ALIGO interferometer sensitivity

[RockyMarciano]

Hi everyone!

One thing that's been bothering me about all the hype and comments related the detection of GWs by the aLIGO team is that given the amazing sensitivity(strains~1.0E-22) achieved by the interferometers nobody has mentioned the possibility that they have become sensitive to not only GWs but also to strains caused by the power radiated by EM waves via radiation pressure. I'm thinking about the kind of energy emitted by GRBs with peak luminosities that range in the order of ~1.0E52 ergs/s that might be delivered in miliseconds enhancing the power in the case of short bursts. In this scenario is it reasonable that such short bursts reaching the Earth and inducing vibrations in the interferometer test masses in the frequency and amplitude the interferometer is designed to respond more efficiently, could mimic the oscillations expected from GWs?

 

[BvU]

Do they also exhibit the chirp characteristics (freq going up) ?

 

[RockyMarciano]

Do they also exhibit the chirp characteristics (freq going up) ?

I wouldn't know for sure. Supposing that a gamma ray reached the detectors with enough energy to "shove" the tests masses just enough to produce the strain amplitude observed, which is the main point of my question above, I've read a member of aLIGO saying that the interferometer has been designed to have the maximum amplitude at the 100-250 Hz frequency, losing sensitivity markedly above 300Hz, but certainly I haven't modeled the response of the interferometer to noise in the form of short gamma-ray bursts effects. Maybe if someone related to the collaboration is lurking might shed light.
I know from the technical papers on the finding that the calibration of the interferometer(in the obvious absence of lab GWs to calibrate the response) is performed with radiation pressure exerted on the mirrors test masses with an infrared laser-they basically point to different parts of the mirror and calculate from the motion using the weight and the torque of the test mass and the power of the laser as input.

 

[RockyMarciano]

Given how recent the GWs announcement was made I was somewhat expecting more interest regarding the device's sensitivity that made it possible.
Is it too far-fetched to think that the advencedLigo interferometer could be sensitive enough to detect transfer of momentum from the most energetic photons that reach the earth? is it physically plausible?
I would guess it is basically a matter of comparing(approximately, giving an order magnitude estimate) the power that can be radiated to the detectors from a GW versus an EM wave's highest energy gamma photons from bursts that we may have reference of.
Or comparing the pressure needed to produce a strain h(t) of the order of the one obtained by aLigo in the mirror test masses with the pressure of reflection the approximate irradiance from a short GRB from a given angle would produce in the interferometer mirror considered as a perfectly planar reflecting surface.
I have my estimate but I thought it would be better to gather some other estimates from more knowledgeable people.
Also with respect to Bvu question about the waveform chirp shape, all I could find is this from the LIGO FAQ:"LIGO is very sensitive to vibrations from our environment. It is possible for a passing truck or a dropped hammer near the detector to make the mirrors inside vibrate in such a way that it "looks" like a gravitational wave." Wich made me think again that the chirp waveform is favoured by the interferometer's enhanced sensitivity to a specific window of frequencies, but I was not able to confirm it from aLIGO.

 

[mfb]

The mirrors are shielded very well, they don't get hit by radiation from outside in any relevant amount. A gamma ray burst could at most generate some completely negligible seismic effect. If it would, seismometers would detect it.

 

[RockyMarciano]

The mirrors are shielded very well.

Can you point me to where in the aLIGO technical papers they refer to specific shielding to gamma radiation?. The ones I've read, dealing with the detector and EM noise sources don't mention any shielding. They discuss the possible effects of magnetic fields and RF signals as noise sources on the interferometer and don't mention any pecific electromagnetic shielding. And when I discussed the influence of EMFs on the interferometer with aLIGO authors of the papers they never mentioned any specific shielding, rather referred to the intrinsic weakness of such fields and that any strong field would have been detected by the PEM sensors(magnetometers and radio receivers) located along the interferometer arms.

 

[mfb]

The atmosphere is a very good shield. Every building is a shield. And an impact of gamma rays would have no effect anyway (compare it to shot noise of the intense lasers). Not even if you explode a nuclear weapon in the atmosphere, which gives orders of magnitude more radiation (but the seismic effect of a nuclear explosion would be relevant of course).

 

[RockyMarciano]

The atmosphere is a very good shield. Every building is a shield.

The atmosphere is indeed a good shield, not perfect though.

And an impact of gamma rays would have no effect anyway (compare it to shot noise of the intense lasers).

What an aLigo colaborator explained to me is that the lasers, that indeed are intense, (specially after their power build-up in the Fabry-Perot cavity, the starting power is between 10 and 20 W) deliver an steady pressure to the mirrors that is already taken into account as baseline, any motion that is detected is added over that constant pressure, in other words it couldn't produce transients, it only adds quantum mechanical noise in the form of radiation pressure from random photons which is an important fraction of the total noise that can be seen as baseline strain oscilations.
If you think about the fact that the mirrors are sensitive to random quantum mechanical photons, I would tend to think that if photons as energetic as those coming from a putative gamma-ray that managed to make it through the atmosphere to the detector, they could maybe have some influence above the noise. I'm also aware this is quite a remote possibility, I just think it is worth discarding it with solid facts.

 

[mfb]

The laser has a constant average pressure, but you still have statistics - it leads to some noise. For frequencies between 10 and 100 Hz, radiation pressure is the dominant source of noise, see this plot. Above that, the photon statistics (shot noise) is dominant. Increasing laser power would increase radiation pressure but reduce shot noise, so there is some tradeoff.

they could maybe have some influence above the noise. I'm also aware this is quite a remote possibility, I just think it is worth discarding it with solid facts.

That's not the way it works. You have to show some study that discusses it as possible relevant thing. We cannot discuss personal speculations here.

