Wednesday, June 25, 2014

Perturbation Theory - Quantum Mechanics

Due to Lack of LATEX compatibility, I am attaching a PDF of this blog here. I will continue to try to fix the issue in the meantime. My apologies for inconvenience.

$$\usepackage{physics}$$ $$\usepackage{amsmath}$$ $$\usepackage{amsfonts}$$ $$\usepackage{amssymb}$$ $$\usepackage{mathtools}$$ $$\usepackage{graphicx}$$ $$\usepackage{xspace}$$ $$\usepackage{color}$$ $$\newcommand{\figref}[1]{Fig.~\ref{#1}}$$ $$\renewcommand{\eqref}[1]{Eq.~\ref{#1}}$$ $$\newcommand{\hn}{\hat{H}_0}$$ $$\newcommand{\hp}{\hat{H}^\prime}$$ $$\newcommand{\phis}[2]{\ket{\phi_{#1}^{#2}}}$$ $$\newcommand{\phib}[2]{\bra{\phi_{#1}^{#2}}}$$ $$\newcommand{\psis}[2]{\ket{\psi_{#1}^{#2}}}$$ $$\newcommand{\uc}[2]{\underbrace{#1}_{\mathclap{#2}}}$$

Results

Complete derivation of time-independent perturbation theory in Quantum Mechanics. The equation we are trying to solve.

Zeroth order
$E_n^{(0)} = E_n^0 \text{ and } \psis{n}{(0)} = \phis{n}{0}$
First Order
$E_i^{(1)} = \hp_{ii} = \ev{\hp}{\phi_i^0}$ and $\psis{n}{(1)} = \sum_{i \neq n} \frac{\mel{\phi_i^0}{\hp}{\phi_n^0}}{E_n^0 - E_i^0} \phis{n}{0}$



Basic Time-Independent Non-Degenerate Perturbation


Here, we will derive the correction in a most basic way.

The known system

Suppose we know everything about $\hn$, and by everything I mean its eigenvalues $E_n^0$ and corresponding eigenstates $\phis{n}{0}$.

\begin{equation}
\hn \phis{n}{0} = E_n^0 \phis{n}{0}
\end{equation}

This known hamiltonian will usually be harmonic oscillator, square well or hydrogen atom.

The Perturbing Hamiltonian

Suppose we add a small perturbing hamiltonian, $\hp$, to the original known hamiltonian $\hn$, and by small I mean the expectation value being small $\qty\big(\ev*{\hn} >> \ev*{\hp})$.

Including the perturbation, the total hamiltonian is $\hat{H} = \hn + \lambda \hp$. Our goal is to find its engenvalues ($E_n$) and corresponding eigentstates (\psis{n}{}). In other words, solve its eigenvalue equation.

\begin{equation}
\hat{H} \psis{n}{} = \qty(\hn + \lambda \hp) \psis{n}{} = E_n \psis{n}{}
\label{Eq H ES}
\end{equation}

Expansion of Eigenvalues and Eigenstates

Since $\lambda$ is assumed to be very small, we can expand both $E_n$ and $\psis{n}{}$ in terms of different powers of $\lambda$.

\begin{gather}
\psis{n}{} = \sum_m \lambda^m \psis{n}{(m)} = \psis{n}{(0)} + \lambda \psis{n}{(1)} + \lambda^2 \psis{n}{(2)} + \dots
\label{Eq psi exp}
\\
E_n = \sum_m \lambda^m E_{n}^{(m)} = E_n^{(0)} + \lambda E_n^{(1)} + \lambda^2 E_n^{(2)} + \dots
\label{Eq E exp}
\end{gather}


Let's rewrite the eigenvalue equation (\eqref{Eq H ES}) using the expansions from \eqref{Eq psi exp} and \eqref{Eq E exp}.

\begin{equation}
\qty(\hn + \lambda \hp) \underbrace{\psis{n}{}}_{\text{Expand}} = \underbrace{E_n \psis{n}{}}_{\text{Expand Both}}
\end{equation}

\begin{equation}
\underbrace{\qty(\hn + \lambda \hp) \qty(\psis{n}{(0)} + \lambda \psis{n}{(1)} + ...)}_{\text{Expand All}} =
\underbrace{\qty(E_n^{(0)} + \lambda E_n^{(1)} + \dots) \qty(\psis{n}{(0)} + \lambda \psis{n}{(1)} + ...)}_{\text{Expand All}}
\end{equation}

