What are Gravitational Waves?
Gravitational waves are 'ripples' in the fabric of space-time caused by some of the most violent and energetic processes in the Universe. Albert Einstein predicted the existence of gravitational waves in 1916 in 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 (like the movement of waves away from a stone thrown into a pond). Furthermore, these ripples would travel at the speed of light through the Universe, carrying with them information about their cataclysmic origins, as well as invaluable clues to the nature of gravity itself.
The strongest gravitational waves are produced by catastrophic events such as colliding black holes, the collapse of stellar cores (supernovae), coalescing neutron stars or white dwarf stars, the slightly wobbly rotation of neutron stars that are not perfect spheres, and the remnants of gravitational radiation created by the birth of the Universe itself.
Though gravitational waves were predicted to exist in 1916, actual proof of their existence wouldn't arrive until 1974, 20 years after Einstein's death. In that year, two astronomers working at the Arecibo Radio Observatory in Puerto Rico discovered a binary pulsar--two extremely dense and heavy stars in orbit around each other. This was exactly the type of system that, according to general relativity, should radiate gravitational waves. Knowing that this discovery could be used to test Einstein's audacious prediction, astronomers began measuring how the period of the stars' orbits changed over time. After eight years of observations, it was determined that the stars were getting closer to each other at precisely the rate predicted by general relativity. This system has now been monitored for over 40 years and the observed changes in the orbit agree so well with general relativity, there is no doubt that it is emitting gravitational waves.
Since then, many astronomers have studied the timing of pulsar radio emissions and found similar effects, further confirming the existence of gravitational waves. But these confirmations had always come indirectly or mathematically and not through actual 'physical' contact.
That was the case up until September 14, 2015, when LIGO, for the first time, physically sensed distortions in spacetime itself caused by passing gravitational waves generated by two colliding black holes nearly 1.3 billion light years away! LIGO and its discovery will go down in history as one of the greatest human scientific achievements.
Lucky for us here on Earth, while the origins of gravitational waves can be extremely violent, by the time the waves reach the Earth they are millions of times smaller and less disruptive. In fact, by the time gravitational waves from the first detection reached LIGO, the amount of space-time wobbling they generated was thousands of times smaller than the nucleus of an atom! Such inconceivably small measurements are what LIGO was designed to make.
Sources and Types of Gravitational Waves
Any object with mass that accelerates (which in science means changes position at a variable rate, and includes spinning and orbiting objects) produces gravitational waves, including humans and cars and airplanes etc. But the gravitational waves made by us here on Earth are much too small to detect. In fact, it isn’t even remotely possible to build a machine that can spin an object fast enough to produce a detectible gravitational wave – even the world’s strongest materials would fly apart at the rotation speeds such a machine would require.
Since we can’t generate detectable gravitational waves on Earth, the only way to study them is to look to the places in the Universe where they are generated by nature. The Universe is filled with incredibly massive objects that undergo rapid accelerations (things like black holes, neutron stars, and stars at the ends of their lives). In order to understand the types of gravitational waves these objects may produce, LIGO scientists have defined four categories of gravitational waves, each with a unique “fingerprint” or characteristic vibrational signature that the interferometers can sense and that researchers look for in LIGO’s data. These categories are: Continuous Gravitational Waves, Compact Binary Inspiral Gravitational Waves, Stochastic Gravitational Waves, and Burst Gravitational Waves. Each of these kinds of gravitational-wave generators is described below.
Continuous Gravitational Waves
Continuous gravitational waves are 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 generate gravitational waves as the star spins. If the spin rate of the star stays constant, so too do the properties of the gravitational waves it emits. That is, the gravitational wave is continuously the same frequency and amplitud (like a singer holding a single note), hence, “Continuous Gravitational Wave”. Researchers have created simulations of what they think an arriving continuous gravitational wave would sound like if the signal LIGO would detect was converted into a sound (based on the frequency of LIGO's laser's flicker).
