1
vote
2answers
144 views

eigenvalue staircase and hamiltonians

Let two Hamiltonians $H_{1}$ and $H_{2}$ be defined in such a manner that their eigenvalue staircases satisfy $ N_{1} (E) = N_{2} (E)+A +O(E^{-1})$ What can we say about their potentials $ V_{1} ...
5
votes
5answers
377 views

What are colors?

If you go to any course about photography, you learn that all colors are made up of the colors red, green and blue (RGB). (If you mix red, green and blue light, you will get white light, or any other ...
3
votes
2answers
4k views

What is 2 $\theta$ in X-ray powder diffraction (XRD)?

Why are we taking 2$\theta$ instead of $\theta$ in X-ray powder diffraction (XRD). I have found the forum post 2 theta in X-ray Powder Diffraction (XRD), but there is no answer. What is the ...
1
vote
3answers
2k views

Geometry of wireless signal strength

How does wireless signal strength correspond to distance? RSSI lies between -100 and 0 (at least, on my computer). Let's say I walk a distance x towards the router, and my RSSI goes from -60 to -50. ...
1
vote
0answers
130 views

An electron is subjected to an electromagnetic field using the canonical equations solve

So I was given the following vector field: $\vec{A}(t)=\{A_{0x}cos(\omega t + \phi_x), A_{0y}cos(\omega t + \phi_y), A_{0z}cos(\omega t + \phi_z)\}$ Where the amplitudes $A_{0i}$ and phase shifts ...
4
votes
1answer
4k views

Why we cannot use Gauss's Law to find the Electric Field of a finite-length charged wire?

One of my physics books has a nice example on how to use Gauss's Law to find the electric field of a long (infinite) charged wire. However, at the very end of the example, the author ends by saying ...
5
votes
3answers
932 views

Entanglement spectrum

What does it mean by the entanglement spectrum of a quantum system? A brief introduction and a few key references would be appreciated.
9
votes
5answers
945 views

Superposition of electromagnetic waves

The superposition of two waves is given by $$\sin(\omega_1 t)+\sin(\omega_2 t)=2\cos\left(\frac{\omega_1-\omega_2}{2}t\right)\sin\left(\frac{\omega_1+\omega_2}{2}t\right).$$ For sound waves, this ...
2
votes
1answer
253 views

Electric dipole transitions/expectation value of position

Part of a homework question asks to show that for $\ell=0$ in both $\Psi_i$ and $\Psi_f$, we have $$ \int \Psi_i^\ast \vec{r} \Psi_f \; d\tau = 0 $$ for the position vector $\vec{r}$. (This is for ...
4
votes
1answer
2k views

Why are electrons and quarks 0-dimensional?

Disregarding String Theory, which says that electrons and quarks are 1-dimensional, why are electrons and quarks 0-dimensional? The way I look at it is that they have mass, and if we were the same ...
-1
votes
1answer
288 views

Bridge Trusses? [closed]

Which kind of truss would produce the best result (aka. best distribution of force): i) An equilateral, isosceles or a scalene triangle ii) 3 small triangles or 2 big triangles (the overall masses of ...
1
vote
1answer
2k views

Trajectory of projectile thrown downhill

I'm teaching myself mechanics, and set out to solve a problem determining the optimum angle to throw a projectile when standing on a hill, for maximum range. My answer seems almost plausible, except ...
0
votes
2answers
230 views

When Physicists thought neutrinos were faster than the speed of light [duplicate]

Possible Duplicate: Superluminal neutrinos I remember not too long ago hearing very much speculation about a discovery that perhaps neutrinos are faster than the speed of light. I've heard ...
5
votes
1answer
296 views

How do physicists know that mass of possible Higgs particle is limited between two values?

How do physicists know that mass of possible Higgs particle is limited between two values 90 GeV/c$^2$ and 145 GeV/c$^2$?
-2
votes
2answers
633 views

Why does the higher pressure of air underneath an aeroplane wing keep it flying?

With aeroplane flight, the wings are shaped so that the air that goes over the top of the wing has to travel faster than the air that goes below the wing. This means that the air below the wing has ...
3
votes
1answer
168 views

Current string-theory version of Big Bang

I always wondered what the description of Big Bang would be like in string theory. How is it different from QFT version of Big Bang? Thanks.
1
vote
2answers
324 views

How can particles being closed strings in String Theory create solidity in objects?

