Frequently Asked Questions
- What are gravitational waves?
- Why do we want to detect them?
- Were we confident that gravitational waves existed, even before we detected them directly?
- What was the first gravitational wave source that we detected?
- How do we know that GW150914 was a black hole merger?
- Are we sure that GW150914 was a real astrophysical event?
- Which direction on the sky was GW150914?
- What is LIGO?
- How does LIGO work?
- If a gravitational wave stretches the distance between the LIGO mirrors, doesn't it also stretch the wavelength of the laser light?
- What types of gravitational wave source is LIGO looking for?
- Was it surprising that Initial LIGO didn't detect any gravitational waves?
- Did we expect LIGO's advanced detectors to make a discovery, then?
- What's so different about LIGO's advanced detectors?
- What spinoff technologies or other direct benefits have come from LIGO instrument research?
- Is anything opaque to gravitational waves?
- Could gravitational waves tell us anything about the early universe?
- Could gravitational waves tell us anything about dark matter or dark energy?
- What is data analysis and why is it important for LIGO?
- Why does the LSC need powerful computer clusters to find gravitational waves?
- How do you know what the signals look like?
What are gravitational waves?
Gravitational waves are 'ripples' in the fabric of space-time caused by some of the most powerful processes in the universe - colliding black holes, exploding stars, and even the birth of the universe itself. Albert Einstein predicted the existence of gravitational waves in 1916, derived from his general theory of relativity. Einstein's mathematics showed that massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt space-time in such a way that waves of distorted space would radiate from the source. These ripples travel at the speed of light through the universe, carrying information about their origins, as well as clues to the nature of gravity itself.
Why do we want to detect them?
Detecting and analyzing the information carried by gravitational waves will allow us to observe universe in a way never before possible. This will open up a new window on the universe.
Historically, scientists have relied primarily on observations with electromagnetic radiation (visible light, x-rays, radio waves, microwaves, etc.) to learn about and understand phenomena in the universe. (In recent years, subatomic particles called neutrinos have also been used to study aspects of the heavens.) Each of these kinds of sources of information gives scientists a different and complementary view of the universe, with each new window bringing exciting new discoveries.
Were we confident that gravitational waves existed, even before we detected them directly?
Although gravitational waves were predicted to exist by Albert Einstein in 1916, observational evidence for their existence didn't come until 1974 when two astronomers, Russell Hulse and Joseph Taylor, discovered a binary pulsar system (two neutron stars orbiting each other). When the astronomers measured how the orbits of the two stars were changing over time, they found that the changes agreed so well with the predictions of General Relativity, it left no doubt that the system was radiating energy in the form of gravitational waves. Since then, more astronomers have studied the timing of pulsar radio emissions and found similar effects consistent with the existence of gravitational waves.
But these confirmations were indirect, from observing the effects of 'unseen' gravitational waves on remote astrophysical objects and not through actual physical effects here at the Earth. In that sense, the observation of gravitational waves from GW150914 was the first direct detection of gravitational waves - finally confirming Einstein's prediction. It was also the first direct evidence for the existence of black holes, and that black holes can exist - and merge together - in binary pairs.
What was the first gravitational wave source that we detected?
On September 14, 2015 at 09:50:45 Greenwich Mean Time the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) - arguably the most sensitive scientific instruments ever built - detected a gravitational-wave signal, referred to as GW150914, from the collision and merger of a pair of black holes more than a billion light years from the Earth. Our analysis revealed that the masses of these two black holes were 29 and 36 times the mass of the Sun respectively. We estimated the mass of the post-merger black hole to be 62 times that of the Sun.
If we compare the masses of the pre- and post-merger black holes, we see that the coalescence converted the equivalent of about three times the mass of the Sun (or nearly six million trillion trillion kilograms) into gravitational-wave energy, most of it emitted in a fraction of a second. In fact, the gravitational-wave power radiated by GW150914 was more than ten times greater than the combined luminosity (i.e. the light power) of every star and galaxy in the observable Universe.
How do we know that GW150914 was a black hole merger?
The two components of GW150914 were only a few hundred kilometers apart just before they merged, and black holes are the only known objects compact enough to get this close together without merging. Also, based on our estimated total mass for the two components, a pair of neutron stars would not be massive enough, and a black hole-neutron star pair would have already merged at a lower frequency than the value observed by LIGO.
