# All Questions

3k views

### Ultrasonic wave through air

I am not a physicist but I am extremely interested in this area. The simple version of my question is: "What is the maximum range of an ultrasonic wave traveling through air?" Now, I know it depends ...
292 views

### Does the electron have spin in its own reference frame?

In our atomic physics class, we saw that the spin-orbit coupling term arises from the scalar product of the magnetic moment of the electron (proportional to its spin), and the magnetic field created ...
271 views

### Swimming and forces

I was told that the total integral of the stress over the surface of a swimmer (i.e. the total force exerted by the swimmer on the fluid) always vanishes, because there are no external forces applied ...
879 views

### Does light induce an electric current in a conductor?

I know that electromagnetic waves induce electric currents in conductors and that's the basis for radio, wi-fi etc. I also know that light is also an electromagnetic wave. So, can light induce a ...
398 views

### Hydrogen atom in quantum mechanics

I have problems following the calculations in Griffiths' Introduction to Quantum Mechanics (Chapter 4.2.1): If you apply the Schrödinger equation to the Coulomb potential you get the following ...
121 views

### Black holes and entropy

1)I'm told that for Black Holes, when they radiate (Hawking radiation) particles and anti-particles) split at the event horizon, one going to infinity, the other into the BH. They then lose mass. How ...
4k views

### Could negative dimension ever make sense?

After some quick check I found that negative dimensions are not used. But we have negative probability, negative energy etc. So is it so likely that we won't ever use negative dimension(s) ? Update ...
235 views

### Nonpertubative renormalization in quantum field theory versus statistical physics

I am trying to work my head around how renormalization works for quantum field theory. Most treatments cover perturbative renormalization theory and I am fine with this approach. But it is not the ...
431 views

### What are the mathematical prerequisites to understand this paper? [closed]

What are the mathematical prerequisites to understand this paper? Blumenhagen et al. Four-dimensional String Compactifications with D-Branes, Orientifolds and Fluxes. Phys. Rept. 445 no. 1-6, pp. ...
833 views

### Since there are gravitational lenses, are there gravitational mirrors?

Gravitational lensing is an observed phenomenon. Can one have a gravitational mirror? A slightly unrelated question: Can gravitational waves be reflected?
14k views

### Newtonian gravity vs. general relativity: exactly how wrong is Newton?

Is there a simple function I can use to describe the difference between simple Newtonian dynamics and the actual observed motion? Or maybe some ratios for common examples of, say, the motion of stars ...
86 views

### Photons emitted by radioactive source

Can we calculate the number of photons emitted per second by a radioactive source (gamma) ? we can take 100g of barium-133 as an example
434 views

### Why lagrangian is negative number?

In the special relativistic action for a massive point particle, $$\int_{t_i}^{t_f}\mathcal {L}dt,$$ why is the Lagrangian $$\mathcal {L}=-E_o\gamma^{-1}$$ a negative number?
1k views

### First Chern number, monoples and quantum Hall states

The first Chern number $\cal C$ is known to be related to various physical objects. Gauge fields are known as connections of some principle bundles. In particular, principle $U(1)$ bundle is said to ...
2k views

### Does light change color on its way through a window? [duplicate]

Looking at the refractive index of glass, it's around $1.6$. Then the speed of light $x$ through light should be given by $$1.6 = \frac{3.0\times10^8}{x},$$ so $x$ is about ...
330 views

### Relationship between Mars and Earth rotation

Is it by pure random chance that Mars and the Earth have nearly the same day duration (Mars day is barely 40 minutes longer, which is just 3% difference), or there is some causal relationship between ...
68 views

### A Mathematician Who Wants to Learn Particle Physics [duplicate]

Possible Duplicate: Book recommendations I'm a grad student in pure math, wrapping up a thesis in Lie theory. After years of talking to mathematicians and physicists, I've decided that it's ...
181 views

### Relation between Casimir and hydrophobic effects

Background Some years ago I was studying "Fundamentos de biología" (Biology fundamentals) and learned how the lipids create a bilayer due to the water repulsion. Some time later I learned that this ...
238 views

### Confusion about time shift in special relativity

I have never really found a way to comfortably comprehend the idea of time shift even though I know its not the hard part of relativity theory. In that light, can someone point out what is wrong or ...
179 views

### How do the “tidal forces warming moons” theories hold when apart from heating from expansion, there may be also cooling from contraction?

