Chas­ing the ghost par­ti­cle

A cen­tury-old mys­tery is solved at the South Pole

All About Space - - Contents -

Re­ported by Ben Gilliland

For nearly 4 bil­lion years a ghostly trav­eller hur­tled to­wards our planet at nearly the speed of light – pass­ing through gal­ax­ies and skirt­ing alien worlds, but never once de­vi­at­ing from its course.

When its jour­ney be­gan, the dis­tant Earth had been a life­less lump of rock or­bit­ing a young star. The ghost’s jour­ney was so vast that as it trav­elled, the first sin­gle-celled life evolved on Earth, sin­gle­celled life be­came mul­ti­cel­lu­lar, com­plex life blos­somed in the seas and con­quered the land, the di­nosaurs rose and fell and hu­mans took their first ten­ta­tive steps out of Africa.

Through the en­tirety of the his­tory of life on Earth, the ghost had jour­neyed un­hin­dered through the cos­mos. Then, on 22 Septem­ber 2017, it was de­tected on Earth. And, deep be­neath the South Pole it per­ished. In do­ing so, it im­parted a se­cret.

This is the story of the hunt for that ghost and the cen­tury-old mys­tery its demise is help­ing to solve.

At the turn of the 20th cen­tury physi­cists were busy ex­per­i­ment­ing with the ra­dioac­tive el­e­ment ra­dium (ra­di­a­tion it­self had only re­cently been dis­cov­ered) when they started to no­tice some­thing strange – even when the ra­dium was re­moved their in­stru­ments were still reg­is­ter­ing the pres­ence of en­er­getic par­ti­cles.

Most sci­en­tists of the time be­lieved that the ra­di­a­tion must have been com­ing from min­er­als in the ground. But in 1910, Ger­man physi­cist Theodor Wulf, took an elec­tro­scope to the Eif­fel Tower and tested ra­di­a­tion lev­els at ground level and at the top of the tower. He found that the ef­fect was ac­tu­ally stronger at al­ti­tude – the ra­di­a­tion wasn’t com­ing from the ground.

The mys­tery was picked up by an Aus­trian physi­cist, Vic­tor Hess, who, in a se­ries of dar­ing bal­loon flights in 1911 and 1912, took ra­di­a­tion read­ings from ground level to an al­ti­tude of 5,300 me­tres (17,390 feet). He found that al­though ra­di­a­tion lev­els ini­tially dropped off, as he gained

al­ti­tude the read­ings in­creased un­til, at the high­est al­ti­tudes, the ra­di­a­tion lev­els were many times greater than they were at ground level. Hess con­cluded that “a ra­di­a­tion of very high pen­e­trat­ing power enters our at­mos­phere from above”. Or, to put it an­other way, the par­ti­cles were com­ing from space. But just how far out in space were they com­ing from?

One ob­vi­ous can­di­date was the Sun, but Hess ruled that out by per­form­ing one of his bal­loon ex­per­i­ments dur­ing a to­tal eclipse in 1912 – had par­ti­cles been com­ing from the Sun the read­ings would have dropped off as the Moon cov­ered it up, but the lev­els re­mained the same. The only pos­si­ble ex­pla­na­tion was that they came from fur­ther out in space. Hess had dis­cov­ered cos­mic rays.

Space is flooded with cos­mic ra­di­a­tion in the form of high-en­ergy, charged par­ti­cles, such as pro­tons and atomic nu­clei. Th­ese par­ti­cles can be pro­duced in all sorts of astro­nom­i­cal pro­cesses, such as the nu­clear re­ac­tions in stars.

But then there are the high­est en­ergy cos­mic rays. Th­ese are par­ti­cles that are im­bued with hun­dreds of mil­lions, or per­haps even bil­lions, of times more en­ergy than those cre­ated in hu­man­made par­ti­cle ac­cel­er­a­tors. In fact, physi­cists would

need to build a par­ti­cle ac­cel­er­a­tor the same size as the or­bit of the planet Mer­cury – about 360 mil­lion kilo­me­tres (224 mil­lion miles) – to reach the en­ergy of the par­ti­cles in cos­mic rays.

What mech­a­nism could ac­cel­er­ate th­ese par­ti­cles to the sort of en­er­gies that make the Large Hadron Col­lider look like a par­ti­cle pea shooter? They must surely come from some of the most vi­o­lent and least un­der­stood ob­jects and events in the uni­verse, such as a mas­sive star dy­ing in a su­per­nova ex­plo­sion, or from an ac­tive black hole in the cen­tre of some dis­tant galaxy. It is a mys­tery that has en­dured for more than a cen­tury.

Cos­mic rays are the frag­ments of atoms, such as pro­tons, elec­trons and atomic nu­clei, that have been torn apart and ac­cel­er­ated in vi­o­lent cos­mic events. The trou­ble with th­ese par­ti­cles is they all carry an elec­tric charge, which means as they fly through space they in­ter­act with the mag­netic fields of stars and other ob­jects. Th­ese mag­netic fields at­tract or de­flect the charged par­ti­cles, which al­ters their tra­jec­tory and makes them swerve around space like a cos­mic drunk driver. This means that by the time they reach Earth they can tell us very lit­tle about where they came from.

“Space is flooded with cos­mic ra­di­a­tion in the form of high-en­ergy, charged par­ti­cles, such as pro­tons and atomic nu­clei”

Ghost par­ti­cles are el­e­men­tary Ac­cord­ing to the Stan­dard Model of par­ti­cle physics, a neu­trino is an el­e­men­tary par­ti­cle. The el­e­men­tary par­ti­cles are build­ing blocks of the uni­verse.

A ren­der­ing of the track de­tected by IceCube’s sen­sors on 22 Septem­ber 2017. It tracks the course of a muon cre­ated when the neu­trino col­lided with an atom in the ice. The colours show its di­rec­tion of travel – from red (first de­tec­tion) to green and blueThey are born from vi­o­lence Neu­tri­nos are cre­ated as a by-prod­uct of nu­clear fu­sion in stars and in vi­o­lent astro­phys­i­cal events like su­per­novae.

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