WHAT’S UP WITH GRAV­ITY?

Some prob­lems are so im­por­tant that sci­en­tists have been try­ing to solve them for cen­turies. No mat­ter if they are ever found, the search for the an­swers pro­vides us with new knowl­edge.

Science Illustrated - - CONTENTS -

Grav­ity is pre­dictable in its ef­fects, but we still don’t know ex­actly how or why it works. It’s ar­guably sci­ence’s great­est mys­tery.

It keeps our feet planted solidly on the ground and plan­ets or­bit­ing the Sun at dis­tances of mil­lions of km. Nev­er­the­less, grav­ity is very much weaker than the other forces of na­ture, and it re­mains a mys­tery to sci­en­tists. The search for the par­ti­cle which car­ries its force has been in vain so far. Per­haps the an­swer to the mys­tery of grav­ity is not a par­ti­cle, rather it is hid­den in seven so far un­seen di­men­sions.

Afew days af­ter as­tro­naut Jack Lousma had re­turned to Earth af­ter spend­ing two months in the US Sky­lab space sta­tion, he put down his af­ter­shave bot­tle in the air be­side him. A loud bang and lots of bro­ken glass sud­denly re­minded him that it was a bad idea. In the space sta­tion, he had got­ten used to ev­ery­thing fly­ing about in a state of weight­less­ness. But in a bath­room on Earth, other rules ap­ply. We are sub­jected to grav­ity, and al­though it is not par­tic­u­larly “loud”, it rules ev­ery­thing – in­clud­ing af­ter­shave.

When you lift your cof­fee cup from the ta­ble, you sense the in­vis­i­ble force. When your smart­phone hits the as­phalt, it is due to grav­ity. And when you step down from your bath­room scales, it de­cides the num­ber of kg in­di­cated. In­deed, Earth and the uni­verse would not ex­ist, if it were not for grav­ity.

Al­most 14 bil­lion years ago, af­ter the Big Bang, grav­ity made sure to con­tract mat­ter, so stars and plan­ets were formed. The fact that Earth is cir­cu­lar, is also due to grav­ity. Grav­ity tries to at­tract ev­ery­thing that plan­ets are made of to their cen­tres, but as the ma­te­rial can­not be to­tally com­pressed, they are shaped like balls.

In short, grav­ity is the ruler of the uni­verse. But al­though it might seems ev­i­dent, it is one of the ma­jor sci­en­tific mys­ter­ies. Ev­ery time sci­en­tists have man­aged to lift a bit of the veil, they have run straight into new prob­lems. The ma­jor ques­tion that still re­mains unan­swered is how grav­ity is car­ried. Some sci­en­tists as­sume that a par­ti­cle car­ries the force, but al­though they have al­ready named the par­ti­cle – a gravi­ton – they have never man­aged to cap­ture it in spite of per­sis­tent ef­forts.

Thanks to ge­niuses such as Isaac New­ton and Al­bert Ein­stein, we now know how grav­ity works be­tween Earth and a rocket, etc., and how it makes plan­ets or­bit their stars. But how it works at the atomic level re­mains a mys­tery, which sci­en­tists are still struggling to solve. If they are suc­cess­ful, we might get the very “man­ual” of the uni­verse – from the tini­est of ele­men­tary par­ti­cles to the largest of gal­ax­ies.

Rocks and wa­ter long to be back on Earth

Around 1600 AD, Galileo Galilei of Italy climbed to the top of a tower to throw down two metal balls. That was the be­gin­ning of sci­en­tific re­search con­cern­ing grav­ity. Galileo was highly scep­ti­cal of the ex­ist­ing view of the world, which dated back to around 350 BC.

