( ( ( ( ( If ( ( the ( test masses ) ) ( are ( subjected ( to ( a ( gravitational wave ) ) ) ) ) ) ) ( , ( ( their positions ) ( will ( change ( in ( a ( ( ( ( small , ) but ) well-defined ) way ) ) ) ) ) ) ) ) ; ) ( it ( is ( ( this ( resulting motion ) ) ( that ( LIGO ( is ( designed ( to detect ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (SBAR (IN If) (S (NP (DT the) (NN test) (NNS masses)) (VP (VBP are) (VP (VBN subjected) (PP (TO to) (NP (DT a) (JJ gravitational) (NN wave))))))) (, ,) (NP (PRP$ their) (NNS positions)) (VP (MD will) (VP (VB change) (PP (IN in) (NP (DT a) (ADJP (ADJP (JJ small)) (, ,) (CC but) (ADJP (JJ well-defined))) (NN way)))))) (: ;) (S (NP (PRP it)) (VP (VBZ is) (NP (NP (DT this) (VBG resulting) (NN motion)) (SBAR (IN that) (S (NP (NNP LIGO)) (VP (VBZ is) (VP (VBN designed) (S (VP (TO to) (VP (VB detect))))))))))) (. .)))
If the test masses are subjected to a gravitational wave, their positions will change in a     small, but well-defined way; it is this resulting motion that LIGO is designed to detect.
(ROOT (S (PP (IN In) (NP (NN science))) (, ,) (NP (NN motion)) (VP (VBZ is) (VP (VBN defined) (PP (IN as) (NP (NP (DT a) (NN change)) (PP (IN in) (NP (NN position))))))) (. .)))