# FAQ

#### Below are answers to some of the most frequently asked questions about LIGO.

If a gravitational wave stretches the distance between the LIGO mirrors, doesn't it also stretch the wavelength of the laser light?

While it's true that a gravitational wave does stretch and squeeze the wavelength of the light in the arms ever so slightly, it does NOT affect the fact that the beams will travel different distances as the wave changes each arm's length. And the only thing that matters to LIGO is how far the beams travel in each arm before being merged once again.

LIGO is designed so that as long as the distance the laser beams travel is exactly the same in both arms, they will make their trips in exactly the same time. When recombined, the beams totally destructively interfere with each other. In other words, they cancel each other out and no light emerges from the instrument. When this is occurring, we know the interferometer and its components are stable and the Universe is quiet.

Suddenly, a gravitational wave passes! What happens?

A gravitational wave causes each of LIGO's arms to change length in an opposite fashion, i.e., when one arm gets longer, the other gets shorter. Then they switch--the longer arm becomes the shorter arm and the shorter arm becomes the longer arm. This opposite oscillation in length occurs for as long as the waves pass, one getting longer while the other shorter, then vice versa, and so on, until the waves dissipate.

As this is happening, the laser beams are making their way through the interferometer unaware of the fact that the distance they have to travel before meeting up again is changing. A beam traveling through a longer arm, therefore, takes a little longer to return to the merger point than the beam in the shorter arm, which means, when they do meet up again, their waves won't necessarily just cancel each other out. Instead, they pass through each other, sometimes canceling out, but other times adding together to make a brighter light. Back and forth the waves pass through (*interfere with*) each other as the arms themselves change length, causing light interference that ranges fully between totally destructive to totally constructive. In other words, instead of nothing coming out of the interferometer, a flicker of light appears.

While this sounds like a mess, it's not. In fact, during a gravitational wave's passage, the resulting interference pattern itself changes in-step with the changing lengths of the arms. Looking at the changing interference pattern, LIGO computers decipher exactly how the arms of the interferometer itself must have changed over the time of the wave's passage to create the patterns that emerge. In this elegant way, the changing times of arrival of the laser beams from each arm cause a changing interference pattern that betrays changes in the lengths of the arms caused by the gravitational wave!

This varying pattern of misaligned laser beams observed and recorded over the time it takes for the gravitational wave to pass tells us two things: 1. How much the arms changed length during the light-beams' journeys and, 2. the frequency at which, or how quickly the arms changed lengths (longer then shorter then longer, etc.) in response to the gravitational wave. This information can tell us what generated the gravitational wave in the first place.

This is why the wavelengths of light have no bearing on the all-important interference pattern. The effects of the length changes in the arms far outweigh any change in the wavelength of the laser, so we can virtually ignore it altogether.

How will LIGO know that a signal in the data really came from an event in space? Can you ever be 100% certain, even when multiple labs measure the same vibration?

LIGO uses several techniques to sift through the din of vibrational noise to confirm a gravitational wave detection. Some of these are:

• Measuring all known noise sources (e.g., earthquakes, winds, ocean waves, traffic, farming activities, even molecular vibrations in LIGO's mirrors) with seismometers, magnetometers, microphones, and gamma ray detectors, and then filtering out the vibrations caused by these sources from our data.
• Looking for identical, simultaneous signals from multiple detectors world-wide (LIGO, Virgo, GEO600). This rules out noise sources which are local to a given detector. The more detectors that feel the same vibration at "the same time" (accounting for travel-time between detectors), the more certain we are that the source of the vibration was not local.
• Constantly comparing the signals coming out of the interferometers with theorized patterns of gravitational waves generated by known phenomena (these patterns are derived directly from the equations of general relativity).
• Comparing the time of arrival of a possible gravitational wave event with an electromagnetic (EM) event seen by EM observatories.

These methods have proven hugely successful. Perhaps most significantly, the last technique mentioned above was first used in August, 2017 when LIGO detected its first pair of merging neutron stars. The arrival time of the gravitational waves coincided with a short gamma ray burst detected by an orbiting observatory. Within tens of minutes, data from LIGO, Virgo, and the gamma ray detectors were combined and the source of the event localized to a very small patch of sky. Within just a few hours, astronomers, systematically imaging the galaxies in that patch of sky, spotted a new star-like object in a galaxy 130-million light years away, marking the first time a gravitational wave event was also observed by electromagnetic astronomers.

As further test of LIGO's abilities, LIGO's estimate of the distance to this event (information also encoded in the interference pattern) was in near-perfect agreement with estimates independently-acquired by EM astronomers.  This provided irrefutable evidence that LIGO is truly detecting gravitational waves from distant regions of the Universe. Our confidence will only increase with our numbers of detections, especially as more are also observed by our electromagnetic partners.

