Partially Coherent

One thing that we are taught very early on in physics lectures is that the speed of light in vacuum, c, is the fastest anything can ever move, period. But is that really true? Well, the short answer is yes, and the long answer has some ifs and buts. Probably the most popular reasoning behind this statement is that Einsteins theory of relativity does not allow for anything to exceed the speed of light. But what it actually says is that information or energy cannot travel faster than light. Of course, even this is a very strict condition, since every particle, physical system and any radiation can be used to carry information and/or energy, which means that none of those can travel faster than light. To achieve a speed faster than c, you need something that doesn't contain information or energy, but what could that possibly be? One entity that potentially carries neither, is the group velocity of light. In optical telecommunication, group velocity is the speed of the

While I was writing my  first article , I noticed that there were almost no models that describe how the radioactive fallout settles to the ground (or deposits, as they say). The models that I found were either extremely primitive or simply mathematical fittings to data, with next to none predictive power! The crudest model is that you take the radioactive concentration (Becquerel/cubic meter) and multiply it with the amount of rain (millimeters). I have no idea what motivated this (maybe that the dimensions work out to be correct?), and it's just as reliable as it sounds. The most advanced model I found was actually quite reliable in estimating deposition, but as it was just a mathematical fit to measured data, it did not offer any insight to the physics behind it. So, I started drafting a simple model to introduce in the first paper. I had a small notebook where I would scribble whatever I had come up with. The notebook filled up quickly and I realized that there was n