My book states the following, "when a particle, in a stationary state, is bound the expectation value of its momentum is zero" but it never elaborates why.    I know that $$\int\Psi^*(x,t)\hat{Q}\Psi(x,t)dx$$  but why would that necessarily mean that the expectation value of its momentum is zero?
Is it logically correct to assert that the expectation of the momentum $$\langle \hat p \rangle=0$$ for any bound state because it is bound to some finite region? What is the physical interpretation of the fact that $$\langle \hat p \rangle=0$$ in an energy eigenstate $\psi_n(x,t)$  but $$\langle \hat p \rangle\neq0$$ in some superposition state $$\psi(x,t)=c_m\psi_m(x,t)+c_n\psi_n(x,t)~?$$ Here $\psi_n(x,t)$ the eigenstates of the Hamiltonian, for example, in the problem of particle in a box (say).
Show that:  If $U$ is a principal ultrafilter, then the canonical inmersion $j$ is an isomorphism between $\mathcal A$ and $Ult_{U}\mathcal A$ Can I add some HTML in my SO Profile 'About me' text box to show some button from linkedin? How are the AES S-Boxes calculated? I'm trying to understand how the AES S-Boxes are calculated. I understand how the multiplicative inverse is calculated over $GF(2^8)$, but I'm confused by the description of the affine transformation. I haven't been able to Google a good explanation of how the S-Box values are calculated. Can someone explain how this works, starting after the calculation of the multiplicative inverse? Regression Proof that the point of averages (x,y) lies on the estimated regression line How do you show that the point of averages (x,y) lies on the estimated regression line? Block quote with big quotation marks For gravity, we have    General Relativity, which is a geometric theory for gravitation.  Is there a similar analog for Electromagnetism? How did the Wampa freeze Luke's feet into the ceiling of the cave? Example where $f\circ g$ is bijective, but neither $f$ nor $g$ is bijective Can anyone come up with an explicit example of two functions $f$ and $g$ such that: $f\circ g$ is bijective, but neither $f$ nor $g$ is bijective?  I tried the following: $$f:\mathbb{R}\rightarrow \mathbb{R^{+}} $$ $$f(x)=x^{2}$$  and  $$g:\mathbb{R^{+}}\rightarrow \mathbb{R}$$ $$g(x)=\sqrt{x}$$  $f$ is not injective, and $g$ is not surjective, but $f\circ g$ is bijective  Any other examples? PHP script not executing on Apache server Object on curve wont stay flat! I'd like to install both Ubuntu 12.04 and Windows XP on a Dell laptop, and I was thinking about using a dual boot structure, and using the bulk of my hard drive as empty hard drive space to share files between the two operating systems (so choice of file system type is very important in this set-up).     The kind of partitioning structure I would like to use is  Partition 1 - Ubuntu 12.04 (root) (20GB)  Partition 2 - Ubuntu /home        (20GB)  Partition 3 : Free Space          (560GB)  Partition 4 : Windows XP          (35GB)  Partition 5 : SWAP                (3GB)  (Total Hardrive Capacity is ~640GB)    My question is; what is the best way to go about setting up this kind this system?   Should I install Windows XP first and setup the partitions, and then install Ubuntu which I believe will install the GRUB bootloader for OS booting choice   or   Do I install Ubuntu first, setting up the available partitions and then perform a WIndows install?  Please let me know if there is anything in this setup that I have left out and should know about, including things related to setting particular partitions as logical or primary, and whether the boot partition and the filesystem partition should actually be two separate partitions. My kids are playing Minecraft on 2 PCs, creative mode.  They are connected by the way of one of them starts a single player game on the other one than joins that game over the LAN.  Can I, using Minecraft PE on my iPad, join their game as well? Is one time pad cipher reuse and random key secure? Sorting grid cells in fishnet grid by location and number using QGIS? I am using QGIS 3.2.0 on a Mac (and also have QGIS 2), and I have a fishnet grid (probably created in Arc) that comprises of nearly 2 million grid cells. As a result, every change I make to the attribute table takes hours, so I can't experiment as I would usually.  I need to label each grid cell such that the north-westernmost cell is "1", and the numbers proceed by rows, until the last number goes into the south-easternmost cell. Using the row-number doesn't work, as that's not how the table is sorted. I've done this before, but cannot remember how. It always takes a  and money to find the next . Why is it so important to do this work and what is the application those numbers? How to make different materials for different objects? When I change one material, it changes it on another object. Why and how do I fix it? is there some link between the objects?    Here is my blender file of Iron Man's faceplate.  Here is my whole screen with the main portion of the face selected. On the right, you can see the material, and I can tell you that that is the same as the eyes part: Keep values selected after form submission "Mixed declaration and code" warning, is it worth addressing? Suppose that $A$ is an integral domain. If $a,b$ are element of $A$ and $a$ is a unit, then how come the change of variable $X\mapsto aX+b$ extends to a unique automorphism of the polynomial ring $A[X]$, which equals the identity map when restricted to $A$?  Thanks for any ideas or ways to get started. How can I unlock a password locked rar file without knowing the password? I have .rar file that is password protected, but I accidentally have forgotten the password. The file extremely important to me.  How can I get my file back? In windows, there was many programs that could remove the password of such a file. Is there one for Ubuntu?