Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
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Online QFT video lectures I'm aware of Sidney Coleman's 1975/76 sequence of 54 lectures on Quantum Field Theory. Are there any other high-quality QFT lecture series available online?
| Although the recordings of lectures from the Perimeter Institute has been done some years now, the Perimeter Institute launched the PSI Online program in 2018, which as of now contains three full courses (lecture videos + problem sets), two in Quantum Field Theory and one in advanced Condensed Matter physics.
An overvi... | {
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Gravitational time dilation at the earth's center I would like to know what happens with time dilation (relative to surface) at earth's center .
There is a way to calculate it?
Is time going faster at center of earth?
I've made other questions about this matter and the answers refers to:
$\Delta\Phi$ (difference in Ne... | The rule I mentioned in another question, that the time dilation factor is $1+\Delta\Phi/c^2$, applies here. The derivation (found in various textbooks) depends only on the assumptions that fields are weak and matter is nonrelativistic, both of which are true for the Earth.
Modeling the Earth as a uniform-density spher... | {
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Are many-worlds and the multiverse really the same thing? Are many-worlds and the multiverse really the same thing?
Not too long ago, Susskind and Bousso uploaded the article "The Multiverse Interpretation of Quantum Mechanics" with the thesis that the many-worlds interpretation and the multiverse of eternal inflation ... | In Figure 6 of their article, they draw a causal diamond, and divide the future null boundary into $B^+$ and $B^-$. They implicitly assumed that the Hilbert space of $B$ is the tensor product of the Hilbert space of $B^+$ with the Hilbert space of $B^-$. If $B$ were a spacelike surface, that would be true, but it's not... | {
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What are the conditions for decoherence to be irreversible? Spin echo experiments have been able to reverse the motions of all the molecules in a gas in statistical mechanics in the manner of Loschmidt. The Fermi-Ulam-Pasta model has solutions with a single mode dispersing, only to recohere after quite some time has el... | An article you might be interested in: http://www.physics.arizona.edu/~cronin/Research/Lab/some%20decoherence%20refs/RBH97.pdf
| {
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Is the wave function objective or subjective? Here is a question I am curious about.
Is the wave function objective or subjective, or is such a question meaningless?
Conventionally, subjectivity is as follows: if a quantity is subjective then it is possible for two different people to legitimately give it different val... | According to the Pusey-Barrett-Rudolph theorem, if scientists Alice and Bob disagree about their beliefs of the wave function, at least one of them has to be wrong. By your definition of objectivity, this makes the wave function "objective".
Note the PBR theorem doesn't apply to mixed density states, as in Wigner's fri... | {
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Graduate Physics Problems Books Need to brush up on my late-undergrad and early-grad physics and was wondering if anyone can recommend books or lecture notes (hard copy, or on-line) that also have solutions.
Two that I have come across are:
Princeton Problems in Physics with Solutions - Nathan Newbury
University of Ch... | Thinking Like a Physicist: Physics Problems for Undergrads: I love this book because it fosters a real sense of physical understanding, so it's not just mathematics, but actual physical reasoning. Plus, I found the problems challenging and interesting.
Then, there is always the MIT Open Course Ware in Phsyics, which ... | {
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Do quantum states contain exponentially more information than classical states? Do quantum states contain exponentially more information than classical states? It might seem so at first sight, but what about in light of this talk?
| This is exactly the question to which I was seeking with a colleague of mine to give some sort of answer. We considered a game played by a team of two - say Alice and Bob - in which the value of a random variable $x$ is revealed to Alice only, who cannot freely communicate with Bob. Instead, she is given a quantum $n$-... | {
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Why don't waves erase out each other when looking onto a wall? If I stand exactly in front of a colorful wall, I imagine the light waves they emit, and they receive should randomly double or erase out each other.
So as a result, I imagine I should see a weird combination of colors, or a full-black/full-white/very ligh... | If you look at the time domain and draw different waves with very different frequencies , and also draw some waves with very short peak, you can imagine that the sum of these low peak waves do not interfere that much in the final result of the bigger peak wave.
in digital communications, we say that the signal to noise... | {
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Understanding the cause of sidebands in Amplitude Modulation I've read it many places that Amplitude Modulation produces sidebands in the frequency domain. But as best as I can imagine it, modulating the amplitude of a fixed-frequency carrier wave just makes that "louder" or "quieter", not higher-frequency or lower-fr... | You are correct that the modulated signal touches a peak every 1/f and a trough every 1/f. But if you looked at the waveform of a carrier being amplitude modulated by a much lower frequency pure sinewave, it is intuitive that there is another frequency component. (That is the "feel" part.) The math in other posts sh... | {
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Why doesn't a phone charge faster, rather than slower when it is in use In Physics class, we were building parallel circuits, and as more lights were attached in parallel, they got brighter (as more power was being provided to the lights, and the resistance decreases). So, when I charge a phone, why doesn't the battery... | When you add components in parallel, the voltage across each component is the same. Thus, in a simple AC circuit, when you add an extra light bulb in parallel, the brightness of the light bulbs should stay the same, not increase.
This might seem wrong because there is less total resistance in the parallel circuit, an... | {
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Does a magnetic field do work on an intrinsic magnetic dipole? When you release a magnetic dipole in a nonuniform magnetic field, it will accelerate.
I understand that for current loops (and other such macroscopic objects) the magnetic moment comes from moving charges, and since magnetic fields do no work on charges ($... | Yes, of course that if a field - magnetic field - is able to make a bar magnet rotate or move, it is doing work. The statement that magnetic fields don't do any work only applies to point-like pure electric charges.
Magnetic moments may be visualized as objects with a forced motion of charges (solenoids have the same m... | {
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Is 3+1 spacetime as privileged as is claimed? I've often heard the argument that having 3 spatial dimensions is very special. Such arguments are invariably based on certain assumptions that do not appear to be justifiable at all, at least to me. There is a summary of arguments on Wikipedia.
For example, a common argume... | Science fiction writer (but also published physicist) Greg Egan has put quite a bit of work into investigating a universe with 4+0 dimensions: Orthogonal. Some of it is quite ingenious, eg. assuming a compact universe guarantees that the (modified) wave equation doesn't have exponentially growing solutions and time app... | {
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How do laser rangefinders work when the object surface is not perpendicular to the laser beam? I find the functioning of a laser rangefinder confusing.
