4 ms·

> usually created by the merging of two black holes — have dramatically increased their sensitivity since the first identification was made in 2015. The growin

> usually created by the merging of two black holes — have dramatically increased their sensitivity since the first identification was made in 2015. The growing data set is helping astronomy That graph looks like a hockey stick. Is it really accelerating or are we just getting better at detecting it (this makes more sense)? One question for me is how do they know which collision is BH+BH, BH+star, Or star+star? That sounds incredibly interesting detail

m4r35n357 6y ago
Looks like things started to get _much_ better after Virgo joined. I assume that is because of the increased "baseline" (angular resolution) and because two different instruments at different locations makes it easier to detect & reject spurious signals.
- the8472 6y ago
  > and because two different instruments at different locations Virgo is the 3rd actually. LIGO consists of 2 separate facilities. But yes, having more detectors decreases the degrees of freedom when calculating the origin of the signal.
tsimionescu 6y ago
> One question for me is how do they know which collision is BH+BH, BH+star, Or star+star IANAA, but I imagine that they can detect the source of the waves and check electromagnetic observations from the same area. BH+BH = no electromagnetic radiation, star+star = electromagnetic radiation from 2 stars. Of course, it's not easy to detect such fine measurements and correlate between different instruments.
- jessriedel 6y ago
  No, the distinction between BH and NS can be made by just looking at the shape of the gravitational wave (stress as a function of time). A BH and a NS of the same mass will have different densities, so they produce very different ripples as they merge. Of course, this can be corroborated with electromagnetic observations. But, to my knowledge, not all NS-NS mergers are witnessed in telescopes for various technical limitations.
  - astro123 6y ago
    I don't think that is right, though I'm not a GW expert so please tell me if I'm wrong. I think they know the difference just by looking at the mass. i.e. we think it is hard to form black holes smaller than ~3.3ish solar masses and we don't think neutron stars can be more massive than 2.2 solar masses. This is why we get articles like [1] where there is an issue when we think we've found something between those numbers. And yes, the foolproof way of checking whether a NS was involved is to follow up with telescopes. But the constraints on position from GW aren't always good and so you can't always find it. [1] https://www.sciencemag.org/news/2020/06/gravitational-waves-reveal-lightest-black-hole-ever-observed https://www.sciencemag.org/news/2020/06/gravitational-waves-...
    - jessriedel 6y ago
      Yikes, on reflection I think you're right. Sorry about that. Thanks for the correction!
    - jessriedel 6y ago
      Ok, I'm actually only 90% wrong! It turns out that there is in fact some information about the BH-NS distinction in the gravitational signal itself, but it is very weak and hasn't been detected yet. It is expected to be detected in the future though. https://twitter.com/di_goldene_pave/status/1322322977984143360 https://twitter.com/di_goldene_pave/status/13223229779841433...
      - astro123 6y ago
        I didn't know about that, that's really cool! Thanks for letting me know.
privong 6y ago
> That graph looks like a hockey stick. Is it really accelerating or are we just getting better at detecting it (this makes more sense)? I suspect it is a piecewise combination of two linear functions. There was probably a ~fixed sensitivity across Runs 1-2, with some upgrades before Run 3[0], which resulted in an improved sensitivity for that run. So in effect the slope of detections/time is steeper with the improved sensitivity. [0] https://indico.cern.ch/event/577856/contributions/3422625/ https://indico.cern.ch/event/577856/contributions/3422625/
deleted 6y ago
[deleted]
framecowbird 6y ago
surely the latter, and that is what the sentence implies: "observatories [...] have dramatically increased their sensitivity"
ojnabieoot 6y ago
For the first question: I think a big part is that other detectors came online, providing much more confidence that LIGO measurements were real gravitational waves. For the second question: they can infer the masses of the objects in question from the gravitational waves. Generally if the mass is < 2.5 solar masses it has to be a neutron star, and if it’s > 5 it must be a black hole. Keep in mind this isn’t rigorous! The theoretical maximum for neutron stars is a little less than 3 solar masses, and the estimated minimum for a stellar black hole is much more empirical - I think the smallest known is about 3.5. There’s a lot of interesting physics between 2.5 and 5 solar masses - perhaps this is how we’ll discover “quark stars.” I believe our detectors are not sensitive enough to detect the acceleration of normal stars in any physically plausible scenario - I think they would have to be accelerating way too fast to generate waves that are indistinguishable from noise.
- AtlasBarfed 6y ago
  Could a gravity whip of a large star around the a galactic central black hole make a big enough ripple?
  - ojnabieoot 6y ago
    I am not sure and am only a layperson on this (math grad school, physics undergrad) - conceptually you could think of a blue giant orbiting at 0.99c, but a) the orbit would be so large that the actual acceleration is fairly small, or b) the centripetal forces of a small, relativistic orbit would rip the star into pieces. In particular this would dramatically reduce the effective density of the accelerating mass. Having enough gravitational force to retain coherent structure while accelerating quickly enough to generate gravitational waves seems like a tough circle to square.
  - simonh 6y ago
    The material in stars is too spread out over a large volume, and stars move too slowly for these systems to detect them. Black hole collisions events last on the order of about 10 milliseconds and emit up to about 40% of their combined mass in gravitational energy. Nothing a star can do is fast or energetic enough to register at those scales.
wolfram74 6y ago
Sensitivity is getting much better, vibrational isolation, processing, this run I believe even includes elements that reduce the noise produced from quantum uncertainties [0]. The uncertainty principle means the product of two measurements (positionmomentum, timeenergy, conjugate variables in general) can't be more precise than a certain threshold, so if you only care about one value of that pair, you can dump all the uncertainty into the unmeasured quantity and get past what might otherwise be considered a barrier. [0]https://astronomy.com/news/2019/12/new-technology-improves-gravitational-wave-detectors-by-cutting-quantum-noise https://astronomy.com/news/2019/12/new-technology-improves-g...
jorpal 6y ago
The number of detected events depend on the volume of space surveyed. So, for some increase in detector sensitivity x you get x^3 more events, roughly speaking. We’re just getting better at detecting them.

