Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Decoherence inside black holes I have a question about decoherence. Assume there is a macroscopic black hole floating around and you have some macroscopic object with you with a huge number of internal degrees of freedom. Conventional decoherence theory predicts decoherence. If this macroscopic object is dumped into th... | Grow up and be a no-nonsense physicist. Translation - become a positivist. Ask what are the empirical experimental observations that can be made here. Any more metaphysics is mere sophistry and illusion, and has to be committed to the flames.
Any experimenter jumping into the black hole to measure the macroscopic objec... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/17136",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
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Spin Echo Experiment I'm doing an spin echo experiment for different dilutions of Glycerol as an undergraduate physics lab experiment. At the end of my experiment I will need to extend it and do some initiatives! I have a few potential extensions in mind. But I was wondering what other cool stuff one can do with typica... | You could try bilateral experiments opposed to mechanical ones which are more fun if you into watching things happen by themselves without computing the control mechanisms. Start with some type of "conversion plane" which could be any material that interacts with the experimental magnets, then modify the levels of plan... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/17190",
"timestamp": "2023-03-29T00:00:00",
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What type of substances allows the use of the Ideal Gas Law? I know that I can use the ideal gas law with pure gases or pure liquids. But can I also use the ideal gas law at saturated gases and saturated liquids as long as they aren't two phase substances?
| Aside from the classical cases where the ideal gas law applies, it also applies to describe the exact entropy of a dilute solution, even if that solution is in a dense liquid. The reason is that the entropy of a dilute solution in a dense liquid is exactly the same as the entropy of a dilute gas, the number of possible... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/17362",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
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How would nucleosynthesis be different if the neutron were stable? If the strong nuclear force were just 2% stronger, the neutron would be a stable particle instead of having a half life of about 13 minutes. What difference would that have made to Big Bang nucleosynthesis, to the growth of structure, to the formation o... | Assuming that the proton is heavier than the neutron, by more than the mass of the electron (plus the mass of a neutrino, plus the ionization energy of hydrogen), this is easy to answer, it would just make hydrogen unstable to decay to a neutron an an electron positron pair, so that a mostly hydrogen universe will deca... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/17431",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
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Why can't $ i\hbar\frac{\partial}{\partial t}$ be considered the Hamiltonian operator? In the time-dependent Schrodinger equation, $ H\Psi = i\hbar\frac{\partial}{\partial t}\Psi,$ the Hamiltonian operator is given by
$$\displaystyle H = -\frac{\hbar^2}{2m}\nabla^2+V.$$
Why can't we consider $\displaystyle i\hbar\frac... | Answer to the top question is actually very short. Time is an external parameter in conventional QM; parametrizing a unitary evolution. It, as well as $i\partial_t$, nothing has in common with operators, observables etc. In other words $t$ in $\psi(t)$ does not enumerate some basis vectors of an observable like $x$ doe... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/17477",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "105",
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Radar Frequency Bandwidth I've come across an interesting question in the course of doing some exam review in a quantum mechanics book and thought I'd share it here.
"What must be the frequency bandwidth of the detecting and amplifying stages of a radar system operating at pulse widths of 0.1usec? If the radar is used... | Since you're talking about a quantum mechanics book rather than a radar design engineering book, I assume that they're talking about a basic textbook radar system, rather than one with all the advanced features.
The receiver needs to have a bandwidth great enough to receive most of the energy in the reflected pulses. ... | {
"language": "en",
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What is the physical sense of the transition dipole moment? So if the states are the same we achieve the expectation value of the dipole moment for a given state. I mean
$ \langle \mathbf{\mu} \rangle = \langle \psi \vert \hat{\mathbf{\mu}} \vert \psi \rangle$
But I don't feel the physical sense in the case of transiti... | I never thought of it this way, but off-diagonal components of an operator means that this operator is... Well, non-diagonal. Which means that neither of $| \psi \rangle$ realizes a definite value of this operator.
The dipole operator comes from the term which couples charged particle with electric field. It would be ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/17594",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "12",
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Is it possible to recover Classical Mechanics from Schrödinger's equation? Let me explain in details. Let $\Psi=\Psi(x,t)$ be the wave function of a particle moving in a unidimensional space. Is there a way of writing $\Psi(x,t)$ so that $|\Psi(x,t)|^2$ represents the probability density of finding a particle in classi... | You can recover Schroedingers equation from the path integral formulation of Quantum mechanics by Feynman. In the path integral picture the classical trajectories are the stationary points of the integrand. So in the stationary phase approximation, they are the contribution of $0$-th order in $\hbar$. Of course that is... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/17651",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Can stable nuclei theoretically fission through quantum tunneling? As I understand it, an unstable nucleus is going to randomly fission because the forces binding it together are momentarily weaker than the electrostatic repulsion of the protons.
Given that some nuclei are really unstable, and others are stable for bil... | Stability of nuclei is a matter of the binding energy and conservation of quantum numbers: number of baryons is conserved. If ( and it is a large if) the proton decays, i.e. baryon number has a small but finite probability of not being conserved, then also stable nuclei might decay spontaneously. At the moment the life... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/17752",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Is there a limit to the resolving power of a mirror telescope? Like, if you grabbed the asteroid 16 Psyche and hammered it out into a disc of 1 mm thick iron foil and curved it into a telescope mirror with 2.4x the radius of the Sun, could you resolve details on the surface of an exoplanet? At what resolution?
Could... | The resolving power of a device is limited by the so called Rayleigh criterion (commonly known as the diffraction limit). This is applicable for "classical" states of light (i.e light that has a coherent state representation). For non-classical light (squeezed light or entangled light), you can beat the diffraction lim... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/17881",
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What is escape velocity? In reality, how can something no longer be under the gravitational influence of something else? Isn't G a continuous function and although you leave the immediate vicinity of the earth with an escape velocity won't it always exert a force, however small it may be. Won't that force eventually pu... | It's true that it doesn't matter how far away you get from the Earth, there will always be an attractive force, and that if you are at rest gravity will pull you towards the Earth.
However, if you start at Earth's surface with that speed, you can show that your trajectory will never take you back to Earth. It will be a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18070",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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When has the speed of light been measured, recently? Yes, it is weird, absurd, but I can't stop thinking that the would-be superluminal
neutrino speed has been computed by an arithmetic operation (space/time) and not by direct comparison with a simultaneous light ray running in "parallel".
So the "unspeakable", outrag... | There has been a lot of coverage of the faster-than-light neutrinos and therefore reasonable to think about the subject. There are some theories that suggest that the speed of light is not a constant, but that change would be over the lifetime of the Universe. In any case, there is no evidence for such changes in the s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18149",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Do multiple permanent magnets aggregated together approach the same strength as a single magnet of the same size? Here's an applied physics question. ;) If I buy some cube or sphere magnets like these, can I aggregate them together to create a stronger magnet (almost as strong as a single magnet)?
| To expand on Omar's Answer: KJMagnetics has an FAQ that answers this question: http://www.kjmagnetics.com/FAQ.asp#stack.
