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134988 | How can I write a Gaussian state as a squeezed, displaced thermal state | I would like to write a Gaussian state with density matrix $\rho$ (single mode) as a squeezed, displaced thermal state: \begin{gather} \rho = \hat{S}(\zeta) \hat{D}(\alpha) \rho_{\bar{n}} \hat{D}^\dagger(\alpha) \hat{S}^\dagger(\zeta) . \end{gather} Here, \begin{gather} \rho_{\bar{n}} = \int_{\mathbb{C}} P_{\bar{n}}(\a... | Given a single mode Gaussian state described by the density matrix \(\rho = \hat{S}(\zeta) \hat{D}(\alpha) \rho_{\bar{n}} \hat{D}^\dagger(\alpha) \hat{S}^\dagger(\zeta)\), where \(\rho_{\bar{n}} = \int_{\mathbb{C}} P_{\bar{n}}(\alpha) |\alpha\rangle\langle\alpha|d\alpha\) and \(P_{\bar{n}}(\alpha) = \frac{1}{\pi \bar{n... | 21,981 | cqadupstack-physics |
100469 | How would charge distribute if electrons were balls? | In a conductor, any excess charge will distribute itself evenly over the surface of the conductor. Because of quantum mechanics, this is possible with small charges (i.e. 1e). But if electrons were balls, they wouldn't always be able to distribute themselves evenly over the surface of the conductor. For example, if the... | What configuration of charges would you observe on the surface of a spherical conductor if it had an excess charge of 3e, considering the discrete nature of electrons? Would they be able to distribute evenly, and if not, describe the possible arrangement. | 28,384 | cqadupstack-physics |
131536 | How much power you might need to generate dangerous EMP shock? | While playing with voltage multiplication and capacitors, I began to think if my experiments could damage my electrical equipment (eg. mobile phone) by creating strong magnetic fields over a short duration. (for example when experimenting with coil gun technology) So far, I have restrained myself to human-safe voltages... | What is the minimum power output required to generate a magnetic field strong enough to induce harmful electrical currents in nearby electronic devices, and what factors influence this threshold? | 4,504 | cqadupstack-physics |
90211 | Bound states of the Schrödinger equation on $S^1\times \mathbb{R}^3$ | I wana study the $\textbf{bound states}$ of a particle in the space $\mathbb{R}^3\times S^1$ which is under the effect of a time independent potential $V(\vec{r})$ but is free on $S^1$. So, I begin by writing the Schrödinger equation $i\hbar\frac{\partial}{\partial{}t}\psi=\left(\frac{-\hbar^2}{2m}[\nabla^2+\frac{\part... | Given the Schrödinger equation for a particle in the space \(\mathbb{R}^3 \times S^1\) under a time-independent potential \(V(\vec{r})\), how do you determine the conditions for bound states? Specifically, should you consider solutions with \(\omega\) negative or those with \((\omega - \omega_1)\) negative? Additionall... | 5,939 | cqadupstack-physics |
31429 | How long was a day at the creation of earth? | Since the earth is slowing its rotation, and as far as I know, each day is 1 second longer every about 1.5 years, how long was an earth day near the formation of earth (4.5 billion years ago)? I wouldn't assume to just do 4.5b/1.5 and subtract, because you would think the rate of change is changing itself, as seen here... | Given that the Earth's rotation slows by approximately 1 second every 1.5 years, estimate the length of a day on Earth, in hours, 4.5 billion years ago, assuming a constant rate of change over that time period. | 14,069 | cqadupstack-physics |
130406 | What is the physical importance of the commutation relations of angular momentum? | What is the physical meaning of these commutation relations: $$[L_{z},L_{\pm}]=\pm\hbar L_{\pm}\tag{1}$$ and $$[L_{+},L_{-}]=2\hbar L_{z} ~?\tag{2}$$ | Given the commutation relations $$[L_{z},L_{\pm}]=\pm\hbar L_{\pm}$$ and $$[L_{+},L_{-}]=2\hbar L_{z}$$, calculate the expectation value of the operator \(L_{z}\) in the state \( |L, m\rangle \) where \(L\) and \(m\) are the quantum numbers corresponding to angular momentum. What is the value of \(L_{z}\) in terms of \... | 34,823 | cqadupstack-physics |
83617 | Local nature of a surface charge density | Boundary S of a cavity in a very large (perfect) conductor is a connected compact (smooth) surface. A positive point charge +q is placed inside this cavity. From Gauss' law we know that the _total_ induced charge on the surface S is -q. However, is the surface charge density on S necessary _locally_ negative (at every ... | Given a positive point charge +q placed inside a cavity of a very large perfect conductor, which induces a total charge of -q on the boundary surface S of the cavity, what is the charge density on a specific point of the surface S if the distance from the point to the charge is r? Prove whether the local charge density... | 11,836 | cqadupstack-physics |
25350 | What physical interactions actually make single stars leave their binary companions at formation? | From an interesting ScienceDaily article, I read this > Before the groups of stars disperse, binary stars move through their birth > sites and the group studied how they interact with other stars > gravitationally. "In many cases the pairs are torn apart into two single > stars, in the same way that a pair of dancers m... | Given a stellar nursery with a density of 10 stars per cubic parsec, calculate the expected number of binary star systems that will interact with other stars and potentially split into single stars, assuming the initial population consists of 30% binary systems. How does this compare to a nursery with a density of 50 s... | 37,524 | cqadupstack-physics |
78307 | What does "define analytically" mean? | I'm trying to do my homework in physics but here I have found something that I am not understanding. It says: > Define analytically the terms that should stationary flow accomplish. I know that in a stationary flow the velocity, pressure, and density don't change, but I don't understand what "define analytically" means... | What are the mathematical definitions and characteristics of stationary flow in fluid dynamics, specifically regarding how velocity, pressure, and density behave over time? | 24,519 | cqadupstack-physics |
57883 | Why does tea rises in the pot but water don't? | I was wondering when I boil water in a pot it only shakes too much while boiling. But I could not figure out why tea rises in the pot when we boil it. it is also a liquid but it starts rising up till it jumps out of the pot and tries to kill the flame. I think this happens due to change in the adhesive properties of th... | What is the cause of the boiling behavior of a tea mixture compared to pure water, and how do the properties of the added ingredients (milk, sugar, and tea leaves) affect the boiling point and behavior of the liquid in the pot? | 3,500 | cqadupstack-physics |
23463 | Laws of aerodynamics after breaking the sound barrier | my knowledge of physics is not very extensive, so I hope my question isn't too stupid. I know that when (for instance) a plane breaks trough the sound barrier, the laws of the aerodynamics change. But I don't know why because the plane is still being carried by the same air, only it's travelling much faster and this cr... | What changes occur in the aerodynamic forces acting on a plane as it approaches and surpasses the speed of sound, specifically regarding air pressure and density, and how do these changes affect the stability and control of the aircraft? | 15 | cqadupstack-physics |
27179 | Is ghost-number a physical reality/observable? | One perspective is to say that one introduced the ghost fields into the Lagrangian to be able to write the gauge transformation determinant as a path- integral. Hence I was tempted to think of them as just some auxiliary variables introduced into the theory to make things manageable. But then one observes that having i... | What are the implications of introducing ghost fields on the symmetry of a quantum field theory, and how can one experimentally verify the existence of a global $U(1)$ ghost number symmetry in a laboratory setting? | 36,400 | cqadupstack-physics |
