[time-nuts] LIGO detects gravitational waves

[Bill Byrom]

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 at 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.
>  
>  
>  
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