Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Suggested reading for renormalization (not only in QFT) What papers/books/reviews can you suggest to learn what Renormalization "really" is?
Standard QFT textbooks are usually computation-heavy and provide little physical insight in this regard - after my QFT course, I was left with the impression that Renormalization ... | And I have written another pedagogical article about renormalizations and IR divergences. I created a Google research group "QED Reformulation" and I run a blog on this subject. It is an alternative view of the problem, and I think, it is much more physical than the mainstream one. It is always useful to see the proble... | {
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Is Dr Quantum's Double Slit Experiment video scientifically accurate? I'm fascinated by the fundamental questions raised by the Double Slit Experiment at the quantum level. I found this "Dr Quantum" video clip which seems like a great explanation. But is it scientifically accurate?
| The video is horrifyingly bad. It shows a single-slit electron pattern over here, and then puts a second slit in, and shows the pattern from the second slit over there. Then it says: what if you have both slits open at the same time?
In fact, since the pattern from the first slit is separated from the pattern of the s... | {
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Books that every physicist should read Inspired by How should a physics student study mathematics? and in the same vein as Best books for mathematical background?, although in a more general fashion, I'd like to know if anyone is interested in doing a list of the books 'par excellence' for a physicist.
In spite of the ... | Paul Dirac - Principles of Quantum mechanics
Robert Griffiths - Consistent Quantum Theory
A. Zee - Quantum Field Theory in a Nutshell
V. Mukhanov - Physical Foundations of Cosmology
| {
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Relativistic Cellular Automata Cellular automata provide interesting models of physics: Google Scholar gives more than 25,000 results when searching for "cellular automata" physics.
Google Scholar still gives more than 2.000 results when searching for "quantum cellular automata".
But it gives only 1 (one!) result when ... | Check out Mark Smith's PhD thesis titled Cellular automata methods in mathematical physics, specifically Chapter 4: Lorentz Invariance in Cellular Automata.
The conclusion part of the chapter:
Symmetry is an important aspect of physical laws, and it is therefore desirable to identify analogous symmetry in CA rules. Fu... | {
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Lightning strikes the Ocean I'm swimming in - what happens? I'm swimming in the ocean and there's a thunderstorm. Lightning bolts hit ships around me. Should I get out of the water?
| Here's a crude way to look at the problem:
Suppose there are $N$ wires. Each has resistance $R$, common potential difference $V$ and are connected in parallel. So the current through each wire is $I = \frac{V}{NR}$.
Let's imagine a hypothetical wire formed by sea water which has a length, $L$ and cross sectional area,... | {
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How does gravity escape a black hole? My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. It would seem to me that the gravitational attraction caused by a black hole carries information ab... | We can think of gravity at distance as an energy level. Then The question how gravity escapes black hole is irrelevant!
| {
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How relevant is LHC to quantum gravity? Premise: the LHC is obviously mapping unseen territory in high energies, and therefore it's always possible to imagine far out results.
Excluding completely unexpected outcomes - is the LHC performing any experiment that could help with string theory or m-theory? For example:
*... | This article from CERN Courier is about string-theory and experimental tests of it, and about LHC. It is not really technical but it also links to various references and articles.
And for more technical info, you can browse the list of String Theory Seminar of the TH department at CERN and search for "LHC"; very intere... | {
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Common false beliefs in Physics Well, in Mathematics there are somethings, which appear true but they aren't true. Naive students often get fooled by these results.
Let me consider a very simple example. As a child one learns this formula $$(a+b)^{2} =a^{2}+ 2 \cdot a \cdot b + b^{2}$$ But as one mature's he applies t... | The concept that quantum mechanics undermines determinism. The Schrodinger wave equation evolution is completely deterministic. The results of measurements are probabilistic, but this does not mean that the various superposed states do not have causes. This is not the same thing as a hidden variable theory. The probabi... | {
"language": "en",
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Home experiment to estimate Avogadro's number? How to get an approximation of Avogadro or Boltzmann constant through experimental means accessible by an hobbyist ?
| Your best bet is to try to replicate the experiments of Perrin who first measured Avogadro's constant. This is a common lab in "Advanced Lab" courses in undergraduate or graduate courses, so you can probably find writeups and such via google.
The principle is to observe Brownian motion under a microscope and measure t... | {
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What's the difference between helicity and chirality? When a particle spins in the same direction as its momentum, it has right helicity, and left helicity otherwise. Neutrinos, however, have some kind of inherent helicity called chirality. But they can have either helicity. How is chirality different from helicity?
| Helicity is easy to define; chirality is more subtle.
The helicity of a particle is the normalized projection of the spin on the direction of momentum. If the spin is more along the same direction of the momentum than against it, then the helicity is positive; otherwise it is negative.
Chirality is to do with the way t... | {
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How do I figure out the effects of wind on flight? For a school project, I'm trying to make an automated flight planner, considering mainly the differences in flight paths according to wind.
Now I've heard that flying with/against the wind affects airspeed. But I really don't know any specific(even empirical) laws on d... | Wind speed affects ground speed, not air speed. The airplanes fly at a specified IAS (indicated air speed). Add the IAS with the wind speed in the direction of travel and you get the ground speed. Using simple algebra
$$ u_{ground} = u_{IAS} + u_{wind} $$
So to travel a distance $S$ with head wind takes $t_{A\rightarr... | {
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Are gauge choices in electrodynamics really always possible? If $B$ is magnetic field and $E$ electric Field, then
$$B=\nabla\times A,$$
$$E= -\nabla V+\frac{\partial A}{\partial t}.$$
There is Gauge invariance for the transformation
$$A'\rightarrow A+{\nabla L}$$
$$V'\rightarrow V-\frac{dL}{dt}.$$
Now, we can write:
... | Wikipedia on gauge fixing seems to imply that gauge fixing always works in abelian Yang-Mills theory, of which electrodynamics is the standard example. But that this does not always work in non-abelian Yang-Mills theory. There you have to restrict to submanifolds of the base spacetime to get the gauge fixing. Usually ... | {
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Utility of displacements potentials in geophysics In the elasticity theory, you can derive a wave equation from the fundamental equation of motion for an elastic linear homogeneous isotropic medium:
$\rho \partial^2_t \overline{u} = \mu \nabla^2 \overline{u} + (\mu+\lambda) \nabla(\nabla \cdot \overline{u})$
But in th... | Also, adding to what j.c. states:
If you have a complicated constitutive relation between the electric field and the electric displacement field, e.g. nonlinear and in terms of a convolution (no instantanious reaction of the medium), it might be very complicated to find a representation in terms of potentials obaying a... | {
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Best example of energy-entropy competition? What are the best examples in practical life
of an energy-entropy competition which favors entropy over energy?
