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The correlation functions found in Barouch and McCoy's paper (PRA 3, 2137 (1971)) for the XX spin chain use a method which uses Wick's theorem. For the zz correlation function, this gives $\langle \sigma_l^z \sigma_{l+R}^z \rangle = \langle \sigma_l^z \rangle^2 - G_R^2$ where for $R=1$, $G_1 = -\langle \sigma_l^x \sig...
Dear Community, recently, Transformation optics celebrates some sort of scientific revival due to its (possible) applications for cloaking, see e.g. Broadband Invisibility by Non-Euclidean Cloaking by Leonhard and Tyc. Willie Wong explained to me in another thread how a so-called material manifold could be used to incl...
Every time we have to stop the car, it is costing us extra money, because we have to then accelerate to full speed again. I would like to know how much. In order to simplify the situation, we can make these assumptions. we disregard cost of car maintenance (brake pads wearing out etc.). This is focused entirely on fue...
I need "educated" reasons whether it is safe to use any wireless device during a lightning storm. Most people said don't use it but they cannot explain why.
I'm going to be teaching a course on gauge/gravity duality (aka AdS/CFT) in the winter. The focus will be on applications in particle theory including $N=4$ SYM, the viscosity/entropy bound, and aspects of large $N$ QCD. I would like to include at least one application to condensed matter physics. There has been a lot ...
Consider a spherical rigid stone rotating with angular velocity $\omega$ being dropped vertically onto a horizontal rigid surface with the coefficient of friction $\mu$. Can the stone roll on the surface? If it can, what type of the motion is it? Constant velocity, constant accelaration, or varying acceleration? Ignore...
I know that it's possible to make an object levitate using an electromagnet to hold it up. But is it also possible to do this with regular magnets? Is there a special kind of magnet I need in order to have one powerful enough to hold an object up? I'm asking because I have this idea in mind where I want to make a deco...
If the earth's gravity exerts a net downward gravitational force on all air molecules, how come the molecules don't eventually lose their momentum and all settle down? How is the atmosphere is still miles thick after billions of years?
Something I have read multiple times that I've never intuitively understood is that "heavier" particles are harder to detect than "lighter" ones... For example, I quote from Stephen Hawking's "The Grand Design" in relation to supersymmetry: But various calculations that physicists have performed indicate that the pa...
How is the following classical optics phenomenon explained in quantum electrodynamics? Reflection and Refraction Are they simply due to photons being absorbed and re-emitted? How do we get to Snell's law, for example, in that case? Split by request: See the other part of this question here.
At the moment I am studying operator algebras from a mathematical point of view. Up to now I have read and heard of many remarks and side notes that von Neumann algebras ($W^*$ algebras) are important in quantum physics. However I didn't see where they actually occur and why they are important. So my question is, where...
Surely group theory is a very handy tool in the problems dealing with symmetry. But is there any application for ring theory in physics? If not, what's this that makes rings not applicable in physics problems?
In the standard model of particle physics, there are three generations of quarks (up/down, strange/charm, and top/bottom), along with three generations of leptons (electron, muon, and tau). All of these particles have been observed experimentally, and we don't seem to have seen anything new along these lines. A prior...
How is the following classical optics phenomenon explained in quantum electrodynamics? Color According to Schroedinger's model of the atom, only particular colors are emitted depending on the type of atom and the state of its electrons. Then, how it is possible that the same type of matter changes color in relation t...
Possible Duplicate: Is it safe to use any wireless device during a lightning storm? We often hear the tale that a person gets stuck by lighting when he is unfortunate enough to use a cell phone outdoor. But why is this so?
From Wikipedia entry on Kinetic Theory The temperature of an ideal monatomic gas is a measure of the average kinetic energy of its atoms. Now if I remove all the particles from the box shown below will the temperature be zero?
Anyone have good mnemonics for remembering standard packets of data in physics? Any field within physics would be welcomed. Examples of such "packets": data in the standard model of particle physics charges/masses of common subatomic particles and light atoms cosmological data -- numbers, masses, luminosities (I reca...