The explanation usually goes like this: "you shine a laser beam onto the object, the laser beam gets reflected and gets back to the device and time required for that is... | The amount of laser energy reflected back will be the limiting factor of its effective range. However, since the laser's radiation is of a specific wavelength, it won't be confused by extraneous radiation from ambient sources.
| {
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Why are smaller animals stronger than larger ones, when considered relative to their body weight? I am interested in why many small animals such as ants can lift many times their own weight, yet we don't see any large animals capable of such a feat.
It has been suggested to me that this is due to physics, but I am not ... | To lift anything, a life form on this planet needs muscles. If you want to lift heavier things you need
*
*more muscles and
*a stronger body/legs to support that additional weight.
Stronger muscles and bones need to be supplied with more oxygen, nutrion and so on which leads to needing a stronger heart, a better ... | {
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Experimental evidence showing the kinetic energy of an electron changes in a static non-uniform magnetic field? In a previous question, Does a magnetic field do work on an intrinsic magnetic dipole?, one highly rated answer suggested that static magnetic fields do work on intrinsic magnetic dipoles in a non-uniform mag... | There is no evidence that the kinetic energy of an electron changes in a static magnetic field. You'd end up with a perpetual motion machine otherwise.
| {
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Can the effects of gravity be broken by jumping? I was having a debate the other day with a work colleague where I explained that gravity is a weak force because it is easily broken. Then I remembered a lecture by someone, I forget who, that explained gravity is very weak because you can break its influence just by jum... | I think what you heard in that lecture is this argument:
Gravitation is by far the weakest of the four interactions. Hence it is always ignored when doing particle physics. The weakness of gravity can easily be demonstrated by suspending a pin using a simple magnet (such as a refrigerator magnet). The magnet is able t... | {
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Nuclear decay rate affected by sun and quantum randomness If nuclear decay rate were affected by sun,
then emission probabilities would be subject to sun state and its influence,
so quantum randomness would depend on it,
Would it still be truly random?
One could argue that randomness would keep intact when consider th... | If decay rates are found could be reduced to 0, then we are thoroughly wrong in what we thought about nuclear decay, and that is unlikely to happen. The term "nuclear decay" would probably be obsolete in that case. Quantum mechanics, itself, may have answering to do given the strong form of what you are speculating a... | {
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Is the fine structure constant actually a constant or does its value depend on the energy scale? The value of the fine structure constant is given as
$$
\alpha = \frac{e^2}{4\pi\varepsilon_0\hbar c} = \frac{1}{137.035\,999..}
$$
It's value is only dependent on physical constants (the elementary charge $e$, speed of l... | Do not worry, it is a constant.
There is a sloppiness and misunderstanding in physics when the energy dependence of the cross sections is wrongly attributed to the "fundamental constant" whereas it is a cross section feature.
| {
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Calculating diffraction-limited resolution for a lens setup Supposed a lens arrangement is prepared where light from an object is collimated, focused and recollimated etc. before entering a CCD array. Given that we can calculate the diffraction-limited resolution for each lens in the system, how do we measure the diffr... | In the spot diagrams of optic designs the ray aberrations are always compared to the size of diffraction limited spot.
In order to do this you have to find the smallest aperture in your system. This can be a deliberately placed mechanical aperture but it can also be the circumference of a lens.
I copied the following s... | {
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significance of maxima and minima of time varying kinetic energy of a system Consider a system of particles where the kinetic energy of the system is varying with time. I'd like to know the significance (or meaning) of the time derivative of the kinetic energy being zero at a point. What is the significance of time ins... | As Anna pointed out in her comment, it really depends on the system. In general, there's not much you can say except that the minimum of kinetic energy corresponds to the minimum speed and the maximum of kinetic energy corresponds to the maximum speed.
There are many systems in which the minimum speed (and thus the min... | {
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What do we consider "Perpetual Motion" I know this is a bad question to most serious Physics but I have a question about what is considered “Perpetual motion.” The Foucault pendulum in the UN consists of sphere that passes directly over a raised metal ring at the centre that contains an electromagnet, which induces a c... | From Wikipedia: "Perpetual motion describes hypothetical machines that operate or produce useful work indefinitely and, more generally, hypothetical machines that produce more work or energy than they consume, whether they might operate indefinitely or not."
The key is not moving indefinitely, but DOING WORK. People di... | {
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what cools bottle of water faster: ice or snow Imagine you have a pile of snow and a pile of ice shards. You put a soda bottle which has a room temperature into both piles. Which bottle is going to cool down faster?
| This depends on contact area between bottle & ice/snow, and consistency of snow.
If there is not much air in the snow, it should have bigger contact area with bottle, and thus heat will be transferred faster.
Ice will contact with the bottle mainly at shards edges, so contact area is small.
PS. Adding water will change... | {
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Length of a curve in D dimensional euclidean space In a book I am reading on special relativity, the infinitesimal line element is defined as $dl^2=\delta_{ij}dx^idx^j$ (Einstein summation convention) where $\delta_{ij}$ is the euclidean metric. Next, if we have some curve C between two points $P_1$ and $P_2$ in this s... | Well, $\mathrm{d}l$ represents an infinitesimal length along the curve. $\mathrm{d}t$ also represents an infinitesimal length along the curve, although if the parametrizations $t$ and $l$ are different, the two infinitesimal lengths are not going to be the same. You can write the identity $\mathrm{d}l = \frac{\mathrm{d... | {
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(Almost) double light speed Let's say we have $2$ particles facing each other and each traveling (almost) at speed of light.
Let's say I'm sitting on #$1$ particle so in my point of view #$2$ particle's speed is (almost) $c+c=2c$, double light speed? Please say why I am incorrect :)
EDIT: About sitting me is just exa... | You are both correct and wrong.
If - sitting on one photon - you would measure the velocity of the approaching photon, the figure received would be exactly c.
However, if two photons separated by the distance of 1 light-year are sent toward each other, they will meet after exactly six months and exactly in the middle o... | {
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How to find the principal point in an image? I need to find the principal point in an image. Its a point where the principal axis intersects the image sensor.
Due to misalignment this point is not at the center of image always(or image sensor). I need to precisely determine its location using any of the optical methods... | To do this you would need to take an image of some sort of reference object, like a regular grid pattern. You would also have to know that your imaging system has some measurable field aberrations which are radially symmetric, such as distortion. Then you could do some simple image processing to locate the center of th... | {
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How is thermodynamic entropy defined? What is its relationship to information entropy? I read that thermodynamic entropy is a measure of the number of microenergy states. What is the derivation for $S=k\log N$, where $k$ is Boltzmann constant, $N$ number of microenergy states.
How is the logarithmic measure justified?