My initial thought is a clogged drain -- too unlikely for the
hair/spaghetti to align itself along the pipe -- but this is probably
far from an optimal example. Cu... | Blackbody radiation: anything hotter than its environment radiates energy thus increasing the entropy of the universe. Entropy wins :-)
The Sun :-) The Sun's energy does not increase the Earth's total energy!
In fact, the Earth radiates almost exactly the same amount of energy as it receives from the Sun. What we reall... | {
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Searching books and papers with equations Sometimes I may come up with an equation in mind, so I want to search for the related material. It may be the case that I learn it before but forget the name, or, there is no name for the equation yet. In this case, I may be able to recall a reference book. Searching in Interne... | There was a similar question at Mathoverflow. I think it contains some useful references and discussion so it's definitely worth checking out.
Usually I would post this as a comment under the question but probably nobody would notice it anymore. And seeing that the other answer is also quite short I hope this is fine.... | {
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Is there a name for the derivative of current with respect to time, or the second derivative of charge with respect to time? This measurement comes up a lot in my E&M class, in regards to inductance and inductors.
Is there really no conventional term for this?
If not, is there some historical reason for this omission?
| Nope, not as far as I know. It's just "rate of change of current" or something like that.
I suppose it's possible someone has given it a name in some paper or textbook, but if so, it's not widely used.
| {
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Is it possible to obtain gold through nuclear decay? Is there a series of transmutations through nuclear decay that will result in the stable gold isotope ${}^{197}\mathrm{Au}$ ? How long will the process take?
| Last time I did the sum, 201Hg to 197Au plus 4He did not need external energy:
200.970277 - (196.9665516 + 4.0026032) = 4.0037254 - 4.0026032 = 0.0011222.
Caveats:
*
*I did not check the error term of atomic weigths, and
*The first decay, alpha to 197Pt, should be awesomely slow. In fact
*201Hg is considered sta... | {
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Which experiments prove atomic theory? Which experiments prove atomic theory?
Sub-atomic theories:
*
*atoms have: nuclei; electrons; protons; and neutrons.
*That the number of electrons atoms have determines their relationship with other atoms.
*That the atom is the smallest elemental unit of matter - that we ca... | No experiments prove any theory. Experiments can only refute theories.
| {
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Does the recent re-count of stars in elliptical gallaxies affect our understanding of the universal mass balance? I've seen several popular reports of a new count of low-mass stars in elliptical galaxies (here's one).
Edit: Pursuant to several correct comments I've changed the title to agree with the actual report whic... | The fraction of baryonic matter to dark matter is not deduced only from galactic dynamics. It is also derived from big bang nucleosynthesis and from the higher multipole acoustic peaks in the CMB spectrum. I would say that the element abundance is a far more important indicator of the fraction between baryonic and dark... | {
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What's the difference between running up a hill and running up an inclined treadmill? Clearly there will be differences like air resistance; I'm not interested in that. It seems like you're working against gravity when you're actually running in a way that you're not if you're on a treadmill, but on the other hand it s... | I think the most significant difference between work done on an inclined treadmill and work done on a real incline is the gain in potential energy on the real incline. There is no real delta mgz on a treadmill, whereas if you fell back to your starting height from a real incline, you'd certainly notice a large amount o... | {
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"source": "stackexchange",
"question_score": "27",
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Mechanics around a rail tank wagon Some time ago I came across a problem which might be of interest to the physics.se, I think. The problem sounds like a homework problem, but I think it is not trivial (i am still thinking about it):
Consider a rail tank wagon filled with liquid, say water.
Suppose that at some moment... | As the problem is initially described, the nozzle is located on the left bottom side of the tank with the nozzle exit facing downward. if this is the case, there will be no horizontal force to act as a thrust to start the tank in a horizontal motion. Any thrust that may be developed by the water exiting the nozzle wil... | {
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Is it possible to accelerate air to supersonic speeds? What would it look like? The speed of sound is the rate that disturbances in air propagate through it.
Is it possible to have a wind that itself is moving at supersonic speeds relative to stationary winds around it?
Or perhaps a fluid flowing through a pipe at a sp... | The facility to accelerate wind to supersonic speed is called wind tunnel. Here is the pamphlet of a hypersonic (Mach 7 and 8) wind tunnel in Japan. It is explained how it works and you can even see some photos of hypersonic flow.
| {
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Why doesn't air freeze? I am in no way experienced in the Physics field so this question may seem a bit silly but i'd appreciate an answer :)
Why doesn't air freeze?
| At the normal pressure, 99.9% of air (nitrogen, oxygen and argon) will solidify in 55K (where the oxygen does). Below about 15K also hydrogen (0.000524% of normalized air) solidifies leaving only helium which will be a liquid up to 0K (yet will change into a superfluid about 2.17K).
| {
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Does a towel that's spread out cool faster than one that isn't? I was thinking about how they say those sails on top of some dinosaurs helped regulate their body temperature. If a dinosaur didn't have that sail, would it really make any difference?
If you heated up two towels (large ones) to 50 degrees Celsius, and spr... | Heat loss is largely proportional to surface area, so your spreadout towel more more efficiently cool.
About those dinosaurs. It is know that elephants large ears do get used for cooling. With a good blood supply those sails certainly could have dissipated at lot of heat. That doesn't prove that that was their primary ... | {
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Why are physicists interested in graph theory? Can you tell me how graph theory comes into physics, and the concept of small world graphs?
(inspired to ask from comment from sean tilson in):
Which areas in physics overlap with those of social network theory for the analysis of the graphs?
| One context in which graphs can be useful in physics is in the discrete representation of spacetime in quantum gravity, where events are represented by the nodes of a type of poset (partially ordered set) called a causet and causal relationships are represented by the edges. This is particularly suited to a graph-theor... | {
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Why CAN we see the new moon at night? I understand that the Moon's phases are determined by its position in orbit relative to the Sun. (See: Full Story on the Moon). The "shadow" is not cast by the Earth (a common misconception - this is actually a lunar eclipse), but by the moon's body itself.
