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18472 | How did they measure the speed of light observing Jupiter's moons, centuries ago? | I am interested in the pratical method and I like to discover if it is cheap enough to be done as an experiment in a high school. Thank you. | What are some practical and cost-effective experiments that high school students can perform to explore fundamental concepts in chemistry, biology, or physics? Please provide examples with estimated costs and required materials. | 5,220 | cqadupstack-physics |
128814 | Does Free Will Theorem imply that quantum mechanics plays crucial role in our brain’s functioning (consciousness)? | Is opposite statement of Free Will Theorem right: If elementary particles have a certain amount of free will, then so must we? Because to me elementary particles does have a bit of free will – quantum mechanics guarantees that nobody can predict what one is going to do, say in double slit experiment. So Penrose was rig... | What is the relationship between free will as described in quantum mechanics and human consciousness, and can you provide a quantitative example or evidence supporting the influence of quantum effects on decision-making processes? | 13,659 | cqadupstack-physics |
63224 | Holomorphic coupling as a source for gaugino condensation | On the top of page 23 of hep-th/03061119, it is pointed out that treating the holomorphic gauge coupling $\tau$ as a background (spurion) superfield allows one to think of its $F$-term, $F_\tau$ as the source for the gaugino condensate, $\lambda^a\lambda^a$. This is because in the supersymmetric gauge kinetic term, $\m... | Why is the holomorphic gauge coupling $\tau$ not considered a source for the gluon condensate, while the $F$-term $F_\tau$ is considered as a source for the gaugino condensate $\lambda^a\lambda^a$ in supersymmetric gauge theory? | 37,842 | cqadupstack-physics |
133677 | Ward identities and operator product expansions | Polchinski's (2.3.11) gives the Ward Identity $$i\epsilon[Res_{z\rightarrow z_0}j(z)\mathcal A(z_0,\bar z_0)+\bar {Res}_{\bar z\rightarrow \bar z_0}\tilde j(\bar z)\mathcal A(z_0,\bar z_0)]=\delta\mathcal A(z_0,\bar z_0).$$ Then he says on p46 that by this identity, $T(z)\mathcal A(0,0)$ contains the terms $$\frac{h}{z... | In the context of conformal field theory, given the Ward Identity from Polchinski, can you explain how the terms \(\frac{h}{z^2}\mathcal A(0,0)\) and \(\frac{1}{z}\partial \mathcal A(0,0)\) are derived? Specifically, how do these terms arise from the identity and why does the first term not include the conformal dimens... | 16,964 | cqadupstack-physics |
29387 | What are the increasingly sophisticated ways to perform a Lorentz transformation? | Since Einstein first derived the Lorentz transformations, their generalisation and execution has changed over the century. So starting with those first derived by Einstein: What are the main, increasingly sophisticated ways, of carrying out a Lorentz transformation today? What are their advantages/disadvantages over th... | What are the mathematical expressions used to perform Lorentz transformations, and how do the advantages and disadvantages of at least three modern approaches compare to the original transformations derived by Einstein? | 21,399 | cqadupstack-physics |
64882 | Penetration of light in the atmosphere | While I was considering an answer to this question, I wondered how much light that enters the atmosphere reaches the ground without colliding with air molecules—if any. I've taken a good bit of physics course (not optics, yet), but I'm not really sure how light interacts with gases or other transparent material. I've a... | What fraction of light that enters the Earth's atmosphere reaches the ground without colliding with air molecules, and how does this fraction change with different wavelengths of light? | 16,060 | cqadupstack-physics |
82287 | A question on Bernoulli's principle | Which is more appropriate regarding Bernoulli's principle * fast moving air causes low pressure or * lower pressure causes fast moving air. | According to Bernoulli's principle, what is the relationship between the speed of a fluid and the pressure exerted by that fluid? | 27,643 | cqadupstack-physics |
135188 | Sound pressure and velocity pressure concepts vs loudness | Here is my understanding and question: Atmospheric pressure is around 101325 Pa. Our ear drum is exposed to this pressure from both sides so total pressure is zero. But when we hear sound, ear drum starts vibrating and the effective pressure on ear drum is calculated for many scenarios. For example a jet engine produce... | How can the intensity of sound, measured in pressure (Pa), cause pain at a lower threshold (63.2 Pa) compared to a steady wind pressure of 140 Pa, which does not induce the same pain despite being greater in magnitude? What is the underlying physical principle that explains this difference in perception? | 13,425 | cqadupstack-physics |
105726 | The Lagrangian as a metric | My question is, _can the (classical) Lagrangian be thought of as a metric_? That is, is there a meaningful sense in which we can think of the _least- action_ path from the initial to the final configuration as being the _shortest_ one? Then the equations of motion would be geodesics - not in physical space, but in phas... | Is there a specific example of a classical mechanical system in which the Lagrangian can be interpreted as a metric, such that the least-action path corresponds to the shortest path in a generalized phase space? If so, calculate the length of the least-action path using the Lagrangian for a simple harmonic oscillator. | 18,804 | cqadupstack-physics |
119772 | Illustrate the transition from linear to rotational kinetic energy | I wish to find an everyday situation that illustrates the following: A rod is moving in a direction perpendicular to its axis. One end "gets caught" and the rod starts rotating around this end. THE SPEED OF THE OTHER END OF THE ROD WILL THEN INCREASE BY A FACTOR SQRT(3). Here is an illustration. Can someone please thin... | In a scenario where a horizontal rod of length \( L \) is moving with a speed \( v \) perpendicular to its axis, one end of the rod suddenly gets caught, causing it to rotate around that end. Given that the speed of the other end of the rod will increase by a factor of \( \sqrt{3} \), what will be the final speed of th... | 29,361 | cqadupstack-physics |
21739 | A quanta of time | A question of Quantum Time: Does a minimum interval of time cause wave-like behavior? If we think about the uncertainty principle, could it derive from a quanta of time? Does plank’s constant somehow derive from the quanta of time? Hypothesis: When delta t is larger than a quanta of time, then particles behave like par... | If time has a minimum value, or a quantum of time, how does this influence the measurement of speed for objects traveling faster than this minimum time interval? Can you derive an expression that relates this quantum of time to the limit of the speed of light? | 31,501 | cqadupstack-physics |
108087 | Expectation value of Hamiltonian on number state | Hamiltonian is defined by $H_I = \hbar \omega (\hat{a}^+ \hat{a} + 1/2)$ What is the expectation value of the energy on the number state $$\vert \psi \rangle = \frac{1}{\sqrt{2}} ( \vert 1 \rangle + \vert 2 \rangle )$$ So I think that its $$\langle E \rangle = \langle \psi \vert H_I \vert \psi \rangle$$ $$ = \hbar \ome... | Given the Hamiltonian defined by \( H_I = \hbar \omega (\hat{a}^+ \hat{a} + 1/2) \) and the number state \( \vert \psi \rangle = \frac{1}{\sqrt{2}} ( \vert 1 \rangle + \vert 2 \rangle ) \), calculate the expectation value of the energy \( \langle E \rangle \). What is the final expression for \( \langle E \rangle \) in... | 17,467 | cqadupstack-physics |
97706 | Idea of integrable systems | I do not quite understand the idea an integrable dynamical system. Does it mean that the EOMs are analytically and exactly solvable? What are the necessary and sufficient conditions such that a system is integrable? | What are the necessary and sufficient conditions for a dynamical system described by ordinary differential equations (ODEs) to be classified as integrable, and can you provide an example of such a system? | 14,259 | cqadupstack-physics |
