Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
How did we get the formula $d U = nCvdT$? Our teacher taught us that for any thermodynamic process, dU=nCvdT where Cv is molar specific heat capacity at constant volume and dU is change in internal energy. How did we get this formula and why is it valid for all processes
| $dU=nC_{v}dT$ is only valid for all processes in the case of an ideal gas where the internal energy is considered to be purely kinetic depending on temperature only. We get this formula for ideal gases by combining the first law, the ideal gas law. and the general definition of the molar specific heat at constant volum... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/747357",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 1
} |
Can red and blue light interfere to make fringes in young’s double split experiment? Supposing in young’s double split experiment, I cover one slit with red filter and the other slit with blue filter. The light coming out from the first slit would be red and the second slit would be blue. Would there be any interferenc... | You would have two single slit diffraction patterns overlapping on the screen.
The two pattern have different spacings between their fringes. For example the blue spacings are smaller than the red spacings. Like any beat pattern you will find points where bright fringes from blue coincide with bright fringes from red. ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/747560",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 1
} |
WKB approximation derivation for $EI understand that we can write any complex wavefunction on polar form $A\exp(iθ)$ with both $A,θ$ real. Following the logic of Griffiths on WKB (here, page 291):
*
*We write the energy wavefunction in the previous form.
*For $E>V$, we insert the previous form in S.E and demand $A^"... | OP has a point: The polar form of the complex wavefunction $\psi$ is not useful in the classically forbidden region$E<V$ because the complex TISE then doesn't separate into 2 real equations.
Alternatively, consider the approach of Ref. 1 (which happens to be problem 8.2 in Ref. 2). Here the wavefunction is assumed to b... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/747714",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
How does this derivation of the proper time derivative of a covariant vector work? Define the operator $\frac{D}{D\tau}$ by its action on an arbitrary contravariant vector $A^\lambda$:
$$\frac{DA^\lambda}{D\tau} = \frac{dA^\lambda}{d\tau} + \Gamma^\lambda_{\mu\nu} \frac{dx^\mu}{d\tau} A^\nu$$
(with the motivation that ... | This essentially follows from that $\frac{D}{D\tau}=\nabla_{\dot{x}}$ and from how a connection $\nabla$ acts on tensors.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/747869",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
In an optical system, does the Point Spread Function apply to all light? Or only Point light sources? So in optics, the Point Spread Function (PSF) describes how an optical system responds to a point source of light. My understanding is that this is due to diffraction and the wave-like nature of Light.
This would lead... | For a point source the spread varies as inverse square of the distance. However for a linear source it varies as receprocal of the distance near the source. So the variation of intensity depends on the geometry of the source specially at close distance.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/748020",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
How do non-periodically varying currents produce electromagnetic waves? Electromagnetic radiation is created by the varying/accelerating of a system of charges and currents. Suppose that the time dependence of the charges and currents are $\rho(x,t)$ and $J(x,t)$. Then the subsequent radiation will have the same time d... | The confusion is between Fourier series, which is expansion for periodic functions, and Fourier transform - which is an expansion for arbitrary functions (satisfying certain mathematical conditions.)
Fourier transform can be though of as a generalization of Fourier series. Some would probably even say that Fourier ser... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/748085",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
How can a grain of sand be "spaghettified" when nearing a black hole? I have a hard time wrapping my head around this "spaghettification" process that apparently takes places when getting close to a black hole.
Gravity is proportional to the distance of the object exerting gravity. Everything in space is unimaginably H... | Spaghettification does not always happen outside the event horizon. You could fall through the event horizon of a supermassive black hole, like the one at the center of our galaxy, without suffering any immediate harm. But soon enough (quite soon, in fact) you would get close enough to the singularity to be spaghettifi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/748241",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "16",
"answer_count": 6,
"answer_id": 2
} |
Does work done by a non-conservative force involve distance rather than displacement? I am a new physics teacher and struggling to piece out the nuance of work calculations for my Advanced Placement (AP) students.
I feel like after a fruitful year of distinguishing between vector and scalar quantities for the use of ki... | Both conservative and nonconservative forces do work as the path integral $\int _L \vec F \cdot d\vec s$.
If force and path are antiparallel (as for friction*) and force is constant in magnitude along the path, since the dot product of antiparallel vectors is negative the product of their magnitudes, we can replace the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/748386",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 0
} |
Husband and Wife with Metal and Fabric Parachutes Let's suppose: an open parachute made of fabric with a lady hanging below, starting from stand still, falls from 5000 meters. Another parachute made of thick metal, with the same size and shape as the fabric one and with the lady's husband hanging below, is also falling... |
If the two parachutes have the same acceleration from the gravitational pull, then their velocity would remain the same because the air resistance is also the same.
You are mixing force and acceleration in this statement. They have the same gravitational acceleration (different gravitational forces) and they have the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/748517",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
How does Bolztmann Brain explain experience of time? If a Boltzmann Brain existence is fleeting due to the absence of supporting organs and environment, how does the thought experiment account for the passage of time it experience in its brief existence? Even if the brain pops into existence complete with a lifetime wo... | Well, if you think about it carefully, you will realize you only ever have the experience of the passage of time in the current moment. You never have any experience of passage of time in a past moment, you only have a memory of said experience, even if the experience seems to have taken place just a split second ago. ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/748849",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
How can QFT perturbation be used for electron-positron scattering? I'm studying scattering and perturbation theory in QFT from Peskin and Schroeder book. After all the calculations and theory developing, they made a calculation for $$e^-e^+\to\mu^-\mu^+$$ scattering. Even though I understood the calculations, one thing... | According to the path integral approach (qualitatively), they take all possible paths, which presumably includes arbitrary closeness...which may mean up to some UV limit in the renormalization scheme. How those infinities cancel is a question for experts.
Moreover: Feynman diagrams are an expansion in momentum space, s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/749057",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Potential due to line charge: Incorrect result using spherical coordinates Context
This is not a homework problem. Then answer to this problem is well known and can be found in [1]. The potential of a line of charge situated between $x=-a$ to $x=+a$ ``can be found by superposing the point charge potentials of infinit... |
Adjusting from [1] $(b→a)$, the answer to the problem below is
$$\Phi{\left(r, \frac{\pi}{2},\pi\pm \frac{\pi}{2}\right)}
=
\frac{1}{4\,\pi\,\epsilon_o}
\frac{Q}{ a}
\left[
\ln{\left(\frac{ a+ \sqrt{a ^2 + r^2}}{ -a+ \sqrt{a ^2 + r^2}}\right)}
\right]$$
This is wrong by a factor of 2, in the original answer they... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/749536",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
How to test resonance frequency of spring using sound? I've done experiment with spring and mass to determine the natural frequencies of 4 springs. The first experiment went well but I had some problem when I want to test the resonant frequency. I'll explain the context first.