... | I think that the best way to justify the logarithm is that you want entropy to be an extensive quantity -- that is, if you have two non-interacting systems A and B, you want the entropy of the combined system to be
$$
S_{AB}=S_A+S_B.
$$
If the two systems have $N_A,N_B$ states each, then the combined system has $N_A... | {
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Clebsch-Gordan Identity I'm trying to take advantage of a particular identity for the sum of the product of three Clebsch-Gordan coefficients, however, the present form of my equation is slightly different. Is there a symmetry relation that will allow me to change:
$\sum_{\alpha\beta\delta}C_{a\alpha b\beta}^{c\gamma}... | Notice that $C^{22}_{1111}=1$ but $C^{11}_{2211}=0$. I don't think that this is true unless $a=d$ and the sums over $\alpha$ and $\delta$ have the same range.
| {
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How to calculate concentration of vapor at the surface of a water drop I'm reading a paper that examines the evaporation rates of water. In the final formula, it has the following constant:
$c_s - c_\infty $ where $c_s$ is the concentration of the vapor at the sphere surface and $c_\infty$ is the concentration of the ... | So, it turns out that the answer is incredibly simple. Which is what I suspected given the mechanism: Question seems super easy, but can't seem to get answer of the top of my head. So I go searching for an answer, but because the solution is so obvious, no book has it explicitly. Ask people, again, no one seems to ... | {
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Batman spotlight in the sky I have noticed that obstructing a spotlight typically results in a blurred shadow unlike the crisp batman symbol in the comics of batman. Is there a way to create a spotlight with a crisp batman symbol?
| You need to have a focusing lens built into your projector. After that, it's just like any other movie projector sort of thing, you can focus it provided you have an object to project it onto (like a flat cloud). Physically, the clarity of the image is limited by two or three things.
(1) The size of your optics (the l... | {
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If you view the Earth from far enough away can you observe its past? From my understanding of light, you are always looking into the past based on how much time it takes the light to reach you from what you are observing.
For example when you see a star burn out, if the star was 5 light years away then the star actuall... | Yes, you can. And you do not even need to leave the Earth to do it.
You are always viewing things in the past, just as you are always hearing things in the past. If you see someone do something, who is 30 meters away, you are seeing what happened $(30\;\mathrm{m})/(3\times10^8\;\mathrm{m}/\mathrm{s}) = 0.1\;\mu\mat... | {
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Is spacetime simply connected? As I've stated in a prior question of mine, I am a mathematician with very little knowledge of Physics, and I ask here things I'm curious about/things that will help me learn.
This falls into the category of things I'm curious about. Have people considered whether spacetime is simply conn... | I suppose there are many aspects to look at this from, anna v mentioned how Calabi-Yao manifolds in string theory (might?) have lots of holes, I'll approach the question from a purely General Relativity perspective as far as global topology.
Solutions in the Einstein Equations themselves do not reveal anything about gl... | {
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Calculating time for a fully charged UPS I have a UPS of 1000 Volts connected with 2 batteries each of 150 Amp. How much time it will take to consume the whole UPS (after fully charged) when a device of 1Amp is getting electricity form that UPS.
Please also explain me the calculation.
| There are some missing data in your question.
*
*What voltage does the batteries have, I'm going to assume 12V since it is common.
*Battery capacity, you typed it as 150A but I guess it was 150Ah. Please note that a normal car battery on a car like a VW Golf has approx 60Ah so 150Ah is a quite big battery.
*Output... | {
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How come an anti-reflective coating makes glass *more* transparent? The book I'm reading about optics says that an anti-reflective film applied on glass* makes the glass more transparent, because the air→film and film→glass reflected waves (originated from a paraxial incoming wave) interfere destructively with each oth... | The wave reflected from the air-film interface continues happily traveling back, as you say, but so does the wave reflected from the film-glass interface. Since they are the same frequency but in antiphase, they interfere destructively as long as they keep going. Since the superposition principle states that they do no... | {
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Is there any way to increase a rubber-bands lifetime? Rubber-bands are simple, yet very useful. Old rubber bands(5 years?) get brittle? Why is that?
| If a rubber band is usable for 5 years, it's a very good one.
There is not much you can do to prevent brittleness other than storing them in a dark and cool place. Light and oxygen are the most dangerous enemy of rubber.
The biggest influence on lifespan is the manufacturer, because they can choose
to use a persisten... | {
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What happens to light in a perfect reflective sphere? Let's say you have the ability to shine some light into a perfectly round sphere and the sphere's interior surface was perfectly smooth and reflective and there was no way for the light to escape.
If you could observe the inside of the sphere, what would you observe... | Just another perspective:
Since the sphere is non-ergodic, your observation depends on your and the source locations inside the sphere. For ergodic shapes (ellipsoid, etc), you will see an evenly lit world.
| {
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Simplest interferometer I want to build simplest interferometer which should be able to measure movements down to fraction of wavelength.
What is the simplest scheme for that, and what are the requirements for a laser?
I have a bunch of laser diode-based ones, and I guess they might be not coherent enough...
Are green... | Well, one problem you are likely to encounter is that your setup will likely be vibrating with amplitude on this order.. do you have a floating optical table?
Coherence is probably not that important. At a minimum you will need a beamsplitter, two mirrors and a diode or some other way to measure the interference patter... | {
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Software for simulating 3D Newtonian dynamics of simple geometric objects (with force fields) I'm looking for something short of a molecular dynamics package, where I can build up simple geometric shapes with flexible linkages/etc and simulate the consequences of electrostatic repulsion between surfaces. Something, sa... | I've not tried it myself, but you may want to check out the Elmer multiphysics package, which looks like it can do time dependendent 3D finite element mechanics + electrostatics.
Here's a nice pic they have of a beam being deflected by electrostatic attraction:
It's F.O.S.S. They have windows binaries and it's in the ... | {
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Do eyeballs exhibit chromatic aberration? Fairly straightforward question. If not, why not?
I suspect that if they do, it is not perceived due to the regions of highest dispersion being in one's region of lowest visual acuity.
| Yes, it does. We don't see it because our brain automagically 'correct it' because it always see the same aberration from the childhood.
Our eye focuses on 'green' wavelength as it's its peak sensitivity, so red and especially violet lines are usually slightly out-of-focus.
| {
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'Getting in' to research physics? I'm going to be choosing a university course soon, and I want to go into a branch of physics. A dream job for me would be to work in research, however, I do realise that this isn't for everyone and is difficult to reach. So what is the best way to go about achieving this aim? What thin... | I'd say study hard, but also learn about the fundamentals of experiment design, learn how to write proper scientific papers, and learn how publish your work. These things aren't (in my experience) typically taught in the undergraduate or even graduate level coursework, but are absolutely crucial to "success" in a rese... | {
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Solve spring system I am not physics in training so if anything is unclear please request clarification.
