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What is the dominant form of heat transfer between warm water and cold air?
If a $100 mg$ drop of water falls through $-40 C$ air, how quickly could it freeze?
Is it credible that in very cold weather spit freezes in the half a second it takes to reach the ground?
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In the book "Vectors, Tensors, and the Basic Equations of Fluid Mechanics" by Rutherford Aris I read the following:
If the fluid is such that the torques
within it arise only as the moments of
direct forces we shall call it
nonpolar. A polar fluid is one that is
capable of transmitting stress couples
and bei... |
I recently found out that a PET scan stands for a positron emission tomography. Are there any other practical uses of antimatter in the present?
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Let's suppose there is a straight rigid bar with height $h$ and center of mass at the middle of height $h/2$. Now if the bar is vertically upright from ground, how long will it take to fall on the ground and what is the equation of motion of the center of mass (Lagrangian)?
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Is randomness based on lack of knowledge or behavior of universe is true random?
Or in other words,
are the allegation by EPR about hidden variable in the QM theory justifiable? What evidence can disprove/prove EPR?
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Given what we know about space, time and the movement of galaxies, have we or can we determine what our position is in relation to the projected location of the Big Bang? I've read some introductory papers on the superstructure and galaxy cluster movements, but none of them specifically mentioned space in terms of rela... |
A big argument by the nitrogen-in-the-tire crowd is that:
Nitrogen atoms are bigger and thus less likely to escape the tire, bringing stability to your tire pressure.
If Earth's atmosphere is %78.084 percent nitrogen, then the non-Nitrogen composition is ~22%. If this 22% is more likely to seep out of the tire, and N... |
In 2009, Rowan University released a paper claiming to replicate Blacklight Power's results on energy generation using hydrino states of the hydrogen atom. The paper (link now dead) appears to describe the procedure in every detail as far as my untrained eye can tell.
The press release 11/29/10 states:
Cranbury, NJ (N... |
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 it getting some type of charge... |
For an experiment I will hopefully be soon conducting at Johns Hopkins I need two different lenses.
The first needs to allow all wavelengths above 500 nm to pass (thus a high pass filter) and cut off everything else.
The second needs to allow all wavelengths below 370 nm to pass (thus a low pass filter) and cut off eve... |
Of course, assuming your grandmother is not a theoretical physicist.
I'd like to hear the basics concepts that make LQG tick and the way it relates to the GR. I heard about spin-networks where one assigns Lie groups representations to the edges and intertwining operators to the nodes of the graph but at the moment I ha... |
Does the force of kinetic friction increase with the relative speed of the objects involved?
I have heard and read that the answer is no. This is counter intuitive, and is a big part of why the "Airplane on a Treadmill" question is so interesting.
What phenomena are at work with kinetic friction, and why does it not... |
A type-I superconductor can expel almost all magnetic flux (below some critical value $H_c$) from its interior when superconducting. Light as we know is an electromagnetic wave. So what would happen if we shine a light deep into a type-I superconductor? Suppose the magnetic field of the light is significantly lower tha... |
From my understanding objects do not fall but are pulled to the earth from gravity.
With this in mind, I can't understand why if helium filled balloons are not pulled by gravity then shouldn't they be stationary in the sky (or drift like objects in space with no gravity) rather than actively move away from the earth?
I... |
In its simplest form, equivalence principle states that the inertial mass and the gravitational mass should be the same. This is easy to understand.
But why is it so important to the formulation of General Relativity? To be more specific, I don't understand how the gravitational field equation:
$$G_{\mu\nu}+\Lambda g_{... |
I've always heard that watering plants if the temperature goes a few degrees below freezing will help prevent them from freezing, but I've never quite understood the physics behind it. Can you guys help me to understand this a bit better? Thanks!
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I've heard that turning on faucets to a slight drip will prevent pipes from freezing, but I've never understood why this is the case. Can anyone out there help me to understand? Thanks!
