All Questions
12,537 questions
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Origin of mass from electromagnetism and other forces
Special relativity tells us that interactions with forces contributes to the masses of objects. In the case of electromagnetic interactions this causes the slight difference in mass between protons ...
- 439
-2
votes
0
answers
34
views
Elapsed times mismatch in special relativity problem [closed]
Triplets A, B, C synchronized clocks when being in a reference frame in uniform motion and not moving relative to each other.
Immediately after synchronization B and C started moving with acceleration ...
- 97
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votes
0
answers
70
views
Photons are massless then how do they exist? [duplicate]
If something exists, then it surely has some mass. If photons are massless then they simply should not exist. But they not only exist but have momentum(p) also despite the fact that p=mv and if m=0 ...
5
votes
5
answers
744
views
Intuitively, why do farther events happen sooner from a moving perspective?
A stationary observer sees two boxes A and B explode at the same time. A is close to the stationary observer and B is far away. For a moving observer moving towards the two boxes, they will see B ...
0
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0
answers
21
views
Exhaust mass flow rate calculation for a thruster whose exhaust velocity is close to the speed of light
Let's say that we have a thruster whose exhaust velocity is very close to the speed of light. So when we calculate the mass flow rate to calculate the thrust do we take into consideration the ...
0
votes
1
answer
68
views
Why Gravitational Time Dilation Equals SR Time Dilation for Objects Free-Falling from Infinity?
In general relativity, the gravitational time dilation at a certain distance from a massive object matches the time dilation predicted by special relativity for an object free-falling from infinity to ...
- 686
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0
answers
57
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Would it mean that the product $kT$ under Lorentz boost transforms as $\gamma kT$?
I consider the Boltzmann/Gibbs distribution of occupation probabilities for energy levels $E_i$ which in the reference frame are given by $\sim exp(-\frac{E_{i}}{kT})$.
I think that the probabilities ...
- 1
0
votes
1
answer
114
views
Mandelstam variables
For a while now I have been struggling with a problem I am facing when considering the 2 to 2 process (Mandestam variables), and I would like to have an understanding of what is going on.
Considering ...
- 1,628
0
votes
0
answers
95
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Wightman distribution is complex Lorentz invariant
I asked in Math.SE (here), but I have not gotten answer, so I ask in Phys.SE.
How do I prove that Wightman distribution's analytic continuation is invariant under complex proper Lorentz transformation ...
-1
votes
1
answer
67
views
Particle A decays to particle B and massless particle C [closed]
I'm having a hard time figuring out how to solve this.
Particle A with mass MA and vA=0, decays into particle B with mass MB and some vB !=0, and massless particle C.
We are supposed to solve for the ...
- 107
0
votes
0
answers
58
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Spinor indices, raising indices of pauli matrices
How do we raise indices for a Pauli matrix. For example let $\left(\sigma^{y}\right)_{a\dot{a}} = \begin{pmatrix}
0 & -i \\
i & 0 \\
\end{pmatrix}$. How can I raise the index using 2D levi-...
- 257
-13
votes
0
answers
128
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Is the propagation speed of light in the vacuum really an absolute? [closed]
The part described by many that "propagation speed of light is always an absolute", IMO is wrong. Moving gun firing a bullet example, compared to moving flashlight are the same exactly if ...
- 4,376
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votes
0
answers
40
views
Paper on Dependent time coodinate in spacetime relation [closed]
I want to write a research paper on the topic mentioned above using a mathematical framework of my own (stil in progress).
Is this idea novel or already explored?
Are there gaps in my reasoning?
2
votes
5
answers
229
views
In special relativity, can a transfer of energy increase only the mass of a point particle and not its speed?
According to the mass-energy equivalence, if we have a point particle without internal degrees of freedom, then the energy content of this particle includes contributions from the mass as well as the ...
- 3,451
8
votes
4
answers
573
views
What kind of tensor is the electromagnetic field tensor (Faraday tensor)
I've seen the EM field tensor mostly written with 2 upper indices, $F^{\mu\nu}$. Does this imply that it's necessarily a (2,0)-tensor (that is, it has 2 contravariant components and 0 covariant ones)? ...
- 95
0
votes
0
answers
56
views
Energy to produce particles in different frames
I am a fourth year undergraduate student taking a course in Nuclear and Particle physics.
When asked nuclear related questions like "how much energy is produced [in the LAB frame] in the decay $X ...
- 860
3
votes
2
answers
277
views
Pebble dropped on a stationary pond with a non-perpendicular angle of impact to help conceptualize the Michelson-Morley experiment
I am trying to better understand the logic behind the famed 1887 Michelson-Morley experiment...and want to comprehend what they EXPECTED to see. As I understand it, the motivation behind this ...
