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

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Dirichlet boundary conditions in space-time?

In the context of string theory, and world sheets the Dirichlet boundary conditions can be written as: $$\frac{\partial X^\mu(\tau,\sigma_1)}{\partial \tau}=0$$ where $\sigma_1$ is the value of the ...
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How fast does gravitational information travel? [duplicate]

Imagine two objects with equal mass in empty space attracting each other. One of these objects moves tangentially with a very high speed (lets say 0.9c). (p1 = (0, 0) p2 = (1, 0) v1 = (0, 0) v2 = (0, ...
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relation between speed and temperature in relativistic gas

What is the correct formulation of the relation between the speed and temperature in the relativistic kinetic gas? I have found one mentioned in problem 3.24 of the third edition of Pathria's ...
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What's the fastest you can slingshot from one black hole to another? [closed]

Let's say you have your spaceship orbiting around a black hole. Its orbital velocity is arbitrarily close to the speed of light (or you can just set it to c*0.99 if it makes it easier.) There is ...
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Why was general relativity needed to explain gravity?

I was thinking about this recently and I think this is a reasonable question to ask. SR can handle forces just like Newtonian physics can; the difference is that the four-force is defined a bit ...
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Why does time slow down the closer you are to a mass?

When ever i look this up all I get is sites saying how its because general relativity says "-" why does it do it though? it is because there is more motion near gravity than further away? Or is it ...
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What are the dimensions of the stress energy tensor in relativity? [duplicate]

Can anyone tell me what the dimension of the stress energy tensor is? Also, if it represents energy density, will calling it kinetic or potential energy be appropriate?
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Doesn't dating the universe violate the concept of spacetime's inseparability?

It would seem that measuring an age of the universe from the big bang requires separating spacetime into a 3D coordinate system and a time track. I fail to understand why it is appropriate to take ...
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What is the pace of absolute time

How fast or slow compared to my alarm clock time does time passes in a place where there is no gravitational pull and where you are not moving. I don't know if my question is clear, but is there some ...
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If time stops for an object, does that object feel gravity?

As far as I understand The GTR, it is said that Mass bends space-time which causes gravity. So every Mass in this universe is flowing through space-time example earth is moving along with space-time ...
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Can we find actual rest mass of things on Earth

Earth moves around the Sun and the Sun moves around the galaxy and the galaxy moves with unknown speed and direction. We have speed so the mass of us all altered. Can we know the real rest mass? If ...
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Perpetual Mobile and Gravitation

I have fundamental question about what is called the “law of conservation of energy”. We all hear about the tidal power stations which using the tidal power. The source of the tidal power came from ...
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Since the Earth is moving through space, do we experience time dilation?

What I mean is, the Earth orbits around the sun at around 30km/s, while I understand that this isn't a very high velocity does time dilation still occur to say a person who is completely stationary in ...
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Which is the corrispondent of the Lorentz's transformation in general relativity?

The Lorentz's transformations tell us how space and time change in a flat case? There are a more general and powerfull transformation for general relativity?
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122 views

How can you explain objects of unequal masses falling at the same rate using GR?

Isn't gravity caused by the curvature of space-time, and the stronger it's curved the stronger the attraction? it makes more sense to me that if a heavier object is falling on earth it would fall ...
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2answers
51 views

Did at least one photon go straight and never interact, and is that the one true clock?

This is a late night thought, and I apologize if it's something many have already thought of. Did at least one photon go straight out from the origin of the big bang and continue at C until now? If ...
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132 views

Question about basic formalism of GR and the metric tensor

I really don't know much about GR, but I've come across a few rough sketches of its formalism in my DG books. I'm trying to piece it together to get a very basic intuition of what spacetime is in GR. ...
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1answer
91 views

Proper acceleration of a stationary object

What is the proper acceleration measured for a stationary object? is it 9.8 or 0? Thank you.
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what are the relativistic/cosmological implications on a long rigid body? [duplicate]

This is my first question. Hope it's in respect of quality standards. Let's suppose to have a "long" rigid almost unbreakable bar of fixed negligible diameter . (we can imagine it as a rigid wire) ...
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129 views

Meaning of Proper time

Sorry for a bit of a basic question, but want to clarify things in my head. Is proper time quantified by the amount of physical process that an object, or physical system undergoes, for example the ...
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Does quantum mechanics have any insight into why and how photons get adsorbed into atoms? [duplicate]

Relativity holds that energy and mass are equivalent via the equation $E=mc^2$ also I know in quantum mechanics we talk about electrons jumping into bigger orbitals if they gain just the right amount ...
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Does a fixed amount of gas have slighty more mass when at a higher temperature?

According to relativity, energy and mass are equivalent. Does this mean that the energy added to a fixed amount of gas when it is heated adds slightly to the mass of the gas? The difference would be ...
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2answers
174 views

Relativity conclusions from Michelson-Morley Experiment

This is what my textbook says about the Michelson-Morley experiment: "This invariance of the speed of light between inertial reference frames means that there must be some relativity principle that ...
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Is there a name for the linear quantity corresponding to the (quadratic) “interval $\Delta s^2$”?