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This is a second question (in what will probably become a series) in my attempt to understand LQG a little. In the previous question I asked about the general concepts behind LQG to which space_cadet provided a very nice answer.
Let me summarize that answer so that I can check that I've understood it properly. In LQG, ... |
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 if I do drink it), and I can't mix it with alcohol in case I get ... |
I apologise in advance if this question doesn't appeal to the advanced questions being asked in this Physics forum, but I'm a great fan of the Stack Exchange software and would trust the answers provided here to be more correct than that of Yahoo! Answers etc.
A car is travelling with a constant speed of 80km/h and pas... |
I'm looking for primer material on the modelling the physics of liquids. In particular I want to make a small simulation (I'm a programmer by profession) of throwing ink at a board, much like this.
http://www.pond5.com/stock-footage/670934/ink-splatter.html
I'm researching ways to approach the problem before I start ba... |
Regarding general relativity:
What is the physical meaning of the Christoffel symbol ($\Gamma^i_{\ jk}$)?
What are the (preferably physical) differences between the Riemann curvature tensor ($R^i_{\ jkl}$), Ricci tensor ($R_{ij}$) and Ricci scalar ($R$)? For example why do the Einstein equations include the Ricci tens... |
I saw this mind boggling result that if the tires don't slip then the work done by an engine to move a car is zero. Why is this true? Moreover, what does this truly mean?
Update: Sorry about not being clear, but I was talking about an idealised case where air resistance is negligible and the tires are perfectly rigid a... |
What happens to the light [energy] after it enters an eye and hits the rods and cones? I presume the energy becomes electrical, and it must be near 100% perfect, else our eyes would heat up? Or am I missing something?
The motivation of this question is solar panel technology.
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As per Newton objects with mass attract each other, and per Einstein this is further explained by saying that mass warps space-time. So a massive object makes a "dent" into space-time,
a gravity well. I have taken to visualizing this as placing a object on a rubber sheet and the
resulting dent, being the gravity field.... |
Reading a previous closed question an interesting variation has come to my mind.
Suppose that time travel to the past was possible:
I wait for an atom to decay and measure the time, $t_{1a}$
I travel back in time at $t_0<t_{1a}$
I wait for the same atom to decay and measure the time, $t_{1b}$
Let's think about the t... |
Some friends and I were at Disneyland this past week. We spent an hour arguing back and forth whether it is possible for someone to throw an NFL football over the Matterhorn? I think it is, but no one else really thought it was possible.
What do you guys think? Is this humanly possible?
The Matterhorn is 146 feet (4... |
Does the weak nuclear force play a role (positive or negative) in nuclear binding?
Normally you only see discussions about weak decay and flavour changing physics, but is there a contribution to nuclear binding when a proton and neutron exchange a $W^\pm$ and thus exchange places? Or do $Z$ exchanges / neutral currents... |
Is it possible to counter-act g-force for a jet-pilot, by him putting on a scuba-diving suit
and filling the cockpit with water? On earth we are constantly pulled down, or accelerated with one g. In this situation, if we put the jet-pilot in a pool, he would neither sink nor float.
If we could increase Earth's gravity ... |
What happens if you blow bubbles into a glass of lemonade on the international space station?
Since you are weightless in orbit, there's no up, down, left nor right. We define down on the Earth surface as the direction things tend to fall, down. On Earth bubbles rise if blown into a glass of lemonade (with a straw and... |
If you could travel to the center of the Earth (or any planet), would you be weightless there?
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Possible Duplicate:
Do current models of particle physics explain the chemical properties of elements/compounds?
Is there a rule/pattern, or multiple rules and/or patterns, which describe the properties of an element given the number of protons and neutrons it has. For example a carbon atom which has 6 protons and 6... |
What is the significance about the bell shape, when its hit at the rim it rings/produces sound better than other shaped objects? If so could anyone explain a little bit on it.
