# All Questions

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### The Quantization of Photon Energies [duplicate]

Despite Planck's constant being in $E=hf$, it would appear to me that energy is still not discrete, as frequency can be an fraction of a Hertz that one wants. How does this imply that electromagnetic ...
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### Laser divergence/convergence over large distance

I have hit a roadblock in my simulation, where there are 2 authors with contradicting equations on the model. So I have simulated both, and drew it out for a few diameters. My dish size is 40m. I am ...
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### Density of Air in a balloon

A balloon with volume 2800 m³ is heated up to 60 °C. If the outer temperature is 12 °C and the air has a pressure of 960 hPa, what is the density of the air inside the balloon and on the ...
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### In famous Einsteins Photoelectric effect, Why does intensity of light doesn't raise the kinetic energy of the emitting electrons?

The work function of any metal is no doubt constant for it is related to electromagnetic attraction between electrons and protons. However on increasing the intensity of any light source the kinetic ...
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### How much physics a mathematician needs to know to study GR? [duplicate]

I'm intending to study General Relativity on my own. The thing is, my physics background is not very strong. I know classical mechanics and I know some electromagnetism. I'm familiar with Gauss' law, ...
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### Does a magnetically suspended frog feel weightless?

Magnetic levitation has been used to suspend frogs in midair. I was wondering: Does the animal still feel gravitational pull? I mean: Does the frog feel like an astronaut on the ISS, or does he feel ...
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### How would we see a near-lightspeed object emitting light?

Consider an object travelling near the speed of light relative to us (let it be a spaceship or a star), which is emitting light (consider it monochromatic resulting from a two level electronic ...
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### Decomposition of representations of the Virasoro algebra under $sl(2)$

The Virasoro algebra has a finite $sl(2)$ sub-algebra generated by $L_{-1}$, $L_0$ and $L_{+1}$. Let's consider a unitary highest weight representation of the Virasoro algebra with conformal weight ...
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### Moving electron - finding the wavefunction

On our modern physics class my professor did a problem: Write down a wavefunction of an electron which is moving from left to right and has an energy $100\text{ eV}$. At first i said: "Oh i know ...
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### Fermi's golden rule and Probabilities in QM

In Fermi's golden rule $$P_{ab}(t)=2\pi t/\hbar \left|\langle\psi_b|V|\psi_a\rangle\right|^2 \delta(E_f-E_i)$$ for transition probability from state $a$ to $b$, how can the probability grow with ...
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### Kinematics problem

I'm on earth and we have two spaceships $A,B$. They can instantaneously accelerate to certain velocities $v_A,v_B$. We want to travel to another planet which is $\Delta x$ lightseconds away, and we ...
588 views

### Why does the wind direction vary locally?

If the air is flowing with the earth, as it rotates, how do we feel a breeze and strong winds at times, as everyone has experienced, the wind shifts as if from many directions.
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### Diagonalization of a hamiltonian for a quantum wire with proximity-induced superconductivity

I'm trying to diagonalize the Hamiltonian for a 1D wire with proximity-induced superconductivity. In the case without a superconductor it's all fine. However, with a superconductor I don't get the ...
766 views

### QM - calculating expectation value for velocity of an electron

How do we calculate the expectation value for speed? I have heard that we must first calculate the expectaion value for kinetic energy. Someone please explain a bit what options do we have.
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### Center of Mass of two planets

The formula for calculating the center of mass is $$r_{center} = \frac{m_1 \cdot r_1 + m_2 \cdot r_2}{m_1+m_2}$$ Why can't I use it to calculate the barycentre of two planets? I understand how ...
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### Polarisation of Light and Atomic Excitation

How does an atomic transition between ground and excited states depend upon the direction of polarisation of incident light?
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### Deriving entanglement entropy from Renyi entropy

My questions are based on this paper - http://arxiv.org/abs/0905.4013 Firstly I want to know as to whether some assumptions are needed about the relationship between the systems $A$ and $B$ for the ...
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A wavefunction is inherently a multi-particle function. If you have a container that is perfectly isolated from the external universe (not possible, but just imagine it) and filled with $n$ ...
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### Energy-Momentum Tensor in Conformal Field Theory

