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Recently J.Webb submitted paper which has extraordinary claim - that Fine Structure Constant is different in different directions in space! He (with others) measured $\alpha$ using quasar spectrum absorption method in different directions and found small variation. There is huge debate about this finding, but lets supp...
My friend thinks it's because she has less air resistance but I'm not sure.
This is rather a technical question for experts in General Relativity. An accessible link would be an accepable answer, although any additional discussion is welcome. GR has well known solutions relating to single Black Holes: Schwarzchild, Rotating & Rotating with Charge. These solutions demonstrate some non-trivial G...
I'm trying to simulate an RLC circuit using transfer function. Circuit is there: https://i.stack.imgur.com/MC8ME.png (I'm a new user therefore I cannot post images) But I can, L.Motl... Main current (I) is the output and V is the input. So far, I'm stuck at this: TF = (Q1(s) + Q2(s)) / ((30 + s)(Q1(s) + Q2(s)) + 20 *...
As an exercise I sat down and derived the magnetic field produced by moving charges for a few contrived situations. I started out with Coulomb's Law and Special Relativity. For example, I derived the magnetic field produced by a current $I$ in an infinite wire. It's a relativistic effect; in the frame of a test charge,...
recently, I stumbled accross a concept which might be very helpful understanding quasiparticles and effective theories (and might shed light on an the question How to calculate the properties of Photon-Quasiparticles): the spectral function $$A\left(\mathbf{k},\omega \right) \equiv -2\Im G\left(\mathbf{k},\omega \right...
In Landau and Lipshitz's introductory book on Quanum Mechanics, "Quantum Mechanics Non-Relativistic Theory, Third Edition: Volume 3", chapter XIV (page 433 in the edition on Amazon) is "Addition of Angular Momenta". Interestingly, the first page is footnoted to the effect that the theory covered only applies in the lim...
This question on binary black hole solutions, led to me think about the similar question from the perspective of what we know about the Hydrogen atom. Prior to quantum mechanics, it was not understood what led to the stability of the Hydrogen atom against collapse of the electron orbits due to Bremsstrahlung, i.e. the ...
This paper seems to show that $d=4, N=8$ supergravity is finite. Yet the paper only has three citations in spires, and I certainly haven't heard talk of a new candidate theory of gravity. Why isn't perturbative supergravity with some supersymmetry breaking principle, coupled with the standard model considered a poss...
Meaning, why is it the exact number that it is? Why not $2\times10^8$ m/s instead of $3$? Does it have something to do with the mass, size or behavior of a photon? To be clear, I'm not asking how we determined the speed of light. I know there isn't a clear answer, I'm really looking for the prevailing theories.
Can someone give a simple expose on Coleman Mandula theorem and what Mandelstam variables are? Coleman-Mandula is often cited as being the key theorem that leads us to consider Supersymmetry for unification. An overview discussion with sufficient detail is missing in many popular texts. So how does Coleman Mandula th...
Is it possible to describe the Local F-theory models which seems to be relevant for particle physics phenomenology using Gauged Linear Sigma Models (GLSM), this includes describing the decoupling limit using the parameters of the GLSM? Actually, I am not sure if my question makes sense. Another related question is: Can...
This question asks what constraints there are on the global topology of spacetime from the Einstein equations. It seems to me the quotient of any global solution can in turn be a global solution. In particular, there should be non-orientable solutions. But does quantum physics place any constraint? Because it seems to ...
Possible Duplicate: Why does space expansion not expand matter? Does cosmological expansion have any effects at the atomic level?
In all the canonical approaches to the problem of quantum gravity, (eg. loop variable) spacetime is thought to have a discrete structure. One question immediately comes naively to an outsider of this approach is whether it picks a privileged frame of reference and thereby violating the key principle of the special rela...
Under what conditions does a system with many degrees of freedom satisfy the equipartition theorem?
I'm asking for a qualitative explanation if there is one. My own answer doesn't work. I would have guessed it's because when a gas has pressure the kinetic energy adds to the rest mass of a given quantity of the gas, so the pressure contribution would be equal to whatever energy density it contributes. But that can...
