1
vote
1answer
250 views

Angular Diameter - Perceived size of Objects [closed]

I'm currently in the process of writing a 2.5D application that should display the perceived size of an object. For example, When I have a ball that has a diameter of 1 meter, how big would it appear ...
4
votes
1answer
1k views

Air flow coming out of a fan feels much stronger than air flow coming in. Why? [duplicate]

Possible Duplicate: Why is exhaling more forceful than inhaling? If I put my hand behind my computer's system block, I can feel a very strong flux of air coming out, as it is being pushed ...
15
votes
4answers
2k views

Does a photon in vacuum have a rest frame?

Quite a few of the questions given on this site mention a photon in vacuum having a rest frame such as it having a zero mass in its rest frame. I find this contradictory since photons must travel at ...
4
votes
1answer
302 views

Laser frequency tripling

I've heard that common way to get 354nm laser is to triple 1064nm Nd:YAG one. But how to make non-linear crystal to do tripling instead of doubling? What are the best crystals for tripling and what's ...
3
votes
3answers
688 views

Defining a Riemannian manifold - made easy?

In the context of GTR spacetime, I'm trying to get the basic idea of a Riemannian manifold clear in my mind. Apologies for the longwindedness. Question 1. Is this a reasonable, simplified summary of ...
4
votes
1answer
543 views

Sparks from van de graff generator

I was at a lecture yesterday and there was a demo of a van de graff generator. He held the smaller metal globe which is attached by a wire, about 4cm from the main globe. This created a spark between ...
2
votes
2answers
2k views

What is the potential difference between point X and point Y?

Here is the problem: In the above figure I want help on finding the potential difference between X and Y. It is getting quite confusing due to the battery in the middle. I found the current in both ...
18
votes
2answers
892 views

Kähler potential vs full effective potential

In evaluating the vacuum structure of quantum field theories you need to find the minima of the effective potential including perturbative and nonperturbative corrections where possible. In ...
3
votes
2answers
305 views

Why higher frequencies in Fourier series are more suppressed than lower frequencies?

One can expand any periodic function in sines and cosines. When calculating the coefficients $a_0$, $a_n$, and $b_n$ one find that $a_1>a_2>...>a_n>...$, similarly for $b_n$. Is there an ...
0
votes
1answer
1k views

Simple Harmonic Motion Question [closed]

I have two formulae: Displacement = Amplitude * Cos(Angular Frequency * Time) Velocity = - Amplitude * Angular Frequency * Sin(Angular Frequency * Time) OR $x = Acos(wt)$ $v = -A.w.sin(wt)$ And ...
2
votes
2answers
754 views

Orbital radius of Geo-stationary satellite

Could you please tell, Why all the geo-stationary satellites are to be dropped at same height from earth? Why can't it be closer or away from its regular orbit(ie, 35,000 km)? If all satellites are ...
1
vote
3answers
508 views

Problem with an electricity / thermodynamics assignment

I've been trying to figure this one out for a while on my own, so I'd like to ask for your help if you could offer some. The task states: A heater made out of a wire with a diameter $R = ...
9
votes
7answers
4k views

How does Newtonian gravitation conflict with special relativity?

In the Wikipedia article Classical Field Theory (Gravitation), it says After Newtonian gravitation was found to be inconsistent with special relativity, . . . I don't see how Newtonian ...
9
votes
1answer
138 views

Is there a background independent closed string field theory?

Analogous to the background independent open string field theory by Witten. If there isn't, what are the main stumbling blocks preventing its construction?
7
votes
2answers
1k views

Recommendations for time-line and road map in graduate school towards specializing in Maldacena's conjecture

This question was asked on Theoretical Physics Stackexchange and was grossly misread and closed. I am again posting the question here hoping to get some valuable insights. Also some people were ...
2
votes
3answers
3k views

Bullet trajectory at different angles

I've read from different sources (can't find them online) that firing a bullet at different angles from horizontal will result in differing amount of bullet drop. For example, a gun is fired at a ...
7
votes
2answers
185 views

How do the LASERs in LIGO realize that space has expanded as a gravitational wave passes by?

I read an article on LIGO, and I heard it mentioned that it is a nontrivial argument to say that the effect can be measured by interferometry. What happens to space as the wave passes? Does the light ...
25
votes
2answers
13k views

How can airplanes fly upside down?

