Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
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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... | The short answer is that physicists/astronomers want to avoid fine tuning wherever possible. Creating a universe where the temperature everywhere was essentially the same requires exceptional fine tuning. Creating a universe where the temperatures were random in different parts of space and had an opportunity to come i... | {
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Why does Jupiter emit more energy than it receives? I hear that Jupiter and Saturn emit more energy than they receive from the Sun.
This excess energy is supposedly due to contraction.
*
*Is this accepted as fact (or is it controversial)?
*Does this mean that Jupiter is shrinking a little bit (its diameter decrea... | The book Jupiter: The Planet, Satellites and Magnetosphere, edited by Fran Bagenal, Timothy Dowling, and William McKinnon has, in its third chapter, the text: "[Jupiter] is still contracting at a rate of ~3 cm per year while its interior cools by ~1 K per million year."
The chapter does not give a specific source for ... | {
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Why did the WMAP mission last so much longer than Planck? NASA endorsed 9 years of data taken with the Wilkinson Microwave Anisotropy Probe (WMAP). The High Frequency Instrument aboard the Planck satellite ran out of coolant at the start of 2012, after about two and a half years of operation. Even if the Low Frequency ... | From Wikipedia:
Passive thermal radiators cool the WMAP to ca. 90 degrees K; they are
connected to the low-noise amplifiers. The telescope consumes 419 W of
power. The available telescope heaters are emergency-survival heaters,
and there is a transmitter heater, used to warm them when off. The
WMAP spacecraft'... | {
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Where can I find information for how to create amateur radio telescope? Where can I find good source of information for how to create amateur radio telescope.
Particularly interested in creating Fresnel Zone Plate Antennas (aka flat dish).
*
*design, diagrams, electronics.
*discussions about probes in production e... | Taking a step back, what do you want to study with your telescope? The telescope is just one component in your DIY astronomy lab. You are also going to need a plan for your study, a way to acquire signals from your telescope, and a way to process those signals.
Some telescope / antenna stuff -
*
*A guy who built a t... | {
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Is it possible to make a hydrogen-alpha solar scope? Is the construction of an etalon / Fabry-Pérot interferometer within the reach of amateur telescope makers? Are there any resources pointing to such projects?
| You can buy etalons on eBay cheap, they even have some 1" 1047nm for $35 now. I am a holographer, and I know how to use these in a laser. What confuses me is how they are used in a telescope with converging, rather than parallel rays.
Can you just put mylar and a long pass filter (maybe a couple lighting gels) in front... | {
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What is behind the cosmological horizon barrier? I'm wondering what is behind the cosmological horizon barrier?
| The optical horizon is calculated to be about 46 Gyrs away if we presume a flat Universe, from the simple relationship R_h = 3ct, where R_h is the optical horizon and t time. This relationship can be found in P.J.E.Peebles book, Principles of Physical Cosmology, and in the book by J.B. Hartle, Gravity. So the 46 Gyrs m... | {
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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... | In the first case, in regards to star measurement, I believe you're thinking of how the diameter of very large stars are measured using interferometry. Because light waves from the edges of these stars arrive at us in parallel, and they are waves, we can determine the diameter of the star by measuring the interference ... | {
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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... | If light is interacting with ions/atoms and neutrinos do not, that would mean that light has a variable speed no? Therefore neutrinos are more constant at the "speed of light". If this is true, then can we devise an experiment that slows light? It is energy and has mass, why not? If this is correct, then wouldn't nuetr... | {
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Where are the Voyagers going? Given enough time, where are the Voyager spacecrafts heading? (Assuming some alien civilization doesn't pick them up.)
Will they pass by any interesting stars on the way to the black hole at the center of our galaxy or will it perhaps leave the galaxy?
What are the highlights on their jour... | So basically voyager 2 is not really moving at all. Our solar system is traveling away from the center of the galaxy at approximately 35,000 miles per hour and voyager is traveling toward the center of the galaxy or away from the our sun at the same speed.
| {
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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 mass of these single stars is sometimes determined by the effect of Gravitational Lensing. In General Relativity, it is stated that light is bent when it is influenced by a Gravitational Field, so actually the angle of bending of any light coming from near a single star, would give us the magnitude of its Gravitati... | {
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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?
| They are much brighter than the Milky Way - I have once seen one bright enough to read by, think it was back in '76 or '77 and it was so powerful it was also audible in Orkney (59 degrees north.) Unfortunately haven't seen this week's one as it has been overcast here.
A full moon will drown out a lot of the detail to t... | {
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Can the Hubble telescope bring any star into focus? Lets say I am talking about a view like this supernova - 13 billions light year away. In short can Hubble bring any star into focus in the entire universe? And if so, to what definition?
I also wonder, how much time would time would it need to focus on a distant star... | Telescopes do not focus. The objects they view (even the Moon) are so far away compared to the aperture and focal length of a telescope that they are all at infinite focus, so telescopes use a fixed focus.
Whether a telescope can resolve a distant object or not is dependent on aperture and brightness. Brightness in tur... | {
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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... | Since no one actually lives on the Moon, there is no call to have special names.
A day and a month on the Moon are the same length, 29.5 Earth days. Each new lunar month is called a "lunation" and numbered. Lunation 1094 started on 2011 Jun 01 and Lunation 1095 starts on 2011 Jul 01.
| {
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Why is a new moon not the same as a solar eclipse? Forgive the elementary nature of this question:
Because a new moon occurs when the moon is positioned between the earth and sun, doesn't this also mean that somewhere on the Earth, a solar eclipse (or partial eclipse) is happening?
