• A real "song" - codified in so-called gravitational waves - makes it possible to identify the finest details of the black holes that emit it, according to our partner The Conversation.

  • In fact, all binary stars “sing” gravitationally, but only those made up of very compact objects (black holes, neutron stars, white dwarfs, etc.) sing “loud enough” to be detected.

  • The analysis of this phenomenon was carried out by José Luis Jaramillo, professor of universities at the Institute of Mathematics of Burgundy (University of Burgundy - UBFC).

Black holes dance, and when they dance, they do so as a couple.

This is also the norm in the Universe: most stars evolve in so-called binary systems, formed by two objects orbiting one around the other.

VIDEO:

Simulation of the merger of two supermassive black holes (NASA Goddard)

Not only do they dance, but while they dance, black holes also sing.

This singular song does not take the form of sound, light, or other electromagnetic waves.

It is nevertheless a radiance with its rhythms, its tones and its harmonics, even its melody and its different "voices" ...

A real song, codified in so-called gravitational waves, which makes it possible to identify the finest details of the black holes which correspond to it and of their orbital dance: in the manner of the ornithologist who recognizes in the song of birds their species and their characteristics, astrophysicists extract from gravitational waves the properties of each of the black holes and their orbit.

Disturbance in the curvature of space-time

The existence of these waves, extremely difficult to detect, was predicted by Albert Einstein in 1916, just after his formulation of general relativity, which is none other than the theoretical description we use today to explain gravity.

This theory explains the gravitational phenomenon in terms of what is referred to as "curvature of space-time".

The waves emitted by the binaries of black holes, of gravitational nature, are then disturbances of this curvature of space-time which propagate on the proper space-time.

Similar to waves in a pond, which are disturbances to the surface of the water as they propagate over it.

On September 14, 2015, the Ligo gravitational antenna directly detected these waves for the first time.

Since then, around fifty detections have followed until the present date, initiating a real new stage in the study of the Universe: the astronomy of gravitational waves.

An operation similar to that of the tides

But to describe these waves as disturbances of the curvature of space-time is rather cryptic.

A more intuitive approach uses the more familiar notion of the tide, including the rise and fall of oceans twice a day.

These are produced for the gravitational action of the Moon and the Sun, which deform the surface of the oceans into a sort of ellipsoid.

Artist's vision of the binary black hole at the origin of the source of gravitational waves GW170104 © LIGO / Caltech / MIT / Sonoma State (Aurore Simonnet)

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 to say that its shape do not change.

It is the rotation of the Earth, whose crust (more rigid) is not deformed by the tides, which means that a given coast will pass twice a day through a bump of the water ellipsoid (high tides ) and twice a day by a trough (low tides).

This is the familiar phenomenon of the tides.

Gravitational waves as propagating tides

What would happen if, suddenly, the Sun and the Moon were no longer there?

The oceans would no longer have any reason to be deformed and would recover a more spheroidal shape.

But this process is subject to two constraints: 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 the special relativity of Einstein).

On the other hand, the relaxation of the oceans towards its state without deformation takes place in oscillations around the spheroid.

VIDEO:

Gravitational waves, what are they?

(Futura, 2016)

A "gravitational wave" is the physical phenomenon which informs the changes of a gravitational source (in the example, the Moon and the Sun) by means of a signal which propagates at a finite speed and which induces oscillations in the shape of the bodies. found on its way (in the example, the oceans).

In a literal sense, gravitational waves are dynamic tides propagating in space.

This gravitational song is a "silent" song, it is expressed by the changes of "forms".

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 quickly or a binary system of compact astrophysical objects.

This leads to an apparent paradox: if any system that deforms over time emits these waves, why are we not surrounded by these tides which in turn deform any object found in their path?

In reality, they are there, but too weak to be perceptible.

This is the case with me when I wave my arms.

Only very massive objects or with speeds comparable to that of light, are able to produce appreciable signals, like the binary of compact objects.

Therefore, we need to look beyond Earth to identify the right sources.

And this is where binary black holes, with their large masses and orbital speeds close to that of light, come into play.

Break through gravitational silence

Now let's come back to our initial assertion which advanced the ability of black hole binaries to "sing".

In fact, all binary stars “sing” “gravitationally”, but only those made up of very compact objects (black holes, neutron stars, white dwarfs…) sing “loud enough”.

The others make their melody resonate too "low" to be detected: if all the songs of binary systems are "silent", some are more so than others ...

It is therefore thanks to a real technological tour de force that the astrophysicists have succeeded in breaking through this "gravitational silence".

The development of Laser interferometers, true gravitational antennas, has enabled the direct detection of these waves and access to their astrophysical and cosmological information.

A network of interferometers on Earth

These interferometers are formed by two perpendicular "arms" of exactly the same length, subjected to oscillations (stretching and compressions) when a gravitational wave passes through them.

Optical “interferometry” makes it possible to measure very precisely the relative change in the length of these arms, thus identifying the passage of a wave.

Since 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 (4 km in LIGO in the United States).

Today, there is an extensive network of interferometers on Earth, the simultaneous operation of which is crucial for the analysis of these waves.

To study the most massive objects, like black holes in galactic centers, it will be necessary to build interferometers in space, which is the heart of the Lisa space program.

We now have interferometric ears to listen to and decipher the silent gravitational song.

And its melody is rich.

A major field of research

The discovery of gravitational waves was a first-rate scientific event that resulted in the 2017 Nobel Prize in Physics. In fact, the study of gravity is going through a particularly sweet time: three of the last five Nobel Prizes were awarded for research carried out in the context of gravitation.

Our "BLACK HOLES" folder

In 2017, gravitational waves were awarded, and in 2019 the turn of physical cosmology and the discovery of exoplanets.

The 2020 Nobel Prize dedicated to the theoretical prediction of black holes and its direct observation at galactic centers.

At the moment, 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 to us.

Science

Black hole fusion: What is this phenomenon (of incredible violence) that generates storms of gravitational waves?

Science

Astronomy: Why are Saturn's moons so different from each other?

This analysis was written by José Luis Jaramillo, Professor of Universities at the Institute of Mathematics of Burgundy (University of Burgundy - UBFC).


The original article was published on The Conversation website.

Declaration of interests

José Luis Jaramillo does not work, advise, own shares, receive funds from any organization that could benefit from this article, and has not declared any affiliation other than his research organization.

  • stars

  • Astrophysics

  • Space

  • Black hole

  • Video

  • The Conversation

  • Science

  • 0 comment

  • 0 share

    • Share on Messenger

    • Share on Facebook

    • Share on twitter

    • Share on Flipboard

    • Share on Pinterest

    • Share on Linkedin

    • Send by Mail

  • To safeguard

  • A fault ?

  • To print