Would an object like a wooden bed interfere or block the signal coming from a 802.11 wireless router?
We found that water with salt, sugar, or baking soda dissolved in it cools faster than pure water. Water has a very high specific heat; how do these solutes lower it? We heated a beaker (300ml) of water to 90° C and let it cool, checking the temperature every 5 minutes. We repeated the experiment adding a tablespoon o...
Let's look at the measurement problem in the orthodox interpretation of quantum mechanics as an inconsistency between inner and outer treatment of the measurement apparatus. You can always push your boundaries of treating the evolution of your system as unitary further and further. You can say OK, the universe as a who...
'tis the season as they say! It seems to me obvious that it's better to drive in high gear on snowy roads to reduce the torque. However, there are completely opposite advices being given on different sites: weather.com says "Use low gears to keep traction" bbc.co.uk says "Stay in a higher gear for better control" Who...
I searched and couldn't find it on the site, so here it is (quoted to the letter): On this infinite grid of ideal one-ohm resistors, what's the equivalent resistance between the two marked nodes? With a link to the source. I'm not really sure if there is an answer for this question. However, given my lack of experti...
It is said that Klein-Gordon equation is a relativistic version of the Schrodinger equation. In Schrodinger equation, it is straightforward to include potential energy. But for K-G eqn things seem to be more complicated. To put it specifically, how can one find the energy eigenfunctions of a particle in a finite square...
I am looking for good introductory papers and/or books on the principles of lasers. In particular, I am interested in pulsed laser technology. I understand that Gould, R. Gordon (1959). "The LASER, Light Amplification by Stimulated Emission of Radiation" was one of the principal papers published by the disputed inv...
Suppose I tie one end of a rope to my ceiling and the other end to a spot on my floor directly underneath it. Because the rope has some mass, the tension varies along the rope, from highest at the ceiling to lowest at the floor. If a wave packet begins propagating down the rope, will its shape change? If so, it is po...
Closely related to this question on traveling waves on a hanging rope, I would also like to know what the normal modes are on a rope that hangs vertically, fixed at both ends. Tension in the rope increases with height, so I expect that the wavelength of standing waves decreases as we move up, and nodes are clustered s...
A while ago someone proposed the following thought experiment to me: A horse attached to a cart is resting on a horizontal road. If the horse attempts to move by pulling the cart, according to the 3rd Newton's Law, the cart will exert a force equal in magnitude and opposite in direction, cancelling each other out and t...
When you stick ice in a drink, AFAICT (the last physics I took was in high school) two things cool the drink:The ice, being cooler than the drink, gets heat transferred to it from the drink (Newton's law of cooling). (This continues after it melts, too.)The ice melts, and the cool liquid mixes with the drink, which mak...
I have looked at other questions on this site (e.g. "why does space expansion affect matter") but can't find the answer I am looking for. So here is my question: One often hears talk of space expanding when we talk about the speed of galaxies relative to ours. Why, if space is expanding, does matter not also expand? If...
To suck water through a straw, you create a partial vacuum in your lungs. Water rises through the straw until the pressure in the straw at the water level equals atmospheric pressure. This corresponds to drinking water through a straw about ten meters long at maximum. By taping several straws together, a friend and I...
What is the cross section of the smallest object that can be detected with say visible light ($\lambda$ ~380 - 750 nm) or X - band radar ($\lambda$ ~20 - 50 mm). Does the object need to have one side larger then $\frac{\lambda}{2}$ or something like that to get a reflection?
I want to compare the much-talked about Cheeta running prosthesis to a the normal running process in terms of force and energy, but I don't know where to start. How would you start the comparison? A mathematical model would be much appreciated.
Is is possible to set a swing into oscillations without touching the ground? This occurred to me while watching the second pirates movie. There is a scene where the ship's crew is suspended in a cage from a bridge in between two cliffs. They escape by swinging the cage towards one of the cliff. Is that even possible? ...