Some key results of our analysis of GW150914, comparing the reconstructed gravitational-wave strain (as seen by H1 at Hanford) with the predictions of the best-matching waveform computed from general relativity, over the three stages of the event: inspiral, merger and ringdown. Also shown are the separation and velocity of the black holes, and how they change as the merger event unfolds. Credit: LIGO.
Are we sure that GW150914 was a real astrophysical event?
The short answer is "yes", but of course this is a crucial question and the LIGO Scientific Collaboration and Virgo Collaboration have together made a huge effort to address it, carrying out a variety of independent and thorough checks.
The Hanford and Livingston signals showed a very similar pattern, and were strong enough to 'stand out' against the background noise around the time of the event - like a burst of laughter heard above the background chatter of a crowded room.
Understanding this background noise is an essential part of our analysis and involves monitoring a vast array of environmental data recorded at both sites: ground motions, temperature variations and power grid fluctuations to name just a few. In parallel, many data channels monitor in real time the status of the interferometers - checking, for example, that the various laser beams are properly centred. If any of these environmental or instrumental channels indicated a problem, then the detector data would be discarded. However, despite exhaustive studies, no such data quality problems were found at the time of the event.
(Adapted from Figure 1 of our publication). The gravitational wave event GW150914 observed by the LIGO Hanford (H1, left panel) and LIGO Livingston (L1, right panel) detectors. The two plots show how the gravitational wave strain (see below) produced by the event in each LIGO detector varied as a function of time (in seconds) and frequency (in hertz, or number of wave cycles per second). Both plots show the frequency of GW150914 sweeping sharply upwards, from 35 Hz to about 150 Hz over two tenths of a second. GW150914 arrived first at L1 and then at H1 about seven thousandths of a second later - consistent with the time taken for light, or gravitational waves, to travel between the two detectors. Credit: LIGO.
Which direction on the sky was GW150914?
We can use the difference in arrival time of gravitational waves at different detectors to 'triangulate' the position of a gravitational-wave source on the sky. However, with only the two LIGO detectors our estimate of the sky position is typically very crude and for GW150914 we could only localize its position to within about 600 square degrees on the sky - about 2500 times the area of the full moon. Similarly, GW151226 could only be localized to within about 800 square degrees.
Fortunately, the precision of this triangulation method will be greatly improved with the addition of more gravitational-wave detectors in future - beginning in the next few months with the Advanced Virgo detector in Italy.
The approximate locations of the two gravitational-wave events detected so far by LIGO are shown on this sky map of the southern hemisphere. The colored lines represent different probabilities for where the signal originated: the outer purple line defines the region where the signal is predicted to have come from with a 90 percent confidence level; the inner yellow line defines the target region at a 10 percent confidence level. Image credit: LIGO/Axel Mellinger.
What is LIGO?
LIGO (Laser Interferometer Gravitational Wave Observatory) is the world's largest gravitational wave observatory and one of the world's most sophisticated physics experiments. LIGO consists of two laser interferometers located thousands of kilometers apart, one in Livingston Louisiana and the other in Hanford Washington State. LIGO uses the physical properties of light and of space itself to detect gravitational waves. It was funded by the US National Science Foundation, and it is managed by Caltech and MIT. Hundreds of scientists in the LIGO Scientific Collaboration, in many countries, contribute to the astrophysical and instrument science of LIGO.
How does LIGO work?
An interferometer like LIGO consists of two perpendicular "arms" (in LIGO's case each one is 4km long!) along which a laser beam is shone and reflected by mirrors at each end.
When a gravitational wave passes by, the stretching and squashing of space causes the arms of the interferometer alternately to lengthen and shorten, one getting longer while the other gets shorter and then vice-versa.
Stretching and squeezing of the interferometer arms. Image: Wikimedia Commons.
As the interferometers' arms change lengths, the laser beams traveling through the arms travel different distances - which means that the two beams are no longer "in step" and what we call an interference pattern is produced. (This is why we call the LIGO instruments "interferometers".)
Now the effect of this change in arm length is very small -- for a typical passing gravitational wave we expect it to be about 1/10,000th the width of a proton! But LIGO's interferometers are so sensitive that they can measure even such tiny amounts. LIGO scientists can look for the pattern of arm length changes that we expect from different types of gravitational wave source: if they see the pattern, they'll know a gravitational wave has passed by!
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. So the laser light is acting not so much as a ruler, but as a stopwatch. Read more about this interesting question.
What types of gravitational wave source is LIGO looking for?
LIGO scientists have identified four categories of gravitational waves, each with a unique pattern that the interferometers can sense.