The Schwarzschild metric reduces to the Minkowski metric in the limit of vanishing $M$, but the Hawking temperature which is proportional to $1/M$ diverges in the same limit. This would imply that flat spacetime has infinite rather than zero temperature. What am I missing? EDIT: This question is back up on the front pa...
1) Can a state be entangled without also being a superposition? (Please give an example.) 2) Can a state be a superposition without being entangled? (Again, an example please.) 3) And what about a cat state? I am royally confused, and a little bit of googling didn't help. This question is partly motivated by the ...
Question: If there are two conducting spherical shells and the inner shell is grounded, what will be >the charge density in the inner shell if there is a charge Q placed on the outer shell? Yes, this is a HW problem, but I am not asking you guys to solve it for me... just show me the way :D If there is a charge on t...
I am not a professional physicist, so I may say something rubbish in here, but this question has always popped in my mind every time I read or hear anyone speak of particles hitting singularities and "weird things happen". Now to the question at hand, please follow my slow reasoning... As far as I've learned, to reach...
This problem is giving me a lot of problems. So $E=k*q/d^2$. We'd want to find the distance from q1 to P, which is .1 meters (not cm) using pythagorean thereom. So we know k, which is just $9x10^9$ times q1 which is $-2.4u$ where $u=10^{-6}$ divided by $r^2$ which is just $.1^2$. Then I get $-216,000,000$. To get x co...
The method of image charges is a well-known and very useful tool for solving problems in electrostatics. Unfortunately, when I was taught this method, it was presented simply as an algorithm. No real physical justification was given for its usage, and there was a complete absence of rigorous mathematics. The method has...
Below is attached for reference, but the question is simply about whether vectors used in physics in a vector space can be represented by complex numbers and whether they can be divided. In abstract algebra, a field is an algebraic structure with notions of addition, subtraction, multiplication, and division, satisfyi...
What substance will have the largest specific heat capacity integrated from T=0 to, say, room temperature? In other words, given a finite amount of mass, what object or collection of objects has the largest number of degrees of freedom that can be excited as it absorbs energy starting from T=0? Would it be a complica...
The heterotic string is a combination of right-moving excitations from a $\mathrm{D} =10$ superstring and left-moving excitations from a $\mathrm{D} =26$ bosonic string, with the left-movers behaving as if the extra 16 dimensions are compactified. The heterotic string is also derived from $\mathrm{D} =11$ M -theory , a...
Berry-Pancharatnam phase is the phase that quantum systems exhibit when they pass through a sequence of states and return to their original state. It's a complex phase and it is different from the usual complex phases in that it does not depend on the arbitrary complex phases present in quantum states. For an encyclope...
I am puzzled by some data I see for a compressor. From what I read, the maximum discharge pressure reachable with a reciprocal compressor would vary depending on the nature of the gas used, and its molecular mass (maximum pressure lower with a heavier gas). This does not seem to fit with my quickly jotted notes from a ...
Have some trouble finding a useful reference to answer the following questions: What's a typical electron-hole recombination time of semiconductors? And how does it depend on the temperature and the band gap? If there's no formula, examples will do. What's the probability of an electron being kicked into the conductio...
This is a classic trick to do with a IR camera: Bu why is the plastic bag transparent, while the glasses aren't? I've also heard that water is not transparent in IR light. What causes this phenomena?
A previous Stack question (before I joined) asking about continuity in GR received replies which suggested that Curvature would be discontinuous at say a planetary boundary (assume no atmosphere for simplicity). I will analyse some basics of this and then return to that question. It is true that the Stress-Energy Tenso...
In Physics class we were doing the two slit experiment with a helium-neon red laser. We used this to work out the wavelength of the laser light to a high degree of accuracy. On the piece of paper the light shined on there were patterns of interference, both constructive and destructive. My question is, when the part of...
Topological insulators are materials known to have bulk insulator and metallic surface states. But, what is the origin of these metallic surface states? And how the topology of band could help the understanding of these states?
I know they have their adherents, but do more or less esoteric branches of mathematics such as Category Theory and/or Quantum Logic provide powerful tools for new theory development or are they just occasionally-useful presentational frameworks? More particularly, is it worth investing the time to study these formalism...
