About GW190814 Artistic Interpretation

Although GW190814 Artistic Interpretation was widely spread around May 2020, I never really gave detailed explainations on how it was made. Here is an overview of the scientific elements, the parts of artistic interpretation as well as some technical insights.

The video

Inception

Jérôme Degallaix is a CNRS researcher at IP2I. A previous collaboration in 2019 led to the Variations Physiques vol. 2 virtual reality film, inspired by the formation of stars, gravitational waves and emergence.

Jérôme also works at the Virgo interferometer based in Pisa, Italy, and run by the European Gravitational Observatory.

He wrote to me something like : « Hey, there is this cool unique event in the gravitational wave world, the discovery will have a public release and we’re looking for an artistic image. We don’t have a penny for that one, you can reuse former images, we’ll understand if you say no of course. »

I finally said OK because this is way bigger than us, and that was the start of my voluntary work on the topic.

Start

At the time I was on a backpack & train trip from Paris to Dijon (France) on the reversed steps of scientist Henri Navier, made famous by the well-known Navier-Stokes equations in physics.

We are early March 2020, and it seems that I am being chased by the spread of a special virus coupled with an overall feeling of panic…

In Metz, Nancy, Thann, Mulhouse, Belfort, Besançon, Dijon, and then Lyon where I was locked down, I started spending hours on our work.

© OpenStreetMap contributors

After looking at existing representations of blackholes, reading a bit of science on the topic and meditating on the uniqueness of the GW190814 event, I started working a few of iterations.

On representing a blackhole

The first immediate solution I came up with was to use my own favourite visual effects tool and display 2 rings in the scene around the blackhole, one on a horizontal plane, the other facing the camera. This way, we would see the lensed part of the ring (what should be behind is distorted and visible on top) and the overall look was full of glittering matter particles all around.

Although a certain number of scientific key points were already taken into consideration, the visual coherence of putting together matter particles and a swallowing black sphere was quite hard to achieve.

On the iteration below – and despite the fact that the velocities at stake might not be comparable to the cosmic expansion – you can see for instance how redshift is highlighted: particles going away from the camera are redder than those getting closer to the camera.

The approach phase was not sufficiently visible on both video steps shown above. Also I was not fully satisfied with the double-ring trick for lensing…

Then I spent hours looking at technical tutorials of some dudes mimicking a blackhole effect with shaders. I tried to implement the existing solutions, but I didn’t fully manage. Above all, the end result worked on the final image layer, with only one distortion effect (called lensing) that couldn’t be shifted away from the center of the screen.

Yet again: frustration.
Intense thinking…

Aha!

At last I had a hint: an immense force field in the shape of a tube that would push away all particles behind the blackhole, so the background remains dark.

Then giving more thinking into it, the force field became a visual distortion field, and I worked on some math functions to achieve lensing effect around the blackhole.

Here below is a drawing I attempted to make during our first call with Virgo Outreach team. You can ignore the leftmost sketch which just represents the approach phase.

@credits MS Paint

It’s a fairly simple trick : just imagine a cone that pushes away all visual elements. Starting from the eye of the viewer, aiming at the blackhole.

We are simply adding some extra displacement vector to where a particle should be.

With 2 different cones (one lensing effect for each of the massive objects), and some special treat to what is in front of the blackhole, which should have no distortion. The end result is a flexible displacement function in the (x, y, z) space, which you can apply to any set of particles within this VFX Graph tool.

The code behind the blackhole effect

Unity VFX Graph of the blackhole effect
(one for the unidentified object, one for the background stars)

The whole animation was made within the Unity 3D environment.

I used to code with C#, and seldom with a shader laguage such as GLSL.
When VFX Graph came out, all I had to do is transfer the mental math mechanics of my plain code into this new visual way of chaining operations.

The underlying math are fairly simple, involve Gaussian functions, power laws and cross products. A few millions of particles dance around, controlled by a tailor-made set of equations.

