0
votes
0answers
6 views

Relativity of simultaneity example in Resnick

My question is a follow up to this question about simultaneity. I would have posted it as a comment to the replies for that question, but I wasn't allowed to. When Resnick introduces relativity of ...
1
vote
0answers
11 views

Which cosmological dark matter theory is most compatible with current particle physics?

This occurred to me while I was writing Why hasn't warm dark matter replaced cold dark matter as the standard model of cosmology? but I think this deserves a question of its own. The background ...
2
votes
0answers
14 views

Why hasn't warm dark matter replaced cold dark matter as the standard model of cosmology?

The $\Lambda\rm CDM$ (cold dark matter with cosmological constant) is the current standard model of cosmology because the model comes with a long list of phenomena successfully explained by it. ...
1
vote
0answers
27 views

Sufficient conditions for the energy to be not conserved?

I'm almost embarrased to ask this question because I thought I was by now very confident with classical mechanics. Someone has stated that given a mechanical system with a Lagrangian $L$ s.t. ...
0
votes
4answers
45 views

If the velocity vector of a moving particle is always perpendicular to the position vector, is the path a circle?

A Newtonian physics question: If the velocity vector of a moving particle is always perpendicular to the position vector, is the only possible path a circle? What if the magnitude of the velocity ...
0
votes
1answer
17 views

Thermal Velocity

What is thermal velocity? What is it's physical significance? Wikipedia says: The thermal velocity or thermal speed is a typical velocity of the thermal motion of particles which make up a gas, ...
0
votes
1answer
17 views

Yang Mills theory and SU(N) groups

Trying to get a better understanding of the relation between a SU(N) Yang Mill theory and its number of "color" space. Most of the description I've found so far are either way to complex/specific. ...
0
votes
0answers
8 views

How to calculate average temperature of Nichrome 80 resistance wire?

I know the following: I need the resistance wire to hold a temperature of 400C with a steady current The resistance wire is 1cm in length I have a 1500 mAh battery with 3.2V available What gauge ...
1
vote
0answers
47 views

Recent missed opportunities à la Freeman Dyson

There is an excellent paper by Freeman Dyson from 1972 (here) and therein the author cites old talks by Hilbert (here) and Minkowski (chapter 2 here) speaking about similar topics, namely how ...
0
votes
1answer
24 views

Would someone versed in relative motion consider it more accurate to say that a car slammed into, or collided with, a wall?

I've read that it's just as correct to think of A moving toward B (ie. A doesn't move, but B does) as it is to think about B as moving toward A (again, A doesn't move, but B does) or as the two moving ...
0
votes
0answers
21 views

Can observables with discrete and continous eigenvalues be commuting?

In Dirac's The Principles of Quantum Mechanics he often uses the notion of "a complete set of commuting observables". This means a set of observables $\{\xi_1, \xi_2, \ldots, \xi_v, \xi_{v+1}, \ldots, ...
0
votes
0answers
34 views

Could Particles Really Spin in the Really Real World? The video in the link says they do

In the video bellow the presenter indicates that particles really do spin. He also claims to calculate the energy release from a U 235 fission reaction. Could this be real? ...
0
votes
0answers
13 views

Why infrared absorption is a nonlinear technique?

I am looking for a good explanation and/or reference quotation explaining why infrared absorption technique is essentially nonlinear (eg. for carbon monoxide quantification). When using ...
9
votes
1answer
299 views

Why don't the black holes appear black in color in images of galaxies taken from HST?

According to NASA a black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area According to the documentary Space Unraveling The Cosmos about ...
0
votes
1answer
16 views

Flow rate is calculated only using the parallel component of the velocity vector

Flow rate is calculated using only the parallel component of the velocity vector to the area vector. Why is this? How can I mathematically prove this? Namely, how do I prove any perpendicular ...
0
votes
0answers
16 views

Finding the stabilizer group given a state

Consider general pure state $|\psi\rangle$ in some hilbert space $\mathcal{H}$ (which could be a tensor product of other Hilbert spaces) I would like to know whether there is a way to ...
2
votes
1answer
31 views

How fast do molecules move in objects?