At that time, Greek philoso­pher Aris­to­tle re­alised that ob­jects fall­ing to­wards the ground had to do so for a rea­son – and ac­cord­ing to Aris­to­tle, the ex­pla­na­tion was ob­vi­ous. Ob­jects fall to­wards the ground, be­cause they try to get back to the place they orig­i­nally came from. A rock comes from Earth, and so, a fall­ing rock will try to get back to Earth. The same is true for wa­ter, which is also na­tive to our planet. Fire and air, on the other hand, are not earthly, so they will rise, ac­cord­ing to Aris­to­tle. More­over, he was con­vinced that the heav­ier an ob­ject is, the faster it will re­turn to its start­ing point to be re­united with its ele­ment. In short, heavy ob­jects will fall faster than light ones, ac­cord­ing to the Greek philoso­pher.

The the­ory seemed so ev­i­dent that about 2,000 years passed, be­fore any­one ques­tioned it. That was when Galileo en­tered the scene. Among the pro­fes­sors of the Univer­sity of Pisa, Galileo Galileo had a rep­u­ta­tion for be­ing a very bright stu­dent, but he was also very stub­born in­deed. He ques­tioned ev­ery­thing. In 1582, at the age of 17, he be­gan to study medicine, but af­ter all, he loved math­e­mat­ics and me­chan­ics more, and some­thing was both­er­ing him. Ev­ery time his teach­ers talked about Aris­to­tle’s the­o­ries, Galileo ob­jected. He re­fused to ac­cept that the weight of an ob­ject has any­thing at all to do with the speed of its fall. In a vac­uum, in which there is no air re­sis­tance, any body will fall at the ex­act same speed – and a rock will not fall any faster than a feather, ac­cord­ing to Galileo.

Around 1600, he de­cided to put the­ory into prac­tice. He car­ried a heavy and a light metal ball up the stairs of a tower – the Lean­ing Tower of Pisa, ac­cord­ing to the myth – to make an ex­per­i­ment. Hun­dreds of cu­ri­ous peo­ple came to the base of the tower to watch the re­bel­lious Galileo make a fool of him­self.

Heavy and light balls fall at the same speed

The crowd stared at the dar­ing sci­en­tist, as he let go of the two metal balls, mak­ing them fall freely from the top of the tower. Peo­ple cheered, as con­trary to ex­pec­ta­tion, the heavy and the light ball hit the ground at the ex­act same time, prov­ing Galileo right.

With this and a long se­ries of sim­i­lar ex­per­i­ments, Galileo pulled the rug from un­der the ex­ist­ing the­o­ries, demon­strat­ing over and over again that grav­ity is char­ac­ter­ized by the fact that that all bod­ies, dis­re­gard­ing their masses, fall at the same speed un­der its in­flu­ence.

If he had lived for about 400 years, he would no doubt have cel­e­brated an ex­per­i­ment which Amer­i­can Apollo 15 as­tro­naut David Scott made dur­ing a lu­nar land­ing in Au­gust 1971. A few hours be­fore the re­turn, Scott took a fal­con feather from his pocket and made the 30 g feather and a 1.3 kg ham­mer fall from the same alti­tude in the vac­uum as a trib­ute to Galileo. And just as the late Ital­ian had pre­dicted, the feather and the ham­mer hit the moon dust at the very same time.

“Noth­ing like a lit­tle sci­ence on the Moon,” David Scott en­thu­si­as­ti­cally said from his out­post ap­prox­i­mately 400,000 kilo­me­tres from Earth.

At the time when Galileo was en­gaged in his ex­per­i­ments with bod­ies in a free fall, Ger­man as­tronomer Jo­hannes Ke­pler made a sur­pris­ing dis­cov­ery. Fol­low­ing many years of ob­ser­va­tions of plan­e­tary po­si­tions in the sky, he had to ac­knowl­edge that the plan­ets travel in el­lip­ti­cal or­bits, not in per­fect cir­cles, such as schol­ars used to be­lieve.

That one body should act upon another... with­out the me­di­a­tion of any­thing else is so great an ab­sur­dity that no man suited to do sci­ence... can ever fall into it... ISAAC NEW­TON in a let­ter to a friend in the 1690s

Jo­hannes Ke­pler in­tro­duced a se­ries of laws con­cern­ing the way in which plan­ets travel around the Sun, but he was un­able to ex­plain the rea­son why they travel in the fash­ion which they do.

