My MSc thesis: following light and matter around black holes
I have just wrapped up my MSc thesis, Behavior of Null and Timelike Geodesics around Black Holes. I had been working on it since December, and the whole idea for the thesis came from my MSc thesis supervisor. I am grateful for that, because it pulled me into the field I ended up doing research in.
What the title means
In general relativity, gravity isn't a force pulling on things; it is the shape of spacetime itself. Objects simply follow the straightest possible paths through that curved spacetime. Those paths are called geodesics.
- Null geodesics are the paths of light.
- Timelike geodesics are the paths of anything with mass: particles, planets, or a spaceship.
So the thesis was really about one question: how do light and matter move around very dense objects?
What I learned about
- Black holes, both non-rotating (Schwarzschild) and spinning (Kerr), where I wrote Python notebooks to trace particle paths.
- Naked singularities, extremely dense objects without a horizon to hide them.
- Wormholes, including a metric proposed by Juan Maldacena and Alexey Milekhin in their paper on humanly traversable wormholes. It was a thrill to work with a spacetime from a paper by one of the biggest names in theoretical physics.
Shadows and precession
Two ideas from the thesis have stayed with me because they connect theory to things we can actually observe.
Shadows. Light that passes close to a black hole can be bent, trapped, or swallowed. The result is a dark patch surrounded by a bright ring, which is exactly what the Event Horizon Telescope photographed. The size and shape of that shadow depend on what the central object really is, so a naked singularity or a wormhole could leave a different shadow from a black hole.
Orbital precession. An orbit around a massive object doesn't close perfectly; the whole ellipse slowly rotates. Mercury does this around the Sun, and it was one of the first tests of Einstein's theory. Stars orbiting the centre of our galaxy do it too, and the amount of rotation depends on the geometry close to the central object.
Put together, shadows and precession give us a way to ask a deep question with real data: is the thing at the centre of our galaxy really a black hole, or something that only looks like one?
This thesis is where my research journey really started, and it led directly to my first paper a few weeks later.