112081 | Significant error conversion | So here is my question: Say we have measured something to be 15,67 mm and the significant error is $\pm 0,01$mm. then we convert the measurement to meter to be 0,01567m would the significant error then be $\pm 0.00001$m? | If a measurement is recorded as 15.67 mm with a significant error of ±0.01 mm, what would be the correct conversion of the measurement to meters, and what would be the associated significant error in meters? | 20,258 | cqadupstack-physics |
60280 | Math of eigenvalue problem in quantum mechanics | I learned the eigenvalue problem in linear algebra before and I just find that the quantum mechanics happen to associate the Schrodinger equation with the eigenvalue problem. In linear algebra, we always gives the matrix of certain size (e.g. 4x4) so to find the eigenvalues is identical to solve the related secular equ... | Consider the Hermitian operator \( A \) with eigenstates \( |a\rangle \) and \( |b\rangle \) corresponding to eigenvalues \( a \) and \( b \) respectively. Given the Hamilton operator defined as \( H = |a\rangle\langle b| + |b\rangle\langle a| \), what are the eigenvalues of the Hamiltonian \( H \)? | 29,976 | cqadupstack-physics |
77824 | Global Chern-Simons forms and topological gauge theories | I am reading the classic Dijkgraaf and Witten paper on topological gauge theories and something struck me that I didn't understand. For a trivial bundle $E$ on smooth 3-manifold $M$ with compact gauge group $G$ the Chern- Simons form is of course $$S(A)=\frac{k}{8\pi^2}\int Tr (A\wedge dA + \frac{2}{3}A\wedge A\wedge A... | In the context of topological gauge theories and Chern-Simons actions, can you explain under what conditions a Lie algebra valued 1-form can represent a connection on a non-trivial principal bundle over a smooth 3-manifold? Specifically, what implications does the non-triviality of the bundle have on the global represe... | 31,296 | cqadupstack-physics |
130963 | If MOND theory doesn't explain gravitational lensing, couldn't well placed black holes explain it? | When I first heard about star velocity as a function of its distance from the center of the galaxy and the difference between prediction and observation I immediately thought "there must be a threshold where gravity reacts differently". Then I discovered MOND theory and that it can't explain gravitational lensing. But ... | What is the specific mass range of black holes that would need to exist to account for the observed effects typically attributed to dark matter, considering their gravitational influence on stellar motion in galaxies? | 5,514 | cqadupstack-physics |
28957 | $\hbar$, the angular momentum and the action | Is there anything interesting to say about the fact that $\hbar$, the angular momentum and the action have the same units or is it a pure coincidence? | What is the relationship between the units of reduced Planck's constant $\hbar$, angular momentum, and action, and how does this relationship reflect the principles of quantum mechanics? Calculate the units for each and discuss any physical significance of their similarity. | 32,803 | cqadupstack-physics |
30524 | Does a interstellar spacecraft traveling at relativistic velocity require continous thrust to maintain velocity? | Assuming completely empty space, does a spacecraft traveling at 0.5 C require continuous thrust to avoid deceleration? If the spacecraft is traveling at 0.5 C, does it's relativistic mass act upon objects that it passes by? It is understood that space is not truly empty. The question was designed to consider only relat... | If a spacecraft is traveling at 0.5 C in a vacuum, what kind of thrust is required to maintain its velocity, and does its relativistic mass have any effect on nearby objects as it passes by? Additionally, when the spacecraft begins to decelerate, should the thrust applied be based on its relativistic mass or its rest m... | 1,730 | cqadupstack-physics |
68002 | Confusion with derivation of fictitious forces | I read the mathematical derivation of fictitious forces on Wikipedia and I'm having trouble understanding it. I went on a few other sites looking for a better derivation, but they're all basically the same. I've also seen this answer already. What I'm having trouble understanding is that, in every derivation, at some p... | In the context of non-inertial frames of reference, can you explain why the position vector in the accelerating frame is represented as $\vec{x}_A = \sum_{i=1}^3 \hat{u}_i x_i$ instead of simply $(x_1, x_2, x_3)$, and how this relates to the transformation between the inertial frame and the non-inertial frame? | 1,617 | cqadupstack-physics |
70916 | What is electron density and its unit in plasma physics? | I can not find (through Google search engine) that what means electron density and what is its unit? and what is there relation between electron density and electrical conductivity? | What is the definition of electron density, what are its units, and how is it quantitatively related to electrical conductivity in materials? | 20,388 | cqadupstack-physics |
89076 | Causes of hexagonal shape of Saturn's jet stream | NASA has just shown a more detailed picture of the hexagonal vortex/storm on Saturn: > http://www.ibtimes.com/nasa-releases-images-saturns-hexagon-mega-storm-may- > have-been-swirling-centuries-1496218 > https://en.wikipedia.org/wiki/Saturn_hexagon Is that theoretically understood what is the cause behind this eye-c... | What are the physical principles and mathematical equations that explain the formation of Saturn's hexagonal vortex, particularly in terms of nonlinear dynamics such as Rossby waves and the Coriolis effect? | 27,412 | cqadupstack-physics |
25903 | Will Neptune change Pluto's orbit some day? | My seven-year-old son loves astronomy-- so much so that we read space books before bed time. One of our books talks about Pluto's orbit crossing over Neptune's orbit and will be (or is) closer to the Sun than Neptune for a period of years. I'm assuming the orbits actually never have a collision path, but what will happ... | What is the minimum distance between Pluto and Neptune at their closest approach, and how would the gravitational influence of Neptune affect Pluto's orbit during that time? | 17,972 | cqadupstack-physics |
110213 | How to define the dilatation rate (strain) between two particles knowing the instantaneous positions and velocities? | Provided that I have the instantaneous positions and velocities of two particles that can separate in 3D with respect to time. I would like to characterize the change in length. What could be a good formulation for that? Thanks. | Given the instantaneous positions \(\mathbf{r_1}(t)\) and \(\mathbf{r_2}(t)\) of two particles in 3D space at time \(t\), how would you calculate the rate of change of the distance \(d(t)\) between the two particles over time? Provide the mathematical formulation for \(d(t)\) and its derivative with respect to time. | 5,470 | cqadupstack-physics |
15282 | Quantum entanglement faster than speed of light? | recently i was watching a video on quantum computing where the narrators describes that quantum entanglement information travels faster than light! Is it really possible for anything to move faster than light? Or are the narrators just wrong? Regards, | Is there any scientific evidence or principle that supports the idea that information can travel faster than the speed of light, specifically in the context of quantum entanglement? | 7,780 | cqadupstack-physics |
54130 | Technical detail in the solution of the hydrogen atom | I'm trying to do an exercise in which you solve the Schrödinger equation for the hydrogen atom. Through the exercise, I've already shown that the wavefunction is: $$ \psi_{n\ell m}(r,\theta,\varphi) = R_{n\ell}(r)Y^m_\ell (\theta,\varphi)$$ and that $Y^m_\ell (\theta,\varphi)$ are the spherical harmonics. Then, when so... | Using Stokes's theorem, show that \( \nabla^2 \left ( \frac{1}{r} \right ) = -4\pi\delta({\mathbf r}) \) applies in the context of the Schrödinger equation for the hydrogen atom, and demonstrate that this result leads to the conclusion that \( \lambda = \ell + 1 \) for \( \ell = 0 \). What implications does this have o... | 30,743 | cqadupstack-physics |