m4r35n357 6y ago

Looks like things started to get _much_ better after Virgo joined. I assume that is because of the increased "baseline" (angular resolution) and because two different instruments at different locations makes it easier to detect & reject spurious signals.

the8472 6y ago
> and because two different instruments at different locations Virgo is the 3rd actually. LIGO consists of 2 separate facilities. But yes, having more detectors decreases the degrees of freedom when calculating the origin of the signal.

tsimionescu 6y ago

> One question for me is how do they know which collision is BH+BH, BH+star, Or star+star IANAA, but I imagine that they can detect the source of the waves and check electromagnetic observations from the same area. BH+BH = no electromagnetic radiation, star+star = electromagnetic radiation from 2 stars. Of course, it's not easy to detect such fine measurements and correlate between different instruments.

jessriedel 6y ago
No, the distinction between BH and NS can be made by just looking at the shape of the gravitational wave (stress as a function of time). A BH and a NS of the same mass will have different densities, so they produce very different ripples as they merge. Of course, this can be corroborated with electromagnetic observations. But, to my knowledge, not all NS-NS mergers are witnessed in telescopes for various technical limitations.
- astro123 6y ago
  I don't think that is right, though I'm not a GW expert so please tell me if I'm wrong. I think they know the difference just by looking at the mass. i.e. we think it is hard to form black holes smaller than ~3.3ish solar masses and we don't think neutron stars can be more massive than 2.2 solar masses. This is why we get articles like [1] where there is an issue when we think we've found something between those numbers. And yes, the foolproof way of checking whether a NS was involved is to follow up with telescopes. But the constraints on position from GW aren't always good and so you can't always find it. [1] https://www.sciencemag.org/news/2020/06/gravitational-waves-reveal-lightest-black-hole-ever-observed https://www.sciencemag.org/news/2020/06/gravitational-waves-...
  - jessriedel 6y ago
    Yikes, on reflection I think you're right. Sorry about that. Thanks for the correction!
  - jessriedel 6y ago
    Ok, I'm actually only 90% wrong! It turns out that there is in fact some information about the BH-NS distinction in the gravitational signal itself, but it is very weak and hasn't been detected yet. It is expected to be detected in the future though. https://twitter.com/di_goldene_pave/status/1322322977984143360 https://twitter.com/di_goldene_pave/status/13223229779841433...
    - astro123 6y ago
      I didn't know about that, that's really cool! Thanks for letting me know.

