# All Questions

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### Gravitational PE lost v Elastic PE gained in mass - spring

If a spring has a load $m$ added to it (and so is extended by $x$), the gravitational potential energy lost by the mass will be $mgx$. The elastic potential energy gained by the spring is ...
1k views

### What really is a Dirac delta function?

Yesterday a friend asked me what a Dirac delta function really is. I tried to explain it but eventually confused myself. It seems that a Dirac delta is defined as a function that satisfies these ...
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### What determines the Bloch wavepacket length scale?

We know in the semiclassical Boltzmann theory, there are several length scales: the lattice constant $a$, the Bloch wavepacket spread $\xi$, other length scales such as the mean free path $l$, the ...
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### Could there be ways to form hypergiant stars, quasars or supermassive black holes at the beginning of the Dark Ages?

Original title: Heat loss in collapsing parcels of gas or weak plasma at the beginning (not the end) of the cosmological dark ages Star formation from collapsing parcels of primordial gas in the late ...
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### Photon gas, recovering state function from empirical result

Given that for a photon gas the following is true: $$U=3pV$$ $$\mu=0$$ I want to recover a state function $U(S,V,N)$ (or $S(U,V,N)$ or $F(T,V,N)$ etc.) in the most direct, simple way. So far I got ...
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### Would the detection of advanced waves be possible in curved space-time?

In the transactional interpretation of quantum mechanics reference 1, inspired by the Wheeler–Feynman absorber theory, a transaction is formed between the emitter and absorber by a superposition of ...
185 views

### Why is the Spin of the photon neglected?

We know photons have spin s=1. However, in Nuclear physics, the conservation of angular momentum in case of Gamma transitions is employed as follows: $$\vec J_i=\vec J_f+\vec L$$ where $J_i$ is the ...
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### What is the time evolution operator in quantum mechanics [duplicate]

I'm curious about what happen to a system when the configuration of the system changes. If we have a system in a state $|\psi_{\textrm{in}}\rangle$ and we change the configuration of the system, the ...
159 views

### Is it possible to mix a luminescent drink? [closed]

Is it possible to create a drink that glows? In other words, is it possible to create a non-poisonous drink that fluoresces? Are there known compounds suitable for that? If not, then what about a ...
125 views

### Why not a magnetic field local maximum?

Reading a brief on magnetic traps I've read that you can't have a magnetic field local maximum. I believe this is linked to the divergence of B being 0 in vacuum, but I don't see why one can have a ...
320 views

### Conservation of momentum in a baseball

Conservation of momentum: A thought experiment. A baseball is placed on top of a baseball holder, the kind used to train young batters. A batter hits the stationary ball perfectly horizontal, sending ...
130 views

### Does orbital angular mometum has no meaning for single photons?

In the quantization of free electromagnetic field, it is found that the left-circularly polarised photons corrsponds to helicity $\vec{S}\cdot\hat p=+\hbar$ and right-circularly polarised photons ...
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### objects and particles react to wave

Is it possible for a wave to be have a wavelength so small that it doesn't interact/affect an object? Like if it is smaller than the atoms itself will it still affect them? I am talking about a wave ...
136 views

### Inaccuracy at measuring gravity constant with Cavendish experiment

For a scientific work for school I decided to measure the gravity constant with the Cavendish experiment. I set up a structure like the one suggested on this website: ...
83 views

### Could this be a valid reason for why the universe could be infinite?

So, special relativity says that every frame is as good as any other frame, and there is no absolute frame of reference. All good. Suppose there is a finite universe (let's just assume the universe ...
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### Quantum entanglement correlations [closed]

I have a vague idea of what entanglement is but I am stuck at one place: we all know that entanglement is destroyed with time. this is called decoherence. I wish to know if there is any correlation ...
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### How to explain the upgrade from Particles to Fields between Relativistic QM->QFT?

It is strange that all books I walked through, non of them explains or motivates how physicists realised that we need to deal with fields instead of particles. Maybe the closest thing I found is the ...
39 views

### Is there a current in this situation?

If a wire moves through a magnetic field a current is induced in the wire. If you drop a piece of wire through a magnetic field, the wire is of a finite length and the whole length is shorter than ...
81 views

### Higgs mechanism in 1+1 spacetime dimensions

I know there are problems with Goldstone's theorem, as stated by Coleman in http://projecteuclid.org/download/pdf_1/euclid.cmp/1103859034. What about Higgs mechanism? Can I possibly think of a ...
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### Multiple Definition For Gravitational Potential Energy?

This may just be a simple Misconception Question, here goes: Definition for Gravitational Potential Energy: The work done by gravity to pull an object to the ground. ...
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### Usefulness in microstates that belong to the same macrostate?

