See the main paper in Physical Review Letters: *Observation of Gravitational Waves from a Binary Black Hole Merger* http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.116.061102
In figure 3 on page 4 you see a simplified diagram of the interferometer setup. The text at the end of page 3 describes how the 20 W laser output is increased to 700 W at the main beam splitter (by the power recycling partially reflecting mirror) resulting in a 100 kW recirculating in the two interferometer arm cavities. Strain on space-time at the detector arms by the transverse gravitational wave tends to produce out of phase length changes in the interferometer arms (depending on the polarization and orientation of the wave relative to the detector arms). The high finesse of the cavities causes the phase change due to a gravitational wave at the beam splitter to be about 1,000 times greater than what you would expect for a 8 km round-trip cavity length. The cavity response time is probably around 26 us, so the phase build-up from repeated reflections in the cavity can produce an effect within the bandwidth of the signals being measured (under 1 kHz). The system is carefully adjusted so that the two interferometer legs produce a null at the beam splitter. A transverse gravitational wave of the appropriate orientation and polarization produces a lengthening of one leg and a shortening of the other, and after phase multiplication by the cavity finesse the beam splitter is then slightly out of null and emits a small signal. The next half cycle of the gravitational wave produces another out of null condition in the opposite phase. The finesse of the cavities makes the effective round-trip length of each arm close to 8,000 km (rather than the actual 8 km round-trip length). The reason for the power recycling is to improve the S/N at the beam splitter. However, too much resonant power at various locations in the system produces small movements of the mirrors due to very small amplitude variations in the laser output. I'm sure that the actual system is much more complex than the simplified diagram, and the years of work which has been done improving these detectors has finally improved the sensitivity so that a BBH (binary black hole) merger was finally detected. Neutron star close binary orbits might also be detected by this system. For nearby objects (small redshifts), the volume of space in which such a BBH merger could be detected is improved by the third power of the detector sensitivity. Work is underway to improve the S/N by a factor of 3, which would increase the volume of space in which such objects could be detected by a factor of 27. The current S/N with a matched filter was 24 for the GW150914 event, so I think they are very excited about detecting many events per year. The current two-detector GW telescope has poor angular resolution, since it can only tell that the GW source was near a cone at a certain angle from the axis of the two detectors. When three of more detectors with similar characteristics are in service they can triangulate to give a single direction for the source. Hopefully they will be able to correlate the results with X-ray, optical, and/or radio telescopes in the future to identify the actual source of the gravitational waves. -- Bill Byrom N5BB On Sun, Feb 14, 2016, at 02:26 PM, Paul Boven wrote: > Hi PHK, everyone, > > On 2016-02-14 0:30:22, Poul-Henning Kamp wrote: >> -------- >> In message <1E75A9592178425ABD11390EB725D060@pc52>, "Tom Van Baak" writes: >> >>> Yes, the interferometer is 4 km in length but they bounce the beam back >>> and forth 400 times so the effective length is more like 1600 km. They >>> keep the mirrors stationary to "picometers". They use hundreds of clever >>> tricks to pull this off. >> >> It's actually more amazing than that, each arm is a resonant cavity, >> so while the actual laser is only about 10W, they have about 20 kW of >> photons inflight at any one time. >> >> With 20 kiloWatt of light safety-glasses are not __that__ important any more. > > This number keeps getting repeated, but I have some doubts there. > > The 'finesse' of the cavity is about 1000. The view that the photons > keep bouncing back and forth seems a bit simplistic, wouldn't it be more > like a standing wave? > > The cavity acts as a resonator, and although the instantaneous power > would indeed be 20kW, as soon as you load that cavity, its stored energy > would be dissipated in whoever was unlucky enough to end up in the beam. > Given a length of 4km, it would take no more than 13 us to empty the > cavity. And 13us times 20kW gives an energy of only 0.27 J. > > The part that I am still having trouble understanding is why the two > cavities in the arms of the interferometer help increase the > sensitivity. Are they modulating the reflectivity of the mirrors on the > inner testmasses so they can 'dump' both beams at the same time back > into the half-silvered mirror? > > Cheers, Paul - 73 de PEaNUT. > > > > _________________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.