-2
votes
0answers
33 views

Why does the centrifuge work backwards? [on hold]

Let's suppose, there is a big round recipient spinning in the air. If we throw some dirt in it, all the dirt will fly around in the recipient until all the heavy parts will stick on the walls of the ...
1
vote
0answers
15 views

Measuring the growth of the Hubble sphere

I have a question which I believe will greatly help my understanding of the Hubble sphere and what it means physically. Say we have a flat ($k=0$) Universe with no dark energy ($\Lambda=0$) during ...
0
votes
0answers
18 views

What's the Propogater in the Free Particle Case? (Path Integrals with Source Term)

If I take the Lagrangian to be, $$L(t)=\frac{1}{2}m \dot q(t)^2$$ The Euclidean Path Integral is supposed to be, $$K=\int D[q(t)] \ e^{-\int L(\dot q) d \tau}$$ If I add a source term $J(\tau)$ we ...
0
votes
0answers
15 views

watch lcd-display turns blank, time halts. [on hold]

My wrist-watch has been acting a bit funny lately, and I am looking for possible explanations of it's behavior. Several times the lcd-display on my watch has turned blank. At the push of any arbitrary ...
0
votes
0answers
11 views

Is it correct that the polarized scatter of a polarized light source is max. orthogonal to the light source?

First of all, is the statement above correct? And if so, is there a constant gradient, with no polarized scatter parallel to the polarized source up to fully polarized at 90 degrees?
0
votes
1answer
55 views

Is it true that $\frac{d}{dt}\int_S \mathbf{B} \cdot d \mathbf{a}$ goes to zero if the amperian loop delimiting $S$ contracts indefinitely?

I suppose to have an ordinary magnetic field: in the answer I'm not interested to involve Dirac delta: the integral goes to zero. I want to focus on another point: an infinitesimal physical quantity ...
2
votes
1answer
31 views

Why does the classical Doppler formula make a distinction between movement of the source and movement of the receiver?

I've tried rewriting the Doppler formula to include only the relative velocity between the source and the receiver of sound waves. However, when I compare the results with the results of the formula ...
1
vote
2answers
41 views

Finding relation between angular acceleration and liner acceleration directed in one direction

In the above given configuration what could be the relation between the angular acceleration of the rod $\alpha$ and the acceleration $a$ of the $2m$ block. My attempt : What I thought is that the ...
0
votes
0answers
15 views

New approach to cyclic Big Bang [on hold]

I am trying to explain the Big Bang without the need to create a full-fledged universe from nothing in a fraction of a second. I would like to ask the readers of this page if they find any ...
3
votes
1answer
52 views

Relativistic correction to Hydrogen atom - Perturbation theory

Given the relativistic correction $$ H_1' = - \frac{p^4}{8m^3 c^2} $$ to the Hamiltonian (i.e. a perturbation), what does it mean when $[H_1', \mathbf{L}] = 0$? The book I'm reading says this implies ...
0
votes
0answers
16 views

Project Orion pusher plate computer modeling

Linked from the space.stackexchange forum: http://space.stackexchange.com/questions/15051/orion-project-npp-pusher-plate-computer-models-and-survivability Project Orion is discussed as if it is a ...
0
votes
0answers
31 views

How to evaluate $[L^2,x_{j}]$ [on hold]

I want to evaluate the following commutator: $[\vec L²,x_{j}]$ where $\vec L= \vec x\times \vec p$, $(L_{i}=\epsilon_{ijk}x_{j}p_{k})$ and $\vec L²=L_{i}L_{i}$, here is my work so far: ...
4
votes
2answers
139 views

How does the inclusion of vacuum energy modify Newtonian gravity equation of motion?

The equation of motion (in the center of mass frame) due only to gravitational forces between two point masses is: $$\frac{d^2r}{dt^2} = -\frac{GM}{r^2}$$ How does the equation get modified when a ...
2
votes
0answers
23 views

Physical motivation for one dimensional SHM superposition

Are there any real life, simple and mechanical system which motivate the study of Simple Harmonic Motion (SHM) superposition in one dimension? I am preparing a lecture about it but I have not seen any ...
0
votes
1answer
35 views

What happens when two polarized lights of the same wavelength interfere at 90 degrees with each other?

am I right in assuming that if I cross two polarized lights of the same wavelength the result would be destructive interference? I don't mean 90 degrees as in 'orthogonal polarization', but the two ...
9
votes
6answers
2k views

Light's inverse square law: Does it require a minimum distance from the source?

