Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
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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... | If you are interested, there is an audio recording of the IAU General Assembly session on the definition of a planet http://www.jodcast.net/archive/200608IAU/
| {
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"timestamp": "2023-03-29T00:00:00",
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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... | From Wikipedia:
The finite speed of light is important in astronomy. Due to the vast distances involved, it can take a very long time for light to travel from its source to Earth. For example, it has taken 13 billion (13×109) years for light to travel to Earth from the faraway galaxies viewed in the Hubble Ultra Deep ... | {
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How does a spacecraft's orientation get determined What method are employed to determine a spacecraft orientation with respect to a known orientation or plane?
Are gyroscopes reliable enough to make this determination, for example in Voyager spacecraft.
| Most satellites, at least that I know about, use a series (usually three) of small star tracking telescopes. They are oriented in different directions and locate and lock on to known bright stars. Given the angles to the stars relative to the spacecraft body, the spacecraft knows its orientation in space.
| {
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"url": "https://physics.stackexchange.com/questions/25244",
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Video of Earth spinning? If the Earth is spinning or rotating at a really fast speed, why haven't we seen any videos from space of it spinning when we get a lot of photos of it?
| You know I have been asking this same question, why aren't there videos of the earth's rotation from space? I've searched and only have found the opposite. Dish network had a station called "dish earth" that was up for about 3 years. Now if you select the 'dish earth' channel it will give you a different channel/statio... | {
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Why does the homogeneity of the universe require inflation? They say inflation must have occured because the universe is very homogeneous. Otherwise, how could one part of the universe reach the same temperature as another when the distance between the parts is more than light could have traveled in the given time?
Why... | What Big Banged To Produce The Universe
From : http://universe-life.com/2011/12/10/eotoe-embarrassingly-obvious-theory-of-everything/
A commonsensible conjecture is that Universe Contraction is initiated following the Big-Bang event, as released moving gravitons (energy) start reconverting to mass (gravity) and eventu... | {
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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 ... | It is certainly possible to break the gravitational bind of a planets gravity. To do so you permanently you need to achieve escape velocity, which is an energetic statement that says a particle with enough kinetic energy will never 'come back down'. To calculate the associated velocity we equate the gravitational pot... | {
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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?
| In current cosmological models, the Milky Way resides in a 'halo' of dark matter. Halo is a technical term - in this case, it means a spherically symmetric collection of dark matter. Since dark matter is not self-interacting and does not interact with other matter, it doesn't experience any sort of collisions or fric... | {
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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 Warp Drive hasn't been proved theoretically impossible; although requiring an amount of energy equivalent to several solar masses appears to be one of the less difficult engineering challenges that would confront anyone attempting to build one.
| {
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If neutrinos travel faster than light, how much lead time would we have over detecting supernovas? In light of the recent story that neutrinos travel faster than photons, I realize the news about this is sensationalistic and many tests still remain, but let's ASSUME neutrinos are eventually proven to travel "60 ns fast... | I would think that if neutrinos travel faster than light the first thing one would need to know is their velocity.
Yesterday or today the Opera folks announced that they had found a loose cable connection and had calculated that the error it caused was the same as the discrepancy between the expected time of arrival ... | {
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Which direction before dawn to look for Comet Lovejoy (C/2011 W3)? Various websites today are reporting with photos and videos of Comet Lovejoy. However, I can't seem to find a definition of which direction to look for it tomorrow morning. I'm in Christchurch, New Zealand (roughly 43°S 173°E).
| According to Starry Night, Comet Lovejoy will rise this morning in the Southeast around 4 a.m. in Christchurch. Not a chance of seeing it here in Canada, alas!
| {
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How do we know the masses of single stars? I have recently read that we can only know the masses of stars in binary systems, because we use Kepler's third law to indirectly measure the mass. However, it is not hard to find measurements for the mass of stars not in binary systems. So how is the mass of these stars deter... | The Hertzsprung–Russell diagram is the key to determining masses of individual stars. For stars on the main sequence, their properties are essentially determined by their mass. Age and metallicity are also interrelated factors, but of considerably less importance than mass. That is, if you tell me the mass of a star on... | {
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How bright are auroras (aurorae)? Digital cameras are making the recent auroras look magnificent, but what are they like to the naked eye? Are they comparable in surface brightness to the Milky Way?
| Your eye can see them without having doubts about what it is. Is different than clouds and they are as magnificent as in the pictures. Usually the pictures are made with long exposures but not very long, between 2-30 seconds. The photo that I am posting is made with a 4 seconds exposure in a zone with a lot of light po... | {
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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?
| I originally had something about the constellations changing in the sky to show that the Earth orbits the sun, but that would still be the case if the Sun orbited the Earth instead. Now that I think about it, there is one thing that conclusively proves that the Earth orbits the sun: parallax. Over the course of one yea... | {
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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... | Proabably not, at least not that I've ever heard of. Since no one has ever lived there :), there has never been any sort of calendaring system needed. Even the longest Apollo missions were only there a few days.
I'm sure if there was ever a permanent base (or bases) there, some sort of time keeping system would be de... | {
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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... | At the equator the dark side of the Moon grows down whereas by the poles it grows to either left or right. That's because, when you are at the opposite side of the planet, you are standing upside down. So the direction of the darkness can tell you how far north/south you are.
Regarding east and west.
At any given time,... | {
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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... | It really depends on the range. A star going supernova is going to absolutely obliterate any planet that's closely orbiting it. For solely ozone/atmospheric damage, you're going to have to be several light years away. Probably anything within 10 light years is going to suffer a severe extinction event at the very least... | {
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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?
| For statistical purposes, the difference between the odds of two people dying and the odds of one person dying are not signficant so, depending on who you reference...
1 in 200,000
1 in 700,000
1 in 40,000 << This seems pretty high, considering that this is about the rate of death by chicken pox. However this also inc... | {
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Why did the asteroid belt between Mars and Jupiter form as it did? I'm curious about why the asteroid belt wasn't pulled by Mars's or Jupiter's gravity or formed into either moons or planets. Why did it form into an asteroid belt instead?
| The original mass of the solar disk at that position in the Solar System is speculated to be about the same as Earth. Due to gravitational perturbations of Jupiter and Mars, the Asteroid Belt was too chaotic to allow a planet to fully form. Instead of relatively gentle collisions, allowing them to accrete, the impacts ... | {
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What will we see between the CMB and the current oldest object seen? The cosmic background radiation (CMB) is estimated to be from 13.7 billion years ago (BYA), and very shortly after the big bang compared to that time frame. The oldest coherent objects we've detected are around 500 million years after the big bang, m... | Well, we have to rely on theory here. The $\lambda$CDM model predicts that there should be lots of hot optically opaque hydrogen forming into the first stars and galaxies. Particular objects of interest will be the Population III stars which formed and died very quickly.