I have a set of objects to be placed on a line, linked with springs of known lengths (L) and stiffnesses (K). I want to solve the equilibrium positions of each object (x).
o---------o--------------o
x0 x1 ... | I'm assuming you want to solve such problems automatically on a computer. If you want to solve them individually by hand, the best method will obviously be different.
Without the constraint that the objects can't pass through each other, the problem is an unconstrained quadratic optimization problem, which is solved si... | {
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Roughly how many atoms thick is the layer of graphite left by a pencil writing on paper? Actually I can't expand much as the question pretty much explains the query. I would be interested in the method of estimating an answer as well as a potential way to measure it experimentally. Thanks.
P.s. I'm not sure what tags t... | Although I don't know anything about this, using some rough estimates I think I can get the right order of magnitude:
*
*Volume of graphite in a pencil: $10 cm$ cylinder of $1 mm$ thick = $0.314 mm^3$ (error: ~factor 2)
*Maximum surface a pencil can write: $50 km$ $\times$ $1$ mm = $10 m^2$ (error: ~factor 5)
*Thi... | {
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Why is 55-60 MPH optimal for gas mileage of a passenger car? My driver's education teacher back in high school said 55 MPH is optimal for gas mileage of a passenger car. Just last week, I read an article in a magazine saying 60 MPH is optimal. These numbers are pretty close, so there's some validity in the statement. W... | It's really an engineering question. Cars are designed for efficiency nowadays; especially in Europe the taxes are directly related to the MPG ratings on standardized test cycles. But even in the US, there are standard test cycles for fuel effiency. Those test cycles typically have large segments driven at ~55 mph.
Now... | {
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Could a human run horizontally inside a Wall of Death? A popular circus stunt is for a motorcycle rider to ride inside a bowl shaped depression called a "Wall of Death." The rider goes higher and higher up the wall until they are actually horizontal. I wonder if a human could do the same.
| The speed you need (explained http://physics.mut.ac.th/PhysicsMagic/wall.htm)
$$
V^2 > R g/u
$$
$v$ = the velocity (m/s)
$R$ = Radius of the pit (m)
$g$ = acceleration of gravity (9.8m/s^2)
$u$ = coeff of static friction
So if u is 1 (the maximum possible), and you are an olympic athlete that can run 100m in 10s
Th... | {
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Books that every layman should read To add to Books that every physicist should read:
A list of popular physics books for people who aren't necessarily interested in technical physics.
(see also Book recommendations)
| I can recommend the following physics books. They are all somewhat different, but collectively they provide a good picture of where ideas in physics are today and how modern theories developed.
The Strangest Man: The Hidden Life of Paul Dirac, Quantum Genius, Graham Farmelo
Quantum Enigma, Bruce Rosenblum and Fred ... | {
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Is the earth expanding? I recently saw this video on youtube:
http://www.youtube.com/watch?v=oJfBSc6e7QQ
and I don't know what to make of it. It seems as if the theory has enough evidence to be correct but where would all the water have appeared from? Would that much water have appeared over 60 million years? Also what... | Although considered a viable alternative hypothesis in the past, the expanding earth hypothesis is now generally considered to be obsolete, given the overwhelming evidence in support of plate tectonics. The following paper uses geologic evidence and a classical physical analysis of the earth-moon system to show there ... | {
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Creation of matter in the Big Bang I appreciate your patience to my neophyte question. I am working on my dissertation in philosophy (which has nothing or little to do with physics) about the "problem of naming." Briefly what I am arguing is that when we name something, we stop it from being anything or everything else... | The closer to the Big Bang you look, the more fundamental are the particles that constitute the universe (quarks, leptons, etc.) - go far back enough and you reach the point where only a field description makes sense because of the curvature of the universe.
But after the 3rd minute and up to around the 20th minute, th... | {
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Smoothed particle hydrodynamics in cosmological N-body simulations What is the role of smoothed particle hydrodynamics (SPH) in cosmological N-body simulations like the Millenium Run (performed with Gadget-2)?
| To answer your specific question: absolutely none.
The Millenium run is a "dark matter-only" simulation. In this sort of simulation gas physics is taken to play a negligible role. All the gas (and stars, indeed all "baryonic matter" as it's called in the jargon) is removed and replaced with additional dark matter. The ... | {
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How much solar energy hits a rooftop in Israel? I was asked to today about the available plans (in Israel) to install photo-voltaic receptors on the roof and sell the energy. This is a legitimate plan and there are several companies that do this. The claim is that the initial investment is repaid after roughly 5 years ... | For the middle east, typically around 2000 KWh/m^2/year
A good place to start is wiki page for insolation (technical term for sunlight arriving)
| {
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What does the quantification of causes and effect look like, for clouds in offshore wind turbine wakes? At Horns Rev windfarm off the coast of Denmark, sometimes in winter, clouds appears in the wake of the turbines. I've only seen photos of the phenomenon when the wind direction is exactly aligned with the grid layout... | Here is a paper published in Energies, freely accessible:
http://www.mdpi.com/1996-1073/6/2/696
| {
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In dimensional analysis, why the dimensionless constant is usually of order 1? Usually in all discussions and arguments of scaling or solving problems using dimensional analysis, the dimensionless constant is indeterminate but it is usually assumed that it is of order 1.
*
*What does "of order 1" mean? 0.1-10?
*Is... | Notable exception: Reynolds number.
| {
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Why does the weighing balance restore when tilted and released I'm talking about a Weighing Balance shown in the figure:
Press & Hold on onside of the horizontal beam and then release it. It makes some oscillations and comes back to equilibrium like shown in the figure.
Both the pans are of equal equal masses. When th... | It behaves this way because that's how it was built. By adjusting the mass distribution, we could make a scale that flops to one side, is roughly balanced at all angles, etc. However, those scales would not be useful, so the scale isn't built that way.
It might be assumed from the left/right symmetry of the picture t... | {
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Must the action be a Lorentz scalar? Page 580, Chapter 12 in Jackson's 3rd edition text carries the statement:
From the first postulate of special relativity the action integral must be a Lorentz scalar because the equations of motion are determined by the extemum condition, $\delta A = 0$
Certainly the extremeum con... | First, observe that although the non-relativistic Lagrangian is not invariant. It changes by a total derivative, thus the equations of motions remain invariant.
