• 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…

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  • 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…

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  • 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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  • Hadronic emission from the jets of X-ray binaries?

    X-ray binaries with a Galactic black hole (BHXRBs) have recently been detected in GeV and TeV γ-rays. The radiative mechanism responsible for this emission is not yet fully understood and both leptonic and hadronic processes seem viable. In the former scenario, electrons are accelerated to high energies and dominate the non-thermal spectrum with synchrotron and inverse Compton scattering. In the latter case, protons are also accelerated as well and contribute (or even dominate) in the most energetic regime of the spectrum via neutral pion decay. In the past few years, I have been working on understanding these physical processes, focusing mostly on the hadronic ones. But allow me first to…

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  • Warps and twists around black holes

    Twisted jets? Warped disks? It’s all possible in black holes! Hi all! Today I will talk a bit about our recent paper on the different types of disks you can have around black holes. As perhaps you would know, in 2019, the Event Horizon Telescope collaboration (EHTC) released the first-ever photos of the inner regions of the galaxy M87 (see link here). The EHTC studied an extensive suite of theoretical models of accreting black holes and tried to see which models looked similar to the observed images. These models simulate how the gas accretes onto a spinning or non-spinning black hole and then calculates how photons of light produced by…

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  • What cooling can do for Sagittarius A*

    Making an assumption is inevitable and reasonable in many research projects since it is practically impossible to handle all physics we know in a single study; even if it is feasible, it would rather squander resources, which are given to us. However, it is always required to evaluate carefully whether the assumptions are applicable for the particular project. Otherwise, the results may not be able to convince the community, even when the causal connection is well proven to be true. In this post, I will introduce my recent work talking about one assumption people used to make for the study of Sagittarius A*: radiative cooling can be ignored. The supermassive…

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  • War of the Clouds: Relativistic jet-cloud interactions

    The powerful jets launched by a supermassive black hole feeding on material at the centre of a galaxy, comprise some of the most powerful and highly energetic sources of sustained energy in the known Universe. Launched with velocities close to the speed of light, jets transport large amounts of energy from the central regions of their host galaxies and deposit it into their surroundings. Jets emit radiation across the entire electromagnetic spectrum, but emit radiation particularly strongly at radio frequencies. As the jets travel outwards from the central region of their host galaxy they can come into contact with a variety of obstacles, alongside complex and even hostile environments, which…

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  • Things need to change

    This is going to be a hard post for me to write, and for everyone to read. It will not discuss science, because today I want to talk about something that I think is far more important.TW: sexual assault discussion below. My main hobby is PC gaming, and many of my favorite games (Starcraft 1 and 2, Dota 2, World of Warcraft) in the past few years have turned into full blown electronic sports (or e-sports) – they are now played competitively by full time e-athletes, and the competition is as fierce, enthralling and exciting as any sport. In the past week or so, more and more people (mostly involving…

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  • The Cubic Kilometer Neutrino Telescope or KM3NeT

    Can you smell this? It’s the fragrance of gunpowder! Let us then investigate the “smoking gun”! Oh, I love this expression! But why do we relate the expression, “smoking gun”,  to astrophysical neutrinos? The answer is simple. We cannot observe particles being accelerated to high energies in situ. To be more precise, we cannot observe protons/nuclei gaining energy of the order of 1015 eV (or even 1020 eV sometimes) in the distant Universe. Then, how can we study these kinds of particles? Indirectly of course! Just let these protons/nuclei gain enough energy so as to be able to interact with their ambient medium and voila! The interactions of accelerated protons/nuclei…

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  • An extraordinary burst overshone the radio sky

    Fast radio bursts are enigmatic transient sources that attracted many studies in the past 13 years. They are radio bursts that release a huge amount of energy within milliseconds time. For many years, they were only observed from very distant galaxies. Given the large distances, those extragalactic fast radio bursts remain mysterious events (find here a review by Emily Petroff). On Apr 28, 2019, an extremely bright millisecond-duration radio burst was caught from a magnetar in our Miky Way! The radio bursts were detected by two radio telescopes — Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Survey for Transient Astronomical Radio Emission 2 (STARE2).  They are powerful radio arrays…

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