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Q&A

General Q&A about physics of any type and at any level

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81%
+7 −0
Unusual way to write spacetime coordinates/metric: Is there any downside?

In special relativity, spacetime coordinates are normally given as $$(ct, x, y, z) \tag{S}$$ with the metric being either $$g = \operatorname{diag}(-1,1,1,1) \tag{+}$$ or $$g = \operatorname{d...

1 answer  ·  posted 3y ago by celtschk‭  ·  last activity 3y ago by dmckee‭

80%
+6 −0
How does probability conservation work in Dirac's original formulation of relativistic QM?

I asked this question on Stack Exchange, and didn't get an answer, but maybe someone here will be able to help. In non-relativistic quantum mechanics, the normalization condition for position eige...

1 answer  ·  posted 3y ago by Technically Natural‭  ·  last activity 3y ago by Derek Elkins‭

80%
+6 −0
How do constraints work in Lagrangian systems?

I have a question about the discussion of constrained Lagrangian systems in the book Mathematical Aspects of Classical and Celestial Mechanics by Arnold et al. (section 1.2.5). The Lagrangian syst...

1 answer  ·  posted 3y ago by Technically Natural‭  ·  last activity 3y ago by Derek Elkins‭

80%
+6 −0
What is inflation in cosmology?

I am having trouble understanding what a cosmic inflation is. Please try to suffice an explanation for non physicists / non formal students for physics / general audience. I understand that an ...

2 answers  ·  posted 3y ago by deleted user  ·  last activity 2y ago by deleted user

77%
+5 −0
Why is it forbidden for two photons to turn into one?

In the context of quantum field theory, why is it impossible for two photons (or other massless bosons like gluons) to collide and produce a single photon? This kind of a process is supposed to be ...

2 answers  ·  posted 10mo ago by Technically Natural‭  ·  last activity 10mo ago by Derek Elkins‭

77%
+5 −0
Should I always write units in equation no matter if it looks like variable?

I am not sure if it is suitable in the site. I am asking the question here cause Olin and Celtschk said it here. They told me to use unit in every "single line" (Nope! They didn't say like it. Oli...

2 answers  ·  posted 2y ago by deleted user  ·  last activity 2y ago by Canina‭

75%
+4 −0
What is the meaning that the universe is flat?

I have heard the physicist Laurence Krauss claiming that he believes that the universe is flat. Okay, but if we take four astronomers and we put one in the north pole, one in the south pole, one i...

2 answers  ·  posted 2y ago by deleted user  ·  last activity 2y ago by celtschk‭

75%
+4 −0
How are the assumptions behind two ways of deriving the Rayleigh-Jeans law related?

The Rayleigh-Jeans law does a good job of describing the spectral radiance of a black body at low frequencies: $$B_{\nu}(T)=\frac{2kT\nu^2}{c^2}$$ with $T$ the temperature and $\nu$ the frequency...

1 answer  ·  posted 2y ago by HDE 226868‭  ·  last activity 2y ago by Derek Elkins‭

75%
+4 −0
If you're stopped and about to be hit from behind, should you brake or release the brake?

The title is a framing for a theoretical question; I'm not asking for practical advice. A friend was recently in this situation and my attempts to apply what I remember of a couple semesters of co...

1 answer  ·  posted 1y ago by Monica Cellio‭  ·  last activity 1y ago by Olin Lathrop‭

75%
+4 −0
What's the meaning of "outdated" in physics?

I was recently reading some questions here and there saying that "relativistic mass is outdated". I saw someone saying that "outdated" doesn't mean the concept is wrong. My question is what physici...

1 answer  ·  posted 2y ago by deleted user  ·  edited 10mo ago by Reinstate Monica on Stack Exchange‭

75%
+4 −0
Is it possible to derive the Dieterici equation starting from assumptions about microstates?

I was introduced to a somewhat novel derivation of the ideal gas law that starts by thinking about the number of microstates of an ideal gas, $\Omega$. Say we have a gas with a single particle in a...

0 answers  ·  posted 2y ago by HDE 226868‭

71%
+3 −0
Calculate Center of Thrust

If I have a rocket with some known number of engines $n$, each producing thrust with arbitrary direction $T_n$ and center of thrust $r_n$, the total thrust amount and direction $T_{total}$ is equal...