After the spring test, the spring was test... | The resonant frequency depends just as much on mass density as on stretch and spring constant. You also have to consider whether you are looking at a transverse wave or a longitudinal wave. Blasting from the side is a transverse wave, similar to a wave on a string. A mass hanging from the spring is a longitudinal ef... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/750269",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Mass-Energy Equivalence and First Law of Thermodynamics Einstein showed mass can be converted into energy and vice versa.
$E=mc^2$
However, in school we are taught that according to the First Law of Thermodynamics, energy can neither be created nor destroyed.
Are they not contradicting each other? I already tried findi... | Mass is not converted to energy. Mass is energy. Sometimes mass energy can be converted into heat energy or light energy, or vise versa (as in chemical or nuclear reactions). Thermodynamics always applies regardless to these transfers or conversion of energy. In most thermodynamics and chemistry contexts, the amoun... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/750429",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
How does the magnetic field strength (in Teslas) change when two cylindrical magnets are pulled appart? I have two cylindrical magnets aligned such that the opposite poles are facing each other (N-S N-S).
I am trying to find a mathematical relationship that models the change in the magnetic field strength (B - measured... | A simple model of your scenario would be to replace both your magnets with ideal magnetic dipoles, which is a decent approximation and quite useful because the fields of finitely sized permanent magnets are difficult to write down. Electromagnetism has an important and useful property known as superposition. The tota... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/750809",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 0
} |
Confusion regarding Relation between Frequency and Loudness The mathematical relation between intensity and loudness is:
$$ I_{L} = 10 \log_{10}\left( \frac{I}{I_{0}} \right) $$
where $I$ is the sound intensity and $I_{0}$ the reference intensity. The unit of sound intensity is expressed in decibels (dB).
The mathemati... | Personally, I am not sure where exactly you found that loudness is not frequency dependent. Loudness is a psychoacoustic quantity (measure) and according to the well known Fletcher-Munson curves (Equal Loudness Contours) there is strong frequency dependence on loudness.
You may be referring to the Sound Intensity Level... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/751016",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why does magnetic force only act on moving charges? I don't understand why the magnetic force only acts on moving charges. When I have a permanent magnet and place another magnet inside its field, they clearly act as forces onto one another with them both being stationary. Also, I am clearly misunderstanding something.... | As a simple(ish) special case of the principle set out by @Cleonis, imagine two initially stationary charges, starting a short distance apart. Under the Coulomb (electrostatic) force, they will accelerate apart (if they're like charges) or together (if they're opposite charges).
Now think about this same pair of charg... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/751358",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 7,
"answer_id": 4
} |
On Bohr's response to EPR If I understand correctly, the EPR paper (1935) points out that quantum mechanics is incomplete theory if it describes individual particles and measurements. This is true by the mathematical formalism. But already in 1926 quantum mechanics had its statistical interpretation, and in 1930 Heisen... | Because that would be conceding to Einstein's view. Einstein believed quantum theory is statistical and not fundamental, and the EPR provides an argument for that view. Bohr believed quantum theory is fundamental, and thus did not agree with Einstein's characterization of the theory as incomplete.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/751512",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Is the temperature of the hottest star's core known? WR 102 is believed to be the hottest star in the observable universe, whose surface temperature is $210,000 ^\circ K$. But the related wikipedia entry does not say anything about the temperature of its core. So,
*
*Is the core temperature of this star known?
*Is ... | The core temperature of this star isn't known, but can be estimated from theoretical models of stellar evolution, using the observed luminosity and photospheric temperature as constraints.
These suggest that WR102 is currently (it will have lost a lot of mass during its earlier life) about 16 times the mass of the Sun ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/751911",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Why is electromotive force in magnetohydrodynamics a vector quantity? In the mean-field dynamo theory in magnetohydrodynamics, I frequently came across a quantity;
$\langle v'\times B' \rangle$, which is termed as the mean electromotive force. I want to know that why is it termed as electromotive force, if it is a vect... | Strictly speaking, you are right. However, in magnetohydrodynamics, it happens that people refer to the force per unit charge $ \frac{{\bf J}}{e}\times {\bf B}$ as emf, with the implicit assumption that its line integral over a path provides the real emf.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/752083",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 2
} |
Motivation behind introducing creation/annihilation operators into the Dirac equation When studying the Klein-Gordon equation, the introduction of creation/annihilation operators was justified by recognizing a harmonic-oscillator-like equation which we know how to quantize. Is there a similar justification when introdu... | In general the reason is that it was desired to find an operator solution to the equation, so that the fields could be operators in analogy to the observable operators that we have in nonrelativistic quantum mechanics.
I don't have an explicit proof of it, but a professor of mathematical physics who I spoke to told me ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/752179",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Is there anything truly "stationary" in the universe? Ok, so I read this question and it got me thinking about something. Is there anything genuinely stationary in our universe? What does it mean to be stationary or devoid of any motion? If there isn't anything stationary, can there be a time when a thing is stationar... | The laws of physics do not allow us to distinguish who in a pair is moving and who is stationary. As such, the question of whether or not there is anything in our world that is truly stationary is meaningless, because we have no way at all of determining whether anything in our world is stationary to begin with.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/752328",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 0
} |
Question on the relation between the Hubble constant and the absolute magnitude of Type Ia supernovae I would like to ask a question about the relation between the Hubble constant and the absolute magnitude of supernovae
I have read that supernovae alone cannot fix $H_0$ and that there is a degeneracy between $H_0$ an... | Observation of Type Ia supernovae yield a redshift and an apparent magnitude at peak brightness. If you don't know the absolute magnitude at peak brightness then the apparent magnitude does not give you its distance. A measurement of distance, as well as redshift, is required to determine the Hubble parameter.
The abso... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/753639",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
How do you account for all the photons and plethora of quantum particles in the box between the double slits and the back wall? Does the particle being shot not interact with the all the particles that must be consuming the space in the box before the back wall? How do we know it’s the same photon traveling the distanc... | You take the "particle" characteristic of Photons too far, they are not like atoms or molecules in your box. T do not bump in each other. For the double slit experiment you have to think of light as a wave.And it has nothing to do with the double slit experiment. If you look at some object, the light comes in your ey ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/753828",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
What experiment would disprove string theory? I know that there's big controversy between two groups of physicists:
*
*those who support string theory (most of them, I think)
*and those who oppose it.