\begin{equation}
\underbrace{\hn \psis{n}{(0)} + \lambda \hn \psis{n}{(1)} + \lambda \hp \psis{n}{(0)} + \lambda^2 \hp \psis{n}{(1)} + \dots}_{\text{Collect Like Terms}} =
\underbrace{E_n^{(0)} \psis{n}{(0)} + \lambda E_n^{(0)} \psis{n}{(1)} + \lambda E_n^{(1)} \psis{n}{(0)} + \lambda^2 E_n^{(1)} \psis{n}{(1)} + \dots}_{\text{Collect Like Terms}}
\end{equation}

\begin{equation}
\underbrace{\hn \psis{n}{(0)}}_{0^{th} \text{ power of } \lambda} +
\lambda \underbrace{\qty(\hn \psis{n}{(1)} + \hp \psis{n}{(0)})}_{1^{st} \text{ power of } \lambda} +
\lambda^2 \underbrace{\hp \psis{n}{(1)} + \dots}_{\text{Higher Powers}} =
\underbrace{E_n^{(0)} \psis{n}{(0)}}_{0^{th} \text{ power of } \lambda} +
\lambda \underbrace{\qty(E_n^{(0)} \psis{n}{(1)} + E_n^{(1)} \psis{n}{(0)})}_{1^{st} \text{ power of } \lambda} +
\lambda^2 \underbrace{E_n^{(1)} \psis{n}{(1)} + \dots}_{\text{Higher Powers}}
\label{Eq Expansion}
\end{equation}

For this equation to hold true, the different powers of $\lambda$ have to equal to each other individually.


$0^{th}$ order correction


The $0^{th}$ order terms on both sides have to equal to each other.

\begin{equation}
\underbrace{\hn \psis{n}{(0)} = E_n^{(0)} \psis{n}{(0)}}_{\text{This is an eigenvalue equation for \hn. So, $E_n^{(0)}$ and \psis{n}{(0)} are eigenvalues and eigenstates of \hn.}}
\end{equation}

\begin{equation}
E_n^{(0)} = E_n^0 \text{ and } \psis{n}{(0)} = \phis{n}{0}
\end{equation}


$1^{st}$ order correction


The $1^{th}$ order terms on both sides have to equal to each other.

\begin{equation}
\hn \psis{n}{(1)} + \hp \underbrace{\psis{n}{(0)}}_{\mathclap{=\phis{n}{0}}} = \underbrace{E_n^{(0)}}_{=E_n^0} \psis{n}{(1)} + E_n^{(1)} \underbrace{\psis{n}{(0)}}_{\mathclap{\text{Known from $0^{th}$ order = }\phis{n}{0}}}
\end{equation}

\begin{equation}
\hn \underbrace{\psis{n}{(1)}} + \hp \phis{n}{0} = E_n^0 \underbrace{\psis{n}{(1)}} + E_n^{(1)} \phis{n}{0}
\end{equation}

We can represent this state \psis{n}{(1)} as a linear combination of the eigenstates of \hn as \psis{n}{(1)} = $\sum_m c_{n,m}^{(1)}$ \phis{m}{0}. If we can determine the value of each $c_{n,m}^{(1)}$, then we can construct the state \psis{n}{(1)}.

\begin{equation}
\underbrace{\hn \qty(\sum_m c_{n,m}^{(1)} \phis{m}{0})}_{\hn \phis{m}{0} = E_m^0 \phis{m}{0}} +
\hp \phis{n}{0}
= E_n^0 \qty(\sum_m c_{n,m}^{(1)} \phis{m}{0}) +
E_n^{(1)} \phis{n}{0}
\end{equation}

\begin{equation}
\underbrace{\sum_m c_{n,m}^{(1)} E_m^0 \phis{m}{0}} +
\hp \phis{n}{0}
= E_n^0 \underbrace{\sum_m c_{n,m}^{(1)} \phis{m}{0}} +
E_n^{(1)} \phis{n}{0}
\label{Eq Ci}
\end{equation}

In order to pick out one of the $c_{n,m}^{(1)}$, multiply above equation (\eqref{Eq Ci}) by $\phis{i}{0}$. This will pick out the $i^{th}$ component of c's (i.e. $c_{n,i}^{(1)}$). Once we know all $i$'s, we can reconstruct \psis{n}{(1)}.

\begin{equation}
\phib{i}{0} \qty\Bigg[\sum_m c_{n,m}^{(1)} E_m^0 \underbrace{\phis{m}{0}}_{\mathclap{\braket{\phi_i^0}{\phi_m^0} = \delta_{i,m}  }} +
\hp \phis{n}{0}
= E_n^0 \sum_m c_{n,m}^{(1)} \underbrace{\phis{m}{0}}_{\delta_{i,m}} +
E_n^{(1)} \underbrace{\phis{n}{0}}_{\delta_{i,n}}]
\end{equation}

\begin{equation}
\underbrace{\sum_m c_{n,m}^{(1)} E_m^0 \delta_{i,m}} + \mel{\phi_i^0}{\hp}{\phi_n^0} =
E_n^0 \underbrace{\sum_m c_{n,m}^{(1)} \delta_{i,m}} + E_n^{(1)} \delta_{i,n}
\end{equation}

