When tapped at any of the four cardinal points \u2013 North, South, East or West \u2013 a teacup with its handle towards the North emits the same ringing sound. When the tap is displaced by 45 degrees, the note is about a semitone higher. <\/p>\n
The tapping causes microscopic distortions in the circular rim, which is rapidly stretched and squeezed in perpendicular directions, like a pulsating ellipse [in simplistic terms, an oval].<\/p>\n
The difference in pitch is due to the inertia of the handle: with taps at the cardinal points, the handle oscillates back and forth. <\/p>\n
But with taps at the secondary points, the handle remains unmoving at a node or static point, so the oscillations are more rapid and higher in pitch.<\/p>\n
The ringing teacup has two polarisation modes, which we may call the plus and cross mode. This is also a feature of gravitational waves \u2013 ripples in spacetime generated by cosmic cataclysms. <\/p>\n
[\u00a0Background hum in the cosmos: astrophysicists hail \u2018the start of something big\u2019Opens in new window<\/a>\u00a0]<\/p>\n Shake an electric charge and you generate radio waves. Shake a mass and you generate gravitational waves \u2013 transverse waves with plus and cross polarisation modes \u2013 just like the teacup. <\/p>\n Gravitational waves are detectable using instruments called interferometers, which measure tiny compressions and elongations of space as the waves pass.<\/p>\n From Albert Einstein<\/a>\u2019s general relativity theory, several new phenomena became apparent. Black holes were anticipated by early solutions of his equations. <\/p>\n Einstein also showed that, just as matter can bend space, accelerating matter can cause spacetime to oscillate, generating ripples that spread out at light speed. Einstein believed that they would be too weak to be observed. Indeed, a century elapsed before these gravitational waves were detected.<\/p>\n When two massive black holes collide, they trigger what might be called a space-quake, radiating waves that travel at light speed through the cosmos. <\/p>\n The Laser Interferometer Gravitational-Wave Observatory (LIGO) comprises two interferometer arrays, both in the United States<\/a> \u2013 one in Louisiana and one in Washington State \u2013 which can detect the distortions of spacetime resulting from gravitational waves. <\/p>\n Each spacecraft carries two telescopes and two lasers pointing at the other two spacecraft. When gravitational waves pass the array, the lengths of the arms vary due to spacetime distortions caused by the waves<\/p>\n LIGO has two laser beams and two detectors 3,000km apart. In September 2015, both arrays observed gravitational waves from the merger, more than a billion years ago, of two black holes. <\/p>\n The scope of the earthbound LIGO observatory is limited by its physical size. To detect a broader range of gravitational waves, a much larger array is required. The Laser Interferometer Space Antenna (LISA) mission is led by the European Space Agency (ESA)<\/a>, with Nasa<\/a> as a major partner. This space-based array will be able to detect gravitational waves in a lower frequency band than LIGO. LISA is scheduled to launch in 2035.<\/p>\n