# Tagged Questions

The union of special (SR) and general (GR) relativity. Use this tag if both SR and GR apply.

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### Boundary conditions of fields from the stationary action principle

First principle of stationary action Consider a real Klein-Gordon scalar field $\phi$ living in a $D$ dimensional flat spacetime. The field is considered off shell (the on shell condition is defined ...
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### Topics required before studying Tensor Calculus [closed]

Which topics do I need to know before going through tensor calculus?
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### Spacelike interval and length contraction

I am having a serious trouble finding an agreement between the two facts in Special Relativity: (1) given two events $A$ and $B$ in spacetime separeted by a spacelike interval $s$, the distance in ...
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### Simple special relativity problem [closed]

A body at rest in a frame of reference $S$ disintegrates into two pieces moving in opposite directions. The masses of each fragment are $3.0 \ kg$ and $4.0 \ kg$ and their velocities $0.8 \ c$ ...
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### What if a light clock travels perpendicular to mirrors that make up the clock?

I'm guessing you're all familiar with the classic intuitive way of explaining time dilation: with a light clock traveling at velocity v directed at a parallel direction to the mirrors that make up the ...
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### Textbooks to teach myself physics [duplicate]

I'm currently studying mathematics in Switzerland and only have a couple hours of General Physics a week. I would like to teach myself physics at a more advanced level but I don't know what textbooks ...
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### Why is special relativity still in use, if general relativity is more broad?

I always thought special relativity is only there because it is the building block for "The General theory of Relativity", until recently I encountered a text from my course book given below. A ...
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### Graduate level orbital mechanics book

I recently finished an undergraduate course in classical mechanics and really enjoyed the subject, particularly the sections regarding the mechanics of orbits. I am considering pursuing a graduate ...
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### Transformation of energy of a photon

I'm new to the forum so excuse me if I'm doing anything in a wrong format. My question is this: A photon fired from a spaceship at rest has energy $E$, if the spaceship starts moving with speed ...
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### 4-velocity in an accelerated frame

I am trying to check some calculations of the displacement tensor from a paper that has been given to me by my advisor. And it seems I am stuck on the form of the 4-velocity in an accelerated frame. ...
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### is relativity an illusion , a measuring error? [closed]

In the proof of relativity through lorenz transformation it looks like the relativity is essentially a mis of accuracy to measure the time and lengths. Am I true?
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### Michelson–Morley experiment's aether; Why is the time gained from travelling downwind less than that lost travelling upwind?

From the Wikipedia article on the Michelson–Morley experiment, explaining one of the concepts behind the landmark experiment: "If the Earth is traveling through an aether medium, a beam reflecting ...
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### Are fully raised/lowered versions of Kronecker delta tensors?

I am confused. I have two textbooks contradicting each other, at least, it seems to me so. The first one – "Field theory" by Landau & Lifshitz says that by lowering or raising one index of ...
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### Explicit calculation using metric tensor

I'm trying to calculate the following quantity: S_{\bar{\alpha} \bar{\beta}} \equiv \left( \Lambda^{-1} \right)^\alpha_{ \ \bar{\alpha}} \left( \Lambda^{-1} \right)^\beta_{ \ \bar{\beta}} ...
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### Do things move towards each other in space?

I watched a video once explaining space-time curvature, and it said that one of the things that made Einstein think of it is because objects in free fall move radially inward, so two objects falling ...
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### How does a flash of light appear to a high speed observer

Imagine two dark chambers separated by a light-proof partition each with a 30m high ceiling. Chamber A has a letter A painted in white on its black ceiling, chamber B has the letter B also in white on ...
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### proper time interval

We have two frames s and s' , s' moves with a constant velocity 'v' with respect to s in positive x direction, a particle B is thrown downwards from s' , and it collides with a particle A , the point ...
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### Why does the light beam moves diagonally?

In almost all articles that explains Relativity or Time dilation. They start by saying the the Light is absolute and isn't affected by the inertial frames of reference yet when they start to explain ...
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### In a system of two charges, if one charge disappears, does the force on other charge vanish instantaneouly?

It is commonly asked that if the sun dissappears, will earth shoot off in a tangent instantaneously or after some time. We know from the theory of relativity that the gravity waves travel at the speed ...
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### Can we actually have null curves in Minkowski space?

I know that this sounds really stupid but, when I think of the Minkowski space I cannot imagine a null curve, only null lines. For me, the only possible way to have one is to change the basis of the ...
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### Does the magnetic force depend on the reference frame?

Force due to a moving charge = qvB Immagine a charge moving with some velocity on earth and I calculate the force due to its magnetic field with Earth as reference frame for me. An astronaut in space ...
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### How are the turbulent spectra determined in relativistic turbulence?

In a non-relativistic compressible fluid, the turbulent energy spectra are well-understood and appear to follow the Kolmogorov hypothesis. It would also appear that relativistic turbulence also ...
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### Relativistic time dilation on Mars compared to Earth?

What is the time dilation in Mars, compared to earth? Can we accurately calculate it? What information is needed to do these calculations?
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### Time dilation and reference frame in an orbiting context [closed]

I have looked for other answers, but most confused me more than clarified and did not provide specific calculations. So I am still struggling with this simple question. Consider two persons, $P_a$ ...
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### Relativity and “light years”

As I understand relativity, time is relative to your velocity, meaning your watch moves slower relative to those who are stationary (relatively) when moving at great speeds. So if that's true then ...
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### Would Magnetic field be Zero if measured while moving along with charges?