New­ton’s ap­ple tree trav­els into space

Hardly any­body else in the world would have paid at­ten­tion to how ma­ture fruit falls to the ground, but 23-year-old Isaac New­ton was an un­usu­ally gifted young man. Due to the plague, which caused havoc in Europe, he had fled Cam­bridge, where he was study­ing, for the coun­try­side. One day in the late sum­mer of 1666, he was sit­ting in the gar­den of his child­hood home, drink­ing tea in the shadow of an ap­ple tree, as his mind trav­elled. Sud­denly, an ap­ple fell and landed at his feet.

This very or­di­nary phe­nom­e­non made New­ton won­der, why ap­ples al­ways fall ver­ti­cally? Why do they not rise or move side­ways, he thought. He imag­ined that some sort of at­trac­tion was at work. Earth at­tracted the ap­ple and all other bod­ies near it, and per­haps the at­trac­tion had an even longer reach – as far as to the Moon and fur­ther into the uni­verse. The re­al­iza­tion was to have far-reach­ing con­se­quences and take up all New­ton’s time for many years to come.

Ever since he was a child, New­ton had im­pressed peo­ple around him with his bril­liant ideas. When he was a boy, he had in­vented a grain grinder pow­ered by mice, he had de­signed clever clocks that mea­sured time by means of wa­ter, and by watch­ing his own shadow, he could im­me­di­ately tell what time of day it was.

More­over, if he had had the abil­ity to look into the fu­ture, Isaac New­ton would have known that the very ap­ple tree which let go of one of its fruits in the late sum­mer of 1666 in the gar­den of his child­hood home, Wool­sthorpe Manor, would at some point in the fu­ture be known as the Grav­ity Tree. He would also have known that a hand­ful of seeds from the same, ex­tremely die-hard ap­ple tree would one day in De­cem­ber 2015 be launched with a rocket to es­cape the very force that had once made the ap­ple fall down at New­ton’s feet.

The seeds formed part of an ex­per­i­ment at the In­ter­na­tional Space Sta­tion, ISS, where New­ton’s fel­low coun­try­man, as­tro­naut Tim Peake, stud­ied how space travel af­fected their growth.

Any­thing with a mass has at­trac­tion

In­spired by the fallen fruit, New­ton thought about link­ing Ke­pler’s laws of plan­e­tary mo­tion with Galileo’s laws con­cern­ing fall­ing ob­jects. The forces ap­ply­ing on Earth must also gov­ern the uni­verse, New­ton re­al­ized. The force that makes the ap­ple fall from the tree must be the ex­act same one which keeps the Moon in its or­bit around Earth and the plan­ets in their or­bits around the Sun. And the rea­son why the plan­ets do not crash into the Sun is that they travel so fast that they en­ter into an or­bit around it.

In 1687, Isaac New­ton pub­lished his ground-break­ing the­ory of grav­ity in the Prin­cipia mas­ter­piece, which would be known as one of the most im­por­tant sci­en­tific

A feather is much lighter than an ap­ple, so it will fall more slowly, as it is slowed more down by air re­sis­tance. In a vac­uum such as on the Moon, ap­ple and feather will fall at the same speed.

works ever writ­ten. In it, New­ton in an in­tel­lec­tual tour de force in­tro­duced not only a math­e­mat­i­cal the­ory of grav­ity, but also three laws that de­scribe the mo­tion of bod­ies.

Ac­cord­ing to New­ton, grav­ity is a force be­tween two bod­ies. All ob­jects with weight at­tract each other. The ex­tent of the at­trac­tion de­pends on the masses of the ob­jects and the dis­tances be­tween them, ac­cord­ing to the the­ory, which, New­ton in­sisted, had to ap­ply to all bod­ies in the en­tire uni­verse, and so he named it the law of uni­ver­sal grav­i­ta­tion.