John Cramer’s transactional interpretation of quantum mechanics (TIQM) is billed as resolving the fuzzy agnosticism of the Copenhagen interpretation while avoiding the alleged ontological excesses of the Many Worlds Interpretation. Yet it has a low profile. Is this because no-one care anymore about ontology in physics,...
I've got a basic understanding of these facts: The Universe is a little over 13 billion years old. Our Galaxy is almost that old. Our Solar System is roughly 4.6 billion years old. The heavier elements (carbon, oxygen, etc...) are only produced by stars. When our Solar System coalesced, the matter that makes up the ...
I've been very impressed to learn about kaluza-klein theory and compactification strategies. I would like to read more about this but in the meantime i'm curious about 2 different points. I have the feeling that there are no precise answers to these questions at the current moment but nonetheless i feel obligued to ask...
I've been looking at the, now very popular, graph of the SM Higgs decay branching ratios: (source: fnal.gov) You see that the ZZ branching ratio has a funny dip around the $170\, GeV$, very different from the WW counterpart. It's true that graphs with Z will always have the $\gamma$ interference and, perhaps, this is ...
How much information is contained in one qubit? A qubit is defined in Wikipedia as $a\left\|0\right> +b\left\|1\right>$, where a and b are complex numbers subject to $a^2 + b^2 = 1$. One complex number is equivalent to two real numbers, which suggests that a qubit is equivalent to four real numbers worth of information....
What are the current most important theoretical problems on quantum entanglement? What is that we don't yet understand about how it works? (Not considering interpretation etc problems)
I was reading this interesting recent review on arXiv about particle identification: "Particle Identification" by Christian Lippmann (2011), arXiv:1101.3276 In figure 2, there is an interesting comparison between the CMS and ATLAS calorimeter performances. I hope the paper author won't mind if I reproduce the image her...
The title says it all. Why are snowflakes symmetrical in shape and not a mush of ice? Is it a property of water freezing or what? Does anyone care to explain it to me? I'm intrigued by this and couldn't find an explanation.
Whereas I can calculate the Chern number of a quantum state (or band) from the integration of the Berry curvature in all space. How can I infer the topology of the quantum state from this result? What is the physical meaning of a quantum state with non-zero Chern number?
Watching quantum mechanics lectures and it was mentioned that it is pointless/meaningless to try to talk/question things that can not be tested/measured. Is this a principle? And if so what is it's name? Also does this apply to questions other than Quantum mechanics? E.g. does it make sense to ask if earth was the only...
Cold fusion is being mentioned a lot lately because of some new setup that apparently works. This is an unverified claim. See for example: http://hardware.slashdot.org/story/11/01/24/1550205/Italian-Scientists-Demonstrate-Cold-Fusion http://www.physorg.com/news/2011-01-italian-scientists-cold-fusion-video.html http://...
Proton cuircuits and proton motive force are part of standard discussion in biology and processes involving photosynthesis. The sort of proton currents discussed in biology are obviously slightly different than the magnetically confined proton currents in an acclerator like LHC, however, although we are very familar w...
I need a solution to the heat equation that shows temp increase in an object, e.g. a cube or sphere, in sunlight. The object is assumed to be exposed on all sides except one. It is a solid object with a certain surface emissivity and heat capacity. In other words, I don't care what is inside the object. It absorbs ...
Assuming I have a body travelling in space at a rate of $1000~\text{m/s}$. Let's also assume my maximum deceleration speed is $10~\text{m/s}^2$. How can I calculate the minimum stopping distance of the body? All the formulas I can find seem to require either time or distance, but not one or the other.
Given that the electrical force is so much stronger than gravitational force at atomic levels, why is it that it's the gravitational force between you and the earth that keeps you on the ground rather than the electrical force between you and the earth?
One of the great unheralded advances made in the history of science was the ability to determine the age of Earth based on the decay of isotopic uranium. Based on the apparent abundance of uranium in the early Earth, what conclusions can be drawn about the star that preceded the Sun?
Could you please help me understand how one can measure output frequency of free electron laser (provided that we know size of magnetic domains and electron energy)? This should be a function of magnetic domains size & electron velocity? But electron velocity should drop due to photon emission - does that means that FE...
Imagine opening a water tap in order to have a smooth and cylindrical outflow and then slowly decrease the flow by adjusting the knob. At a certain moment, the side profile of the flow will become uneven, and even if you decrease the scale of the faucet, the water shall start to drip. It is possible to determine, based...