- 167
0
votes
0
answers
38
views
Center of mass of the electromagnetic field [closed]
I’m studying the electromagnetic tensor.
Im trying to derive the equation for the rate of change of the position R of the center mass: $$\frac{\mathrm{d}\mathbf{R}}{\mathrm{d}t}=\frac{\mathbf{G}c^2}{E}...
- 1
0
votes
1
answer
135
views
Does this tensor identity hold in any kind of generality?
Assuming Minkowski spacetime, I am given an antisymmetric tensor $F^{\alpha\beta}$, and am asked to prove the following identity:
$$ F_{\mu}{}^{\alpha}{}_{,\nu}F^{\nu}{}_{\alpha} = -F_{\mu\alpha,\...
- 3,303
5
votes
6
answers
2k
views
What is the meaning of universal speed limit?
When it is said that no object can exceed the speed of light $c$ in vacuum, I have some misunderstanding about this statement. Does exceeding the speed of light mean exceeding the speed of light ...
- 129
0
votes
1
answer
70
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How does a mirror lose mass during photon reflection?
When a photon is reflected by a mirror, it transfers momentum to the mirror, causing the mirror to gain kinetic energy. However, the photon's energy does not change. This seems to imply that the ...
- 11
1
vote
1
answer
62
views
Relativistic corrections to Mandelstam variable $t$
a book (Quantum Electrodynamics by Akhiezer and Berestetskii, 2nd ed. pp. 522) claims that when evaluating a Feynman diagram for Moller scattering, the $\frac{1}{q^2}$ in the matrix element $(\bar{u}_{...
- 13
0
votes
2
answers
81
views
Time dilation question [closed]
I understand that an object moving at high velocity relative to an observer will appear to have time move slower. But what if the high velocity object was traveling in a circle and able to communicate ...
- 3
0
votes
3
answers
100
views
Does length contraction "break the speed limit"? Part 2 [closed]
This is a follow-up to another question I asked here:
Does length contraction "break the speed limit"?
The answer to that question was "nothing is moving faster than light, the observer ...
- 301
2
votes
0
answers
62
views
Derivation of transfer matrices for accelerator elements
I am trying to derive the transfer matrices for different elements in particle accelerators. For simplicity, let's consider the drift space. The Hamiltonian in the standard Frenet-Serret coordinates ...
1
vote
5
answers
177
views
Entanglement collapse and relative order of events
According to QM (and many confirming experiments) outcomes (e.g. spin) of entangled particles measurement are non-locally correlated (i.e. can’t be pre-configured for such correlations).
It seems that ...
- 93
1
vote
0
answers
103
views
Stress-energy tensor for a point particle
I am trying to find the stress-energy tensor for a free particle. Let's start by finding the Lagrangian.
$$ S = -mc \int \sqrt{u^\mu u_\mu} \, ds = -mc \int ds \sqrt{u^\mu u_\mu} \int \delta^{(4)}(x^\...
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1
vote
0
answers
59
views
Field tensor for monochromatic EM wave
I’m having some troubles computing $F^{\mu\nu}F_{\mu\nu}$ for a 4-potential $A^\mu(x) = \overline{A}^\mu \exp\left(-ik_\alpha x^\alpha\right)$, that is derived under the assumptions $\partial_\mu A^\...
- 21
1
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1
answer
26
views
Power radiated by moving charged particles: linear vs circular emissions
I'm trying to prove a statement that relates to the emission of charged particles in linear and circular accelerators:
...hence, for the same applied force $\left(\frac{d\vec{p}}{dt}\right)$, one has ...
- 151
1
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0
answers
37
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Resources to understand problems posed to Newtonian mechanics by Maxwell equations [closed]
Einstein undertook writing his paper on special relativity in response to the CRISIS that emerged in physics when trying to do mechanics for fast-moving bodies in the light (pun intended) of Maxwell's ...
1
vote
2
answers
96
views
Lorentz invariants of electromagnetic tensor
I`m stuck in what seems like a very basic derivation.
I`m having my very first contact with the covariant formulation of Electrodynamics, and I want to show that
$$\epsilon^{\mu\nu\rho\sigma}F_{\mu\nu}...
- 151
0
votes
0
answers
18
views
Current attracting moving/static particle, the relativity approach [duplicate]
I just saw a video that tries to explain magnetism (to an extent) though relativity. The idea is that you have a moving particle that is attracted to a wire with current. The explanation is that, in ...
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19
votes
3
answers
2k
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Does length contraction "break the speed limit"?
I've been trying to learn some special relativity, and while trying out a thought experiment I hit a paradox that I don't understand how to resolve. Can someone help me understand where I'm going ...
- 301
2
votes
0
answers
60
views
Degeneracies of the Dirac Equation
When one computes the solutions of the Dirac equation by employing the plane-wave ansatz, one obtains 4 solutions of the Dirac equation: with two solutions being positive energy solutions with same ...