EDIT: From the suggestions in the comments, clarification for the term "sound better": Sound more effective for the purpose which bells are crea... |
I admit, it's been a few years since I've studied physics, but the following question came to me when I was listening to a talk by Lawrence Krauss.
Is there any knowledge of from where matter that exists today originated? I recall that the law of conservation of mass asserts that matter cannot be created nor destroyed,... |
Except for Mercury, the planets in the Solar System have very small eccentricities.
Is this property special to the Solar System? Wikipedia states:
Most exoplanets with orbital periods of 20 days or less have near-circular orbits of very low eccentricity. That is believed to be due to tidal circularization, an effect... |
Earth's perihelion passed about nine hours ago. How accurately do we know the moment of closest approach of the Earth to the center of the sun? How do we make this measurement?
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in recent questions like "How are classical optics phenomena explained in QED (Snell's law)?" and "Do photons gain mass when they travel through glass?" we could learn something about effective properties of matter interacting with a force field in terms of the path integral and quasiparticles.
Surely, both approaches ... |
Below is a picture of Giant Water Lily. Scientific Name: Victoria Amazonica. Leaves of some of these could be as big as 3 m diameter and carry a weight of 45kg spread evenly and can support a child. Now the problem:
Suppose that a leaf of such flower with a child is floating freely on water. Child crawls along the ed... |
I am trying to get a common understanding from these two previous questions:
Why does the mass of an object increase when its speed approaches that of light?
What happens if light/particles exceeded the speed of light for a particular medium (sic)
Does the increase of mass occur only if the particle approaches c (sp... |
A friend of mine asked me what is the flux of the electric field (or any vector field like
$$
\vec r=(x,y,z)\mapsto \frac{\vec r}{|r|^3}
$$
(where $|r|=(x^2+y^2+z^2)^{1/2}$) through a Möbius strip. It seems to me there are no ways to compute it in the "standard" way because the strip is not orientable, but if I think ... |
Is there any reason to believe that any measure of loudness (e.g. sound pressure) might have an upper boundary, similar to upper limit (c) of the speed of mass?
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Could you please recommend any good texts on algebraic geometry (just over the complex numbers rather than arbitrary fields) and on complex geometry including Kahler manifolds that could serve as an informal introduction to the subject for a theoretical physicist (having in mind the applications in physics, e.g. in the... |
What is the advantage for rockets to have multiple stages?
Wouldn't a single stage with the same amount of fuel weigh less?
Note I would like a quantitative answer, if possible :-)
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I know that string theory is still under heavy development, and as such it still can't make predictions (or not that many predictions anyways).
On the other hand, it is clear from the number of years this theory has been under development and from the large number of theoretical physicists studying it, that it is consi... |
As a trade I am a software architect. It does happen that I have to design sites where the servers are located in different parts of the world.
Now an essential parameter to consider is latency: that is the time it takes between when a packet is sent from one server, and when it is received on the other side.
Of course... |
I don't know much about light-emitting diodes, but I imaging if you had a panel of RGB diodes you could produce any wavelength of color within the visible light spectrum. However, if I
also wanted to generate specific wavelengths of UVA or UVB (anywhere from 290 to 400nm), could I also accomplish this using diodes? E... |
I read an article which tells power consumption by many devices. It say that a desktop computer (computer and monitor) use 400 to 600 watt.
While when i checked my computer and monitor with meter, it was about 60 + 60 = 120 watt (computer + 17" CRT monitor) after loading windows xp and running an application. The volta... |
Mass m is connected to the end of a cord at length R above its rotational axis (the axis is parallel to the horizon, the position of the mass is perpendicular to the horizon). It is given an initial velocity, V0, at a direction parallel to the horizon. The initial state is depicted at position A in the image.
The force... |
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)?
|
What are the different death scenarios for a black hole? I know they can evaporate through Hawking radiation - but is there any other way? What if you just kept shoveling more and more mass and energy into the black hole?
|
Imagine a circuit with a voltage source, a switch and an inductivity all connected in series.
