All Questions

0
votes
1answer
37 views

Does a black hole have thrust? Could they be acting as galactic engines? [on hold]

I identify four possible kinds of thrust below (there may be more or less). I don't know how to do the math to test them though. Direct transfer of kinetic energy as thrust. The energy of objects ...
0
votes
0answers
40 views

What is the co-moving distance to a point, at which the late-time acceleration of the universe began?

First of all sorry for my English - it is not my native language. According to the following wonderful diagram made by Pulsar, the late-time acceleration of the universe began at $t_{acc}=$ 7.7 ...
0
votes
0answers
48 views

Relativity and Quantum Mechanics

I have been thinking about the problem of relativistic path integrals and I encountered several difficulties. Let's assume we have a particle initially a position $x_i$ at $t_i$ in a certain reference ...
-1
votes
0answers
12 views

Direction of magnetic field of current carrying arc

To find the direction of the magnetic field, for example if we have the following scenario: When the current moves around a circular arc, how do we determine the magnetic field direction of this ...
-1
votes
1answer
55 views

Where does the centripetal force come from? How to calculate?

When a bucket filled with water rotates, the water surface will be parabolic. Where does the centripetal force of a particle of water come from? From horizontal thrust? Or does it come from the water ...
2
votes
3answers
1k views

Can the electrostatic force be infinite in magnitude?

The magnitude of the electrostatic force between two charges $Q$ and $q$ separated by a distance $r$ is given by $$F=\frac{kqQ}{r^2}$$ but the minimum value of $r$ must be $10^{-15}\ \rm m$. Therefore,...
-2
votes
0answers
24 views

Induced Electric Field Due To An Infinite Wire

Consider an infinite wire of negligibe thickness. It carries a current I which increases with time. Let dI/dt=c=a positive constant. Will there be an induced electric field around it in space? ...
0
votes
0answers
47 views

Deriving the Pauli-Schrödinger equation from the Dirac equation

Since the Schrödinger Pauli equation describes a non-relativistic spin ½ particle. This equation must be an approximation of the Dirac equation in an electromagnetic field. I was trying to derive this ...
0
votes
0answers
20 views

Compute a model, based on the satellite velocity at 300 km orbit

I wanna test a special sensor which will be the payload of a satellite. For this, I need to fix a camera (with the sensor inside) at a distance h from a rotative cylinder (radius: r) to model a ...
0
votes
1answer
20 views

Binding Energy and Energy Released

Let's say that we have a nucleus with 7 units of mass. And let's say that the combined mass of its constituents is 10 units. The 3 units contribute toward the "binding energy." However, we do know ...
2
votes
1answer
23 views

Necessary photon energy for electronic excitation

What happens if a photon incident on an atom has greater energy than the difference of energies between successive energy levels corresponding to electronic excitation(say between ground state and ...
0
votes
0answers
26 views

Is the warping of spacetime proportional to the mass/energy/momentum of an object in GR [duplicate]

Just wondering if the warping of spacetime proportional to the mass/energy/momentum of an object in general relativity?
-1
votes
1answer
56 views

Why is there a factor of $\sqrt2$ listed for the neutral pion?

I was reading the Wikipedia page on pions. At the bottom of the page, there is a listing for the neutral pion ($\frac{\rm u\bar u+d\bar d}{\sqrt{2}}$). Why are they over the square root of 2? There ...
0
votes
1answer
24 views

Possible decay modes of Pion

I am aware that the following decay mode may happen: $$\pi^- → e^-\overline\nu $$ My question is why is the following decay mode not possible $$\pi^- → e^-\nu $$
0
votes
2answers
22 views

Is an obsidian plate safe to use as a Sun viewer?

Is a 0.5 cm thick obsidian plate good to see the Sun directly?
5
votes
1answer
550 views

When is separating the total wavefunction into a space part and a spin part possible?

