# All Questions

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 ...
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 ...
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 ...
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 ...
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?
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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: ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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?
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. ...
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 ...
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? ...
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 ...
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 ...
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 ...
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 ...
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
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 ...
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 ...
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 ...
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 ...
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 ...