Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Strange modulation of radioactive decay rates with solar activity Recently I found out this strange article about nuclear decay rates somehow showing seasonal variations with a high correlation with sun activity.
*
*Has this been experimentally confirmed/disproved? An experiment using neutrinos from a fission reacto... | A quite detailed article (in german though) discussing this "effect" can be found here:
*
*Radioaktivität: Wer hat an der Uhr gedreht?
At the end of the article there is a list of interesting related papers and literature. According to this article first experiments trying to use artificially produced neutrinos to ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/9421",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
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In what order should the subjects be studied in order to get to String Theory I know:
*
*Quantum Mechanics (Griffiths Level, currently doing Sakurai Level)
*Mechanics (Newtonian+ Lagrangian/Hamiltonian but at level lower than Goldstein/Landau)
*Classical Electrodynamics (Griffiths + electro/magnetostatics from Ja... | I would say Special Relativity, then General Relativity, and finally QFT.
Books:
Special Relativity -- I would recommend Wheeler & Taylor and Woodhouse
General Relativity -- Woodhouse
QFT -- Zee and Aitchison & Hey
There's also an undergrad physics book [junior level] on String Theory by Zwieback that's highly recommen... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/9468",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Laplace's equation: Why is $\ell$ to be a non-negative integer? I have got some mathematical difficulties in the following exercise:
Calculate the potential of the polarized sphere along the $z$-axis. There are no free charges.
For this, we need to solve Laplace's equation, by using the method of separation of variab... | It need not be positive but can also be zero. So let's suppose you're asking why it has to be non-negative.
The answer to this question is that Laplace's operator $\Delta$ splits into radial and angular parts (these are precisely the operators that can be found in your equations, if you discard the $\phi$ dependence). ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Is this a weather phenomenon or an instrumental artifact? The radar image of the midwest provided by Weatherunderground at 10:30 PM Central time, May 8 2011 has odd patterns.
Are these patterns real? Perhaps caused by large scale convection over cities? Or are they artifacts of radar placement?
Here is the image that ... | I have a master's degree in meteorology so I think I can clear this up for you!
This is simply ground clutter. You will see this sort of thing happening on evenings where the relative humidity is very high, more so when the mixing ratio is high also. The radar beam can actually start to interact with water droplets... | {
"language": "en",
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$cm^3/g$ as a unit of adsorption I recently saw $cm^3/g$ as a unit for amount adsorbed. Usually, you see either $\mathrm{kg_{adsorbate}/kg_{adsorbent}}$ or $\mathrm{mole_{adsorbate}/kg_{adsorbent}}$. Does anyone know the meaning of this unit?
| In the case of $cm^3/g$, the quantity of gas adsorbed (the adsorbate) is expressed as its volume in gaseous form at STP. These units are still commonally used although the International Union of Pure and Applied Chemistry (IUPAC) recommends that the quantity adsorbed be described in units of $mol/g$. The quantity of a... | {
"language": "en",
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Why is $\frac{dx}{dt}=0$ in this average momentum calculation? In the following excerpt from S. Gasiorowicz's Quantum Physics, he derives an expression for the average momentum of a free particle. $\psi(x,t)$ is the wave function of a free particle, $\psi^*$ denotes its complex conjugate.
We try the following: since c... | This $x$ is a position in the reference frame's coordinate system, which is just static by design. You can imagine it as a ruler, with a probability cloud in a foreground; the ruler stays on its place while the cloud moves and deforms changing its mean position.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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What is "pure energy" in matter-antimatter annihilation made of? I used to read the term "pure energy" in the context of matter-antimatter annihilation. Is the "pure energy" spoken of photons? Is it some form of heat? Some kind of particles with mass?
Basically, what does "pure energy" in the context of matter-antim... | All the three above answers basically are correct and saying almost the same thing, i.e. the end product of matter-antimatter collision is photon at low energies and massive particles such as mesons... at high energies. However,by thinking of the reverse action namely production of matter from a form of energy ( radiat... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Feedback on the paper, 'CCC-predicted low-variance circles in CMB sky and LCDM' by V. G. Gurzadyan and R. Penrose Ref: CCC-predicted low-variance circles in CMB sky and LCDM
To all cosmology / theoretical physics / related or similar researchers and academics,
Are there some updates concerning the issue of these concen... | It's very easy to go wrong in analyzing microwave background maps for this sort of signal. In particular, unless you're quite used to doing this sort of analysis, it's easy to make mistakes in calculating the probability of getting a false positive -- that is, of seeing patterns like the ones you're looking for, even i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/9850",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
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If dark matter is a new type of particle, what does that imply? My understanding is that dark matter cannot be (or is at least highly unlikely to be) an exotic form of any known particle. On the other hand, articles about particle accelerators seem to say that the Higgs is the last piece missing in the Standard Model j... | Let me address this part of the question, as the physics part is covered by Ted Bunn.
If dark matter is determined to be some form of new particle, what are the certain implications? Might such a discovery "stand to the side" of the Standard Model or would it certainly change the foundations?
Up to now, progress in... | {
"language": "en",
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Online QFT video lectures I'm aware of Sidney Coleman's 1975/76 sequence of 54 lectures on Quantum Field Theory. Are there any other high-quality QFT lecture series available online?
| The Perimeter Institute has recorded all the courses of their one-year Master's in Theoretical Physics. The courses are given by professors and lecturers from different universities.
For a full list of the courses they recorded in 2010/w011 go to Pirsa.org, and under "4. Collections" select "Course" and "2010". The cou... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/10021",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "43",
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Is there any simple proof of the no-ghost theorem? Is there any simple proof of the no-ghost theorem in string theory?
| The proof using DDF formalism involves constructing a set of operators that commute with the Virasoro operators, and when applied to the ground state, they give all possible physical states. These operators $A^{i}_{n}$, where $i$ runs over $d-2$ transverse dimensions of spacetime and $n$ is an arbitrary integer generat... | {
"language": "en",
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Gravitational time dilation at the earth's center I would like to know what happens with time dilation (relative to surface) at earth's center .
There is a way to calculate it?
Is time going faster at center of earth?
I've made other questions about this matter and the answers refers to:
$\Delta\Phi$ (difference in Ne... | The CENTER of the earth will not have more gravity, but less. This is because half the mass will be "above" half "below" ( Regardless of orientation)...Sort of less g and in different direction/vectors. The thing is mass is not concentrated at a spot in the center with more and more g as one moves closer to the center... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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What are the conditions for decoherence to be irreversible? Spin echo experiments have been able to reverse the motions of all the molecules in a gas in statistical mechanics in the manner of Loschmidt. The Fermi-Ulam-Pasta model has solutions with a single mode dispersing, only to recohere after quite some time has el... | Decoherence in a quantum mechanical system appears because the phase information is lost. If it were recorded/known then, as in your first example, one could reverse it. The answer for me seems to be complexity.