If you wander round the KJMagnetics site, they give a lot of technical information (I bought some magnets off them 2-3 years ago).
Their FAQ answer describes the situation for stacking flat magnets v... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18206",
"timestamp": "2023-03-29T00:00:00",
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Define Pressure at A point. Why is it a Scalar? I have a final exam tomorrow for fluid mechanics and I was just looking over the practice exam questions. They do not provide solutions. But pretty much I have to define pressure at a point and also say why pressure is scalar instead of a vector.
I am thinking pressure at... | Because it doesn't behave like a vector -- for example, the standard vector norm of the pressure doesn't mean anything, but the sum of pressure components does. Just because something has a bunch of components doesn't mean it's a vector. Pressure refers to the diagonal components of the stress tensor (actually their su... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18255",
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"source": "stackexchange",
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Heat transfer, Cold vs. Hot If you boil water inside a pot the outer rim bubbles first, I imagine because its hotter. Does that same concept apply for the inside of a refrigerator for example. Is the outer rim always more extreme, or at least at first?
| When you are boiling water bubbles typically form first at the bottom of the pot, and then at the walls. As you guessed, this is because the bottom is usually at higher temperature than the walls, since it's in direct contact with the heat source. The reason that bubbles don't appear sooner in the bulk of the water is ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18332",
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Physics Paradox about Newtons Second Law $F=ma$
*
*If force equals mass times acceleration, wouldn't a basketball dropped from the top of the Eiffel tower exert the same force on the ground as a basketball dropped a foot off the ground? They both have the same mass, and they both are accelerating towards the ground a... | This is the case of jerk in physics , rightly pointed above, so when the ball hits ground it change in acceleration is 2a in very less time. This is the reason why you get more injured when you fall from high tower.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18387",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
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How does gravity work underground? Would the effect of gravity on me change if I were to dig a very deep hole and stand in it? If so, how would it change? Am I more likely to be pulled downwards, or pulled towards the edges of the hole? If there would be no change, why not?
| The other answers provide a first-order approximation, assuming uniform density (though Adam Zalcman's does allude to deviations from linearity). (Summary: All the mass farther away from the center cancels out, and gravity decreases linearly with depth from 1 g at the surface to zero at the center.)
But in fact, the E... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18446",
"timestamp": "2023-03-29T00:00:00",
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Deterministic quantum mechanics I came across a very recent paper by Gerard 't Hooft
The abstract says:
It is often claimed that the collapse of the wave function and Born's rule to interpret the square of the norm as a probability, have to be introduced as separate axioms in quantum mechanics besides the Schroedinge... | One place to look is the homepage of Antony Valentini now at Clemson University. He claims that Born's probability rule is only an approximation. David Bohm first made this claim. One can show that entanglement can be used for faster-than-light and even retro-causal back-from-the-future delayed choice signaling once th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18586",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "31",
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Do interaction-free measurements require a physical collapse or splitting in order to be truly interaction free? Interaction-free quantum experiments like Renninger's experiment or the Elitzur-Vaidman bomb tester are often taken to be examples of interaction-free measurements of a system. Unfortunately, such assumption... |
The Elitzur-Vaidman bomb tester isn't really an interaction free measurement. Analyze it using consistent histories. Suppose initially, for the three possible bomb states, we start off with the mixture diag(p, 1-p, 0) for dud, workable but unexploded, and exploded respectively. Let $P_c$ correspond to the projector of... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18617",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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Complex numbers in optics I have recently studied optics. But I feel having missed something important: how can amplitudes of light waves be complex numbers?
| With any simple harmonic oscillator there are two quantities we are interested in, the phase and the amplitude. Complex numbers are an easy way to represent both of these in a single value, especially as a complex number can be written in the form $Ae^{i\theta}$ where $A$ is the amplitude and $\theta$ is the phase. Thi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18659",
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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} $ ??
| Yes, such theories exist. They're known as "conformal quantum mechanics". See
http://scholar.google.com/scholar?q=%22conformal+quantum+mechanics%22&hl=en&lr=&btnG=Search
http://scholar.google.com/scholar?q=ads2-cft1&hl=en&lr=&btnG=Search
There is an $SL(2,{\mathbb R})$ symmetry in them, or its (e.g. supersymmetric) e... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18709",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
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How do we visualise antenna reception of individual radiowave photons building up to a resonant AC current on the antenna? I am a chemical/biological scientist by trade and wish to understand how quantum EM phenomena translates to our more recognizable classical world.
In particular, I want to get a mechanistic picture... | You cannot understand how a radio antenna works by counting the number of photons that strike the copper wire. This number is many orders of magnitude too small to account for the actual power absorbed by an antenna. An antenna would not work if it depended on physically intercepting photons. I explain all this in my b... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18823",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
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Reducing General Relativity to Special Relativity in limiting case I understand that general relativity is applicable to gravitational fields and special relativity is applicable to case when there is no gravity. But is there a derivation on how to reduce General Relativity to Special Relativity in limiting case, much ... | Other answers have already addressed the relationship between General Relativity and the Minkowski metric, but it seems you are most interested in getting from the Minkowski metric to the Lorentz transformation. So let's do that.
Given a set of coordinates in which the metric takes the standard Minkowski form
$ds^2 = ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18904",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 5,
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Software to simulate and visualize atoms? Not sure if this is a physics or chemistry question. But if the motion of atoms and it's particles can be described by quantum mechanics, then is there a software that simulate full atoms and it's boundings, in a way you can visualize them, and that can be used, for instance, t... | quantum espresso is a package that contains many different packages
for electronic structure calculations.
It is now available in a docker container and therefore very easy to use..
Installed docker on linux, windows or mac,
you just need to run the docker image that is available on dockerhub :
docker run -d rinnocente... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18969",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
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What frequency photons are involved in mediating physical force? If the force felt when pushing an object is mediated by the electromagnetic interaction and hence photons, what is their frequency?
| The electrostatic force is mediated by virtual photons and one could say that they are not physical photons and they do not have a frequency, i.e. one could claim your question is invalid.
However, one may also determine the frequency from the energy $E$ of the virtual photons (via $E=hf$) which actually is determined ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/19015",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
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How much energy is in a lightning strike? According to Wikipedia an average lightning strike has $1$ TW power, the whole world used $16$ TW of power in 2006.