The reason of the difference between the Lorentzian and the Galilean cases is that the group action of the Lorentz group on the classical variables (positions ... | {
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Does the wavelength always decrease in a medium? I was studying a GRE Physics Test problem where optical light with a wavelength of 500 nm travels through a gas with refractive index $n$.
If we look at the equations for wave motion and index of refraction
$$c=\lambda_0\nu\quad\text{(in vacuum)}$$
$$v = \lambda\nu\quad\... | The answer to your first question
Is this a consistent property at all frequencies and for all mediums with refractive index real and greater than 1?
is Yes.
Not only in optics but in other wave mechanics too.
Refraction of light is the most commonly observed phenomenon, but any type of wave can refract when it in... | {
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Why did my liquid soda freeze once I pulled it out of the fridge and opened it? This isn't a duplicate to "Why did my liquid soda freeze once I pulled it out of the fridge?". My question is why soda froze after it was opened. Opening a can or bottle seems to have a larger effect than just jostling it.
Is it because of ... | Farenheit calibrated his thermometer to have 0 degrees be the freezing point of salt water. Fresh water freezes at 32F. Water with stuff dissolved in it freezes at a lower temperature than water with less stuff dissolved in it.
Another hypothesis for why soda freezes after opening is that when you open it some CO2 ou... | {
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With potential $V(x)= ax^6$ the quantized energy level $E$ depends on which power of $n$? A particle in one dimension moves under the influence of a potential $V(x)= ax^6$, where $a$ is a real constant. For large $n$, what is the form of the dependence of the energy $E$ on $n$?
| You might not be aware, but the quantum number "n" has a classical interpretation as the action variable "J". The action variable measures the area in phase space of the classical orbit,
$$J = \oint p {dx\over dt} dt$$
And the correspondence between J and n was known before quantum theory was developed. It is easy to w... | {
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Physical interpretation of describing mass in units of length I'm working in Taylor and Wheeler's "Exploring Black Holes" and on p.2-14 they use two honorary constants: Newton's constant divided by the speed of light squared e.g. $G/c^2$ as a term to convert mass measured in $kg$ to distance.
Without doing the arithme... | They represent the scale on which general relativisic effects dominate physics related to bodies of that mass.
For instance if you were to create a (un-rotating, uncharged) black hole of 1 Earth mass it's event horizon would have a radius of about $9\text{ mm} = 2 * M_\text{Earth}$ in those units.
For scales much, much... | {
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How can it be that the beginning universe had a high temperature and a low entropy at the same time? The Big Bang theory assumes that our universe started from a very/infinitely dense and extremely/infinitely hot state. But on the other side, it is often claimed that our universe must have been started in a state with ... | Let me show you that there is no contradiction by pointing out e.g. that for ordinary expansion periods (that is away from first order phase transitions, decouplings...) the total entropy is actually constant in time while the universe is getting bigger and cooler. Or, going back in time, the universe is getting hotte... | {
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Collision of Phobos Mars has two moons: Phobos and Deimos. Both are irregular and are believed to have been captured from the nearby asteroid belt.
Phobos always shows the same face to Mars because of tidal forces exerted by the planet on its satellite. These same forces causes Phobos to drift increasingly closer to Ma... | Dynamical models over a likely timescale (say $10^6$ to $10^9$ years) would have significant error bars as mentioned above, and therefore one off predictions about individual moons have little validity.
| {
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Experimental evidence for parallel universes/multiverses My idea of physics is that it is a collection of mathematical laws relating observables. And that one can perform alot of mathematical derivations on these laws to produce new laws between observables. My question is how does one translate a mathematical equation... | There is no evidence that another universe exists. There are various physical theories that allow for the existence of parallel universes, and as far as I know there are no widely accepted theories that prohibit their existence outright, so strictly speaking, I suppose it's possible. But it's unlikely that we would eve... | {
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Why Do Hurricane Balls Spin So Fast? I was wondering if anyone could offer an explanation as to why the balls described in this video spin so fast.
Here's the setup: Two metal balls are wielded together. When spun with air, they acquire a massive amount of rpm.
| Look at around 0:34 in the video. I want to point out something relevant to the question here. The end of the tube is narrowed. That is, in technical terms, a nozzle. Nozzles are extremely common in engineering and they work as a form of mechanical leverage just like a lever. I should also note that the straw itse... | {
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Could the Schrödinger equation be nonlinear? Is there any specific reasons why so few consider the possibility that there might be something underlying the Schrödinger equation which is nonlinear? For instance, can't quantum gravity (QG) be nonlinear like general relativity (GR)?
| In addition to the classic Weinberg paper cited above, there's this shorter version, and then follow ups by Peres 1989 on how it violates the 2nd law, by Gisin on how it allows superluminal communications, and by Polchinski on how it would allow for an 'Everett' phone.
More recently, there's this mathematical argument ... | {
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Why is the string theory graviton spin-2? In string theory, the first excited level of the bosonic string can be decomposed into irreducible representations of the transverse rotation group, $SO(D-2)$. We then claim that the symmetric traceless part (i.e. the 35 rep) is the spin-2 graviton - but isn't the label "spin-2... | I don't know anything about string theory, but the graviton has been described as a spin-2 particle well before string theory. In his "Lectures on Gravitation" Feynman explains why the graviton must be integer spin, then explains why it can't be 0 or 1, then proceeds to attempt to build the quantum field theory of a sp... | {
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Why does gravity forbid local observables? I heard in a conference that gravity forbids to construct local gauge invariants like $\mathrm{Tr}\left\{−\frac{1}{4} F_{μν}^{a}F_{a}^{μν}\right\}$ and only allows non-local gauge invariant quantities like Wilson Loops: $\mathrm{Tr}\mathcal{P}\exp\left[\oint_{\gamma} A^{a}dx_{... | General coordinate invariance lets you arbitrarily set the values of the metric and it's first derivative at any one point-- Fermi coordinates . Since you can do this, constructions like the maxwell term you describe above will be necessarily coordinate-dependent, and thus, not local observables.
| {
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Meaning of dimension in dimensional analysis I was wondering what dimension can mean in physics?
I know it can mean the dimension of the space and time.
