4
votes
2answers
783 views

What is the formal definition of a stellar day?

I'm having trouble understanding precisely what a stellar day is. Neither the USNO nor the IERS sites provide a definition. And Wikipedia's description as the "rotation period relative to the fixed ...
4
votes
1answer
68 views

Do error checking costs of quantum computing shrink BQP?

BQP is the set of problems solvable in polynomial time for a given error tolerance, and it is suspected to be larger than P (and BPP, which is probably equal to P). However, inability for the gates to ...
1
vote
2answers
272 views

Conservation of Linear Momentum with respect to a given direction

Is linear momentum conserved in any direction? More specifically, if you project all momentum vectors in a system onto another vector, will momentum be conserved? I know that momentum is conserved ...
2
votes
3answers
243 views

Zero Point Fluctuations

The total energy of a mode in a quantum mechanical resonator is given by $E_n ~=~ (n+ 1/2)hf$ where $n$ is the number of modes. So when there are no modes or vibrations, i.e. $n=0$, the energy is ...
0
votes
0answers
115 views

Naive Uncertainty principle for string theory

Is it possible, in some sense, that a naive uncertainly principle for string theory could be expressed as : $$ \Delta x_i \Delta p_j \Delta \sigma ~=~ \delta_{ij} \hbar \ell_s$$ where $\ell_s$ is ...
3
votes
1answer
337 views

No cloning theorem and exclusive-or (XOR) operator

According to IBM's website, [...]where we would [classically] have done an assignment (x=y), we instead initialize the target (x=0) and use exclusive or (x^=y). This sounds like x is a copy ...
1
vote
3answers
2k views

Long/short-range interaction

A potential of the form $r^{-n}$ is often considered long-range, while one that decays exponentially is considered short-range. Is this characterization simply relative/conventional, or is there a ...
2
votes
1answer
142 views

What are the properties and impediments of a liquid air fueled engine?

I recently came across a very interesting article that suggested the possibility of using liquified gases like air, nitrogen, or oxygen as a power source for cars. It appears that this company is ...
3
votes
2answers
2k views

Widom-Larsen Theory

Does the Widom-Larsen theory pose a credible underpinning for Low Energy Nuclear Reactions, often incorrectly referred to as "Cold Fusion"? http://arxiv.org/abs/cond-mat/0505026
0
votes
1answer
88 views

Could someone introduce books or reviews on electron-electron interaction to me? [duplicate]

Possible Duplicate: Books for Condensed Matter Physics Could someone introduce books or reviews on electron-electron interaction to me? Especially its effects on screening and transport?
3
votes
3answers
1k views

When Hamiltonian and the total energy are the same

In which condition, the Hamiltonian is the same as the total energy of the system, or say $H=T+V$?
-1
votes
3answers
1k views

Good books about waves and optics [duplicate]

Possible Duplicate: What’s a good textbook to learn about waves and oscillations? Where is a good place to learn (classical) optics? I'm looking for a good book about waves and optics, ...
-1
votes
1answer
262 views

I need help with this question on Heat Capacity

A calorimeter has a Heat Capacity of $70 J/K$. There is $150g$ water with a temperature of $20^oC$ in this calorimeter. In this, you put a metal cube of $60g$ with a temperature of $100^oC$. The ...
1
vote
1answer
539 views

Velocity required for Horizontal Rain

Related to my previous question - Change in appearance of liquid drop due to gravity... Ok, I think we all have noticed this practical phenomena (a kind of illusion though)... During rain (while ...
3
votes
1answer
1k views

Area under a $pV$ diagram

What does the area under a Pressure volume diagram equal? I read in my textbook it equals 'external' work done, but why is this? First of all, what exactly is external work? Can you get it ...
1
vote
2answers
510 views

Finding an equation relating the mass of a blade of a wind turbine to its velocity

I'm writing up my physics coursework and I thought I'd try and find an equation described in the title. This is my attempt: Is it correct?
5
votes
1answer
233 views

Is it possible to make statements about bosonic/fermionic systems by taking the limit $\theta\to \pi$ or $\theta\to 0$, of an anyonic system?