In some sense there exists a single quantum mechanical state function of the universe. To record it one wou... | {
"language": "en",
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Is the wave function objective or subjective? Here is a question I am curious about.
Is the wave function objective or subjective, or is such a question meaningless?
Conventionally, subjectivity is as follows: if a quantity is subjective then it is possible for two different people to legitimately give it different val... | A wave function is as objective as a photo picture of somebody. It encodes an objective information about something. It consist, as any information, of many bits of information.
At the same time is is subjective because it depends on our perception. For one some information tells nothing, for somebody else it may be ve... | {
"language": "en",
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"source": "stackexchange",
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What happens if you connect a hot resistor to a cold resistor? Kind of an extension to this question:
If you heat up an object, and put it in contact with a colder object, in an ideal insulated box, the heat from one will transfer to the other through thermal conduction and they will eventually reach an equilibrium tem... | Actually, that's exactly what happens: heat is transported by freely moving electrons. - but you wouldn't call it noise current, you would call it heat transfer, regardless of whether it's phonons or electrons which carry the heat.
The whole thing really has nothing to do with the resistors, it would work equally well ... | {
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Graduate Physics Problems Books Need to brush up on my late-undergrad and early-grad physics and was wondering if anyone can recommend books or lecture notes (hard copy, or on-line) that also have solutions.
Two that I have come across are:
Princeton Problems in Physics with Solutions - Nathan Newbury
University of Ch... | Some review/problem set books that I like are:
Solid State Physics: problems and solutions - Laszalo Mihaly and Michael C. Martin
Problems in Quantum Mechanics with Solutions - Gordon Leslie Squires (The reviews are bare bones but I find the questions to be very good at making you think)
Hope this helps. I am intereste... | {
"language": "en",
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"source": "stackexchange",
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Do quantum states contain exponentially more information than classical states? Do quantum states contain exponentially more information than classical states? It might seem so at first sight, but what about in light of this talk?
| Experts agree that one qubit stores no more information than one bit.
But a zillion identical qubits (which you can prepare or manufacture to your specifications, even though you can not clone an unknown qubit) contains about a zillion times more information than a zillion identical bits which still contains only a si... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/10364",
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Why don't waves erase out each other when looking onto a wall? If I stand exactly in front of a colorful wall, I imagine the light waves they emit, and they receive should randomly double or erase out each other.
So as a result, I imagine I should see a weird combination of colors, or a full-black/full-white/very ligh... | I will like to point out that the walls do not emit any light but instead they reflect what shines on them. You may also be interested in reading more about diffuse reflection which roughly and non mathematically attempts at explaining some of the physics at work here. If you are looking for a more mathematical treatme... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/10410",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Why does the road look like it's wet on hot days? Often, I'll be driving down the road on a summer day, and as I look ahead toward the horizon, I notice that the road looks like there's a puddle of water on it, or that it was somehow wet. Of course, as I get closer, the effect disappears.
I know that it is some kind of... | It is a mirage: in particular it is caused by hot air near the road and less hot air above it creates a gradient in the refractive index of the air and so making a virtual image of the sky appear to be on or below the road. Air currents make this shimmer, similar to a reflection of the sky on water, hence causing the ... | {
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Why doesn't a phone charge faster, rather than slower when it is in use In Physics class, we were building parallel circuits, and as more lights were attached in parallel, they got brighter (as more power was being provided to the lights, and the resistance decreases). So, when I charge a phone, why doesn't the battery... | Assuming your phone is typical, the charger can only generate a limited current. When the phone is off, all the current from the charger goes to the battery. When the phone is on, the current from the charger goes partly to charging the battery and partly to running the phone. Hence the battery takes longer to charge.
| {
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How do laser rangefinders work when the object surface is not perpendicular to the laser beam? I find the functioning of a laser rangefinder confusing.
The explanation usually goes like this: "you shine a laser beam onto the object, the laser beam gets reflected and gets back to the device and time required for that is... | Some laser rangefinding uses a retroreflector, which will bounce the laser light back in the direction it came regardless of orientation.
Otherwise, lasers operate at a very specific frequency, so the signal/noise ratio only needs to be strong enough to be detectable at that frequency.
If you shine a normal laser point... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Can heat be transfered via magnetic field in a vacuum? Say you want to store hot coffee in a container surrounded by a vacuum. To remove all sources of conductive energy loss the container is suspended in the vacuum by a magnetic field and does not have a physical connection to the sides of the vacuum chamber,
My quest... | No, a static magnetic field does not conduct heat.
The only way for thermal energy to cross a vacuum is by radiation. Of course, radiative heat transfer actually does consist of electric and magnetic fields: hot objects give off electromagnetic waves, which travel through the vacuum and are absorbed by, say, the walls ... | {
"language": "en",
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Why photons transfer to electrons perpendicular momentum? Linear antenna directed along z, photons (EM waves) propagate along x. Momentum of photons have only x component. Why electrons in antenna have z component of momentum?
| There are 2 different effects of the EM waves:
*
*Acceleration of charges due to EM field
*Radiation pressure of the EM field
If the EM wave propagates along x direction and your antenna is along the z direction, then electrons will be driven along the antenna by the electric field. On the other hand, the antenna... | {
"language": "en",
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Can the effects of gravity be broken by jumping? I was having a debate the other day with a work colleague where I explained that gravity is a weak force because it is easily broken. Then I remembered a lecture by someone, I forget who, that explained gravity is very weak because you can break its influence just by jum... | Well, keep in mind gravity isn't being "broken" when you jump, it is still exerting a force of approximately F_g=G * m_1 * m_2 / r^2, where G is the gravitational constant (6.677 * 10^(-11)), m_1 and m_2 are the two masses, and r is the distance between them.
The idea of gravity being a weak force, relative the electro... | {
"language": "en",
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Will the sun cool and produce a disrupting EMP?
Scientists say rare drop in sunspot activity could cause global cooling
http://www.globalpost.com/dispatch/news/business-tech/science/110615/science-news-solar-flares-sunspots-global-warming
Last week's event is almost certain to be just the start of a cycle that is ex... | The first is front line research announced at a conference, and is news, and closer to the current truth as far as the way cycle 24 is manifesting.
The probability of a large solar eruption is another story, one large eruption was manifested even though the cycle is not yet at its maximum; there is no direct contradic... | {
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"url": "https://physics.stackexchange.com/questions/11120",
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Analyzing the motion of a ball rolling without slipping inside a hemispherical bowl Consider a solid ball of radius $r$ and mass $m$ rolling without slipping in a hemispherical bowl of radius $R$ (simple back and forth motion). Now, I assume the oscillations are small and so the small angle approximation holds. I wish ... | The answer you got in first method was wrong.
