# All Questions

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### Is there a way to create an artificial solar eclipse?

I heard this story, where they celebrated the birthday of the now defunct North Korean dictator Kim Il-sung in the 1970s and as a birthday present they created through some very complex artillery ...
807 views

### Why is spacetime near a quantum black hole approximately AdS?

In this link, one of the answers contains the statement If you examine the space-time near a finite area quantum black hole, you will see an approximate AdS space. Presumably "approximate" ...
203 views

### Does a Lorentz-contracted object float or sink?

Consider the following thought experiment: Imagine an object of a certain mass density which allows it to float in water. Now if this object is viewed from a moving frame with high speed, it will ...
317 views

### Why is it concluded that the cosmos is expanding when in fact the observations are for events further back in time?

Why is it concluded that the cosmos is expanding at an ever increasing rate when looking further out and therefore further back in time? Surely, if the time were reversed to play forwards, the ...
695 views

### Photon spin projection to arbitrary axis

For photons (and any massless particle) we consider only a spin projection into the direction of motion (helicity). Why it's meaningless to talk about projection of photon's spin into some arbitrary ...
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### Physical meaning of the energy density of an electrostatic field

I understand the physical meaning of electrostatic energy of a system of charges (or a distribution with given density) as the energy stored in the system while working to carry the charges from ...
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### Differentiation of the action functional

In the QFT book by Itzykson and Zuber, the variation of the action functional $I=\int_{t_1}^{t_2}dtL$ is written as: $$\delta I=\int_{t_1}^{t_2}dt\frac{\delta I}{\delta q(t)}\delta q(t)$$ How is ...
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### Is it possible to create a hologram using X-Rays?

Is it possible to create a hologram using X-Rays (or other penetrating EM-waves)?
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### Determining cloud cover from observer to near the horizon

Does there exist a Clear Sky Chart with the following enhancements?: 1 - Actual Cloud Cover (Offered Visually and not just Colors with a Legend, Over Time/Past & Predictive) 2 - Simulate/Predict ...
2k views

### reflection, refraction and diffraction occur in radio waves, which one occurs the most?

One of the things about Radio waves is that they undergo different behaviors such as reflection, refraction and diffraction when in contact with obstacles such as walls, buildings etc. what i want to ...
116 views

### Certain material heating water in a recipient

I don't know how to resolve a problem, but I don't want the answer since I'm almost going to have it resolved. What the problem says is we have 85 liters of water at 7ºC in an iron pot of 29kg. We ...
15k views

### Why can't energy be created or destroyed?

My physics instructor told the class, when lecturing about energy, that it can't be created or destroyed. Why is that? Is there a theory or scientific evidence that proves his statement true or ...
408 views

### Horizontal Speed of Descending Parachute

I understand how to get the vertical (descent speed) of a parachute and its payload, but how could one find the horizontal speed/velocity of this parachute depending on the speed of the wind? (yeah I ...
508 views

### Derivation of the basic equation for Witten diagrams

I could understand the derivation of the "bulk-to-boundary" propagators ($K$) for scalar fields in $AdS$ but the iterative definition of the "bulk-to-bulk" propagators is not clear to me. On is ...
577 views

### Advanced topics in string theory

I'm looking for texts about topics in string theory that are "advanced" in the sense that they go beyond perturbative string theory. Specifically I'm interested in String field theory (including ...
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### Tolerance of Natural Frequency & Resonance?

I'm writing a report at the moment about natural frequency, driving frequency and resonance - and I was wondering, is there a typical % tolerance inside which the driving frequency will cause ...
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### What is the mass of a photon in non-empty spaces?

It is a well known result of the special theory of relativity that the photon has no rest mass, because for a particle to attain the speed of light, it must have zero rest-mass. I will not dig into ...
111 views

### String landscape in different dimensions

For D = 11 large (uncompactified) spacetime dimensions, the only "string theory" vacuum is M-theory For D = 10, there are 5 vacua. Or maybe it's more correct to say 4, since type I is S-dual to ...
302 views

### Uniqueness of the 5 string theories

This question combines several sub-questions, the common theme being: why the known 5 string theories are unique? Firstly, regarding heterotic theory. I understand the only allowed gauge groups are ...
4k views

### Two axes for rotational motion

I understand that angular momentum is a vector, etc.. But, what really happens when some object, say a ball for example, is set to rotate along two axes? What would the resulting motion look like?
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### How to construct a 2D conformal field on a mirrored annulus with a looping pattern?

I would like to construct a 2D conformal field on an annulus in which the inner and outer boundaries are like mirrors, and can be approximated by regular polygons (with the same number n of mirror ...
166 views

### How Relative Is Relativity With Respect To Visual Effects?

This might seem naive, but here goes. Imagine Albert and Rick, of equal mass, accelerate together to a significant fraction of light-speed (call it v) relative to Earth, enough to clearly see the ...
3k views

### Why can't echoes be heard inside a room?

If I go camping and shout anywhere, in the forest or on a cliff, I usually hear the echo of my voice. Why when I shout in my room I do not hear any echoes?
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### Why, intuitively, must a solution in physics be unique?

