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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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Is rotation absolute?

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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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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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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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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(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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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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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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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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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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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 ...
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Composite particles and Weinberg Witten (WW) theorem

I am quite familiar with the proof of Weinberg Witten (WW) theorem. One major result which follow from WW is that the graviton cannot be a composite particle. I have 2 questions here: How do we tell (...
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Why does time pass slow in an orbiting satellite relative to an observer on the ground?

I know that according special relativity, if an object travels at a very high constant velocity relative to an observer, the observer will see the object's clock running slow. I also know that ...
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Using relativity to calculate the radius of a cyclotron when accelerating protons to 0.99c

If I need to find the minimum radius necessary to have a fixed target collision of $10^{10}eV$ with two protons, and the traveling proton is going 0.99c in a cyclotron of 8 Tesla. Do I need to ...
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Relativistic Resistors

I would like to simulate what happens if you move electric circuits at relativistic speeds. At first, I would like to check the resistor. If I move a wire in the simplest case with speed $v$ along ...
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Why am I not getting the right values for the time it takes a spaceship to get to 0.9c? [duplicate]

I'm attempting this problem: Rocket undergoes constant acceleration from rest until it reaches a speed of 0.9c. The astronaut in the rocket feels a constant "induced gravity" of 1g. How long does it ...
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2answers
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How to do dimensional analysis?

$$mgh = \frac{mc^2}{\sqrt{1-(v/c)^2}}-mc^2.$$ In dimensional analysis do we just ignore the square root? Or do we solve what’s inside first then we do the square root? Do we say $(v/c)^2$ is 1 as ...
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Is the relativistic increase in (relativistic) mass same for a freely falling body as compared to body which is accelerated to the same velocity?

For a similar mass either falling toward a black hole or accelerated by a force is there a comparable increase in mass for similar velocity? Suppose a mass of one kilogram is accelerated to 99% of the ...
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What precisely does it mean for “information to not travel faster than the speed of light”?

This is something that's been bothering me for a while. The way we usually first hear about causality is that "nothing travels faster than $c$". But then you learn that phase velocities can ...
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How time dilated is the average particle relative to the big bang? [duplicate]

Particles created during the being bang have experienced time dilation by means of gravitation and general acceleration for the past 13.8 billion years. On average, how time dilated is the average ...
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How does gravitational redshift not violate the Second Law of Thermodynamics?

Suppose we had two black bodies at distinct heights in an uniform gravitational field. They are connected by an insulating tube that only allows the exchange of electromagnetic radiation. Since ...
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2answers
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Strange super script notation $^{(4)}R$ in the textbook Numerical Relativity

In Numerical Relativity by Thomas W. Baumgarte and Stuart L. Shapiro. There are bunch of superscript $(4)$ over $T,\Gamma, R$ i.e. $^{(4)}\Gamma^a_{bc}$ _ $^{(4)}R_{abcd}$ _ $^{(4)}R$ ... (...
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Is relativity really relative?

Because of the relativity of linear motion, you can't tell the difference between whether your spaceship is moving and the stars are standing still or whether you're still and the stars are moving the ...
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1answer
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Does relativistic mass really increase with velocity? [closed]

Suppose there are two masses on the ground $M$ and $m$. The distance is $d$. So according to newton's law the gravitational force between them is $F=\frac{GMm}{d^2}$. Now suppose that I am going ...
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2answers
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Do planets experience time dilation as they orbit the sun and if so what effect would this have on their orbit

Do the planets experience time dilation as they increase / decrease speed around the sun and are experiencing higher and lower gravitational forces as they move from a closer position i.e. the ...
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Why does the warm air rises up?

Warm air has more energy than cold air. This means that according to the Einstein equation $E = mc^2$ the warmer air has a greater mass than the cold one. Why is the warm air rising, if it has a ...
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1answer
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Non-compatibility between relativity and quantum mechanics [closed]

Is the discrepancy between quantum mechanics and relativity only in the math involved or is it much deeper? That is, do the same interactions have different and non-comparable interpretations in both, ...
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How does the extra time taken by photon to travel becomes the same extra time the clock has advanced in 'Rear Clock Ahead' question of relativity?