I understand how particles with certain masses can form to make atoms, which create solidity in objects due to Pauli's Exclusion Principle and what have you. These particles actually have mass and to ...
3
votes
3answers
288 views

Why must the excitation of closed strings in String Theory be spin-2?

In String Theory it is predicted that as a result of the closed strings we have spin-2 gravitons. 1) How do we know there must be an excitation of spin-2 particles? 2) Why does a spin-2 particle ...
3
votes
5answers
219 views

Light refraction and causality

One way how to look at refraction by a dielectric medium like water or glass is that (phase) velocity of light decreases because it is the wavelength rather than the frequency of the light which ...
1
vote
1answer
543 views

How would I calculate the convection coefficient in transient convection?

So I have faced a problem dealing with transient conduction and I need a little help with the problem solving concepts. I need to determine how long it would take to reach the final temperature but I ...
4
votes
3answers
2k views

How much lift does an airplane get from its wings, vs the rest of the airframe?

Consider a big commercial airliner, like a 727, 747, or a 787. At cruising altitude, under standard conditions, how much of the lift of the aircraft comes from the wings, and how much from the rest ...
5
votes
2answers
280 views

Poincare Symmetry in QFT

Given that spacetime is not affine Minkowskispace, it does of course not possess Poincare symmetry. It is still sensible to speak of rotations and translations (parallel transport), but instead of ...
3
votes
2answers
87 views

A charged particle moves in a plane subject to the oscillatory potential

A charged particle moves in a plane subject to the oscillatory potential: $U(r)=\frac{m\omega^2 r^2}{2}$ There is also a constant EM-field described by: $\vec{A}=\frac{1}{2}[\vec{B}\times\vec{r}]$ ...
5
votes
3answers
3k views

How does Higgs field relate to Aether theories?

I am an amateur learning about the Higgs because I was interested in what the LHC's purpose is. I read that as a particle passes through space, it is actually passing through a Higgs field and there ...
-1
votes
3answers
205 views

Why aren't stratospheric balloons compressing their helium to descent? [closed]

I can't find a reference of a working stratospheric balloon mounted with a compressor to control its altitude. Is there a big physical difficulty (such as compressor weight/power ratio, or power ...
0
votes
1answer
163 views

Factor 2 and equations for the weak gravitational field

By using some axioms people derives equation for Lorentz force and, then, Maxwell's equations from the Coulomb's law and Lorentz transformations. When I used analogical methodology for Newton's law of ...
0
votes
2answers
445 views

Find vector position in time give this graph?

Graph is position x position. There are 3 points, $A$, $B$ and $C$. $A(0,2)$ $B(4,2)$ $C(6,0)$ Particle travels from $A$ to $B$ and from $B$ to $C$ at constant $v = 2 ~m/s$. Find vector ...
0
votes
2answers
1k views

Operators Uncertainty

$\hat A$ is an operator. The uncertainty on $\hat{A}$, $\Delta A$ is defined by: $$\Delta A=\sqrt{\langle\hat A^2\rangle - \langle\hat A\rangle^2}$$ what is difference between $\langle\hat ...
2
votes
1answer
205 views

No attraction radially in an cylinder of spherical magnets

I have a set of small magnetic spheres the size of ball bearings. When many of them are built into a cylinder such that they are hexagonally packed, there is no magnetic attraction radially (between ...
0
votes
2answers
441 views

Calculating eccentricity from semi-major axis, position and velocity of particles

I'm trying to calculate the eccentricity of a binary system's body knowing the bodies' velocities, positions, masses and the semi-major axis. I am only trying to calculate the eccentricity for one of ...
1
vote
1answer
962 views

Find the dispersion of the slab?

I've been having trouble with this for a few days and already used up all my tries on the homework for it, but have the final is coming next week. Thus I would like to know how do this, however I ...
1
vote
2answers
383 views

Explain these graphs of rotation and velocity of pucks on air hockey board

I've been tasked to do a simple experiment on the elasticity of collisions. For this experiment I used two "pucks" (very light circular metal pieces of certain height but hollow) and a table that ...
2
votes
0answers
37 views

Baryon asymmetry [duplicate]

Possible Duplicate: Baryon asymmetry Baryon asymmetry refers to the observation that apparently there is matter in the Universe but not much antimatter. We don't see galaxies made of ...
1
vote
0answers
263 views

moment of tennis racket & volley

I've been looking for the relation between moment of a tennis racket and its maneuverability. If I define maneuverability as the easiness to get in position for quick reaction strokes like volleys and ...
2
votes
1answer
444 views

Validity of Einstein Coefficient Derivations

Consider a two-level energy gap with electronic energy states $E_1$ & $E_2$ and associated population densities $n_1$ and $n_2$ with $E_2>E_1$. In the derivation of the Einstein coefficients ...
7
votes
1answer
299 views

Where is the “true” Higgs if the LHC 125 GeV signal is rather a higher dimensional radion than a SM Higgs?

