February 18th 2021, 12:55 PST. A room in California erupts in cheers and applause as mission control watches its robot land on another planet. After years of preparations and almost seven months of space travel, Perseverance arrived safe and sound on the Red Planet, ready to study its environment for a planned duration of two years. It will join NASA’s Mars Exploration Program lineage of four martian rovers, making it the fifth since 1997 to explore this curious planet. But why do we care about Mars so much? Mars has always been an object of wonder, from the idea that a civilization could inhabit it to Elon Musk’s plans for…
Read More >>To be MAD or not to be MAD: problematic naming conventions in astronomy
Last week’s blog post was entitled To be MAD or not to be MAD. In it, my colleague Doosoo Yoon did a good job of discussing two models used to describe the accretion disks of matter flowing onto a black hole. These models started a discussion in our research group meeting this week for a different reason: the names of the two models MAD and SANE are pretty ableist. As Doosoo pointed out, MAD stands for Magnetically Arrested Disk and SANE stands for Standard And Normal Evolution. Now the MAD model which was created first, about 20 years ago, has a simple acronym and is widely used in the astronomy…
Read More >>To be MAD or not to be MAD
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…
Read More >>The curious case of FR0s: The relatively new class of radio galaxy
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…
Read More >>MasterChef Universe : Spaghetti(fication) with Black Holes (Part II)
This is second part in the series of posts about Spaghettification. In this part we go over the idea of how gravity squeezes objects. We will also briefly discuss how to make tidal forces stronger. Squeezing Now let’s rotate the rod so that it is oriented with both ends equally distant from the Earth, while the rod’s mid point is still located at a distance R from the Earth’s center. Unlike the case when the rod was vertical, here the Earth pulls at both these ends with an equal force directed towards the Earth’s center (white arrows in Figure 2). The force at the left end (point A) can be…
Read More >>MasterChef Universe : Spaghetti(fication) with Black Holes (Part I)
Can black holes make spaghetti? Like Obama, black holes would reply ‘Yes we can’! To top that off they can make spaghetti out of anything – using their ‘gravitational machinery’. In this three part series we will see how black holes employ gravity to do so! Before we go exploring black hole’s ‘spaghetti skills’, in part I, first let us look closer to home, at our very own Earth’s ‘spaghetti skills’ and its connection to the concept of tidal forces. Gravity and Tidal Force Let’s begin with the familiar. Earth produces a gravitational field by the virtue of its mass, and objects immersed in this field feel a gravitational force.…
Read More >>Can you Discriminate between Space and Time? (Part III)
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…
Read More >>Can you Discriminate between Space and Time? (Part II)
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…
Read More >>Can you Discriminate between Space and Time? (Part I)
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…
Read More >>The best plot in astrophysics
In these last few years many fields in astrophysics have been advancing at an amazing rate. One of the fastest has been the study of the so-called “fast radio bursts” (or FRBs). As the name implies, FRBs are flashes of radio emission that happen extremely fast – down to a few milliseconds. This short duration makes them very challenging to detect, so the first FRB was only detected in 2007 at a facility called the Parkes Observatory. To complicate the picture, earlier on the Parkes Observatory detected another type of never seen before transient, called “perytons”. Early on it became clear that while it was possible for FRBs to be…
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