Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Current from induced emf If the induced emf in a circuit is negative, and current from this emf is the emf over the resistance, what happens to the negative sign in the induced emf when solving for the current? Surely there's no such thing as "negative" current?
For example, if we have a constant magnetic field that is... | Negative current just means it flows opposite to the chosen direction.
Take your example, you have a loop with increasing area and downward pointing B-field
x x x x x x x x x
+-------------|----------- ^
x | x x x x | x x x | y
| | ---> h ^
x | x x x x | x x x ... | {
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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... | There are two reasons a new moon can never be seen at night. The first is that by definition, new moon means 100% of Moon's unilluminated side faces Earth. The second is that when the first condition is met, Moon is sufficiently close (in angular separation) to Sun that it cannot be above the horizon at local nighttime... | {
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How long does it take an iceberg to melt in the ocean? This is a quantitative question. The problem is inspired by this event:
On August 5, 2010, an enormous chunk of ice, roughly 97 square miles (251 square kilometers) in size, broke off the Petermann Glacier along the northwestern coast of Greenland. The Petermann Gl... | Heat enters through the surface of the iceberg. This surface area obeys $$A \propto r^2,$$ with $r$ some linear measure of the size of the iceberg (e.g. the radius). The heat that has entered therefore obeys $$Q \propto r^2 t,$$ with $t$ the time for melting.
The heat required to melt an iceberg depends on mass, which ... | {
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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... | Energy is being expended maintaining it in position. Earth's gravity is applying a force downwards, the book is being accelerated down gravitation force.
A force is being applied to the hand and arm which must be resisted and thus energy expended.
The arm and book are not a closed system.
| {
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Can I levitate an object without using an electromagnet? I know that it's possible to make an object levitate using an electromagnet to hold it up.
But is it also possible to do this with regular magnets? Is there a special kind of magnet I need in order to have one powerful enough to hold an object up?
I'm asking bec... | Earnshaw's theorem proves that using only static ferromagnetism (like refridgerator magnets, which have a very slowly changing magnetic field) it is impossible to stably levitate against gravity.
http://en.wikipedia.org/wiki/Earnshaw%27s_theorem.
Diamagnetic materials might be used to levitate objects.
| {
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Driving on snowy roads 'tis the season as they say!
It seems to me obvious that it's better to drive in high gear on snowy roads to reduce the torque.
However, there are completely opposite advices being given on different sites:
*
*weather.com says "Use low gears to keep traction"
*bbc.co.uk says "Stay in a higher... | The issue is that the available friction coefficient is highest when the tire is not sliding.
Once it starts sliding, it has friction, but less than when it was not.
For example, once the tire starts sliding, the heat of sliding friction melts a thin layer of water, which acts as a lubricant.
The advantage of using a h... | {
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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?
| Laser light is spatially and temporally coherent. The stimulated emission is mainly responsible for the temporal coherence.
So the answer is yes, you can create an electromagnetic beam that is spatially but not temporally coherent by placing a pinhole close to the source, and then another pinhole in the far field of th... | {
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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... | Here is an interesting site about this idea and similar ideas:
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3
Essentially the problem with this idea is there are no such thing as perfectly rigid bodies. So as you push, it sends a little compression wave through the material, which travels at t... | {
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Aspherical lenses - perfect analytical shape It's known that single spherical lens cannot focus parallel beam of monochromatic light into single (diffraction-limited) point, so it has to have aspherical shape to achieve that.
Is perfect analytical lens shape is known that is able to focus light into a single point (aga... | Thorlabs has a little information on aspheric lens design on their product page (scroll down below the product pictures and click on "Lens Formula"). Note that no traditional lens can focus light down to a single point; the minimum size is subject to the diffraction limit and is in the order of the wavelength.
| {
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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 ... | IMO this is only obtained with an electromagnet inside this cone structure. The cuts only help to spin the fluid moving up/downwards.
Here is a "more verbose" example of such a structure: http://www.youtube.com/watch?v=OE2pB1pyZN0
| {
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How can I prevent drinking water from freezing in cold weather? I'd like to keep a container of potable water in my car for when I get stuck, but in the winter it gets cold enough for bottled water to freeze.
I can't put salt in it, because then it won't be drinkable anymore (or at least, will make my condition worse i... | It may not be the answer you are looking for but I recommend you get a thermos or a well insulated flask. These are what mountaineers use and you do not have to change the chemical composition of water this way.
| {
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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)?
| This answer provides a variation of @Marek's answer
that tries to address
@vonjd's concern about the non-inertial observer (in green)
trying to be considered "inertial" (instead of the blue inertial observer).
First, as others have already said, general relativity is not needed to analyze this problem. Special relati... | {
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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... | When you lose weight, it is mostly fat that disappears from under your skin (or someone else's skin). How does the fat get out? Most of it doesn't directly evaporate. Instead, it is burned much like fuel in your car.
Fat contains mainly carbon, hydrogen, and oxygen, much like typical organic compounds. When it's "idea... | {
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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... | Theoretically the answer is yes. That is because the sun is not a blackbody emitter, there is an excess of UV radiation. So if you were able to achieve radiative equilibrium with only UV light (which is maybe 1% of BB radiation at those temps), you could do it. Practically, I'd think it would be just about impossible, ... | {
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Magnetron limits What are the practical limits on generated wavelength in a Magnetron?
