**Name:** Valery Vermeulen**Occupation:** Scientist, researcher, musician, producer **Nationality:** Belgian**Recent release:** Valery Vermeulen's *Mikromedas AdS/CFT 001* is out via Ash International. It is part of Vermeulen's ongoing research and interest into data sonification and the exploration of cosmic phenomena.

The seeds for the Mikromedas performance series were sown while he was studying mathematics at Ghent University: *"I learned about black holes in the General Relativity courses. I also still remember that the technical details of gravitational waves were covered,"* Vermeulen remembers, *"At that time, I found these topics super intriguing, especially the math, as it is closely related to the domain of geometry."*

It wasn't until the start of the Mikromedas project, however, that he would actually turn towards making space a topic for his music. Mikromedas began in 2014 as an umbrella for different musical performance series focusing on topics coming from astrophysics and theoretical physics. It was only a matter of time before black holes started making an appearance: *"I wanted to expand Mikromedas towards the field of theoretical physics. What emerged from my research was that during the last two decades the study of black holes has become a key element in theoretical physics. So, it was a very natural consequence to focus the next Mikromedas series on black holes.”**Mikromedas AdS/CFT 001*, the subject of this first of two interviews we conducted with Valery, presents the first results of this scientific and sonic journey. What do black holes sound like? You've never come closer to the answer to that question than right here. *If you enjoyed this interview with Valery Vermeulen and would like to dive deeper into his work, visit his official homepage. He is also on Instagram, and Facebook.* **Can we actually hear black holes? **

A reaction people have often had when reading that the album was made using black holes, is the logical thought that this is not possible.

The main reason is that there is no sound in space as space is a giant vacuum without air molecules. And air molecules are needed for any sound to be able to exist. And this is a fact that is completely true.

So from a perspective of physics, black holes do not naturally generate sound. Although recent research shows that there is a way a black hole can generate a sort of sound waves. But for this, you would need a jet stream, i.e. a stream of gas moving close to the speed of light, produced by the black hole that slams into hot gas pervading a giant galaxy in our universe.

But apart from this, to listen to a black hole, you need to find ways to get information or data from it. And this can either be done by using computer simulations or by observing indirect astrophysical phenomena caused by the dynamics of a black hole. Once such data is obtained, data sonification offers various methods to listen to this data. This then offers a way to listen to a black hole.**What kind of data did you include for the project? It wasn't just black hole data, correct?**

No, correct, we included gravitational wave data, particle trajectory data of both massive and massless particles near various types of black holes, white dwarf data and data from black branes. Which are higher dimensional generalizations of black holes. **Black holes, in many respects, create effects beyond human comprehension. What do you personally think of when you think of a black hole? **

The inner part of a black hole and how it behaves bends all our current understanding of reality. Up until now, there is no theoretical model for how reality would look or feel in this inner section.

Most black holes divide the space surrounding them into two parts: the inner part and the outer part. The theoretical boundary between the inner and outer parts is also known as the event horizon. If you were to travel to a black hole, this event horizon is not visible. But once you have passed the event horizon, reality changes to an unimaginable extent.

By construction, it is also impossible to get any information or measurements out of the inner part of a black hole. So it's really a hidden black box that we will never be able to experience.

When I think about black holes, I directly associate them with the mystery and secret of their inner part and what it would be like to travel to these unexplored inner regions.**I believe there are different types of black holes?**

That's right. The theory of general relativity shows that black holes have 3 fundamental properties. Namely their mass, their charge, and their angular momentum which is connected to their possible spinning movements. Knowing the values of these 3 properties completely determines the structure and behavior of any black hole. This is the famous “no hair” theorem.

As a result of this theorem there are 4 types of black holes; Schwarzschild, Riessner-Nordström, Kerr-Newman, and Kerr-type black holes. White dwarfs data was also included in the data sets used for the album. **[For an in-depth explanation of the different types of black holes, head over to Nicolus Rotich's article at Cantors Paradise]**

The reason is that *Mikromedas AdS/CFT 001* also focuses on astrophysical bodies with extreme gravitational fields. And white dwarfs fall exactly into this category. **Can you shortly expand on the theory behind white dwarfs?**

White dwarfs are stellar core remnants of stars, with a certain mass, that has exploded. The study and understanding of white dwarfs are also closely related to black hole astrophysics. A major breakthrough in our understanding of white dwarfs was due to legendary physicist S. Chandrasekhar.

According to his findings, it turned out that for a dying star to turn into a white dwarf, its mass has to be smaller than 1.4 solar masses. This is the so-called Chandrasekhar limit. A result that baffled the whole astrophysical community at the time. **What was so baffling about it?**

As white dwarfs are remnants of the stellar core they are unimaginably dense. A teaspoon of white dwarf matter weighs up to 15 tons. And as mass is connected to gravity the gravitational field they produce is enormous.

