A Rumors of Gravitational Wave Inspiral at Advanced LIGO | Sept 2015 Launch

[mfb]

I have a question, which was partially discussed in other places on the web, for anyone to comment on: can gravitational waves be red-shifted?

They are redshifted in the same way light is. The distance gives a redshift of ~9%. The effect of galactic gravitational potentials is negligible.

 

[Buzz Bloom]

the apparatus was functioning as it needed to be in order to detect a real signal, and that happened to be when they got this signal.

Hi bcrowell:

Thanks for the explanation. I did not get a chance to see the press conference.

Of course we don't know how frequently LIGO will find more gravitational waves in the future, but it seems like wonderful luck they had LIGO on the air at that time. The kind of event LIGO detected may well be quite rare, and it LIGO had had its engineering run a day later, it might have been maybe years before any detection occurred. On the other hand, perhaps LIGO will find more BH pair crashes near daily and be overwhelmed with the need to verify so much data.

Regards,
Buzz

 

[Buzz Bloom]

I have a question about the location of the source galaxy.
From http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.116.061102

With only two detectors the source position is primarily determined by the relative arrival Time and localized to an area of approximately 600 deg² (90% credible region).
z = 0.09^+0.03_-0.04.​

If my math is OK, this means that the source can be located with 90% confidence to a region of the sky with an angular diameter of about 14 deg. Combining that with the z value, how many galaxies are candidates for the source?

 

[DiracPool]

What was the eventual fallout from the BICEP2 experiment, btw. Was that totally debunked?

 

[mfb]

I have a question about the location of the source galaxy.
From http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.116.061102

With only two detectors the source position is primarily determined by the relative arrival Time and localized to an area of approximately 600 deg² (90% credible region).
z = 0.09^+0.03_-0.04.​

If my math is OK, this means that the source can be located with 90% confidence to a region of the sky with an angular diameter of about 14 deg. Combining that with the z value, how many galaxies are candidates for the source?

Too many. ~500 million light years uncertainty for the distance, and the distance of ~1.3 billion light years gives ~400 million light years for 14 degrees. So roughly a volume of (400Mly)^3. Tens of millions of galaxies I guess.

What was the eventual fallout from the BICEP2 experiment, btw. Was that totally debunked?

The updated measurement sets an upper limit that excludes the previous value. Gravitational waves could still be there in a sizeable amount, but BICEP2 didn't see them.

 

[fresh_42]

Too many. ~500 million light years uncertainty for the distance, and the distance of ~1.3 billion light years gives ~400 million light years for 14 degrees. So roughly a volume of (400Mly)^3. Tens of millions of galaxies I guess.

But how could they know then the masses of the merging BH and that it was a merger?

 

[PAllen]

But how could they know then the masses of the merging BH and that it was a merger?

From analyzing the waveform of the signal.

 

[Tphysics]

I have a question about how they were able to detect the gravitational wave. They say they have two different sites approx. 4000 km away from one another. If the size of the wave is one tenth of a electron-mass how did both sites detect the wave came through?

 

[Vanadium 50]

If the size of the wave is one tenth of a electron-mass

I don't know what that means, but the wavelength varies from 10,000 miles to about 1000 miles.

 

[Tphysics]

I don't know what that means, but the wavelength varies from 10,000 miles to about 1000 miles.

So the wavelength is so large that the wave might encompass half of earth? So there was a chance per say that only one site could detect the wave because the two sites were thousands of miles apart?

 

[Ibix]

So the wavelength is so large that the wave might encompass half of earth? So there was a chance per say that only one site could detect the wave because the two sites were thousands of miles apart?

No. Think of ripples on a pond. The black holes are the stone dropped into the water; the LIGO detectors are two rocks sticking out of the water some distance away. The wavelength is the spacing between one ripple and the one following it and the strength of the signal is the height of the wave. Whatever the wavelength is, and however tiny the height of the wave is, the wave washes over both rocks.

Gravitational waves are different from water waves in a number of ways, but they will always pass through both detectors.

 

[Tphysics]

No. Think of ripples on a pond. The black holes are the stone dropped into the water; the LIGO detectors are two rocks sticking out of the water some distance away. The wavelength is the spacing between one ripple and the one following it and the strength of the signal is the height of the wave. Whatever the wavelength is, and however tiny the height of the wave is, the wave washes over both rocks.

