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46034 | Characteristics of bloch electron in a priodic potential | Effective mass of a Bloch electron in a periodic potential is negative why ? | What is the quantitative relationship between the effective mass of a Bloch electron in a periodic potential and the curvature of the energy band structure near the k-point, and how does this relationship lead to a negative effective mass? | 29,110 | cqadupstack-physics |
92471 | Wightman axioms and gauge symmetries | I have a basic understanding of the Wightman axioms for QFT. I was reading the about the Mass Gap problem for simple compact gauge groups and was wondering how the gauge group is supposed to be implemented into the framework. The official problem description (pdf) says: > _To establish existence of four-dimensional qua... | What are the implications of the Wightman axioms for defining a quantum field theory with a compact gauge group, and how can one verify the existence of a mass gap in such theories? | 16,100 | cqadupstack-physics |
96579 | Determining the minimum pull force of a magnet required to hold it in place | If I place a magnet on the underside of a metal object so that it is held there by its attraction to the metal, how can I determine the minimum pull force the magnet needs to have in order to keep it attracted to the metal and not fall off? Would the required pull force be equivalent to the magnet's weight? | What is the minimum pull force a magnet must exert to hold a 500 gram metal object against the force of gravity, assuming the gravitational acceleration is 9.81 m/s²? Would this required pull force differ from the weight of the magnet itself? | 22,151 | cqadupstack-physics |
41203 | Is physics very dependent on equipment? | I always had the impression that physics depends a lot on particle accelerators and heavy machines for experimentation of new theories, I know there's the field called _theoretical physics_ but until now I have no idea on how _TF_ relies on these machines. Does theoretical physics rely a lot on this machinery? | To what extent does theoretical physics utilize data from particle accelerators and heavy machinery for the validation of its theories? | 4,310 | cqadupstack-physics |
66780 | Human power on treadmill | On an elliptical treadmill a regular person can easily burn 1000 calories in one hour (treadmill reports calories burnt). This translates into: $$(1\times 10^3\mathrm{cal/hr}\times 4.2\times10^3\mathrm{J/cal})/3.6\times 10^3\mathrm{s/hr} \approx 1.2 \; \mathrm{kW} \approx 1.5 \; \mathrm{hp}$$ On the other hand, Wikiped... | What is the maximum mechanical power output in watts that a trained athlete can sustain for one hour while performing high-intensity exercise on an elliptical treadmill compared to cycling, and how does this reflect on the efficiency of different forms of exercise? | 32,786 | cqadupstack-physics |
119190 | Why $SU(3)$ and not $U(3)$? | Is there a good reason not to pick $U(3)$ as the colour group? Is there any experiment or intrinsic reason that would ruled out $U(3)$ as colour group instead? | What are the physical implications of selecting $U(3)$ as the color group in quantum chromodynamics, and how does it compare to the standard $SU(3)$ group in terms of particle interactions? | 28,579 | cqadupstack-physics |
95064 | Same partition functions, different theories | I am reading the book "Basic Concepts of String Theory" by Blumenhagen, Lust and Theisen and in page 290 they say: "It follows that the $E8\times E8$ and the $SO(32)$ heterotic string theories have the same number of states at every mass level which are however differently organized under the internal gauge symmetries.... | What is an example of a specific correlation function in the $E8 \times E8$ heterotic string theory that differs from a corresponding correlation function in the $SO(32)$ heterotic string theory, and how do these differences arise from the distinct internal gauge symmetries of the two theories? | 38,028 | cqadupstack-physics |
114575 | details for the double slit experiment | In the double slit experiment with electrons, are all electrons going through the slits? If the electron gun is directed between two slits, than it should hit the central part between the slits, isn't it? So if one would count ( **not watch** the flying electrons, to not collapse the wave function) the electrons hited ... | In a double slit experiment with electrons, if an electron gun is aimed precisely between the two slits and we count the total number of electrons that hit the detection screen without observing their paths, will the number of detected electrons be equal to the number of electrons emitted from the gun? Additionally, co... | 34,699 | cqadupstack-physics |
2808 | Lagrangian density of linear elastic solid | I need the general expression for the lagrangian density of a linear elastic solid. I haven't been able to find this anywhere. Thanks. | What is the general expression for the Lagrangian density of a linear elastic solid, and how does it relate to the deformation gradient and strain energy density? | 23,092 | cqadupstack-physics |
52303 | What does the graviton-matter Feynman scattering diagram look like? | The question is in the title really. I know gravity + QFT is on shaky ground but I'm not looking for Feynman rules, just a schematic, if that is possible. Edit 1: added possible diagram for clarification.  **Edit 2: Actually, isn't _this_ diagram the most important... | What is the expected cross-section for a hypothetical graviton interacting with a proton at low energies, given that the graviton has a mass of \(10^{-25}\) eV/c² and the interaction can be approximated by a Yukawa potential? | 28,461 | cqadupstack-physics |
41934 | What does the wind speed have to be to blow away a person? | With the approaching hurricane, I am curious about what would happen if I go outside, in particular whether the wind gusts might be fast enough to blow me away. How fast would the wind have to be to blow away a person? | What is the minimum wind speed, in meters per second, that could potentially lift and blow away an average adult human, considering their weight and body surface area? | 26,599 | cqadupstack-physics |
88567 | Analog of Klein-Gordon equation from Proca action | What would be the general form of Lagrange Equation when instead of a scalar field we have a vector potential? has anyone derived the klein gordon equation for a corresponding vector potential Lagrangian? | What is the Lagrangian density for a vector potential \( \mathbf{A} \) that leads to the Klein-Gordon equation, and how does it compare to the scalar field case? | 37,596 | cqadupstack-physics |
6545 | Why are Saturn's rings so thin? | Take a look at this picture (from APOD http://apod.nasa.gov/apod/ap110308.html):  I presume that rocks within rings smash each other. Below the picture there is a note which says that Saturn's rings are about 1 km thick. Is it ... | What is the average mass of a rock in Saturn's rings if they are about 1 km thick and made up of particles ranging from tiny grains to larger boulders, with an estimated total mass of the rings around 1/1000th that of the Moon? | 30,090 | cqadupstack-physics |
95327 | Pole and Barn Paradox | I'm having trouble reconciling two different versions of the Pole and Barn paradox. Version 1: Consider a pole 10 m long and a barn 5 m long with a front and rear door. A runner carrying the pole (frame S') moving with respect to the barn and a farmer (frame S) runs into the barn. The farmer will see that the pole fits... | In both versions of the Pole and Barn paradox, what is the maximum speed at which the runner can move such that the 10 m pole is not broken when trying to fit it into the barn, considering the relativistic effects of length contraction? | 14,686 | cqadupstack-physics |