But there is dimensional analysis. How is this dimension related to and different from the previous one? How is it related to and different from units (e.g. kilometer)... | A dimension (in dimensional analysis) is defined by the transformation law of an object under changes in scale. If I have an object which is twice as big, it has 4 times the surface area and 8 times the volume, so the surface area has dimension of length-squared, and the volume has dimension of length-cubed. Dimensiona... | {
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How and why will the Milky Way collide with the Andromeda galaxy? Hubble's law says that the universe is expanding. How come the Milky Way and the Andromeda galaxies are on a collision course? How will they end up colliding with each other?
| The large-scale structure of the Universe is expanding. However, gravity still works, and it's especially powerful if the distance is small. E.g., the Earth is still pulling your body closer to it, even though the Universe is expanding. The Earth and the Moon still attract each other, even though the Universe is expand... | {
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Paramagnet: Negative specific heat? for a simple paramagnet ($N$ magnetic moments with values
$-\mu m_i$ and $m_i = -s, ..., s$) in an external magnetic field $B$, I have computed the Gibbs partition function and thus the Gibbs free energy $G(B,T)$ and obtained
a simple approximation for small $B$ fields, which has t... | It is the constant magnetization assumption which causes this. If you increase the temperature at constant $B$, then the magnetization would reduce, so to keep magnetization constant, you have to increase the magnetic field. The energy, $E=-MB$, so as you increase $B$ at constant $M$ you are making energy more negati... | {
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Why are color values stored as Red, Green, Blue? I learned in elementary school that you could get green by mixing blue with yellow.
However with LEDs, TFTs, etc. you always have RGB (red, green, blue) values?
Why is that? From what you learned in elementary yellow would be the 'natural' choice instead of green.
| When you mix colors using Watercolors, then they mix as "Subtractive Colors". However, Light itself mixes as "Additive Colors".
Even though it might seem strange why the inherently same thing works so differently, it makes sense if you think about Watercolors, etc. as absorbing everything but that specific color.
| {
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Naïve relativistic schrodinger equation
Possible Duplicate:
Why are higher order Lagrangians called 'non-local'?
Bjorken and Drell presents the equation:
$$i\hbar\frac{d\psi}{dt}=H\psi=\sqrt{p^2 c^2+m^2 c^4}\psi=\sqrt{-\hbar^2 c^2 \nabla^2+m^2 c^4}\psi$$
The squareroot can be expanded to obtain an equation with all ... | To see why the theory is nonlocal, consider the effect of the derivative operator... I like to put things on a lattice, so I will: $\psi_i=\psi(x_i)$, then the derivatives (in 1D, for simplicity) become
$$\nabla^2 \psi_i \propto (\psi_{i-1}-2\psi_i + \psi_{i+1})$$
Now, you can see what happens as you continue to apply ... | {
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How would you store heat? Um .. naive question perhaps but if somebody wanted to store heat, how would they go about it? Can heat be stored?
I'm told that decomposing kitchen waste in a closed vessel results in a rise in temperature on the body of the vessel. I'm just wondering whether it could be stored for later use... | If you want to store heat in a battery-like device, you could use the heat to power a turbine, generate electrical energy, and store it as chemical energy in a battery. This is extremely inefficient, but I think this is most analogous to what you are asking.
You could also find a high-energy chemical reaction in equil... | {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 3,
"answer_id": 1
} |
Graphene space elevator possible? I just read this story on MIT working on industrial scale, km^2 sheet production of graphene.
A quick check of Wikipedia on graphene and Wikipedia on space elevator tells me
Measurements have shown that graphene has a breaking strength 200 times greater than steel, with a tensile stre... | @lurscher of course I understand it's from GEO, the fact that GEO is the net zero apparent acceleration point is the reason it would be "unfurled" from GEO. If your point behind the stages is that it could be carried up in segments, then yes, no one ever argued otherwise. The only thing your $k^N$ mathematics shows i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15052",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 6,
"answer_id": 3
} |
Why is it important that Hamilton's equations have the four symplectic properties and what do they mean? The symplectic properties are:
*
*time invariance
*conservation of energy
*the element of phase space volume is invariant to coordinate transformations
*the volume the phase space element is invariant with res... | I think the properties 3 and 4 are important because in these way the probability
distribution in phase space is conserved and the information is also conserved.
Some systems dont have this property, these are chaotic. In these systems the volume
in phase space could increase until filling the complete phase space in ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15195",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
What does it take to understand Maxwell's equations? Assume I want to learn math and physics enough to reach a level where I understand Maxwell's equations (The terms and reasoning in the equations I.e. why they "work"). What would I have to learn in order to have the tools I need to make sense out of it?
I'm kindof lo... | I learned undergraduate electrostatics from Wangsness' Electromagnetic Fields. The text is one of the clearest and most comprehensible I've seen at this level and I highly recommend it. The first chapter covers only vector calculus and provides an excellent basis for the necessary math. We got to Maxwell's equations... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15249",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
"answer_count": 5,
"answer_id": 2
} |
Is an electron/proton gun possible? In the 1944 SF story “Off the Beam” by George O. Smith, an electron gun is constructed along the length of a spaceship. In order to avoid being constrained by a net charge imbalance, it is built to also fire the same number of protons in the other direction, dissipating the mass of t... | An example of a real-life electron/proton beam is the Neutral Beam Injector used in magnetically confined fusion devices. They're used to inject faster particles into the fusion plasma, raising the overall temperature. However an ordinary (charged) proton/deuteron beam will be bent away by the strong magnetic fields su... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15290",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 3
} |
Open quantum systems and measuring devices The Copenhagen interpretation by Niels Bohr insists that quantum systems do not exist independently of the measuring apparatus but only comes into being by the process of measurement itself. It is only through the apparatus that anything can be said about the system. By necess... | Q: Can quantum mechanics be applied to closed systems where the measuring apparatus is itself part of the system? | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15547",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 6,
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How does one calculate the force applied on an object by a magnetic field? I've tried very hard to find an answer to this question, and every path leads me to an abstract discussion of fundamental forces. Therefore, I will propose two very specific scenarios and see if they yield the result that I am looking for.
Scena... | I am consider Scenario One:
For given ball and magnet the magnitude of the interaction force between them, depending on the distance $x$, follows the formula:
$$F=\frac{const}{x^7}$$ where $const$ depends on the ball's radius, on the magnetic dipole moment of the magnet and on the magnetic permeability(expected to ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15578",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 1
} |
Why do ships lean to the outside, but boats lean to the inside of a turn? Small vessels generally lean into a turn, whereas big vessels lean out.
Why do ships lean to the outside, but boats lean to the inside of a turn?
For example, a boat leaning into a turn:
Image Source.
And a ship leaning out:
Image source
| The answer to the question is; leaning out or leaning in is a result of the vessel's rudder's position relative to the keel. If the rudder is ABOVE the keel, the vessel will lean OUT; if the rudder is below the keel, the vessel will lean IN. ("Rudder"- the method or mechanism by which the flow of water under the vessel... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15631",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "31",
"answer_count": 7,
"answer_id": 5
} |
Mathematical Universe Hypothesis What is the current "consensus" on Max Tegmark's Mathematical Universe Hypothesis (MUH) which claims every concievable mathematical structure exists, including infinite different Universes etc.