Consider an infinite square grid, where each side of a square is a spring following Hooke's law, with spring constant $k$. What is the relation between the force and displacement between two points? If they are proportional, what is the equivalent spring constant between the origin and the point $(x,y)$ (integers) ? Ed...
Dark energy is introduced as a constant inside Einstein's equations. Its primary purpose, from what I understand, is to make Einstein's equations compatible with the accelerating expansion of the universe. As a consequence, of the "predictions" of dark energy is the expansion of the universe according to Hubble's law. ...
Can someone please explain what a potential is? Example. velocity potential in ideal flows, acoustic potential (gradient of which gives the particle velocity in a sound wave). Whenever I see potential functions complex analysis is applied to compute integrals, complex functions for conformal mapping. I vaguely understa...
This is not another question about faster-than-light travel or superluminal communication. I totally appreciate the speed limit capped by physical laws (or theories.) Just curious, since there is no limit for phase velocity, is there any way to generate a wave with controllable phase velocity higher than speed of ligh...
I have always pictured volume element as a small cuboid in with volume $dx dy dz$. however in curvilinear system, how would the shape of this volume element be? I mean in spherical polar coordinate system, how the shape of this volume element be visualized (is it a small sphere whose integration give the volume of obje...
Laser beams are said to have high "spatial coherence". This means that the beam is highly concentrated even at long distances (low spread). Can this be achieved with radio waves (much longer waves) or is it due to laser's stimulated emission?
I remember a while ago my father dropped a glass lid and it smashed. It looked something like this. When that happened, for about 5 minutes afterwards, the glass parts were splitting, kind of like popcorn, and you could hear the sound. I was just wondering why this happened, and the particles didn't just sit quietly in...
I essentially have three questions concerning weapons based on EM waves or more generally. Focusing on the weapons using radio-waves and/or micro-waves, what power do these types of weapons need to radiate to generate the described effects? How easy or difficult would it be to make your own weapon or to acquire one. A...
I have a 4' hose that is closed at one end and connected to a Airdata Test Set (precise control of pressure) and a high accuracy pressure monitor on the other end with a T and valve. The valve allows the Airdata Test Set connection to be closed off resulting in a hose connected to the pressure monitor and closed at th...
Could you suggest any if it's possible to deposit a layer of $\rm SiO_2$ via PVD? Why doesn't it widely used? Is that correct that I need $~1600\ C$ in vacuum?
I just saw this video which was kind of nifty. What principles govern this? Is it simply that 700 lbs of air pressure are exerted from that little 1 HP blower? What would you have to take into consideration to adopt the same principles to move something like a car on a turntable? Is moving something with air more econo...
I read somewhere that according to relativity, it is possible - involving black holes and other stuff - to jump into the past. Is it possible for anything to go back in time either continuously or by jumping?
There are many videos on youtube in which people arranged magnets in circle and rotated one placing in middle of that circle on a shaft, and the magnet (magnet motor) starts madly and continues its movement. Do they really rotate infinitely? If so, do they from where they get that extra energy to move so fast and infin...
In the classic metaphor, a light beam bends for the same reason that a wagon getting one wheel stuck in the sand does...the wheels travel at uneven speeds, and the wheel on the smoother surface travels faster. But the key to the wagon scenario is the axle - if the two wheels were not bound, the faster wheel would sail ...
I am a physics novice. Google tells me that electron microscopes work much like their optical counterparts -- but the analogy falls apart for me when I think about what I'm "viewing." Obviously, you can see light through the lenses, but what is the "image" analog for electron microscopes? Is it at-all like spraying an...
This question has arisen from a wish to understand an end-of-universe scenario: heat death. Are time and mass intrinsically linked? If so, does time "run slower" (whatever that may mean) in a universe with less mass? And ultimately, in a universe without any mass, just space and energy (photons), then the universal clo...
