24
votes
2answers
2k views

How light speeds up after coming out of a glass slab?

As I learned today in School, my teacher told me that when light enters a Glass Slab it slows down due to the change in density and it speeds up as it goes out of the Glass Slab. This causes a lateral ...
2
votes
0answers
45 views

Time ordering, interaction Lagrangian calculation, QED

I am trying to compute $$ \langle 0| \, T\left\{\phi^\dagger(x_1) \phi(x_2) \exp \left[i \! \int{L_1(x) \, \mathrm{d}x} \right] \right\}|0 \rangle $$ for $$ L_1(x) = ...
3
votes
3answers
341 views

What's the difference between “numerical methods” & “mathematical analysis” as said by Feynman in his lectures?

While reading his lectures, I came to these lines: On the basis of Newton's second law of motion,which gives the relation between the acceleration of any body & the force acting on it,any ...
2
votes
2answers
57 views

Can string theory get rid of randomness in quantum processes?

I am not a physicist, but I am very much into popular science, especially string theory. I would like to know if it is conceivable that string theory might be able to get rid of the randomness ...
1
vote
0answers
7 views

Explain materials with 4 fold symmetry having same reflectance when shone with LCP and RCP

This is my first post here. I am currently reading "Optical planar chiral metamaterial designs for strong circular dichroism and polarization rotation" by Do-Hoon Kwon, Pingjuan L. Werner, and ...
3
votes
1answer
90 views

What does Weinberg–Witten theorem want to express?

Weinberg-Witten theorem states that massless particles (either composite or elementary) with spin $j > 1/2$ cannot carry a Lorentz-covariant current, while massless particles with spin $j > 1$ ...
0
votes
0answers
22 views

Fast and slow modes in renormalization group of nonlinear sigma model

A general nonlinear sigma model can be expressed as \begin{equation} S[g] = \frac{1}{\lambda} \int d^dr\ \text{tr}[\triangledown g\triangledown g^{-1}] \end{equation} where $g$ takes value in a matrix ...
0
votes
1answer
20 views

Does doping silicon affect its coefficient of thermal expansion (CTE)?

Everywhere I have searched I always see the CTE of silicon listed as 2.6*10e-6 °C^-1. However, I have silicon that was phosphorus-doped to an ion concentration of of 2.41*10e11 atmos/cm^3, and I would ...
4
votes
1answer
59 views

How much pressure would be needed to contain a 1 gigaton nuclear bomb explosion within a sphere of one meter radius?

How much pressure would be needed to contain the largest human exploitable nuclear bomb within a sphere of one radius? Also would it be possible to create a magnetic field that controlled some ...
0
votes
1answer
99 views

Angular momentum needn't always change in multiples of $\hbar$?

I read the following claim in Slichter's popular book, Principles of Magnetic Resonance (after Fig. 4.3, it's p100 in this version.). Despite the title, the author claims it in a quite general manner ...
1
vote
1answer
40 views

Practical Book on Hamiltonian and Lagrangians? [duplicate]

Are there any terse, accessible books that are geared specifically at learning these two formalisms and how to effectively use them? So far I've only see either topic introduced as a part of another ...
3
votes
4answers
212 views

Why do physics students find vectors so difficult to deal with? [on hold]

When I teach introductory physics to undergraduates, I find that although the classes are frequently split into "algebra-based" and "calculus-based" sections, the most difficult concept for any of ...
0
votes
1answer
25 views

How do I calculate the apparent brightness of a mirror reflecting the sun at a given distance?

I've stumbled around trying to figure this out, but it just isn't my area and I haven't gotten far. This is for writing about an idea of a field of mirrors on the moon, on the terminator when the moon ...
0
votes
0answers
21 views

Elastic scaterring of neutrinos - Why does the electron neutrino dominate this channel

Could anyone add more detail to this statement off wikipedia In the elastic scattering interaction, a neutrino collides with an atomic electron and imparts some of its energy to the electron. ...
0
votes
2answers
29 views

How are different wavelength components collected by the same detector in a spectrometer?

Let's think, we have a detector array (128x1 and each CMOS detector responds 400 to 1000 nm TSL1401CL that way, each detector has 4.6875 nm interval). Then, basically in a spectrometer a prism reflect ...
3
votes
1answer
110 views

About the nonlocality of QM and faster-than-light/backward-in-time machines

The fact the quantum mechanics is nonlocal is known already for a long time, since the Bell works (1966 and later) and the Aspect's group experiments confirming the Bell-type CHSH inequality (1980 ...
0
votes
3answers
69 views

In special relativity, do explanations involving clocks require that the clocks are ticking and that $c$ is fixed?

