5 ms·

Why would LIGO (https://www.ligo.caltech.edu/ https://www.ligo.caltech.edu/) not detect these signals?

mokus 7y ago
Speculation here, but from the article’s description I suspect these are too high in magnitude (they would likely saturate the sensors) and not the right “mode” or frequency. The LIGO method is looking for very small fluctuations in a more complex field (certain components of the “metric tensor” which amounts to measuring tiny time-dependent variations in the speed of light in two different directions). By contrast, this signal would be a change in the direction and strength of gravity’s local pull and is probably one of the things they work hard to filter out at LIGO. As such, the main instrument probably wouldn’t be able to measure them accurately, but I’d expect the instruments used to maintain and isolate the main instrument would see it pretty well. Similar in concept to how a radio antenna isn’t the best way to measure a shift in the local magnetic field. It’s the “same” thing, but the instrument is sensitive to a very different part of the space of “magnetic field changes”, including in this case the fact that the antenna may actually be measuring the field by looking at different data (electric field) and relying on the way the two interact in the “far field”. Edit: sorry for the abuse of scare quotes, I’m on mobile and it’s the easiest way I can think of right now to indicate that I’m oversimplifying certain technical digressions that are deep subjects in their own right
- jcims 7y ago
  I did some stubby pencil work a while back and if I recall correctly, the mass energy conversion of 1kg of material (i.e. nuclear test) on the other side of the planet was orders of magnitude 'louder' than anything that LIGO has detected so far. It still suffers from having no chirp before that you get from two orbiting bodies that merge, but I think it would technically be detectable as a pop in the signal.
magicalhippo 7y ago
Some quick googling led me to this[1] paper which says the PEGS signals are loudest in the 0.002 to 0.03 Hz range, while Advanced LIGO is primarily sensitive in the 10 to 10kHz range[2]. Reading into the aLIGO paper, seismic and similar noise sources are the main reason it's sensitivity is limited below 10 Hz. [1]: http://www.ipgp.fr/~vallee/PUBLICATIONS/2019_Vallee_Juhel_New_PEGS_JGRAccepted.pdf http://www.ipgp.fr/~vallee/PUBLICATIONS/2019_Vallee_Juhel_Ne... [2]: https://arxiv.org/abs/1604.00439 https://arxiv.org/abs/1604.00439
- hnuser123456 7y ago
  This makes it evident that any gravity wave sources fundamentally slower than 10hz (SMBH merger?) might be undetectable, despite being one of the potentially most interesting events. Then again, who knows how common SMBH mergers are.
  - magicalhippo 7y ago
    That's why they want LISA, as it can sense much lower frequencies[1]. [1]: https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna#/media/File:LISA_and_eLISA_noise_curves.png https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...
    - black_puppydog 7y ago
      Well, that's gonna be of limited use for (terrestrial) seismology. :D

Speculation here, but from the article’s description I suspect these are too high in magnitude (they would likely saturate the sensors) and not the right “mode” or frequency. The LIGO method is looking for very small fluctuations in a more complex field (certain components of the “metric tensor” which amounts to measuring tiny time-dependent variations in the speed of light in two different directions). By contrast, this signal would be a change in the direction and strength of gravity’s local pull and is probably one of the things they work hard to filter out at LIGO. As such, the main instrument probably wouldn’t be able to measure them accurately, but I’d expect the instruments used to maintain and isolate the main instrument would see it pretty well. Similar in concept to how a radio antenna isn’t the best way to measure a shift in the local magnetic field. It’s the “same” thing, but the instrument is sensitive to a very different part of the space of “magnetic field changes”, including in this case the fact that the antenna may actually be measuring the field by looking at different data (electric field) and relying on the way the two interact in the “far field”. Edit: sorry for the abuse of scare quotes, I’m on mobile and it’s the easiest way I can think of right now to indicate that I’m oversimplifying certain technical digressions that are deep subjects in their own right

jcims 7y ago
I did some stubby pencil work a while back and if I recall correctly, the mass energy conversion of 1kg of material (i.e. nuclear test) on the other side of the planet was orders of magnitude 'louder' than anything that LIGO has detected so far. It still suffers from having no chirp before that you get from two orbiting bodies that merge, but I think it would technically be detectable as a pop in the signal.

magicalhippo 7y ago

Some quick googling led me to this[1] paper which says the PEGS signals are loudest in the 0.002 to 0.03 Hz range, while Advanced LIGO is primarily sensitive in the 10 to 10kHz range[2]. Reading into the aLIGO paper, seismic and similar noise sources are the main reason it's sensitivity is limited below 10 Hz. [1]: http://www.ipgp.fr/~vallee/PUBLICATIONS/2019_Vallee_Juhel_New_PEGS_JGRAccepted.pdf http://www.ipgp.fr/~vallee/PUBLICATIONS/2019_Vallee_Juhel_Ne... [2]: https://arxiv.org/abs/1604.00439 https://arxiv.org/abs/1604.00439

hnuser123456 7y ago
This makes it evident that any gravity wave sources fundamentally slower than 10hz (SMBH merger?) might be undetectable, despite being one of the potentially most interesting events. Then again, who knows how common SMBH mergers are.
- magicalhippo 7y ago
  That's why they want LISA, as it can sense much lower frequencies[1]. [1]: https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna#/media/File:LISA_and_eLISA_noise_curves.png https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...
  - black_puppydog 7y ago
    Well, that's gonna be of limited use for (terrestrial) seismology. :D