Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Why singularity in a black hole, and not just "very dense"? Why does there have to be a singularity in a black hole, and not just a very dense lump of matter of finite size? If there's any such thing as granularity of space, couldn't the "singularity" be just the smallest possible size?
| Because otherwise general relativity would contradict itself. The event horizon of a black hole is where not even light can escape. Below the horizon all photons must fall. In relativity theory all observers measure the speed of light the same, c; that's a postulate of the theory. Then all physical things (including ob... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/18981",
"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... | Part of the problem in confirming physics theories with experiments is that we don't know all of it. Most likely, there are unthought of circumstances and margins of error in the "neutrino experiments". If discrepancies occur in experiments of such well tested theories, more rigorous testing remains before failure of t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/19142",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Why is spacetime near a quantum black hole approximately AdS? In this link, one of the answers contains the statement
If you examine the space-time near a finite area quantum black hole,
you will see an approximate AdS space.
Presumably "approximate" means this is only true to some order in the distance from the ho... | The answer you quoted was invalid in most respects, including this one. The quantum character of a black hole has nothing to do with the AdS geometry; the AdS geometry is the near-horizon geometry of a classical black hole (or black brane). Which black hole? It has to be an extremal black hole, i.e. it must have the ma... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/19239",
"timestamp": "2023-03-29T00:00:00",
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Why doesn’t gravity break down in a large black hole? By popular theory gravity didn’t exist at the start of the Big Bang, but came into existence some moments later. I think the other forces came into existence a little latter.
When a black hole crushes matter to a singularity (infinite density), at some point should... | A black hole is in many ways like a time reversed version of the Big Bang. If there were several stages of symmetry breaking as the Big Bang evolved, then it's certainly possible that an observer falling into a black hole would see the symmetries restored as they approached the singularity. This wouldn't really mean gr... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Have experiments ever suggested two different values to the same divergent series? I believe to have understood that some physical experiments suggest finite values to divergent series (please correct me if I'm wrong, my understanding of these matters is limited).
I heard, for example, that the "equality"
$$ \sum_{n=... | No physical experiment ever predicts the result of a mathematical formula. A physical experiment may determine whether a certain model, described in the language of math, applies to a particular physical phenomenon.
That being said, divergent series can come up when working within the mathematical framework of quantum ... | {
"language": "en",
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Entropy of a mass arrangement around the earth An mind experiment, taking the entire Earth as an isolated system
Then this is the initial state:
N masses are distributed around the earth, at different height.
(for example we can use a single grain of sand from one building of every city of the world)
Heights are not sp... | An always attractive force, Gravity, seems to be antagonistic with an always disipating "force", Entropy, I mean the mere existence of clumps of matter (planets, stars..) again seems to propitiate the concentration of energy (the opposite of Entropy)..
Where is the trick? (Based on Zephyr/UnbanRonMaimon and Greg P comm... | {
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Why are geons unstable? Are there other problems with geons? I read in various places geons are "generally considered unstable." Why? How solid is this reasoning?
Is the reason geons are not studied much anymore because we can't make more progress without better GR solutions or a better theory of quantum gravity, or i... | The stability argument is as follows--- the Geon system will have some mass, and it is made out of massless fields orbiting in closed orbits, so if you make the geon a little smaller with the same total energy, you expect the gravity to win and the massless fields to collapse into a black hole, and if you make the geon... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Solutions to SHMO equation in Feynman's Thesis I'm reading Feynman's Thesis, and have some background in math and physics, but I'm not sure where Feynman gets his solution to his harmonic oscillator equation. He gives three different formulations. The first includes an integral. Is this standard Fourier Analysis stu... | This comes from linearity, the solution of the homogenous equation, plus the response to a delta-function kick.
The homogenous equation gives the first two terms. The equation
$$ \ddot{x} + \omega^2 x = 0 $$
is solved by a combination of cos and sin which reflect the initial position and velocity
$$ x(t) = x_0 cos(\ome... | {
"language": "en",
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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... | If $\vec{p}$ the vector connecting the center of mass of b1 to the center of mass of b2 then you must have
$$ \vec{v}_2 = \vec{v}_1 + \vec{\omega}_1 \times \vec{p}
\\ \vec{\omega}_2 = \vec{\omega}_1 $$
$$ \vec{a}_2 = \vec{a}_1 + \vec{\alpha}_1 \times \vec{p} + \vec{\omega}_1 \times \vec{\omega}_1 \times \vec{p}
\\ \v... | {
"language": "en",
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What is the meaning of speed of light $c$ in $E=mc^2$? $E=mc^2$ is the famous mass-energy equation of Albert Einstein. I know that it tells that mass can be converted to energy and vice versa. I know that $E$ is energy, $m$ is mass of a matter and $c$ is speed of light in vacuum.
What I didn't understood is how we wil... | A previous answer has provided a beautiful explanation of what $c$ represents, and that is not necessarily related to light. This answer just adds a bit from the nuclear reaction perspective.
You start with a large nucleus, say uranium. Once it splits, it forms two smaller nuclei. The masses of these smaller nuclei put... | {
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Where to find cross section data for $e^{-}$ + $p$ $\longrightarrow$ $p$ + $e^{-}$? Where to find cross section data for $e^{-}$ + $p$ $\longrightarrow$ $p$ + $e^{-}$ ?
PDG's cross-section data listing does not include it.
| you can search the HEPDATA database at http://www.slac.stanford.edu/spires/hepdata/ with the query string
[reac = e- p --> e- p]
and the first result will be:
"Jefferson Lab. Measurement of the elastic electron-proton cross section in the Q*2 range from 0.4 to 5.5 GeV*2"
| {
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Close electric field lines in wave guides In a wave guide, graphics of propagation of Transversal Magnetic modes show closed field lines for the electric field.
For example, for a rectangular guide:
$E_x (x,y,z) = \frac {-j\beta m \pi}{a k^2_c} B_{mn}\cos\frac{m\pi x}{a}\sin\frac{n\pi y}{b}e^{-j(\beta z + \omega t)}$
... | Yes, it is possible. Maxwell's equations say
$$
\oint_l \vec{E}\; d\vec{l} = -\frac{1}{c}\int_{S(l)}\frac{\partial \vec{B}}{\partial t} d\vec{S}.
$$
The electric field of this closed line is proportional to the rate of the change of the magnetic flux.