116613 | How to understand the equivalence between Andreev reflection and Cooper pair injection? | It is well know that Andreev reflection dominates the subgap transport at the normal metal-superconductor interface. An incident electron can be reflected as a hole in the Nambu space, which effectively describes two electrons transmit into the superconductor and form a Cooper pair. It seems reasonable and intuitive th... | What is the total momentum of two electrons injected from a normal metal into a superconductor during the Andreev reflection process, and how does this compare to the momentum of the Cooper pair they form? | 29,117 | cqadupstack-physics |
76931 | How does a gas of particles with uniform speed reach the Maxwell-Boltzmann distribution? | Take an empty container and fill it with $N$ gas particles (ideally a monoatomic gas), each having the same kinetic energy $E$, then isolate the container. Since initially the speeds don't follow the Maxwell-Boltzmann distribution, such a system cannot be in thermodynamic equilibrium. On the other hand, assuming perfec... | If you have a container filled with \( N \) monoatomic gas particles, each with the same initial kinetic energy \( E \), and you ensure that the system is isolated, what will be the total heat content of the system as it evolves towards thermodynamic equilibrium, and how can you explain the transition to the Maxwell-Bo... | 22,349 | cqadupstack-physics |
95840 | Why is Clapeyron equation so important? | **Context** : I'm studying basic thermodynamics. My textbook has a chapter on the _Clapeyron_ equation which, as a reminder, is given by the following formula: \begin{equation} \frac{dP}{dT} = \frac{\Delta H_{a,b}}{T\Delta V} \end{equation} (a,b being the different phases) **Question** : The equation describes the tang... | Using the Clapeyron equation, derive the relationship between the change in vapor pressure of a substance at a given temperature and its enthalpy of vaporization. Given that the enthalpy of vaporization of RDX is approximately 300 kJ/mol, calculate the change in vapor pressure when the temperature increases from 25°C t... | 32,131 | cqadupstack-physics |
48634 | How do I account for the direction of friction acting on a spring? | I would like to set up the equations of motion for a simple spring oscillator. Let's have a spring lying horizontally; we attach a small mass $m$ to the (massless) spring. The force of the spring on the mass is $$F_\text{spring} = - k x$$ where $k$ is the spring constant, and $x$ is the displacement from the position o... | Given a mass \( m = 0.5 \, \text{kg} \) attached to a spring with a spring constant \( k = 200 \, \text{N/m} \) and a coefficient of kinetic friction \( \mu = 0.1 \), calculate the acceleration \( a \) of the mass when it is displaced \( x = 0.1 \, \text{m} \) from the position of rest. Assume that the normal force \( ... | 7,411 | cqadupstack-physics |
46414 | Influence of choice of statistics on gas kinetics | In the derivation of distributions over energy states, a common assumption made is that under normal conditions (normal from a fluid dynamics standpoint, so > 300K typically) the energy states are very sparsely populated and so the vast majority of them remain empty. Because of this sparsity, the choice between **_Bose... | In conditions where the Boltzmann limit is not satisfied, how does the use of Fermi-Dirac statistics instead of Boltzmann statistics affect the specific heat coefficients and the equations of state for a system of fermions? Additionally, would macroscopic phenomena such as shock waves or chemical non-equilibrium behave... | 27,424 | cqadupstack-physics |
57039 | Estimate the difference between two sets of atoms | I've been working on amorphous structures derived from a crystalline one (using MD) containing $N$ atoms. I want to prove that these structures are different and to **quantify** their "differentness". One could think of simply displaying the structures, but: * I actually have 20 of them and I don't want to show them ... | What alternative metric or method can I use to quantify the differences between multiple amorphous structures derived from a crystalline one, considering the challenges with RMS distance calculations due to outliers and the need for rotational and translational alignment? | 15,590 | cqadupstack-physics |
83394 | Dark matter and SO(10) grand unification | $SO(10)$ grand unified theories nicely accommodate a massive $\sim 10^{14-15}\; GeV$ sterile neutrino. Would this be a viable dark matter candidate? I haven't found any _specific_ material regarding this possibility and I was wondering if there is some simple reason to exclude it. GUT's based on $SU(5)$ have mostly bee... | Is a sterile neutrino with a mass of approximately \(10^{14-15}\; \text{GeV}\) a viable candidate for dark matter, and what factors could be considered in assessing its viability? Also, would the absence of proton decay in experiments such as Hyper-Kamiokande provide evidence against \(SO(10)\) grand unified theories? | 31,629 | cqadupstack-physics |
86219 | Understanding the Eötvös experiment | The aim of the Eötvös experiment was to "prove" that for every (massive) particle, the quotient $\frac{m_g}{m_i}$ is constant, where $m_g$ is the gravitational mass and $m_i$ is the inertial mass. **The experiment:** Consider two objects with coordinates $x(t)$, $y(t)$ and with masses $M_i$, $M_g$, $m_i$, $m_g$ (on the... | In the Eötvös experiment, if two objects are connected by a rod of length \( r \) and subjected to the gravitational field \( \mathbf{g} \) such that their accelerations are given by \( \ddot{\mathbf{x}}(t) = -\frac{M_g}{M_i} \mathbf{g} \) and \( \ddot{\mathbf{y}}(t) = -\frac{m_g}{m_i} \mathbf{g} \), what is the expres... | 31,251 | cqadupstack-physics |
45730 | Theoretically speaking what are the limits for super-capacitors in terms of their storage capability? | According to wikipedia, commercially available super-capacitors in 2010 can store 30Wh/kg, with 85Wh/kg achieved in a lab in 2011. Have any theoretical calculations been done to show what can be achieved as an upper limit to super-capacitors? | What is the theoretical energy density limit for supercapacitors based on current research in materials science and electrochemistry? | 30,133 | cqadupstack-physics |
28086 | Did Galileo offer an explanation why his law of falling bodies is independent of mass? | Naively, (and endorsed by Aristotle) one expects heavier things to fall more quickly, Galileo Galilei showed in fact this is not correct, and that in fact how they fell as independent of mass (if air resistance remained negligible). Did he offer an explanation of this observation? | What experimental setup did Galileo Galilei use to demonstrate that the acceleration of falling objects is independent of their mass, and what were the key observations he made during this experiment? | 2,778 | cqadupstack-physics |
69604 | Heisenberg Uncertainty Principle: Which formula is correct? | Some websites and textbooks refer to $\Delta x \Delta p \geq \frac{\hbar}{2}$ as the correct formula for the uncertainty principle whereas other sources use the formula $\Delta x \Delta p \geq \hbar$ . The latter is used in the textbook "Physics II for Dummies" (German edition) for several examples and the author also ... | What is the correct expression for the uncertainty principle in quantum mechanics, and how does the derivation of the expression $\Delta p \Delta y \geq \frac{h}{2 \pi}$ lead to the conclusion that $\Delta x \Delta p \geq \hbar$ rather than $\Delta x \Delta p \geq \frac{\hbar}{2}$? Please provide a clear explanation of... | 13,730 | cqadupstack-physics |
