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41816 | Why does a tire need to be filled with air? | The **Mini 4WD** 's tires aren't full of air, and it can run. Also, the tank doesn't have tires with air. So, the question is: why do real cars on the road need to be filled with air? What is the idea behind it?  | What is the role of air pressure in the performance and safety of tires in real cars compared to the tire-less design of a Mini 4WD? Provide a quantitative comparison of tire pressure requirements for optimal performance in full-sized vehicles. | 34,551 | cqadupstack-physics |
107887 | Why aren't there more natural satellites orbiting the Earth than just the Moon? | Considering that there's a lot of debris in space and that impacts fling out rocks into space all the time, why do we only have one large natural satellite - the Moon? Shouldn't there be all kinds of rocks in all shapes and sizes orbiting the Earth? | What factors contribute to the formation and stability of a planet's natural satellites, and how does the Earth's gravitational influence affect the likelihood of capturing and maintaining multiple significant-sized objects in orbit? | 8,219 | cqadupstack-physics |
7849 | fermions and quantum gates | Say that I have 2 qubits \- 2 spin half fermions. my initial condition is $|00\rangle$ in the spin-wave function and some anti-symmetrical spacial wave function. I'm wondering about what happens when applying a NOT gate. The Fermions total wave function still has to be inverted when swapping the particles. so does it m... | What is the effect of applying a NOT gate to the spin state of two identical spin-1/2 fermions in the initial state $|00\rangle$ while considering the requirements of antisymmetry in the total wave function, particularly regarding the spatial wave function? | 8,291 | cqadupstack-physics |
41591 | Why photons are emitted because of changes to electron behavior | Explanations I have read of why photons are emitted from atoms mention electrons being 'excited' to another energy level, and then returning to their base level, releasing a photon. I have also seen the occasional mention of 'fields' and I vaguely expect from what I've read that interaction of the electrons with these ... | What are the various theories explaining the emission of photons from atoms due to electron transitions, and what experimental evidence supports these theories? Specifically, how do disturbances in electromagnetic fields and additional particles depicted in Feynman diagrams influence electron behavior and photon emissi... | 25,476 | cqadupstack-physics |
97889 | Why is it more likely that the particle with negative mass falls into a black hole? | If particle-antiparticle pairs pop into existence at the event horizon, and presumably their orientation to the black hole is random, why is it the particle with negative mass is more often (always?) the one that falls in, leading to hawking radiation with positive mass? Why is it not equally likely for positive mass a... | In the context of Hawking radiation, if particle-antiparticle pairs form near the event horizon of a black hole, how does the conservation of energy influence the likelihood of a negative mass particle falling into the black hole compared to a positive mass particle? What is the expected ratio of negative mass particle... | 5,363 | cqadupstack-physics |
23243 | Intuitive picture for spin-fluctuations contribution to specific heat of He3 | Usually when discussing Fermi liquid theory, it is stated that due to the quasiparticles effectively behaving like a free electron gas with effective mass, the specific heat is linear in $T$ at small temperatures. However, it turns out the Helium-3 has also a dependence of type $T^3 \ln T$. I want to understand where t... | What is the dependence of the specific heat on temperature for Helium-3 at low temperatures, and how do spin fluctuations contribute to this behavior in contrast to the linear specific heat observed in typical Fermi liquids? | 24,765 | cqadupstack-physics |
44764 | Quantum mechanics textbooks that use path integrals | I'm looking for a textbook in quantum mechanics that relies heavily on Green functions and the path integral formalism to supplement my QM books. I want to do some calculations using alternative methods (path integrals, green functions, Lagrangians etc). | What is the value of the path integral for a free particle in one dimension, and how does it relate to the quantum mechanical propagator expressed using Green's functions? | 22,132 | cqadupstack-physics |
95238 | HOMEWORK: Minimum angle to start the mop moving | Situation: I'm a janitor who wants to clean the floor with a mop. I need to find a minimum angle theta so that the mop starts moving.  First, I constructed the Newton's equations: $$ma_x = F_\text{push} \sin(\theta) - F_f$$ $$ma_y = F_n - F_g - F_\text{... | What is the minimum angle \(\theta\) required for a mop to start moving on a floor, given that the coefficient of static friction is \(\mu_{\text{static}}\), and how does this angle relate to the assumption that the applied force \(F_p\) is extremely large? | 37,505 | cqadupstack-physics |
27908 | Pseudo-Superluminal Motion and the Synchronization of Clocks | Let's consider two points A an B separated by a finite distance in curved space time. A light ray flashes across an infinitesimally small spatial interval at B. Metric: $$ds^2=g_{00}dt^2-g_{11}dx^2-g_{22}dy^2-g_{33}dz^2$$ ----------- (1) We may write, $$ds^2=dT^2-dL^2$$ Where, $dT= \sqrt{g_{00}}dt$ and $dL=\sqrt{ g_{11... | Given the relationships between local and non-local speeds of light and particles as defined by the metric coefficients \( g_{00} \) at points A and B, how does the non-local speed of light \( c_{non-local} \) differ from the local speed of light \( c \) when \( g_{A(00)} = 0.9 \) and \( g_{B(00)} = 1.1 \)? Calculate t... | 11,501 | cqadupstack-physics |
35018 | Reflection of Electromagnetic Waves | Visible light - Being an Electromagnetic wave is reflected by glass (take mirror). Would all other waves in the electromagnetic spectrum be reflected in the same way by our simple mirror... For highly intense X-Rays & gamma rays, take that the mirror is more **stronger** such that it could resist for at least 30 second... | If a highly intense beam of X-rays is reflected off a specially designed mirror for 30 seconds, how do the intensity and energy of the X-ray beam change after 30 reflections, considering that some photons scatter in different directions with each reflection? | 5,870 | cqadupstack-physics |
123020 | How does AC current flow in an open circuit? | In common house hold wiring we have the hot lead, neutral and ground. If the hot lead in electrical wiring contacts earth ground (perhaps though a short circuit in the chassis of a device) then the current shorted to earth. This however doesn't appear to me to be a complete closed circuit. My intuition tells me current... | In a household electrical system, if the neutral wire is not grounded at the service panel and a hot lead contacts earth ground due to a short circuit, can you calculate how much current would flow into the ground, assuming the resistance of the ground is known? If so, what is the formula to determine the current, and ... | 8,207 | cqadupstack-physics |
31820 | use of Mobile phone on Petrol pump | I was trying to find out the cause behind the Explosion at petrol pump due to the Mobile phones but eventually it turns out it is not Radio Frequency which may cause the explosion rather it is totally the static electricity which may cause serious explosion and ignition of vapors, The other reason I found is, due to th... | Is there a difference in the amount of static electricity generated by mobile phones compared to other common devices, and how does this relate to the safety regulations at petrol stations? | 18,974 | cqadupstack-physics |