This sum is non-zero only when $m=i$. So, $\sum_m c_{n,m}^{(1)} E_m^0 \delta_{i,m} = c_{n,i}^{(1)} E_i^0$. Similar for the other sum term. Also, we can rewrite $\mel{\phi_i^0}{\hp}{\phi_n^0}$ as the matrix element of \hp. So, $\mel{\phi_i^0}{\hp}{\phi_n^0}=\hp_{in}$

\begin{equation}
c_{n,i}^{(1)} E_i^0 + \hp_{in} = c_{n,i}^{(1)} E_n^0  + \underbrace{E_n^{(1)} \delta_{i,n}}_{\mathclap{\text{To find $E_i^{(1)}$ set $n=i$.}  }}
\label{Eq First Order}
\end{equation}

\begin{equation}
c_{i,i}^{(1)} E_i^0 + \hp_{ii} = c_{i,i}^{(1)} E_i^0 + E_i^{(1)}
\end{equation}

\begin{equation}
E_i^{(1)} = \hp_{ii} = \ev{\hp}{\phi_i^0}
\end{equation}

We need to find $c_{n,i}^{(1)}$ in order to find the $n^{th}$ eigenvector correction. Let's go back to \eqref{Eq First Order} and plug in $E_n^{(1)} =\hp_{nn} = \ev{\hp}{\phi_n^0}$.

\begin{equation}
\underbrace{c_{n,i}^{(1)}} E_i^0 + \hp_{in} = \underbrace{c_{n,i}^{(1)}}_{\mathclap{\text{Solve for } c_{n,i}^{(1)} }} E_n^0  + \hp_{nn} \delta_{i,n}
\end{equation}

\begin{equation}
c_{n,i}^{(1)} = \frac{
\overbrace{\hp_{nn} \delta_{i,n} - \hp_{in}}^{\mathclap{\text{Note $i \neq n$, because we get 0/0.}} } }
{E_i^0 - E_n^0}
\end{equation}

Using this we can reconstruct \psis{n}{(1)} because \psis{n}{(1)} = $\sum_m c_{n,m}^{(1)} \phis{n}{0}$ = $\sum_i c_{n,i}^{(1)} \phis{n}{0}$.

\begin{equation}
\psis{n}{(1)} = \sum_{i \neq n} \frac{\hp_{nn} \delta_{i,n} - \hp_{in}}{E_i^0 - E_n^0} \phis{n}{0} = \sum_{i \neq n} \frac{\mel{\phi_i^0}{\hp}{\phi_n^0}}{E_n^0 - E_i^0} \phis{n}{0}
\end{equation}


$2^{nd}$ Order Correction


We need to find all the $\lambda^2$ terms from \eqref{Eq Expansion}. Note that I have not written all the terms in \eqref{Eq Expansion}, so I suggest you write all the terms upto $\lambda^2$. You should get the following.

\begin{equation}
\hn \psis{n}{(2)} + \hp \uc{\psis{n}{(1)}}{} = \uc{E_n^{(0)}}{\text{We know many of these terms from $0^{th}$ and $1^{st}$ order corrections.}} \psis{n}{(2)} + \uc{E_n^{(1)}}{} \uc{\psis{n}{(1)}}{} + E_n^{(2)} \uc{\psis{n}{(0)}}{}
\end{equation}




Saturday, June 14, 2014

Three body simulation in Mathematica

Dear Classmates,

Lately, I have been very interested in chaotic motions. To explore properties of chaotic motion, I have looked at chaotic pendulum and three particles under 1/r^2 forces.

If we include air-friction and some force then simple pendulum is not very simple. The motion is very chaotic and not predictable. 
Here are some phase-space diagrams:

Here I plotted amplitude vs. force (at normal frequencies)
Here is a zoomed in version
As we can see, even the most chaotic pendulum has patterns in some phase-space diagrams.

How is this relevant to Astrophysics?

The N-body simulations are also chaotic. I have started my research to find any patterns in  the behavior of 3-bodies under gravitational force. I do this by setting up the differential equations of motion in Mathematica and using random values as initial conditions. I try to plot different phase-spaces to see any kind of patterns. 
Here is one of the video that shows the motion of three-body over time.