15005 | Does the ruling out of TeV scale SUSY breaking disfavor grand unification? | One of the arguments in favor of TeV scale SUSY breaking is that it leads to the appropriate running of the gauge coupling strengths leading to grand unification, i.e. $k_Y = \frac{5}{3}$ instead of $k_Y = \frac{4}{3}$. With the LHC ruling out TeV scale SUSY breaking, what is the current consensus on grand unification?... | What is the estimated Bayesian posterior probability for the existence of grand unification in the current theoretical physics landscape, given the constraints from recent experiments and observations, such as those from the LHC? | 8,094 | cqadupstack-physics |
94087 | How to determine the instantaneous acceleration of each box right after the connecting thread is cut? | Consider the following diagram. The system is in equilibrium. When the red thread is cut, determine the magnitude and direction of instantaneous acceleration of each box? Neglect the mass of the thread and spring.  In my mental model, the $M$ will be ac... | Consider a system in equilibrium consisting of two boxes, one with mass \( M \) and the other with mass \( m \), connected by a spring and a thread. When the red thread is cut, what are the magnitudes and directions of the instantaneous accelerations of both boxes \( M \) and \( m \)? Assume \( m \) is hanging vertical... | 10,220 | cqadupstack-physics |
121366 | Can the new results (about photonic time travel) make quantum computers feasible? | New results published about photonic time travel, reference here make quantum computers a reality in the near future? These results seem to indicate that there can be qubits that can exhibit nonlinear behaviour, and this in turn may have deep implications to quantum cryptography, and possible profound effects on the na... | What are the potential implications of nonlinear qubit behavior on quantum cryptography and the nature of unitary interactions in quantum computing? Quantify how these implications could affect the efficiency or security of quantum algorithms. | 16,141 | cqadupstack-physics |
61239 | How the CMB anisotropy is linked to the existence of cold dark matter and dark energy? | After the data from the cosmic microwave background has been collected by WMAP or Planck, what types of analysis is needed to conduct in order to deduce the cold dark matter density and the distribution of matter in the universe? In other words - How the CMB anisotropy measurements giving us evidence for the existence ... | How can the anisotropies in the cosmic microwave background (CMB) be quantitatively analyzed to estimate the density of cold dark matter in the universe, and what specific statistical methods or models are used in this analysis? | 6,944 | cqadupstack-physics |
133463 | Dark matter a medium for light propagation | Is dark matter a candidate to fill void left by luminiferous ether as a medium for light travel? | What are the key differences between dark matter and luminiferous ether in terms of their proposed roles in the propagation of light? | 14,579 | cqadupstack-physics |
14212 | Collision of Phobos | Mars has two moons: Phobos and Deimos. Both are irregular and are believed to have been captured from the nearby asteroid belt. Phobos always shows the same face to Mars because of tidal forces exerted by the planet on its satellite. These same forces causes Phobos to drift increasingly closer to Mars, a situation that... | How can I calculate the rate at which Phobos is drifting closer to Mars in order to estimate the time until a potential collision occurs, given that this collision is predicted to happen in about 50 to 100 million years? | 26,317 | cqadupstack-physics |
13141 | Can pockets of air exist underwater? | If yes, why don't they fill up with water, and can you breathe the air there? Like, it's not exactly atmosphere there, but an underwater cave with higher ceiling. P.S. Possible that it has a different name than air pocket, but due to my limited knowledge of English, I have no idea. Thanks in advance! | What is the average partial pressure of oxygen in an underwater cave with a ceiling that does not fill with water, and how does it compare to the atmospheric pressure at sea level? | 3,091 | cqadupstack-physics |
79692 | Why Negative refractive index is negative | $n=\sqrt{\epsilon \mu }$ Negative refractive index happens when permittivity ($\epsilon $) and permeability ($\mu $)of a material is negative. My question is, if permittivity and permeabilitity of a material is negative then the end product, according to the above equation, should be positive. If two negative numbers m... | If the permittivity ($\epsilon$) and permeability ($\mu$) of a material are both negative, what is the mathematical interpretation of the equation $n = \sqrt{\epsilon \mu}$, and how can it result in a negative refractive index? | 18,932 | cqadupstack-physics |
133974 | Solving differential equations without approximations? | In physics, many problems start with a mathematical relationship of the physical phenomenon at hand, and then, in many occasion, always only leave whatever in the first order to get a nice and solvable differential equation. Then there may be terms of higher order considered later, by as far as I know, it rares goes to... | What are the implications of retaining all orders of terms in a Taylor series expansion for the solutions to the equations of motion in classical mechanics, and how would this impact the analysis of a simple harmonic oscillator? | 31,743 | cqadupstack-physics |
5235 | Visibility of moon-based laser from Earth | If someone (or some robot) on the moon were to point a laser at the Earth, how many watts would the laser need, to be easily seen with the un-aided eye from Earth? Obviously this depends on a number of factors. Assume the beam is made divergent enough to cover latitudes 60S to 60N, wavelength 532nm, and the beam origin... | What is the minimum output power in watts required for a laser targeting Earth from the moon, with a wavelength of 532 nm, in order to be visibly seen by the unaided eye, assuming the laser beam diverges sufficiently to cover latitudes 60°S to 60°N and the observation occurs in complete darkness? Please include calcula... | 23,054 | cqadupstack-physics |
111365 | What should I look for at SAR levels for cell phones? | I've been trying to wrap my head around SAR levels when comparing cell phones. Considering that I spent most of my time not talking on the phone but carrying it around near my body, would a phone **lower body** value but **higher head** value be preferable one with the reverse combination? Or, suppose one phone has a v... | If one cell phone has a specific absorption rate (SAR) value of 1.2 W/kg for the head and 0.5 W/kg for the body, while another phone has SAR values of 0.4 W/kg for the head and 1.0 W/kg for the body, which phone would generally be considered safer to carry during daily use, and how much higher is the total SAR exposure... | 14,879 | cqadupstack-physics |
32196 | Are there any well-known theories successfully unifying the inertial and gravitational mass? | From what little I know of general relativity, the equality of inertial and gravitational mass is an axiom of the theory. I suspect that this precludes GR from unifying them in the same sense as electric, magnetic and weak interactions are unified as the electroweak force. But since the two appear to be equal under sta... | What are some prominent theoretical frameworks that attempt to unify inertial and gravitational mass, and how do these theories address the discrepancies that arise from their failure to correspond to our observed universe? | 7,540 | cqadupstack-physics |
58102 | microcanonical distribution | We know that in an isolated system, the density matrix is the microcanonical distribution matrix. That this the possibility for all the states with energy in a certain interval is a constant? But how can I deduce this from the postulate of equal probability? | In an isolated system, if the energy states are uniformly distributed within a specified interval such that each state has an equal probability, how does this imply that the density matrix represents the microcanonical distribution? Please provide a mathematical explanation. | 3,228 | cqadupstack-physics |
