2
votes
1answer
435 views

Can 3 photons be combined to give a spin-0 projection?

Motivation: The neutral pion decays to 2 photons ($\pi^0\to\gamma\gamma$) most of the time. For the decay of the neutral to 3 photons ($\pi^0\to 3\gamma$) we have an upper limit on the branching ...
2
votes
4answers
140 views

The motion of a spring

I have a question about the force set by this spring, I saw many times that $\overrightarrow{F}=-Kx\overrightarrow{i}$. I'm asking why not using $\overrightarrow{F}=Kx\overrightarrow{i}$ without the ...
4
votes
4answers
578 views

Comparing Static Frictions

In this figure, which of the static frictional forces will be more? My aim isn't to solve this particular problem but to learn how is static friction distributed . Since each of the rough-surfaces ...
0
votes
1answer
131 views

Would an antimatter beam create multiple matter-antimatter explosions?

To expand upon the question - in an atmosphere, would a beam of pure antimatter (disregarding technical difficulties creating such a beam or passing it through a medium) interact with the matter in ...
1
vote
1answer
93 views

weight of fully loaded RAM

I had read a post Is a hard drive heavier when it is full? It states, there is some difference in the weight of fully loaded hard drive and an empty hard drive due to different orientation of ...
4
votes
3answers
4k views

Moment of Inertia (triangular plate)

I want to generalize the formula for the MOI of a triangular plate (sides $a,b,c$) about an axis passing through mid point of one sides and perpendicular to it's plane . The mass of plate $M$ is ...
1
vote
2answers
1k views

Massless string Paradox

If we introduce the notion of a massless string to denote the fact that net force on a massless string will always be $0$, since it is massless . How can these massless strings ever accelerate when ...
0
votes
2answers
80 views

Finding radioactive nucleus given table of values

Question Measuring the number of decays per minute $N(t)$ of a radioactive source every four days we have that: $N(t=0):=N_0=200$, $N(t=4)=141$, $N(t=8)=100$, $N(t=12)=71$, where $t$ is measured in ...
0
votes
1answer
2k views

Work done by Static friction

Here $v1$ is relative to the block on which sphere is pure rolling but static friction isn't $0$ as of now . In the following diagram, is work done by static friction $0$ ?, since the point of ...
9
votes
3answers
5k views

Is it viable or possible to make your own transistor?

Just wondering if it is possible/viable to construct your own transistor, not small like todays, but the same scale as the one created at Bell Labs.
2
votes
3answers
457 views

Two dimensional elastic collisions with varying angle of incident

If in an elastic collision I know all initial values and that mass for each object remains constant throughout the collision (but different from one another) how can I determine their final velocity ...
1
vote
2answers
160 views

Grover algorithm $R_D$ Circuit

I need sketch two circuits to understand Grover algorithm. The first is the operator $R_f$ and another is the operator $R_D = H^{\otimes n}(2|0\rangle\langle0|-I)H^{\otimes n}$. I get the first ...
0
votes
2answers
6k views

Derivation of the Biot-Savart Law [duplicate]

Can someone provide a derivation of the Biot-Savart law for electromagnetic induction? To be clear, $$ d\vec{B}~=~\frac{\mu_0}{4\pi}\frac{I d\vec{\ell}\times \vec{r}}{r^3}. $$ Is there a simple way ...
3
votes
3answers
268 views

Energy conserved… or not? Confused!

I am confused. Could someone kindly explain what's going on in this question? A particle of mass $m$ and charge $e$ moves in the $x,y-$ plane. There is a constant magnetic field $B$ that points in ...
3
votes
1answer
159 views

Boundary conditions on wave equation

I am having trouble understanding the boundary conditions. From the solutions, the first is that $D_1(0, t) = D_2(0, t)$ because the rope can't break at the junction. The second is that ...
0
votes
1answer
290 views

Does brown but transparent swimming pool water heat significantly faster than western style highly chlorinated pools?

Eastern European swimming pools are often brown tinted water. i was told it was the color of the chemical to keep the pools clean, but who knows. These pools did not smell unsanitary and may have even ...
8
votes
3answers
968 views

How can string theory work without supersymmetry?

