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77199 | Questions about gravitational and inertial mass | 1. What differences between gravitational mass and inertial mass? I cannot tell the differences between them. 2. In history, which concept was put forward firstly? 3. Are there some experiments to prove they are identical? | What experimental evidence supports the equivalence of gravitational mass and inertial mass, and what is the numerical ratio of gravitational force to inertial resistance in free-fall experiments? | 29,429 | cqadupstack-physics |
1734 | How much can a laser's position be fine-tuned? | Suppose you wanted to do a time-resolved experiment with a molecular beam traveling at, say, 300 m/s involving a mobile excitation (pump) laser that scans across the length of the molecular beam and a stationary detector. So the time-resolution of the experiment depends on how precisely the position of the excitation l... | What is the minimum position fine-tuning precision required for a pump laser in a time-resolved experiment with a molecular beam traveling at 300 m/s to achieve a time resolution of 1 nanosecond? Additionally, what are some common methods to achieve such precision in laser tuning? | 26,063 | cqadupstack-physics |
11835 | Invariance and forms of the Lagrangian | I have been doing Landau and due to its concise style been facing a few problems. I hope you can help me out here somehow. 1)Does the "homogeneity of space and time" essentially talk about the invariance of the Lagrangian with co-ordinate system or time invariance? Is this the basis of Landau's proof of the Law of Iner... | Consider a closed system of particles where the Lagrangian \( L \) is defined as a function of the generalized coordinates \( q_i \) and velocities \( \dot{q}_i \). If the Lagrangian is invariant under a transformation of the coordinates, explain how this invariance relates to the conservation of momentum in the system... | 4,323 | cqadupstack-physics |
114022 | What is the minimal symmetry required for a spin Hamiltonian to describe a spin-liquid ground state? | Let's restrict to the case of spin-1/2 system. As we know, a spin-liquid (SL) state is the ground state of a lattice spin Hamiltonian with no spontaneous broken symmetries (sometime it may spontaneously break time-reversal symmetry and is called a chiral SL), where two _essential_ symmetries of a SL state are **lattice... | What is the minimal spin-rotation symmetry required for a spin Hamiltonian to describe a spin-liquid ground state, and does the $Q_8$ group qualify as this minimal symmetry? | 34,716 | cqadupstack-physics |
27841 | Temperature of the CMB when the Earth formed and the faint young Sun paradox | The cosmic microwave background (CMB) has a modern temperature of about 2.7 K. At the time of the origin of the CMB, about 13.6 billion years ago, it had a temperature of about 3000 K. There is a well-known problem that the Sun should have been too faint at the time the Earth formed, about 4.6 billion years ago, to all... | What was the temperature of the cosmic microwave background (CMB) 4.6 billion years ago, and how would this temperature affect the equilibrium temperature of the Earth, considering its role as a heat sink in the context of the faint young Sun paradox? | 21,365 | cqadupstack-physics |
104966 | The angle to shoot moving object | I am obliged to count very simple problem (at least it seemed that it is simple, I hope it isn't to simple for this site). So i got observer who is standing $H$ below the object. The object is fired horizontaly with the speed $V_{0}$. Assuming that the observer has the reaction time that equals $T$, on what angle he sh... | An observer standing a vertical distance \( H \) below a horizontally fired object, moving at speed \( V_0 \), needs to know the angle \( \alpha \) at which to fire a bullet, traveling at speed \( V_1 \), to intercept the object. If the observer has a reaction time \( T \), derive the expression for the required angle ... | 22,958 | cqadupstack-physics |
19552 | Would a light or a heavy ball roll fastest down a slope? | A small, light ball and a larger, heavier ball are released from the top of a slope. Which will move further? which will come down faster? | If a small, lightweight ball and a larger, heavier ball are released from the same height on a slope, which ball will reach the bottom first, and how far will each ball travel if they roll off the edge of the slope? Assume there is no air resistance and both balls roll without slipping. | 21,761 | cqadupstack-physics |
104050 | Quantum entanglement: does it necessarily imply superluminal information transfer? | From what I understand, information is communicated _instantly_ between two quantum-entangled particles regardless of the spatial distance between them. However, does this necessarily imply superluminal data transfer? If the "distance" between the particles is simply assumed to be measured in our common 3D euclidean sp... | What experimental evidence exists to support or refute the possibility of faster-than-light communication via quantum entanglement, and how might additional dimensions or alternative geometries affect this understanding? | 10,234 | cqadupstack-physics |
36122 | Temperature measurement, precision | I am having problems with one of the problems I have done (a) which is basically $\theta = 273.16K (19/15) = 346.00K$ For (b) I am given that the length is 15.00cm when the thermometer is incontact with water at its triple point. But I have no idea what... | Given that the thermometer has a length of 15.00 cm when it is in contact with the water at its triple point (TP), and using the formula \(\theta = 273.16(X/X_{tp})\), where \(X\) is the length of the mercury column at temperature θ and \(X_{tp}\) is the length at the triple point, what would be the length of the mercu... | 27,164 | cqadupstack-physics |
117034 | Guides for solving E&M problems | I am an undergraduate physics student in a university and I am looking for a good guide or a handbook with lots of questions and full answers in subjects such as Gauss' law and Maxwell equations to study for my test. | What are some recommended textbooks or handbooks for undergraduate physics that include numerous problems and comprehensive solutions specifically focused on Gauss' law and Maxwell's equations? | 27,567 | cqadupstack-physics |
60590 | Is it ever necessary to extend an analysis of Grover's algorithm beyond $k/N = 1/2$? | Is it ever necessary to extend an analysis of Grover's algorithm beyond $k/N = 1/2$, where $k$ is the number of "hits" in a total of $N$ possible values for $|\,x\rangle$? If we know $k$, and know that it is greater than $1/2$, can't we just redefine hits as misses by modifying the oracle to produce $$-(-1)^{f(x)}\,|\,... | If Grover's algorithm is analyzed at the point where the ratio \( k/N = 1/2 \), how does the probability of successfully finding a "hit" change as the ratio \( k/N \) varies from greater than \( 1/2 \) to approaching \( 1 \)? Specifically, for a given number of hits \( k = 10 \) in a total of \( N = 20 \) possible valu... | 6,567 | cqadupstack-physics |
4247 | Solving a Young Laplace equation for a meniscus against a flat plate | This is more of a math question and one, furthermore, that I know the final answer to. What I am asking is more of a "how do I get there" question as this question was generated during a self study situation. So, for a flat plate with a meniscus on it at some contact angle the form is: a y''$/(1+(y')^2)^{3/2}$ + b g y ... | Given the differential equation for a flat plate with a meniscus:
\[
a \frac{d^2y}{dx^2}{(1 + (dy/dx)^2)^{-3/2}} + bgy = 0
\]
where \( a \) is the surface tension, \( b \) is the density, and \( g \) is the acceleration due to gravity (9.8 m/s²), derive the dimensionless form of this equation and outline the steps ne... | 32,824 | cqadupstack-physics |
69038 | Indistinguishability in Quantum Mechanics | When describing the defining characteristics of bosons and fermions, I have a problem with the idea of "label switching" - whereby you have the wavefunction for two particles and the particles' labels in the wavefunction are switched and we look at the effect. The wavefunction remains the same: particles are bosons. If... | In a system of two indistinguishable particles, how does the symmetry of the wavefunction differ between bosons and fermions, and what is the mathematical representation of each case? | 29,381 | cqadupstack-physics |
