Below are answers to some of the most frequently asked questions about LIGO.
What Are Gravitational Waves?
Good question! So good that we have an entire webpage dedicated to the answer. Visit What are Gravitational Waves? to learn more.
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 how fast the light beams travel (the speed of light). And the only thing that matters to LIGO is the time it takes each laser beam to travel through its arm before being merged with the beam from the other arm.
LIGO is designed such 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, and recombine nicely when merged. When recombined, the light waves actually completely cancel each other out in a phenomenon called "total destructive interference". When this is occurring, we know the interferometer and its components are stable.
So what happens when a gravitational wave passes?
A gravitational wave causes LIGO's arms to alternately get longer and then shorter. As an analogy, imagine a buoy bobbing up and down on the water when a wave from a speed boat passes. In LIGO's case, rather than bobbing up and down, the arms oscillate back and forth, one getting longer while the other shorter, then vice versa, and so on, for as long as the gravitational wave passes.
As this is happening, the laser beams are making their way through the interferometer at a regular pace (the speed of light) unaware of the changing lengths of the arms. They expect to meet up again and destructively interfere with each other. But when they come together, that doesn't happen. As the arms changed length, so too did the distance each laser had to travel in its respective arm. The beam traveling through a longer arm arrives at the merger point after the beam in the shorter arm! Now, instead of the laser waves syncing nicely to destructively interfere, their light-waves are out of sync and something else happens. Try to imagine this happening over the time it takes for the gravitational wave to pass. Since the lengths of the arms are constantly changing as the gravitational wave passes, so too does the amount of out-of-sync-ness of the light waves!
While this sounds like a mess, it's not. In fact, over the course of the gravitational wave's passage, the resulting interference pattern changes in-step with the changing lengths of the arms. LIGO scientists can look at the changing interference pattern and decipher exactly how the arms of the interferometer itself must have changed over time to create the patterns that they observe. In this elegant way, the changing times of arrival of the laser beams from each arm cause a changing interference pattern that in turn 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.
In this scenario, the actual wavelengths of the beams of light have no bearing on the much more 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 constant vibrational noise we encounter in order 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. In short order, data from LIGO, Virgo, and the gamma ray detectors were combined and the source of the event pinpointed in a galaxy 130-million light years away. Within just a few hours, the galaxy was imaged by a ground-based telescope, revealing the unmistakable light from a cataclysmic event. This marked the first time a gravitational wave event was also observed by electromagnetic astronomers.
Even more impressively, LIGO's estimate of the distance to this event 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.
It bears noting, however, that no measuring device is 100% accurate or precise. For LIGO, we'd like to be more than 99.9999% sure that a possible detection wasn't just noise before we announce a 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 delicate, even a few molecules bouncing off of LIGO's mirrors could induce vibrations that could 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 does not travel in a vacuum (no molecules means no sound), LIGO's vacuum environment prevents sound waves from causing vibrations on the mirrors. The vacuum provides a super quiet, nearly molecule-free environment in which the detector can operate and 'listen' only for gravitational waves.
2. To reduce light vibrations. 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. It is analagous to a twinkling star. Stars twinkle becuase the delicate light from that star gets refracted as it passes through the atmosphere, encountering pockets of air of varying density (caused by different temperatures and humidity levels). The photons get scattered, reflected, refracted etc. 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 impeded the photons. In the same way, if LIGO's beam tubes had air in them, the photons in the laser beams would bounce around (in essense, twinkle), making it impossible for us to distinguish a gravitational wave 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.
Passive Vibration Isolation
LIGO’s Passive Vibration Isolation system prevents vibrations from reaching the crucial mirror or "test mass" that reflects the laser beam that tells us whether or not a gravitational wave has passed. It does this using some fairly basic physics: principles of pendulums, and the Law of Inertia.
You can illustrate at home why a pendulum is so effective at reducing vibrations. Using the image at left as a guide, tie four heavy washers or bolts together in a line, each one separated by an equal length of string. Hold the string at the top and rapidly shake the top back and forth by a small amount (you're simulating vibrations from the environment around LIGO). You'll see that the lowest 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 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 tied together in the pendulum. This is where the Law of Inertia comes into play. The Law of Inertia describes how the heavier something is, the more effort or energy it takes to move it. To take advantage of this effect, each 'node' in LIGO's quad suspension chain is a hefty mass in itself; possessing a lot of inertia. Simply by its inertia, each mass can prevent much of the vibration coming from above 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 is much more stable than the one above. How effective can this really be? In LIGO's suspensions, this process results in the magnitude of the vibrations that reach LIGO's critical 'test masses' being 100-million times smaller than the vibrations that 'shook' up the very top of the suspension system! Now, it's important to point out that 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 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 reach the passive system. This 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.
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. Up 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 and a black hole in orbit around each other
- 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. Who knows what more breakthroughs we will discover!
- 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, we hope that LIGO’s data will contain the key pieces of information needed to answer these big questions:
- How many neutron stars and black holes reside in a typical galaxy?
- How do binary systems with these objects evolve and how often do they collide?
- Are these systems the source of the observed but mysterious short gamma ray bursts we often see? (LIGO has already contributed to an answer to this question! Our 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
- 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 soemthing that was not achieved in 8 years of previous operation! That bodes well for future detections.
On average, LIGO's detections have been made at a rate of one every 2.4 months, and this is before we reach so-called "design sensitivity" (which we expect to reach in 2020). The detections we've 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 per day, 7 days per week, 365 days per 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 (and as of this writing, continues to be monitored by radio astronomers). We predict that once LIGO reaches its most sensitive state we could detect about 40 merging neutron stars per year.
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 made several 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 our most sensitive, we estimate that LIGO could detect merging neutron stars out to about 650-million light years (that's 5 times farther away than the August 2017 event). Of course, this doesn't mean that we won't hear 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 we 'sample'. 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 listen to, we expect to detect (on average) about 40 neutron-star mergers per year once we reach 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
(b) that merging neutron stars are, in fact, responsible for generating the quantities of heavy elements that we observe througout 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. To learn even more, 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 collicing 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 astounding. 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.