privong 6y ago

> That graph looks like a hockey stick. Is it really accelerating or are we just getting better at detecting it (this makes more sense)? I suspect it is a piecewise combination of two linear functions. There was probably a ~fixed sensitivity across Runs 1-2, with some upgrades before Run 3[0], which resulted in an improved sensitivity for that run. So in effect the slope of detections/time is steeper with the improved sensitivity. [0] https://indico.cern.ch/event/577856/contributions/3422625/ https://indico.cern.ch/event/577856/contributions/3422625/

deleted 6y ago

[deleted]

framecowbird 6y ago

surely the latter, and that is what the sentence implies: "observatories [...] have dramatically increased their sensitivity"

ojnabieoot 6y ago

For the first question: I think a big part is that other detectors came online, providing much more confidence that LIGO measurements were real gravitational waves. For the second question: they can infer the masses of the objects in question from the gravitational waves. Generally if the mass is < 2.5 solar masses it has to be a neutron star, and if it’s > 5 it must be a black hole. Keep in mind this isn’t rigorous! The theoretical maximum for neutron stars is a little less than 3 solar masses, and the estimated minimum for a stellar black hole is much more empirical - I think the smallest known is about 3.5. There’s a lot of interesting physics between 2.5 and 5 solar masses - perhaps this is how we’ll discover “quark stars.” I believe our detectors are not sensitive enough to detect the acceleration of normal stars in any physically plausible scenario - I think they would have to be accelerating way too fast to generate waves that are indistinguishable from noise.

AtlasBarfed 6y ago
Could a gravity whip of a large star around the a galactic central black hole make a big enough ripple?
- ojnabieoot 6y ago
  I am not sure and am only a layperson on this (math grad school, physics undergrad) - conceptually you could think of a blue giant orbiting at 0.99c, but a) the orbit would be so large that the actual acceleration is fairly small, or b) the centripetal forces of a small, relativistic orbit would rip the star into pieces. In particular this would dramatically reduce the effective density of the accelerating mass. Having enough gravitational force to retain coherent structure while accelerating quickly enough to generate gravitational waves seems like a tough circle to square.
- simonh 6y ago
  The material in stars is too spread out over a large volume, and stars move too slowly for these systems to detect them. Black hole collisions events last on the order of about 10 milliseconds and emit up to about 40% of their combined mass in gravitational energy. Nothing a star can do is fast or energetic enough to register at those scales.

wolfram74 6y ago

Sensitivity is getting much better, vibrational isolation, processing, this run I believe even includes elements that reduce the noise produced from quantum uncertainties [0]. The uncertainty principle means the product of two measurements (positionmomentum, timeenergy, conjugate variables in general) can't be more precise than a certain threshold, so if you only care about one value of that pair, you can dump all the uncertainty into the unmeasured quantity and get past what might otherwise be considered a barrier. [0]https://astronomy.com/news/2019/12/new-technology-improves-gravitational-wave-detectors-by-cutting-quantum-noise https://astronomy.com/news/2019/12/new-technology-improves-g...

jorpal 6y ago

The number of detected events depend on the volume of space surveyed. So, for some increase in detector sensitivity x you get x^3 more events, roughly speaking. We’re just getting better at detecting them.