The lightning strike lasts for $30$ microseconds. Does this mean that you get $100$ TW of energy in just $30$ microseconds?
| You are confused about units. Watt is a unit of power (energy/time), Watt-hour is a unit of energy. 16TW is an estimate of the continuous average power usage of the world - which is about 140,000 TWh each year. If the lightning has a peak power of 1TW for 30 microseconds, this corresponds to an energy content of about ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/19082",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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What fundamental principles or theories are required by modern physics? We have been taught that speed of light is insurmountable but as we know an experiment recently tried to show otherwise.
If the experiment did turn out to be correct and confirmed by others, would it make physics to be rethought of? What other con... | Not very much !
The rest of physics still has to work, finding that the speed of light can be exceeded in certain circumstances doesn't suddenly change the results in other experiments or allow perpetual motion machines to start working.
There have been some discoveries where things that were classically 'impossible' ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Why can't echoes be heard inside a room? If I go camping and shout anywhere, in the forest or on a cliff, I usually hear the echo of my voice.
Why when I shout in my room I do not hear any echoes?
| You just haven't tried a big enough room...try a large, empty gymnasium or something similar (but not a concert hall as they are usually designed to suppress echos.).
The speed of sound is roughly $v_s = 340\text{ m/s}$ (1100 feet per second), and hearing an echo requires at least
*
*a perceptible time between the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/19195",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
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If microwave ovens and WiFi both operate on the same frequency, why doesn't WiFi cook things? If we ignore 5GHz WiFi, then both microwaves and WiFi create photons at ~2.4GHz but one of them will boil water in a few seconds but the other doesn't have any effect. So what's the difference?
Is it simply the number of photo... | Power - your wifi router puts out about 0.1 - 1.0 W, your microwave oven puts out 1000W.
It would take a lot of wifi routers to cook a turkey - more than you think because the antennea on the router is designed to spread the power evenly around the room rather than concentrate it on the center of the oven.
There is a d... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/19285",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Physics related Podcasts
Possible Duplicate:
Are there any good audio recordings of educational physics material?
In the same way that was already asked about good books of Physics in this StackExchange, I would like to know good physics podcasts! What are the most informative and enjoyable to hear?
| I could collect the sources from various websites and place it here, browse through their content to get what you want
Are there any good physics podcasts?
Physics General Interest Seminar Podcasts
Physics Podcasts
Podcasting the mysteries of the universe
The first link has some good collection . Enjoy
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/19354",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Could gravity hold electron charge together? Could the gravitational force be what holds the charge of the electron together? It seems to be the only obvious possibility; what other ideas have been proposed besides side-stepping the issue and assuming a "point charge"? How would this affect the electron "self-energy" p... | Yes, it can. Here is a toy model using Newtonian gravity.
V/c^2 = e^2/mc^2r - /\r^2
e^2/mc^2 = rc (classical electron radius)
with SSS metric
g00 = 1 + 2V/c^2 = - 1/grr
g/c^2 = -dV/dr = + rs/r + 2/\r
We can get g = 0 with /\ < 0 i.e. AdS metric
In a vacuum where the w = -1 virtual electron positron pairs surrounding th... | {
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how to represent the effect of linking rigid-bodies together? I have 2 rigid-bodies (b1,b2) if i linked one to the other (as if they are conjoined together) , how to represent b1 effect on b2 and b2 effect on b1
Is there any LAW that affect the position/orientation of the other body ?
notes :
*
*i am using Quater... | The laws you are looking for are conservation of momentum and conservation of angular momentum. If you stick the two bodies together both laws still must be fulfilled (inelastic effects neglected). In the end you will have a compound single object. With the parallel axis theorem (wikipedia) you can calculate the mass m... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/19724",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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What is the connection between Poisson brackets and commutators? The Poisson bracket is defined as:
$$\{f,g\} ~:=~ \sum_{i=1}^{N} \left[
\frac{\partial f}{\partial q_{i}} \frac{\partial g}{\partial p_{i}} -
\frac{\partial f}{\partial p_{i}} \frac{\partial g}{\partial q_{i}}
\right]. $$
The anticommutator is defined as... | Regarding the significance of the observables momentum and position there are many similarities between Classical and Quantum mechanics. Some of the algebraic relations have been pointed out.
In the end, there is still an important difference, which is obvious by the fact that the function algebra generated by classic... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/19770",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "71",
"answer_count": 6,
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What is the meaning of speed of light $c$ in $E=mc^2$? $E=mc^2$ is the famous mass-energy equation of Albert Einstein. I know that it tells that mass can be converted to energy and vice versa. I know that $E$ is energy, $m$ is mass of a matter and $c$ is speed of light in vacuum.
What I didn't understood is how we wil... | c is a priori not the speed of light. It is the speed of massless particles. The way it comes about is as follows: You construct the Lorentz-transformations as the symmetry transformations of Minkowski space. The group has one parameter, that's c. You have to fix it by physical means. You can look at the dynamics of ma... | {
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"answer_id": 2
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Rotating fluid under gravity, fluid dynamics question An incompressible inviscid fluid is rotating under gravity g with constant angular
velocity $\Omega$ about the z-axis, which is vertical, so that $u = (−\Omega y, \Omega x, 0)$ relative
to fixed Cartesian axes. We wish to find the surfaces of constant pressure, and
hen... | In the Physics Forums you should find the answer to your exact problem 'Fluid dynamics - finding pressure for a rotating fluid'. Another answer is here (the point is that Bernoulli's law is applicable only along a streamline so that we must use a rotating frame and add the centripetal acceleration).
From another point ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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For someone who only studied electromagnetism, what is the modern way to explain electromagnetic fields? After reading most of the electromagnetism chapters of Feynman's lectures on physics, I would like to understand in more detail, at least an idea, of what causes the electromagnetic fields. Not sure where to go.
For... | The force acts between bodies, the electric field determines the force into the equations of motion of bodies. The body interaction is generally retarded so the field depends on time in a retarded way. The total electric field of a given charge is a sum of a "near field" that depends on time, but decays with distance a... | {
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Why don't electrons crash into the nuclei they "orbit"? I'm having trouble understanding the simple "planetary" model of the atom that I'm being taught in my basic chemistry course.
In particular,
*
*I can't see how a negatively charged electron can stay in "orbit" around a positively charged nucleus. Even if the ... | Think a little further. When the electrons get accelerated closer to the nucleus they radiates some energy away which fills the vacuum and get scattered by other electrons, accelerating them. This finally becomes an equilibrium condition. This was calculated by Puthoff in 1987.
Ground state of hydrogen as a zero-point... | {
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Can the charge of particles spontaneously flip from positive to negative or vice versa? I'm thinking of matter antimatter annihilation, are there reactions where normal matter converts to antimatter?
| Short, short answer: no.
Short answer: That would violate various conservation laws.