The total wavefunction of an electron $\psi(\vec{r},s)$ can always be written as $$\psi(\vec{r},s)=\phi(\vec{r})\zeta_{s,m_s}$$ where $\phi(\vec{r})$ is the space part and $\zeta_{s,m_s}$ is the spin ...
1
vote
1answer
56 views

Communication between observers in gravity wells

Suppose two friends live 100 light years apart. Can they use gravitational time dilation to reduce the 200-year roundtrip lag they experience during their phone calls? Specifically, how would the lag ...
-1
votes
0answers
53 views

Behavior of photons in magnetic field [on hold]

Initially I wanted to emulate the Penrose experiment (extracting energy from the black hole), so I decided to use the electrons with electric field that could act as black hole. But later realized ...
0
votes
1answer
14 views

re loss/gain of energy by particle/wave photon/electron to slit grating

Although an electron or photon wave is meant to just diffuse and interfere, when it hits the screen in a certain diffraction pattern at the end, the particle version would be considered to have ...
2
votes
0answers
32 views

Feynman-Wheeler absorber theory

Firstly, I’m sorry if this is not a question that should be here. I would like to ask if anyone could link or guide me to where I could find resources (articles, books, etc.), about Feynman-wheeler ...
3
votes
1answer
44 views

$\phi R$ term for scalar field in a curved background

Condider the following action for a free scalar field $\phi$ in a curved background $$S=\int dx\Big( \frac12g^{\mu\nu}\partial_\mu\phi\partial_\nu\phi+\gamma \phi R\Big)$$ Here $g_{\mu\nu}$ is a ...
0
votes
0answers
33 views

AdS/CFT correspondence and gravitational singularities

While Einstein‘s equation breaks down at singularities, the question arises: which statements/answers can be provided by the corresponding conformal field theory (CFT) in this case?
0
votes
1answer
22 views

Inverse proportional to cosmological scale factor

How can I show using calculation that the temperature of the universe is inversely proportional to the cosmological scale factor? I am just curious, as my textbook states this fact but does not show ...
0
votes
0answers
17 views

Difference between diffusion and interface mobility?

I have had this question when reading about the growth of ferrite (in steel) that is controlled by carbon diffusion and by interface mobility of Carbon. I would like to know the difference between ...
0
votes
0answers
27 views

Lie Algebra in Particle Physics

In his book " Lie Algebra in Particle Physics" Georgie directly put the relation $$(1-P)D(g)(1-P)=D(g)(1-P)...(1)$$ This came from the two previous relations: $$PD(g)P=D(g)P$$ $$PD(g)P=PD(g).$$ where ...
0
votes
0answers
37 views

Finding moment of inertia of square plate [on hold]

How to find Moment of Inertia of a horizontal lying square plate about an axis making some angle theta with the vertical.
-1
votes
0answers
22 views

Time dilation part c [on hold]

Baboodok is a star located 5 light years from Earth. (a) Calculate this distance in metres. [2] (b) If a spaceship can travel at a speed of 50 km/s, calculate how long this spaceship will take to ...
1
vote
0answers
19 views

Time interval to closest approach between two sattelites

Two artificial satellites are in circular earth orbits in the same plane. The orbit radius of one of the satellite is $r$ and that of the other is slightly greater than the other i.e $r + \Delta r$. ...
0
votes
1answer
53 views

How is partial trace well-defined?

When I define the partial trace as below, how can I prove it well-defined? I understand that I have to indicate $Tr_k(\rho)$ does not depend on how to take the ONB of $\mathbb{C}^2$ $$n\in \mathbb{Z}_{...
0
votes
1answer
28 views

Conceptual doubt regarding energy loss of a system

Suppose I have an ideal spring with a block of mass M attached to it at one end and the other end of the spring on a wall. The spring is executing SHM. I now gently place a small mass m on the block M ...
0
votes
1answer
26 views

Total charge for Dipole and Quadrupole moment

I have been struggling on what to put for $Q(total)$ in the equations for the dipole and quadrupole moment for the potential: $$ V_{quad}(\mathbf r) \approx \frac{2Qd^2}{4\pi\epsilon_0} \left( \frac{...
1
vote
0answers
28 views

On rotational kinetic energy

Why is the expression KE of rotation =angular momentum squared over 2 times the moment of inertia true only for objects symmetric along the axis of rotation?
-2
votes
0answers
34 views

The Implication of $G$ in the Planck Mass [on hold]

Why is it considered appropriate to derive an expression for the Planck mass [mP] using the gravitational constant G simply because the resulting units are of 'mass' when it is far from clear what the ...
0
votes
0answers
6 views

Radiation Pressure Noise in Gravitational Wave Detection

I am trying to work out where equation 9 comes from in Martynov et al. (2016), who discuss the radiation pressure noise in the LIGO detector. $$L(f) = \frac{2}{cM\pi^2 f^2} \left(h \nu G_{-} P_{\rm ...
0
votes
0answers
83 views