I guess it depends on the heat or the type of the material but can you give some examples or formulas to calculate it ? The best example would be the average speed of the air molecules (all types in ...
0
votes
0answers
18 views

Mathematical derivation of interference pattern for electrons?

One of the most famous experiments in quantum mechanics in the context of wave-particle duality is certainly passing a beam of electrons through two slits, which results in an interference pattern ...
0
votes
0answers
37 views

Philosophical thoughts on Reconciling Different Physical Theories—Example of Entropy

I always like to reason through the motivation behind equations, and during this process I began thinking about Wigner's 'unreasonable effectiveness of mathematics', and whether it can sometimes lead ...
0
votes
1answer
26 views

(Level: Undergrad) Continuity Conditions on the Wavefunction and Initial Values

I know that a physically meaningful $\Psi$ needs to be continuous. However, recently I came across a problem in which they were considering a wavefunction for the infinite square well potential and ...
3
votes
1answer
57 views

If I'm running away from chasing gamma photons, I can see them, right?

Suppose Bruce Banner goes back in time, convinced that the Hulk is a stupid menace. Let's say he also borrows Quicksilver's speed abilities (maybe the Flash is more appropriate, but based on the ...
4
votes
1answer
36 views

How do geocentric theories explain seasons?

Did the geocentric theorists also use the fact that the earth had an axis tilt? Or was their explanation of some other kind?
2
votes
1answer
22 views

2 entangled electrons in QFT

In field theory, by quantizing a dirac field, we can obtain a creation operator for a single electron of definite momentum, of definite spin up or down, these respectively are: ...
6
votes
2answers
283 views

How would wormhole-based FTL violate causality?

We already have an answer why physically traveling faster than light would violate causality (the clock on board our hypothetical FTL spaceship would tick backwards to some outside observers). ...
0
votes
2answers
21 views

If outside a cylindrical solenoid exist an electrical field what does that mean to the Aharonov-Bohm Effect?

To the question "What is the electric field outside a cylindrical solenoid when inside is turned on a magnetic field" the answer is that outside exists a electric field. Does that mean that the ...
1
vote
1answer
49 views

Can speed be defined in the complex plane?

This question cropped up while I was playing with the equation for time dilation. If I set the speed to be $i$ (imaginary unit) the answer from the equation still makes sense, but does that matter if ...
0
votes
0answers
13 views

What is the most general definition of a bosonic Gaussian state?

I am reading this paper where the definition of the bosonic state is mentioned on page 2 here :- http://arxiv.org/pdf/0806.1625.pdf . From a general definition of any density operator in terms of ...
0
votes
0answers
11 views

Where does $R/\gamma^3$ come from in relativistic E&M?

I am starting to read many papers on electron beam physics; more specifically its motion in a magnet i.e. curved motion. The term $R/\gamma^3$ comes up a lot, but I don't know where they are coming ...
1
vote
0answers
12 views

Quasicrystals - Projections from higher dimensional regular crystals [on hold]

Why are Quasicrystals Projections from higher dimensional regular crystals?
0
votes
0answers
4 views

Determining porosity in metals

I am in need of a book (preferably) or other reference material which would help technicians determine whether porosity exists in the fracture face of a tensile test bar for a variety of different ...
1
vote
1answer
51 views

Geomagnetic reversal?

I have always been told that the geomagnetic field acts "as if" there were a bar magnet inside the Earth. I now this is not true, but I feel that knowing what is actually going on would help me ...
0
votes
2answers
40 views

Can stimulated emitted photons be absorbed?

Typically a stimulated photon will be one of a pair with its stimulating photon. If the leading photon is absorbed by a particle in the ground state, will it then be re-emited by the stimulated ...
0
votes
1answer
7 views

All Andreev phenomena

Do You know any other Andreev connected phenomena apart from this listed below: Andreev reflection, Andreev bound states, Multiple Andreev reflection, Crossed Andreev reflection, Reflectionless ...
3
votes
1answer
27 views

Why do we need to prove the gauge invariance of QED (or all of the gauge theories) on the Feynman diagrams language?