Discussion:
To begin with I want to exclude from discussion those particles that are their own anti-particles--the force carrying bosons and the compound particles like the neutral pion that are symmetric under the charge-exchange oper... | {
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How Does Dark Matter Form Lumps? As far as we know, the particles of dark matter can interact with each other only by gravitation. No electromagnetics, no weak force, no strong force. So, let's suppose a local slight concentration of dark matter comes about by chance motions and begins to gravitate. The particles wo... | In fact, three and more body purely gravitational interactions can form clusters and clumps by concentrating some particles and expelling others, as mentioned in a previous answer. Astronomers have discovered that this seems to happen quite rapidly. They call it "violent relaxation". Google "violent relaxation" for m... | {
"language": "en",
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Physics book for 15 year old boy
Possible Duplicate:
List of good classical physics books
my name is Bruno Alano. As stated in the title, I'm 15 years old (I'll do 16 on 7 of Feb) and much love Computer Science (C, C++), Mathematics and Physics.
Some information may have been unnecessary, but my question is: What is... | It is not a physics books, though this book will make you a physics lover.
Surely You are joking Mr. Feynman
If you can get books for highschool that would be a good start, don't rush yourself, learning physics takes time and hard work.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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How are the Pauli matrices for the electron spin derived? Could you explain how to derive the Pauli matrices?
$$\sigma_1 = \sigma_x =
\begin{pmatrix}
0&1 \\ 1&0
\end{pmatrix}\,, \qquad
\sigma_2 = \sigma_y =
\begin{pmatrix}
0&-i\\ i&0
\end{pmatrix}\,, \qquad
\sigma_3 = \sigma_z =
\begin{pmatrix}
1&0\\0... | Yet another way is from almost purely experimental considerations. Consider a Stern-Gerlach set up; the identity is
$$\mathbb{I}=|\uparrow\rangle \langle \uparrow |+|\downarrow\rangle \langle \downarrow | $$
Now experimentally we can only get two different numbers, $\hbar/2$ and $-\hbar/2$, and regardless of how we ar... | {
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Would a thinner atmosphere (such as high altitude) yield better solar panel energy generation? I am curious. If you were to put a solar panel up in high altitude or on the surface of the moon, both of which have much less atmosphere to reflect/refract light, would that solar panel produce more energy?
I am curious beca... | There is a real problem operating equipement at high altitude , such as observatories in the Chilean high deserts and in Antarctica (> 15000ft) - UV radiation destroys most plastics very quickly. I don't know what it would do to silicon solar panels but making anything else work there is a real pain
| {
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What is the physical definition of causality? Maxwell's equations give a physical relationship between the electric and magnetic fields $\vec E$, $\vec B$ at the same time, which some interpret as changes in one causes changes in the other etc. I find this confusing because to me, the cause of both is charge and cause ... | I may add to the above already-excellent answer, that the reasoning behind the introduction of magnetic field can be explained a little less mathematical.
Basically, first thing to consider is an electric charge. When it does not move in our reference frame it does not produce any magnetic field, just electric one. No... | {
"language": "en",
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Calculating lagrangian density from first principle In most of the field theory text they will start with lagrangian density for spin 1 and spin 1/2 particles. But i could find any text where this lagrangian density is derived from first principle.
| Try Steven Weinbergs comprehensive The Quantum Theory of Fields (Vol. 1, "Foundations"). He follows a very systematic approach from "first principles", i.e. from Wigner's classification of unitary irreducible representations of the Poincaré group, over free fields for different mass/spin configurations (including spin ... | {
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What are the practical applications of decoherence? Let me clarify this question somewhat. I know decoherence is ubiquitous in nature and explains the emergence of a classical world from quantum physics. My question is really about how a knowledge of how decoherence actually works can be put to use in a practical appli... | Since very weak interactions are sufficient to significantly decohere a quantum system, quantum systems can potentially be used as very sensitive force sensors if their decoherence is monitored. This monitoring can take the form of interferometric measurements in which the fringe visibility is measured as a function of... | {
"language": "en",
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Static plane in an inertial frame of reference Suppose we are given a mechanical frame consisting of two points. How can we prove that assuming any initial conditions there is an inertial frame of reference in which these points will be in a static plane?
| Here is one interpretation of the question(v1). In the framework of non-relativistic Newtonian mechanics, let us consider an initial frame and two point point masses with initial positions ${\bf r}_1(0)$ and ${\bf r}_2(0)$; and initial velocities ${\bf v}_1(0)$ and ${\bf v}_2(0)$. (By a translation of the initial frame... | {
"language": "en",
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What is the velocity area method for estimating the flow of water? Can anyone explain to me what the Velocity Area method for measuring river or water flow is?
My guess is that the product of the cross sectional area and the velocity of water flowing in a pipe is always constant. If the Cross sectional area of the pipe... | I am assuming that you want the relation for a noncompressible fluid ($\rho$ is constant).
Well, what we are doing is conserving the volume(i.e. mass) of fluid entering and leaving the pipe in unit time. Thus, $\frac{dV}{dt}=const$ (more correctly $\frac{\partial V}{\partial t}$). As $V=Ax$, $\frac{dV}{dt}=A\frac{dx}{d... | {
"language": "en",
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Could a Dyson sphere destroy a star? Freeman Dyson proposed that the power needs of an advanced civilisation would eventually require the entire energy output of a star to be collected, so that the star would end up surrounded by a dense network of satellites extracting power from the radiation. In science fiction thi... | Of course not. A star doesn't care what energy is collected from its
radiation.
| {
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How can a human eye focus on a screen directly in front of it? I am asking this question here because I think the answer has something to do with the way light is bent as it's captured through the eye.
I saw a show a while ago about tiny screens on contact lenses to pull up data on objects you see in the real world, I ... | Sadly I don't have a pair of the Google glasses to play with, but I have seen similar things over the years, and these worked as projectors not screens.
You're quite right that it would be impossible to focus on a screen in the lens of your spectacles as it's far too close to the eye. However a projector mounted on the... | {
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Electrodynamics textbooks that emphasize applications Please recommend undergraduate-level textbooks on electrodynamics which emphasize practical applications and real life examples. Please describe the book's level and contents and its intended audience in as much detail as possible.
Please provide both applications ... | A text that I use as a supplement for my undergraduate E&M class is:
Zangwill, Modern Electrodynamics, 2013
This is definitely a more advanced textbook than the usual upper-class E&M texts; I'd put it somewhere between Griffiths and Jackson. Where it excels is in the range of applications provided for the upper-lev... | {
"language": "en",
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What formulations of QM are there? It is usually said that there are different formulations of QM, for example historically there was Schrodinger's (wave mechanics), and Heisenberg's (matrix mechanics), then Dirac's (which showed they are equivalent)
Since they are all physically equivalent I have a few questions:
1-Is... | 2) Path integral formulation sometimes called another (Fynman) formulation of quantum mechanics, as opposed to Dirak and Heisenberg ones.
Density matrices are to my knowledge never called another formulation.
