3
votes
2answers
1k views

Can an asteroid/meteroid/meteor/meteorite be stopped in its way to Earth?

I was just studying meteorology and had a doubt about the motion of meteors towards earth. Can a meteor be stopped while it is still under under the gravitational influence of Earth? As far as I know ...
0
votes
1answer
63 views

Does a Force on the block $M_1$ in this diagram have an effect on the block $M_2$?

Suppose we have two blocks: There is no friction. Does $F$ has the same effect on $M_2$?
10
votes
2answers
918 views

Does dark energy have gravity?

So I keep running into this idea on the Internet that only mass has gravity, but isn't mass simply contained energy? Energy bounces around, or travels at the speed of light, because it is not in the ...
-2
votes
1answer
137 views

Why Does Gravity Work? [closed]

I'm trying to work out if energy has gravity, because if it does, then it will all get sucked into black holes, including dark energy, meaning that there will be no pushing effect left, and that the ...
1
vote
1answer
44 views

Time dependency of the phase of a single photon

I am wondering if a wave packet of a single photon in the time domaine $$ \psi(t)=|\psi(t)|\; \text e^{\text i \varphi(t)} $$ can have a different $t$ dependence in phase than the simple phase ...
6
votes
3answers
479 views

Integrating the generator of the infinitesimal special conformal transformation

(c.f Di Francesco, Conformal Field Theory chapters 2 and 4). The expression for the full generator, $G_a$, of a transformation is $$iG_a \Phi = \frac{\delta x^{\mu}}{\delta \omega_{a}} \partial_{\mu} ...
1
vote
2answers
117 views

Divergence of conservative electric field

I have a little doubt about the following: according Gauss law in the form of Maxwell's equation, we know that: $$ {\rm div} (D)~=~ \rho(v) $$ This just tells us that the electric field has nonzero ...
1
vote
2answers
90 views

Weird Kinematics Homework [closed]

Here's another "hard" homework question I was given on how printers work. (I find it more confusing than it is hard.) I'm supposed to find the horizontal velocity of a charged droplet as it leaves ...
5
votes
3answers
687 views

Is the electrostatic field inside of any closed, uniformly charged surface zero?

We know that a simple application of Gauss's law tells us that the field inside of a uniformly charged spherical shell is zero. Does this hold for all uniformly charged closed surfaces? If so, how ...
3
votes
2answers
85 views

Will two perpendicular orbits settle into a disc?

Scenario: one "fixed" object (like the sun...) of mass X two "planets" (P1 and P2) of mass Y P1's orbit is perpendicular to P2's orbit, and the sun is the center of both orbits P1 and P2 will never ...
17
votes
1answer
1k views

Is the Uncertainty Principle valid for information about the past?

My layman understanding of the Uncertainty Principle is that you can't determine the both the position and momentum of a particle at the same point in time, because measuring one variable changes the ...
0
votes
1answer
102 views

Uncertainty Principle tricked - so why not Newtonian Determinism?

Recently I read that some results are obtained in directions of tricking the uncertainty principle. The relevant link is here: http://www.caltech.edu/content/tricking-uncertainty-principle , and the ...
0
votes
1answer
117 views

Deriving Lorentz transformation - Serway

in Serway, the derivation of Lorentz transformation started from these two equations $$x′=\gamma(x−ut),$$ $$x=\gamma(x′+ut′).$$ The book derived the first equation and then deduced the second. Is ...
1
vote
1answer
81 views

Duality in arbitrary finite dimension using the Levi-Civita tensor

In 4-D flat metric E&M context, given a rank $p$ tensor, one can construct dual of $4-p$ rank tensor by Levi-Civita tensor. Here dual is not in the same sense of mathematical dual. I do not know ...
3
votes
1answer
33 views

Why do we still get sharp scattering spots with quasi-crystal?

In a quasi-crystal, there is no translational invariance. This means there is no delta-function in the Fourier transform. But to get a sharp scattering spot, we need a delta function. Physically, ...
0
votes
0answers
32 views

Angular diameter distance in an inhomogeneous universe?