Some weeks ago, there was lots of talk about this CDF paper: Evidence for a Mass Dependent Forward-Backward Asymmetry in Top Quark Pair Production where they measured a much higher asymmetry than the one predicted in the SM. In trying to understand the paper, I had some question which I thought would fit in a discussio...
I'm looking for simple problems in theoretical mechanics that are impossible or unreasonably difficult to solve by means of "brute-force" numerical integration of Newton or Euler-lagrange equations. I'm interested in these beacuse I noticed that kind of "computer-nihilism" point of view is getting popular (at least am...
Some explanations of the device base it on a simple echo of light: "The camera transmits invisible near-infrared light and measures its “time of flight” after it reflects off the objects. Time-of-flight works like sonar: If you know how long the light takes to return, you know how far away an object is." (wired.com). I...
When I dip a paper towel in a cup of water the water gets drawn up due to capillary action. How is this reconciled with conservation of energy, as it would seem on the surface that the potential energy of the system has gone up with no work being done?
Dirac came up with a general theory of constraints, including second-class constraints. To quantize such systems, he first computed the Dirac bracket classically, and only then "promoted" the classical Dirac bracket into a commutator. However, this leads to operator ordering ambiguities over and above what already exis...
The so called consistent histories interpretation is claimed to be a correction of the Copenhagen Interpretation. One of its aim, as much as I can see is to show that observers don't have any special role and a history is represented as a sequence of projection operators in the Heisenberg picture. It has been convincin...
I feel this is somehow a stupid question, but I don't know the true answer. What happens to an astronaut who's floating in a spaceship in space when it begins to move? Will the astronaut not move until he smashes onto a wall in the spaceship? Or will he move with the spaceship due to gravity? Or does it depend on the s...
I have never seen a “causality operator” in physics. When people invoke the informal concept of causality aren’t they really talking about consistency (perhaps in a temporal context)? For example, if you allow material object velocities > c in SR you will be able to prove that at a definite space-time location the phys...
I am trying to calculate $\sqrt{-g}$ in terms of a background metric and metric perturbations, to second order in the perturbations. I know how to expand tensors that depend on the metric, but I don't know how to expand the metric determinant. Does anyone know how to to this or (even better) know of a source that does ...
I was just cooking some noodles and staring at the pot waiting for them to cook made me wonder... Will my food cook quicker when the water is boiling and bubbling, or when it is at a temperature where it is about to start boiling but there are no bubbles in the water yet? My instinct would be that food will cook quicke...
When a quantum system have a double degenerescence at one point, the Hamiltonian should be proportional to Pauli matrices near this point (also known as diabolic point) [Ref.]. But, why the Hamiltonian have this form?
$:\!\!\hat\phi(x)^2\!\!:$, for example, constructed from the real Klein-Gordon quantum field. For a Wightman field, the Wightman function $\left<0\right\|\hat\phi(x)\hat\phi(y)\left\|0\right>$ is a distribution, which is certainly the case for the real Klein-Gordon quantum field --- call it $C(x-y)$ in this case. In cont...
I was thinking of, for example a Schwarzchild metric at r=0, i.e. the gravitational singularity, a point of infinite density. I realise that there are different types of singularities--timelike, spacelike, co-ordinate singularities etc. In a short discussion with Lubos, I was a bit surprised when I assumed they are ide...
By now everybody knows that gravity is non-renormalizable, what is often lacking is a simplified mathematical description of what that means. Can anybody provide such a description?
The textbooks I have available explain that due to the infinite degrees of freedom of a field, the relevant object in QFT is the Lagrangian density. A Lagrangian is then obtained for the field by integrating over space. I find the justification for this procedure unclear. In classical mechanics, the Lagrangians of two ...
Does anyone have a good explanation of the physics and vectors of force involved in the skateboarding trick the ollie (where the skater jumps and causes the skateboard to rise off the ground with him)?
Trying to secure a wall hanging using magnets; me and a coworker came up with an interesting question: When the hanging is hung using 1 magnet, the weight of it causes it to quickly drag the magnet down and the hanging drops. Using n magnets retards this process; causing it to fall more slowly, but does there exist a ...