22159 | Spectrum of Free Strings | > **Possible Duplicate:** > Spectrum of Free Strings As far as I understand, both in bosonic and superstring theory one considers initially a free string propagating through D-dimensional Minkowskispace. Regardless what quantization one uses at the end one arrives at a spectrum, where the excitations are classified ... | In the context of string theory, how does the mass of excitations change when transitioning from a D-dimensional Minkowski background to a different background, and is there a known method to define mass in non-Minkowskian backgrounds? | 33,475 | cqadupstack-physics |
72589 | What does string theory say about the metric expansion? | Specifically, what happens to those small intertwined hidden dimensions? Do those expand too? | If the universe expands, how does this expansion affect the size of hidden dimensions theorized to be compactified in string theory? Calculate the potential change in size of these dimensions based on a known rate of cosmic expansion. | 17,985 | cqadupstack-physics |
114897 | Can we reach absolute zero one day, and if so, can we measure it? | I was reading this question, and I had this doubt, if we can ever reach absolute zero, how can we measure it? I ask this because as far I know, any measurement could change (increase) the temperature. **EDIT** Reach absolute zero is a hypothesis. My doubt is, if hypothetically one day we can get to absolute zero, have ... | If we could hypothetically reach absolute zero (0 K or -273.15 °C), what experimental methods or theories could be employed to verify that a system has indeed attained this temperature, considering that conventional measurements might alter the system's thermal state? | 31,682 | cqadupstack-physics |
4031 | Superconformal theories | * Can anyone tell me where can I read about the notion of "short" and "long" representations? Like what they are etc. * From where can I learn the arguments which show that the bosonic subalgebra of $\cal{N}=2$ and $\cal{N}=3$ Lie superalgebra are given by $SO(3,2)\times SO(2)$ and $SO(3,2)\times SO(3)$ respectively... | What are the unitarity bounds for primary states in supersymmetric algebras, and how do the values of the scaling dimension \((\Delta)\), spin \((j)\), and \(R\)-charge \((h)\) relate to whether a representation is considered short or long? Specifically, can you explain the inequalities that govern these relationships,... | 15,828 | cqadupstack-physics |
78739 | Commutator problem | I am trying to calculate the following commutator $$[\mathcal{H}_0(r',t'),\psi(r,t)]_-$$ where $\mathcal{H}_0 = (\frac{1}{2m}\nabla^2 + e\mathbf{A}(r',t'))^2 + e\phi(r',t') - \mu$, and $\mu$ is the chemical potential. It is supposed to be $0$, but I don't understand why it is not a a delta- function $\delta(r-r')[\math... | Can you explain why the commutator $$[\mathcal{H}_0(r',t'), \psi(r,t)]_-$$ is equal to zero, given that $$\mathcal{H}_0 = \left(\frac{1}{2m}\nabla^2 + e\mathbf{A}(r',t')\right)^2 + e\phi(r',t') - \mu$$ and $\mu$ is the chemical potential? Is there a condition under which this commutator results in a delta function inst... | 28,965 | cqadupstack-physics |
65397 | Quantum mechanics and everyday nature | Is there a phenomenon visible to the naked eye that requires quantum mechanics to be satisfactorily explained? I am looking for a sort of quantic Newtonian apple. | What is a visible phenomenon that can only be fully understood through the principles of quantum mechanics, and what are the key quantum mechanical concepts involved in its explanation? | 24,140 | cqadupstack-physics |
55053 | Jacobi's matric formulation for tensors | Hi I am trying to derive the E field equation and am stuck using the Jacobi formula, is this correct: $$\delta \det g_{\mu \nu} = Tr(ADJ(g_{\mu\nu}\delta g_{\mu\nu})=\det(g_{\mu\nu})Tr(g^{\mu\nu}\delta g_{\mu \nu}) $$ Then how can we remove the trace, or should it be: $$\delta \det g_{\mu \nu} =\det(g_{\mu\nu})Tr(g^{-1... | What is the relationship between the determinant of the metric tensor \( g_{\mu \nu} \) and its variation \( \delta g_{\mu \nu} \), and how can we express \( \delta \det g_{\mu \nu} \) in terms of the components of the metric tensor and its inverse, specifically using the trace operation? | 24,462 | cqadupstack-physics |
31192 | Lorentz transformations of the polarization vector |  Let $\bf{n}'$ be a unit vector in the direction of a wavevector in the plasma rest frame and $\bf{B'}$ be a unit vector along the magnetic field in the plasma rest frame. The electric field of a linearly polarized electromagnetic wave is directed along... | Given the equation for the normalized electric field vector $\hat{e'}$ in the explosion frame as
\[
\hat{e'} = \frac{\bf{b} \times \hat{B'}}{\Gamma (1-\bf{n}\cdot\bf{v})} + \frac{1+\Gamma (1-\bf{n}\cdot \bf{v})}{(1+\Gamma(1-\bf{n}\cdot \bf{v}))},
\]
where $\Gamma = \frac{1}{\sqrt{1-v^2/c^2}}$, determine the conditio... | 17,835 | cqadupstack-physics |
78744 | Why do organisms accumulate potential energy? | I can understand that animals need some battaries to run. But, we learn that plants serve like batteries for animals because they accumulate the sun energy in the first place! You can eat them or burn to extract the accumulated energy. This means that they consist of structures that they want to blow-up and disintegrat... | How does the process of photosynthesis create hydrocarbons, given that these molecules are inherently unstable and tend to degrade, and what implications does this have for the energy balance in living organisms? | 21,466 | cqadupstack-physics |
128208 | Why the involution condition is imposed in the definition of integrability? | For an $N$-degree-of-freedom system to be integrable, the usual definition imposes the existence of $N$ independent conserved quantities, which must be in involution to each other, i.e., $$\\{ F_i, F_j \\} ~=~ 0\tag{1}$$ Why this extra condition? Why is $$\\{ H, F_i \\}~=~0 \tag{2} $$ insufficient? | In a Hamiltonian system with \( N \) degrees of freedom, if you have \( N \) independent conserved quantities \( F_i \) that satisfy the condition \( \{ H, F_i \} = 0 \), what additional criteria must these quantities meet to ensure the system is integrable? Specifically, explain why the condition \( \{ F_i, F_j \} = 0... | 24,670 | cqadupstack-physics |
1942 | Does the cube stay on the loop-de-loop? | Suppose a cube of mass $m$ enters a loop-de-loop of radius $r$, travelling halfway around and reaching the top. At the top, it has velocity $v$, and the second half of the loop is frictionless. How can I determine whether or not it will continue around the loop, or fall off? According to the notes I took during class I... | If a cube of mass \( m \) enters a loop of radius \( r \) and reaches the top with a velocity \( v \), under what condition will the cube continue moving around the loop without falling off? Also, explain how this condition changes if there is friction on the second half of the loop. | 28,238 | cqadupstack-physics |
38905 | Field theory:functional derivative involving Fourier Transform | I have to solve the following functional derivative $$ \frac{\delta}{\delta \Lambda(\mathbf{x})}\log[A-\mathbf{k}^2\Lambda(\mathbf{k})] $$ where $\Lambda(\mathbf{k})$ is the Fourier transform of $\Lambda(\mathbf{x})$, namely $$ \Lambda(\mathbf{k})=\int d\mathbf{x\prime} e^{-i\mathbf{k}\cdot\mathbf{x\prime}}\Lambda(\mat... | In the functional derivative derivation of $$ \frac{\delta}{\delta \Lambda(\mathbf{x})}\log[A-\mathbf{k}^2\Lambda(\mathbf{k})] $$, I arrive at $$ \frac{-\mathbf{k}^2}{A-\mathbf{k}^2\Lambda(\mathbf{k})} e^{-i\mathbf{k}\cdot\mathbf{x}} $$, while another source presents it as containing a positive exponent, $$ e^{i\mathbf... | 1,662 | cqadupstack-physics |
118842 | Why is $I_0\sin\left(\dfrac{\pi}{2}-\omega t\right)$ not used as a solution of the purely capacitive AC circuit? | Why is $I_0\sin\left(\dfrac{\pi}{2}-\omega t\right)$ not used as a solution of the purely capacitive circuit? **My Research** I've searched on Google, Phys.SE and many school level Physics books but couldn't find the answer. If the reader thinks that the question can be answered with a quick google search then please g... | In a purely capacitive circuit, why is the expression $I_0\sin\left(\dfrac{\pi}{2}-\omega t\right)$ not typically used to represent the current, and what is the standard expression for current in such a circuit? | 23,172 | cqadupstack-physics |