First, the switch is open and there's no current and no magnetic field around. If we close the switch, the potential difference of the voltage source is instantaneously applied to the inductivity. Lenz's Law tells us that the ... |
Suppose a black hole were to suck up a massive amount of surrounding oxygen and hydrogen. Would this ignite inside the black hole and turn it back into a star?
|
Many light sources like LEDs and lasers only emit a single wavelength of light.
Is there a light source that emits all wavelengths of visible light at the same time?
|
Quite a while ago I read about a series of experiments that basically suggested that a certain kind of particle/atom/(something) were "intelligent" and could appear in two places at once, or essentially could "tell the future" when it came to navigating a "maze" ...I think it might have involved lasers or mirrors?
Does... |
EDIT: I edited the question to reflect Moshe's objections. Please, look at it again.
It's apparently a black hole time around here so I decided to ask a question of my own.
After a few generic questions about black holes, I am wondering whether string theory is able to provide something beyond the usual semiclassical ... |
I'm guessing this isn't a great physics question, but I just can't find an answer with Google.
If the galactic plane is perfectly horizontal at what angle and rotation will the ecliptic plane of our Solar System be?
|
It is true for fermions in the same potential that the total wavefunction of two particles must be antisymmetric with respect to exchange of electrons. Which means the spin wavefunction is given by
$\chi=\frac{1}{\sqrt{2}}[\chi_+ (1)\chi_- (2)-\chi_+ (2)\chi_- (1)] $
which looks very much like the bell state,
$\beta_... |
On one hand, in the classical electrodynamics polarization of transparent media yields in lowering the speed of light by the factor of $n=\sqrt{\epsilon_r \mu_r}$ (refractive index).
On the other, in the quantum field theory the vacuum polarization does not decrease the speed of light. The thing it does is increase of ... |
A lot of string-theory questions have been asked here. This one is, hopefully, different in that this inquiry is specific and the expected answer would be more mathematical than philosophical in character.
To give some context, I'm familiar with LQG, QFT etc. I'm a beginner in CFT and string-theory. On reading papers o... |
shortly after the invention of quantum electrodynamics, one discovered that the theory had some very bad properties. It took twenty years to discover that certain infinities could be overcome by a process called renormalization.
One might state the physical reason behind this that we are only aware of effective theorie... |
I understand that energy conservation is not a rule in general relativity, but I'd like to know under what circumstances it can still be possible. In other words, when is it possible to associate a potential energy to the gravitational field, so that the energy is constant in the evolution of the system?
Here are some ... |
In a recent discussion about black holes, space_cadet provided me with the following paper of Rovelli: Black Hole Entropy from Loop Quantum Gravity which claims to derive the Bekenstein-Hawking formula for the entropy of the black hole.
Parts of his derivation seem strange to me, so I hope someone will able to clarify ... |
I'm in the process of trying to understand X-Ray Absorption Fine Structure (XAFS) spectroscopy, but I think this question is more fundamental than merely as it pertains to XAFS. I think I don't entirely understand the interplay between electronic, vibrational, and rotational excitations in molecules. I understand tha... |
If you take two ideal gases at different temperatures, and allow them to share energy through heat, they'll eventually reach a thermodynamic equilibrium state that has higher entropy than the original. The time evolution of that entropy increase is easy to predict, because we know the time evolution of $T$ and $U$ for ... |
Background: I have written a pop-science book explaining quantum mechanics through imaginary conversations with my dog-- the dog serves as a sort of reader surrogate, popping in occasionally to ask questions that a non-scientist might ask-- and I am now working on a sequel. In the sequel, I find myself having to talk a... |
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 heat but I knew this is wrong, since heat is simply ... |
Having read the Wikipedia page on superfluids I'm still not sure if stuff like liquid helium at the lambda point actually have surface tension or not. Is superfluidity the same thing? And are there any liquids with no surface tension at room temperature?
|
In elasticity theory, general equations of motion are:
$$\rho \partial^2_t \overline{u} = \mu \nabla^2 \overline{u} + (\mu+\lambda) \nabla(\nabla \cdot \overline{u})$$
where $\overline u$ are displacements.
