DOES SPACE HAVE A FI­NAL DI­MEN­SION?

There could be more to the uni­verse than meets the eye with ex­tra facets be­yond space and time

All About Space - - Contents - Re­ported by Colin Stu­art

Cu­ri­ouser and cu­ri­ouser!” cried Alice. In Lewis Car­roll’s sur­real Vic­to­rian story the epony­mous char­ac­ter goes on ad­ven­tures in Won­der­land. There she meets a cast of quirky char­ac­ters in a world that’s so alien it de­fies be­lief and com­mon sense. To reach this oth­er­worldly place she dis­ap­pears down a rab­bit hole and into an­other di­men­sion.

As won­der­ful and wacky as Alice in Won­der­land is, some physi­cists be­lieve some­thing sim­i­lar might be at play in our uni­verse. “It all starts with the hi­er­ar­chy prob­lem,” says Kris Pardo from the De­part­ment of Astro­phys­i­cal Sciences at Prince­ton Univer­sity. Grav­ity, it seems, doesn’t play by the rules, par­tic­u­larly when you com­pare it to the other ma­jor forces. “It’s just so much weaker,” says Pardo. It’s ten thou­sand tril­lion tril­lion tril­lion-times fee­bler than the strong nu­clear force that helps bind atomic nu­clei to­gether, for ex­am­ple. To see just how weak grav­ity really is, re­mem­ber that you can jump into the air and tem­po­rar­ily over­come the col­lec­tive grav­i­ta­tional pull of six tril­lion tril­lion kilo­grams of the Earth be­neath your feet.

The puz­zle of why grav­ity is so puny com­pared to its sib­ling forces is one of the great­est mys­ter­ies in physics. It has led some re­searchers to sug­gest that grav­ity must do the as­tro­nom­i­cal equiv­a­lent of Alice and dis­ap­pear down a ce­les­tial rab­bit hole. What if grav­ity isn’t really weaker – we only per­ceive it that way be­cause it leaks into ad­di­tional di­men­sions? If you could be an all-see­ing eye, ca­pa­ble of ob­serv­ing ev­ery di­men­sion at once, you wouldn’t en­counter a hi­er­ar­chy prob­lem at all. Such an idea might sound far-fetched but, thanks to re­cent break­throughs, Pardo has been able to test it.

In 2015, physi­cists de­tected grav­i­ta­tional waves for the first time. These rip­ples in the very fab­ric of space were pre­dicted by Al­bert Ein­stein 100 years ear­lier. Calami­tous events in the uni­verse send out rolling waves, much like those cre­ated when a stone is dropped into a pond. When the waves pass through the Earth they can be de­tected by ex­per­i­ments like the Laser In­ter­fer­om­e­ter Grav­i­ta­tional-Wave Ob­ser­va­tory (LIGO) in the US.

Its four-kilo­me­tre (2.5-mile) arms are sen­si­tive to changes in space equiv­a­lent to one-ten-thou­sandth the width of a pro­ton. The dis­cov­ery was so mon­u­men­tal that the No­bel Prize in Physics was awarded to the founders of the fa­cil­ity in 2017.

The first grav­i­ta­tional waves de­tected by LIGO came from the col­li­sion of two black holes about

1.3 bil­lion light years away. But in Au­gust 2017 an­other type of col­li­sion was picked up: two neu­tron stars smash­ing to­gether 130 mil­lion light years away. This event – known as GW170817 af­ter the date it was first de­tected – pre­sented a unique op­por­tu­nity to test the idea of grav­ity leak­ing into ex­tra di­men­sions. That’s be­cause a neu­tron star merger pro­duces a sear­ing flash of light in the form of gamma rays along with the grav­i­ta­tional waves. Black holes, on the other hand, fa­mously gob­ble up light. So GW170817 be­came the first event ever de­tected to emit both light and grav­i­ta­tional waves.

Pardo and his col­leagues were able to com­pare the flash of gamma rays with the grav­i­ta­tional waves. “We think we know how much en­ergy is re­leased in the form of grav­i­ta­tional waves af­ter an event like the one that we saw,” Pardo says. “And we

Grav­ity may dis­ap­pear into ex­tra di­men­sions, just as Alice dis­ap­peared down the rab­bit hole in Alice in Won­der­land

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