1 answer  ·  posted 2y ago by Josh Hyatt‭  ·  last activity 2y ago by Olin Lathrop‭

71%
+3 −0
How to find position of a particle at a time given a position dependent force

If we have a force which changes depending on the position of a particle, how can we find the position of the particle at some time $t$? We can find its velocity if it has travelled a given distan...

1 answer  ·  posted 2y ago by MissMulan‭  ·  edited 1y ago by Trilarion‭

71%
+3 −0
How exactly do eddy currents slow down objects moving though a magnetic field

When a pendulum made of a conducting material moves through a magnetic field, it's a well-established fact that it experiences a retarding force, thus slowing it down, however, I'm unable to unders...

1 answer  ·  posted 1y ago by esrdtfghjk‭  ·  last activity 1y ago by Olin Lathrop‭

71%
+3 −0
What is "order" and "disorder" in entropy?

What is "disorder" in entropy? Entropy is measurement of "disorder". (Some says it's not "disorder") I had read "disorder" and "order" of entropy in my book. But I was wondering what "order" ...

2 answers  ·  posted 2y ago by deleted user  ·  last activity 2y ago by Derek Elkins‭

71%
+3 −0
What is virtual work?

$$\sum_i F_i \cdot \delta r_i$$ is virtual work when internal force is $0$. For that reason, $$\sum_i F_i \cdot \delta r_i = 0$$ Here internal force stands for what? When a object's displacement ...

1 answer  ·  posted 2y ago by deleted user  ·  last activity 2y ago by deleted user

71%
+3 −0
How can the kinetic energy equation be intuitively understood?

Momentum is proportional to an object's velocity, and kinetic energy is proportional to the square of its velocity $\dfrac{mv^2}{2}$. It's pretty intuitive that if object B is going twice as fast a...

3 answers  ·  posted 2y ago by gmcgath‭  ·  last activity 2y ago by celtschk‭

71%
+3 −0
Why series springs behave like parallel? A comparison between parallel resistance and series spring.

I know that equation for parallel resistance is $$\frac{1}{r_{tot}}=\sum_i \frac{1}{r_i}$$ But i wonder to see equation of series spring constant. If we add multiple spring in series. Then their ...

1 answer  ·  posted 2y ago by deleted user  ·  last activity 2y ago by Olin Lathrop‭

71%
+3 −0
Calculating Max humidity for a house

I want to write some code to control my whole house humidifier. I want my code to calculate the percentile relative humidity above which dew will form on my windows. I have sensors for air tempera...

0 answers  ·  posted 2y ago by re89j‭

71%
+3 −0
Interaction terms in Srednicki's proof of spin-statistics theorem

In Srednicki's proof of the spin-statistics theorem for scalar fields ("Quantum Field Theory", section 4), he considers interaction terms, added to the free Hamiltonian, that are Hermitian function...

1 answer  ·  posted 3y ago by Technically Natural‭  ·  last activity 3y ago by Mithrandir24601‭

66%
+2 −0
Motion of charged particle inside a magnetic field

We have place a charged particle of 2C with mass 2kg, 1mm above a current-carrying wire of 1A.The charged particle has an initial velocity of 100m/s The magnetic field of the wire for simplicity w...

0 answers  ·  posted 2y ago by MissMulan‭  ·  edited 2y ago by MissMulan‭

66%
+2 −0
Does an ultrasonic air humidifier use energy less energy than a steam humidifier?

There are a couple different types of indoor air humidifiers. Most notably, the naive steam humidifier with a heating element, and then ultrasonic humidifiers. Now, heating water to create steam o...

1 answer  ·  posted 5mo ago by Iizuki‭  ·  last activity 5mo ago by Olin Lathrop‭

66%
+2 −0
Applying Young-Laplace equation on meniscus formed due to rise of liquid on a single plate

Let's say we have a single plate with liquid on both sides rising up due to surface tension. The meniscus formed has a radius of curvature $R$. I'm trying to find the excess pressure, i.e, the pre...

0 answers  ·  posted 3y ago by TripleFault‭

66%
+2 −0
Osmosis in a U-shaped tube with selective permeable membrane

Starting from the classical osmosis experiment, a U-shaped tube with a semi-permeable membrane, I would like to consider the case when the solute added to one of the compartments (labelled A) is co...

0 answers  ·  posted 8mo ago by Joce‭

66%
+4 −1
Why should there not be water below 6kms in the Earths crust?

In 1970, the USSR began drilling at the Kola Super-deep Borehole. The target depth was 15,000 meters; The stated areas of study were the deep structure of the Baltic Shield, seismic discontinuit...