What, then, is the difference between... | - This is because of the of moon’s tilted orbit around Earth with respect to the earth’s orbital plane (ecliptic).
*
*Solar and lunar eclipses happen only during an eclipse season when the plane of the Earth's orbit
around the Sun crosses with the plane of the Moon's orbit around the Earth.
*If the orbit of the Ear... | {
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How could I translate a field of view value into a magnification value? When I zoom in with Stellarium, it indicates a field of view (FOV) value in degrees, but most binoculars and telescopes are advertised with value like "nX magnification power."
How could I translate this value so I get an idea of what I will see wi... | The magnification on a telescope can be calculated using its focal length divided by the focal length of the eyepiece you are using, this means, using a 150mm focal length scope with a 5mm eyepiece would give you roughly 30x but also a 1200mm FL telescope with a 40mm eyepiece will.
If you can get the values of aperture... | {
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Mass of a galaxy via luminosity Is there a general formula for calculating the mass of a galaxy, or even a nebula from the luminosity? Or, is there a way of calculating the total mass of a galaxy from its energy output?
Is there a Hertzsprung–Russell diagram equivalent for galaxies?
I know about gravitational lensing o... | There is not a straightforward relation between a galaxy's luminosity and its mass. The luminosity depends on how much present and recent star formation there has been. Some very massive elliptical galaxies have little star formation going on, so they are not particularly luminous for their mass.
To understand why this... | {
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In astronomy what phenomena have theory predicted before observations? As far as I know, astronomy is generally an observational science. We see something and then try to explain why it is happening. The one exception that I know of is black holes: first it was thought of, then it was found.
Einstein's relativity is ... | Aristotle (4th century BCE) first hypothesized that the Earth is geographically divided into three types of climatic zones based on their distance from the equator (circles of latitude). In my opinion, it's the basis of one of the greatest science experiments of antiquity that never was. If the ancient Greek astronomer... | {
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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... | You could orient yourself using the CMB fluctuations as your compass--- given a detailed WMAP picture of the CMB flutuations, you could tell which way is which anywhere in the local galactic region, and how fast you are going relative to the CMB.
| {
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Why are stars, planets and larger moons (approximately) spherical in shape (like, the Sun, the Moon, the Earth, and other planets)? Why are stars, planets and larger moons (approximately) spherical in shape (like, the Sun, the Moon, the Earth, and other planets)?
| I'm not sure if this is the "done" thing, but the question is cross-posted from Physics.SE, so I'm cross-posting my answer...
In short, it's because gravity is "round". That is, it only depends on the distance between objects. All objects that are at a particular distance are attracted with the same acceleration, so we... | {
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Why does Omega Centauri have a distinct chemical signature from the rest of the Milky Way? In answering a question about the orbital path of Omega Centauri, I learned that it has a distinct chemical signature from the rest of the Milky Way. Basically, it is very rich in s-process elements, which I think are primarily p... | After looking through a few papers, in particular Chemical Abundances and Kinematics in Globular Clusters and Local Group Dwarf Galaxies and Their Implications for Formation Theories of the Galactic Halo and references therein, I think I have a reasonable answer. Omega Centauri's chemical abundance seems to be most eas... | {
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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... | Rain Today provide a rain cloud image down to 15 minutes prior. But rain cloud doesn't always equate to total cloud cover.
I think the major drawback is that clouds can arise from clear air as the moisture condenses, and predicting where this is going to happen becomes too complicated to resolve at fine enough detail ... | {
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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?
| I assume you mean measure the apparent angular diameter of a galaxy too distant to be resolved, in which case I would say yes. (If you can resolve it, the HBT effect is unnecessary, of course!) First of all, stars are extended objects, just like galaxies, so there should be no fundamental difference. Secondly, the Wiki... | {
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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... | The key reason why there will never be a next-generation optical telescope in space is: adaptive optics.
In 1978 when funding for the HST started, adaptive optics was in its infancy. Only in the 1990's when computer technology had sufficiently advanced, did adaptive optics really take off. Modern earth-bound telescope... | {
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Can black holes actually merge? If time stops at the event horizon, can we ever detect two black holes merging? In other words, if you are a short distance away, would you encounter a spherically symmetric gravitational field, or a dipole field?
| As a distant observer we can watch the shadow of the black holes forming in front of the background stars. According to a nice little paper by Daisuke Nitta, Takeshi Chiba, and Naoshi Sugiyama ("Shadows of Colliding Black Holes, 2011") the answer is yes. To a distant observer in a finite span of time two black holes f... | {
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What is exactly the density of a black hole and how can it be calculated? How do scientists calculate that density? What data do they have to calculate that?
| There might be no full-fledged theory of quantum gravity, but we can speculate a little on results from whatever the true theory is. Quantizing gravity usually implies quantizing spacetime- in other words, the entire universe is grainy. It is likely that you can pack no more than about one Planck mass into each Planck ... | {
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What if our Sun were located in the middle of a globular cluster? Say you took our current solar system and relocated it deep in the heart of a globular cluster such as Omega Centauri. What would the night sky look like? Would the starshine of nearby stars be enough to turn the sky blue or cast shadows or are they stil... | I just have to say that our planets would probably never get life because other suns would no doubt destroy the planets. however if we did live in a place like that night would be as bright as day.
| {
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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... | The virtual particle/antiparticle explanation is common, but (from what I understand) not very accurate; see e.g. this explanation by John Baez. To summarize it in less technical terms, spacetime near the black hole's event horizon is so strongly curved that what a nearby observer would call "absolute zero" (i.e. zero... | {
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What is this shadow of the Sun on the Moon? I was reading the article Moon Phases on HowStuffWorks. In the picture, each moon has a dark green area which represents the shadow of the Sun. How is this shadow formed and why is this important?
| That is a very odd diagram - the area of the moon shown as dark is not necessarily dark: it just represents the side we cannot see.
