You might follow the news—which you may find depressing these days—but maybe you don’t follow the science news, which is plenty exciting! Gravity is making waves these days, literally: gravitational waves. In fact, several weeks ago, the LIGO scientific collaboration announced the release of a catalog of gravitational wave sources.
Gravitational-wave detection began just a few short years ago, and only starting in 2017, scientists began to get the first really interesting observations. These most recent events have enabled us to understand fundamental physics better by ruling out or circumscribing a few grand theories of everything, and they have allowed us to see the universe in ways we never could before.
Yet the events themselves are mere “inspiral chirps” which happen in less than a second and usually at a remove of a billion parsecs or more.1 How can something so brief and so distant, be so meaningful? That’s what today’s explainer is all about.
To get the big deal about gravitational waves,2 let’s review what we know about gravity.
Gravity happens because mass warps space and time. All mass does this, and the warping effect stretches out across the universe infinitely. Even you, as you sit reading this, cause a very tiny change in the space in your vicinity and in the passage of time, and that extends beyond you out into space forever, however faintly.
Two things are important to note, though. First, the degree to which any object warps space around it weakens very sharply as you move away from it. This is because the farther you get from it, the amount of space there is to warp grows exponentially, so the warpage falls off in turn. (This is a kind of conservation law, known as the inverse-square law.) Second, that warpage—that gravity—doesn’t travel instantly. It’s bound by the same speed limit as everything else, just as special relativity requires.
So given these facts, we know that gravity spreads out in much the same way light does. How does it form waves? Waves result from any cyclical phenomenon which traverses a distance. Think of a cork bobbing in a pond. The cork, stationary, merely moves up and down, but the ripples move out in waves. Waves made of gravity can therefore ripple outward anytime a source of mass changes in a repeating way.
This is what gravitational waves are—cyclical changes in gravity. The ones we can detect result from vastly large masses spiraling in toward one another extremely rapidly and colliding. Their circling motion causes the gravity from them to ripple out in a pattern of repetitive change—in waves—as the masses revolve around each other. This spiraling pattern causes the masses to alternate positions quickly, sometimes lining up or sometimes sitting side-by-side (from our point of view). As they revolve, they also draw closer and closer to one another. Finally, when the masses collide, the wave source stops in a sudden “inspiral chirp”—so called because the gravitational wave is so rapid and stops so suddenly, it sounds like a chirping sound when played as audio.3
We detect gravitational waves with a lasers, of course. Actually, it’s a bit more complicated. There are multiple lasers. And we bounce them down tunnels (called “arms”) over four kilometers long—long enough that the Earth curves downward by a meter over their length—and back.
When a gravitational wave ripples through a LIGO facility, the phenomenon literally causes those arms to change shape and size according to general relativity. The facility is a vast instrument called an interferometer which causes the lasers to interfere with one another in a very specific and measurable way.
The idea is that a pair of lasers are fired over a very great distance (through the arms) and bounced back to the detector, which is a specialized kind of digital camera. When they bounce back, they’re meant to interfere with each other in a very precise way because of how they overlap when they hit the detector. However, minute vibrations upset the delicate interference pattern, and the detector can see that.
The lasers have to be so long because they’re directly measuring very tiny warpages in the shape of spacetime itself. Gravitational waves ripple out from violent but brief, distant events, and so these instruments must be extraordinarily sensitive. LIGO reports that at its most sensitive, it can detect a change in distance ten-thousandth the width of a single proton. The facility in Hanford, Washington, detects vibrations so sensitive that it can pick up ocean waves crashing on the beach several hundred miles away.
Using multiple facilities located in different locations, it’s possible to detect gravitational waves very quickly using advanced, purpose-made software (used to separate the data from the noise) and roughly locate the source in the sky.
Gravitational-wave detection is one of the newest and most profound breakthroughs in recent observational cosmology. Even merely detecting a gravitational wave is a feat not to be understated—it signifies that we have directly measured a ripple in the fabric of spacetime itself and further cemented the theory of general relativity. It took nearly a century after their first theoretical prediction to achieve a direct detection.
Gravitational-wave astronomy gives us our first look at the universe beyond electromagnetic radiation (light, infrared, x-rays, and so on). We are finally able to see the ripples of the pond in which we all live, not just the specks of light. Gravity behaves differently than EM radiation in several important ways, so it promises new insights into massive phenomena like neutron stars, supermassive black holes, and the like—all at incredible distances difficult to observe otherwise. The promise of revelations into the formation of galaxies, exotic phenomena, dark matter, or even the creation of the universe all await.
Already, though, we’ve seen the birth of a new form of astronomy altogether called multi-messenger astronomy which combines both gravitational wave observations along with traditional radio or optical telescopic observations of the same event. Until now, humanity has only ever been able to see the light from the stars and make educated guesses about distance, mass, and so on. What’s more, we still have more questions than answers about how EM radiation and gravity relate to one another. The most fundamental explanations of all of creation, from the subatomic level to the cosmic level, depend on answers to these questions.
The first event observed via both gravity and light was called GW170817. Gravitational waves from this event was detected by three detector facilities in real time,4 and a corresponding gamma-ray burst (the most violent kind of explosion in the universe) was found at the same location in the sky by dozens of observatories. This event, which is thought to be two neutron stars colliding, has already taught us new things and begun to constrain models of fundamental physics.
For example, since it was observed via both light and gravity, we can compare the time it took for both to reach us and see what differences may exist. Some grand unifying theories of everything thought that perhaps gravity would take longer to cross the distance to us because it had to travel differently (through hidden, “compactified” spacetime dimensions,5 for example). Since that didn’t happen, those theoretical physicists will have to go back to the drawing board.
Gravity travels unattenuated by dust and unscattered over vast distances. Events like GW170817 travel over distances only affected by other masses, allowing us to “see” the universe in a different and maybe clearer way. Some scientists hope that we may even find primordial gravitational waves leftover from the earliest epochs of the universe, before even light could emerge because matter was too dense. Gravitational waves may let us pierce the wall of creation’s primordial fire and look beyond into nearly the very earliest moments of the universe itself.
Gravitational-wave astronomy and multi-messenger astronomy are extraordinarily young sciences. The data from the events we’ve observed are still being pored over by scientists as they attempt to make or break new theories and find new signals in the noise.
In the future, we may be able to put extraordinarily large interferometers into space which extend over massive distances and which would not be subject to earthly vibrations such as trucks, oceans, footfalls, or earthquakes. One such planned project is called LISA. We would be able to observe many more sources of waves with such a detector, even ones within our own galaxy. Perhaps we will even find sources of gravitational waves we never even expected. We’re standing at the verge of a whole new universe.