Computing the angular diameter distance $D_{A}$ is a well known academic exercise in an homogeneous Universe. But now suppose that we are in an inhomogeneous Universe and that I am interested in ...
2
votes
0answers
55 views

What to do when you are living in very cold environment? [closed]

I am on my way to a shimla, and as it is one of the coldest places of India. Previously I lived in a warm environment of southern india. What would be the correct way to live in a cold environment? I ...
1
vote
0answers
26 views

Explain polarization in RF in which the conductor is stationary

Consider a metal rod parallel to $x$-axis moving with velocity $\vec v =(0,v,0)$ perpendicular to magnetic field $\vec B=(0,0,B)$. Lorentz force will give rise to the electric field $\vec E = - ...
0
votes
1answer
71 views

How is that the life of a virtual particle duo is shorter than Planck Time?

I have always thought that the time is discrete (jumping), not continuous and the shortest time is Planck Time. One reason for that was the information on ...
8
votes
0answers
425 views

Intuition for when the replica trick should work and why it works

I am a graduate student in mathematics working in probability (without a very good background in physics honestly) and I've started to see arguments based on computations derived from the replica ...
2
votes
1answer
66 views

Why 21cm line observations is more helpful compared to other radiation?

Why 21cm line observations is more helpful compared to other radiation? Is it only because of its abundance in whole universe or anything else?
9
votes
2answers
459 views

New version of double-slit experiment

I am trying to find a definite answer whether the following version of the double-slit experiment has ever been performed. Calculate/observe what interference pattern should appear by emitting ...
0
votes
0answers
82 views

Quantum mechanics and probability

I've done an intro course on QM and I'm now hoping to understand exactly how to use probability theory rigorously in solving problems. My question is: How do I do the same thing, or the closest thing ...
0
votes
1answer
48 views

Is the field generated by an electromagnet always proportional to its current?

Imagine that I use a long wire to create an electromagnet. Let's also assume that the current flowing along the wire is constant, and that the wire is winded on the vacuumm. Is the magnetic field ...
1
vote
1answer
77 views

Lev Landau's proof of molecular motion at absolute zero?

I am reading the book "The man they wouldn't let die" by Alexander Dorozynski, which is an biography of Russian theoretical physicist Lev Landau, and I have encountered this passage Absolute zero ...
0
votes
2answers
51 views

Dual particle-wave behavior

If electrons and photons, and possibly more particles, exhibit dual character, why don't physicists create a new classification for them? Why describe them as both waves and particles. Why not rather ...
-2
votes
4answers
719 views

Can a “bowl” shaped liquid half-bubble be free-floating in the air?

Intent: Not looking to make-up something, I seek explanations which are possible to duplicate Description of Phenomenon: The halved side of the bubble would be horizontal to Earth's surface, and the ...
1
vote
2answers
203 views

How do different fields interact with each other?

Recently I've seen a few talks and lectures about Quantum Field Theory. They explained what a "particle" means in a field, and that a large enough excitement in a certain field can excite another ...
4
votes
3answers
1k views

Why does the speed of the propellant limit the speed of a space ship in open space?

Isn't speed a relative thing in space? If so, why would the speed of a propellant matter? Why can't a space ship accelerate infinitely?
0
votes
2answers
192 views

List of experimental band gap

does anyone know where one can find a list, database, book of experimental bandgap values of semiconductors? Is there such a collection? Or do I need to scour papers one at a time to get the values?
5
votes
3answers
153 views

Integral in different coordinate systems

In Griffiths' electrodynamics book, he uses the equation, $$\nabla^2\mathbf{A}=-\mu_0 \mathbf{J},$$ to state that $$\mathbf{A}(\mathbf{r}) = ...
6
votes
2answers
2k views

Why isn't water running faster hotter?

I was running the washing up water this morning, and started to think about why the cold tap isn't hot, and why the water doesn't get hotter the faster it is flowing (if anything, the cold tap gets ...
2
votes
1answer
206 views

power counting and (superficial) non-renormalizability

Comment: This stuff is new to me so it doesn't entirely make sense (yet). Question: As I understand from Peskin and Schroeder chap 10 if you have a theory with interaction terms $\lambda \phi^n$ in ...
3
votes
3answers
779 views

Should a Faraday cage block a radio's signal?