The green area is meant to represent the side unlit by the sun, leaving the lit section as that which we can see from Earth and lit by the sun.
It is obviously done this way to help you un... | {
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Counting complete sets of mutually unbiased bases composed of stabilizer states Consider $N$ qubits. There are many complete sets of $2^N+1$ mutually unbiased bases formed exclusively of stabilizer states. How many?
Each complete set can be constructed as follows: partition the set of $4^N-1$ Pauli operators (excluding... | For finite dimensional systems, R. Buniy and T Kephart in 1012.2630 quant-ph provide a tool for defining a set of equivalence classes for entanglement states based on their algebraic properties. Your answer should be in there.
| {
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Values of SM parameters at one certain scale The general question is:
What are the values of Standard Model parameters (in the $\bar{MS}$ renormalization scheme) at some scale e.g. $m_{Z}$? As its parametrization in Yukawa matrices is not unique - what are the values of gauge couplings, fermion masses and CKM matrix?
T... | See Appendix A of my PhD thesis: http://arxiv.org/abs/0905.1425
I worked out the values of the parameters to use in RGE at $\mu=M_Z$.
| {
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Physical interpretation to the category of CFTs This question comes from reading Andre's question where I wandered whether that question even makes sense physically. In mathematics, VOAs form a category, does this category as a whole have a physical interpretation?
| The category of CFT's (and related 3D TQFT) has been studied by
Kapustin and Saulinas in their recent paper Topological boundary conditions in abelian Chern-Simons theory.
The quote "we obtain a classification of such theories up to isomorphism" from their abstract refers to a notion of isomorphism that is clearly hig... | {
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Unitarity of S-matrix in QFT I am a beginner in QFT, and my question is probably very basic.
As far as I understand, usually in QFT, in particular in QED, one postulates existence of IN and OUT states. Unitarity of the S-matrix is also essentially postulated. On the other hand, in more classical and better understood ... | Unitarity of the S-matrix can be checked perturbatively. Bound states tend to be non-perturbative effects, so may not show up naive perturbative calculations. Unfortunately, the datailed proof is not discussed in many places. One book that has it is Scharf's book on QED. When looking through other books you should loo... | {
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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... | There are plenty of chiral CFTs with central charge 16 and nice properties studied in the mathematics literature. A nice example in this context would be chiral differential operators on a 8-manifold. If you want modularity of the character so that you want a holomorphic vertex algebra then the reference is
"Holomorphi... | {
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Examples of heterotic CFTs I'm trying to get a global idea of the world of conformal field theories.
Many authors restrict attention to CFTs where the algebras of left and right movers agree. I'd like to increase my intuition for the cases where that fails (i.e. heterotic CFTs).
What are the simplest models of heterot... | I just found by incidence a simple example in some proceedings of Böckenhauer and Evans below. Namely for $\mathrm{Spin}(8\ell)_1$ (so $D_{4\ell}$ lattice) with $\ell=1,2,\ldots$ there exist modular invariants, which should give rise to heterotic models (by Rehrens paper).
see section 7 in
http://books.google.de/books... | {
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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... | In any supersymmetric theory, a BPS state is a state which preserves some of the supersymmetry.
If we take as a definition of a supersymmetric theory, some theory (classical or quantum) which admits a Lie superalgebra of symmetries, then a BPS state (or configuration) of such a theory is one which is annihilated by som... | {
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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
$$ \text{Tr}(\mathcal O \, \Phi[\rho]) = \text{Tr}(\Phi^*[\mathc... | This is probably not exactly what you had in mind, but how about the channel that discards its input and always outputs the state corresponding the the minimum eigenvalue of $\mathcal{O}$?
| {
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Rigorous proof of Bohr-Sommerfeld quantization Bohr-Sommerfeld quantization provides an approximate recipe for recovering the spectrum of a quantum integrable system. Is there a mathematically rigorous explanation why this recipe works? In particular, I suppose it gives an exact description of the large quantum number ... | Yes, it can be made precise and corresponds to the leading order of the semiclassical expansion (WKB approximation) in $\hbar$. See Faddeev-Yakubovsky's "Lectures on quantum mechanics for mathematics students" (§20, formula (13)). An approach inspired by geometric quantization is explained in chapter 4 in Bates-Weinste... | {
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Question from Schutz's In q. 22 in page 141, I am asked to show that if
$$U^{\alpha}\nabla_{\alpha} V^{\beta} = W^{\beta},$$
then
$$U^{\alpha}\nabla_{\alpha}V_{\beta}=W_{\beta}.$$
Here's what I have done:
$$V_{\beta}=g_{\beta \gamma} V^{\gamma},$$
so
$$U^{\alpha} \nabla_{\alpha} (g_{\beta \gamma} V^{\gamma})=U^{\alp... | I don't think you need metric compatibility to prove this although you can use it. There is a much simpler way with repeated use of (any) metric to lower the index.