The best eyes on this scene in the near future ... | {
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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... | On Earth the north and the south are defined by the south and north magnetic poles respectively of the Earth's Magnetic field. In space there is a thing called the Galactic Magnetic field which permeates galaxies, including the Milky Way (http://arxiv.org/abs/astro-ph/0207240). However, the strength of the Galactic mag... | {
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Can CMEs disturb a planet orbit? Yesterday (March 6), a massive X-class solar flare erupted from the Sun.
I was wondering if this kind of solar flares can affect in some way the orbit of a planet if aimed directly. Let's say, for example, Mercury.
Can this happen at all?
| CMEs can't disrupt planetary orbits. When you consider a collision you should compare the momenta of the colliding masses and while the speed of CMEs is up to two orders of magnitude larger than orbital speeds of the planets, its total mass is more than ten orders of magnitude smaller. Moreover, since CME's wave spread... | {
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Determining cloud cover from observer to near the horizon Does there exist a Clear Sky Chart with the following enhancements?:
1 - Actual Cloud Cover (Offered Visually and not just Colors with a Legend, Over Time/Past & Predictive)
2 - Simulate/Predict Cloud Cover taking into account the direction from Observer to Obse... | It sounds as if you are not aware of the enormous amount of work which has been put into the Clear Sky Charts we have by two extremely generous and knowledgeable gentlemen: Alan Rahill of Environment Canada, who teased their supercomputer into generating the weather maps, and Attilla Danko, who persuaded Alan's maps to... | {
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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?
| In theory, yes, In practice its a lot trickier...
Within a simple interferometric measurement (1 baseline) You are only measuring the spatial frequencies parallel to that baseline, with a nice symmetric star you can assume a simple physical model of a circle and whatever angular diameter you recover that would be your ... | {
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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... | A big reason is that through subsidized servicing missions the Hubble has been substantially upgraded over the years. Very few of the original HST instruments remain, and that has resulted in a dramatic expansion of the science gathering capabilities of Hubble over time.
Also, there has been a Hubble successor in the w... | {
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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 concept behind velocity dispersion comes from statistical mechanics, in which you're generally describing gases or fluids. In gases, you characterize the energy of the system by the temperature, which is a measure of the average thermal (random) motion of particles.
Astronomers do the same thing for galaxies. The... | {
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Vision vs. limiting magnitude Does anyone know how the acuity of your vision translates to a difference in limiting magnitude?
e.g., the kind of answer I'm looking for would be
"For each factor of 2 improvement in your vision (20/80 to 20/40, 20/40 to 20/20, 20/20 to 20/10, etc., your personal limiting magnitude for po... | There are two causes of limiting magnitude. First is pure sensitivity, where no matter how dark the sky is, one can't see stars fainter than about 6.5. If your vision loses acuity and blurs things out, it may not at first have much effect on limiting magnitude, especially if you use averted vision where things are not ... | {
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Has a human ever perished in space? Apollo 13 returned safely. The Challenger was leaving when it exploded. The Columbia was coming back when it burned up, as was that Russian guy who was profiled on National Public Radio (NPR) and that recent book. Has any human ever died in space?
| There have only been 3 recorded deaths that occurred in space (that is, greater than 60 miles above the Earth). The crew of the Russian capsule Soyuz 11, died when their capsule depressurised during preparations for re-entry. It wasn't known they had died until the re-entry capsule was opened on Earth as communications... | {
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Can extraterrestials detect our messages? We transmitted several messages to the space and listening to space for signs of intelligent life for years (SETI).
Assuming they have at least the same technology we have, could they detect these messages we sent out?
If not, how powerful transmitter is needed to make our wor... | Here's a somewhat technical document talking about insterstellar beacons. A beacon would act as a "searchlight", sweeping across the sky, so that the time spent on each target star would be rather short. With a beam aperture able to illuminate 1% of the sky and working at 0.5 Hz they calculate that 6.9 GW power will be... | {
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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?
| Part of the reason is a matter of ratios.
We're not talking about a fraction of the total mass that can be observed. We're talking multiples. There's about 10x as much dark matter in any large galaxy than there is normal matter that can be observed. If this was all in black holes, then there would be 10x as many black ... | {
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Limitations in using FLEX as a DMFT solver When using the fluctuating exchange approximation (FLEX) as a dynamical mean field theory (DMFT) solver, Kotliar, et al. (p. 898) suggest that it is only reliable for when the interaction strength, $U$, is less than half the bandwidth. How would one verify this? Also, is the... | The criterion you mention is roughly the threshold for the formation of the Coulomb gap in the Hubbard model or the local moment in the Anderson model. It is a common break-down region for many approaches starting from one of the limits (insulator/local moments versus conductor/mixed valence).
For perturbation theory ... | {
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What is the Holevo-Schumacher-Westmoreland capacity of a Pauli channel? Suppose you are given an $n$-qubit quantum channel defined as $\mathcal{E}(\rho) = \sum_{i} p_i X_i \rho X_i^\dagger$, where $X_i$ denotes an $n$-fold tensor product of Pauli matrices and $\{p_i\}$ is a probability distribution. The Holevo-Schumach... | Finding the HSW capacity is an optimization problem which I believe is moderately tractable. There is an iterative numerical method outlined in this paper of mine ("Capacities of quantum channels and how to find them."). A different, although somewhat similar, method was detailed in the paper "Qubit channels which requ... | {
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Uniqueness of supersymmetric heterotic string theory Usually we say there are two types of heterotic strings, namely $E_8\times E_8$ and $Spin(32)/\mathbb{Z}_2$. (Let's forget about non-supersymmetric heterotic strings for now.)
The standard argument goes as follows.
