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

[Orodruin]

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 then 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.

I agree with this. It is my naive expectation as well. Do the math on a Minkowski metric background and the perturbation satisfies the wave equation in flat space. It is a reasonable expectation that doing the perturbation in a curved background you might end up with the wave equation in the curved space. Then again, I have not done the math either.

 

[Borg]

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

Ah, I see the difference. Gravity waves would not be blocked but they still might be lensed. Thanks for your responses.

 

[sanman]

Ah, I see the difference. Gravity waves would not be blocked but they still might be lensed. Thanks for your responses.

So Gravity is able to act like a waveguide for its own waves? Is there any other precedent or analogy for this in nature?

 

[PAllen]

I agree with this. It is my naive expectation as well. Do the math on a Minkowski metric background and the perturbation satisfies the wave equation in flat space. It is a reasonable expectation that doing the perturbation in a curved background you might end up with the wave equation in the curved space. Then again, I have not done the math either.

And indeed, they've already thought of that, and the answer is yes. The references in this paper list similar work as well:

http://arxiv.org/abs/1309.5731

 

[PAllen]

So Gravity is able to act like a waveguide for its own waves? Is there any other precedent or analogy for this in nature?

At the moment, I can't think of any analog. Certainly not EM - a charge does not deflect light.

 

[Dr. Courtney]

Is the speed of gravitational waves a constant?

I mean is there anything analogous to a refractive index that can slow them down, or is their speed truly absolute?

This question came up in a recent meal with my students where I was joking that me trying to hula hoop might be detected by the Livingston facility.

 

[PAllen]

Is the speed of gravitational waves a constant?

I mean is there anything analogous to a refractive index that can slow them down, or is their speed truly absolute?

This question came up in a recent meal with my students where I was joking that me trying to hula hoop might be detected by the Livingston facility.

Well, any time you have path bending you can model it as speed slow down, but, in GR, this is considered a coordinated dependent feature (as are all speeds in GR). What makes the EM case invariant is the ability to compare light through a medium to light through a vacuum on 'nearly the same path'. As with the twin scenario versus coordinate dependent time dilation, the ability to do this comparison is what gives you an invariant effect.

Thus, lensing is not going to provide an answer, as you can't have an unbent and bent GW on the same path.

So, the question boils down to whether, e.g. a dust cloud can slightly slow GW. I haven't heard of this, off the top of my head.

 

[DaTario]

Thank you , Vanadium 50.

How often such apparatus will be able to confirm these measurements?
How often such experimental conditions are fullfilled in nature?

Best wishes,

DaTario

 

[mfb]

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 ?

Not just from now on. Luckily we already have a theory that can handle this: General Relativity.

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?

3 solar masses within ~0.2 seconds, with an estimated peak power of 3.6*10⁴⁹ W, more power than the luminosity of all stars in the observable universe combined.

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.

Right, forgot about that part.

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?

GEO600 wouldn't be sensitive enough for a clear detection, and I doubt it would have seen it at all.
Gravitational waves can be influenced by matter as discussed above, but this effect is tiny. To a very good approximation, they just spread out forever. The wave that passed us in September is now about 5 light months away from us.

How often such apparatus will be able to confirm these measurements?
How often such experimental conditions are fullfilled in nature?

That is one of the questions LIGO tries to anwer. We'll have to wait until more data is available.

 

[vartanieno]

If the amplitude of the discovered gravity waves less than the size of atomic nuclei by the time it reached us, I wonder what the amplitude was at the moment of collision right next to these two black holes.

 

[mfb]

Less than the size of a nucleus over a distance of 4 kilometers.

Right next to the black holes, the deformations were of order 1 - like 1 meter per meter. But there you don't have a nice flat spacetime you could take as baseline, and the deformations don't come from the waves but from the near gravitational fields.

If we go a bit away (like thousands of kilometers), it gets easier: strain at a distance of 1.3 billion light years was 10^-21, and it scales inversely with distance. At a distance of 5,000 km, it was 0.002. Probably enough to be visible in a standard videocamera video with some careful analysis and at least 50 frames per second.