61393 | Scaling arguments for the Contact mechanics between two elastic spheres | I am studying a bit granular dynamics and I have seen that two spheres of radius $R$ in contact with a contact area of radius $a$ would need an applied force $F$ on this two spheres that is nonlinear in the depth of deformation $\delta$ as it goes as: $F \sim \delta^{3/2}$ To be honnest, I am not really interested in t... | In the context of two spheres in contact, if the area of contact has a radius \( a \) and the spheres have a radius \( R \) with \( a \ll R \), how does the strain \( \epsilon \) relate to the vertical deformation \( \delta \) such that \( \epsilon \) can be expressed as \( \epsilon \sim \delta/a \)? Could you explain ... | 29,460 | cqadupstack-physics |
97958 | How to estimate a person's mass from a sensor? | I'm trying to estimate the person's weight from some available sensors and I have an accelerometer, a gyrometer and a magnetometer. The triaxial accelerometer is fixed in a band in the person's chest, so it can measure the body acceleration. While the person is still in its tiptoes, as the figure below, he's asked to l... | Given a person of known height \( h \) who is falling from a height \( h \) and is modeled as a mass-spring-damper system, if the peak vertical acceleration measured by the accelerometer is \( a_{max} \) m/s² and the natural frequency of the system is \( f_n \) Hz, can you derive an equation to estimate the person's ma... | 16,670 | cqadupstack-physics |
51506 | Reflection of sound waves | I was doing a physics experiment, and i encountered a question which i couldnt answer. The experiment was about using a radar technique to measure the speed of sound. The apparatus was a plastic tube 90cm long and the sound source was a speaker connected to a function generator. The generator was sending a square wave ... | In a physics experiment using a 90 cm long plastic tube to measure the speed of sound, a square wave sound source generates sound pulses that are observed as echoes on an oscilloscope. When the tube is closed at one end, why do sound waves reflect back, and what principle explains the behavior of the sound waves reflec... | 6,302 | cqadupstack-physics |
36143 | picture of newtonian gravity and black hole |  We assume that the earth planet is a black hole. When a light beam is fired. The only way that even light can move is A and B which means this is impossible that light move on other ways C, D, E Isn't it true? | If Earth were to behave like a black hole, what would be the Schwarzschild radius of Earth, given that its mass is approximately \(5.97 \times 10^{24} \) kg? | 1,693 | cqadupstack-physics |
99982 | How Would an Alcubierre Drive "ride" a Wave of Spacetime? | The colloquial explanation is that the spacetime in front of a ship contracts and the spacetime behind expands. I see how one could _think_ that this would bring you closer, but I don't see that it actually does. Let's look at the front of the bubble. So spacetime "contracts". If you look at a differential volume being... | If the Alcubierre drive contracts spacetime in front of a ship and expands it behind, what is the total effective distance the ship would need to travel compared to the distance the ship would travel without the drive, assuming the curvature of spacetime is uniform? | 30,951 | cqadupstack-physics |
1479 | Why is the mapped universe shaped like an hourglass? | I've watched a video from the American National History Museum entitled The Known Universe. The video shows a continuous animation zooming out from earth to the entire known universe. It claims to accurately display every star and galaxy mapped so far. At one point in this video [3:00 - 3:15 minutes] it displays this t... | Why do the mapped galaxies in the universe appear to form an hourglass shape around Earth, and how does this relate to the limitations of our observational capabilities, such as the speed of light and the mapping choices made by astronomers? | 18,355 | cqadupstack-physics |
12786 | Any interesting physics theroies/equations to be used in a video game? | I am designing a little game based on Newtons Graviational Law. Are there any theories that I can use to create games? thanks | How can I calculate the gravitational force between two objects using Newton's Law of Universal Gravitation, and what would be the gravitational force between a 5 kg object and a 10 kg object separated by a distance of 2 meters? | 21,839 | cqadupstack-physics |
61165 | Workdone in an equipotential surface is zero? | An equipotential surface is one in which all the points are at the same electric potential. If a charge is to be moved between any two points (say from point A to point B) on an equipotential surface, according to the formula $dW = q\cdot dV$, the work done becomes zero. My question is, how to move a particle without d... | If an electron is moved from point A to point B on an equipotential surface with a potential difference of 0 volts, what is the work done on the electron during this movement, and why? | 14,501 | cqadupstack-physics |
93649 | Surface charge density relation with the radius of curvature at the surface of a conductor | In a text book it was given that the surface charge density of a conductor at a particular region on its surface is inversely proportional to the radius of curvature at that region. I didn't understand it. I want proof for that can anyone please post the proof. | How does the surface charge density of a conductor relate to the radius of curvature at a point on its surface? Can you derive a formula that demonstrates this inverse proportionality? | 26,545 | cqadupstack-physics |
68995 | How can we describe the electrons of multi-electron atoms (i.e. not Hydrogen) when equations/analytic solutions only exist for Hydrogen? | I've been digging into emission spectra of different elements and found that such things as the Rydberg equation, Bohr's model, and quantum mechanics can only fully describe the single electron in the Hydrogen atom. How did we then make the leap to s,p,d,f shells of multi-electron atoms? How accurate is our analysis of... | Using the Rydberg equation $$\frac{1}{\lambda}=R_H\left( \frac{1}{n_1^2}-\frac{1}{n_2^2}\right)$$ for hydrogen, calculate the wavelength of the emitted photon when an electron transitions from the n=4 energy level to the n=2 energy level. Additionally, how does the Rydberg equation apply to multi-electron atoms, and wh... | 531 | cqadupstack-physics |
29789 | Critical bathymetric profile to maximize surge and minimize breaking? | Reading about storm surge, I found it fascinating that the gradual slope of the Gulf Coast of Florida resulted in a much higher storm surge but much lower energy release in breaking waves. Is there a critical bathymetric profile to the ocean floor that would result in virtually all wave energy (of various frequencies) ... | What is the ideal bathymetric profile of the ocean floor that maximizes the conversion of wave energy into storm surge elevation, minimizing the energy released in breaking waves? Provide a mathematical model or equation to support your answer. | 15,261 | cqadupstack-physics |
69087 | Are there any other mechanisms that can make virtual particles 'real' other than Hawking Radiation and Universe Births? | As I understand it, if virtual particles do not recombine within the plank time they become 'real'. This is proposed to happen in Hawking Radiation, where one virtual particle crosses the black hole's event horizon and the other does not, where it robs some energy from the black hole. Lawrence Krauss also proposes in h... | What are the theoretical mechanisms, if any, by which virtual particles can transition to real particles, and how do these mechanisms ensure the conservation of energy within the context of quantum field theory? | 35,821 | cqadupstack-physics |
5829 | Why are some solitons formed from bosonic fields fermionic? | Some topological solitons formed from bosonic fields have fermionic statistics. Why? | What is the relationship between the properties of bosonic fields in topological solitons and the emergence of fermionic statistics, and can you provide an example that illustrates this phenomenon? | 11,136 | cqadupstack-physics |