Are the explanations involving clocks only valid if the clocks are ticking when light hits? Is it true that these thought experiments experiment could only be valid due to the invariance of $c$?
0
votes
0answers
42 views

How is energy converted within the body? [on hold]

I guess that this question is going to be on hold but: how does the energy in food is transmitted to our cells and we gain energy? what is there in the food which is transmitted as the energy to our ...
3
votes
1answer
143 views

What is the average mass of galaxies according to Hubble Deep and Ultra Deep field observations?

It is very widely known among people interested in astronomy that there are 100-400 billion stars in the Milky Way galaxy and there are ~ 100 billion galaxies in the observable universe, which is ...
1
vote
4answers
222 views

Why drops form spheres?

Consider a drop of water floating in an inertial frame in STP air (e.g., the ISS). Intuitively, the equilibrium shape of the drop is a sphere. How would one prove that? Is it equivalent to showing ...
4
votes
1answer
42 views

What is the temperature of the clear night sky from the surface of Earth?

Before you all jump in with 2.73 K or thereabouts, this is more of an experimental question. It will obviously depend on humidity and radiation being scattered back towards the surface of the Earth. ...
4
votes
1answer
21 views

Why could the Homestake experiment only detect electron neutrinos

The Homestake experiment measured the incoming electron neutrino flux via $$\nu_{e}+ Cl^{37} \rightarrow Ar^{37} +e^{-}$$ Why does this reaction not apply to the other neutrino flavours? i.e. what ...
0
votes
1answer
24 views

Kittel solid state physics handbook - Plasma oscillation of a ball - Am I solving this right?

I'm self learning nanotechnology undergraduate and I'm trying to solve a problem from chapter "plasmons, polaritons and polarons". This is it: Frequency of uniform mode of plasmons in a ball is ...
-5
votes
3answers
48 views

Net force of the following? [on hold]

The following objects are attached to one another and have difference force directions: Where is the direction of force? And What is the net force? I'm trying to calculate it using their angles, ...
0
votes
1answer
21 views

When conserving angular momentum, about which point(s) should it be calculated?

In my physics problem I have a ball fired at a non fixed bar, which is moving at some velocity, causing them both to stick together and the bar-ball pair to rotate about the new center of mass. About ...
3
votes
1answer
34 views

Can the speed of a vehicle be determined based upon the extent of injuries to a pedestrian?

While driving at night in a Prius, at varying speeds of 5-20 MPH in heavy, stop-and-go traffic on a 4 lane city street, two pedestrians in dark clothing were struck (not in a crosswalk). One died of ...
1
vote
0answers
40 views

Can spin angular momentum be understood as orbital angular momentum in extra dimensions?

It is to my understanding that in Kaluza-Klein theories the mass of particles can be understood as linear momentum in the extra dimensions. Let's consider in $\mathbb{R}^{1,3}\times{}B$ space-time a ...
1
vote
1answer
98 views

Electromagnetism Boundary condition

The dielectric slab surrounded by air as shown in diagram has permittivity $\epsilon$ different from air and permeability almost same as air. Electric field everywhere within the slab is given to be ...
6
votes
1answer
52 views

Cosmic rays and neutrinos

I'm reading Gaggero's Cosmic Ray Diffusion in the Galaxy and Diffuse Gamma Emission and he makes the claim, ...the definitive proof of [Cosmic Ray proton acceleration in supernova remnants] would ...
0
votes
0answers
37 views

Experimental physics book suited for a math student [on hold]

I am currently a Master's student in Mathematics who is very much interested to learn advanced physics (QFT, Particle Physics, GR,Cosmology). The main focus of my undergraduate programme was on ...
-1
votes
1answer
46 views

Norm of summation of vectors

If we have a vector $\partial_v$ and we want o find its norm, we easily say (According to the given metric) that the norm of that vector is:$ g^{vv}\partial_v\partial_v$. My question what if we have ...
2
votes
1answer
36 views

Can asymmetrical Lorentz forces account for Relativistic affects near the speed of light?

The underlying thought here is that at low relativistic speeds all objects are subjected to emf radiation from all directions. This is basically the sum of all the radiation (light, infra-red, ...
0
votes
1answer
64 views

If I take a bottle of air into space, and open it, then close it and come back to earth and open it one last time, what would happen?