70440 | How to derive Eq. (1.2.17) in Polchinski? | I have a super stupid question about deriving Eq. (1.2.17) in Polchinski's string theory, vol 1. The book seems to derive from $$\tag{1.2.16} h_{ab}=\frac{1}{2} \gamma_{ab} \gamma^{cd} h_{cd} $$ to $$\tag{1.2.17} h_{ab} (-h)^{-1/2} = \gamma_{ab} (-\gamma)^{-1/2}.$$ I cannot figure out how to manipulate the determinants... | What are the values of the determinants \(\det(\gamma)\) and \(\det(h)\) in terms of their matrix elements for the following 2x2 symmetric matrices, given \( \gamma = \begin{pmatrix} a & b \\ b & c \end{pmatrix} \) and \( h = \begin{pmatrix} d & e \\ e & f \end{pmatrix} \)? | 35,198 | cqadupstack-physics |
95398 | Calculating angle of projectile motion | I realize that similar questions have been asked but I'd like to put my problem here and what I have to make sure everything is right. So my problem is: I've got a rock $2.5$ meters above the ground and I shoot it out of a slingshot. It hits another rock which is $3.6$ meters above the ground, $14$ meters away from it'... | Given a rock shot from a height of 2.5 meters at an initial velocity of 15.9 m/s, which hits another rock located 3.6 meters above the ground and 14 meters horizontally away from the launch point, calculate the angle θ at which the rock was launched. Assume the acceleration due to gravity g is approximately 9.81 m/s². | 13,640 | cqadupstack-physics |
67199 | why no derivation in scientific papers? | Is there a reason why scientific papers rarely show derivation of an equation? I fail to see the any advantages ( besides savings latex hours for the authors) of this approach which is the standard way. Do editors get to see full derivations when one send papers to them? Why so secretive? Do these scientists live in th... | What are the benefits of presenting final equations without full derivations in scientific papers, and how does this practice impact the reproducibility of research findings? | 10,363 | cqadupstack-physics |
101207 | Modeling a 2-dimensional mass spring system | First of all, I am unfortunately not an expert in physics, so please be indulge with me. I am trying to model a $2$-dimensional mass-spring system with $1$ mass and $3$ springs to solve a dynamics problem in frequency domain. I've been looking for a solution for a similar problem but I couldn't find anything useful. Ar... | In a two-dimensional mass-spring system where a mass \( m \) is connected to three identical springs \( k_1 = k_2 = k_3 = k \) that are fixed at their endpoints and arranged at \( 120^\circ \) angles to each other, how will the natural frequency of the mass-spring system change if the spring constant \( k \) is doubled... | 32,653 | cqadupstack-physics |
126443 | Regarding luminosity monitor of particle collider | I am sorry if my question is completely wrong or foolish I would feel glad to be corrected On the wiki page for Bhabha Scattering, it is mentioned that it is used as luminosity monitor in many collision experiments .Can anyone explain how they do it and what is the advantage of higher luminosity in collision experime... | How can Bhabha scattering be used to measure luminosity in particle collision experiments, and what advantages does higher luminosity provide for these experiments in terms of event rate and statistical significance? | 30,982 | cqadupstack-physics |
31930 | What does a non-perturbative theory mean? | I'm a science writer and I'm having difficulty understanding what a non- perturbative approach means. I thought I understood what perturbative meant, but in looking for explanations of non-perturbative, I'm just confused about both concepts. Please explain as simply as possible. | What is the difference between perturbative and non-perturbative approaches in quantum field theory, and can you provide an example of a physical phenomenon where a non-perturbative approach is necessary? | 19,282 | cqadupstack-physics |
7780 | Can a water bubble be frozen into ice bubble? | To maintain the surface tension which formed our original bubble (in order to keep the bubble from breaking), we may change the temperature/pressure of air on both sides of the bubble varyingly, with different rates. Is such a configuration possible? Examples of any such system? | What would be the effect of increasing the temperature and decreasing the pressure on both sides of a soap bubble on its surface tension, and at what specific temperature and pressure conditions could the bubble remain stable without breaking? | 6,976 | cqadupstack-physics |
96211 | Why does a cooling motorcycle make popping noises? | I was sitting in my parked car tonight when a Harley-Davidson motorcycle parked next to me. With my window down, I could hear that as the motorcycle was cooling down, it was giving off some type of “popping noises.” My first guess was that as the metal of the exhaust pipes cooled, thermal contraction occurs such that i... | What physical processes could be responsible for the popping noises emitted by a cooling motorcycle exhaust, and how can the magnitude of the temperature change of the exhaust pipes be quantitatively related to the frequency of the popping sounds? | 4,972 | cqadupstack-physics |
14095 | What is the origin of the Dirac delta term in the dipole electric field? | I am a bit lost how one has deduced the formula for electric field with electric dipole because of some inconsistency between different sources. The Wikipedia article contains a delta function in the formula, but this paper gives only the formula without the delta function, $$\bar{E} = -\nabla \phi = \frac{-p}{4\pi \ep... | What is the correct expression for the electric field generated by an electric dipole, and how does it relate to the potential associated with the dipole? Please explain the significance of the delta function in the context of the electric dipole formula, and how it affects the resulting electric field. | 17,783 | cqadupstack-physics |
118728 | Is this a photograph of an electron-positron annihilation? | With degrees in Mechanical and Electrical engineering but no advanced education in physics, I submit a query based on ellipsometric macro photography of TEMS supplied by FDA/NIH. In one TEM a triangle of RNA/DNA indicates ionic activity. PHOTO 1.  This piqu... | What is the theoretical ratio of electron collisions to photon interactions in the context of electron microscopy, given that electrons convert silver halide over 10,000 times more efficiently than photons? | 24,497 | cqadupstack-physics |
105068 | How does a ducted fan suck in air? | I am thinking of a simple ducted fan with a single set of rotating blades, no stator vanes behind. If the effect of the fan is to raise the pressure of the air, how is there flow into the fan against this pressure gradient? I understand that the rotating fan blades have the effect of increasing the stagnation enthalpy ... | In a simple ducted fan system with a single set of rotating blades that increases the stagnation pressure of the air, how can airflow be achieved into the fan against the established pressure gradient? Explain the role of kinetic energy and the difference in static pressure and velocity across the fan in your answer. | 28,199 | cqadupstack-physics |
80507 | Expression for combining several probabilities in a single discriminator | I'm doing an analysis of some experiment. Details are not very important, but I've stumbled on an expression that I cannot make sense of. I have several simultaneous measurements of a certain parameter $x_i$. I also have precomputed pdfs, $\rho_i(x_i)$, for that parameter to be "bad". Based on these, I'm computing a co... | What does the expression $$P = Q \sum_i \frac{(-\ln Q)^{i-1}}{(i-1)!}$$ represent in the context of combining confidence intervals from multiple measurements, and how can it be interpreted in terms of probabilistic significance in experimental data analysis? | 37,361 | cqadupstack-physics |