The answer in second is nearly reached, we have to take $R-r$ instead of $r$ because you have to take $r$ from centre of the sphere.
The correct answer was $2\pi\sqrt{7(R-r)/5g}$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/11227",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
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what cools bottle of water faster: ice or snow Imagine you have a pile of snow and a pile of ice shards. You put a soda bottle which has a room temperature into both piles. Which bottle is going to cool down faster?
| For one you must define room temperature. because if the bottle is worm enough to to slightly melt the show first and then refreeze then this one will cool 1st of not the air in the snow can act as an insulator and this one will take longer. Now the shape and sise of the ice will make a large contribution/small to the ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Non-Dimentionalization of equations I am trying to understand a paper in which they mention;
" We non-dimentionalize the problem hereafter by selecting mass, length and time units such that the fluid density
$\rho\equiv1$
the gravitational acceleration g
$\equiv1$
and initial tank depth
$h_0\equiv1$
".
The reson for d... | As you note, the quantity on the vertical axis of the plot is $P/\rho g h_0$. So if this quantity equals 20, then
$$
P/\rho g h_0=20,
$$
or
$$
P=20\rho g h_0.
$$
The units on both sides of this expression match, so this expression is correct in any system of units. If you want to know the pressure in SI units (pas... | {
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Ascent rate and size of balloon I am part of a school project, Project Stratos to send a balloon to the edge of space (the closer side :P) and was wondering how you would work out the accent rate of a large balloon (roughly 1m^3 of helium with 100g of mass) and the size of it as it increases its Altitude. I am creating... | Ascent rate of a balloon (assuming spherical symmetry) depends on the following forces:
(1) The upward buoyant force $F_B=\frac{4}{3}\pi r^3\rho_{air}g$
(2) The gravitational pull downwards: $F_G=\frac{4}{3}\pi r^3\rho g$
(3) The drag force acting: $F_D\, =\, \tfrac12\, \rho_{air}\, v^2\, C_D\, A$
On a first glance it ... | {
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Direct exposure to the vacuum of space I was watching a few sci-fi movies and was wondering the real science explaining what would happen if you were to be subject to the conditions of outerspace.
I read the wikipedia article on space exposure, but was still confused. If a person was about the same distance from the su... | They would freeze.
They wouldn't freeze to death - since they would die of something else (lack of oxygen) first
Although a small part of you is facing a hot sun at 6000K most of your surface is facing cold dark space at 3K. You can work out what temperature you will reach - it depends only on how reflective you are.
... | {
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"timestamp": "2023-03-29T00:00:00",
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Why is the gravitational force always attractive? Why is the gravitational force always attractive? Is there another way to explain this without the curvature of space time?
PS: If the simple answer to this question is that mass makes space-time curve in a concave fashion, I can rephrase the question as why does mass ... | The inverse square is apparently a consequence of conservation of momentum. For two particles in orbit, Newton showed that the orbit is planar, and Bertrand https://en.wikipedia.org/wiki/Bertrand%27s_theorem showed that the forces between the two have to be either of the inverse square k/r^2 or space spring/Hook's law.... | {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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How to find the principal point in an image? I need to find the principal point in an image. Its a point where the principal axis intersects the image sensor.
Due to misalignment this point is not at the center of image always(or image sensor). I need to precisely determine its location using any of the optical methods... | I suggest shining a narrow laser beam along the optical axis. Just don't fry your sensor! Hopefully, the antireflective coating of the lens is not perfect, and you will be able to see on the object side two reflections of the beam: one from each side of the lens. Align the laser in such a way that both reflections come... | {
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Clebsch-Gordan Identity I'm trying to take advantage of a particular identity for the sum of the product of three Clebsch-Gordan coefficients, however, the present form of my equation is slightly different. Is there a symmetry relation that will allow me to change:
$\sum_{\alpha\beta\delta}C_{a\alpha b\beta}^{c\gamma}... | In general you cannot make the change you suggest because of the condition on projections. In your first equation, the projections in your last CG must satisfy
$\delta +\phi=\alpha$
whereas in your second equation, the projections in your last CG must satisfy
$\alpha+\delta=\phi$.
Thus, unless there is further symme... | {
"language": "en",
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On a bicycle, why does my back tyre wear so much more quickly than the front? This question is cross-posted from Bicycles.SE, but it is really one for those that know a bit about physics.
Why does the back tyre of a bicycle wear out quicker than the front tyre?
I have my uneducated suspicions but I would appreciate an ... | When braking, most of your weight is going to be on the front tire (due to your forward momentum). This is why the front brake is so much more effective than the rear brake. That also means that, if you use the rear brake, you are pretty likely to skid (which only rarely happens on the front tire), because the weight o... | {
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How to calculate concentration of vapor at the surface of a water drop I'm reading a paper that examines the evaporation rates of water. In the final formula, it has the following constant:
$c_s - c_\infty $ where $c_s$ is the concentration of the vapor at the sphere surface and $c_\infty$ is the concentration of the ... | Very close to the liquid surface, you assume that the vapor and liquid are in diffusive equilibrium (equal chemical potentials).
If the droplet is very very large, you can start with the ordinary saturation vapor pressure and figure it out as in pballjew's answer.
If the droplet is small enough, you need to use the Kel... | {
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If you view the Earth from far enough away can you observe its past? From my understanding of light, you are always looking into the past based on how much time it takes the light to reach you from what you are observing.
For example when you see a star burn out, if the star was 5 light years away then the star actuall... | No, I don't think the concept of time works quite like that. Yes, if a star is 5 light years away and it burned out - yes it burned out 5 years ago for that point in time. Space time is relevant for that time period.
In your example:
You are 27 Years old (Present Day Earth Time)
You are teleported 27 Light Years A... | {
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Is spacetime simply connected? As I've stated in a prior question of mine, I am a mathematician with very little knowledge of Physics, and I ask here things I'm curious about/things that will help me learn.
This falls into the category of things I'm curious about. Have people considered whether spacetime is simply conn... | Benjamin Horowitz's answer covered a lot of the key points, but it's worth adding that the question of the topology of the universe has been investigated by astrophysical observations. If the universe is multiply connected, and if the length scale is shorter than the horizon scale, then we should be able to see evidenc... | {
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Calculating time for a fully charged UPS I have a UPS of 1000 Volts connected with 2 batteries each of 150 Amp. How much time it will take to consume the whole UPS (after fully charged) when a device of 1Amp is getting electricity form that UPS.