Thanks to New­ton’s equa­tions, it had fi­nally be­come pos­si­ble to cal­cu­late plan­e­tary or­bits in the So­lar Sys­tem and the Moon’s or­bit around Earth ex­tremely ac­cu­rately. New­ton could even ex­plain tides and Earth’s shape. Tides are caused by the at­trac­tion of the Moon and the Sun, and as a re­sult of Earth’s ro­ta­tion around its own axis, the world must be flat at the poles, New­ton proved the­o­ret­i­cally. Since then, the as­ser­tion has been fully con­firmed by a wealth of mea­sure­ments, pho­tos from space, and radar and satel­lite data.

New­ton has proved to be just as durable, when it comes to de­ter­min­ing the or­bits of plan­ets and comets. By means of New­ton’s for­mu­las, as­tronomers can cal­cu­late the mo­tions of heav­enly bod­ies thou­sands of years into the fu­ture or back in time and pre­dict fu­ture so­lar eclipses or state the time of past ones very ac­cu­rately.

New­ton’s law of grav­ity can also ex­plain why Galileo’s two balls fell at the same speed, al­though one was heav­ier than the other. Ac­cord­ing to his grav­ity equa­tion, the at­trac­tion which Earth ex­er­cises on the heavy ball is greater than its at­trac­tion on the light one. On the other hand, it takes more force to move the heavy ball as far as the light one, so the two fac­tors can­cel each other out.

Planet af­fects Uranus

Ac­cord­ing to New­ton’s the­ory, grav­ity ex­ists through­out the uni­verse, and this very as­sump­tion was some camel to swal­low for con­tem­po­rary schol­ars. The fact that the forces of at­trac­tion can be ex­er­cised over mil­lions of km and reach all the way from the Sun to Earth seemed com­pletely con­trary to na­ture to them.

New­ton was claimed to work with oc­cult forces, but in 1846, the crit­i­cism ceased once and for all. Up un­til then, all plan­ets had been dis­cov­ered by ac­ci­dent, but based solely on New­ton’s the­o­ries, two as­tronomers, John Couch Adams and Ur­bain Le Ver­rier, in­de­pen­dently pre­dicted the ex­is­tence of an un­known planet, Nep­tune. Both had no­ticed ir­reg­u­lar­i­ties of Uranus’ or­bit, which, they con­cluded, had to be due to the grav­i­ta­tional pull of an un­known planet out­side Uranus’ or­bit. The anal­y­sis proved cor­rect. In the po­si­tion which the two as­tronomers had pre­dicted us­ing pen and pa­per, Jo­hann Galle of Ger­many in 1846 used his tele­scope to ob­serve the planet of Nep­tune.

But al­though Isaac New­ton was the "fa­ther" of the law of grav­ity, he did not be­lieve that he had found an ex­pla­na­tion of the na­ture of grav­ity – he had not dis­cov­ered how it works, he had only found the for­mula.

“That one body may act upon another at a dis­tance through a vac­uum, with­out the me­di­a­tion of any­thing else . . . is to me so great an ab­sur­dity, that I be­lieve no man, who has in philo­soph­i­cal mat­ters a com­pe­tent fac­ulty of think­ing, can ever fall into it,” New­ton wrote about his dis­cov­ery in a let­ter to a friend in the 1690s.

Con­se­quently, he passed the task of dis­cov­er­ing the "soul" of grav­ity on to his de­scen­dants – specif­i­cally, it turned out, a Ger­man by the name of Al­bert Ein­stein, who in the early 1900s worked as a clerk in a patent of­fice in Bern, Switzer­land.

Are we on Earth or in­side a space­craft?

Space is bent, the man with the un­ruly locks and the vivid gaze claimed – and the planet of Mer­cury proves that Ein­stein was right, when it came to his epoch-mak­ing recog­ni­tion.

