# All Questions

311 views

### What techniques can be used to prove that a spacetime is not asymptotically flat?

The modern coordinate-indepenent definition of asymptotic flatness was introduced by Geroch in 1972. You can find presentations in Wald 1984 and Townsend 1997. The definition is in terms of existence ...
275 views

### Fine-Tuning, the Hiearchy Problem, and Mass in the Standard Model

In Chapter 1 of his book String Theory in a Nutshell, Kiritsis states the following. The [Standard M]odel is unstable as we increase the energy (hierarchy problem of mass scales) and the theory ...
125 views

### Is there a better alternative to assuming $a=0$ and then calculating acceleration? (Exam question: radiated energy after Coulomb scattering)

My classmates and I are having some debate about how to solve this problem which came up on one of our comprehensive exams. A full, step-by-step solution is not needed, an outline is enough. A ...
5k views

### Can single DC 1.5V battery generates 1500 Watts power?

Suppose there is a DC $1.5\mathrm{V}$ battery connected with $1.5\times10^{-3}\mathrm{\Omega}$ resistor. Then the amount of circuit current is $I=V/R=10^3\mathrm{A}$. (according to Ohm's law) I know ...
2k views

### Why does a ball bounce forever?

In short: Do bouncing balls keep bouncing forever? If not, does it have to do more with energy than velocity? Learner.org: Energy If energy is conserved, why do bouncing balls, ...
58 views

### Modeling Acoustic Lubrication

I thought about posting this question in the Biology StackExchange site but really it is just the application of my question that applies to biology not the core of the question itself. Can anyone ...
581 views

### The magnetic field of a magnetic monopole

Let us define the magnetic field $$\vec{B} = g\frac{\vec{r}}{r^3}$$ for some constant $g$. How can we show that the divergence of this field correspond to the charge distribution of a single magnetic ...
474 views

### How to theoretically determine the angular momentum of an atom?

To determine if an atom is a boson or a fermion I have to count the fermions that constitute the atom (protons, neutrons and electrons). My question is: How to theoretically (as opposed to ...
217 views

165 views

### Is there a fundamental limit to the temporal resolution of signals from space?

In Earth-based experiments, we can measure phenomenon very rapidly in an experiment given appropriate equipment. Clearly if something takes a long exposure to see (due to a weak signal), then the ...
378 views

### Projectile motion of a charged particle in a homogeneous electric field

I'm reading an article about projectile motion, but I'm having some trouble with how the author found the equations of motion when a homogeneous electric field is considered. To allow an immediate ...
162 views

### A naive look at the band of stability suggests a stable isotope of every element. Why is this intuition false?

Looking at the band of stability, my first intuition is to conclude (erroneously) that there is a stable isotope of every element that lies close to the belt of stability. Why is this false? (For ...
167 views

2k views

### Finding a differential equation to for the amount of chemicals in a body of water

I am taking a differential equations course, and most of the problems relate to physical phenomenon. The calculus is not giving me trouble, but the way of approaching the problems is hanging me up. ...
350 views

How does one see that that the Euclidean AdS is the same as the hyperbolic space at the same dimension ie $EAdS_n = \mathbb{H}_n = SO_0(n,1)/SO(n)$? Or is this to be seen as the definition of ...
198 views

### Magnetic moment and other invariants of Lorentz System

It's a standard fact that the quantity $\mu = v_{\perp}^{2}/B$ is approximately conserved in the system $d_{t}\mathbf{v} = \gamma (\mathbf{v}\times\mathbf{B})$. The paper Magnetic Moment to Second ...
199 views

### Uncertainty principle and multiple observers

My understanding is that an observer can measure the precise location of a particle so long as the corresponding uncertainty in momentum measurement is not an issue and vice-versa. Say there is ...
311 views

### Behavior of black holes in higher- and lower-dimensional space-times

The behavior of black holes in 3+1 dimensional space-time as our own is rather well known: formation, event-horizon size, mass, spin, radiation etc. However, my question is what would black holes ...
5k views

### What prevents this magnetic perpetuum mobile from working?

As a child, I imagined this device, which may seem to rotate indefinitely. I have two questions. Is this perpetual motion machine already known? If it is, could you please give some references? What ...
431 views

### Peltier heating/cooling of small surface area [closed]

I am looking at peltier heaters/coolers, and all of them are thin wafers like the one pictured below. This works fine for some things (ie. a microchip or a beer coaster) but not for others (ie. ...
4k views