Possible Duplicate: Hamilton's Principle The Lagrangian formulation of Classical Mechanics seem to suggest strongly that "action" is more than a mathematical trick. I suspect strongly that it is closely related to some kind of "laziness principle" in nature - Fermat's principle of "least time", for example, seems a ...
In this review of the QHE, Steve Girvin makes the following statement (bottom of pg. 6, beginning of Sec. 1.1.1): As one learns in the study of scaling in the localization transition, resistivity (which is what theorists calculate) and resistance (which is what experimental- ists measure) for classical systems (in the...
I have read in a couple of places that $\psi(p)$ and $\psi(q)$ are Fourier transforms of one another (e.g. Penrose). But isn't a Fourier transform simply a decomposition of a function into a sum or integral of other functions? Whereas the position and momentum wavefunctions are essentially different but related. The...
Previous posts such as this ask about types of stationary point in Hamilton's Principle. There is, however, another aspect to discuss: the question as to whether the extremal path is unique. One geometric way to envisage this is to assume that multiple paths are simultaneously extremal. I believe that this is an explan...
In our Physics lab we have a 1 milliwatt (0.001W) helium neon laser. Despite the low power, we were cautioned not to even look at reflections of the beam as it could cause permanent eye damage - why is such a low power level hazardous? I have a 3W LED which I can look directly into which is more than 3,000x more powerf...
One of the key elements of any quantum mechanical system is the spectrum of the Hamiltonian. But what about in quantum field theory? It seems as if nobody ever discusses the spectrum of a system at all -- or have I missed something? Just what role does the spectrum of any given potential play in field theory? To pu...
I read that if we have a system with two co-existing phases with chemical potentials $\mu_1$ and $\mu_2$, respectively, then, at the equilibrium, the concentrations $X_1$ and $X_2$ are related by the equation: $$X_1=X_2\exp\left(\frac{-(\mu_1-\mu_2)}{kT}\right).$$ Is that true? How can I derive this expression?
In somes references of Relativistic Quantum Mechanics, the one-particle states are given by representation theory of Poincaré algebra. Could I mimics this for the non-relativistic case? States in non-relativistic quantum mechanics could be seen as representation of Galilei algebra (or Galilei group)?
This question arose in a seminar today about the solar wind... This is my vagueish understanding of the problem - please correct if you see errors! The 'classical' picture of atmospheric electricity is that the Earth as a whole is neutral, but that thunderstorms maintain a voltage of around +300kV at the electrosphere ...
The heliopause is now estimated to be something around 100 AU (1 AU = Astronomical unit = about the earth sun distance). See the wikipedia article: http://en.wikipedia.org/wiki/Heliosphere From reading a book on NASA's Voyager mission, I learned that before the launch of these spacecraft, the expectation was that the h...
The one difficulty I see with LQG is that it requires an enormous number of degrees of freedom, e.g. these spin variables in the net. This is in contrast to stringy holographic theory where the fields in a space are equivalent to fields on a boundary or a horizon of one dimension lower. In this setting entropy of a b...
There exists this famous Haag's theorem which basically states that the interaction picture in QFT cannot exist. Yet, everyone uses it to calculate almost everything in QFT and it works beautifully. Why? More specifically to particle physics: In which limit does the LSZ formula work? Can someone give me an example of...
Newton's Bucket This thought experiment is originally due to Sir Isaac Newton. We have a sphere of water floating freely in an opaque box in intergalactic space, held together by surface tension and not rotating with respect to the distant galaxies. Now we set the box and water to rotate about some axis and we notice t...
Why, in somes cases, such as spin 1/2 systems like in the Stern-Gerlach experiment, the representation of some kets has a term with an complex exponential? For example: $ \vert S_{x}; + \rangle = \dfrac{1}{\sqrt{2}} \vert + \rangle + \dfrac{1}{\sqrt{2}} \exp [i \delta] \vert - \rangle $
In a couple of letters dated 1923, Pauli writes to Sommerfeld and Lande about Zeeman effect, and he describes (a piece of) quantisation in a peculiar way: the substitution of the "Differentialquotient" $$ {d \over dj} {1 \over j} $$ ($j$ is called a impulsquantenzahlen? is it an angular momentum, isn't it?) by the "Dif...
