Why can't LIGO detect collisions of super massive black holes?

[mitosis]

Does anyone know, and is there an instrument that could?

 

[PeterDonis]

Why do you think LIGO can't detect collisions of supermassive black holes? What do you think LIGO has been detecting?

 

[phyzguy]

I think the OP is considering supermassive black holes to be black holes with masses of millions or billions of solar masses, and is asking why LIGO doesn't detect collisions of them. The collisions LIGO has detected so far have been stellar mass black holes. The answer is that LIGO would detect a SMBH collision if it occurred, but they are very rare. I don't know the exact number, but they probably only occur in the observable universe on million year time scales.

 

[Vanadium 50]

We are indulging in that favorite PF activity "guess what the poster meant". Maybe we should let him or her tell us what he or she means.

 

[phyzguy]

We are indulging in that favorite PF activity "guess what the poster meant". Maybe we should let him or her tell us what he or she means.

I disagree. The phrase "super massive black hole" has a well-defined astrophysical meaning. For example, Wikipedia says:

"A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, containing a mass of the order of hundreds of thousands, to billions times, the mass of the Sun (M☉)."

 

[mitosis]

I disagree. The phrase "super massive black hole" has a well-defined astrophysical meaning. For example, Wikipedia says:

"A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, containing a mass of the order of hundreds of thousands, to billions times, the mass of the Sun (M☉)."

 

[mitosis]

Exactly right, they have been recording stellar black hole collisions. I was hoping it was just the frequency/ timing issue but I thought I heard a LIGO scientist in an interview say they couldn't measure a super massive black hole collision, so I wanted to check with this wonderful community! I appreciate that this exists.

 

[Vanadium 50]

but I thought I heard

Can you tell us where?

 

[mitosis]

I thought it was on a segment of Science Friday on NPR.

 

[phyzguy]

Well, the frequency of the gravitational waves emitted by a SMBH collision will be much lower than a stellar mass black hole, so I guess it is possible that it is outside of the LIGO bandwidth. One of the things that LISA hopes to detect is SMBH inspirals, that is the oscillations that occur before the collision. I would think that an actual coalescence of SMBHs would produce a huge burst of gravitational waves, and that part of it would be within the LIGO bandwidth, but I really don't know for certain.

 

[Ibix]

Figure 3 in this paper suggests that LIGO isn't sensitive to signals from the inspiral of black holes above about 1000 solar masses (the chirp mass isn't the same as individual masses, but you get the idea). I think it's simply the frequency of the waves emitted by that size of binary black hole. Like any antenna, LIGO only works well for picking up waves in a limited range of frequencies.

 

[mitosis]

Figure 3 in this paper suggests that LIGO isn't sensitive to signals from the inspiral of black holes above about 1000 solar masses (the chirp mass isn't the same as individual masses, but you get the idea). I think it's simply the frequency of the waves emitted by that size of binary black hole. Like any antenna, LIGO only works well for picking up waves in a limited range of frequencies.

I guess if you had to pick one range to measure it is logical to select the one to record the much more frequently occurring events. It still blows my mind that LIGO could be made to be sensitive enough to measure such small changes.

 

[jedishrfu]

Heres an article on it

https://www.simonsfoundation.org/2017/11/13/gravitational-waves-supermassive-black-hole-merger/
They say it happens too fast relative to what is detected today.

 

[Ibix]

Heres an article on it

https://www.simonsfoundation.org/2017/11/13/gravitational-waves-supermassive-black-hole-merger/
They say it happens too fast relative to what is detected today.

I think that isn't quite the same issue. That article is proposing using an indirect detection - observing the effect of gravitational waves on the detected tick rate of distant pulsars. And they are proposing looking for much much longer times before the final inspiral - millions of years, not seconds. I don't think this methodology would have the sensitivity to pick up what LIGO looks for. On the other hand, I think it can detect enormously lower frequencies. Which is why they're looking at SMBHs (huge power output) early in the inspiral (enormous wavelength).

If I understand correctly, anyway...

 

[mitosis]

I've also read about the "final parsec problem" about how the final mergers of binary smbh can be very drawn out.

 

[mitosis]

Figure 3 in this paper suggests that LIGO isn't sensitive to signals from the inspiral of black holes above about 1000 solar masses (the chirp mass isn't the same as individual masses, but you get the idea). I think it's simply the frequency of the waves emitted by that size of binary black hole. Like any antenna, LIGO only works well for picking up waves in a limited range of frequencies.

Thanks for solving IBEX!

 

[mitosis]

Sorry, Ibix!