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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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Thought experiment - travelling one hour at the speed of light [duplicate]

I'm new to physics and I've been debating a thought experiment with my friends. I think something similar has been answered on this site but to be honest I don't understand the responses - if anyone ...
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Einstein's Theory of Relativity Time Dilation

My friend claims that this proves that time moves faster on a mountain than at sea level. He says that since both of these people start and end at the same angular position and are both traveling at ...
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Relativity Confusion

In the time dilation formula $\Delta t=\gamma\Delta t_0$, I am confused about what $\Delta t$ and $\Delta t_0$ measure. If we have two people, Tom and Bob, moving at constant velocity relative to ...
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Time dilation in a gravitational field

A friend and I were talking about relativity and we had different opinions about this situation: If I choose a frame of reference on the surface of a planet, would time "run" differently between the ...
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1answer
71 views

Toy almost everywhere flat metric

I was looking into this paper from 2003 by Krasnikov - “The quantum inequalities do not forbid spacetime shortcuts” - and apart from the fact that I find it very scetchy without many details and hard ...
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It is said that an object cannot exceed the speed of light, because high speed makes the object closer to infinite mass, but speed relative to what?

That is, the speed of an object, when getting closer to speed of light, makes the mass of the object closer to infinite, but isn’t speed relative? So a man on earth feels a metal ball on the table has ...
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1answer
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Question (mostly) About Relativity

Before I get into the actual question, I'd like to say that my only knowledge on Einstein's Theory of Relativity (both Special and General) have been obtained through about an hours worth of videos ...
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Can we consider speed of light as escape velocity? [closed]

Consider following hypothetical scenario: A space ship starts to accelerate from planet Earth towards Alpha Centaury at a very pleasant 9,8m/s2.The astronauts onboard the ship keeps the acceleration ...
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Charged particle and magnet, can this event vary relatively?

suppose we take a charged particle and a magnet and place them at some particular distance apart .Now let's take 2 frame of reference. [the charged particle and the magnet are in rest with respect ...
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3answers
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Questions regarding photons and distance contraction

I've got a few question regarding photons and lorentz contraction. they may be really stupid questions (I am not a physicist). But they have been bothering me for a while. 1. Is Lorentz distance ...
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1answer
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Is the answer to the question “How many moments in time exist between any two particular moments?”, infinitely many? [duplicate]

This question is related to Xeno's paradox, and to the discrete/continuous time debate, but it comes from a slightly different angle. If the answer is infinitely many, why would this not be a ...
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Does $A_\mu$ transform with the coordinates, or as a vector in a tangent space?

Does the vector potential $A_\mu$ transform when we merely relabel events in space-time (coordinate transformation), or does it transform with the basis vectors of a tangent space in which it lives? ...
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A vector that is both parallel and perpendicular to another vector

The following is an excerpt from "Einstein Manifolds" by Besse Suppose we have a freely falling test particle, described by a timelike geodesic $\gamma$. A nearby (infinitesimally close) freely ...
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Speed that a galaxy-sized newton's cradle would propagate motion

This doesn't necessarily have to be a newton's cradle, but the important point is that the length of the chain of spheres is exceptionally large (on the order of light years). Ignoring the effects of ...
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How would a massless object behave on Earth? [closed]

I wonder how a massless object would behave on Earth. To make things concrete, consider the object as a sphere of radius R, and zero mass. It also interacts with matter like a wood pencil would. My ...
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Why does a charged particle experience lorentz force while moving in a uniform magnetic field?

Firstly, when a charged particle moves in an infinite uniform linear magnetic field, it experiences a force.But,how does the charge or how do we differentiate that the relative position of charge is ...
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What if there was a perfect tangential road built on the surface on the earth that extended into space

What would it look and feel like as one traversed the road from a perceived level point at the surface of the earth towards space?
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Covariance of applying a four-force to a stress-energy tensor with forward Euler

I've run into a great deal of confusion on what I expected to be a very simple issue of covariance. I have an equation $$T^{\mu\nu}_{~~~~;\mu} = -G^{\nu}.$$ This is manifestly covariant; so far so ...
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Could magnetic fields really be completely substituted by relativity and electric fields?

In many textbooks (especially those for undergraduate level), magnetic fields are described merely as a relativistic side product of electric fields when considering frames in motion relative to ...
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Relativistic Electric Potetnial

Suppose you have a particle of charge q circling the z axis in the xy plane. The orbit is centered at the origin and has a radius of $r_0$. The angular frequency of the orbit is $\omega$. Position of ...
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Electromagnetic field conversion in non-inertial frame

I found these equations for electromagnetic field conversion in inertial frame for non relativistic case. If I have permanent static magnetic field with 3 components $B_x, B_y, B_z$ and permanent ...
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1answer
54 views

Electromagnetic field tensor and antisymmetry

Why does the inner product between the four force (caused by the electromagnetic field tensor) and the four velocity equaling zero imply that the electromagnetic field tensor is antisymmetric? This ...
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0answers
51 views

Does the proper four-acceleration $A^{\mu} = (0,0)?$

Let the proper four-position vector $x^{\mu}(\tau) = (0, \tau)$. Differentiating this successively wrt $\tau$ I get the four-velocity $u^{\mu}(\tau) = (0, 1)$ and then the four-acceleration $A^{\mu}(\...
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Can a heat engine be made with vacuum as the working material?

