Think you un­der­stand grav­ity? Think again. The ev­ery­day force is a con­stant source of ques­tions for physi­cists - and some are look­ing to re­place the the­ory

All About Space - - Contents - Re­ported by Abi­gail Beall

Grav­ity is every­where, and you can see the ef­fects every­where you look. It’s the rea­son the planet or­bits around the Sun, the Moon or­bits Earth and your feet stay planted firmly on the ground. It’s one of the first as­pects of physics we en­counter at school, for ex­am­ple, with the tale of New­ton and his ap­ple fall­ing off his head. Yet we know sur­pris­ingly lit­tle about this 'in­vis­i­ble force'.

Grav­ity be­longs to a fam­ily of four forces that be­tween them can an­swer pretty much any ques­tion along the lines of ‘why does that hap­pen?’ when it comes to physics. Along with the strong force, the weak force and the elec­tro­mag­netic force, the grav­i­ta­tional force gov­erns the way ev­ery­thing in­ter­acts with each other in the known uni­verse.

Grav­ity has been a source of won­der for hu­mans since 1687 when New­ton re­alised a force was needed to ‘keep the Moon in her Orb’. Be­cause of this long his­tory, it would be un­der­stand­able to as­sume grav­ity is the most well-un­der­stood of the forces, but it’s not that sim­ple.

Some of the great­est sci­en­tists to have lived, notably in­clud­ing Ein­stein, have ded­i­cated their lives to un­der­stand­ing the force of grav­ity. New­ton de­scribed grav­ity as a force, but Ein­stein went fur­ther, show­ing grav­ity to be the re­sult of how mass and en­ergy dis­tort space and time. As a re­sult, we are clued up on how it works on a clas­si­cal scale in mod­er­ate grav­i­ta­tional fields – think of how satel­lites or­bit the Earth, or how plan­ets or­bit the Sun, all of which can be de­scribed by New­ton’s equa­tions of mo­tion.

The ex­cep­tion was Mer­cury’s or­bit around the Sun, which New­ton could not ex­plain. Mer­cury, be­ing so close to our near­est star, ex­pe­ri­ences a stronger grav­i­ta­tional field and it took Ein­stein and his Gen­eral The­ory of Rel­a­tiv­ity in 1915 to ex­plain how that stronger grav­ity in­flu­ences the swift world's path.

What is more, Ein­stein’s the­o­ries pre­dict how mas­sive ob­jects like stars dis­tort space-time to the ex­tent that they bend light around them, mag­ni­fy­ing it in a process known as grav­i­ta­tional lens­ing. Pre­dic­tions Ein­stein made in equa­tions al­most 100 years ago are still be­ing tested in the most ex­treme grav­i­ta­tional en­vi­ron­ments that we know of, such as around black holes and bi­nary neu­tron stars, and those equa­tions are turn­ing out to hold true now. For ex­am­ple, the first ev­i­dence show­ing how the mo­tions of stars or­bit­ing a su­per­mas­sive black hole are af­fected in ac­cor­dance with gen­eral rel­a­tiv­ity was pub­lished in Au­gust of last year.

But there are a few key prob­lems with grav­ity that still need to be ex­plained.

“There are many mys­ter­ies about grav­ity” says Paul Sutter, as­tronomer at Ohio State Uni­ver­sity.

“For one, why is it so weak? It's far, far weaker than any of the other forces.”

Grav­ity might not feel fee­ble when you are walk­ing up a steep hill, or fall­ing over. Yet the fact that you can over­come the force of grav­ity from a mass as large as the Earth’s and stand up straight high­lights how weak it is, com­par­a­tively. Grav­ity is around 1040 times weaker than the other forces, in­clud­ing the elec­tro­mag­netic force that holds atoms to­gether. It’s even weaker than the ‘weak force’ that causes ra­dioac­tive de­cay. Maybe grav­ity has been given the wrong name.

