1
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1answer
393 views

Commutators with a density matrix

The equation describing the evolution of our system is as follows: $ \dot{\rho} = u_1(t)(a^\dagger a \rho - 2a\rho a^\dagger +\rho a^\dagger a) + u_2(t)(a a^\dagger \rho - 2a^\dagger\rho a +\rho a ...
1
vote
1answer
426 views

Open Ended/ Close Ended instruments?

Close ended instruments have twice the wavelength, because the wave must travel twice the distance to repeat itself. Why must a wave reach a lower density medium (air in this case) to repeat? When ...
2
votes
2answers
778 views

Why is the atmosphere transparent in the visible spectrum?

One of the great coincidences in physics is that the Sun happens to shine most brightly at exactly the wavelengths our eyes can see; it's an easy explanation that our eyes evolved to make the most out ...
2
votes
1answer
101 views

DFT breakdown for effects that depend on phase?

Given the Hohenberg-Kohn theorem, a given ground-state density uniquely defines all ground state properties, because in principle the external potential $V_{ext}$ can be understood as a functional of ...
2
votes
3answers
811 views

Would a Helium powered balloon vehicle work on Mars?

Would a Helium powered balloon vehicle work on Mars? Things that might need to be answered: How much liquidHelium would you need to pack in order to fill a balloon with enough helium gas to travel ...
1
vote
1answer
1k views

Yukawa Coupling of a Scalar $SU(2)$ Triplet to a Left-Handed Fermionic $SU(2)$ Doublet

Suppose we have a field theory with a single complex scalar field $\phi$ and a single Dirac Fermion $\psi$, both massless. Let us write $\psi _L=\frac{1}{2}(1-\gamma ^5)\psi$. Then, the Yukawa ...
-2
votes
2answers
25k views

How exactly does mass affect speed?

I konw that mass affects weight (force), so how does that relate to speed? F=ma. so how does all this affect speed?
3
votes
1answer
312 views

Wasn't the Hawking Paradox solved by Einstein?

I just watched a BBC Horizon episode where they talked about the Hawking Paradox. They mentioned a controversy about information being lost but I couldn't get my head around this. Black hole ...
0
votes
1answer
184 views

The discrepancy between the two values in units of sigmas?

Assuming gaussian error distributions, how to calculate the discrepancy between the two values in units of sigmas?
12
votes
1answer
3k views

What would happen if the Earth was tidally locked with the Sun?

I'm thinking of writing a short story set on a version of Earth that is tidally locked to the Sun. I'm not exactly sure how to research the topic. Here's a number of questions about what would happen: ...
1
vote
2answers
180 views

Using CO2 to air condition a room

I'm trying to determine how much dry ice or liquid nitrogen I would need to cool 3300 cubic feet, about 90,000 liters of air, from about 100F (37.78C or 310K) to about 90F (26.67C or 299.81K). I'm ...
1
vote
2answers
642 views

Where can I find beginner's information about quantum mechanics? [duplicate]

Possible Duplicate: Popular books on QM I hope that this question is suitable for this site. I am interested in reading up on quantum theory so that I can reasonably understand the ...
36
votes
6answers
4k views

Why is information indestructible?

I really can't understand what Leonard Susskind means when he says that information is indestructible. Is that information that is lost, through the increase of entropy really recoverable? He ...
8
votes
3answers
402 views

What is the microscopic picture for warm air rising?

The usual explanation for warm fluids rising past cooler ones is that the warmer fluid has a lower density. I'm trying to understand what this looks like at a molecular scale. The density seems to ...
1
vote
0answers
46 views

What is the fluid approximation for BCS action?

For a system with free fermion gas at finite density (i.e. free Dirac action), we have an ideal fluid description with rho, p, n. What is the fluid description for BCS action? Suppose we have a box ...
0
votes
1answer
114 views

Does General Relativity require that Spacetime must be a orientable? [duplicate]

Possible Duplicate: Can spacetime be non-orientable? Apart from the constraints put on the topology of spacetime by QFT (Parity For example), if the global topology of a universe is that of ...
1
vote
3answers
282 views

Is there a point in universe that is observable at present?

We know that we can see distant galaxies only billions years before now. We can observe the nearest stars just several years before the present. Something on the Moon can be observed only some seconds ...
0
votes
1answer
404 views

Is it possible to calcualte the yield-stress of a fluid by measuring the smalles bubble that rises through it?