We know that Magnetrons could be used efficiently for generating microwaves for water heating, or for radar applications, but what are the achivable wavelength limits?
| I interpret the question as "which frequencies/wavelengths can be generated by cavity magnetrons?"
As far as the physics is concerned, I do not believe there are fundamental limits that constrain the frequencies generated. These frequencies are dictated by the cavity dimensions (see http://en.wikipedia.org/wiki/Cavity_... | {
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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?
| Yes, gravitational waves can be shielded. As gravitons are the particles which can interact with matter, the matter can absorb gravitons.
This would depend on the wavelength however. For uniform substance the rate of absorption would generally decrease with the wavelength. The rate of absorption also can be increased ... | {
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Lagrangian density of linear elastic solid I need the general expression for the lagrangian density of a linear elastic solid. I haven't been able to find this anywhere. Thanks.
| the Lagrangian density is $L=T-U$, the difference between the kinetic and potential energy density, as we're used to in all of mechanics.
The kinetic energy density is $T = \rho v(x,y,z)^2/2$ where one has to calculate the density $\rho$ properly. The potential energy is more general and complicated,
$$U = \frac{1}{2}... | {
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Accuracy of the Boltzmann equation I have had this question for some time now. Hopefully someone can answer it.
I know that the Boltzmann equation is widely regarded as a cornerstone of statistical mechanics and many applications have been explored with a linearized version.
I also know that it's extremely hard to obt... | The Boltzmann equation is an approximation. It is obtained by closing the so-called BBGKY hierarchy that links N-particle distribution functions to N+1 particle distribution functions. The closing is obtained by introducing an assumption, the 'molecular chaos' assumption. In short, molecular chaos asumes the absence of... | {
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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? ... | I think it might be clearest to first consider the case of a rope NOT connected to anything; a free, straightened rope in empty space. Each point along the rope feels the same effective force due to Hubble expansion; each point along the rope is slightly repelled by each of its neighboring points. Assuming the expansio... | {
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Where can I find a good classification for phase transitions? I'm having a hard time to find a good (and modern) classification scheme for phase transitions and related universality classes. Can someone recommend a paper/book/site? Detailed mathematical aspects are very much welcome.
| If you already know something about phase transitions and universality classes and want to know what kind of phenomena are similar to each other, I would recommend the following article:
Géza Ódor, Universality classes in nonequilibrium lattice systems, REVIEWS OF MODERN PHYSICS, VOLUME 76, JULY 2004
This contains lots... | {
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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,... | A simple explanation:
Firstly, the vortex becomes stable after you stop shaking the cup because as you shake the cup in the conventional way, the forces acting on the whole fluid are uneven, when you stop shaking the cup the vortex is able to evenly apply force to the fluid. (Friction between the sides of the cup and ... | {
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Why did the universe not collapse to a black hole shortly after the big bang? Wasn't the density of the universe at the moment after the Big Bang so great as to create a black hole? If the answer is that the universe/space-time can expand anyway what does it imply about what our universe looks like from the outside?
| I don't think that the question "what does the universe look like from the outside?" is very meaningful. Just because there is not outside for the universe. As for the black hole why should high density i.e. a lot of mass in little volume, cause the creation of a black hole? If you are thinking about the Schwarzschild ... | {
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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... | No that is not how it works. A 360 rotation multiplies the wave function by a factor -1 which by itself is not observable. It does not switch up and down spins.
An experiment which demonstrated the effect would have to involve forming interference patterns between streams of electrons where one stream was being rotated... | {
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Gauss vs $\rm mW/cm^2$: same thing? A friend of mine is concerned about electromagnetic field negatively affecting their health and got a "DMF meter" to measure the field strength in their house in units of milliGauss. They are trying to compare the reading to a background level measured in $\rm mW/cm^2$.
I am skeptica... | Are you sure they aren't webers?
$1 \ \mathrm{mWb/cm^2}$ is $10$ Tesla. Which is ridiculously huge. $1$ milligauss, by comparison, is $10^{-7}$ Tesla.
Using @David's conversion, we would have a flux density
$B_0$ = $\sqrt{\left(\frac{\mu_0}{2c} × 1.26 × 10^{-7} mW/cm^2 \right)}$
$\ \ \ \ \ \ \approx \ 5×10^{-11} T$
whi... | {
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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... | The Reference Frame
it is the only one that is challenging.
| {
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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're identical. Typically, we use complex fields if we have a $U(1)$ symmetry, or some more complicated gauge group with complex representations.
Incidentally, the same comment applies to whether we use Majorana spinors or Weyl spinors.
| {
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can a system with nested building blocks (atoms,cells) NOT be "fine tuned"? Never mind whether the universe is "fine tuned" for anything in particular,just the idea that there is a nested hierarchy seems incredibly constraining on the outcome, and anything but accidental.
| You have to define exactly what you mean by 'nested hierarchy', but I suppose you deliberately pose your question in loose terms, and just wonder about systems that can be meaningfully described in markedly different ways at a hierarchy of levels.
Let's take a simple system as example: Conway's 'Game Of Life' (GoL). W... | {
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Linear sigma models for F-Theory Is it possible to describe the Local F-theory models which seems to be relevant for particle physics phenomenology using Gauged Linear Sigma Models (GLSM), this includes describing the decoupling limit using the parameters of the GLSM? Actually, I am not sure if my question makes sense.... | concerning the first question, I don't think so. If I understand you well, you want to use GLSM's for a world sheet description of the vacuum. But a world sheet description of any string theory is only possible if the coupling constant in almost all of space is weak because the world sheet is the history of a (weakly c... | {
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Charge density in concentric spheres Question:
If there are two conducting spherical shells and the inner shell is grounded, what will be >the charge density in the inner shell if there is a charge Q placed on the outer shell?