27643 | Constructing a Hamiltonian (as a polynomial of $q_i$ and $p_i$) from its spectrum | For a countable sequence of positive numbers $S=\\{\lambda_i\\}_{i\in N}$ is there a construction producing a Hamiltonian with spectrum $S$ (or at least having the same eigenvalues for $i\leq s$ for some $s$)? Here by 'Hamiltonian' I understand a polynomial of $p_i$ and $q_i$ (or equivalently - $a_i$ and $a_i^\dagger$)... | Can you construct a Hamiltonian for the spectrum \( S = \{3, 5, 7, 9, \ldots\} \) that has the same eigenvalues for the first \( s \) values? Provide the general form of such a Hamiltonian and explain how it meets the criteria for the specified spectrum. | 33,069 | cqadupstack-physics |
78478 | Confusion With How Dimensions Work | Form what I understand if you have an equation such as: $$v = v_0 + at$$ then the dimensions must match on both sides i.e. $L/T = L/T$ (which is true in this case), but I have seen equations such as 'position as a function of time' $x(t) = 1 + t^2$, and obviously time is in $T$, but apparently the function gives you po... | In the equation \( x(t) = 1 + t^2 \), where \( x \) represents position and \( t \) represents time, how can we verify the consistency of dimensions on both sides of the equation, knowing that \( x \) has dimensions of length \( [L] \) and \( t \) has dimensions of time \( [T] \)? Specifically, what are the implication... | 247 | cqadupstack-physics |
82529 | Ways of achieving parallel rays of light? | I am thinking of ways of achieving parallel rays of light. Let's say I have an object that emits light, for instance the display of my mobile phone. The display emits rays of light in every direction. Is there a "filter" I can put in front of the screen that only lets rays of light traveling in a certain direction thro... | What optical device can be used to convert divergent rays of light from a point source, such as a mobile phone display, into a set of parallel rays, specifically those traveling orthogonal to the display surface? | 4,821 | cqadupstack-physics |
132020 | Is there an equivalent to wetness for air? | I was wondering if there was something equivalent to the property of being wet with water, but with air instead. For example, if I drop water on my shirt, I'll notice by its appearance and feel that it is wet, so in a sense its properties were changed by being exposed to water. So I'm wondering if similarly, by being e... | If a shirt is exposed to ambient air for an extended period, what measurable changes in its physical properties (such as weight, moisture content, or fabric integrity) might occur compared to the same shirt kept in a vacuum, and how can these changes be quantified? | 16,013 | cqadupstack-physics |
122781 | Evaluating low-temperature dependence of the BCS gap function | How does one go about evaluating the behavior of the BCS gap $ \Delta = \Delta(T) $ for $ T \to 0^+ $ under the weak coupling approximation $ \Delta/\hbar\omega_D \ll 1 $? In Fetter & Walecka, _Quantum theory of Many-Particle Systems_ , Prob. 13.9 it is said that the starting point is $$\tag{1} \ln\frac{\Delta_0}{\Delt... | Evaluate the integral $$ I(\Delta, \beta) = \int_0^{+\infty}{\frac{\mathrm d x}{\sqrt{x^2 + 1}}\frac{e^{-\beta\Delta\sqrt{x^2 + 1}}}{1 + e^{-\beta\Delta\sqrt{x^2 + 1}}} } $$ under the weak coupling approximation for $\Delta \approx \Delta_0$ and $\beta \Delta \to \infty$. What is the asymptotic behavior of the integral... | 30,286 | cqadupstack-physics |
112963 | Tidal Forces from the Moon on the Earth's Crust | How do we determine how much the surface of the Earth deflects due to tidal forces from the Moon (and Sun)? | What is the approximate maximum height of the tidal bulge produced on Earth's surface due to the gravitational pull of the Moon, given that the average distance from the Earth to the Moon is about 384,400 km? | 21,382 | cqadupstack-physics |
5736 | How do you derive Noether's theorem when the action combines chiral, antichiral, and full superspace? | How do you derive Noether's theorem when the action combines chiral, antichiral, and full superspace? | What is the relationship between symmetries in a supersymmetric theory and conserved quantities, as established by Noether's theorem, when considering an action that includes chiral, antichiral, and full superspace components? | 14,468 | cqadupstack-physics |
59213 | What are the various physical mechanisms for energy transfer to the photon during blackbody emission? | By conservation of energy, the solid is left in a lower energy state following emission of a photon. Clearly absorption and emission balance at thermal equilibrium, however, thermodynamic equilibrium is a statement of the mean behaviour of the system, not a statement that the internal energy is constant on arbitrarily ... | What are the dominant physical processes mediating energy transfer during blackbody emission, and how do these processes vary across different energy ranges, such as visible light, infrared, and gamma rays? | 24,205 | cqadupstack-physics |
72073 | Black magic "Hartree" approximation | The question is about an unusual looking version of the Hartree or mean field approximation. The context is several papers I've been reading recently about the out of equilibrium dynamics of phase transitions in the early universe [1-3]. The procedure is to shift a (scalar) field $\Psi \to \psi + \langle \Psi \rangle$,... | What is the systematic method for deriving the numerical coefficients in the Hartree approximation for bosonic fields when replacing cubic and quartic terms with their simplified quadratic forms, such as in the expressions \( \psi^4 \to 6 \langle \psi^2 \rangle \psi^2 - 8 \langle \psi \rangle^3 \psi + 6 \langle \psi \r... | 35,203 | cqadupstack-physics |
10352 | Gyrochronology, the formula is empirical ? | The formula used in Gyrochronology that relates a star's Period of Rotation- Mass-Age is empirical? This news How to Learn a Star’s True Age > "“A star’s rotation slows down steadily with time, like a top spinning on a > table, and can be used as a clock to determine its age," pointed to this paper THE KEPLER CLUSTER S... | Is the relationship between a star's period of rotation and its age represented by the formula where the period is proportional to age raised to the power of 1/2 a confirmed empirical result, or is it influenced by other factors like mass loss due to radiation? Additionally, what experimental data can validate or refut... | 2,816 | cqadupstack-physics |
98886 | Imaginary Eigenvalue Of A Hermitian Operator | The eigenfunctions of a Hermitian operator are real. But consider a function $\psi(x)=e^{-\kappa x}$, $x\in\mathbb{R}$, where $\kappa$ is a real constant. Then, $$\hat p \psi(x)=-i\hbar \frac{d}{dx}e^{-\kappa x}=i\kappa \hbar \psi(x).$$ This gives a pure imaginary eigenvalue. Is it not a contradiction? Or am I missing ... | If the function \(\psi(x) = e^{-\kappa x}\) (where \(\kappa\) is a real constant) is an eigenfunction of the momentum operator \(\hat{p}\) given by \(\hat{p} \psi(x) = -i\hbar \frac{d}{dx} e^{-\kappa x}\), resulting in an eigenvalue of \(i\kappa\hbar\), why does this not violate the property that eigenvalues of Hermiti... | 20,728 | cqadupstack-physics |
10955 | What is quantum discord? | What is quantum discord? I stumbled upon this term on Quantum Computing: The power of discord, but have never heard of it before. Can you give a bit more mathematical explanation of the term here? | What is the mathematical definition of quantum discord, and how can it be quantified for two-qubit systems? | 1,775 | cqadupstack-physics |
