# All Questions

118 views

### GR limit with massless particles and strong fields

What the mechanics arises if to take limit of general relativity with massless particles interacting with strong fields? Suppose there a system of attracting particles that have zero rest mass. What ...
1k views

### Elastic collision in two dimensions

Suppose a particle with mass $m_1$ and speed $v_{1i}$ undergoes an elastic collision with stationary particle of mass $m_2$. After the collision, particle of mass $m_1$ moves with speed $v_{1f}$ in a ...
193 views

### Why is the planck function continuous and not discrete?

If we imagine a object made up of Hydrogen gas that is optically thick to all radiation, and is in thermal equilibrium, then, microscopically, photons will be emitted and absorbed as ...
121 views

### super-jump air balloon [closed]

We have the following objects: a 80 kg person a rope of negligible weight a balloon filled with helium, which can lift for around the same weight, 80kg. My question is, which of the following ...
213 views

### Logic behind topological orders

Long-range entanglement (LRE) is the main feature of topological orders. The string-net condensation model was constructed to exhibit LRE. But the many-body systems of such models do not look like ...
283 views

### Lagrangian for Goldstone mode + topological excitation

The XY-model Hamiltonian is the following, $${\cal H}~=~-J\sum_{\langle i,j\rangle} \cos (\theta_i -\theta_j).$$ The Goldstone mode corresponds to term $(\nabla \theta)^2$ in the effective ...
3k views

### Can you speed up radioactive decay of plutonium?

We all know the issue of deep geological repositories for fuel rods. Is there a currently feasible way to speed up the rod's decay to render them harmless in less than 10 years?
343 views

### Does General Relativity encompass Special Relativity?

Can all of the predictions made in Special Relativity (SR) also be made in General Relativity (GR)?
100 views

### Low-scale supersymmetry

One of the major reasons SUSY was adopted in particle physics is to Naturally have a Higgs boson (a fundamental scalar) at the weak scale. If we abandon this argument, what other motivation for ...
1k views

### What is background independence and how important is it?

What is background independence and how important is it? In order to be a theory of everything, will the final string-theory/m-theory have to be background independent? Does the current lack of ...
532 views

### why are two higgs doublets required in SUSY?

I can't really understand why two higgs doublets are required in SUSY. From the literature, I have found opaque explanations that say something along the lines of: the superpotential W must be a ...
521 views

### Wavefunction collapse and gravity

If gravity can be thought of as both a wave (the gravitational wave, as predicted to exist by Albert Einstein and certain calculations) and a particle (the graviton), would it make sense to apply ...
263 views

### Tachyon vertex operator (Polchinski's book)

I would like to know how does Polchinski in his book "derive" what is the "tachyon vertex operator" (..as say stated in equation 3.6.25, 6.2.11..) I can't locate a "derivation" of the fact that ...
366 views

### Why is $F = mg - T$ in this case?

The situation is as follows: I am told that $F_{net} = mg - T$ in this case, but doesn't that not take into account that $T$ isn't applied to the center of mass? Newton's second law is defined for ...
125 views

### Characteristics of bloch electron in a priodic potential

Effective mass of a Bloch electron in a periodic potential is negative why ?
1k views

### Can you measure the speed of water coming out of a hose by its arc?

Water comes out of a horizontally stationed hose and creates an arc as it heads towards the ground. Can I determine the speed the water was traveling in when it exited the hose by the measuring the ...
141 views

### Order of magnetic phase transitions

Is there any phase transition occur in paramagnetism to diamagnetism transitions state. What should be the order and how will I calculate the order?
38 views

### How does spectroscopy determine what depth of the remote atmosphere a particular gas is at?

This article mentions "...there is less methane than expected in the upper atmosphere" of an exoplanet. How does spectroscopy determine what depth of the atmosphere a particular gas is at? Is it a ...
78 views

### about wavefunction and vector entries

I am beginer of physics and I am studying some very fundamental idea of quantum mechanics by myself. In the introducing book I am reading, there is an example to show a particle diffraced by a slit or ...
4k views

### Do you feel gravity?

I have been reading a few articles about the question why we don't feel/notice gravity in everyday life, but I couldn't understand why exactly we don't feel/notice it, that is, why we don't feel a ...
4k views

### Why quantum mechanics?

Imagine you're teaching a first course on quantum mechanics in which your students are well-versed in classical mechanics, but have never seen any quantum before. How would you motivate the subject ...
77 views

### What's a compact scientific answer to question “(Why there is) / (what is before) the Big Bang?” [duplicate]

Possible Duplicate: Did spacetime start with the Big bang? on causality and The Big Bang Theory Before the Big Bang What's a compact scientific answer to question "(Why there is) / ...
188 views

### Gaussian integration and dimension argument

I made a mistake recently regarding the Gaussian density, by putting the determinant of the variance to the power $\frac{d}{2}$. Would the following argumentation be valid to highlight it should be to ...
83 views

### Not working brakes: just another energy conservation problem

A car is driving down a mountain ($v=90 km/h=25 m/s$, when the driver realizes that brakes aren't working. He try to lose velocity going up an inclined ($20°$) plane, with a friction coefficient of ...
153 views

### Why does the electron energy distribution from muon decay peak near the kinematic maximum?

I'm trying to understand why when you have a muon decay event, the energy of the electron peaks near the maximum kinematically allowed value. Is there an intuitive explanation for why this is the case ...
154 views

### Nothing escapes BHs, gravitons mediate gravity, so why do BHs gravitate? [duplicate]

Possible Duplicate: How does gravity escape a black hole? Nothing escapes black holes, gravitons mediate gravity, so why do black holes gravitate? My question is, "where is the hole (no ...
4k views