According to Einstein's relativistic interpretation magnetic field caused by charges moving inside a wire is caused only by the relative speed between charges and the observer. So, if electrons are ...
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### Big Bang: Initial velocity of all matter in four dimensions? [closed]

Consider this hypothesis: Two objects A and B, are able to maintain their spatial distance. Object A is near a neutron star, and thus it clock is running at a different speed than object B, and both ...
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### Special Relativity and Gravity

As Einstein was seeking a relativistic theory of gravity, he thought that special relativity should be upgraded to general relativity thus promoting the Minkowski space to curved pseudo-Riemannian ...
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Here is the twin paradox with a twist. Scenario 1: An observer (A) leaves from the equator of the earth and travels with an acceleration of $9.8\,\mathrm{m/s^2}$ in a north direction ( i.e. in the ...
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### Are there any well-known large body objects moving at a significant fraction of the speed of light relative to earth?

Introduction I've been reading a book called PsychoPhysics, part of the "Illuminati God Series" by Mike Hockney (a pseudonym). Without getting into too much detail, they basically believe that the ...
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### Equal and opposite reaction at light speed

My knowledge of physics is very limited. And this question is only really to help me put to rest a question i have had for some time. I would like the answer to be a simple one as i am not really in ...
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### Michelson-Morley expression for relative phase

I am trying to find the relative phase when ether is propagating vertically versus when ether is propagating horizontally. The question is written below When the light (of wavelength) is split at the ...
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### Why does photon have to travel in sideways to hit the light clock?

From this article, Let’s say Alice is holding a light clock, and Bob is watching her run by, while holding it, with speed V. Alice is standing still (according to Alice), and the time,$\tau$, ...
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### Can a black hole move at speed of light?

Black hole comes in all sizes ranging from microscopic black hole on a quantum scale to supermassive black hole that resides in the center of probably every galaxy. Photon have no rest mass hence they ...
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### Do Quantum Mechanics At The Macro Scale Disprove General Relativity or Prove Something New?

We all know about the discrepancy between relativity and quantum physics at the scale relative to particles. Wouldn't the fact that recent experiments show quantum effects at the macro-scale in some ...
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### Index Placement for Spinors in Relativity

This may ultimately be a silly question, but a pedantic mind like mine gets tied into knots over differing notation. (Disclaimer: I'm a mathematician.) Let $\mathbb{W}$ be a complex two-dimensional ...
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### Metric Tensor and Strain Rate Tensor- Comparison of Units

Is there any way the metric tensor can have a dimension in general relativity? I ask because there is an equation where the strain rate tensor of continuum mechanics is expressed as a difference of ...
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### can a spaceship decelerate relativistically? [closed]

Say I have a spaceship, and on board is a laboratory set to accomplish some sort of experiment, simulation, or computation which might take upwards of a few hundred years or more to complete. Instead ...
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### Can all of physics be described by simple math? [closed]

Recently I was browsing through A Dynamical Theory of electromagnetic field by Maxwell and wondered because the paper did not seem to include any vector calculus or any vectors. I thought of the ...
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### Acceleration of objects with speeds comparable to $c$. Can we find acceleration of the frame by treating $v$ in $1/\sqrt{1-v^2/c^2}$ as variable?

We know that the $v-t$ graph for constant acceleration is a straight line. But when the speed becomes comparable to $c$, the speed of light, relativistic effects come into play. Suppose if a rocket on ...
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### Why Space Element of Four Momentum Vector is 3 Momentum Vector?

For example, if we look at the relation between space element of 4-velocity and 3-velocity; and space element of 4-force and 3-force $V = v\cdot\gamma(v)$ $F = f\cdot\gamma(v)$ But ...
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### Is This Educational Model of Relativity Right? [closed]

After watching and reading about Special Relativity I am still left with some questions unanswered. Can you please tell me if this educational model of relativity, simplified to level of a cartoon, ...
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### At what rate would I travel back in time at $2c$? [closed]

I was thinking about Einstein's equations and the solutions using imaginary mass and imaginary time. How fast would I have to travel over $c$ to be traveling back in time at minus one meter per ...
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### How to literally slow time [closed]

Let's say there is this car in a bubble of some sort. I want to slow time for that car in the bubble, let's say, by half so if the car is going 40 mph, outside of the bubble, people see it as going 80 ...
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### Two synchronised clocks accelerated perpendicular to Earth's gravitational field [duplicate]

If I have two synchronized (at the station while waiting for me) clocks on a train, one at front and the other one at back, and I start to accelerate the train perpendicular to Earth's gravitational ...
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### Do objects' velocities slow down going into black holes? Would they appear to?

I always hear that time slows down as you enter a black hole, and when I hear this, I always mentally imagine all of a person's motions, including their net velocity relative to the black hole, ...
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### Relativistic Effects of Radioactive Decay in Carbon Dating

Theoretically if in early years the earth was moving at an astronomically different pace (whether in orbit or rotation, whatever sounds better) wouldn't that alter the science of carbon dating? As I ...
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### Black Holes and Einstein Field Equations

I recently watched a video where Michio Kaku argues that there must be something fundamentally wrong with the theory of General Relativity because it gives time t=0 as a solution at the centre of a ...