From the mid-1800s, it was clear that New­ton’s law of grav­ity could not ex­plain Mer­cury’s or­bit around the Sun. For ev­ery or­bit, the el­lip­ti­cal or­bit shifts slightly, which is in­con­sis­tent with New­ton’s the­ory. The dis­cov­ery turned physi­cists' hair grey and trig­gered a large-scale search for an un­known planet, which was able to in­flu­ence Mer­cury’s or­bit. But in spite of per­sis­tent ef­forts, the planet was never dis­cov­ered. For very good rea­sons, as it does not ex­ist.

In 1905, the young clerk Al­bert Ein­stein in­tro­duced his spe­cial rel­a­tiv­ity the­ory, ac­cord­ing to which time and dis­tance are rel­a­tive fac­tors that de­pend on how fast the ob­server is mov­ing. Space and time can­not be seen as sep­a­rate phe­nom­ena, but must be con­sid­ered as one: space­time. The spe­cial rel­a­tiv­ity the­ory can ex­plain a lot about the uni­verse, but not grav­ity. One au­tumn day in 1907, as Ein­stein was sit­ting in his of­fice in Bern, star­ing out the win­dow, he had his “best idea ever”. He thought that if a man falls from a roof, he will not feel grav­ity in the free fall – he will be weight­less. The man will not feel that he ac­cel­er­ates, for if he drops his ham­mer or some­thing else, it will ac­cel­er­ate at the ex­act same speed be­side him.

In a mo­ment of clear-sight­ed­ness, Ein­stein re­al­ized that there had to be a con­nec­tion be­tween grav­ity and ac­cel­er­a­tion. It is im­pos­si­ble to make an ex­per­i­ment that de­ter­mines, if you are on the sur­face of Earth or in a space­craft ac­cel­er­at­ing at a speed of 9.8 m/s2 – the ac­cel­er­a­tion of ob­jects in a free fall at Earth’s sur­face, also known as the ac­cel­er­a­tion of free fall. In prac­tice, ac­cel­er­a­tion and grav­ity are the same.

This in­sight put Ein­stein on the track of a new, ground­break­ing the­ory, the gen­eral rel­a­tiv­ity the­ory, which he in­tro­duced in 1915. Ac­cord­ing to his spe­cial rel­a­tiv­ity the­ory from 1905, dif­fer­ences of speed make space and time change. Ac­cel­er­a­tion is a change of speed, and as ac­cel­er­a­tion and grav­ity are ba­si­cally the same, it is clear that space­time changes around all ob­jects with a mass. In his gen­eral rel­a­tiv­ity the­ory, Ein­stein de­ter­mines that grav­ity is sim­ply space­time bends. The heav­ier an ob­ject, the greater the bend around it. Space­time can be com­pared to a rub­ber sheet, on which the Sun, etc., is ly­ing like a heavy iron ball. The weight of the ball makes the rub­ber sheet give in, wear­ing it down to

com­pleted We have this land­mark ex­per­i­ment test­ing Ein­stein's uni­verse, and Ein­stein sur­vives. AS­TRONOMER FRANK DYSON at a meet­ing of the Royal So­ci­ety and Royal Astro­nom­i­cal So­ci­ety in London on 6 Novem­ber 1919

form a kind of fun­nel, and when a lighter ball such as Earth rolls across the sheet, it is forced to change di­rec­tion.

So­lar eclipse puts Ein­stein to the test

New­ton had un­der­stood grav­ity as an enig­matic force be­tween two bod­ies, but in his gen­eral rel­a­tiv­ity the­ory, Ein­stein claimed that grav­ity is a char­ac­ter­is­tic of space it­self – and with his ground-break­ing the­ory, he was able to solve the old mys­tery of Mer­cury’s strange or­bit.

Mer­cury is main­tained in its or­bit around the Sun, be­cause the Sun’s pow­er­ful grav­i­ta­tional field causes a bowl-shaped bend of space, in which the small planet is rolling about like a ball in a game of roulette. This means that for ev­ery or­bit, the an­gle of the path changes rel­a­tive to the Sun. Mer­cury is the So­lar Sys­tem planet which is or­bit­ing the clos­est to the Sun, and so, it is sub­jected to the most pow­er­ful grav­i­ta­tional pull. In the case of such strong grav­i­ta­tional fields, New­ton’s law of grav­ity is in­ad­e­quate.