$
U^\alpha\nabla_\alpha V^\beta=W^\beta
$
$
\Rightarrow U^\alpha g^{\beta\gamma}\nabla_\alpha V_\gamma=g^{\beta\gamma}V_\gamma
$
$
\Rightarrow U^\alpha g_{... | {
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Hilbert-Schmidt basis for many qubits - reference Every density matrix of $n$ qubits can be written in the following way
$$\hat{\rho}=\frac{1}{2^n}\sum_{i_1,i_2,\ldots,i_n=0}^3 t_{i_1i_2\ldots i_n} \hat{\sigma}_{i_1}\otimes\hat{\sigma}_{i_2}\otimes\ldots\otimes\hat{\sigma}_{i_n},$$
where $-1 \leq t_{i_1i_2\ldots i_n} \... | A good starting point, I have checked just chapter 4 but there is more, is
R. R. Puri, Mathematical Methods of Quantum Optics, Springer (2001) (see here).
| {
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How to deterministically distinguish the following quantum states? (1) How to deterministically distinguish the following quantum states:
$$\frac{1}{\sqrt{2}}[|+0\rangle|0\rangle+|-1\rangle|1\rangle$$,
$$\frac{1}{\sqrt{2}}|-0\rangle|0\rangle+|+1\rangle|1\rangle$$,
$$\frac{1}{\sqrt{2}}|-0\rangle|0\rangle-|+1\rangle|1\ra... | If you want a deterministic local-operations-and-classical-communication protocol that will unambiguously determine which of the four states is present, it isn't possible. Alice can perform a degenerate measurement that collapses onto the spans of $|+0\rangle,|-1\rangle$ and $|-0\rangle,+1\rangle$, but after that the q... | {
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Given entanglement, why is it permissible to consider the quantum state of subsystems? Quantum entanglement is the norm, is it not? All that exists in reality is the wave function of the whole universe, true? So how come we can blithely talk about the quantum state of subsystems if everything is entangled? How is it ev... | If you have two subsystems that are entangled and forming an (ideal) pure state, it is still meaningful to consider a subsystem experimentally. If I entangle two atoms and give one to you, nothing prevents you from making measurements on "your" atom only.
By repeating this same experiment many times, with atoms prepare... | {
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Which main physics journals publish the main types of physics papers? I'm thinking of writing a paper on a new way of deriving the conservation of energy from symmetry principles and the Galilean transformations, but I'm not sure where to publish. Taking a look at AJP, it tends to be there for teachers at universities ... | If your argument is nice pedagogically, and very original, it might be suitable for the "American Journal of Physics". But in general, if you just want to get it out, you can just put in on Arxiv under "General Physics", as various people have done with pedagogical innovations. Or else you can just write it in a blog, ... | {
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Would you die if you put your hands on a powerline? You know how birds perch on powerlines without getting electrocuted? What if by some chance that I find myself falling and I grab on one of them? Let's say both of my hands are on the same line, would i get electrocuted?
I am thinking I won't because the current won't... | There actually is a current that moves through your body, albeit it not enough to hurt you. You don't need to actually touch the powerline, just standing near it will cause a current to flow in your body. If we model a single powercable hanging a height H above the ground as an electric charged cable of infinite length... | {
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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 can't see what's the physical meaning of this length scale, and, most of all, I can't see how this length scale can come up while deriving the 2D Coulomb potential by means of a Fourier transform:
zakk, the constant L in the logarithm is the integration constant which is present for any differential equation and mea... | {
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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... | Quadrupole magnet
Quadrupole magnets are mostly used for beam focusing.
http://en.wikipedia.org/wiki/Quadrupole_magnet
| {
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A Query on the Trapped Null Surface A a compact, orientable, spacelike surface always has 2 independent forward-in-time pointing, lightlike, normal directions. For example, a (spacelike) sphere in Minkowski space has lightlike vectors pointing inward and outward along the radial direction. The inward-pointing lightl... | The surface is codimension two in spacetime, it is two dimensional inside a four dimensional space. This means that the tangent space to the surface does not consist of all vectors perpendicular to the normal to the surface, just as in three dimensions, the one-dimensional vector tangent space is not the collection of ... | {
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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?
| I assume you are thinking along the lines of why is the sky blue?
Its because of the interaction of light with matter. In this case the interaction is called Rayleigh scattering.
The intensity of Rayleigh scattering is proportional to $$(Energy\;of\; the\; Photon)^4$$ or more completely
$$I = I_0\frac{8 \pi ^4 \alpha ... | {
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Time Reversal Invariance in Quantum Mechanics I thought of a thought experiment that had me questioning how time reversal works in quantum mechanics and the implications. The idea is this ... you are going forward in time when you decide to measure a particle. The particle then collapses to the observed state. Now if p... | Just a few pointers for you to explore more on this. Check out Aharonov's paper the time symmetric formulation of quantum mechanics: http://arxiv.org/abs/quant-ph/9501011
Tony Leggett talks about this: http://www.youtube.com/watch?v=IGim9uzcumk
It's a nice video and quite simple to understand.
| {
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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... | All you need is quantum mechanics, i.e. that nature in the microcosm is dual,sometimes it can manifest wave properties and sometimes particle properties.
It depends on the measurement/experiment if the wave or the particle nature will manifest itself. Electrons manifest this duality: in the two slit experiment their wa... | {
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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?
| Magnetic field, and interactions of atoms with magnetic field, conserve energy, i.e. no net energy gets created at any point. There is not a lot of energy in the field of even a strong permanent magnet. One could, in principle, construct a 'motor' that would demagnetise the magnets somehow, converting the field energy ... | {
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Origin of the Higgs field Are there any attempts in the literature at addressing the origin of the Higgs field? And, which lines of research that find it inevitable to address this question?
| In quantum field theory, the Higgs field is – much like the electromagnetic field, the W-boson field, the electron's Dirac field, and other fields – an elementary entity that can't be decomposed to anything simpler.
