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?
A gravitational wave does stretch and squeeze the wavelength of the light in the arms. But the interference pattern doesn't come about because of the difference between the length of the arm and the wavelength of the light. Instead it's caused by the different arrival time of the light wave's "crests and troughs" from one arm with the arrival time of the light that traveled in the other arm. To get how this works, it is also important to know that gravitational waves do NOT change the speed of light.
With that in mind, imagine now that you and a friend want to compare how long it takes you to drive to the end of the interferometer arms and back. Just like LIGO's laser light waves, you leave the corner station at exactly the same time, take different paths, and travel at precisely the same speed. You expect to meet up again at the same time. But if a gravitational wave passes while you are on your journey, one of you will end up traveling down the longer arm, and one of you will travel down the shorter arm. Since you're still going the same speed, one of you will take longer to return than the other!
The arrival times change because when the arms of the interferometer change lengths, so too do the distances the light waves travel before exiting the interferometer. What gravitational waves do not change, however, is the speed of light. This means that a wave of light that happens to be in a longer arm during a gravitational wave has to travel farther before exiting, so it takes longer to leave than the beam that was in the shorter arm. The light waves no longer match up when they exit, so they interfere with each other. The laser light acts not as a ruler, but as a stopwatch.
But what if suddenly the length of one path got longer while the other route got shorter? One of you would have to travel a little farther and take longer to reach your destination than the other; you would no longer arrive at the same time! Furthermore, by precisely measuring the difference in arrival times, and knowing your rates of speed, you could actually calculate how much farther or less far you each had to drive in order to arrive when you did
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?
This is a huge portion of the work that is done by many of the scientists and engineers in the LIGO Scientific Collaboration -- separating a gravitational wave vibration from all the other vibrations the detectors feel (LIGO calls any non-gravitational wave vibration "noise"). To confirm a detection, we use several techniques to help sift through the noise, including:
- Measuring all known noise sources (e.g. earthquakes, winds, ocean waves, trucks driving by on nearby roads, farming activities, even molecular vibrations in LIGO's mirrors) with seismometers, magnetometers, microphones, and gamma ray detectors, and then filtering out the signals caused by these noise 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 a gravitational-wave's travel time between detectors), the more certain we are that the source of the vibration was not local.
- Using sophisticated analysis techniques to filter out and separate noise from a potential signal
- Comparing the signals received with theorized patterns of gravitational waves generated by known phenomena
- Confirming the timing of the possible gravitational wave event with astronomical observatories, hoping to see a coincident electromagnetic event on the sky (e.g. light from a supernova explosion).
Despite these precautions, however, no measuring device is 100% accurate or precise, so no result of an experiment is ever 100% certain. For LIGO, we'd like to be more than 99.9999% sure that a possible detection wasn't just noise.
With these methods, LIGO was able to confirm that the signals received at the Livingston and Hanford observatories on September 14th, 2015 were generated by an astrophysical event--in this case, the merger of two massive black holes, 1.3 billion light years away! This first EVER confirmed detection of gravitational waves demonstrates that our efforts to understand noise sources and the designs of the observatories themselves have paid off.
Once we start to see signals on a regular basis in conjunction with other observations and other observatories around the world, our confidence that we are truly detecting gravitational waves will grow until any uncertainties will be too small to worry about.
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. When a gravitational wave passes, the mirrors move and that causes the frequency of the laser light to fluctuate ever so slightly. Unfortunately, light (including lasers) can be scattered, refracted, and (potentially) absorbed by air molecules. This means that even a few oxygen or nitrogen molecules in the path of one of LIGO's laser beams could cause the beam to fluctuate and drown out any passing gravitational wave signal. It is analagous to a twinkling star. Anyone who has seen a twinkling star knows what air can do to light. Watching a star twinkle and jump around can make you think that the star itself is jumping around or changing brightness. But it's just the delicate light from that star being refracted as it passes through different-density pockets of the atmosphere. In space, where there is no air, stars don't twinkle. In the same way, if LIGO's laser beam tubes had air in them, the laser beams would bounce around (in essense, twinkle) making it impossible to distinguish between twinkling caused by air and that caused by a real gravitational wave. To reduce the chances of twinkling, LIGO’s vacuum tubes have 8 to 10 times fewer particles than the vacuum of space. This ensures that any observed vibrations in the laser light are at least not caused by refraction by air molecules.
How do you prevent the mirrors from detecting vibrations of the Earth?
LIGO uses two basic strategies to shield the detectors from vibrations of the Earth. They are referred to as “passive” and “active” vibration isolation systems.
LIGO’s passive vibration isolation system absorbs vibrations before they reach the all-important mirrors. One way these vibrations are absorbed is through a pendulum suspension system: LIGO’s main mirrors are suspended at the bottom of 4 pendulums--each node in the pendulum being a hefty mass (LIGO's mirrors alone each weigh 40 kg, or 88 lbs.). Since heavy things don’t like to move (that’s Newton’s Law of Inertia), by their sheer weight, each mass at the top of each step of the 4-step pendulum absorbs vibrations from the mass above it until nothing reaches the lowest, all-important hanging mass: LIGO’s laser-reflecting mirror. To illustrate the effect for yourself, tie four heavy washers or bolts together in a line, each one separated by an equal length of string (see illustration at left). 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 just won’t move much at all compared to how much you're moving the top mass because each mass in the chain absorbs or ‘damps’ the vibration, isolating the last one from a lot of noise above it.