The de­ci­sive test of Ein­stein’s rel­a­tiv­ity the­ory came dur­ing a to­tal so­lar eclipse in 1919. Ein­stin had pre­dicted that the light from a re­mote star pass­ing closely by the Sun would be bent by the star’s bend of space, and now, his pre­dic­tion would be put to the test.

Dur­ing the so­lar eclipse of 29 May 1919, Bri­tish as­tronomer Arthur Ed­ding­ton pho­tographed a star close to the Sun, and at a meet­ing of the Royal So­ci­ety and Royal Astro­nom­i­cal So­ci­ety in London on 6 Novem­ber of the same year, the ten­sion was fi­nally re­lieved:

“We have com­pleted this land­mark ex­per­i­ment test­ing Ein­stein's uni­verse, and Ein­stein sur­vives,” as­tronomer Frank Dyson said at the meet­ing.

The Sun had in­deed bent the light of the star. And Ein­stein had "de­feated" New­ton with his gen­eral rel­a­tiv­ity the­ory, which made the head­lines of news­pa­pers through­out the world in the days that fol­lowed:

“Sci­en­tific revo­lu­tion. New the­ory about the uni­verse. New­ton’s ideas have been de­feated,” it said on the front page of the Times of London. “The light is off course in the sky,” the New York Times wrote, adding: “Sci­en­tists are more or less be­side them­selves due to eclipse ob­ser­va­tions. Ein­stein’s the­ory wins.”

Satel­lite mea­sures Earth’s bend of space

Ein­stein’s gen­eral rel­a­tiv­ity the­ory is now the best con­cern­ing grav­ity, but New­ton’s law of grav­ity still func­tions quite well, when it comes to cal­cu­lat­ing rocket paths as they are launched from Earth, where the bend of space is min­i­mal, and Al­bert Ein­stein him­self doubted that it was re­ally pos­si­ble to mea­sure the ef­fect of Earth’s rel­a­tively weak grav­ity on space. But in 2011, like an echo from 1919, NASA sci­en­tists de­clared, that Al­bert Ein­stein’s the­ory also holds wa­ter in this re­spect.

When we are in a free fall, such as on a steep roller coaster, we are weight­less and do not feel that we are ac­cel­er­at­ing, be­cause ev­ery­thing within our reach is ac­cel­er­at­ing at the same pace. If we drop a ball, it will "fly" in the air next to us. This made Al­bert Ein­stein re­al­ize that it is im­pos­si­ble to dif­fer be­tween ac­cel­er­a­tion and grav­ity.

By means of four ul­tra-ac­cu­rate gy­ro­scopes – i.e. de­vices used to mea­sure di­rec­tion – the Grav­ity Probe B satel­lite had tested Ein­stein’s the­o­ries in its or­bit 640 km above Earth. The mea­sure­ments con­sisted of fol­low­ing the axes of ro­ta­tion of the four gy­ro­scopes in­side the probe, whose tele­scope was aimed at one sin­gle star, IM Pe­gasi. As the di­rec­tion to the star was fixed, tiny changes of the gy­ro­scopes’ axes of ro­ta­tion could be mea­sured by mag­netic quan­tum de­tec­tors. Ac­cord­ing to Ein­stein, the axes of ro­ta­tion of Grav­ity Probe B’s gy­ro­scopes were to grad­u­ally change due to Earth’s mass and ro­ta­tion, and when the sci­en­tists an­a­lysed the mea­sure­ment re­sults, they found an an­gle change of the gy­ro­scopes’ ori­en­ta­tions. In other words, the data defini­tively re­vealed that Earth’s grav­i­ta­tional field bends space in the same way as a ball wears down the rub­ber sheet of a tram­po­line.

