“Sonifying” black holes

Professor Scott A. Hughes, Department of Physics, Massachusetts Institute of Technology

An essay like this gives me the opportunity to make a confession: I love black holes. I love their weirdness, I love their simplicity, I love that Nature appears to produce them in abundance. And yet I must acknowledge that the evidence for black holes in Nature is not as solid as one might imagine. We have very strong evidence that a large number of very dense, very compact objects in our universe are probably black holes, but we have not yet established that these objects have all the properties that the laws of physics require black holes to have.

This is perhaps surprising to many readers — after all, many articles appear every year describing discoveries involving black holes and their interactions with stars and other “normal” objects. In all of these cases, the so-called “black hole” is more properly a black hole candidate. When astronomers talk of a “black hole,” they know that the object is really massive (usually at least 10 times more massive than the sun, often millions or even billions of times more massive) and that it is very compact: All that mass is crammed into a space so small that it either must be a black hole, or it is something even more bizarre and surprising.

Many of us have dedicated part of our careers to investigating this mystery, with the aim either to make the case for black holes even stronger, or to find evidence that Nature makes objects even more bizarre and surprising. Astronomy provides the tools to investigate these mysteries, and, as we all know, astronomy’s main tool is the telescope. However, investigating black holes with telescopes is challenging. Their gravity is so strong that not even light can escape them, so, by definition, they do not emit any light and telescopes cannot see them.

Fortunately, black holes do not exist in isolation. They are members of a crowded universe, and we find our candidates paired up with stars, or eating gas that falls into them from nearby stars in their host galaxy. Over the past two decades, the evidence we have built by studying stars and other matter moving near candidate black holes has become quite strong, allowing us to nail down both the black holes’ extreme masses and extreme compactness with great precision. Measurements can now begin to test whether these candidates have the properties that the laws of physics demand of black holes. Do they have event horizons — surfaces of “no escape,” defining where gravity becomes so strong even light is trapped? Black holes should rotate, with a speed limit set by their mass. Do the candidates we observe obey this speed limit? Spinning black holes should have a shape determined by their rotation speed, much as a rapidly spinning planet is squashed by centrifugal forces. Do they? Does that shape agree with what the laws of physics demands it should be? Telescopes now coming on line will be able to test these properties by carefully measuring light that comes from gas falling into the black hole and emitting a final burst of radiation before disappearing forever.

Wonderful as these measurements are, one can protest that this is circumstantial evidence: We are not seeing the black hole itself, we are inferring the black hole’s properties from its action on other matter. Might it be possible to directly probe the nature of the black hole, and not just infer its action from what it does to its neighbors?

Because black holes are creatures of gravity, a direct probe of a black hole must be a gravitational probe. We now have such a probe. Electromagnetic waves, like light or radio, are produced whenever electric charges are accelerated. In an analogous way, gravitational waves are produced whenever masses are accelerated. These waves are weak and quite difficult to measure. To get waves strong enough that our detectors can measure them, we need to find extremely massive objects moving very fast. Fortunately, Nature produces such objects, and they are some of the most fascinating things in the cosmos — things likeneutron stars (the remnant cores left behind when massive stars explode in supernovae) and black holes. The imprint of gravitational waves has already been inferred in the evolution of certain binary star systems. New detectors, like those of the American LIGO (Laser Interferometer Gravitational-wave Observatory) project, are now sensitive enough that we will soon measure these waves directly. When we measure gravitational waves from black holes and neutrons stars, we will have a new tool to study these amazing things. We are on the verge of a new epoch in probing our universe.

Over the past two or so decades, my colleagues and I have developed tools to use gravitational waves as a new way of doing astronomy. We have found that these waves encode an enormous amount of information, such as the masses of the objects that generate the waves, how rapidly they spin, whether they are black holes or are made of some kind of superdense fluid.

One of the things we have realized is that the best way to understand gravitational waves is like a kind of sound. This is, of course, an analogy — sound waves are vibrations of pressure propagating through a gas or fluid; gravitational waves are vibrations of gravity propagating through space and time — but it is a remarkably useful analogy. In our research, we make models for the gravitational waves from an astronomical system. We have found that encoding the wave as a sound gives us a remarkably clear way to convey what the waves tell us. A binary that contains rapidly spinning objects is modulated in a distinctive way. A highly eccentric binary sounds very different — more “buzzy” — than a circular binary. If the final state of the binary is a single black hole, the last few cycles of the waves “ring” in a distinctive way. This “sonification” of the wave we seek to measure gives us an incredible way to understand how the properties of a source are imprinted on the gravitational waves it generates, and guides our quest to measure these waves as our instruments begin operating.

Over the past few months, it has been an enormous pleasure to work with Rebecca Kamen and Susan Alexjander on this installation. I’ve had great fun letting them in on some of the excitement that drives me to think about things like black holes and gravitational waves. I am stunned by what they have made. I was particularly struck by how Rebecca synthesized the notion of the “Light Cone” with the nearly chaotic motion we see in some black hole orbits, as well as the outward propagating waves the orbit generates. It is a perfect merging of the many elements that go into the events that my colleagues and I study. Susan’s accompanying soundscape subtly and evocatively layers the sounds we have made into a wonderful companion piece. I hope you enjoy their creations as much as I have enjoyed working with them.