50114 | Question regarding the direction of electric field | Suppose that there are negative charges (e.g. electrons) only. There are more negative charges on left than on right. How would electric field be constructed? (So, What would be the direction?) And how would electrons flow (because of electric field)? The text says that it flows from left to right, but I am not getting... | In a system with an unequal distribution of negative charge, where there are more electrons on the left side than on the right side, what is the direction of the electric field, and what would be the resulting flow of electrons in response to that electric field? | 35,875 | cqadupstack-physics |
41943 | Why does the law of increasing entropy, a law arising from statistics of many particles, underpin modern physics? | As far as I interpret it, the law of ever increasing entropy states that "a system will always move towards the most disordered state, never in the other direction". Now, I understand why it would be virtually impossible for a system to decrease it's entropy, just as it is virtually impossible for me to solve a Rubik's... | What is the quantitative probability of a system, such as a gas in a closed container, spontaneously decreasing its entropy in a given time frame, and how does this probability illustrate the concept of entropy in thermodynamics? | 28,842 | cqadupstack-physics |
34075 | Why is the tunnel effect of solid matter not observable in macroscopic objects? | Assume I place a tea cup on a table (say, about a centimetre thick). Quantum mechanics tells us that the wave function for the nuclei and electrons of the cup is not zero below the table (while being terribly small, though). I'm constantly measuring that system. So, with a very small (but not zero) probability, the par... | If a tea cup is placed on a one-centimeter thick table, what is the approximate time it would take for the cup to completely tunnel through the table due to quantum tunneling, given the probabilities associated with the particles in the cup? | 1,508 | cqadupstack-physics |
48075 | When are leap seconds added in various time zones? | I understand that technically when a leap second is added, it is added after midnight UTC, but I'm unclear how the addition is handled in other timezones. For precise reckoning of course (e.g. astronomical ephemeris), it must be added at the same instant (e.g. at 1 AM in France, or 7 PM in in New York), but for many lo... | How does the implementation of a leap second at midnight UTC affect local timekeeping across different time zones, and are there any standard practices on how different regions manage this addition? | 10,022 | cqadupstack-physics |
99929 | What would Planck's Radiation Law look like if there were only two modes of oscillation, $n = 0, 1$? | I have an idea that the law (since it's based on average energy, number of modes per volume, and a constant) would be shortened to something that would eliminate the summation terms in average energy. My problem is I can't seem to make the law both frequency AND temperature dependent if I simplify that way. For referen... | How can you modify Planck's Law to express the dependence on both frequency \( v \) and temperature \( T \) without the summation terms in the average energy expression, while still maintaining the integrity of the law? Provide a mathematical form of your modification and explain the implications of your changes. | 27,894 | cqadupstack-physics |
41174 | Does every wavenumber of IR result in a different kind of vibration? | Does every wavenumber of IR result in a different kind of vibration? If that is true, what if a molecule absorb 2 different wavenumbers (which cause different rocking and symmetrical stretching for example ) in the same time? | If a diatomic molecule absorbs infrared radiation at two different wavenumbers, resulting in rocking and symmetrical stretching vibrations, what will be the combined effect on the molecule's vibrational energy levels? Quantify the change in energy if one wavenumber is 1500 cm⁻¹ and the other is 1600 cm⁻¹. | 4,143 | cqadupstack-physics |
46737 | Internal energy according to the van der Waals equation | I am trying to derive the internal energy of a gas which obeys the van der Waals equation. I have however encountered some problems. I calculate the integral of $dU$ from $V=0,T=0$ to $V=V, T=\infty$ to $V=V,T=T$. I can calculate the work: $$\left(p+\left(\frac{a}{n}\right)^2\right)(V-nb)=nRT \implies p=\frac{nRT}{V-nb... | How can I calculate the change in internal energy, \( \Delta U \), for one mole of a van der Waals gas undergoing an isothermal expansion from an initial volume \( V_1 \) to a final volume \( V_2 \), and what expression would involve the van der Waals coefficients \( a \) and \( b \)? | 12,243 | cqadupstack-physics |
93611 | Can Massless Particles Have Dimensions? | We have been taught that anything occupying space/volume has mass. Light has both a particle nature-photon and wave nature. Since It has an momentum of h/λ, energy of hc/λ. Since a photon carries energy and experiments have put an upper limit to its mass of 1e-18 eV/c^2, does it possess dimensions, a shape ? | Considering that a photon has an upper mass limit of \(1 \times 10^{-18} \, \text{eV/c}^2\), can you calculate the corresponding equivalent mass in kilograms and discuss whether this mass contributes to the photon having dimensions or a shape? | 30,662 | cqadupstack-physics |
109877 | Is there no radioactive decay between nuclear fusion and solid material formation? | I'm aware my question might be considered a duplicate of this one: Radio- dating and the age of the earth I read that one and I looked everywhere and I still can't find my piece of mind. I would really like to understand the following points: * does one uranium nucleus start the radioactive decaying process the insta... | If a uranium nucleus begins its radioactive decay process with a half-life of 4.5 billion years, and it becomes part of a zircon crystal after having existed for 0.99 of its half-life, what is the probability of that nucleus decaying within the next million years? | 11,676 | cqadupstack-physics |
111189 | When photons are emitted, do they accelerate to reach the speed of light? | Photons are considered mass-less particle with a specific velocity but according to the electromagnetic theory, a photon is considered to have both energy and momentum. So what happen when they are emitted, do they accelerate to reach the speed of light or is it somehow instantaneous? | If a photon is emitted from a source, what is its velocity at the moment of emission, and how does this velocity relate to the speed of light in a vacuum? | 30,624 | cqadupstack-physics |
24313 | In the known universe, would an atom not present in our periodic table exist? | I have watched this movie Battleship. In it the researchers say this piece of metal is alien because we cant find this metal on earth. So that would mean somewhere else in the universe any of the following should be true? 1. Atoms' composition is not similar to that as on earth (nucleus, electrons, anything else) ... | What properties would a hypothetical element with an atomic number above 120 need to have in order to be stable, and how do these properties differ from the known elements in the periodic table on Earth? | 30,833 | cqadupstack-physics |
47547 | Why do many people say vector fields describe spin-1 particle but omit the spin-0 part? | We know a vector field is a $(\frac{1}{2},\frac{1}{2})$ representation of Lorentz group, which should describe both spin-1 and spin-0 particles. However many of the articles(mostly lecture notes) I've read, when they talk about the relation between types of fields and spins of particles, they'll always say something li... | What is the significance of the $(\frac{1}{2}, \frac{1}{2})$ representation of the Lorentz group in relation to spin-0 and spin-1 particles, and why is it typical to associate vector fields specifically with spin-1 particles rather than acknowledging their potential description of spin-0 particles? | 28,248 | cqadupstack-physics |