Does the inverse square law begin to take effect the moment light leaves its source? For example, does light's intensity decrease, i.e. does the area in which the photons might land increase, at a few ...
1
vote
1answer
27 views

Density Matrix representation of excited atoms

I'd like to get an answer to this question from someone who knows his density matrix theory. I want to compare two different systems and ask how their density matrix representation looks. First look ...
0
votes
0answers
17 views

How Do I find the Thermal increase of a ideal gas flowing through a magnetic field? [on hold]

I am thinking of turning dissociated 2H_2 and O_2 into plasma by forcing it through a electromagnet field. what are the equations to determine the thermal energy added to the gas. I'm thinking to ...
0
votes
1answer
49 views

Why are there e.m. vacuüm fluctuations?

According to the rules of qft there are virtual photons in the vacuüm. But how can this be if for the production of photons you need an electric charge?
0
votes
1answer
70 views

Can the fact that the vacuum energy in curved spacetime is not boost invariant be explained without mathematics?

I read in an answer to a question if Hawking radiation can be explained without too much mathematics that this is impossible insofar the vacuum energy is not boost invariant in a curved spacetime. ...
-2
votes
0answers
42 views

Diffraction and wavelength [duplicate]

Why is it that when the slit-width is much larger compared to the wavelength of light, there is little diffraction or spreading-out of the light? And why is it that the diffraction effects are ...
1
vote
0answers
35 views

What are the conditions for the string of a pendulum to become slack? [on hold]

Suppose the bob of a stationary pendulum is given a sharp hit to impart it some horizontal speed. Now what are the conditions for the string to become slack after rotating a particular angle? ...
-2
votes
3answers
42 views

How do Calories (kcal) relate to watts?

In thinking about exercise and "burning calories" it occurred to me that there should be some fairly intuitive correlation between the biological energy conversion going on in a person's body with the ...
0
votes
0answers
26 views

How to draw Feynman diagram for K^- + p -> Sigma^0 + pi^0 — superposition in diagram

I'm trying to draw the Feynman diagram for the reaction $$K^- +p \rightarrow \Sigma^0 + \pi^0$$ I have two problems: How to draw the $\pi^0 = (u\bar u + d\bar d)/\sqrt 2$, a superposition of ...
0
votes
0answers
52 views

Would it be correct to state that all the forms of everyday matter (mountains, tables, buildings, people, etc.) are in a metastable state?

We had a fire going in the backyard of a friends house this weekend, and after thinking about the chemical reactions occurring in the wood (and anything else we threw into the pit), I started to ...
-1
votes
3answers
50 views

Right Hand Rule on the magnetic field not working? [on hold]

Background: The magnetic field is going into the page and the velocity of the particle is going towards the right. What is the sign of the particle? I keep getting this wrong. The magnetic field ...
-5
votes
0answers
19 views

Power formulas? [duplicate]

Are different power formulas in circuits used for different reasons? For Example Power = V^2/R Power = I^2/R Power = VI
0
votes
0answers
16 views

Electronics - Series/Parallel Circuit Question [on hold]

Im working on this question but I've confused myself more by watching videos. I'm not sure if the circuit is in series and parallel or just parallel. I understand the 2.5k Ohms and 1.4k Ohms are in ...
1
vote
0answers
45 views

(Causal) Set notation round brackets vs square brackets? [on hold]

In many (quite old) papers & books I have been reading recently in the causal theory of general relativity (e.g. On the structure of causal spaces, Kronheimer & Penrose, 1967) I find sets ...
0
votes
0answers
29 views

Is it possible to approximately know the wattage/luminosity of a lamp knowing distance and using a DSLR?

I'd like to know if it is possible to approximately know the wattage/luminosity of a lamp knowing distance and using a DSLR (digital single lens reflex camera) mounted on tripod. Maybe, when shooting ...
0
votes
0answers
45 views

Galileo's ship experiment with a laser pen instead of a rock

Ship's mast experiment with the boat moving at speed v: If a rock is dropped from the mast, then for an observer inside the boat they will see this: For an observer outside the boat and on the ...
0
votes
1answer
45 views

Understanding work with rotational momentum/moment of inertia

Apologies for the basic question but between the vectors and the spinning, I'm getting confused. If I have a wheel with moment of inertia $I$ spinning at some rate $\vec\omega$ around an axis, what ...
0
votes
0answers
29 views

Interpretations of uncertanity principle used for approximation

Just a short question regarding an interpretation of the Heisenberg uncertainty principle $\sigma_x \sigma_p \geq \frac{\hbar}{2}$. Question: Why is it also sometimes that $\Delta x$ and $\Delta ...
0
votes
1answer
45 views