AFAIK, other more advanced approaches are not called another formulations partly due to the fact that they ar... | {
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Does EM radiation (any, i.e. RF), or sound, radiate everywhere at once? I am having trouble understanding electromagnetic radiation (or waves in general, be it EM or sound). If I have a 1 Watt speaker, is it infinitely divided and spread out so that everyone in every direction around the speaker can hear it?
I do not b... | Electromagnetic or acoustic waves cannot be "infinitely divided", but the minimal "portions" of the waves (quanta - photons or phonons) typically have very low energy. Radiation from an acoustic speaker or an electromagnetic antenna propagates in all directions, but if the speaker or the antenna is directional, the so ... | {
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Degree of freedom paradox for a rigid body Suppose we consider a rigid body, which has $N$ particles. Then the number of degrees of freedom is $3N - (\mbox{# of constraints})$.
As the distance between any two points in a rigid body is fixed, we have $N\choose{2}$ constraints giving $$\mbox{d.o.f} = 3N - \frac{N(N-1)}{2... | You're double counting here. Lets take three particles. You're counting $\binom{3}{2}=3$ DOFs, right? But fixing the vector distance between particle 1 and two, and then fixing it between 2 and 3 includes fixing it between 1 and 3. Mathematically, $\vec{d}_{1,3}=\vec{d}_{1,2}+\vec{d}_{2,3}$
The easier way to count DOFs... | {
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Finding distance when the force is a function of time I'm having trouble with this homework question
A mysterious rocket-propelled object of mass 49.0 kg is initially at rest in the middle of the horizontal, frictionless surface of an ice-covered lake. Then a force directed east and with magnitude $F(t) = (16.3\text{ ... | Your mistake here is that you're using the three equations of motion ($v=u+at$ et al), even though they are only applicable for constant acceleration.
The correct way is this (i'm writing 16.3 as $k$) :$$F=ma=kt$$
$$\therefore a=kt/m$$
$$\therefore \frac{dv}{dt}=kt/m$$
$$\therefore dv=\frac{ktdt}{m}$$
$$\therefore \int... | {
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Looking for the name of a particular device Please move this if it's not in the right location.
I'm looking for the name of a device that I frequently see in many scenarios, specifically that of an office/library which can be described as having multiple rings that rotate in various directions. I was thinking it was a ... | The device you are describing is a Cardan suspension.
| {
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Calculating Impact Velocity Given Displacement and Acceleration
Assume a car has hit a wall in a right angled collision and the front bumper has been displaced 9 cm. The resulting impact is 25g. Also, it is evident by skid marks that the car braked for 5m with an acceleration of 1.5m/s^2. What is the impact velocity i... | I always look for the easiest way to describe questions like this, and I think the easiest way is to do it in reverse. Start with the car stationary and accelerate it at 25g for 9cm, then accelerate it at 1.5ms$^{-1}$ for 5m. You've correctly identified the relevant equation of motion:
$$v^2 = u^2 + 2as$$
So for step 1... | {
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Mechanism by which electric and magnetic fields interrelate I read that force due to electric field on some particle in one reference frame can exhibit itself as force due to magnetic field in some other reference frame and that electric and magnetic fields are two aspects of same underlying electromagnetic field.
My ... | https://ocw.mit.edu/courses/materials-science-and-engineering/3-a08-attraction-and-repulsion-the-magic-of-magnets-fall-2005/assignments/mag_relativity.pdf
Have a look at this link - it will probably be better than any of our explanation.
| {
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What determines color -- wavelength or frequency? What determines the color of light -- is it the wavelength of the light or the frequency?
(i.e. If you put light through a medium other than air, in order to keep its color the same, which one would you need to keep constant: the wavelength or the frequency?)
| For almost all detectors, it is actually the energy of the photon that is the attribute that is detected and the energy is not changed by a refractive medium. So the "color" is unchanged by the medium...
| {
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What is the origin of the naming convention for position functions? In physics, position as a function of time is generally called $d(t)$ or $s(t)$. Using "$d$" is pretty intuitive, however I haven't been able to figure out why "$s$" is used as well. Is it possibly based on another language?
| As commenters have pointed out, it's German Strecke.
Note that $s$ is for displacement, whereas $d$ is for distance. Distance is the distance along the path traveled by a body, whereas displacement is the birds-eye distance traveled. Displacement can also be negative in 1-D, depending upon your reference positive direc... | {
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Example in the book: A simple accelerometer A simple accelerometer
You tape one end of a piece of string to the ceiling light of your car and hang a key with mass m to the other end (Figure 5.7). A protractor taped to the light allows you to measure the angle the string makes with the vertical. Your friend drives the ... | Acceleration and force are not the same thing.
So first simplify the problem by just talking about acceleration.
For example, suppose the car is accelerating forward at a rate of 1.0 meter per second per second. It also feels an "acceleration" due to gravity, which is 9.8 m/(s^2).
If you want to know the angle at whic... | {
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Have red shifted photons lost energy and where did it go? I think the title says it. Did expansion of the universe steal the energy somehow?
| The short answer is "yes". The energy lost from the photons is taken up by the energy in the gravitational field. Of course energy is a relative concept but if you take the simplest case of a spatially flat homogeneous cosmology with no cosmological constant then the equation for energy in an expanding volume $V(t) = a... | {
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How to find the value of the parameter $a$ in this transfer function? I am given a transfer function of a second-order system as:
$$G(s)=\frac{a}{s^{2}+4s+a}$$
and I need to find the value of the parameter $a$ that will make the damping coefficient $\zeta=.7$. I am not sure how to do this but I might have found somethi... | You have the answer.
Consider $2*\zeta*\omega_n = 4$. $\zeta = 0.7$. $\omega_n^2 = a$ What value of $\omega_n$ (or $a$ in your case) satisfies this?
Roughly 8.18 is your answer.
| {
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Expected Energy Production From High Efficiency Solar Cells First, a bit about my thoughts. I believe we have the capability today to provide energy, water, food, education, and transportation to every man woman and child on the planet. To that end, I would like to become a force that brings about this change.
In try... | Have a look at this innovative solar panel technology from CERN.
The ultra-high
vacuum provides the panels' heat chambers with exceptional insulation, vastly reducing heat loss and greatly improving efficiency. "We've had temperatures of 80°C inside the panel when the panels were covered in snow", says Benvenuti.T... | {
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"source": "stackexchange",
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Irreducible representation in physics I am confused about something.