122118 | A question on spin algebra | In scattering theory, one can form a lorentz invariant quantity by $\epsilon_{\mu 1 2\nu}P^{\mu}_{1}P^{\nu}_{2}$ which is really $1\otimes 1$ 's spin 0 state. Is there such a kind of argument to show $\epsilon_{\alpha\beta\mu\nu}$ is also invariant under lorentz transformation without showing the determinant of lorentz... | How can we demonstrate that the Levi-Civita symbol \(\epsilon_{\alpha\beta\mu\nu}\) is invariant under Lorentz transformations using more advanced tensor analysis methods, without resorting to the determinant property of Lorentz transformations being equal to 1? | 3,256 | cqadupstack-physics |
4384 | Physical meaning of Legendre transformation | I would like to know the physical meaning of the Legendre transformation, if there is any? I've used it in thermodynamics and classical mechanics and it seemed only a change of coordinates? | What is the significance of the Legendre transformation in thermodynamics and classical mechanics, and how does it relate to the concept of changing coordinates between different physical quantities? Can you provide an example demonstrating its application? | 16,753 | cqadupstack-physics |
76750 | How widespread is the meme "QM is counterintuitive" in academic physics? | I have recently entered university -- studying CS -- and I have spoken to many physics students on campus. Most of these -- when propmted -- will gladly proclaim that QM is counterintuitive, and not something you are supposed to understand. To my knowledge, no one proclaims similar things about, e.g. Relativity, that i... | What are the historical reasons behind the perception that quantum mechanics (QM) is inherently counterintuitive compared to other areas of physics, such as relativity, which is seen as more mathematically challenging but not necessarily counterintuitive? | 36,619 | cqadupstack-physics |
68041 | Best EM/Photon rocket using avalable tech? | Given 10-1000 Watts of electrical power (and no other consumables), what is the current best way to turn it into thrust? Just running it through a heater on an insulating pad would result in an IR thruster, but has bad focus. A laser has good focus but only for a small percentage of the energy. | What is the maximum thrust that can be generated by an infrared thruster using an electrical power input of 500 Watts, assuming ideal conditions and a conversion efficiency of 50%? | 7,629 | cqadupstack-physics |
106871 | Simple question about circular motion | The picture below shows a page of my mechanics (M2) textbook. At the top of this page, you'll see that it says that "a=-ω^2r". Why is it then that all the answers to the question in exercise 6B (also on the page in the picture attached) are positive? Surely the minus sign in the equation for 'a' should man that all the... | In the context of circular motion, if the centripetal acceleration is given by the formula \( a = -\omega^2 r \), where \( \omega \) is the angular velocity and \( r \) is the radius, how can the magnitude of centripetal acceleration be expressed as a positive value despite the negative sign in the equation? What is th... | 27,356 | cqadupstack-physics |
69397 | An approachable example of a field with a "mass gap" | **Preamble** : I have come to believe that alot of difficulties in explaining physics to people of all levels comes from the relatively mundane idea of a wave equation with a mass gap $$\left(-\partial^2_t +\nabla^2 -m^2\right)\phi = 0$$ or more generally a field that does not have propagating modes in some frequency b... | Can you provide an example of a physical system that demonstrates gapped wave behavior, including a clear distinction between propagating and non-propagating frequencies, that would be suitable for teaching purposes? | 14,643 | cqadupstack-physics |
45647 | Rayleigh-Benard Convection | I found this nice paper about RB convection. However I am confused by what is going on page 6. In particular why we are suddenly using Helmholtz equation to find spatially periodic solutions. Aren't we working with convection, so why are we looking at it from a wave like point of view? Or maybe I'm just missing the poi... | How can the Helmholtz equation be applied to analyze spatially periodic solutions in the context of Rayleigh-Bénard convection, and what would be the expected relationship between the temperature gradient and the onset of convection in a heated glass tube filled with olive oil? | 17,481 | cqadupstack-physics |
72808 | Can't understand a thing about emissivity concept | I can't understand something about emissivity. Emissivity is defined as the ratio of the radiant energy of an object, to radiant energy of blackbody at a temperature $T$. So, The vegetation has approximate emissivity of $0.99$ and concrete smaller than this like $0.95$. But in thermal images vegetation appears darker t... | Why does vegetation, which has a higher emissivity of 0.99 compared to concrete's emissivity of 0.95, appear darker in thermal images if it emits more radiant energy? | 35,745 | cqadupstack-physics |
92687 | Is it physically realistic to have an electric field and polarisation density but no displacement field? | Given a Lagrangian density that describes a classical dielectric in interaction with the EM field, I found the Euler-Lagrange equations, and in the case of the electric field, worked through to find that $\vec{P} = -\epsilon_0 \vec{E}$. For some reason alarm bells are going off in my head but I'm not sure why. This imp... | In a classical dielectric material described by the equations of electromagnetism, if the polarization density \(\vec{P}\) is found to be equal to \(-\epsilon_0 \vec{E}\), what does this imply for the electric displacement field \(\vec{D}\), and how would you mathematically verify whether \(\vec{D}\) can indeed be zero... | 30,503 | cqadupstack-physics |
27262 | Mermin-Wagner theorem in the presence of hard-core interactions | It seems quite common in the theoretical physics literature to see applications of the "Mermin-Wagner theorem" (see wikipedia or scholarpedia for some limited background) to systems with hard-core interactions; for example to conclude that genuine crystal phases for a system of hard disks (with possible additional inte... | Given the Patrascioiu-Seiler model where $$ V(S_i,S_j) = \begin{cases} -S_i\cdot S_j & \text{if }|\delta(S_i,S_j)|\leq\delta_0,\\\ +\infty & \text{otherwise,} \end{cases} $$ with $\delta_0 < \pi/4$, what is the critical angle $\delta_0$ at which rotation invariance of the phases starts to break down, and how does this ... | 27,016 | cqadupstack-physics |
47791 | What do massive photons have to do with superconductivity? | I keep reading that the idea of massive photons leads to an explanation of the Meissner effect but I fail to see how photons are involved with the repulsion of fields inside a superconductor. How can a photon effect the electromagnetic fields inside a superconductor? | How does the theoretical concept of massive photons contribute to the understanding of the Meissner effect in superconductors, and what are the implications for the electromagnetic fields within such materials? | 9,219 | cqadupstack-physics |
7396 | What all is needed to solve for the metric in GR? | Einstein's field equations are: $R_{ab} - {1 \over 2}g_{ab}\,R + g_{ab} \Lambda = {8 \pi G \over c^4} T_{ab}$ And since the Ricci curvature tensor is "less information" than the Riemann curvature tensor because: $R_{ab} = R^c{}_{acb}$ and the Riccie curvature scalar is even "less information" than the Ricci curvature t... | Given the Einstein field equations, \( R_{ab} - \frac{1}{2}g_{ab} R + g_{ab} \Lambda = \frac{8 \pi G}{c^4} T_{ab} \), and the relationship between the Ricci curvature tensor and the Riemann curvature tensor, what additional conditions or constraints, if any, must be specified to uniquely determine the evolution of the ... | 31,290 | cqadupstack-physics |
