Sources and Types of Gravitational Waves
Any object with mass that accelerates (i.e., changes position at a variable rate) produces gravitational waves. This includes humans and cars and airplanes etc., but the gravitational waves made by us here on Earth are much too small to detect.
Since we can’t generate detectable gravitational waves on Earth, the only way to study them is to look where they are generated by nature. The Universe is filled with incredibly massive objects that undergo rapid accelerations (things like orbiting pairs of black holes and neutron stars, or stars blowing up at the ends of their lives). Based on these different sources, LIGO scientists have defined four categories of gravitational waves: Continuous Gravitational Waves, Compact Binary Inspiral Gravitational Waves, Stochastic Gravitational Waves, and Burst Gravitational Waves. Each category generates a unique set of “fingerprints” or characteristic vibrational signatures that LIGO's interferometers can sense, and that researchers can look for in LIGO’s data. Each is described in more detail below.
Continuous Gravitational Waves
Continuous gravitational waves are thought to be produced by a single spinning massive object like a neutron star. Any bumps on or imperfections in the spherical shape of this star will generate gravitational waves as it spins. If the spin-rate of the star stays constant, so too do the gravitational waves it emits. That is, the gravitational wave is continuously the same frequency and amplitude (like a singer holding a single note). Naturally, then, these are called “Continuous Gravitational Waves”. Researchers have created simulations of what an arriving continuous gravitational wave would sound like if the signal LIGO detected was converted into a sound. Click on "Continuous Gravitational Wave Signal" below to hear what the gravitational waves from a spinning neutron star would "sound" like to LIGO.
(Credit: SXS Collaboration, http://www.black-holes.org)
Compact Binary Inspiral Gravitational Waves
The next class of gravitational waves is Compact Binary Inspiral. Compact binary inspiral gravitational waves are produced by orbiting pairs of massive and dense ("compact") objects like white dwarf stars, black holes, and neutron stars. There are three subclasses of "compact binary" systems in this category of gravitational wave generators:
- Binary Neutron Star (BNS)
- Binary Black Hole (BBH)
- Neutron Star-Black Hole Binary (NSBH)
Each binary pair creates a characteristic series of gravitational waves, but the mechanism of wave-generation is the same across all three: "inspiral".
Inspiral occurs over millions of years as pairs of dense compact objects revolve around each other. As they orbit, they emit gravitational waves, which removes some of the system's orbital energy. Over eons, the objects inch closer and closer together. Unfortunately, moving closer causes them to orbit each other faster, which causes them to emit stronger gravitational waves, which causes them to lose more orbital energy, inch ever closer, orbit faster, lose more energy, move closer, orbit faster... etc. The stars are now doomed, inescapably locked in a runaway accelerating spiraling embrace.
This accelerating spin process is analogous to a spinning figure skater. Imagine that the skater's outstretched fists are neutron stars or black holes, and the skater's body is the force of gravity binding them together. As the spinning skater pulls their fists in toward their body (i.e., as the objects orbit closer and closer), they spin faster and faster.
Unlike the skater, however, the pairs of stars or black holes cannot halt their rotation. The process of emitting gravitational waves and orbiting closer and closer sets off an unstoppable sequence of events that can only end with the two objects colliding, generating one of the Universe's most violent explosions.
Compact Binary Inspiral gravitational waves vary in duration depending on the masses of the objects involved. Colliding black holes produce short gravitational waves on the order of fractions of a second, whereas neutron stars (being less massive than black holes) generate signals several tens of seconds long. In both cases, the signal frequency increases rapidly as the objects spiral into each other.
LIGO has detected both merging black holes and merging neutron stars, and the differences in their signals is quite striking. LIGO's first black hole merger detection produced a signal just two-tenths of a second long. The signal was converted into an audible sound we call a "chirp". Click on the video below to experience the moment when two black holes collide!
On the other end of the scale, the neutron star merger that LIGO detected in August 2017 generated a signal that was seen in LIGO's detectors for over 100 seconds. The video below plays the "chirp" generated by these two neutron stars as they finally spun into each other.
These two examples of actual gravitational waves illustrate how different systems of merging objects display unique signatures in the interferometers. Shorter signals mean more massive objects, like black holes, were involved; longer signals suggest lower-mass objects, like neutron stars.
To date, LIGO has published the detection of gravitational waves generated by 10 pairs of merging black holes and two pairs of colliding neutron stars. Since April 1, 2019, dozens more detections have been made, keeping LIGO scientists busy analyzing the data to understand the true natures of those detections. This success belies the fact that LIGO's instruments must strain to sense anything over the constant din generated by everything on Earth from internal fluctuations of the laser beam itself, to traffic on nearby roads, to weather, and earthquakes happening all over the world. For a more realistic sense of what LIGO contends with in its search for gravitational waves, click on the link below to hear how a simulated neutron star merger would sound like buried in all that noise. You'll have to listen very carefully to hear the chirp:
(Credit: SXS Collaboration, http://www.black-holes.org)
Stochastic Gravitational Waves
Astronomers predict that there are so few significant sources of continuous or binary inspiral gravitational waves in the Universe that LIGO doesn't worry about the possibility of more than one passing by Earth at the same time (producing confusing signals in the detectors). However, we do presume that many small gravitational waves are passing by from all over the Universe all the time, and that they are mixed together at random. These small waves from every direction make up what is called a “Stochastic Signal”, so called because the word, 'stochastic' means having a random pattern that may be analyzed statistically but not predicted precisely. These will be the smallest and most difficult gravitational waves to detect, but it is possible that at least part of this stochastic signal may originate from the Big Bang. Detecting relic gravitational waves from the Big Bang will allow us to see farther back into the history of the Universe than ever before. Click on the image at left to hear a simulated "Stochastic Signal" (Audio simulation credit: SXS Collaboration, http://www.black-holes.org).
Burst Gravitational Waves
The search for 'burst gravitational waves' is truly a search for the unexpected—both because LIGO has yet to detect them, and because there are still so many unknowns that we really don’t know what to expect! For example, sometimes we don’t know enough about the physics of a system to predict how gravitational waves from that source will appear.
We also expect to detect gravitational waves from systems we never knew about before. To search for these kinds of gravitational waves, we cannot assume that they will have well-defined properties like those of continuous and compact binary inspiral waves. This means we cannot restrict our analyses to searching only for the signatures of gravitational waves that scientists have predicted.
Searching for burst gravitational waves requires being utterly open-minded. For these kinds of gravitational waves, scientists must recognize a pattern of signals even when such a pattern has not been modeled (what we think a signal may look like) before. If you don’t know what you’re looking for, it’s really hard to find it. While this makes searching for burst gravitational waves difficult, detecting them has the greatest potential to reveal revolutionary information about the Universe.