All Questions

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2 views

Why is the transverse Doppler shift “difficult to measure”?

I'm currently reading a book on optics, and have encountered a curious section: $$\nu = \nu'\sqrt{1-\frac{u^2}{c^2}} = \nu'\left(1-\frac{u^2}{2c^2}+\ldots\right)$$ This is the formula for the ...
0
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0answers
5 views

Is there a cosmological model where the observable universe is infinity bi-ended and also bi-limited

Is there a cosmological model where the observable universe continues to shrink as time decreases, and continues to expand as time increases, and such that at both directions it never reaches the ...
-1
votes
0answers
8 views

Quantum mechanics (Hamiltonian)

the wave function of a particle sci =1/√2×{phoi0+phoi1} is given by in terms of eigen state phoi0 and phoi1 corresponding to the ground state energy and first excited state energy E0 and E1 ...
0
votes
1answer
14 views

What would happen if I put two identical particles close enough?

Is there a repulsive force between these two particles to prevent them from being in the same point? I mean, in order to obey Pauli exclusion principle?
0
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0answers
12 views

Renewable energy resources

A butyl rubber bag of total volume 3.0 m3 is used as a biogas digester. Each day it is fed an input of 0.20 m3 of slurry, of which 4.0 kg is volatile solids, and a corresponding volume of digested ...
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0answers
5 views

Charges in different mediums

Generally we read or solve problems about charges in the same medium. I wonder what would happen if the two charges were placed in different mediums. eg.) One is placed in vaccum and other is placed ...
0
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0answers
10 views

Uncertainty from the generalised uncertainty principle

The Robertson-Schrodinger or the generalized Uncertainty relation is given by $$\sigma_A^2\sigma_B^2 \geq \left| \frac{1}{2}\langle\{\hat{A}, \hat{B}\}\rangle - \langle \hat{A} \rangle\langle \hat{B}\...
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0answers
8 views

How to write BdG equation for generalize Hamiltonian?

I am reading this parer Majorana Corner Modes in a High-Temperature Platform,its model Hamiltonian is $H(\vec{k})=M(\vec{k})\sigma_z\tau_z+A_xsin(k_x)\sigma_xs_z+A_ysin(k_y)\sigma_y\tau_z+\Delta(\vec{...
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0answers
7 views

The value of a quantity resembling Berry phase

Let us consider a two level system with eigenenergies $E_0$ and $E_1$ and corresponding normalized eigenfunctions $\psi_0$ and $\psi_1$. All the functions periodically depend on a parameter $\lambda$. ...
0
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0answers
12 views

Material science [on hold]

Is there any possible method to calculate electron positron projected range if we know CSDA and total stopping power . Can we just simple divide CSDA with density to get projected range . Also what is ...
0
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1answer
16 views

Intensity of an Electromagnetic Wave

What happens to the intensity of an electromagnetic wave if the amplitude of the electric field is halved?
0
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0answers
7 views

Why Li is not added as contact by ion implantation method in p type HPGe detector?

n+ contact of p type Ge is ~700µm thick compared to p+ contact of n type Ge only ~0.3µm. The process of adding the contact is different. n+ contact Li is added by diffusion process but p+ contact B is ...
0
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0answers
6 views

How to explain the phenomena of minimum deviation position of prism-spectrometer?

https://youtu.be/M07I1ompX54 Why does the spectral line change direction at minimum deviation position even if the prism table is rotated in the same direction?
0
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0answers
11 views

How does the inertia of granules within a rotating cylinder affect its inertia?

If a cylinder is partially filled with granules and is rolling down an inclined plane, is there any relationship between the rotational motion of the granules and the motion of the cylinder? I'm ...
0
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0answers
12 views

Radiation pressure at a surface which is perfectly refracting

Radiation pressure at perfectly reflecting surface = 2I/c Radiation pressure at perfectly absorbing surface = I/c What will be the radiation pressure at a perfectly refracting surface? Will it be ...
1
vote
2answers
23 views

Can we recover a state on a composite system from a state on the subsystem?

$\renewcommand{\ket}[1]{\left \lvert #1 \right \rangle}$ $\renewcommand{\bra}[1]{\left \langle #1 \right \rvert}$ I'm wondering if we can recover the state of a composite system from the information ...
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0answers
24 views

A doubt in Modified gravity

I am new to dark matter and modified gravity so excuse and inform me if I am wrong. If changes are made in the Friedman equations then there wouldn't always be an underlying action action principle. ...
0
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1answer
16 views

Hubble’s law distance and time dependence

Hubble’s law states that the recessional velocity is proportional to the distance. The further the observed distance, the further back in time one is observing. I want to know (generally) how this ...
2
votes
0answers
42 views

Theoretically, if I swish a laser back and forth fast enough/at the right frequency on a distant surface, will I get an interference pattern?