Is it possible for information (like 1 and 0s) to be transmitted faster than light? For instance, take a rigid pole of several AU in length. Now say you have a person on each end, and one of them starts pulling and pushing on his/her end. The person on the opposite end should receive the pushes and pulls instantaneou...
As I understand it (and admittedly it's a weak grasp), a computer processes information irreversibly (AND gates, for example), and therefore has some minimum entropy increase associated with its computations. The true entropy increase is much greater, and comes from the conversion of electrical energy to heat. How eff...
Hey guys, I have a final tomorrow and I am going over some assignments. One of the questions from the assignment was: A photon having 37 keV scatters from a free electron at rest. What is the maximum energy that the electron can obtain? Now, I got the right answer (4.67 KeV) but I can't seem to figure it out n...
How does the optical zoom works.. Does it brings the light closer to the object... Or Does it just enlarges the picture of the object. In other words... Suppose I have telescope with 1 light-year zoom capacity, could I see stars 1 year ahead of its light reaching earth... Ps : I'm not physics graduate, but interested...
Background: I'm in a legal academic discussion about the status of electronic 'goods' and whether they qualify as 'goods' in the same way a chair and a pen do. In this context (and specifically at the exact circumstance under discussion), it matters if electricity is 'tangible'. So far, most authors have blindly assume...
It's known that single spherical lens cannot focus parallel beam of monochromatic light into single (diffraction-limited) point, so it has to have aspherical shape to achieve that. Is perfect analytical lens shape is known that is able to focus light into a single point (again, light is monochromatic)? Is there a unive...
Another interesting infinite lattice problem I found while watching a physics documentary. Imagine an infinite square lattice of point masses, subject to gravity. The masses involved are all $m$ and the length of each square of the lattice is $l$. Due to the symmetries of the problem the system should be in (unstable) ...
If you take a hardboiled egg and put it on a table and start to spin it, if you spin it fast enough it will start to spin in an upright position. What is the angular velocity needed for this transition to occur? Is energy conserved in the transition? Must there be some imperfection in the egg for the transition to occu...
How big could an array of space telescopes acting as an interferometer be ? How big would it have to be to resolve exoplanet surface detail the size of Iceland at a distance of 100 light years ?
I recently heard about jet quenching concerning data taken by the experiments at the LHC. Apparently it is related to the existence to the quark-gluon plasma. As far as I understood this interpretation is an analogy to hydrodynamics (the jet is "blocked" by the plasma). I would like to understand that in more details: ...
I'm interested in the meaning of the phrase "continuum limit" specifically as it is often used in expressions relating to the ability of a quantum gravity theory to recover GR in the continuum limit. I believe I understand the meaning but want to make sure I am not missing some important part of the precise definition ...
What ever happened to "action at a distance" in entangled quantum states, i.e. the Einstein-Rosen-Podolsky (EPR) paradox? I thought they argued that in principle one could communicate faster than speed of light with entangled, separated states when one wave function gets collapsed. I imagine this paradox has been resol...
It's a Christmas time and so I hope I'll be pardoned for asking a question which probably doesn't make much sense :-) In standard undergraduate nuclear physics course one learns about models such as Liquid Drop model and Shell model that explain some properties of nucleus. But as I understand it these models are purel...
I'm interested in learning physics. I do realize that the subject is large and that it would be easier if I had a specific area of interest. However, I do not. I suppose I want to learn about the fundamentals of it all; the axioms that combine all physics fields. Or, in other words, a high school physics class. Specif...
Theory predicts that uniform acceleration leads to experiencing thermal radiation (so called Fulling Davies Unruh radiation), associated with the appearance of an event horizon. For non uniform but unidirectional acceleration the shape of the experienced radiation changes from thermal to other spectral densities, bu...
I heard somewhere that quarks have a property called 'colour' - what does this mean?
As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects being affected by gravity. However, momentum is the product of mass and velocity, so, by this definition, massless photons ...
I was told that the Galilean relative velocity rule does not apply to the speed of light. No matter how fast two objects are moving, the speed of light will remain same for both of them. How and why is this possible? Also, why can't anything travel faster than light?