- 698
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votes
0
answers
74
views
Light's speed RE relativity [duplicate]
If we used to think the earth was the center of the solar system but now know otherwise. Light's wavelength is in the middle of the spectrum. If it's not just because we see in it's wavelength. Why ...
- 15
0
votes
0
answers
34
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For an $O(N)$ symmetric function, is the 4-derivative simply equal to the derivative w.r.t the $N$-dimensional norm?
I am working on an $O(4)$-symmetric instanton which has the Lagrangian: $$L = \frac{M^6}{4E^2T^2} \left[ \frac{1}{2}(\partial_\mu\Phi)^2 - \frac{1}{2} \Phi^2 + \frac{1}{2}\Phi^3 - \frac{\alpha}{8} \...
- 179
0
votes
2
answers
85
views
Lorentz scalar Lagrangian in curved spacetime
This question might be very simple but I guess I'm missing something. We know that Lagrangian has to be a Lorentz scalar. I can see why that should be the case when dealing with inertial frames of ...
- 387
1
vote
1
answer
50
views
Variable mass term in $4$-force
Defining the $4$-momentum of a system of mass $m$ like $P^\mu=mU^\mu=(m\gamma c, \mathbf p)$, when computing the $3$-force,
$$\mathbf f=\frac{\mathrm d\mathbf p}{\mathrm dt}=\frac{\mathrm d(m\gamma\...
- 420
2
votes
2
answers
64
views
Change of simultaneity with speed and direction
I am a physics enthusiast. I was watching a video by Professor Brian Greene on YouTube. He said that when an alien in a distant galaxy is at rest relative to Earth, he will see the present of earth. ...
- 129
6
votes
6
answers
2k
views
How do different observers decide if they are looking at the same thing?
Note: I added some more descriptions, so that anyone who reads can benefit possibly more. I suspect this question become somewhat popular, because there may be many people confused with SR, but how ...
- 435
9
votes
4
answers
3k
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Does the twin paradox hold in a universe that's empty except for the twins?
I recently obtained a used copy of Shadowitz's obscure book on Special Relativity, in which he makes the following astounding claim (which I have not seen elsewhere) concerning the twin paradox with ...
- 107
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1
answer
42
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Relativity and wave (function) “squeezing”
Seem like nature have max speed limit so to conserve energy/momentum some of the speed increase goes into increasing wave frequency which is kind of “squeezing” the wave.
For example, light-wave fired ...
- 93
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1
answer
71
views
Are Stokes parameters Lorentz invariant?
Are the Stokes parameters for polarization relativistic? i.e. Lorentz invariant? And if it is so, then how to show it?
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0
answers
62
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Why Wick rotate at all? Why not just use $\mathbb{C}^4$?
I always thought it was strange that in a supposedly symmetric ++++ starting space, only the $s$ coordinate is Wick rotated to $t = is$. If some $A \in \mathrm{SO}(4)$ were applied first before Wick ...
0
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1
answer
58
views
What is the general energy expresion for a massless particle?
The general energy-momentum relation for a massless particle is $E=cp$.
I know that for a photon we have $E=hf$. Is this relation valid for any massless particle?
- 416
2
votes
0
answers
25
views
Auroral Kilometric Radiation (AKR) Peak Frequency Calculation
Auroral Kilometric Radiation (AKR) occurs due to emission from charged particles moving in helical paths around planetary magnetic field lines at or around the electron gyro frequency and/or its lower ...
0
votes
1
answer
70
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Why are Weyl's Equations composed of only first-order derivatives?
I'm studying the Weyl's Equations from Section 1.5 of Perkins' Introduction to High Energy Physics.
The author says this:
Dirac set out to formulate a wave equation symmetric in space and time, ...
- 1,637
1
vote
0
answers
35
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Why do relativistic length contraction explanations of magnetic fields seem inconsistent in these scenarios?
I have heard about how magnetic fields can be explained as electric fields that arise from length dilation of moving charges in a conductor. Intially this seemed convincing but there are two things ...
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votes
2
answers
89
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Can I see the origin of light before it reaches the final point, so do I see the origin at the exact moment it is created?
Questioning the Perception of Light and Its Path
I have a question about the perception of light and its propagation. The main question is:
"Can I see the origin of light before it reaches the ...
0
votes
1
answer
44
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How to find 4-acceleration scalar product in terms of $ds$ spacetime interval?
We know 4-velocity $$U^i =dx^i/ds$$ where $$ds=\sqrt{dx^idx_i}$$ so we have 4-acceleration $$A^i=dU^i/ds$$
Then we have $$A^iA_i=\dfrac{dU^i}{ds}\dfrac{dU_i}{ds}$$
How should I proceed to find this ...