63156 | Impact parameter in scattering process | Let's consider classical two body scattering process. Is the impact parameter the same in the laboratory and inertial frame of references? | In a classical two-body scattering process, how does the impact parameter differ when measured in the laboratory frame of reference compared to an inertial frame of reference where the total momentum is zero? | 28,716 | cqadupstack-physics |
21037 | resonance frequency | A string has a mass per unit length of 9 10–3 kg/m. What must be the tension in the string if its second harmonic has the same frequency as the second resonance mode of a 2m long pipe open at one end? The length of the string is 1.5 m. this is my solution http://screensnapr.com/e/u4kvtH.png i just want to ask if my sec... | What is the tension in a string with a mass per unit length of 9 × 10⁻³ kg/m and a length of 1.5 m, if its second harmonic frequency is equal to the frequency of the second resonance mode of a 2 m long pipe that is open at one end? | 36,139 | cqadupstack-physics |
91417 | Phasor form of Maxwell's Equations | I'm interested in the transformation from the standard Maxwell's equations to their phasor equivalents. From the literature, this means injecting: \begin{equation} E = Re(\boldsymbol{E}e^{j\omega t}) \end{equation} into \begin{equation} \nabla \times E = -\frac{\partial B}{\partial t} \end{equation} to deduce \begin{eq... | What assumption is commonly made when transitioning from the phasor representation of Maxwell's equations to their real-time equivalent, allowing for the neglect of the imaginary components in the derivation? Additionally, under what specific conditions can the imaginary part be ignored in this context? | 20,527 | cqadupstack-physics |
49919 | How can the Moon have such a strong effect on the ocean? | The gravitational acceleration on Earth is approximately $ 10 \mathrm{m}/\mathrm{s}^2 $. Compared to this, the tidal effect of the Moon's gravity gives a local variation in the acceleration of approximately $ 9 \cdot 10^{-7} \mathrm{m}/\mathrm{s}^2 $, that is, seven orders of magnitudes less. The level of the water can... | Given that the gravitational acceleration due to the Moon causes a variation of approximately \( 9 \times 10^{-7} \, \mathrm{m/s^2} \), calculate the force exerted by the Moon's gravity on a 1 kg mass located at the Earth's surface. How does this force compare to the force exerted by Earth’s gravity on the same mass? | 16,227 | cqadupstack-physics |
937 | How does gravity escape a black hole? | My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. It would seem to me that the gravitational attraction caused by a black hole carries information about the amount of mass within the black ... | If a black hole's event horizon prevents light from escaping, how does the gravitational influence of a black hole convey information about its mass to the surrounding space, considering that no information can travel faster than the speed of light? Additionally, do theoretical gravitons, if they exist, propagate at a ... | 5,450 | cqadupstack-physics |
102814 | Velocity in a pseudo-Riemannian manifold | Suppose we have a pseudo-Riemannian manifold (modeled by $\mathbb R^{p,q}$) with signature $$(\underbrace{+,+,\cdots,+}_p,\underbrace{-,-,\cdots,-}_q)$$ Meaning we have $q$ spacelike dimensions and $p$ timelike dimensions. For a given observer in this manifold, we can define the position vector $\mathbf S$ of an object... | Given the relationship \( ds = \beta^* \cdot dw \) and the equation \( \nabla (s - \beta^* w) \cdot d\vec T = 0 \), how would you express the Jacobian matrix \( \mathbf B \) in terms of the gradients \( \nabla s \) and \( \nabla w \) to demonstrate that the spatial and temporal variations are consistent with the observ... | 30,355 | cqadupstack-physics |
23742 | Why are we forced to choose a specific value for $\pi$ field in Nambu-Goldstone phenomenon? | In the sigma-model of spontaneous symmetry breaking, we have degenerate vacuum states. But if we don't pick up a particular value of VEV, we won't have any symmetry breaking. As I read from a book, in field theory at finite volume, there would be no problem; but at infinite volume, we are forced to choose a value of th... | In the context of the sigma-model of spontaneous symmetry breaking, how does the energy density of the vacuum depend on the choice of vacuum expectation value (VEV) for a field at infinite volume compared to finite volume, and what implications does this have for the stability of the vacuum states? | 11,939 | cqadupstack-physics |
93324 | From where does a particle get the energy to tunnel? | When a particle is made to confine more and more to a particular position it breaks the energy barrier to get out because of the uncertainty principle. But, from where does the particle get the energy required to do so? Does its mass get converted into energy? In that case consider a particle in a potential well equal ... | In quantum mechanics, how does a particle escape from a potential well, and what role does the uncertainty principle play in this process? Additionally, can you explain whether quantum tunneling occurs during this escape and how energy is conserved when a particle transitions between confined and free states? | 22,540 | cqadupstack-physics |
44241 | Poincaré group on quantum Klein-Gordon field (C*-algebraic scenario) | on the same topic as this question, I have been trying to fool around with the free real K-G field in flat spacetime on the C*-algebraic scenario (Haag- Kastler axioms, Weyl quantization, etc). Since I'm talking about the free (linear) Klein-Gordon field, the C*-algebra is taken as the CCR algebra generated by the unit... | Consider the free Klein-Gordon field in flat spacetime represented by the C*-algebra generated by the Weyl operators \( W(f) = \exp(i\Phi(f)) \), where \( \Phi(f) \) are field operators. If we apply a Lorentz transformation \( \Lambda \) and a translation \( a \) from the Poincaré group, what is the transformation of t... | 28,274 | cqadupstack-physics |
74708 | Angular momentum contained in E&M fields | I am trying to do this question as a prep for my quals and find my solution too simplistic. I fear I am missing something important. "Iron atoms (atomic mass 56) contain two free electron spins that can align with an external magnetic field. An iron wire 3 cm long and 1 mm in diameter is suspended vertically and is fre... | What is the change in the angular velocity of a 3 cm long and 1 mm diameter iron wire that is suspended vertically and free to rotate, when subjected to a strong magnetic field aligned with its axis, given that the density of iron is 8 g/cm³ and the atomic mass of iron is 56? | 12,204 | cqadupstack-physics |
70320 | Quantum Hall Effect and Edge States | In quantum hall effect we measure the hall conductance (in transverse direction) which is quantized. My question how do they take care of the edge states that are in the longitudinal side? | What is the quantized value of the Hall conductance in the Quantum Hall effect, and how do edge states contribute to this measurement? | 19,066 | cqadupstack-physics |
104742 | Matrix Representations of Galilean group | The general group element (in the vector representation) $$ \left [{ \begin{array} {c} \bar x^1 \\\ \bar x^2 \\\ \bar x^3 \\\ \bar t \\\ 1 \\\ \end{array} } \right] = \left[ {\begin{array}{ccccc} a_{11} & a_{12} & a_{13} & V_1 & a_1\\\ a_{21} & a_{22} & a_{23} & V_2 & a_2\\\ a_{31} & a_{32} & a_{33} & V_3 & a_3\\\ 0 & ... | What is the form of the generators for the Galilean group that satisfy the commutation relations, specifically ensuring that the commutator between the boost generators \( B_i \) and the momentum generators \( P_i \) equals zero, given that the parameter for boosts \( B_i \) is not velocity \( V_i \)? Can you define wh... | 14,595 | cqadupstack-physics |
77648 | What is the highest useful magnification todays largest telescopes can offer? | I know that the maximum (useful) magnification is limited by the diffraction limit, but I was not able to find numbers for the highest useful magnification factors using modern large telescopes. How much magnification is possible, e.g. using the E-ELT? | What is the maximum useful magnification achievable with the Extremely Large Telescope (E-ELT) considering the diffraction limit? | 5,290 | cqadupstack-physics |
