# All Questions

135,118 questions
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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,...
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### 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? ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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?
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### 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 ...
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### 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 $$\pi^- → e^-\nu$$
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### 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?
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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 ...
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### $\phi R$ 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 ...
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### 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?
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### 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 ...
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### 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 ...
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### 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 ...
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### 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.
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### 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 ...
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### 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 $r + \Delta r$. ...
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### 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?
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### 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 ...
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### 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 ...
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### 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 ...
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### 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....
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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 ...
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### 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, ...
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### 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 ...
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### 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?