I was reading this post, but no one seem to go further as say what would happen if you brought the bottle back to Earth? Guess what another thing I'm wanting got know is, what would now be in the ...
0
votes
1answer
59 views

Can Einstein's 'Theory of General Relativity' exist in Harmony with 'Quantum Mechanics'? [on hold]

From the Book 'In Search of Schrodinger's Cat': Coordinates in space-time represent position; causality depends on knowing precisely where things are going, essentially on knowing their momentum. ...
1
vote
2answers
79 views

What is the point of the reduced Planck's constant $\hbar$ (h-bar)? - Why don't we just have Planck's Constant $h$?

I know that $ħ$ is $h / 2π$ - and that $h$ is the Planck Constant ($6.62606957 × 10^{-34}$ $Js$). But why don't we just use $h$ - is it that $ħ$ is used in Angular Momentum Calculations?
12
votes
3answers
837 views

What is the most compelling evidence of General Relativity in the presence of matter and energy?

The most oft-cited triumphs of GR are things such as the shifting perihelion of Mercury, gravitational redshift experiments, and gravitational lensing. But, as far as I know, these are only ...
1
vote
0answers
27 views

How Do we Know How Old the Universe Is? [duplicate]

It is common knowledge that the universe is 13.8 ± 0.037 billion years - but how was this calculated?
1
vote
0answers
15 views

Will Ice Cubes Form Quicker when Made from Hot Water or Cold Water? [duplicate]

When you put water in the freezer you can make Ice Cubes. But does the time taken for these Ice Cubes to form decrease or increase when the water which is used is hotter or colder?
0
votes
1answer
29 views

Why is there not much research into nuclear physics with plasmas?

There is a lot of research and theory around fusion reactions with plasma, but is there a reason why plasmas cannot be used for researching other nuclear reactions, either in the lab or from ...
2
votes
1answer
47 views

Does irregular reflection form images?

In one of my test papers at school, we had a true-false question which said "Irregular reflection can form an image." I marked it true, however I was not given any marks for the question. I asked my ...
0
votes
0answers
17 views

What does a completely negative Greens function in frequency mean?

What can a Greens function of frequency mean when it is always negative? The Greens function is for the photons as the following: (It's derived by Matsubara method to enter the thermal effects and the ...
0
votes
1answer
31 views

Gravitation - hole to center of Earth [on hold]

Question: Assume that a tunnel is dug across the Earth (radius = $R$) passing through its centre. Find the time taken for a particle to reach the centre if it is projected into the tunnel from the ...
0
votes
0answers
36 views

Viscosity calculation of a rarefied gas

I am studying the rotational-translational relaxation of a diatomic gas (like oxygen) using a GPU in order to accelerate the calculations; during the calculations I get the translational temperature, ...
14
votes
2answers
625 views

Symbols of derivatives

What is the exact use of the symbols $\partial$, $\delta$ and $\mathrm{d}$ in derivatives in physics? How are they different and when are they used? It would be nice to get that settled once and for ...
1
vote
1answer
56 views

Prove the relation between relative velocity ,momentum and energy

In Chapter 8 of F.Mandl's book Quantum field theory, during the derivation of the differential cross section, the following relation is used: $$E_1E_2v_{rel}=\sqrt{(p_1p_2)^2-m_1^2m_2^2} \,\, ,$$ ...
1
vote
1answer
34 views

Are superoperators (CPTPM) equal if they are equal on all density operators?

$\DeclareMathOperator\tr{tr} $Is the following statement true? Conjecture: Let $\cal E_1,\cal E_2$ be completely positive trace-preserving maps (quantum superoperators). Assume that for any positive ...
0
votes
1answer
35 views

A falling steel beam - Maximum induced voltage [on hold]

I'd appreciate it if someone could explain how to answer this question: A 2 m long steel beam is falling from a height of 12,5 m. During the fall, the beam is oriented in an east-west direction. ...
-1
votes
0answers
24 views

Goldstino wave function

I'm a novice in SUSY and I'v got a question concerning spontaneous supersymmetry breaking and goldstinos. In Martin's review on page 68 there is a proof of a statement about existence of massless ...
1
vote
1answer
61 views

Amplitude $\langle0|e^{-iHT}|0\rangle$ in A. Zee's QFT In A Nutshell

In his Quantum Field Theory In a Nutshell, in page 12, (Second Ed), A Zee says that conventionally, the amplitude $\langle0|e^{-iHT}|0\rangle$ is denoted by $Z$. In the next paragraph, he considers ...
2
votes
2answers
42 views

Another condition of calculating work

Let's imagine that there is a box placed at the corner of a table, and I push it so that my applied force makes an angle of 30° from the table's surface. The box would move and, due to the effect ...

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