There are a mul­ti­tude of the­o­ries as to why grav­ity is so weak, but no clear lead­ing con­tender. “I don't think any cur­rent op­tions look very promis­ing right now,” says Sutter. “We're floun­der­ing when it comes to ex­tend­ing our knowl­edge of physics, and we're only now be­gin­ning to de­sign what we hope to be the right kinds of ex­per­i­ments to give us a clue as to what na­ture is think­ing.”

How­ever, the an­swer could lie in solv­ing the prob­lem that not only eluded Ein­stein, but ev­ery physi­cist since – the uni­fi­ca­tion of grav­ity, which de­scribes things on very larger scales, with quan­tum physics, which is fa­mous for de­scrib­ing be­hav­iour on very tiny scales.

If grav­ity is quan­tised on the small­est scales, like light is, then it must have a par­ti­cle that car­ries its force through quan­tum fields. For light, this par­ti­cle is the pho­ton. Grav­ity’s force car­rier would be the gravi­ton, but it re­mains merely hy­po­thet­i­cal at this point in time.

Two of the most pop­u­lar at­tempts to de­rive a quan­tum the­ory of grav­ity are String The­ory and Loop Quan­tum Grav­ity. String The­ory pro­poses that all of mat­ter is made up of tiny vi­brat­ing strings, and that one of these vi­bra­tional states rep­re­sents that of a gravi­ton.

String The­ory also re­quires ex­tra di­men­sions be­yond the four of space and time that we are fa­mil­iar with, and some sce­nar­ios sug­gest that the grav­ity we ex­pe­ri­ence is weak be­cause it is leak­ing in to our four-di­men­sional space-time from an­other di­men­sion that is curled up so tight that we can’t oth­er­wise de­tect it.

Mean­while, Loop Quan­tum Grav­ity tack­les the prob­lem from a dif­fer­ent an­gle by say­ing that space it­self is quan­tised - that is, it can be bro­ken down into the small­est pos­si­ble pieces called quan­tum loops of grav­ity. How­ever, it’s still a fairly un­de­vel­oped the­ory, since it does not pre­dict the gravi­ton and it hasn’t been shown that it is re­lated to gen­eral rel­a­tiv­ity.

Like elec­tro­mag­netism, grav­ity has an in­fi­nite range, which means it only gets weaker and weaker the fur­ther the two ob­jects are apart, and never goes com­pletely to zero. This is un­like the weak and strong forces, which only work within a spe­cific range. This it­self is not a prob­lem when it comes to un­der­stand­ing grav­ity, but it does make grav­ity a strong driver of the uni­verse around us.

Grav­ity is an at­trac­tive force only, and be­cause of this it can­not be can­celled out. This

“Ein­stein threw a span­ner in the works when he de­scribed grav­ity as not a force, but a con­se­quence of the struc­ture of space-time”

makes it very dif­fer­ent to elec­tro­mag­netism, which can at­tract and re­pel. The fact it is only at­trac­tive means grav­ity per­me­ates across the vast dis­tances in the uni­verse with noth­ing get­ting in the way. But this is also an­other rea­son it makes no sense, ac­cord­ing to some sci­en­tists.

Martin Ta­j­mar, Pro­fes­sor of Physics at the Tech­ni­cal Uni­ver­sity Dres­den, in Ger­many, says the strangest thing about grav­ity is “that there is only pos­i­tive mass around us with no neg­a­tive coun­ter­part as in elec­tro­mag­netism”.

If an elec­tro­mag­netic force is at­tract­ing an ob­ject, ap­ply­ing an op­po­site force would bal­ance the orig­i­nal out, mean­ing the net force on the ob­ject would be zero. In what would po­ten­tially be the equiv­a­lent of this phe­nom­e­non but for grav­ity, Ta­j­mar is look­ing into ways to ‘counter’ grav­ity, an ‘anti-grav­ity’ of sorts. He hopes come up with an op­po­site kind of force that would bal­ance the grav­i­ta­tional pull of an ob­ject.

