Gravitational waves: the silent song of black holes

Black holes dance, and when they do, they do it in pairs. This is the norm in the universe: most stars evolve in so-called binary systems, made up of two objects that orbit each other.

Not only do they dance, but while they dance the black holes also sing. This unusual song does not take the form of sound, light, or other electromagnetic waves. However, it is a radiation with its rhythms, tones and harmonics, even its melody and its different voices…

A song, encoded in the so-called gravitational waves, that allows to identify the finest details of the corresponding black holes and of their orbital dance: in the manner of an ornithologist who recognizes their species and characteristics in the song of birds, the Astrophysicists extract the properties of each of the black holes and their orbits from gravitational waves.

Disturbance in the curvature of space-time

The existence of these waves, extremely difficult to detect, was predicted by Albert Einstein in 1916, right after his formulation of general relativity, which is the theoretical description we use today to explain gravity. This theory explains gravitational phenomena in terms of what is known as “curvature of space-time.”

The waves emitted by binary black holes, of a gravitational nature, are then perturbations of this curvature of space-time that propagate. It is similar to the waves in a pond, which are disturbances of the water’s surface as it spreads.

On September 14, 2015, the Ligo gravitational antenna directly detected these waves for the first time. Since then, there have been about 50 detections, starting a new stage in the study of the Universe: gravitational wave astronomy.

Similar to the tides

But describing these waves as disturbances of the curvature of space-time is quite cryptic. A more intuitive approach uses the more familiar notion of tides, that is, the rise and fall of the oceans twice a day.

They are produced by the gravitational action of the Moon and the Sun, which distort the surface of the oceans into a kind of ellipsoid.

Given a relative position of the Earth-Moon-Sun (which defines what is called a “day in a month”), this ellipsoidal deformation of the oceans is “stationary”, that is, its shape does not change. It is the rotation of the Earth, whose crust (more rigid) is not deformed by the tides, which makes a certain coast pass twice a day through a hump in the ellipsoid of the waters (high tides) and twice a day by a depression (low tides).

It is the well-known phenomenon of the tides.

Gravitational waves like spreading tides

What if the Sun and the Moon suddenly disappeared? The oceans would no longer have a reason to be distorted and would regain a more spheroidal shape.

But this process is subject to two limitations: on the one hand, the information of the disappearance of the Moon and the Sun must propagate at a finite speed (nothing can travel faster than light, according to Einstein’s special relativity). On the other hand, the relaxation of the oceans to their undistorted state is done by oscillating around the spheroid.

A “gravitational wave” is the physical phenomenon that informs of the changes in a gravitational source (in the example, the Moon and the Sun) by means of a signal that propagates at a finite speed and induces oscillations in the shape of the bodies that are they meet on their path (in the example, the oceans).

In a literal sense, gravitational waves are dynamic tides that travel through space. This gravitational chant is a chant silent, is expressed by changes in shapes.

The origins of gravitational waves

What are the physical systems that produce these propagating tides? In other words, what are the sources of these waves? The answer is simple: any system whose shape changes over time is a source of gravitational waves. It could be me waving my arms rapidly or a binary system of compact astrophysical objects.

This leads us to an apparent paradox: if any system that deforms in time emits these waves, why are we not surrounded by these tides that in turn deform any object that is in its path? They are actually there but they are too faint to be noticeable. This is my case when I wave my arms. Only very massive objects or objects with speeds comparable to that of light are capable of producing appreciable signals, such as the binaries of compact objects.

Therefore, we have to look beyond Earth to identify the proper sources. And this is where binary black holes come in, with their great masses and orbital speeds close to light.

Breaking the gravitational silence

Now let’s go back to our original claim that binary black holes can to sing.

In fact, all binary stars they sing gravitationally, but only those formed by very compact objects (black holes, neutron stars, white dwarfs …) sing loud enough. The others make their melody sound too low to be detected: if all the chants of the binary systems are silent, some are louder than others …

Therefore, thanks to a tour de force technological, astrophysicists have managed to break this gravitational silence. The development of laser interferometers, true gravitational antennas, has allowed the direct detection of these waves and access to their astrophysical and cosmological information.

A network of interferometers on Earth

These interferometers are made up of two arms perpendiculars of exactly the same length, which are subjected to oscillations (stretching and compression) when a gravitational wave passes through them. Optical interferometry makes it possible to measure with great precision the relative change in the length of these arms, thus identifying the passage of a wave.

As these gravitational waves are tidal phenomena and their effect is stronger the larger the size of the deformed object, the arms of the interferometers are several kilometers long (4 km in LIGO, in the United States).

Today, there is a vast network of interferometers on Earth, the simultaneous operation of which is crucial for the analysis of these waves. To study the most massive objects, such as black holes at galactic centers, interferometers will have to be built in space, which is the core of the Lisa space program. We now have interferometric ears to hear and decipher the silent gravitational song. And its melody is rich.

An important field of research

The discovery of gravitational waves was a great scientific event that was reflected at the 2017 Nobel Prize in Physics. In fact, the study of gravity is going through a particularly sweet moment: three of the last five Nobel prizes have been awarded to research carried out in the field of gravitation.

In 2017 gravitational waves won and in 2019 physical cosmology and the discovery of exoplanets. The 2020 Nobel Prize was awarded for the theoretical prediction of black holes and their direct observation at the galactic centers.

Today, the synergy between different disciplines is breaking new ground in cosmology, astrophysics, and fundamental physics. In return, the gravitational universe sings to us to reveal its mysteries.

Jose Luis Jaramillo, Professeur des Universités, Institut de Mathématiques de Bourgogne (IMB), University of Bourgogne – UBFC

This article was originally published on The Conversation. read the original.