In this article, Lumo introduces and explains the idea (presented by the original authors in this paper) that the LHC signal at about 125 GeV could alternatively be interpreted as a higher ...
3
votes
1answer
325 views

What is the return probability for Brownian motion in three dimensions?

I would like to know the probability of return to the initial point in three dimensional Brownian motion. Does someone know an expression for the diffusion constant? (Suggestions of books on this ...
4
votes
4answers
5k views

Infinitely charged wire and Differential form of Gauss' Law

I have tried calculating the potential of a charged wire the direct way. If lambda is the charge density of the wire, then I get $$\phi(r) = \frac{\lambda}{4 \pi \epsilon_0 r} \int_{-\infty}^\infty ...
-2
votes
4answers
834 views

A person weighing 100 N stands on some bathroom scales in a lift. If the scales show a reading of 110 N, which way is he going [closed]

Can someone answer and explain it please. Here are the options: Moving downwards and decelerating. Moving downwards with a constant velocity. Moving upwards and decelerating. Moving upwards with a ...
4
votes
2answers
652 views

Again about all-win lottery

I suggest the following thought experiment that describes a machine which makes everybody happy. Suppose a lottery is conducted. The winner is awarded a billion dollars plus the title of eternal ...
-2
votes
2answers
674 views

Transient radiation--heating a slab

Hey guys I really need help on this problem. A ceramic slab of dimentions 5cm x 10 cm x .25 cm has to be heated to $177\,^{\circ}{\rm C}$. The ceramic slab travels on a conveyor belt traveling at ...
3
votes
1answer
435 views

Relation between external magnetic field intensity H, magnetisation M and the entropy?

How are the external magnetic field intensity H, magnetisation M and the entropy related to each other? i.e. if I change the magnetic field intensity by dH what will be the change in entropy dS in ...
3
votes
2answers
1k views

Connection between entropy and energy

An isolated system $A$ has entropy $S_a>0$. Next, the isolation of $A$ is temporarily violated, and it has entropy reduced $$S_b ~=~ S_a - S,\space\space\space S\leq S_a.$$ Is it true to say: the ...
2
votes
2answers
321 views

How does gravitation propagate along curved spacetime?

In this wikipedia article it is described how a beam of light, with its locally constant speed, can travel "faster than light". That is to say it travels a distance, which, from a special relativistic ...
0
votes
0answers
34 views

What is the support for the suggestion that reality is a computer simulation? [duplicate]

Possible Duplicate: Does Quantum Physics really suggests this universe as a computer simulation? I know the title sounds far-fetched, but the idea was first brought to my attention by a ...
3
votes
1answer
1k views

Why/How is this Wick's theorem?

Let $\phi$ be a scalar field and then I see the following expression for the square of the normal ordered version of $\phi^2(x)$. $$T(:\phi^2(x)::\phi^2(0):) ~=~ 2<0|T(\phi(x)\phi(0))|0>^2 $$ ...
0
votes
0answers
83 views

Is it necessary to use all solutions when calculating an expectation value in a spin state?

I'm given an spinor $\Psi$ which is solution of the Free Dirac equation, such that is an eigenfunction of $\hat{\vec{p}}$ and has positive energy. Then I'm asked to calculate the expectation value of ...
2
votes
1answer
231 views

Why is Hawking's No Boundary condition described in terms of an instanton if there's no tunneling?

Regarding his take about combining No Boundary proposal with inflation theory (Hawking and Turok), he talks about the "pea instanton". To my very limited understanding I thought instantons were only ...
0
votes
1answer
513 views

Feynman's sum over histories?

The concept requires all possible path's to be mapped out, and any remaining paths not cancelled out represent the most probable path of the object. Considering this: i) If "infinite" paths are ...
8
votes
3answers
383 views

Does a UV completion of gravity necessarily need to be so drastic as String Theory or LQG?

First of all I, it is my understanding that the problems one encounters with the non-renormalizability of gravity are very similar (if not the same) as one encounters in any non-renormalizable theory. ...

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