29546 | Aerodynamic drag on a cannonball? | I'm trying to build a ballistics simulation where I shoot a cannonball. I want to allow for drag and am trying to work out the math to do so. I can work the drag out using $F = C_d\times S\times V^2\times A/2$. What I need to know is how to find the $x$ and $y$ parts of that. I use $V\cos(A)$ and $V\sin(A)$ to work out... | How can I calculate the drag force acting on a cannonball in both the horizontal (x) and vertical (y) directions when considering air resistance, given that the drag force is described by \( F_d = C_d \times S \times V^2 \times \frac{A}{2} \), and the initial velocity components are \( V_x = V \cos(A) \) and \( V_y = V... | 8,906 | cqadupstack-physics |
5702 | Can we transport energy over infinite distances through vacuum using light? | I know that light (or electromagnetic radiation in general) attenuates in intensity as the square of the distance it travels. * Why does it attenuate? * Are the photons being scattered by the medium they pass through? I also know that the energy carried by light _quanta_ is a function of their frequency only. * ... | If the intensity of light decreases with the square of the distance it travels due to scattering, how does the distance traveled by light in a vacuum differ from that in a medium, and what implications does this have for transmitting information over long distances? | 31,166 | cqadupstack-physics |
44976 | BEC for holography? | I am spending some time reading about Bose-Einstein condensation. I want to know if it is possible to use atom lasers to realize the kind of holography traditionally associated with nano-fabrication. Many papers say that the motivation for BEC is the aforementioned holography, but I can't find if they were able to actu... | What are the key challenges and advancements in using atom lasers for holography, particularly in the context of Bose-Einstein condensates, and has there been any successful realization of this application since the work done by Japanese scientists in the 1990s? | 8,357 | cqadupstack-physics |
47968 | Levi Civita Symbol and contravariance vs covariance | I have a question regarding the Levi-Civita symbol and contravariance vs covariance. Some of this was asked in a previous post, but I think I need more clarification. Consider the magnetic field: \begin{equation} B^k = \tilde{\varepsilon}^{ijk}\partial_i A_j \end{equation} Qn 1: In the formula I wrote it as the Levi-Ci... | Given the expression for the magnetic field in terms of the Levi-Civita symbol and the vector potential, \( B^k = \tilde{\varepsilon}^{ijk} \partial_i A_j \), and knowing that in Cartesian coordinates this corresponds to the curl operation \( \vec{B} = \nabla \times \vec{A} \), what would be the correct expression for ... | 32,059 | cqadupstack-physics |
69069 | Tale of two magnets | Please help. My example: You take a sheet of steel and suspend it in the air. Then you take a permanent magnet (i.e. a neodymium magnet) and attach it to the sheet of steel and from that you suspend a 1kg weight. The weight and magnet will stay put providing that the room doesn’t change and that gravity is still acting... | Why does a permanent magnet maintain its attraction to a piece of steel when a 1 kg weight is hung from it, while an electromagnet requires continuous electrical power to achieve the same effect, resulting in battery drainage? | 30,729 | cqadupstack-physics |
83907 | How to calculate upward force of a container filled with a less dense gas than the surroundings where it is placed? | I'm trying to determine the calculation which would state the upward force of a container where the density inside the container is less then that in which it sits. Does this calculation change with shape? I.e. sphere vs cube? | What is the upward buoyant force acting on a submerged container with a density of 800 kg/m³ when placed in water with a density of 1000 kg/m³, and does this force vary with the shape of the container (e.g., spherical vs. cubic)? | 30,363 | cqadupstack-physics |
24028 | How do I calculate DC motor speed for a given load? | Suppose I have a robot of a given mass, and I'm choosing between 2 different wheels and 2 different motors to put on it. For each wheel I have the diameter, and for each motor I know the stall torque and free speed. How would I figure out which motor and wheel combination will make the robot move the fastest? My calcul... | How can I calculate the optimal wheel and motor combination for my robot to achieve the maximum speed, given the diameters of the wheels, the stall torque, and free speed of the motors? | 1,030 | cqadupstack-physics |
47016 | Could an NEO strike the moon with sufficient velocity for the shrapnel to escape Lunar gravity, and be attracted by Earth's? | Between the news item of an asteroid giving Earth a close shave, and another news item of the impending GRAIL impact ; I find myself wondering whether a NEO could be a hazard to Earth _via_ the moon. I'm not sure this scenario is realistic. Perhaps such a large body may naturally be captured by Earth gravity instead of... | What is the minimum size and velocity an Near-Earth Object (NEO) would need to impact the Moon and have fragments large enough to escape lunar gravity and potentially reach Earth, causing significant loss of life or property? | 12,663 | cqadupstack-physics |
131275 | Angular momentum in an accretion disk | I need to plot the time evolution of the total angular momentum in an accretion disk. This confuses me because I thought this should be constant, since angular momentum has to be conserved? I'm given the angular velocity $\Omega=(GM/R^3)^\frac{1}{2}$ where $M$ is the mass of the central object, and that the disk is mad... | Given the formulation for the angular velocity \(\Omega = \left(\frac{GM}{R^3}\right)^{1/2}\) of the accretion disk and the mass of an annulus of matter as \(dm = 2\pi R \Delta R \Sigma(R, t)\), where \(\Sigma(R, t)\) is the surface density, calculate the expression for the total angular momentum \(L\) of the disk in t... | 31,236 | cqadupstack-physics |
105871 | Two coffee beans in a cup of water + vibrations = gravity? | If you float two coffee beans in a cup of water that is being vibrated by your subwoofer (go to a sin wave generator online), the two beans will either be attracted to the sides of the cup or if positioned right will be attracted to each other in an accelerating manner. What phenomenon is this, and could it be related ... | What is the phenomenon that causes two floating coffee beans in a vibrating cup of water to either move towards the sides of the cup or accelerate towards each other, and how might this phenomenon be connected to the concepts of gravity and movement in a microgravity environment? | 17,774 | cqadupstack-physics |
92302 | What shape would an explosion be in orbit? | In the Gravity movie discussions we learn that objects accellerated towards an object in the same orbit miss it and move to a higher orbit. Objects accellerated away from an object in the same orbit move to a lower orbit. What shape would an explosion be in orbit? Edit. Compared to an explosion in space that is not in ... | If an explosion occurs in a stable circular orbit, how will the trajectories of the resulting shrapnel evolve over time compared to an explosion that occurs in the vacuum of space? Specifically, describe the changes in trajectory shape and distance from the explosion source for both scenarios. | 14,737 | cqadupstack-physics |
77098 | Finding superpotentials and central charges in $AdS_3$ | In text "Covariant theory of asymptotic symmetries, conservation laws and central charges" is given an example of finding central charges and superpotential (among other things). I am interested in $AdS_3$ case, since there is a lot of literature on this space-time, and I'm going to need to preform the same analysis fo... | Given the expression for the variation of the Lagrangian density \(\mathcal{H}^{\mu\nu}[h;\bar{g}]\) in the context of asymptotic symmetries and boundary conditions for \(AdS_3\), how can one compute the leading order behavior of \(\mathcal{H}^{tt}\) as \(r \to \infty\) given that \(h_{\mu\nu} \to \mathcal{O}(r^{m})\) ... | 13,384 | cqadupstack-physics |