Please also explain me the calculation.
| Battery capacity is normally rated in ampere-hours (Ah), not ampere. This means what it sounds like - if you have 2 * 150 Ah of batteries, you can (ideally) pull 1A for 300 hours like in your question (*), or 300 amps for 1 hour. This is an intuitive value of battery capacity but is not strictly speaking a measurement ... | {
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whats the rate of energy increase required for constant acceleration between 0.0c and 0.99c? I was wondering how much energy would be required to accelerate 1000kg to 0.99c at 1G.
What I don't understand is what the rate of increase of energy is required as velocity increases. I was looking at the Lorentz factor curve ... | The energy of the mass at rest is
$$
E_0 = m c^2
$$
At speed $v = 0.99 c$ it becomes
$$
E = \frac{m c^2}{\sqrt{1-v^2/c^2}}
$$
So, the increase in energy is
$$
E - E_0 = m c^2\left(\frac{1}{\sqrt{1-v^2/c^2}} - 1\right) \approx 6.09 m c^2.
$$
irrespective of the acceleration.
| {
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What happens to light in a perfect reflective sphere? Let's say you have the ability to shine some light into a perfectly round sphere and the sphere's interior surface was perfectly smooth and reflective and there was no way for the light to escape.
If you could observe the inside of the sphere, what would you observe... | The interaction of the light with any particles present (there can be no transfer of energy even light without matter) would lead to an increase of particle density and a vacuum until the sphere collapses in on itself. The idea that the light would just stop if the source was "switched off is nonsense. How we see the l... | {
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What is sector decomposition What is sector decomposition and how can it be used to 'disentangle' UV and IR divergences?
I have read about it in the paper SecDec: A general program for sector decomposition,
but I have no idea if, with a suitable change of variable $x=1/q$ on each coordinate, we can change the IR diverg... | UV divergence is due to self-action. It exists even in statics. No self-action is introduced, no UV divergence appears in calculation. Renormalizations discard the self-action contributions perturbatively rather than exactly, at the beginning.
IR divergence is due to too distant initial approximation, distant from the ... | {
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Simplest interferometer I want to build simplest interferometer which should be able to measure movements down to fraction of wavelength.
What is the simplest scheme for that, and what are the requirements for a laser?
I have a bunch of laser diode-based ones, and I guess they might be not coherent enough...
Are green... | Lasers aside, the mirror translation stages are likely to be your biggest hassle. If you want to measure sub-optical-wavelength distances, then you should expect to need translation stages that are stable to this sub-micron range or your measurement will wash out. That is, you can have static interference patterns, but... | {
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Software for simulating 3D Newtonian dynamics of simple geometric objects (with force fields) I'm looking for something short of a molecular dynamics package, where I can build up simple geometric shapes with flexible linkages/etc and simulate the consequences of electrostatic repulsion between surfaces. Something, sa... | I think that the 'agent based' approach is good for your purpose. I've already used MASON to play a little with 'a sort of' gravity.
(you will have to know java - easier if you work on top of the examples- and the documentation is very good)
MASON
is a fast discrete-event multiagent simulation library core in Java, d... | {
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Do eyeballs exhibit chromatic aberration? Fairly straightforward question. If not, why not?
I suspect that if they do, it is not perceived due to the regions of highest dispersion being in one's region of lowest visual acuity.
| I have a 405nm laser pointer, which emits light right on the edge of the visible spectrum. It is easy to see that this wavelength of light appears out of focus through the eye likely due to chromatic aberration. I would imagine this is because our eyes are not made to see that kind of light. I've also noticed that me... | {
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What are the mathematical problems in introducing Spin 3/2 fermions? Can the physics complications of introducing spin 3/2 Rarita-Schwinger matter be put in geometric (or other) terms readily accessible to a mathematician?
| Physics complications of "introducing" spin 3/2 matter are the same as for spin 1/2 and spin 0 - the initial approximation in the corresponding interaction theory is physically wrong and calculations give too big (= just wrong) perturbative corrections. It is a complete failure of physics description and it cannot be c... | {
"language": "en",
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'Getting in' to research physics? I'm going to be choosing a university course soon, and I want to go into a branch of physics. A dream job for me would be to work in research, however, I do realise that this isn't for everyone and is difficult to reach. So what is the best way to go about achieving this aim? What thin... | First of all, if you consider science as an ordinary career choice, then the effort and the risk is not worth it.
But if you are genuinely interested in science, and really want to do research for your living, here they are:
*
*stay curious,
*learn hard, especially the things you are interested the most,
*particip... | {
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How beam focusing looks like in electron microscope? I mean I know there are electrostatic/electromagnetic lenses which does focus the beam, but I am not sure how it is possible to foсus beam down to a few 10nm while emitter might be 1mm thick while having large focusing distance (especially when looking at chromatic a... | The answer is a combination of two things:
*
*The "source size" (the first crossover size, in fact) is about 50 μm even for thermionic cathodes. They are able to do that by using hairpin cathodes and a negative electrode known as the Wehnhelt cylinder (see this image for a diagram of a thermionic electron gun).
*Th... | {
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Why is 55-60 MPH optimal for gas mileage of a passenger car? My driver's education teacher back in high school said 55 MPH is optimal for gas mileage of a passenger car. Just last week, I read an article in a magazine saying 60 MPH is optimal. These numbers are pretty close, so there's some validity in the statement. W... | There are two things to consider.
*
*The amount of energy required to maintain constant speed. This starts at a low value and increases. Energy rate can be measured in Watts (power) and it is proportional to $v^3$.
$$ P = F_{drag}\,v $$
*The amount of fuel supply needed to make the above energy rate. This is called... | {
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Could a human run horizontally inside a Wall of Death? A popular circus stunt is for a motorcycle rider to ride inside a bowl shaped depression called a "Wall of Death." The rider goes higher and higher up the wall until they are actually horizontal. I wonder if a human could do the same.
| Short answer: No.
Longer answer: I found it easiest to approach this by asking the question in a different way ---> If a runner were to run around in circles on an inverted conical ramp, how fast would he have to run to remain perdendicular to the ramp's surface? The answer depends on three things: radius for the ru... | {
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The philosophy behind the mathematics of quantum mechanics My field of study is computer science, and I recently had some readings on quantum physics and computation.