This claim of course assumes that the Higgs field isn't composite. The mass of the Higgs boson around 12... | {
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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 ... | Perception of an event by a brain is a process distributed over a large part of brain's neural network (rather than being a process performed by a single special neuron cell). Thus, for perception to occur a part of the network must reach a state distributed across multiple neuron cells. Due to the limited speed at whi... | {
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When would the proposed black hole at the centre of Milky Way gulp in our solar system? I've heard and read that our solar system lies near to the peripheral region of the Galaxy. Then accordingly we would have a greater probability of sustaining to eventual gulping down by the super-massive black hole. But how long ?
| We don't have to worry about falling into the black hole because we have far too much angular momentum from our motion about the galactic center. This can be made quantitative by considering the sign of the effective potential of our orbit. For reference, the sun is about 27000 light years from the galactic center, and... | {
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Rotational speed of a discus I was wondering whether the rotational speed of a discus has any influence on the flight of the discus. Would slowing the rotation or speeding it up change the trajectory in any way or would the flight simply become unstable when slowing down?
| Given the physical conditions, this seems like an appropriate explanation: The faster the discus rotates, the more violently and quickly it displaces the air around it. Now the absence or scarcity of air causes a reduction in air friction or viscosity around the discus and this allows it to move onward in the direction... | {
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Is it possible to have incommensurable but equally valid theories of nature which fits all experimental data? Is it possible to have mutually incommensurable but equally valid theories of nature which fits all experimental data? The philosopher of science Paul Feyerabend defended this seemingly outrageous thesis and ma... | The philosopher Saul Kripke has come up with a solution using modal logic and the possible worlds semantics. For each model or theory which fits all experimental data, assign a possible world. If a proposition P is true in all possible worlds, $\square P$. If a proposition Q is only true in some possible worlds, $\Diam... | {
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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?
| Lenses are expensive, and have an upper limit on their size (when they get too big they get really expensive, and you start seeing a lot more aberration effects). Really cheap telescopes are usually refractors (at least from what I've seen), but if you want a respectable telescope, refractors climb in price rather quic... | {
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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 ... | Electrostatic pressure is the tension developed inside the sphere due to mutual repulsion among the charges of the same sphere. Its same like a rubber band is stretched from all the points outwards so a stress is developed in it
| {
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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... | Your statement is true.
Proof:
Let $\rho$ be the mass density of the rigid body.
Remember that the tensor of inertia $I$ is given by:
$$
\vec{v}^t I \vec{w} = \int d^3b\, \rho(\vec{b}) (\vec{v} \cdot \vec{w} - (\vec{v}\cdot \vec{b})(\vec{w}\cdot \vec{b}))
$$
for all $\vec{v},\vec{w} \in \mathbb{R}^3$.
Now take an or... | {
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Power dissipated in resistor Suppose one has a circuit consisting of an inductor $L$ and resistor $R$ in series where $L$ and $R$ are known, passes an alternating voltage of frequency $\omega$ through it and that one wishes to calculate the mean power dissipated in the resistor.
Let the RMS voltage across the series co... | okay, This was really cool and I got some help from my physics professors on this one (apparently I won't learn this until next semester) and to find the magnitude of the square of a complex number you take it times it's complex conjugate. So in this case $$\frac{V_0}{i \omega L +R}$$ is multiplied with $$\frac{V_0}{-i... | {
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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.
| This is a hand waving answer.
You ask in the comments:
I see that that particle gains/loses energy, but aren't there other ways to do that?
Down in the particle world everything is quantum mechanical and the only things that exist are the standard model particles, which may sometimes act as waves according to strict QM... | {
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Would the rate of ascent of an indestructible balloon increase as function of it's altitude? Assume a balloon filled with Hydrogen, fitted with a perfect valve, and capable of enduring vacuum (that is to say, it would retain it's shape and so well insulated that the extremes of temperature at high altitudes and in spac... | There are several things to note here. First, the atmosphere stops being buoyant around 100-150 km above the surface of Earth (I believe the mesopause is roughly where this occurs). Balloons simply cannot float beyond this point, no matter how fluffy (lower density per unit volume) they are. Second, past the tropopause... | {
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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... | Currently all life lives off of the sun's energy, at least indirectly. One possible exception is some types of bacteria that need only heat energy plus some inorganic chemicals that abound in the ocean - those would be fine as long as the Earth is geologically active.
For us, however, it would be curtains, within mont... | {
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At what g is terminal velocity not terminal? How weak would gravity need to be in order for a human to reliably survive the terminal velocity of falling through air?
(Context: watching scifi on a space station with a variety of artificial gravities, it occurred to me that medium-strength gravity would have some advanta... | The smallest body that we know has an atmosphere is Titan, which has about 1/7 of Earth's surface gravity ($1.4\ m/s^2$) but an atmospheric pressure of 1.45 Earth's ($146.7\ kPa$).
Using $pV=nRT$ where $T=95K$ and a mean molar mass of Titan's atmosphere being $28.6\ g/mol$ allows us to calculate an atmospheric density ... | {
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Show that Bell states cannot be decomposed as tensor products of single qubits' states I'm trying to learn about the Bell state $\frac{1}{\sqrt{2}}|00\rangle+\frac{1}{\sqrt{2}}|11\rangle$. Question 10.1 in Algorithms asks us to show that this cannot be decomposed into the tensor product of two single qubits' states.