105605 | Electric arc due to static discharge in a T-Shirt possible? | Yesterday, when I came home, I went to the bathroom (lights off) and i took off my T-Shirt (100% cotton) which I wore under a Shirt (50% cotton, 50% polyester). I _believe_ to have seen a small but well visible light arc. **Question:** Is the amount of static discharge in T-Shirt big enough to produce a visible light a... | What is the minimum amount of static charge (in nanocoulombs) required to produce a visible light arc in a dark environment, and how does the static charge generated by removing a 100% cotton T-shirt compare to that threshold? | 34,266 | cqadupstack-physics |
78818 | Parallel Transport of a 4-vector |  Why does the parallel transported $4$-vector change from $X^a(x)$ to $X^a(x) + \bar{\delta}X^a(x)$ ? This is also discussed in D'Inverno's relativity book [page - 72]; but the reason is not clear. | What is the mathematical expression for the change in a parallel transported 4-vector \(X^a(x)\) as it is modified from \(X^a(x)\) to \(X^a(x) + \bar{\delta}X^a(x)\)? Please provide the conditions under which this change occurs in the context of general relativity. | 24,829 | cqadupstack-physics |
45161 | What is the importance of electrodynamics and magnetism in physics as a whole? | At my university the second half of a year long sequence in basic calculus based physics focuses on electrodynamics and magnetism. I am wondering what is the significance of these topics to physics in general? How does it relate/prepare you to study the more advanced subjects like quantum mechanics and the like? | What are the fundamental differences between electric fields and magnetic fields in terms of their mathematical representation and physical significance, and how do these concepts lay the foundation for understanding electromagnetic waves? | 29,814 | cqadupstack-physics |
82153 | What is the difference between broken and unbroken sypersymmetry? | In a physics article I read recently, the author introduces the notion of supersymmetry by saying basically that the system described by the Hamiltonian $H$ is supersymmetric if $H$ can be decomposed as $H=QQ^\dagger + Q^\dagger Q$ with $Q^2=0$. This condition then leads to a nice splitting of the underlying Hilbert sp... | What are the physical implications of broken versus unbroken supersymmetry in a quantum system, and how does the presence or absence of ground states influence the properties of the Hamiltonian and the associated Hilbert space? | 17,450 | cqadupstack-physics |
29132 | Is there a tb meson? | I was wandering around the particle date group page for meson and couldn't find a meson for top-bottom, which from symmetry you would expect. Q1: Is this because it hasn't been found? Q2: There is an underlying reason why it can't exist? | Is there a theoretical framework within the Standard Model of particle physics that predicts the existence of a top-bottom meson, and if so, what are the reasons for its absence in experimental observations? | 12,664 | cqadupstack-physics |
7672 | live Kp index data | I need to create an online service displaying latest Kp index. Where I can take the data? The data should be in machine-readable format, i.e. text files, XML, or CGI gateways, for instance. No graphical plots! * * * I found this: http://www.swpc.noaa.gov/wingkp/wingkp_list.txt Is Est. Kp what I need? I compared the dat... | What factors could cause discrepancies between the Kp index values reported in text files and those represented in graphical plots? | 11,045 | cqadupstack-physics |
24727 | Why don't Turbojet engines use the De Laval Nozzle? | Why don't Turbojet engines use the De Laval Nozzle? In fact, it seems that in a typical turbojet, the output nozzle contracts, not expands. As the temperature of exhaust gases is high -- it should be possible to extract more energy out of it in this case. | What is the thermodynamic efficiency of a turbojet engine that uses a converging nozzle compared to one that employs a De Laval nozzle, assuming all other conditions remain constant? Provide the relevant equations and calculations to support your answer. | 8,664 | cqadupstack-physics |
39401 | Is Hyperbolic Space $H3$ the best representation space for momenta, in momentum-scale invariant theories? | The motivation is the following: For each particle of mass m, we could write : $- (E/m)^2 + (p_x/m)^2 + (p_y/m)^2 + (p_z/m)^2 = -1$, which is nothing than a equation for a point in the hyperbolic space $\mathbb H3$, which can be seen, with Poincaré representation (Poincaré ball), as a (full) sphere (with hyperbolic dis... | In the context of hyperbolic space \( \mathbb{H}^3 \) and the behavior of particles, how does the introduction of a tiny mass for photons affect their representation within this space, and what implications does this have for momentum-scale invariant theories? Please provide a mathematical explanation or derivation. | 12,265 | cqadupstack-physics |
30235 | What would it feel like to touch a frictionless surface | Assume there is a completely frictionless surface. Would touching such a surface be possible? If so, what would it feel to the touch? | If a person were to touch a completely frictionless surface, what forces would act on their hand, and how would those forces affect their sensation of touch? | 24,964 | cqadupstack-physics |
58136 | Does the sun rotate? | As implied from the question, does the sun rotate? If so, do other stars not including the sun also rotate? Would there be any consequences if the sun and other stars didn't rotate? Me and my friends have differing views on this, and would like some clarification. Thanks! | What is the rotational period of the Sun, and how does this compare to the average rotational periods of other stars in our galaxy? Additionally, what are some consequences for star formation and stability if stars did not rotate? | 22,732 | cqadupstack-physics |
88311 | Geometriclength and effective length | > Why is effective length of a bar magnet shorter than its geometric length? > Blockquote in a textbook, author wrote that, effective length is equal to 0.85 times geometric length. But didn't mention the mathematics behind it. I don't understand which keyfactor can determine the differences between geometric length an... | Calculate the effective length of a bar magnet if its geometric length is 20 cm. Use the relationship that the effective length is 0.85 times the geometric length. What is the effective length? | 12,835 | cqadupstack-physics |
87631 | How do you define an aperture for seismic data processing? | Lets consider this in marine seismic processsing. I am assuming you know what a first order primary reflection is, if not, I can define it. Ok, so if we were to have a source $(S)$ and receiver $(R)$ lying directly on the $x$ axis seperated with a distance of $n$ meters, and a horzontial reflective surface, then the re... | If a source \( S \) and a receiver \( R \) are separated by a distance of \( n \) meters along the \( x \)-axis, and there is a dip of \( \theta \) degrees in the reflector's surface in the \( x \)-direction, what is the range of the "aperture" for the reflection point, assuming that the angle of incidence is equal to ... | 36,683 | cqadupstack-physics |