Yes, this is a HW problem, but I am not asking you guys to solve it for me... just show me... | Firstly assume that you know the charge on the inner shell $q_{in}$. Then calculate the electric field between the shells $E_{in}$ and the field outside shells $E_{out}$ by using Gauss law. Know by knowing that the potential at the $\infty$ is 0 and that when conductor is grounded has the same potential, you can write ... | {
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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... | It goes to the brighter strips. In these regions there is a constructive interference, which is actually brighter than would be with just one source.
| {
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Examples of circuitry using proton currents Proton cuircuits and proton motive force are part of standard discussion in biology and processes involving photosynthesis. The sort of proton currents discussed in biology are obviously slightly different than the magnetically confined proton currents in an acclerator like ... | One cannot use the example of lightning since that has nothing to do with proton current. The way lighting works is when to clouds cross each other and "rub" against each other one cloud removes a lot of electrons from the other. One cloud will have extra electrons and one cloud have missing electrons.The one missing t... | {
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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... | The gravitational force of the spaceship on the astronaut is tiny, effectively zero. It would take a really huge object (e.g. a small moon) to have any appreciable effect on an astronaut or anything else.
If an astronaut is in a spaceship and the spaceship accelerates, for all practical purposes, the astronaut will not... | {
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Mechanical energy problem Question goes: "An anvil hanging vertically from a long rope in a barn is pulled to the side and raised like a pendulum 1.6 m above its equilibrium position. It then swings to its lowermost point where the rope is cut by a sharp blade. The anvil then has a horizontal velocity with which it sai... | Just a quick comment:
It is always best to give every quantity a symbol and perform all algebraic manipulations on the symbols; you then substitute in numerical factors at the very end.
In this particular case, you can call $h=1.6\textrm{ m}$ and $H=10\textrm{ m}$, which makes your calculation look like:
$$mgh=\frac{1}... | {
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Why are there only derivatives to the first order in the Lagrangian? Why is the Lagrangian a function of the position and velocity (possibly also of time) and why are dependences on higher order derivatives (acceleration, jerk,...) excluded?
Is there a good reason for this or is it simply "because it works".
| Well, the usual physics in classical mechanics is formulated in terms of second-order differential equations. If you are familiar with the process of deriving Euler-Lagrange equations from the Lagrangian then it should be natural that the kinetic term must be proportional to $(\partial_t x)^2$ to reproduce that.
If you... | {
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Why beauty is a good guide in physics? Dirac once said that he was mainly guided by mathematical beauty more than anything else in his discovery of the famous Dirac equation. Most of the deepest equations of physics are also the most beautiful ones e.g. Maxwell's equations of classical electrodynamics, Einstein's equat... | Take a preference for "beautiful" theory as a meta-rule like Occam's Razor.
It seems to work a lot of the time, but like the Razor, it must give way to data.
| {
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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... | My brother, an electrical engineer, used to carry around a 1 megaohm resistor
during the dry winter months when you easily get a shock after walking across a carpet and touching a light switch or another person. If you hold one lead of the resistor in your hand and touch the light switch or whatever else you are touchi... | {
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Can one make an axial mode circularly polarized radio wave filter with purely conducting materials? Can one construct a filter out of nothing but pure metal that will pass only right circular polarized radiation and reflect left circularly polarized radiation?
What would it look like? A helical spring in a tube?
What ... | From the way your question is written, I believe that you already know how to make a wave guide that will split horizontal from vertical waves. I'm assuming that you also know how to combine and split radio waves. Furthermore, I'm going to assume that you can build a "splitter", that is a device where radio waves come ... | {
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Does entropy apply to Newton's First Law or does "acted upon" always require an external factor?
First law: Every body remains in a state of rest or uniform motion (constant velocity) unless it is acted upon by an external unbalanced force. This means that in the absence of a non-zero net force, the center of mass of ... | Yes, it will (in the classical picture) continue forever, even with its entropy increasing, the entropy increase just means that some of the potential energy within the body will turn into heat (=kinetic energy), however the center of mass motion is unaffected.
Entropy increase says nothing about slowing down, rather t... | {
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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... | In the RNS formalism the vertex operators for Ramond fields are not polynomials in the worldsheet fields. If you want to include them as part of the background, you'd have to deal with non-polynomial actions on the worldsheet. This is technically difficult.
One way to deal with this difficulty is to use a different me... | {
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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... | Another reason is that if n did not equal 2, some of the symmetries of the Newton equation would be lost. For example, classically, physics on a microscopic scale is time reversal invariant. We can see this from the Newton equation because if x(t) is sent to x(-t), the 2nd time derivative ensures that the negatives can... | {
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Why are the lighter halogens gases? In the periodic table of the elements, only a few of the elements are gases at standard temperature and pressure. Those elements include the noble gases some of the halogens, and a few of the elements next to them, i.e. oxygen and nitrogen.
So why are these elements gases?
| It is basically for the reason Lagerbaer states. They have one open p-orbital available and bind in diatomic molecules. The perturbation on the other closed p-orbitals is small so there is little dipolar interaction. You have to make then very cold to get a liquid.