Can someone explain (as rigorously as possible) what is involved in analytically continuing, say, the Schwarzschild solution to the Kruskal manifold? I understand the two metrics separately but I'm not sure how analytic continuation is used, since I can't really see how the process of extending the domain of a complex ...
Last month, a puzzle here in physics.stackexchange asked to calculate the dynamics of the outflow in a loose (over rails, no friction) tank using Torricelli's law as a simplification. The simplification turned out to be problematic; one would prefer some solution where the height $h(0)=h_0$ starts decreasing with the u...
Recently I saw some physical problems that can be modeled by equations with fractional derivatives, and I had some doubts: is it possible to write an action that results in an equation with fractional derivatives? For example, consider a hypothetical physical system with the principle of least action. Is there a "wave ...
FYI: I imagine the physics of thunderstorms involves wind stripping electrons from condensed water cells (raindrops or snowflakes). Like a Van De Graaff generator, sort of; I'm also using Wilson's cloud chamber concept of an ion at the core of a droplet. Not sure if global warming promises higher winds, but I think so....
Yesterday I looked underwater with my eyes open (and no goggles) and I realized I can't see anything clearly. Everything looks very, very blurry. My guess is that the eye needs direct contact with air in order to work properly. With water, the refraction index is different, and the eye lens are not able to compensate f...
My question is the following. Are there solutions to the Einstein field equations, which have the property that there is a hypersurface of constant time and to the past of that surface space is empty (Minkowski space-time) and to the future it is not (non-vanishing stress-energy tensor)? At first thought it seems stran...
Measuring a qubit and ending up with a bit feels a little like tossing out infinities in renormalization. Does neglecting the part of the wave function with a vanishing Hilbert space norm amount to renormalizing of Hilbert space?
LQG formulations have a minimum length/area. Since say, a Planck area can always be boosted, any minimum area in space can be shrunk. Do LQG proponents worry about local Lorentz invariance violation, and if not, why not? In LQG, does considering length to be a quantum operator really get rid of the boost problem?
Can anyone tell me where can I read about the notion of "short" and "long" representations? Like what they are etc. From where can I learn the arguments which show that the bosonic subalgebra of $\cal{N}=2$ and $\cal{N}=3$ Lie superalgebra are given by $SO(3,2)\times SO(2)$ and $SO(3,2)\times SO(3)$ respectively? I...
I find myself greatly fascinated by physics and astronomy, though I really do not understand most of it. For example, I avidly watch programs and read books that discuss our current search for a Unified Theory which will unite the major forces (Strong Interactive, Weak Interactive, Electromagnetism on the one side, wit...
Now that an equivalence of Navier Stokes and Einstein equations has been established, and it is known solutions to Einstein–Maxwell–Boltzmann exist and are unique, and it is known that Einstein equations in a wave gauge with electromagnetic source with a Lorentz gauge also have unique solutions that exist, what are the...
A barrel/drum with a diameter of 60cm is rotating at 20RPM to get a good mixing of the fluid contained (type thick oil). At what RPM should a barrel of 30cm rotate to get the same mixing efficiency with the same fluid? To get the same peripheral velocity, the RPM should be 40RPM, but will this give the same mixing of ...
How the notion of weak measurement resolves Hardy's paradox?
Consider a particle in a potential well. Let’s assume it’s a simple harmonic oscillator potential and the particle is in its ground state with energy E0 = (1/2) ℏω0. We measure its position (measurement-1) with a high degree of accuracy which localises the particle, corresponding to a superposition of momentum (and the...
The Feynman Checkerboard Wikipedia article states: "There has been no consensus on an optimal extension of the Chessboard model to a fully four-dimensional space-time." Why is it hard to extend it to more than 1+1 dimensions?
What is the meaning, mathematical or physical, of the anti-commutator term? $$\langle ( \Delta A )^{2} \rangle \langle ( \Delta B )^{2} \rangle \geq \dfrac{1}{4} \vert \langle [ A,B ] \rangle \vert^{2} + \dfrac{1}{4} \vert \langle \{ \Delta A, \Delta B \} \rangle \vert^{2},$$ where $\Delta A, \Delta B, A$ and $ B$ are ...

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