87220 | Are $G$, $F$ and $H$ (thermodynamics potentials) extensive quantities? | Internal energy $E$ is an extensive quantity for most systems. But energy extensivity is not valid in systems with long-range interactions, like gravity (e.g. in astrophysical systems). For extensive systems: $$E=TS+\bf J. \bf x +\bf{\mu}.\bf N\tag{1}$$ where $\bf J$ and $\bf x$ are intensive and extensive thermodynami... | In the context of non-extensive systems such as galaxies, how do the thermodynamic potentials \( G \), \( F \), and \( H \) differ in their physical meanings compared to their interpretations in extensive systems? Can you provide an example that illustrates the implications of this difference? | 33,557 | cqadupstack-physics |
92856 | 'Polar Vortex' Boiling Water to Snow is Mpemba Effect? | I am based far away from the icy storm currently blanketing the US - the 'polar vortex'. However, I have seen in the TV news footage of reporters throwing boiling water into the air, the water freezing immediately to form snow. I'm curious as to whether this would also work with cold water. If not, I presume that it is... | Does cold water freeze instantly when thrown into extremely cold air, similar to how boiling water does, or is the phenomenon exclusive to boiling water, and what scientific principles could explain the difference in behavior? | 10,221 | cqadupstack-physics |
130739 | Will my car consume more if I plug in my laptop? | I drive a Volvo v40 with a 110hp diesel engine. It usually use 4.6L/100km. If I plug my laptop which is 85W, will this change the fuel need? If so, by how much? | If I drive a Volvo V40 with a 110 hp diesel engine that consumes 4.6 L/100 km, how much additional fuel would I need to account for when using my 85 W laptop during a 100 km trip? | 12,996 | cqadupstack-physics |
70054 | Why do electrons emit radiation? | I know how you can emit light with an alternating current, running back and forth, creating an electric field in addition to the magnetic field. But why does an electron emit light when it gets accelerated, there is no positive end to create an electric field. Further more, as opposed to talking about currents, talking... | What is the relationship between the acceleration of an electron and the emission of light, and how does this phenomenon relate to the concepts of electric and magnetic fields? | 2,860 | cqadupstack-physics |
17157 | Is spherical coordinates enough to calculate the exact position of a point with rotations included? | With "Geographic coordinate system" we only get the location, no rotations or anything. Translated into _latitude_ and _longitude_. Where with "Spherical coordinate system", we should be able to get the exact position, including rotations. > The spherical coordinates of a point P are defined as follows: > > * the rad... | Calculate the spherical coordinates (radius, inclination, and azimuth) of a point located at sea level on the surface of the Earth, given that the radius from the center of the Earth to sea level is approximately 6,371 kilometers. Assume the inclination is measured directly from the vertical at the surface and the azim... | 34,964 | cqadupstack-physics |
59663 | How to define the order parameter of the q-state Potts model? | The order parameter of Ising model can be defined as $m=\frac{N_1-N_2}{N}$, if $N$ is the total number of lattice points, $N_1$ and $N_2$ is the number of lattice points spin up and down respectively, $N=N_1+N_2$. But I am not able to write down the order parameter of the q-state Potts model. Any help will be appreciat... | What is the expression for the order parameter in the q-state Potts model, considering it is defined in a similar way to the Ising model? | 29,781 | cqadupstack-physics |
113669 | Isn't $E-U = K$ in Schrodinger's Equation? | I'm studying quantum mechanics in its most basic level (I don't even know if Physicists call this already quantum mechanics) and I have one doubt in Schrodinger's equation. The book presents the equation for the special case where the solution is of the form $\Psi(x,t)=\psi(x)e^{-i\omega t}$ and says that Schrodinger's... | In the context of Schrödinger's equation where the total energy \( E \) is considered constant, how can we ensure that the total energy remains constant even when the potential energy \( U(x) \) varies with position? Can you clarify the distinction between the potential energy term in the equation and the one that cont... | 17,063 | cqadupstack-physics |
107703 | What is the probability of quantum tunneling occurring in this CPU? | You may have noticed over the last few years that Moore's law is no longer applying to the real world. This observation states that over the history of computing hardware, the number of transistors on integrated circuits doubles approximately every two years. However, as microprocessors in computers have continued to b... | What is the probability of quantum tunneling occurring in a semiconductor that is 5 nm in length, given a voltage of 1.2V and an amperage of 63A? | 31,150 | cqadupstack-physics |
18222 | How does the speed of an incoming pitch affect the speed of a baseball after it's hit? | Which will go further if a batter manages to hit it with a baseball bat: a baseball thrown to the batter at 90 miles per hour or one thrown at 60 miles per hour? | If a baseball batter hits two pitches, one thrown at 90 miles per hour and the other at 60 miles per hour, assuming all other factors are equal, which baseball would theoretically travel further after being hit? Calculate the expected distance each baseball would travel after impact based on their initial speeds and th... | 16,502 | cqadupstack-physics |
54283 | A classically trivial quantum field theory of electromagnetism | Presumably there is a field theory of electromagnetism that classically gives trivial equations of motion, but when quantized shows interesting topological phenomena. I am talking about the Lagrangian density $$\mathcal{L}=\epsilon^{\mu\nu\lambda\sigma} F_{\mu\nu}F_{\lambda\sigma}\propto \vec{E}\cdot\vec{B}$$ where $F_... | What are the implications of the Lagrangian density $$\mathcal{L}=\epsilon^{\mu\nu\lambda\sigma} F_{\mu\nu}F_{\lambda\sigma}$$ in the context of electromagnetism, particularly regarding its quantization and any resulting topological phenomena? | 12,021 | cqadupstack-physics |
101913 | Why does a remote car key work when held to your head/body? | I was trying to unlock my car, but I was out of range. A friend of mine said that I have to hold the transmitter next to my head. It worked, so I tried the following later that day: * Walked away from the car until I was out of range * Put key next to my head (it worked) * Put key on my chest (it worked) * Put ... | What is the optimal position for a car key fob to maximize its signal range when attempting to unlock a car, and how might the human body affect this signal transmission? | 28,451 | cqadupstack-physics |
76680 | Potentiometer voltage change | I'm trying to understand simple things about electricity reading allaboutcircuits.com web. This chapter includes image which´s principle I don't understand. Here it is:  When supplying constant voltage to circuit where resistance and current change, how... | A circuit consists of a 12 V battery and a resistor with a variable resistance. If the resistance is decreased from 10 ohms to 5 ohms, what will be the current flowing through the circuit, and how will the voltage across the resistor change? | 27,733 | cqadupstack-physics |
62167 | Missing step while (classically) deriving Friedmann equation | I'm trying to understand the classical derivation of Friedmann equation but I'm missing one step. So, I start with accelerations, where $a$ is a scale factor $\ddot{a}=-\frac{GM}{a^{2}}$ $\ddot{a}=-\frac{4\pi G}{3}\rho a$ Now multiply both sides by $\dot{a}$, and here is the question: Why LHS is $\dot{a}\ddot{a}=\frac{... | Can you explain why multiplying the acceleration, \(\ddot{a}\), by the velocity, \(\dot{a}\), results in the expression \(\dot{a}\ddot{a}=\frac{1}{2}\frac{d}{dt}\left(\dot{a}^{2}\right)\), and how this relates to the energy conservation in cosmological models like the Friedmann equation? | 31,428 | cqadupstack-physics |