I don’t know what liquid Cl would be like, but some... | {
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What are the most important discoveries/breakthroughs in physics recently? Can you provide a list of the most important discoveries/breakthroughs in physics recently?
By recent, I mean the past decade or so. All branches of physics are welcome.
Basically, I am interested in major physics breakthroughs/discoveries which... | 1) Black hole radiation (Hawking radiation)
2) Discovery of the dark energy
3) Evaluation of the BH entropy from string theory.
| {
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What causes gravity? What causes gravity? Why is there attraction between masses? Is it due to time or space distortion?
| Wikipedia (on Aether Theory) cites Isaac Newton:
"Doth not this aethereal medium in passing out of water, glass, crystal, and other compact and dense bodies in empty spaces, grow denser and denser by degrees, and by that means refract the rays of light not in a point, but by bending them gradually in curve lines? ...... | {
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What is the wavefunction of the observer himself? I am aware about different interpretations of quantum mechanics out there but would mostly like to see an answer from the perspective of Copenhagen interpretation (or relative quantum mechanics if you wish).
Let an observer being a man with brain consisting of molecules... | Paraphrasing from a book I read a long time ago (Quantum Measurement by Braginskii):
It is possible to write down a wave function for a human, but it will not be comprehensible. It is necessary to define the initial states of all the elementary particles from which the observer is made. However, the amount of informa... | {
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What does it mean for a Hamiltonian or system to be gapped or gapless? I've read some papers recently that talk about gapped Hamiltonians or gapless systems, but what does it mean?
Edit: Is an XX spin chain in a magnetic field gapped? Why or why not?
| Gapped or gapless is a distinction between continuous and discrete spectra of low energy excitations. For a Hamiltonian $H$ with gapped spectrum, the first excited state has an energy eigenvalue $E_1$ that is separated by a gap $\Delta > 0$ from the ground state $E_0$. For example, a dispersion relation of the form $E ... | {
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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... | Since the word "supersymmetry" did not appear in your list of forbidden words let me give you this answer:
Because Calabi-Yau manifolds leave unbroken some part of the original supersymmetry, which is advantageous for model building.
But there are alternatives to Calabi-Yaus, like flux compactififcations or large extra... | {
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What is the 4th dimension? I have heard before that the 4th dimension is time, however, another theory makes a lot more sense to me. This is that the 4th dimension is the third dimension stacked on top of each other in a similar in which 3d objects are just many 2d planes.
I have seen many articles related to the 4thdi... | It is all in what you want to describe mathematically. You can have an $N$ dimensional space and yes, you could "visualize" the analogue of two dimensions going into three. These are Euclidean spaces, i.e. the metric is $ds^2=dx^2+dy^2+dz^2+\dots$ up to $N$ terms.
Time is the fourth dimension in current physics because... | {
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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
*
*... | Obviously, the option 3 is the right option. There is no violation of the 2nd law, since the emission rate is higher than the absorption rate for the hotter body.
Consider a closed system of two objects. One is hotter and the other is colder. What will happen? Both will radiate energy and both will absorb energy from t... | {
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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 ... | Without having heard this argument before I would guess that the plan it to reduce the degree to which the head rattles around.
Most of the brain damage (short term and long term) associate with a punch comes from the brain bouncing off the skull a few times as the head whips back and forth. Minimize the motion, minimi... | {
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Cooling a cup of coffee with help of a spoon During breakfast with my colleagues, a question popped into my head:
What is the fastest method to cool a cup of coffee, if your only available instrument is a spoon?
A qualitative answer would be nice, but if we could find a mathematical model or even better make the experi... | With the movement of the spoon you give energy to the particles and molecules.In this way the molecules increase their kinetic energy and many of them escape from the cup of the tea.Therefore the use of a spoon for cooling a cup of tea is valid!
| {
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What does it take to become a top physicist? What does it take to become a top physicist?
Why do so many extremely talented young upstarts totally flop as they move to more advanced physics?
| *
*How to become a good theoretical physicist by Gerard 't Hooft (Nobel Laureate)
*Hang out here :-)
*Always remember the reason why are you doing physics.
Here is my reason (and it also happens to be Richard Feynman's too :)
Physics is like sex. It may produce some practical results. But that is not why we do it.... | {
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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... | dmckee has given a good, thorough answer. I'd bet that there are lots of other interactions that haven't been seen. How about $\gamma+\gamma\to Z+\overline Z$, or $\gamma+\gamma\to t\overline t$? To see those reactions, you'd need high-energy photons, and the cross sections should be very low. I doubt that we've produc... | {
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What is the medium that allows magnetic fields *or any forcefield* to exist? Magnetic fields are obvious distortions.. of.. something, but what exactly are they distortions of? Massive objects produce curvatures/gradients in space-time resulting in what we observe as gravity.. what is the equivalent explanation for ma... | The short answer is nothing at all - fields are a fundamental part of the universe.
What dmckee is teasing you about in the comments is that your intuition saying that there must be a luminiferous aether, which is an idea that was actually conclusively disproven in the late 19th century by Michelson and Morley in one ... | {
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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?
| I suspect the statement was meant to be about electrical discharges via dielectric breakdowm. Such as lightening bolts etc. As such it has partial validity, in that say a tall tree is more likely to be hit than a short one. But the reality is that dielectric breakdown is a chaotic process, which is why lightening appea... | {
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A die versus a quantum experiment Let suppose you roll a die, and it falls into a hidden place, for example under furniture.
