0
votes
0answers
61 views

Poynting Vector for a Laser

How do you calculate the Poynting vector for a laser given it's power? I know for a sphere you can just take the power, and divide it by 4$\pi R^2$, but I don't know what I would do for a laser. Would ...
6
votes
1answer
119 views

Material implementations of the holographic principle

I'm afraid this question is a little too open-ended, but bear with me while I find a better formulation. carbon allotropes (like fullerenes and graphene) are regular patterned. Conduction bands of ...
1
vote
2answers
343 views

why F is to be defined as ma?

First, I would like to apology for my lousy English used here, as being a non-native english speaker, I am currently using Classical Mechanics as a tool to learn how to construct a theory as an end. ...
4
votes
1answer
892 views

When does a planet become a star?

How big can a gas giant become before it's considered a star? How does the theoretical maximum compare the observed maximum?
6
votes
1answer
95 views

Are there rogue planets between Sun and Proxima Centauri?

Would we be able to detect (via emitted radiation, or its gravity) a rogue planet between us and Proxima Centauri? How big would that planet need to be?
2
votes
1answer
67 views

Has a system of natural units been designed for human-like scales?

A long time ago I was thinking about how the Imperial system of measurements is arbitrary and annoying, and I decided to design the best system of units ever (I wasn't very old then). I worked on this ...
1
vote
2answers
366 views

Conservation of linear momentum magnitude along a trajectory

I was once criticized for "taking angular momentum as momentum going in a circle". I was loosely trying to state, in classical mechanics, that in using conservation of momentum, one can switch between ...
2
votes
1answer
66 views

Does the sun's bearing at a given time vary, and if so, how?

More an astronomy question, but Physics is the closest stack exchange category I could find and Google doesn't have the answer. Granted, the sun's elevation changes for a given time of day throughout ...
2
votes
0answers
39 views

Does FTL signaling allow signaling in the past at the cost of arbitrary precision?

In a recent paper (http://link.springer.com/chapter/10.1007/978-3-642-30870-3_53#page-1), G. Szekely and P.Nemeti wrote the following. Let us consider two reference frames A and B initially, at event ...
8
votes
1answer
225 views

link between real particles, representation of algebra and Young tableau

I know that different representations of this algebra correspond to different spin. One can sort the representation according to the casimir. For any simple Lie algebra, the operator $$ T^2 = ...
0
votes
2answers
292 views

3 Questions about modern Physics [closed]

First i'd like to apologize for both my writting skills (i'm not english) and for my physics knowledge (being them very basic and/or naive). With general relativity from Einstein, gravity is no ...
5
votes
1answer
228 views

Why does one of the extra dimensions of F-Theory have to be a temporal dimension?

F-Theory, as I understand it, is a realisation of Type IIB String Theory as a 12-dimensional theory in such a way that the $SL(2,\mathbb Z)$ symmetry becomes natural because Type IIB String Theory is ...
8
votes
1answer
256 views

What are the details of the renormalization of Chern-Simons theory?

What is a good, simple argument as to why Chern-Simons theory' is renormalisable? Any good books/references dealing with this effectively? Why does the $\beta$-function vanish? Thanks!
1
vote
2answers
171 views

How can time be a dimension?

In physical terms, dimension refers to the constituent structure of all space and its position in time. Time is same throughout the universe. So, how can time be a dimension?
2
votes
2answers
428 views

Einstein Equation at the Singularity. Understanding the Dr. Michio Kaku's explanation

I'm trying to understand what Dr. Michio Kaku is exaplaining in this video: http://www.youtube.com/watch?v=hydDhUNvva8 I'm just able to rewrite this two equations. I think I've probably made ​​some ...
1
vote
2answers
2k views

Why can't we destroy energy?

From a wikipedia article: In physics, the law of conservation of energy states that the total energy of an isolated system cannot change—it is said to be conserved over time. Energy can be neither ...
8
votes
3answers
443 views

Are black holes really singularities?

A popular assumption about black holes is that their gravity grows beyond any limit so it beats all repulsive forces and the matter collapses into a singularity. Is there any evidence for this ...
2
votes
2answers
97 views

Factor of two differences for free field Green's functions in conformal field theory

I have a question about the expressions for free field Green's functions in conformal field theory. It comes from three origins 1) In Polchinski's string theory volume I p36, it is given $$ ...
3
votes
2answers
169 views

Space translation of operators, states, and particle densities

In Sidney Coleman's Lectures he talked about space translations such that $$\tag{1} e^{ia P}\rho(x) e^{-ia P} ~=~ \rho(x-a),$$ but when I expanded the exponentials and used the commutation relation ...
0
votes
0answers
34 views

How to calculate the binding energy of electron of hydrogen atom by using uncertainty principle? [duplicate]

How to calculate the binding energy of an electron of hydrogen atom using uncertainty principle?
2
votes
0answers
43 views

Degenerate electron gas state

What is degenerate in degenerate electron gas state? Why is it called degenerate?
11
votes
2answers
388 views

Why is a hard sphere gas correlated?