There is no problem with energy conservation. An electron moving a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/19972",
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How Does Dark Matter Form Lumps? As far as we know, the particles of dark matter can interact with each other only by gravitation. No electromagnetics, no weak force, no strong force. So, let's suppose a local slight concentration of dark matter comes about by chance motions and begins to gravitate. The particles wo... | I suggest dark matter loses energy by proxy though gravitational interactions with ordinary matter that is losing energy through radiative processes. As ordinary matter loses energy by radiation allowing it to clump gravitationally, dark matter clumps along with it by losing energy to the cooling ordinary matter.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/20097",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
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Why does electron-positron annihilation prefer to emit photons? If gravitons are also massless, and neutrinos nearly so, why aren't pairs of either of them normally expected outcomes of electron-positron annihilations? Are they possible but simply unlikely, or is there actually some conserved quantity prohibiting their... | It's possible, just very unlikely. You can get a clue of the relevant probabilities by looking at the Feynman diagrams for different kinds of $e^+e^-$ annihilation. Here's $e^+e^-\to\gamma\gamma$:
The probability of this occurring (actually, the cross section) is proportional to a factor of $g_\text{EM}$ for each vert... | {
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How are the Pauli matrices for the electron spin derived? Could you explain how to derive the Pauli matrices?
$$\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... | That certainly depends on what exactly you mean. I take your question as "how do you see that the (non-relativistic) electron spin (or more generally, Spin-1/2) is described by the Pauli matrices?"
Well, to start, we know that measuring the electron spin can only result in one of two values. From this we see that we ne... | {
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"url": "https://physics.stackexchange.com/questions/20201",
"timestamp": "2023-03-29T00:00:00",
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How does one pronounce this particle's name? How would you read the following particles' names in a conversation in English? I am looking for some "proper" way of doing it. Say, imagine you are reading a technical description in a semi-formal occasion that you would like to avoid being lousy or overly simplistic.
$$\... | For questions about resonances and particles the Particle Data Group is the best reference.
One can find the whole Delta resonance family and remind oneself what each number is standing for, and thus know how to pronounce the symbol.
The number in parenthesis is the mass in MeV. The superscript is the charge of the p... | {
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How is Planck's law defined? Now, I found three different definitions of Planck's law:
$$
P_1(\nu,T) = \frac{8 \pi}{c}\frac{h \nu^{3}}{c^2} \frac{1}{e^{h\nu/kT}-1}
$$
$$
P_2(\nu,T) = 2\frac{h \nu^{3}}{c^2} \frac{1}{e^{h\nu/kT}-1}
$$
$$
P_3(\nu,T) = \frac{h \nu^{3}}{c^2} \frac{1}{e^{h\nu/kT}-1}
$$
Which of these is corr... | Your second equation, $P(\nu,T) = \frac{2 h {\nu}^3}{c^2}$ $\frac{1}{\exp\bigl(\frac{h \nu}{kT}\bigr) - 1}$ is what is commonly referred to as Planck's law for radiation, although a more standard symbol used is $B_\nu(T)$. This is the energy radiated per time, per area, per frequency interval, per steradian. It is a fo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/20328",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
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Calculating lagrangian density from first principle In most of the field theory text they will start with lagrangian density for spin 1 and spin 1/2 particles. But i could find any text where this lagrangian density is derived from first principle.
| The "first principle" for any Lagrangian is the corresponding equation. If you advance, for any particular reason, an equation, you may construct its Lagrangian knowing the structure of the Lagrange equations:$$\frac{d}{dt}\frac{\partial L}{\partial \dot {\phi}}=\frac{\partial L}{\partial {\phi}}$$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/20353",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
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Is a world with constant/decreasing entropy theoretically impossible? We can imagine many changes to the laws of physics - you could scrap all of electromagnetism, gravity could be an inverse cubed law, even the first law of thermodynamics could hypothetically be broken - we've all imagined perpetual motion machines at... | The microscopic laws are reversible in time (if you also change chirality and the sign of all charges). Thus one cannot prove what you'd like to prove.
Statistical mechanics, which is the discipline in which one derives the second law from microphysics, always makes one or the other assumption that induces the directio... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/20401",
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"source": "stackexchange",
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How is gradient the maximum rate of change of a function? Recently I read a book which described about gradient. It says
$${\rm d}T~=~ \nabla T \cdot {\rm d}{\bf r},$$
and suddenly they concluded that $\nabla T$ is the maximum rate of change of $f(T)$ where $T$ stands for Temperature. I did not understand. How gradi... | Have a look at http://en.wikipedia.org/wiki/Del.
Del, or $\nabla$ , is a generalisation of the gradient to more than one dimension. In one dimension $\nabla$ is the same as the gradient.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/20568",
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"source": "stackexchange",
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Can a photon be emitted with a wavelength > 299,792,458 meters, and would this violate c? Just curious if the possibility exists (not necessarily spontaneously) for a photon with a wavelength greater than the distance component of c to be emitted, and would this inherently violate the scalar c?
| See http://en.wikipedia.org/wiki/Ultra_low_frequency
EM frequencies below 1Hz, and therefore with a wavelength longer than c meters can be observed in nature. This does not violate relativity since those waves still propagate with velocity c (in vacuum).
| {
"language": "en",
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How do I find average temperature given a temperature distribution? I was told to find the temperature distribution of a wire with a current going through it. So I found $$T(x)=T_{\infty}-\frac{\dot{q}}{km^{2}}[\frac{cosh(mx)}{cosh(mL)}-1]$$
I need to find the average temperature in the wire using this formula. I know ... | The average of any quantity $s$ is $\frac{\sum\limits_{r=0}^ns_r}{n}$. If the distribution is continuous, lets say as a function of x, then it becomes $\lim\limits_{n\to\infty}\frac{\sum\limits_{r=0}^ns_r}{n}$. This can be rewritten as $\frac{\int s(x)dx}{\int dx}$, taking limits as the length of the wire. In your form... | {
"language": "en",
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Why do we use only nonrelativistic equations in nuclear physics? What is limit between relativistic and non-relativistic equations? Which conditions do we have to use one of these?
| It is useful in beam-based experimental nuclear physics (as opposed to the nuclear power context that Zassounotsukushi discusses) to use energies up to a few GeV. At those energies electrons are highly relativistic, and nucleons are fast enough to that one has to treat them relativistically, but heavy nuclei are genera... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/20958",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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17 Joules of Energy From a Mouse Trap Do you think it would be possible to get 17 joules out of a standard size mouse trap. By my math, it is a torsion coefficient of 3.45 or so out of the spring.
| I haven't used a mousetrap for several decades, but as I recall the moving arm is about 5cm long, so the tip moves 0.05$\pi$ or about 0.16m. To get 17J of work the force at the tip of the arm would need to be 100N. I'm fairly sure the force isn't anything like that great. I remember being able to pull the arm back with... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/21017",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Riemann Tensor Calculation trick(number of element) When we calculate Riemann Tensor for different curvature we have lots of components. However, there are many components that are zero. How can we argue, based on the symmetry of connection , that those elements are zero?