### Is energy the ability to do work?

Here was my argument against this, the second law of thermodynamics, in effect says that, there is no heat engine that can take all of some energy that was transferred to it by heat and do work on ...
101 views

### Hamilton operator in absence of causal order?

I hope, this question isn't too broad or vague. In a recent paper, Ognyan Oreshkov et al. worked out a theory of quantum correlations in absence of any causal order, dropping the assumptions of a ...
116 views

### Energy problem: What's wrong here?

A car ($m=540\,\text{kg}$) engine, has a power of $60\,\text{kW}$. The static friction coefficient between wheels and road is $k=0.6$. How long does it take to reach the speed of ...
148 views

### Definitions of Lagrange points: $L_4$ and $L_5$

We have the the five Lagrange points (let consider Earth and Sun): $L_1$ - lie between Sun and Earth; $L_2$ - beyond the Earth; $L_3$ - beyond the Sun; And what's the difference between $L_4$ and ...
358 views

### Displacement with zero velocity

I know that we can rotate a deformable object using internal forces only in space. Thus we can cause an angular displacement without the use of any external forces. The following youtube video shows ...
493 views

### Is the step of analytic continuation unavoidable or can you model around it?

One sometimes considers the analytic continuation of certain quantities in physics and take them seriously. More so than the direct or actual values, actually. For example if you use the procedure ...
201 views

### What is the energy of interaction between two infinite charged wires?

This is related to my previous question. I assume that the two wires have the same charge density with the same sign ($\lambda_1 = \lambda_2 = \lambda$) If I want to take the same approach, the ...
274 views

### Appearing To Reverse Object's Rotation

Can it be done, and if so, how does one you explain mathematically the ability to cause a rotating object to appear to change the direction of rotation? I believe it has something to do with angular ...
164 views

### Measuring nucleons using electron beams

sorry if the question is too elementary. From: The Britannica Guide to Particle Physics: The sizes of atoms, nuclei, and nucleons are measured by firing a beam of electrons at an appropriate target. ...
264 views

### Does the kinetic theory of gases means gases mix almost instantaneously?

This theory has bugged me ever since my first physics class on the subject. If this (http://en.wikipedia.org/wiki/Kinetic_theory) is true, it leads me to a few weird conclusions. Opening the rear ...
2k views

### How do I approach this conservation of energy problem, symbolically

In the absence of nonconservative forces such as friction and air resistance, the total mechanical energy in a closed system is conserved. This is why that when I toss an object directly upwards, the ...
1k views

### Do black holes have infinite areas and volumes?

How to calculate the area / volume of a black hole? Is there a corresponding mathematical function such as rotating $1/x$ around the $x$-axis or likewise to find the volume?
187 views

### Splitting light into colors, mathematical expression (fourier transforms)

I am trying to solve a problem that includes a function of the light hitting a certain area. My question is, how would I change a function $G(x)$ of photons hitting a certain area to include just ...
206 views

### Can I estimate the long-term surface heat transfer coefficient based on a temperature difference between the air surface and ground surface?

I want to estimate the long-term or annual heat transfer coefficient for the earth's surface in a particular area where the mean annual air surface temperature is about 13$^\circ$C, and the mean ...
1k views

### What is the force of friction between two bodies given their masses and a force pulling them as a unit accross a surface?

Where a force of 200N pulls two blocks together(as one system) across a horizontal table top(µ=0.800) $m_A$ = 5.00kg, $m_B$ = 10.0kg Find the acceleration of the system. Find f$_k$ between B and A ...
2k views

### Projectile Motion with Air Resistance and Wind

I am wondering how the general kinematics equations would change in the following situation. If an object were fired out of a cannon, or some sort of launcher, so that it had both an initial velocity ...
293 views

### Are the Møller wave operators $\Omega_\pm$ related to $\lim_{t\rightarrow\infty}U(t)$ from field theory?

When we want to compute correlation functions $\langle\Omega|\,T\hat{\phi}(x_1)\ldots|\Omega\rangle$ in an interacting quantum field theory, we relate it to the free-field objects $|0\rangle$ and ...
851 views

### Need for a side book for E.Sopers Classical Theory Of Fields.

I am reading now E Soper Classical Theory Of Fields now and sometimes it is very hard to follow the equations.So I need a side book to read it comfortably.Landaus book is not helping as its content ...
284 views

### It seems to me that superpotentials can be defined in a theory with or without supersymmetry. Is this true?

I recently read "An Introduction to Supersymmetry in Quantum Mechanical Systems" by T. Wellman (amongst other sources) in an effort to find out what a superpotential actually is and how it relates to ...
1k views

### Evaluating propagator without the epsilon trick

Consider the Klein–Gordon equation and its propagator: $$G(x,y) = \frac{1}{(2\pi)^4}\int d^4 p \frac{e^{-i p.(x-y)}}{p^2 - m^2} \; .$$ I'd like to see a method of evaluating explicit form of $G$ ...
846 views

### How did we find out the shape of our own galaxy without going out of it?

As far as I know, scientists have been able to see a lot of differently shaped galaxies in our visible universe through modern age telescopes. But I was wondering how it was possible to know how our ...
666 views

### Action for a point particle in a curved spacetime

Is this action for a point particle in a curved spacetime correct? $$\mathcal S =-Mc \int ds = -Mc \int_{\xi_0}^{\xi_1}\sqrt{g_{\mu\nu}(x)\frac{dx^\mu(\xi)}{d\xi} \frac{dx^\nu(\xi)}{d\xi}} \ \ d\xi$$