LIGO mirror coatings get upgrade

November 10, 2017 by Vicky Stein, SLAC National Accelerator Laboratory

Stanford scientists will lead a new national cooperative effort, the LIGO Scientific Collaboration Center for Coatings Research, to improve detection of gravitational waves at the twin LIGO facilities.

LIGO, the Laser Interferometer Gravitational-wave Observatory, has a problem of scale: galaxy-shaking events like the recently revealed collision of two neutron stars happened so far away that the echoes took 130 million years to travel to our planet. A collision of black holes detected in 2015 was even farther, 1.3 billion light years away.

By the time the effects of these massive events reach Earth, they are tiny enough that they can only be detected using the most sensitive equipment scientists could devise. Changes in distance (as detected over the sprawling four-kilometer arms of LIGO) caused by , said Stanford researcher Riccardo Bassiri, are "a thousand times smaller than the size of an atomic nucleus."

Any "noise" or molecular disarray introduced by the mirrors can completely obscure the faint signals from distant gravitational wave sources.

"It's quite amazing, this four-kilometer, massive piece of machinery – and the coatings on the mirrors play this key role in how many gravitational-wave events we can observe," Bassiri said. In the end, the sensitivity of LIGO's massive interferometers is limited by atomic-scale vibrations of molecules in the mirrors that reflect the facilities' powerful lasers. These vibrations are known collectively as Brownian thermal noise. According to Bassiri, it will be the dominant noise source limiting LIGO's sensitivity, and a major challenge to future generations of the facilities.

The goal of the new center, comprising 10 US institutions and led at Stanford by Martin Fejer, professor of applied physics, will be to improve LIGO's sensitivity with better coatings for its interferometers. Researchers hope to have new materials ready in for the next update to the LIGO facilities in as soon as three years. If they are successful and halve the amount of thermal noise from the mirror coatings, they could expand the volume of the universe that LIGO can observe eight times over current capabilities.

The coatings in question are comprised of multiple layers no larger than a few hundreds of nanometers in thickness each – hundreds of times thinner than a human hair. In the past, researchers have followed an iterative process, creating a new and then testing it, hoping to improve on previous versions.

Through the new center, Stanford will be leading researchers and facilities across the country in what they hope will be a more targeted approach. For example, working with collaborators at the SLAC National Accelerator Laboratory's Stanford Synchrotron Radiation Lightsource, scientists can inspect newly devised mirror coatings at an atomic level.

With this critical mass of funding and participation, "rather than following this trial-and-error Edisonian approach, we can come to a materials-by-design process," Bassiri said. "Ultimately, the reward of developing better coatings for LIGO will be to further enable exploration of the universe through gravitational wave astronomy."

Explore further: X-ray studies could help make LIGO gravitational wave detector 10 times more sensitive

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1 / 5 (2) Nov 10, 2017
Does anyone have any data involving a ringdown signal arising in one LIGO but not the other(s)? If we had such data we could calculate the probability of these 'orphan' ringdowns arising in more than one at a randomly coinciding separation time which may be interpreted by analysis procedures as being one actual rindown being detected by more than one LIGO, while it may be just coincidence (which must eventually happen if there are many 'orphan' ringdown signals arising from systemic rather than 'merging event' sources). It would help minimize spurious 'detections' claims, and help the confidence levels of those 'events' that cannot be explained by this random/coincidence 'orphan' signals. If anyone has relevant data/info I would be very interested Thanks.
Uncle Ira
5 / 5 (4) Nov 10, 2017
@ Really-Skippy. How you are Cher? I am fine and dandy and would feel guilty if I felt any better, thanks for asking.

Does anyone have any data involving a ringdown signal arising in one LIGO but not the other(s)?
Yeah Cher. I got him but not on this computer so you will have to wait until I get back home next week. I suppose if it is an emergency I could ask Mrs-Ira-Skippy if she could send him to me so I can send him to you. (Assuming she would be able to understand what that is and how to find him on my at home computer.)

The real scientist-Skippys have already been keeping track of that sort of thing which is why even the ignorant/troll/bot like me knows about such a thing. Since you are not the scientist you probably didn't know that is something they been talking about for a couple of years now, the single spur of the moment rings on only one detector.
Da Schneib
5 / 5 (3) Nov 10, 2017
@RC, what makes you think there has ever been a ringdown signal detected at a single gravitational observatory and not detected at the others? Do you have some sort of evidence or mechanism in mind, or are you just speculating? This is a highly distinctive signature, not duplicated by any known or observed seismic phenomenon, and even if it were seismic disturbances are filtered out in the mirror suspension using the same technique, in broad strokes, as is used in noise cancelling headphones. See here: search the page for "seismic."

You can see details of the suspension design here: https://www.ligo....solation This design uses two innovative concepts: the active noise cancelling referred to above, and a four-element pendulum design which reduces sway in the fourth element to almost nothing in the first place.
Da Schneib
5 / 5 (3) Nov 10, 2017
In addition to all of this, seismic events are easily detected and filtered out with simple seismometers, and several other types of interference are also detected and filtered out including magnetic, sonic, and gamma-ray noise. See the FAQ linked above and search on magnetometer.

In short, extreme precautions have been taken to isolate the LIGO mirrors from vibration, and further precautions have been taken to detect and filter out noise from local sources. Nothing is ever 100% certain, but they have 99.9999+% certainty, which is a one in 100,000 chance of it being anything but a GW, and that's only for one detection. For five detections, it's one in 10,000,000,000,000,000,000,000,000. And that's not even including the chances of observing one and then doing a follow-up in visible light and finding something, like the latest one.

This argument is pretty much over.
3.5 / 5 (8) Nov 10, 2017
It would be some seismic event if they picked it up in Italy at the same precise instant as in the U.S.! Instantaneous P & S waves!
Da Schneib
5 / 5 (4) Nov 10, 2017
@jones not to mention that P and S waves not only arrive at different times but also aren't ringdown waves; they're at a mostly constant frequency. The ringdown waves detected at LIGO are specifically waves that increase in frequency over time.

And that's not even discussing the facts of vibration isolation and seismic wave filtering shown by the links I provided above.

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