Then although the experiment has already been made (the die already has a number to show), that value can not be known, so the experiment was not fully realized.
Then till you see the die's top s... | A somewhat similar (yet inverted) question was posed, infamously, by Einstein, Podolsky, and Rosen as an argument against the same phenomenon you are challenging.
The basic argument was that such quantum wave collapses are indistinguishable from dice falling under couches, but under opposite grounds as yours -- both mu... | {
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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... | I can answer for experimental particle physics. The answer is yes.
Seniors are instrumental in motivating and, as you say, getting the money, overseeing that the experiment runs in group decisions, since experiments involve many groups. If they are any good in physics they also have ideas on analysis, but the bulk of ... | {
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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... | Speaking of patterns, you may want to read Shive's Similarities in Physics. It discusses common behaviour patterns such as filtering, dispersion, resonance, interference, exponential decay, noise, etc which can be observed across acoustics, optics, mechanics and electronics.
There is a book by Pikovsky et al Synchro... | {
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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.
| The DIY aluminum foil approach if you have a HV supply:
http://wiki.4hv.org/index.php/Nitrogen_TEA_laser
http://www.sparkbangbuzz.com/tealaser/tealaser7.htm
http://www.repairfaq.org/sam/lasercn2.htm
This is based on the TEA approach with nitrogen from the air (or from other sources) as the lasing material.
| {
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Find drop-off rate of magnetic interference from a mass of pure iron on a magnetic compass
How can I find the magnetic interference of a stationary $35000 \ \mathrm{kg}$ block of $100\%$ pure iron would have on a magnetic compass and what the drop off rate of the interference would be?
So if said $35000 \ \mathrm{kg... | Specific density of iron is $7.87$ times water, so
$35000 \ \mathrm{kg}$ of iron $= 35/7.87 = 4.45 \ \mathrm{m}^3$
Assuming the block is spherical, this is a sphere with radius $1.02 \ \mathrm{m}$. So I'm assuming you mean $1 \ \mathrm{m}$ from the surface of the block.
At these distances you can more or less approxim... | {
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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?
| Just to note that people could leave research because they may not have "research grant funding" skills or aptitude.
A researcher who has invested a few years in a sub-field X may become almost a world expert in an aspect of X - maybe in one of the experimental/computational/calculational aspects. Unfortunately fashio... | {
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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.... | I personally don't know of particular books dedicated to the subject covering all areas of physics (maybe "Mathematical Methods for Physics and Engineering" by Riley, Hobson and Bence isn't quite what you're looking for), but if you happen to come across the subject of Quantum Field Theory then I suggest you have a loo... | {
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Arguments for and against Many Worlds? I would like to hear the best arguments for and against the Many Worlds interpretation of QM.
| In addition to the many good answers already posted, Scott Aaronson points out that the MWI interpretation is rather "brittle", in the sense that the discovery of pretty much any tiny experimental deviation from the Schrodinger equation would probably require completely scrapping the MWI interpretation. It's hard to im... | {
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Neutron star references? I'm looking for contemporary reviews on neutron stars. Seems like this area is pretty active, so even reviews from five or ten years ago are somewhat lacking, though certainly not worthless. Does anyone have recommendations? Newer is better.
Books are okay too, but I'd prefer one of those 70-is... | There are rather new lecture notes (1 year old) by my colleague Andreas Schnmitt focusing on the microscopic aspects of compact stars (formerly known as neutron stars): see here.
It is more 140-ish than 70-ish, but the layout is rather generous with space, so it might be exactly what you were looking for.
| {
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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... | A nice set of books have been made available through project Gutenberg, are made available through the Open Ebooks Library, including:
Handbook of Formula and Physical constants
Space, Time and Gravitation - An Outline of the General Relativity Theory, Sir Arthur Stanley Eddington (beware of this book)
Relativity : the... | {
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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... | I do not know any chemistry-for-physicists textbook, but Chemistry: The Central Science is a good textbook that gives a general picture of chemistry. I do not know what do you mean by "that takes advantage of thermodynamics, statistical mechanics, and quantum mechanics".
| {
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Does a photon interfere only with itself? I sometimes hear statements like:
Quantum-mechanically, an interference pattern occurs due to quantum interference of the wavefunction of a photon. The wavefunction of a single photon only interferes with itself. Different photons (for example from different atoms) do not inte... | No. Even though such claim is in Dirac's classical work it is not true.
See e.g. Hong-Ou-Mandel interference, when exactly two photons interfere (they can be even from different sources). For quotation of Paul Dirac, and some more analysis, see:
*
*Andrzej Dragan, Paweł Ziń, Interference of Fock states in a single m... | {
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Does hot air really rise? "Heat rises" or "warm air rises" is a widely used phrase (and widely accepted phenomenon).
Does hot air really rise? Or is it simply displaced by colder (denser) air pulled down by gravity?
| The Action force is simultaneous with the Reaction force. One can not happen without the other.
Archimedes settled that a less dense fluid move on top of a denser fluid (see Buoyancy - Archimedes' principle) .
(and vice-versa: a denser..moves..to bottom..)
The Rayleigh-Taylor instability describes the evolution of the... | {
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I need help with finding distance traveled How do I find the distance traveled of an object if the speed is not constant?
| You use integral calculus. The distance traveled is the integral of the speed over time.
If the speed were constant, the distance traveled would be the speed multiplied by the time.