Gravitational waves are different from water waves in a number of ways, but they will always pass through both detectors.

I guess I was sort of thinking about them as a particle instead of a wave. But duh this makes total sense. Shows how sensitive these machines were.

 

[Vanadium 50]

So there was a chance per say that only one site could detect the wave because the two sites were thousands of miles apart?

Why would you think that? The wavelength for FM radio is about 10 feet, yet people can tune it in all around the city.

 

[sanman]

Once again, I'd like to ask about Gravitational Waves and Dark Matter - would Dark Matter potentially be detectable/identifiable through Gravitational Waves? What sort of criteria would the signal have to meet in order to indicate Dark Matter?

 

[PAllen]

Gravitational lensing is already a major piece of evidence for dark matter. I don't really see any way GW would help unless there were a model of a large mass of dark matter with changing acceleration - totally unlikely. Matter (dark or not) between us and a GW source is invisible to the GW. Short answer: GW will not help with dark matter, but GR has helped a lot via lensing.

 

[sanman]

Gravitational lensing is already a major piece of evidence for dark matter. I don't really see any way GW would help unless there were a model of a large mass of dark matter with changing acceleration - totally unlikely. Matter (dark or not) between us and a GW source is invisible to the GW. Short answer: GW will not help with dark matter, but GR has helped a lot via lensing.

Hmm, so Gravitational Wave observation can only be useful for observing violent cataclysmic phenomena in space? What about for example the rotation of galaxies, for which Dark Matter has been hypothesized as an explanation on why such galaxies don't fly apart? If we could observe some waves from rotating galaxies, this might tell us how much mass/matter is in those galaxies, which we could then cross-reference against visible light observations. Gravitational Waves should at least afford us proper mass measurements of galaxies and other large entities.

I'm even wondering if SETI could use Gravitational Wave observation to look for signs of intelligent life. Perhaps some advanced civilizations use Gravitational Waves for communication, since they pass through everything instead of being absorbed. Perhaps any exotic "FTL" propulsion would likewise generate some telltale Gravitational Wave signature. Perhaps large artificial constructs like Dyson Spheres could also exhibit peculiar characteristics.

What I really see an opportunity for is the further development of Atom Interferometry (and even molecular interferometry) for better detection of Gravitational Waves. They might be able to afford a sensitivity that goes far beyond LIGO. Combine that with "Big Data" analytics and you might be able to parse out very detailed signals on much smaller astrophysical phenomena. Maybe this too could lead to Dark Matter detection.

 

[bcrowell]

If we could observe some waves from rotating galaxies, this might tell us how much mass/matter is in those galaxies, which we could then cross-reference against visible light observations.

This isn't going to work. The rate of radiation of gravitational waves is proportional to the frequency raised to the 6th power. The period of rotation of a galaxy is on the order of 10^8 years, so the frequency is extremely small.

 

[sanman]

Would it at least be possible to artificially generate Gravitational Waves - very tiny ones, obviously - and detect them using a detector like LIGO or like an Atom Interferometer? Even though Man-made Gravitational Waves would be far, far smaller than those from astrophysical phenomena like Black Holes, at least any attempts to measure them would be done at distances far, far smaller than the distant Black Holes that LIGO has been getting signals from. Wouldn't the far lower distance offset the fact of the smaller amplitudes, to make detection feasible? I was just imagining that if Gravitational Waves could be harnessed for communication purposes, then there'd be no need for satellite relays and such. We could have direct communication between Earth and Mars even when both are on opposite sides of the Sun.

 

[PAllen]

Would it at least be possible to artificially generate Gravitational Waves - very tiny ones, obviously - and detect them using a detector like LIGO or like an Atom Interferometer? Even though Man-made Gravitational Waves would be far, far smaller than those from astrophysical phenomena like Black Holes, at least any attempts to measure them would be done at distances far, far smaller than the distant Black Holes that LIGO has been getting signals from. Wouldn't the far lower distance offset the fact of the smaller amplitudes, to make detection feasible? I was just imagining that if Gravitational Waves could be harnessed for communication purposes, then there'd be no need for satellite relays and such. We could have direct communication between Earth and Mars even when both are on opposite sides of the Sun.

I have never heard of a small detector (e.g. atom interferometer) being able to detect GW. Do you have any source for this? So far as I know, the bigger the detector, the better for GW.