It... | This property of the tensor product has really nothing to do with its interpretation in quantum mechanics or your specific example. Given two vector spaces $U,V$ the tensorproduct $\phi: U \times V \to U \otimes V$ is characterized by the property (up to isomorphism), that for any bilinear map $b \colon U \times V \to ... | {
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Changing the Half-Life of Radioactive Substances Is there a way to extend or reduce the half-life of a radioactive object? Perhaps by subjecting it to more radiation or some other method.
| Have a look at the paragraph "radioactive decay" .
The half life is characteristic of each radioactive nucleus and depends on the basic interactions holding the nucleus together.
It depends on the quantum mechanical probabilities of transition from one energy level to another, sometimes changing element in the period... | {
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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... | By definition we'll never be able to observe other universes directly, because if we could they'd be in our universe.
However suppose (for example) string/M theory does get developed into a theory that is easily testable, and suppose we find that all the predictions string theory makes about our universe are experiment... | {
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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?
| White hole is an impossible object in universe.
Mathematically it is a black hole under inverted time. This can be interpreted as a black hole in an universe where second law of thermodynamics is inverted, that is the entropy always diminishes.
Since second law of thermodynamics has probablistic nature, one can see a ... | {
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Number of bits needed to express physical laws? What is the minimum number of bits that would be needed to express a given physical law, like the law of universal gravitation? How many bits are needed to express each of the four fundamental forces? Is there a pattern here?
| How many bits would it take to define the standard model plus gravity plus massive neutrinos plus dark matter and dark energy? It's an excellent question.
Let's consider the "new minimal standard model", which is a little out of date but still provides a template for the study of this question. The elements of the lag... | {
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"timestamp": "2023-03-29T00:00:00",
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"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
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?
| A magnetic field is a essentially a cloud of virtual photon "place-holders" in a state of flux; it's what the electrons that produce the field "owe" to other nearby electrons (which have gained real photons), for having their spin-charge moments aligned in the same direction.
A magnetic field is even more fundamental a... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
"answer_count": 7,
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In what sense is SUSY a spacetime symmetry? Clearly the SUSY anti-commutation relations involve momentum, and thus the generator of translations in spacetime:
$$\{ Q_\alpha, \bar{Q}_{\dot{\beta}} \} = 2 (\sigma^\mu)_{\alpha \dot{\beta}} P_\mu . $$
So I would say that naively SUSY has 'something to do with spacetime' s... | SUSY has something to do with spacetime since its generators $Q$ carry spin angular momentum, so they change the spin of the state they act on, hence SUSY is a spacetime symmetry. And this kind of generators is called fermionic generators. (compare with the generators of gauge symmetries, which are unphysical symmetrie... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "12",
"answer_count": 3,
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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... | Try looking for uniform circular motion in google. It is not hard to prove it if you know something about vectors and what taking a derivative of vector function means. Force is a vector it is proportional to acceleration. Acceleration is change in velocity(remember a vector) divided by time(really shot period of time)... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30614",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
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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?
| It really does depend on the place.
Sometimes it will be to minimise the (real or imagined) damaging effect of the light and/or heat from the flash, sometimes because it annoys or disturbs other visitors.
In a very popular place it may be to avoid triggering a photosensitive epileptic fit. Theoretically that's possible... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30835",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
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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?... | It depends on who you ask.
If you ask someone with solid-state physics background, they will probably answer along the lines of Colin McFaul or John Rennie: The fermi level is the same as chemical potential (or maybe one should say "electrochemical potential"), i.e. the energy at which a state has 50% chance of being o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/30922",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "33",
"answer_count": 5,
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} |
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 ... | The Ghirardi-Rimini-Weber Model is such a theory.
See for instance http://arxiv.org/abs/quant-ph/0406094.
| {
"language": "en",
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"source": "stackexchange",
"question_score": "4",
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"answer_id": 0
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How to compute the pure extensions of a given mixed state? Let us consider any pure state $|\psi\rangle\in\mathbb{C}^n\otimes \mathbb{C}^n\otimes \mathbb{C}^n$. Its reduced bipartite density matrix represent a pure state or mixed state depending on whether $|\psi\rangle$ is entangled or not (exactly how it is entangled... | *
*The fact that every mixed state $\rho$ acting on a finite dimensional Hilbert spaces can be viewed as the reduced state of some pure state $|\psi\rangle$ on a bigger Hilbert space is known as purification, see this Wikipedia page, where also the algorithm is given.
*In OP's case of $$\rho~\in~\mathcal{B}(\mathbb{... | {
"language": "en",
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"source": "stackexchange",
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Intrinsic Viscosity I'm presently undergoing an Experiment for the determination of the viscosity of Ficoll-70 using Ostwald viscometer to calculate the time and a digital weighing balance to determine the weight of a specified volume of the solution at different concentration. I have reached a result but unable to cor... | Your viscometer measures the viscosity of your Ficoll-70 solution. This viscosity is made up partly from the viscosity of the water and partly from the viscosity of the Ficoll-70 dissolved within it, and obviously the viscosity you measure increases with the concentration of Ficoll-70.