One of the arguments of the second group is that there's no way to disprove the correctness of the string theory.... | String excitations, rspt lack thereof. Problem is however that unless you believe in a theory with a lowered Planck scale, you'll have to go to energies we'll never be able to reach in the lab to test this regime. But at least in principle it's falsifiable.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/15",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "313",
"answer_count": 11,
"answer_id": 0
} |
Where do magnets get the energy to repel? If I separate two magnets whose opposite poles are facing, I am adding energy. If I let go of the magnets, then presumably the energy that I added is used to move the magnets together again.
However, if I start with two separated magnets (with like poles facing), as I move them... | Magnetic field in this case (a set of magnets in space, no relativity involved) is conservative, which means it has a potential -- each positional configuration of charges (or dipoles in this case) has its fixed energy which does not depend on history or momenta of charges. So, the work you put or get from displacing t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/35",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "26",
"answer_count": 6,
"answer_id": 2
} |
Is it possible that all neutron stars are actually pulsars? I'm assuming that what I've been told is true:
We can only detect pulsars if their beams of electromagnetic radiation is directed towards Earth.
That pulsars are the same as neutron stars, only that they emit beams of EM radiation out of their magnetic poles.
... | Pulsars are a label we apply to neutron stars that have been observed to "pulse" radio and x-ray emissions. Although all pulsars are neutron stars, not all pulsars are the same. There are three distinct classes of pulsars are currently known: rotation-powered, where the loss of rotational energy of the star provides th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/90",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "17",
"answer_count": 3,
"answer_id": 1
} |
Book about classical mechanics I am looking for a book about "advanced" classical mechanics. By advanced I mean a book considering directly Lagrangian and Hamiltonian formulation, and also providing a firm basis in the geometrical consideration related to these to formalism (like tangent bundle, cotangent bundle, 1-for... | Although not specifically answering the needs of Cedric H. (symplectic geometry) I find Introduction to Dynamics by Percival & Richards one of the best (and simplest) introductions to Lagrangian and Hamiltonian dynamics, particularly canonical transformations and so on. I point out this book because it is probably not ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/111",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "28",
"answer_count": 9,
"answer_id": 5
} |
Is acceleration an absolute quantity? I would like to know if acceleration is an absolute quantity, and if so why?
| In standard Newtonian mechanics, acceleration is indeed considered to be an absolute quantity, in that it is not determined relative to any inertial frame of reference (constant velocity). This fact follows directly from the principle that forces are the same everywhere, independent of observer.
Of course, if you're do... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/173",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 5,
"answer_id": 2
} |
Why can you "suck in" cooked spaghetti? We all know that there is no "sucking", only pushing. So how are cooked spaghetti pushed into your mouth? The air pressure applies orthogonal on the spaghetti surface. Where does the component directed into your mouth come from?
| Since I'm not entirely content with the answers to date, here's my take - everyone seems to agree on the basics of forces generated by pressure differentials. If you took a rigid uncooked spaghetto with a cross section of $A$, the case is quite clear - on the cylinder's base in your mouth, a force of $p_{in} A$ is tryi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/217",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "32",
"answer_count": 4,
"answer_id": 2
} |
What property of objects allow them to float? I used to think that the shape of an object determines its ability to float (boat-shaped objects are more likely to float, and spheres tend to sink). But my friend, who is fond of making me look stupid, took me to the local lake showed me a sphere that floated and a boat-s... | DENSITY
It is because of densities of the object that is floating and the liquid in which it is floating.
If an object have density lower than a fluid it will float otherwise it will sink.
Density of entire object [mass / volume] should be
taken into account and not merely the
density of material it is made up of.... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/239",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 9,
"answer_id": 1
} |
Why don't spinning tops fall over? One topic which was covered in university, but which I never understood, is how a spinning top "magically" resists the force of gravity. The conservation of energy explanations make sense, but I don't believe that they provide as much insight as a mechanical explanation would.
The hyp... | This is a nice example which shows understanding does not come automatically after completing a calculation. But calculation still serves the (perhaps the most) important guide. Nobody in the above has mentioned the discussions given in \ittext{Landau & Lifshitz, Mechanics (BH, 3rd ed.), page 112}. I think these discus... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/271",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "31",
"answer_count": 7,
"answer_id": 6
} |
Why does water make a sound when it is disturbed? When I disturb a body of water, what causes the familiar "water moving" sound?
| As your question is very general, I can suggest a general answer: when a water wave is hitting a wall for example, you can "trap" an air bubble between the wave and the walls. This bubble can be compressed, the pressure will be higher and when the water moves, this bubble "explode" emitting a sound (which is nothing el... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/303",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 6,
"answer_id": 2
} |
Angular Momentum and Average Torque
Refer to number 6. This is the one I'm stuck on. So angular momentum is conserved right, so initial angular momentum is equal to final angular momentum. Initial is 7.87 so final must be 7.87, right? And so average torque is just change in angular momentum / change in time, so 0/7=0.... | The angular momentum of the rod is 0 at the beginning because it is not rotating.
I would proceed like that:
*
*by conservation of angular momentum, calculate the final rotational speed of the rod
*with that given, calculate the final angular momentum of the rod
*You have that the torque gives the variation (with ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/326",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Home experiments to derive the speed of light? Are there any experiments I can do to derive the speed of light with only common household tools?
| Those laser tape-measures operate in an interesting way, that relies on the speed of light to determine distance. So conversely, if you have a known distance, then with the same equipment you should be able to estimate c.
What the tape measures do is modulate the intensity of the outgoing laser according to the intensi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/357",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "58",
"answer_count": 11,
"answer_id": 2
} |
Books for general relativity What are some good books for learning general relativity?
| Spacetime Physics by Taylor/Wheeler
(a red book, with white text on it)
and this book as well
General Relativity, Black Holes, and Cosmology by Andrew J. S. Hamilton 4 December 2021 Available here (free)
*
*https://jila.colorado.edu/%7Eajsh/courses/astr5770_21/text.html
footnotes
my view on it | Really a great intr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/363",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "126",
"answer_count": 21,
"answer_id": 20
} |
What software programs are used to draw physics diagrams, and what are their relative merits? Undoubtedly, people use a variety of programs to draw diagrams for physics, but I am not familiar with many of them. I usually hand-draw things in GIMP which is powerful in some regards, but it is time consuming to do things ... | There is an add-in for Microsoft Word called Science Teacher's Helper.
http://www.helpscience.com
SmartDraw is also an excellent program for creating diagrams.
http://www.smartdraw.com
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/401",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "92",
"answer_count": 20,
"answer_id": 16
} |
Can extra-solar gamma rays reach the Earth's surface? Can gamma rays of high enough energy entering our planet's atmosphere reach the surface (50% probability)?
Or, in other words, is there a window for extremely high-energy gamma rays like for the visible spectrum and radio?