All that being said, it bears noting that no measuring device is 100% accurate. At LIGO, we like to be more than 99.9999% sure that a signal wasn't just caused by some Earthly noise before we announce a true detection to the world.

Why do you need to create a vacuum to get accurate data?

There are two main reasons why LIGO needs to operate in a pristine vacuum:

1. To reduce sound vibrations. Sound waves are vibrations of molecules which, when they bounce off of obstacles, cause other things to vibrate (like your eardrum). Because the signals that LIGO needs to feel are so unimaginably delicate, even a few molecules bouncing off of LIGO's mirrors could cause vibrations that could mimic or mask a gravitational wave signal. This is why so much effort goes into getting rid of as much air as possible with LIGO's vacuum system. Since sound cannot travel in a vacuum (no molecules means no sound), LIGO's vacuum environment prevents sound waves from inducing vibrations in the mirrors. The vacuum provides a super quiet, nearly molecule-free environment in which the detector can operate and 'listen' for gravitational waves.

2. To reduce light scattering. LIGO uses lasers to measure the distance between the hanging mirrors (we call them "test masses"). Unfortunately, laser light can be scattered, refracted, and (potentially) absorbed by molecules. Even a few oxygen or nitrogen molecules in the path of one of LIGO's laser beams could cause the beam to scatter and create a 'flicker' that could mimic a gravitational wave signal. Think of twinkling stars. Stars twinkle because the light from the star gets bounced around as it encounters pockets of air of varying density in our atmosphere (caused by different temperatures and humidity levels). The photons themselves get scattered, reflected, and refracted as they travel to your eye, which perceives the effect as twinkling. In the vacuum of space, stars don't twinkle because there's no atmosphere to impede the photons. So if LIGO's beam tubes had air in them, the photons in the laser beams would bounce around (in essence, twinkle), making it impossible for us to distinguish a gravitational wave flicker from just some scattered light. To eliminate this problem, LIGO’s vacuum tubes have 8 to 10 times fewer atoms and molecules than the vacuum of space! This ensures that any observed vibrations in the laser light are not caused by scattering by atoms or molecules in the beam tube.

How do you prevent the mirrors from detecting vibrations of the Earth?

LIGO uses two basic strategies to shield the detectors from Earthly vibrations. They are referred to as “Passive” and “Active” vibration isolation systems. Each one has many components, but the basics are pretty simple.

Basic "quad" pendulum demonstrating passive damping through suspension

Passive Vibration Isolation

LIGO’s Passive Vibration Isolation system prevents vibrations from reaching the mirrors ("test masses") that reflect the laser beam, which tells us whether or not a gravitational wave has passed. It does this using some fairly basic physics concepts: principles of pendulums, and the Law of Inertia.

Pendulums are excellent vibration isolators. Try this at home: Using the image at left as a guide, tie four heavy washers together in a line, each one separated by an equal length of string (this is analogous to LIGO's "quad" (i.e., 4-segment) suspension). Hold the string at the top and rapidly shake it back and forth by a small amount (you're simulating vibrations from the environment around LIGO). Notice that the lowest mass (in LIGO this is the test mass) moves very little, if at all, compared to the top one. This is because each segment in the pendulum absorbs the vibration it 'feels' from above and prevents it from being transmitted to the segment below. In this way, this system "isolates" the bottom mass from all the "noise" you created above it.

Now, there's another factor that can affect how still the bottom mass remains as you shake the top: the weights of the things you tied together in your pendulum. This is where the Law of Inertia comes in handy. The Law of Inertia states that the heavier something is, the more energy it takes to move it (try pushing a bicycle uphill with and without a person on it!). To take advantage of this effect, each 'node' (bolt or washer in your homemade suspension system) in LIGO's quad suspension chain is a hefty mass in itself; possessing a lot of inertia. Simply by its inertia, each mass can absorb vibrations coming from above and prevent them from reaching the mass immediately below. A little may leak through to the mass below, but then its inertia absorbs much of those vibrations and prevents it from reaching the next one, and so on. The result is that each subsequent mass in the chain is much more stable than the one above.

How effective can this really be? In LIGO's suspensions, this process reduces any vibrations present at the top of the suspension 100-million times smaller by the time they reach the test mass! Yes, there are other ways within the quad suspension that LIGO engineers have found to reduce vibrations. But in large part, it is achieved through the processes described above.