If the speed is changing, we don't know what speed to use. The solution is to break time up into little chunks - one minute, say. How f... | {
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Are water waves (i.e. on the surface of the ocean) longitudinal or transverse? I'm convinced that water waves for example:
are a combination of longitudinal and transverse. Any references or proofs of this or otherwise?
| In deep waters, the fluid particles describe circles when a wave passes by. So, in a sense, these waves are neither transverse nor longitidinal. For a demonstration, see for example Howard Georgi's book (chapter 11).
In very shallow waters the particles go essentially back and forth. In the intermediate cases they foll... | {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 6,
"answer_id": 2
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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... | Since long ago there exists Relativistic Theory of Gravity (RTG) by A.A. Logunov with co-authors where gravity is a physical field in the Minkowski space-time acting on matter as an effective metric of an effective Riemann space-time. It carries energy-momentum and is different from pure geometry. In this theory there ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
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"answer_id": 6
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Why is the decibel scale logarithmic? Could someone explain in simple terms (let's say, limited to a high school calculus vocabulary) why decibels are measured on a logarithmic scale?
(This isn't homework, just good old fashioned curiousity.)
| Human senses, nearly all, work in a manner and obey Weber–Fetcher law, that response of the sense machinery is logarithm of an input. It is true at least for hearing, but also for eye sensitivity, temperature sense etc. And of course, in areas where it works normally. Because in extreme, there are other processes such ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6588",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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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... | A slightly different answer to this is "neutralisation". That is the free positive ions around (in gas clouds nearby maybe) will neutralise the charge. This is generally assumed to keep the charge of a Black Hole near to zero in astrophysical contexts.
| {
"language": "en",
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"source": "stackexchange",
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"answer_id": 4
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Angular momentum and EM wave Is there any sense in saying that circularly polarized EM waves have angular momentum?
| Yes. Electromagnetic waves carry energy and momentum, and can carry angular momentum.
A linearly polarized wave packet doesn't carry any angular momentum (measured about an axis through its center -- a linearly polarized wave packet moving past you off to one side has angular momentum about an axis located where you a... | {
"language": "en",
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"source": "stackexchange",
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How can earthquakes shift the earth's axis? One often comes across news articles that claim that an earthquake shifted the earth's axis.
http://news.google.com/?q=earthquake%20shifted%20OR%20shifts%20earth%27s%20axis
If you ignore the influence of other celestial bodies, an internal event like an earthquake surely ca... | There is a confusion in the words "axis of rotation". These words have been used for the vector "angular momentum", which doesn't change in absence of external torque or forces. The other vector is the vector "instantaneous rotation", which can change its orientation during the movement, even if the rotating solid does... | {
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Find true anomaly given period, eccentricity and time Is it possible to find the true anomaly of an object in a Kepler orbit given the orbital period of the object, the orbital eccentricity and the time? Assuming a two body system and the mass of the orbiting body is negligible.
I'm doing this computationally so I'd li... | Kepler's equation: $M=E-\varepsilon\cdot \sin( E)$, you need to solve for $E$.
The inverse problem may be solved (according to wikipedia) as follows:
$E = \begin{cases} \displaystyle \sum_{n=1}^{\infty} {\frac{M^{\frac{n}{3}}}{n!}} \lim_{\theta \to 0} \left( \frac{\mathrm{d}^{\,n-1}}{\mathrm{d}\theta^{\,n-1}} \left( \f... | {
"language": "en",
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"source": "stackexchange",
"question_score": "3",
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The size of the planets What calculations were done to discover the size of all the planets?
| For exoplanets, the technique used depends on the aspect of "size" being measured.
Masses of exoplanets are estimated by using doppler spectroscopy to determine the acceleration of the planet's parent star caused by the orbiting planet, while diameters are estimated by observing the percentage of a the parent star's li... | {
"language": "en",
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Giant string in space I saw an interview with Ed Witten, where he said one way to confirm string-theory is to observe a giant string floating in space, left over from the Big-Bang.
How does one observe such a string, does it have thickness?
Is this string any different then the strings that (hypothetically) make up ele... | This paper: http://arxiv.org/PS_cache/astro-ph/pdf/0302/0302547v1.pdf was a speculative look at a possible double galaxy image. They considered that it had been mirrored down the centre by a cosmic string to repeat the galaxy. It turned out to be untrue however when looked at in more detail by hubble.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6898",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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"answer_count": 3,
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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 question amounts to whether this thing can be buried away. Yes, and that will have to happen, and the sooner the better. The complex of 4 reactors is what is called in the military a cluster f**k --- a total loss and disaster. Gorbachev in the wave of the Chernobyl meltdown ordered the military to fly helicopter... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Why does the light side of the moon appear not to line up correctly with the evening sun? I live at roughly $(52.4^\circ,-2.1^\circ)$. On sunny evenings I've often looked at the Moon and the Sun and noticed that the light part of the Moon does not appear to line up with the Sun. For example, at about 17:00 GMT on 13 Ma... | I think it is a parallax effect/optical illusion, and I'm not confident of explaining this clearly but here goes!
The normal vector to the illuminated portion of the moon is pointing generally away from the Earth/moon system towards a point over our horizon. At low altitudes (evenings) the sun will be close to the hor... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/6969",
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"source": "stackexchange",
"question_score": "13",
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Long time deviations from exponential decay in radioactivity Are there any examples of common substances whose decay is not exponential?