This figure, from Electromagnetic Spectrum,... | I found a reference through Google Books, Very high energy gamma-ray astronomy by Trevor Weekes, which says that the atmosphere is essentially opaque to high-energy gamma rays, equivalent to a meter-thick wall of lead. We are able to do gamma-ray astronomy with ground-based telescopes by detecting the decay products of... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/426",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 0
} |
Why is it thought that normal physics doesn't exist inside the event horizon of a black hole? A black hole is so dense that a sphere around it called the event horizon has a greater escape velocity than the speed of light, making it black. So why do astronomers think that there is anything weird (or lack of anything In... | For example, for starters, outside a Schwarzschild black hole horizon, particles can move in any direction, but time only goes one way. i.e. forward. Inside the horizon, particles can only move inward toward the central singularity, i.e. one way, but time can go either forward or backwards. This is a result of the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/461",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 2
} |
Quantum Field Theory cross sections integrals Where can I find some examples of cross sections calculations in QFT done step-by-step? Those integrals are a little horror. For example - a simple scalar+scalar -> scalar+scalar at the tree level in a theory scalar $\phi^4$ ?
| Peskin and Schroeder tends to be the book used in most introductory QFT courses, so you'll definitely find all things there done in a pretty detailed way.
Warren Siegel has an online book which is also pretty good, Fields.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/494",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 1,
"answer_id": 0
} |
Unambiguous distinguishing of quantum states by local measurement Let's have two orthogonal n-particle quantum states: $|\psi \rangle$ and $|\phi \rangle$. In theory it is always possible to make an unambiguous measurement.
However, things get complicated when one restricts oneself to a certain class of measurements. W... | (Edited to correct TeX typo)
I complete Tim Goodman's answer in answer to get something more systematic. A local measurement has to be written as a tensorial product of two observables A⊗B. And it can only distinguish (with probability 1) its eigenstates. The states |ϕ⟩ and |ψ⟩ of Tim's example cannot be written as eig... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/523",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "17",
"answer_count": 2,
"answer_id": 0
} |
Can the coefficient of static friction be less than that of kinetic friction? I was recently wondering what would happen if the force sliding two surfaces against each other were somehow weaker than kinetic friction but stronger than static friction. Since the sliding force is greater than the maximum force of static f... | This answer is speculative - not based on my experience with friction.
Logically, there's no reason kinetic friction has to be velocity-independent. You could have kinetic friction that increases with velocity. That way, if you push on something with more force than static friction, the thing would accelerate up to s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/541",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "24",
"answer_count": 6,
"answer_id": 0
} |
How do neutron stars burn? Is it decay or fusion or something else?
*
*What makes a neutron star burn, and what kind of fusion/decay is happening there?
*What is supposed to happen with a neutron star in the long run? What if it cools, then what do the degenerated matter looks like after it cools? Will
the gravitat... | Neutron stars can actually "burn" and leave quark matter as an ash, according to some hypotheses.
The Bodmer-Witten hypothesis states that quark matter made of up, down, and strange quarks might be the most stable form of matter. If one could find a way to melt neutrons and protons into stable, strange quark matter, t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/567",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "12",
"answer_count": 5,
"answer_id": 1
} |
Two-point correlation function for planar Potts model Fastest known method for computing Potts model partition function (Bedini and Jacobsen's "A tree-decomposed transfer matrix for computing exact Potts model partition functions for arbitrary graphs, with applications to planar graph colourings") uses a "tree decompos... | *
*First, what we clam is that our method is the best for arbitrary planar graphs, due to the fact that the treewidth for a planar graph of size N scales a O(N^1/2). For the particular case of the square lattice, our method is not different from the traditional TM, since a strip of width L has a tree decomposition of ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/613",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Why does one experience a short pull in the wrong direction when a vehicle stops? When you're in a train and it slows down, you experience the push forward from the deceleration which is no surprise since the force one experiences results from good old $F=m a$. However, the moment the train stops one is apparently pull... | You are pulled wrong way by your own hands and legs, which were stressed to keep you decelerating together with the vehicle.
When the vehicle suddenly stopped accelerating (this is how friction works: it is opposite to speed, and then speed suddenly reaches zero) then this tension keeps pulling you until you react and ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/629",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "43",
"answer_count": 17,
"answer_id": 1
} |
logarithmic wind speed profile Under some atmospheric stability condition, over flat terrain, it has been observed for a while that the ratio between wind speed at height $h_1$ above the earth and the wind speed at height $h_0$ is $\log\frac{h_1}{h^*}/\log\frac{h_0}{h^*}$ where $h^*$ is related to the terrain (called r... | My guess is this is a property of turbulent boundary layers over rough surfaces. You may be able to associate the roughness and windspeed to a Strouhal number, and together with the Raynolds number give you at what situation you have this $\log()$ law be dominant.
As for a derivation of this behavior, I am hoping someo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/639",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Learning physics online? I'm thinking of following some kind of education in physics online. I have a master degree in Computer Science and have reasonable good knowledge in physics. I would like a program of 1-2 years and I'm more interested in particle physics.
Is there any good online program that offer something s... | Here are a few that don't appear to have been mentioned:
[EDIT: I will list one per answer, as the spam filter won't allow me to post more than one link at a time.]
*
*I second MIT's OCW as the very best resource for self-learners.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/679",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "28",
"answer_count": 16,
"answer_id": 5
} |
Mathematics of AdS/CFT To date, what is the most mathematically precise formulation of the AdS/CFT correspondence, and what are the most robust tests of the conjecture?
| when one talks about "truly rigorous" mathematical physics, there is really no good treatment of ordinary quantum field theory yet. So of course, there is no "truly rigorous" framework to discuss AdS/CFT whose one side is a quantum field theory and another side is something even more complicated - a description of a qu... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/756",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "22",
"answer_count": 3,
"answer_id": 0
} |
Books that every physicist should read Inspired by How should a physics student study mathematics? and in the same vein as Best books for mathematical background?, although in a more general fashion, I'd like to know if anyone is interested in doing a list of the books 'par excellence' for a physicist.
In spite of the ... | *
*Nonlinear Optics - Robert Boyd
*Photons-Atom Interactions - Cohen Tannoudji
*Photons - Cohen Tannoudji
*Classical Field Theory - A.O Barout (Dover book)
*Problems in General Physics -I.E Irodov (Undergrad/Highschool problem sets)
*Classical Field Thory - Jan Rezwuzki (extremely rare!, Polish academy of science... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/884",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "29",
"answer_count": 24,
"answer_id": 10
} |
Books that develop interest & critical thinking among high school students I heard about Yakov Perelman and his books. I just finished reading his two volumes of Physics for Entertainment. What a delightful read! What a splendid author. This is the exact book I've been searching for. I can use it to develop interest fo... | try
The Cartoon Guide to Physics by Larry Gonick
funny and smart!