One might naïvely write the (anti-)commutation relations for bosonic/fermionic ladder operators as limits $$ \delta_{k,\ell} = \bigl[ \hat{b}_{k}, \hat{b}_{\ell}^\dagger \bigr] = ...
6
votes
2answers
504 views

Is there an intuitive explanation for the Southward force caused by the Coriolis Effect on rotating spheres?

From the Coriolis Effect article on Wikipedia, the following with regard to the Coriolis Effect on a rotating sphere: By setting vn = 0, it can be seen immediately that (for positive φ and ω) a ...
7
votes
1answer
279 views

Parton showering in Pythia 6 Monte Carlo generator

I have Pythia Monte Carlo (MC) samples where I can't understand the parton showering model. If I print out full decay chains from the events, each event contains multiple string objects with pdgId 92. ...
1
vote
1answer
80 views

Ultracold atoms and General theory of Relativty

I am looking for good reviews for the subject of Ultracold atoms and it's application in test of General theory of relativity. I am planning to pick up this topic as a semester project, can somebody ...
1
vote
1answer
126 views

Time dilation $t'=(1-\frac{v}{c})t$ instead of $t'=\frac{1}{\gamma} t$

Suppose we have a train moving. When the origin of train's frame coincides with the origin of observers frame; the the time is set to zero. At that very instant, a photon is emitted from train towards ...
1
vote
1answer
131 views

How can I name this 3D rotation

First I would like to be clear: this is a noob question, I need a simple answer if such an answer exists. In a three-dimentional space, how can I name this rotation? See an animation Let's assume X ...
1
vote
1answer
197 views

Why does renormalization need an unbroken symmetry?

Common wisdom is that for a QFT to be renormalizable it must be invariant under a symmetry transformation. Why does renormalization need an unbroken symmetry? Which is the first publication that ...
0
votes
2answers
904 views

Are holograms like in this video possible? [duplicate]

http://www.youtube.com/watch?feature=player_detailpage&v=jfySIeS1XZE#t=118s If so, how are they created (roughly speaking). Earlier in the video he says that a laser scanned the surface of the ...
4
votes
1answer
499 views

What would jumping into a pool and feeling cold be called? Conduction, or convection?

This was another question from my son's workbook. It said: ...
3
votes
1answer
602 views

Finding the diameter a parachute must be to land a probe on Mars at specific speed

Hi I'm trying to solve this textbook example but I don't know where to begin; NASA has decided to send an experimental probe to Mars. Its weight on earth is $40 kg_f$. When the probe is near ...
5
votes
1answer
423 views

Basis transformation between eigenstates of harmonic oscillators with different frequency

Given two harmonic oscillators with frequencies $\Omega$ and $\Omega'$, the eigenstates themselves are exactly known. Let's call them $\Psi_n$ and $\Psi'_n$. Is there a compact expression for the ...
3
votes
3answers
4k views

Two people pushing off each other

If there are two people on ice skates named $A$ and $B$, and $A$ pushes on $B$ with a force $F_A$, then both of them will experience a force of magnitude $F_A$ in opposite directions. I'm confused as ...
3
votes
1answer
340 views

Mathematical definitions in string theory

Does anyone know of a book that has mathematical definitions of a string, a $p$-brane, a $D$-brane and other related topics. All the books I have looked at don't have a precise definition and this is ...
3
votes
1answer
318 views

How do eigenstates of harmonic oscillators with different frequencies compare?

Suppose I have a harmonic oscillator with frequency $\Omega_1$ and another one with frequency $\Omega_2$. Is there a simple relationship between the eigenstates of the two? Especially, how would the ...
3
votes
3answers
2k views

Cause of buoyant force?

Can you explain to me what causes the buoyant force? Is this a result of a density gradient, or is it like a normal force with solid objects?
0
votes
1answer
1k views

Metric tensor under coordinate transformation x to y(x)

Say I have a metric representation $g_{\mu\nu}$ in a coordinate system $x$ and I want to find the representation of the metric in a new set of coordinates $y = y(x)$. I know how to do this if you are ...
3
votes
2answers
184 views

Can wind currents form in the mesosphere?