“By means of this ground-break­ing ex­per­i­ment, we have tested Ein­stein’s uni­verse, and Ein­stein holds wa­ter,” one of the sci­en­tists, Fran­cis Everitt from the Stan­ford Univer­sity, said dur­ing a press con­fer­ence on 4 May 2011.

Five years later, in Fe­bru­ary 2016, Ein­stein’s idea of the bend of space­time was once again con­firmed. Physi­cists from the Amer­i­can Laser In­ter­fer­om­e­ter Grav­i­ta­tion­alWave Ob­ser­va­tory sen­sa­tion­ally pub­lished that they had mea­sured waves in space­time, i.e. grav­i­ta­tional waves, that rip­ple through space, spread­ing like rings on the wa­ter. The rip­ples of time and space came from two black holes that had col­lided, caus­ing – just as Ein­stein had pre­dicted – waves in space­time.

Is grav­ity car­ried by a par­ti­cle?

Al­though sev­eral astro­nom­i­cal ob­ser­va­tions have con­firmed Ein­stein’s rel­a­tiv­ity the­ory, sci­en­tists still get blank ex­pres­sions, when they are to ex­plain, how grav­ity works. They now know that grav­ity ex­ists, as space bends. But how the force is car­ried – how masses at­tract each other – they can­not say. Presently, the most likely ex­pla­na­tion is that grav­ity is car­ried by a spe­cial – so far only imag­i­nary – par­ti­cle know as a gravi­ton. And the as­sump­tion does not come out of the blue, as the ex­act same prin­ci­ple ap­plies to the other forces of na­ture.

Grav­ity is one of four fun­da­men­tal forces of na­ture which gov­ern our world. If atoms are the build­ing blocks of the uni­verse, the forces of na­ture are the glue and mor­tar that do not only hold the atoms to­gether, but also tell ma­te­ri­als how to be­have. Two of the forces, grav­ity and the elec­tro­mag­netic force, have eter­nal reaches. All mass in the uni­verse at­tracts other mass via grav­ity, and the elec­tro­mag­netic force can be ob­served even from re­mote gal­ax­ies in the shape of light. The two other forces of na­ture, the strong and the weak nu­clear forces, only ap­ply in­side atoms, where the first one holds the atomic nu­cleus to­gether, whereas the other one is re­spon­si­ble for ra­dioac­tive de­cay.

Of the four forces of na­ture, grav­ity is the one which sci­en­tists know the least about, which could seem to be a para­dox, as we feel its ef­fect any­where. How­ever, the prob­lem is that grav­ity is ex­tremely much weaker than the other forces of na­ture – even a fridge mag­net will eas­ily over­come Earth’s grav­ity to pick up a nee­dle from the floor.

In ex­per­i­ments, physi­cists have proved the ex­is­tence of the par­ti­cles that carry force in the cases of both the elec­tro­mag­netic force and the weak and strong nu­clear forces. Small pack­ets of en­ergy are sent and re­ceived, which physi­cists have named quanta. The best known ex­am­ples are light quanta, called pho­tons, which carry the elec­tro­mag­netic force. When this ap­plies to the three other forces of na­ture, why would the last force, grav­ity, not also func­tion by means of quanta, they ar­gue.

How­ever, the prob­lem is that all physi­cist ef­forts to find the imag­i­nary grav­ity par­ti­cles have been in vain. But at the Euro­pean Or­ga­ni­za­tion for Nu­clear Re­search, CERN, in Switzer­land, sci­en­tists are work­ing hard to find it. In the fu­ture, physi­cists hope to be able to de­tect the gravi­ton in ex­per­i­ments in the world’s largest par­ti­cle ac­cel­er­a­tor, the 27-km-long, un­der­ground Large Hadron Col­lider. In the ac­cel­er­a­tor, pro­tons are fired at speeds close to that of the light, and when they col­lide, par­ti­cles re­sult, which do not ex­ist un­der nor­mal cir­cum­stances.