Active Vibration Isolation

As good as it is, on its own, the "Passive" system is still not good enough to enable LIGO to detect gravitational waves. To get to that level, the passive system is itself contained within an "Active" isolation system. As the name implies, active isolation is a process whereby a number of sensors in and near LIGO's interferometers detect vibrations from the environment (wind, earthquakes, traffic, etc) and send feedback signals to actuators that physically move the passive isolation system around to cancel out as many vibrations as possible before they even affect the passive system. It is the same basic principle by which noise-cancelling headphones operate. This active system cannot remove all vibrations, however, and what little makes it to the quad suspension is taken care of by the passive isolation system described above.

Working together, the Active and Passive Isolation Systems keep LIGO’s mirrors perfectly still and primed to detect gravitational waves. Impressively, all of this effort has made LIGO's interferometers the most sensitive measuring devices ever built.

For more about how these isolation systems work, read Vibration Isolation.

How does LIGO use the data that it collects?

In 1916, Albert Einstein predicted the existence of gravitational waves in his theory of general relativity. And until September 14, 2015 we only inferred that they existed. Today, LIGO has made enough detections to know without a doubt, gravitational waves are real! The more detections LIGO makes, the more questions it will help to answer about physics and the Universe in general. Since each source of gravitational waves plays a unique "tune", the first thing we’ll learn is which amazing event in the Universe generated the wave. The known possibilities are:

• Merging (coalescing) black-holes, or neutron stars, or a neutron star merging with a black hole
• The gravitational waves detected by LIGO on September 14, 2015 were generated by the merger of two massive black holes. Less than two years later, on August 17, 2017, LIGO detected colliding neutron stars. These two events in particular, made scientific history.
• The rotation of a bumpy neutron star
• The explosion of a lumpy star (if a star isn't perfectly spherical when it explodes), called a supernova
• Motions of matter and energy that occurred right after the Big Bang

And there's always a chance that we'll detect something we can't yet explain.

LIGO’s results will greatly augment the knowledge-base of astronomers who observe in the electromagnetic spectrum (e.g., radio waves, infra-red, X-rays, gamma rays). In some cases, LIGO’s data may contain key pieces of information needed to answer these big questions:

• How many neutron stars and black holes reside in a typical galaxy?
• How are binary systems containing these objects created in the first place, and how often do they collide?
• Are these systems the source of the observed but mysterious short gamma ray bursts we often see? (LIGO's detection of colliding neutron stars in August 2017 provided clear evidence that at least some short gamma ray bursts are, in fact, caused by colliding neutron stars.)
• How "lumpy" are supernova explosions and neutron stars?
• Do black holes really affect space and time in the way Einstein's theory predicts?
• Can gravitational waves tell us anything about the first instants after the Big Bang?

In addition, although LIGO will be looking for gravitational waves caused by astronomical phenomena, LIGO's data will also contribute to the broader physics and engineering communities and will help to answer fundamental questions of physics, such as:

• What are the properties of gravitational waves?
• Is general relativity the correct theory of gravity?
• Is general relativity valid under extremely strong gravity conditions?
• Are nature's black holes the black holes of general relativity?
• How does matter behave under extremes of density and pressure?

Last, but not least, LIGO is already making contributions outside of astronomy and astrophysics by advancing knowledge in the following fields:

• Quantum measurement/high-precision spacetime metrology
• Optics/quantum optics/laser systems
• Space science and technology
• Geology and geodesy
• Materials science and technology
• Cryogenics and cryo-electronics
• Computing
• Methods in theoretical physics
• Chemistry (LIGO's detection of colliding neutron stars resulted in a completely new understanding of the origins of heavy elements in the periodic table, answering a long-standing question about how such elements were formed in the quantities we observe in the Universe at large.)

Although LIGO was designed to listen for one elusive, almost undetectable phenomenon, its broader impact on science has already been significant.

How often do gravitational waves that LIGO can detect pass by the Earth?

Nobody really knows yet. Strong gravitational waves are believed to occur rarely enough that LIGO did not detect any in its first years of operation between 2002 and 2010. However, after a major upgrade that took 4 years to complete, LIGO detected its first gravitational wave on September 14, 2015, within days of turning on the new and improved detector. In other words, in a few days, LIGO accomplished something that was not achieved in 8 years of previous operation! That bodes well for future detections.

On average, over its first two observing runs, LIGO's detections have been made at a rate of one every 2 months, and this is before we reach so-called "design sensitivity" (which we expect to reach in 2020). The eleven confirmed detections made so far suggest that these kinds of events are relatively common. Once LIGO's detectors reach their maximum sensitivity, they could be detecting gravitational waves at a rate of one per week. The only way to refine the estimates of how often detectable gravitational waves pass, is to keep making detections!

To that end, it is expected that within 10 years, five gravitational wave detectors like (and including) LIGO will be operating around the world. When that happens, we should be ready to detect gravitational waves 24 hours a day, 7 days a week, 365 days a year. Who knows what multitude of wonders await!