There is no currently conceivable method to detect gravitational waves that can be produced locally. Note that the total power output of GW from all sources in the solar system is estimated to be enough to power a few light bulbs - distributed over the volume of the solar system.

You seem to have the syndrome "if you have a hammer everything looks like a nail". This detection is fantastic, and more will come, but GW are not some magic tool that solves a broad range of problems.

 

[sanman]

I have never heard of a small detector (e.g. atom interferometer) being able to detect GW. Do you have any source for this? So far as I know, the bigger the detector, the better for GW.

There is no currently conceivable method to detect gravitational waves that can be produced locally. Note that the total power output of GW from all sources in the solar system is estimated to be enough to power a few light bulbs - distributed over the volume of the solar system.

You seem to have the syndrome "if you have a hammer everything looks like a nail". This detection is fantastic, and more will come, but GW are not some magic tool that solves a broad range of problems.

Since the atom is more massive than the photon, it has a much lower DeBroglie wavelength, thus allowing much greater precision in interferometry. Photons were turned into coherent light long ago with the invention of the laser. The more recent invention of the BEC has led to the "atom laser" and atom interferometry.





The atom interferometer can be a much more compact device than the huge LIGO, and could be launched into space (perhaps to a LaGrange Point). It can have far greater precision/sensitivity, and would also be much less expensive to build.

 

[sanman]

More:



It seems like atom interferometers are the best possible tool to perform Gravitational Wave observations.
(Researchers are working on applying the same idea to molecules, which are even heavier than atoms, and thus have an even shorter DeBroglie wavelength which could be used for even more precise interferometry)

 

[fresh_42]

The true sensation of the discovery is the engineering masterpiece!

 

[Vanadium 50]

This isn't going to work. The rate of radiation of gravitational waves is proportional to the frequency raised to the 6th power. The period of rotation of a galaxy is on the order of 10^8 years, so the frequency is extremely small.

Additionally, because the frequency is so small, one needs to listen for hundreds of millions of years to see a wave. So this is impossible on two grounds: there's not enough energy to see it. and there's not enough time to see it.

And (to sanman):

You seem to have the syndrome "if you have a hammer everything looks like a nail".

+1

It's interesting how, right after a discovery, there are a bunch of messages of the form, "I know how to do this better!"

 

[mfb]

I have a question about how they were able to detect the gravitational wave. They say they have two different sites approx. 4000 km away from one another. If the size of the wave is one tenth of a electron-mass how did both sites detect the wave came through?

The wave hit the whole galaxy (and much larger structures) in a uniform way. The amplitude (the relative change in distance) is tiny, but it is the same amplitude everywhere.
Comparing the amplitude of gravitational waves to masses does not make sense.

Would it at least be possible to artificially generate Gravitational Waves - very tiny ones, obviously - and detect them using a detector like LIGO or like an Atom Interferometer? Even though Man-made Gravitational Waves would be far, far smaller than those from astrophysical phenomena like Black Holes, at least any attempts to measure them would be done at distances far, far smaller than the distant Black Holes that LIGO has been getting signals from. Wouldn't the far lower distance offset the fact of the smaller amplitudes, to make detection feasible? I was just imagining that if Gravitational Waves could be harnessed for communication purposes, then there'd be no need for satellite relays and such. We could have direct communication between Earth and Mars even when both are on opposite sides of the Sun.

The strain from artificial gravitational waves at the boundary of what can possibly be produced on Earth is something like 15 orders of magnitude smaller than what LIGO can measure. I don't say it is impossibe, but it is way beyond the capabilities of even the keenest proposals today.

 

[PAllen]

Since the atom is more massive than the photon, it has a much lower DeBroglie wavelength, thus allowing much greater precision in interferometry. Photons were turned into coherent light long ago with the invention of the laser. The more recent invention of the BEC has led to the "atom laser" and atom interferometry.





The atom interferometer can be a much more compact device than the huge LIGO, and could be launched into space (perhaps to a LaGrange Point). It can have far greater precision/sensitivity, and would also be much less expensive to build.

Since the atom is more massive than the photon, it has a much lower DeBroglie wavelength, thus allowing much greater precision in interferometry. Photons were turned into coherent light long ago with the invention of the laser. The more recent invention of the BEC has led to the "atom laser" and atom interferometry.





The atom interferometer can be a much more compact device than the huge LIGO, and could be launched into space (perhaps to a LaGrange Point). It can have far greater precision/sensitivity, and would also be much less expensive to build.

