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Astrophysicists unveil glut of gravitational-wave detections

nature.com

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Re: Astrophysicists unveil glut of gravitational-wave detections

#71

Post by foo92691 »

secondaryacct wrote: post #66
foo92691 wrote: post #58

Earlier quoted context omitted.

You can think of each individual detector producing a single-channel audio signal. By combining the signals from multiple detectors it's possible to determine where the signal is coming from. But the output is neither a picture nor a single pixel: it's a brief blip of a few seconds of audio-frequency time series. A decent analogy is to think of each LIGO detector not as a camera but a microphone.

Or a 3 pixels camera whose pixel color changing is what matters :D The only difference between a camera and a mic is the number or vibrating thing it cares about (mic only cares about the vibration of its single membrane, cameras create millions of membranes sensitive to photon vibration on a grid) LIGO 3 interferometers care about the time-variation of the difference of distance measure in 3 groups of 2 mirrors. So…

The microphone analogy is particularly apt, because the signals are also audio-frequency. You can listen to them. Sometimes in the control room we play the output on a loudspeaker. It can help in tuning the instrument (most of what you hear is noise).

Re: Astrophysicists unveil glut of gravitational-wave detections

#73

Post by jliptzin »

foo92691 wrote: post #62
jliptzin wrote: post #41

How do they know it’s black holes colliding and not just large stars?

Extensive computer modeling is used to predict the expected waveforms, and then we fit the observed waveforms to the expected ones.

Cool thx

Re: Astrophysicists unveil glut of gravitational-wave detections

#74

Post by jjoonathan »

nharada wrote: post #12

LIGO blows my mind whenever I hear news about it. The sensitivity alone is insane -- it can detect a change in distance between its mirrors 1/10000th the width of a proton. This is the equivalent of measuring the distance to the nearest star (4.2 light years) to an accuracy of 1 human hair . Those are bonkers numbers. How the hell did we even come up with this thing?

Speaking of which, I can understand how interferometry gets you to, say, 1/1000th of a wavelength, but the wavelength is 1000nm. How do they go from 1nm to 1/10000th the width of a proton? What's the trick?

Is it an integral transform thing, like how spectrum analyzers can claim super low noise floors if you sort of gloss over the "noise is proportional to badwidth" part and look in a tiny bandwidth without normalizing?

Re: Astrophysicists unveil glut of gravitational-wave detections

#75

Post by sdenton4 »

ww520 wrote: post #18

LIGO is a huge milestone. Turning it on is like the moment Galileo pointed his telescope to the moon. This is a new class of instruments observing the universe in a medium that was never utilized before. Using gravitational waves can observe things that cannot be seen before, like stars behind the dust clouds. It opens a new window to the world. We might finally be able to “see” dark matter. May be able to see the gr…

Here's one (negative!) result on trying to detect dark matter with LIGO:

https://www.ligo.org/science/Publication-O3DarkPhotons/

Re: Astrophysicists unveil glut of gravitational-wave detections

#76

Post by sleavey »

jjoonathan wrote: post #74
nharada wrote: post #12

LIGO blows my mind whenever I hear news about it. The sensitivity alone is insane -- it can detect a change in distance between its mirrors 1/10000th the width of a proton. This is the equivalent of measuring the distance to the nearest star (4.2 light years) to an accuracy of 1 human hair . Those are bonkers numbers. How the hell did we even come up with this thing?

Speaking of which, I can understand how interferometry gets you to, say, 1/1000th of a wavelength, but the wavelength is 1000nm. How do they go from 1nm to 1/10000th the width of a proton? What's the trick? Is it an integral transform thing, like how spectrum analyzers can claim super low noise floors if you sort of gloss over the "noise is proportional to badwidth" part and look in a tiny bandwidth without normalizi…

Cavities. We trade off bandwidth for peak sensitivity by sending the same light back and forth between mirrors in the arms of the interferometer hundreds of times. As the gravitational wave passes, the same light samples it over and over and picks up additional phase shift, enhancing the signal. The downside is that we can't see gravitational waves at signals far above the cavity pole frequencies at a few 10s of kHz, but the most promising sources we aimed at when the detectors were designed were considered to be below that.