I can understand a temporary heating, from the tital forces exerted on the moon but wouldn't there be cooling as well eventually when particles "give in" to contraction? Wouldn't they eventually net a ...
2k views

### Physical significance of Killing vector field along geodesic

Let us denote by $X^i=(1,\vec 0)$ the Killing vector field and by $u^i(s)$ a tangent vector field of a geodesic, where $s$ is some affine parameter. What physical significance do the scalar quantity ...
192 views

### Spring extensions

A certain spring has attached to it a mass of 25 units: on increasing the load by 6 units it extends 2.5 cm. a) What is the time of oscillation under the original load? b) What will be ...
5k views

In graphene, we have (in the low energy limit) the linear energy-momentum dispersion relation: $E=\hbar v_{\rm{F}}|k|$. This expression arises from a tight-binding model, in fact $E =\frac{3\hbar ... 1answer 79 views ### How feasible is Laser Plasma Acceleration for the post-LHC world? I was wondering how feasible Laser Plasma Acceleration for the post-LHC world is. It seems very promising. 1answer 707 views ### Electrostatic adhesion instead of glue. Is it possible? I am thinking about the way to attach the printed photographs to the wall but not using the frame. And the most interesting idea for me is the use of electrostatics. In addition I have found the ... 1answer 115 views ### what is use of relativistic action? this is relativistic action: $$S=\int_C \mathcal {L}dt$$ where the$\mathcal{L}$is$-m_oc^2\gamma^{-1}$what is use of relativistic action!? 2answers 1k views ### Where does the mass term come from in the Proca Lagrangian? There are many good books describing how to construct the Lagrangian for an electromagnetic field in a medium. $$\mathcal{L}~=~-\frac{1}{16\pi}F^{\mu\nu}F_{\mu\nu}-\frac{1}{c}j^{\nu}A_{\nu}$$ ... 1answer 2k views ### Ways to create controlled small bubbles in water within plastic tubing What is a practical way in an engineering or physics laboratory to create bubbles of a specified size in water within plastic tubing? The tubing is a few mm inner diameter. We'd like to make bubbles ... 2answers 132 views ### Is a large system just a set of smaller systems? All particles exhibit wave-particle duality. And I have a strange question. Why does a larger system, liken an atom that is just a set of smaller systems, itself exhibit wave-particle duality? In ... 1answer 369 views ### Photosensitivity of Photodiodes, lower wavelength -> lower sensitivity? I saw a diagram of the photosensitivity (Current per Power) of a photodiode. So there is this diagonal stating the 100% quantum efficiency. I wondered why the sensitivity for bluer light lower ... 1answer 462 views ### Lamb-Chaplygin dipole There is an interesting issue of hydrodynamics: whirlpools. I want to learn about the Lamb-Chaplygin dipole. Lamb-Chaplygin dipole corresponds to a steady solution of the two dimensional Euler ... 2answers 1k views ### Problem of Landau, Lifshitz - Mechanics - Integration of the Equations of motion [closed] I'm studying Landau, Lifshitz - Mechanics. Could someone help me with this problem ? =) Problem 2 (Page 27 3rd Edition) Determine the period of oscillation, as a function of the energy, when a ... 1answer 89 views ### Exciton energy splitting in indirect semiconductor Let's take an indirect bulk semiconductor and imagine the exciton that build up at the indirect gap. Since we have a bulk semiconductor we should find three p-like orbitals:$p_x,p_y,p_z$. Would their ... 2answers 326 views ### Do scientists literally believe the Schröedinger's cat though experiment? I've heard the Schrödinger's cat "paradox" (although there's nothing particularly paradoxical about it, just counterintuitive), but I've never been clear on whether or not it's meant to be taken with ... 3answers 641 views ### What are the differences between indistinguishable and identical? What is the difference between indistinguishable particles and identical particles? 