14102 | What happens to the absorbed light energy? | When light comes across with a solid material, some of it is reflected, some of it passes through and some of it is absorbed. I understand the reflection and passing through, but I don't understand what happens when light is absorbed. Suppose that there is an enclosed box. Its material is thick enough so that no light ... | In an enclosed box made of a solid material that prevents any light from escaping, if a LED diode continuously emits light with all its power converted to light energy (with no heat loss), what steady state scenario will occur over time, and how does this relate to the absorption of light within the box? Additionally, ... | 16,746 | cqadupstack-physics |
90472 | When is quasiparticle same as elementary excitation, and when is not? | Can anyone shed light on the comparison between these two concepts? | What are the key differences in the energy transfer mechanisms between photosynthesis and cellular respiration, and how are these differences quantitatively represented in their respective energy yield? | 2,316 | cqadupstack-physics |
106050 | Can electrons coincidentally flow along a circuit to cause current? | My understanding of circuits which are not supplied an e.m.f. is that the electrons randomly just flow about in random directions, and since there's so many of them, probability dictates that any forwards or useful movement is cancelled almost perfectly by the negative or unwanted movement. If this understanding is cor... | What is the theoretical possibility of generating a net current in a circuit with no supplied e.m.f. solely due to random electron motion, and how would you calculate the likelihood of such an event occurring? | 16,531 | cqadupstack-physics |
89525 | Where does the divergence in the $g\phi^3$ $d=4$ 3 point one loop diagram (three external legs) come from? | $g\phi^3$ , $d=4$ , 3 point One loop diagram (three external legs) Divergence I am trying to find where the divergence factor/pole is on the following diagram in 4 dimensions so that I can use minimal subtraction...  I know it is proportional to... $$ \... | What is the divergence factor of the one-loop diagram with three external legs in four dimensions for the interaction term proportional to \(g\phi^3\), and how does it relate to the renormalization of the coupling constant \(g\)? Specifically, can you evaluate the integral involving Feynman propagators to find the cond... | 21,636 | cqadupstack-physics |
47828 | Fighting a black hole: Could a strong spherical shell inside an event horizon resist falling in to the singularity? | As a thought experiment imagine an incredibly strong spherical shell with a diameter a bit smaller than the event horizon of a particular large black hole. The shell is split into two hemispheres, and the two hemispheres are released from either side of the black hole such that they will meet and join inside the event ... | If a spherical shell made of a theoretical maximally densely packed neutronium-like material is constructed to have a diameter just smaller than the event horizon of a massive black hole, what minimum tensile strength would this shell need to theoretically resist the gravitational forces inside the event horizon and ma... | 35,425 | cqadupstack-physics |
115078 | Magnetic field in materials with non-constant magnetic susceptibility | I'm quite lost what $B$ and $H$ is. It seams to me that most of the texts I read do quite poor job in explaining them properly. They are explained only in cases when magnetic susceptibility is constant in the space. First _law_ says that $M = \chi H$, $M$ is magnetization and to my understanding $H$ is something like e... | How do you calculate the magnetic field \( B \) around a paramagnetic material considering that magnetic susceptibility \( \chi \) may not be constant, and what are the implications on the divergence of \( B \)? | 20,430 | cqadupstack-physics |
69100 | Meaning of "Gradient with respect to coordinates of particle" in SPH | I'm currently trying to implement a simple SPH simulation based on a variety of papers. However as I'm not a trained physicist nor mathematician I have a small issue with the following notation and formulation repeated in a number of different papers and internet sites: > and $\nabla_a$ denotes the gradient taken with ... | In a Smoothed Particle Hydrodynamics (SPH) simulation, when the notation $\nabla_a$ is used to denote the gradient with respect to the coordinates of particle $a$, what is the mathematical interpretation of this notation? Specifically, how does this relate to vector calculus in terms of partial derivatives across diffe... | 33,392 | cqadupstack-physics |
71024 | Where does $p^i/p^+$ come from in the EOM of an open string? | I have a stupid question about Eq. (1.3.22) in Polchinski's string theory volume 1. In the equation of motion for an open string, Eq. (1.3.22), $$X^i (\tau, \sigma) = x^i + \frac{ p^i}{p^+} \tau + i \bigl(2 \alpha'\bigr)^{1/2} \sum_{\substack{n= -\infty,\\\n\neq0}}^{\infty} \frac{1}{n} \alpha_n^i \exp\biggl( -\frac{ \p... | In the equation of motion for an open string given by $$X^i (\tau, \sigma) = x^i + \frac{ p^i}{p^+} \tau + i \bigl(2 \alpha'\bigr)^{1/2} \sum_{\substack{n= -\infty,\\n\neq0}}^{\infty} \frac{1}{n} \alpha_n^i \exp\biggl( -\frac{ \pi i n c \tau}{ l}\biggr) \cos \frac{ \pi n \sigma}{l},$$ what is the physical significance ... | 38,064 | cqadupstack-physics |
41237 | Is there a device that could measure the speed of time? | Is there (or can there be) a device that could measure the speed and acceleration of time? | What physical principle could be used to theoretically measure the change in the perceived rate of time under different gravitational fields, and how would you quantify this effect using the equations of general relativity? | 17,305 | cqadupstack-physics |
55912 | The number of mean free paths which a particle traverses is independent of the material traversed | This is a manual from a Monte Carlo physics manual: Computing the occurrence of a process Can anyone tell me why the number of mean free paths of a particle is independent of the material which it traverses? It does not make sense to me. | How can you derive the mean free path of a particle in a gas and demonstrate that it remains constant regardless of the specific gas composition? | 31,416 | cqadupstack-physics |
113133 | Atomic nucleus consisting of only neutrons? | Is it true that the nucleus of all atoms (including radioactive isotopes) contain at least one proton? Is there an atomic nucleus consisting entirely of neutrons? (Let's exclude neutron stars for the moment.) If so, how does one name them? (since Periodic Table starts from atomic number 1, not 0.) | What is the name of the hypothetical atomic nucleus that contains only neutrons and no protons, and can such a nucleus exist under normal conditions, excluding scenarios like neutron stars? | 19,217 | cqadupstack-physics |
98904 | Rod Falling on Frictionless Surface | A rigid rod with mass m is initially held at an angle with respect to a frictionless plane by a string. Then the string is cut. What happens to the rod? My intuition suggests that the rod should slide to the left but I cannot figure out how this is supposed to happen when the only forced involved are vertical (reaction... | A rigid rod of length \( L \) and mass \( m \) is initially held at an angle \( \theta \) to a frictionless horizontal plane by a string. When the string is cut, what is the trajectory of the center of mass of the rod immediately after the string is cut, and what angle does the rod make with the horizontal plane just b... | 30,194 | cqadupstack-physics |