For some reason this video doesn't work. Here is a link to youtube link to this video.
If you like to play around with numbers, I have included the Mathematica source code. 


Why spend time?

Astrophysicists wants to find out how a star forms, how it dies, and the complicated phases it goes through during its evolution. Many scientists have made simulations of these events, but they take months to render in world's fastest super computers. 
If we can find alternative way to calculate motion of N-bodies (alternative to newton's motion), then we can run this simulations at very low costs and more efficiently. This would also help us understand complicated star evolution processes.

Special thanks to Mathematica

Friday, June 13, 2014

Harmony of Spheres



Harmony of Spheres, also known as Music of Spheres or Musica Universalis, comes from ancient times.  It is mathematical and philosophical concept that expresses tones of energy resulting from numbers, angles, and shapes within pattern of proportion.  


The Pythagoras were the first to identify that pitch of musical note is proportional to length of string that makes it and intervals between frequencies form numerical ratios.  Back in these ancient times, musical notes were assigned to seven heavenly bodies in symbolic arrangements.  It was thought that sun, moon and planets have orbital resonance due to their orbits.


Then Kepler came along, interested in Harmony of Spheres and wanted to calculate them.  He noticed that ratios between planets’ extreme angular velocities were all harmonic intervals.  He wrote book called Musica Universalis that showed relationships between geometry, cosmology, and harmonics.

I thought this topic was cool because it combines geometry, physics and music.  Three things I enjoy.


Sources:
http://en.wikipedia.org/wiki/Harmony_of_the_Spheres
http://en.wikipedia.org/wiki/Harmonices_Mundi

Thursday, June 12, 2014

Dust in interstellar atmospheres

Dust affects our view of the distant universe!
The red ring around the galaxy is dust

How and why does dust matter?

Stars convert hydrogen into heavier elements and release them into the interstellar medium (via supernova, etc). These particles absorb light at certain wavelengths, which creates a big problem to the study star forming regions.

Dust particles will absorb high energy light that is emitted from star forming regions (the H-alpha regions). The luminosity that we receive is the luminosity from star formation - the absorption by dust. It is important to correct for the dust affects. It helps us correctly calculate the initial mass function and star formation rates. If we knew how much dust is in the interstellar atmosphere, then we could calculate the true luminosities of star forming regions. This data could help theorists better approximate the initial mass function and star forming rates.

How to measure the amount of dust?

Dust emits light in IR range. Here is a plot of luminosity of distant star forming galaxies.


As you can see, our IR detectors are not sensitive enough to calculate the luminosity from dust. This doesn't mean that we have to give up. There is a very interesting method to estimate the amount of dust know as stacking method.

We use stacking method to cancel out the effects of random disturbances to see any peaks (in IR) in star forming regions. Basic idea is to average luminosities from different IR images to see any peaks.

Procedure:
Look at different regions of sky and find as many star forming regions as you can from H(alpha) break from UV/Visible pictures.
Take IR images of these regions and cut out star forming areas. Now average fluxes from these cutouts and hope that you see some peak. Doing this should make all the disturbances cancel out, and reveal the small peak in the flux.




Special Thanks to Dr. Naveen Reddy











Wednesday, June 11, 2014

Asteroid Belt


Asteroid Belt

Beyond Mars, which marks end of inner solar system, lies a particularly huge space.  On the other side is massive planet Jupiter.  It is in this huge space that Asteroid Belt exists.  Thousands of large and small tumbling rocks is what Asteroid Belt is made up of.  There are four types of asteroids that current Asteroid Belt consists of, carbonaceous asteroids, silicaceous asteroids, metallic asteroids and rare basaltic asteroids.

Carbonaceous asteroids are most common type and make up large percentage of Asteroid Belt and its outer regions.  They are carbon rich and consist of organic compounds.  An interesting article can be found at http://www.psrd.hawaii.edu/April11/amino_acids.html

Silicaceous asteroids are second most common type and make up majority of inner region” of Asteroid Belt.  These types asteroids are mainly composed of iron and magnesium silicates and a very few even show hint of organic compounds.  This tells us that their “materials have been significantly modified from their primordial composition, most likely through melting and reformation”.

Metallic asteroids are third most common type of asteroids.  Their compositions are not fully known, but many consist of nickel, iron and a little stone.  This class of asteroids is not fully understood and it appears there may be several different types.

Basaltic asteroids are considered rare and are mystery in Astrophysics.  They are composed of extrusive volcanic rock and mineral olivine, which is magnesium iron silicate.  If you are interested, you can read more about these mysterious types of asteroids here.