This is surely a basic question for the physics researcher, but the answer helps me a lot to get a better understanding of the formulas, rather than rega... | Scott Aaronson, himself a (quantum) computer scientist, thinks and writes about a number of these subjects in his paper Is Quantum Mechanics An Island In Theoryspace? - at least the "why complex numbers and not the reals or the quaternions?", and I'm pretty sure he mentions it in his 'Democritus' lectures as well.
| {
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Is the earth expanding? I recently saw this video on youtube:
http://www.youtube.com/watch?v=oJfBSc6e7QQ
and I don't know what to make of it. It seems as if the theory has enough evidence to be correct but where would all the water have appeared from? Would that much water have appeared over 60 million years? Also what... | I don't know as a fact if the earth is expanding or not. But way is it so hard to believe it is. On the same principal that ice expands when it mixes whit gases. Lava expands when it cools specialty when water is present. It fells whit gas bobbles and increases in volume 3 fold. On the same principal way is it not poss... | {
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Creation of matter in the Big Bang I appreciate your patience to my neophyte question. I am working on my dissertation in philosophy (which has nothing or little to do with physics) about the "problem of naming." Briefly what I am arguing is that when we name something, we stop it from being anything or everything else... | Hydrogen and helium (and lithium) were formed in the big bang, but other elements arose later through a variety of different routes, including stellar and supernova nucleosynthesis, as well as fission and spallation. So only some of your atoms are truly primordial.
| {
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Creating the opposite of an optical lattice Is it possible to create periodic potentials that instead of creating a well for an atom to be trapped in, repulsed by that specific location?
If yes, can we use this as a means to make artificial objects that we can feel, with let's say our hands, by putting lots of these re... | Atoms in an optical lattice are attracted to high/low field states depending on the quantum state the atom is in. Atoms in states that would be shifted down in energy by the application of an electric field are drawn to low-field regions and repulsed from high-field regions, and vice versa for atoms in states that woul... | {
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At what speed does our universe expand? Conceivably it expands with the speed of light. I do not know, but curious, if there is an answer. At what velocity, does our universe expand?
| The rate of expansion of the universe (the fact that all objects are receding from each other and more so if they're farther away from each other) is given by the Hubble constant $H_0= 69.32 ± 0.80 (km/s)/Mpc$ 1
Check out this plot from Wikipedia
On the y axis you have the velocity with which the object is receding f... | {
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In dimensional analysis, why the dimensionless constant is usually of order 1? Usually in all discussions and arguments of scaling or solving problems using dimensional analysis, the dimensionless constant is indeterminate but it is usually assumed that it is of order 1.
*
*What does "of order 1" mean? 0.1-10?
*Is... | To add to the other answers, it should be noted that when formulating laws and relationships it is custom to use only unitary values of the relevant physical quantities.
$F=ma$, $V=IR$ etc could just as well have had a constant inserted, and if you shift from SI units you will need to include some. Therefore lack of n... | {
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Why does the weighing balance restore when tilted and released I'm talking about a Weighing Balance shown in the figure:
Press & Hold on onside of the horizontal beam and then release it. It makes some oscillations and comes back to equilibrium like shown in the figure.
Both the pans are of equal equal masses. When th... | Nice question! if the following analogy applies : imagine a seesaw on a half-sphere fulcrum (top of the picture). if it inclines e.g. to left side (bottom) - the length from the right edge to fulcrum ($L_2$) increases, the lever rule kicks in ($F_2>F_1$) and the weight of the right side brings the seesaw back to equili... | {
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Must the action be a Lorentz scalar? Page 580, Chapter 12 in Jackson's 3rd edition text carries the statement:
From the first postulate of special relativity the action integral must be a Lorentz scalar because the equations of motion are determined by the extemum condition, $\delta A = 0$
Certainly the extremeum con... | Yest, it must. It does not guarantee that the equations have physical exact solutions but at least everything looks relativistic.
| {
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GUT that includes all 3 particle families into a large group? Explaining SU(5) GUTs (using the first particle family as an example) in the last SUSY lecture 10, Lenny Susskind mentioned that there are at present no ideas how to combine simultaneously all 3 particle families into a large GUT theory.
I somehow dont belie... | Jacob Bourjaily derives three Standard Model generations from an E8 singularity here and here. (See comments by Lubos, 1 2 3.)
| {
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Introducing emf of a chemical cell as a hint towards quantum mechanics Today I had a discussion with a colleague who teaches electricity and magnetism to 2nd year undergraduate physics students. He is seeking the best way to explain how is the emf generated inside a battery with a minimal appeal to physics beyond class... | What you're describing is electrochemistry, which is summarised here as
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor (a metal or a semiconductor) and an ionic conductor (the electrolyte), and which involve electron tr... | {
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Why did my liquid soda freeze once I pulled it out of the fridge and opened it? This isn't a duplicate to "Why did my liquid soda freeze once I pulled it out of the fridge?". My question is why soda froze after it was opened. Opening a can or bottle seems to have a larger effect than just jostling it.
Is it because of ... | this is very simple. when a gas loses pressure it cools. when the top is removed from a carbonated drink gas is released which expands cooling the liquid enough to freeze. some time open the valve on a scuba tank. the air coming out will be cold, it is expanding. the same tank will get very hot while being filled. that... | {
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How does the internuclear repulsion vary in Hydrogenic atom collision? Hydrogen fusion requires two hydrogen nuclei to get close enough (typically a few fm) to fuse. Much of the problem of creating a fusion reactor is overcoming the Coulomb repulsion between a pair of nuclei - the millions of degrees for Maxwellian dis... | Your last sentence is exactly right: the energy cost for fusion is almost all in those last few femtometers, at which electronic effects are negligible. Although there is in principle a difference between colliding neutral atoms and nuclei, at the energies required for fusion the effect is tiny.
The energy differences ... | {
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Physical interpretation of describing mass in units of length I'm working in Taylor and Wheeler's "Exploring Black Holes" and on p.2-14 they use two honorary constants: Newton's constant divided by the speed of light squared e.g. $G/c^2$ as a term to convert mass measured in $kg$ to distance.
Without doing the arithme... |
Earth is 0.444 cm; and of the sun is 1.477 km
It corresponds to half of the respective Schwarzschild radius.
The $\frac{G}{c^2}$ is covered there and also in Adam’s answer.
| {
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Can superconducting magnets fly (or repel the earth's core)? If a superconducting magnet and appropriate power supply had just enough $I\cdot s$ (current $\cdot$ length) so that when it was perpendicular to the earth's magnetic field, the force of the interaction was just enough to excede the force exerted on the objec... | The entire craft wouldn't need to be rotating, only a hollow torus containing sufficient mass (of a heavy superfluid?), rotating at a sufficient speed.
It would also make an incredibly good flywheel, assuming you could easily put energy in and take it out again. (super-ferrofluid?)
| {
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What is the physical significance of dot & cross product of vectors? Why is division not defined for vectors? I get the physical significance of vector addition & subtraction. But I don't understand what do dot & cross products mean?