122582 | Can diffusion produce energy? | A friend and I recently got into a silly argument where I stated > pure diffusion can't produce energy since **diffusion are a part of passive > transport**. He stated if we > If we have tinny turbine which allow to transfer only one molecule at a time > due to the different concentration the molecule will make turbine... | Can passive diffusion create usable energy if a semi-permeable membrane allows the flow of molecules from a high concentration to a low concentration, and how does this process relate to the principles of thermodynamics? | 6,019 | cqadupstack-physics |
43827 | Car accident, put in park or neutral? | I was waiting on a red light the other day and was wondering. If I'm in my car, not moving and I see a car that's going to hit me from behind. Would I (my body) be safer if I put on the break or if I put the car in neutral? I assume there's no car in front of me and there's no posibility to be hit by an other car. Also... | If a stationary car is hit from behind by another vehicle traveling at a speed of 30 m/s, what would be the difference in the force experienced by the driver if the car is in neutral compared to if the brakes are applied? Assume that the mass of the stationary car is 1,200 kg and that the impact lasts for 0.1 seconds. ... | 24,668 | cqadupstack-physics |
53682 | What causes ballpoint pens to write intermittently? | After a while, a ball point pen doesn't write very well anymore. It will write for a little distance, then leave a gap, then maybe write in little streaks, then maybe write properly again. It seems to be worse with older pens, but I have observed this with new pens right out of the box too. Experiments I have done: *... | What is the average time it takes for a standard ballpoint pen to start writing again after being left ball-down for different intervals, and how does this time change with variations in ink viscosity? | 1,241 | cqadupstack-physics |
100512 | Understanding Counter - EMF? | When an coil rotor is moving around a magnetic field there is -V induced to resist the input V. Let's take an example, a 12 V DC motor induces -10V, and the actual running voltage is 2V. If there we're no -V in the process, the motor would preform a lot better? What is the advantages and disadvantages of counter EMF? I... | What is the relationship between the induced counter EMF and the actual running voltage of a 12 V DC motor that exhibits a counter EMF of -10 V, and how does this affect the motor's efficiency and performance? | 15,206 | cqadupstack-physics |
93855 | Blimp challenge. Doubts about volume at different heights | I want to design and build a small semi-rigid airship/dirigible. It will be controlled with RC so I will install motors, arduino, battery etc. Filled with helium. Weight of the equipment is about 2kg and weight of the dirigible itself (envelope, light internal scaffold...) is unknown since I don't know how big it has t... | What is the minimum volume of helium that my semi-rigid airship needs to displace in order to achieve neutral buoyancy at an altitude of 3000 meters, given the air density of 0.91 kg/m³ and helium density of 0.1256 kg/m³? Additionally, how would this volume change if the air temperature at that altitude significantly a... | 20,243 | cqadupstack-physics |
5265 | Cooling a cup of coffee with help of a spoon | During the breakfast with my colleagues, a question popped into my head: _**What is the fastest method to cool a cup of coffee, if your only available instrument is a spoon?_** A qualitative answer would be nice, but if we could find a mathematical model or even better make the experiment (we don't have the means here ... | What is the quantitative effect of stirring a cup of coffee with a metal spoon on its cooling rate compared to leaving the spoon inside without stirring, considering factors such as heat transfer and surface area? | 4,790 | cqadupstack-physics |
126144 | Optically thick media | What does "optically thick media" mean? Does this property depend on the length of an enclosure? Why is the diffusion approximation for the radiative transfer equation applied to optically thick media? | In a medium with a known absorption coefficient of 0.5 m⁻¹, at what minimum thickness would you classify the medium as "optically thick" if the criteria for optical thickness is defined as τ > 1? | 24,828 | cqadupstack-physics |
49790 | Why does weak equivalence principle say gravity is equivalent to acceleration? | I am told that the weak equivalent principle, that $m_i=m_g$ (inertial and gravitational masses are equivalent) is equivalent to the statement that in a small system you can't tell whether you are in a uniform gravitational field, or in an equivalent accelerating frame. My question is about light: Special relativity do... | In a rocket ship accelerating at 9.81 m/s², if a beam of light is emitted horizontally from the center of the ship, how far will the light travel relative to an observer inside the rocket before the rocket moves a distance of 1 meter? Assume the effects of special relativity can be ignored, and compute the time it take... | 28,556 | cqadupstack-physics |
104955 | Is every electromagnetic radiation considered "light"? | Somebody mentioned on Freenode chatroom for physics that All Electromagnetic Radiation are delivered in form of Photons not just light. Is it true? Does that mean if we get a THF electrical oscillator to emit the frequencies that are visible to the eye, do we see light coming out of the circuit? | If an electrical oscillator is tuned to emit frequencies within the visible spectrum (approximately 400 to 700 nm), what type of electromagnetic radiation does it produce, and how does this relate to the emission of photons? | 27,033 | cqadupstack-physics |
63810 | Under what conditions are the renormalization group equations "reversible"? | As I understand it, the renormalization group is only a semi-group because the coarse graining part of a renormalization step consisting of 1. Summing / integrating over the small scales (coarse graining) 2. Calculating the new effective Hamiltonian or Lagrangian 3. Rescaling of coupling constants, fields, etc. ... | Under what specific theoretical conditions can the renormalization group transformations be considered invertible, allowing for the integration of renormalization group equations backward in time towards the ultraviolet (UV) regime, particularly in the context of coupling constant unification? | 32,023 | cqadupstack-physics |
82511 | Details of the new solutions to the three-body problem | > This question is part of this week's Journal Club session. * * * These systems look ridiculously fun to construct. Could someone explain the particulars? What are the various types of solutions, and what are their dynamics? How are they different from previously discovered solutions? ![enter image description here](h... | What are the different types of solutions for the three-body problem, and how do their dynamics compare to those of previously discovered solutions? Specifically, can you quantify the number of new periodic orbits identified in the 2013 paper by Šuvakov and Dmitrašinović? | 26,783 | cqadupstack-physics |
36292 | Is it true looking at an object from a great distance with a telescope will show the past version of the object? | Once again from my son's workbook. It discussed standing on a planet 65 million light years away from Earth, with an extremely powerful telescope pointed at Earth. It claimed that then you could see the dinosaurs if that was the case. Why? | If a powerful telescope on a planet 65 million light years away from Earth could clearly observe the Earth, how many years would it take light from Earth, during the time of the dinosaurs, to reach this planet? What implications does this have for the visibility of events on Earth from that distance? | 3,183 | cqadupstack-physics |
113289 | role of entropy in gibbs free energy intuition | I have looked at various derivations of the Gibbs free energy equation and the underlying definition of the gibbs free energy. However I have been unable to attain direct insight/intuition over the significance of entropy in the equation. Specifically, lets take the case of 2 gases reacting to form a solid (the reactio... | How does an increase in temperature affect the spontaneity of a reaction where two gases react to form a solid, considering the negative entropy change associated with the transition from gas to solid? Specifically, what role does entropy play in this scenario and why might high temperatures inhibit the reaction despit... | 22,932 | cqadupstack-physics |