*
*To have a supersymmetric heterotic string the... | I think that the two solutions are the only modular-invariant chiral CFTs with the right central charge. They have the right transformation law under $\tau\to\tau+1$ and especially (and less trivially) $\tau\to-1/\tau$ where $\tau$ is the complex structure of the world sheet torus. That's needed for a consistent path i... | {
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Bogomol'nyi-Prasad-Sommerfield (BPS) states: Mathematical definition What is the proper mathematical definition of BPS states?
In string theory the BPS states correspond either to coherent sheaves or special Lagrangians of Calabi-Yau manifold depending upon the type of string theory considered. but in SUSY quantum fiel... | The BPS bound was discovered independently of supersymmetry, but it was then better understood as general feature of the supersymmetry algebra. Look at the original paper by Witten and Olive. BPS states are states which saturate the BPS bound, forming "short" representations an extended supersymmetry algebra. Such repr... | {
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Quantum mechanics as classical field theory Can we view the normal, non-relativistic quantum mechanics as a classical fields?
I know, that one can derive the Schrödinger equation from the Lagrangian density
$${\cal L} ~=~ \frac{i\hbar}{2} (\psi^* \dot\psi - \dot\psi^* \psi) - \frac{\hbar^2}{2m}\nabla\psi^* \cdot \nabla... | I am no expert in classical fields, but I guess you have no entanglement there, that is, there is no difference with a single particle, but there is a big difference with more than one.
| {
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Constructing a CP map with some decaying property Given some observable $\mathcal O \in \mathcal H$ it is simple to construct a CP (completely positive) map $\Phi:\mathcal{H}\mapsto \mathcal{H}$ that conserves this quantity. All one has to observe is that
$$ \text{Tr}(\mathcal O \, \Phi[\rho]) = \text{Tr}(\Phi^*[\mathc... | The formal condition---the correspondent of $\Phi^*[\mathcal{O}]=\mathcal{O}$ in the other case---is $\Gamma^*[\mathcal{O}]\leq \mathcal{O}$. For $\mathcal{O}>0$ (all eigenvalues strictly positive), if one multiplies on the right and on the left by $\mathcal{O}^{-1/2}$, one can see that this condition is equivalent to ... | {
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"url": "https://physics.stackexchange.com/questions/27445",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Rigorous proof of Bohr-Sommerfeld quantization Bohr-Sommerfeld quantization provides an approximate recipe for recovering the spectrum of a quantum integrable system. Is there a mathematically rigorous explanation why this recipe works? In particular, I suppose it gives an exact description of the large quantum number ... | Here's a very basic way to see this easily:
This is Action, search for the "Abbreviated Action", and it has the SI unit of Joule-second.
This is the equation that Planck (and later, Einstein) used:
$$E=nhf$$
for $n=1,2,3...$ and $f$ in frequency, in unit of 1/second).
This means that the Planck's constant has also an ... | {
"language": "en",
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"source": "stackexchange",
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which letter to use for a CFT? In math, one says "let $G$ be a group", "let $A$ be an algebra", ...
For groups, the typical letters are $G$, $H$, $K$, ...
For algebras, the typical letters are $A$, $B$, ...
I want to say things such as "let xxx be a conformal field theory" and "let xxx $\subset$ xxx be a conformal inc... | Ben-Zvi & Frenkel denote vertex algebras $V$,$W$,... They're using the labels specifically for the spaces of states, but one could also use them to refer the whole package.
Alternately, one sometimes sees all caps abbreviations: $YM_2$, $SYM_{4,G}$,...
There is not to my knowledge any conventional notation for morphi... | {
"language": "en",
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Convert state Vectors to Bloch Sphere angles I think this question is a bit low brow for the forum. I want to take a state vector $ \alpha |0\rangle + \beta |1\rangle $ to the two bloch angles. What's the best way? I tried to just factor out the phase from $\alpha$, but then ended up with a divide by zero when tryin... | $\phi$ is the relative phase between $\alpha$ and $\beta$ (so the phase of $\alpha/\beta$). You will only get zero or divide-by-zero when $\alpha=0$ or $\beta=0$. But in that case, $\phi$ is arbitrary. And when $\alpha$ or $\beta$ are close to zero, you are near the poles of the Bloch sphere, and $\phi$ doesn't real... | {
"language": "en",
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How to write a paper in physics? I really like to do research in physics and like to calculate to see what happen. However, I really find it hard to write a paper, to explain the results I obtained and to put them in order. One of the reasons is the lack of my vocabulary.
*
*How do I write physics well? I think tha... | In addition to the Joe's answer, a bunch of good advices is here:
*
*G. M. Whitesides, Whitesides' Group: Writing a Paper (Adv Mat 2004), doi:10.1002/adma.200400767
Its two main points are:
*
*Start writing a draft as soon as you have some results, not - when the research is complete (as the later may never come)... | {
"language": "en",
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"source": "stackexchange",
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Electricity & Magnetism - Is an electric field infinite? The inverse square law for an electric field is:
$$
E = \frac{Q}{4\pi\varepsilon_{0}r^2}
$$
Here: $$\frac{Q}{\varepsilon_{0}}$$
is the source strength of the charge. It is the point charge divided by the vacuum permittivity or electric constant, I would like v... | The Landau Pole is not a problem for QED because at scales much smaller than it (the Planck scale, which is smaller than the Landau pole by 260 orders of magnitude) the (negative) gravitational self-energy of the particle will more than cancel out its electromagnetic self-energy. So string theory is not necessary in th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/27820",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Fermi statistics and Berry phase When the positions of two fermions are exchanged adiabatically in three-dimensional space, we know that the wave function gains a factor of $-1$. Is this related to Berry's phase?
| The answer is yes.
The angular momentum of a (scalar) particle moving in the background of a Dirac magnetic monopole of an odd magnetic charge becomes half integral. (The magnetic charge must be an integer by the Dirac quantization condition.)
Under these conditions, the phase acquired by the wavefunction through a 36... | {
"language": "en",
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Did anyone claim that quantum theory meant lasers would never work I've been reading 'How the Hippies saved Physics', which describes a design for a superluminal communication device, of which the crucial part was a laser which duplicated an incoming photon many times. The reason this won't work is what is now known a... | Yeah, Neils Bohr and John Von Neumann were skeptics:
Many prominent physicists thought it could not even work, based on their knowledge of physical principles. In quantum mechanics, the uncertainty principle developed by Einstein, says that the energy (and therefore the frequency, by E=hv) of a photon can't be known t... | {
"language": "en",
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'Applications' of surface tension
What are some common applications, uses, exploitations of the properties of surface tension?