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 (hence "compact") objects like white dwarf stars, black holes, and neutron stars. There are three kinds of "compact binary" systems in this category of gravitational wave generators:
- Binary Neutron Star (neutron star-neutron star) or BNS
- Binary Black Hole (black hole-black hole) or 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; it's called, "inspiral".
Inspiral occurs over millennia as pairs of these dense compact objects revolve around each other. As they orbit, they send off gravitational waves, which remove some of the system's orbital energy (energy which keeps them from colliding). Over aeons, as the objects revolve and lose this energy, they inch closer and closer together. Unfortunately, moving closer causes them to orbit each other faster, which causes them to emit more and 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 process is analagous 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 at the end of which the two objects will collide, causing one of the Universe's most cataclysmic events. This is why the process is called “Inspiral”.
Compact Binary Inspiral Gravitational waves are characteristically short in duration (several seconds to less than a second long) and increase in frequency as the stars orbit ever-faster. The expected gravitational wave signals from merger of neutron stars and black holes have been modeled into audible signals based on the frequencies of the gravitational waves as they would arrive at LIGO's detectors. They're called “chirps”.
Lucky for LIGO, astronomers no longer have to wait to hear what real gravitational waves 'sound' like. On September 14, 2015 LIGO made scientific history by detecting gravitational waves for the very first time. The signal received by LIGO was converted into an audible sound just like the above examples. Click on the video below to hear the sound of the very first gravitational waves ever detected:
This first detection is a spectacular discovery: the gravitational waves were produced during the final fraction of a second of the merger of two black holes with masses about 29 and 36 times that of the Sun into one single, more massive spinning black hole 1.3 billion light years away! This kind of collision had been predicted but never observed.
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 know 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 we call a “Stochastic Signal”, so called because the word, 'stochastic' means, having a random pattern that may be analyzed statistically but may not be predicted precisely. These will be the smallest (i.e. quietest) 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 the relic gravitational waves from the Big Bang will allow us to see farther back into the history of the Universe than ever before.
Burst Gravitational Waves
The search for 'burst gravitational waves' is truly a search for the unexpected—both because we’ve never detected them directly before, and because there are still so many unknowns that we really don’t know what to expect or what we might find. 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 find 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 the continuous and compact binary inspiral signals do. 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 is an exercise in being utterly open-minded. For these kinds of gravitational waves, scientists must maintain an ability to recognize when a noticeable pattern of signals arrives, even when such a signal has not been predicted or 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 that we may never have learned any other way.
Why Detect Them?
Detecting and analyzing the information carried by gravitational waves will allow us to observe the Universe in a way never before possible. It will open up a new window of study on the Universe, give us a deeper understanding of these cataclysmic events, and usher in cutting-edge research in physics, astronomy, and astrophysics.
Historically, scientists have relied primarily on observations with electromagnetic radiation (visible light, x-rays, radio waves, microwaves, etc.) to learn about and understand objects and phenomena in the Universe. (In recent years, even subatomic particles called neutrinos have begun to be used to study aspects of the heavens.) Each of these sources of information provides scientists with a different and complementary view of the Universe, with exciting new discoveries occurring as each new 'window' has been discovered, introduced, and utilized.
Gravitational waves are not electromagnetic radiation. They are a completely different phenomenon, carrying information about cosmic objects and events that is not carried by electromagnetic radiation. Colliding black holes, for example, emit little or no electromagnetic radiation, but the gravitational waves they emit will cause them to "shine brightly" like beacons on an utterly dark cosmic sea. More importantly, since gravitational waves interact very weakly with matter (unlike electromagnetic radiation), they travel through the Universe virtually unimpeded giving us a clear view of the gravitational-wave Universe. They carry information about their origins that is free of the kinds of distortion or alteration suffered by electromagnetic radiation as it traverses intergalactic space. With this completely new way of examining astrophysical objects and phenomena, gravitational waves will truly open a new window on the Universe, providing astronomers and other scientists with their first glimpses of previously unseen and unseeable wonders, and greatly adding to our understanding of the nature of space and time itself.
Source : LIGO