SONIFICATION SHOWCASE
In Depth

In this sonification we wanted to demonstrate how a particle (such as a clump of gas), spirals down towards a black hole and eventually becomes part of the black hole. It is part of our feature-length show, "A Black Hole's Journey". For the show, we only provide a relatively simple description of the science and sonification approach. However, underpinning this sonification is a detailed theoretical model which accurately determines both the sound representation and the animation! 

The spiral simulation uses analytic formulae for the free-fall velocity of a particle with angular momentum equal to that of a particle in circular orbit at the innermost stable circular orbit (ISCO) radius. This is a special class of orbit in general relativity, representing the innermost orbit where the gradient of the gravitational attraction is shallow enough to permit stable orbits - that is, orbits that do not collapse and fall into the black hole if perturbed by a small amount. Below the ISCO, in the so-called 'plunging region', matter is in free-fall and spirals inwards towards the black hole.

When observed by a distant observer, lensing effects from the spacetime curvature generated by the black hole distort how the spiral would be observed, such that when the point is e.g. 'behind' the black hole it actually appears to be above it! The observer also measures an energy shift due to the relative motion of the particle (the Doppler effect), and light emitted by the particle appears blue-shifted as it moves towards the observer and red-shifted as it moves away. There is also a contribution from the gravitational redshift, as photons close to the black hole must climb out of the potential well, gaining potential and losing kinetic energy, and therefore appear redder.

The effects of time dilation due to curvature are here also important, and appear to slow the motion of the particle towards the black hole, even though it is actually accelerating towards the black hole. In reality, the particle would only ever appear to asymptotically approach the black hole, as it would take an infinite amount of time for the distant observer to see the particle touch the event horizon. Locally for the particle, time would behave as normal and they would pass over the horizon with no problem. However, waiting an infinite amount of time to see the particle cross the horizon would be academic at the very least, as the particle would also become infinitely red-shifted, so that any light emitted from it would be so dim that we would lose sight of the object much sooner. In practical terms, it does become part of the black hole, and adds mass to it!

This sonification is combined with an animation which was created by Tommy Howell. The positions of the particle and the size of the black hole shadow are based on the model, described above.