Just curious. I’m kinda stumped and am interested in how one would go about visualizing/calculating this.
0
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0answers
18 views

Meaning of Shannon Information

Is there any (simple!) explanation as to which of the following pieces of paper tape, each with the same number of holes, contains information: A simple coding of a Shakespeare sonnet The same ...
1
vote
2answers
55 views

Why is it necessary to keep UTC within 0.9 seconds from UT1?

Why is it necessary to keep UTC within 0.9 seconds from UT1? I thought that UTC in fact more accurate than UT1, so why should it be based on UT1 within that range? Another question would be, why isn'...
1
vote
1answer
33 views

Electric field of point charge that has circular motion

We know that moving charge in constant velocity has deformed equation of electric field. $$ E = k \frac{1-\left(\frac{v}{c}\right)^2}{\left[1-\frac{v^2 \sin^2\theta}{c^2}\right]^{3/2}} \frac{q}{R^2}$...
0
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2answers
32 views

Is it possible (either in practice or in principle) to produce images with a LIGO-type gravitational “telescope”?

I was reading an article in the newspaper today that referred to LIGO (and gravity-wave measurements in general) as "a new type of telescope". That got me thinking -- as I understand it, LIGO uses ...
0
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1answer
27 views

How to calculate the derivative of scale factor as a function of conformal time from the solution of Friedmann equation

For the flat geometry of lamda CDM model, the solution for Friedmann equation is $$ a(t) = \left\{ \frac{Ω_{m,0}}{Ω_{Λ,0}} \sinh^2 \left[\frac{3}{2} \sqrt{Ω_{Λ,0}} H_0(t - t_0)\right] \right\}^{1/3},...
0
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1answer
21 views

Method of Images conundrum

My lecturer and I have found separately valid solutions to Poisson's equation in the region of interest for the following problem: Here is my interpretation of the boundary conditions: $$V(x,y,z \to ...
0
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0answers
9 views

How would one go about deriving the ideal rocket equation for a multi stage rocket?

In staged rockets, each stage is jettisoned back to Earth after it completes its burn. Derive an expression for the total ΔV that a rocket can provide as a function of the number of stages, given a ...
0
votes
1answer
31 views

Calculate speed of an object using angle and time

I haven't had any physics class or anything close to that in a long time, and I'm currently confronted with an exercise I don't remember the formula(s) to solve it. Assuming I have an object of ...
0
votes
1answer
57 views

Is it speed or acceleration that leads to time dilation? [duplicate]

Is it acceleration or speed that leads to time dilation or both? For example, when a spaceship accelerated to (almost) the speed of light, then reversed course to come back towards earth, does this ...
1
vote
1answer
38 views

Is naturalness meaningful for non-fundamental theories?

Naturalness has been a guiding philosophy for particle physics for a long time, but a few years ago I heard a talk by Nima Arkani-Hamed where he pointed out that it seems to have failed us as it ...
1
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0answers
12 views

Pulsar distance estimation

This is quite an interesting problem in astrophysics so I thought it would be a good idea to ask here so we can archive the solution for future reference. Consider a pulsar that emits pulses of ...
-3
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0answers
18 views

Laser and Xray Wavelength [on hold]

Calculate the wavelength of the xray emitted from a molybdenum target (z=42) when an electron moves from the L shell(n=2) to the K shell(n=1)
0
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0answers
16 views

Unit strength Wiener process

I am not much aware of Brownian motion and/or the Wiener process. Recently I have been reading a research paper where they mention a variable $dW$ as a "unit strength Wiener process". My question is ...
1
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0answers
23 views

What exactly is delta-14C and why are there periodic plateaus in the delta-14C curve?

I downloaded the IntCal13, IntCal09, and IntCal04 datasets (parent site), and when I plotted the last 24k years was surprised to see a plateau or even reversal appear every ~5300 years. Apparently we ...
0
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0answers
15 views

Tunnelling probability for a quantum particle with the special energy-momentum relation

In quasi-classical approach the tunnelling probability through a barrier is given by the formula $$\text{prob}\sim e^{-\int p(x)dx}.$$ I left out the coefficients, with $p(x)\sim \sqrt{E-V(x)}$ being ...
3
votes
2answers
37 views

Why can we factorize the state of a particle?