Thanks, very interesting. However, it changes nothing else I said.

 

[DiracPool]

Since the atom is more massive than the photon, it has a much lower DeBroglie wavelength, thus allowing much greater precision in interferometry.

That's what I thought, too. Hence the comment in my post #21:

My guess is that they measure these tolerances so closely because they are using an interferometer running an extremely high frequency/small wavelength and that they have a technique to measure extremely small phase discrepancies.

But then after reading the article it looks as though they optimize the readings at extremely low frequencies/long wavelengths, as in roughly 100 Hz.

https://dcc.ligo.org/public/0122/P150914/014/LIGO-P150914_Detection_of_GW150914.pdf

"In their most sensitive band, 100–300 Hz, the current LIGO detectors are 3 to 5 times more sensitive to strain than initial LIGO [67]; at lower frequencies, the improvement is even greater, with more than ten times better sensitivity below 60 Hz."

Maybe I'm reading this wrong, but I'm guessing the frequency doesn't matter so much as it is their ability to detect minute differences in the phase. If you took at the timeplot of the inspiral event, it too looks to be on the order of 100 Hz or less.

 

[maline]

A maximum-entropy big bang, which is the most overwhelmingly likely possibility, would have had its gravitational degrees of freedom equilibrated with all the other degrees of freedom

I have been wondering: to what extent is this consensus science, and to what extent Roger Penrose's pet ideas?

 

[bcrowell]

I have been wondering: to what extent is this consensus science, and to what extent Roger Penrose's pet ideas?

You could compare, for example, with this article:

Callender, Craig, "Thermodynamic Asymmetry in Time", The Stanford Encyclopedia of Philosophy (Fall 2011 Edition), Edward N. Zalta (ed.), http://plato.stanford.edu/archives/fall2011/entries/time-thermo

This is a topic on which many people on the internet spout a huge amount of nonsense, and that may have contributed to your doubts. Among people who actually understand the topic, AFAIK there is no controversy on the description I gave in #96. If you want to have a serious discussion on this, we should really start a separate thread.

 

[wolram]

In the Arxiv's today.
arXiv:1602.03842
The Rate of Binary Black Hole Mergers Inferred from Advanced LIGO Observations Surrounding GW150914
B. P. Abbott, R. Abbott, T. D. Abbott, M. R. Abernathy, F. Acernese, K. Ackley, C. Adams, T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya, C. Affeldt, M. Agathos, K. Agatsuma,N. Aggarwal, O. D. Aguiar, L. Aiello, A. Ain, P. Ajith, B. Allen, A. Allocca, P. A. Altin, S. B. Anderson, W. G. Anderson, K. Arai, M. C. Araya, C. C. Arceneaux, J. S. Areeda, N. Arnaud, K. G. Arun, S. Ascenzi, G. Ashton, M. Ast, S. M. Aston, P. Astone, P. Aufmuth, C. Aulbert, S. Babak, P. Bacon, M. K. M. Bader, P. T. Baker, F. Baldaccini, G. Ballardin,S. W. Ballmer, J. C. Barayoga, S. E. Barclay, B. C. Barish, D. Barker, F. Barone, B. Barr, L. Barsotti, M. Barsuglia, D. Barta, J. Bartlett, I. Bartos, R. Bassiri, A. Basti, J. C. Batch,C. Baune, V. Bavigadda, M. Bazzan, B. Behnke, M. Bejger, C. Belczynski, A. S. Bell, et al. (901 additional authors not shown)
Comments: 16 pages, 6 figures, submitted to ApJL
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); General Relativity and Quantum Cosmology (gr-qc)
A transient gravitational-wave signal was identified in the twin Advanced LIGO detectors on September 14, 2015 at 09:50:45 UTC (GW150914). To assess the implications of this discovery, the detectors remained in operation with unchanged configurations over a period of 39 d around the time of the signal. A search of 16 days of simultaneous two-detector observational data found GW150914 to have a false alarm probability (FAP) of $2 \times 10^{-7}$. Parameter estimation followup on this trigger identifies its source as a binary black hole (BBH) merger with component masses $(m_1, m_2) = 36^{+5}_{-4}, 29^{+4}_{-4} \, M_\odot$ at redshift $z = 0.09^{+0.03}_{-0.04}$. Here we report on the constraints these observations place on the rate of BBH coalescences. Considering only GW150914, assuming that all BBHs in the universe have the same masses and spins as this event, imposing a false alarm threshold of 1 per 100 years, and assuming that the BBH merger rate is constant in the comoving frame, we infer a 90% credible range of $2-53 \, \mathrm{Gpc}^{-3} \, \mathrm{yr}^{-1}$ (comoving frame). Incorporating all triggers that pass the search threshold while accounting for the uncertainty in the astrophysical origin of each trigger, we estimate a higher rate, ranging from $6-400 \, \mathrm{Gpc}^{-3} \, \mathrm{yr}^{-1}$ depending on assumptions about the BBH mass distribution. All together, our various rate estimates fall in the conservative range $2-400 \, \mathrm{Gpc}^{-3} \, \mathrm{yr}^{-1}$.