It would appear that, in... | You can't see the moon during the new moon phase because the sun, moon, and earth are in a line in that order so the sun is lighting up one side of the moon and the side facing earth is shadowed and can't be seen./Users/randyhopkins/Desktop/Photo on 1-13-14 at 5.50 PM.jpg
| {
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"source": "stackexchange",
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Why does holding something up cost energy while no work is being done? I read the definition of work as
$$W ~=~ \vec{F} \cdot \vec{d}$$
$$\text{ Work = (Force) $\cdot$ (Distance)}.$$
If a book is there on the table, no work is done as no distance is covered. If I hold up a book in my hand and my arm is stretched, i... | $F=ma$ means that every force is applied to a mass and produces an acceleration. Okay. Acceleration is $a=\frac{\Delta v}{\Delta t}$. If you put this $\Delta v$ into ${\frac{1}{2}m(\Delta v)^2}$ you discover the energy which have been necessary to let that mass accelerate. Since energy is neither created nor destroyed,... | {
"language": "en",
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Why do we think there are only three generations of fundamental particles? In the standard model of particle physics, there are three generations of quarks (up/down, strange/charm, and top/bottom), along with three generations of leptons (electron, muon, and tau). All of these particles have been observed experimental... | Gell-Mann's Baryon Decuplet can get enhanced and it can be shown that the 3 Upper Quarks and the 3 lower Quarks are exactly the points of Gravity of the 6 Triangels. There ist no more Place.
There are also 6 Gluons double colored on the horizontal 1 Spin boson circle, but the multicolored 2 Gluons take the perpendicula... | {
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On this infinite grid of resistors, what's the equivalent resistance? I searched and couldn't find it on the site, so here it is (quoted to the letter):
On this infinite grid of ideal one-ohm resistors, what's the equivalent resistance between the two marked nodes?
With a link to the source.
I'm not really sure if t... | Nerd Sniping!
The answer is $\frac{4}{\pi} - \frac{1}{2}$.
Simple explanation:
Successive Approximation! I'll start with the simplest case (see image
below) and add more and more resistors to try and approximate an
infinite grid of resistors.
Mathematical derivation:
$$R_{m,m}=\frac 2\pi \left( 1 + \frac 13 + \f... | {
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How long a straw could Superman use? To suck water through a straw, you create a partial vacuum in your lungs. Water rises through the straw until the pressure in the straw at the water level equals atmospheric pressure. This corresponds to drinking water through a straw about ten meters long at maximum.
By taping se... | If we are looking at this from a purely suction related problem then superman the maximum height sumperman could lift water in a straw would be equal to the pressure being exerted on the water he is drinking. If drinking from sea level then he could lift or suck the water about 10 m. Theoretically he could create a c... | {
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Can radio waves be formed into a pencil beam? Laser beams are said to have high "spatial coherence". This means that the beam is highly concentrated even at long distances (low spread).
Can this be achieved with radio waves (much longer waves) or is it due to laser's stimulated emission?
| It depends on how big a pencil you're thinking about. There's no fundamental reason why radio waves can't be collimated in the same sort of way that visible light beams are. In fact, some radar systems send out fairly collimated beams at radio frequencies.
If you want to make a radio-wave beam that is the same size as ... | {
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Is it possible for information to be transmitted faster than light by using a rigid pole? Is it possible for information (like 1 and 0s) to be transmitted faster than light?
For instance, take a rigid pole of several AU in length. Now say you have a person on each end, and one of them starts pulling and pushing on hi... | A simple explanation why the speed of sound can never be faster than the speed of light:
Consider two atoms $A$ and $B$. Give the nucleus of $A$ a slight push. As we know, this push will carry over to $B$, but why? It's due to their electrostatic repulsion. So for $B$ to even react, you first need at least an electroma... | {
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How does the temperature of the triple point of water depend on gravitational acceleration? Suppose I do two experiments to find the triple point of water, one in zero-g and one on Earth. On Earth, water in the liquid or solid phase has less gravitational potential per unit mass than water in the gas phase. Therefore... | There is no difference; phase transitions does not change gravitational potential.
| {
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How cold does it need to be for spit to freeze before hitting the ground? What is the dominant form of heat transfer between warm water and cold air?
If a $100 mg$ drop of water falls through $-40 C$ air, how quickly could it freeze?
Is it credible that in very cold weather spit freezes in the half a second it takes ... | Consider a spherical drop of water, initial temp 40C, radius 3mm, mass 0.1g
To get it down to 0C, you need to remove 4.18 (J/gK) * 0.1 g * 40 K = 17 J
then, to freeze it solid, you need to remove latent heat of fusion 333 (J/g) * 0.1 g = 33 J
for a total of 50 J.
The heat conductivity equation is
$H=\frac{\Delta Q}{\De... | {
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Dynamic ferrofluid sculptures http://www.youtube.com/watch?v=UJJuq_pcyIQ
What exactly is going on in the video example? I understand the phenomena occurs because of magnetism but I am trying to figure out the mechanics behind this sculpture. There obviously is a magnet underneath but what is it doing? Is it moving? Is ... | I think I can tackle the mechanics aspect. What you can see is an inverted metallic cone with a helical groove running down and around its surface. On zooming in, you the perforations in the cone's surface. There is a pump which pumps the fluid back to the top as it flows down. As the empty cone begins to refill, the f... | {
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Solving straight-line motion question for time I apologise in advance if this question doesn't appeal to the advanced questions being asked in this Physics forum, but I'm a great fan of the Stack Exchange software and would trust the answers provided here to be more correct than that of Yahoo! Answers etc.
A car is tra... | Your mistake is in the equation
$$22.22t = 27.78t - 236.1$$
Everything up to there made good sense, but if the police officer has already traveled 236 meters, you should add that to his distance traveled, not subtract it. You'll also need to account for the way the police officer only began traveling at full speed 15 ... | {
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Would you be weightless at the center of the Earth? If you could travel to the center of the Earth (or any planet), would you be weightless there?
| You would not be weightless at the center of the Earth. In other words, the Earth does not follow a geodesic. Let me explain.
The Earth is not spherical, it is an oblate spheroid. The acceleration of a uniform non-spherical body in a spherical gravitational field does not follow an inverse square law. The acceleration ... | {
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Is there a limit to loudness? Is there any reason to believe that any measure of loudness (e.g. sound pressure) might have an upper boundary, similar to upper limit (c) of the speed of mass?
| Yes - there is a sound pressure limit for undistorted sound. Over that limit we have a shock wave. It depends on the environmental pressure, but there is a theoretical limit to loudness which you can find here.