For example if I am calculating the Riemann Te... | The number of independent components for the Riemann curvature tensor $R_{ijk\ell}$ for the Levi-Civita connection is greatly reduced because of symmetries. The last two indices $k\neq \ell$ have to be different, because of antisymmetry
$$R_{ijk\ell}~=~-R_{ij\ell k}.$$
Interchange symmetry
$$R_{ijk\ell}~=~R_{k\ell ij}... | {
"language": "en",
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What's the difference between "boundary value problems" and "initial value problems"? Mathematically speaking, is there any essential difference between initial value problems and boundary value problems?
The specification of the values of a function $f$ and the "velocities" $\frac{\partial f}{\partial t}$ at an initia... | When there is only one spatial variable then mathematically the two are indistinguishable. But often boundary value problems are solved over a higher dimensional domain. For example, a common problem in physics is to solve Laplace's equation over a spatial region of three dimensions, with a two dimensional surface prov... | {
"language": "en",
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What conditions must be met for a ball to roll perfectly down an incline without slipping? What conditions must be met for a ball to roll perfectly down an incline without slipping? A mathematically rigorous definition, please.
I honestly don't know where to begin with answering this problem.
| The formula is $$\mu_s \geq \frac{g\tan\theta}{1+\frac{k^2}{r^2}}$$ where $\mu_s$ is static friction coefficient for the ball-incline interface. $\theta$ is the angle of the incline, and $k$ is the radius of gyration of the ball (for a solid uniform spherical ball, $k=R\sqrt{\frac{2}{5}}$). R is the radius of the ball.... | {
"language": "en",
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What longest time ever was achieved at holding light in a closed volume? For what longest possible time it was possible to hold light in a closed volume with mirrored walls?
I would be most interested for results with empty volume but results with solid-state volume may be also interesting.
| The lifetime of a photon in a resonant cavity is pretty trivial to compute, given the cavity length, internal losses, and mirror reflectivity. Switching momentarily to a wave description, we will let $L$ be the cavity length, $R_1$ and $R_2$ be the reflectivity of mirrors 1 and 3 respectively, and $T_i$ be the loss in ... | {
"language": "en",
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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?)
| As FrankH said, it's actually energy that determines color. The reason, in summary, is that color is a psychological phenomenon that the brain constructs based on the signals it receives from cone cells on the eye's retina. Those signals, in turn, are generated when photons interact with proteins called photopsins. The... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/21336",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "76",
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Does sending data down a fiber optic cable take longer if the cable is bent? Ok, so, my simplified understanding of fiber optics is that light is sent down the cable and it rebounds off the sides to end up at its destination. Which got me thinking, if it has to bounce more times (and having a shorter travel between eac... | One of the main reasons fiber optics is a great engineering tool to distribute light is because virtually all of the light is internally reflected. Meaning there is very little light lost from transmission out of the cylinder. The main reason for this is because firber optics cables are small.
Larger optical fibers cau... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/21378",
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Dielectric in Parallel Plate Capacitor Given a parallel plate capacitor of width $w$, length $l$, with a dielectric moving along the length $l$. Let the dielectric be from $x$ onwards.
The capacitance will be $\frac{w \epsilon_0}{d} (\epsilon_r l - \chi_e x)$. Griffiths (p. 195) says that the total charge $Q$ in the $C... | If the plates of the capacitor are isolated then the total amount of free charge on the capacitor plates cannot change.
Put another way, if the capacitor pales are isolated where could more/less free charge come from / go to?
The situation would be different if there was a voltage source connected to the plates.
In tha... | {
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What happens when a supersonic airplane flies through a cloud? What happens when a supersonic airplane flies through a cloud? Will it punch a hole or is it more like a bullet through water (= hole closes immediately after the aircraft has passed)?
Is there some special effect because of the supersonic speed? Or maybe t... | Couldn't find a cloud image but this is interesting
edit: Apparently this is due to the drop in pressure immediately behind the shock wave of a supersonic aircraft. (Like a moving cloud chamber?)
photo is by US navy and therefore public domain.
There are a whole set of similar images here
| {
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Do the trigonometric functions preserve units? I saw an exercise where you had to calculate the units of $C_i, i=1,2$ from an equation like this:
*
*$v^2=2\cdot C_1x$ and
*$x=C_1\cdot \cos(C_2\cdot t)$
where
*
*$x$ means meters,
*$t$ means seconds and
*$v$ means velocity.
For $C_1$ I got $C_1=m/s^2$. But c... | Trigonometric functions don't "preserve" units. The expression under a trigonometric function must be dimensionless and so is the value of a trigonometric function.
Thus, C2 in your equations is in units of frequency: Hz or 1/s.
There is an error in one of the equations, perhaps a missing constant.
| {
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Noise amplitude increases as sample rate increase I am testing the material properties of some very low stiffness materials.
I'm using a force probe connected to software, sensing at about a hundredth of a gram of force.
Now, what's interesting is when my sample rate is 1/sec I get a smooth line, as expected. If I inc... | From your description of the experiment (please correct me if my assumptions are wrong), it sounds like your apparatus consists of the application of a controlled stress to the sample (and the sensor), and the resulting strain in the sensor is measured. Whenever the stress applied by your apparatus changes, it will tak... | {
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How would you explain spectrum and spectral analysis to grandma? E.g. what the light or sound spectrum is, what it's useful for - in very simple terms that a grandmother or a child would understand.
| To a child you have to show pictures.
A diamond in your ring or a wedge of glass or crystal, then the Sun light, or ..
Then the rainbow and the clouds, all the colors in the cristal, etc ...
Use the filters in the equalizer of your hi-fi (or software media player) to show that the sound is composed of many distinct sou... | {
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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 ${}^{87} \mathrm{Rb}$... | Though it's too late to answer, I was looking for answer and saw your question. I observed the same thing through my experiments today. Zeroing the imposed magnetic field leads to vanish the dark state due to a circularly polarized light.
My reasoning is related to the most basic concepts of quantum mechanics; based on... | {
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What will be the relative speed of the fly? It has happened many times and i have ignored it everytime.
Yesterday it happened again .