Could anyone prove or disprove my proposition below? (about quantum information) [on hold]

I'm a begginer at the quantum information field and I'm thinking about the construction of quantum deletion error correcting codes. From my experiences, I found that the following proposition holds. ...
0
votes
0answers
8 views

Semi-classical Hydrogen Atom Angular Momentum under Magnetic Field

Suppose we have a semiclassical hydrogen atom in its ground state at the x-y plane with the proton being at the origin. Let there be a magnetic field $\vec{B}=B\hat{z}$. By deriving the orbital ...
0
votes
0answers
38 views

Why is my focal length wrong? [on hold]

This is a lens system given in my textbook: Where there are two convex(red) lens. Between them is water which makes a concave lens. Focal lenth of convex lens is given as $f=20 cm$ (both lens are ...
1
vote
1answer
14 views

Velocity of efflux when fluid is itself moving

In efflux velocity given by Torricelli equation: $$ v^2=2gh $$ what is reference frame for this $v$. Is it fluid or ground? I am just a high school level student, so please forgive my ignorance.
0
votes
1answer
23 views

Can I make a box full of molecules weigh more and then weigh less?

Gee I hope this is clear. Say I have a box full of some element. Helium or whatever. If I could break all the nuclei apart the individual protons will have more mass because of the binding energy, ...
-4
votes
1answer
78 views

Are photons blinking? [on hold]

Is a photon passing through a point in space blinking relative to that point? If the photon for some reason destructively interfere with it self at that point, what happen to it? is it off or ...
-1
votes
1answer
32 views

Work done by a force where displacement varies with time [on hold]

I am a student and I recently came across a problem: A force acts on a $2\,{\rm kg}$ object so that its position is given as a function of time $$x(t) = 3t^2 + 5$$ Find work done by the ...
0
votes
0answers
13 views

Similarities and analogies between the $E, P$ and $D$ fields with the $B, M$ and $H$ fields and their limitations

In the course of my learning electromagnetism, I’ve noticed there are a striking amount of symmetries in electrostatics and magnetostatics, almost down to replacing divergence operators with curl ...
2
votes
0answers
26 views

What's the difference between cylindrical geometry and disk with a hole for topological chiral $p_x\!+\!i p_y$ superconductor?

We know that for a topological chiral p-wave superconductor with a cylindrical geometry, i.e. one conserved momentum $k$ and one open boundary direction, there exists edge modes with opposite ...
-2
votes
0answers
14 views

Finding radius of curvature of a point on a rolling disc [on hold]

What is the radius of curvature of a point at let's say r/2 units away from the center of a disc of radius r units rolling with velocity v,when the given point is directly above the center of the disk....
0
votes
1answer
38 views

Ampere's law paradox

Imagine I have two current-carrying wires with the current going in the same direction. The magnetic field looks like the sketch below on the left: Now imagine I squish the wires very close together ...
0
votes
0answers
24 views

How to determine the kinetic energy of a rocket after its engine is connected? [on hold]

I need some help with this problem. I need to calculate the kinetic energy of a rocket that was at rest after the engine is connected. In two rockets, one of which moves and the other is at rest, ...
0
votes
0answers
14 views

Does the measuring instrument which counts the number of quantum particles exist?

I have recently been thinking about the quantum deletion error correcting codes, but the primary problem is whether the receiver can detect the loss of quantum particles or not. So my question is that ...
0
votes
1answer
20 views

Can quantum deletion error correcting codes be constructed?

I'm wondering whether or not we can construct quantum deletion error correcting codes. The quantum deletion error is defined by the partial trace. If we can, could anyone give an example?
0
votes
3answers
36 views

If you were to push a block in space in an ideal vacumn scenario with zero friction, what would be the final velocity of that object

Say you push a mass with 10 N force(just once and let it go) of 10kg object in space with zero friction and nothing in the way of its path. What would be the velocity of that object? I would assume ...
0
votes
0answers
30 views

Work done by frictional force when rolling without slipping [duplicate]

When an object is rolling (no slipping) on a slope there should be a frictional force which causes rotation. But when we use power insulation philosophy, we haven't considered about an energy loss ...

15 30 50 per page