Consider a yield-stress liquid in gravity. I assume that the buyouncy of a small gas-bubble will not be enough to overcome the yield stress (so the liquid doesn't behave liquid), thus leaving the ...
3
votes
0answers
539 views

Goldstone modes and Heisenberg model

The ideia is to show that, because of Goldstone modes, 2d systems are quite different from 3d ones. So, considering the Heisenberg model, I'll post here what I'm asked to and my current thoughts on ...
4
votes
2answers
258 views

What is behind recoherence?

I am quite familiar with the concept of decoherence, and I heard that a system that has decohered could recohere after that, I was wondering what could cause the the coherences that have leaked into ...
1
vote
1answer
202 views

Is every QFT non-local in the U.V.?

As much as I understand the renormalization group transformation and the concept of relevant/irrelevant operators, I'd say that if we push the reasoning of only looking at relevant operators when we ...
0
votes
1answer
467 views

Time dilation at the speed of light

Does a massless particle travelling at the speed on light in a vacuum (c) experience an infinite time dilation effect? i.e. Would the time dilation extend from the perspective of the massless ...
3
votes
6answers
806 views

Is quantum entanglement an objective or subjective property?

Imagine the following gedankenexperiment. Observer Alice is right here on Earth. Observer Bob is at say Alpha Centauri. A pair of maximally entangled qubits is formed with one qubit handed over to ...
5
votes
1answer
236 views

The bijective correspondence between a symmetric polynomial and edge excitation of the fractional quantum hall droplet

I am recently reading Xiao-Gang Wen's paper (http://dao.mit.edu/~wen/pub/edgere.pdf) on edge excitation for fractional quantum hall effect. On page 25, he claimed that it is easy to show that there ...
26
votes
8answers
4k views

Is there an intuitive explanation for why Lorentz force is perpendicular to a particle's velocity and the magnetic field?

The Lorentz force on a charged particle is perpendicular to the particle's velocity and the magnetic field it's moving through. This is obvious from the equation: $$ \mathbf{F} = q\mathbf{v} \times ...
3
votes
1answer
178 views

Is there a tb meson?

I was wandering around the particle date group page for meson and couldn't find a meson for top-bottom, which from symmetry you would expect. Q1: Is this because it hasn't been found? Q2: There is ...
5
votes
2answers
1k views

What causes a Phase-Transition?

A phase transition occurs when for example, heat is applied continuously to a liquid and after a certain time it converts into a gas. How does this process work in detail? Is their a chain reaction ...
2
votes
1answer
224 views

Will photon's energy be exactly same after million years?

If photon will travel for million years without collisions, what subtle effects can be accumulated ? Gravity fields affect trajectory, but is energy completely intact after fly by ? Photon has its ...
1
vote
2answers
563 views

Motion of a rod struck at one end

Imagine a strong metal rod of uniform density and thickness floating in a weightless environment. Imagine it lies on an X-Y plane, with one end (A) lying at 0,0, and the other end (B) at 0,1. Then it ...
1
vote
1answer
684 views

What forces act on a person sitting in a chair with wheels when they push off another stationary object

What is slowing me down when I push off my desk? What are the "Major" contributors? and Does how hard I push with one hand (or with two hands) make a significant difference?
0
votes
1answer
416 views

Four momentum of a photon squared, trouble with combining wave-particle duality and energy-frequency relation

Since $p^2=E^2-\vec{p}^2=m^2$ and $E=h\nu=\frac{hc}{\lambda}$ and $|\vec{p}|=\frac{h}{\lambda}$ we have that $p^2=\frac{h^2c^2}{\lambda^2}-\frac{h^2}{\lambda ^2}$ If I go to Planck-units ...
2
votes
3answers
235 views

Should any theory of physics respect the principle of conservation of angular momentum or linear momentum?

Is it possible that a theory that can describe the universe at the planck scale can violate things that we now consider fundamental in nature?For example can it violate rotational and translational ...
1
vote
1answer
645 views

Origin of the Higgs field

Are there any attempts in the literature at addressing the origin of the Higgs field? And, which lines of research that find it inevitable to address this question?
2
votes
1answer
266 views

Why $\frac{d}{dt}r_{a}\nabla_{a}U_{ab}+\frac{d}{dt}r_{b}\nabla_{b}U_{ba}=\frac{d}{dt}U_{ab}?$

In classical mechanics for two mass particles $a$,$b$ we assume the symmetric potential arising from $F_{ab}$ and $F_{ab}$ given by $$U_{ab}(r)=-\int^{r}_{r_{0}}F_{ab}(r')dr'$$ and ...
3
votes
3answers
1k views

Does the Breit Wigner formula apply to intermediate virtual particles?