I was travelling in a train and saw a fly (insect) flying near my seat.
Train was running at a speed of around of 100 km/hr.
So according to the physics rules , my speed will also be 10... | Have a look at http://en.wikipedia.org/wiki/Galilean_invariance. This is not too mathematical and explains what's going on.
The basic idea is that there is no such thing as absolute motion. For example, because the earth is rotating as I sit here typing I'm moving at about 800 miles per hour. Why am I not splattered ag... | {
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Piston movements in four stroke cycle? I was reading about a four stroke cycle. Here's what I understood:
*
*In the first stroke, the piston starts at the top and moves down.
*In the second stroke, the piston moves upwards.
*In the third stroke, the piston moves down due to the combustion by spark plug.
*In the... | In an internal combustion engine, we have multiple cylinders. They are attached to a shaft in an alternating manner such that when one set of the cylinders have combustion, they drive the shaft to move down in the other set.
See http://commons.wikimedia.org/wiki/File:Cshaft.gif
| {
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Does it take significantly more fuel to fly a heavier airplane? I was reading in the papers how some-airline-or-the-other increased their prices for extra luggage, citing increased fuel costs.
Now I'm a bit skeptical. Using the (wrong) Bernoulli-effect explanation of lift, I get this:
More luggage$\implies$more lift ne... | Lift is roughly proportional to angle of attack, and to speed squared. As a pilot, you instinctively balance these two.
ADDED: Like if you suddenly drop a heavy weight, making the plane lighter, its lift isn't any less, so it starts to accelerate upward (climb). You notice this and either push the nose down with the tr... | {
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What are the calculations for Vacuum Energy? In wiki the Vacuum Energy in a cubic meter of free space ranges from $10^{-9}$ from the cosmological constant to $10^{113}$ due to calculations in Quantum Electrodynamics (QED) and Stochastic Electrodynamics (SED).
I've looked at Baez and references given on the wiki page bu... | You can understand the origin of these numbers from a simple consideration of dimensional analysis, and the cosmological data available. This keeps the answer intuitive, and any more complicated derivation will not change the answer substantially.
The first of your numbers, $10^{-9}$ Joules per cubic meter, is simply a... | {
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Magnetic force and work If the magnetic force does no work on a particle with electric charge, then:
How can you influence the motion of the particle? Is there perhaps another example of the work force but do not have a significant effect on the motion of the particle?
| Work performed by forces acting on a particle is equal to the change in particle's energy. If the forces acting on a particle perform zero work on it, particle's energy does not change.
In particular, whenever a force acting on a particle is perpendicular to the particle's displacement as is the case with magnetic comp... | {
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Double slit experiment and indirect measurements In the classic Young double slit experiment, with slits labeled as "A" and "B" and the detector screen "C", we put a detector with 100% accuracy (no particle can pass through the slit without the detector noticing) on slit B, leaving slit A unchecked. What kind of patter... | Dreelich, you might want to get hold of a copy of the Feynman Lectures on Physics and take a look at Vol. III, Chapter 37, Section 1-6 "Watching the electrons". The sections leading up to that one are also relevant.
In addition to being a great read (well, if you like that sort of thing, but that's likely a safe assu... | {
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Is it possible to fly like a bird using semi-motorized wings?
On his website http://www.humanbirdwings.net/ the dutch engineer Jarno
Smeets claims to have successfully build a set of 17 m^2 bird-like
wings from material of a kite. It is claimed that it uses sensors
taken from Wii controllers and a smart phone as... | http://articles.latimes.com/1986-05-18/news/mn-20955_1_pterodactyl-flight
A motorized "life-sized" pterodactyl model did fly by flapping a few times before it crashed.
A little bit bigger wing span, a little more powerful motor... who knows?
Maybe you also need a much smarter computerized controller. But your conclus... | {
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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... | Because if the surface is not equipotential then it would mean that there is a tangential component of electric field along the surface.
This component will result in motion of electrons, but since we have static fields this is not possible. Thus by contradiction we can say that surface must be equipotential.
| {
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Areas of research and their transferable skills I've noticed that ads for postdoctoral positions emphasize the skill set that one must have for a particular position. That said, what are the areas of research to avoid because they give you few transferable skills and hence limit your range of possible postdoctoral posi... | Coming from a computer science standpoint, I don't know so much about which fields to avoid.
However, I will highly encourage serious physicists to take up general GPU programming. On my side, the boon of physics is obvious. However on research especially dealing with sensors and data, there are so many applications w... | {
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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... | We don't know whether the light source shoots photons or not. We know that if we turn off power to the light source the interference pattern disappears, and that if we turn down the light intensity enough we eventually start seeing individual events, if we have the right sort of measurement apparatus. Again, if we turn... | {
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Flow rate of a syringe Suppose a syringe (placed horizontally) contains a liquid with the density of water, composed of a barrel and a needle component. The barrel of the syringe has a cross-sectional area of $\alpha~m^2$, and the pressure everywhere is $\beta$ atm, when no force is applied.
The needle has a pressure ... | I've already modelled this case and you'll find that the flow is indeed laminar and for a medical syringe (say 5ml) with a 26 or 27G needle you'll get a Re value of under 100. This situ changes if the liquid is more or less viscous e.g. due to temperature.
Typically forces at the plunger are between 2 to 20N.
When usin... | {
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Faster-than-light communication using Alcubierre warp drive metric around a single qubit? The Alcubierre warp drive metric has been criticized on the points of requiring a large amount of exotic matter with negative energy, and conditions deadly for human travellers inside the bubble. What if the Alcubierre metric is u... | The problem is not the availability of exotic matter. It is the manipulation of it in a practical manner that is not known. The stuff of exotic matter (quantum fluctuation) is everywhere. The Casimir Effect is only a demonstration of its existence in a practical manner. To do more with that so called negative energy th... | {
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What is lambda R in Richardson's Law? I've got to calculate the thermionic emission through a diode, so I need to use Richardson's Law. However, one thing's got me confused - according to the Wikipedia page:
$$J = A_GT^2e^\frac{-W}{kt}$$
I could live with that, but for $A_G$. Apparently, I'm not the only one; Wikipedia... | The quantity $\lambda_R$ is the dimensionless extracted tunneling/nucleation amplitude for electrons to get out of the metal. It is not simple to compute because it is an average over the thermal motion of the electrons of the probability of the electron getting far enough away from the metal in order to escape to infi... | {
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Conceptual quantum field theory Often papers and books give some bold(deep physical insight) statements in quantum field theory which are not backed by mathematics, and seldom by citing papers. Being a student I don't grasp the real meaning of those statements, making me think that I don't really understand QFT. I have... | E. Zeidler, Quantum Field theory I Basics in Mathematics and Physics, Springer 2006. http://www.mis.mpg.de/zeidler/qft.html
is a book I highly recommend. It is the first volume of a sequence, of which not all volumes have been published yet. This volume gives an overview over the main mathematical techniques used in qu... | {
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Maximum efficiency for a counter-current heat exchanger (double flux controlled motorized ventilation) I am not sure if I can explain the question correctly because I don't know the name of this mechanism in English.