58127 | Strings and QFT: particles moving backward in time? | New question: In string theory and QFT, do particles travel back in time? Not related to antimatter: Do they travel back and forth in time in reality or are these just interpretations of mathematical formulas used to make sense of calculations? This is a different question (from this one) as it concerns strings and not... | In the context of string theory and quantum field theory, quantify the conditions under which particles could theoretically travel back in time, and discuss how this concept is mathematically represented. | 16,781 | cqadupstack-physics |
103836 | Which form of Maxwell's equations is fundamental, in integral form or differential form? | I am not sure which form of Maxwell's equations is fundamental, integral form or differential form. Imagine an ideal infinitely long solenoid. When a current is changing in time, can we detect classical effects outside a solenoid, for example generating a circular current around solenoid by Faraday's law. If the differ... | If we have an infinitely long solenoid carrying a time-varying current, what is the magnitude of the induced electromotive force (EMF) around a circular loop of radius \( r \) located outside the solenoid, given that the solenoid has \( n \) turns per unit length and the current \( I(t) \) changes with time according t... | 25,937 | cqadupstack-physics |
132158 | Power to suspend a mass in air | How much power is needed to suspend a mass in the air? The four parts below deal with the same problem. I post them all here, should there be some subtle things I didn't see. ## The textbook problem A 9th edition college physics textbook has this as an example problem in its energy chapter (details cleaned-up): An elev... | What is the minimum power required for a motor to keep a 1800 kg elevator car suspended in the air at a constant height, assuming there is no acceleration and 100% efficiency? | 27,593 | cqadupstack-physics |
132639 | Computationally solving bodies that push and pull | Is there a way to find a solution for the positions (as a function of time) of multiple free bodies that push and pull on each other? Say for instance I have a collection of cells which can individually contract or expand, and are joined to one another. For simplicity, I'm assuming two dimensions; no gravity, friction,... | Given a system of N circular cells in two dimensions, where each cell can contract or expand by changing its radius and exerts a force on connected cells during these changes, how can we mathematically model the positions of these cells as a function of time? Specifically, how can we approximate their motion considerin... | 34,756 | cqadupstack-physics |
98941 | $\tau$ pair production question | There's a question on my homework about the process $e^{-} e^{+} \rightarrow \tau^{+} \tau^{-}$. Specifically, it is claimed that the minimum energy required of the colliding positron and electron beams is slightly less than twice the $\tau$ mass, and I am asked to explain this deviation and compute it. In the context ... | What is the minimum energy required for the collision of an electron and a positron to produce a tau-antitau pair, and how does the Coulomb interaction between the electron and positron affect this energy requirement? Please compute the deviation from the predicted value considering the masses of the particles involved... | 19,083 | cqadupstack-physics |
9516 | How close can two extremal black holes with the same charge can get? | Here's a puzzle I have been pondering over. If we have two extremal black holes with the same charge, the electrostatic repulsion between them ought to cancel the gravitational attraction between them. Without any net attraction or repulsion between them, how close can we bring these two black holes to each other witho... | Given two extremal black holes with the same charge, how close can they approach each other without merging, considering the balance between electrostatic repulsion and gravitational attraction? Specifically, what is the distance between their event horizons, and how does the extremal nature of their charge influence t... | 24,688 | cqadupstack-physics |
71732 | A column falls, how will it break? | I'm not expecting a definitive answer. But I would like someone to explain which are the main forces that interact in this situation: An ideal cylindrical column that is at first vertical is pushed just enough so it loses its balance and falls on its side. At every moment there's a point in the circumference of the bas... | When an ideal cylindrical column made of a brittle material falls on its side and comes into contact with a much harder floor, how can we determine the number of pieces it will break into, and what physical properties or constants (such as material strength, height of the column, and angle of impact) are relevant in an... | 12,834 | cqadupstack-physics |
23211 | Representations of Pauli matrices involving outer product of qubit states | Let $| 0 \rangle$ and $| 1 \rangle $ be the states of qubit. Let $\hat{\sigma_x}$, $\hat{\sigma_y}$, $\hat{\sigma_z}$ be Pauli matrices: $$ \hat{\sigma}_{x} = \left( \begin{array}{cc} 0 & 1 \\\ 1 & 0 \end{array} \right), \;\;\; \hat{\sigma}_{y} = \left( \begin{array}{cc} 0 & -i \\\ i & 0 \end{array} \right), \;\;\; \ha... | Verify the representations for the Pauli matrices $\hat{\sigma}_x$, $\hat{\sigma}_y$, and $\hat{\sigma}_z$ in terms of the outer products $|0\rangle \langle 0|$, $|1\rangle \langle 1|$, and $|0\rangle \langle 1|$, $|1\rangle \langle 0|$. Are the following equalities correct?
1. $\hat{\sigma}_x = |1\rangle \langle 0| ... | 2,030 | cqadupstack-physics |
81635 | why does water do that? | I found recently that water fills a bottle quicker through only observation if I hold the tube away from the filling water than if I leave the tube inside the bottle. why does that happen? does potential energy have anything to do with this?  | When filling a bottle with water using a tube, how does the position of the tube in relation to the bottle affect the flow rate of the water, and what factors can be quantified to explain this difference? For instance, how does the height difference between the water source and the bottle influence the flow rate due to... | 8,638 | cqadupstack-physics |
19629 | How does repeater amplify the radio signals? | When you send some kind of radio signals at some frequency say 5 GHz to a repeater and it transmits it back at higher power, what change is made to the radio signal? For example: Will it be transmitted back at higher frequency or in what terms it is said to be amplified? How do you measure its strength? | When a radio signal at a frequency of 5 GHz is transmitted to a repeater that amplifies it, what specific changes occur to the signal's power and frequency? Additionally, how is the strength of the amplified signal measured in terms of decibels (dB)? | 35,615 | cqadupstack-physics |
29281 | Exist some relationship between irradiance units and wavelenght of the incident sunlight? | Exist some relationship between irradiance units and wavelength of the incident sunlight? What about irradiance? I want to establish a relationship between wavelength and irradiance, because I would try to model photosynthesis on Vensim. | How does the irradiance of sunlight vary across different wavelengths, and what is the quantitative relationship between specific wavelength ranges and their corresponding irradiance values? | 31,274 | cqadupstack-physics |
98066 | Experimental proof for Conservation of Total Energy | > In physics, the law conservation of energy states that the total energy of > an **isolated system** cannot change—it is said to be conserved over time. > (Source : wikipedia(conservation of energy) And > Truly isolated systems cannot exist in nature, other than **allegedly** the > universe itself, and they are thus h... | How can we design a practical experiment to demonstrate the conservation of energy in a truly isolated system, accounting for all forms of energy transfer, and what would be a measurable outcome that confirms or challenges the conservation of energy? | 4,118 | cqadupstack-physics |