Whenever a star has a mass larger than the Chandrasekhar limit, it explodes and can turn into a neutron star or black hole. So by this white dwarfs are one of the forms of a star when it reaches its final stage.**You mentioned "black branes" before. Even after looking the term up, I'm somewhat uncertain as to what these are. How would you explain them in simple terms?**

Black branes are objects that arise from string theory.

String theory is a domain in physics in which researchers are trying to find the building blocks of space and time. The core idea is that the fundamental building blocks of our reality are strings. The theory arose in the 1960s when researchers were trying to understand the behavior of hadrons i.e. the family of elementary particles built out of quarks. Quarks are the smallest known building blocks of matter to date.

One of the strange features of string theory is that it proposes a multi-dimensional picture of reality. **Strange in so far as there are many more than just the three or four we take for granted?**

Yes. In classical string theory, there are, for example, 10 dimensions, i.e. 1 time dimension and 9 spatial dimensions. As the domain of string theory evolved gradually, different versions of the theory appeared, each describing different possible models of our everyday world.

To create clarity in the growing jungle of such theories, a new revolutionary theory, called M-Theory was developed and proposed by physicist Ed Witten in 1995. And one key ingredient in M-Theory are branes.

You can see branes as higher-dimensional generalizations of the particle concept that is key to the field of elementary particle physics. So branes can be 2 dimensional, 3 dimensional but also 5 dimensional physical entities.

In this setting, a black brane can then be viewed as a higher dimensional version of a black hole. So a black brane can be, for example, a 4 dimensional structure that exerts an enormous gravitational field on its surroundings in a 10 dimensional representation of our reality.**What, would you say, can we learn from studying black holes and branes?**

Black holes are extreme objects that arise from Einstein's theory of general relativity. One of the key lessons from studying black holes is the relative nature of time and the interchangeability of space and time.

For example, looking at the equation of the simplest black hole, theoretically discovered by Karl Schwarzschild in 1915, it is not difficult to see that space and time become interchangeable close to the center of the black hole. **That's a key part of relativity that most people have come to remember – the identity of space and time. And yet, it's not something we can imagine in an easy or practical way.**

I agree! But is a scientific fact. They also directly confront us with the implications of the theory of special relativity on the nature of time. A concrete example to illustrate this is the following.

Suppose you would be able to travel by spaceship near a black hole where the gravitational forces are much larger than on Earth. Let's say you stay there one hour and then return to Earth. Because of the gravitational pull and the intertwining between space, time, and gravity, time flows much much slower when your spaceship was stationed near the black hole than on Earth.

So the time interval of one hour for you in your spaceship might mean that a couple of million years might have passed in the meantime on Earth. If you then return to earth you might find yourself millions and millions of years into the future. This scenario might sound like science fiction or something from the “Back to the Future” franchise. But it is a reality. **So it's gravity that is slowing time down?**

Exactly. We refer to that in physics as time dilation. The principle of time dilation is used by almost all of us daily. Namely, it is of crucial importance for all GPS systems.

So without taking time dilation into account accurate and reliable GPS, a functional app such as google maps would just not be possible.**You've spent many years researching and thinking about black holes. I'm curious if the project yielded moments of surprise for yourself? Were there aspects that sonification revealed that you may not have been aware of before?**

One of the lessons I learned from working on the album *Mikromedas AdS/CFT 001* is that there are possibilities out there to sonify the sometimes very abstract models used in theoretical physics. Working on black hole astrophysics has been for me a living proof of this fact. This was also one of the big challenges and open questions I had when starting the *Mikromedas AdS/CFT 001* project.

As a concrete example I can mention here the new ways to sonify gravitational waves I found during the production process. This was the result of a collaboration I did with Thomas Hertog in the light of a commissioned lecture performance. Thomas Hertog is a Belgian physicist who was a long-time collaborator of Stephen Hawking. The collaboration was organized by co-production partner Concertgebouw Brugge. For this lecture-performance, I was asked to make music using simulated gravitational wave data.

At the time I knew about previous sonications of gravitational waves. But most of them were not inspiring or I didn't see ways to use or re-use them. So I wanted a new approach to using this data. After a meeting with Thomas, the solution was found. The key element was provided by programming code to represent the gravitational waves as evolving 3 dimensional objects.

This representation was completely new to me and gave me a lot of new options to sonify the waves and their evolution. Moreover, these newly obtained sonifcations also offered a lot of new possibilities to use them for compositional purposes.