This is my explanation attempt: In a house, a tube is expelling the air from the inside to the outside,... | It sounds as if you are describing a countercurrent heat exchanger.
The theoretical efficiency of these can reach 1, though note that for heat exchangers efficiency doesn't mean the same as for heat engines i.e. heat converted to work. For heat exchangers an efficiency of 1 just means the incoming air is heated to the ... | {
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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 ... | As we all know that matter in our environment is made up of basic element "atoms" well silk is obtained from cocoons that are living thing thus made from "amino acid" that is the fundamental compound of living being and the components of amino acid are H2NCHRCOOH thus we see that R letter then requires to gain electro... | {
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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 (... | Interesting question. I had a Google around and came across http://silver.neep.wisc.edu/~lakes/BME315ScalingWalk.html, which seems a reasonable discussion of the mechanics (very simplified). The conclusion is that the walking speed is proportional to the square root of leg length, so taller people do walk faster but th... | {
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What is magnetic conductivity? I searched google for the meaning of magnetic conductivity but couldn't figure out what is it?
electric conductivity is usually means that there is the electric field parallel to the interface is continuous between to interfaces, what about magnetic conductivity material ?
| This is what they told us about magnetic circuits and magnetic resistence at engineering school.
Magnetic reluctance (resistance) is similar to the concept of resistivity of simple resistors. Magnetic relucance is used when calculating a magnetic circuit e.g. transformer magnetic core, electromotor magnetic core, gener... | {
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Baryon asymmetry Baryon asymmetry refers to the observation that apparently there is matter in the Universe but not much antimatter. We don't see galaxies made of antimatter or observe gamma rays that would be produced if large chunks of antimatter would annihilate with matter. Hence at early times, when both were pres... | The only source of asymmetry in the Standard Model is from CP violation, and although there is CP violation in the Standard Model it is not large enough to account for the observed asymmetry. It's expected that the asymmetry will be explained by some extension to the standard model, but at the moment we don't know whic... | {
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Commutating Annihilators with a beamsplitter I am reading Nielsen and Chuang on P. 291, for anyone interested in the origin of my question.
Given an annihilator $a$ and its corresponding creator $a^\dagger$ such that $[a,a^\dagger] = 1$ and another annihilator $b$ with creator $b^\dagger$, an argument in a proof claims... | This question was answered in a now-deleted comment by Luboš Motl:
Hi, just use $[XY,Z]=XYZ-ZXY = XYZ-XZY+XZY-ZXY = X[Y,Z]+[X,Z]Y$ and the basic commutators $[a,a^\dagger]=1$ and similarly for $b$ while other commutators vanish. You will see that from the right hand side, only one term survives and it gives you what y... | {
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A charged particle moves in a plane subject to the oscillatory potential A charged particle moves in a plane subject to the oscillatory potential:
$U(r)=\frac{m\omega^2 r^2}{2}$
There is also a constant EM-field described by:
$\vec{A}=\frac{1}{2}[\vec{B}\times\vec{r}]$
where B is normal to the plane.
This produces the ... | $\newcommand{\er}{\hat e_r} \newcommand{\et}{\hat e_\tau} \newcommand{\d}{\dot} \newcommand{\m}{\frac{1}{2}m} $
This one gave me a feeling of déjà vu, since I's already answered a similar one. Here's the relevant part of the derivation:
My $\theta$ is your $\phi$\ (usually $\phi$ is used for the azimuthal angle in sph... | {
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Geometry of wireless signal strength How does wireless signal strength correspond to distance? RSSI lies between -100 and 0 (at least, on my computer). Let's say I walk a distance x towards the router, and my RSSI goes from -60 to -50. Now, lets say instead I walk a distance 2x towards the router. Would this imply that... | From http://en.wikipedia.org/wiki/Received_signal_strength_indication:
There is no standardized relationship of any particular physical parameter to
the RSSI reading. The 802.11 standard does not define any relationship between RSSI value and power level in mW or dBm. Vendors provide their own accuracy, granularity... | {
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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... | Well, meteorite minerals like iridium and all aren't really found on Earth in appreciable quantities.
What you're looking for are exotic atoms. These certainly exist, but are too unstable. And, for certain exotic atoms like onia, atomic number isn't even defined.
The binding forces cannot be different since the couplin... | {
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Why do objects follow geodesics in spacetime? Trying to teach myself general relativity. I sort of understand the derivation of the geodesic equation $$\frac{d^{2}x^{\alpha}}{d\tau^{2}}+\Gamma_{\gamma\beta}^{\alpha}\frac{dx^{\beta}}{d\tau}\frac{dx^{\gamma}}{d\tau}=0.$$ which describes "how" objects move through spacet... | The result that a freely falling test particle(a particle whose effect on space-time can be neglected) in a gravitational field moves along a geodesic can be deduced(just like a theorem of maths) from the equivalence principle, which is a hypothesis of the general theory of relativity.
| {
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Calculate relativistic boost to COM frame from two arbitary velocities? Looking in Goldstein's book, there doesn't seem to be a standard formula to calculate the COM frame velocity for two particles, from their relativistic velocities in the lab frame, although it is done for the case where one particle is initially at... | The center of mass 4-momentum is the sum of the 4-momenta of the particles (no vector symbol or index, but the v's are four-component vectors) using the masses as the weights:
$$ P_\mathrm{CM} = m_1 v_1 + m_2 v_2 $$
The length of this is the mass of the combined system, (mostly minus metric)
$$ M^2 = |P|^2 = m_1^2 + m... | {
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A practical deceleration question My friend is a U.S. Army paratrooper. Today, through an unfortunate series of events, he was jerked out of a C-17 traveling at 160 knots by his reserve parachute. First-hand accounts describe it as he was instantly gone.
