Infalling on to the black hole may drive materials crazy. How on earth can they keep their sanity while all bodies undergo extreme gravity? These days, most researchers who are simulating an accreting flow around a black hole choose the condition of the accretion disc between the “SANE” and “MAD”. Now you may think that the infalling materials would be CRAZIER in the “MAD” disc than the other. In fact, the component for dividing “SANE” and “MAD” is the strength of poloidal magnetic fields. “SANE” stands for Standard and Normal Evolution, and “MAD” stands for Magnetically Arrested Disc. Over the decades, the magnetic field structures around the central black hole…

Radio galaxies are a class of active galactic nuclei that shine very brightly at radio frequencies but also radiate across the entire electromagnetic spectrum too. Powered by accretion of matter onto a central fast-spinning supermassive black hole, radio galaxies can display extended radio structures- jets. Jets emit radiation strongly at radio frequencies and can propagate hundreds of kiloparsecs (1 kiloparsec = 3.086 x 1016 km) or more beyond the central black hole that powers them. Jets travel with relativistic velocities, carrying large amounts of energy with them to regions well outside the confines of their host galaxy. Radio galaxies which display these jets can be classified based on their morphology…

In part II we saw that in a non-rotating black hole, the spatial and temporal coordinates are exchanged. Now we can take a step further, to consider the case of a rotating black hole. The line element in such spacetime has very complicated coefficients, but we don’t need the specific expressions here. So let’s just represent them in terms of the metric components: From the specific forms of those coefficients, we know that the dt2 and dϕ2 terms are always positive while the dtdϕ term is negative. What differs from the non-rotating case is, here the gtt and grr don’t switch signs at the same place, but at rH and…

In part I we showed that space and time coordinates lead to different types of interval, spacelike and timelike respectively. Hence if we are given a line element expression in terms of any coordinates, we can figure out which of the coordinates are spatial and temporal, by looking at the sign of their coefficients. The coordinate t is temporal not because of the variable used, but the timelike interval it creates. Now we may ask: if a coordinate is spatial or temporal, does it always keep this identity? The short answer is, no. More rigorously, it depends on the structure of spacetime. For flat spacetime as shown in the last…

What is the universe? All of space and time along with their contents. The unity of space and time is well-embodied in its Chinese term: Yu Zhou, where “Yu” means all of space and “Zhou” means all of time. However, what is space and what is time, and what is their difference? This is an ultimate puzzle for anyone exposed to special and general relativity. According to our daily sense, space and time are a set of concepts to quantify the events in the world. Every event occurs at a place in space and a moment in time. With such space and time coordinates, we can compare different events and…