 

[mfb]

If you replace the $ by ##, TeX gets rendered properly.

 

[gjonesy]

One question looms in my mind, now that GR has gained this validation, what does that mean for other existing theoretical frame works? Will science focus more on expanding on GR or continue looking for other theories to complete the standard model?

Just curious about others opinions/ thoughts.

 

[atyy]

There is a class of quantum gravity approaches (based on a variant way of handling spin 2 quantization, put forward by a few Russian physicists), that had a specific prediction (yeah!) that collapse would stop signficantly before horizon formation (as I recall, close to the photon sphere of classical GR). Presumably this result (along with increasing reliability of horizon / mass observations) completely kills this approach (the downside of a specific prediction). It probably kills most any approache that suggests the QG effects diverge from classical GR outside the horizon.

Or the effects outside the non existent horizon may just be be small, eg. Mathur's fuzzball.

"7.8 Do fuzzballs look the same as the traditional black hole outside the horizon?
...
...
Thus outside the fuzzball the non-sphericality effects fall off very fast, and we see an effectively spherical geometry."

http://www.physics.ohio-state.edu/~mathur/faq2.pdf

 

[atyy]

One question looms in my mind, now that GR has gained this validation, what does that mean for other existing theoretical frame works? Will science focus more on expanding on GR or continue looking for other theories to complete the standard model?

Just curious about others opinions/ thoughts.

GR has long been validated at low energies, but remains problematic at high energies. It is at high energies that theories are still needed to complete GR and the standard model.

The direct detection of gravitational waves is first of all a beautiful experiment, because the prediction is beautiful and the measurement at the limits of precision. Secondly, it will allow us to see many more low energy events that were previously hard to see, such as black hole mergers.

 

[PAllen]

Or the effects outside the non existent horizon may just be be small, eg. Mathur's fuzzball.

"7.8 Do fuzzballs look the same as the traditional black hole outside the horizon?
...
...
Thus outside the fuzzball the non-sphericality effects fall off very fast, and we see an effectively spherical geometry."

http://www.physics.ohio-state.edu/~mathur/faq2.pdf

I agree. There is no impact on a theory like this, and I quite like the fuzzball idea.

 

[Islam Hassan]

Does the data underlying LIGO's detection of gravitational waves allow us to say anything new about gravitons?

IH

 

[mfb]

Does the data underlying LIGO's detection of gravitational waves allow us to say anything new about gravitons?

IH

It allows to set an upper limit on the mass, but indirect constraints from the effect of gravity on large scales are still better than those upper limits.

 

[Orodruin]

It allows to set an upper limit on the mass, but indirect constraints from the effect of gravity on large scales are still better than those upper limits.

All under the assumption that gravitons exist ...

 

[DaTario]

Hi All,

Very excited with this result, but with some basic doubts.

1) do gravitational waves travel with the velocity of light?

2) was the velocity of the gravitational wave determined in this experiment?

3) was it that the staff at LIGO have brought the supposition of merging BH after analysing the oscillations observed in the interferometers? Was it then a plausible cause for the oscillations?

4) If one is stuck in the Newton's formula and accepts the relativity postulate of velocities no larger than c, this person would agree that fastly oscillating the position of a highly massive object would imply the propagation of a "gravitational" wave, measurable at great distances. Is this correct?

Best wishes,

DaTario

 

[mfb]

All under the assumption that gravitons exist ...

Well, sure, but it seems likely.

1) do gravitational waves travel with the velocity of light?

All measurements are pointing towards this, but there is no direct speed measurement yet.

