Nobel Prize goes to Gravitational Waves

In Einstein’s Gen­eral the­ory of rel­a­tiv­ity, grav­i­ta­tion is no force any­more, but a man­i­fes­ta­tion of the geom­e­try of space­time which is curved. A mass rep­re­sents the source of such a space­time cur­va­ture. Large masses like black holes curve space­time. If non-​spherically sym­met­ric masses move accel­er­ated, they pro­duce defor­ma­tions in space­time which have to adjust to the new con­fig­u­ra­tion. These per­tur­ba­tions, rip­ples in space­time are prop­a­gat­ing with the veloc­ity of light in the form of a wave.

While elec­tro­mag­netic waves pro­duced by accel­er­ated charges prop­a­gate in space­time, grav­i­ta­tional waves gen­er­ated by the accel­er­a­tion of masses are waves of the space­time struc­ture itself. And other than elec­tro­mag­netic waves, grav­i­ta­tional waves inter­act only very weakly with mat­ter and can pen­e­trate every­thing with­out los­ing inten­sity. Thus, very far­away regions and objects in the uni­verse can be stud­ied and very vio­lent processes like the col­li­sion and merg­ing of black holes.

Due to the prop­a­ga­tion of a grav­i­ta­tional wave, a peri­odic stretch­ing and com­pres­sion of space per­pen­dic­u­lar to the prop­a­ga­tion direc­tion arises. The dis­tances between objects change, but these changes in length are tiny. With the help of high-​precision laser inter­fer­om­e­ters, these can be mea­sured.

LIGO’s multi-​kilometer-​scale grav­i­ta­tional wave detec­tors use laser inter­fer­om­e­try to mea­sure the rip­ples in space­time caused by pass­ing grav­i­ta­tional waves from cos­mic sources such as the merg­ers of pairs of neu­tron stars or black holes, or by super­novae. LIGO con­sists of two widely sep­a­rated inter­fer­om­e­ters within the United States - one in Han­ford, Wash­ing­ton and the other in Liv­ingston, Louisiana - oper­ated in uni­son to detect grav­i­ta­tional waves.

At each obser­va­tory, the 2 1/​2-​mile (4-​km) long, L-​shaped LIGO inter­fer­om­e­ter uses laser light split into two beams that travel back and forth down the arms (four-​foot diam­e­ter tubes kept under a near-​perfect vac­uum). The beams are used to mon­i­tor the dis­tance between mir­rors pre­cisely posi­tioned at the ends of the arms. Accord­ing to Einstein’s the­ory, the dis­tance between the mir­rors will change by an infin­i­tes­i­mal amount when a grav­i­ta­tional wave passes by the detec­tor. A change in the lengths of the arms smaller than one-​ten-​thousandth the diam­e­ter of a pro­ton (10^-19 meter) can be detected.