 

[Titan97]

I might sound dumb. But how did LIGO detect the collision of two black holes? I thought it could only detect gravitational waves. How did the scientists conclude that the source was the collision of two black holes?

 

[PeterDonis]

How did the scientists conclude that the source was the collision of two black holes?

By the pattern of the detected waves. Scientists have done detailed numerical simulations of the gravitational wave patterns we should expect from various events. The pattern detected by LIGO from this event matched the pattern the simulations gave for a black hole collision. The patterns for other types of events are different.

 

[strangerep]

At the moment, I can't think of any analog. Certainly not EM - a charge does not deflect light.

An EM field can deflect an EM wave -- an extremely weak QED phenomenon known as Delbruck scattering.

 

[Dave Vrona]

Is production of a GRB expected for this type of event?

If so, was one detected by the satellites?

 

[Haelfix]

By the pattern of the detected waves. Scientists have done detailed numerical simulations of the gravitational wave patterns we should expect from various events. The pattern detected by LIGO from this event matched the pattern the simulations gave for a black hole collision. The patterns for other types of events are different.

This part is actually quite the delicate undertaking. I've been trying to understand the attribution and validation methods and needless to say, it is technically challenging for anyone who isn't an expert.

See the following paper here:

https://dcc.ligo.org/LIGO-P1500218/public/main
Which bases a lot of the numerical work on a set of papers starting with this one:
http://arxiv.org/abs/gr-qc/0507014

What's a little difficult to understand, is how LIGO manage to pinpoint the parameters of the system so well. As far as I can see, they analyze a large amount of different models, each with different assumed parameters (mass, spin, orientation, orbital eccentricity etc (there are 17 parameters in total) and then compute the likeliness of each given the observed data, and then tabulate the best fits through a straightforward Bayesian analysis.

I personally find that the error bars on the analysis, especially on the secondary mass and other inferred parameters which aren't able to be read off in a straightforward manner really quite strong, which indicates a great deal of trust in the numerical methods being utilized... I find this rather remarkable if it holds up to more scrutiny, given how difficult the system it that's being analyzed.

 

[PAllen]

An EM field can deflect an EM wave -- an extremely weak QED phenomenon known as Delbruck scattering.

Very interesting. I was, of course, thinking classically, but anyway wasn't familiar with this.

 

[ProfChuck]

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?

Considering that G waves transport energy, or else LIGOS wouldn't work, they must transfer some of that energy to the objects that they move. however, this question is important from the perspective of possible quantum gravity theories. If gravity is quantized and if gravitons exist then they will not transfer their energy in a continuum but in quanta. This is a whole new area of research enabled by this discovery.

 

[PeterDonis]

If gravity is quantized and if gravitons exist then they will not transfer their energy in a continuum but in quanta. This is a whole new area of research enabled by this discovery.

Even if gravitational waves do transfer energy in quanta, LIGO will not be able to detect this. The quanta are way too small.

 

[mfb]

The gravitational wave had a peak intensity of about 240 mW/m² here on Earth. That is roughly the intensity of artificial light in buildings (as it hits walls, floor and so on).

 

[lpetrich]

The event's peak gravitational strain at the Earth was about 10^-21. Since (strain) ~ 1/(distance), we can extrapolate it back to where it was roughly 1. The distance to the event's source is roughly 1.2*10²⁵ m. So the strain = 1 distance is 10⁴ m or 10 km.

The Sun has a Schwarzschild or black-hole radius of about 3.0 km, and the final black hole thus has one of about 190 km. Thus, the maximum G-wave strain near there was about 1/20.

 

[DaTario]

Not just from now on. Luckily we already have a theory that can handle this: General Relativity.

Ok, it is the Einstein's prediction part, I see. But concerning GR, does it have a well defined prediction for the GW's velocity?

 

[PeterDonis]

concerning GR, does it have a well defined prediction for the GW's velocity?

Yes, it predicts that GWs in vacuum travel at the speed of light.