In a thin homogeneus rod with tractions and compressions in the x direction, stress tensor $S_{ij}$ is:
$$ \pmatr... |
Let's suppose I have a Hilbert space $K = L^2(X)$ equipped with a Hamiltonian $H$ such that the Schrödinger equation with respect to $H$ on $K$ describes some boson I'm interested in, and I want to create and annihilate a bunch of these bosons. So I construct the bosonic Fock space
$$S(K) = \bigoplus_{i \ge 0} S^i(K)$... |
What is the difference between thermodynamics and statistical mechanics?
|
I got a question concerning the band strucutre of solids.
The reference I'm using is the book on solid state physics by Ashcroft/Mermin.
My problem is that I don't completely understand the reason why there exists a
band-index n. On the one hand Ashcroft gives a quite plausible explanation
saying that the general sol... |
And expansion of space is equal to expansion of vacuum?
|
So, I remember in college physics the prof using liquid nitrogen in a demonstration. When he was done, he threw the container of LN at the front row, and of course it evaporated (or whatever) before it got to the students.
I am watching a cooking show now and they are using LN -- if they touched that, what would happe... |
This story about the use of battery/freewheel based Frequency Regulators confused me about how the 60hz frequency of the North American power grid was set--saying that it was kept at that frequency by balancing load and supply. I used to think that it was only voltage which was affected by this balance, and that the fr... |
I've always assumed/been told that Newton's 2nd law is an empirical law — it must be discovered by experiment. If this is the case, what experiments did Newton do to discover this? Is it related to his studies of the motion of the moon and earth? Was he able to analyze this data to see that the masses were inversely re... |
I've heard it from a few people, and I've seen it popup here in the site a couple of times. There seems to be speculation (and studies?) towards this idea, and this is what I've picked up so far:
Living organisms are low entropy systems.
The second law of thermodynamics is somehow strongly connected to the origin of l... |
In loop quantum gravity, the canonical (Ashtekar) variables are chosen to be the densitized inverse triad $\mathbf{E}$ and some rotation connection field $\mathbf{A}$. To get the ordinary triad from $\mathbf{E}$, we need to take its "square root". But we know as long as the volume factor, i.e. the square root of the de... |
A sun jar is an object that stores solar energy in a battery and then releases it during dark hours through a led.
Assume:
a $65cm^2$ solar panel
a 12h/12h light/dark cycle
insolation of $2.61kWh/m^2/day$
perfectly efficient components (i.e. without violating Entropy laws or other theoretical limits)
light is emitted... |
I'm doing magnification and lens in class currently, and I really don't get why virtual and real images are called what they are.
A virtual image occurs the object is less than the focal length of the lens from the lens, and a real image occurs when an object is further than focal length.
By why virtual and real? What'... |
Initially Wheeler and Feynman postulated that, the electromagnetic field is just a set of bookkeeping variables required in a Hamiltonian description. This is very neat because makes the point of divergent vacuum energy a moot point (i.e: an example of asking the wrong question)
However, a few years later (1951), Feyn... |
You've probably ridden on the fairground ride called the "Tilt-A-Whirl", or--as Disneyland calls it--the "Spinning Teacups", as well as other fairground rides that employ epicycles. You can really feel the centrifugal force strongly when your car spins complementary to the spin of the main rotor.
Now, can that extra f... |
Consider that we have a system described by a wavefunction $\psi(x)$. We then make an exact copy of the system, and anything associated with it, (including the inner cogs and gears of the elementary paticles, if any, aswell as the fabric of spacetime), but where all distances are multiplied by a number $k$, so $\psi(x)... |
Why, when we calculate the total cross section, we make the average other initial states and the sum over final states?
|
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 about 6000K. The target will then heat up to 6000K and s... |
Assume a static metric with (known) components $g_{\mu\nu}$. I'd like to know what is the gravitational pull $g$ of a test particle placed on an arbitrary point $X$.