More specifically,
*
*Why is it that dot product of vectors $\vec{A}$ and $\vec{B... | In addition to nibot's answer: division something is finding a part of something. In case of a vector, its part has the same direction but a smaller length. So it is natural to divide vectors by numbers, not by vectors.
Those dot and cross products are not simple products because they depend not only on lengths but als... | {
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Nomenclature of radial solutions to the Schrodinger Equation For the free particle with quantum number $l=0$, the regular solution to the radial Schrodinger equation is $R_0 (\rho)=\frac{\sin{\rho}}{\rho}$ while the irregular solution is $R_0 (\rho)=\frac{\cos{\rho}}{\rho}$. Is there a reason for this nomenclature -- (... | The reason for this nomenclature is the behavior at $r=0$:
$$\lim_{r\to0^{+}}\frac{\sin r}{r} = 1,$$
$$\lim_{r\to0^{+}}\frac{\cos r}{r} = \infty .$$
$\frac{\sin r}{r}$ is regular at $r=0$ while $\frac{\cos r}{r}$ is irregular.
| {
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Collision of Phobos Mars has two moons: Phobos and Deimos. Both are irregular and are believed to have been captured from the nearby asteroid belt.
Phobos always shows the same face to Mars because of tidal forces exerted by the planet on its satellite. These same forces causes Phobos to drift increasingly closer to Ma... | Because Phobos is already within the Roche Limit, within which it should disintegrate due to tidal forces, "appropriate data" would have to include quite a bit of detail about Phobos's structure and composition (information which we currently lack), which would let you determine not so much the "time at which" it will ... | {
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Why Do Hurricane Balls Spin So Fast? I was wondering if anyone could offer an explanation as to why the balls described in this video spin so fast.
Here's the setup: Two metal balls are wielded together. When spun with air, they acquire a massive amount of rpm.
| He says that the balls are spinning at 2000 Hz, and that they are two half inch balls welded together. That says that their outer surface is moving at most something like $2\pi 0.5$ inches in 0.0005 seconds, for a speed of 6300 inches per second or 523 feet per second or 356 miles per hour. This is a difficult to belie... | {
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Since the universe is expanding, does anything ever occupy the same point in space? Let's say we can observe expansion in a supercluster.
We define origin of our frame of reference at the center of the supercluster.
We observe an object/atom at point A at time T. The object is motionless relative to the origin.
We wait... | In the classical description of general relativity, the spacetime points constitute a smooth manifold with local coordinates $x^{\mu}$. In order to compute distances and intervals between points, an extra piece of information is needed, namely the metric tensor field $g_{\mu\nu}$(x). Space expansion can be thought of... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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The energy carried from one winding of a transformer to another, in quantum terms I have read in wikipedia this statement
"The energy carried from one winding of a transformer to another, in quantum terms is carried by virtual photons, not real photons" (wikipedia src: virtual particle)
Of course anything in wikipedia ... | In your case it is a near field that stands for "virtual" photons. The near field does not propagate like EMW in the sense it is "attached to the charges and currents. In fact, it is the Coulomb and quasi-static magnetic interactions of charges and currents. Despite being time-dependent, the near field decays with dist... | {
"language": "en",
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Particle antiparticle annihilation-do they have to be of the same type? I read that a particle will meet its antiparticle and annihilate to generate a photon. Is it important for the pairs to be of the same type? What will happen when for example a neutron meets an antiproton or a proton meets a positron? Are there any... | Yes, there are rules that depend on the quantum numbers carried by the particles under question and the energy available for the interaction.
In general we label as annihilation when particle meets antiparticle because all the characterising quantum numbers are equal and opposite in sign and add and become 0, allowing ... | {
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How long does a permanent magnet remain a magnet? I have a bunch of magnets (one of those game-board thingies) given to me when I was a school-going lad over 20 years ago, and the magnets feel just as strong as it was the day it was given.
As a corollary to this question Do magnets lose their magnetism?, is there a wa... | Some magnets, e.g. AlNiCo, require a keeper (essentially, a piece of iron placed between the poles) to keep magnetic flux lines concentrated inside the magnet, to keep from spontaneously demagnetizing (the material reaches an unstable point in the intrinsic B-H curve). They can also be demagnetized by mechanical means ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "22",
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Why are color values stored as Red, Green, Blue? I learned in elementary school that you could get green by mixing blue with yellow.
However with LEDs, TFTs, etc. you always have RGB (red, green, blue) values?
Why is that? From what you learned in elementary yellow would be the 'natural' choice instead of green.
| What @Michael said, plus your retina has sensors for roughly red, roughly green, and roughly blue, and not for any other colors.
(BTW, the green sensors are more sensitive, so it takes less green to make the same brightness.)
When you see something yellow, it's in between red and green, so it excites both of them, and ... | {
"language": "en",
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"source": "stackexchange",
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Wind turbine: Single large propeller or multiple small propellers? Assuming smaller propellers would spin even in very low wind, whereas a large propeller would stall;
Would it be efficient to use a bunch of propellers from axial fans geared to a single motor instead of a single large propeller?
| The power output of a wind turbine is proportional to the square of the blade length $L$ (specifically, the "swept area" enclosed by the tips, which will be $\pi L^2$). A single, large turbine is more efficient because it has fewer moving parts than several smaller ones. If you added up the friction from every bearing ... | {
"language": "en",
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An example of a quantum system for which Wigner function transitions to negative values I want to check my understanding of the Wigner transform and try to understand why and how exactly the probabilistic interpretation drops down as the function goes to zero and then to negative values
So, suppose we have a free quant... | The times that the Wigner function is positive does not mean it should be interpreted as a probability distribution. (What events would it be the probability distribution of? Certainly not a fuzzy joint measurement of position and momentum, e.g, with coherent states; that's the Husimi Q function.)
Is there a simple s... | {
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How are neutrino beams emitted at CERN? As far I know they come from accelerator collisions, but I have read confusing things like magnetically focused. How could neutrinos be guided magnetically if they aren't affected by
the electromagnetic field?
I would like to have a better idea of how neutrinos are emitted.
| The basis of all neutrino beams is a less exotic (protons most of the time) beam smashing some mundane target and making scads of assorted particles---many of them charged. Those charged particles are focused (and possible subjected to a second filtering for energy by using collimators and more magnets, though this ste... | {
"language": "en",
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"source": "stackexchange",
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Graphene space elevator possible? I just read this story on MIT working on industrial scale, km^2 sheet production of graphene.