Intrinsic viscosity is a rather d... | {
"language": "en",
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"source": "stackexchange",
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Could someone remind me of what we mean by zero electric field "inside" a conductor? If I have a spherical conductor (perhaps a shell) and "inside", as in the hollow area there is nothing. The electric field is 0. But what happens if there is a charge "inside" (not like inside the conductor, but in the hollow region) o... | For a static situation (i.e. no charges moving or current flowing) the net electric field is always zero inside of a conductor. Where by 'inside' I mean actually inside the material itself, not a region of free space that is simply enclosed by the material. The reason for this is, say there is an electric field $\vec{E... | {
"language": "en",
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Dimension of vector resulting from tensorial product I'm quoting what I found in a book about quantum computation:
Individual state spaces of $n$ particles combine quantum mechanically through the tensor product. If $X$ and $Y$ are vectors, then their tensor product $X\otimes Y$ is also a vector, but its dimension is ... | The tensor product is the natural extension of the ordinary product
$(a+b)(c+d)=ac+ad+bc+bd$.
If you have two vector $x,y$ of dimension $n$ the tensor product become
$$
x_\mu \otimes y_\nu= x_\mu y_\nu=\Theta_{\mu\nu}
$$
where $\Theta_{\mu\nu}$ is a matrix of dimension $n\times n$.
The vector product of two vectors ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31354",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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} |
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... | Actually we only gain 1.3 milliseconds every 96-100 years, not 1 second every 1.5 years! :) the shortest known Earth day was 6 hours and the longest is 24 hours & 2.5 milliseconds (today's current day), in 1820 the day was exactly 24 hours, but since it's been nearly 200 years we've gained 2.5 milliseconds to our day. ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31429",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 3,
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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) ... | To elaborate a bit: when Einstein published the General Relativity papers, he included a calculation of the precession of the orbit of Mercury which showed GR made up for the rest of the corrections needed to account for the observed values, which was the first empirical evidence for GR. As a second piece of evidence, ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31485",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
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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... | Mechanical friction is a perfectly fine example. The coefficient of friction between two materials does not approach zero at absolute zero.
Electrical resistance (as pointed out by Alexander) is another example. Some materials (superconductors) have zero resistance at absolute zero, but by no means all of them!
I would... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31533",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
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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 mmf due to a current is determined by the current through the surface bounded by the closed path along which the magnetic field is integrated. A closed path within a cross-section of a conductor with, say, a uniform current density, will have a non-zero mmf associated with it and thus, a non-zero magnetic field ex... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31598",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
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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... | When a baby cries, the sound of his/her cry becomes lighter. But actually the frequency of the baby's vocal-cord arises. Due to the limitation of our ears (20-20000Hz) we failed to feel the loudness of the baby's cry.
Same way, when the bomb closer to ground the frequency of bomb reaches near 20000Hz (with respect to o... | {
"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": 4
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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 ... | A higher temperature will cause the atoms in the material to vibrate more, increasing the number of microstates available to the material. Thus, the entropy also increases, since the (microscopic) definition of entropy is
$$ S = k \log \Omega $$
where $k$ is the Bolzmann constant, and $\Omega$ is the number of microsta... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31764",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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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... | Yes, here is an example where they made a bullet proof vest by soaking Kevlar fabric in a non-newtonian fluid:
http://www.youtube.com/watch?v=LlEo5MbcaX0
The video is from 2006, I don't know if this has been developed further.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/31833",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 4,
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Is there an intuitive description of vacuum entanglement? People often refer to the fact that the vacuum is an entangled state (It's even described as a maximally entangled state).
I was trying to get a feeling for what that really means. The problem is that most descriptions of this are done in the formalism of AQ... | For a non-interacting quantum field, the whole mathematical structure of purely Gaussian VEVs that is the vacuum state is contained in the 2-point VEV, which for the KG field is the distribution
$$\left<0\right|\hat\phi(x+y)\hat\phi(y)\left|0\right>=\frac{m\theta(x^2)}{8\pi\sqrt{x^2}}\left[Y_1(m\sqrt{x^2})+\epsilon(x_0... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32092",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "21",
"answer_count": 4,
"answer_id": 3
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Phase shift of 180 degrees of transversal wave on reflection from denser medium Can anyone please provide an intuitive explanation of why phase shift of 180 degrees occurs in the Electric Field of a EM wave, when reflected from an optically denser medium?
I tried searching for it but everywhere the result is just used.... | Mathematical explanation: The is because the boundary is rigid and the
disturbance must have zero displacement at all
times at the boundary. By the principle of
superposition, this is possible only if the reflected
and incident waves differ by a phase of π, so that
the resultant displacement is zero.
Using Newton's law... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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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'm not completely sure, but I think they are introduced by Gibbs, and that book (available for download) is of historic importance.
The word ensemble really just means "set" in French, you consider the space of canonical coordinates of the detailed mechanics = microstates and you impose statistics by the fundamental ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32225",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
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} |
About an electrostatics integral and a delta-function kernel I'm having trouble with an integral and I would like some pointers on how to "take" it:
$$
\int \limits_{-\infty}^{\infty}\frac{3\gamma a^{2}d^{3}\mathbf r}{4 \pi \left( r^{2} + \frac{\gamma^{2}}{c^{2}}(\mathbf r \cdot \mathbf u)^{2} + a^{2}\right)^{\frac{5}{... | Set the $z$ axis along the direction of $\mathbf{u}$ and use spherical coordinates, which reduces your integral to something like
$$\int_0^\infty dr\int_0^\pi d\theta\int_0^{2\pi}d\phi\frac{r^2 \sin(\theta)}{\left(a^2 +r^2(1+\frac{\gamma^2}{c^2}\cos^2(\theta))\right)^{5/2}}.$$
Do the $\phi$ integral first and then the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32359",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
Measurement and uncertainty principle in QM The Wikipedia says on the page for the uncertainty principle:
Mathematically, the uncertainty relation between position and momentum arises because the expressions of the wave function in the two corresponding bases are Fourier transforms of one another (i.e., position and m... | Any measurement in physics is in general described by a probability distribution of different outcomes. This distribution depends both on the state of the system being measured and on the measurement apparatus, which are two different things. In quantum mechanics states are described by vectors in Hilbert space $\left|... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32421",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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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 ... | Trying to separate electric and magnetic parts of a wave is not possible (Maxwell's equations couple them for propagation), so I will ignore your first two paragraphs.