93109 | Work done by Lorentz Force in case of motional emf | In the classical example of the slidewire generator where the rod slides on a U-shaped conductor in a magnetic field, we get a charge separation due to the Lorentz force. The way the induced emf is usually derived is the following: $\mathcal{E}=W_l/q=vBL$ where $W_l$ is the work done by the magnetic (Lorentz) force. Bu... | In a slidewire generator, if a rod of length \( L = 0.5 \) m moves at a speed \( v = 2 \) m/s through a magnetic field of strength \( B = 0.1 \) T, what is the induced electromotive force (emf) in the circuit? Additionally, if the resistance \( R \) of the circuit is \( 4 \, \Omega \), calculate the heating effect (Jou... | 10,644 | cqadupstack-physics |
27473 | Continuum theory from lattice theory | I am looking for references on how to obtain continuum theories from lattice theories. There are basically a few questions that I am interested in, but any references are welcome. For example, you can obtain the Ising chiral CFT from a lattice theory. How does this work exactly? Intuitively is is clear that one should ... | How can the Ising model be rigorously derived from a lattice theory as the lattice spacing approaches zero, and what specific conditions must be satisfied for this transition to a continuum theory to be valid? Please provide references to mathematical treatments of this process. | 37,330 | cqadupstack-physics |
134619 | Mathematical interpretation of Poisson Brackets | Lets say we are working in a classical scalar field theory and we have two functional $ F[\phi, \pi](x)$ and $G[\phi, \pi](x)$. In most of the references, starting with two functional the Poisson bracket is defined as $$\\{F(x),G(y)\\} = \int d^3z \left( \frac{\delta F(x)}{\delta \phi(z)}\frac{\delta G(y)}{\delta \pi(z... | Can you derive the Poisson bracket of two functionals \( F[\phi, \pi](x) \) and \( G[\phi, \pi](y) \) in a scalar field theory using distribution theory, and show under which conditions this bracket can be expressed as a convolution of functional derivatives? Additionally, can you discuss the implications of formulatin... | 26,233 | cqadupstack-physics |
17885 | What material can a lens be made from? | I need to make a camera lens that can easily withstand temperatures of about 1000 Fahrenheit. What should I make the lens out of? | What materials can withstand temperatures of 1000 degrees Fahrenheit while maintaining optical clarity for a camera lens? | 34,772 | cqadupstack-physics |
81750 | Electrical circuits problem | Batteries have a circuit which looks like this :  The electrons go around the circuit and then return through the battery where they get charged again and flow around. My issue is, what about alternating current? For example main power, simply it looks ... | In an AC circuit, how do the electrons behave in terms of direction and flow? Specifically, when there is a short-circuit, why do the electrons seem to preferentially flow to the ground instead of primarily returning through the neutral wire? What factors contribute to this behavior? | 3,360 | cqadupstack-physics |
53221 | Difference between spinor and vector field | How do we distinguish spinors and vector fields? I want to know it in terms of physics with mathematical argument. | How can we mathematically demonstrate the distinctions between spinors and vector fields in the context of quantum mechanics, particularly regarding their transformation properties under Lorentz transformations? | 32,371 | cqadupstack-physics |
98474 | Relation between speed of sound and compressibility | We know that $c^2=\frac{\partial p}{\partial ρ}$ The adiabatic compressibility is defined as: $\beta_S=-\frac{1}{V}\frac{\partial V}{\partial p}$ such that the subscript "S" stands for "adiabatic" How can I show that $c^2=\frac{1}{\rho \beta_S}$ ? I tried replacing $V$ by $\frac{m}{\rho}$ but I get for $\beta_S=-\rho \... | Given the relationship \( c^2 = \frac{\partial p}{\partial \rho} \) and the definition of adiabatic compressibility \( \beta_S = -\frac{1}{V}\frac{\partial V}{\partial p} \), can you derive the expression \( c^2 = \frac{1}{\rho \beta_S} \) and explain each step of your reasoning? | 5,612 | cqadupstack-physics |
75580 | Is there a better alternative to assuming $a=0$ and then calculating acceleration? (Exam question: radiated energy after Coulomb scattering) | My classmates and I are having some debate about how to solve this problem which came up on one of our comprehensive exams. A full, step-by-step solution is not needed, an outline is enough. A solution does not need to be relativistic, as we are interested in both cases. > Find the total energy radiated in the collisio... | What is the total energy radiated during the collision of a relativistic particle with charge \( q \), mass \( m \), and velocity \( v \) (in the lab), against a fixed target of charge \( Q \), assuming small deflections with an impact parameter \( b \)? Additionally, how would you express the condition for "small defl... | 1,390 | cqadupstack-physics |
107488 | Thermodynamic relations | I'm asked to establish the following relations: $\left( \frac{\partial C_V}{\partial V} \right)_{T,N} = \frac{T}{N} \left( \frac{\partial^2 P}{\partial T^2} \right)_{V,N} $ $\left( \frac{\partial (\beta F)}{\partial \beta} \right)_{V,N} = U $ where $\beta = \frac{1}{k_B T}$ I'm a bit puzzled by the first one since I ca... | Can you derive and simplify the expression for \( C_V \) in terms of pressure \( P \) and temperature \( T \) based on the relation \( \left( \frac{\partial C_V}{\partial V} \right)_{T,N} = \frac{T}{N} \left( \frac{\partial^2 P}{\partial T^2} \right)_{V,N} \), and clarify how keeping the number of particles \( N \) and... | 8,556 | cqadupstack-physics |
103461 | Query on Polarization | Describe the state of polarization represented by (here w = omega and TT = pi) a) $E_x = E\sin(kz - \omega t)\,\& \,E_y = E\cos(kz - \omega t)$ b) $E_x = E\cos(kz - \omega t) \,\&\, E_y = E\cos(kz - \omega t + \pi/4) $ c) $E_x = E\sin(kz - \omega t) \,\&\, E_y = E\sin(kz - \omega t)$ | Determine the state of polarization for each of the following cases of electric field components \(E_x\) and \(E_y\):
a) \(E_x = E\sin(kz - \omega t)\) and \(E_y = E\cos(kz - \omega t)\)
b) \(E_x = E\cos(kz - \omega t)\) and \(E_y = E\cos(kz - \omega t + \frac{\pi}{4})\)
c) \(E_x = E\sin(kz - \omega t)\) and \(E_... | 13,826 | cqadupstack-physics |
37935 | Why are topological solitons present in some phases for lattice models? | Over a spatial continuum, it is easy to see why some topological solitons like vortices and monopoles have to be stable. For similar reasons, Skyrmions also have to be stable, with a conserved topological density. The reason is nontrivial homotopy. Surprisingly, in some phases, but not all phases, the analog of topolog... | In the context of lattice models, what conditions or parameters lead to the emergence of stable deconfined vortices and monopoles, given that there is no nontrivial homotopy over a lattice? | 34,906 | cqadupstack-physics |
23474 | What makes the difference between ionic and covalent bonds? | Backstory: When I learned about chemical reactions, there were two types of molecular bonds: Ionic, where an atom or compound molecule with a low valence number loses those valence electron(s) to one or more other compounds with a high valence number, which it then bonds with; and Covalent, where valence electrons are ... | What are the key differences in the properties of ionic and covalent bonds, and is there a quantitative method to classify a bond as ionic or covalent based on factors like electronegativity difference or bond polarity? | 33,714 | cqadupstack-physics |
8476 | Scale set by cosmological constant | Following on Jim Graber's answer to: Can "big rip" rip apart an atomic nucleus? If the cosmological constant is large enough, even the ground state of a hydrogen atom can be affected. So why is the energy scale for quantum gravity set by the planck mass and not by the cosmological constant? Is it because the cosmologic... | What is the relationship between the cosmological constant and the Planck mass in terms of their influence on quantum effects, particularly in the context of hydrogen atom stability and potential implications of a large cosmological constant? | 6,652 | cqadupstack-physics |