This question is inspired from reading Mitchell Porter's nice answer here to a question asking why supersymmetry should be expected naturally. Among other things, he explains that since weak scale ...
4
votes
1answer
257 views

Asymptotic Freedom - Qualitative Explanation

I am doing a (mostly qualitative) course on Particle Physics, and am confused about the concept of asymptotic freedom. The lecture notes basically say that a quark may experience no force/be "unbound" ...
4
votes
2answers
186 views

5D Ricci Curvature

As part of a hw problem for a class, we're supposed to be deriving the equivalence given in equation 2.3 of this paper http://arxiv.org/abs/1107.5563. I was wondering if there is some special ...
0
votes
2answers
3k views

Voltmeter forming a closed circuit

A battery is connected to a 10Ω resistor as shown in Figure 2. The emf (electromotive force) of the battery is 6.0 V. When the switch is open the voltmeter reads 6.0 V and when it is closed it ...
5
votes
3answers
557 views

Why does quantum cryptography give us uncrackable codes?

Why does quantum cryptography give us uncrackable codes? What makes it 'uncrackable'? Articles in for example pop science magazines always claim QC produces uncrackable coded, however I highly doubt ...
3
votes
1answer
48 views

Origin of interaction in inelastic neutron scatting

In solid state physics, inelastic neutron scattering is a commonly-used experimental technique for probing the energy spectrum of phonon and magnon excitations. This technique relies on the ...
0
votes
1answer
57 views

Diagonal matrix in k-space

I'm having some trouble with an integration I hope you guys can help me with. I have that: ${{\mathbf{v}}_{i}}\left( \mathbf{k} \right)=\frac{\hbar {{\mathbf{k}}_{i}}}{m}$ and ...
3
votes
1answer
85 views

Some sort of conservation equation

As far as I know, in General Relativity, an expression of the kind $\nabla_{\mu} X = 0$ states that, associated to $X$, there exist a charge which is conserved. The first example that comes to mind is ...
2
votes
2answers
91 views

What is the derivation for the exponential energy relation and where does it apply?

Very often when people state a relaxation time $\tau_\text{kin-kin}, \tau_\text{rot-kin}$,, etc. they think of a context where the energy relaxation goes as $\propto\text e^{-t/\tau}$. Related is an ...
6
votes
3answers
15k views

How does an earthen pot keep water cool?

I understand that evaporative cooling takes place thanks to small pores contained in the pot and that allow some water to go through and evaporate. However I couldn't understand clearly whether water ...
2
votes
1answer
296 views

What are the implications of the Nesvizhevsky experiment and followup experiments with ultracold neutrons?

I remember reading about the groundbreaking experiment by Nesvizhevsky (et al. 2001) some 12 years ago using ultra-cold neutrons which showed the first experimental evidence of quantum gravity. It is ...
2
votes
1answer
132 views

Topological vs. non-topological noetherian charges

What (if any) is the relationship between the conserved (non-topological) noetherian charges and topological charges? Namely, is there any "generalization" of the Noether's first theorem that includes ...
5
votes
0answers
189 views

Gravitational redshift of Hawking radiation

How can Hawking radiation with a finite (greather than zero) temperature come from the event horizon of a black hole? A redshifted thermal radiation still has Planck spectrum but with the lower ...
4
votes
3answers
2k views

Do laser beams interfere noticeably with each other?

I want to arrange some red lasers along a 90 degree arc of a circle, aimed towards the center of the circle. When their beams hit the opposite side of the circle, will I notice any interference ...
1
vote
3answers
4k views

Air pressure in balloon

I have to calculate the air pressure inside of an hot air balloon. After some searching I found out that I can use the ideal gas law: PV = nRT (from Wikipedia) So to get the pressure in the balloon I ...
1
vote
2answers
167 views

Question on the Hagedorn tower in Type I string theory

In a previous question (Mass spectrum of Type I string theory), I had asked about the mass spectrum of Type I string theory. I got a response saying that it is a Hagedorn tower. However, my source ...
5
votes
1answer
3k views