The gravitational pull being defined as the acceleration the particle suffers as measured by an observer sitting in a reference frame fixed at the origin.... |
We're all familiar with basic tenets such as "information cannot be transmitted faster than light" and ideas such as information conservation in scenarios like Hawking radiation (and in general, obviously). The Holographic Principle says, loosely, that information about a volume of space is encoded on its two-dimension... |
I'm wondering what some standard, modern references might be for continuum mechanics. I imagine most references are probably more used by mechanical engineers than physicists but it's still a classical mechanics question.
This came up in a conversation with my father (who is a mechanical engineer). I was curious to ... |
According to Noether's theorem, all conservation laws originate from invariance of a system to shifts in a certain space. For example conservation of energy stems from invariance to time translation.
What kind of symmetry creates the conservation of mass?
|
Here's the situation: a friend is working on a civil engineering project to make some equipment to decompress liquefied natural gas before pumping to households. I don't know what the proper term for this bit of kit is calld but I will call it the decompresser
Apparently what happens in these stations is that the gas ... |
It is known that any accelerated observer is subject to a heat bath due to Unruh radiation. The principle of equivalence suggests that any stationary observer on the surface of a massive body should also experience heat bath. Indeed by substituting surface gravity g into the formula for Unruh radiation one can find the... |
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?
|
Over and over I'm getting into the same trouble, so I'd like to ask for some help.
I need to solve some basic electrodynamics problem, involving magnetic fields, moving charges or currents.
But I forgot all this rules about "where the magnetic field should go if the current flows up". I vaguely remember that it abou... |
I was searching for the eigensolutions of the two-dimensional Schrödinger equation
$$\mathrm{i}\hbar \partial_t \mid \psi \rangle = \frac{\mathbf{p}^2}{2m_e}\mid \psi \rangle + V \mid \psi \rangle$$
where the potential is given by
$$V(\rho, \varphi)=\begin{cases} V_1 & \rho < R \\ -V_2 & \rho \geq R \end{cases}$$
u... |
What are the main practical applications that a Bose-Einstein condensate can have?
|
In industry, it's common to cut steel using 50-200 W $\mathrm{CO}_2$ laser.
Meanwhile, for making holes in PCB (35 $\mu$m of Cu), DPSS 355 nm laser is usually used and secondary $\mathrm{CO}_2$ to drill through glass.
Why $CO_2$ cuts steel easily, but cannot be used to make holes in Copper?
UPDATE: Will it help to add... |
I've seen foliation used in the context of "foliation of spacetime" here and elsewhere in papers and such. Generally defined in reference to a "sequence of spatial hypersurfaces." But I don't know what that means either.
Again, I can imagine what these terms mean because of the English language meaning of the words. Bu... |
Suppose you have to specify the moment in time when a given event occurred, a "zero time". The record must be accurate to the minute, and be obtainable even after thousands of years. All the measures of time we currently have are relative to a well defined zero, but the zero is not easy to backtrack exactly.
One possib... |
I have been taught that the $\pi^0$ particle contains either an up quark and an anti-up quark or a down and an anti-down. How can these exist without annihilating?
Also, it is its own antiparticle, but it doesn't make sense that the up version and down version would annihilate when they meet.
Or is what I've been taugh... |
Given today's revelation of the detection of terrestrial gammay-ray flashes (TGF) produced by thunderstorms and the associated pair production of an electron and a positron, how feasible is it to set up ground-based gamma ray detection station on the cheap using a Geiger-Muller tube?
The electron/positron pair travels ... |
Another Noether's theorem question, this time about electrical charge.
According to Noether's theorem, all conservation laws originate from invariance of a system to shifts in a certain space. For example conservation of energy stems from invariance to time translation.
What kind of symmetry creates the conservation of... |
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