Scientific elements visible in the video

  • Real-time “Order 2” mirrored gravitational lensing on both bodies
  • Timing scale of the merger
  • Neutron star matter streams of orbiting particles with unequal masses (result of initial density gradient)
  • Slightly inhomogeneous sky with stars oscillating in accordance with GW amplitude and frequency (~t² square law over time during the approach phase) to picture distortion of spacetime
  • Imaginary Neutron Star // Black Hole massive object with electrified particles on 4 spherical layers (concentration gradient + seismic activity + internal electric currents in a Neutron Star) and the lensend heart of a Black Hole
  • Gentle tidal forces that don’t rip out the unidentified object too quickly during the approach phase
  • Gravitationally coherent system (matter streams)
  • Newtonian rotation of matter streams around a center of mass (not a Kerr blackhole, just standard orbits)
  • Event textual information during the introduction
  • Redshifted colors in the matter streams (slightly visible on leftmost and rightmost particles that are aligned with the camera-object axis)
  • Prescribed circular kinetic waltz (not the natural 2-body problem due to rapid divergence with Newtonian 1/r² depite use of fake viscosity) with excentricity corresponding to GW190814 mass ratio

And this one is purely artistic: 4 matter streams evoke the 4 strings of a violin or a cello.

Recording audio

I was very lucky I could stay at a couple of friends’ appartment during the first Covid lockdown in France. There was a keyboard.

The soundcrack chords can be heared in a song by Charles Aznavour: Trousse Chemise.

If you listen carefully to the notes during the approach phase on GW190814 Artistic Interpretation, you will notice that only a few keys are pressed, on the same chord. As the wave expands, more and more notes are played, starting at the center and radiating away to the low and high pitched notes until the climax in intensity. The keyboard acts as a one-dimensional space in which musical waves are emitted from the center to the edges.

The still image shooting

Instant shot with fixed camera in the virtual environment

Final scientific validations

Sending the video to Jérôme, it then reached the Virgo Outreach team and we had online meetings. Also with LIGO.

We were talking about scientific accuracy. Overall it was a question of preserving the quality of the animation while not inducing public misunderstanding. We had debates on the introductory texts and the naming of the unidentified creature. I thought it was fun to propose Neutron Hole, but according to the team the press may have gotten crazy about a seemingly newfound object, while it was probably nothing of a new nature. We ended up with Unidentified massive compact object.
The BBC took care of the extra creativity:

Most importantly, discussion with LIGO collaborators (global team, in charge of the 2 gravitational wave detectors located in the USA) led to a disambiguation text at the very end of the video: The streams and lensing around the compact objects are an artistic invention. GW190814 is thought to have been electromagnetically dark, unlensed, and free of ejected matter.

Release and media reception

Very soon after the public release I received emails and phonecalls from press. It was mostly for permission to use image and video content. One or two interviews for local press too (Lyon).

I think all the feedback I had was positive and encouraging.
Although some did, I was also surprised that some well-known publishers just didn’t ask for permission to use content.

For instance the New York Times just took a screenshot of the video and used it as it was for an article that would remain at the front page for a little while, although I had a high-definition image pack for Press available on demand. Some others chose to capture a specific frame of the video. Quite amusing.

I also discovered that some online media websites just work by copypasting content from other sources.

Some publishers politely wrote to me for permission. SKA magazine even paid a little for their front cover. Matt O’Dowd from PBS Spacetime also wrote to me, and some video extracts are used in his presentation.

It’s a nice one which I recommend viewing

Some links

Here are some links where I found some GW190814 Artistic Interpretation content:

Some keynote drafts

Most notes below were jotted down during the making of GW190814 Artistic Interpretation

Outcomes

In October 2021 I had the chance to visit the Virgo detector and discuss with researchers and technicians in the control room – quite impressive hotspot – as well as with the Outreach team on ideas for possible future collaborations.

Acknowledgements

Virgo – EGO https://www.virgo-gw.eu/
LIGO
https://www.ligo.org/
Dorian Mignérat for improving recorded sound quality
Antoine Brun & Estelle Huet

Thanks for reading!