2answers 609 views ### Order of phase transitions I got to read things like In case of a first order phase transition, the volume and temperature change in a discontinuous manner. However for phase transitions of higher order the change in ... 3answers 919 views ### How do I correctly express the work required to put an object with positive buoyancy down into a certain depth (of water)? It seems to me that I can express the work required to put an object under water in the same way that I express lifting an object up against gravity. I. e., in both cases I increase the potential ... 2answers 9k views ### What does y with a line over it represent? I've been asked to complete this chart and have never come across this symbol before, nor can I find anything about it on google: What does the y with the line over it represent? And also, does the ... 4answers 3k views ### What is the physical meaning/concept behind Legendre polynomials? In mathematical physics and other textbooks we find the Legendre polynomials are solutions of Legendre's differential equations. But I didn't understand where we encounter Legendre's differential ... 2answers 188 views ### Can I see the light from 10 million km away? If light is switched ON, only for a second, and the distance between the observer and the light source is 10 million kilometers, can I still see the light spark? For example, let's assume that the ... 1answer 238 views ### Gauss-Bonnet theorem in the Hawking/Ellis book At the page 336 of Hawking, Ellis: The Large Scale Structure of Space-Time, the Gauss-Bonnet theorem is stated as $$\int_H \hat{R}\ d\hat{S} = 2\pi \chi(H) \qquad (1)$$ with $$\hat{R} = R_{abcd} ... 2answers 4k views ### Is it possible to speed up radioactive decay rates? [duplicate] Possible Duplicate: Do some half-lives change over time? Would it be possible to considerably speed up the decay rate of an isotope? Considerably meaning more then a 1 or 2% increase in ... 1answer 389 views ### Effect of electric field of plasma sheaths How will an electric fields affect the formation of plasma sheaths? 1answer 148 views ### Understanding that nothing can go beyond the speed of light [duplicate] Possible Duplicate: Is the “How to break the speed of light” minute physics video wrong? Radius of the moon - 1737 km. Thus circumference of the side that can been seen from earth - pi * ... 3answers 9k views ### What is the speed of time? [closed] When we measure the speed of a moving element we do it with the help of a reference frame. Now if we need to measure the speed of time, is it possible? Does time actually have a speed? 1answer 99 views ### What the effect of gravity in the center of a planet feels like [duplicate] Possible Duplicate: Would you be weightless at the center of the Earth? The issue of weightlessness at the center of the Earth has obviously already been discussed, however I am curious as ... 1answer 2k views ### Physical Significance of Fourier Transform and Uncertainty Relationships What is the physical significance of a fourier transform? I am interested in knowing exactly how it works when crossing over from momentum space to co ordinate space and also how we arrive at the ... 3answers 455 views ### The viscous force between the layers of liquid is same, then why there is variation in the velocities of its layers? I have learned in my textbook that when the liquid flows the bottom layer of the liquid never moves because of friction, but the upper layers move with increasing velocities how it is possible if the ... 2answers 288 views ### Proof for p=\gamma_Pmu As I'm reading about Relativistic Momentum, my book states the following:$$p=m \frac{\Delta x}{\Delta t}=m\frac{\Delta x}{\sqrt{(1-u^2/c^2)}\Delta t}=\frac{mu}{\sqrt{1-u^2/c^2}}=\gamma_Pmu$$... 1answer 319 views ### Theories with non-vanishing commutators outside the lightcone I'm reading Weinberg's new book on Quantum Mechanics, and in Chapter 8.7 "Time-Dependent Perturbation Theory" he derives the usual Dyson series for the$S\$ matrix when the interaction Hamiltonian ...

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