The number of microstates can be huge, numbers with millions of digits, but they can all represent the same macrostate. IS there any information that can be associated with each microstate ...
690 views

### How can length contraction result in electron circular motion in a magnetic field?

If you ask around about magnetic fields, you will read seemingly-authoritative articles which say magnetism is a consequence of length contraction. This is widely taught and is repeated in answers ...
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### Penetration depth of thermal IR into building and insulation materials

What are the values of the penetration depth (or the attenuation coefficient) of thermal infrared radiation (say, 5-20 $\mu$m wavelength) into common building and insulation materials (brick, stone, ...
13 views

### Tuning the frequency in graphene

As is well known, the frequency of transverse optical phonons in single layer graphene ranges from $10^{11}$ to $10^{12}$ Hertz. How can one "tune" the frequency to a specific value?
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### Alternate Formulation of Kinetic Energy for Lagrangian

So I have a question regarding this system: It is supposed to be a simple model of an aircraft with the fuselage idealized as a concentrated mass $M_0$ and the wings modeled as rigid bars carrying ...
44 views

### Why do we need any model beyond SM to fine-tune the Higgs mass?

ATLAS and CMS combined their results recently and it shows that higgs mass is ~125 GeV. Theoretical physicists try to figure out the "low mass" of the Higgs which causes hierarchy problem at high ...
385 views

### The uncertainty in angular momentum

It is known that the different spatial components of the angular momentum don't commute with each other. $$[L_x,L_y] \propto L_z \\ [L_y,L_z] \propto L_x \\ [L_z,L_x] \propto L_y$$ Also it is known ...
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### Are we looking at single stars, or solar systems?

When looking at the night sky, the stars we can see just look like single dots. Are they alone in space- each one a single object? Or, when we look at a star, are we actually looking at a distant ...
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### In perturbation theory, how do I determine the order of an approximation?

The title says it all: I'm confused about the various approximations and their orders. In time-independent perturbation everything is quite explicit and obvious, but, for example, how would it be with ...
93 views

### Must a universal motor always rotate at the frequency of the AC current?

So here I have a picture of a motor (I think it is a universal motor but I am not sure). My question is if this motor is connected to an A.C. supply, does it have to rotate at the frequency of the ...
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### How do we calculate the adjustment factor for hyperbolic secant pulse for calculation of peak power in ultrashort laser pulses?

In the formula for peak power it is equal to total energy divided by FWHM pulse width multiplied by an adjustment factor 'a' the value of which depends on the shape of the pulse. For hyperbolic secant ...
119 views

### Calculating Reynolds number of ellipse in fluid

I know Reynolds number is calculated from the formula by including length or diameter of an object in fluid. I have a case where there is ellipse with 2a major and 2b axis. I hv learnt when the ...
65 views

### Why does the discussion in a certain reference frame gives us the correct physics?

The question: whenever you are dealing with quantum optics you will encounter a lot of unitary transformations very soon. A question that is in my mind for quite a long time is: Why does the ...
156 views

### Path of a light ray in a medium of varying refractive index [closed]

How do we get the equation for the path of a light ray in a medium of varying refractive index. I can draw a rough sketch, but don't know how do we get the exact curve. I encountered this while ...
236 views

### Force between two charges when the dielectric constant of the material varies?

If two point charges $q_{1}$ and $q_{2}$ are seperated by a distance $l$ apart, and the space between them is filled with a variable dielectric constant. Near $q_{1}$, the dielectric constant is ...
374 views

### Pulley system on a frictionless cart

Let's say you have a pulley set up as below on a cart, with a massless pulley and string. The mass hanging off the side is attached via a rail, and all surfaces & pulley are frictionless except ...
29 views

### What is the difference between hydraulic radius and geometrical radius in a cylindrical pore?

What is the difference between hydraulic radius and geometrical radius in a cylindrical pore? I was going through the 'stacks' present in an thermoacoustic refrigerators there I came across the ...
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### What is the specific source(s) of sliding kinetic friction

In simplistic (K-8) physics classes, it seems to be generally instructed that the friction between two moving surfaces is due to the unevenness of each surface and the microscopic roughness. However, ...
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### How do amorphous objects emit blackbody/thermal radiation? [duplicate]

How do amorphous objects emit blackbody/thermal radiation when such objects don't have optical phonons?
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### Does a crucible need to be fired in a kiln before use?

Say I made a crucible out of aluminum oxide primarily. Won't it be fired by normal use of it or do you have to pre-fire it? I understand they do that to get a consistent mass measurement. But does ...
56 views

### Can you predict the path of a particle headed for 2 slits?

In the Double Slit experiment, particles are shot at 2 slits. We know that the particles will show a striped pattern on the screen if unobserved, but show up directly behind the slits if observed. I ...
33 views

### Why is wave-number $k$ taken real & not complex when dealing with infinite line of atoms in a lattice?

This is the following excerpt from 13–2States of definite energy where Feynman tells why $k$ must be real: [...] Notice that we have been assuming that the number $k$ that we put in our trial ...
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### The confusion regarding friction [duplicate]

I have read that friction is caused due to electrical force of attraction between the two surfaces. Rougher the surface, the number of contact points is less. Therefore I think on smoothing the ...
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### Would it be possible to have a planetary system where one planet is in continuous shade?

When I ask this I'm thinking of a system similar to ours, but the orbits are perfectly staggered and all planets involved have orbital periods such that when the planet (or small moon, whichever works ...
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### Can tidal forces significantly alter the orbits of satellites?

I would assume that there are other larger, more significant, forces acting on artificial satellites, but can tidal forces drastically alter the orbit of a satellite over time? I was thinking this ...