As it suggests, “active” isolation is an active process whereby a set of sensors feels vibrations and sends signals to “force actuators” that generate counter-forces to cancel out vibrations. This is the same basic principle by which noise-cancelling headphones operate. In LIGO's case, the active isolation process actually occurs outside of the suspension system, removing the largest vibrations before they reach the first step in the pendulums described above (as the name implies, LIGO's suspensions are also 'suspended' below the active seismic isolation platforms). The suspensions then remove the smaller vibrations, leaving LIGO’s mirrors perfectly still and primed to detect gravitational waves.
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 knew from indirect observations that they exist. Today, with LIGO's historic direct detection, we know without a doubt that they are out there! Once LIGO begins detecting gravitational waves on a regular basis, the data will be used to answer outstanding questions 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:
- The merging (coalescence) of two black-holes, or two 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. Not only was this the world's first detection of gravitational waves, but it was also the first time black holes were directly 'observed', the first time black holes of this particular size were observed, and also the first confirmation that binary black holes (two black holes orbiting each other) exist at all! With one detection, LIGO has already uncovered and solved some mysteries of the Universe!
- The vibration or rotation of a bumpy neutron star
- The explosion of a lumpy supernova (if a star isn't perfectly spherical when it explodes)
- Motions of matter and energy right after the Big Bang
And there's always a chance that we'll detect something we've never thought of before.
LIGO’s results will be added to the knowledge of astronomers who observe in the electromagnetic spectrum (these days, astronomers study the universe not only with visible light, but also with radio waves, infra-red light, and even x-rays, all of which are part of the electromagnetic spectrum.) 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 coalesce?
- Are these systems the source of the observed but mysterious short gamma ray bursts we often see?
- 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
Although LIGO was designed to listen for one elusive, almost undetectable phenomenon, its impact on science overall will be 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 3.5 years of operation between 2005 and 2010. However, after a major upgrade, where LIGO was outfitted with extra-special noise-cancelling headphones, higher-power lasers, and larger mirrors, the Advanced LIGO detectors made their VERY FIRST OBSERVATION of gravitational waves on September 14, 2015, within days of becoming fully operational! This means either LIGO got lucky, or these kinds of events are relatively common. Now that we know LIGO can detect gravitational waves, more detections will enable astronomers to answer this question more definitively.
At present, aLIGO is still not at its 'design sensitivity', so it's mostly capable of "hearing" only the loudest gravitational wave-producing events in the Universe. This is precisely what we heard: two massive black holes colliding 1.3 billion light years away (that means it actually happened 1.3 billion years ago!)
Multitudes of fainter gravitational waves are produced in the Universe all the time, but these still appear to lie below our current sensitivity. With further planned upgrades to improve LIGO's sensitivity over the next several years, we expect to detect fainter events with some frequency. But it will still be extremely challenging. To illustrate the point, imagine standing in the middle of a field a few acres in size. A handful of people are scattered across the field; none are very close to you. One or two are shouting, some are talking, and some are whispering. The sound waves from all of them along with all the other noises in the environment are passing by your ears, but will you be able to decifer all of the conversations? No -- you're likely to hear only the shouters, whose voices are much louder than the others and the surrounding environmental noise. That's what LIGO heard in its first gravitational wave detection--big, loud 'shouting' colliding black holes, over a billion light years away. We can't yet hear the closer whispers.
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?
We predict that once LIGO reaches its most sensitive state we could detect about 40 per year, but that's just from merging neutron stars. There will be even more if we detect other sources like supernovae or more merging black holes (which is precisely what LIGO detected for the very first time on September 14, 2015).
You might now ask, "how is this possible if such events are so rare in our galaxy?"
If LIGO couldn't hear anything outside of our own galaxy (say beyond 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, listening for gravitational wave vibrations from galaxies hundreds of millions of light years away.
Still, estimating how often we might hear something is difficult because there are a lot of unknowns about sources of detectable gravitational waves. But one possible source that we know exists is binary neutron stars (two neutron stars in orbit around each other). For this reason, we use our knowledge of these systems to estimate the sizes of gravitational waves they would emit, which then gives us a reasonable estimate of how far away these objects can be in order to be heard by LIGO. Doing the math, at design sensitivity we estimate that LIGO will detect merging neutron stars as far away as 650-million light years. Of course, this doesn't mean that we won't hear 'louder' objects (the "shouters" discussed in the previous question) from farther away--like the merging black holes that LIGO has already detected (which were about 1.3 BILLION light years away...that's twice as far as our neutron stars estimate).
In terms of volume of space, 650-million light years distance translates into a volume of over one-BILLION cubic light years! That's a lot more space than occupied by our own galaxy, or even our local group of galaxies. A volume of one-billion 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 gravitational waves 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 our very first detection, such as
(a) binary black holes actually exist, and
(b) black holes with masses about 30 times that of the sun also exist
Neither of these facts were known before LIGO's historic detection!
The knowledge that astronomers gain from measuring gravitational waves could also improve our understanding of space, time, matter, energy, and the interactions between all of these things. In so doing, this field of study 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 that a person weighing 150 lb (68 kg) on Earth would weigh 21,000,000,000,000 pounds (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 such as supernovae and gamma ray bursts. Someday, gravitational waves might even allow us to listen to 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 us on Earth with little interference from the matter in the Universe. For a longer list of ways in which LIGO data will contribute to science, read the answer to the above question, "How does LIGO use the data that it collects?"
What discoveries does LIGO hope to make?
LIGO's historic 2015 detection of two colliding black holes will open 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 (extreme because we’re studying extreme forces of gravity, extreme explosions, and extreme collisions) that we stand to gain is astounding. Even better, as with any science, the best rewards come from discovering things we never knew before nor could even have imagined. 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 surprised and intrigued by what we didn’t expect to find once gravitational wave astronomy becomes its own genuine field of inquiry.