# All Questions

463 views

### Why do we still not have an exact definition for a kilogram?

I read that there is an effort to define a kilogram in terms that can exactly be reproduced in a lab. Why has it taken so long to get this done? It seems this would be fairly important. Edit Today I ...
514 views

### Glass - paper: Stevin's Law

In my understanding the well-known experiment of the glass full of water in equilibrium with a piece of paper, the atmospheric pressure acts on a small layer inside the glass (on the top) and under ...
230 views

### Where do you get $I_p=\frac{P}{4 \pi r^2} sin \alpha$?

This problem is about color science and it is short but tricky: Where do you get $I_p=\frac{P}{4 \pi r^2} sin \alpha$, where $I_p$ is radiant intensity(color science: radiometric quantities). Look at ...
205 views

### light absorbing diodes characteristics

Recently, there was a publication regarding some new materials that would let thru light in one direction and either absorb it or reflect it in the other direction; the publication does not clarify on ...
1k views

### What are the most realistic ways of high speed space propulsion?

Liquid and solid chemical fuels in rockets are very expensive and inefficient. I have heard of solar sails but what are the most realistic space travel fuels that will be used in the future to get ...
45 views

### Experimental tests of Cluster Decmposition

How tight are experimental and astrophysical tests on whether Cluster Decomposition is satisfied at various space-like separations? Is there a review paper or a standard reference on the question? I ...
62 views

### Concave Sphere Habitat characteristics

I'm writing a short story set in an artificial planet-sized sphere with an ecosystem in its inner surface, whose "gravity" is created through spinning. Energy sources aside, what other interesting ...
243 views

### Subjectivity of decoherence

I read that quantum decoherence is subjective, in the sense that two observers may not have the same "environment" and after each one has traced over those degrees of freedom they will end up with a ...
484 views

Why does a table fan eventually stop circulating air? I noticed this with every fan I ever used. The fan will work great for about 2 hours blowing air at full speed, then I notice that, even though ...
411 views

### Adding rotation to internal coordinates

I'm optimizing the geometry of a system composed of several interacting masses (a molecule). The energy of the system depends on the relative position of the masses, and all velocities are zero. In a ...
230 views

### What are some mechanics examples with a globally non-generic symplecic structure?

In the framework of statistical mechanics, in books and lectures when the fundamentals are stated, i.e. phase space, Hamiltons equation, the density etc., phase space seems usually be assumed to be ...
349 views

### Hermitian operator ?? is this possible

we know that the operator $H= - \hbar ^{2} \frac{d^{2}}{dx^{2}}+ V(x)$ is hermitian isn't it ?? however what would happen if the potential were still real but it depends on the Wave function, for ...
2k views

### What future technologies does particle physics and string theory promise? [closed]

What practical application can we expect from particle physics a century or two from now? What use can we make of quark-gluon plasmas or strange quarks? How can we harness W- and Z-bosons or the Higgs ...
78 views

### Smooth trajectory on a smooth manifold

Physicists talk about a smooth trajectory of a particle on a smooth manifold and they label it as q(t) where q_1(t)....q_n(t) are component functions coming from the homeomorphism. I don't see how we ...
488 views

### Why were the SI base quantities chosen as such?

The reasons for choosing length, mass, time, temperature, and amount as base quantities look (at least to me) obvious. What I'm puzzling about is why current (as opposed to resistance, electromotive ...
359 views

### Relative Speed vs speed of light [duplicate]

Possible Duplicate: Travelling faster than the speed of light Someting almost faster than light traveling on something else almost faster than light I've got two questions which are ...
138 views

### Deducing from the double slit experiment that electrons mostly behave like particles

I saw this video of a lecture by Feynman where he said that electrons behave like particles when there is a photon source to detect which slit they pass through. Does this imply that electrons mostly ...
479 views

### From where does the force/energy of action-reaction comes from? (Newton's Third Law of Motion)

I was wondering, from where does the opposite force and thus energy comes from when we apply force. For example, lets say there are two persons (P1 and P2) on the universe, and no force is applied to ...
981 views

### How to capture electomagnetic radiation/waves?

If I wanted to find out what kind of electomagnetic waves "travel" through my room at which frequency, what kind of equipment would I need? Suppose I want to view frequencies from 0 Hz to 6 GHz.
391 views

### Does the Higgs boson (or the 125 GeV boson, if it's not exactly the Higgs as predicted) occur in nature?

If the Higgs boson mediates mass interactions, do they exist in nature? Are there Higgs bosons flying around all the time? Or do they only exist for a tiny fraction of a second while they mediate ...
337 views

### Electromagnetic field of unpolarized light

I need help in finding an expression for the instantaneous electric and magnetic field of unpolarized light in order to write down and evaluate the time-averaged norm of the Poynting vector (i.e. the ...
131 views

### Are there convective movements similar to those in clouds everywhere in the atmosphere?

I heard there were powerful convective movements in clouds which were responsible for increasing the size of water droplets or ice crystals. My question is: do the same movements appear outside of ...
545 views

### Why regularization?

In quantum field theory when dealing with divergent integrals, particularly in calculating corrections to scattering amplitudes, what is often done to render the integrals convergent is to add a ...
316 views

### Significant Figures [closed]

Subtract 0.2 J from 7.26 J. Express your answer to the correct number of significant figures for the given data. I think its 7, but the answer is 7.1. HOW?
273 views