0 answers  ·  posted 3y ago by Conrado‭  ·  edited 3y ago by Conrado‭

66%
+2 −0
What is Ether theory? (I think the book I read is misinterpreting Ether)

I was reading about Ether, Maxwell and Young-double-slit experiment. I got a question about Ether law. Let me say what I read about Ether Law. Suppose, a boat is traveling with Adverse of Water ...

2 answers  ·  posted 2y ago by A person who doesn't know anything‭  ·  edited 2y ago by A person who doesn't know anything‭

66%
+2 −0
What is semiholonomic?

What is semiholonomic is? Holonomic actually means two point can be expressed by a equation. Non-holonomic can be expressed but, not properly. Holonomic e.g. $r^2-a^2=0$ Non-holonomic e.g. $r^2-a...

1 answer  ·  posted 2y ago by Anonymous‭  ·  last activity 2y ago by HDE 226868‭

66%
+4 −1
What does Lagrangian actually represent?

$L=T-U$ Here, $L$ is Lagrangian. T is kinetic energy. U is potential energy. But, what Lagrangian actually is? I know what Holonomic and non-holonomic is. But, I was thinking what the Lagrangian re...

2 answers  ·  posted 2y ago by deleted user  ·  last activity 2y ago by deleted user

66%
+2 −0
What happens if an electron collides with a proton?

What will happen if an electron and a proton collide? They attract each other inside an atom. But, why they don't collide inside atoms? Does the "boundary" of the nucleus push them away? depends...

2 answers  ·  posted 2y ago by deleted user  ·  edited 2y ago by deleted user

66%
+2 −0
Why we can't find a particle accelerating unless there's some other particle accelerating somewhere else?

I was reading "Introduction to classical Mechanics" by David Morin. In that book they wrote that The third law says we will never find a particle accelerating unless there’s some other particle ...

3 answers  ·  posted 2y ago by deleted user  ·  last activity 2y ago by Olin Lathrop‭

66%
+2 −0
Magnetization as a function of temperature in ferromagnets

Suppose a ferromagnetic material with initial magnetization $M_o$.Is there some specific formula which calculates the total magnetization $M$ as a function of $M_{o}$ and the Curie temperature $T_{...

1 answer  ·  posted 1y ago by MissMulan‭  ·  last activity 10mo ago by Anyon‭

66%
+2 −0
Should Earth's moon colonies being deep underground ensure natural heating?

Should Earth's moon colonies being deep underground ensure natural heating? By natural I mean "non artificial" i.e. not from "artificial" air conditioning. The question is based on the thought th...

1 answer  ·  posted 3y ago by deleted user  ·  last activity 3y ago by Dave Tweed‭

60%
+1 −0
What happens if an electron collides with another electron?

We know that electrons always repel electrons. But, if somehow they collide, then what will happen? I found a related question of above one in Quora. An incoming electron can instead make them ...

0 answers  ·  posted 2y ago by deleted user

60%
+1 −0
Is there an expected upper bound on the processing abilities of quantum computers?

A computer can store some number $n$ of bits in memory, and it can perform some number $m$ of essential computational operations, such as addition or multiplication, and rewrite those bits to memor...

1 answer  ·  posted 1mo ago by Julius H.‭  ·  last activity 33m ago by ariyadanesh‭

60%
+1 −0
Conductivity with angle of light

In the lab I changed the angle the light hits a photoresistor and it doesnt obey Lambert's cosine law the conductivity of the photoresistor drops fast from +-20 to +-30 degrees angle.Why?

1 answer  ·  posted 2y ago by MissMulan‭  ·  last activity 2y ago by Olin Lathrop‭

60%
+4 −2
Would we need Alternating Current if superconducting wires existed?

The major advantage of Alternating Current is that it can be transmitted to large distances without significant losses, which is not possible in Direct Current. Had economical superconducting wire...

2 answers  ·  posted 3y ago by theabbie‭  ·  last activity 3y ago by celtschk‭

60%
+1 −0
May it be that there are more than 8/9 planets in our solar system?

May it be that there are more than 8/9 planets in our solar system which aren't detectable with the current technologies? Is there any theory suggesting that our solar system has more planets, all...

1 answer  ·  posted 2y ago by deleted user  ·  last activity 2y ago by Canina‭

60%
+1 −0
Is that electron which jumps from one stationary state to another?