782 | How to calculate a ballistic trajectory for a suborbital flight? | I'm trying to calculate the initial launch angles and velocity of a projectile (atmosphere's effects can be neglected), assuming that I know the lat/lon coordinates of both the launch and the destination points. Can you recommend any software libraries or reading materials that could help? Thank you. | Given the latitude and longitude coordinates of a launch point and a destination point, calculate the optimal initial launch angle and velocity required for a projectile to reach the destination, assuming that there are no atmospheric effects. Provide the equations used and the values for the launch angle and velocity. | 20,297 | cqadupstack-physics |
55513 | Empirical bound on sum of electron and proton charge | Followup to "Why do electron and proton have the same but opposite electric charge?". It is argued that even a tiny residual charge would result in huge amounts of electricity in bulk matter, everything would be different, etc. I do not find that a convincing answer: suppose $n$ protons plus $n+1$ electrons are neutral... | In a hypothetical scenario where every $n$'th hydrogen atom is a negative ion, what is the minimum value of $n$ such that the presence of these negative ions does not lead to a detectable bulk charge imbalance in a macroscopic sample of hydrogen? | 13,850 | cqadupstack-physics |
69096 | I'm reading this in a senior year physics text book, need to know if this is true, and if so then how? : | "Even if a magnet is broken into atoms, each atom shall be a complete magnet. If the atom is further broken into electrons, protons, neutrons, etc. even then each particle shall behave like a complete magnet." | If a bar magnet is cut into 8 equal pieces, how many individual magnetic poles will be present in total among all the pieces, and what is the expected behavior of each piece when it comes to magnetism? | 28,856 | cqadupstack-physics |
61967 | If there's a light ray and it's turned to a new location by a certain angle | Imagine that there's a light ray, with source at point **A** , and it's directed towards point **B** (which is very far from point **A** ) and it continues for a huge distance. How will an observer at point **B** perceive the light ray if it gets moved be a certain angle so that it's now directed towards point **C**. I... | If a light ray originating from point **A** is directed toward a distant point **B** and then deflected at an angle to point **C**, how will the time it takes for the light to reach point **B** compare to the time it would take if the light were uninterrupted? Assume the distance **AB** is large enough that relativisti... | 10,935 | cqadupstack-physics |
6367 | Specific heat and temperature | This is a two-part question... Firstly, models of the specific heat capacity $C$ (i.e. Debye, Einstein) in relation to the temperature $T$ give $C$ as steadily increasing with $T$. I assume that the change in $C$ is due to the heat the system gains being stored in its degrees of freedom- so why doesn't it increase in s... | In a system where the specific heat capacity \( C \) increases steadily with temperature \( T \) according to models like Debye and Einstein, how can the specific heat capacity be defined when the temperature changes from \( T_1 \) to \( T_2 \)? If \( C \) is temperature-dependent, should we consider it as \( C(T_1) \)... | 1,283 | cqadupstack-physics |
98689 | Resources for QCD colour factors | I'm looking for resources on the computation of (I think) very simple Feynman diagrams for QCD processes. The processes are q'qbar' -> qqbar and gg->qqbar (only at tree level, no loops involved) and my main problem is the computation of the colour factors. I'm not interested in the absolute value of the cross-section, ... | What are the color factors for the QCD processes \( q' \bar{q}' \to q \bar{q} \) and \( gg \to q \bar{q} \) at tree level, and how do they influence the angular distribution in terms of \( \cos \theta \)? | 18,813 | cqadupstack-physics |
92435 | Why 1 Month LHC Magnet Cooling Times | An often quoted figure is that the LHC magnets take a month to completely cool and a month to warm. There is never an explanation as to why that is. I can conjure any number of reasons (slow changes to prevent stress, very low temperature deltas, gremlins can hold their breath for 29 days, etc) but I can't find any fac... | What are the thermodynamic principles and design considerations that explain why the cooling and warming process of the LHC magnets takes approximately one month instead of a shorter time, such as 14.6 hours? | 28,268 | cqadupstack-physics |
23302 | How to calculate gamma radiation shielding? | A device emits 0.2 μSv/h of gamma rays. How thick does an aluminum sheet need to be to completely stop radiation from coming out ? What equation is to be used to calculate this ? | How thick must an aluminum sheet be to completely shield against a gamma radiation emission of 0.2 μSv/h? Which equation is applicable for calculating the thickness required for complete attenuation? | 23,425 | cqadupstack-physics |
46353 | Is observable universe an explanation against Olbers' paradox? | First of all, let me tell you that I'm not a physicist but rather a computer scientist with a mere interest in physics at nowhere near a professional level so feel free to close this question if it doesn't make any sense. I remember a physicist friend mentioning me about an argument about the finiteness of the universe... | What is Olbers' Paradox, and how does it contribute to the argument regarding the finiteness of the universe? Additionally, could you explain how this paradox relates to the concept of the observable universe containing approximately \(10^{80}\) atoms? | 37,205 | cqadupstack-physics |
47839 | Elastic potential | I have a doubt: elastic potential energy is given by: $U=\frac{k}{2}x^2+K$ but does elastic potential exist? (for example: potential gravitational energy is given by $U=mgz+K$ and gravitational potential is given by $V=-U$) ($K$ is the constant of integration) | Is elastic potential energy given by the formula \( U = \frac{1}{2}kx^2 + K \) always positive, or can it be negative depending on the choice of \( K \)? Additionally, how does this compare to the gravitational potential energy equation \( U = mgz + K \) where \( K \) can also be chosen arbitrarily? | 33,490 | cqadupstack-physics |
35758 | The vacuum light speed: Is it really constant, i.e., independent of location in space-time? | I am by no means an expert in this field, however something puzzles me about the speed of light and the relativity of time and space (space-time). Is is universally acknowledged that the speed of light (299,792,458 m/s) is the universal speed limit, and that nothing can travel faster than light? That is a measurement b... | If time on the surface of a black hole passes 10 times slower relative to Earth, does the speed of light remain constant at 299,792,458 m/s from the perspective of an observer on Earth, or does it change when measured at the surface of the black hole? | 2,607 | cqadupstack-physics |
64444 | Why is the Coulomb potential in pseudo-2D experiments proportional to the logarithm of distance? | Inspired by this question, I ask another. Theoretically, Coulomb potential in 2D is proportional to the logarithm of distance; In experiments, though electrons are constrained in a pseudo-2D environment, they are actually existing in 3-dimentional space. Why is the Coulomb potential in pseudo-2D experiments still propo... | In a pseudo-2D environment where electrons are constrained, if the Coulomb potential is still proportional to the logarithm of distance, what is the expression for the Coulomb potential \( V(r) \) as a function of distance \( r \), and how does it compare to the 3D Coulomb potential? | 20,390 | cqadupstack-physics |
131717 | Why should multiplication of a ket vector by a complex number change only its "direction"? | Dirac argues on page 17 of his book, The Principles of Quantum Mechanics, that multiplication of a ket by a complex number shouldn't change the state this ket represents. But then concludes: > Thus a state is represented by the direction of a ket vector, and any length > one may assign to the ket vector is irrelevant. ... | How does the concept of a unit vector in infinite-dimensional spaces relate to Dirac's assertion that the direction of a ket vector, rather than its magnitude, defines the state it represents in quantum mechanics? | 16,384 | cqadupstack-physics |