Group theory books written for physicists say that any reducible representation can be decomposed in terms of irreducible representations (so correct me if I am wrong, to me irreducible representations are like the unit vectors i j k in terms of whi... | The physical motivation is pretty simple. In quantum mechanics this means that if Hamiltonian is invariant under all $g\in G$ you may use the fact that the solutions of this Hamiltonian form a (reducible) representation of this group (as any other full system). Here comes representation theory. It is pretty easy to sho... | {
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What do the dimensions of circulation mean, and how is circulation related to action? The dimensions of circulation $\int_C \vec{v}\cdot d\vec{r}$ seem strange, but if you include
(even a constant) density $\rho$, then $\int_C \rho\vec{v}\cdot d\vec{r}$ has dimensions
the same as action/volume. Is there any significan... | Don't confuse circulation with the vortex that produces it. Circulation is irrotational flow, vortices are rotational and are found in the boundary layer. The flux of the vorticity is circulation. In aerodynamic applications, I find it helps to understand its physical meaning by dividing the circulation by the distance... | {
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Are there more bosons or fermions in the universe? The question is in the title: are there more bosons or fermions in the universe? Or is there the same number of bosons and fermions?
I think there is the same number but I don't know why exactly.
| I suppose that the question is ill-posed for at least three reasons.
Since Fermions and Bosons can be generated from each other, there is a certain dynamics involved. Therefore: When would you count those numbers? The problem I see is connected with defining the (global?) time in which to count - given a lack of simult... | {
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Why does optical pumping of Rubidium require presence of magnetic field? The optical pumping experiment of Rubidium requires the presence of magnetic field, but I don't understand why.
The basic principle of pumping is that the selection rule forbids transition from $m_F=2$ of the ground state of ${}^{87} \mathrm{Rb}$... | There are two kinds of optical pumping that are possible. Hyperfine Optical pumping and Zeeman Optical Pumping. The latter is under zero magnetic field and former requires a magnetic field. The spin-wave (ground state coherence) you create depends mainly on the polarization of the light being used and its intensity. If... | {
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How can people do music with Tesla coils? I saw a lot of videos of Tesla coils doing music on YouTube. And I wonder how can they do that sort of things.
How they can calculate what tone it is going to do? And what are the factors to consider?
| In a simplified picture a Tesla coil is just a transformer which creates a high voltage, high frequency current between the top part and the grounded bottom.
This transformer receives it's energy from an AC source. If you modulate the AC source (amplitude modulation) the sound created from the sparks of the Tesla coil... | {
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Do neutron stars reflect light? The setup is very simple: you have a regular ($1.35$ to $2$ solar masses) evolved neutron star, and you shine plane electromagnetic waves on it with given $\lambda$. Very roughly, what shall be the total flux of absorbed/scattered EM radiation?
Shall the result change if the neutron star... | A neutron star will have a thin layer of normal matter at the surface, and of course this reflects light just like any other normal matter.
But I guess you're really asking if neutronium reflects light, and that's a very good question that a quick Google failed to answer. EM radiation generally interacts with dipoles o... | {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Why is the conductor an equipotential surface in electrostatics? Since the electric field inside a conductor is zero that means the potential is constant inside a conductor, which means the "inside" of a conductor is an equal potential region.
Why do books also conclude, that the surface is at the same potential as we... | This result can be understood mathematically. Suppose the system has reached equilibrium and all charges have stopped moving so that electrostatics applies. Then the potential is a harmonic function $\Delta \varphi = 0$ in $\mathbb{R}^3$ and the conductor is a closed and bounded region in $\mathbb{R}^3$.
A general prop... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/22776",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "15",
"answer_count": 5,
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Decoherence when no one is looking? I understand that in the single-electron-at-a-time double slit experiment, if a detector is placed before the slit, the interference pattern vanishes.
Suppose I left the detector on, but put a bag over its screen (I can't tell what state the electron is before it passes the slits), d... | No. The electron doesn't care at all whether you look at the screen or don't. It's a common misunderstanding from taking the notion of "observer" too seriously. In quantum mechanics, the "observer" is anything that causes decoherence. In your example, that's the detector. Putting a bag over the screen doesn't change an... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/22824",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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What shape is needed to contain a blade made of plasma? Although this may stray into the subject of fiction, this question requires physics expertise.
If one were able to create a strong enough magnetic field to contain a blade of plasma, what shape would be needed to contain it in a loop?
| The problem with using magnetic fields to contain plasma is that the charged particles move at right angles to the field you're applying. This makes it exceedly difficult to contain them. Tokamak reactors manage it by holding the plasma in a loop so that when the particles move at right angles to the field they just go... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Slit screen and wave-particle duality In a double-slit experiment, interference patterns are shown when light passes through the slits and illuminate the screen. So the question is, if one shoots a single photon, does the screen show interference pattern? Or does the screen show only one location that the single photon... | The answer is yes to both questions: yes, the screen does show one location for one particle and yes, the accumulated picture after repeating the experiment many, many times does show the interference pattern.
There is a set of beautiful pictures and a video of the double slit experiment in one-particle-per-time mode t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/22923",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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How can a Higgs decay to heavier products than its mass? How is it possible that a higgs at ~125 GeV can decay into 2 W bosons @ ~ 80 GeV a piece (for example)? Shouldn't a particle only be allowed to decay to lighter particles + energy?
Diagram copied from this question
| The decay channels of the Higgs boson on your graph contain decays to virtual particles as well. In particular, one talks about $ZZ^*$ and $WW^*$ decays, too. The asterisk means that the particular particle which carries the asterisk is virtual. Its being virtual means that its energy and momentum don't have to obey
$... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/22974",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
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Are we inside a black hole? I was surprised to only recently notice that
An object of any density can be large enough to fall within its own
Schwarzschild radius.
Of course! It turns out that supermassive black holes at galactic centers can have an average density of less than water's. Somehow I always operated u... | Only when you're not looking at it. When you are looking, presuming it's expanding, it is a white hole.
Seriously, if we are inside a true singularity, then all of time is included within it, so issues of redshifting, movement, even gravity, etc., are "red herrings" -- artifacts of the observer's frame of reference t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/23118",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "62",
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Is chaos theory essential in practical applications yet? Do you know cases where chaos theory is actually applied to successfully predict essential results? Maybe some live identification of chaotic regimes, which causes new treatment of situations.
I'd like to consider this from the engineers point of view. By this I ... | I don't have a very detailed list of applications, but off the top of my head, I would say the Stability of Solar System is an essential prediction. Yes, I understand that it is not extensively essential, because I can't imagine what we would have done if, on paper, the System turned out to be unstable.
As far as the a... | {
"language": "en",
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"source": "stackexchange",
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Where is the flaw in deriving Gauss's law in its differential form? From the divergence theorem for any vector field E,
$\displaystyle\oint E\cdot da=\int (\nabla\cdot E) ~d\tau$
and from Gauss's law
$\displaystyle\oint E\cdot da=\frac{Q_{enclosed}}{\epsilon_0}=\int \frac{\rho}{\epsilon_0}~d\tau$
Hence,
$\displaystyle... | The equation
$$\displaystyle\int_{V}\frac{\rho}{\epsilon_0}d\tau=\int_{V}(\nabla\cdot E)~d\tau$$
is true for all region $V$ in space the integration is performed over. That is why it follows that the integrands are equal.
