# Notes on Physics

> Updates on my research and expository papers, discussion of open problems, and other physics or astro related topics. By Aronno Mirdha


## Posts

- [A Few Words on Cornell](https://aronnomirdha.com/statement/1400): I usually write about physics here, but I felt I had to say something about what has recently been reported at Cornell University.
- [Why MHD Works in Weakly Collisional Astrophysical Plasmas](https://aronnomirdha.com/note/1371): Here is a number that should bother you.
- [From the Equivalence Principle to Curved Geometry of Spacetime](https://aronnomirdha.com/note/1302): Newtonian mechanics contains two masses with no obvious reason to be related.
- [The Eddington Luminosity in Spherical Accretion and Beyond](https://aronnomirdha.com/expository/1238): The Eddington luminosity is the point at which the radiation coming out of an object pushes on the surrounding gas as hard as the object's gravity pulls it in.
- [How a Molecular Cloud Collapses Into a Star](https://aronnomirdha.com/expository/1203): Making a star means compressing gas by something like twenty-four orders of magnitude.
- [Why $R_{\mu\nu}=0$ does not imply $R_{\mu\nu\rho\sigma}=0$](https://aronnomirdha.com/note/1155): Almost everyone makes this mistake once, and it’s a good one to make. You learn the Einstein field equations.
- [Derivation of the Cosmic-Ray Diffusion Coefficient from Pitch-Angle Scattering](https://aronnomirdha.com/expository/1173): Cosmic rays arrive at Earth from every direction almost perfectly evenly, to about one part in a thousand.
- [The rubber sheet gets the geometry right and the mechanism wrong!](https://aronnomirdha.com/note/1149): You have seen this demonstration. Somebody stretches a rubber sheet over a frame, drops a bowling ball in the middle, and the sheet sags.
- [A Scientist Before “Scientist”](https://aronnomirdha.com/note/1146): Mary Somerville is so difficult to put into words that it's funny. To be honest, who was she?
- [What are Quarternions?](https://aronnomirdha.com/expository/1093): I used to think complex numbers were the end of the line.
- [So, what is a vector?](https://aronnomirdha.com/expository/1036): In my first year of undergraduate physics, one of my professors walked into the lecture hall, placed his notes on the desk, looked around the room, and asked a question that, at first, seemed almost too easy.
- [Deriving the Euler–Lagrange Equation for Particles, Fields, and Curved Spacetime](https://aronnomirdha.com/expository/1013): A good way to understand the laws of motion is not to memorize a separate equation for each situation, but to notice there is really only one trick, and every equation of motion is what falls out when you apply it.
- [So what are tensors, really?](https://aronnomirdha.com/expository/990): I want to start with a confession. When I first heard the word "tensor," I assumed it was one of those words that exists to make physicists sound clever.
- [On physics, visibility, and why I stayed](https://aronnomirdha.com/diary/971): I remember being in high school and telling people I wanted to study physics. The response was almost always the same.
- [The mathematics behind a "wormhole”](https://aronnomirdha.com/note/925): Let me tell you something that will sound ridiculous at first. Take a piece of paper. Draw a dot on the left and a dot on the right.
- [How many solutions of Einstein’s equations are there?](https://aronnomirdha.com/note/908): How many solutions does the most beautiful equation in physics have? More than you’d think… probably infinitely many, and most of them will never have names.
- [On the Consistency of Published M87* Mass Measurements](https://aronnomirdha.com/expository/894): A useful way to test a black hole spacetime is not only to ask whether one observational method agrees with Kerr, but to ask whether several independent methods agree with each other.
- [Some remarks on quasinormal modes for Euler–Heisenberg black holes in a PFDM background](https://aronnomirdha.com/expository/755): One of the recurring themes in black hole perturbation theory is that many apparently complicated dynamical questions eventually reduce to a rather geometric spectral problem.
- [Aristotle on Motion](https://aronnomirdha.com/diary/800): Aristotle divided motion into two classes: natural motion and violent motion.
- [Recent notes on covariance-weighted consistency tests for Kerr parameter estimates](https://aronnomirdha.com/note/753): A recurring issue in strong-field tests of General Relativity is the question of how one should compare parameter estimates inferred from genuinely independent observational sectors.
- [Vega Through 600 Frames of Starlight](https://aronnomirdha.com/project/astrophotography/712): Tonight I pointed the Seestar S50 toward Vega and let it run for a while. Vega is one of those stars that almost feels too bright to photograph properly.
- [Letter to the crew of Artemis II](https://aronnomirdha.com/statement/673): Dear Artemis II crew, Good luck on your mission. What you are about to do is more than a flight.
- [Inspiral-merger-ringdown consistency tests and the reconstruction of Kerr geometry](https://aronnomirdha.com/note/761): One of the more conceptually interesting developments in gravitational wave astronomy is the inspiral-merger-ringdown (IMR) consistency test.
- [Traversable wormholes and the geometry of effective exoticity](https://aronnomirdha.com/expository/765): One of the useful lessons of general relativity is that the Einstein equations are not, by themselves, especially conservative about the kinds of geometries they permit.
- [A Dark Halo That Almost Became a Galaxy](https://aronnomirdha.com/expository/369): One of the cleanest ideas in modern cosmology is also one of the easiest to overlook.
- [A Consistency Test for Kerr Black Holes via Orbital Motion, Ringdown, and Imaging](https://aronnomirdha.com/note/361): Kerr Trisector Closure (KTC) is a consistency test for the Kerr hypothesis that tries to stay honest about what is actually being inferred from data.
- [Towards a derivation of the metric tensor in general relativity](https://aronnomirdha.com/note/289): One of the central tasks in differential geometry is to make precise the notion of length and angle on a smooth manifold.
- [Overdetermined parameter interference in physics](https://aronnomirdha.com/expository/180): In many areas of physics, a system is described by a small number of fundamental parameters, while the available observations greatly exceed this number.
- [The Kerr Trisector Closure: A project on internal consistency tests of General Relativity](https://aronnomirdha.com/expository/43): General Relativity describes gravity as the curvature of spacetime.
- [Spacetime as a Lorentzian Manifold](https://aronnomirdha.com/note/297): One of the central tasks in differential geometry is to make precise the notion of length and angle on a smooth manifold.

## Pages

- [My Projects](https://aronnomirdha.com/projects): A lot of tests have shown that General Relativity is true, but each test only looks at a small part of the theory.
- [About](https://aronnomirdha.com/about): I study theoretical physics and a little astronomy, with a focus on General Relativity, differential geometry, astronomy, and quantum field theory.

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