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Questions tagged [relativity]

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

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2answers
45 views

Transforming covariant and contravariant derivatives

The covariant derivative of a contravector field is given by: \begin{equation} D_{k} A^{i} \equiv A^{i}_{\parallel k} = A^{i}_{\mid k} + \Gamma^{i}_{kp} A^{p} \end{equation} With $A^{i}_{\mid k} = \...
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Will the track length increase to keep the light velocity constant?

Assuming light travels at 3 km/sec(for calculation purposes), what happens when a 1 km long train moving at 2 km/sec along a straight track switches on its headlight at 30 km distance from a wall.So ...
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4answers
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We all accept/believe in $c$. But what happens when light travels away from the center of gravity of a heavy star? Does it slow down?

I'm basically trying to understand a mysterious characteristic of the universe. Why light has to travel at $c$? I understand and accept that from experiment. Not arguing that it does not. Just saying ...
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How the universe look like if it did not obey relativity theory? [closed]

I have tried to think about a way to do FTL communication and sometime there was a statement that "We can only pick 2 from 3, Relativity | Causality | FTL" In our universe is not possible to do FTL. ...
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Undertsanding gravitational time dilation in terms of four-vectors

I am struggling to understand how to calculate gravitational time dilation using four-vectors, based on my understanding of them. Specifically, I would appreciate if someone could point out the ...
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Affect of walking upto and beyond lightspeed [closed]

Setup: a giant planet rotating at the speed of light(L) + 1m/s at its equator (E) Assume at E ± 1m the speed of rotation is L Also, at both the South and North Pole the rotation is 0m/s such that ...
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when consider what kind of problems shoule we consider the theory of relativity? [closed]

Why we should not consider this theory when consider about something related to electron or photons and so forth?
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2answers
230 views

Moving objects and gravitational radiation

I have read that GR predicts that moving, massive objects emit some of their energy as gravitational waves. In reality, the energy loss is negligible and undetectable, but in some systems, like PSR ...
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3answers
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In SR length contraction, does matter flatten because the space around it flattens or does the structure of the matter flatten?

Most people believe length contraction is a real spatial phenomenon and if so there's nothing to contract in empty space and since matter contains mostly empty space then it is only the structure of ...
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4answers
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Frequency of light versus frequency of electron vibration

I've been trying to understand photons and light, although entirely conceptually (layman with not much of a background here, but I really want to understand this a bit better) and there's a couple of ...
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1answer
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Could theory of Relativity be overlooking a relationship between time and force? [closed]

Does Time dilation effect correspond to a property of the world that is suited to the term's "modulated force" or "force dilation"? It seems a simple truth. A modulation in the rate of force-driven ...
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In the twin paradox does the returning twin also come back permanently length contracted flatter than the twin on Earth?

This video from Brian Greene suggests this is so: https://www.youtube.com/watch?v=2sZUNud6rRw&list=PLj6DWzIvBi4PFDXCCV1bNhVUgDLTwVbFc&index=60 It shows if you stop a pole in the barn (...
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An uncertainty principle modification of GR? [closed]

Can we modify the GR field equations to contain uncertainty and then use an assumption that the uncertainities are somehow related? Are there any known attempts that try to extend the established ...
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4answers
547 views

Very Massive Relativistic Body [duplicate]

You're observing a massive object (probably a neutron star), and it is moving at a significant fraction of the speed of light relative to you. The mass of the object is just below the mass necessary ...
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Two solenoids of co-axis in relativity

Lets think about two solenoids sharing their axis. Their radius is almost same, so you can ignore the difference of the area of the cross-section. Two solenoid has same magnitude of current, but ...
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6answers
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Is a light year a different distance if measured from a moving object?

The speed of light is absolute, but time is relative. So would a light-year for us on earth be a different distance from a light-year on a different uniformly moving object? Why or why not?
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1answer
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How to find a normal to an hypersurface

I have to apply the Israel junction conditions in a region in which a hypersurface with O(3) symmetry separates two spacetime with Schwarzschild metric (with masses $M_+$, the exterior one, and $M_-$, ...
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Problem of now. How physics deals with it? [duplicate]

i know this is kind of off topic. from my understanding in relativity spacetime is frozen, nothing changes, there's no "time" as we understand it in everyday life. is there anything in physics that ...
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2answers
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How does $p\cdot u$ relate to observed energy and momentum for a massive particle?