The mirror conductivity is the key. The electric field from light reaches the mirror's
metal and thereby causes a current to flow (which actually gener... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32483",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 2,
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} |
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... | Diffusion is a stochastic process where a single particle can move in each direction with the same probability.
Another description of the diffusion coefficient is the following equation:
$$D = x^2/(2t)$$
where $t$ is the time and $x^2$ is the mean squared displacement of the particles at this time.
The mean squared di... | {
"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": 0
} |
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... | I agree. Light can not exist in 2D spacetime as well as a $\vec{B}$ component that has to be perpendicular to $\vec{E}$. Also, the Gauss law requires
$$E \propto \frac{q}{r^{D-1}}$$ where D is the number of spatial dimensions.
Therefore it is absent in 2D.
However, 2D or 3D spacetimes still can have the speed of light ... | {
"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": 2
} |
What does symplecticity imply? Symplectic systems are a common object of studies in classical physics and nonlinearity sciences.
At first I assumed it was just another way of saying Hamiltonian, but I also heard it in the context of dissipative systems, so I am no longer confident in my assumption.
My question now is,... | Symplectic geometry is may be the cornerstone of the geometrization of physics. In addition to the very known fact that classical mechanics can be described by symplectic geometry, given some other structures, symplectic spaces can be quantized to produce quantum mechanics as well. A subclass of symplectic geometries n... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32738",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 5,
"answer_id": 2
} |
Motion in the body-fixed frame? This is really basic, I'm sure: For rigid body motion, Euler's equations refer to $L_i$ and $\omega_i$ as measured in the fixed-body frame. But that frame is just that: fixed in the body. So how could such an observer ever measure non-zero $L$ or $\omega$?
| I have yet to find a physics book that doesn't make this really confusing. If one has a vector fixed in inertial space, its components as viewed in a moving frame are obtained by the dot product of the vector with the moving unit triad fixed to the body but moving relative to inertial space. While the inertial frame wo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32787",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 6,
"answer_id": 5
} |
Is it theoretically possible to reach $0$ Kelvin? I'm having a discussion with someone.
I said that it is -even theoretically- impossible to reach $0$ K, because that would imply that all molecules in the substance would stand perfectly still.
He said that this isn't true, because my theory violates energy-time uncerta... | For a temperature to be definable and measurable the distribution of the kinetic energies of the molecules in the medium under discussion should be known.
The process of cooling involves removing thermal energy from a system. When no more energy can be removed, the system is at absolute zero, which cannot be achieved... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32830",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 8,
"answer_id": 1
} |
Can I study Quantum Computing or Quantum Mechanics with an Engineering background? I am currently studying Electrical & Electronic Engineering. I wish to pursue Quantum Mechanics or Quantum Computing as my research subject. Is it possible for me to do my M.Tech. and then pursue my research subject? What are the prerequ... | Yes, it is possible. Your first step would be to learn Quantum mechanics from s standard textbook such as Modern Quantum mechanics: by sakurai, or Principles of Quantum Mechanics: by Shankar. There is nothing such as research in pure quantum mechanics, though there are a small handful of people working on the foundati... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32894",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 2,
"answer_id": 1
} |
Explanation for $E~$ not falling off at $1/r^2$ for infinite line and sheet charges? For an infinite line charge, $E$ falls off with $1/r$; for an infinite sheet of charge it's independent of r! The infinitesimal contributions to $E$ fall off with $1/r^2$, so why doesn't the total $E$ fall off the same way for the infi... | Loosely speaking, as we walk away from a sphere it looks smaller, as we walk away from a cylinder just the radius looks smaller, but not the infinite length, and finally as we walk away from an infinite sheet of charge it never looks any smaller (we can never 'get away' from an infinite sheet).
At more mathematical le... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/32948",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 3,
"answer_id": 2
} |
Reflectance vs. Thin Metal film Thickness Graph Is there formula that gives reflectance of very thin film of given metal (tens of nanometers) to the visible light of given wavelength(808nm) ? Which properties of metals are needed for the formula ?
I would like to draw a plot of reflectance that is a function of titaniu... | Have a look at my answer to Make a semi transparent mirror with copper. To a good approximation the transmission falls exponentially with thickness. Just work out what tranmission you need, e.g. if you want 80% reflectance choose 20% transmission, and work out the film thickness accordingly.
You can find the optical co... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/33049",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
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What would happen if a hydrogen bomb were to explode in Saturn's atmosphere? Purely hypothetical since any kind of testing in atmosphere/space is banned by international legislation/agreement.
The humans have already bombed Luna so ... what could be expected to happen on Saturn if a hydrogen bomb were to explode in it... | Nothing devastating would happen. When the comet Shoemaker Levy hit Jupiter, with considerably more energy than an H-bomb, it made a big bang but Jupiter is still there.
Saturn's atmosphere can't burn because there is no free oxygen present. In fact there is regular lightning on Saturn, so if the atmosphere was going t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/33117",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
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