2) was the velocity of the gravitational wave determined in this experiment?

No.

3) was it that the staff at LIGO have brought the supposition of merging BH after analysing the oscillations observed in the interferometers? Was it then a plausible cause for the oscillations?

They computed signals for hundreds of thousands of different scenarios in advance, and then compared the observed signal to those calculations. Two merging black holes fit, all other things do not fit. They also checked this based on the signal afterwards.

4) If one is stuck in the Newton's formula and accepts the relativity postulate of velocities no larger than c

Newton's formula and a finite speed of light are incompatible. They just do not work together.

 

[DaTario]

I agree that Newton's formula doesn´t have included this principle. But is it plausible to speculate that our calculations with gravity must deal, from now on, with retarded potentials or similar resources ?

 

[Islam Hassan]

We'll know more in a week.
A loss of three solar masses in the merger would correspond to a power above 10⁴⁶ W (probably much higher), the most powerful event we ever saw.

The combined luminosity of all stars in the observable universe is about 10⁴⁹ W.

Is this equivalent to the annihilation of three solar masses worth of matter in an instant? Was such energy release predicted by present models of BH merger/collision or is this a new phenomenon?

IH

 

[phyzguy]

Is this equivalent to the annihilation of three solar masses worth of matter in an instant?

Yes.

Was such energy release predicted by present models of BH merger/collision or is this a new phenomenon?

It was predicted. Without these enormous energies, the signal wouldn't be strong enough for LIGO to see, and we would never have built it.

 

[fresh_42]

All under the assumption that gravitons exist ...

Where does the spin 2 prediction come from?

 

[Vanadium 50]

I'm going to disagree a little with mfb on point (2). He's correct that this experiment doesn't measure the speed of gravity. However, there is still information. The speed of gravity has to be less than the distance between Richland and Livingston, which is about 1850 miles, divided by the difference in time, which is less than 10 ms. So that gives as a ballpark estimate of within a factor of a few of 185,000 miles per second.

So, while a single measurement is not very constraining, it shows that the speed of gravitational radiation is of the same order as the speed of light.

 

[Edgardo]

This nature article mentions that other interferometers such as Geo600 in Germany and Virgo in Italy were not operating at the time . Is it known whether Geo600 would have detected the gravitational waves?

What happens with gravitational waves? Do they exist forever or can they be absorbed or transformed?

 

[Borg]

What happens with gravitational waves? Do they exist forever or can they be absorbed or transformed?

I've been lurking on the thread about this fascinating announcement but this question got me to wondering. Can gravity waves be theoretically affected by gravitational lensing? My guess would be that they are but I don't know the details enough to know.

Edit: It seems that I'm wrong.

Matter (dark or not) between us and a GW source is invisible to the GW.

 

[nikkkom]

Is this equivalent to the annihilation of three solar masses worth of matter in an instant?

This depends on what you mean by "annihilation".
Three solar masses worth of energy were converted to the energy of gravitational waves, which are "invisible" (they interact very weakly).
The "annihilation" as in matter-antimatter reaction would generate electromagnetic radiation (gamma rays), not gravitational radiation.

 

[Islam Hassan]

This depends on what you mean by "annihilation".
Three solar masses worth of energy were converted to the energy of gravitational waves, which are "invisible" (they interact very weakly).
The "annihilation" as in matter-antimatter reaction would generate electromagnetic radiation (gamma rays), not gravitational radiation.

True; what I was trying to gauge was the value of energy release. From Phyzguy's reply I take it that the energy is the same as that released if we were indeed taéking about matter-antimatter annihilation.

IH

 

[Orodruin]

I've been lurking on the thread about this fascinating announcement but this question got me to wondering. Can gravity waves be theoretically affected by gravitational lensing? My guess would be that they are but I don't know the details enough to know.

Edit: It seems that I'm wrong.

His statement is not about the gravitational lensing. It is about how the GW would interact with matter and dark matter.

 

[PAllen]

His statement is not about the gravitational lensing. It is about how the GW would interact with matter and dark matter.

Right. I have no knowledge of an analysis of lensing for GW; it is a great question. A purely heuristic argument to expect it is that if GW travel at c, and can be treated similar to the EM geometric optics approximation of treating a piece of the wave front as having a world line, then that world line ought to be a null geodesic. Then, the lensing would be basically the same as light. But this is just a general argument - I would not be very confident in it without more analysis or information.