 

[spacejunkie]

Is there any significance in the spin of the final black hole being 2/3c ? I have a vague recollection of reading that this is a natural limit of some kind but I can't pin it down.

 

[Yashbhatt]

Can anyone explain in simple terms the way in which the LIGO managed to keep the mirrors so still?

 

[mfb]

The event's peak gravitational strain at the Earth was about 10^-21. Since (strain) ~ 1/(distance), we can extrapolate it back to where it was roughly 1. The distance to the event's source is roughly 1.2*10²⁵ m. So the strain = 1 distance is 10⁴ m or 10 km.

The Sun has a Schwarzschild or black-hole radius of about 3.0 km, and the final black hole thus has one of about 190 km. Thus, the maximum G-wave strain near there was about 1/20.

At that distance, you have to include nonlinear near-field effects.

Can anyone explain in simple terms the way in which the LIGO managed to keep the mirrors so still?

The mirrors are suspended by a set of 4 consecutive pendulums, which provide passive damping. In addition, seismic motion is actively canceled by moving the point where they are suspended.

 

[lpetrich]

At that distance, you have to include nonlinear near-field effects.

True, but I was concerned about getting some rough approximation.

I will now consider the question of predictions of alternatives to general relativity. So far, most alternatives to GR have been ruled out because their post-Newtonian predictions disagree with observations. http://relativity.livingreviews.org/Articles/lrr-2014-4/ (Clifford Will, 2014) discusses several of them. The most plausible survivor is the Generalized Brans-Dicke theory, but it contains some parameters that can be adjusted to make it arbitrarily close to GR + (noninteracting scalar field).

Black holes in the Brans-Dicke Theory of Gravitation - Springer by Stephen Hawking.

It is shown that a stationary space containing a black hole is a solution of the Brans-Dicke field equations if and only if it is a solution of the Einstein field equations. This implies that when the star collapses to form a black hole, it loses that fraction (about 7%) of its measured gravitational mass that arises from the scalar interaction. This mass loss is in addition to that caused by emission of scalar or tensor gravitational radiation. Another consequence is that there will not be any scalar gravitational radiation emitted when two black holes collide.

Clifford Will's paper also agrees. However, papers like [gr-qc/9811012] New Black Hole Solutions in Brans-Dicke Theory of Gravity claim that there do exist nontrivial solutions, those with a varying scalar field.

According to Stephen Hawking and Clifford Will, the recent black-hole merger observation does not distinguish between GR and GBD -- the scalar field is constant. So one has to look to systems with at least one white dwarf or neutron star to test GBD's predictions of gravitational waves.

 

[websterling]

Is there any significance in the spin of the final black hole being 2/3c ? I have a vague recollection of reading that this is a natural limit of some kind but I can't pin it down.

The spin of the final black hole, listed as 0.57 to 0.72, is a dimensionless number known as the spin parameter. It is a measure of the angular momentum of a Kerr (rotating) black hole. It has a range of 0 to 1 with 0 being non-rotating and 1 corresponding to a hole with maximum angular momentum. It is defined as $a = \frac{cJ}{GM^2}$

The is no significance to the value for this event other than it being the result of the spins of the original holes and the dynamics of the inspiral and merger.

 

[Terry Coates]

Surely gravitational waves have been observed from the time of Adam, in that the ocean tides are caused mainly by the gravitation of the moon.
Assuming that gravity waves travel at the speed of light, I make the wavelength of diurnal tides to be 2.682 x 10^13 meters.
Detecting gravitational waves of distant sources is of course quite an achievement

 

[PAllen]

Surely gravitational waves have been observed from the time of Adam, in that the ocean tides are caused mainly by the gravitation of the moon.
Assuming that gravity waves travel at the speed of light, I make the wavelength of diurnal tides to be 2.682 x 10^13 meters.
Detecting gravitational waves of distant sources is of course quite an achievement

Gravity waves and gravitational waves are completely different things. What you refer to is a simple Newtonian effect. Gravitational waves are not an aspect of Newtonian gravity at all. This is unfortunately confusing terminology, but we are stuck with it.