Here is what I mean. A water strider can walk on water, that is a consequence of surface tension. This is a consequence, but it is not human made.
On the other hand, I heard that in the cons... | The waterproofing of tents (or of Gore-Tex that maybe more of us will be familar with) is really to do with wetting of the fabric rather than surface tension. Though having said that, the wetting i.e. contact angle, is a play-off between the water-air, fabric-water and fabric-air interfacial tensions, so it is related ... | {
"language": "en",
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Angular momentum operator and expectation values I was reading some notes and it says that $\langle L_z^2\rangle=\langle L^2\rangle$ IFF the system is radially symmetric. I can see that in order that the LHS of the statement implies that $\langle L_x^2\rangle=0=\langle L_y^2\rangle$. But I am not sure how to rigorously... | I am assuming you're talking about the expectation values of these operators on hydrogenlike wave functions?
Consider the eigenvalues of $\hat{L_z}$ and $\hat{L^2}$ operating on some eigenstate $|\psi>$:
$L_z|\psi>=\hbar m|\psi>$ and $L^2|\psi>=l(l+1)\hbar^2|\psi>$.
So applying $\hat{L_z}$ twice to get $\hat{L_z^2}$ gi... | {
"language": "en",
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A certain regularization and renormalization scheme In a certain lecture of Witten's about some QFT in $1+1$ dimensions, I came across these two statements of regularization and renormalization, which I could not prove,
(1) $\int ^\Lambda \frac{d^2 k}{(2\pi)^2}\frac{1}{k^2 + q_i ^2 \vert \sigma \vert ^2} = - \frac{1}... | Let's just look at the integral
$$\int \frac{d^2k}{(2\pi)^2} \frac{1}{k^2+\alpha^2}.$$
The other integrals should follow from this one.
Introduce the Pauli-Villars regulator,
$$\begin{eqnarray*}
\int \frac{d^2k}{(2\pi)^2} \frac{1}{k^2+\alpha^2}
&\rightarrow& \int \frac{d^2k}{(2\pi)^2} \frac{1}{k^2+\alpha^2} - \int... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/28194",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Is the environment around an asteroid harsher than in interplanetary space? In the wikipedia article about NEAR Shoemaker it is mentioned that the craft stopped operating under these conditions:
At 7 p.m. EST on February 28, 2001 the last data signals were received from NEAR Shoemaker before it was shut down. A final ... | A spacecraft in space should only lose heat by radiation to space, since it's not in direct contact with anything.
NEAR Shoemaker actually soft-landed on the surface of Eros. After the landing, its contact with the surface meant that it would lose heat by direct conduction, making it more difficult to maintain the temp... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/28264",
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What does the Fermi Energy really signify in a Semiconductor? In understanding the behavior of semiconductors, I'm coming across a description of the Fermi Energy here and at Wikipedia's page (Fermi Energy, Fermi Level). If I understand correctly, the Fermi Level refers to the energy state at which there's a 50% chance... | The reason for this apparent contradiction is that you have two "separate" quantum effects.
*
*Fermi-Dirac distribution describes the energies of single particles in a system comprising many identical particles that obey the Pauli exclusion principle. Distribution is calculated for potential-free space and is tempe... | {
"language": "en",
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The speed of sound is proportional to the square root of absolute temperature. What happens at extremely high temperatures? The speed cannot increase unboundedly of course, so what happens?
| It's a bit misleading to simply say the speed of sound is proportional to $\sqrt{T}$ because life is a bit more complicated than that. You've probably seen http://en.wikipedia.org/wiki/Speed_of_sound and this does indeed say in the introduction that the speed of sound is a function of the square root of the absolute te... | {
"language": "en",
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"source": "stackexchange",
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Impervious nature of solid matter due to quantum degeneracy pressure On Wikipedia the following statement is made without reference:
Freeman Dyson showed that the imperviousness of solid matter is due to
quantum degeneracy pressure rather than electrostatic repulsion as had
been previously assumed.
Can anyone fin... | Wikipedia to the rescue!
FJ Dyson and A Lenard: Stability of Matter, Parts I and II (J. Math. Phys., 8, 423-434
(1967); J. Math. Phys., 9, 698-711 (1968) ); FJ Dyson: Ground-State Energy of a Finite
System of Charged Particles (J.Math.Phys. 8, 1538-1545 (1967) )
I found the reference in ref 6 of http://en.wikip... | {
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Coulomb potential in 2D I know that the Coulomb potential is logarithmic is two dimensions, and that (see for instance this paper: http://pil.phys.uniroma1.it/~satlongrange/abstracts/samaj.pdf) a length scale naturally arises:
$$ V(\mathbf{x}) = - \ln \left( \frac{\left| \mathbf{x} \right|}{L} \right) $$
I can't see wh... | I don't know if you have solved it or not, but my suggestion is to add a convergence factor $$e^{-a\,r}$$ and get the integral, and then let $a \to 0$ (the same trick with yukawa potential)
You can do this integral by the mathematica.
| {
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"source": "stackexchange",
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How do we recognize hardware used in accelerator physics When I see a new accelerator in real life or on a picture, I always find it interesting to see how many thing I can recognize. In that way, I can also get a small first idea of how the accelerator is working.
Here is a picture, I have taken of LEIR at CERN
Help m... | Sextupole magnet
Sextupole magnets are mainly used to correct for chromaticity.
http://en.wikipedia.org/wiki/Sextupole_magnet
| {
"language": "en",
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On constancy of cometary orbits how are the comets able to keep to a nearly fixed orbital period, though they lose a certain amount of mass during their perihelion?
| As seen from Kepler's laws the orbit is not dependent of the orbiting object's mass. Also, if you do derivation for the orbit (usually in polar coordinates), coordinates $\theta$ and $r$, are orbiting object's mass independent:
$$ \ddot{r} - r \dot{\theta}^2 = -\frac{GM}{r^2}$$
$$ r \ddot{\theta} + 2 \dot{r} \dot{\the... | {
"language": "en",
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If blue light has a higher energy than red light, why does it scatter more? As $E=hf=\frac{hc}{\lambda}$, blue light - with a smaller wavelength - should have a higher energy. However, it is the case that blue light scatters the most. Why is it that higher energy rays scatter more?
| In general, the scattering of light from some object depends on the how close the wavelength of light is to the size of the object.