Gravitation by Charles W. Misner, Kip Throne and John Wheeler page 65 Exercise 2.5. The book defined "energy" for a photon $E=-\mathbf p\cdot \mathbf u$ for subsection 2.8, which later explained as a ...
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2answers
51 views

Gravitational time dilation with a unit of time corresponding to the period of a light wave

How is it that time runs faster at regions further out from the center of a gravitational field if: A clock's arm is made to make one cycle for a complete wavelength of a particular light beam. ...
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Normal to an hypersurface and junction conditions

I'm looking for applying the Israel junction conditions to the collapse of a spherical region filled with pressure-free dust. The metric inside is a k=1 Friedmann model $$ds^2_{-}=−d\tau^2 +a^2(\tau)�...
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1answer
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The speed of electron when neutron converts into proton and electron [closed]

Neutron converts into an electron and a proton. After the reaction all of the remaining energy $E=782\ \mathrm{keV}$ is converted to the kinetic energy of the electron. Electron mass is $9.11\cdot 10^...
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What am I missing about the paradox between relativity and quantum mechanics? [duplicate]

Please help me understand what the paradox or disconnect is between quantum mechanics and general relativity. With my very simple understanding of each, there appears to be harmony to me. ...
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3answers
132 views

In relativity can a single frame encompass two relatively stationary points such as Earth and Proxima Centauri 4 light years away?

I can understand you can't determine who's moving between a ship passing earth at a constant relative velocity of .8c on to Proxima Centauri because of reciprocity of time and distance perspectives. ...
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1answer
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In femtosecond Raskar photography of a light pulse, why is the light pulse not length contracted?

If length contraction is a physical phenomenon, as many (not including myself) believe, then at a trillion frames per second, Raskar photography should be able to pick up the length contraction of an ...
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3answers
76 views

Time dilation calculation in the relativistic rocket

Rindler coordinates are useful for describing objects moving at constant acceleration: \begin{equation} \mathbf{x} = \begin{pmatrix} t \\ x \end{pmatrix} = \begin{pmatrix} \xi\sinh [\alpha \zeta] \\ \...
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3answers
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2 Different Results for Length Contraction

Consider a frame of reference S' moving with a velocity $v$ along x-axis with respect to a frame of reference S. Using Lorentz transformation, $x'=\gamma(x-vt)$, where $\gamma=\frac{1}{\sqrt{1-\frac{...
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1answer
62 views

Time dilation, mass-energy equivalence, and implications for the passage of time [closed]

I'm an amateur physics enthusiast, and there is a question that's been in the back of my mind for some time that I haven't been able to answer on my own. First, I want to define a couple of terms and ...
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1answer
36 views

What do the anti-symmetric matrices in quantum mechanics represent?

Anti-symmetric matrices have a trace 0. One of the three Pauli matrices, $\sigma_y$ is anti-symmetric. I also see anti-symmetric matrices pop up in Relativity. So is there any special conceptual ...
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1answer
33 views

Covariant relativistic photoelectric effect

General textbooks introduce the photoelectric effect as the Einstein's formula $$hf=hf_0+E_c(max)$$ and where $E_c=mv^2_e/2$ is the kinetic energy maximum value, the work function is $eV_0=hf_0$, and ...
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1answer
102 views

Do superluminal worldlines constitute closed time-like curves under the right conditions?