When solving Laplace's equation or Poisson's equation say, we require that the solution must be unique, which can be shown. In general, what is the physics behind seeking a unique solution? Are ...
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I am confused by most discussions of analog Hawking radiation in fluids (see, for example, the recent experimental result of Weinfurtner et al. Phys. Rev. Lett. 106, 021302 (2011), ...
628 views

### Gears and efficiency/speed

For Christmas me and my brother got an AR Drone, the propellers are on a large gear, which is rotated by a small gear that is connected to the motor. This means the small gear (and therefore the ...
380 views

### A quantum particle in a box (with a catch)

I am reading Shankar's Quantum Mechanics and I am looking at the case where there is one particle inside a box, where the potential is zero inside the wall and abruptly goes to infinity outside the ...
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Consider the one-dimensional Ising model with constant magnetic field and node-dependent interaction on a finite lattice, given by $$H(\sigma) = -\sum_{i = 1}^N J_i\sigma_i\sigma_{i + 1} - h\sum_{i = ... 0answers 147 views ### Is the axial gauge with a \xi term useful in Yang-Mills theory? i) Do people use axial gauge with a \xi term? When \xi\neq 0, ghosts do not decouple, but maybe it's still useful? ii) Is it proved that the term \frac 1 {2\xi}(n.A)^2 in the Lagrangian does ... 4answers 12k views ### Understanding the relationship between electricity and magnetism I keep on hearing that magnetism is just another form of electricity and vice versa. If that's the case why can't we use magnets as batteries, and why aren't my batteries magnetic? 2answers 536 views ### How far does physics influence russian roulette? Prompted by this discussion on the math exchange My thought was that the added mass of a bullet in an otherwise empty revolver would bias the chamber spin such that the bullet would remain in one of ... 7answers 873 views ### Why do two ends of a long conducting wire have the same electric potential? I am not seeing the "big picture" here. If I have two conducting spheres separated by a long conducting wire, why would the spheres share the same electric potential? I think of the spheres as point ... 2answers 3k views ### Relating milliampere-hours to watt-hours for batteries I've seen many batteries that are measured in milliampere hours (mAh), while others are measured in watt hours (wh). How can I convert them between each other so that I can actually compare them? It's ... 4answers 485 views ### Would a person not be able to distinguish left from right in outer space? Is it true that human in outer space can't differ right side and left side, with no other objects for reference? 1answer 5k views ### How much sky do we see at any one moment? When we look at any particular point the sky, what percentage of the celestial sphere do we see? This question arises from the notion that on average there passes one meteor per hour overhead. So ... 1answer 301 views ### Can non-free forces change the rest mass? While reading Hobsen et al.'s "General Relativity: An Introduction for Physicists", I came across a bit confusing derivation. Multiplying the 4-force and 4-velocity, the following derivation can be ... 4answers 944 views ### How to locate the Orion Nebula? I was trying to see Orion Nebula for few days now and I simply can't spot it. I'm using Celestron PowerSeeker 127EQ ( 5" , Newtonian , 1000m ). I'm pretty sure I have the right spot as I saw many ... 1answer 296 views ### What force is acting on the charge in the dielectric? For example I have a dielectric solid with a small charged ball in it. And I have external electric field E. So what force is acting on this ball? The field in dielectric is \frac{E}{\epsilon}, ... 4answers 800 views ### How does water carve rock? How does water carve rock? And more generally, how does a soft material carve a hard material? Obviously it happens, but is it a continual process (every drop of soft water carries away a minute ... 2answers 626 views ### Gravity of a disc What proof is there that the center of gravity of a disc is at its center when the gravitational field is not uniform, such as objects close to the edge of the disc, or lying within the body of the ... 1answer 325 views ### Why dynamic Casimir effect does not appear in static gravity field? Dynamic Casimir effect tells us that a constantly-accelerated mirror should emit radiation due to interaction with vacuum. Following principle of equivalence, a similar mirror placed in static ... 3answers 188 views ### Quality loss when adding more components to a telescope? I would think this would be inherently true, but I'm curious: (Assuming the highest quality amateur instruments are used.) Is quality lost when, say, adding a T-mount adapter, diagonal or any other ... 4answers 416 views ### What fundamental principles or theories are required by modern physics? We have been taught that speed of light is insurmountable but as we know an experiment recently tried to show otherwise. If the experiment did turn out to be correct and confirmed by others, would ... 1answer 430 views ### Is it a total or an explicite time derivative in the Schrödinger equation? I am always dubious when I need write Schrödinger equation: do I write \partial / \partial t or d/dt ? I suppose it depends on the space in which it is considered. How? 2answers 2k views ### How to distinguish female and male voices via Fourier analysis? What makes one, without looking, be able to identify the gender of the talker as male or female? I mean if we Fourier analysed the voice of males and females, how the 2 spectrums are different which ... 3answers 2k views ### Conservation of electric charge in Feynman diagram Here is a Feynman diagram showing the mutual annihilation of a bound state electron positron pair into two photons: Is the electric charge conserved at the point A (or B)? What is the "charge" of ... 2answers 346 views ### Renyi entropy in physical systems We know that the Shannon entropy H(P)=- k_{\mathrm{B}}\sum_i p_i \ln p_i is mostly the entropy of the thermodynamic systems. Does the Renyi measure H_{\alpha}(P)=\frac{1}{1-\alpha}\log \sum ... 1answer 382 views ### What is the information geometry of 1D Ising model for a complex magnetic field? Consider the one-dimensional Ising model with constant magnetic field and node-dependent interaction on a finite lattice, given by$$H(\sigma) = -\sum_{i = 1}^N J_i\sigma_i\sigma_{i + 1} - h\sum_{i = ...
Background: In Bose-Einstein stats the quantum concentration $N_q$ (particles per volume) is proportional to the total mass M of the system: $$N_q = (M k T/2 \pi \hbar^2)^{3/2}$$ where k ...