Continuous gravitational waves are thought to be produced by a single spinning massive object, like an extremely dense star called a neutron star. Any bumps or imperfections in the spherical shape of this star will continuously generate gravitational waves as the star spins.
Continuous gravitational waves from an asymmetric neutron star. Image: Centre for Gravitational Physics/Australian National University.
Compact binary coalescence gravitational waves are produced by two massive stars, like neutron stars or black holes, orbiting each other. As the two stars revolve around each other emitting gravitational waves, they lose orbital energy. Over time, this causes the stars to move closer together and revolve around each other faster and faster until they eventually merge together, or coalesce, in one of the universe's most violent events.
Gravitational waves produced by 2 orbiting black holes. Image: Henze, NASA.
Astronomers predict that there are so few significant sources of Continuous or Compact Binary Coalescence gravitational waves in the universe that we don't worry about more than one passing by Earth at exactly the same time. But there could be a background signal, like the background hum of voices in a crowded room, coming from all over the universe that all combine together into what we call the stochastic background of gravitational waves. It's possible that at least part of this stochastic signal may originate from the Big Bang, and detecting those relic gravitational waves would let us see farther back into the history of the universe than ever before.
Crab Nebula supernova remnant. Such a core-collapse supernova could produce a burst of gravitational waves. Image: Hester, Loll, NASA, ESA.
The fourth category is known as burst gravitational waves. Here's where we are searching for the truly unexpected. Of course if you don't know what you're looking for, it's really hard to find it, but looking for burst signals may offer the best prospects for discovering completely new things about our universe.
Was it surprising that Initial LIGO didn't detect any gravitational waves?
Not really. We study the behavior of our detectors intensely and, when we are taking data, we perform what we call "blind injections". This is when a few people in our collaboration secretly add a fake signal into the data coming out of the detector, to test whether we can find the signal and that we understand the instrument properly. During a science run of the initial LIGO experiment, a signal was seen that looked like a binary merger at a distance of about 60 - 180 million light years, coming from the direction of the constellation Canis Major (the signal was later called the "Big Dog"). The collaboration proceeded to do the full analysis and check everything over and over, then wrote a paper claiming the first detection. Only then was it revealed that the signal was a blind injection! While that might be a little disappointing, it shows that we were prepared and understand the instruments enough to be confident that they work as expected.
Strength of the 'Big Dog' blind injection. The signal sweeps upwards in frequency (known as a "chirp") as the stars spiral into one another, approaching merger. Image: Caltech/MIT/LIGO Lab.
So, blind injections aside, should we have expected a detection during one of the initial LIGO science runs? After taking into account that we don't know precisely where, when and how often the major source events happen in the universe, our best guess is "no". So the fact that Initial LIGO didn't detect any sources wasn't surprising. But it's important to realise that Advanced LIGO would have been impossible without LIGO. If you think of LIGO as a proof of concept and technology (that may have gotten lucky) then the project was a great success.
Did we expect LIGO's advanced detectors to make a discovery, then?
Good question! The expected event rates for different kinds of gravitational wave source can be worked out, but these calculations involve a lot of complicated physics so the predictions are very uncertain. Even so, we have tried to carefully take account of all of that by calculating "realistic", "optimistic" and "pessimistic" rates -- based on different assumptions. (The results of those calculations can be found in a paper published in 2010 that is freely downloadable from http://arxiv.org/abs/1003.2480).
Take binary neutron star mergers, for example. Initial LIGO was able to look for these within about 15 megaparsecs (Mpc -- a parsec is about 3.25 light years, so 15 Mpc is about 50 million light years). Initial LIGO didn't see any, but that was in line with our best guess of the expected rate for these events. On the other hand, once it reaches its full design sensitivity Advanced LIGO should be able to see binary neutron star mergers out to about 200 Mpc. This factor of more than 10 increase in the distance reach corresponds to a factor of over 1000 times more volume of space. For full-sensitivity Advanced LIGO, therefore, binary neutron star mergers should be detected with event rates between 0.4 to 400 times a year -- with about 40 per year believed most likely.