To make an analogy, if a tidal wave with a wavelength of several kilometers hits a telegraph pole with a radius of 15 cm it isn't going to scatter very much. On the other hand, waves with ... | {
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The Planck constant $\hbar$, the angular momentum, and the action Is there anything interesting to say about the fact that the Planck constant $\hbar$, the angular momentum, and the action have the same units or is it a pure coincidence?
| The dimensions of
*
*the Planck constant $\hbar$,
*the action $S$, and
*the angular momentum,
are constrained by the following important facts:
*
*A conjugate pair of two observables is quantum mechanically related to the Planck constant $\hbar$ via a Heisenberg uncertainty relation.
*A conjugate pair of... | {
"language": "en",
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How can a Photon have a "frequency"? I picture light ray as a composition of photons with an energy equal to the frequency of the light ray according to $E=hf$. Is this the good way to picture this? Although I can solve elementary problems with the formulas, I've never really been comfortable with the idea of an object... | It is the quantum wave field $\psi$ (probability amplitude) for a particle which may have oscillations. So for a wave pulse advancing smoothly in some direction the oscillations inside the pulse correspond to the photon frequency and wavelength meanwhile the general movement of the pulse would be associated with the sm... | {
"language": "en",
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Has any permanent magnet motor been proven to run? I have read lots of articles about permanent magnet motors, some of which claim the possibility and other which refute it. Is it possible to have a permanent magnet motor that runs on the magnetic force of permanent magnets?
| Why, here you go, an example of permanent magnet motor - a Curie engine.
Motors that combine permanent magnets and electromagnets are common too.
Thing is magnetic field as a means of storage of energy is subject of the same thermodynamics laws as the rest of the universe. If you want the motor to move, you must change... | {
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Should any theory of physics respect the principle of conservation of angular momentum or linear momentum? Is it possible that a theory that can describe the universe at the planck scale can violate things that we now consider fundamental in nature?For example can it violate rotational and translational invariance and... | In once sense, the quantities you mention get violated all the time in quantum mechanics. Furthermore, it happens at levels that are far less ferocious, inaccessible, and hypothetical (if you can't get there it's hypothetical, by definition) than the Planck scale. Such violations rely on quantum uncertainty and result ... | {
"language": "en",
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Is there a point in universe that is observable at present? We know that we can see distant galaxies only billions years before now. We can observe the nearest stars just several years before the present. Something on the Moon can be observed only some seconds in the past.
Continuing this scale, is there an object in ... | An answer in more or less the same spirit as Adam's:
You need to consider the time involved for the neural impulses to get to your brain and for your brain to make sense of the information. Neural impulses travel considerably slower than light, and you'd expect a number of neural impulse "bounces" to take place for any... | {
"language": "en",
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Can every particle be regarded as being a combination of Black holes and White holes? Can the statement be regarded as true? That every particle, or element in the universe can be regarded as a combination of black hole and white hole in variable proportion.
| Over the years there have been suggestions that elementary particles may be black holes. However no-one has ever been able to make this quantitative and I doubt anyone believes it these days. There was some discussion of this in what is the difference between a blackhole and a point particle, and Googling will find you... | {
"language": "en",
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Gamma Ray Bursts What is the maximum frequency of the Gamma Rays produced during supernovae? And how are these detected by telescopes without getting some serious damage done?
| There is the ground based observatory ( nice picture) Veritas.
VERITAS (Very Energetic Radiation Imaging Telescope Array System) is a major ground-based gamma-ray observatory located at the basecamp of the Fred Lawrence Whipple Observatory in southern Arizona, designed to observe and study very-high-energy (VHE) gamma... | {
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Simple projectile motion problem A person wants to throw an object from the top of a tower $9,0m$ high towards a target which is $3,5m$ far from the place where the person is launching the object. Suppose that this object is thrown horizontally.
What I want to find out is:
*
*What initial speed does the object need ... | You are right about the second question.
for the first one. Ask yourself how long (time) does it have before it hits the ground (and what is this dependent on)? Some thought will reveal that the time it's in the air has to do only with its motion in the $y$ direction. So, if we only throw the ball horizontally, then ... | {
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Lenses (refractor) or mirrors (reflector) telescope? What differentiates, in terms of practical quality, not technical implementation, a refractor from a reflector telescope?
Why would one prefer a refractor over a reflector, when reflectors come with such large diameters at a smaller price?
| First of all, Carson is correct in that refractor lenses climb exponentially in price as the size of the lens grows.
Reflecting telescopes are very useful for astronomy, and it's easy to spend a small amount of money and end up with a hefty tube that can gather a ton of light with very few drawbacks.
On the other hand,... | {
"language": "en",
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Electrostatic Pressure Concept There was a Question bothering me.
I tried solving it But couldn't So I finally went up to my teacher asked him for help . He told me that there was a formula for Electrostatic pressure $\rightarrow$
$$\mbox{Pressure}= \frac{\sigma^2}{2\epsilon_0}$$
And we had just to multiply it to the ... |
Take the case of 3-D conductor and derive pressure exerted on a plate slightly separated from that conductor.
First try to derive it on your own if not comfortable take help from uploaded picture.
| {
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"question_score": "11",
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Rotationally invariant body and principal axis Suppose a rigid body is invariant under a rotation around an axis $\mathsf{A}$ by a given angle $0 \leq \alpha_0 < 2\pi$ (and also every multiple of $\alpha_0$).
Is it true that in this case the axis $\mathsf{A}$ is a principal axis of the rigid body?
If so, how to prove... | At least as phrased ("by a given angle $\alpha_0$"), axis $\mathsf{A}$ can be trivially shown not to necessarily be a principal axis. If $\alpha_0$ is a multiple of $2\pi$, every rigid body is invariant under a rotation about any axis.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/29614",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
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How does universal inflation fit with the Planck length? If the universe is undergoing inflation, and there is a minimum scale that things can exist at (the Planck length), does that mean that new Planck-sized domains have to be continuously popping into existence? If not, does that mean that the Planck length is const... | This Wikipedia article on the Metric expansion of space might provide a better answer, but here goes my understanding, good or bad.
I don't think it makes sense to think that new Plank domains come into existence. After all, the Plank distance is just a unit of measure. The distance between objects in the Universe does... | {
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Calculating the Uncertainty for an Average Value How would I calculate the uncertainty for the average of this set?