A quick check of Wikipedia on graphene and Wikipedia on space elevator tells me
Measurements have shown that graphene has a breaking strength 200 times greater than steel, with a tensile stre... | Though we may be a while away from the ability to make the all important material, we could in fact start working towards this now. A while ago I penned a small idea for building 2 elevators, the first of which would be a "Lunar Elevator". With greatly reduced gravity, the tensile strength of the cable would be reduc... | {
"language": "en",
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"source": "stackexchange",
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How do you find the uncertainty of a weighted average? The following is taken from a practice GRE question:
Two experimental techniques determine the
mass of an object to be $11\pm 1\, \mathrm{kg}$ and $10\pm 2\, \mathrm{kg}$.
These two measurements can be combined to
give a weighted average. What is the unce... | Since other questions as well as Google point to this question, I would like to extend the already existing answer from luksen by a motivation from the stochastic for using the error propagation equation.
Let's assume we have $n$ random measurements $X_i$, typically denoted by $E\{X_i\} \pm \sigma_i$ (or $x_i \pm \Delt... | {
"language": "en",
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What does it take to understand Maxwell's equations? Assume I want to learn math and physics enough to reach a level where I understand Maxwell's equations (The terms and reasoning in the equations I.e. why they "work"). What would I have to learn in order to have the tools I need to make sense out of it?
I'm kindof lo... | Most books have a detailed section on the level of the text, with the assumed background. Pick up a book on the subject, and read this section carefully to understand the prerequisites. This section is usually in the forward, before the actual text begins. There are frequently course maps in these sections as well, ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Why Is String Theory Called A Theory
Possible Duplicate:
Laws of Atomic Theory - how is this possible?
Generally in science, a theory is effectively a fact. The theory of evolution is not a guess, or a hypothesis. It's currently understood to be a fact, backed up by evidence from numerous scientific fields spannin... | These terms are not used consistently, nor in a way consistent with the way people who talk about science interpret them. For example, the thing called the "standard model" is not really a model anymore (except for the Higgs sector), but an excellent theory, perhaps even a fact of nature, but it is still called the "st... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15303",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 1,
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What is the difference between electric potential, electrostatic potential, potential difference (PD), voltage and electromotive force (EMF)? This is a confused part ever since I started learning electricity. What is the difference between electric potential, electrostatic potential, potential difference (PD), voltage ... | Anyway the simple answer is e.m.f. is not a force in the mechanical sense. It measures the amount of work to be done for a unit charge to travel in a closed loop of a conducting material.
Let's make it more clear. In static case (ignoring time variation of any magnetic field), electric field at a point can be derived s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15402",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "30",
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Numerical software to manipulate a light beam in its plane wave representation? Any light field can be expressed as a sum of plane waves. Such an ensemble of plane waves is called the plane wave spectrum of the light field. The plane wave spectrum is the Fourier transform of the light field in the real space representa... | You could try MEEP. It is an opensource tool for light manipulation. It automatically does all the FFTs and has a built-in Gaussian source and it also produces nice plots of 1D, 2D, 3D numerical simulations. It also has a python wrapper.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
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} |
Open quantum systems and measuring devices The Copenhagen interpretation by Niels Bohr insists that quantum systems do not exist independently of the measuring apparatus but only comes into being by the process of measurement itself. It is only through the apparatus that anything can be said about the system. By necess... | Definitely, yes. Sometimes it is even necessary, such as in the case when you place an atom ("system") in between two mirrors ("apparatus"). The resulting quantum mechanical model is that of cavity quantum electrodynamics. Now if you want to know what's going on there, you have to bring in a second measuring apparat... | {
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How does one calculate the force applied on an object by a magnetic field? I've tried very hard to find an answer to this question, and every path leads me to an abstract discussion of fundamental forces. Therefore, I will propose two very specific scenarios and see if they yield the result that I am looking for.
Scena... | I would measure the force using a well calibrated weighing scale. You will see the weight decrease as a function of distance. The difference is the magnetic force. Note that the weighing scale gives kilogram force, 1 kgf being about 9.81 Newton.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15578",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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Why do ships lean to the outside, but boats lean to the inside of a turn? Small vessels generally lean into a turn, whereas big vessels lean out.
Why do ships lean to the outside, but boats lean to the inside of a turn?
For example, a boat leaning into a turn:
Image Source.
And a ship leaning out:
Image source
| The answer is much simpler than explained above. Small boats turn with a change in the direction of the propeller. Large boats turn with a rudder. To turn left, the turn of the propeller applies a force in the back of the boat that makes it lean to the left. This does not happens with a rudder.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15631",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "31",
"answer_count": 7,
"answer_id": 6
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The topology of a "closed" universe - is it really closed? The spatial part of the positive curvature FRW metric has the form
\begin{equation}
ds^2=\frac{dr^2}{1-(r/R)^2}+r^2d\Omega^2
\end{equation}
or
\begin{equation}
ds^2=R^2(d\chi^2+\sin{\chi}^2d\Omega^2)
\end{equation}
This is described as "closed", as it has the m... | The first metric form uses coordinates that are valid only in a patch of the whole space. This is a fairly common situation. Consider the example of a 2-sphere projected onto a tangent plane from the center. This is the gnomonic projection which maps all great circles into straight lines. The $(x,y)$ coordinate sys... | {
"language": "en",
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Can an arbitrary RLC-circuit network have non-minimum phase zeros? I am working with certain input-output maps that can be thought of as large RLC-networks. I thought maybe this might be a place to get some thoughts/ideas/answers.
My basic question is, given some large connected RLC network (all linear and ideal eleme... |
More generally, is it true that every transfer function representing
an RLC-circuit network is minimum phase?
I suspect the answer is true, but I am having a hard time proving it.
It's not true because you can have an RLC all-pass filter. To see a more specific example, let's analyse a lattice phase equaliser topol... | {
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How does a state vector be projected onto an eigenspace after measurement In http://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics#Degenerate_spectra, it is said that
If there are multiple eigenstates with the same eigenvalue (called degeneracies),..., The probability of measuring a particular eigenvalue is t... | Suppose you are in the state
$$|\Psi \rangle = a|\alpha_1\rangle + b|\alpha_2\rangle + c|\beta \rangle$$
$|\alpha_1\rangle$ and $|\alpha_2\rangle$ are eigenvectors of the observable $A$, both with eigenvalue $\alpha$. $|\beta\rangle$ is also an eigenvector of this observable but with the different eigenvalue $\beta$.