The limit is basically equal to the pressure.
Theoretical limit for undistorted sound at 1 atmosphere enviro... | {
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How is the classical twin paradox resolved? I read a lot about the classical twin paradox recently. What confuses me is that some authors claim that it can be resolved within SRT, others say that you need GRT. Now, what is true (and why)?
| The problem is the symmetry break caused by the traveling twin when he changes his direction. This break makes the two twins distinguishable from each other. Before the traveling one changes his direction, both think of their partner to be the younger one (because the time in your own system is always the fastest possi... | {
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How does mass leave the body when you lose weight? When your body burns calories and you lose weight, obviously mass is leaving your body. In what form does it leave? In other words, what is the physical process by which the body loses weight when it burns its fuel?
Somebody said it leaves the body in the form of h... | I recently lost a good deal of weight and would say it seems the water mostly leaves through urination, not exhalation. Basically, I could tell when my weight was going to be decreasing because I'd have a lot of pee in the middle of the night, without having much to drink before bed.
As the other answers say respirati... | {
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What temperature can you attain with a solar furnace? A solar furnace is a device that concentrates the sun's light on a small point to heat it up to high temperature. One can imagine that in the limit of being completely surrounded by mirrors, your entire $4\pi$ solid angle will look like the surface of the sun, at ab... | According to wikipedia, it can reach 3500-4000 °C
You can increase the temperature by choosing material which is black at visible light range and white at IR range.
| {
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"Magnetic mnemonics" Over and over I'm getting into the same trouble, so I'd like to ask for some help.
*
*I need to solve some basic electrodynamics problem, involving magnetic fields, moving charges or currents.
*But I forgot all this rules about "where the magnetic field should go if the current flows up". I v... | To get the right answer, you should use the right hand, which happends to be the right hand, it cannot possibly get simpler
| {
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How fast a (relatively) small black hole will consume the Earth? This question appeared quite a time ago and was inspired, of course, by all the fuss around "LHC will destroy the Earth".
Consider a small black hole, that is somehow got inside the Earth. Under "small" I mean small enough to not to destroy Earth instant... | Since I have much better answer from Vagelford -- I'll write my own version.
When matter falls on the black hole it gets fractioned and radiates. As far as I know (correct me if I'm wrong) one can estimate the radiated energy as $\simeq 0.05mc^2$. Where $m$ is the mass of the falling matter.
The Earth's matter is ... | {
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Is it theoretically possible to shield gravitational fields or waves? Electromagnetic waves can be shielded by a perfect conductor. What about gravitational fields or waves?
| One potential solution, related to a problem discussed by Kip Thorne, would be to construct an enormous array of resonant bar detectors that attenuate the amplitude of the GWs as they pass through. If it is big enough, and the frequency is correct, you could absorb all the energy.
A more active device would be like noi... | {
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Why does my wife's skin buzz when she's using her laptop? When my wife uses her laptop, if I touch her skin, I can feel a buzz. She doesn't feel the buzz, but she can hear it if I touch her ear.
So I'm guessing it's a faulty laptop, and she's conducting an electrical current.
But why would she not feel anything, and w... | This effect occurs very often when touching electronic devices that are connected to the power mains.
You can verify that it is the connect to the power mains: unplug the power adapter and all other connections to other devices connected to the power mains and try again. The effect will be gone away.
It can best be fe... | {
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Tricky spring on a surface question I have this relative simple-looking question that I haven't been able to solve for hours now, it's one of those questions that just drive you nuts if you don't know how to do it.
This is the scenario:
I have a spring that is on a flat surface, the springs details are like this:
sprin... | I assume for simplicity that the spring constant has a quite a high value so that the settling down of the spring under its own weight is insignificant.
Designations:
$x$-vertical displacement of the center of mass of the spring from its equilibrium position.
$l$-vertical displacement of the top of the spring fro... | {
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How Does Hubble's Expansion Affect Two Rope-Tied Galaxies? Suppose we have two galaxies that are sufficiently far apart so that the distance between them increases due to Hubble's expansion. If I were to connect these two galaxies with a rope, would there be tension in the rope? Would the tension increase with time? ... | Against all the above opinions:
The rope will shrink
and maintain the normal tension between atomic structure
A space expansion scenario is dual (almost) to a shrinking matter scenario.
Unless we have a way to decide I will support my point.
Already explained here:
A relativistic time variation of matter/space fits bot... | {
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Vortex in liquid collects particles in center At xmas, I had a cup of tea with some debris at the bottom from the leaves. With less than an inch of tea left, I'd shake the cup to get a little vortex going, then stop shaking and watch it spin. At first, the particles were dispersed fairly evenly throughout the liquid,... | I think the leaves congregate in the center as the tea decelerates due to the flow pattern established by the initial rotation. In effect the flow pattern in the tea will be a toroid flowing up the centre and down the outside effectively driving the denser particles (tea) at the bottom on the fluid into a pile in the m... | {
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Basic Spin or Double Cover Experiment We know that Spin is described with $SU(2)$ and that $SU(2)$ is a double cover of the rotation group $SO(3)$. This suggests a simple thought experiment, to be described below. The question then is in three parts:
*
*Is this thought experiment theoretically sound?
*Can it be c... | I'm surprised to encounter this old question without what I'd consider the correct answer: that the change of sign of a spinor under one rotation has been experimentally observed!
The experiment was performed using a neutron interferometer. A beam of polarized neutrons is divided, steered, and recombined by diffraction... | {
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Your favorite Physics/Astrophysics blogs? What are the Physics/Astrophysics blogs you regularly read? I'm looking to beef up my RSS feeds to catch up with during my long commutes. I'd like to discover lesser-known gems (e.g. not well known blogs such as Cosmic Variance), possibly that are updated regularly and recently... | John Baez's Stuff
It is more mathematics, but a lot of physics/mathematical physics related "stuff" also.
| {
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What is the difference between a complex scalar field and two real scalar fields? Consider a complex scalar field $\phi$ with the Lagrangian:
$$L = \partial_\mu\phi^\dagger\partial^\mu\phi - m^2 \phi^\dagger\phi.$$
Consider also two real scalar fields $\phi_1$ and $\phi_2$ with the Lagrangian:
$$L = \frac12\partial_\mu... | they are equivalent from a physics point of view and can be mapped into each other.
| {
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Two slit experiment: Where does the energy go? In Physics class we were doing the two slit experiment with a helium-neon red laser. We used this to work out the wavelength of the laser light to a high degree of accuracy. On the piece of paper the light shined on there were patterns of interference, both constructive an... | Try integrating the power in the resulting interference pattern; you will find that energy is indeed conserved. In the case where the slit is illuminated by a plane wave and observed in the far-field, this is very easy: the interference pattern is simply the Fourier transform of the pattern of slits, and conservation... | {
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Why are snowflakes symmetrical? The title says it all. Why are snowflakes symmetrical in shape and not a mush of ice?