72501 | Lagrange Multipliers Versus Generalized Coordinates | When forced to explain to someone why one could either set up a general Lagrangian & then incorporate constraints using Lagrange multipliers, as opposed to just setting up a Lagrangian with generalized coordinate built in from the start I found I couldn't do it - I don't actually know why one can use either method othe... | What are the conditions under which the method of Lagrange multipliers and the direct incorporation of constraints into the Lagrangian yield equivalent results in classical mechanics? | 18,631 | cqadupstack-physics |
7327 | Is it possible to restart formal higher education in physics at a later age? | I would begin by apologizing for a soft question. Reading the FAQ, I have tried to formulate my question precisely in the last paragraph and you may skip the following paragraph where I make subjective observations, which I think may be relevant and any comments would be more than helpful. By formal education I mean re... | What is the average age of PhD completion for physicists, and how does it compare to the age at which significant contributions to the field, such as major discoveries or theories, are typically made? Additionally, what factors could contribute to a successful career in physics for someone who begins their formal educa... | 34,717 | cqadupstack-physics |
759 | Does Quantum Physics really suggests this universe as a computer simulation? | I was reading about interesting article here which suggests that our universe is a big computer simulation and the proof of it is a Quantum Physics. I know quantum physics tries to provide some explanation about unpredictable behavior of universe at atomic scale but could not believe that thing has anyway relates to pi... | What evidence from quantum physics could be interpreted as supporting the idea that our universe functions like a computer simulation, and how might this relate to the concept of "pixels" at the atomic level? | 36,691 | cqadupstack-physics |
119529 | Intersecting Wilson loops in 2D Yang-Mills | I am currently trying to understand 2D Yang-Mills theory, and I cannot seem to find an explanation for calculation of the expectation value of intersecting Wilson loops. In his _On Quantum Gauge Theories in two dimensions_, Witten carries out a curious calculation: For three reps $ \alpha,\beta,\gamma $, we fix a basis... | What is the relationship between the 6j Wigner symbol and the associator of the tensor product in the context of calculating the expectation value of intersecting Wilson loops in 2D Yang-Mills theory? Additionally, can you provide a method or reference for proving this relationship? | 23,689 | cqadupstack-physics |
129472 | Flywheels and work | I'll start with a scenario for some control in the concepts I'm asking about today. Forgive me in advance, I never had the chance to explore physics in school, but I'd be happy to learn something new here. Let's say I have 3 different fly wheels, all 3 of them have a handle in the middle to hand-crank them. The first t... | How does the distribution of weights on flywheels affect the torque required to rotate them, specifically comparing a flywheel with weights at the center to one with weights at the edges? Additionally, what impact does the delayed deployment of weights from the center to the outer edge have on the overall work done dur... | 16,182 | cqadupstack-physics |
60372 | Question about a finite time interval step in the derivation of the Feynman path integral in Sakurai | This may be a possible errata but Sakurai (pp 126 in the 2nd Edition) states that starting with $$S = \int dt \,\,\scr{L_{\mathrm{classical}}}$$ Looking at a finite-time-interval of the action: \begin{equation} S(n,n-1) = \int\limits_{t_{n-1}}^{t_n} dt \left( \frac{m \dot{x}^2}{2} - V(x)\right)\,\,\,\,\,\,(1) \end{equa... | Why does the potential energy term in the action approximation change from \( V(x_n - x_{n-1}) \) to \( V\left(\frac{x_n + x_{n-1}}{2}\right) \) when using a straight-line approximation for the trajectory in classical mechanics? | 17,586 | cqadupstack-physics |
3579 | What is Time like Quantum Entanglement? | I came across a New Study at : http://arxiv.org/PS_cache/arxiv/pdf/1101/1101.2565v1.pdf . Which talks about Time like quantum entanglement. What does that mean? **Comment added by L.Motl** : The same preprint has been discussed in this question and will be discussed in this question in the future. | What are the implications of time-like quantum entanglement for the speed of information transfer in quantum communication, and under what conditions can it occur according to current theories in quantum mechanics? | 35,568 | cqadupstack-physics |
86846 | Problem with Velocity of efflux | I am stuck in this problem-  I need to find the velocity of efflux at the hole of the container. [We can assume that the area of the hole is negligible in comparison with the base area of the container]. Here's my approach Velocity of liquid at the upper-surface = $v_2$... | What is the correct expression for the velocity of efflux at the hole of the container according to Bernoulli's principle, assuming the densities of the fluids are \(\rho_1\) and \(\rho_2\), and the height of the liquid column is \(h_1\)? | 34,426 | cqadupstack-physics |
110698 | Spin-Position Coupling/Interaction | Firstly, is there a general procedure/algorithm for attacking Hamiltonians having spin-position coupling, e.g., $$\frac{\mathbf{\sigma_1}\cdot\mathbf{\sigma_2}}{|\mathbf{r_1}-\mathbf{r_2}|}$$ I do not know where to begin. Secondly, I would like you to recommend a textbook which discuss such a kind of systems. I looked ... | What methods can be used to solve for the energy levels of a two-spin system with a Hamiltonian that includes spin-position coupling, such as $$H = \frac{\mathbf{\sigma_1}\cdot\mathbf{\sigma_2}}{|\mathbf{r_1}-\mathbf{r_2}|}$$, and could you recommend a textbook that covers this topic in detail? | 10,549 | cqadupstack-physics |
45352 | Is this $W_{r} = \frac{1}{2}kx^2 $ procedure correct | I have a $0.40 Kg$ of mass that has $v= 0.5 m/s$, and then crashes with a spring with constant $k = 750N/m$ Whats the distance that the spring was compressed? I was using the equation $W_{r} = \frac{1}{2}kx^2 $ so $\frac{1}{2}(0.4)(0.5*0.5) = \frac{1}{2}(750)(x^2)$ so $x = 0.011 m$ Is this correct? what is the solution... | A mass of 0.40 kg moving at a velocity of 0.5 m/s crashes into a spring with a spring constant of 750 N/m. Using the work-energy principle, what is the maximum compression of the spring during the collision? Provide the correct calculations and answer with appropriate units. | 15,469 | cqadupstack-physics |
116315 | Feynman rules for coupled systems | I have the following system of two coupled real scalar fields $\sigma$ and $\phi$: $S[\phi,\sigma]=\int{d^4x[-\frac{1}{2}\partial_\mu\phi\partial^\mu\phi-\frac{1}{2}m^2\phi^2-\frac{1}{2}M^2\sigma^2-\frac{\lambda}{2}\sigma\phi^2]}$. What would the Feynman rules be for this system? I realise that something will be differ... | What are the Feynman rules for the coupled scalar fields $\sigma$ and $\phi$ described by the action $S[\phi,\sigma]=\int{d^4x[-\frac{1}{2}\partial_\mu\phi\partial^\mu\phi-\frac{1}{2}m^2\phi^2-\frac{1}{2}M^2\sigma^2-\frac{\lambda}{2}\sigma\phi^2]}$, particularly considering the implications of the absence of a kinetic ... | 20,141 | cqadupstack-physics |
79794 | How is black hole affected by gravity of nearby bodies? | I run into wikipedia articles about ergosphere of rotating black holes. What if some massive body passes nearby some black hole: is something like ergosphere produced, or is the event horizon distorted by the gravity of the passing body? | If a massive body passes near a rotating black hole, how would the spacetime geometry around the black hole, particularly the ergosphere and event horizon, be affected in terms of gravitational distortion? Can you provide a quantitative description of the changes in the ergosphere's boundary? | 9,238 | cqadupstack-physics |
