Refractive Substructure from Interstellar Scattering
The interstellar medium contains regions of dilute, ionized plasma. These regions are turbulent and irregular and scatter radio waves. This can cause the appearance of distant radio sources to become distorted, just like looking though water or a flame. For a long time it was thought that the scattering will just blur out an image, but it actually has the opposite effect, introducing fractal substructure in the scattered image.
I've developed the theory behind this substructure, with an emphasis on cases where the scattered image can be resolved using VLBI, and have also created tools to numerically simulate these effects of scattering. These effects have turned out to be especially important for imaging at the highest angular resolutions with the Event Horizon Telescope and with RadioAstron (Earth-space VLBI). See here for a paper summarizing the simulations and theory behind this work. See here for the first detection of refractive substructure, in images of our Galactic Center supermassive black hole, Sgr A*, at 22 GHz.
I gave a 20-minute intro to some topics in scattering at the Perimeter Institute in 2014. You can view it here.
Although the scattering looks completely random, it actually consists of many small copies of the source that have been distorted and magnified. For example, here you can see a movie comparing the effects of refractive scattering for a Gaussian source and a ring (download here). The same scattering screen has a very different appearance depending on what's behind it!
Feel free to contact me if you are interested in obtaining code and running some scattering simulations yourself!