I spent last week at Columbia University attending the Edoardo Amaldi Conference, the largest annual international meeting on gravitational waves. Short synopsis: GWs have not been found.
I was reminded that I still owe CV readers a discussion of gravitational waves, following-up from an ancient post on their convoluted theoretical history. While the theoretical community was arguing about the existence of gravitational waves, the observational community was essentially non-existent. The wave strengths expected at Earth are extraordinarily weak, with the most promising sources being the inspiral and merger of stellar-mass compact binary systems (e.g., neutron stars and/or black holes). It took great courage and vision to propose instruments to detect the waves at all. One of the pioneers of gravitational wave detection was Joe Weber, who first invented and built so-called Weber bar detectors. As the gravitational waves pass through a large cylindrical bar of material, they excite resonant modes of the bar, which might then be detectable. In 1969 Weber published an article in Physical Review Letters announcing the detection of gravitational waves by noting coincidences between two bar detectors separated by a thousand kilometers. These observations generated tremendous excitement, especially given that they suggested wave strengths greatly in excess of what was expected. Unfortunately, as other experimenters built ever more sophisticated follow-up detectors, they were unable to reproduce his findings. Over three decades later, and with many orders-of-magnitude improvement in detector sensitivity, gravitational waves have yet to be directly detected. An unfortunate side-note is that Weber continued to claim that he was seeing gravitational waves, even in the face of compelling counter evidence, right up until his death in 2000. Weber was an experimental master (having independently invented the maser and the laser), and is widely credited as the father of gravitational wave astronomy, but in his last decades he was an outcast of the very community he helped found.
A profound development in gravitational waves detection came in 1974 with the discovery by Hulse and Taylor of a binary pulsar. This system consists of a 59 millisecond pulsar in orbit with another star, with a period of 7.75 hours. The pulsar provides an exceedingly precise clock, allowing us to measure the spin-down of the binary system due to the emission of gravitational waves (using the same quadrupole formula mentioned in the previous post). Theory and observation agree spectacularly well, and Hulse and Taylor were awarded the Nobel prize for this indirect detection of gravitational waves. The community is now breathlessly awaiting the first direct detection of gravitational waves. So why am I nattering on about all this? Because of this:
This is a sensitivity plot of LIGO, the Laser Interferometer Gravitational Wave Observatory. LIGO is composed of two power-recycled Michelson interferometers with 4-km long Fabry-Perot arms, located in Hanford, Washington and Livingston, Louisiana. Each curve shows the noise floor, with LIGO sensitive to sources falling above the curve. The curves represent official science runs, and the steady progression shows the improvement since 2002 (1st science run). LIGO has been in development for decades, and has in the last few years (the red curve) reached its original design sensitivity (the solid black curve). LIGO has finished a year-long science run (S5), which constitutes by far the deepest look we’ve ever had at the gravitational wave Universe. It hasn’t seen anything yet; only upper limits. (At least, that is the public stance. Given the convoluted history detailed above, there will be a long period of double and triple checking before any sort of public announcement is made. Although it’s hard to imagine rumors of a first detection won’t leak out, and my rumor well is dry.) At the moment LIGO is being “Enhanced” (a factor of 2 improvement), with the installation of more powerful lasers. It should be back up and running within the year, will run for a year, and then will undergo a major upgrade. By 2014 LIGO will come back online at “Advanced” sensitivity (another factor of 5, which translates into a factor of 1000 in volume compared to today), at which point the first direct detection of gravitational waves is widely anticipated.
There is lots to say about LIGO, but I’d like to focus on one point: the scale on the y-axis of the plot above (which represents strain, the fractional change in the length of the LIGO arms). LIGO is sensitive, over a wide range of frequency, to a strain of better than 1 part in 1022. In other words, it measures changes in the relative length of its 4km arms to better than a thousandth of the size of a proton. This plot should absolutely blow your mind. If not, perhaps I’m being too abstract? This is the equivalent of monitoring changes in the distance between New York and San Francisco to better than one ten billionth the width of a human hair. LIGO is a technical tour-de-force. It is one of the most amazing instruments humankind has ever built.
N.B.: In the comments, Brian137 points out that LIGO will be back on starting tomorrow (7/7/09) for a month-long run! (From the LIGO blog.) And nicolas points out that I was remiss in neglecting to mention Virgo, which is a French/Italian GW detector currently operating in Italy. It is easily as impressive as LIGO, since it achieves similar sensitivity with 3km arms (instead of 4km for LIGO).