We're used to thinking about radioactivity in terms of half-lives. This is a concept that makes sense only for a decay that is exponential. However, there are plenty of physics articl... | http://arxiv.org/abs/1304.6885 There are many papers on apparent sinusoidal modulation of overall beta-decay. One explanation is competing pathways with different kinetics whose rates add to beats. The obvious non-exponential decay case is electron-capture decay of a fully ionized atom.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7008",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "24",
"answer_count": 3,
"answer_id": 2
} |
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?
| But it does have a critical mass. Otherwise it would not become critical.
What you avoid in a well designed power reactor is 'prompt' criticality where an active core becomes supercritical on prompt neutrons only. Normally a critical reactor is stable on a fraction of prompt neutrons (neutrons released at the instant o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7149",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "12",
"answer_count": 9,
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What properties do you need for building a tower? When I was a boy I used to daydream about building a tower so tall that the top of it would project into near space.
There would perhaps be a zero gravity area in the penthouse where my friends and I could bounce around and play space versions of various earth-based gam... | Re: the original question:
If you wanted "zero gravity" at the top of the tower, you'd have to build a tower tall enough to reach the height of geostationary orbit: a point at which the orbital period of an object in freefall matches the time it takes the earth to rotate once. As other commenters have pointed out, tha... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7441",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 7,
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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?... | All physics that we know obeys the principle of relativity, which states that it is impossible to tell whether the train is moving at a constant speed or not without looking outside. In a real train, you can tell, but only because the train ride is bumpy and the train changes speeds. In a perfectly-smooth train, it w... | {
"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": 1
} |
Mech stability through gyroscope I recently read up about gyroscopes, angular momentum and mechs (the big Cockpit controlled robots) and was wondering if it would be possible to get a stable walking mech (only as example, not meant to be a serious attempt http://autopixx.de/bilder/lM23mgzq/battle-mech.jpg) by adding a ... | Short answer: I believe, yes, it is possible.
But what you likely will want is a Control Moment Gyroscope (CMG) - multiple "gimbaled" gyros operating in tandem, with their orientation controlled by small actuators and a computer. With this approach you can not only help keep things balanced but shift large amounts o... | {
"language": "en",
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"source": "stackexchange",
"question_score": "4",
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"answer_id": 1
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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... | Probably not, lighting uses a decreasing proportion of the power load these days.
In the UK the grid used to carefully monitor TV, during the commercial breaks in popular soaps the power draw would go up by giga-watts as everybody made tea! This was in the days when 20M people watched the same show and in a country wit... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7576",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Internal Energy and entropy in a open system Here http://en.wikipedia.org/wiki/Internal_energy we can read
$$U= TS-PV+\sum_i \mu_i N_i$$
Let's suppose i=1 and a ideal gas. We know:
$$U=N/N_A c_v T$$;
$$PV=NKT$$;
$$\mu=\frac{\partial U}{\partial N}= c_v T/N_A$$;
so $S=\frac{U+PV-\mu N}{T}$ would be $
NK
$
but it's no... | The problem here is that the thermodynamic potentials are functions of three thermodynamic variables each. Now, each thermodynamic potential has a set of natural variables. For the internal energy $U$, these are S, V and N.
Now, your partial derivatives should explicitly state which other variables are held constant. ... | {
"language": "en",
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What's the relevance of Efimov physics? Is it something unexpected?Why universality in cold atomic gases is important?What researches are looking for?Can this be useful for topological quantum computers?
Can we expect a whole myriad of these states?
http://arxiv.org/abs/1012.2698
thanks
| Efimov physics is interesting because it's a neat bit of fundamental physics describing something we have a tendency to neglect: 3-body quantum scattering problems. As for being unexpected -- I'm not sure what you want; if Efimov states were expected then V.N. Efimov wouldn't have had to discover them (already more tha... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7733",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
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Space-like Killing vector of Robertson-Walker metric? In the book "Kinetic theory in the expanding Universe" (J. Bernstein, 1988, Camb. Univ. Press), it was stated that
"for nonstationary Robertson-Walker
matrixes [sic] there is no spacelike
Killing vector."
(page 6, footnote.)
But we know this is not true, sinc... | The de Sitter metric
$$
ds^2~=~dt^2~-~e^{\sqrt{\Lambda/3}t}(dr^2~+~r^2d\Omega^2)
$$
has this time dependent factor. This prevents a time-like Killing vector, for any vector formed by $\sqrt{g_{rr}^{-1}}\partial/\partial t$, or something similar, will not constant on a timelike spacetime vector. The same holds for ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/7818",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 1
} |
Salt water conduction Does salt water conduct mostly by the ions travelling through the solution, or by electrons collectively flowing or hopping through the solution like in metals?
| Let's look at pure Water with pure Sodium Chloride in it. We know that in Sodium Chloride, the Sodium atoms give one electron to the Chlorine. This results in both atoms having closed shells.
For electrons to collectively flow/hop through a material, you need to have electron states that are delocalized over many atom... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
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Massless charged particles Are there any massless (zero invariant mass) particles carrying electric charge?
If not, why not? Do we expect to see any or are they a theoretical impossibility?
| Suppose such a particle existed.
Question is what would happen if it was to enter an electric field?