26816 | Do bipartite spin glasses have simple relaxation dynamics? | From what I gather, a Boltzmann machine (BM) is essentially a spin glass with no applied field evolving under Glauber dynamics (if this is at all mistaken, I don't think it will be off enough to materially affect the following, but I would welcome elaboration anyway). For background, see, e.g. Amit _et al_., PRA **32**... | What characteristic timescales are observed in the relaxation dynamics of restricted Boltzmann machines (RBMs), and how do these timescales compare to those in traditional spin glasses? | 19,992 | cqadupstack-physics |
54617 | Question about Ryder text (Generating functional) |  The second equality in (6.88) he says was obtained by expanding the denomitator by the binomial theorem. It is probably very dumb but I'm not following. I see how the 1 and the vacuum term in the numerator cancel with the denominator and give a 1. But ... | How would you use the binomial theorem to expand the denominator in the expression given by (6.88) and derive the resulting terms that contribute to the overall simplification? Specifically, could you show step-by-step how the numerator terms cancel with the denominator to yield a final simplified expression? | 21,176 | cqadupstack-physics |
17534 | what is it called: box potential with one infinite wall | The finite square well and the infinite square well problem are well known, however is there a reason that there is almost no reference to the one sided infinite square well? > Consider a particle with mass $m$ moving in the one dimensional potential > $V(x)= \infty$ for $x<0$ , $V(x) = -V_{0}$ for $0\le x \le L$; $V(x... | Consider a particle of mass \( m \) moving in a one-dimensional one-sided infinite square well potential defined as \( V(x) = \infty \) for \( x < 0 \), \( V(x) = -V_0 \) for \( 0 \leq x \leq L \), and \( V(x) = 0 \) for \( x > L \).
1. Can you derive the dimensionless parameter \(\lambda\) that allows this problem t... | 17,675 | cqadupstack-physics |
65334 | Calculating vector components from other vectors | I'm busy studying for my Physics exam this evening and I've come across a vector problem that I cant quite solve. Any help would be appreciated! We are given the following vectors: A = 5i - 6.5j B = -3.5i + 7j We are also told that a vector C lies in the xy-plane and is perpendicular to A. The sc... | Given the vectors \( \mathbf{A} = 5\mathbf{i} - 6.5\mathbf{j} \) and \( \mathbf{B} = -3.5\mathbf{i} + 7\mathbf{j} \), find the components of vector \( \mathbf{C} \) that is perpendicular to \( \mathbf{A} \) and satisfies the condition \( \mathbf{B} \cdot \mathbf{C} = 15 \). How can we solve for the components of vector... | 19,698 | cqadupstack-physics |
81841 | Large object is pushed through a small wormhole, what happens? | A simple question about wormholes: What happens if a large object is pushed through a much smaller wormhole? For example, what happens if I push an elephant through a palm-sized wormhole? My intuition is that depending on the magnitude of the force applied, either a part of the elephant is ripped out, or the elephant s... | If an elephant is pushed through a palm-sized wormhole, what is the minimum force required to either completely pass the elephant through or to deform it significantly, assuming the wormhole can maintain its stability under extreme conditions? | 19,753 | cqadupstack-physics |
37947 | Implications of Unruh-inertia to theories of gravity | If it turns out to be true that the galaxy rotation curves can be explained away by Unruh modes that become greater than the Hubble scale at accelerations around $10^{-10} m/s^2$ as proposed in here, what modifications would have to be done to the effective General Relativity equations? Also, if anyones knows at this e... | What modifications to the effective General Relativity equations would be necessary if Unruh modes become significant at accelerations around \(10^{-10} \, m/s^2\) to explain galaxy rotation curves, and what implications would this have for gravity theories that depend fundamentally on the equivalence principle, such a... | 28,528 | cqadupstack-physics |
44773 | Vector Nature Of Angular Velocity | I am currently reading about angular position, angular velocity, and angular acceleration. I came across this paragraph that was particularly confusing, and was wondering if someone could perhaps help me. Here is the paragraph: "If a particle rotates in the $xy$ plane, the direction of $\vec{\omega}$ for the particle *... | Given a particle rotating in the $xy$ plane, if it completes one full counterclockwise rotation in 4 seconds, what is the angular velocity $\vec{\omega}$ of the particle in radians per second? Additionally, if the particle's angular speed increases at a constant rate of 2 rad/s², what will be the angular acceleration $... | 36,959 | cqadupstack-physics |
12700 | How is angular momentum conserved when a spinning top finally stops spinning? | Where does the top's angular momentum get transferred to? Does it _very slightly_ change the angular momentum of the table, and then the angular momentum of the Earth? | When a spinning top is placed on a table, what is the relationship between the top's angular momentum and the resulting changes in the angular momentum of the table and Earth? Quantify the expected change in angular momentum of the table if the top has an angular momentum of 0.5 kg·m²/s and remains on the table for 10 ... | 28,015 | cqadupstack-physics |
70981 | Dimensional analysis of magnetic energy: dimensions of µ0 and H | When calculating the energy difference between the normal and the superconducting state in a superconductor at zero magnetic field, the result is as follows:  Now I'm quite confident of this result, as it is the same as my textbook tells me it is. The p... | What is the derived energy difference between the normal and the superconducting state in a superconductor at zero magnetic field, given that the magnetic field strength \( H \) is 1 T and the vacuum permeability \( \mu_0 \) is \( 4\pi \times 10^{-7} \, \text{T m/A} \)? | 24,776 | cqadupstack-physics |
76350 | Physical structures that trap information | I labeled this question "soft" because it might not make any sense. Anyway, what I'm wondering is if there's a notion in physics that deals with the ability of matter to trap information. For instance, if I get in the way of a red light and see it, then my eye and my brain have taken the information about the wavelengt... | What is the concept in physics that describes the ability of a physical system to store and process information, and how does this relate to the fundamental characteristics of living systems? | 6,894 | cqadupstack-physics |
54467 | Online magnetic field solver | Is there a site that allows you to give the approximate specs of a set of magnets and then it produces the graph of the magnetic field for you? Here is a situation I try to understand: I have a set of rectangular magnets about 1*0.75*.25 inches with north and south on large faces. The magnets are glued in two long line... | What is the approximate strength of the magnetic field measured in teslas between two parallel rectangular magnets, each measuring 1x0.75x0.25 inches, placed 1 inch apart with their north poles facing upwards? | 27,755 | cqadupstack-physics |
108019 | One proof of linear momentum of the system |    I copy and paste all the proof here, but I'm confused about the last step, which is that the summation of the intern... | In a system where four forces \( F_1, F_2, F_3, \) and \( F_4 \) are acting on an object, if the internal force can be divided into two summations such that \( \sum F_{\text{internal}} = \sum F_{1,2} + \sum F_{3,4} \), how would you express the total internal force in terms of the individual forces \( F_1, F_2, F_3, \)... | 28,155 | cqadupstack-physics |