The idea of coun­ter­ing grav­ity is not only im­por­tant for mak­ing grav­ity be­have like all the other forces. It may have prac­ti­cal ap­pli­ca­tions too. “It would be a game-chang­ing tech­nol­ogy like the use of elec­tro­mag­netism that cre­ated the mod­ern world” says Ta­j­mar. “Per­haps, it could open up new ways for space travel too.”

But in order to come up with a way to counter grav­ity, we need to go be­yond the the­o­ries we cur­rently have.

“To­gether with my stu­dents, I’m look­ing into the­o­ries that pre­dict de­vi­a­tions from our cur­rent un­der­stand­ing of grav­ity” he says. “We are per­form­ing ex­per­i­ments to test and ver­ify those very con­cepts.”

For an al­ter­na­tive the­ory of grav­ity to be taken se­ri­ously, it must be pos­si­ble to re­duce it to gen­eral rel­a­tiv­ity in cer­tain sce­nar­ios. Be­cause of this, it is nec­es­sary to test the lim­its of the gen­eral rel­a­tiv­ity to see where it de­vi­ates from re­al­ity and into the ter­ri­tory of a com­pletely new the­ory.

Some of these bound­aries in­volve strange links be­tween grav­ity and other phe­nom­ena in physics, like su­per­con­duc­tiv­ity. In 2003, Ta­j­mar wrote a pa­per propos­ing grav­i­ta­tional ef­fects could be re­spon­si­ble for a dif­fer­ence be­tween the mea­sured mass of pairs of elec­trons at low tem­per­a­tures, found in su­per­con­duc­tors, com­pared to the the­o­ret­i­cal value. This the­ory is known to physi­cists as grav­it­o­mag­netism.

“Grav­it­o­mag­netism is an ap­prox­i­ma­tion to Ein­stein’s the­ory of gen­eral rel­a­tiv­ity for low ve­loc­i­ties in flat space-time” says Ta­j­mar. “It al­lows us to eas­ily il­lus­trate the ef­fect of space-time ‘frame­drag­ging’, for ex­am­ple by the ro­tat­ing Earth.”

Frame-drag­ging oc­curs when a mas­sive ro­tat­ing ob­ject drags nearby space-time – and any ob­jects, such as satel­lites, in that space-time – around with it. In the the­ory of grav­it­o­mag­netism, when the Earth ro­tates it gen­er­ates a field sim­i­lar to the mag­netic field gen­er­ated by a ro­tat­ing charge. This, in turn, af­fects the mo­tion of masses sim­i­lar to the way elec­trons are de­flected if they move at right an­gle to a mag­netic field.

This has been mea­sured, says Ta­j­mar. “Satel­lites in polar or­bit around the Earth are slightly de­flected by the ro­tat­ing Earth in line with Ein­stein’s the­ory,” says Ta­j­mar. “How­ever, the ef­fect [of this frame­drag­ging] is so small that there are no prac­ti­cal ap­pli­ca­tions so far.” There is much more re­search needed be­fore it can be used as a way to start coun­ter­ing grav­ity, it seems, or even ex­plain­ing why grav­ity only at­tracts and does not re­pel.

Grav­ity is es­pe­cially im­por­tant when it comes to the struc­ture of the uni­verse. It is re­spon­si­ble for the for­ma­tion and evo­lu­tion of gal­ax­ies and black holes and, it also acts against the ex­pan­sion of the uni­verse, try­ing to slow it down in the face of dark en­ergy, which is ac­cel­er­at­ing the ex­pan­sion. Whether grav­ity or dark en­ergy wins will de­cide the fate of the cos­mos.