29111 | Should any theory of physics respect the principle of conservation of angular momentum or linear momentum? | Is it possible that a theory that can describe the universe at the planck scale can violate things that we now consider fundamental in nature?For example can it violate rotational and translational invariance and subsequently momentum wouldn't be conserved ?Should one consider these invariance principles to be fundamen... | Given a hypothetical theory that operates at the Planck scale, what are the implications for the conservation of momentum if this theory allows for violations of rotational and translational invariance? Provide a quantitative example where momentum conservation is calculated in a scenario that violates these invariance... | 1,811 | cqadupstack-physics |
100529 | Getting the Lagrangian from the action in curved spacetime | Suppose I have this action: $$ S = \int \mathrm d^4 x\sqrt{-g}\times \text{something}$$ where $g$ is the determinant of the metric. Should I take the Lagrangian to be: $$ \mathcal L = \sqrt{-g} \times \text{something} $$ or: $$ \mathcal L = \text{something}$$ instead? Yes, this is a stupid question. | Given the action \( S = \int \mathrm{d}^4 x \sqrt{-g} \times \text{something} \), where \( g \) is the determinant of the metric, determine how the choice of Lagrangian \( \mathcal{L} \) affects the equations of motion derived from this action. Specifically, if one were to choose \( \mathcal{L} = \sqrt{-g} \times \text... | 27,210 | cqadupstack-physics |
29955 | Would a fast inter-stellar spaceship benefit from an aerodynamic shape? | Some (generous) assumptions: * We have a spaceship that can reach a reasonable fraction of light speed. * The ship is able to withstand the high energies of matter impacting at that speed. Given the amount of matter in inter-stellar space, at high speed, would it encounter enough of it and frequently enough that an... | At what speed does a spaceship traveling through interstellar space begin to encounter significant drag from matter, and how does the density of that matter impact the fuel efficiency of the ship? | 3,106 | cqadupstack-physics |
133815 | Total Vs Partial in Lagrange density? | I have a question regarding the red term below. This is the integration by parts during the derivation of the Euler-Lagrange equation for continuous systems. Why is this not the time derivative $d/dt$.... why do we use the partial? I have seen the equation expressed in both ways and wondered what the physicality's are.... | In the context of the Euler-Lagrange equation derived from the action principle in classical mechanics, consider the Lagrangian density given by \(\mathcal{L} = \mathcal{L}(\phi, \dot{\phi}, \partial_x \phi)\). Why is the term \(\frac{\partial}{\partial t}\) used in the integration by parts instead of the total time de... | 38,290 | cqadupstack-physics |
104336 | Can a theory be proven by an experiment (or series of) that sustains it? | This is a question that I've always had but it wasn't until the recent news regarding the BICEP2 experiment that I really gave thought about it (please note that the question is general and not particular to the theories involved in that experiment). I think a series of experiments or experimental data that conforms to... | To what extent can a scientific theory be considered validated based on repeated experimental confirmations, and how is this quantifiable in terms of statistical confidence levels? Specifically, what is the minimum percentage of agreement from independent experiments needed to elevate the confidence in a theory from "h... | 4,203 | cqadupstack-physics |
129668 | How Chern-Simons gauge field transform fermion to scalar? | In A.Zee's "QFT in a nutshell"book Page 324, after he wrote down the general Lagrangian, he said "in previous chapter, we learned that by introducing a Chern-Simons gauge field we can transform $\psi$ to a scalar field" However, I didn't found any clarification about this argument in previous chapter. Any comment or re... | What is the relationship between Chern-Simons gauge fields and the transformation of spinor fields to scalar fields, and can you provide an example or application of this concept in quantum field theory? | 30,902 | cqadupstack-physics |
122649 | Block on an inclined plane | If you take moments about the centre of mass of a block positioned on an inclined plane so that the gravitational force can be drawn from the centre of mass of the block to one corner of the block, why is there no turning effect due to the reaction? | If a block is positioned on an inclined plane, at what angle of inclination would the gravitational force acting through the center of mass begin to exert a turning effect on the block, assuming it is uniform and its center of mass is at its geometric center? | 13,286 | cqadupstack-physics |
73265 | Orthogonality of the wavefunctions on an subinterval? | Lets say that functions (eigenfunctions) $\psi_0$ and $\psi_1$ are orthogonal on an interval $-d/2 < x < d/2$. Are they also orthogonal on any subinterval inside the interval $-d/2 < x < d/2$? For example if I take subinterval $$d/4 < x < d/2$$ Can I state that: $$ \int\limits_{d/4}^{d/2}\overline{\psi}_0\psi_1\, dx= 0... | If two functions $\psi_0$ and $\psi_1$ are orthogonal over the interval $-d/2 < x < d/2$, can you provide an example showing whether they are necessarily orthogonal over the subinterval $d/4 < x < d/2$? Specifically, what conditions must be met for the integral $$ \int\limits_{d/4}^{d/2}\overline{\psi}_0\psi_1\, dx = 0... | 34,131 | cqadupstack-physics |
97837 | Majorana equation in two forms | Let's have two forms of Majorana equation. First form (standart or spinor representations of gamma-matrices). $$ i\gamma^{\mu} \partial_{\mu}\Psi - m\Psi = 0, \quad \Psi = \Psi_{c} = \hat {C} \bar {\Psi}^{T} = \begin{pmatrix} \Psi_{a} \\\ \bar {\Psi}^{\dot {a}}\end{pmatrix}, \quad \hat {C} = diag (\varepsilon_{\alpha \... | In the context of the Majorana equation and the transformation between different gamma matrix representations, if the charge conjugation operator for Majorana fermions is defined as $$ \Psi^{c} = \hat {C} \gamma_{0}^{T} \Psi^{*} $$ and the Majorana representation leads to $\hat {C}^{Majorana} = \begin{pmatrix} -i & 0 \... | 2,260 | cqadupstack-physics |
111732 | Is it possible to have $\Delta T\neq 0$ with no heat exchange $Q=0$, no work $W=0$ done, and no change in internal energy $\Delta E=0$? | Is it possible to have $Q=0$, $\Delta E=0$ and $W=0$, but $\Delta T\neq 0$? In particular, if there is no change in internal energy, doesn't that imply it is an isothermal process, and therefore that $\Delta T=0$? | Can a system undergo a process where the heat transfer (Q), work done (W), and change in internal energy (ΔE) are all zero, while still resulting in a change in temperature (ΔT) greater than zero? If so, provide a physical explanation for how this can occur. | 29,957 | cqadupstack-physics |
53175 | How do I determine where I can no longer hear sound? | A teacher stands well back from an outside doorway that is 95 cm wide and blows a frequency of 600 Hz. The door faces north. Assume the speed of sound is 340 m/s and that there are no reflections. At what angles (from the line normal to the doorway) is it not possible to hear the whistle outside on the playground? I've... | A teacher blows a whistle at a frequency of 600 Hz through a doorway that is 95 cm wide. Given that the speed of sound is 340 m/s, at what angles from the line normal to the doorway will destructive interference occur, resulting in areas where the whistle cannot be heard outside? | 24,711 | cqadupstack-physics |
9818 | Logical positivism and black hole interiors | A black hole exists. Eventually, it will completely evaporate away. Alice falls into the hole. Imagine you are Alice. According to logical positivism, the interior of the black hole exists. But you will die at the singularity. You can't measure anything after you're dead, so any event outside after the black hole has v... | If Quentin measures his qubit and finds it in the state |0⟩, what is the event probability of him encountering the singularity as he jumps into the black hole, assuming he cannot return to the observable universe? | 31,886 | cqadupstack-physics |