Your counterexample is invalid, because the integrals are equal only when the domain of integrati... | {
"language": "en",
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Why do books have dog ears? I googled the question and found no explanation. It seems that dog ears are inevitable (for paperbacks, notably) even if you've always been careful. From my experience, they are about equally likely to appear on the top corners as on the bottom corners (for both the beginning pages and the e... | I think this is primarily about plastic (non-reversible) deformation. Plastic deformation appears when the stress of material is large enough. Stress is a generally speaking ratio between "force" and "dimension" of the object. At the corners, this ratio is larger, even for the same force, as "dimension" is smaller, ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/23485",
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"source": "stackexchange",
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Why does a glass rod when rubbed with silk cloth aquire positive charge and not negative charge? I have read many times in the topic of induction that a glass rod when rubbed against a silk cloth acquires a positive charge. Why does it acquire positive charge only, and not negative charge?
It is also said that glass ... | Well this can be explained by the work function of materials. Due to rubbing, heat is generated which supplies energy for removal of electrons. As the work function of the glass rod is smaller than the silk cloth, it easily loses electrons to the silk cloth which then releases energy (electron gain enthalpy) and thus e... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/23515",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "26",
"answer_count": 5,
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What is the fate of a 3-Torus universe? Since it is flat, will it expand forever like a flat and open universe or collapse like a closed and curved universe?
| Starting with $\mathbb{T}^3$ with the standard metric, it is just $\mathbb{R}^3/\mathbb{Z}^3$. In particular, taking the FLRW ansatz $\mathrm{d}s^2 = -\mathrm{d}t^2 + a(t)^2 \mathrm{d}\Sigma^2$ where $\mathrm{d}\Sigma^2$ is the flat Euclidean metric, you see that modding out the spatial slice by translations you get im... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/23575",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
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How to combine the error of two independent measurements of the same quantity? I have measured $k_1$ and $k_2$ in two measurements and then I calculated $\Delta k_1$ and $\Delta k_2$. Now I want to calculate $k$ and $\Delta k$.
$k$ is just the mean of $k_1$ and $k_2$. I thought that I would need to square-sum the error... |
$k$ is just the mean of $k_1$ and $k_2$
No, the best value of k is calculated using a weighted mean, weighting by the reciprocals of the squares of the respective individual uncertainty values. An accurate measurement must contribute more to the best value than an inaccurate measurement.
I thought that I would need... | {
"language": "en",
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"source": "stackexchange",
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Is the EmDrive, or "Relativity Drive" possible? In 2006, New Scientist magazine published an article titled Relativity drive: The end of wings and wheels1 [1] about the EmDrive [Wikipedia] which stirred up a fair degree of controversy and some claims that New Scientist was engaging in pseudo-science.
Since the origina... | No. In special relativity, 4 momentum is exactly conserved. The first component of 4 momentum is total mass/energy, but the next 3 are given by:
p = m*γ(v)*v
m is the invariant mass, how much inertia it has when you are moving at the same velocity of it.
This is Newton except now momentum is a non-linear function of ve... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/23725",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "27",
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Is it possible to mathematically derive the formula for resistance? Resistance is given by $\rho L/A$, where $\rho$ is the material constant, $L$ is the length, and $A$ is the area.
Is there any way that this can be derived mathematically, or is the only way experimentally?
Personally, I think experiment is the only wa... | The answer is "yes", if you take for granted that $R$ is defined by the relation $\Delta V=IR$. In fact it is derived from (the real) Ohm's Law.
Ohm's law states that, for some materials (the so-called "Ohmic" materials) the current density vector $\vec{J}$ (current per unit area) is parallel to the electric field $\ve... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/23813",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
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Why there is a $180^{\circ}$ phase shift for a transverse wave and no phase shift for a longitudinal waves upon reflection from a rigid wall? Why is it that when a transverse wave is reflected from a 'rigid' surface, it undergoes a phase change of $\pi$ radians, whereas when a longitudinal wave is reflected from a rigi... | When a transverse wave travel in a medium the particle velocity is in upward direction &wave velocity is in forward direction.When a transverse wave meet the surface of the wall,it exert force in upward direction,because particle velocity of wave is in upward direction.so wall also exert force in downward direction(New... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/23847",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
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Why would it be true that people with longer legs walk faster than ones with shorter legs? When a person walks, the only force acting on him is the force of friction between him and the ground (neglecting air resistance and all). The magnitude of acceleration due to this force is independent of the mass of the object (... | I think the simplest model that may be useful here is to treat the legs as simple pendula. In "steady state" comfortable walking, it is reasonable to assume that the legs oscillate close to their natural frequency. That is, the forward contacting leg lifts allowing the rear to swing forward freely over the stride. For ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/23921",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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The equivalent electric field of a magnetic field I know that Lorentz force for a charge $q$, with velocity $\vec{v}$ in magnetic field $\vec{B}$ is given by
$$\vec{F} =q \vec{v} \times \vec{B}$$
but there will exist a frame of reference where observer move at same velocity with that of charge $q$, so according to him ... | I haven't read them, but this, this, this and this thread (I thank a diligent Qmechanic) are related and clear up the but why-questions you might have.
The transformation of the quantities in electrodynamics with respect to boosts are
$$
\begin{alignat}{7}
\mathbf{E}'&~=~ \gamma \left(\mathbf{E} + \mathbf{v} \times \m... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/24010",
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"source": "stackexchange",
"question_score": "10",
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Is the Avogadro's constant equal to one? Question: Is the Avogadro's constant equal to one?
I was tasked with creating a presentation on Avogadro's work, and this is the first time I actually got introduced to the mole and to Avogadro's constant. And, to be honest, it doesn't make any mathematical sense to me.
1 mole =... | Unfortunately, I can't post comments, so I have to write it this way. The statement 1 mole = 6.022 * 10^23 (which you use to show that N_A=1) simply doesn't hold - at least it didn't hold when they taught chemistry in my class.
It's $1 \text{ mol} = 6.022\cdot10^{23}/N_A$, isn't it?
P. S. And what exactly did I do to ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/24034",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "17",
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Why is there a 90˚ phase angle between particle velocity and sound pressure in spherical waves? My text says that in a plane sound wave (or in the far field), particle velocity and pressure is in phase. As we move closer to the sound source (to near field and more spherical waves), the phase angle between these two qua... | It's a little easier to understand if you look at the electrical case with cylindrical geometry: a long wire carrying AC current. If you are far from the wire, the E and B fields are in phase, because they represent radiated power flowing away from the wire. But close to the wire, you are in the range where inductive c... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/24091",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 3,
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Operators Uncertainty $\hat A$ is an operator.