$32.5 \pm 0.1$
$32.0 \pm 0.1$
$32.3 \pm 0.1$
| You can callculate the standart deviation as show in this link http://en.wikipedia.org/wiki/Standard_deviation#Generalizing_from_two_numbers
The standart deviation $\sigma=\sqrt{\frac{\sum_{i=1}^n a_i^2}{n}-\left(\frac{\sum_{i=1}^n a_i}{n}\right)^2}$, where $a_i$ is the $i$-th number in your set and $n$ is the number o... | {
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Why/how does an electron emit a photon when decelerating? I've had two special relativity courses so far but none really gave me a clear description of the process.
| Charged particles are permanently coupled to the electromagnetic field, it's an experimental fact and the very essential feature of charges. As any coupled (compound) system, the system (electron + EMF) has its center of mass variables and the "relative motion" (or "internal") variables. Generally, when you act on one ... | {
"language": "en",
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Gravity on the International Space Station We created a table in my physics class which contained the strength of gravity on different planet and objects in space. At altitude 0 (Earth), the gravitational strength is 100%. On the Moon at altitude 240,000 miles, it's 0.028%. And on the International Space Station at 4,2... | The astronauts are just floating around because they are each in orbit. Because they are at the same altitude as the ISS they are in the same orbit and moving at the same speed as it and so appear weightless.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/29929",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "25",
"answer_count": 7,
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Why do magnetic field lines go from North to South? Why magnetic lines comes from north to south out side of the magnet
is any magnetic lines comes from south to north if so in which direction
What is the reason of magnetic lineS
| Making magnetic field lines go from north to south is just a convention. An equally valid convention could have been magnetic field lines going from south to north. The magnetic field lines are a conceptual tool to visualize the magnetic field.
An interesting point about magnetic fields is that they are divergenceless... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30046",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Is there any way to survive solarwinter like in Sunshine - movie? Is there any way to survive solarwinter like in Sunshine - movie?
Solar winter is where for some reason sun looses its capasity to produce radiation( heat etc.). It doesn't loose everything but some of its radiation energy( say 50 %) That causes earth to... | Food could be grown using UV lights, powered by nuclear fission.
We could probably do it.
But it would be the spece equivalent of a human being on a life support machine - all our time and energy would be consumed just with survival, so while humans as a species might survive, our society, culture and science would p... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30095",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Ideal gas with two kinds of particles, Grand canonical partition function Consider an ideal gas contained in a volume V at temperature T. If all particles are identical the Grand canonical partition function can be calculated using
$$Z_g(V,T,z) := \sum_{N=0}^\infty z^N Z_c(N,V,T)$$
where $z$ is the fugacity, and
$$Z_c... | Here's my shot at it and my whole thought process so we are checking each other.
If we put in the Hamiltonian into the exponent and talk it through, we get the following:
$$
\int e^{-\beta( \sum_i p^{2}_i/2m_1 +\sum_j p^{2}_j/2m_2)}
$$
but what is the measure of integration? well they are non interacting, so we have to... | {
"language": "en",
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Does this relation about direction of particles make sense? Maybe I've just stared at this statement too long and I've missed something obvious. Nevertheless, here's the problem: Landau-Lifshitz vol. 1§16, problem 1.
Consider (classical) collision of two particles in center of mass coordinates. Before the collision, t... | The difference comes from the picture--- the $\theta_1$ and $\theta_2$ in the original statement are both relative to the positive x-axis, while in the solution $\theta_2$ is the final angle relative to the initial velocity of the corresponding particle, so if the velocity is along the x-axis, $\theta_1$ is the angle r... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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How do we prove the existence of a multiverse? How do we prove that a multiverse exists?
Scientists are talking about our universe not being the only universe, but even if that is true, how can we prove the existence of multiverse? We are being 'confined' in this universe and there is no way we can know what is happeni... | You don't. That's because Multiverse is not a scientific hypothesis. It speculates that there are numerous universes out there. Those are not observable. They would have each their own set of laws of physics much different than our own. As conceptualized, the Multiverse theory can’t be tested. For this reason, the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30361",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 1
} |
Collision of a black hole & a white hole A black hole and white hole experience a direct collision.
What happens? What shall be the result of such a collision?
| In an entirely time symmetric situation, and assuming the universe does not contain any other fields besides the two B&W holes, the only way to determine what is the preferred time direction is by looking which of the two holes have the bigger entropy.
Each hole has an entropy proportional to the square of its mass. i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30406",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
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"answer_id": 3
} |
What are electromagnetic fields made of? I am trying to understand electromagnetic fields so I have two question related to them.
*
*What is a electromagnetic field made of? Is it made of photons / virtual photons?
*How about a static electric or magnetic field?
| The most fundamental thing in physics, is the way that we conduct phyiscs. And physics will be as good as we are conducting it.
We conduct physics by using 1) logic and 2) the scientific method.
In that context - a scientific theory is just us Humans, trying to give an accurate description of our unerstanding of our re... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30517",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
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} |
Path traced out by a point While studying uniform circular motion at school, one of my friends asked a question:
"How do I prove that the path traced out by a particle such that an applied force of constant magnitude acts on it perpendicular to its velocity is a circle?" Our physics teacher said it was not exactly a v... | If you don't want or know how to solve a pair of simultaneous differential equations, try this more elementary approach using complex numbers and ordinary time derivatives.
Consider the arbitrary path, with parameter t, in the complex plane:
$r(t)e^{i\theta(t)}$
The "velocity" is the time derivative:
$[\frac{dr}{dt} + ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30614",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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"answer_id": 2
} |
Does $p=mc$ hold for photons? Known that $E=hf$, $p=hf/c=h/\lambda$, then if $p=mc$, where $m$ is the (relativistic) mass, then $E=mc^2$ follows directly as an algebraic fact. Is this the case?
| According to Special Relativity the relativistic energy for a particle is: $E^2= m^2c^4+p^2c^2$
The invariant quantity under relativistic transformations is the rest mass $m$ of the particle.
For a photon $m=0$
Using some simple algebra it is found $E=pc$ for a photon.
You will see this preserves the frequency and ene... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30764",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 5,
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Why isn't it allowed to use a flash when taking pictures in a certain place? When I go to, for example, a museum I try to take some pictures.