... | {
"language": "en",
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How can a conductor be grounded yet there are induced charges on it? A classic example for the method of images is the following, quoted from Griffiths's Introduction to Electrodynamics, page 121:
Suppose a point charge $q$ is held a distance $d$ above an infinite grounded conducting plane. Question: What is the poten... | Here grounded means V=0; V=0 means, there is no work to be done to bring a positive charge near the plane of the grounded conductor. This means that there are no charges on the plane. Had there been any charges on the plane it would either push or pull the test charge and V would not be zero. I think this particular ex... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
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why making a surface "super" smooth increases the coefficient of friction? I read that:
If you take a rough surface and make it smooth, the coefficient of friction decreases. But if you make it super smooth, then the coefficient of friction increases. How come?
| If I think about friction geometrically, i.e. ignore for now chemical interactions, (although they are very important) you get an argument about the surface area in contact. If you take two flat smooth surfaces, they will contact along a plane. If you roughen up, one or more of the surfaces, contact will only take plac... | {
"language": "en",
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Does the Opera result hint to a discrete spacetime? Could the Opera result be interpreted as some kind of hint to a discrete spacetime that is only seen for high enough energy neutrinos?
I think I've read (some time ago) something like this in a popular article where among other things tests of quantum gravity theories... | Searching on Google there is nothing new . Considering the plethora of arxiv papers coming out with theoretical comments on the superluminal result I would think that if the LQG model had something to say, it would have said it, particularly if it were vindicated.
So the answer is "no" . For the nonce.
Because if one r... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Problem with an electricity / thermodynamics assignment I've been trying to figure this one out for a while on my own, so I'd like to ask for your help if you could offer some.
The task states:
A heater made out of a wire with a diameter $R = 0.2\text{ mm}$, length $4\pi\text{ m}$ and electrical resistivity of $0.5\ti... | You may consider this question from perspective of energy view: the electric energy is consumed by the resistor and convert this energy to the thermal energy (the source of heat that heat up the water). So from this point of view, if you can assume the 100% electric energy converting to thermal energy, and usually, thi... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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The visibility of air For pilots of gliders or sailplanes, the 'thermal' is the most important phenomena of the air. A thermal is classically described as an upward flow of air caused by ground level heating of air that rises in bubbles or a connected stream of warmed air. Given sufficient velocity of the rising air,... | Mirages are a visible effect of the inhomogeneous, temperature-dependent refractive index of air. They're visible because of the relatively large heat gradient close to the earth's surface, and the excellent reference line provided by a uniform, flat horizon. I think those conditions would be difficult to replicate in-... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16046",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
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Is the cooling rate of a (very) cold object, sitting next to an AC higher or lower? In more detail:
If i have two soda cans, both are cooled to exactly 4 degrees celsius,
And i put one in a 25 degrees room, and the other next to an AC vent set to 16 degrees.
After three minutes, which one should be colder than the othe... | AlanSE's answer is pretty good. I'll try to add to it by keeping it math free and strictly using everyday experiences.
Imagine you're in a pool of cool, still, water. If you keep still you're not too cold but as soon as you move a bit you get very cold. A blanket of warm water forms around you that protects you. As so... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16076",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 5,
"answer_id": 4
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Will the water added to an ice piece freeze? Water, at room temperature is poured into a hole made of a block of melting ice(kept at room temperature).I was wondering if the water will ever freeze?
Thank you.
| Ice coming from the freezer will typically be around -19 deg. celsius, and can only be stored for a limited time at room temperature. As soon as the ice is heated to 0 deg. or above, the ice will melt into liquid water. Liquid water coming into contact with ice will be cooled, and if cooled below 0 deg. it will also fr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16124",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 5,
"answer_id": 0
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Dependence of Friction on Area Is friction really independent of area? The friction force, $f_s = \mu_s N$. The equation says that friction only depends on the normal force, which is $ N = W = mg$, and nature of sliding surface, due to $\mu_S$.
Now, less inflated tires experiences more friction compared to well inflat... | When you say underinflated tires experience more friction, do you mean static friction (i.e., resistance to slipping) or rolling resistance, which is something quite different?
Afaik the origin of the friction law is very much phenomenological, and has it's limits of applicability (especially at the static - dynamic tr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16213",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "22",
"answer_count": 4,
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Are specific heat and thermal conductivity related? Are there any logical relationship between specific heat capacity and thermal conductivity ?
I was wondering about this when I was reading an article on whether to choose cast iron or aluminium vessels for kitchen.
Aluminium has more thermal conductivity and specific ... | Imagine a substance in the size and form of an ice cube. If you could keep shooting it with a photon of say energy $1$ and you shot $10$ of these photons and noticed that the substance had gained a temperature difference say from $25$ to $26^\circ\mathrm{C}$, then its specific heat capacity would be $10$. (Specific he... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16255",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "26",
"answer_count": 8,
"answer_id": 3
} |
Inverted Harmonic oscillator what are the energies of the inverted Harmonic oscillator?
$$ H=p^{2}-\omega^{2}x^{2} $$
since the eigenfunctions of this operator do not belong to any $ L^{2}(R)$ space I believe that the spectrum will be continuous, anyway in spite of the inverted oscillator having a continuum spectrum ar... | The QHO does not permit analytic continuation, because its energies and wavefunctions depend not on $\omega$, but on $|\omega|$. Thus, their dependence on $\omega$ is not analytic and $\omega$ cannot be simply replaced by $i\omega$.
Moreover, this Hamiltonian is not Hermitian. Still, just like few other interesting cas... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16299",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 2,
"answer_id": 0
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How do we perceive hotness or coldness of an object? Some objects, especially metallic ones, feel cold on touching and others like wood, etc. feel warm on touching. Both are exposed to same environment and are in their stable state, so some kind of equilibrium must be being reached. What is this equilibrium?
And how do... | sensation of heat activates an electrical impulse which is passed through neurons and synapsis as electrical an chemical reactions and reaches our brain which interprets the message as heat some times if the heat is unbearable the message is interpreted by the spinal chord
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16333",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 6,
"answer_id": 5
} |
The shape of the earth$\ldots$ ....is an oblate spheroid because centrifugal force stretches the tropical regions to a point farther from the center than they would be if the planet did not rotate. So we all learned in childhood, and it seems perfectly obvious. However...
I am at $45^\circ$ north latitude. Does that... | 1, At 45 deg (N) latitude you are closer to the North pole, to picture this just draw the Earth as a much more extreme oblate spheroid.
2, The shape of the Earth is set by the outward rotational force exactly balancing the inward gravitational force at every point (except for local geography). So the overall potential ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16382",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
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Compound lenses and virtual images Ok so I have this problem where I have a system of two lenses. All I know is that the distance between the object and the first lens is 30cm, the distance between the object and the final image is 70cm, the focal distance of the first lens is 20 and the total magnification is -0.666.... | Most likely, your problem is the sign of the distance from the image of the object through the first lens, to the second lens. When you use the formula for the ideal thin lens you must be really careful about the convention used.
For example, I usually use $\frac{1}{f} = \frac{1}{o} + \frac{1}{i}$, where $f$ is the len... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/16434",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
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