119534 | What to work on? | (I am not exactly sure if this question is supposed to go here, but it seems like the appropriate place for it.) Hello Scientific Community, I am soon graduating from high school and, as everyone does, I am searching for something to study at college. I have always wanted to study straight forward physics, but recently... | What are the most common career paths and job opportunities for someone with a degree in physics, particularly in countries with limited investment in scientific research? | 34,941 | cqadupstack-physics |
101976 | Is TRUE invisibility possible? | First off, this question may seem highly stupid at first, but it arose from past cartoon memories. In the cartoon Tom & Jerry from the 1940s, there was already hypothesizing over cream that, when applied, makes you invisible. I have looked into invisibility, and I see that nothing surprising has been done. For example,... | Is it scientifically possible to create a material or mechanism that allows a cloak to render a person completely invisible in visible light without any additional technology, means, or external factors, such that they cannot be detected or sensed by the human eye? | 18,262 | cqadupstack-physics |
100498 | What is the "discrete" analogue to "continuum" mechanics? | If I wanted to explore a discrete mathematics approach to continuum mechanics, what textbooks should I look into? I suppose a ready answer to the question might be: "computational continuum mechanics", but usually textbooks that discuss such a subject are usually focused upon applying numerical analysis to continuous t... | What are the distinguishing features and key mathematical methods of discrete mathematics approaches in continuum mechanics compared to traditional continuous theories? | 15,350 | cqadupstack-physics |
40975 | growth condition for the potential | what growth conditions should the potential inside the Hamiltonian $ H=p^{2}+ V(x) $ has in order to get ALWAYS a discrete spectrum ?? for example how can we know for teh cases $ |x|^{a} $ , $ exp(b|x|) $ and so on with real parameters a,b and c how can we prove that for theses potentials we will have only a discrete s... | What conditions must the potential \( V(x) \) in the Hamiltonian \( H = p^{2} + V(x) \) satisfy to ensure a discrete spectrum of eigenvalues? Specifically, for the potentials \( V(x) = |x|^a \) and \( V(x) = e^{b|x|} \) with real parameters \( a \) and \( b \), how can we prove that they only yield a discrete set of ei... | 10,289 | cqadupstack-physics |
22825 | Does a negative line tension in a 3-fluid immiscible interface make sense? | This question is inspired by this question/answer pair: Is this formula for the energy of a configuration of 3 fluids physically reasonable? Consider three immiscible fluids forming contact surfaces, where none of the three can make a lubrication layer for the other two (the surface energy between fluid 1 and fluid 2 i... | Can you explain the conditions under which a three-phase fluid system can exhibit negative line tension, and whether experimental evidence supports the concept that such systems renormalize to zero line tension at long wavelengths? | 19,571 | cqadupstack-physics |
56059 | Is the crystallization of a heat pack the same as the crystallization of freezing water? | Is the crystallization of a heat pack the same as the crystallization of freezing water? If so, why does one generate heat where the other does not? | Compare the crystallization process of a heat pack to that of freezing water. Specifically, what is the enthalpy change associated with the crystallization of a heat pack, and how does it differ from the enthalpy change during the freezing of water? | 352 | cqadupstack-physics |
47440 | Thermal imaging camera | I'm making a tea bag experiment (Make a hollow cylindrical tube from light paper e.g. from an empty tea bag. When the top end of the cylinder is lit, it takes off.) I need to know how hot air is moving so please help me if you have thermal imaging camera or with out it write your opinion | What is the maximum height a hollow cylindrical tea bag can reach when the top end is ignited, and how does the temperature difference between the hot air inside and the cooler ambient air affect the ascent? | 20,884 | cqadupstack-physics |
2000 | Can a multipass x-ray absorption cell be constructed? | I've been trying to understand the various concepts behind x-ray optics compared to standard visible/IR optics like mirrors and such. However, the x-ray mirrors I've been finding typically have optimal angles of reflectance on the order of a few milliradians, particularly for hard x-rays. I think I mostly understand th... | Can a multipass absorption cell for x-rays be constructed using fiber optics, and if so, what are the key design considerations to ensure efficient x-ray transmission and minimal absorption? | 8,808 | cqadupstack-physics |
78353 | What are anticipated events and experimental uncertainty? | I am working on this lab that involved gathering data from two different sources. It involved gathering reaction times from a device and from a web application which was put into our data sets. It is asking about anticipated events and if they agreed with the experimental uncertainty. After Googling these terms, I stil... | How can I calculate the average reaction time from data collected from two different sources and determine if the observed results fall within the range of experimental uncertainty? | 21,370 | cqadupstack-physics |
37743 | 273 + degree Celsius = Kelvin. Why 273? | Temperature conversion: 273 + degree Celsius = Kelvin Actually why is that 273? How does one come up with this? My teacher mentioned Gann's law (not sure if this is the one) but I couldn't find anything related to this, which law should it be? | What is the relationship between Celsius and Kelvin in terms of their temperature scales, and why is the zero point of the Kelvin scale defined as 273.15 degrees Celsius? | 20,679 | cqadupstack-physics |
51368 | Dirac Equation in General Relativity | Dirac equation for the massless fermions in curved spase time is $γ^ae^μ_aD_μΨ=0$, where $e^μ_a$ are the tetrads. I have to show that Dirac spinors obey the following equation: $$(−D_μD^μ+\frac{1}{4}R)Ψ=0\qquad(1)$$ where $R$ is the Ricci scalar. I already know that $[D_\mu,D_\nu]A^\rho={{R_{\mu\nu}}^\rho}_\sigma A^\si... | What is the expression for the commutator of the covariant derivatives acting on a Dirac spinor, $[D_\mu, D_\nu] \Psi$, in terms of the curvature components, and how does it relate to the equation $(-D_\mu D^\mu + \frac{1}{4} R) \Psi = 0$? | 34,282 | cqadupstack-physics |
105498 | Toy helicopters at very high altitude | Because air is thin up there, toy helicopters usually have a maximum operational altitude. If the toy brought up by a real aircraft, higher than its intended maximum altitude, would it be able to hover inside the real aircraft? If so, what would happen if the real aircraft would now vanish magically, so that the toy he... | If a toy helicopter has a maximum operational altitude of 1,000 meters and is brought to an altitude of 2,000 meters inside a real aircraft, what would happen if the aircraft suddenly vanished? Would the helicopter be able to hover, and if not, at what altitude would it lose its ability to maintain lift? | 23,341 | cqadupstack-physics |
109894 | Modified gravity and dark energy | Is there a model of _modified gravity_ that either was close to or have succeeded in modelling the problem of dark energy, and suffers from no instabilities and explains the observational data. Also has the approach to use modified gravity to explain the problem of dark energy been abandoned? Needed to write a review a... | What specific models of modified gravity have been proposed to address dark energy, and how do they compare in terms of stability and alignment with observational data? Please provide quantitative examples where applicable. | 21,072 | cqadupstack-physics |
