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Gravitational waves have some similar properties to light. They move at the same speed in a vacuum – and with a certain frequency and amplitude. Where they differ from light is that they are not scattered or absorbed by matter, in the way that light is.
Thus, it’s likely that primordial gravitational waves, that are speculated to have been produced by the Big Bang, are still out there waiting to be detected and analyzed.
Gravitational waves have been indirectly detected via observations of pulsar PSR 1913+16, a member of a binary system, the orbit of which decays at the rate of approximately three millimetres per orbit. The inspiraling of the binary (i.e. the decay of its orbit) can only be explained by an invisible loss of energy, which we presume to be the result of gravitational waves transporting energy away from the system.
Direct observation of gravitational waves currently escapes us – but seems at least feasible by monitoring the alignment of widely separated test masses. Such monitoring systems are currently in place on Earth, including LIGO, which has test masses separated by up to four kilometres – that separation distance being monitored by lasers designed to detect tiny changes in that distance, which might result from the passage of a gravitational wave initiated from a distant point in the universe.
The passing of a gravitational wave should stretch and contract the Earth. This is not because it strikes the Earth and imparts kinetic energy to it – like an ocean wave hitting land. Instead, the Earth – which sits within space-time – has its geometry altered, so that it continues to fit the momentarily stretched and then contracted space-time within which it sits, as a gravitational wave passes.
Gravitational waves are thought to be unaffected by interaction with matter and they move at the speed of light in a vacuum, regardless of whether or not they themselves are in a vacuum. They do lose amplitude (wave height) over distance, but only through attenuation. This is similar to the way that a water wave, emanating from the point of impact of a pebble dropped into a pond, loses amplitude proportionally to the square of the radius of the growing circle that it forms.
Gravity waves may also decline in frequency (i.e. increase in wavelength) over very large distances, due to the expansion of the universe – in much the same way that the wavelength of light is red-shifted by the expansion of the universe.
Given all this, the exceedingly tiny effects that are expected of the gravitational waves that may routinely pass by Earth create a substantial challenge for detection and measurement – since these tiny space-time fluctuations must be distinguished from any background noise.
The noise background for LIGO includes seismic noise (i.e. intrinsic movements of the Earth), instrument noise (i.e. temperature changes that affect the alignment of the detection equipment) and a quantum-level noise, also known as Johnson-Nyquist noise – which arises from the quantum indeterminacy of photon positions.
Kip Thorne, one of the big names in gravity wave theory and research, has apparently ironed out that last and perhaps most troublesome effect through the application of quantum non-demolition principles – which enable the measurement of something without destroying it, or without collapsing its wave function.
Nonetheless, the need for invoking quantum non-demolition principles is some indication of the exceedingly faint nature of gravitational waves – which have a generally weak signal strength (i.e. small amplitude) and low frequency (i.e. long, in fact very long, wavelength).
Where visible light may be 390 nanometres and radio light may be 3 metres in wavelength – gravitational waves are more in the order of 300 kilometres for an average supernova blast, up to 300,000 kilometres for an inspiraling black hole binary and maybe up to 3 billion light years for the primordial echoes of the Big Bang.
So, there’s a fair way to go with all this at a technological level – although proponents (as proponents are want) say that we are on the verge of our first confirmed observation of a gravitational wave – or otherwise they reckon that we have already collected the data, but don’t fully know how to interpret them yet.
This article follows a public lecture delivered by Kip Thorne at the Australian National University in July 2011 – where he discussed plans for LIGO Australia and also the animated simulations of black hole collisions described in the paper below – which may provide templates to interpret the waveforms that will be detected in the future by gravitational wave observatories.
Further reading: Owen et al (including Thorne, K.) Frame-Dragging Vortexes and Tidal Tendexes Attached to Colliding Black Holes: Visualizing the Curvature of Spacetime.