Since he came through it relatively unscathed, I'm curious to kno... | The forces experienced by your friend will be the same as if he was free falling at 160 knots and opened his parachute. The fact he was in the plane when the parachute deployed makes no difference, because in both cases he is slowed by the parachute from a high speed relative to the air to whatever the speed of a parac... | {
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How can I use sound/resonance to clean sewers? This probably doesn't fit into the realm of regular questions ; it is more of an applied rather than theory/math question ...
Anyway, I'm curious whether a metre diameter speaker fitted over a manhole may dislodge any blockage using the principle of resonance. Obviously bl... | I'd be very surprised if a lump of sludge blocking a sewage pipe had any useful resonance.
The idea of using a resonance is that the amplitude of oscillation builds up rapidly in response to the sound. However this will only happen if the oscillation has a high Q i.e. if it doesn't dissipate much energy. For a wine gla... | {
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what gives the vermiculite it's insultative properties i know that vermiculite is used in insulation applications.
i found this notion of the R-value of vermiculite, that i don't know if its true.
basically i want to know if the attribute of the vermiculite, of expanding in a temperature of 870 degrees Celsius, is what... | Vermiculite is a type of clay. Clays in general form structures composed of sheets made from two layers of tetrahedrally bonded silica with octahedrally co-ordinated metal ions in the middle. See http://en.wikipedia.org/wiki/Montmorillonite for a typical structure (montmorillonite is an archetypal clay much beloved of ... | {
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Will a stone thrown in space move forever? If I throw a stone in space, in a place where gravity is equal zero, and the space had no end, and no objects to collide with, will the stone move forward forever, because no air, so no friction?
| From the perspective of General Relativity, assuming we can ignore interactions with intergalatic gas and the CMB then a thrown stone follows a curve called a geodesic. In general geodesics go on forever so your stone will be moving forever, just as Lev said in his answer.
However there are circumstances in which geode... | {
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What is the probability that a star of a given spectral type will have planets? There is a lot of new data from the various extrasolar planet projects including NASA's Kepler mission on extra-solar planets. Based on our current data what is the probability that a star of each of the main spectral types (O, B, A, etc) w... | This question was asked a couple of years ago and things have changed since then.
We now know that small planets are found around stars across a broad range of metallicities and that it is only the existence of giant planets that are affected by low metallicity. Nature article here.
It was previously thought that small... | {
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A method to estimate the relative magnitude of a star using nearby stars I remember a method to make a reasonable estimate of the magnitude of a given star X by using two other stars of known magnitude as references. The method used evaluation phrases like "the star X and the reference star appear to have the same brig... | This method sounds way too complicated to me. I'm an experienced variable star observer, and use the methods recommended by the American Association of Variable Star Observers. The AAVSO publishes detailed charts for thousand of variable stars with comparison stars marked to the nearest tenth of a magnitude. The bright... | {
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What causes millisecond pulsars to speed up? Millisecond pulsars are supposed to be old neutron stars. However, they are spinning even more rapidly than newly formed pulsars. Since pulsars slow down as they age, something must have caused these older pulsars to "spin up" and be rotating as fast as they are. What is ... | There are a couple of pieces of observational evidence that support the explanation Jeremy provided. Many millisecond pulsars have been found via x-ray or gamma ray observations, and is interpreted as accretion in a disk on to the surfaces of the pulsars. The falling material speeds up the rotation of the pulsar due to... | {
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Mixing and matching across eyepiece designs / manufacturers? Right now, I am considering moving from 1.25" eyepieces to 2". While I'm convinced of the quality of the premium eyepieces, it would take me years to afford a complete set and, if I go that route, I will necessarily pick them up piecemeal. Would I be wise to ... | Regarding your "lumpy" distribution of eyepiece quality: Think about which eyepieces do you use most of the time. I recommend filling the low quality dips piecemeal with your desired type of premium eyepiece, first replacing the eyepiece you dislike but use often.
Upgrade from the middle out. The eyepieces of the e... | {
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How large is the universe? We know that the age of the universe (or, at least the time since the Big Bang) is roughly 13.75 billion years. I have heard that the size of the universe is much larger than what we can see, in other words, much larger than the observable universe. Is this true? Why would this not conflict w... | It's easy to underestimate the size of the Universe because we are concentrating on what we see. What we see when we look at the Cosmic Microwave Background is a look back 13.8 billion years ago to when the Universe was only 1/1000 as large as it is now.
Take a piece of the sky as visualized on maps of the cosmic backg... | {
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Optimal Angular Field of View (AFOV) Given the rather huge price differences between eye pieces at the same focal length. How exactly does the AFOV affect the view seen through the eyepiece?
Are higher / lower AFOV better for certain situations? or is higher always better?
| There are actually two different fields of view to consider. The apparent field of view is the apparent view you see in the eyepiece, typically 35° to 110°. The actual field of view is how much of the sky you are actually seeing, typically from a few degrees to a few arc minutes. The actual field of view is the apparen... | {
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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... | I don't really care what the IAU voted. Pluto will always be a planet in my book. Astronomy is full of historic inaccuracies that we perpetuate for tradition sake. Some examples that come to mind are early/late-type galaxies, Population I/II/III stars, and brown dwarfs.
| {
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Seeing cosmic activity now, really means it happens millions/billions of years ago? A Recent report about a cosmic burst 3.8 billion light years away. It is written as though it is happening now. However, my question is, if the event is 3.8 billion light years away, doesn't that mean we are continuously looking at hist... | This is true, but you should not forget that comparison of times depends on the frame of reference by relativity theory. In particular, from the point of view of the travelling light, no time has passed.
You might want to look at the cartoon at
http://xkcd.com/811/
Don't forget to read the mouse-over text.
| {
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Does Mercury have a balmy spot? From Wikipedia:
Although the daylight temperature at the surface of Mercury is generally extremely high, observations strongly suggest that ice exists on Mercury.
Does that mean there could be a spot on Mercury where a person could stand and it would be a balmy 80° F (27° C)?
Obviously... | The observations of ice are at the poles, in permanently shadowed craters, much as people think there may be stable, solid ice in permanently shadowed regions of the lunar south pole. While one atom that's -50°C right next to another that's +200°C would equilibrate almost immediately, there will be a very tiny transit... | {
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How can I stabilize an unstable telescope? I have an 80 mm refractor telescope on a tripod, but it shakes on every touch. It's very hard to see via 6 mm (x120) ocular. Even a little wind causes the image to become too unsteady.
How can I make my tripod more steady?
| First make sure all your screws are tight, and that there isn't any shaking because of slack in any areas where things connect to each other.
Another thing you can do is buy vibration dampening pads to put your tripod on.