67548 | Solving differential Equation for the Two-Body Problem | So, I'm following the derivation in D. Morin, _Introduction to Classical Mechanics,_ of the equations for a two-body system. I understand all of it, aside from this one step. When he's talking about solving Lagrangian for $r(\theta)$, I don't follow the step. $$\tag{*}L = \frac {1}{2} m\dot{r}^{2}+ \frac{\ell^2}{2mr^2}... | Given the Lagrangian for a two-body system,
$$L = \frac {1}{2} m\dot{r}^{2}+ \frac{\ell^2}{2mr^2}+u(r),$$
derive the expression
$$(\frac{1}{r^2} \frac{dr}{d \theta})^2 = \frac{2mE}{\ell^2}- \frac{1}{r^2} - \frac{2mu(r)}{\ell^2}$$
starting from the Euler-Lagrange equations. Assume conservation of energy \(E\) an... | 30,667 | cqadupstack-physics |
47151 | Is the induced electric field due to time varying magnetic flux always circular? | According to Faraday's law, changing magnetic flux induces an electric field. Is that electric field always circular? | Given Faraday's law of electromagnetic induction, determine whether the induced electric field from a changing magnetic flux is always circular in nature, and if so, provide a mathematical justification for this characteristic by deriving the expression for the induced electric field in a region with a uniform magnetic... | 16,652 | cqadupstack-physics |
75759 | what is "work"? (of a force) | I would like to know why two forces (of different magnitudes but same direction) that we'd use sequentially on an object to move it by the same amount twice, will produce two different values for the work quantity. Why is there a mismatch between the maths and the observed physical change? What exactly is the work of a... | If two forces with different magnitudes but the same direction are applied sequentially to move an object the same distance, how can you quantify the work done by each force, and why do they result in different values of work? Additionally, how is the concept of work defined in physics without circular reasoning? | 1,069 | cqadupstack-physics |
90775 | How is the probability of intersystem crossing influenced by small differences in energy levels? | Say I have some dye molecule in some low level triplet state, e.g. $T_1$, and it's decaying slowly to the ground state via phosphorescence. However, there are also events where the dye in the $T_1$ state can undergo intersystem crossing to transition to the first singlet excited state, $S_1$, which then decays rapidly ... | How does the probability of intersystem crossing, \( P(T_1 \to S_1) \), change with respect to the energy gap, \( \Delta G \), between the triplet state \( T_1 \) and the singlet state \( S_1 \), according to the Boltzmann approximation \( P(T_1 \to S_1) \approx P_c \times e^{\left(\frac{\Delta G}{k_B T}\right)} \)? Ca... | 36,129 | cqadupstack-physics |
72496 | Is sub critical plutonium "safe" to handle? | Apparently, in Los Alamos scientists handled sub critical masses of plutonium (for example the demon core) with little or no protection. Richard Feynman and others mentioned that plutonium spheres were warm to the touch, which seems to imply that they actually touched them. Slotin (one of the victims of the demon core)... | What is the temperature range of plutonium isotopes when they are in a subcritical state, and how does this relate to their handling safety? | 23,089 | cqadupstack-physics |
33243 | What is the shortest run needed for a "dropped skateboard" rider to reach 160 mph? | I just saw a great clip on the fastest skateboarder to date. He wants to reach 160 mph from a dropped board, but says his speed is limited by the roads currently available. What is the shortest run required for a "dropped skateboard" rider to reach 160 mph? Is it even possible? | What is the minimum distance required for a skateboard rider, starting from a drop, to accelerate from rest to a speed of 160 mph, assuming a constant acceleration and neglecting air resistance and friction? | 16,194 | cqadupstack-physics |
48027 | Geometric quantization of a hydrogen atom | I want to know how to quantize a hydrogen atom as an example of geometric quantization. Apparently there is a derivation in the book "Geometric Quantization in Action: Applications of Harmonic Analysis in Quantum Statistical Mechanics and Quantum Field Theory" by N.E. Hurt, but I have no access to it. An article even p... | What are the quantized energy levels of a hydrogen atom, and how can these levels be derived using the principles of geometric quantization? | 1,065 | cqadupstack-physics |
108045 | If the current is increased, is there more charge flowing or is it moving quicker? | **Problem** Current is the amount of charge that is flowing through a component per unit of time. For a given voltage, Ohm's law tells us that if we increase the resistance, then the current must decrease. But _what's actually happening to decrease the current?_ **My reasoning so far - is it correct?** More resistance ... | How does an increase in resistance affect the flow of charge in a circuit, and what specific mechanisms contribute to the decrease in current according to Ohm's law? Provide a quantitative example to illustrate your explanation. | 10,924 | cqadupstack-physics |
119611 | Limitation of Gauss's Law | We can use Gauss's law to find out the electric field $\vec{E}(\vec{r})$ due to an infinite cylinder of charge.  But if the cylinder is of finite length then it is said that $|\vec{E}(\vec{r})|$ is valid at $\vec{r}$ when $r<< R$. Why this is so? And ho... | What is the expression for the electric field magnitude \( |\vec{E}(\vec{r})| \) at a point outside a finite cylinder of charge, given the cylinder has a radius \( R \) and a linear charge density \( \lambda \), under the condition that \( r \ll R \)? Please derive the mathematical proof showing how this condition affe... | 29,811 | cqadupstack-physics |
79775 | Nature of frictional force | I was thinking about a situation where a person in standing on the ground with some friction. The frictional force is directly proportional to the normal reaction acting on him by the ground. Assume that he leaned forward i.e his center of mass is not vertically up the point of contact of him with the ground.Then, does... | If a person leans forward while standing on a surface with friction, how does the frictional force acting on them change compared to when they are standing upright? Specifically, if the person has a mass of 70 kg and the coefficient of friction between their shoes and the ground is 0.4, what is the maximum static frict... | 33,709 | cqadupstack-physics |
15728 | Photoelectric effect: Experimental Physics | Suppose you are doing an experiment to determine the work function of a metal. You get $KE_1$, $\nu_1$ and $KE_2$, $\nu_2$. We know that $KE = h\nu - W$ but when you solve the simultaneous equations, we have a $h$ slightly different from the textbook value. Question: In such a situation should you 1. Use the textbook... | If you determine the work function \( W \) of a metal using two sets of data: \( KE_1, \nu_1 \) and \( KE_2, \nu_2 \), with a measured Planck's constant \( h' \) that differs from the textbook value \( h \), what is the more effective method to calculate the work function: 1) Use the textbook value \( h \) for both cal... | 29,357 | cqadupstack-physics |
122310 | Does light bend in a vacuum? | I'm familiar with gravitational lensing but still I'm wondering if there is experiments (conducted here on Earth) which show that light bends due to gravity. For example mirrors setup to hold the light or something like that. My question is inspired by this bounty question. | What experimental setup could be used to demonstrate the bending of light due to gravity on Earth, and how would the results quantify the deflection angle of light in a gravitational field? | 19,028 | cqadupstack-physics |