### Why are electrons defined to have negative charge? [duplicate]

We normally think of the "default" or "root" state of things as being on the positive side of the spectrum. For example, we don't normally use a + symbol to ...
9k views

### Why does boiling water in the microwave make a cup of tea go weird?

When I boil water in the kettle, it makes a nice cup of tea. Sometimes I need to use a microwave because a kettle isn't available. I boil the water in the mug and it looks pretty normal, but when I ...
6k views

### Why do chimneys have these spiral “wings”?

While walking around I noticed something very peculiar. Many chimneys had spiral "wings", while others didn't. I came up with two possibilities: The wind circles around the chimney upwards which ...
406 views

### How do gravitons impact on general relativity?

As I'm reading about GR a lot lately, I was wondering: how do gravitons (if they exist ofc) impact the general relativity? Since in GR, when we look at particles moving in space-time, we are only ...
113 views

### Two interacting particles on sphere drift to sphere poles

Suppose we have two particles which can move on sphere of radius $r$, and they attract to each other so that their potential energy is $g(d)=ad$ where $d$ is distance between them. I've found ...
215 views

### Developement of modern condensed matter physics [closed]

Do you know any resources describing historical aspects of developments of modern condensed matter physics (many body physics etc)? Thanks.
523 views

### Symmetric transverse traceless tensors of rank $s$ and $(s,0,0,..,0)$ representations of $SO(n)$

Can someone help see this connection as to why a spin $s$ (an Integer) particle is to be thought of as a symmetric transverse traceless tensor of rank $s$ and that they lie in the $(s,0,0,..,0)$ ...
535 views

### Why time difference is permanent and mass increase is temporary?

When a clock is transported here and there into space and then brought to the same place it differs with the other clock. When particles are accelerated with high speeds and then brought to rest their ...
478 views

### Why does string theory have such a huge landscape?

I was browsing through Foundations of Space and Time, a compilation of essays on various theories of quantum gravity. The following passage in the introduction intrigued me: Each compactification ...
799 views

### Are Stokes' theorem and Gauss's theorem examples of the Holographic Principle?

Before I write this question, I'd want to say that I've read this question , and Lubos Motl's answer to it (I found it through the "Questions that may already have your answer"). ...
847 views

### What does the height refer to in the Bernoulli equation?

What does the height refer to in the Bernoulli equation? I'm trying to determine the pressure on a surface where the length is short enough that the velocity should be constant across it. Fan -> ...
574 views

### What is the 1977 Wow signal? [closed]

Can someone explain about the 1977 Wow signal? I know that it is a signal received from space in 1977, in the form of a series of code "6EQUJ5". I would like to know if there is any thing or event ...
5k views

### why does headlights dim when a starter motor is turned on

okay first of all, what exactly does the starter motor do? does it increase the current flow and decrease voltage? and does starter motor have its own internal resistance? i understand how there is a ...
750 views

### Collision of two balls [duplicate]

Two balls, with masses $m_1$ and $m_2$ are falling from the heigh $h$. All the collisions are perfectly elastic. Do not consider the size of the balls. $m_1 < m_2$ and lighter ball is on the top. ...
304 views

### Characteristic length of a triangle [closed]

I am reading a paper on collision detection in cloth simulation, please help me understanding following lines written in the paper : To check if a point x4 is closer than some thickness h to a ...
398 views

### Perturbation theory

I am puzzled with perturbation theory when studying quantum mechanics and solid theory. What I learn about perturbation is, from my ignorant point of view, just mathematics, or even simpler, matrix ...
153 views

### Is it better to call the doppler effect a change in wavelength or frequency?

Why is it preferable to say that the doppler effect causes a shift in frequency rather than a shift in wavelength? I often read on websites that they define the doppler effect as a change in ...
42 views

How do I calculate sky irradiance from radiance (L) from a hemisphere above a surface which is tilted relative to the normal (x=0,y=0,z=1). I have L as a function of zenith (0 to 180deg) and azimuth ...
2k views

### Frequency of small oscillation of particle under gravity constrained to move in curve $y=ax^4$

How to find the frequency of small oscillation of a particle under gravity that moves along curve $y = a x^4$ where $y$ is vertical height and $(a>0)$ is constant? I tried comparing \$V(x) = \frac ...

15 30 50 per page