Finally, you can add counter weights and pendulum weights to the tripod to give it more mass to wi... | {
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Why can't you escape a black hole? I understand that the event horizon of a black hole forms at the radius from the singularity where the escape velocity is $c$. But it's also true that you don't have to go escape velocity to escape an object if you can maintain some kind of thrust. You could escape the earth at 1 km/h... | It is actually possible to get out of the horizon. To get out one only have to reach speed higher than c.
But no object that bears information can reach that speed, this would violate casualty. So only things that bear no information can get out from inside the horizon.
One such thing is the Hawking radiation (not only... | {
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How can Voyager 1 escape gravity of moons and planets? I think this one is pretty simple so excuse me for my ignorance. But since most planets in our solar system are very well tied to their orbit around the sun or orbit around their planet (for moons), I was wondering how can a really small spacecraft such as Voyager ... | You are absolutely right.
Considering $r$ constant, Newton's law of universal gravitation, $ F = \dfrac{Gm_1m_2}{r^2}$, says that the force of attraction between two objects with mass $m_1$ and $m_2$, depends on the product of their masses. Hence, though one object is having a larger mass (here planets), the force can ... | {
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Which is the heaviest present day lifter (rocket)? And is it comparable to the Saturn V rocket? I know of the Ariane 5 ECA, the Delta IV rocket and a few more, but which of the present day's rockets is the top heavy lifter, say, to low Earth orbit (LEO)?
Although it is not a certain fact, I would imagine that a very he... | The Saturn V payload mass to LEO was 118,000 kg. Wikipedia has a decent comparison of Super-heavy launch systems with a payload mass to LEO of 50,000 kg or more. None are in current use, and only two systems are in development.
There is also a "Heavy" lift launch system list which includes the Delta IV and Ariane 5 you... | {
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Is dark matter around the Milky Way spread in a spiral shape (or, in a different shape)? Dark matter doesn't interact with electromagnetic radiation, but it, at least, participates in gravitational interactions as known from the discovery of dark matter. But does dark matter exist in a spiral shape around our galaxy?
| Disclaimer: I don't do dark matter, I don't work with anyone whose does and I haven't read enough papers to make an difference. I have seen a colloquium and a couple of seminars by people who do do dark matter.
Take what follows with that in mind.
Dark matter either not having been observed or having served up exactly... | {
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Are we going to be able to travel trough space deforming the space-time? I'm not talking about the speed of the spaceship. If we can deform space-time we needn't any type of propulsion. And how can the travel affect to it's pilots? Can they survive?
| the Alcubierre drive has problems far worse than the energy violations; you need to distribute the exotic matter in a space-like direction before you can ride over it.
Even if you could produce enough exotic matter, you will have to distribute it first by conventional travel. No amount of exotic matter will avoid this... | {
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How to measure the diameter of a star? I am thinking about something I read somewhere (if only I could find it again) in a textbook. It is about the size of a star and its ER peaks. It has to do with the waves coming off the edge (maybe) and arriving later than those from "head on" and therefore you can know somethin... | To calculate the linear diameter of a star, we need only to know its effective temperature, the bolometric correction, and its absolute magnitude. And if, instead of the absolute magnitude, we know the apparent diameter can then calculate the angle.
The formula used to determine the size of a star appeared in Article S... | {
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How much energy does a super nova generate? For a scene in a SciFi book, I want to know: Is it possible to estimate how much energy per m² an object would receive that hides behind an in-system planet when the sun goes nova?
| Is it possible to estimate? Yes. I'll give it a quick try. But the details of whether the planet will be incinerated and so on will make the reality much more complicated.
As a ballpark, I think supernovae release about $10^{53}$ erg of energy. Spread over a sphere of, say, 1 AU gives $3.55\times10^{22}$J.m$^{-2}$. Thi... | {
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What is the simplest way to prove that Earth orbits the Sun? Assume you're talking to someone ignorant of the basic facts of astronomy.
How would you prove to them that Earth orbits the Sun? Similarly, how would you prove to them that the Moon orbits Earth?
| So called 'stellar aberration',the shifting of the apparent positions of stars by up to 20 arc seconds towards the direction the Earth is going in its orbit, was the first method available to 'simple'equipment in the 19th century, namely transit telescopes.
By the early 20th century, the annual variations of the Dopple... | {
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Do days and months on the Moon have names? On Earth we have various calendars, for example,
Days: Monday, Tuesday, Wednesday, etc., etc.
Months: January, February, March
Does the Moon have names for its "daily" rotations, etc.?
It sounds like a silly question, and I am not sure if I've asked it using the correct termin... | As 1 lunar day is roughly 27 earth days, I doubt they'd have names of days. Most likely if there was ever a permanent base or something like the ISS, they would use earth days and months. In other words Monday on the moon would still be 24 hours long and would not have roughly half day and night like the majority of ea... | {
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Given a photo of the Moon, taken from Earth, is it possible to calculate the position of the photographer's site? Given a photo of the Moon, taken from Earth, is it possible to calculate the position (Earth longitude and latitude) of the photographer's site?
I am thinking about photos taken with a normal camera lens an... | This is a great question to tie in the quest to determine longitude as a driver of astronomy.
Briefly, your question is exactly in line with the thinking of any number of ambitious 18th century scientists, inventors, and natural philosophers. And @BradC's fantastic movie of the libration of the moon gives an idea of ex... | {
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Do nearby gamma ray busts/supernova damage more than just the ozone layer? So we know that many people are putting hard constraints on the galactic habitability zone based on the presence of nearby supernova/gamma ray bursts. But if they only affect the ozone layer, then I doubt that it's as hard of a constraint as man... | Dr Phil Plait covers the effects of a supernova near a habited planet extensively in his book Death from the Skies. Basically, it would have to happen at a distance closer than 25 light years. Given that constraint, and looking at our system, no stars are candidates for a supernova explosion that would wipe us out. ... | {
"language": "en",
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What are the chances that a deadly asteroid will hit Earth in the next decade? What are the chances that an asteroid that will kill multiple people will hit Earth in the next decade?
| Hm. To answer that question, one would simply count such occurrences for a given time in the past, and divide it through the timespan. Then do some corrections for the increased number of cities in the last centuries.
The only bigger impacts which where reported from people was the Tunguska-Event 1908 and 1490 in Chin... | {
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Is there such a thing as "North" in outerspace? On Earth, North is determined by the magnetic poles of our planet. Is there such a thing as "North" in outerspace? To put it another way, is there any other way for astronauts to navigate besides starcharts? For instance, if an astronauts spaceship were to be placed somew... | Actually, North on the Earth is determined not by the Earth's magnetic field, but by the apparent motion of stars as the Earth rotates. Magnetic North is not toward the northern end of the spin axis of the Earth, in general.