Breit Wigner Formula describes the cross section for interactions that proceed dominantly via a intermediate particle (O*) A+B → O* → C + D: $$σ = \frac{2\Pi}{k^{2}}\frac{Γ_{i}Γ_{f}}{(E-E_{o})^{2} + ...
5
votes
2answers
480 views

Equivalent Rotation using Baker-Campbell-Hausdorff relation

Is there a way in which one can use the BCH relation to find the equivalent angle and the axis for two rotations? I am aware that one can do it in a precise way using Euler Angles but I was wondering ...
1
vote
1answer
92 views

Rainfalls and critical phenomena

By definition, rainfalls are transitions from vapor state to liquid state of water. I can say that "by definition" rainfalls must viewed as critical phenomenon?
4
votes
2answers
213 views

Why is latent heat required?

Why is latent heat required for a system to change its state? (I.e. liquid water to ice) Why the "extra" energy?
7
votes
4answers
23k views

Why can't helicopters reach mount everest?

Is there a reason why people can't just take the helicopter to mount Everest? Why is it that helicopters can't reach that high?
7
votes
6answers
443 views

A database for review literature in theoretical physics

Is there a database somewhere of the review literature (especially the most cited papers) in theoretical physics? I know about Spires but unfortunately it is not working. When I open a page it says ...
2
votes
1answer
609 views

Combined Gravitational Force Vectors in a Spherical Coordinate System

Asking a question here is quite intimidating for me -- while I love Physics, my high-school understanding only allows me to go so far... I've been trying to solve this problem for the last couple of ...
15
votes
7answers
43k views

Would time freeze if you could travel at the speed of light?

I read with interest about Einstein's Theory of Relativity and his proposition about the speed of light being the universal speed limit. So, if I were to travel in a spacecraft at (practically) the ...
0
votes
1answer
148 views

Say we're driving a bike and suddenly hold the brakes?

It's easy for me to imagine that if we brake the front wheel then there is a chance that I'll flip. On the other hand if I brake the back wheel, there is no way it'll happen no matter how fast I ...
-1
votes
2answers
2k views

Regarding possibility of infinitely many spatial dimensions

Recently I was taking a look at a video explaining the existence of fourth spatial dimension and thereupon that infinitely many spatial dimensions are possible. Also it showed that what Einstein told ...
2
votes
1answer
177 views

Is the classical Doopler Effect, for light shift, $1-v/c$, exact? What is it an approximation of?

Is the classical doopler effect for light shift equal to $1-v/c$ exact or an approximation of a classical formula? I know that it is an approximation of the relativistic formula, but what was the ...
1
vote
2answers
31k views

Has any permanent magnet motor been proven to run?

I have read lots of articles about permanent magnet motors, some of which claim the possibility and other which refute it. Is it possible to have a permanent magnet motor that runs on the magnetic ...
7
votes
4answers
475 views

How can there be a quantum field theory that predicts all particle masses?

Say I have a theory with only one (energy) scale, e.g. one given by the fundamental constants $$\epsilon=\sqrt{\dfrac{\hbar c^5}{G}}.$$ In this case, where I can't compare to something else, is ...
2
votes
1answer
240 views

Loudspeaker frequency response dip

I'm trying to determine the reason for a dip in the frequency response curve of my loudspeaker. I have a loudspeaker mounted on the inside of an enclosure, and a 100mm ID tube mounted in-front of the ...
6
votes
1answer
1k views

What is the relationship between string net theory and string / M-theory?

I've just learned from this one of Prof. Wen's answers that there exists a theory called string net theory. Since I've never heard about this before it picks my curiosity, so I`d like to ask some ...
4
votes
3answers
710 views

Thermodynamics of cooking rice

The question is whether (1) cooking rice in a pot of boiling water for an hour and then leaving the pot (of water and rice) to cool to room temp heats my house the same amount as compared to (2) just ...

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