I've noticed that an airplane appears to have more lift when it's almost touching the ground then it has 100 feet or more in the air. What causes this to occur?
Pardon me for my stubborn classical/semiclassical brain. But I bet I am not the only one finding such description confusing. If EM force is caused by the exchange of photons, does that mean only when there are photons exchanged shall there be a force? To my knowledge, once charged particles are placed, the electromagne...
It seems like among the electrical conductors there's a relationship between the ability to conduct heat as well as electricity. Eg: Copper is better than aluminum at conducting both electricity and heat, and silver is better yet at both. Is the reason for this known? Are there materials that are good at conducting ele...
I've been reading this review on Giant Magnetoresistance, and something about it is bothering me. The basic effect is that, using a special "stack" of layers (alternating between ferromagnetic and not magnetic) you can cause great variations on the electrical resistance of the material by applying a small magnetic fiel...
What is the theoretical difference between the physical elementary interaction that causes an e+ to attract an e− when they exchange a virtual photon? Why is this exchange different from an e-e- scattering (which produces repulsion)? This question is related to but more specific than the recently posted The exchange of...
Take a glass of water and piece of toilet paper. If you keep the paper vertical, and touch the surface of the water with the tip of the paper, you can see the water being absorbed and climbing up the paper. This climbing up is intriguing me. Where is the energy coming from? Supposedly, a lot of water molecules are movi...
I am working on a project that requires some amount of number crunching. I require some basic data about the galaxy M82 and our own Milky Way (particularly luminosities). My advisor is a nut and he blurts out numbers from his head that seem to be right. All the more, I would like to be sure. So is there any place where...
In an astronomy forum that I frequent, I have been having a discussion where the state of quantum gravity research came up. I claimed that Loop Quantum Gravity theories couldn't prove GR in the continuum limit, nor could they compute Mercury's perihelion because they didn't have any matter, at this point. Another poste...
Why in the irradiament mulipoles of Lienard-Wiechert's potential we say that electric quadrupole give a contribute of the same order of the magnetic dipole? How can we see it from their equations? And is there a physical reason for this? Sorry for my trivial question.
Say that the airplane is going in 1000 km/h. On the side of the airplane, there is a 10x10 cm window. How much friction would this window cause. For the sake of the calculation, imagine that the airplane is infinitely long. The window is also perfectly level with the rest of the wall, and there is no turbulence on the ...
This question was inspired by the answer to the question "If the universe were compressed into a super massive black hole, how big would it be" Assume that we have a matter with a uniform density $\rho$. Some mass of this matter may forms a black hole with the Schwarzschild radius: $\large{R_S=c*\sqrt{\frac{3}{8\pi G\...
We are taught that the universe began as a singularity - an infinitely small and infinitely dense point. At the beginning of time there was a 'Big Bang' or, more accurately, 'Inflation'. The main evidence for this is the observation of the red shift of all of the galaxies. This shows us that as time increases, the univ...
Possible Duplicate: Faster than light information Hey there, I just want to make clear that I'm new here and I don't know whether this is the right place to ask such a hypothetical question. So if it's not, I'd be glad if someone could tell me where to ask. :) Imagine you and a friend of yours were very far away fro...
Using purely gravitational potential energy, what is the highest efficiency one can achieve with a trebuchet counter-weight type of machine? Efficiency defined here as transformation of potential energy in the counterweight to kinetic energy of the trajectory. Edit: To be more specific, we can use the following ideali...
Our ability to store data on or in physical media continues to grow, with the maximum amount a data you can store in a given volume increasing exponentially from year to year. Storage devices continue to get smaller and their capacity gets bigger. This can't continue forever, though, I would imagine. "Things" can only...
As an example of such a historical myth that we all learned, there is the story that in his confrontation with Cardinal Bellarmine in 1632, Galileo had all the evidence on his side, won the intellectual argument by the weight of that evidence, and was defeated only by the ignorant prejudices the trial judges. However...