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/893",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "26",
"answer_count": 11,
"answer_id": 1
} |
PSF Measurements In Fluorescence Imaging Quite a technical question!
I have measured the Point Spread Function of 100nm fluorescent breads with my Olympus scanning head. I'm two-photon exciting the beads with a wavelength of 800nm and focused in the sample with a 100x with N.A.: 1.4
The theory suggests me the resolutio... | I've found the solution to my problem. First of all, I had a factor 2 discrepancy due to the pixel dimension and, more important from an optical point of view, the laser beam was underfilling the entrance pupil of the objective. This means the focusing was worse!
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/946",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 0
} |
Explanation: Simple Harmonic Motion I am a Math Grad student with a little bit of interest in physics. Recently I looked into the Wikipedia page for Simple Harmonic Motion.
Guess, I am too bad at physics to understand it. Considering me as a layman how would one explain:
*
*What a Simple Harmonic motion is? And why... | I guess this thread shows everyone has their own tastes when it comes to this topic!
First of all, it's called harmonic motion because sine and cosine are the elementary harmonic functions. Recall that in general a harmonic function is a solution of Laplace's equation (which shows up everywhere in physics), and in we ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1018",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 9,
"answer_id": 1
} |
Common false beliefs in Physics Well, in Mathematics there are somethings, which appear true but they aren't true. Naive students often get fooled by these results.
Let me consider a very simple example. As a child one learns this formula $$(a+b)^{2} =a^{2}+ 2 \cdot a \cdot b + b^{2}$$ But as one mature's he applies t... | Notion of simultaneity. Because of speed of light is so big, it looks true in our day to day affairs.
But it really is a non existent thing [due to special relativity].
2 people in 2 different places can't say "at the same instant".
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1019",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "53",
"answer_count": 49,
"answer_id": 24
} |
How do contact lenses work? I understand how telescope, microscope and glasses work.
But how do contact lenses work?
| Contact lenses work in the same way as glasses, by adding or subtracting wavefront curvature.
This basically adjusts where the focal point of light entering the eye is, glasses and contact lenses are designed to adjust it so that the focal point of the light lies on the retina.
The main differences I can see between gl... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1037",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 2
} |
Home experiment to estimate Avogadro's number? How to get an approximation of Avogadro or Boltzmann constant through experimental means accessible by an hobbyist ?
| To get a rough estimate of the Avogadro number, one can also use a method similar to that used by Loschmidt ([1] http://iweb.tntech.edu/tfurtsch/Loschmidt/LOSCHMID.HTML). Gas viscosity can be measured (see, e.g., http://www.phywe.com/index.php/fuseaction/download/lrn_file/versuchsanleitungen/P3010201/e/LEC01_02_LV.pdf ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1075",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "19",
"answer_count": 4,
"answer_id": 3
} |
What's the difference between helicity and chirality? When a particle spins in the same direction as its momentum, it has right helicity, and left helicity otherwise. Neutrinos, however, have some kind of inherent helicity called chirality. But they can have either helicity. How is chirality different from helicity?
| Helicity and chirality are not the same thing in the massless limit. They are unrelated. Helicity is an extrinsic physical property related to the alignment of spin and momentum; chirality is related to weak interactions. Chirality is more akin to electric charge or strong color charge than it is to momentum.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1111",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "40",
"answer_count": 5,
"answer_id": 3
} |
What is the difference between "kinematics" and "dynamics"? I have noticed that authors in the literature sometimes divide characteristics of some phenomenon into "kinematics" and "dynamics".
I first encountered this in Jackson's E&M book, where, in section 7.3 of the third edition, he writes, on the reflection and r... | In mechanical systems, I would say the difference is whether the forces involved are due to static or quasi-static situations in which the forces are due to weight/gravity, springs, etc. If the forces result from accelerations then we have a dynamic system, whereas the former would be a kinematic system.
In the transmi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1135",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "84",
"answer_count": 14,
"answer_id": 11
} |
Why is it hopeless to view differential geometry as the limit of a discrete geometry? This is a follow-up question to Introductions to discrete space-time:
Why is this line of thought hopeless?
Classical mechanics can be understood
as the limit of relativistic mechanics
$RM_c$ for $c \rightarrow \infty$.
Classical... | Not exactly sure what you are asking. For a cubic lattice, the limit as the lattice spacing goes to 0 recovers all of classical physics, and, if you want to discretize time, then as $\Delta{}t$ approaches 0, then finite difference equations become differentials.
For example, Low energy sound waves in a crystal don't se... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1251",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 1
} |
What sustains the rotation of earth's core (faster than surface)? I recently read that the earth's core rotates faster than the surface.
Well, firstly, it's easier to digest the concept of planetary bodies, stars, galaxies in rotation and/or orbital motion.
But, what makes a planet's core rotate? And in the earth's cas... | On a planetary scale the earths surface is a good insulator and we do not lose significant net heat to space or gain net heat from the sun. So heat generated may build up over time. Radioactive decay heat and gravitationally produced friction would tend to melt the interior of a sufficiently large rocky planet. Heavy ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1336",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 6,
"answer_id": 2
} |
Best example of energy-entropy competition? What are the best examples in practical life
of an energy-entropy competition which favors entropy over energy?
My initial thought is a clogged drain -- too unlikely for the
hair/spaghetti to align itself along the pipe -- but this is probably
far from an optimal example. Cu... | If you are willing to go down to microscopic scales, a nice example of "entropy winning" is the phenomenon of depletion forces. Large particles in a suspension of smaller ones feel an effective attractive force, even if the interaction between all particles is just hard-wall. The attractive force arises because the vol... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1354",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 7,
"answer_id": 5
} |
Is it possible to separate the poles of a magnet? It might seem common sense that when we split a magnet we get 2 magnets with their own N-S poles. But somehow, I find it hard to accept this fact. (Which I now know is stated by the magnetic Gauss's Law $\vec{\nabla}\cdot \vec{B} =0.$)
I have had this doubt ever since r... | I suspect your problem is you may want to think about it rhetorically. Magnetic poles are really just a mental shortcut useful to provide a bit of intuition to something that is inherently just math. We don't have physical entities called mag poles, we have a magnetic field, and it works as if it were generated by curr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1402",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "15",
"answer_count": 8,
"answer_id": 1
} |
How efficient is an electric heater? How efficient is an electric heater?
My guess: greater than 95%. Possibly even 99%. I say this because most energy is converted into heat; some is converted into light and kinetic energy, and possibly other forms of energy.