62468 | Light Ray Reflection from concave mirror | Suppose a ray of light hits a concave mirror and is parallel to principle axis but far away from it such that it doesn't follow _paraxial ray approximation_. Will it pass through focus or between focus and radius of curvature or between pole and focus? Here pole, focus and radius of curvature mean the same thing as in ... | If a ray of light that is parallel to the principal axis of a concave mirror but far from it strikes the mirror, will it converge at the focus, between the focus and the radius of curvature, or between the pole and the focus? Provide a clear explanation of the ray's path based on the geometry of the mirror. | 11,303 | cqadupstack-physics |
64578 | Is there a way to see light frequencies invisible to the human eye without the use of electronic sensors? | I wonder if it's possible to be able to see light frequencies that cannot be normally seen by human eye, without using sensors designed specifically for detecting a certain spectrum range, but rather using some combination of lenses or something else that could transform the light without causing a noticeable latency. | Is it possible to design an optical system using lenses that can shift the frequency of light into the visible spectrum, allowing a human to perceive normally invisible light (such as infrared or ultraviolet) without any significant latency? If so, what optical principles would be involved in this transformation, and w... | 24,275 | cqadupstack-physics |
67051 | How could we see microwave radiation with our eyes? | A few years ago I read a short little article about how big our eyes would have to be to observe microwaves (or any long-wave radiation for that matter). I don't remember enough about the article, or know enough about observing EM waves to reconstruct the equations, and I can't find the article again. So my question is... | What is the minimum diameter of an optical instrument, such as a telescope, needed to effectively observe electromagnetic waves with a wavelength of 1 millimeter, and how does this relate to the principles of diffraction? | 35,735 | cqadupstack-physics |
102259 | What is Electromotive force (EMF)? How is it related to potential difference? | What is Electromotive force (EMF)? How is it related to potential difference? Is it the creation of potential difference in any conductor??Is it a process?Why is it called force? Does writing emf instead of voltage make a difference in AC circuits...I find some books using emf while others voltage(whle studying Alterna... | Calculate the electromotive force (EMF) produced by a generator if it successfully generates a potential difference of 120 volts across its terminals and operates with an internal resistance of 2 ohms, drawing a current of 10 amperes. | 36,826 | cqadupstack-physics |
74539 | Justification for smeared fields in the Wightman axioms? | I just started reading _PCT, Spin and Statistics, and All That._ Can someone explain why we use operator valued distributions to describe fields? I read somewhere that it would take infinite energy to measure an observable at a single point. Why don't we instead use functions from some collection of subsets (i.e. the o... | What is the relationship between the width of test functions used in operator valued distributions and the uncertainty principle in quantum mechanics, and how does this influence the physical interpretation of measurements in quantum field theory? | 13,270 | cqadupstack-physics |
133030 | How far is the range of an ultrasonic sensor as far as it can transmit? | How far can ultrasonic waves propagate in the air? (if the energy is enough), how far can ultrasonic waves propagate in the ground (concrete floor)? Are there any articles or experiments about attenuation of ultrasonic waves in different mediums? | What is the typical distance that ultrasonic waves can travel in air and in concrete, and how does attenuation differ between these two mediums? | 3,839 | cqadupstack-physics |
75589 | Polar ice caps and thermal radiation | I was reading an article on global warming and it said that the polar ice caps, because they are white, reflect a lot of the sun's radiation. The article also has a picture of some houses in England that are painted all white and supposedly these houses keep cooler in the summer and warmer in the winter. I understand t... | What are the thermal properties of a white-painted surface compared to a black-painted surface when exposed to infrared radiation, and how does the wavelength of the radiation affect the reflectivity and absorptivity of different colored surfaces? | 34,299 | cqadupstack-physics |
70813 | Calculate force of impact on running dog | I'm going to hold a dog protection tournament. I'd like to give the audience information about the force of the impact of the running dog when it hit(attacks) the decoy on a huge screen. I'm a software developer student so I have the neccesary skills to development the software/electronics for the project. However, i'm... | How can I calculate the force of impact in newtons when a 40 kg dog runs at a speed of 30 km/h and collides with a stationary decoy? | 19,197 | cqadupstack-physics |
57942 | Experimental perspective in understanding the Heisenberg Uncertainty Principle | I need to confirm whether or not I understand Heisenberg Uncertainty Principle. So the crucial thing is that you need an **"ensemble" of measurements** : $$\delta x \delta p \ge \frac{\hbar}{2}.$$ If I were to conduct an experiment trying to validate this with particles, then I would first measure, say, the position in... | If an experiment is conducted where you measure the positions of 1000 particles and find that the standard deviation of the position measurements, \(\delta x\), is 0.01 m, what is the minimum possible value of the standard deviation of the momentum measurements, \(\delta p\), according to the Heisenberg Uncertainty Pri... | 5,955 | cqadupstack-physics |
5804 | Does time have a special status in general relativity? | In a lot of laymen explanations of general relativity it is implied that the four dimensions of the space-time are equivalent, and we perceive time as different only because it is embedded in our human perception to do so. My question is: is that really how general relativity treats the 4 dimensions? If so - what are t... | In the context of general relativity, how does the treatment of time as a fourth dimension compare to the three spatial dimensions, and what are the implications of this on causality within the framework of the theory? Additionally, can general relativity accommodate the existence of more than one time dimension, and w... | 16,499 | cqadupstack-physics |
65302 | what sort of set up would i need to create an electro magnetic chair with no legs | I am a furniture design student therefore please keep it simple. a system strong enough to hold the avergae male of say 90 kg is this possible even.... | What materials and structural designs could be used to create a furniture system strong enough to support an average male weight of 90 kg, and how much weight can different materials typically support per unit area? | 30,809 | cqadupstack-physics |
127480 | Instantons in Witten's supersymmetry and Morse theory | I'm reading Witten's paper on supersymmetry and Morse theory and am confused about the details of the instanton calculation which he uses to define a Morse complex (beginning at page 11 of the pdf) . Witten writes down the relevant supersymmetric Lagrangian and then states > Instanton solutions or tunneling paths in th... | In Witten's discussion of instanton solutions in supersymmetry, he mentions that nonzero eigenvalues cancel between bosons and fermions due to supersymmetry, leaving only the zero eigenvalues of the fermions. Can you explain how this cancellation occurs, particularly in the context of the index of the Dirac operator re... | 13,840 | cqadupstack-physics |
27119 | Physical interpretation of different selfadjoint extensions | Given a symmetric (densely defined) operator in a Hilbert space, there might be quite a lot of selfadjoint extensions to it. This might be the case for a Schrödinger operator with a "bad" potential. There is a "smallest" one (Friedrichs) and a largest one (Krein), and all others are in some sense in between. Considerin... | What is the role of the Friedrichs and Krein extensions of a Schrödinger operator in quantum mechanics, and how do their associated unitary groups differ in terms of physical interpretations or applications? | 35,163 | cqadupstack-physics |