What is the point of having 45 degreed blade in Guillotine? After all, the pressure-Force over Area-blade imposes to the neck is the same either way.
Suppose I do two experiments to find the triple point of water, one in zero-g and one on Earth. On Earth, water in the liquid or solid phase has less gravitational potential per unit mass than water in the gas phase. Therefore, the solid and liquid phases should be favored slightly more on Earth than in zero-g. In a ...
When judging if relativity is important in a given phenomenon, we might examine the number $v/c$, with $v$ a typical velocity of the object. If this number is near one, relativity is important. In optics, we examine $\lambda/d$, with $\lambda$ the wavelength of light involved, and $d$ a typical size-scale for the obj...
Could you please help me in this problem? I have 3 independent variables, (T,H,t) , as inputs and one output P ( I have all data for these inputs and the output, done experimentally measured every hour during one year). I want to find a formula of this form: P=f(T,H,t) where t is the time in hours and it is always i...
From wikipedia: When a space-like interval separates two events, not enough time passes between their occurrences for there to exist a causal relationship crossing the spatial distance between the two events at the speed of light or slower. Generally, the events are considered not to occur in each othe...
Including air resistance, what is the escape velocity from the surface of the earth for a free-flying trajectile?
I was reading up on Tesla's Wikipedia page last night, and I came across this: When he was 81, Tesla stated he had completed a "dynamic theory of gravity". He stated that it was "worked out in all details" and that he hoped to soon give it to the world. [75] The theory was never published. I was wondering...
This is my first question on this site so forgive me for the awkward wording of the question. Basically, my question is why does light from, say, a sparkler, seem to remain where it just came from to you EYE? I attached a sample photograph and i understand that cameras work differently in that they're designed to "reta...
While looking at some exercises in my physics textbook, I came across the following problem which I thought was quite interesting: It is possible for the electron beam in a television picture tube to move across the screen at a speed faster than the speed of light. Why does this not contradict special relativity? I ...
Recently a "U" shaped scratch appeared on my car's front tire. It looks like someone rolled past a curb and scraped it. This has led me to the following problem: Generate a plot (ideally polar) of a scratch pattern on a tire given tire radius and the ground height of a protrusion. Assume the protrusion is a single poin...
Is there an equation I can use to calculate the temperature (as a function of time) of an object which is gaining or losing heat by convection? Or equivalently, the rate of energy transfer from the object to the surrounding fluid (or vice-versa)? It can involve constants representing properties of the object and the su...
At the AGU, I presented a poster on vocabulary for discussing data systems, and someone left a note at my poster stating: You have a bias here towards observational data. Need to recognize that a lot of data comes from models and analyses. And I completely agree; I didn't mention models and the values that come from...
This is a question motivated by Angry Birds. When playing the game, I notice that if the initial velocity is constant, the way to land a bird furthest away from the launching point is by launching the bird at 45 degrees from the ground. Is it possible to obtain a derivation and a proof of this?
I know a lot about Theoretical Physicists from Sheldon Cooper of Big Bang Theory. I know about parallel universes (and I wonder if one codingtales in one of the parallel universes believes if parallel universes do not exist). I'm sorry, but this is just for fun. But I've real questions: Are all the equations that appe...
Inspired by a similar post, I wanted to know what video games, past or present, most accurately employ the laws of physics. I believe this is an appropriate question for this site, since the users are far more qualified to answer it than anyone on typical gaming sites, who think they know a lot more about physics than...
Does time stretch all the way back for infinity or was there a point when time appears to start in the universe? I remember reading long ago somewhere that according to one theory time began shortly before the creation of the universe. Does time have a starting point of note?
I recently became really interested in learning about physics and cosmology, but I still know very little. Hopefully someone with more knowledge will be able to shed some light on my questions. Here are my presumptions (please correct me if I'm mistaken): $t_{0}$: Big Bang. The Universe expands (and thus cools) suffic...

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