Is it a property of water freezing or what? Does anyone care to explain it to me? I'm intrigued by this and couldn't find an explanation.
| K Libbrecht has a nice paper that answers your question in considerable detail and has some nice pictures-- his homepage:
http://www.its.caltech.edu/~atomic/publist/kglpub.htm
Scroll down to the article in American Scientist in his publications list
"The Formation of Snow Crystals," K. G. Libbrecht, American Scientist... | {
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Solar heating of an object in air I need a solution to the heat equation that shows temp increase in an object, e.g. a cube or sphere, in sunlight. The object is assumed to be exposed on all sides except one. It is a solid object with a certain surface emissivity and heat capacity. In other words, I don't care what ... | To the extent that convection is important, LOL. Temperature differences drive the fluid flows, which drive the heat loss. It would be a lot easier with a sphere than a cube, but the real problem is relating surface temperature with heat loss. If you put it in a vacuum, so you only had radiative losses, it would be eas... | {
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What happens to an astronaut who's floating in a spaceship (in space) when it begins to move? I feel this is somehow a stupid question, but I don't know the true answer. What happens to an astronaut who's floating in a spaceship in space when it begins to move? Will the astronaut not move until he smashes onto a wall i... | Dear huy, when a spaceship is flying to the Moon or when the International Space Station is orbiting the Earth, both the spaceship and the astronaut are moving by the same speed, so the relativity velocity is zero. Moreover, gravity determines the acceleration of all of them. The principle of equivalence implies that w... | {
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Will tensile strength keep a cable from snapping indefinitely? Trying to secure a wall hanging using magnets; me and a coworker came up with an interesting question:
When the hanging is hung using 1 magnet, the weight of it causes it to quickly drag the magnet down and the hanging drops. Using n magnets retards this p... | Your two questions are not really related, in my thoughts.
The first one is about friction of some magnets clutched to a ferromagnetic wall.
The second is about failiure of some "wire".
Both are strange and unnessecary mixtures of idealized classical
mechanics and some real world problem.
So, first Question is real... | {
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How to avoid getting shocked by static electricity? sometimes I get "charged" and the next thing I touch something that conducts electricity such as a person, a car, a motal door, etc I get shocked by static electricity.
I'm trying to avoid this so if I suspect being "charged" I try to touch something that does not con... | Carry some metal in your pocket. When you suspect you are carrying an electric charge, take the metal (a coin?) out of your pocket and touch it to something grounded.
| {
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Polarization of the gluon I think that, by now, it's understood that the gluon propagator in QCD has a dynamically generated mass. Ok, so my question is the following: where does the extra polarization degree of freedom come from? Or, asking in another way: suppose you try to define an S matrix for QCD, apart from the ... | The asymptotic states of QCD are gauge invariant. They can include mesons which are quark-anti quark bound states and glueballs (which are roughly speaking bound states of gluons) but not gluons themselves. It doesn't really make any sense to say that the gluon propagator has a dynamically generated mass as this is a v... | {
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R-R fields in RNS formalism In string theory I came across the fact that there are difficulties in describing the coupling of R-R fields with world sheets in RNS formalism and it can be done in GS formalism only. Can someone explain the reason(s) behind that?
Also, I think if you have a space time where you can quantiz... | The easiest way to write the curved background action is to covariantize the vertex operator. But in the RNS version of the superstring, to write the RR vertex you need to break worldsheet supersymmetry and mix ghost and matter fields. You can look at that famous paper by Friedan, Martinec and Shenker if you don't know... | {
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What would happen if $F=m\dot{a}$? What would happen if instead of $F=m \frac{d^2x}{dt^2}$, we had $F=m \frac{d^3x}{dt^3}$ or higher?
Intuitively, I have always seen a justification for $\sim 1/r^2$ forces as the "forces being divided equally over the area of a sphere of radius $r$".
But why $n=2$ in $F=m\frac{d^nx}{dt... | It is because the evolution of mechanical system is fully determined by initial coordinates and speeds. Therefore, your equation must be of second order, otherwise setting initial accelerations and "speeds of increase of accelerations" would be necessary.
| {
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Homework about spinning top I have a top of unknown mass that has a moment of inertia $I=4\times 10^{-7} kg \cdot m^2$. A string is wrapped around the top and pulls it so that its tension is kept at 5.57 N for a distance of .8 m.
Could somebody help me derive some equations to help with this? Or to get me in the right ... | You should be able to calculate the work done by pulling the string.
You should also be able to write down an equation for the amount of work necessary to accelerate an object with a given MOI to some arbitrary angular velocity.
That should be a good start.
| {
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Does Wick rotation work for quantum gravity? Does Wick rotation work for quantum gravity? The Euclidean Einstein-Hilbert action isn't bounded from below.
| I am a firm believer of notion of wick rotation. As Lubos nicely pointed out, the most clear indication of wick rotation comes from quantum cosmology. In fact, many new recent results in different models of quantum gravity such as loop quantum gravity$^1$ (LQG), causal dynamical triangulation$^2$ (CDT), non-commutative... | {
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Why (in relatively non-technical terms) are Calabi-Yau manifolds favored for compactified dimensions in string theory? I was hoping for an answer in general terms avoiding things like holonomy, Chern classes, Kahler manifolds, fibre bundles and terms of similar ilk. Simply, what are the compelling reasons for restricti... | We can have compactifications over 7D manifolds with a $G_2$ holonomy, or an 8D manifold with an $SO(7)$ holonomy. We can have orbifolds, or flux compactifications. We can have warped compactifications like $AdS_5 \times S^5$.
| {
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Mathematical background for Quantum Mechanics What are some good sources to learn the mathematical background of Quantum Mechanics?
I am talking functional analysis, operator theory etc etc...
| You will have more than enough math for the first two semesters of quantum mechanics if you are taking functional analysis in a mathematics course.