The force is hid­ing in in­vis­i­ble di­men­sions

If they do one day con­firm the ex­is­tence of the gravi­ton, physi­cists will have come a gi­ant step closer to one of the great­est aims of sci­ence: a the­ory of ev­ery­thing. The the­ory is to ex­plain both the largest and the tini­est of phe­nom­ena in the uni­verse – from stars and gal­ax­ies to atoms and mol­e­cules – and hence solve the great­est of all mys­ter­ies: what caused the Big Bang, the ex­plo­sive birth of space about 13.7 bil­lion years ago, and what hap­pened dur­ing the pe­riod im­me­di­ately af­ter the Big Bang?

In the search for a the­ory which can ex­plain all phe­nom­ena, sci­en­tists have, through­out his­tory, been on the look­out for sim­ple laws of na­ture to de­scribe a com­plex world. But grav­ity is the eter­nal prob­lem and the only one of the four forces of na­ture that can­not be ex­plained by means of quan­tum me­chan­ics – the the­ory which de­scribes na­ture at the small­est scales of en­ergy lev­els of atoms and sub­atomic par­ti­cles – but only by means of Ein­stein’s rel­a­tiv­ity the­ory.

“Our prob­lem in physics is that ev­ery­thing is based on these two dif­fer­ent the­o­ries, and when we com­bine them, we get non­sense.”

These words were said by Amer­i­can physi­cist Ed­ward Wit­ten. The for­mu­las of quan­tum me­chan­ics and the rel­a­tiv­ity the­ory are math­e­mat­i­cally in­com­pat­i­ble, but Wit­ten rep­re­sents the so far most promis­ing the­ory of ev­ery­thing – a the­ory that could com­bine Ein­stein’s gen­eral rel­a­tiv­ity the­ory and quan­tum me­chan­ics. Wit­ten, who has been named the most gifted physi­cist of his gen­er­a­tion, has been work­ing on the string the­ory since 1975. The the­ory aims for a co­her­ent un­der­stand­ing of mat­ter and forces of na­ture, and the essence of the the­ory is that ev­ery­thing in the uni­verse – all mat­ter and all four forces of na­ture – were

Our prob­lem in physics is that ev­ery­thing is based on these two dif­fer­ent the­o­ries and when we com­bine them, we get non­sense. ED­WARD WIT­TEN, the physi­cist be­hind the string the­ory about the in­com­pat­i­bil­ity of quan­tum me­chan­ics and the rel­a­tiv­ity the­ory.

pro­duced from in­cred­i­bly tiny, vi­brat­ing strings, which are the tini­est build­ing blocks of the uni­verse. The strings are to be un­der­stood as threads of en­ergy that vi­brate in no less than 11 di­men­sions: the three spa­cial ones and the time di­men­sion plus seven other di­men­sions that are curled up so we can­not see them.

Ac­cord­ing to the su­per­string the­ory, grav­ity is not weaker than the other forces of na­ture, al­though that seems to be the case – we just do not feel its full ef­fect, as it is spread across the ex­tra di­men­sions.

The su­per­string the­ory lives up to all physi­cists’ re­quire­ments for the long sought the­ory about ev­ery­thing – but fails big time, when it comes to doc­u­men­ta­tion. So far, the the­ory is only a math­e­mat­i­cal con­struc­tion and pure imag­i­na­tion. The strings and the ex­tra di­men­sions are so tiny that we can never spot them. So, the the­ory can­not im­me­di­ately be tested – un­less the Large Hadron Col­lider pro­duces a mir­a­cle.

If the par­ti­cle ac­cel­er­a­tor de­tec­tors sud­denly spot an un­ex­pected guest in the shape of an un­known par­ti­cle, it might prove to be the long sought gravi­ton, which shows signs of life, be­fore it dis­ap­pears into the in­vis­i­ble di­men­sions. If it hap­pens, sci­en­tists will, in spite of grav­ity, have ma­jor dif­fi­cul­ties keep­ing their feet on the ground.

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