94544 | Proving Lorentz invariance of Maxwell equations | I've read somewhere that one does not need to prove Lorentz invariance of the Maxwell equations $F_{\mu\nu,\sigma}+F_{\nu\sigma,\mu}+F_{\sigma\mu,\nu}=0$ because it is "manifestly Lorentz invariant" or "because they are tensor equations"? What is meant by that? I've read that this could mean that space and time are tre... | What does it mean for the Maxwell equations to be "manifestly Lorentz invariant," and how does this property relate to the concept of spacetime treating space and time "on equal footing"? Can you provide an example illustrating this invariance? | 29,638 | cqadupstack-physics |
73879 | Magnetic moment precession around magnetic field | I have a question regarding the magnetic moment of an atom. If you just have one atom, and you apply an external magnetic field in lets say the z-direction. Then the magnetic moment will precess around the direction of the magnetic field applied, and depending on the quantum numbers it can precess in different quantize... | What is the formula that describes the precession frequency of the magnetic moment of an atom in an external magnetic field, and how does this frequency depend on the strength of the magnetic field and the magnetic moment itself? | 17,343 | cqadupstack-physics |
41740 | Is there a quantum state for a large system | My understanding of quantum mechanics is that the state of a system is represented by a vector in multidimensional complex vector space. Is there, in principal, a state vector that represents a large, classical object such as, say, a cheeseburger, at an instant in time? If so, what is the physical meaning of that "stat... | Can a classical object, such as a cheeseburger, be represented by a quantum state vector, and if so, what properties would this state vector encompass in relation to the cheeseburger's physical characteristics? | 36,025 | cqadupstack-physics |
4782 | Trying to model pinball physics for game AI | I'm working on an AI for a pinball-related video game. The ultimate goal for the system is that the AI will be able to fire a flipper at the appropriate time to aim a pinball at a particular point on the table. We are using an engine that will handle the underlying physics interactions when the ball is hit (Unreal). I'... | Given a pinball of mass m rolling towards a flipper at an initial velocity \( v_0 \) with an angle \( \theta \) relative to the horizontal, and a flipper that can rotate to an angle \( \alpha \), applying a torque \( \tau \) at a distance \( d \) from the pivot, calculate the resulting velocity \( v_f \) of the ball im... | 15,473 | cqadupstack-physics |
56378 | current electricity and voltaic cell | let us consider that i have a voltaic cell in an open circuit that has copper plate at higher potential w.r.t zinc plate. Now my query is, since the electrolyte separating them is a conducting medium, the positive and negative charges on the electrodes must get neutralised immediately(i think so)...But it doesnt happen... | In a voltaic cell consisting of a copper plate at a higher potential and a zinc plate at a lower potential, how is it possible for the cell to maintain a steady potential difference even when the circuit is open, given that the electrolyte is a conducting medium? What prevents the immediate neutralization of charges on... | 32,761 | cqadupstack-physics |
35726 | Stationary Solutions | An unbelievably basic question, but it's something I've never been taught. Am I right in thinking that the following defines a stationary solution? Let $\phi$ be some dynamical variable satisfying a differential equation $D(\phi)=0$ with respect to some parameter $t$. Then $\phi$ is called a **stationary solution** or ... | Given a dynamical variable \(\phi\) that satisfies the differential equation \(D(\phi) = 0\) with respect to a parameter \(t\), under what conditions can \(\phi\) be classified as a stationary solution? Additionally, if \(\phi\) is defined such that its derivatives with respect to time \(t\) are all equal to zero, what... | 791 | cqadupstack-physics |
94207 | Proper time along path in Minkowski Space | Consider the path $x^\mu(u)$ in Minkowski space; such that: $$t = \frac{a}{c} \sinh(u) , \quad x = a \cosh(u) ,\quad y = 0 ,\quad z = 0 $$ where $a$ is a positive constant and $u$ is a parameter Use equation: $$ c \nabla \tau = ds = \sqrt{\eta_{\mu\nu} \dot{x}^\mu \dot{x}^\nu} du $$ to find the proper time elapsed alon... | Calculate the proper time elapsed along the path described by the equations \( t = \frac{a}{c} \sinh(u) \) and \( x = a \cosh(u) \) for \( y = 0 \) and \( z = 0 \), starting from \( u = 0 \). Express your answer as a function of \( u \) using the proper time relationship given by \( c \nabla \tau = ds = \sqrt{\eta_{\mu... | 15,605 | cqadupstack-physics |
13165 | oscillations of blocks connected by a spring | Imagine two blocks of masses $m_1$ and $m_2$ joined together by a spring of spring constant $K$. Now let the spring be stretched by a distance $X$ and then the system is released. suppose during the stretching the block of mass $m_1$ moves towards left by a distance $A$ and the block of mass $m_2$ by a distance B. Now ... | Given two blocks of masses \( m_1 = 2 \, \text{kg} \) and \( m_2 = 3 \, \text{kg} \) connected by a spring with a spring constant \( K = 150 \, \text{N/m} \), if the spring is stretched by a distance \( X = 0.1 \, \text{m} \), and assuming that the block of mass \( m_1 \) moves a distance \( A = 0.04 \, \text{m} \) to ... | 28,620 | cqadupstack-physics |
33578 | Bound states in QCD: Why only bound states of 2 or 3 quarks and not more? | Why when people/textbooks talk about strong interaction, they talk only about bound states of 2 or 3 quarks to form baryons and mesons? Does the strong interaction allow bound states of more than 3 quarks? If so, how is the stability of a bound state of more than 3 quarks studied? | What is the maximum number of quarks that can form a stable bound state, and how is the stability of such a multi-quark state quantitatively assessed in quantum chromodynamics (QCD)? | 5,667 | cqadupstack-physics |
1096 | Gravity and Collision of two continuous mass distributions | How could one explain the collision of two continuous mass distributions in view of gravitation (Newtonanian and General relativity) ? | Calculate the gravitational force experienced by two spherical continuous mass distributions of equal mass (M) and radius (R) separated by a distance (d) in the Newtonian framework. Assume d > 2R. Then, compare this force to the prediction made by General Relativity for the same setup. What differences would you expect... | 14,197 | cqadupstack-physics |
32155 | What is a clock? | Relationalists love to define time operationally as what clocks measure, but this begs the question of what counts as a clock. Clearly, it's a measuring instrument and what it measures is supposedly time. We need to avoid circular definitions of time. There are many possible measuring devices, and most don't measure ti... | What are the essential characteristics that a measuring instrument must possess to be classified as a clock, and how do these characteristics ensure that it provides a linear, real-valued representation of time? | 23,842 | cqadupstack-physics |
34063 | What is the maximum surface charge density of aluminum? | I understand that the maximum free charge carrier density for aluminum has been measured using the Hall effect (in the case of electric current). However, I'm not clear how to determine the maximum surface charge density to which aluminum (or any conductor) can be charged, assuming the neighboring medium does not break... | What is the maximum surface charge density that aluminum plates in a parallel plate capacitor can be charged to, assuming the neighboring medium can withstand an infinite potential without breakdown? How would you calculate this value based on the material's properties? | 16,685 | cqadupstack-physics |
