas,” says as­tronomer Prof Stacy McGaugh of Case Western Re­serve Uni­ver­sity, Ohio. “And it takes a long time for such things to set­tle out.”

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