### A question on intereference experiment with water waves as given in the Feynman Lectures on Physics

I have a question related to the interference (thought)experiment with water waves given in the book Feynman Lectures on Physics Vol.3. When only one hole (hole 1) is open the measured wave intensity ...
330 views

### Why are particles different sizes?

Is it correct in saying that a particles size is it's rest energy, and that particles don't actually have size (in the way you get different size objects)? What defines what sizes a particle can be? ...
355 views

### shifting from mathematics to physics

I am a postgraduate in mathematics. I studied physics during my B.Sc.studies.I want to go for further studies in physics particularly in theoretical physics. I am in a job and cant afford regular ...
350 views

### Basic Concepts of Water Pressure for Plumbers

I am learning how to install water pipes and I was told that to check if a pipe is leaking water I need to check the water pressure. But I don't understanding how water pressure works inside the ...
127 views

### What is the rate of B violation expected in the standard model during high energy collisions?

In a recent question Can colliders detect B violation? I asked about detecting B violation in collisions. Here I am interested in the theory aspect. (I asked both questions originally in the same ...
140 views

### How can a deSitter space have finite size?

a deSitter space is a maximally symmetric solution of Einstein equations, I have some problem picturing one thing: this space is past and future (time) infinite but spatial slices have finite size, ...
61 views

### Should we expect reliable nano-particle engines within the next 5 years?

"Regarding engines" what are the current theories and examples of ongoing progress in nanoparticles? References: National Nanotechnology Initiative electric motor made from one molecule 18 atoms ...
138 views

### X-ray diffraction from crystals and time-averaging

To calculate the elastically scattered intensity of x-rays from crystals, one classically considers scattering from a free electron first and then one sums up the scattered em-fields of all electrons ...
117 views

### Reference request for low entropy big bang

There is a somewhat widely accepted argument that the second law of thermodynamics exists because the universe began in a low-entropy state. I'm writing a paper that mentions this (and must be ...
4k views

### Are matrices and second rank tensors the same thing?

Tensors are mathematical objects that are needed in physics to define certain quantities. I have a couple of questions regarding them that need to be clarified: 1-Are matrices and second rank tensors ...
159 views

### What is the origin of nonconservative force?

My understanding about conservative force is a force that its work is independent of path such that we can construct another form of the work called potential to make our life easier. For friction, ...
688 views

### Why is the (free) neutron lifetime so long?

A neutron outside the nucleus lives for about 15 minutes and decays mainly through weak decays (beta decay). Many other weakly decaying particles decay with lifetimes between $10^{-10}$ and $10^{-12}$ ...
202 views

### What equations govern the formation of droplets on a surface?

When some smooth surface (like that of a steel or glass plate) is brought in contact with steam (over e.g. boiling milk) then water is usually seen to condense on that surface not uniformly but as ...
544 views

### The required energy to accelerate a particle from one velocity to another

Say I have a charged molecule running along a linear evacuated tube (so no wind resistance). In the laboratory frame, we can measure that the particle is moving at some speed $v_1$. Provided that ...
86 views

### random triggers

I understand that a small proportion of events at the LHC that would not trigger on any deterministic trigger are saved on what might be called a random trigger, so that, amongst other uses, proposed ...
2k views

### Relation between Newtons and Kilograms

Work is expressed as W=F*d, where the F is in Newton, d is in meters and result ...
72 views

### Reference request: FLRW with k>0, dust, and positive cosmological constant

The exact solution representing a FLRW universe with $k>0$ and dust (p=0), and $\Lambda=0$, is described by a cycloid. What is the exact solution for dust, in the presence of a positive ...
223 views

### Why must $\Psi (x,t)$ go to zero faster than $\frac{1}{\sqrt{|x|}}$?

Why must $\Psi (x,t)$ go to zero faster than $\frac{1}{\sqrt{|x|}}$ as $|x|$ goes to $\infty$? According to Griffiths' Introduction to Quantum Mechanics, it must. I don't understand why, and this is ...
4k views

### Calculate quadrotor propeller torque due to aerodynamic drag

I'd like to know the right formula to calculate torque of a quadrotor propeller (propeller pitch is constant) due to aerodynamic drag in terms of angular velocity $\omega$. For the sake of my ...
728 views

### How do I show that all Brillouin zones have the same volume?

I have read in a few books that all Brillouin zones have the same volume, and I can vaguely see how it works, but have not been able to think up a formal proof. Help?
220 views

### Why conserved quantities in quantum mechanics are experimentally interesting?

Part of the QM lecture notes of Prof. Biswas, available online, on the harmonic oscillator reads: "Now it is easy to see from theorem 4.2 that neither P nor X is conserved. The only conserved ...
502 views

### How important is the cosmic censorship conjecture?

I would like to know how important the cosmic censorship conjecture is? Should a quantum theory of gravity must obey this? It was never rigorously proved in classical GR too. What would be the ...
118 views

### Why does the cosmic censorship conjecture hold so well?

Why does the cosmic censorship conjecture hold so well? Penrose proposed spacelike singularities and closed timelike curves are always hidden behind event horizons in general relativity. His ...
321 views

### How can a body be transparent?

How can a body be transparent? I need the theoretical explanation for the same. Has anybody succeeded in doing that practically?