132823 | Entropy of an oscillator in Einstein's solid | This is a homework problem and I need help with it. A solid's (Einstein's model) oscillators are in the first excited state on average. How much entropy does one oscillator have? What I've tried so far: So, the statistical sum is $$Z=\sum_n \exp(\phi(n+1/2))$$ where $$\phi = \frac{h\nu}{kT}$$ And the entropy can be exp... | Given that the average occupation number \(\bar n\) of a solid's oscillators in the first excited state is 1, calculate the entropy \(S\) per oscillator using Einstein's model. You may express your answer in terms of constants \(k\), \(h\), \(\nu\), and \(T\). | 6,371 | cqadupstack-physics |
13804 | Cross-field diffusion from Smoluchowski approximation | I'm reading _An Introduction to Stochastic Processes in Physics_ by Don S Lemons. Problem 10.2 leads to a pair of equations: $dV_x = -\gamma V_xdt+V_y\Omega dt-V_y\sqrt{2\gamma dt}N_t(0,1)$ $dV_y = -\gamma V_ydt-V_x\Omega dt+V_x\sqrt{2\gamma dt}N_t(0,1)$ ($\gamma$, $\Omega$ constants, $X$, $Y$ position, $V_x$, $V_y$ ve... | How do the terms involving the velocities \( V_x \) and \( V_y \) in the stochastic differential equations transform under the Smoluchowski approximation, specifically for the noise terms \( V_y\sqrt{2\gamma dt}N_t(0,1) \) and \( V_x\sqrt{2\gamma dt}N_t(0,1) \)? If we apply this approximation and set \( dV_x = 0 \) and... | 15,784 | cqadupstack-physics |
34339 | How would a physicist move? | Let's assume that Johnny Physicist has decided to move from his poor dingy second story apartment, into his much deserved home. Without making modifications to the existing structures? What would be the most work efficient way of moving all of his possessions? [Lets assume that the apartment is outside facing, and has ... | What is the minimum amount of work required to move Johnny Physicist's belongings, assuming he can carry a maximum of 20 kg at a time and each stair step is 0.2 meters high, while considering that he has 10 boxes weighing 15 kg each and 5 larger items weighing 40 kg each that require extra effort to lift? | 21,020 | cqadupstack-physics |
2916 | Shape of the universe? | 1. What is the exact shape of the universe? I know of the balloon analogy, and the bread with raisins in it. These clarify some points, like how the universe can have no centre, and how it can expand equally everywhere in all directions. 2. But they also raise some questions, like if you are on the surface of a ballo... | What is the current understanding of the large-scale geometric shape of the universe, and how might its topology allow for properties such as returning to a starting point after traveling in one direction? | 33,415 | cqadupstack-physics |
88187 | Can a quantum state with infinite variance of photon number be found in nature or artificially created? | Suppose we have a quantum state $\rho$ and let's denote the photon number operator $\hat{n}=\hat{a}^\dagger\hat{a}$ where $\hat{a}$ is the annihilation operator. Let mean photon number $\bar{n}=\operatorname{Tr}(\rho\hat{n})<\infty$ obviously be finite. Can $\rho$ have infinite photon number _variance_ , which we defin... | Can a quantum state $\rho$ exist such that it has a finite mean photon number $\bar{n} = \operatorname{Tr}(\rho \hat{n}) < \infty$ but an infinite photon number variance $\operatorname{Var}(\rho) = \operatorname{Tr}(\rho \hat{a}^\dagger \hat{a}^\dagger \hat{a} \hat{a}) - \bar{n}^2$? If such a state exists, how can it b... | 11,531 | cqadupstack-physics |
60431 | Given expectation values for E and B, can you find an associated state? | When we quantize the electromagnetic field, we develop the concept of the field operator $A(\vec{r},t)$ and the simultaneous eigenstates of momentum and the free field Hamiltonian (i.e., each eigenstate is given by specifying the number of photons with momentum $k$ and polarization $\mu$). We can then construct the ope... | Given the expectation values of the electric field \(E(\vec{r})\) and magnetic field \(B(\vec{r})\) that are time-independent and satisfy Maxwell's Equations, can you determine whether it is possible to construct a quantum state with these specific expectation values? Furthermore, is this state unique, or could there b... | 3,313 | cqadupstack-physics |
17368 | Special conformal transformations and locality | In the conformal symmetry, used in some QFT theories, the infinitesimal generators, applying to space-time, are all linear (translations, rotations, boosts, dilatation), except the special conformal transformations, which are not linear. I feel that this non - linearity could cause a conflict with locality, in the foll... | Can you explain how the non-linearity of special conformal transformations in quantum field theory could potentially lead to violations of locality, particularly in the context of wave packets separated by space-like intervals? | 7,210 | cqadupstack-physics |
87328 | Do photons age in a medium? | According to special relativity, time starts to slow down as we increase our speed and eventually stops once we get to the speed of light. By that logic, photons don't age in a vacuum state as, to us, the time stops for them. However, in a medium, their speed decreases, that means time is 'stationary'. Does that mean t... | If photons travel slower in a medium compared to their speed in a vacuum, how does their effective aging compare when measured in terms of distance traveled and the time taken within that medium? | 28,146 | cqadupstack-physics |
2219 | Beginner Physics Resources? | 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. Specifi... | What are the fundamental principles or axioms of physics that unify the different fields, and can you recommend accessible resources such as textbooks or video series that cover these principles suitable for a high school level? | 36,573 | cqadupstack-physics |
97694 | Why does a picture of a person seem to be looking in the same direction irrespective of the angle of observation? | If you observe a picture of a person hanging on a wall who seems to be looking directly towards you always seems to be looking at you even though you change your angle of observation to the extremes. The same can be observed in a television. If a television is watched by many people from different angles all observe th... | What is the geometric principle behind the phenomenon where a figure in a painting or on a screen appears to be looking directly at the viewer, regardless of the viewer's position, and how does this relate to perspectives in 3D film projection? | 37,679 | cqadupstack-physics |
24917 | How can the Earth's day increase and its rotation slow down at the same time? | I heard that the Earth rotation is slowing down, but I also heard the Earth's length of day is increasing. Does the two theories go together or conflict with each other? | If the Earth's rotation is slowing down, how much does the length of a day increase on average per century, and what are the implications of this change for long-term timekeeping? | 29,271 | cqadupstack-physics |
53475 | A fly in an accelerating car | A fly is flying around in a car, the fly never touches any surface in the car only fly’s around in the air inside the car. The car accelerates. does the fly slam in to the rear window. or does the fly continue to fly uninterrupted? | If a fly is flying freely inside a car that suddenly accelerates forward, what forces act on the fly, and will it remain unaffected or potentially collide with the rear window? Provide a detailed explanation of your reasoning and the physical principles involved. | 9,456 | cqadupstack-physics |
95373 | N level nucleon photon emission in a infinite potential well | Little question : Consider a nucleon in a infinite square potential well. At any given instant the nucleon goes from energy $E_n$ to $E_1$, emitting a photon with energy $3E_1$. In what circumstances can we be sure that before the transition the nucleon was on energy level $E_2$? And in what way will it be affected by ... | In an infinite square potential well, a nucleon transitions from energy level \(E_n\) to \(E_1\) while emitting a photon with energy \(3E_1\). Given that the emission of this photon corresponds to specific energy levels, under what condition can we confirm that the nucleon was originally at energy level \(E_2\)? Additi... | 22,066 | cqadupstack-physics |
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