Anyone other opinions? (This is not homework. I am just cu... | It was a good answer Mark. Of course by drawing a lot of current some Joule heating will happen outside the house as well, in the transmission lines and transformers especially. So the efficiency will get lower depending upon where you draw the (electrical box). Some of the energy from the TV and refrigerator will also... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1493",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "27",
"answer_count": 7,
"answer_id": 0
} |
Is it possible to obtain gold through nuclear decay? Is there a series of transmutations through nuclear decay that will result in the stable gold isotope ${}^{197}\mathrm{Au}$ ? How long will the process take?
| I guess you are really looking for this wikipedia page : http://en.wikipedia.org/wiki/Synthesis_of_noble_metals#Gold .
In short, there are gold synthesis technique, but they apparently all need some external energy (either $\gamma$-ray or neutron capture) and are not restricted to nuclear decay. One of them has for int... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1530",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 7,
"answer_id": 4
} |
Which experiments prove atomic theory? Which experiments prove atomic theory?
Sub-atomic theories:
*
*atoms have: nuclei; electrons; protons; and neutrons.
*That the number of electrons atoms have determines their relationship with other atoms.
*That the atom is the smallest elemental unit of matter - that we ca... | I think that the points made about Einstein's theoretical explanation for the observed Brownian motion and the observed Perrin experiments on it are quite valid. But perhaps one could quibble that actually the forces on the pollen were produced by molecules...not by atoms... and perhaps one could resist the point by w... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1566",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "17",
"answer_count": 6,
"answer_id": 4
} |
Spectral Line Width and Uncertainty principle so I've been at this for about 3 - 4 hours now. It is an homework assignment (well part of a question which i've already completed). We did not learn this in class. All work is shown below.
An atom in an excited state of $4.9 eV$ emits a photon and ends up in the ground st... | Okay so a buddy helped me out.
You had to use the following formula:
$$ \Delta \lambda = hc \( \frac{1}{E_1} - \frac{1}{E_2} \) $$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1610",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
What's the difference between running up a hill and running up an inclined treadmill? Clearly there will be differences like air resistance; I'm not interested in that. It seems like you're working against gravity when you're actually running in a way that you're not if you're on a treadmill, but on the other hand it s... | For sake of argument I will compare a climber maintaining constant speed up a hill and a treadmill runner. If the climber suddenly stopped spending any amount of energy climbing the hill, gravity will tug on him as $mg sin \theta$ while additionally, friction acts in the same direction to slow him down. If a person on ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1639",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "27",
"answer_count": 10,
"answer_id": 9
} |
Ice skating, how does it really work? Some textbooks I came across, and a homework assignment I had to do several years ago, suggested that the reason we can skate on ice is the peculiar $p(T)$-curve of the ice-water boundary. The reasoning is that due to the high pressure the skates put on the ice, it will melt at tem... | The assertion that the skate does not exert enough pressure to melt ice is wrong. Imagine that the skate is lowered vertically until it touches a perfectly flat surface of ice. The initial contact area (before the blade starts to sink into the ice) would be incalculably small and the initial pressure incalculably lar... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1720",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "41",
"answer_count": 7,
"answer_id": 2
} |
Where can we find information of International Physics/Astrophysics conferences? Where do you check and put them usually?
Let's make a wiki ~
| There is a number of sites which lists conferences, e.g.
*
*Conference Listing in Mathematics, Physics and Chemistry - Mandl (this one is good and aesthetic; once I attended a conference I had found there)
Also:
*
*Associations/societies list their conferences
*Some research groups homepages have list on confe... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1796",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 4,
"answer_id": 1
} |
Hawking radiation and quark confinement The simple picture of Hawking radiation is that a pair-antiparticle pair is produced near the event horizon, then one falls into the black hole while the other escapes. Suppose the particles are quarks-antiquarks, which experience quark confinement thanks to QCD. If one of them i... | Most calculations of the Hawking effect assume free quantum fields. This assumption breaks down for strongly coupled quantum chromodynamics. As the Hawking temperature is much lower than the QCD deconfinement temperature, there isn't enough energy to hadronize "virtual" quark-antiquark pairs. Instead, the particle just... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1843",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "15",
"answer_count": 3,
"answer_id": 2
} |
Why are physicists interested in graph theory? Can you tell me how graph theory comes into physics, and the concept of small world graphs?
(inspired to ask from comment from sean tilson in):
Which areas in physics overlap with those of social network theory for the analysis of the graphs?
| Graph theory is very useful in design and analysis of electronic circuits.
It is very useful in designing various control systems.
E.g. Signal Flow Graphs and Meson's Rule make your life a lot easier while trying to find transfer functions.
Also, while solving differential equations numerically Graph Theory is used for... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1876",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "24",
"answer_count": 5,
"answer_id": 4
} |
What does it mean that the universe is "infinite"? This question is about cosmology and general relativity.
I understand the difference between the universe and the observable universe. What I am not really clear about is what is meant when I read that the universe is infinite.
*
*Does it have infinite mass or is it... | If the basic question is how we define whether the universe is finite or infinite, then the most straightforward answer is that in a finite universe, there is an upper bound on the proper distance (which is defined as the distance between two points measured by a chain of rulers, each of which is at rest relative to th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1915",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
"answer_count": 3,
"answer_id": 0
} |
What is the difference between a white object and a mirror? I was taught that something which reflects all the colors of light is white. The function of a mirror is the same, it also reflects all light. What's the difference?
Update:
But what if the white object is purely plain and does not scatter light? Will it be a ... | White color is associated with reflected (not absorbed) light. White paint usually includes a titanium oxide component, whose absorption is in UV.
The difference between a shiny surface and a Lambertian surface is its roughness, if light is reflected in a collective manner it look shiny. Any smooth surface is shiny giv... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1957",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "48",
"answer_count": 7,
"answer_id": 1
} |
What is terminal velocity? What is terminal velocity? I've heard the term especially when the Discovery Channel is covering something about sky diving. Also, it is commonly known that HALO (Hi-Altitude, Lo-Opening) infantry reaches terminal velocity before their chutes open.
Can the terminal velocity be different for... | You can find a good article here: http://en.wikipedia.org/wiki/Terminal_velocity
In the context you provide, terminal velocity is the maximum speed that an object in free fall reaches in the atmosphere.
When an object is falling, or in free fall, there are two forces that determine whether it will accelerate downwards ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1989",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 4,
"answer_id": 1
} |
How come gas molecules don't settle down? If the earth's gravity exerts a net downward gravitational force on all air molecules, how come the molecules don't eventually lose their momentum and all settle down? How is the atmosphere is still miles thick after billions of years?
| The other answers are correct but to understand them you have to get an idea of how much thermal energy does an average molecule have.