How Special Relativity causes magnetism

So my physics teacher assigned us an article about how special relativity causes magnetism in a wire with a current, even with the low drift velocities of electrons in a current. It seemed that the ...
0
votes
0answers
78 views

The most general entropy

What is the most general (likely non-extensive) entropy allowed/realized in Nature and its symmetry group of transformations? We know Shannon-Gibbs-Von Neumann entropy, Tsallis entropy, ...
4
votes
2answers
241 views

Superluminal particles with causality

What kind of CLASSICAL theories would allow to true (non-apparent) superluminal particles (beyond speed of light, BSOL) agreeing with causality to exist? I mean, are causal superluminal classical ...
-1
votes
2answers
131 views

Proof $\left[ {\hat H,{{\hat p}_i}} \right] = - \frac{\hbar }{i}\frac{{\partial \hat H}}{{\partial {{\hat q}_i}}}$ [closed]

I have a problem with the Hamiltonian, I don't think anything to solve it!! So could you give me some hints! Knowing that: $$\left[ {{{\hat p}_i},{{\hat q}_k}} \right] = \frac{\hbar }{i}{\delta ...
-2
votes
2answers
6k views

what is the density of natural gas at 293K and 700 kPa? [closed]

I am trying to calculate the density of natural gas to define the amount of natural gas flow through an orifice. I am coming up with 5.45 kg/m3 Is this correct?
3
votes
1answer
1k views

Why doesn't soda fizz at high pressure?

In this video, a can of soda is opened in an underwater station at 2.5 atm. The demonstration is that shaking the soda doesn't cause it to explode like it would at normal pressures. Why does this ...
2
votes
2answers
619 views

How do we measure velocity field?

How do we measure velocity field u(r,t)? i know that how to measure ordinary velocity. $v=\frac{dr}{dt}$ but what about velocity field? what is difference between them?
1
vote
1answer
114 views

Gyro vs Accelero : Why is that assessment false?

Can we say that the Accelerometer's values are somewhat the integral of the Gyroscope's values ? I think that because I know that the velocity is the derivative of the position with respect to time ...
0
votes
0answers
90 views

Relativistic canonical transformation

What is relativistic canonical transformation? I need every piece of information about it. Does anyone know a reference or an article about relativistic canonical transformation? For example, in ...
2
votes
4answers
1k views

Potential energy sign conventions

Almost every book on physics that I read have some weird and non-clear explanations regarding the potential energy. Ok, I do understand that if we integrate a force over some path, we'll get a ...
3
votes
3answers
310 views

Precision of Coulomb's law

Up to which precision has the coulomb law proven to be true? I.e. if you have two electrons in a vacuum chamber, 5 meters appart, have the third order terms been ruled out? Are there any theoretical ...
-1
votes
1answer
128 views
0
votes
3answers
120 views

Why is the void devoid of an electric field?

For a region devoid of charge, maxwell's equation yields $\nabla \cdot \mathbf{E} = 0$ which still allows a constant field. So why is in electrostatics for the vacuum always $\mathbf{E} = 0$ assumed? ...
6
votes
1answer
733 views

Theta parameter of QCD: Very small, periodic in $2\pi$ or both?

I've been learning about the theta parameter of QCD and I'm confused about the fact that it's supposed to be very small but at the same time some sources say that the Yang-Mills theory should be ...
2
votes
2answers
4k views

Does inertia increase with speed?

I have heard that when the speed of the object increase, the mass of the object also increase. (Why does an object with higher speed gain more (relativistic) mass?) So inertia which is related to ...
1
vote
1answer
57 views

Speed of air through valve

Right now I'm working on a hot air balloon in Physics formulas. At the moment it's flying up like it should, but once it is at a certain height we want it to go back down. After some research about ...
2
votes
6answers
728 views

Centripetal Force Acceleration

In uniform circular motion, acceleration is $\frac{v^2}{r}$ and time which it acts $\rightarrow 0$. So $\Delta v = 0$, but then why/how does direction change, when the acceleration should be ...
2
votes
4answers
4k views

How can we detect X-rays?

I know that X-rays can be detected by various ways, like ionizing of air particles. Is there a way to detect X-rays,which are photons, by detecting? Can something absorb the energy of the X-rays and ...

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