The immense majority of quantum mechanics books will have the requisite math in an appendix. That is true whether or not you want the more sophisticated mathematical treat... | {
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Colder surface radiates to warmer surface When radiation from a colder source arrives at a warmer surface there is some debate about what happens next. To make the question more concrete lets say that the colder source is at temperature 288K. The warmer surface is at 888K and has emissivity of 1.
3 possibilities
*
*... | The option three is valid -- the outgoing radiation is only dependent on the radiating object. Second law of thermodynamics is not violated since it must be applied to the whole system (and so to the net radiative exchange).
| {
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Resistance between two points on a conducting surface Suppose we have a cylindrical resistor, with resistance given by $R=\rho\cdot l/(\pi r^2)$
Let $d$ be the distance between two points in the interior of the resistor and let $r\gg d\gg l$.
Ie. it is approximately a 2D-surface (a rather thin disk).
What is the resis... | I think the answer is infinite because of the singular nature of a point. If we assume a steady state situation then the divergence of the potential is zero, by symmetry in a full 2D model the current density J will scale like 1/r. This implies the voltage scales like log(r), which diverges as r goes to zero. In the 3D... | {
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Should you really lean into a punch? There's a conventional wisdom that the best way to minimize the force impact of a punch to the head is to lean into it, rather than away from it.
Is it true? If so, why?
EDIT: Hard to search for where I got this CW, but heres one, and another. The reason it seems counter-intuitive ... | The only possible reason I can see is that the earlier, one faces the punch the less amount of momentum he would face. A punch builds its momentum during its journey towards destination and it reaches its maximum at the end of it. It the target comes close then he would naturally encounter less momentum and hence less ... | {
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Difference between electric field $\mathbf E$ and electric displacement field $\mathbf D$ $$\mathbf D = \varepsilon \mathbf E$$
I don't understand the difference between $\mathbf D$ and $\mathbf E$.
When I have a plate capacitor, a different medium inside will change $\mathbf D$, right?
$\mathbf E$ is only dependent f... | $\mathbf E$ is the fundamental field in Maxwell equations, so it depends on all charges. But materials have lots of internal charges you usually don't care about. You can get rid of them by introducing polarization $\mathbf P$ (which is the material's response to the applied $\mathbf E$ field). Then you can subtract th... | {
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Has every possible interaction between elementary particles been observed? There are some interactions that are forbidden by conservation laws, e.g. an electron cannot turn into a positron by conservation of charge and a photon cannot turn into a positron electron pair by conservation of momentum.
My question is if eve... | There are some extremely important reactions that have never been observed directly. My favorite example is p-p fusion,
$$\text{p} + \text{p} \rightarrow \text{d} + \text{e}^+ + \nu_e$$
which is the rate-determining step for the main fusion process in the Sun and all other small stars. But it is utterly impossible to o... | {
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} |
References about rigorous thermodynamics Can you suggest some references for rigorous treatment of thermodynamics? I want things like reversibility, equilibrium to be clearly defined in terms of the basic assumptions of the framework.
| There are many presentations that are mathematically rigorous – but they have different underlying philosophies and conceptions of what thermal physics is or should do. So it'd be best if you read as many of them as possible to find those that are closer to your own philosophy, and maybe use them to be critical about y... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/5614",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "25",
"answer_count": 8,
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Electricity takes the path of least resistance?
Electricity takes the path of least resistance!
Is this statement correct?
If so, why is it the case? If there are two paths available, and one, for example, has a resistor, why would the current run through the other path only, and not both?
| The statement is correct if you interpret it to mean that there is a larger current in the path that has a lower resistance, when both paths have the same voltage across them. (This doesn't mean that the path with higher resistance has no current, just less current - as Ted Bunn's example shows)
You can understand this... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/5670",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "30",
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How do you derive Noether's theorem when the action combines chiral, antichiral, and full superspace? How do you derive Noether's theorem when the action combines chiral, antichiral, and full superspace?
| The full superspace terms are the "most general ones" but you may convert the chiral and antichiral terms to the full superspace form, too. In particular,
$$\int d^2\bar\theta = \int d^2\theta\, d^2 \bar\theta\, \theta_1\theta_2$$
and similarly for its complex conjugate. Sorry if a sign is wrong. Note that $\theta_1\th... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Do senior physicists actually conduct research? Senior physicists constantly complain they spend too much time on administration, teaching, getting grants, serving in committees, peer-reviewing articles, supervising, etc. . Do senior physicists conduct research by getting their post-docs and graduate students to do all... | Physics is top sport. When sports(wo)men age, they generaly discover that it becomes more and more difficult to compete. Some keep training and stay in the game till late age, but sooner or later they discover that the golden years are long past. No problem: these seniors carry a wealth of knowledge, and a position as ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/5775",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 1
} |
How does one build up intuition in physics? How does one build up an intuitive gut feeling for physics that some people naturally have? Physics seems to be a hodgepodge of random facts.
Is that a sign to quit physics and take up something easier?
Thanks for all the answers. On a related note, how many years does it tak... | I think the intuitive sense (your 'intuitive gut feeling'mentioned above)that one develops for physics starts with a passion for the subject itself. In my case that's where it all started. As a child, I recalled asking my father why his airplane flies. Later, when I was able to study physics, I could recall some relati... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/5819",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 6,
"answer_id": 5
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Do all light rays from a point passing through a thin lens converge at the image? I have often seen diagrams, like this one on Wikipedia for a thin convex lens that show three lines from a point on the object converging at the image. Do all the other lines from that point on the object that pass through the lens conver... | No broadly speaking. Yes in a narrow context. Chromatic dispersion guarantees that it is impossible to construct a lens which will be free of caustics and other optical aberrations for all wavelengths. Individually lenses can be engineered to provide nearly arbitrary precision for narrow bands of wavelengths. However a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/5865",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 2,
"answer_id": 0
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How to build a laser in the garage? So I wonder if it is any how possible to build laser at home. A powerful one to melt brick.
| I believe that the answer to your question is no. Of course if you order all special components like resonator mirrors, high voltage power supply, etc., then it doesn't matter where you build the laser. But you cannot build a laser with items from a DIY-shop.