90670 | Some small questions about quantum spin and rotations | I'm studying about quantum-spin (in a syllabus about non-relativistic quantum- mechanics though), but I have some trouble understanding everything. So I would like to ask some small questions, which may perhaps clear up the picture as a whole. So they are trying to determine the influence of a spatial rotation, to the ... | What conditions must the hermitian operator \( Â \) satisfy for the inner products \( \langle \chi | \chi \rangle \), \( \langle \chi' | \chi' \rangle \), and \( \langle \chi' | \chi \rangle \) to remain invariant under a spatial rotation in a two-dimensional spin space? Additionally, why does this require that \( Â = ... | 21,309 | cqadupstack-physics |
62426 | What are "packets"? | Paraphrased from Wikipedia: > Infrared sensing in snakes depends on a kind of natural thermography, by > which tiny **packets** of cellular water are raised in temperature by the > infrared radiation. What is "packets" refering to in this is example? | What specific structures or components in snakes' biology are referred to as "packets" of cellular water that are involved in infrared sensing? | 21,378 | cqadupstack-physics |
17685 | If the entire universe is the same age, can one assume that any potential life elsewhere is at the same stage of development as us? | There was some discussion in another question regarding whether you can consider all parts of the universe to have existed for the same length of time, and the conclusion seemed to be that, because of relativity, it depends. I'd like to know, in a sort of practical sense, if we imagine life to exist elsewhere in the un... | If we consider the age of the universe to be approximately 13.8 billion years, how many years would it have taken for life to develop on another planet, similar to Earth, assuming it followed a comparable evolutionary timeline? Calculate the possible evolutionary development stages of life on that planet if it began at... | 34,898 | cqadupstack-physics |
41056 | Why minibands are formed in superlattices? | In a single, finite quantum well, there are energy levels defined by the eigenstates - the solutions of the Schroedinger's Equation. The corresponding wavefunctions leak to the barrier because of its finite height. If we bring two identical QWs together, the wavefunctions of corresponding states will mix and, as far as... | What is the quantitative relationship between the number of adjacent quantum wells in a superlattice and the number of minibands formed as a function of the width of the energy barrier? | 19,264 | cqadupstack-physics |
128396 | Is the plasma state always visible? | All the examples of plasma I have come across are visible. Is there any plasma which is not visible? For example, during a dark lightening we don't see the radiation because its gamma radiation. Is plasma formed during a dark lightening? | Is it possible for plasma to exist in an invisible state, such as during dark lightning events, and if so, what are the specific conditions under which this invisible plasma can form? | 11,395 | cqadupstack-physics |
41951 | Weinberg angle measurement methods | I was reading up on the history of $W/Z$ bosons today and I got a little puzzled. I always assumed that people measured $M_Z$ and $M_W$ and then derived the Weinberg angle. But it appears that they knew what the Weinberg angle was by the late 70's (but not with a lot of precision). How do you measure the Weinberg angle... | How can the Weinberg angle be experimentally determined without directly measuring the masses of the \(W\) and \(Z\) bosons? | 7,797 | cqadupstack-physics |
47100 | Rotating Empty Sphere around stationary black hole | Would it be possible to create a habitable in terms of gravity planet that would rotate with the black hole acting as a center of gravity? The rotation of the structure would lessen the gravitational force, so it would be much harder to access it from outside. Would the rotation of the planet be sufficient to lessen th... | What minimum rotational velocity would a planet need to achieve in order to counteract the gravitational pull of a black hole of mass \(M\), at a distance \(d\) from its center, so that the effective gravitational force experienced on the planet's surface is equivalent to Earth's gravity (\(9.81 \, \text{m/s}^2\))? | 35,093 | cqadupstack-physics |
102091 | In how big percentage of gravity core does play role in? | If something would take whole core of earth and done this $(((coreearth)-(restofearth)):coreearth)*100=X$ What would `X` be? In short, which part of our planet matters more in gravity, the core, or what's around it? Does the same apply to stars? | If we assume the Earth's core has a mass of approximately \(1.94 \times 10^{24}\) kg and the mass of the entire Earth is about \(5.97 \times 10^{24}\) kg, what percentage of the Earth's total mass does the core represent? Additionally, how does this distribution of mass affect the gravitational influence of the core co... | 11,481 | cqadupstack-physics |
77661 | How would Einstein's later years have been different with modern computers? | This is a historical question partly, and maybe too broad for this site, but would require some familiarity with modern physics research practice so hopefully appropriate here. Einstein's later years, after general relativity, were (as I understand it) taken up predominantly by his effort to find a unified theory that ... | How might access to modern computational tools, like programming languages and symbolic mathematics software, have accelerated Einstein's search for a unified theory of gravitation and electromagnetism, and what specific advancements in theoretical and mathematical physics can be attributed to the use of computers? | 2,967 | cqadupstack-physics |
12596 | Correlation functions in thermal field theory etc | Suppose I am studying a field theory at finite temperature or some black hole formation scenario from boundary theory perspective in the sense of AdS/CFT. How is it possible to gain information about them from say looking at the two point functions (propagators) of operators in field theory? I mean will there be some s... | In the context of AdS/CFT correspondence, how does the behavior of two-point correlation functions (propagators) in a conformal field theory change at finite temperature, especially in relation to pole structures in the Green's function? What quantitative predictions can be made about these changes, and are there speci... | 16,877 | cqadupstack-physics |
103605 | Did the singularity really existed in the begining of the universe? | I read in Hawking's brief history of time the following: > The final result was a joint paper by Penrose and myself in 1970, which at > last proved that there must have been a big bang singularity provided only > that general relativity is correct and the universe contains as much matter > as we observe. There was a lo... | What mathematical theorem did Penrose and Hawking prove in 1970 regarding the existence of singularities in the universe, and what are the current theories that challenge this idea? | 31,803 | cqadupstack-physics |
51865 | Commercial Infrared lights | I purchased an infrared light. It's a 100 W Philips infrared lightbulb. Says it's infrared, but I haven't done any spectrum analysis so I don't know for sure if it's just red or really infrared. As I recall glass is specially opaque to infrared light. Doesn't the light bulb have to be of special glass? (I may be wrong ... | What is the wavelength range of infrared light, and how does its penetration power compare to that of visible light, particularly white light, when considering its interaction with materials? Additionally, is infrared radiation considered harmless to humans? | 30,072 | cqadupstack-physics |
87604 | Strange behaviour of water drops on a heated pan | I did an experiment in my home and I was surprised by the results. I heated a pan without anything in it. After some time it became very hot and I took some water (2 drops) and put it into the pan. It started behaving very strangely. The water is forming droplets, and the droplets are jumping here and there and fizzing... | What phenomenon is responsible for the water droplets behaving like small balls that move and jump around on a hot surface, and what factors influence this behavior? | 12,721 | cqadupstack-physics |
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