I've read many times, including here on this very site that the commonly known explanation of flight is wrong, and that airplanes can fly because the shape of their wings deflects air down. This makes ...
5
votes
1answer
323 views

Do Killing spinors know global information?

The conformal Killing spinor equations on $R\times S^3$ in Minkowski signature are \begin{equation} \nabla_\mu \epsilon=\pm \frac{i}{2}\gamma_\mu\gamma^0\gamma^5\epsilon \end{equation} whose solution ...
4
votes
2answers
4k views

What's a good textbook to learn about waves and oscillations?

I'm taking a course on waves and oscillations using Crawford from the Berkeley series (out of print excluding international copies), and would like to know if anyone has any suggestions for a better ...
12
votes
0answers
163 views

Super Lie-infinity algebra of closed superstring field theory?

Bosonic closed string field theory is famously governed by a Lie n-algebra for $n = \infty$ whose $k$-ary bracket is given by the genus-0 (k+1)-point function in the BRST complex of the string. One ...
5
votes
2answers
154 views

Do asymptotically similar potentials yield similar energy levels asymptotically?

Let there be given two Hamiltonians $$H_1~=~ p^{2}+f(x) \qquad \mathrm{and} \qquad H_2~=~ p^{2}+g(x). $$ Let's suppose that for big big $x$, the potentials are asymptotically similar in the sense ...
5
votes
1answer
563 views

Reverse Sun position algorithm?

To find the sun's position (elevation, azimuth) we can use many algorithms including the PSA algorithm. These algorithm share the fact that the input is the local time, longitude and latitude and the ...
3
votes
3answers
597 views

Does the Opera result hint to a discrete spacetime?

Could the Opera result be interpreted as some kind of hint to a discrete spacetime that is only seen for high enough energy neutrinos? I think I've read (some time ago) something like this in a ...
4
votes
3answers
597 views

Could a ship equipped with Alcubierre drive theoretically escape from a black hole?

Could a ship equipped with Alcubierre drive theoretically escape from a black hole? Also, could it reach parts of the universe that are receding faster than the speed of light from us?
3
votes
1answer
142 views

How does holographic voxel density scale with holographic film metrics?

I'm trying to understand how one can generate bounds on the effective number of voxels (volumetric pixels) in a hologram, or information density, provided various metrics for the two-dimensional ...
19
votes
1answer
761 views

Why does charge conservation due to gauge symmetry only hold on-shell?

While deriving Noether's theorem or the generator(and hence conserved current) for a continuous symmetry, we work modulo the assumption that the field equations hold. Considering the case of gauge ...
3
votes
1answer
675 views

What is the formula for the glug point?

When you pour water out of a bottle, normally you have a smooth stream. However, if you pour it too fast it glugs, which is to say, comes out in quantized bursts. What is the formula for calculating ...
2
votes
2answers
537 views

separation of variables

I'm a math student who's dabbled a little in physics, and one thing I'm a little confused by is separation of variables. Specifically, consider the following simple example: I have a Hamiltonian $H$ ...
1
vote
0answers
790 views

How to use the Coddington shape factor?

We are given $\sigma=\frac{r2+r1}{r2-r1}$, with $\sigma$ being the Coddington shape factor. What I am having problems with is solving for $r1$ and $r2$ when I am just given a focal length and a ...
4
votes
2answers
616 views

If we assume that protons don't decay, then will all matter ultimately decay into Iron-56 or into nickel-62?

Wikipedia says that all matter should decay into iron-56. But it also says Nickel-62 is the most stable nucleus. So could this mean that in the far future, everything could (through quantum ...
4
votes
2answers
982 views

Can an “absolute” frame of reference be determined by measuring the compression of light?

General relativity tells us that there is no absolute frame of reference (actually, it tells us that all frames are relative, which is close but not the same as there is no absolute frame). Special ...
2
votes
2answers
407 views

Air drag coefficient value

Is there any equation that calculates the $C_d$ value in the formula of air drag ? $$F_d = \rho C_d v^2 A$$ I have found that there are tables for this constant but I am looking for the equation that ...
1
vote
1answer
1k views

How does a table fan work?