Of course, the first event that the Advanced LIGO detectors discovered - on September 14th 2015 - was a binary black hole merger. This remarkable discovery was announced to the world on February 11th 2016 and you can read all about it at https://www.ligo.caltech.edu/detection. A second event, discovered on December 26th 2015 and announced on June 15th 2016, was also a binary black hole merger. A second confirmed detection then followed in December 2015, and was announced in June 2016. This was another binary black hole merger - this time with masses 14 times and 8 times that of the Sun - again more than a billion light years distant. The gravitational wave signal from this event, referred to as GW151226, was less strong than that from GW150914, and was harder to see as it was spread over a longer time - lasting 1 second compared with about 0.2 seconds for the first detection. Despite the difficulty in spotting this event by eye, our detection software was able to find the signal in our data.
You can read all about the full results from the first science run of the Advanced LIGO detectors (including a third candidate gravitational-wave signal that we couldn't be sure was real, but may have been another binary black hole merger) in this article.
Schematic showing the much greater volume of the universe to which the Advanced LIGO detectors will be sensitive. Image: Caltech/MIT/LIGO Lab.
What's so different about LIGO's advanced detectors?
As we said above, once they have reached their full design sensitivity within the next few years, the Advanced LIGO detectors should be about 10 times more sensitive than Initial LIGO, capable of measuring gravitational wave strains as small as one part in ten to the power 23. This astounding sensitivity should be a real game changer for gravitational-wave astronomy.
Installing the Advanced LIGO upgrades. Image: Caltech/MIT/LIGO Lab.
Although the Advanced detectors are geographically situated in the same locations as their Initial LIGO predecessors, to achieve their much greater sensitivity required the upgrading of almost every aspect of the Initial LIGO design. These upgrades include:
- Increasing the laser power from the initial LIGO value of 10 W to about 200 W
- Using larger and heavier fused-silica test mass optics, designed to reduce the impact of thermal noise and "radiation pressure" noise, both of which cause very small, random motions of the mirror
- Suspending the test mass optics using fused silica fibers, as opposed to the steel wire sling suspensions used in Initial LIGO, in order to reduce the suspension thermal noise
- Using a four-stage pendulum in the complete suspension system, to improve the seismic isolation of the test mass optics
- Using a measure-and-cancel strategy for reducing the motion of the masses due to the motion of the ground -- this is called "active" seismic isolation
LIGO's advanced detectors bring to fruition years of global research and development in key areas of LIGO technology. A number of U.S. and international institutions have played lead roles in all of this work, and Caltech and MIT (the institutions that operate LIGO on behalf of the NSF and the LIGO Scientific Collaboration) continue to provide critical R&D, testing and integration along with these international partners.
The completion of the advanced detectors, and the beginning of their science operations, in autumn 2015 represents a phenomenal scientific and engineering achievement -- and the dawn of an exciting new era for gravitational-wave astronomy! Read more about the design and construction of LIGO's advanced detectors in Issue 1 and Issue 7 of the LIGO Magazine.
What spinoff technologies or other indirect benefits have come from LIGO instrument research?
The development of the technology necessary to measure the tiny position changes that are necessary for LIGO to detect gravitational waves can have other applications too. Below is a short list of examples of "spinoff" technologies.
- The need for very low loss of light in the LIGO optics lead to the development of a technique to measure such low loss and the company "Stanford Photo-Thermal Solutions" who markets the device developed for LIGO. It sells primarily to the basic optics and homeland security markets.
- The company Lightwave Electronics built the initial laser that LIGO used. It now sells the same laser (and an improved version) for use in materials processing for making light emitting diodes, computer chips, and smart phone circuit boards.
- A typical gravitational wave signal has what is called a "chirp" in amplitude and phase. LIGO developed computer algorithms that are very effective at finding these chirps even when hidden in noise. Similar chirp signals appear in radar, sonar, laser pulses and other situations, and LIGO's algorithm is being used in these applications.
- The heating of LIGO optics can deform the laser beam, making it less effective at measuring position changes. This problem is called thermal lensing, and is shared by other precision optics measurements. LIGO found a solution to this problem by adding additional optics to the system whose temperature can be controlled with heaters. This same technology is now being used on high-power laser systems used for laser radar, welding, cutting and drilling, and other applications.
More information on these and other spinoffs.
Is anything opaque to gravitational waves?
Opaqueness to light occurs because the energy of the light is absorbed by the opaque material. This is essentially because the material has charges in it that are free to move under the influence of the electromagnetic fields that the light is composed of, and their movement absorbs the energy of the light. There have to be enough charges in the material so that almost all of the 'packets' of light energy, called photons, find one and get absorbed.