77467 | Expectation Values in Quantum Mechanics | Why is the expectation value what it is? Why don't you apply the operator, then multiply that by it's conjugate? | In quantum mechanics, if you apply an operator \( \hat{A} \) to a state \( |\psi\rangle \) and then multiply the result by the complex conjugate of the state \( \langle \psi| \), how can you express the expectation value \( \langle A \rangle \) in terms of these operations? What is the formula for calculating \( \langl... | 23,834 | cqadupstack-physics |
119458 | How do I create a direct current with a magnet? The magnet is not to be moved in the direction of the wire | How do I create a direct current with a magnet? The magnet is not to be moved in the direction of the wire. In fact I'm looking for the contrary to the drawing from https://commons.wikimedia.org/wiki/File:RechteHand.png  Update 1st: The background is that in the Germ... | What is the relationship between the movement of free electrons and the magnetic field they generate in a straight wire carrying direct current, and how does this compare to the electromagnetic induction principles? | 37,511 | cqadupstack-physics |
52139 | Relation between Casimir and hydrophobic effects | ## Background Some years ago I was studying "Fundamentos de biología" (Biology fundamentals) and learned how the lipids create a bilayer due to the water repulsion. Some time later I learned that this was called the hydrophobic effect and occurred in some other cellular materials. But something else was in my mind back... | How does the Casimir effect relate to the formation of lipid bilayers, and can you explain the underlying principles that govern the entropy changes in both phenomena? Additionally, if a liquid were affecting the Casimir effect, how would the dynamics compare to those of a lipid bilayer, particularly in terms of shape ... | 29,268 | cqadupstack-physics |
47497 | Physical meaning of Laplace-Beltrami eigenfunctions? | The eigenfunctions of Laplace-Beltrami operator are often used as the basis of functions defined on some manifolds. It seems that there is some kind of connection between eigen analysis of Laplace-Beltrami operator and the natural vibration analysis of objects. I wonder, is my intuition true? What is the physical meani... | What is the physical significance of the eigenfunctions of the Laplace-Beltrami operator in the context of natural vibration analysis, and how do their orthogonality and real nature contribute to their application as a basis for functions on a manifold? | 6,926 | cqadupstack-physics |
106892 | How to deal with non fixed springs? | In every physics book they explain how to compute the force that a spring exert if one of its ends is fixed to a wall (or equivalent) and the other end is compressed or stretched. But how to deal with the problem if both ends are 'free'. For example, suppose you have two blocks attached with a spring, and someone apply... | If a horizontal force of 50 N is applied to two blocks connected by a spring with a spring constant of 300 N/m, how would you calculate the acceleration of each block, assuming there is no friction and the system starts from rest? | 9,171 | cqadupstack-physics |
3653 | Linear sigma models for F-Theory | Is it possible to describe the Local F-theory models which seems to be relevant for particle physics phenomenology using Gauged Linear Sigma Models (GLSM), this includes describing the decoupling limit using the parameters of the GLSM? Actually, I am not sure if my question makes sense. Another related question is: Can... | What are the necessary conditions for a Gauged Linear Sigma Model (GLSM) to accurately describe the IR limit of elliptically fibered Calabi-Yau 4-folds and produce the Weierstrass form of the elliptic fiber in the context of F-Theory compactifications? | 6,566 | cqadupstack-physics |
86976 | What is the use of this formula 1 Tesla = 1 Newton/Ampere/Meter? | What does Newtons/Ampere/Meter stand for? From this formula: 1 Tesla = 1 Newtons/Ampere/meter what can it be used for? To do what? Ampere/meter Is the same unit as a field's intensity H? Or what is it? | What is the relationship between the units of Tesla and the intensity of a magnetic field, specifically in terms of Newtons, Amperes, and meters, and how can this relationship be applied in practical scenarios? | 12,659 | cqadupstack-physics |
107665 | Does the cell phone make dipole particles in our body dance the same way Microwave oven does in food | I am a non-physics person trying to wrap my head around some EM radiation facts. Please help me with this. I have read about Dielectric Heating on Wikipedia. And watched quite a few videos on internet on how Microwave oven works. It all boils down to the following facts. Please correct me if I am wrong on any of them. ... | If microwaves at 2.4 GHz cause water molecules to vibrate rapidly and heat them up, why do UHF frequencies used by cell phones, such as 800 MHz and 400 MHz, not result in similar heating effects in water molecules in our bodies? What is the difference in energy absorption between these frequencies? | 36,032 | cqadupstack-physics |
93223 | Show that the electric field, $\mathbf{\vec{E}}$ cannot be generated by any static distribution of charges | > Show that the electric field $$\mathbf{\vec{E}}=\begin{pmatrix}0 \\\ E_0x > \\\ 0\end{pmatrix}$$ where $E_0$ is a constant, cannot be generated by any > static distribution of charges. I understand that an electrostatic field is irrotational and the divergence of the electric field is the charge density over epsilon ... | Why does the electric field \(\mathbf{\vec{E}}=\begin{pmatrix}0 \\ E_0 x \\ 0\end{pmatrix}\) violate the conditions for being generated by a static charge distribution? Can you show how to apply the divergence condition, \(\nabla \cdot \mathbf{E} = \frac{\rho}{\epsilon_0}\), to prove that this electric field cannot cor... | 33,669 | cqadupstack-physics |
108129 | One question about Weinberg's derivation of arbitrary spin fields expressions | In his book "QFT" (vol. 1) Weinberg writes the expression for an arbitrary spin massive field: $$ \hat {\Psi}_{a}(x) = \sum_{\sigma = -s}^{s} \int \frac{d^{3}\mathbf p}{\sqrt{(2 \pi)^{3}2 \epsilon_{\mathbf p}}}\left( k_{1}F_{a}(\mathbf p) \hat {a}^{\sigma}(\mathbf p )e^{-ipx} + k_{2}G_{a}(\mathbf p) {\hat {b}^{\sigma}}... | Given the expression for an arbitrary spin massive field $$ \hat {\Psi}_{a}(x) = \sum_{\sigma = -s}^{s} \int \frac{d^{3}\mathbf p}{\sqrt{(2 \pi)^{3}2 \epsilon_{\mathbf p}}}\left( k_{1}F_{a}(\mathbf p) \hat {a}^{\sigma}(\mathbf p )e^{-ipx} + k_{2}G_{a}(\mathbf p) {\hat {b}^{\sigma}}^{\dagger}(\mathbf p )e^{ipx} \right) ... | 14,907 | cqadupstack-physics |
110655 | How to compute the propogator for Chern-Simons on a torus? | I'm looking to better understand Chern-Simons theory on a torus. We are given the action $$ S(\phi) = \int_E (\partial \phi)(\overline\partial \phi) + \frac{\lambda}{6}(\partial \phi)^3 $$ which yields (apparently) the propagator $$ P(z) = \frac{1}{4\pi}\wp(z;\tau) - \frac{1}{12}E_2(\tau) $$ (at least for $z \neq 0$) W... | Given the action $$ S(\phi) = \int_E (\partial \phi)(\overline\partial \phi) + \frac{\lambda}{6}(\partial \phi)^3 $$ in Chern-Simons theory on a torus, can you compute the equation of motion derived from the action, and explain the physical significance of the interaction term $(\partial \phi)^3$ in relation to the hol... | 12,623 | cqadupstack-physics |
19165 | Relating milliampere-hours to watt-hours for batteries | I've seen many batteries that are measured in milliampere hours (mAh), while others are measured in watt hours (wh). How can I convert them between each other so that I can actually compare them? It's often the case that computers or smartphones use one or the other and would be great to have a metric. | If a smartphone has a battery rated at 3000 mAh and operates at a voltage of 3.7V, what is the equivalent energy capacity of the battery in watt hours (Wh)? | 26,140 | cqadupstack-physics |
