Five-minute Explainer: What Even Are Gravitational Waves?

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. 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, let’s review what we know about gravity.

What Are Gravitational Waves?

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.

How Do We Detect Them?

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.

What Do We Do With This Information?

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, 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, 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.

What’s Next?

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.

Five-minute Explainer: What Is Gravity?

This essay continues from the previous one in this series, “Five-minute Explainer: Why Is Mass Equivalent to Energy?”

An old story relates that Newton figured out gravity when an apple fell on his head. Newton himself doesn’t mention the apple falling on his head—this appears to be a later embellishment—but he does mention the apple anecdote a couple of times in his dotage. John Conduitt remembered,

In the year 1666 [Newton] retired again from Cambridge to his mother in Lincolnshire. Whilst he was pensively meandering in a garden it came into his thought that the power of gravity (which brought an apple from a tree to the ground) was not limited to a certain distance from Earth, but that this power must extend much further than was usually thought.

Why not as high as the Moon said he to himself & if so, that must influence her motion & perhaps retain her orbit, whereupon he fell a calculating what would be the effect of that supposition.

This anecdote describes a key quality of gravity as understood then: its nature as an occult force—something working mysteriously and unseen across space.

Before Newton, it was known that the planets moved according to well known laws (Kepler’s laws) which allowed their motions to be predictable. It was not understood, however, why they should move in that way. Kepler’s laws merely came from generalizations after many observations.

Philosophers at the time were troubled that the planets appeared to have no reason to move as they did. Aristotelian thought required that something must drive the planets in their motions. If concentric spheres of quintessence did not, what could this be? For a while, we believed space might be full of a kind of fluid which moved in vortices which propelled the planets like clockwork. This explanation was unexpectedly successful for decades precisely because it did not require belief in occult forces—which is to say, it didn’t require something invisible to reach magically over distances and cause a thing to happen without touching it. It pushed instead of pulled.

Newton had looked at the apple and realized nothing had pushed it to the ground. It seemed to have fallen of its own accord. Newton then extrapolated this idea out beyond the garden into the stars. Once he did, a compact set of laws allowed him to explain all the motions of the heavens very tidily. His explanation, eventually known as the Principia, laid the groundwork for fundamental physics for centuries to come. It was a feat on par with Euclid’s Elements and fully completed the Scientific Revolution which Galileo had inaugurated.

From Hypotheses to Theories

In the second edition of the Principia, Newton tacked on some notes by popular demand. In this General Scholium, he explained that he was in no position to explain what gravity could tangibly be. Famously, he said, “Hypotheses non fingo” (“I do not feign hypotheses [of what gravity could be]”). He described nature as he found it, and the explanation worked. That’s how the matter lay for centuries.

One problem is that, over time, we observed that Newton’s explanations were not perfect after all. There were subtle but galling errors which cropped up in very rare circumstances (such as predicting where Mercury would be over time). Another problem was more metaphysical—Newton’s laws only explained how gravity worked, not what it was.

Einstein solved both problems in a single stroke with general relativity. His theory of general relativity followed in the decade after special relativity as a consequence of the latter. The general theory extended the special one to more situations and provided a more fundamental explanation of universal phenomena, particularly gravity.

Equivalence All the Way Down

If you’ve made it this far, you’ve read how energy is an impetus to change over time. Motion can be a form of energy because it can impart motion on another object, accelerating it. Energy is also equivalent to mass, and mass to energy—even at rest. Finally, you’ve seen how motion itself changes energy, space, and time relative to someone observing the motion.

Now we add a new equivalence—one so incredible in its implications that Einstein called it his “happiest thought.” It’s now simply known as the equivalence principle, special enough to stand alone by that name. It states that it’s impossible to distinguish between acceleration and gravity in any real, physical way.

That is to say, if you were trapped in some enclosed box and unable to see outside, you could not devise any instrument which would be able to tell you whether that box were accelerating in some direction steadily (and therefore drawing you toward the floor) or within a gravitational field (which would accomplish the same effect). Therefore, experiencing acceleration is equivalent to experiencing a gravitational field.

Einstein realized this in November 1907. From that point, he realized that energy, mass, space, time, and gravity were all inseparably linked, and he spent the next several years feverishly working toward a general theory of relativity to explain how it all works. The explanation he came up with in 1915 works so well that its predictive power overturned Newton and has held up even to this day.

Motion in a Bottle

As a result of special relativity, we saw that motion warps space and time. We also know that motion relative to an observer represents kinetic energy, which is equivalent to any other form of energy. Finally, we know that energy is equivalent to mass and vice versa. The final piece of the puzzle to put into place here is that, since motion—and therefore energy—warps time and space, so does mass.

Think of mass as bottled motion. Mass–energy equivalence lets us treat mass as energy which has congealed, more or less, into one place. As I said in the last essay, it’s not enough to think of mass and energy as distinct things sharing some properties—they are a single substance. Therefore, all the same properties and consequences which apply to one form also apply to the other. That means that all the warping effects which apply to energy—to motion—also apply to mass.

So mass warps time and space, but what does this actually mean in reality? The result is gravity! Gravity is an emergent consequence of how mass warps time and space, exactly the same way motion warps time and space due to special relativity. Gravity is in fact not a force reaching mysteriously across distances but instead a bending of space and time which changes the paths of objects traveling through that space and time, leading them inexorably closer to one another.

The Conservative Appeal of Gravity

Let’s dispense with the tired bowling-ball-on-a-rubber-sheet imagery and talk about what that last paragraph actually means. We can begin with the classic assumptions about how objects behave. Newton’s laws state that objects in motion tend to stay in motion, or at rest, unless acted on. They also state that there’s always an opposite and equal reaction for every action.

These are, at their heart, conservation laws. For things to behave otherwise would mean creating or destroying energy. An action must impart an opposite and equal reaction, or energy would go missing. An object at rest must stay at rest, or energy would spontaneously appear. An object in motion must stay in motion, or energy would vanish.

In flat space, therefore, moving objects tend to stay the course in order to conserve energy. You can trace the line of how the object moves geometrically as a straight line. Now if we introduce a mass nearby, space and time contract and stretch, respectively, in the vicinity of that mass. The object’s path still needs to conserve energy, and in order to do so, the line we trace now curves closer to the mass. It appears as if the object “falls” inwards toward the mass—exactly as you’d expect from a gravitational field.

Occult Forces and Fictitious Forces

We no longer need an “occult force” to explain the mechanism of gravity. General relativity—which geometrically describes space and time as it bends under the influence of mass and energy—provides the complete picture.

As it turns out, gravity is not a force at all in the ordinary sense. It only appears to exert a force in the way that a merry-go-round in motion appears to make a ball curve through the air when you throw it from one side to the other. Gravity plays a similar trick on us: we’re constantly on a path through time and space which, were it not for the gigantic rock beneath us, would cause us to curve inexorably toward the center of the Earth. Since the Earth itself interrupts our course, we press against it, and it against us, which imparts the force we’re familiar with.

Making Waves

By uniting conservation laws and a handful of postulates, we can fully explain the substance and behavior of gravity. When we combine this knowledge with the speed limit of the universe, we see that even gravity takes time to travel, which means that changes in gravity take time to travel. This allows gravity to ripple across space and time. We’ll now be prepared to look at these waves in the next explainer.