I realize it's more metaphysics than physics and that it is not falsifiable, yet a lot of peo... | According to the metaphysical Church-Turing thesis, all that exists has to be Turing computable. There should also be a unity oneness to all that exists.
So, dare I say, maybe all that exists is ONE universal computer dovetailing over all possible programs a la Schmidhuber, or ONE quantum computer running a superpositi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15666",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "15",
"answer_count": 8,
"answer_id": 7
} |
can radiocarbon dating be used on living things? I have been going through a wiki article about worlds oldest living creature. As a matter of fact its a plant, a shrub to be precise.
Wiki says that the plant age was determined by carbon dating. But what i know is that carbon dating can only be done for dead tissues i.e... | When trees grow, they add successive layers to a central core. After only a year or two, this middle core becomes established and stops growing. The living part of a tree is mostly in the bark and layer immediately below that (as well as leaves and roots). This is why woodpeckers don't kill trees but girdling one wi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15717",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
} |
How can a conductor be grounded yet there are induced charges on it? A classic example for the method of images is the following, quoted from Griffiths's Introduction to Electrodynamics, page 121:
Suppose a point charge $q$ is held a distance $d$ above an infinite grounded conducting plane. Question: What is the poten... | Think of the ground plane as being an infinitely big electrical conductor, initially uncharged.
We bring in a (say positive) point charge close to the surface of the ground plane. An negative image charge is induced on the surface near the point charge, and since the net charge on the ground plane is zero, an opposite... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15827",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 2,
"answer_id": 0
} |
Realistic projectile motion I am working on a project involving a simulation of the motion of a projectile (in 3D) aimed at a moving target. The way projectile motion is analyzed in most introductory physics books is not accurate enough for this project. I would like to know what other influences on the motion of a pro... | If you have time to read a book, I strongly recommend Richard Feynman Lecture on Physics:
http://en.wikipedia.org/wiki/The_Feynman_Lectures_on_Physics
It is everything you want from a book:
*
*fun to read
*never boring
*it will change the way you see the world
*everyone can read it
In your case, I would focus ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15875",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Could a ship equipped with Alcubierre drive theoretically escape from a black hole? Could a ship equipped with Alcubierre drive theoretically escape from a black hole?
Also, could it reach parts of the universe that are receding faster than the speed of light from us?
| In my understanding, which is based on simple facts and admittedly not a big understanding of how black holes would work with regard to this...
A warp bubble would compress space in front of the ship and expand space behind the ship. This should effectively move the black hole away from the ship, descreasing the effort... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15960",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "24",
"answer_count": 4,
"answer_id": 3
} |
Problem with an electricity / thermodynamics assignment I've been trying to figure this one out for a while on my own, so I'd like to ask for your help if you could offer some.
The task states:
A heater made out of a wire with a diameter $R = 0.2\text{ mm}$, length $4\pi\text{ m}$ and electrical resistivity of $0.5\ti... | find m from this equation,
$$
mC_p\Delta T = \frac{V^2}{R}t
$$
where,$$R=\frac{\rho l}{\pi r^2}$$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15992",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
The visibility of air For pilots of gliders or sailplanes, the 'thermal' is the most important phenomena of the air. A thermal is classically described as an upward flow of air caused by ground level heating of air that rises in bubbles or a connected stream of warmed air. Given sufficient velocity of the rising air,... | Air of different temperature and pressure has different refractive index - but the difference is very small.
If you think of putting a piece of glass into water and trying to see it. The difference between glass (1.5) and water (1.33) is pretty large, air at different temperatures has refractive index differences of pa... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16046",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 0
} |
Is the cooling rate of a (very) cold object, sitting next to an AC higher or lower? In more detail:
If i have two soda cans, both are cooled to exactly 4 degrees celsius,
And i put one in a 25 degrees room, and the other next to an AC vent set to 16 degrees.
After three minutes, which one should be colder than the othe... | The air around the soda can effects how fast it transfers heat. If the temperature difference is low it will change gradually because the actual energy it takes to transfer energy is constant. waters specific heat is 4181.3 J/(kg·K), water has the second highest specific heat capacity. This means it takes a certain amo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16076",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 5,
"answer_id": 3
} |
Transmission and reflection What is the transmission amplitude of a wavefunction $\phi(x)=e^{ikx}(\tanh x -ik)$? I would have thought that it is $\tanh x -ik$ since this is the factor associated with the forward travelling $e^{ikx}$ but then since the reflection coefficient is $0$, we have that the reflection probabili... | The clue here is that the coefficients of $\mathrm e^{\mathrm ikx}$ and $\mathrm e^{-\mathrm ikx}$ are to be evaluated at infinity. The terms and concepts you're using apply to a situation where we have an incoming wave proportional to $\mathrm e^{\mathrm ikx}$ for $x\to-\infty$. It interacts with a system around the o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16163",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Dependence of Friction on Area Is friction really independent of area? The friction force, $f_s = \mu_s N$. The equation says that friction only depends on the normal force, which is $ N = W = mg$, and nature of sliding surface, due to $\mu_S$.
Now, less inflated tires experiences more friction compared to well inflat... | The increased 'resistance' of an underinflated tyre is due to mechanical deformation, friction is independent of area as suggested. The simplest explanation for me is that: as area increases the applied force per unit area decreases, but there is more contact surface to resist motion.
Added as per Zass' suggestion bel... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16213",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "22",
"answer_count": 4,
"answer_id": 0
} |
Are specific heat and thermal conductivity related? Are there any logical relationship between specific heat capacity and thermal conductivity ?
I was wondering about this when I was reading an article on whether to choose cast iron or aluminium vessels for kitchen.
Aluminium has more thermal conductivity and specific ... | For metals there is a connection between the thermal conductivity and electric conductivity (Wiedemann–Franz law).
However specific heat is not directly related. This is because electric and thermal conductivity are due to the electrons, however the specific heat is mostly due to the ion vibrations (phonons).
Despite "... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16255",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "26",
"answer_count": 8,
"answer_id": 2
} |
Is decoherence due to coarse graining or coupling with the environment? In the literature, sometimes one reads that decoherence is due to the coupling of the system to the external environment, and sometimes one reads that it is due to coarse graining over the microscopic degrees of freedom. Are these two different cas... | The more conventional way is to describe decoherence as being due to the "coupling to the environmental degrees of freedom" that are traced over. However, the "environmental degrees of freedom" may also include geometrically internal degrees of freedom of a physical system such as a cat – unmeasurably complicated corre... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16297",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
How do we perceive hotness or coldness of an object? Some objects, especially metallic ones, feel cold on touching and others like wood, etc. feel warm on touching. Both are exposed to same environment and are in their stable state, so some kind of equilibrium must be being reached. What is this equilibrium?
