All Questions
12,537 questions
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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 ...
- 344
-1
votes
0
answers
37
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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
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0
answers
31
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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
0
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0
answers
63
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Time reversal operation on $\gamma$-matrices
I know that time reversal is realized as an anti-linear operator. Nevertheless I am quite bewildered by the realization of the $T$ reversal on $\gamma$-matrices.
We assume here a Minkowski metric $\...
- 10.2k
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0
answers
70
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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 ...
- 213k
0
votes
1
answer
113
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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 ...
- 3,335
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2
answers
217
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Reconciling different reference frames in Einstein's train-lightening thought experiment [closed]
In all accounts of Einstein’s train-lightening thought experiment the ground observer sees the two flashes simultaneously because he remains “at rest” with respect to the landscape, while the train ...
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2
votes
5
answers
229
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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 ...
- 3,303
95
votes
9
answers
19k
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Why is there a controversy on whether mass increases with speed?
Some people say that mass increases with speed while others say that the mass of an object is independent of its speed.
I understand how some (though not many) things in physics are a matter of ...
- 3,303
0
votes
1
answer
67
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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 ...
- 22.6k
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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
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0
answers
21
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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 ...
1
vote
3
answers
790
views
Does the Cosmic Microwave Background falsify relativity of velocity?
In special relativity velocity is relative and there is no absolute rest frame . However the cosmic microwave background radiation does have a rest frame. Earth is moving with 328 km/s with respect to ...
1
vote
4
answers
389
views
Are superluminal Lorentz contraction velocities real velocities?
Starting with the equation for Lorentz contraction
$L = \sqrt{1-(v/c)^2}L'$
and making the substitution $v=at$ for small $v$ (see a better substitution in the case of relativistic $v$ two paragraphs ...
- 79
6
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2
answers
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Can non-free forces change the rest mass?
While reading Hobsen et al.'s "General Relativity: An Introduction for Physicists", I came across a bit confusing derivation. Multiplying the 4-force and 4-velocity, the following derivation ...
- 41.4k
3
votes
4
answers
490
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The Ehrenfest Paradox and the Wall of Death
In another question evaluating the reality of length contraction, the circular motion was involved and some answers argued that centrifugal force would negate any possible length contraction. A famous ...
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0
answers
94
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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
3
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4
answers
1k
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Why do we need to account for relativity to describe spin-orbit coupling
I am struggling to understand why one has to rely on relativity to explain spin-orbit coupling.
In fact, I always thought that the very nature of spin arises when one includes relativity into the ...
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5
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1
answer
187
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Stuck on Weinberg's quick derivation of Thomas precession
In Weinberg's Gravitation and Cosmology he has a pretty concise derivation of the Thomas precession formula (Eq. 5.1.13). But I don't get the first step...
A particle with intrinsic spin is under the ...
1
vote
1
answer
469
views
Rest frame of perfect fluid
I learned that dust is a collection of particles at rest with repect to each other. Thus the four velocity field $U^\mu$ of dust will be a constant and it is easy to think of what the rest frame of ...
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8
votes
4
answers
572
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)? ...
- 2,383
22
votes
5
answers
8k
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Inverse and Transpose of Lorentz Transformation
I've seen this question asked a few times on Stack Exchange, but I'm still quite confused why the following "contradiction" seems to arise.
By definition:
$(\Lambda^T)^{\mu}{}_{\nu} = \...
- 3,303
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votes
1
answer
67
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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 ...
- 39k
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-...
- 213k
0
votes
1
answer
129
views
Do Special Relativity and Newton's Law of Unverisal Gravitation hold in flat space?
These principles both seem to apply in flat space. Perhaps they are related through the equation for work (Fs=W)?
-13
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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
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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?
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
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1
answer
267
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Does the Dirac Spinor live in the complexification of the Lorentz group?
In this question I learned that when working with quaternionic representations of (the double cover of) our relevant orthogonal group, we cannot avoid working in complex vector spaces. However it is ...
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1
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203
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Gravitational field of a moving object
Here's a simple scenario:
Let's say, that there are 2 stationary planetoids of equal mass on the Earth's orbital motion path, placed at equal distances from Earth just like on the image below:
Will ...
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5
votes
2
answers
518
views
Interferometers and Earth's motion relative to the Aether
Why was the Michelson and not the Fabry-Perot interferometer used to detect the motion of Earth relative to the Aether?
Maybe the Fabry-Perot was used but we all know that the most famous experiment ...
- 89.3k
0
votes
0
answers
56
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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 ...
- 213k
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 ...
- 42.3k
6
votes
6
answers
2k
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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
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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}...
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5
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6
answers
2k
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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 ...
- 10.1k
2
votes
2
answers
198
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Warping Space Time By Focusing on Getting Smaller Rather Than More Energy?
So all energy/mater warps space-time, right? However, whenever we talk about the human manipulation of space-time it is a far-off goal requiring nothing short of tremendous amounts of energy and ...
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2
votes
2
answers
352
views
A worldline from the perspective of another in special relativity
Suppose we have two worldlines $\mathbf{x}(\alpha)$ and $\mathbf{y}(\alpha)$ parameterised by $\alpha$. What does the first worldline look like in a frame moving along the second? Is the correct ...
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1
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5
answers
177
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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 ...
- 78.1k
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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 ...
- 3,335
1
vote
2
answers
194
views
Validity of Maxwell's laws
I know Lorentz's force changes, according to Wikipedia, when the charges are moving nearly light speed but what about Maxwell's laws? Are they still valid when particles move at light speed?
I ask ...
- 51.6k
1
vote
3
answers
340
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Does the generic wave equation imply the universe has a Minkowski spacetime?
https://en.wikipedia.org/wiki/Wave_equation#Introduction
Does the wave equation
$$
(\partial_t{}^2 -c^2 \nabla^2) u=0
$$
imply the metric of the universe is Minkowski (-like)
$$
g= (+,-c^2, -c^2 ,-c^2 ...
- 78.1k
1
vote
1
answer
62
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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}_{...
- 332
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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 ...
- 39k
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 ...
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votes
0
answers
62
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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 ...
- 3,440
2
votes
1
answer
595
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Deriving the Minkowski metric from homogeneity of spacetime and the isotropy of space
In this wikipedia page, it says that one can derive the spacetime interval between 2 arbitrary events from the second postulate of special relativity, together with the homogeneity of spacetime and ...
- 213k
5
votes
1
answer
463
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Weinberg QFT problem 2.1: transformation of quantum states
I'm solving the following problem in Weinberg's QFT textbook: an observer sees a particle of spin 1 and mass $M$ move with momentum $\mathbf{p}$ in the $y$-direction, and spin $z$-component $\sigma$. ...
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0
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103
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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^\...
- 11
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1
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Relativistic mass and Electromagnetism in particle accelerators?
Dear Physics Stack Exchange,
I've been rather troubled as of late on trying to see the problems or issues inherent in crank scientist or layman views on physics topics about special relativity. One ...
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