So, how does this compare to gravitational waves? Are they carrying energy that can be absorbed? Yes, they are. What can gravity impart energy into? The answer is mass. If the mass is freely floating, it will not absorb the energy of the wave, so we need the mass to have a dissipative force applied to it as well. So, if there are masses with dissipative forces that can absorb energy, we can in principle create something opaque to gravitational waves. Can we put enough of these masses together to absorb almost all the gravitons? It would be extremely difficult: the gravitational waves interact so weakly that the chance that any particular graviton passing any particular absorbing mass would be absorbed is minuscule. So, at the end of the day, answer is "in principle, yes, but in practice, not really."
Could gravitational waves tell us anything about the early universe?
Gravitational waves offer a unique view into the very early universe because they can allow us to see "behind" the Cosmic Microwave Background Radiation (CMBR). You may have seen images of the CMBR which give us an image of the universe about 380,000 years after the beginning of the universe, the 'Big Bang'. This is because before that time, the universe was filled with hot ionized gas (meaning the electrons and nuclei were separate, just free electrons flying around), so photons of light would be scattered wildly by these free electrons, rather like what happens on a foggy day, so they don't carry much information about their original source. Because the universe had been expanding since the beginning, it was cooling, and at around the 380,000 year mark, the universe became cool enough that the gas stopped being ionized: the free electrons combined with protons to form neutral hydrogen (we call this recombination), which is much less effective at scattering photons -- in other words the fog clears! For us, this marks the beginning of the period when photons actually can free-stream directly towards us from the early universe, with minimal scattering, so they can carry information about their origins.
The wider gravitational wave spectrum. Image: Institute for Gravitational Research/University of Glasgow.
On the other hand, gravitational radiation does not care about this epoch of recombination, because atoms and molecules of gas -- whether ionized or not -- have minimal effect on gravitational waves. This means that gravitational waves created before recombination can still stream right towards us without being disturbed along the way, and so could in principle tell us something about that very early phase in the history of the cosmos. The LIGO detectors may not be sensitive to these 'primordial' gravitational waves, but there are other ways we can search for their signature -- e.g. by looking for polarized light in the CMBR.
Could gravitational waves tell us anything about dark matter or dark energy?
Dark matter and dark energy have had a big role in the history of the universe expanding (in fact we think dark energy is now causing that expansion to speed up!) and in the formation of galaxies and clusters of galaxies. But we don't expect dark matter to exist in nearly dense enough 'clumps' to produce gravitational waves that could be detected by LIGO.
In the future, however, astronomers hope to use gravitational wave sources such as compact binary coalescence to map out the cosmos, completely independently of the methods available right now -- using e.g. the CMBR or distant supernovae. So it's possible that future gravitational wave observations could help us to better understand the effects of dark matter and dark energy on the expansion of the universe.
What is data analysis and why is it important for LIGO?
For a scientist, collecting data is just the first step. It's then by examining and studying that data, and comparing it with what we were expecting, that we can learn new things and improve our understanding. This is especially important in gravitational wave astronomy, where we expect the signals we are searching for to be relatively faint and buried in the detector noise, so without data analysis we cannot discover gravitational waves. Dedicated, highly efficient computer algorithms are required to find these signals in the first place and to characterize them afterwards. To discover and characterize all the different signal types, the data are analyzed by several complementary methods. Some use pre-calculated waveforms as 'templates' to search for very specific signals. Others use an "eyes wide open" approach and assume much less about what the waveforms should look like.
Why does the LSC need powerful computer clusters to find gravitational waves?
The different data analysis algorithms must search through very large amounts of data. In order not to miss any possible hidden signal, the data have to be searched extensively with many different templates - in some cases millions of them. Searches like these would take several years on a single desktop computer. These searches can however be 'parallelized', which means running thousands of similar analyses at the same time on many computers. These computers are typically organized in large computer clusters, operated at several sites within the LIGO Scientific Collaboration. On these clusters, the computations can be done within days or weeks, rather than many years.
The high-throughput data analysis computer cluster Atlas at the AEI in Hannover, Germany. Image: M. Fiorito/AEI.
How do you know what the signals look like?
The exact form of the gravitational wave signal is predicted by Einstein's theory of general relativity. However, solving Einstein's equations is very complicated and not always possible to do exactly. In some cases we can solve approximate versions of these equations very fast; in other cases, we can run programs on large supercomputers to calculate the expected waveforms for the complete (i.e. non-approximated) equations. This approach gives us more realistic waveforms but will also take much longer. Sometimes it is possible to cleverly combine these two approaches and get the best of both worlds.