And just a remark - there is no way to melt a brick with a ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/5901",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 4,
"answer_id": 1
} |
What percentage of physics PhDs leave physics? What percentage of physics PhDs leave physics to become quantitative analysts, work in computer science/information technology or business? Is physics that bad that so many people leave? Was it worth it?
| @Tim van Beek To look at it in another way, a lot of people get into physics with the aim of walking away from it after the undergrad, grad or phd. For instance, people may choose to go into environmental science. And a lot of physics phds go into biology because the problems there today are much more interesting than ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6002",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 5,
"answer_id": 4
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Physics for mathematicians How and from where does a mathematician learn physics from a mathematical stand point? I am reading the book by Spivak Elementary Mechanics from a mathematicians view point. The first couple of pages of Lecture 1 of the book summarizes what I intend by physics from a mathematical stand point.... | You might try, now in paperback,
Th. Frankel: The Geometry of Physics, An Introduction, Cambridge U.P. (Cambridge), 1997.
It's a course in differential geometry, actually, but one oriented towards physics, with succinct but comprehensive enough developments of physical theories (mechanics, electromagnetism, thermodyna... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6047",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "24",
"answer_count": 10,
"answer_id": 4
} |
List of freely available physics books I'm trying to amass a list of physics books with open-source licenses, like Creative Commons, GPL, etc. The books can be about a particular field in physics or about physics in general.
What are some freely available great physics books on the Internet?
edit: I'm aware that there... | Quantum field theory
Fields, by W. Siegel
Quantum Field Theory, by Mark Srednicki
Superspace, or One thousand and one lessons in supersymmetry
by S.J. Gates Jr, M.T. Grisaru, M. Rocek and W. Siegel
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6157",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "84",
"answer_count": 24,
"answer_id": 15
} |
Where should a physicist go to learn chemistry? I took an introductory chemistry course long ago, but the rules seemed arbitrary, and I've forgotten most of what I learned. Now that I have an undergraduate education in physics, I should be able to use physics to learn general chemistry more effectively. What resource... | Oregon State University offers a pretty complete gen chem sequence online, as well as organic and inorganic. (No P-chem yet.) There are condensed lab courses for the gen chem sequence, three days each, taught on campus in Corvallis.
I took the first two gen chem courses, as well as the associated labs. I really, really... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6208",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "34",
"answer_count": 7,
"answer_id": 5
} |
The Galileo thermometer: why do the bubbles float in the middle of the tube? If the water were uniform temperature, it would have uniform density, so a bubble should either be all the way at the top (if it's lighter than water) or all the way at the bottom (if heavier). But in reality you don't see this neat separation... | Although I could be wrong, I think the reason is that the density of any fluid, including water, increases very slightly with depth.
There is a well-known relation between pressure and depth,
$$\Delta p = -\rho g \Delta y$$
which says that at the difference in pressure between any two points in the fluid is proportiona... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6279",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 2,
"answer_id": 1
} |
I need help with finding distance traveled How do I find the distance traveled of an object if the speed is not constant?
| It depends on whether you mean to find the final displacement,
$$\mathbf{D} = \int_{t_0}^{t_1}\mathbf{v}\:dt,$$
or quite literally the distance traveled. Think of the difference between the two in this way: if you travel from New York to London and back again, do you consider the length of both legs of the journey, or ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6370",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 7,
"answer_id": 3
} |
Snell's law starting from QED? Can one "interpret" Snell's law in terms of QED and the photon picture? How would one justifiy this interpretation with some degree of mathematical rigour? At the end I would like to have a direct path from QED to Snell's law as an approximation which is mathematically exact to some degre... | Sure. Start with QED, obtain Maxwell's equations, do the paraxial approximation and finally use Fermat's principle.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6428",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 0
} |
Gravity theories with the equivalence principle but different from GR Einstein's general relativity assumes the equivalence of acceleration and gravitation. Is there a general class of gravity theories that have this property but disagree with general relativity? Will such theories automatically satisfy any of the test... | I don't know that there's any general class of theories that compete with GR. If a competitor doesn't agree with experimental tests to date it wouldn't be viable.
The vast majority of tests of GR have been tests of the Schwarzschild metric. It's possible to tweak that metric so that it's compatible with quantum mechani... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6541",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
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What prevents the accumulation of charge in a black hole? What prevents a static black hole from accumulating more charge than its maximum? Is it just simple Coulomb repulsion?
Is the answer the same for rotating black holes?
Edit
What I understand from the answers given so far, is that maximum charge is a moving targe... | Coulomb repulsion it is. Specifically, if a black hole has a lot of charge, then particles with a high charge-to-mass ratio will be repelled. Anything that falls in will contribute "more mass than charge," heuristically, keeping the charge-to-mass ratio of the black hole from getting too big.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6650",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 6,
"answer_id": 5
} |
Coriolis effect on Tsunami The Japanese tsunami, moving at about 700 km/h, affected areas as distant as Chile's coast, 20 hours after the earthquake. How does the Coriolis force affect tsunami? Also, I saw an image of a boat caught within a large whirlpool. Is the whirlpool's rotation due to Coriolis force?
| I don't believe the Coriolis force has much effect on a tsunami because it does only affect moving masses. Coriolis force in fact isn't a force but a movement pattern looking as though a force were involved. It is a result of inertia "driving" the moving masses towards a constant direction in space and at the same t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6754",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
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Readings of the detectors at Japan and Izu-Bonin-Mariana Trenches Given the relevance of this subduction system, I would expect that a wide range of detectors (temperature, vibration, seismometers, whatever) are deployed in the depth of these trenches. What would be the canonical source at which one could access the re... | The idea of seafloor observatories making their data available freely and online is a vision and there are observatories being proposed by several of the major oceanographic institutions. Search for the term "seafloor observatory."
You might also take a look here, or contact the researchers:
Realtime Data from the Deep... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Can a nuclear reactor meltdown be contained with molten lead? If lead can absorb or block radiation, would it be possible to pump molten lead into a reactor core which is melting, so that it would eventually cool and contain the radiation?
Is there something that can be dumped into the core that will both stop the reac... | The heat of vaporization is not a temperature, but a capacity for holding heat (whose units are btu/lb, or some equivalent to that) which is at the boiling (vaporization) temperature of a substance. Yes, water does have one of the highest heats of vaporization of any substance around, the Fukushima reactors have not me... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6928",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 12,
"answer_id": 6
} |
For an accelerated charge to radiate, is an electromagnetic field as the source necessary? For an accelerated charge to radiate, must an electromagnetic field be the source of the force?
Would it radiate if accelerated by a gravitational field?
| The radiation, if considered classically, is independent for the reason Mark Eichenlaub gives. But considered quantum mechanically, it is not independent.