Interpretations and approximations of uncertainty principle

Just a short question regarding an interpretation of the Heisenberg uncertainty principle $$\sigma_x \sigma_p \geq \frac{\hbar}{2}$$. Question: The uncertainty principle is sometimes written as ...
0
votes
0answers
46 views

The force of gravitation [on hold]

Which is greater: attraction of the Earth to 5 kg of iron or attraction of 5 kg of iron to the Earth? give reasons. When an apple falls towards the Earth, the Earth moves up to meet the earth. Is ...
0
votes
0answers
25 views

Expressing charge density in terms of spherical multipoles

How to express the charge density in terms of the full set of the spherical multipole moments? That is, how to invert the relationship $$ Q_{lm}=\frac{4\pi}{2l+1}\int ...
1
vote
0answers
20 views

Diagonalizing the Hamiltonian of the two-site Hubbard model

The Hamiltonian for the two-site Hubbard model is as follows: $H = - t \sum_{\sigma = \uparrow, \downarrow} (c_{1\sigma}^{\dagger} c_{2\sigma} + \text{h.c.}) + U ...
0
votes
0answers
10 views

Methods for producing Molecular C60 (A.K.A Buckminsterfullerene) [on hold]

I wanted to know about how to produce molecular C60 - also known as Buckminsterfullerene/buckyballs. I read online that vaporising graphite in helium gas was how scientists initially discovered ...
0
votes
0answers
9 views

difference between weak and strong topologiccal insulators

Does someone know what the difference is between weak and strong topological insulators? (And do both exist in any dimension?).
-1
votes
0answers
22 views

Can a knot be transformed into another knot via quantum tunneling? [on hold]

Sorry for a non-competent question. In string theory: Can a knot be transformed into another knot via quantum tunneling?
-1
votes
0answers
31 views

String theory: Is the number of particles infinite? [on hold]

Sorry for a non-competent question. In string theory (AFAIK) every knot is a separate particle. And the number of knots is infinite (AFAIK). So in ST there exists an infinite number of particles, ...
0
votes
0answers
13 views

Stationary-state scattering process

In a stationary-state scattering process of an incoming plane wave, the outgoing spherical wave can be described by $\psi(\vec r) = e^{ikz} + f(\theta) \frac{e^{ikr}}{r}$. My question is, how is this ...
0
votes
0answers
26 views

Specific function is a Linear combination of energy eigenstates

My lecturer made a statement: In the infinite square well potential $x\psi_n(x)$ (where $\psi_n$ is just the spatial part of an energy eigenstate) can be expressed as a linear combination of the ...
1
vote
1answer
20 views

Actual spot size = geometric spot size + diffraction spot size?

I read in [1] that for a camera obscura the total spot diameter is equal to the diameter of the spot produced concerning geometric optics only plus the diameter of the spot concerning only ...
9
votes
2answers
731 views

Do stars remain electrically neutral? [duplicate]

How electrically neutral do stars remain through their lifetime? As an example, I could imagine processes such as coronal mass ejections leaving the Sun in a slightly charged state. Are there such ...
0
votes
1answer
56 views

It it true that the Fourier transform of the return probability can be written as the product of the retarded propagator & the advanced propagator?

I know the retarded green's function $G(r,r; E)$ is expressed as $$G(r,r; E)=\sum_n \frac{|\mu_n(r)|^2}{E-E_n+i\eta}$$ with the eigenvectors/eigenvalues $\mu_n(r)/E_n$. This expression of $G(r,r; E)$ ...
2
votes
1answer
24 views

What effects arises due to hybridisation of phonon branches?

The phonon dispersion is usually given as the vibration frequency on its momentum space. Usually, there are 3 acoustic phonon modes and 3(N-1) optical modes, where 'N' is the number of atoms in the ...
2
votes
1answer
38 views

Pion decay: why is the positron electron neutrino decay suppressed?

I'm reading through M. Thomson's Modern Particle Physics to better understand why the positron electron neutrino decay channel is suppressed relative to the muon and muon neutrino decay route. I know ...
0
votes
0answers
15 views

Could artificial atoms not based on quantum dots be created at a similiar size to natural atoms

I'm aware of current pace of quantum dot technology so I am aware they are not close to the smaller size of natural atoms of known elements so I wondered if it's theoretically possible to achieve this ...
-8
votes
0answers
56 views

Meaning of constant $c$ [on hold]

I have a puzzlement over Einstein's expression $E = mc^2$. The constant expressed as $c$ is the measured speed at which Light crosses a vacuuum. BUT it is also used to denote the maximum possible ...

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