18073 | How can I prevent being zapped by static electricity every time I touch a doorknob or handle in the office? | I don't know what it is about this office, but it seems everything I touch (doorknob, bathroom faucet, edge of kitchen sink in the break room), I get zapped by static electricity. It's getting old. I feel like that scene in Office Space. I've worked in other offices and it's not nearly this crazy. This has been going o... | What is the relationship between humidity levels in an office environment and the occurrence of static electricity, and how can one reduce static build-up to minimize shocks from surfaces like doorknobs and faucets? | 29,750 | cqadupstack-physics |
87141 | Confusing Classical Mechanics Question | I don't want this to be a "do my home work question" so please tell me how I can make this question helpful for other people. In my physics assignment I found the question below. I'd think that both FBD's and $F_{net}$s are the same for each scenario; since the track is friction- less the rider doesn't need to input an... | In a frictionless scenario on an inclined track with angle \(\theta\), if a rider maintains a constant velocity while moving down the incline, what is the expression for the net force acting on the rider, and how does it compare to the gravitational force acting down the incline? Use \(g\) for the acceleration due to g... | 19,784 | cqadupstack-physics |
122206 | Choice of basis for Fujikawa method to derive chiral anomaly | I am studying the Fujikawa method of determining the chiral anomalies in a $U(1)$ theory. As we know the basis vectors selected are the eigenstates of the Dirac operator. One of the reasons given is that the eigenstates diagonalize the action which is needed for determining an exact quantity such as Ward-Takahashi iden... | What is the significance of eigenstates of the Dirac operator in the context of the Fujikawa method for determining chiral anomalies, particularly with respect to the diagonalization of the action and its relation to Ward-Takahashi identities? | 14,980 | cqadupstack-physics |
79996 | What are some common errors when it comes to measuring hall voltage of a semiconductor? | What are some common errors when it comes to measuring hall voltage of a semiconductor? I've thought of two errors: 1. Adjusting the potentiometer so that the width of the conductor would start with 0 voltage was very difficult. 2. The magnetic field was strongest in the center but was weak on the sides of the wafe... | What are three potential sources of error when measuring the Hall voltage of a semiconductor, and how could each error quantitatively affect the accuracy of the measurements? | 21,669 | cqadupstack-physics |
71603 | What is the probability density function over time for a 1-D random walk on a line with boundaries? | If a single particle sits on an infinite line and undergoes a 1-D random walk, the probability density of its spatio-temporal evolution is captured by a 1-D gaussian distribution. \begin{align} P(x,t)&=\frac{1}{\sqrt{4 \pi D t}}e^{-\frac{(x-x_0)^2}{4Dt}} \end{align} However, suppose there are impassable boundaries on t... | What boundary conditions allow for a closed-form solution of the probability density function describing the behavior over time of a particle undergoing a one-dimensional random walk with impassable boundaries? Additionally, how does the initial position of the particle affect the form of this solution, and can you pro... | 5,146 | cqadupstack-physics |
15251 | Entropy of radiation emitted into space | In several papers I see something equivalent to the following expression for the entropy of radiation given by an astronomical object such as the Sun (assuming the object can be approximated as a black body): $$ s = \frac{4}{3}\frac{u}{T}, $$ where $u$ is the total flux of radiation energy through a spherical shell sur... | What is the difference in entropy calculation for radiation emitted by a black body like the Sun when considered in a vacuum versus in a closed cylinder, and how does this affect our understanding of the entropy flux given by the formula \( s = \frac{4}{3}\frac{u}{T} \) versus \( s = \frac{u}{T} \)? | 21,675 | cqadupstack-physics |
9287 | Relation between total orbital angular momentum and symmetry of the wavefunction | My question essentially revolves around multi-electron atoms and spectroscopic terms. I understand the idea that the total wavefunction for Fermions should be antisymmetric. Consider as an example, the $2p^2$ electrons in a partially filled p shell; that is, the outer shell of Carbon. The two electrons both have $l=1$,... | In a multi-electron atom, considering the $2p^2$ configuration of Carbon, how many distinct microstates correspond to the following total angular momentum (L) and total spin (S) combinations: \(L=1, S=1\) (triplet) and \(L=0, S=0\) (singlet)? | 24,950 | cqadupstack-physics |
102109 | Numerical Ising Model - Wolff algorithm and correlations | I'm doing some numerical Monte Carlo analysis on the 2 dimensional Ising model at the critical point. I was using the Metropolis 'single flip' evolution at first with success, though it suffers from critical slow down and makes studying large lattices unlikely possible. I'm now looking at cluster flip algorithms, speci... | What is the expected behavior of the two-point correlation function \( <\sigma_i \sigma_j> \) in the two-dimensional Ising model at the critical point when using non-periodic free boundary conditions, and how does it compare to the correlation function's behavior predicted by conformal field theory \( (CFT) \) as given... | 12,260 | cqadupstack-physics |
61851 | Faraday's law for a current loop being deformed | I'm working through Jackson's Classical Electrodynamics text (3rd version, chapter 5.15) about Faraday's law: Faraday's law is pretty familiar: $\int_c E \cdot dl = -\frac{d}{dt}(\int_s B \cdot n da)$ which states that the contour integral of the electric field around a bounded surface is equal to the negative time rat... | If a circular wire loop in a uniform magnetic field is subjected to deformation due to an external force (like hitting it with a hammer), how does this change in shape affect the induced electromotive force (emf) in the loop according to Faraday's law? Specifically, calculate the change in the induced emf if the loop's... | 36,238 | cqadupstack-physics |
59398 | What determines the end of space (universe)? | What determines the end of our universe? Is it defined by the farthest out physical object or is it defined by the farthest out form of energy like light? | What is the farthest observable object in the universe, measured in light-years, and how does its distance compare to the cosmic microwave background radiation? | 6,572 | cqadupstack-physics |
106325 | Advantage of taking qutrits in place of qubits | In general, all the quantum algorithms which I have read so far use qubits (so the space is $\mathbb{C}^2$) and the tensor products of the qubit spaces (space is ${\mathbb{C}^2}^{\otimes n}$). So my question is, are we going to get any advantage if we take space of other dimensions. For instance, what if we do the simi... | What are the potential advantages of using qutrits, as opposed to qubits, in quantum computing, particularly in the context of error-correcting codes and experimental realization? Provide quantitative examples or references that support your answer. | 37,076 | cqadupstack-physics |