We also use techniques called power and signal recycling to enhance this bandwidth-sensitivity tradeoff even more. Combined these techniques give you what remains between your 1/1000th wavelength and the actual sensitivity of LIGO and Virgo.

Re: Astrophysicists unveil glut of gravitational-wave detections

#77

Post by mabbo »

pinko wrote: post #27
mabbo wrote: post #20

The article mentions the new KAGRA detector in Japan joining the group. Does anyone know: how does the accuracy improve as more detectors come online? Will we see a day where we have 20, 50, 100 detectors around the globe and events are near-certain because so many detectors see them? Or is the diminishing returns, and 4 detectors is already too many?

You might be interested in this talk on the future of GW detectors and the resulting science prospects: https://www.youtube.com/watch?v=iet6pS4gxCk (esp. from ~25:40 to the end).

Thanks, that was very intersting!

Re: Astrophysicists unveil glut of gravitational-wave detections

#78

Post by tobinfricke »

jjoonathan wrote: post #74
nharada wrote: post #12

LIGO blows my mind whenever I hear news about it. The sensitivity alone is insane -- it can detect a change in distance between its mirrors 1/10000th the width of a proton. This is the equivalent of measuring the distance to the nearest star (4.2 light years) to an accuracy of 1 human hair . Those are bonkers numbers. How the hell did we even come up with this thing?

Speaking of which, I can understand how interferometry gets you to, say, 1/1000th of a wavelength, but the wavelength is 1000nm. How do they go from 1nm to 1/10000th the width of a proton? What's the trick? Is it an integral transform thing, like how spectrum analyzers can claim super low noise floors if you sort of gloss over the "noise is proportional to badwidth" part and look in a tiny bandwidth without normalizi…

Great question! The precision is not just better than the wavelength of the light. It's also way smaller than the surface roughness of the mirrors! How does it work?!

Like you suggest, and adding to what sleavey mentioned above, I would say the answer is: averaging over time and space. The laser beam is pretty wide, so it averages over a significant area of mirror surface. (The optical system also selects one spatial mode of the laser beam.) And the stated displacement sensitivity ("1/10000 the width of a proton") only occurs when you integrate over the sensitive frequency band.

Re: Astrophysicists unveil glut of gravitational-wave detections

#79

Post by colanderman »

sleavey wrote: post #76
jjoonathan wrote: post #74

Earlier quoted context omitted.

Speaking of which, I can understand how interferometry gets you to, say, 1/1000th of a wavelength, but the wavelength is 1000nm. How do they go from 1nm to 1/10000th the width of a proton? What's the trick? Is it an integral transform thing, like how spectrum analyzers can claim super low noise floors if you sort of gloss over the "noise is proportional to badwidth" part and look in a tiny bandwidth without normalizi…

Cavities. We trade off bandwidth for peak sensitivity by sending the same light back and forth between mirrors in the arms of the interferometer hundreds of times. As the gravitational wave passes, the same light samples it over and over and picks up additional phase shift, enhancing the signal. The downside is that we can't see gravitational waves at signals far above the cavity pole frequencies at a few 10s of kHz,…

That sounds like a block & tackle [1] for light.

[1] https://en.wikipedia.org/wiki/Block_and_tackle

Re: Astrophysicists unveil glut of gravitational-wave detections

#80

Post by baq »

foo92691 wrote: post #65
mrfusion wrote: post #19

Could we be seeing warp drive signatures?

So far all of the signals observed by LIGO are from sources that we expected would exist. The most exciting thing would be to observe an unexpected signal.

did you model how an alcubierre drive would look like on the waveform? (only half joking ;))
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