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Posted by
Akano
,
in
Math/Physics
Jan 31 2014
·
153 views

I made thesetwo images in Mathematica and tidied them up in Photoshop.

They're graphs in the complex plane. The color indicates the phase, or argument, of the complex number, and for this function, curves of equal phase are hyperbolas. To animate it, all I did was let the phase vary linearly in time.

I have posted before about the genius of physicist David J. Griffiths. I thought I'd post a few quotes by him to share why I think he's awesome.

"…You can always tell the particles apart, in principle—just paint one of them red and the other one blue, or stamp identification numbers on them, or hire private detectives to follow them around."

"...And, of course, if you’re in a really bad mood you can create a state for which neither position nor momentum is well defined..."

"It is traditional to write the Bohr radius with a subscript: a_{0}. But this is cumbersome and unnecessary, so I prefer to leave the subscript off."

"If you think this is starting to sound like a mystical numerology, I don’t blame you. We will not be using Clebsch-Gordan tables much in the rest of the book, but I wanted you to know where they fit into the scheme of things, in case you encounter them later on. In a mathematical sense this is all applied group theory—what we are talking about is the decomposition of the direct product of two irreducible representations of the rotation group into a direct sum of irreducible representation (you can quote that, to impress your friends)."

"I’m not at all sure what I’m supposed to say today. Maybe you’re expecting a grand philosophy of education. But I learned very early as a parent that almost any philosophy of childrearing is worse than no philosophy at all, and I am inclined to think the same applies to teaching."

"Personally, I never bring notes to a lecture unless I am egregiously ill-prepared, for they break a very delicate and important bond of trust with the listener: If B really follows from A, how come he has to refer to his notes?"

"There are a thousand ways to get a problem wrong—not all of them bad—and many ways to get a problem right—not all of them good."

"Above all, I think studying science—and especially physics—is a tremendously liberating experience. I don’t happen to know how a carburetor works; I’m not even sure what a carburetor does; let me be frank: I don’t know what a carburetor looks like. But I do know that the behavior of carburetors is perfectly rational; somebody understands them, and if I really wanted to I’m sure I could understand them too. For I have confidence, grounded in the study of physics, that the world is rationally intelligible, and this, to me, is the most important—and most profoundly liberating—idea in human experience. The universe is comprehensible..."

"A colleague of mine in Chemistry likes to boast that ‘‘anyone can teach; the important thing is to attract good researchers.’’ I think it’s exactly the reverse: competent research physicists are a dime a dozen, but good teachers are few and far between. Please don’t misunderstand: I’ve got nothing against research—I do a certain amount of it myself, and I think it goes hand in hand with good teaching. But I regard myself as a professional teacher, and an amateur researcher, whereas most physicists are professional researchers but amateur teachers, and it shows. In my opinion by far the most effective thing we can do to improve the quality of physics instruction—much more important than modifications in teaching technique—is to hire, honor, and promote good teachers."

There are many more wonderful quotes, but I don't remember them/don't have the sources on me. Perhaps I'll add to this in another blog entry.

This is one that I didn't really know much about until recently, so I thought I'd share it. Today's equation is known as the Virial theorem,

or, in component form,

The word "virial" comes from the Latin vis, which means "force" or "energy," and looking at the equation, it makes sense why it's called that. Here the big Σ means sum, the "k" index denotes the k^{th} particle of a system of N particles, V is the potential energy function affecting the k^{th} particle, T is the potential energy of all the particles in the system, and r_{k} is the position of the k^{th} particle. This essentially relates the kinetic energy of all the particles to the positions and forces exerted on each particle (since -grad V is the force when energy is conserved, which is an assumption we are making). The brackets 〈 〉 denote that we're taking an average, so 〈T〉 is the average kinetic energy, etc.

Now, you may be thinking, "okay, that's a cute equation, I guess, but I don't see how it's particularly useful." Okay, here's where the usefulness comes in. Let's say I want to know the mass of some distant galaxy, but I don't have a good galaxy-weighing device on hand. We know that the gravitational potential energy of an object is given by

where m is the mass of the star, M is the mass of the center of the galaxy, and r is the distance from the center of the galaxy. Taking the distance r and multiplying by the gradient of the potential yields...the potential again, with a negative sign out front. So, for gravity,

Plugging this into the Virial theorem above and noting that 2T = mv^2 (where v is speed), we get that, for an object in the gravitational pull of an object of mass M,

Thus, we have at our disposal a way of measuring the mass of something like a galaxy by measuring only the speeds of stars and their distance away from the center. That's pretty incredible.