A closed time-like curve is defined as a word-line that returns to its starting point, in the $x$, $y$, $z$, $t$ coordinates. So, for a chronology respecting observer, an object traversing a closed ...
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1answer
134 views

Lagrangian non-relativistic limit to the non-relativistic action: lagrangian of a free particle

Let be $u=|\bar{u}|$ the speed of a free particle (at constant speed) of mass $m$ that is moving in relation to an inertial frame. Why we initially introduce the term $\epsilon$ to the free lagrangian ...
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0answers
38 views

Definite differences between special and general relativity [duplicate]

So I encountered a physics faculty who asked (during a quantum mechanics class) what the difference between special and general relativity is. When I answered that spacetime in SR is flat while in GR ...
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20 views

Applying a General Lorentz Boost to Multi-Partite Quantum State in Dirac Notation

I would like to apply a General Lorentz Boost to some Multi-partite Quantum State. I have read several papers (like this) on the theory of boosting quantum states, but I have a hard time applying ...
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Transverse Relativistic Doppler effect in Non-inertial reference frame

If I have a Non-inertial reference frame in Earth (rotating camera or just my rotating head with eyes) and I am watching the Moon. If my Non-inertial reference frame in Earth rotating with the speed ...
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3answers
105 views

Reference frames versus coordinate systems

I have the following distinction clear in my mind: Reference frame → state of motion of the observer Coordinate system → set of numbers used to map the space points within a reference frame So for ...
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1answer
93 views

How can light reach its speed? [duplicate]

The mass of a photon is said to be 0, But light (photons) get attracted due to gravity of a black hole, that means photons have a mass which is very very very small. And we know that any object with ...
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2answers
52 views

What rest frame to choose for 𝑣 when computing the momentum in context of energy–momentum relation

In energy–momentum relation 𝑝=𝑚𝛾𝑣. From which rest frame 𝑣 is supposed to be captured?
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55 views

How to calculate the geodesic equation for radial acceleration in general relativity?

how do you calculate $\frac{d^2r}{d\tau^2}$ in the geodesic equation: $$\frac{d^2x^m}{d\tau^2}+\Gamma_{pq}^m\frac{dx^p}{d\tau}\frac{dx^q}{d\tau}=0$$ I can figure out the Christoffel symbols but I get ...
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Are laws of Physics same in frames having zero relative acceleration?

If two frames have the same acceleration, then they'll be moving with a uniform speed with respect to each other. Are laws of Physics the same in these two frames?
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1answer
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Is there any known experimental violation of Einstein's $E=mc^2$?

Note that I am not talking about $E_0= m_0c^2$ but $E=mc^2$, where m is the relativistic mass. I know that one holds for stationary particles, moving particles, even light, I read that it even holds ...
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1answer
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Is the observable universe equivalent to 'our' light cone?

All the objects we can observe (stars, galaxies, ...) must be in our past light cone, since otherwise we couldn't see them. Presumably there are more objects located outside of our observable universe ...
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51 views

Time measured by a person vs Time measured by the same person as observed from another frame

I'm talking about the last few lines in the image. It says : $t_A$ (time measured by A1 as seen by A1) = $t’_A$ (time measured by A1 as seen by Bob) Why are these two times equal? The reason given ...
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1answer
91 views

Spacetime - reasons for “unification” of space and time - why we imply some unity in the term spacetime?

Why do we say spacetime, as though stressing that this is some kind of unity: space together with time? I understand a little that mathematically we have 4 variables in the same (Einstein) equation(s)...
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1answer
65 views

Relative velocity from space-time diagram

Consider two frames of reference, with velocities V1 and V2 relative to an initial ground frame. I have made the space-time diagrams for the three frames (Time represented on Y-axis). As far as I ...
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1answer
77 views

Minkowski space vs De Sitter Space

Can you explain to a layman the differences between the former two?
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3answers
62 views

Relative motion with relativity [closed]

In an inertial frame of reference (let's say a car moving with certain constant velocity in which I am sitting) If I observe the motion of another car through my frame of reference i.e.car, will I be ...
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43 views

Motion of an object “halfway in” a gravitational field

As a thought experiment, imagine a very long girder, moving at a constant velocity through space. At some point, it passes close to a black hole, such that one end of the girder is experiencing time ...
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0answers
28 views

World line of a photon from a distant galaxy in the $\Lambda$CDM model

The title is pretty much explaining the question: I am used to the graphs of Einstein-De Sitter model horizons: But then I don't understand 1) Are we using this kind of diagrams also now to ...
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1answer
89 views

Variable acceleration relativistic rocket

I was wondering how a realistic fusion-or-something rocket would fly... More specifically, I wonder about the time dilation that would be experienced (say in the classic example of two twins). So, of ...