Grav­i­ta­tional lenses are a pretty handy tool for mea­sur­ing the struc­ture and dis­tri­bu­tion of mat­ter in the uni­verse. The more mass there is in a star, or a galaxy, or a clus­ter of gal­ax­ies, the greater the space-time dis­tor­tion and the more pow­er­ful the grav­i­ta­tional lens. There­fore, the mass, and the dis­tri­bu­tion of that mass, can be cal­cu­lated based on the amount of lens­ing. Yet when astronomers look at grav­i­ta­tional lenses formed by huge galaxy

“Grav­ity is re­spon­si­ble for the struc­ture of gal­ax­ies and black holes and slow­ing down the ex­pan­sion of the uni­verse”

clus­ters, the find that the lens­ing ef­fect is greater than the mass of the vis­i­ble gal­ax­ies can ac­count for. The im­pli­ca­tion is that there is some un­seen mat­ter - dark mat­ter - that is pro­vid­ing the ex­tra grav­ity for the lenses. We also see the in­flu­ence of this ex­tra grav­ity in the mo­tions of the gal­ax­ies on the out­skirts of clus­ters, and stars on the edges of gal­ax­ies, which are mov­ing faster than they should be based on the amount of vis­i­ble mat­ter.

What if, rather than there be­ing dark mat­ter to pro­vide ex­tra grav­ity, it is grav­ity it­self, or rather our un­der­stand­ing of it, that is wrong? This is the the­ory put for­ward by Is­raeli sci­en­tist Morde­hai Mil­grom in 1983. He called it Mod­i­fied New­to­nian Dy­nam­ics, or MOND for short, and it ar­gues that in very low ac­cel­er­a­tion en­vi­ron­ments, such as those ex­pe­ri­enced on the out­skirts of gal­ax­ies and galaxy clus­ter, grav­ity does not fol­low the tra­di­tional the­o­ries set out by New­ton and Ein­stein.

Prac­ti­tion­ers of MOND are few and far be­tween, since dark mat­ter is by far the most pop­u­lar the­ory, but the level of pop­u­lar­ity doesn’t nec­es­sar­ily dic­tate that a the­ory is cor­rect or not. MOND has had some successes, par­tic­u­larly in ex­plain­ing the ro­ta­tion of stars around gal­ax­ies, but ef­forts to de­velop a cos­mo­log­i­cal the­ory of MOND, in which it co-ex­ists in the Big Bang and In­fla­tion the­o­ries, have so far failed. Nor does MOND have a the­o­ret­i­cal frame­work from which it can be de­rived, leav­ing many sci­en­tists scep­ti­cal about MOND. How­ever, the the­ory of dark mat­ter has its own prob­lems: no­body knows what dark mat­ter is made of, and searches for dark mat­ter have so far drawn a blank.

MOND does not seek to re­place New­ton and Ein­stein’s the­o­ries, but to com­ple­ment them, since they op­er­ate in stronger grav­i­ta­tional fields to MOND. At the mo­ment, the Gen­eral The­ory of Rel­a­tiv­ity is our best ex­pla­na­tion for grav­ity, but we know it is not the fi­nal an­swer. For now quan­tum grav­ity re­mains mys­te­ri­ous, but who­ever can solve that puz­zle which links the grav­ity of the very large to the grav­ity of the very small could change physics and our un­der­stand­ing of grav­ity for­ever.

Grav­i­ta­tional lens­ing has beenused to map the dis­tri­bu­tion of mass around stars, gal­ax­iesand clus­ters of gal­ax­ies

Grav­ity is cru­cial for life on Earth; with­out it we wouldnot even have a planet The at­trac­tion of grav­ity holds to­gether huge gal­ax­ies like this one, with its bright blue gas at the cen­tre Grav­i­ta­tional waves, rip­ples in space-time, were pre­dicted by Ein­stein and first found in 2015

Al­bert Ein­stein de­scribed grav­ity not as a force, but as a con­se­quence of the shape of space-time

This dig­i­tal col­lage con­tains a highly stylised ren­di­tion of our So­lar Sys­tem

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