And how do... | To the first part of your question: What is this equilibrium? At room temperature the metallic or wooden object is in equilibrium with it's environment. When you touch the object this equilibrium is disturbed. As the skin on your fingers will be quite a bit above room temperature when you touch the object heat will flo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16333",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 6,
"answer_id": 4
} |
why evaluate at lambda = 0 I am trying to understand Herbert Goldstein's introduction to 4-vectors. He describes a 1-D curve in spacetime $ P_(\lambda) $ then he says a 4 vector is defined as the tangent vector to this curve $$ v = \biggr ( \frac {dP} {d\lambda}\biggr)_{\lambda =0} $$
why is $ \lambda $=0? what do... |
he says a 4 vector is defined as the tangent vector to this curve
That is not true in general. A four-vector is not always defined as the tangent vector to a curve. In the book they are computing a tangent vector to a curve in 3+1D spacetime; the tangent vector is just one example of a four-vector.
In particular, the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16432",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
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Is there any anti-gravity material? I want to know if there is any anti-gravity material. I am thinking of making flying vehicles which are made up of anti-gravity material so that they will not experience any gravity on them and can easily take off and be more fuel efficient. Is there any such thing? Or any workaround... | Anti-gravity is impossible, as it would let you build a perpetual motion system, as follows.
Assume we have a system in which we can capture the kinetic energy of a falling mass. For example, a ball that falls onto a scooped wheel to drive it. Take the ball and move a sheet of anti-grav material under it. As the ball n... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16474",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 5,
"answer_id": 4
} |
Should annealed disorder be characterized by the average of the partition function? Most of the literature says that for a quenched average over disorder, an average over the log of the partition function must be taken:
\begin{equation}
\langle \log Z \rangle,
\end{equation}
while for the annealed average, it's
\begin{... | I've found the book I was looking for: Ma - Modern Theory of Critical Phenomena (~ p366).
It says that rather the difference between a quenched and annealed average is the probability distribution it should be averaged over. Both types of average (if I understand it correctly) should be taken over the free energy:
\b... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16523",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 3,
"answer_id": 2
} |
About the Ether Theory acceptance Why was the Ether Theory refused by Modern Physics? If you please explain me, I just wanted to understand it more.
| It's not entirely true that ether theory was refused by modern physics, but seen as superfluous and over-complicated compared to special relativity's notion of time and space. There's a version called Lorentz ether theory which postulates that it's not possible to detect the absolute ether, and gives the same results a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16596",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 5,
"answer_id": 3
} |
Why is beta negative decay more common than beta positive? In simple terms, why is beta negative decay more common than beta positive?
I know it's something to do with occuring inside/outside the nucleus - but I can't find a simple, easy to understand explanation!
| Beta-minus decay occurs in nuclei with an excess of neutrons, while beta-plus decay takes place in neutron-deficit nuclei. A lot of natural background radiation on Earth is due to fission or alpha-decay of heavy radioactive elements. The remains of fission or alpha-decay are neutron-rich nuclei, so beta-minus decay... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16653",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 3,
"answer_id": 1
} |
Can there be black light? I mean is it possible to devise a machine that outputs darkness? I understand there are various colours that light can have. But i was wondering why there is no 'black' light. What is the logical explanation for this? I mean I am expecting an answer that goes beyond mentioning the spectrum det... | In the final boss room of Peper Mario: The Thousand Year Door, the boss turns regular candles into dark candles. I don't think it's possible to have a light sources that's like those candles except that it actually makes the room darker. One way you could have that type of light source is to have a source of particles ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16691",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 14,
"answer_id": 10
} |
Quality factor for a quantum oscillator? I've been reading papers about nanomechanical oscillators, and the concept of quality factor often pops up. I understand to some extent about Q factor in classical sense, but since nanomechanic oscillators are often treated quantumly, what does Q factor mean then?
| There is no difference in interpretation. In both classical and quantum oscillators, if they have any dampening, the Q factor is higher the lower the dampening is. In quantum mechanics, it is common to relate the dampening to the half-life, but as far as I can tell, there is no further difference.
EDIT: the definition ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16724",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
How long does it take for expanding space to double in size? I have been reading about Hubble's constant and trying to make 'sense' of the theory of the expanding Universe. Is is possible that space in the universe expands uniformly? If so, absent of other forces (ie gravity), how long does it take for the distance b... | The way to answer your question is to take the Friedman equation and put in the components you want to. In the Standard Model of Cosmology you'd put in radiation, matter and lamdba. You then solve the equation for the scale factor $a(t)$. (This can be automated with a program like Mathematica.) You'll get an explicit $... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16781",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 4,
"answer_id": 2
} |
Red shift and time distortion Superman throws a light emitting object away from himself fast enough to notice a red-shift. The object passes through a region in which time runs more slowly. From Superman's perspective, does the red-shift change as the object slows or is the light's appearance unaffected?
| The object doesn't slow in this case, it speeds up. The redshift is increased. The clock-slowing-down factor is more traditionally called the gravitational potential, and where clocks are slow, this is close to a massive object. In the extreme limit that the light-emitting-object is approaching a black hole, time stops... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16867",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Theoretical basis for black hole evaporation What is the basis for black hole evaporation?
I understand that Hawking-radiation is emitted at the event horizon, a theoretical result originating in General Relativity and Quantum Field Theory, but it seems to me that additionaly one has to assert an integral conservation ... | The Hawking radiation is calculated by the methods of "quantum field theory on curved backgrounds". One may show that the final state contains the thermal radiation (corresponding to the right, Hawking temperature) for any field that propagates on the black hole background.
Quantum field theory on a curved background t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16924",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Relativistic transformation of the wave packet length Let us suppose we have an excited atom at rest. It has a certain mean lifetime $\tau_0$. If we wait sufficiently long time $t>>\tau_0$, we will find a deactivated atom and a (spherical) electromagnetic wave function of photon with about $\tau_0\cdot c$ long layer wi... | No, the probabilistic density isn't Lorentz-invariant. First of all, the probabilistic density isn't a Lorentz scalar: it is a time component of a 4-vector whose spatial components encode the probability current.
Second, it is obviously not true that the ring will be equally thick in all directions. While it's true th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/17062",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
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