There are main-belt comets that orbit within Asteroid Belt which tend to stay in outer part of belt.  There are also spaces, known as Kirkwood Gaps, cleared where orbital resonances with Jupiter occur.  Orbital Resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on one another.”  The largest gap is at orbital distance which would correspond to period of one third that of Jupiter.

The most massive of all the asteroids is Ceres , comprising over one third of total mass of all of asteroids.  It is considered a dwarf planet, smallest one in our solar system, and only dwarf planet to exist in Asteroid Belt. 

Asteroid Belt is highly unlikely to be remains of a small planet because no sizeable body could ever have formed so close to Jupiter.


Sources:







Tuesday, June 10, 2014

Magnetic Field of a Neutron Stars

A neutron star is stellar remnant that results from the gravitational collapse of a massive star (between 8 to 20 solar masses). Even though neutron stars are composed of almost entirely neutrons, they have very strong magnetic fields.

To understand why a neutron star has a huge magnetic field, let's look at its structure.

The structure

This model hasn't been proven, but it gives very good understanding of neutron stars and reveals many known properties of neutron stars. The surface of a neutron star is composed of ordinary nuclei with a sea of free electrons flowing around them. Most of the nuclei are left over hydrogen, helium and iron. The atmospheres of these stars, according to recent hypothesis, could be at most several micrometers. This is because of the intense gravity on the surface. A calculation of gravity is here. This is approximately 10^12 times stronger than earth's gravity.



A little deeper into the star, the gravity gets strong enough so that neutrons leak out of nuclei and become free neutrons. As we dig dipper and dipper, the nuclei becomes smaller and smaller until the core is reached where nuclei disappear. The composition of the super dense core remains uncertain.

Angular velocity

To get a rough estimate of angular velocity, we can assume that angular velocity is conserved.
Here is a sample calculation. We can get something like several rotations per second!!!

Magnetic Field

Here is a rough calculations of neutron stars' magnetic field assuming charge density is uniform in spherical shell shape. This shell can have a lot of surface charge density since there is a sea of electrons. If we estimate the magnetic field of rotating charged sphere, then the below picture shows that neutron stars can have magnetic fields like 50,000 Tesla. Compare that to earth's magnetic field (0.0001 Tesla) or maximum magnetic field produced on earth (100 Tesla).


Here is a link to the interactive CDF file to explore magnetic field of neutron star with different parameters.
Click here to download

Sources
Wikipedia Neutron Star
Magnetic Field of a rotating Sphere Estimates

Monday, March 17, 2014

Last minute studying

(I have added more stuff)

Dear Classmates,

Here are all the notes that I have been writing throughout the class. Some of them include helpful simulations and diagrams.
Hopefully this will help.

You should be able to open PDF versions in the browser (just click on it). Other formats files will need to be downloaded.

CDF file will open with wolfram CDF player. It is free. Here is a link
.nb file will open with mathematica.


Evolution of post-main sequence star. (This is not perfect. I might have mistakes. Check it out if you are completely lost, like i was a day ago.)
Star Evolution
Death Of Stars

Important! How to calculate flux and luminosity form blackbody equation (plank's equation). PLUS Some equations that are not in the equation sheet, and other short notes about calculating mass in binary and radial velocities.
Notes

Classification of Stars and formation of spectral lines
http://dl.dropbox.com/u/58867372/Astrophysics/Classification%20of%20Stars%2C%20Formation%20of%20Spectral%20Lines.cdf
http://dl.dropbox.com/u/58867372/Astrophysics/Classification%20of%20Stars%2C%20Formation%20of%20Spectral%20Lines.nb
http://dl.dropbox.com/u/58867372/Astrophysics/Classification%20of%20Stars%2C%20Formation%20of%20Spectral%20Lines.pdf

Interiors of stars
http://dl.dropbox.com/u/58867372/Astrophysics/Interior%20of%20Stars.cdf
http://dl.dropbox.com/u/58867372/Astrophysics/Interior%20of%20Stars.nb
http://dl.dropbox.com/u/58867372/Astrophysics/Interior%20of%20Stars.pdf

Star Formation
http://dl.dropbox.com/u/58867372/Astrophysics/Star%20Formation.cdf
http://dl.dropbox.com/u/58867372/Astrophysics/Star%20Formation.nb
http://dl.dropbox.com/u/58867372/Astrophysics/Star%20Formation.pdf

Steller Atmospheres
http://dl.dropbox.com/u/58867372/Astrophysics/Steller%20Atmosphere.cdf
http://dl.dropbox.com/u/58867372/Astrophysics/Steller%20Atmosphere.nb
http://dl.dropbox.com/u/58867372/Astrophysics/Steller%20Atmosphere.pdf


Binary System (This one is not so great. I haven't finished making it.)