5
votes
3answers
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 ...
3
votes
1answer
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 ...
0
votes
1answer
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 ...
2
votes
1answer
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 ...
6
votes
6answers
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 ...
5
votes
0answers
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 ...
0
votes
0answers
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 ...
4
votes
3answers
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 ...
0
votes
2answers
484 views

Fan blade physics

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 ...
3
votes
3answers
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 ...
4
votes
3answers
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 ...
3
votes
1answer
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 ...
7
votes
8answers
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 ...
1
vote
1answer
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 ...
7
votes
5answers
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 ...
1
vote
1answer
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 ...
2
votes
1answer
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 ...
0
votes
2answers
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 ...
1
vote
2answers
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.
3
votes
2answers
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 ...
3
votes
1answer
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 ...
3
votes
3answers
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 ...
8
votes
7answers
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 ...
-3
votes
3answers
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?
2
votes
2answers
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 ...
3
votes
2answers
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? ...
3
votes
3answers
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 ...
2
votes
1answer
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 ...
5
votes
0answers
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 ...
3
votes
1answer
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, ...
0
votes
0answers
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 ...
3
votes
2answers
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 ...
2
votes
1answer
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 ...
17
votes
4answers
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 ...
1
vote
2answers
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, ...
14
votes
2answers
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 $10^{-12}$ ...
4
votes
2answers
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 ...
0
votes
3answers
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 ...
2
votes
1answer
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 ...
-1
votes
3answers
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 ...
1
vote
0answers
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 ...
4
votes
1answer
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 ...
2
votes
2answers
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 ...
0
votes
1answer
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?
3
votes
2answers
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 ...
8
votes
5answers
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 ...
2
votes
1answer
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 ...
8
votes
3answers
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?
3
votes
2answers
232 views

Bell Test statistics

I have some questions about the statistics of Bell Tests. For convenience I'm going to refer to the test described in this paper: http://arxiv.org/abs/quant-ph/0205171 They show a coincidence rate ...
0
votes
4answers
266 views

Is this a valid understanding of Newtonian mechanics?

This is a conceptual understanding of Newtonian mechanics. What the laws mean, how we know they're true, etc. I'm looking for criticism. I know this is really border line on the "don't ask questions ...

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