I read about factorization in this two cases. First, since spin is not coupled with position and time it is possible to factorize the state in a spinor and a wave function. Second, in the 2 particles ...
0
votes
1answer
32 views

Charge density $\rho$ as a delta-function $\delta$

Poisson's equation is $$\boldsymbol{\nabla}^{2}\varphi=-4\pi k_{e}\,\rho, \tag{*}$$ that in the case of a point charge $q$, already with spherical symmetry, has as solution \begin{equation} \varphi(...
-2
votes
1answer
60 views

How isthe transparency of glass explained in physics? [on hold]

I would like a rigorous explanation: a theory and model which describes glass as well as iron, and see why one of them is transparent; a detailed mathematical computation or a detailed reference. And,...
1
vote
0answers
32 views

Where does this form of the induction law come from?

I am currently studying a eddy current disk break and came across this promising paper (PDF, princeton.edu) from 1942. However, in the first formula a suspicious $4\pi$ appears before the current $U(...
0
votes
1answer
37 views

Relation between quantum numbers $j$ and $\ell$

I'm given the following problem: "Consider an atom with orbital angular momentum $l$. What are the possible values of the total angular momentum quantum number $j$? Treat the case of $\ell = 0$ ...
0
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0answers
18 views

What is the power dissipated in a conductance in the transmission line? [on hold]

The electric power dissipated due to a resistance is $P=RI^2$. What is the power due to a conductance?
0
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0answers
8 views

two identical disc each of mass m(=4m) and radius r are rotating in opposite sense with equal angular speed w [on hold]

about vertical axes passing through their centres, (as shown in figure) .a person of mass m sitting on circumference of disc a jumps with tangential relative velocity u w.r.t one rotating disc A jumps ...
0
votes
2answers
40 views

The transformations of Lorentz as a general case of the transformations of Galileo

Starting from the transformations of Lorentz, $$ \left\{\begin{aligned} x&=\gamma (x'+\beta ct)\\ y&=y'\\ z&=z\\ ct&=\gamma (ct'+\beta x')\\ \end{aligned}\right. \quad \tag{*}$$ I ...
2
votes
5answers
54 views

Interpretation of rotating a quantum state

I'm currently taking an introductory quantum mechanics course, where the last covered topic was spin and identical particles. During some reading online, I stumbled across the term "rotating a quantum ...
0
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1answer
22 views

Could light and heat reflecting off of surface ice on Earth have a noticeable impact on the sun?

If so, what could be the impact of these reflections on the sun during an ice age on Earth?
-4
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0answers
43 views

Is Everything Reducable to Geometry? [on hold]

Related to a previous post: Is there a Geometric Algebra for gravity? : I asked the question of whether gravity could be given a geometric interpretation though stated algebra. Because I first tackled ...
1
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0answers
25 views

Acceleration as the logarithmic derivative of the redshift factor $U^\sigma \nabla_\sigma U^\mu = g^{\nu\mu}\nabla_\nu \ln V$

Let $K^\mu = V(x)U^\mu$ be vector proportional to the four-velocity $U^\mu$. (e.g. $K^\mu$ is a normalized time-like Killing vector for an observer at infinity). Then, $V(x) = \sqrt{-K_\nu K^\nu}$ is ...
-1
votes
1answer
36 views

Why the National Ignition Facility try to use 192 laser beams to trigger ignition without heat capsule?

In the discussion of the question "Nuclear Fusion with extremely high pressure and low temperature", the conclusion is only ultrahigh pressure cannot trigger fusion. But why does NIF try to use 192 ...
0
votes
2answers
45 views

Energy-momentum vs charge-current

Is there a simple intuitive way to explain why energy-momentum density requires a tensor, while charge-current density is a vector? $\partial_{\mu} J^{\mu} = 0$ is a statement, in effect, that the ...
-7
votes
1answer
37 views

Could micro-states not have equal probability as assumed? [on hold]

How could micro-states have uniform distribution in a 3D space unless there was 0 Gravity? Gravity violates the assumption of equal probability as it makes matter move in a predictable direction. ...
0
votes
0answers
9 views

How does photoluminescence spectroscopy work and how can it apply to quantum dots?

Does anyone have a clear explanation of how photoluminescence works? I know some of the basics, for example, that electrons are excited by photons to create excitons and when the excitons recombine, ...

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