There are easily identifiable distant galaxies, and pulsars, that can serve very well as navig... | {
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Meteorites from Mars? So I've heard of meteorites "originating from Mars" (e.g. AH84001), but the phrase confuses me. I'm interested in what this means - have these rocks somehow escaped Mars' gravity and ended up here; or were they part of the material that Mars formed from, but did not end up as part of the red plane... | These are chunks of rock that existed as part of the crust of Mars but were ejected into interplanetary space by a very powerful impact and then eventually impacted the Earth. It wasn't until we had sent probes to Mars and began to understand the composition of martian minerals and atmosphere that we started realizing ... | {
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Can the Hanbury-Brown and Twiss effect be used to measure the size of composite objects like galaxies? I know that the Hanbury-Brown and Twiss effect can be used to measure the size of stars. Can it also be used to measure the size of galaxies?
| (2) & (3) are both generally correct. The HBT effect is in essence a property of detection, not emission (the sources are incoherent). H&B observed Sirius, which at 8.6 light years (ly) distance and 1.7 times bigger than the sun, is 6 milli-arcseconds (mas). A 140,000 ly diameter galaxy (like Andromeda) at 10 billion l... | {
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Why don't we have a better telescope than the Hubble Space Telescope? The Hubble Space Telescope (HST) was launched in 1990, more than 20 years ago, but I know that it was supposed to be launched in 1986, 24 years ago. Since it only took 66 years from the fist plane to the first man on the Moon why don't we have a bett... | Money and willpower. With any program (scientific, military, public works, etc.) it all depends on the amount of money someone is willing to put to it, and how much backing and protection that program has from getting money re-prioritized to other projects. You are making a false dichotomy of attempting to present ou... | {
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What does velocity dispersion (sigma) reveal about a galaxy? I'm getting hung up on this term. In studying SMBHs, I see that velocity dispersion strongly correlates with mass. Just what is the velocity dispersion? How can the velocity dispersion of the galaxy be expressed in one figure (sigma) if it has to be measured ... | The fact that sigma is used in statistics to refer to standard deviation is an important clue. There are lots of velocities in a typical galaxy. They have a mean and a standard deviation, which is called the velocity dispersion in this case. Directly, it reveals how fast things are going relative to each other or re... | {
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How did micrometeorite flux change with the age of the solar system? Do we also have functions for the micrometeorite flux on the Moon, Mars, and even any random body in space as a function of solar distance too?
| The short answer to both your question in the subject line (change with solar system age) and your topic (function of solar distance) is "no." We don't even know the impact flux of asteroids and comets as a function of solar distance nor time.
The work that most of us in the field cite whenever discussing crater chron... | {
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Accuracy and assumptions in deriving the Tully-Fisher relation I understand the mathematical derivation of the Tully Fisher relation from basic physics formulas, as shown on this site. However, after using the physics equations, it seems that several assumptions are made from this point on.
First are statistical assump... | The Tully-Fisher relation is first and foremost, and historically, an observational relation. The 'derivation' of the relation, is really more of a 'motivation'. The relation is based on a myriad details of galaxy and star formation, and dynamics---an accurate 'derivation' could only be based on numerical simulations... | {
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What is Hawking radiation and how does it cause a black hole to evaporate? My understanding is that Hawking radiation isn't really radiated from a black hole, but rather occurs when a particle anti-particle pair spontaneously pop into existence, and before they can annihilate each other, the antiparticle gets sucked in... | Simply put, the particles that pop in and out of existence are not matter/anti-matter pairs but rather virtual particles (particle/anti particle pairs) that both have net mass, therefore contributing to the net mass of the universe when one of them is swallowed up by a black hole at the event horizon and the other esca... | {
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How do we determine the mass of a black hole? Since by definition we cannot observe black holes directly, how do astronomers determine the mass of a black hole?
What observational techniques are there that would allow us to determine a black hole's mass?
| It is possible to measure the mass of a black hole from the speed of other objects. If we measure the speed of a star orbiting a black hole and the distance of its orbiting radius we can tell its mass. For example we would first locate a star that is traveling at 100 km/s and has an orbiting radius of 150 megameters so... | {
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Why are there more vertical takeoff than horizontal for spacecrafts? Vertical takeoff requires disposable rockets (would it a satellite rocket), which is a money loss, and also a lot of fuel, because initial velocity is zero. Also vertical takeoff seems risky, involves huge pieces of equipments, launch pads, to diminis... | A 747 - can get you to around 35,000 feet. Still very much within the atmosphere.
So what do you do then? Launching a rocket from that point still requires an awful lot of kit, so while you have reduced your propellant requirements a little, the 747 still has to carry a launch platform, so you're not really getting any... | {
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Why can't dark matter be black holes? Since 90 % of matter is what we cannot see, why can't it be black-holes from early on? Is is possible to figure out that there are no black holes in the line of sight of various stars/galaxies we observe?
| Paolo Pani and Avi Loeb had two recent papers claiming to rule out the remaining window of primordial black holes as dark matter because they would either distort the CMB or destroy neutron stars: arXiv:1307.5176 and arXiv:1401.3025.
| {
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Topological twists of SUSY gauge theory Consider $N=4$ super-symmetric gauge theory in 4 dimensions with gauge group $G$. As is explained in the beginning of the paper of Kapustin and Witten on geometric Langlands, this
theory has 3 different topological twists. One was studied a lot during the 1990's and leads
mathema... | The Kapustin-Witten paper
https://arxiv.org/abs/hep-th/0604151
says (on page 17) that two of the three twists are related to Donaldson theory:
Two of the twisted theories, including one that was investigated in detail in [45: Vafa Witten], are closely analogous to Donaldson theory in the sense that they lead to inst... | {
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} |
Papers and preprints worth reading, Jan-midFeb 2012 Which recent (i.e. Jan-midFeb 2012) papers and preprint do you consider really worth reading?
References should be followed by a summary saying what is the result and (implicitly or explicitly) why it is important/interesting/insightful/...
One paper per answer (and s... | Light-cone-like spreading of correlations in a quantum many-body system reports the first measurements of the speed at which quantum correlations spread in a quantum many-body system. Prior related theoretical works are [2,3,4]. Though the main innovations are plausibly the experimental techniques, I think theorists sh... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/27129",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 6,
"answer_id": 0
} |
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