Probably not the right forum for this question. How high in the atmosphere may an air-current form? Can a wind current form up in the mesosphere?
3
votes
2answers
512 views

How do we know the universe is expanding, and not that its contents are shrinking?

Is there a physical reason why not to think that instead of space expanding, all physical constants and parameters are shrinking (including of course the instruments we use to measure the constants) ...
8
votes
4answers
3k views

Difficulties with bra-ket notation

I have started to study quantum mechanics. I know linear algebra,functional analysis, calculus, and so on, but at this moment I have a problem in Dirac bra-ket formalism. Namely, I have problem with ...
10
votes
3answers
8k views

Why is electric field strong at sharp edges?

I learned about the coronal discharge, and the common explanation is because the electric field is strong where radius of curvature is small. But I haven't found anything yet that explains why ...
2
votes
1answer
54 views

Testing covariance of an expression?

This is something I've been unsure of for a while but still don't quite get. How does one tell whether an expression (e.g. the Dirac equation) is covariant or not? I get it for a single tensor, but ...
9
votes
5answers
2k views

What are the challenges to achieving cold fusion?

I am an absolute neophyte regarding physics. What are the challenges to achieving cold fusion? I'm not sure this is a duplicate of Why is cold fusion considered bogus?, because that question is ...
3
votes
2answers
872 views

Equation of the saddle-like surface with constant negative curvature?

What is the equation for the saddle-like 2d surface (embeded in 3d Euclidean space with cartesian coordinates x, y and z) with constant negative curvature frequently used to illustrate open universe ...
2
votes
3answers
4k views

Dropping an anchor from a boat

A yacht on a lake drops its anchor overboard. What happens to the water level in the lake? It rises very slightly. It falls very slightly. It stays exactly the same. It's impossible ...
3
votes
1answer
132 views

Inelastic Scattering and coherent scatterng

Another Scattering Question So I have this Bravais Lattice of sites R vibrating with some normal mode with a small displacement amplitude $u_o$, some wave vector k and some frequency $\omega$. We can ...
12
votes
3answers
11k views

What is the highest possible frequency for an EM wave?

What is the highest possible frequency, shortest wavelength, for an electromagnetic wave in free space, and what limits it? Is the answer different for EM waves in other materials or circumstances? ...
1
vote
0answers
31 views

Can we create a magnet with only one Pole? [duplicate]

Possible Duplicate: How to make a monopole magnet? No matter how many times you cut a magnet, we always end up with 2 poles. Is there any possibility of creating a monopole magnet?
1
vote
3answers
2k views

Conservation of energy in objects at terminal velocities

In vacuum, object free falling under gravity, the sum of Gravitational Potential Energy(GPE) and Kinetic Energy (KE) is a constant. The GPE is a decreasing side of a quadratic and KE is a increasing ...
1
vote
0answers
88 views

Wigner $3j$ symbols

I am trying to determine the expansion that requires using $3j$ symbols; however, I am running into some conceptual snags. First, the expansion produces symbols that have m's that do not agree with ...
2
votes
1answer
530 views

Are circular polarizations a basis for any light polarization?

I was trying to solve an exercise in classical optics, consisting in finding the type polarization of certain field profiles. And, by analyzing this one: $$E_x(z,t) = |E|sin(kz-\omega t)\\ ...
0
votes
2answers
598 views

Why or how is cross product used?

I do not understand how does the result of two vectors acting on a particle require me to take the cross product to find the resultant. Won't the actual force on the particle be the result of the ...
1
vote
1answer
150 views

Symmetries of spacetime and objects over it

I guess according to mathematical didactic, we first think of spacetime as a set and we reason about elements of its topology and then it's furthermore equipped with a metric. Appearently it is this ...
0
votes
3answers
497 views

Temperature and density

As temperature rise the density become lower,When temperature goes down, density is higher but in higher temperature the body become bigger so why density become lower?
10
votes
0answers
477 views

Why does the moon look bigger at the horizon? [duplicate]

Possible Duplicate: Why does the moon sometimes appear giant and a orange red color near the horizon? Why does the moon look bigger at horizon or skyline than at other times e.g. at ...

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