In a table fan, the blades move around in circular way and we feel air/wind when sitting on front of it. How does this happen?
4
votes
4answers
646 views

Unitary Operator as a complex valued function

A book on Quantum Mechanics by Schwinger states, "A unitary operator can be considered to be a complex valued function of a Hermitian operator." Please give a hint on how to prove this assertion.
6
votes
6answers
5k views

How to Make RF Waves Visible

I understand RF (Radio Frequency) Waves are electromagnetic waves and a mode of communication for wireless technologies, such as cordless phones, radar, ham radio, GPS, and television broadcasts. Most ...
7
votes
1answer
399 views

Integers powers of fields in a QFT Lagrangian

Why can we not have non-integer powers of fields in a QFT Lagrangian, eg. $\phi^{5/2}$? Or if we wanted a charged $\phi^3$ theory, could we not have a $(\phi^*\phi)^{3/2}$ term?
2
votes
1answer
535 views

Induction and Halbach Array

I have recently studied and tried to understand the concept of the Inductrack ( http://en.wikipedia.org/wiki/Inductrack ), a form a magnetic levitation. In the following link they present three ...
3
votes
2answers
490 views

why isn't the variance of Fano noise proportional to the number of photon arrivals?

I've been reading "Photon Transfer" by James Janesick and in Chapter 3 (http://spie.org/samples/PM170.pdf) he describes the various sources of noise in a CCD. Basically, incoming photons interact with ...
1
vote
1answer
326 views

Vertical Motion in Air + Density [closed]

I am having trouble with this confusing question. Please help. The fastest recorded skydive was by an Air Force officer who jumped from a helium balloon at an elevation of 103000 ft, three times ...
2
votes
1answer
470 views

What happens to speed and frequency of a light beam moving in transparent medium when observed from different inertial frame of reference?

Suppose a transparent medium where speed of light is $c/n$, an inertial frame of reference $K$ which is stationary relatively to the medium and an inertial frame of reference $K'$ which is moving ...
3
votes
2answers
361 views

Pressure Inside an Ideal Gas

A non-relativistic ideal gas exerts a pressure at the surface of its container $p = \frac13 \rho \langle v^2 \rangle$ where $\rho$ is the mass density of the gas and $\langle v^2 \rangle$ is the ...
0
votes
2answers
204 views

Relative Synchronicity

Einstein said that the synchronization of two clocks is dependant on the velocity of the observer. But I feel a conceptual contradiction can be made: There are two observers A and B. Observer 'A' ...
1
vote
1answer
330 views

Measuring a Rod in Motion with two Synchronized Clocks

An explanation of special relativity I'm struggling with, goes like this: A rod traveling by a "stationary" observer has its length measured by use of two stationary synchronized clocks (synchronized ...
0
votes
1answer
161 views

Showing that a given matrix provides the desired Lorentz transformation

I am working on problem 5.12 in Halzen and Martin’s Quarks and Leptons. We are asked to show that the matrix $$ S = 1 - \frac{i}{4} \sigma_{\mu \nu} \varepsilon^{\mu \nu} $$ represents the ...
1
vote
1answer
771 views

Converting Notes to Milliseconds

Given a note duration (whole, half, quarter, etc.), a tempo (in beats per minute), and a time signature, how long does that note last in milliseconds?
30
votes
7answers
8k views

Why the Principle of Least Action?

I'll be generous and say it might be reasonable to assume that nature would tend to minimize, or maybe even maximize, the integral over time of $T-V$. Okay, fine. You write down the action ...
11
votes
3answers
459 views

Motivation for Potentials

This is a hypothetical question about "pedagogy". Let's say I am trying to take someone who has just a very small amount of knowledge about Newtonian mechanics and convince them that the Lagrangian ...
6
votes
3answers
600 views

Analogue of Princeton Companion to Mathematics for Physics?

I would like to know if there are compendiums much like the Princeton Companion to Mathematics for physics (especially classical physics: fluid mechanics, elasticity theory, Hamiltonian formalism of ...
4
votes
1answer
1k views

Stellar evolution simulation engine or software

Is there any general purpose stellar evolution simulation engine or software? Something to throw in properties of the star and to watch how (and why) they change along the timeline - with or without ...

15 30 50 per page