If current estimates say that merging neutron stars generate gravitational waves in a galaxy once per 10,000 years, how many events could we currently detect per year?

LIGO detected its first neutron-star merger on August 17, 2017 in an event that was also observed around the world by electromagnetic astronomers. Once LIGO reaches its most sensitive state, about 40 merging neutron stars per year may be detected.

You might now ask, "how is this possible if such events are so rare in our galaxy?"

If LIGO couldn't sense events happening outside of our own galaxy (beyond about 80,000 light years), we would probably have to wait a very long time to detect a gravitational wave. But LIGO’s advanced detectors can hear thousands of times father away than this, primed to detect gravitational waves originating in galaxies as far as hundreds of millions of light years away (the August 2017 event originated in a galaxy 130 million light years away).

Estimating how often we might detect any gravitational wave is difficult. Even though we have now confirmed nearly a dozen detections, we still don't know a lot about sources of detectable gravitational waves beyond what astrophysicists have theorized. One source that we know exists and have now detected, is binary neutron stars (two neutron stars orbiting each other). At its most sensitive, LIGO could detect merging neutron stars out to about 650-million light years (5-times farther away than the August 2017 event). Of course, this doesn't mean that LIGO won't sense more powerful events at much greater distances. Such was the case with LIGO's first detection, generated by two black holes some 1.3 billion light years away.

What's really important to LIGO is the volume of space it 'samples'. 650-million light years in distance from Earth translates into a volume of space of over one-BILLION cubic light years! A volume of one-billion cubic light years contains about one-million Milky Way type galaxies. So with a million entire galaxies to probe, we expect to detect (on average) about 40 neutron-star mergers per year once the detectors reach their design sensitivity.

If gravitational waves exert such tiny changes on Earth, why are they important?

Gravitational waves probably won't be useful in helping us understand processes on the Earth, but they will help us understand processes that occur in outer space, such as the collisions of pairs of black holes. We've already learned a lot from just two of our first detections:

From our first detection of merging black holes we learned:

(a) binary black holes actually exist (they were only theorized before), and

(b) black holes with masses about 30 times that of the Sun also exist (astronomers are working to understand how to make such black holes)

Neither of these facts were known before LIGO's historic first detection.

From our first detection of merging neutron stars we learned:

(a) that at least one source of short gamma ray bursts is merging neutron stars, and

(b) that merging neutron stars are, in fact, responsible for generating the quantities of heavy elements that we observe throughout the Universe. This single detection resulted in rewriting the origin stories of many elements in the periodic table.

The knowledge that astronomers gain from measuring gravitational waves could also improve our understanding of space, time, matter, energy, and their interactions. In so doing, we could revolutionize humanity’s knowledge and understanding of the nature of existence itself. Also, LIGO's impact on science in general will reach far beyond just the fields of astronomy and astrophysics. For more on that topic, visit Why Detect Them? in Learn More.

What kinds of information can gravitational waves provide?

Gravitational waves will provide a test of Einstein's theory of general relativity under extreme conditions of gravity where it has never before been tested. They will also give us more information about the unimaginably dense form of matter that makes up neutron stars. Neutron stars contain more matter than our Sun packed into a sphere the size of a city--about 10 km (6 mi.) across. These objects are so dense and their gravity so immense that a person weighing 150 lbs. (68 kg) on Earth would weigh 21,000,000,000,000 lbs. (9,545,000,000,000 kg) on a neutron star! Packed so closely and densely together, the matter that makes up a neutron star is called "degenerate matter", which is not well understood. LIGO will help improve our understanding of degenerate matter.

Gravitational waves will also tell us about how many objects like black holes and neutron stars exist in the Universe. They will give us insight into what happens during some of the Universe's most violent explosions. Someday, gravitational waves might even allow us to understand what was happening in the earliest moments of the Universe when it was so dense and hot that no light could move around. Any photons emitted during that time were long ago absorbed by a plasma of hot ions, but gravitational waves from that era could travel directly to Earth with little interference from the matter in the Universe.

What discoveries does LIGO hope to make?

LIGO's historic 2015 detection of two colliding black holes, and its paradigm shifting detection of colliding neutron stars in 2017 has opened up a new field of astrophysics. Whether gravitational waves are detected from colliding black holes, supernovae, remnant radiation from the Big Bang, or even just the tiniest imperfections on rapidly spinning ultra-dense neutron stars, the amount of potentially new fundamental knowledge of the extreme Universe that we stand to gain is enormous. Even better, as with any science, the best rewards come from discovering things we never knew about, could never have imagined, or simply do not understand. As with every other time we've looked up at the sky in a different way, be it through infrared, X-ray, or gamma-ray goggles, we will almost certainly be inspired and intrigued by what we didn’t expect to find.