According to Maxwell-Boltzmann distribution, the most probable speed of an air (say, nitrogen) molecule at room temperature is $v_p = \sqrt { \frac{2kT}{m} } = 422 m/s$. Without collisi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2032",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
"answer_count": 10,
"answer_id": 0
} |
Good introductory papers and books on laser physics and pulsed lasers I am looking for good introductory papers and/or books on the principles of lasers. In particular, I am interested in pulsed laser technology.
I understand that
Gould, R. Gordon (1959). "The LASER, Light Amplification by Stimulated Emission of Radi... | This answer contains some additional resources that may be useful. Please note that answers which simply list resources but provide no details are strongly discouraged by the site's policy on resource recommendation questions. This answer is left here to contain additional links that do not yet have commentary.
*
*R... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2084",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 1,
"answer_id": 0
} |
Is it possible for information to be transmitted faster than light by using a rigid pole? Is it possible for information (like 1 and 0s) to be transmitted faster than light?
For instance, take a rigid pole of several AU in length. Now say you have a person on each end, and one of them starts pulling and pushing on hi... | The information about the pushes will be received on the other end with the speed of sound in the substance of the pole. For any real material it is much slower than the speed of light (for a steel rod it would be about 5000 m/s).
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2175",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "117",
"answer_count": 16,
"answer_id": 8
} |
Anti-gravity in an infinite lattice of point masses Another interesting infinite lattice problem I found while watching a physics documentary.
Imagine an infinite square lattice of point masses, subject to gravity. The masses involved are all $m$ and the length of each square of the lattice is $l$.
Due to the symmetrie... | This is not correct, but Newton believed this. The infinite system limit of a finite mass density leads to an ill defined problem in Newtonian gravity because $1/r^2$ falloff is balanced by density contributions of size $r^2\rho$, and there is no well defined infinite constant-mass-density system. The reason is that th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2196",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "20",
"answer_count": 4,
"answer_id": 1
} |
If photons have no mass, how can they have momentum? As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects being affected by gravity. However, momentum is the product of mass an... | The reason why the path of photons is bent is that the space in which they travel is distorted. The photons follow the shortest possible path (called a geodesic) in bent space. When the space is not bent, or flat, then the shortest possible path is a straight line. When the space is bent with some spherical curvature, ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2229",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "203",
"answer_count": 10,
"answer_id": 0
} |
Why are materials that are better at conducting electricity also proportionately better at conducting heat? It seems like among the electrical conductors there's a relationship between the ability to conduct heat as well as electricity. Eg: Copper is better than aluminum at conducting both electricity and heat, and sil... | See http://en.wikipedia.org/wiki/Thermal_conductivity In metals, I think it generally has to do with the higher valence band electron mobility, but it gets more interesting elsewhere.
In metals, thermal conductivity approximately tracks electrical conductivity according to the Wiedemann-Franz law, as freely moving v... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2245",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
"answer_count": 3,
"answer_id": 2
} |
Polar vs non-polar fluid 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... | It is about the stress tensor; it is almost always assumed that it is symmetric to satisfy angular momentum conservation. Yet, there are some fluids capable of creating rotation from squeezing (like those spintops with pistons) and thus have some antisymmetric part in their stress tensors.
Aris just calls those fluids ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2364",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 6,
"answer_id": 1
} |
Do high/low pass lenses exist? 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 l... | If you need extremely sharp filters because your wavelengths in question are either close together or need to be sepearated by a high degree, look at filters from http://www.semrock.com (no affiliation).
Other than that a spectral device (grating or prism based) combined with knife edges as described by hwlau is best,... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2398",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 4,
"answer_id": 2
} |
What is the physical meaning of the connection and the curvature tensor? 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 Ricc... | As for the 'physical meaning' of Christoffel symbols, there is a sense in which they don't have a physical meaning, because the information they encode is not really information about the curvature of space but about the geometry of the coordinate system you're using to describe the space.
As for an intuition about the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2447",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "47",
"answer_count": 4,
"answer_id": 1
} |
Is there a limit to loudness? Is there any reason to believe that any measure of loudness (e.g. sound pressure) might have an upper boundary, similar to upper limit (c) of the speed of mass?
| The only practical limit to sound-pressure might be when the medium were to be compressed into a black hole. Although I do not know about the sound-propagating features of black holes. Long before that the medium would come apart, f.e. into plasma.
After all, you compare two things: an upper limit of a speed with a pow... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2523",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 4,
"answer_id": 3
} |
The final death of a black hole 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?
| Hawking radiation is a very slow process of the black hole losing energy and shrinking. If you counter this by supplying a little bit of matter or energy falling into the black hole you can easily overcome it and sustain the black hole.
Other than Hawking radiation I don't think there is any known process for black ho... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2558",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
"answer_count": 5,
"answer_id": 1
} |
How does mass leave the body when you lose weight? When your body burns calories and you lose weight, obviously mass is leaving your body. In what form does it leave? In other words, what is the physical process by which the body loses weight when it burns its fuel?
Somebody said it leaves the body in the form of h... | I encountered this question by accident and also had the same question a while ago. I found a TED talk about this subject with the title The mathematics of weight loss. The author is Ruben Meerman, he describes himself as:
I am [sic!] surfer with a physics degree who fell in love with science
communication.
The vi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2605",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "119",
"answer_count": 18,
"answer_id": 14
} |
How fast a (relatively) small black hole will consume the Earth? This question appeared quite a time ago and was inspired, of course, by all the fuss around "LHC will destroy the Earth".
Consider a small black hole, that is somehow got inside the Earth. Under "small" I mean small enough to not to destroy Earth instant... | It would take a long time if we do a back of the envelope calculation.
*
*the black hole would exert a force of 1g at around 20 km (assuming 10^20 kg of mass).
*if we can reasonably assume that the mass inside this sphere is going to be absorbed quickly, that would mean the black hole mass increases correspondingly... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2743",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "37",
"answer_count": 7,
"answer_id": 4
} |
Rotate a long bar in space and get close to (or even beyond) the speed of light $c$ Imagine a bar
spinning like a helicopter propeller,
At $\omega$ rad/s because the extremes of the bar goes at speed
$$V = \omega * r$$
then we can reach near $c$ (speed of light)
applying some finite amount of energy
just doing
$$\ome... | In your calculations you assume that your propeller is a rigid body.
You cannot use that assumption, when your speeds are not much smaller than the speed of light. Because "there are no rigid bodies in relativity".
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2774",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "16",
"answer_count": 6,
"answer_id": 0
} |
Why does my wife's skin buzz when she's using her laptop? When my wife uses her laptop, if I touch her skin, I can feel a buzz. She doesn't feel the buzz, but she can hear it if I touch her ear.
So I'm guessing it's a faulty laptop, and she's conducting an electrical current.