Sometimes the museum staffs forbid me to use a flash. Do you know the reason? I don't think it is related to photo-electric effect, right?
| As user9886 explained, the main reason is probably not physical. There are indeed cases where strong flashes can damage pigments that are fairly stable in daylight. I know that some modern documents use rhodopsin based ink that makes it impossible to use an ordinary photocopier to copy them without destroying them. I'm... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30835",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 6,
"answer_id": 4
} |
What's the difference between Fermi Energy and Fermi Level? I'm a bit confused about the difference between these two concepts. According to Wikipedia the Fermi energy and Fermi level are closely related concepts. From my understanding, the Fermi energy is the highest occupied energy level of a system in absolute zero?... | The Fermi energy is as you describe: it is the highest occupied level at absolute zero. The Fermi level is the chemical potential. It is the energy level with 50% chance of being occupied at finite temperature T. The Fermi energy does not depend on temperature; the Fermi level does depend on temperature.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30922",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "33",
"answer_count": 5,
"answer_id": 2
} |
Non linear QM and wave function collapse I heard that there have been some propositions about describing the collapse of the wave-function by adding non-linear terms, but I couldn't anything in any any textbooks or even articles (probably those propositions never reached a good level of consistency). However, I'd like ... | As far as I know, nonlinearities aren't compatible with Lorentz invariance. The overall probability renormalization factor also needs to be rescaled globally, although that might not be a problem if rejecting a probabilistic ontology.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30982",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 4,
"answer_id": 1
} |
Practical matter of the Higgs-Mechanism My maybe very naive question is, of what practical importance will the discovery of the Higgs-Mechanism be for our technological advance in the near future?
| I don't think that's a naive question at all, as there's quite a lot to it.
The immediate answer is "no difference". If you, like many of us, read science fiction as an impressionable youngster you're probably vaguely disappointed that warp engines and interdimensional drives haven't been invented yet, and you probably... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31293",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 2,
"answer_id": 0
} |
How long was a day at the creation of Earth? Since the earth is slowing its rotation, and as far as I know, each day is 1 second longer every about 1.5 years, how long was an earth day near the formation of earth (4.5 billion years ago)?
I wouldn't assume to just do 4.5b/1.5 and subtract, because you would think the ra... |
... each day is 1 second longer every about 1.5 years
That figure is way off.
According to this Scientific American article, the Earth's rotation rate just after the collision that formed the Moon was about once every 6 hours. At that time, the Moon would have been about 25,000 kilometers away. The tidal effect of th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31429",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 3,
"answer_id": 1
} |
Can a huge gravitational force cause visible distortions on an object In space, would it be possible to have an object generating such a huge gravitational force so it would be possible for an observer (not affected directly by gravitational force and the space time distortion) to see some visual distortions (bending) ... | This answer may not fully qualify because it is not seen from afar, but when we observe tides we are basically seeing a huge gravitational force (the moon and sun) causing visible distortions on an object (the ocean).
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31485",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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"answer_id": 2
} |
Friction at zero temperature? By the fluctuation-dissipation theorem (detailed-balance for Langevin equation), $$\sigma^2 = 2 \gamma k_B T$$ where $\sigma$ is the variance of noise, $\gamma$ is a friction coefficient, $k_B$ is Boltzmann's constant, and $T$ is temperature. So in principle, one can have $\gamma\neq 0$ wh... | A simple approximation of your question, from the semiclassical point of view, could be this.
Imagine a ball running over a surface. This surface is made up of other little balls: these balls are considered little respect to the one running over them so they create a flat surface. Imagine that these little balls start... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31533",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
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"answer_id": 1
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Magnetic field inside a charged stream Outside a narrow charged stream (say, a beam of ions or electrons) is the same as observing a current through a conducting wire - there is a circular magnetic field around it.
What would happen inside a charged stream (for example, inside a conducting wire or inside a solar flare)... | The field is linearly proportional to r inside the stream (if the current density is uniform inside the beam) and falls off as 1/r outside. This is a simple application of Ampere's law. This leads to an attractive force compressing the beam, but always less than the electrostatic repulsive force pushing the beam out. T... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31598",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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If a superconductor has zero resistance, does it have infinite amperage? If amps = volts / ohms, and ohms is 0, then what is x volts / 0 ohms?
| the amperage is current flowing through the superconductor. All superconductor have a critical magnetic field they can counter before the superconducting phase breaks. This critical magnetic field also implies a critical electrical current, because all current will generate an associated trasverse magnetic field.
What ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31646",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "12",
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Sound frequency of dropping bomb Everyone has seen cartoons of bombs being dropped, accompanied by a whistling sound as they drop. This sound gets lower in frequency as the bomb nears the ground.
I've been lucky enough to not be near falling bombs, but I assume this sound is based on reality.
Why does the frequency dro... | In my opinion I think the whistling sounds come from a manufacturing defect or an engineering defect in the shape or dynamic balance of any rapidly moving object through the air. Eddy current caused behind the moving object can set up as standing waves creating a whistling noise, such eddies are used in the form of flo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31709",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 6,
"answer_id": 5
} |
Will adding heat to a material increase or decrease entropy? Does adding heat to a material, thereby increasing electrical resistance in the material increase or decrease entropy?
Follow up questions:
Is there a situation were Heat flux ie. thermal flux, will change entropy?
Does increasing resistance to em transfer ... | The resistivity of some materials increases with temperature and with others it decreases. The way we manufacture zero temperature coefficient resistors (stable resistors) is by balancing these effects. But ignoring that, your question is also about entropy, does it always increase with temperature?
An assumption of st... | {
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"url": "https://physics.stackexchange.com/questions/31764",
"timestamp": "2023-03-29T00:00:00",
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Non-Newtonian Fluid Stop a Bullet? I just saw a YouTube video about Non-Newtonian fluids where people could actually walk on the surface of the fluid but if they stood still, they'd sink. Cool stuff.
Now, I'm wondering: Could a pool of Non-Newtonian fluid stop a bullet? Why or why not?
If so, if you put this stuff insi... | Actually non-neutonian fluids are a bit heavy and massive with regard to their bullet stopping efficiency. They could work with other approaches.