116869 | I would like to know, what would happen if a primordial black hole mass of Earth, would impact with the Earth | Sorry for any mistakes, I'm Italian Hello everyone, I'm writing a novel, in which the Earth is devastated by enormous geological cataclysms (something like the end of the world), and to implement my cataclysm, I thought of a primordial black hole with a mass similar to that of the Earth. At first I thought to stabilize... | If a primordial black hole with a mass equivalent to that of the Earth were to pass close to the Earth without colliding, what would be the expected gravitational effects on the planet, particularly in terms of tidal forces and potential geological disruptions? Calculate the tidal force exerted by the black hole at a d... | 18,059 | cqadupstack-physics |
109171 | Degeneracy, spherical harmonics | In a 3D oscillator, the energy levels are known to be $(n_x + n_y + n_z + \frac{3}{2})\hbar \omega = (n + \frac{3}{2})\hbar \omega$. Say for $n = 1$, any of the $n$'s can be $1$ and the rest are $0$. For $n = 2$ and $3$, same concept applies. How do I use this information about degeneracy to suggest values of $\ell$ in... | What are the possible values of the angular momentum quantum number \( \ell \) for a 3D quantum harmonic oscillator at energy levels corresponding to \( n = 1 \), \( n = 2 \), and \( n = 3 \), considering the degeneracy of states in terms of \( n_x \), \( n_y \), and \( n_z \)? | 21,549 | cqadupstack-physics |
90298 | Can sand falling in a floating hourglass cause it to sink? (Follow up to hourglass question) | This is related to this question:Does the weight of an hourglass change when sands are falling inside? At Brigham Young University, there is a display consisting of a sealed off liter bottle with a sunken sealed off hourglass. When one turns over the bottle, the hourglass floats as the sands flow down. Eventually, the ... | What is the change in the buoyancy of a sealed hourglass when the sand within it is flowing down, and how does this affect its overall density as compared to the surrounding water? | 14,218 | cqadupstack-physics |
77812 | What is the height of breeze/wind? | How high does a breeze/wind blow? (If you are out in the open and can feel the breeze blowing, till how high is this breeze felt?) And what are the factors that affect it? | At what altitude can a breeze be felt in an open area, and what are the key atmospheric factors that influence this height? | 31,017 | cqadupstack-physics |
39387 | If you are carrying a magnet, can you tell when you cross the event horizon of a black hole? | By the no hair theorem, a black hole is completely characterized by it's mass, charge and angular momentum. Therefore dipole and higher pole magnetic fields are completely expelled when a black hole is formed. (The magnetic field exponentially decays as electromagnetic fields radiate away the energy of the magnetic fie... | If you were to fall into a supermassive black hole while carrying a bar magnet, how would the presence of the black hole's event horizon affect the magnetic field of your magnet, and would you be able to measure any changes to the magnetic field as you crossed the horizon? | 12,862 | cqadupstack-physics |
21100 | Good quantum physics textbooks | > **Possible Duplicate:** > Learn algebra and interpretation of QM I'm a physics student it's my last year and I find my self struggling with usage of quantum mechanics. Now I'm starting to learn nuclear physics, solid state physics and soon quantum field theory and I see that I have holes in my knowledge. It's not ... | What are some recommended textbooks on quantum mechanics that utilize bra/ket notation and include a variety of solved examples to enhance problem-solving intuition? | 13,052 | cqadupstack-physics |
121770 | Metric expansion of space and Newton's second law | I wanted to ask the following question: If the expansion of the universe is really accelerating, does that mean a certain force applied on the universe? (According to Newton's second law) What kind of force is that? | If the expansion of the universe is accelerating, what is the nature of the force causing this acceleration, and how can it be quantified in terms of Newton's second law? | 300 | cqadupstack-physics |
43338 | How to explain (pedagogically) why there is 4 spacetime dimensions while we see only the 3 spatial dimesions? | I have been asked this question by a student, but I was able and in the same time incapable to give a good answer for this without equations, so do you have ideas how one can explain this in a simple way? (Answers like we can take time as an imaginary, or our space is actually pseudo-Euclidean will be hard to grasp for... | How can you explain why we perceive the universe as a three-dimensional space moving through one-dimensional time, and what implications does this have for our understanding of the physical world? | 31,403 | cqadupstack-physics |
16754 | Plotting Hydrogen's $2P_{x,y,z}$ Probability Densities in MATLAB | I have spent an unreasonable amount of time trying to plot $F(r,\theta,\phi)$ plane slices in MATLAB. I want to look at $x-y,y-z,x-z$ planes. Here's the function, specifically: $$F(\rho,\theta,\phi)~=~4\pi\rho^{2}|\frac{\rho e^{\frac{-\rho}{2}}}{4\sqrt[]{2\pi}}\cos(\theta)|^{2}.$$ I certainly know about `contour()` and... | What are the values of the probability density \( F(r, \theta, \phi) \) for a hydrogen atom in the \( x-y \), \( y-z \), and \( x-z \) planes at a radius \( \rho = 1 \) unit, given the function \( F(\rho, \theta, \phi) = 4\pi\rho^{2} \left| \frac{\rho e^{-\frac{\rho}{2}}}{4\sqrt{2\pi}} \cos(\theta) \right|^{2} \)? Plea... | 30,749 | cqadupstack-physics |
104686 | Why doesn't breath condense around poop of flies on the window's glass? | One day it was raining outside and I was breathing on the glass of my window. I noticed that around the spots of fly poop there is a circle where vapor does not condense. With time the circle becomes larger and larger as the condensation slowly disappears. Why does this happen? Edit: Note that the poop is there from da... | In a scenario where a glass window is fogging up due to condensation, explain the physical principle that causes a circular region around spots of dry material, such as fly poop, to appear where vapor does not condense. If the temperature of the fly poop is not significantly different from the glass, what factors could... | 2,976 | cqadupstack-physics |
62188 | Would HgTe be a topological insulator? | In "Quantum Spin Hall Insulator State in HgTe Quantum Wells", researchers observed a 2D topological insulator by sandwiching HgTe between CdTe. Is the CdTe really necessary? Would Vacuum/HgTe/Vacuum itself be a topological insulator? | If a HgTe quantum well is placed in a vacuum, what are the necessary conditions for it to exhibit topological insulator properties, and how do these conditions compare to when CdTe is used as a barrier material? | 27,019 | cqadupstack-physics |
16038 | Quantum phenomena near black hole event horizon | I wanted to re-open the question of quantum measurements across event horizons. If I set up two slits or more generally a diffraction grating which crosses a black hole event horizon, and I shoot a stream of appropriate particles at the slits/grating, will I observe an interference pattern on a detector placed on the f... | If a stream of photons is fired towards a double slit experiment where one slit is outside of a black hole's event horizon and the other slit is inside, will an interference pattern be observed on a detector placed outside the black hole, and what factors might influence the outcome? | 11,921 | cqadupstack-physics |
41209 | Why has the trace of the energy-momentum tensor to vanish for conserved scaling currents to exist? | In this paper, the authors say that the trace of the energy-momentum tensor has to vanish to allow for the existence of conserved dilatation or scaling currents, as defined on p 10, Eq(22) $$ \Theta^{\mu} = x_{\nu} \Theta^{\mu\nu} + \Sigma^\mu.$$ $\Theta^{\mu\nu}$ is the energy-momentum tensor and $\Sigma^\mu = d_{\phi... | Why is it necessary for the trace of the energy-momentum tensor, \(\Theta^{\mu\nu}\), to be zero in order to preserve the existence of conserved dilatation or scaling currents? | 33,560 | cqadupstack-physics |