But why would she not feel anything, and w... | 1 - Use a voltage detector pen in USB connector GND to see if there is a bad isolation between AC ground and DC ground (this is first option not because probability, but because it could be dangerous). It's normal a little fugue current between primary and secondary of transformer but the AC plug have 3 conectors, 2 f... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2824",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "24",
"answer_count": 5,
"answer_id": 3
} |
Why GPS is at LEO? Why GPS/GLONASS/Galileo satellites are on low earth orbit?
Why geostationary orbit is so bad? Sattelites might be placed there 'statically' and more precise...
The only problem I can see is navigation close to poles, but they have this problem anyway.
| This question is slightly faulty... Part of the US GPS system is geostationary (the WAAS component). It's used in conjunction with the non-geostationary birds for higher precision fixes. While primarily used for aircraft instrument approaches, there are off the shelf USB GPS computer peripherals that use WAAS.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2900",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 4,
"answer_id": 1
} |
What is planetary surface temperature given constant sub-surface temperature? If a planet of radius $R_1$ has a constant sub-surface temperature $T_0$ at $R_0<R_1$, what is the long-term equilibrium surface temperature $T_1$? Say we assume constant thermal diffusivity of the planet material, surface emissivity $1.0$, ... | You want to equate the conductive energy flux at the surface which is a constant times T1-T0 -given your geometry and conductivity you can determine the coupling factor, with the radiative heat flux sigma*T**4. You could also add in the CMB, which is simply the same sigma*T**4 using the CMB temperature. Then you simply... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/3035",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 5,
"answer_id": 0
} |
movement of photons In a typical photon experiment the photon is depicted as moving across the page, say from right to left.
Suppose we were actually able to witness such an experiment, from the side (to position of reader to a page).
If the photon is actually moving from left to right can I, standing at 90 degrees to... | Being very careful, we'll assume you mean a situation in which there exists only a single photon and two detectors placed at right angles to one another, all of which exists in a vacuum and effectively isolated from the rest of the universe: the answer would be 'no'. In practice of course, very few photons and detector... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/3070",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 2
} |
When one thinks of a field of operators in QFT, is it reasonable to think of a matrix being associated with each point in space time? Is it correct to visualize operators existing as matrices parameterized by spacetime coordinates in the context of QFT?
| I'm not quite sure if you're asking the same question in the title of your post as you are in the body of your post. But the answer to the question in your title is yes, you can think of an operator field as having a matrix associated with each point in space. That's what a field is, after all: a mapping that associate... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/3107",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 2
} |
Why is quantum entanglement considered to be an active link between particles? From everything I've read about quantum mechanics and quantum entanglement phenomena, it's not obvious to me why quantum entanglement is considered to be an active link. That is, it's stated every time that measurement of one particle affect... | In fact your view is quite close to the 'official' one; entanglement occurs just because both particles are described with one wave-function; the magic is in our classical habit of thinking that separate objects are described with separate "coordinates".
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/3158",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "115",
"answer_count": 9,
"answer_id": 3
} |
Vortex in liquid collects particles in center At xmas, I had a cup of tea with some debris at the bottom from the leaves. With less than an inch of tea left, I'd shake the cup to get a little vortex going, then stop shaking and watch it spin. At first, the particles were dispersed fairly evenly throughout the liquid,... | I jokingly call it Vortex Physics, as it doesn't appear to obey the normal rules. At high speed, where centrifugal force prevails, the heavier particles are forced to the outside, like in a Dyson vacuum cleaner, or industrial cyclone dust separator, from which he got the idea. But at very low speed when the leaves star... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/3244",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "19",
"answer_count": 9,
"answer_id": 6
} |
Where's the best place to add weight to a Pinewood Derby car? A little background: a Pinewood Derby car is a small wooden car that races down an inclined track, powered only by gravity. You are allowed to add weight to the car up to a certain limit.
Here is a recommendation to add the weight to the upper back of the ca... | UPDATE: Having totally misunderstood what a pinewood derby car was, my previous answer probably wasn't very helpful (I thought it was like a go-cart). Here is a proper answer:
First off, let me say the recommendation you read is still wrong, for reasons that will be seen in a minute.
The car starts off inclined at an... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/3296",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 0
} |
Is there a maximum possible acceleration? I'm thinking equivalence principle, possibilities of unbounded space-time curvature, quantum gravity.
| I like previous answer but:
1) I believe that in the provided formula the mass of the electron should have a power of one (not two)
2) It is valid for electrons only, because it uses their Compton wavelength.
By the way, there is such a thing as "Caianiello’s maximal acceleration". In his 1985 paper Caianiello demonstr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/3334",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "27",
"answer_count": 4,
"answer_id": 1
} |
What would the electromagnetic field of a massless electron look like The Standard Model gives non-zero mass to the electron via the coupling to the Higgs field. Issues of renormalizability aside, this is fundamentally unrelated to the fact that the electron couples to the EM field. However, if the Higgs mechanism did ... | With a massless charged fermion, the electromagnetic coupling strength will be totally screened, with $1/e^2$ increasing logarithmically with the scale $R$. The electric field will be Coulomb multiplied by this logarithmic factor in $R$.
Actually, the massless charged fermion will be travelling at the speed of light, b... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/3394",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 2,
"answer_id": 1
} |
Your favorite Physics/Astrophysics blogs? What are the Physics/Astrophysics blogs you regularly read? I'm looking to beef up my RSS feeds to catch up with during my long commutes. I'd like to discover lesser-known gems (e.g. not well known blogs such as Cosmic Variance), possibly that are updated regularly and recently... | Skulls in the Stars
Exceptional for its extremely clear, basic-level introductions to phenomena in optics I would otherwise never have heard of, and for interesting historical posts resulting from digging around in the archives of old journals and scientists' personal letters.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/3432",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "33",
"answer_count": 24,
"answer_id": 17
} |
Expansion of the space-time metric If the space-time metric is expanding with the expansion of the universe, if I could travel back in time, would I be less dense than the matter in that previous era?
| The answer is 'no'. The Hubble expansion does not enlarge the distance between atoms in your body. It even does not enlarge distances between stars in our Milky Way. (Nor between the Milky Way and Andromeda for that matter.)
This question has been asked here more often. See for instance the answers in: Why does space e... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/3518",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Why does a ballerina speed up when she pulls in her arms? My friend thinks it's because she has less air resistance but I'm not sure.
| A very simple explanation is the following: the arms of the ballerina are pulled outwards by the centripetal force she experiences by spinning. When she pulls her arms in, she is doing work by more than counteracting this force and this is what makes her spin faster.
This is due to the fact that the spin velocity is re... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/3611",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "12",
"answer_count": 5,
"answer_id": 1
} |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.