 

[mfb]

I checked the original publication:

To monitor environmental disturbances and their influ- ence on the detectors, each observatory site is equipped with an array of sensors: seismometers, accelerometers, microphones, magnetometers, radio receivers, weather sensors, ac-power line monitors, and a cosmic-ray detector [65]

Reference 65 is Environmental Influences on the LIGO Gravitational Wave Detectors during the 6th Science Run, and does not mention cosmic rays at all (at least not with the words "cosmic", "ray" or "radiation", didn't read everything).

 

[RockyMarciano]

That's not the way it works. You have to show some study that discusses it as possible relevant thing. We cannot discuss personal speculations here.

No personal speculation here. Radiation pressure is a textbook concept. What you, I and most people consider as a remote possibility (and therefore is not considered by LIGO) is the ocurrence of a cosmic gamma ray reaching the ground. Stating the self-evident fact that unlikely isn't the same as impossible (more so if you consider the inefficiency of gamma ray detectors) is not speculation as far as I can see. And given the almost coincidence with a short GRB signal(apparently detected by FERMI but not by INTEGRAL, which is rather telling about the detection efficiency issue) I think it is prudent to be watching closely and wait for possible coincidences bewteen the other GW signals apparently detected and GRBs.

I checked the original publication:
Reference 65 is Environmental Influences on the LIGO Gravitational Wave Detectors during the 6th Science Run, and does not mention cosmic rays at all (at least not with the words "cosmic", "ray" or "radiation", didn't read everything).

Actually the updated relevant paper is "Characterization of transient noise in Advanced LIGO relevant to gravitational wave signal GW150914". There is a detector at Hanford. It only discusses cosmic rays as possible interference source, not gamma radiation itself.

 

[PeterDonis]

What you, I and most people consider as a remote possibility (and therefore is not considered by LIGO) is the ocurrence of a cosmic gamma ray reaching the ground.

You mean, reaching the ground, then penetrating the ground to enough depth, then getting through the tunnel walls and all the other parts of LIGO that surround the actual mirrors, and still having enough energy to perturb the mirrors to the same order of magnitude as GWs. If you're really concerned about this, perhaps you could try running some actual numbers.

 

[RockyMarciano]

You mean, reaching the ground, then penetrating the ground to enough depth, then getting through the tunnel walls and all the other parts of LIGO that surround the actual mirrors, and still having enough energy to perturb the mirrors to the same order of magnitude as GWs. If you're really concerned about this, perhaps you could try running some actual numbers.

Yeah, the numbers I get pretty much lead to the conclusion it is close to impossible, mostly because of the atmosphere optical depth issue and its impact on the energy any surviving photon might have, rather than the other things you mention. From this point of view something like a TGF would be more plausible than a GRB.
So I'll go back to my first post basic point, which is about the kind of amazing sensitive instrument Ligo scientists have managed to develop, and the technical papers (the one I mentioned above and the one about calibration of the interferometer) make clear that the instruent is sensitive to EM waves,even to those of extremely low frequency, like for instance Schumann resonances from lightning are considered as possible sources of confounding signals.
I think we shouldn't lose sight of how extraordinary the claim of a first ever detection of a GW from a black hole merger at 1.3 bly is, it needs extraordinary proof that other signals(that up to the day of the aLigo finding were the only kind of radiation detected traveling at c) the instrument is sensitive to are discarded. Even if one is previously convinced of the existence of GWs(like in my case).

 

[mfb]

Radiation pressure is a textbook concept.

Sure, but its relevance to LIGO is not.

(and therefore is not considered by LIGO)

LIGO did consider it. Read the paper.

The GRB seen by Fermi was seen significantly after the gravitational wave signal.

 

[RockyMarciano]

Sure, but its relevance to LIGO is not.

http://arxiv.org/abs/1602.03844

LIGO did consider it. Read the paper.

I'm sure you can distinguish between cosmic ray particle showers(mentioned in the possible sources of noise) and gamma waves.

The GRB seen by Fermi was seen significantly after the gravitational wave signal.

http://arxiv.org/abs/1602.03920

 

[mfb]

Sure, but its relevance to LIGO is not.

http://arxiv.org/abs/1602.03844

What exactly is your point?

I'm sure you can distinguish between cosmic ray particle showers(mentioned in the possible sources of noise) and gamma waves.
http://arxiv.org/abs/1602.03920

What exactly in a GRB would make it not "cosmic rays"? It is a burst of rays, and of cosmic origin.

The GRB seen by Fermi was seen significantly after the gravitational wave signal.

http://arxiv.org/abs/1602.03920

Thanks for providing the source, but you could have added a quote from the abstract: It started 0.4 seconds afterwards and lasted 1 second.

Providing random links is not a discussion.

 

[RockyMarciano]

Is providing random one-liners a discussion? It is one thing being a bit lazy but this:

Sure, but its relevance to LIGO is not.

LIGO did consider it. Read the paper.

The GRB seen by Fermi was seen significantly after the gravitational wave signal.

 

[mfb]

http://arxiv.org/abs/1602.09141
It is one line more than a single link. In other words, a link is 100% less discussion.

 

[PeterDonis]

the numbers I get pretty much lead to the conclusion it is close to impossible

In other words, gamma rays from GRBs are not a noise source that we need to worry about for LIGO.

So I'll go back to my first post basic point

Which was about a specific possible source of noise--radiation pressure from GRB photons--not about the general idea that LIGO is very sensitive and so noise sources need to be carefully considered (which is a valid point but too general to base a detailed discussion on). And, as above, you have agreed that that particular source of noise is not worth worrying about for LIGO.

 

[PeterDonis]

The specific issue raised in the OP has been discussed sufficiently. Thread closed.