Sounds absolutely ridiculous, for vacuum is no 'material' in strict sense. But special relativity tells me that length may be contracted or expanded. I find no problem in extending this idea, that ...
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2answers
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Relativity in 2+1 or 4+1 dimensions

Whereas we have experience of relativity working with 3 spacial dimensions and one of time would there be similar rules affecting a two dimensional space and one of time or even with a 4 space and 1 ...
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EMF induced into a conductive loop moving through a conservative electric field

Suppose I had some static distribution of charge which provides a source of non-uniform conservative electric fields. We can suppose that they are attached to some very large mass, so acceleration ...
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1answer
79 views

Does a probabilistic universe require absolute simultaneity? [closed]

Is it true that each and every atom that composes my body has its own reference frame, hence its own simultaneity? Does this mean that every atom that makes up my body has its own "present"? Then what ...
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Time direction in the BH parallel exterior region

In the III region of the maximally extended Schwarzschild solution, described in the Kruskal coordinates , Sch. time runs in the reversed direction as compared to the I - event horizon III-II of the ...
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5answers
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Einstein's Elevator - Constant acceleration eventually reaches $c$. Can't that be used to detect gravity vs acceleration?

Objects with mass that continuously accelerate will eventually approach $c$, but cannot exceed it. So if I find myself in an elevator, unable to determine if I'm in a uniform gravitational field or ...
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1answer
41 views

Mass and Energy conversion in photon

If energy can be converted into mass then why isn't photon's energy being converted into mass?
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2answers
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Turning on headlights near light speed [duplicate]

If i turn headlights on in a car at v = 0.99*c, will the headlight speed be 0.01*c? I know speed of light is always the same but it really confuses me? My question is, does space contract so much at ...
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Time Dilation and Particle Decay

I am teaching myself astronomy and I'm working on a basic problem regarding time dilation around black holes. The question is basically this: if a neutron were ejected from a nucleus at a distance of ...
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3answers
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How to differentiate permanent age difference in the twin paradox from reciprocal time dilation? [duplicate]

I'm hereby editing my previous question about the cause of permanent age difference in the twin paradox example as a result of the links I've been given as answers to my original question. I am trying ...
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Muon catalysed fusion from cosmic muons

If thousands of muons are coming to earth. Why can't we use them for muon catalysed fusion which was proved legitimate
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4answers
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Is rotation absolute? [duplicate]

I was reading an article that rotation instead of linear motion is absolute. Can anyone explain why? Shouldn't an observer (A) moving in a circle around a point in an object that rotates (with respect ...
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relativity in an electron beam [duplicate]

If two electrons are travelling side by side with the same velocity with respect to me, they are standing still with respect to each other. For me, they will exert on each other an electrostatic force ...
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Questions about tear in spacetime [duplicate]

Sorry if there's any mistake in the language. In Einstein's General Relativity Theory gravity is described as the curvature of the spacetime fabric. So can analogous to a real fabric can this ...
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2answers
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Relativity thought experiment - spaceship travelling at speed of light [closed]

I have basis knowledge in special relativity, but I haven't studied it for too long and I'm stuck in a simple thought experiment. Assume that there is a spaceship departing from the earth at speed of ...
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If negative mass should exist, how would conservation of momentum work?

Imagine that a particle with negative mass has been discovered. It is known to obey both the equivalence principle and Newton's second law. This means that the particle is a subject to repulsive ...
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1answer
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Non-relativistic limit of the cosmological constant

Usually, when we apply the non-relativistic limit ($c \rightarrow \infty$) to relativistic equations, the cosmological constant $\Lambda \sim \mathrm{L}^{-2}$ is simply offhandedly neglected by ...
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Trying to make some sense of the cosmological constant

The question is simple: Does the naive reasoning below have some physical sense, or is it pure gibberish junk? (I use $c = 1$ and metric signature $\eta = (1, -1, -1, -1)$) I suppose that the ...
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How is the behaviour of an atom particles as the atom approaches the speed of light? [duplicate]

There are many questions regarding what it means to accelerate the constituents of an atom towards to the speed of light, but I am not able to find any cogent article as to what the dynamics of the ...
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1answer
80 views

Einstein's Elevator thought experiment and relativity

Einstein's elevator thought experiment contains the following premise: An observer in a box will see free bodies moving identically between when the box is uniformly accelerating in gravitational ...
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1answer
56 views

(Lorentz etc) invariant vector fields

(Background: I know some but not much differential geometry, hopefully enough to formulate this post.) I want to ask about what physicists mean when they say scalar, vector, etc. The answer in ...
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1answer
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Can we make a formula that describes the magnetic force as a function of electric field using length contraction equation ?

I know that the magnetism is just electricity and due to length contraction and the change in charge density moving charges experience what we call magnetic force. so my question is can we calculate ...
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2answers
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Questions on non-inertial reference frames

Suppose I am in an arbitrary reference frame and i want to know if it is inertial or not. So I decide to do a test, i take a particle of mass $ m $ and let go of it. I observe that the particle is ...
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4answers
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Understanding the difference between timelike and spacelike separations

From Woodhouse's General Relativity: If $A$ is the origin and $B$ is a nearby event with coordinates $dt, dx, dy, dz$, then, $$ds^2 = dt^2 - dx^2 - dy^2 - dz^2$$ is the same in all local inertial ...
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2answers
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Energy requirements for relativistic acceleration

I want to visualize the energy required to continue accelerating near the speed of light. If I use some amount of energy to accelerate to 0.5c, what speed will I be going if I use the same amount of ...
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4answers
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No FTL information implies no FTL travel?

The general consensus in the scientific community is that it is impossible to transmit information faster than light. There is also speculation that it might be possible to open wormholes or travel ...
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Does converting rest mass to energy instantly change the gravitational attraction it exerts?

Let there be two hydrogen atoms having 1.1 mass each. We use solar fusion to convert the two hydrogen plasma to helium with a mass of 2 and 0.2 gamma radiation. Before the fusion process gravity was ...