Today, I tried creating a very basic Faraday cage by surrounding a radio with two baking trays made out of iron. It didn't seem to affect the radio's signal (AM was being used, not FM). In theory, ...
5
votes
1answer
96 views

Entropy as a state property

The usual "proof" entropy is a state property is like that: "Consider a system which undergoes a reversible process from state 1 to state 2 along path A, and let cycle be completed along path B, ...
5
votes
2answers
217 views

Eigenvalue spectrum of $L_x+iL_y$

Is it possible to find out the generic eigenvalue spectrum of the non-Hermitian operator $L_x+iL_y$, without using any representation?
1
vote
2answers
559 views

Help explain how direction change relates to acceleration [duplicate]

I was doing some simple harmonic motion problems and I came across this picture describing the position, velocity and acceleration of a linear oscillator. At the moment in time when v is 0 the ...
3
votes
4answers
714 views

Why can't the work done by a non-conservative force be zero?

Why can't the work done by a non-conservative force be zero? The displacement along a closed path is always zero. So, whatever be the type of force, variable or constant, the work has to be zero. Why ...
1
vote
0answers
150 views

Question about Atwood Machine and Changing Masses

Say we have an object of X Newtons on the ground called object 1, and it is connected to another object called object 2. If we change the mass of object 2 in intervals until object 1 < object 2, ...
5
votes
3answers
466 views

Will a prism widen a monochromatic beam?

The representation of the experiment which demonstrates that white light consists of many colors, invariably shows that the beam of white light is broadened inside the prism and is some what more ...
1
vote
1answer
106 views

Green functions in QFT

What is the sense of Green function $$ \langle | \hat {T}(u_{1}(x_{1})...u_{n}(x_{n})\hat {S})|\rangle , \quad \hat {S} = \hat{T}e^{i\int \hat {L}(x)d^{4}x} ? $$ How is it connected with scattering ...
0
votes
1answer
43 views

Velocity change of objects

Is it possible for small object (small mass, let's say bullet) to hit large object (big mass, let's say rock) and still move forward (or stop) instead of being reflected (let's say objects don't crush ...
5
votes
4answers
7k views

A circuit with no voltage difference, but current flowing

From Michael on Skeptics Stackexchange: How about a wire that's grounded? Safe to touch, right? WRONG. ...
0
votes
1answer
376 views

Determining fluid flow velocity from experimental volumetric flow rate data

I would be very grateful if someone helps me with my issue. I have a pump pumping water into a tube. My goal is to find velocity of the fluid flow in another tube that will be connected to the first ...
1
vote
2answers
158 views

Electric Potential of a sphere given electric field

I'm studying for a course in electromagnetism, and I've been given an electric field for which I need to find the associated scalar potential. The field is the field generated by a sphere of radius ...
12
votes
1answer
576 views

Can a difference in the “speed of time” introduce acceleration? [closed]

Hypothetically, lets say we have a space divided equally into two adjacent areas where (somehow) in one of the areas time goes by at half the speed as the other area. Or specifically, when a clock in ...
1
vote
0answers
85 views

Forces and acceleration due to an angle that varies over time [closed]

I'm having difficulty figuring out how to set a problem up. The problem is as follows: A $5\ kg$ block is being pulled along a frictionless floor by a cord that applies a force of constant magnitude ...
0
votes
2answers
63 views

Measurement of heat in electrons

Is there any method to measure the heat of an electron and if there is; then is it a constant or a variable.
0
votes
1answer
35 views

When things sink?

Why a heavy object over a less dense medium doesn't fall always to the bottom? Think: stone on balls of polystyrene. Why do the balls stick together and don't let the stone sink to the bottom? - ...
1
vote
1answer
211 views

How do you levitate a foam rock? ('Endless Second' artwork)

I saw this in an art museum: How is this possible? The materials used to make this were foam, wood, stone dust, paint, electromagnets, and metal. I would like to know how this was make and how I ...

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