The Laser Interferometer Gravitational-Wave Observatory (LIGO) is an ambitious project. The experiment is designed to detect and characterize gravitational waves generated by energetic and massive events in the cosmos. What’s more, as LIGO has two stations situated 3000 kilometres (1870 miles) apart, through triangulation, the location of a star collision or black hole event can be deduced in the sky. Completed two years ago, LIGO has been taking data ever since and the time has now come to begin analysing the results, seeing if the theoretical gravitational wave can actually be observed, bringing us into a new era of astronomy, gravitational wave astronomy…
Much like ripples in a space-time pond, gravitational waves are expected to propagate throughout the universe. They are created by any massive object, oscillating, orbiting or colliding, any motion that disturbs the fabric of space-time. Gravitational waves are a consequence of Einsteins general theory of relativity, and should they be detected, a direct observation of a fluctuation in space-time will have been discovered. This is where projects such as LIGO are needed.
So far, astronomers can observe electromagnetic radiation (be it optical, infra-red, X-ray or ultraviolet) and have done since Galileo Galilei’s conception of the telescope in the 17th Century. Present day, we have a whole host of observatories (Earth-based and space-borne) detecting this electromagnetic radiation, probing deep into the Suns atmosphere, seeing through stellar clouds, observing proto-stars and even detecting the faint microwave background radiation created by the Big Bang echoing around the cosmos.
To complement electromagnetic astronomy, we now have neutrino detectors, located deep underground to avoid contamination by cosmic rays and other ambient radiation. Neutrinos very weakly interact with matter, making them notoriously hard to detect. The only practical way to detect these ghostly particles is to create a very big target – in this case millions of gallons of fluid (water or some other reactive agent). As the neutrinos hit the target, a few may interact with the fluid molecules, generating a flash of Cherenkov radiation. Neutrinos are an important part of observing the conditions inside our Sun and also, due to the entire Universe being bathed in neutrinos, hold a vast quantity mass and therefore a source of dark matter.
Now, scientists hope to begin a new type of astronomy, adding to electromagnetic and neutrino observations: the observation of gravity, more precisely, the detection of gravitational waves as they ripple through space-time.
In a new report by Maria Alessandra Papa, results from the LIGO Scientific Collaboration (LSC) are reviewed, and generally the data is very exciting. The LIGO system has surpassed all sensitivity expectations. Having just completed a fifth science run, LIGO and other associated detectors have been pooling their data in the hope of detecting compact binary systems (a compact binary white dwarf star system is visualized by this superb NASA animation – gravitational waves included). LIGO is sensitive to waves of between 50 Hz and 1500 Hz, the expected frequency range of such a system.
The two LIGO installations in Washington and Louisiana are approximately 3000 km apart, and both are in an L-shaped configuration. Each “leg” of the L is 4 km long, encasing an ultra high vacuum where lasers can pass unhindered. Interferometers at the L joint are targeted by the lasers. Interferometery is a technique to compare the phase of two waves and to understand what conditions may affect the nature of the light if the phase changes. As LIGO has such a long baseline (i.e. a long distance for the laser to travel), any changes in the laser will be amplified at the interferometer. In the case of a gravitational wave, should it propagate through local space, the laser light will be influenced, changing the signal at the detector. In theory, as a gravitational wave travels through a LIGO laser, its path will be varied (after all, even laser photon paths can be effected by a ripple in space-time), creating an interferometer signal.
Gravitational waves travel at the speed of light, so once a signal is picked up by one LIGO station, it will be a matter of milliseconds before it is received at the second station 3000 km away. Also, as there are two stations, triangulation may be used to derive the location of the gravitational disturbance.
But there’s a catch. This is all very well and good in theory, but we are not entirely sure what the signature of a gravitational wave looks like, so much more data will need to be acquired before a pattern may begin to form. According to Papa, two years of data acquisition will increase the volume of the gravitational Universe that we can observe by a factor of eight. After six years, this will increase to a factor of 1000. The data collected so far, although encouraging, does not challenge our understanding of the cosmos quite yet as we simply don’t have enough data to understand what a gravitational wave looks like, let alone whether it was generated by a black hole, binary system or a supernova. If in six years, definitive proof of a gravitational wave has not been found, we may have to re-think the nature of gravitational waves and go back to the space-time textbooks…