104258 | Topological insulators: why K-theory classification rather than homotopy classification? | I am reading a 2009 paper by Kitaev on K-theory classification of topological insulators. In the 4th page, 1st paragraph in the section "Classification principles", he says, > Continuous deformation, or homotopy is part of the equivalence definition, > but it is not sufficient for a nice classification. Why is not homotop... | In the context of K-theory classification of topological insulators, explain why conventional homotopy classification is not sufficient for distinguishing different phases of matter. Specifically, what role does the requirement of preserving band gaps play in this classification, and how does K-theory address this limi... | 25,919 | cqadupstack-physics |
8182 | Frequency response of infinite line source | Let's say I have an infinite line source in 3d wave medium, like a pulsating cylinder, emitting a broadband noisy signal. I know that the pressure falls off at 3 dB per doubling in distance, unlike a point source which falls off at 6 dB per doubling in distance, because the cylindrical wavefront is only spreading out a... | How does the frequency of a broadband noisy signal emitted from an infinite line source in a 3D wave medium affect its intensity as perceived by a listener at a distance, and what is the expected roll-off behavior at high frequencies? | 10,832 | cqadupstack-physics |
51904 | What is a $TEM_{900}$ cavity radiation mode? | I am reading a paper by Serge Haroche stating the cavity they use sustains a Gaussian mode of the e.m. field called $TEM_{900}$. I understand what Gaussian means. I found this explaining what TEM means, but if I am working in a cavity, what is the "direction of propagation"? And above all, why three indices instead of ... | In the context of cavity quantum electrodynamics, can you explain the significance of the three indices in the notation $TEM_{n,m,p}$ for the modes of the electromagnetic field, particularly focusing on how they relate to the directions of propagation and any physical characteristics of the mode? | 32,460 | cqadupstack-physics |
87831 | Application of Ampère's law in low $Re_m$ MHD | Ampère's law states that $$ \nabla \times \mathbf{B} = \mu_0 \mathbf{J} \tag{1}$$ Simultaneously, from Ohm's law, we know that $$\mathbf{J}=\sigma\left(\mathbf{E}+\mathbf{u}\times\mathbf{B}\right)\tag{2}$$ When equating both currents $\mathbf{J}$, and applying Faraday's law, and knowing that the magnetic field $\mathbf... | In the context of one-way coupled magnetohydrodynamics at low magnetic Reynolds numbers, how does the condition of an insulating boundary, where \( J_y = 0 \), affect the interpretation of Ampère's law when considering non-uniform magnetic fields? Specifically, how can we reconcile the non-zero current density derived ... | 33,898 | cqadupstack-physics |
76898 | Wavefront RMS errors, mirror surface roughness and Gaussian beam scattering | What I'm interested is in the scattered power of Gaussian beams reflected from mirrors with a given surface RMS. Usually the surface RMS $\sigma_{s}$ of a mirror translates in an error for the wavefront $ \sigma_{w} \sin{\theta} = 2 \sigma_{s}$, where the beam has incidence angle $\theta$. Now, What I' concerned is spe... | Given that a perfect Gaussian beam with far-field divergence of \(\frac{\lambda}{\pi W_0}\) reflects from a mirror with a surface RMS error \(\sigma_s\), what is the relationship between the beam quality factor \(M^2\) and the surface errors, particularly in terms of how they affect the divergence of the reflected beam... | 19,696 | cqadupstack-physics |
106399 | Which thermoelectric effect is better? | I am doing an experiment which requires the thermoelectric effect. I know about the Seebeck and the Peltier effects. Well, from what I have found till now on these topics is that they both can produce electricity through temperature differences and both can produce heat or cold through electricity, but have different w... | What are the typical power outputs and cost differences between Seebeck generators and Peltier modules in practical applications? | 15,287 | cqadupstack-physics |
47122 | Definition of "Quantizing" | Could anyone explain to me what "quantize" means in the following context? > Quantize the 1-D harmonic oscillator for which $$H~=~{p^2\over 2m}+{1\over > 2} m\omega^2 x^2.$$ I understand that the energy levels of a Q.H.O. are quantized. But here it is used as a verb... Aren't the energy levels naturally quantized? | What are the quantized energy levels of a one-dimensional harmonic oscillator described by the Hamiltonian $$H=\frac{p^2}{2m}+\frac{1}{2}m\omega^2 x^2$$, and how are these energy levels derived? | 18,775 | cqadupstack-physics |
110329 | Trouble with proof about operators in QM |  I would like to complete the following exercise: > _Prove that if the operators $P_i$ satisfy $P_i^{\dagger}$ = $P_i$ and > $P_i^2$ = $P_i$, then $P_iP_j=0$ for all $i\neq j$._ From $P_i^2 = P_i$ I conclude that $P_i(P_i - \mathbb{I}) = 0$, where $\mat... | Given that the operators \( P_i \) are Hermitian and idempotent, i.e., \( P_i^{\dagger} = P_i \) and \( P_i^2 = P_i \), prove that for any two distinct operators \( P_i \) and \( P_j \) (where \( i \neq j \)), it holds that \( P_i P_j = 0 \). What are the implications of this result in the context of linear algebra and... | 28,593 | cqadupstack-physics |
127588 | Derivation of force law in special relativity | I've seen force defined in special relativity as the rate of change of 4-momentum $$ {\bf{F}} = \frac{d {\bf{p}}}{dt} $$ Can anyone comment on the following derivation of that relation? Take one dimension of space. If I'm moving with 4-velocity ${\bf U}(t) = \frac{d{\bf x}}{d \tau}$, then I'd experience an acceleration... | In special relativity, how do you derive the relationship between force and the rate of change of 4-momentum, and can you show how to express this relationship using proper time in a momentarily co-moving reference frame? Additionally, if a spaceship experiences an acceleration of \( a = 5 \, \text{m/s}^2 \) and has a ... | 9,255 | cqadupstack-physics |
14216 | How reliable is Radiometric dating? Are there limitations? | Young earth creationists dismiss radiometric dating as unreliable, whats the truth? | What are the principles behind radiometric dating, and how do they provide evidence for the age of the Earth? | 34,413 | cqadupstack-physics |
76464 | Classical models with unbounded particle number | Is there any classical model which deals with the birth, life and death of particles? What application could it have? I am talking about a 'billiard-ball' kind of model, but the kind in which balls keep popping in and out. What would the configuration space of such a system be like if particle number was not conserved?... | Consider a classical billiard-ball model where particles can spontaneously appear and disappear. If the particle number is not conserved, how would you define the configuration space of this system? Additionally, derive an expression for the Hamiltonian of this system and describe how it evolves over time. What implica... | 36,840 | cqadupstack-physics |
43410 | Does the volume of a thermodynamic system always have to change for it to do work? | Does the volume of a thermodynamic system always have to change for it to do work? If yes,could you explain why? And if no, could you provide the example of a system, where it is not neccesary. | In a thermodynamic system, what is the relationship between volume change and work done? Can you provide an example of a process where work is performed without a change in volume, and calculate the work done in that scenario? | 30,218 | cqadupstack-physics |
41385 | What really is Spacetime? | What is a good definition on Space, Time and the most specific topic "Spacetime"? Because someone told me that spacetime is the foundation of the entire universe? And also, Is it possible to create spacetime? | What are the mathematical implications of spacetime being a fundamental structure in the universe, and can you provide an example of a scenario where spacetime is manipulated or altered, such as in theoretical physics? | 23,010 | cqadupstack-physics |