In short, photons are bosons. So the presence of radiation of a particular polarization and frequency will increase the probability of the particle radiating that po... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7014",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 5,
"answer_id": 0
} |
Rigid body dynamics joints I can't seem to find any info on connected rigid bodies by a joint. Can someone explain the basics to me? I'm trying to do a little research to find out how feasible it would be to implement 3d ragdoll physics for my first person shooter game.
| Probably too late for the OP, but for the sake of the next generations or anyone searching about something related to this topic, I'm dropping this link:
http://www.gamasutra.com/resource_guide/20030121/jacobson_01.shtml
This article actually contains everything you need to know about programming cool ragdoll physics e... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7066",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 1
} |
Why does nuclear fuel not form a critical mass in the course of a meltdown? A BWR reactor core may contain up to 146 tons of uranium. Why does it not form a critical mass when molten? Are there any estimates of the critical mass of the resulting zirconium alloy, steel, concrete and uranium oxide mixture?
| If you read Wikipedia page about corium, they say that critical mass can be achieved locally.
But if you are concerned about a critical mass allowing a nuclear explosion, the difficulty in nuclear weapon design, as told here, is to achieve the criticality fast enough. If you do not achieve criticality fast enough, you... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7149",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "12",
"answer_count": 9,
"answer_id": 3
} |
What is the most energy efficient way to boil an egg? Starting with a pot of cold tap water, I want to cook a hard-boiled egg using the minimum amount of energy. Is it more energy efficient to bring a pot to boil first and then put the egg in it, or to put the egg in the pot of cold water first and let it heat up with ... | Break egg into vacuum vessel, lower pressure until egg boils (sorry don't have a phase diagram for eggs handy)
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7196",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 5,
"answer_id": 0
} |
What is energy in string theory? Facts agreed on by most Physicists -
GR: One can't apply Noether's theorem to argue there is a conserved energy.
QFT: One can apply Noether's theorem to argue there is a conserved energy.
String Theory: A mathematically consistent quantum theory of gravity.
Conclusion -
If one can apply... | Within GR, there is a conserved stress energy pseudo-tensor. It is called a pseudo-tensor because it is not a tensorial quantity, it's transformation properties allow you to make the stress-energy of the gravitational field vanish at any point. This quantity can be derived by using Noether's prescription on the Einstei... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7244",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 2,
"answer_id": 0
} |
Can D and H form an 'in materials' version of the electromagnetic tensor? In analogy to the electromagnetic tensor, with the components defined as the electric field $E$ and magnetic field $B$ as such:
$F^{ab} = \begin{bmatrix}
0 & -E_x/c & -E_y/c & -E_z/c \\
E_x/c & 0 & -B_z & B_y \\
E_y/c & B_z & 0 & -B_x \\
E_z/... | The answer is "yes". One has
\begin{equation}
\bar{F}^{ab} = \bar{\epsilon}_{abcd} F^{cd}
\end{equation}
where
\begin{equation}
\bar{\epsilon}^{0i0j} = \epsilon^{ij}
\end{equation}
and
\begin{equation}
\bar{\epsilon}^{ijpq} = (1/\mu)^{kr}
\end{equation}
up to a factor $c$ here and there :-) .
Here $i,j,k$ and $p,q,r$ a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7291",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 2
} |
Are regular light bulbs better for the eyes than CFLs or "tube lights"? I've heard that regular light bulbs with a filament are better for the eyes. Is the spectrum of one worse than the other? If so, are there any regulations for their use in industrial settings for worker safety?
| The "warmer" white-color of the filament (incandescent) bulb is usually considered more relaxing than the "colder" white of tubes and certain white LEDs.
However, the main ergonomic drawback of tubes is that many of them flicker. Some people are more sensitive to this than others. Some tubes have high-frequency modulat... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7395",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 2,
"answer_id": 0
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A Basic Question about Gravity, Inertia or Momentum or something along those lines Why is it that if I'm sitting on a seat on a bus or train and its moving quite fast, I am able to throw something in the air and easily catch it? Why is it that I haven't moved 'past' the thing during the time its travelling up and down?... | Barring the affects of air resistance and such, just think of the object as also "part" of the train moving at the same velocity as you. Essentially there is no way to distinguish between an object at rest or moving at a constant velocity.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7479",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 4,
"answer_id": 2
} |
The Heisenberg limit is not a limit? This new Best-ever quantum measurement breaks Heisenberg limit
PHYSICISTS have made the most accurate
quantum measurement yet, breaking a
theoretical limit named for Werner
Heisenberg.
Nature, DOI: 10.1038/nature09778
When an experiment breaks a theoretical limit we have ... | The use of the term "Heisenberg Limit" is somewhat misleading for outsiders (that is non-quantum Interferometers). If we recall the Heisenberg Uncertainty Principle is a limit on simultaneous measurement of two complementary variables. In the case of (quantum) metrology one is only interested in the measurement of a si... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7511",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
} |
Will Earth Hour do damage to power supply system? There is always a debate around Earth Hour every year, and the opposite side of Earth Hour usually claims that
The (sudden) decrease and increase of the power usage in the start and end of Earth Hour will cause much more power loss (than the save of power), and even do... | The grids can take turning the lights off, by experimental observation: everyday all over the world the lights come up at about the same time for each geographical region, and turn off at about the same time due to the similar sleep schedule of millions.
Total black out might overload the system, but the percentage of ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7576",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
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What future technologies does particle physics and string theory promise? What practical application can we expect from particle physics a century or two from now? What use can we make of quark-gluon plasmas or strange quarks? How can we harness W- and Z-bosons or the Higgs boson? Nuclear physics has given us nuclear p... | Allow me to answer your question with some quotes:
*
*"The energy produced by the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine." —Ernst Rutherford
*"There is not the slightest indication that nuclear energy will ever be obtainabl... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7652",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 8,
"answer_id": 2
} |
Nature of tetragonal distortion in Jahn-Teller effect I am wondering: If I have a regular octahedron as my starting point, oriented along the x-y-z axis, and now Jahn-Teller suggest I elongate or compress along the $z$-axis, what happens along the other axis? I expect that these move in the opposite direction, but by h... | Very generally, Jahn-Teller distortions are indeed volume conserving, and there is an "elastic" cost associated with moving the atoms that make up the octahedron from their equilibrium positions, so if you stretch along the $z$-axis, say, then you have to compress along the $x$- and $y$-axes in such a way as to minimiz... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7686",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
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