Editing Gravitational wave (section)

===Difficulties===
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Gravitational waves are not easily detectable. When they reach the Earth, they have a small amplitude with strain approximately 10<sup>−21</sup>, meaning that an extremely sensitive detector is needed, and that other sources of noise can overwhelm the signal.<ref>{{cite web|title=Noise and Sensitivity|url=http://www.gwoptics.org/ebook/sensitivity_noise.php|website=gwoptics: Gravitational wave E-book|publisher=[[University of Birmingham]]|access-date=10 December 2015}}</ref> Gravitational waves are expected to have frequencies 10<sup>−16</sup>&nbsp;Hz < ''f'' < 10<sup>4</sup>&nbsp;Hz.<ref name="arXiv:gr-qc/9506086">{{Cite journal |last=Thorne |first=Kip S. |author-link=Kip Thorne |date=1995-07-01 |title=Gravitational Waves |url=https://archive.org/details/arxiv-gr-qc9506086 |journal=Particle and Nuclear Astrophysics and Cosmology in the Next Millenium |language=English |page=160 |arxiv=gr-qc/9506086 |bibcode=1995pnac.conf..160T}}</ref>