The uncertainty on $\hat{A}$, $\Delta A$ is defined by:
$$\Delta A=\sqrt{\langle\hat A^2\rangle - \langle\hat A\rangle^2}$$
what is difference between
$\langle\hat A^2\rangle$ and $\langle\hat A\rangle^2$
that leads to Uncertainty Relation between two Operators?
more detai... | *
*$\langle\hat A\rangle$ is the expectation value of $\hat A$.
*$\langle\hat A\rangle^2$ is the square of item 1.
*$\langle\hat A^2\rangle$ is the expectation value of $\hat A^2=\hat A \hat A$.
Item 2 and 3 do not have to be equal.
If $\hat A$ is selfadjoint, then it is possible to show
*
*that the expectation... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/24178",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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Entanglement spectrum What does it mean by the entanglement spectrum of a quantum system? A brief introduction and a few key references would be appreciated.
| If a system S is composed of two subsystems A and B, then a state of S is a vector $$|\Psi\rangle \in H_A\otimes H_B$$ Tracing over the "B degrees of freedom" allows you to define the reduced density matrix $\rho_A$ The entanglement entropy is defined as$$-Tr(\rho_Aln\rho_A)$$
I believe that the entanglement spectrum... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/24245",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
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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?
*
*Atoms' composi... | In fact, some nuclear theorists do believe that there will be relatively stable heavy elements, as per your point 2. The so-called Island of Stability is predicted to occur because stability is maximized at certain so-called magic numbers which correspond to especially stable isotopes when the number of protons and/or ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Proton-Neutron Lattice as a form of matter? Would it be possible for a lattice of protons and neutrons (I'm picturing a plane of hexagons in my head) to exist bound by the strong nuclear force (not gravity)? I know that the strong force losses its power when an atomic nucleus gets to be too large, but in a lattice, it ... | A lone proton in between a crystal lattice could find itself with more negative charge then equilibrium from the neighbouring clouds of electrons. Thus there is a compressive force driven by the lattice and heat, in addition to proton repulsion of the nuclear force.
I expect over time their will be a determination tha... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/24403",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
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Ideal gas in a vessel: kinetic energy of particles hitting the vessel's wall Reading Landau's Statistical Physics Part (3rd Edition), I am trying to calculate the answer to Chapter 39, Problem 3.
You are supposed to calculate the total kinetic energy of the particles in an ideal gas hitting the wall of a vessel contain... | The following calculation gives the correct answer:
$$Z\int_0^{\pi/2}\int_0^\infty 2\pi v \sin\theta\; v\; \mathrm{d}\theta\mathrm{d}v\; e^{-mv^2/2kT}\; v \cos\theta\; \frac{1}{2}mv^2,$$
where $Z$ is such that
$$Z\int_0^{\pi}\int_0^\infty 2\pi v \sin\theta\; v\; \mathrm{d}\theta\mathrm{d}v\; e^{-mv^2/2kT} = n,$$
where ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/24595",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
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Possibility for radiation in dark matter that is not interactive with regular matter? Definition: Radiation in this case does not refer to electromagnetic radiation. It refers to any kind of emission of energy, even energy that does not interact with regular matter.
Just like dark matter does not interact with electro... | Yes, just as you have bosons mediating interactions in the Standard Model sector, you could have dark radiation mediating interactions in the DM sector. Given that more energy is in the form of DM than SM particles, this won't be surprising. Of course, one needs proof for such a species of particles to exist :) A parti... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/24793",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 3
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Is there a good explanation for the observation of Martian canals? Martian "canals" have been observed by independent observers after their first description. Now, they are attributed to "optical illusion", but I think that this is not a good choice of word, because an optical illusion should be visible today as well. ... | The human brain likes to find patterns in what it observes. That's why we see patterns in the stars which we call "constellations." The canals of Mars are similar. For most of them, there is something there, just at the edge of vision, and the human tendency is to "connect the dots" and see lines were only vague streak... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/24863",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 0
} |
What do the colors in false color images represent? Every kid who first looks into a telescope is shocked to see that everything's black and white. The pretty colors, like those in this picture of the Sleeping Beauty Galaxy (M64), are missing:
The person running the telescope will explain to them that the color they ... | The images that are currently taken with the High End, (read: $$$), Astro-cameras, indeed produce complete full color images. A perfect example is http://www.kevindixon.westhost.com/Deep_Sky_CCD-Siciliano.htm. This is one of many by this particular Astrophotographer. None of the color is false at all. Astrophysicist... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/24895",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "24",
"answer_count": 5,
"answer_id": 4
} |
What objects look best in an O-III filter? I've heard that an O-III (Oxygen III) filter is great for planetary nebulae.
Is this true for all planetary nebulae, or just some or most?
What other target types are often improved with an O-III filter?
| The dominant OIII emission line is the $^3P-^1D_2$ line, which is at 2.51 eV. If you want to see a pretty picture, you'll want a wide field of emission of this line, which would mean that you need a gas cloud which is exciting the OIII line thermally. The thermal energy required to excite the 2.51 eV line is around 8,0... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/24987",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
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Recommend good book(s) about the "scientific method" as it relates to astronomy/astrophysics? I am interested in astronomy/astrophysics, but I am not science major (I am a computer science graduate). Facts and results of the field are presented to the public without showing how these facts/results got known or inferred... | You can refer :-
Basics of astronomy
It will help you to understand the basics of astronomy.
The Solar system and stars
It will help you to understand our solar system and stars.
From stars to our Galaxy
It will give you the information about stars formation and About Galaxies.
Expanding Universe
It will give you th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/25030",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "16",
"answer_count": 3,
"answer_id": 2
} |
Are solar physics images of use to the night-time community? I'm actually interested in cases of cross-discipline data re-use.
I know that the SOHO/LASCO coronographs are used for comet finding, that solar telescopes were used to get information about Venus's atmosphere during its solar transit in 2004, and STEREO were... | I've been pondering your question for a couple of days, and the only answer I can think of from my own experience is that Coronal Mass Ejections trigger aurora on Earth, and aurora can seriously interfere with night-time observations of faint objects. I'm thinking of an occasion when I travelled to an extremely dark si... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/25118",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
What is the current status of Pluto? Pluto has been designated a planet in our solar system for years (ever since it was discovered in the last century), but in 2006 it was demoted.
What caused this decision? And is there a chance that it could be reversed?
Edit: well, http://www.dailygalaxy.com/my_weblog/2017/03/nasas... | Pluto has been reclassified as a dwarf planet.
It was reclassified as such because a growing number of objects were found similar to Pluto, which exhibited at least one notable difference from the other planets. The choice would have been to accept these other objects as planets or to develop a new class of object.
Th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/25162",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 7,
"answer_id": 3
} |
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