Exact Statement of Question: Two clocks are positioned at the ends of a train of length L (as measured in its own frame). They are synchronized in the train frame. The train travels past you at speed ...
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1answer
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Are Maxwell's equations valid in a rotating frame?

Maxwell's equations are covariant under Lorentz transformations. Are they covariant under going to a rotating frame and if not how do they look?
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Would Earth clocks tick faster than a clock on a GPS satellite if we disregard gravitational time dilation?

If we disregard the time dilation caused by a GPS satellite being in lower gravity, would earth's clocks tick faster or slower from the satellite's perspective? Edit. From earth, we understand that a ...
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1answer
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Lateral acceleration caused by a spinning frame of reference

Here's a thought experiment. If there's an astronaut infinitely far away from the equator of the Moon and he has a laser pointer pointing perfectly downwards on the surface of the Moon and he throws ...
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2answers
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Covariant and contravariant coordinates - index notation

I am learning about electrodynamics and have recently been introduced to the four vector. I also come fresh to the idea of covariant four vectors and contravariant four vectors. My question concerns ...
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photoelectric equation explanation [closed]

how to determine whether or not relativistic mechanics is needed to verify the photoelectric equation with a 1% uncertainty. While the stopping potentials are few volts.
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1answer
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Less speed increase obtained per amount of energy as approaching speed of light?

I found myself in a dichotomy when I tried to explain someone why we start getting diminishing returns on speed vs the energy investment as the speed of light is approached. Is it because of ...
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2answers
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If we filmed what happens on an extremely time dilated planet in realtime

Say there was a planet that experienced insane time dilation due to orbiting a black hole, say 1hr on there is 7 years on Earth (something like that), and that it was close enough to Earth for light ...
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1answer
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Time and acceleration [duplicate]

I am confused by the explanations of time dilation and gravity vs. acceleration. The consensus (and apparently experiments) is that time dilation in a gravitational field (not free-falling) is "as if" ...
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2answers
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Does the position in universe of a person not matter regarding death?

I might be completely wrong, but watching a documentary about time, the Einstein theory of relativity came up. From their explanation, time "slows down" depending where you are in the universe. I ...
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Why photons do not have mass? [duplicate]

According to Einstein $E= mc^2$ so $m = E/ c^2$. So energy present in an atom contributes to its mass. So if a photon is a carrier of energy shouldn't it have mass since energy contributes to mass of ...
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Symmetry of a tensor

This is from my notes, which I don't fully understand: It is straightforward to check that (anti)symmetry is a coordinate-independent notion, e.g., if the components of a tensor are symmetric in some ...
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3answers
76 views

Meaning and significance of the Levi-Civita symbol

I am recently reading Sean Carroll's Spacetime and Geometry: An introduction to General relativity. I am much of a beginner but am really curios to learn about GR. In the first chapter, after ...
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Would pilot-wave gravity reconcile Bohmian mechanics with relativity? [closed]

I was reading Can Bohmian mechanics be made relativistic? from 2016 that attempts to reconcile Bohmian mechanics with relativity. It’s conclusion says: Is such a theory then fundamentally—and/or ...
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2answers
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Why doesn't Faster Than Sound information transfer imply time travel or violate causality? [closed]

I am struggling to understand the answer to What are some scenarios where FTL information transfer would violate causality? In particular, I have not seen a satisfactory rebuttal to the comment from ...
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1answer
52 views

What particles can move in the light-cone coordinates?

Are only relativistic particles able to move in the light-cone coordinates? Are non-relativistic particles also able to move in the light-cone coordinates?
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50 views

Relativistic action is a constant?

Say that you want to find the equations of motion of a free relativistic massive point particle by minimizing the action $$S=-m\int\mathrm{d}\tau\,\sqrt{\eta_{\mu\nu}\frac{\mathrm{d}x^\mu}{\mathrm{d}\...
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Are matter and energy dependent on each other? [closed]

If matter creates energy and energy creates matter can one exist without the other?