Basically, I would really like it if somebody just explained to me what is going on here. Please use any physics lingo you feel is necessary, but explain what you mean. I am just having trouble ...
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### Running of gauge couplings in the Standard Model [closed]

I'm sure many of us are familiar with the following plot showing the running of the inverse of the fine-structure constants of the SM. (I got the picture from google) At one-loop, the expressions ...
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### Symmetry factor of a second order four point function term of the $\phi^4$ theory

I am reading Cheng and Li. On page 9, it is written that the coefficient $\frac{1}{2 \cdot (4!)^2}$ for the second order term of the four point function becomes just $\frac{1}{2}$ for the following ...
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### Are there solid materials with controllable porosity?

In analogy to piezoelectric materials, where the application of an electrical field creates mechanical deformation in the material, I have the following question. Are there solid materials whose ...
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### How much Energy to create a 'warp field' according to White?

An Alcubierre Drive is a hypothetical device that can move a place someplace else faster than the speed of light without violating known laws of physics. This paper provides some equations as to the ...
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### What type of magnetic fields does a Hall effect semi-conductor pick up on?

What type of magnetic fields does a Hall effect semi-conductor pick up on? AC or DC fields? How would one go about building a device that measures AC Magnetic fields?
342 views

### What is wrong with this downforce calculcation?

I tried calculating the downforce of a spoiler I need for a robot. I used this formula. $$F = \frac{1}{2}Ac_a\rho v^2$$ Where $\rho$ is air density, $v$ is the speed of my robot, $c_a$ is the lift ...
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### What generates such a huge amount of gravitational attraction in a black hole?

I had been fascinated to hear that anything can be swallowed by a black hole. But what causes such a huge amount of gravitational force?
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### Finite potential well problem - calculating the ground state

1. The problem statement, all variables and given/known data Electron of is in a 1-D potential well of depth $20eV$ width $d=0.2 nm$ in his ground state $N=1$. What is the energy of the ground ...
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### What do you call the period after sunrise when the sky is bright?

At sunrise, the sky isn't actually up in the sky yet. Twilight occurs before sunrise, then at sunrise the leading part of the sun crosses the horizon. But, the sky isn't bright yet. It takes some time ...
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### Measuring a quantity using two measuring devices

I would like to ask how I can reduce uncertainty by designing an experiment. I need to measure a quantity( say speed, in m/s). I have to calibrate the device and then move the device to measure ...
1k views

### Projectile Motion Question involving a ball and a ramp inclined at an angle

The question is to finde the initial horizontal velocity of the ball at end of the ramp, where it is released. I know how to do this using gravitational potential energy and kinetic energy ...
147 views

### Colors of light

White light is always said to contain all the different wavelengths of light. Why, then, can we 'make' new colors simply by adding wavelengths? Is it just a matter of our perception, that, when two ...
1k views

### Born's Rule, What is the Reason? [duplicate]

As far as I've read online, there isn't a good explanation for the Born Rule. Is this the case? Why does taking the square of the wave function give you the Probability? Naturally it removes negatives ...
235 views

### Wave packets Group Velocity

For the the group velocity of a wave packet, the group velocity is the partial derivative of omega with respect to the wavenumber, what does this mean? I thought that for some given wave packet both ...
4k views

### Physical difference between Lorenz and Coulomb gauge-fixing conditions

Lorenz and Coulomb gauge-fixing conditions. What is physical difference between these two gauge-fixing conditions? Mathematical expression are clear but how to we choose one of these means what they ...
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### How to introduce the electromagnetic field in Quantum Field Theory?

There are many ways to introduce the electromagnrtic field in Quantum Field Theory(QFT), such as canonical quantization method which introduces the creation and annihilation oprators by treating the ...
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### Church-Turing hypothesis as a fundamental law of physics

The Church-Turing hypothesis says one can not build a computing device which has more computing power (in terms of computability) than the abstract model of Turing machine. So, there is something in ...