Let's have the QED lagrangian. It has explicit gauge invariance, so, by the naive thinking, all of the EM processes must satisfy the property of gauge invariance. So why do we need to recheck of gauge ...
1
vote
0answers
22 views

Analytical solution of transient barometric formula for fluid in one dimension

Consider a column of fluid of length $L$, with initial density $\rho_0$ and initial velocity ($u_0 =0$) everywhere. Now at time $t=0$ gravity is switched on. Zero flux boundary conditions are assumed ...
4
votes
3answers
458 views

Calculate loudness of sound: why am I getting contradictory answers?

I know of events that are happening about 45 KM away from me which are said to be 210 or 213 dB at 75 meters distance from multiple sources. I think that I can hear them, so I did the obvious: $$ ...
0
votes
2answers
25 views

What are experimental techniques to probe band structure of solids?

What are experimental techniques to probe band structure of solids? I am not looking for exhaustive list. Few simple to understand techniques will suffice.
0
votes
2answers
29 views

Straight line on 8-axis [on hold]

How would a straight line appear in a 8-axis co-ordinate system. Will it continue to look like a straight line or will it appear like zig-zag line. We can see that straight line appears like a dot in ...
3
votes
5answers
193 views

About the definition of expectation value in quantum mechanics

In quantum mechanics, the expectation value of a observable $A$ is defined as $$\int\Psi^*\hat A\Psi$$ But in probability theory the expectation is a property of a random variable, with respect to a ...
4
votes
1answer
51 views

AM Receiver and sideband frequencies

How sorry for "dummy" question. I have difficulty to understand how the AM receiver. Being a child I was taught that AM receiver resonates only with the radio frequencies it is tuned to, and "passes" ...
3
votes
3answers
82 views

How to understand the definition of vector and tensor? [duplicate]

Physics texts like to define vector as something that transform like a vector and tensor as something that transform like a tensor, which is different from the definition in math books. I am having ...
0
votes
0answers
30 views

Research Topics In Theoretical Physics [on hold]

Could someone with a broader overview of the subject, and with more academic experience list research topics in Theoretical Physics? I'm a third year undergrad thinking about future research. I don't ...
0
votes
0answers
15 views

Trouble understanding phase matching equation

consider a 2nd order non linear optical material, i.e. a material in which it holds that $ P = \epsilon_0 E + 2dE^2$. In the Born approximation, the non linear contribution to the polarization density ...
0
votes
1answer
52 views

Showing Dirac equation's Lorentz invariance and use of unitary matrix $U$

Dirac equation is $i \hbar \gamma^\mu \partial_\mu \psi - m c \psi = 0 $ To show its Lorentz invariance, we convert spacetime into $x'$ and $t'$ from $x$ and $t$ and then $( iU^\dagger \gamma^\mu ...
0
votes
0answers
14 views

Is there some other name used for “ping rigidity”?

In MTW, p. 398, "Box 16.4 (continued)", there's an interesting sketch (which can also be seen on p. 15 of this excerpt (www.pma.caltech.edu/~ph236/yr2008/readings/MTW_Chapter16.pdf). (It's not the ...
1
vote
0answers
38 views

Defining axes for conservation of energy [on hold]

Problem: A bungee-cord jumper is on a bridge that is $y_1$ above water level $y_2$. The bungee cord obeys Hooke's law with spring constant $k$ and has a relaxed length $L$ The jumper stops before ...
2
votes
4answers
50 views

Is work done by how much velocity is changed or how much displacement is done?

A sailboat is moving at a constant velocity. Is work being done by a net external force acting on the boat? The answer key is "No" according to the work energy theorem about work is done when ...
-1
votes
0answers
18 views

Solve the given differential equation by using Green's function method [migrated]

I am really struggling with the concept and handling of the Green's function. I have to solve the given differential equation using Green's function method $$\frac{d^{2}y}{dx^{2}}+k^{2}y=\delta ...
0
votes
1answer
76 views

Quantum states and state vectors

Does a state vector correspond to only one quantum states and the components in the state vector correspond to different states of this quantum state or is it that the components of the state vector ...
0
votes
0answers
11 views

How much inductance and capacitance generate in copper strip? [on hold]

I have 60m long copper strip and cross-section area of 25mm x 3mm. and also it has some 90 degree turn after 10m distance. So, How much inductance and capacitance will be generated in strip.? and we ...

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