89955 | What causes there to be an offset between the light lensing through the space neighboring galaxy clusters and the galaxy clusters themselves? | http://www.wired.com/wiredscience/2013/01/musket-ball-dark-force/ > The reason this is strange is that dark matter is thought to barely interact > with itself. The dark matter should just coast through itself and move at > the same speed as the hardly interacting galaxies. Instead, it looks like > the dark matter is cr... | What experimental evidence would support the idea of a "dark force" affecting dark matter, and how could this be distinguished from the hypothesis that mass is fundamentally associated with the structure of spacetime? | 6,826 | cqadupstack-physics |
74059 | Why does a doppler red shift prove expansion? | My question boils down to this. If the Universe was contracting the stars closer to the center would move faster to the middle than stars that were further away from the center. That would also produce a red doppler shift. So why is it that the red shift is always linked to expansion? Human optimism? Thanks, Andrew | If the Universe were contracting, how would the velocity of stars towards the center vary with their distance from that center, and what implications would this have for the observed redshift of those stars compared to the redshift observed in an expanding Universe? | 32,963 | cqadupstack-physics |
87527 | Question about exterior derivatives | I know from Carroll that the integration in GR is basically a mapping from n-form to the real number. And it's given that $$d^nx=dx^0\wedge\ldots\wedge dx^{n-1}=\frac{1}{n!}\epsilon_{\mu_1\ldots\mu_n}dx^{\mu_1}\ldots dx^{\mu_n}$$ Now, I have an expression that is given in spherical coordinate system, where I have $$\in... | In a 4-dimensional space, when integrating a 2-form over a surface in spherical coordinates, the integration is described by the expression $$\int_\Sigma f(\theta,\phi) d\theta\wedge d\phi.$$ Given that $d\theta\wedge d\phi$ transforms in spherical coordinates as $$d\theta\wedge d\phi = \sin \theta d\theta d\phi,$$ wha... | 14,211 | cqadupstack-physics |
130533 | Name of battery voltage when load connected/disconnected | If I had a 3V battery, and when no load connected it reads 3.2V, and with a load 2.8V (just a hypothetical example), what is the name for these two terms, with a load or no load? I know the voltage drop occurs due to its internal resistance when a load is connected, however I still am not sure of the terms. Here's a fe... | What are the terms used to describe the voltage readings of a battery when it is unloaded and loaded, specifically the 3.2V reading with no load and the 2.8V reading with a load? | 37,079 | cqadupstack-physics |
2071 | Driving on snowy roads | 'tis the season as they say! It seems to me obvious that it's better to drive in high gear on snowy roads to reduce the torque. However, there are completely opposite advices being given on different sites: * weather.com says "Use low gears to keep traction" * bbc.co.uk says "Stay in a higher gear for better contro... | Given a simplified model of a car where the torque of the engine is controlled through a smooth limited function ($0 \leq T \leq T_{max}$) and transmitted via a gear ratio to the wheels, how does the choice of gear ratio affect the car's acceleration on snowy roads? Specifically, can you derive a relationship that desc... | 17,225 | cqadupstack-physics |
38380 | How is the singularity in Newtonian gravity resolved? | In Who's afraid of a Black Hole? at time ~20:38, Michio Kaku makes a claim that $1/r$ when $r=0$ equals $\infty$ and when physicists see the $\infty$ they see a monstrosity. Coincidentally, Newton's gravitational equation $$PE=-\dfrac{GMm}{r}$$ also has a $1/r$ relationship, which might be problematic, except that in N... | What happens to the gravitational potential energy \( PE = -\frac{GMm}{r} \) as \( r \) approaches zero, and how can this be reconciled with the physical observations of inelastic collisions between two objects? | 21,739 | cqadupstack-physics |
123059 | Usage of the terms Raman, Stokes and anti-Stokes scattering | I am confused over the correct usage of the terminology for "Raman scattering", "Stokes scattering", "anti-Stokes scattering", or even "Stokes- Raman scattering" and "anti-Stokes-Raman scattering". Is Raman scattering the superior term for Stokes and anti-Stokes scattering? Or do they essentially mean the same, and peo... | What are the key differences between Raman scattering, Stokes scattering, and anti-Stokes scattering, and how are they related in terms of energy changes in the scattering process? Additionally, could you provide an example that illustrates the distinction between Stokes and anti-Stokes scattering? | 5,042 | cqadupstack-physics |
19391 | Will Gauss's law still hold in case of deviation from inverse square law? | Assume a spherical metallic shell over which a charge $Q$ is distributed uniformly. Applying Gauss's law $\displaystyle\oint\textbf{E}\cdot d\textbf{a}=\frac{Q_{\text{enc.}}}{\epsilon_0}$ by considering a gaussian spherical surface concentric with the shell but with smaller radius, one can easily show that the electric... | If the electrostatic interaction between charges followed an inverse square law with an additional parameter, specifically $1/r^{2+\epsilon}$ instead of $1/r^2$, how would the electric field behave inside a uniformly charged spherical metallic shell? Calculate the electric field inside the shell as a function of the ra... | 30,632 | cqadupstack-physics |
37656 | Does cosmological horizon grow or decrease as it radiates? | Ron Maimon in many posts claimed that cosmological horizon is like a big black hole. Black holes decrease as they evaporate and their radius decreases as well. So what is with a cosmological horizon? If cosmological horizon is just a black hole centered at the opposite side of the universe, we should see the radius of ... | If the cosmological horizon behaves similarly to a black hole and is subject to Hawking radiation, how would the change in entropy influence the observable characteristics of the cosmological horizon's radius over time? What mathematical relationship governs this change? | 7,402 | cqadupstack-physics |
112423 | Any photon colliders in the past? | People have been thinking about a photon-photon collider (see this and this) as an add-on or to supplement the ILC, the next generation linear collider. My question is, have there been any photon-photon colliders running in the past*, including those running below the $e^+ e^-$ threshold to study non- linear effects in... | What are the key physical principles that govern the operation of a photon-photon collider, and how do they differ from those of electron-positron colliders? Additionally, provide a historical overview of any photon-photon colliders that have been constructed and operated, including their energy levels and experimental... | 35,201 | cqadupstack-physics |
19237 | Why is it concluded that the cosmos is expanding when in fact the observations are for events further back in time? | Why is it concluded that the cosmos is expanding at an ever increasing rate when looking further out and therefore further back in time? Surely, if the time were reversed to play forwards, the conclusion would be that the expansion of the universe is actually deaccelerating, and the need to postulate 'Dark Energy' woul... | Calculate the estimated value of the Hubble constant (H₀) that indicates the current rate of expansion of the universe, based on observations of distant supernovae and cosmic microwave background radiation. Additionally, discuss how variations in this value over time can support or contradict the need for a concept lik... | 26,984 | cqadupstack-physics |
110521 | Stop Motion 3D model of airflow over an airfoil | I am looking for a 3D stop motion image of air represented as molecules over a wing. Does anyone know where I can find an image that I can build a model from? | What is the arrangement and spacing of air molecules around a wing in flight, and how can I represent this in a 3D stop motion model? | 31,847 | cqadupstack-physics |