114666 | Boltzmann constant--why units aren't joules per degree PER MOLECULE | I didn't think the information I looked at did a very good job of explaining the Boltzmann Constant, kB, but once I did this next simple calculation, it seemed clear that what I didn't understand were the units. So I simplified the units only: 8.31... = R = energy per degree/mole NA = molecules/mole, Avogadro's constan... | What are the units of the Boltzmann constant \( k_B \), and how do they relate to the ideal gas constant \( R \)? Additionally, can you explain why \( k_B \) is expressed in joules per degree per molecule, while \( R \) is given in joules per degree per mole? | 12,966 | cqadupstack-physics |
18709 | Do a 1-dimensional conformal theory exist? | can we have in physic or can we speak about 1-d conformal theory in physics ?? for example in this one dimensional theory what would be the generators $ x \partial _{x} $ or $ \partial _{x} $ ?? | What are the conformal generators in a one-dimensional conformal field theory, and how do they relate to the scale transformation represented by \( x \partial_{x} \) and the translation \( \partial_{x} \)? | 7,519 | cqadupstack-physics |
55274 | Explanation on the resulting forces of two positive point charges | Why will the resulting force lines of two positive point charges be like this:  I would expect this:  | Calculate the net electric field at a point located midway between two positive point charges, each of magnitude +Q, separated by a distance d. What is the direction of the electric field at that point, and how does it compare to the electric fields produced by each individual charge? | 3,255 | cqadupstack-physics |
51957 | Spectral luminous efficiency as a function of wavelength | I've come across plenty of figures demonstrating the spectral luminous efficiency as a function of wavelength (meaning the humans eye's sensitivity to different wavelengths) http://www.yorku.ca/eye/photopik.htm but I've been unable to find the equation which gives this function so I can plot it myself. | What is the equation that represents the spectral luminous efficiency of the human eye as a function of wavelength, and how can I use it to plot the sensitivity of human vision across different wavelengths? | 10,159 | cqadupstack-physics |
45907 | Is the mathematical truth 1+1=2 analogous to the conservation of energy? | They seem to express the same concept in different fields. | In what ways do the concepts of energy conservation in physics and the conservation of mass in chemistry illustrate similar principles in their respective fields? Provide a quantitative example for each. | 5,407 | cqadupstack-physics |
100617 | how is feshbach resonance potential term physically produced? | In Feshbach resonance model, a 2*2 potential term with space dependent diagonal and non-diagonal terms is written $\left(\begin{array}{cc} V_{11}(\mathbf{r}) & V_{12}(\mathbf{r})\\\ V_{21}(\mathbf{r}) & V_{22}(\mathbf{r}) \end{array}\right)$ How are these terms physically produced ? An example of Feshbach resonance tha... | How can the spatially dependent diagonal and non-diagonal terms \( V_{11}(\mathbf{r}) \), \( V_{12}(\mathbf{r}) \), \( V_{21}(\mathbf{r}) \), and \( V_{22}(\mathbf{r}) \) in the Feshbach resonance model be derived from fundamental interactions, and what specific physical processes contribute to their formation in a two... | 21,282 | cqadupstack-physics |
102080 | Metallic and glass sphere of same size released at a height | This question was in my exam today : A metallic and glass sphere of same size were dropped at same height. Which sphere would hit the ground first and why? I have thought about several things and cannot come to conclusion. An emf will be developed as electrons will move due to earth's magnetic field. So we have a charg... | If a metallic sphere and a glass sphere of the same size are dropped from the same height in a vacuum, which sphere will reach the ground first, and what principles of physics explain the outcome? | 22,076 | cqadupstack-physics |
107501 | What's the difference between Raman scattering and second harmonic generation in crystal? | As far as I know, the Raman scattering is from the stokes and anti-stokes scattering that the laser light interact with molecular vibrations. So we know that ""laser light interact with molecular vibrations"" in crystal can cause Raman scattering. As for second harmonic generation, the pump waves with frequency w go t... | What is the physical mechanism that leads to second harmonic generation in a crystal, and how does it differ from the process of Raman scattering? | 2,368 | cqadupstack-physics |
54958 | Can infrared light be sent to long distance? | I need to send infrared light from its emitter to a distance of about 10-12 feet. Is this possible? | What is the maximum distance infrared light can travel in a vacuum before its intensity decreases to 1% of its original value, given an initial power of 1 milliwatt? | 688 | cqadupstack-physics |
3863 | What happens to an astronaut who's floating in a spaceship (in space) when it begins to move? | I feel this is somehow a stupid question, but I don't know the true answer. What happens to an astronaut who's floating in a spaceship in space when it begins to move? Will the astronaut not move until he smashes onto a wall in the spaceship? Or will he move with the spaceship due to gravity? Or does it depend on the s... | If an astronaut is floating in a spaceship that suddenly accelerates forward at a constant rate of 2 m/s², what will happen to the astronaut in terms of their motion relative to the spaceship? Will they remain stationary in relation to the spaceship, or will they start moving towards the back wall? Provide a detailed e... | 30,651 | cqadupstack-physics |
86012 | Reversed freezing point | Helium-3's Phase diagram shows that at the right temperature and pressure combination, the solid region dips _downward_ as temperature increases. That means that you can _heat it up and it will freeze_. What is going on? | At what specific temperature and pressure combination does Helium-3 transition from a liquid phase to a solid phase, given that its phase diagram shows that the solid region dips downward with increasing temperature? | 22,470 | cqadupstack-physics |
89338 | Differential equation for speed relate to the homogeneous cosmological background in FLRW metric | How to derive DE for the speed (which relate to the homogeneous cosmological background) of the observer which moves with constant proper acceleration in spatially flat FLRW universe? | What is the differential equation that describes the speed of an observer moving with constant proper acceleration in a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe, and how does it relate to the homogeneous cosmological background? | 29,526 | cqadupstack-physics |
79554 | Simulating the Interference Pattern of Fraunhofer Diffraction by a Single Slit | I'm attempting to simulate the Fraunhofer diffraction pattern due to a single slit. We know that the intensity at an angle $\theta$ is $I(\theta)=I_0 \text{sinc} ^2(\beta)$ where $\beta=k\,b\,\sin(\theta) / 2.$ Considering the setup shown in the image where O is the origin (the precise center of the single slit) and p ... | What is the correct expression for the intensity \( I(\theta) \) in terms of the slit width \( b \), the distance \( z \) from the slit to the screen, and the coordinates \( x \) and \( y \) where the intensity is being measured on the screen, such that the output accurately reflects the expected diffraction pattern fo... | 23,746 | cqadupstack-physics |
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