In classical physics book of kleppner, i read that An atom can "jump" from one stationary state a to a lower b by emitting radiation with $E_a-E_b$. The frequency of the emitted "package of radi...

1 answer  ·  posted 2y ago by deleted user  ·  last activity 2y ago by Olin Lathrop‭

60%
+1 −0
Direction of vector of electric field

Suppose we have a arc of charges with some charge density $\lambda(\theta) = sin\theta$.I am using polar coordinates for convenience.But how can I find the direction of the unit vector of the net e...

0 answers  ·  posted 1y ago by MissMulan‭  ·  edited 1y ago by samcarter‭

60%
+1 −0
Calculating Surface temperatures of a thermal insulator

I'm interested in knowing the surface temperature of both sides of a double-pane or triple-pane window. Given the R-value of the window, and the air temps outside and inside, how can I calculate t...

1 answer  ·  posted 2y ago by re89j‭  ·  last activity 2y ago by Olin Lathrop‭

60%
+1 −0
Maxwell equations EM wave confusion

The general formula for a EM wave (solving for the E field) is: where $\varepsilon = \varepsilon _{r}+j\frac{\sigma }{\omega }$ My professor told me that the conductivity of vacuum is 0 so we...

0 answers  ·  posted 1y ago by MissMulan‭

60%
+1 −0
Clear up confusion on Minkowski signature

All given metrics are for orthonormal-basis. 2 dimensional spacetime : I saw that Minkowski Metric looks like this : $$\pmatrix{-1 & 0 \\ 0 & 1}$$ or $$\pmatrix{1 & 0 \\ 0 & -1}...

1 answer  ·  posted 2y ago by deleted user  ·  edited 2y ago by deleted user

60%
+1 −0
Understand intuitively 4th boundary condition of gravitational wave

Suppose we have a gravitational wave which obeys the equation: $[G_{tt}-c^{2}G_{xx}]h_{\mu\nu}=0$ Lets take the case where $h_{\mu\nu}\ne0$ so we are left with the classical wave equation.Suppose...

0 answers  ·  posted 10mo ago by Volpina‭

60%
+1 −0
What does Laplace operator represent?

I was wondering what's the physical meaning of Laplace operator. $\vec\nabla$ actually represent a field. I had seen that Laplace operator is written as $\vec \nabla \cdot \vec\nabla=\nabla^2=\Delt...

1 answer  ·  posted 2y ago by deleted user  ·  last activity 2y ago by deleted user

60%
+1 −0
Find jerk of time varying force

This gravitational field we move inside has some distance L after which it becomes 0.Before L it is just like any gravitational field. Suppose we move inside that gravitational field.The accelerat...

1 answer  ·  posted 2y ago by MissMulan‭  ·  last activity 2y ago by Mithrandir24601‭

60%
+1 −0
Signal modeling as only digital, only analogue, or as both

In signal processing, are signals modeled in only one way (digitally or analogously) or are there some signals which can be modeled in both ways?

3 answers  ·  posted 2y ago by deleted user  ·  last activity 2y ago by Lundin‭

60%
+1 −0
Superconductivity

In my notes from University the reason a material can exhibit superconducting properties is at really low temperatures electrons form Cooper pairs of which the electrons of the Cooper pairs have lo...

0 answers  ·  posted 2y ago by MissMulan‭

60%
+1 −0
Double slit experiment with 2 electrons fired from a gun

Suppose we perform the double slit experiment , but we fire instead 2 electrons instead of 1. In the double slit experiment performed in the 1920s a interference pattern was observed at the screen...

0 answers  ·  posted 1y ago by MissMulan‭

60%
+1 −0
What's the importance of Poisson brackets?

$$F=F(q,p,t)$$ $$\frac{dF}{dt}=\frac{\partial F}{\partial q}\frac{\partial q}{\partial t}+\frac{\partial F}{\partial p}\frac{\partial p}{\partial t}+\frac{\partial F}{\partial t}$$ $$=\frac{\part...

0 answers  ·  posted 2y ago by deleted user  ·  edited 2y ago by deleted user

57%
+2 −1
SI Units of wavefunction

What are the SI units of the wavefunction Ψ(x).I know that [Ψ(x)]^2 describes the probabilty of finding a quantum object at a certain quantum state but what about the wavefunction instead?

1 answer  ·  posted 1y ago by MissMulan‭  ·  last activity 10mo ago by Anyon‭