This does not mean their tension surface is infinite. They are penetrable and a bullet applies huge surface tension
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31833",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 4,
"answer_id": 1
} |
What's an efficient way to produce graphite on TEM-Grids? I am trying to produce graphene with few layers(<10) on a TEM-Grid. Until now I've been trying this with the scotch-tape-method with slight modifications. Unfortunately it requires a lot of time und there are often TEM-Grids without any flakes of the required th... | There are, in fact, a wide variety of techniques for producing graphene other than the scotch-tape method. A very good review of these techniques can be found in this recent review article:
http://onlinelibrary.wiley.com/doi/10.1002/adma.201202321/abstract
It is extremely difficult to obtain the dimensions you require ... | {
"language": "en",
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Atomic structure and corresponding superpartner behavior If all quantum particles have a superpartner, what happens, if this has been able to be speculated based on theory, to the superpartners when the corresponding partners start forming atoms?
*
*Is there an S-hydrogen, S-helium, etc.?
*Or are superpartners free... | Firstly the correspondence happens at the level of fundamental particles. Quarks imply squarks, leptons imply sleptons and so on. Further, the partners of fermions are bosons and vice versa, so their interaction don't mirror their partners and you should not be thinking about s-nuclons or s-Hydrogen.
Further, it's not ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32021",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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Smooth trajectory on a smooth manifold Physicists talk about a smooth trajectory of a particle on a smooth manifold and they label it as $q(t)$ where $q_1(t)....q_n(t)$ are component functions coming from the homeomorphism.
I don't see how we can meaningfully talk about the whole trajectory this way as it might happen... | Is there some specific problem you have which makes this a physics question and not just a mathematics question?
You essentially ask how we can state a parametrized curve, a one-dimensional submanifold, a smooth function from $t$ to $\mathcal M$. Clearly, if you use an atlas without global coordinates, but a set of coo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32132",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Does gravity slow the expansion of the universe? Does gravity slow the expansion of the universe?
I read through the thread http://www.physicsforums.com/showthread.php?t=322633 and I have the same question. I know that the universe is not being stopped by gravity, but is the force of gravity slowing it down in any way?... | The Friedmann equations for the expansion of space are (assuming flat space for simplicity):
$(1)\ (\frac{\dot a}{a})^2 = \frac{8 \pi G \rho + \Lambda}{3}$
$(2)\ \frac{\ddot a}{a}= -\frac{4 \pi G}{3}(\rho + 3P) + \frac{\Lambda}{3}$
where $a$ is the scale factor (roughly, how "expanded" space is), $\dot a$ is the rate o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32189",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 7,
"answer_id": 2
} |
Where do the terms microcanonical, canonical and grand canonical (ensemble) come from? Where do the terms microcanonical, canonical and grand canonical (ensemble) come from?
When were they coined and by whom? Is there any reason for the names or are they historical accidents?
| I do not know where they come from, but in french a canonical form is an expression which appears "naturally". For instance the canonical basis for the linear space spanned by the second degree polynomial is made of: $\{1,X,X^2\}$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32225",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 1
} |
Introduction to differential forms in thermodynamics I've studied differential geometry just enough to be confident with differential forms. Now I want to see application of this formalism in thermodynamics.
I'm looking for a small reference, to learn familiar concepts of (equilibrium?) thermodynamics formulated throug... | There are two articles by S.G. Rajeev: Quantization of Contact Manifolds and Thermodynamics and A Hamilton-Jacobi Formalism for Thermodynamics in which he reviews the formulation of thermodynamics in terms of contact geometry and explains a number of examples such as van der Waals gases and the thermodynamics of black ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32296",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "21",
"answer_count": 4,
"answer_id": 0
} |
How do mirrors work? My physics professor explained to me that electromagnetic waves are consisted of two components - electric and magnetic - which cause each other.
*
*Which part of the mirror actually reflects the wave?
*Which of those two wave components? Both?
*How come the wave doesn't get heavily distorted ... | The reflection could be viewed as a two step process. The incident wave causes the electrons in the silver to vibrate like in an antenna. Though by vibrating they also emit the same light. So it's the electrons at the surface of the silver that reflect the incoming wave. As you mentioned the wave is part electric and ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32483",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 2,
"answer_id": 1
} |
Which universe had a beginning? The universe or the observable universe? When we say the universe had a beginning, do we mean the entire universe or the observable universe? Or did both of them have a beginning?
| You should never distinguish between the universe and the observable universe. This is a fallacy of assuming that the word "exist" means something more than "something we can measure and interact with". There is no accepted or concievable way to explore other universe, so one must be very careful when talking about the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32517",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 1
} |
Hamiltonian and the space-time structure I'm reading Arnold's "Mathematical Methods of Classical Mechanics" but I failed to find rigorous development for the allowed forms of Hamiltonian.
Space-time structure dictates the form of Hamiltonian. Indeed, we know how the free particle should move in inertial frame of refere... | I tried the following: assuming that the metric only depends on the coordinates, $g(q)$, and the energy only depends on the the coordinates and the spatial momenta, $p_0 (q,p_i)$. I then considered variations like
$$
\delta S=\int \delta \theta=\int \delta(g_{ab}p^a dq^b)=\int \delta(g_{00}p^0dq^0+g_{0i}(p^idq^0+p^0dq... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32583",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 4,
"answer_id": 2
} |
What is the physical meaning of diffusion coefficient? In Fick's first law, the diffusion coefficient is velocity, but I do not understand the two-dimensional concept of this velocity. Imagine that solutes are diffusing from one side of a tube to another (this would be the same as persons running from one side of a str... | Let stick to your example of the people running around in the street.
this would be the same as persons running from one side of a street
If all people are running in the same direction, then this would be convection, and not diffusion.
Suppose you have a large room, with 100 guys running around in random directions.... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32628",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 4,
"answer_id": 1
} |
Can light exist in $2+1$ or $1+1$ spacetime dimensions? Spacetime of special relativity is frequently illustrated with its spatial part reduced to one or two spatial dimension (with light sector or cone, respectively). Taken literally, is it possible for $2+1$ or $1+1$ (flat) spacetime dimensions to accommodate Maxwell... | Light may exist in a 2 dimensional space, but it wouldn't appear the same as ours. With 1 less dimension it would become a point. Add an additional dimension to the point and ,as all dimensions should be perpendicular to each other, you would end up with an electromagnetic wave in 3 dimensional space. This wave would i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32685",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "41",
"answer_count": 4,
"answer_id": 3
} |
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