Consider $p$ ($m = 0$, $q > 0$) entering an electric field $E_i$, on a manifold $M (i,j)$
$$F_i = q E_i \; \; \;\text{but} \; \; \; F_i = m a_i$$
It follows that $F_i = 0$ since $m = 0$ meaning either $q = 0$ or $E = 0$... | {
"language": "en",
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"source": "stackexchange",
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What does "soft" in "soft symmetry breaking" mean? For example it is stated that if supersymmetry breaking is soft then stability of gauge hierarchy can be still maintained.
| I saw the words "soft breaking" used without relation to SUSY. I doubt that there is a strict definition of this, but naively it seems to mean the following thing:
A symmetry of your model can be broken either
*
*Spontaneously -- with some of your fields receiving non-invariant vacuum expectation value.
*Or explici... | {
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What happens to chemical compounds that include radioactive nuclei, when those decay? Say you have a chemical compound made up of one or more radioactive nuclei. If these nucleus decay, does the compound decay as well?
Possible outcomes I can think of:
*
*the compounds continues to exist if a bonding is still possibl... | Take the instance of radioactive material like 131-I. Some atoms called excited atoms emit ionizing radiations. After ionizing radiation emission, those excited 131-I atoms become 131-Xe atoms. These excited remain in between stable iodine atoms. These stable iodine atoms remain unchanged.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/8081",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
"answer_count": 2,
"answer_id": 1
} |
Measuring the spin of a single electron Is it possible to measure the spin of a single electron? What papers have been published on answering this question? Would the measurement require a super sensitive SQUID, Superconductive Quantum Interference Device?
| First you need to be assured of a source of single electrons. A good one is from spontaneous decay called conversion electrons. Then you set up a Stern Gerlach magnet setup. The problem is that one would need to cancel out the transverse Lorentz force, and this can be done with a transverse uniform electric field to... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/8188",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 4,
"answer_id": 0
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Why are mirror images flipped horizontally but not vertically? Why is it that when you look in the mirror left and right directions appear flipped, but not the up and down?
| Try this:
*
*Make a paper cut-out of the letter F (or any easy-to-cut-out letter which is different from its horizontally and vertically flipped images). Shade or mark one side of the cut-out to distinguish it from the other. Get a second piece of paper and write the same letter on it several times.
*Hold the cut-o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/8227",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "363",
"answer_count": 29,
"answer_id": 5
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How much of a star falls into a black hole? http://blogs.discovermagazine.com/badastronomy/2011/04/05/astronomers-may-have-witnessed-a-star-torn-apart-by-a-black-hole/
A lot of the star in the disc, a lot of the star in the jets, precisely how much of the star actually falls into the black hole?
| There is no correct answer for all cases. Of course the upper-bound is 100% but in practice, even if nearly all of the matter would fall in, a huge amount of energy will be radiated as x-rays and $\gamma$-rays due to heating in the accretion disk.
The dynamics of in-falling matter are quite complicated and very sensi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/8294",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
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What are the conditions to be satisfied by a theory in order to be a quantum theory? This is in continuation to my previous question. It is not a duplicate of the previous one. This question arises because of the answers and discussions in that question.
Can we call a theory, quantum theory, if it is consistent with HU... | A quantum theory is a theory that obeys the postulates of quantum mechanics, see e.g.
http://en.wikipedia.org/wiki/Postulates_of_quantum_mechanics#Postulates_of_quantum_mechanics
One may choose a slightly different wording of the conditions, add some detailed ones or unify the conditions above to a smaller list, but ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/8396",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Why do all the planets of the solar system orbit in roughly the same 2D plane?
*
*Most images you see of the solar system are 2D and all planets orbit in the same plane. In a 3D view, are really all planets orbiting in similar planes? Is there a reason for this? I'd expect that the orbits distributed all around the s... | *
*More or less yes. The planets mostly orbit in the same plane but with small deviations compared to the size of the system. The largest relative tilt is around 4 to 6 degrees. This 'flatness' is due to orbital mechanics, where the solar system started spherical it now has 'decayed' into a flat disc. This disc is ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/8502",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
"answer_count": 4,
"answer_id": 0
} |
What exactly is heat? Is it energy?
Is it energy per unit volume?
Is it energy per unit time i.e power?
What is it?
| "Thermal Energy", Wikipedia.
Read about molecular / statistical description of thermal energy (which is not heat) you would really get the feel what it really is.
As far as the rigorous description of heat is in terms of entropy, let me denote it by $q$ and entropy by $s$ ($\mathrm{d}q$ is total differential of $q$), t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/8522",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "41",
"answer_count": 6,
"answer_id": 4
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cosmic radiation muons as a source for muon-catalyzed fusion taking rough numbers from here and here, it seems that with natural cosmic radiation sources, we could sustain $10^6$ muon-catalyzed fusion reactions per square-meter per minute. This would be $14 \times 10^6$ MeV of neutron energy (for D-T reactions, assumin... | OK, so the effin muons are too fast to be useful in the ground frame. How inconvenient?
So, obviously, that means we have to accelerate the h, d, t etc. fuel particles to the same frame as the muons. (Or, alternatively, electrostatically decelerate the muons to the ground frame).
For example, drill a 3 km-deep bore ho... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/8589",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 2
} |
Can a disk like object (like UFO's) really fly? UFOs as shown in movies are shown as disk like objects with raised centers that emit some sort of light from bottom. Can such a thing fly?
My very limited knowledge in physics tell me that a disk like object may not be able to maneuver unless it has thrusters on sides and... | Beam propulsion has already been done here on Earth. :-) http://en.wikipedia.org/wiki/Beam-powered_propulsion
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/8640",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 5,
"answer_id": 0
} |
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