This actually is one of the ways scientists support the idea that there is dark matter in the universe; the Virial theorem gives an average of what speeds the stars in our galaxy should have based on their distance away from the center of the Milky Way, but what we actually observe is startlingly different. Thus, we can conclude that something is wrong with our knowledge of how gravity within a galaxy works. Based on this and other observations, the idea that there's extra stuff that can't be seen that adds to the gravitational force of a galaxy seems to be a reasonable idea.

In my research on diatomic hydrogen (H_{2}), the Virial theorem is used in a different capacity. When figuring out the potential energy of an electron (or two) around the two positively charged protons, the virial has the Coulomb force term (which is just -V, just like gravity) and an additional term that pops up from assuming that the electrons are keeping the protons at equilibrium. I won't go too much into the physics, but the final product is

where E, T, and V are the total energy, kinetic energy, and potential energy of the electron(s), respectively, and R is the distance between the nuclei. This tells us something useful about the energy of the electrons; more specifically, it tells us about how the energy changes as you move the nuclei farther apart or closer together. In other words, since E = T + V,

which is very useful when constructing potential energy curves for hydrogen.

On a slightly related note, our lab's paper got published! Akano is now a for reals, published scientist! 8D

I have absolutely no life to the point that I just spent the better part of my afternoon going back through my blog posts and recording the view count, reply count, and word count of all 185 entries I've made prior to this one. These are the results of my labor:

All time averages: Views: 57.4 ± 45.8 Replies: 2.2 ± 2.6 (LOL) Word Count: 176 ± 242 (BIGGER LOL)

Largest stats: Views: 266 (courtesy of "Ask Akano" blog entry; Fort Legoredo review got 265, a close second) Replies: 24 (again, "Ask Akano") Word Count: 1711 (Vampyre Castle review)

I've bloggedbefore about the physics of bouncing a particle like a neutron on a table or other similar surface. Well, someone at the University of Arkansas has made an animation of how the spread of said particle over space evolves in time if it starts out in a "Gaussian wavepacket," which is a fancy way of saying that the neutron starts out looking more like a particle than a wave by being localized in space. The animation is here, while the full page containing the full Quicktime movie is here.

The red dot is a classical bouncing ball evolving over time (it's pretty boring comparatively). On the left is a plot of the probability of finding the particle at a certain height (the vertical axis) at any given time of the movie. The quantum particle does sort of bounce, become kind of wavy and messy, and then bounces again, but out of phase with the classical ball.

TL;DR: Cool animation of doing mundane physics with a quantum system with results that are anything but mundane. Click the links to have your mind blown.

So, in the last few weeks, my family came to visit (including Tekulo and KK) and we spent the latter half of the week seeing sights and enjoying each other's company. My mom won a game of Trivial Pursuit on a category that was supposed to stump her. Story of my Trivial Pursuit life.

When they left, they abandoned KK with me, which has led to me stepping into the nerd realm of playing Dungeons & Dragons. We're doing a campaign in the land of Hyrule with the races of Hyrule being used as analogs of D&D races. We're currently in the Forest Temple seeking an herb to cure the Great Deku Tree's muteness.

I'm also working in a new physics lab where I'm studying the energy states of the hydrogen molecule (H_{2}). I'm thoroughly enjoying it, since I'm learning computational stuffs and learning my way around Linux. (Emacs rules the school.) The program I'm working with is in Fortran, which is my native programming language but was written by someone else with a lot more skill than I possess.

Proto +2 for Premier Membership +1 from Pohuaki for reporting various things in Artwork

Name: Akano Real Name: Forever Shrouded in Mystery Age: 25 Gender: Male Likes: Science, Math, LEGO, Bionicle, Ponies, Comics, Yellow, Voice Acting Notable Facts: One of the few Comic Veterans still around Has been a LEGO fan since ~1996 Bionicle fan from the beginning Misses the 90's. A lot. Twitter: @akanotoe