80778 | magnetic field boundary conditions for perfect conductor | my question relates to the boundary conditions of different medias, which I could understand fine, until I ran into perfect conductors. I was trying to understand why inside a conductor(or superconductors) a surface current might exist, whereas in a regular dielectric it is zero. Also I do not get why the normal compon... | In the context of electromagnetic theory, explain why the normal component of the magnetic field vector **B** is zero inside a perfect conductor and describe the physical implications this has for the behavior of magnetic fields in such materials. Additionally, how does the presence of surface currents in a perfect con... | 104 | cqadupstack-physics |
46525 | How does a warp field interferometer work? | Assume I have a solid grasp of undergraduate physics. From what I've read the warp field interferometer is supposed to be a sort of Michelson interferometer, except instead of adjusting the displacement of one mirror, you have a toroidal capacitor that... somehow expands space? Edit: I am talking about the work of Haro... | What is the principle behind generating a negative energy density with a toroidal capacitor in the context of an Alcubierre drive, and how does this relate to the operation of a Michelson interferometer? | 6,902 | cqadupstack-physics |
123048 | How coordinate system shifting is related to similarity transformations? | I know that coordinate system shifting can be represented using matrices. But how exactly are similarity transformations related to coordinate shifts ? | How does the mathematical representation of a similarity transformation using matrices relate to the concept of coordinate system shifts, and how can you demonstrate this relationship with a specific example involving a transformation of coordinates? | 33,869 | cqadupstack-physics |
106327 | Colliding particles at speeds aproaching c | (In natural units where $\hbar=c=1$.) Two particles are to be collided. Each of these particles has a rest mass of 0.9 GeV and they will be collided at equal but opposite speeds. What is the minimum speed required for the collision to produce a single particle of rest mass 5.6 GeV? I've written down the relativistic eq... | What is the minimum speed required for two particles, each with a rest mass of 0.9 GeV, to collide and produce a single particle with a rest mass of 5.6 GeV, assuming they are moving towards each other with equal speeds in natural units where \(\hbar = c = 1\)? | 5,734 | cqadupstack-physics |
109416 | Why does the Fermi Surface cross the Brillouin zone boundary at right angles? | I'm not sure why the fermi surface crosses the Brillouin zone boundary at right angles. I understand that this is normally the case, but not necessarily always. I'm aware that the fermi surface is a constant energy surface up to the filling point. The Brillouin zone is in reciprocal space. | What is the significance of the Fermi surface intersecting the Brillouin zone boundary at right angles, and under what conditions might this characteristic breakdown occur? | 14,793 | cqadupstack-physics |
18082 | catalysts as traces in a monoidal category | I was fooling around with string diagrams in a monoidal category and I was thinking about them in terms of chemical reactions. I wondered about bending a wire around and attaching it to the input of the box it came out of. I realized that what I was looking at was a catalyst. I then drew a box with many inputs but only... | In a certain autocatalytic reaction represented by a string diagram, if the input quantity of reactant A is 10 moles and the product formed is also reactant A, how many moles of A will be produced after 5 cycles of reaction, assuming each cycle produces 50% more of A than the previous amount? | 3,595 | cqadupstack-physics |
2598 | Discussion of the Rovelli's paper on the black hole entropy in Loop Quantum Gravity | In a recent discussion about black holes, space_cadet provided me with the following paper of Rovelli: Black Hole Entropy from Loop Quantum Gravity which claims to derive the Bekenstein-Hawking formula for the entropy of the black hole. Parts of his derivation seem strange to me, so I hope someone will able to clarify ... | What is the correct constant of proportionality in the Bekenstein-Hawking formula for black hole entropy, and how does it relate to the area of the black hole in terms of distinguishable states? | 31,541 | cqadupstack-physics |
8942 | What are the best open-access journals in physics? | I would like to support open-access journals by choosing to publish in journals which allow readers free online access. Ideally I would also like to retain copyright instead of signing it over to the journal. What are some of the better open-access journals in physics? I am particularly interested in journals focused o... | What is the impact factor of the top five open-access journals in the field of physics that focus on optics, and how do their publishing policies regarding copyright retention compare? | 20,270 | cqadupstack-physics |
26558 | Has a human ever perished in space? | Apollo 13 returned safely. The Challenger was leaving when it exploded. The Columbia was coming back when it burned up, as was that Russian guy who was profiled on National Public Radio (NPR) and that recent book. Has any human ever died in space? | What is the total number of astronauts who have died in space, defined as being beyond the Kármán line (100 km above sea level)? | 13,979 | cqadupstack-physics |
20996 | Velocity in a viscous fluid | The force $F$ to carry a plate of area $A$ with velocity $v$ in a fluid of depth $d$ is given by $$\frac{F}{A}=\eta\frac{v}{d}.$$ Hence if the depth is $kd$, the force becomes $F/k$. > Do this relations hold for a ship in water? | A ship experiences a drag force when moving through water. If the drag force is given by the relationship \(\frac{F}{A} = \eta \frac{v}{d}\), where \(F\) is the force, \(A\) is the area of the hull, \(\eta\) is the viscosity of the water, \(v\) is the velocity of the ship, and \(d\) is the depth of the water, what will... | 24,738 | cqadupstack-physics |
32974 | Does black hole complementarity apply to white holes? | If physics is time reversal invariant, there ought to be white hole complimentarity as well. Imagine a white hole so enormous that it is possible for life to evolve entirely within it for billions of years before emerging to tell their story about the interior of the white hole. Meanwhile, an external observer can obse... | If a white hole is postulated to exist with an immense volume allowing for life to evolve within it, what would be the gravitational pull on an object located a distance \( r \) from the center of the white hole, assuming it has a mass \( M \)? Provide the formula and calculate the gravitational force if \( M = 10^{30}... | 27,576 | cqadupstack-physics |
60897 | Planes of graphite crystal on diffraction experiments | When doing electron diffraction on graphite (a popular experiment for students at universities) always diffraction at these two planes with distances $d_1$ and $d_2$ are observed:  But a plane parallel to the ones with distance $d_2$ can also be formed ... | Why is it that electrons or X-rays do not scatter at the planes with distance $s$ or any multiples of $s$ when analyzing electron diffraction on graphite? | 30,462 | cqadupstack-physics |
102911 | How does time dilation work in this situation? | It is my understanding that when moving near the speed of light, time slows down relative to other things not moving so fast. Based on this principle, would it be theoretically possible to travel a thousand light years in a year, with the thousand years only having passed on the place you're moving relative to? If some... | If a spaceship could travel close to the speed of light, how fast would it need to go in order for its crew to cover a distance of 1,000 light-years in just one year of their own time, according to the principles of time dilation? | 21,147 | cqadupstack-physics |
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