In stat mech we calculated the radial distribution function (a.k.a. pair correlation function) for a classical gas by using perturbation theory for the BBGKY hierarchy. (I could post more details of ...
4
votes
2answers
201 views

Is entanglement necessary for quantum computation?

Is entanglement necessary for quantum computation? If there was no error in the computation,superposition of states would be sufficient for quantum computation to be carried out.Is this right?
0
votes
2answers
62 views

Does resistance in second circuit changes in transformed circuit?

Let $V_1$, $I_1$, $R_1$ and $V_2$, $I_2$, $R_2$ be voltages, currents, resistance in first, second circuit. And we assume $R_1=R_2$, $I_1\neq I_2$. The conservation of electrical power ...
2
votes
2answers
189 views

Polar ice caps and thermal radiation

I was reading an article on global warming and it said that the polar ice caps, because they are white, reflect a lot of the sun's radiation. The article also has a picture of some houses in England ...
0
votes
2answers
214 views

Is $Q=CV$ still acceptable formular on the capacitor charged by the transformed circuit?

Assume there is a 100V to 1000V transformer and the capacitor connected in the second circuit.(You know, transformers have two circuits) The second circuit's voltage is 1000V, but the amount current ...
4
votes
2answers
1k views

What does the Froude number represent?

While reading on Wikipedia, I read the following The Froude number is defined as: $$\mathrm{Fr} = \frac{v}{c}$$ where $v$ is a characteristic velocity, and $c$ is a characteristic water ...
4
votes
1answer
159 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 ...
2
votes
1answer
156 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 ...
5
votes
1answer
123 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 ...
5
votes
3answers
1k 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 ...
3
votes
2answers
685 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, ...
1
vote
0answers
47 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 ...
4
votes
3answers
394 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 ...
1
vote
1answer
162 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 ...
0
votes
0answers
115 views

Solving the source free Maxwell equations for plane waves

I've been trying to solve the maxwell equations: $$\nabla\cdot\vec{D}=0,\quad \nabla\cdot\vec{B}=0$$ $$\nabla\times\vec{E}=-\frac{\partial \vec{B}}{\partial t},\quad \nabla\times\vec{H}=\frac{\partial ...
1
vote
1answer
161 views

Spinor irreducible reps of the Lorentz group and their algebra

Antisymmetric tensor of rank two can be connected with spinor formalism by the formula $$ M_{\mu \nu} = \frac{1}{2}(\sigma_{\mu \nu})^{\alpha \beta}h_{(\alpha \beta )} - \frac{1}{2}(\sigma_{\mu ...
12
votes
2answers
128 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 ...
1
vote
1answer
255 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 ...
4
votes
1answer
89 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 ...
0
votes
1answer
96 views

Pauli matrices identities

There are some definitions and properties for Pauli matrices and their combinations: $$ \varepsilon^{\alpha \beta } = \varepsilon^{\dot {\alpha} \dot {\beta} } = \begin{pmatrix} 0 & 1 \\ -1 & ...
0
votes
0answers
203 views

Power Delivered By An Electromagnetic Wave Versus Power Delivered By A Transmission Line

Background Electromagnetic Waves As I understand it, an electromagnetic wave, like any other wave, is capable of delivering power from a source to a destination. To quantify how much power is ...
2
votes
4answers
904 views

pendulum is a inertial frame of reference or non inertial frame of reference? [closed]

Example of pendulum is inertial frame of reference or non inertial frame of reference? because if pendulum starts moving its continuously moves without changing there period of time but is changes its ...
4
votes
1answer
157 views

Vacuum energy extraction: What happens to the cosmological constant?

If the vacuum energy is extracted, then what will happen to the cosmological constant? Will it remain the same? .
1
vote
4answers
1k views

Necessary condition for square integrable functions?

I'm studying Quantum Mechanics and I came across this which I don't quite understand: For a vector space of functions $f(x)$ to be square integrable (i.e $\int{|f(x)|^2dx < \infty)}$, the necessary ...
1
vote
1answer
100 views

Dimension analysis in Derrick theorem

The following image is taken from p. 85 in the textbook Topological Solitons by N. Manton and P.M. Sutcliffe: What I don't understand from the above statement: why $e(\mu)$ has minimum ...
1
vote
2answers
460 views

Nuclear reactions and energy conservation

How are nuclear fission and fusion compatible with the law of conservation of energy? During fission $He$ splits into 2 hydrogen atoms along with enormous amount of heat energy and hydrogen also ...
2
votes
0answers
92 views

Differential Equations - Waves (Physics self-study suggestions) [closed]

I apologize ahead of time, in case this post is not allowed. After taking a few courses at a community college, I've taken the fall 2013 semester off (I was accepted into a university for the spring ...
1
vote
1answer
440 views

Bending of energy bands

To me energy band diagram is a one dimensional plot which shows the differences between the energy bands of a material. In case of an insulator an energy band diagram shows a huge gap between the ...
0
votes
2answers
2k views

Lorentz and Galilean transformation

I read about Lorentz and Galilean transformation in a book of modern physics some days back, but couldn't clearly understand the difference between the two? Also it was stated there that maxwell's ...

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