<p>One element of the answer is that they do it in the other direction. They simulate <a href="https://arxiv.org/abs/1904.04831" rel="nofollow noreferrer">many different possible black hole mergers</a> and then look for the nearest matches to what they observe. I imagine there is some scope for interpolating between the simulations as well as running <a href="https://www.ligo.caltech.edu/video/ligo20181203v1" rel="nofollow noreferrer">bespoke simulations</a> to confirm the match. Spin affects the wave because the shape of space around a spinning black hole is different from that around a non-spinning one, which affects both the motion of the black holes and the processes by which the gravitational waves are radiated.</p>

CC BY-SA 4.0
2019-06-26T08:41:22.977Z
2019-06-26T09:13:14.297Z
2019-06-26T09:13:14.297Z
Steve Linton
<p>After recording the ripples of  passing gravity waves I would imagine that some high-power math would be required to derive information about the sizes, spin, and distance of the merging black holes. Is it possible to get some insight into how they figured out one was 31 solar masses and the other was 20? How do you "see" spin from a wave?</p>

CC BY-SA 4.0
2019-06-26T08:30:53.760Z
There are some answers to this in this site and in [Physics SE](https://physics.stackexchange.com/) as well but I don't have time to search for them. Briefly, they run simulations with a large number of different starting conditions (masses, spins, etc.) and for each they calculate what the radiated gravitational waves would look like viewed from different directions relative to the pair's orbital plane, and for different orientations of the GW detectors. All of these get put into a big library of possible waveforms.
uhoh
CC BY-SA 4.0
2019-06-26T08:31:52.610Z
When a real gravitational wave is recorded, they search the library for the best fit as a first pass, and if there is anything even close they go on to refine the simulations to improve the fit. See for example [What does “GPU-accelerated butterfly matched filtering over dense bank of time-symmetric chirp-like templates” mean? (GW170817)](https://astronomy.stackexchange.com/q/28404/7982)
uhoh
CC BY-SA 4.0
2019-06-26T08:38:03.397Z
Also see the many excellent answers to [What did LIGO Actually See? (Gravitational waves discovery](https://astronomy.stackexchange.com/q/13782/7982) and to [Observation of Gravitional waves by LIGO from binary black hole merger](https://astronomy.stackexchange.com/q/20691/7982) and to [LIGO gravitational wave chirp signal frequency](https://astronomy.stackexchange.com/q/15023/7982) and to [“Who saw” the binary neutron star merger first? What was the sequence of events? (GRB/GW170817)](https://astronomy.stackexchange.com/q/23187/7982).
uhoh
CC BY-SA 4.0
2019-06-26T12:27:41.707Z
I've edited the title: Gravity -> Gravitational. Gravity waves are an atmospheric/hydrodynamic phenomenon. The name is easily mixed up with gravitational waves, which are a GR phenomenon.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-06-27T03:21:03.593Z
Thanks so much for the answers and links. I will eagerly research them.
Gowron
<gravitational-waves>
Gravitational waves: how can scientists derrive the sizes and spin and distance of two merging black holes?
Gowron
# Gravitational waves: how can scientists derrive the sizes and spin and distance of two merging black holes?

(Asked by: username_0 on 2019-06-26)

After recording the ripples of passing gravity waves I would imagine that some high-power math would be required to derive information about the sizes, spin, and distance of the merging black holes. Is it possible to get some insight into how they figured out one was 31 solar masses and the other was 20? How do you "see" spin from a wave?

**Tags:** gravitational-waves
> **Comments:**
> **username_1**: There are some answers to this in this site and in [Physics SE](https://physics.stackexchange.com/) as well but I don't have time to search for them. Briefly, they run simulations with a large number of different starting conditions (masses, spins, etc.) and for each they calculate what the radiated gravitational waves would look like viewed from different directions relative to the pair's orbital plane, and for different orientations of the GW detectors. All of these get put into a big library of possible waveforms.  
> **username_1**: When a real gravitational wave is recorded, they search the library for the best fit as a first pass, and if there is anything even close they go on to refine the simulations to improve the fit. See for example [What does “GPU-accelerated butterfly matched filtering over dense bank of time-symmetric chirp-like templates” mean? (GW170817)](https://astronomy.stackexchange.com/q/28404/7982)  
> **username_1**: Also see the many excellent answers to [What did LIGO Actually See? (Gravitational waves discovery](https://astronomy.stackexchange.com/q/13782/7982) and to [Observation of Gravitional waves by LIGO from binary black hole merger](https://astronomy.stackexchange.com/q/20691/7982) and to [LIGO gravitational wave chirp signal frequency](https://astronomy.stackexchange.com/q/15023/7982) and to [“Who saw” the binary neutron star merger first? What was the sequence of events? (GRB/GW170817)](https://astronomy.stackexchange.com/q/23187/7982).  
> **username_2**: I've edited the title: Gravity -> Gravitational. Gravity waves are an atmospheric/hydrodynamic phenomenon. The name is easily mixed up with gravitational waves, which are a GR phenomenon.  
> **username_0**: Thanks so much for the answers and links. I will eagerly research them.  

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## 1 answers

### Answer by username_3 on 2019-06-26. Score: 5

One element of the answer is that they do it in the other direction. They simulate [many different possible black hole mergers](https://arxiv.org/abs/1904.04831) and then look for the nearest matches to what they observe. I imagine there is some scope for interpolating between the simulations as well as running [bespoke simulations](https://www.ligo.caltech.edu/video/ligo20181203v1) to confirm the match. Spin affects the wave because the shape of space around a spinning black hole is different from that around a non-spinning one, which affects both the motion of the black holes and the processes by which the gravitational waves are radiated.


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