83179 | What if a probably non wave equation, satisfies the wave equation? | What does it mean for a function to satisfy the wave equation? Does it necessarily mean that it is a wave equation? Take for instance the function $f(t,z)=(at+b z)^2$. This function satisfies the wave equation. Does this mean that it could describe a wave? | Given the function \( f(t,z) = (at + bz)^2 \), verify whether it satisfies the wave equation \( \frac{\partial^2 f}{\partial t^2} = c^2 \frac{\partial^2 f}{\partial z^2} \) for any constant \( c \). If it does, discuss the implications regarding whether this function represents a physical wave. | 19,330 | cqadupstack-physics |
90096 | Why are there usually grids in front of sound speakers? | Usually, the sound speakers have this metal grid.  However, not all of them have this. Is there any purpose, maybe related to the sound quality, that justifies this? Or is it only for physically protect the speaker? | What effect does the presence of a metal grid on sound speakers have on sound quality, and how does it contribute to the physical protection of the speaker? Provide a quantitative analysis of sound wave interference patterns with and without the grid. | 23,819 | cqadupstack-physics |
64104 | Simple conservation of momentum and frame of reference problem | I'm making a very simple physics engine based on momentum, and I'm solving what response to use for a collision from each involved object's frame of reference. However, something about how I'm interpreting frame of reference isn't quite right... Consider the below example, with a 1g object moving 10 m/s toward a 10g ob... | In a perfectly elastic collision between a 1g object moving at 10 m/s and a stationary 10g object, what would be the final velocities of both objects after the collision, using the principles of conservation of momentum and kinetic energy? | 16,172 | cqadupstack-physics |
116383 | Speed of gravity | Consider two objects presented in the figure below. Objects have equal masses and are separated by a distance of 60 light seconds. Assume that we move left object by 3 light seconds to the left in 30 seconds. This requires energy input, lets say it's equal to X. Change in potential energy is also equal to X. Now, the r... | If two objects with equal masses are initially separated by a distance of 60 light seconds, and the left object is moved 3 light seconds to the left in 30 seconds, while the right object remains stationary for 60 seconds, how much energy is required to move the right object 3 light seconds to the right immediately afte... | 5,844 | cqadupstack-physics |
87160 | Flow of water through sand | Imagine a set up with a column of sand with porosity $\phi$ and permeability $k$ contained in a cuboid container. When one pours water evenly (let's say $V_w/A$ cubic meters per square meter) on the top surface of the sand at the same time, how does the distribution of water (water content at a particular height) of th... | How does the water content at a specific height within a column of sand with porosity $\phi$ and permeability $k$ change over time when water is poured at a constant rate $V_w/A$ until it reaches the bottom, and how does this distribution differ after the water has reached the bottom? | 23,187 | cqadupstack-physics |
72765 | Why does the classical equivalent to a quantum computer take so many bits? | A quantum computer with 10 qubits is classically equivalent to $2^{10}$ bits. How is this equivalence worked out? I understand that a single qubit is a vector in a 2-dimensional hilbert space, whose base we can label as $|0>$ and $|1>$. So, 10 qubits will require a 20-dimensional hilbert space. According to these notes... | If a quantum computer uses N qubits, how many distinct classical states can it represent, and how is this relationship expressed mathematically? | 9,736 | cqadupstack-physics |
48644 | Is Dark Energy converted to Gravitational Potential Energy? | Dark Energy is basically doing work against gravity to accelerate the expansion of universe. So, for time-translational symmetry to be hold, dark energy must be converted to gravitational potential energy. I don't think, it can violate critical mass condition required for Cosmic Inflation because gravitational potentia... | What is the ratio of dark energy density to gravitational potential energy density required for the current expansion rate of the universe, assuming a homogeneous distribution of dark energy? | 25,134 | cqadupstack-physics |
10140 | Are many-worlds and the multiverse really the same thing? | Are many-worlds and the multiverse really the same thing? Not too long ago, Susskind and Bousso uploaded the article "The Multiverse Interpretation of Quantum Mechanics" with the thesis that the many-worlds interpretation and the multiverse of eternal inflation are one and the same thing. The parallel worlds of one are... | What are the implications of the claim that decoherence is reversible in the context of the many-worlds interpretation and the multiverse, and how would this affect the probability of observing conscious observers in different bubbles of eternal inflation? | 5,878 | cqadupstack-physics |
93517 | Why do we use the complexification of the Lorentz group? | I do understand why we are using the double cover, but why exactly do we make the transition to complex Lorentz transformations? Where and why are they needed? To be precise: The double cover of the Lorentz group is given by $SU(2)$. Where and why do we make the transition from $SU(2)$ to $SL(2,C)$? ($SL(2,C)$ is the... | What is the relationship between the double cover of the real Lorentz group, $SU(2)$, and the double cover of the complex Lorentz group, $SL(2,C)$, in terms of their representations and applications in physics? | 37,064 | cqadupstack-physics |
114729 | Chemical effect on gravitation? | We know, that gravitation field of charged black hole is different than one of uncharged. I this true only for objects with singularity or is true for all objects? If true, then may we say, that gravitation field of chemical molecule has footprints of it's chemical composition and state and, in principle can be registe... | How does the gravitational field of a charged black hole differ quantitatively from that of an uncharged black hole, and can this difference be analogously applied to determine the gravitational influence of a chemical molecule based on its composition and state? | 31,275 | cqadupstack-physics |
122473 | Does dark energy have gravity? | So I keep running into this idea on the Internet that only mass has gravity, but isn't mass simply contained energy? Energy bounces around, or travels at the speed of light, because it is not in the form of an atom, it is in the form of light, photons, quarks, neutrinos, and things we haven't discovered yet. Wouldn't i... | Can you explain why dark energy is considered to have a constant density despite the expansion of the universe, and how this affects our understanding of gravity and the fate of the universe? What evidence supports this concept? | 19,091 | cqadupstack-physics |
134955 | Rotational symmetry in integration | Can someone please tell me why $$4\int d^4x \, x^\mu x^\nu ~=~\int d^4x \, g^{\mu\nu}x^2 $$ by some rotational symmetry argument? | Prove that the equality $$4\int d^4x \, x^\mu x^\nu = \int d^4x \, g^{\mu\nu}x^2$$ holds by using rotational symmetry in a four-dimensional spacetime, and determine the value of the integrals if the integration is performed over a symmetric range of coordinates \(x^\mu\). | 17,170 | cqadupstack-physics |
110610 | Electric potential is zero but non zero electric field? | What is the physical significance of such a point where electric field is non- zero but electric potential is zero? I mean, how can we understand this concept without mathematics? | At a certain point in space, the electric field is measured to be 3 N/C directed to the left, while the electric potential at that point is 0 V. What does this indicate about the nature of the electric field and potential in that region, and can you provide a practical example that illustrates this scenario? | 11,949 | cqadupstack-physics |
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