Recently it has been affirmed here (again) that the quantity called "interval (also 'spacetime interval' or 'invariant interval')" is referring to two (in general distinct) events as arguments, such ...
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Relative Movements Agreement on Object speed

I am confused about Relative Motion.Lets suppose we have one train and two observer.One of them inside the train and the other one is outside.Lets think the outside observer see the train moves a ...
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487 views

Is time the property of an object?

I don't know if the title makes much sense, but hopefully it will become clear with the text. Temperature is not a property of a point in the three dimensions, but actually of the object occupying ...
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Influence of spacetime curvature on electromagnetic wave propagation [duplicate]

Classical physics assumes that spacetime is evenly distributed in the sense that Coulomb's Law predicts that a charged particle will create a spherically symmetric electric field around its location. ...
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Did relativity make Newtonian mechanics obsolete?

Did Einstein completely prove Newton wrong? If so, why we apply Newtonian mechanics even today? Because Newton said that time is absolute and Einstein suggested it relative? So, if fundamentals are ...
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Is there any relationship between the $E=mc^2$ equation and the $a_n=\kappa v^2$ formula for the normal component to acceleration?

To clarify, I know very little about physics and don't pretend to have any insight whatsoever into relativity beyond what has entered the popular imagination; my knowledge is more or less at the level ...
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192 views

Negative energy of free particle: classical and quantum picture

Classically, the energy of a free particle consists of only the kinetic energy given by $E=\frac{|\textbf{p}|^2}{2m}$ Since $|\textbf{p}| $is real and $m>0$, $E\geq 0$. However, since ...
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Lorentz Transformations and time of event

Consider two inertial frames, $F$ and $F'$, such that $F'$ moves at $\mathbf{v} = (v,0,0)$ with respect to $F$ (assume $v > 0$). Suppose tat $x = x\prime = 0$, when $t = t' = 0$, where $x,t$ refer ...
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105 views

Is it possible for someone traveling at the speed of light to see at all? [closed]

I know that the speed of light is the same for all observers regardless of your speed, But this question is really worth thinking about. If someone let's say managed to reach the speed of light, ...
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620 views

Momentum and spacetime

My apologies up-front for the naive question and my rudimentary understanding, but I should be delighted if someone would enlighten me :) The question has to do with the application of the ...
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Is gravitational attraction increased by relativistic mass as well as rest mass? [duplicate]

Is gravitational attraction increased by relativistic mass as well as rest mass? If so what would happen if an observer travelled at relativistic speeds? Would it appear to him that the whole ...
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241 views

Finding wave-fuctions of a Dirac particle for given 4-momentum and spin 4-vector

I've been reading through various materials on relativistic quantum mechanics, but I find the lack of simple examples disturbing. I'm acquainted with the general form the solutions to the Dirac ...
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Relativity - Diffraction Of Light

Following thought experiment. Inside a rocket, right in the middle, we place two lasers pointing in opposite directions. Both emitting the same wavelength. The Left wall of the rocket is L1 while ...
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Time dilation, reference frame

If we have two objects (clocks) in (empty) space and they separate (the distance between them increases) at relativistic speeds then get back together (just for easier comparison of their times), what ...
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Simultaneity and quantum indeterminism

How can Relativity talk about simultaneity planes when the events are actually quantum in nature so they are indetermined ? How can you say that if you walk on the street with 1m/s you are ...
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Will 1 gram of matter moving at relativistic speeds completely annihilate a larger quantity of stationary antimatter?

This is a question about the relativistic mass concept which I am having trouble understanding, mainly because of the scenario below. Simple scenario: Suppose 1 gram of matter is accelerated to 99% ...
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62 views

Relative motion at almost the speed of light

My title might not be precise but I didn't know how to decribe it better. I've been reading about the special and general theories of relativity for a short while and I started wondering. First, ...
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133 views

Derivation of the energy-momentum tensor for an imperfect fluid

In chapter 7 of the "Physical Foundations of Cosmology" Mukhanov uses this energy-momentum tensor for an imperfect fluid: $T^\mu_\nu = (\rho + p)u^\mu u_\nu - p\delta^\mu_\nu - \eta(P^\mu_\gamma ...
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Is simultaneity testable?

I was reading about Andromeda paradox, and I started wondering. How can we know that the situation in the Andromeda paradox is real ? How can you know that simultaneity is real ? How can you say that ...
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What would a nuclear explosion look like at relativistic speeds where time dilation and length contraction become significant?

Suppose a rocket carrying a nuclear weapon is moving past our planet at very near the speed of light while a stationary observer watches from here on Earth. As the rocket passes directly overhead, an ...
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space contraction near lightspeed

I read a lot of the Time-Life Science books when I was a kid (well, mostly the pictures and captions). One of the things that always stuck in my mind was an explanation of the twin paradox, using ...
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Which is more fundamental,''constancy of speed of light'' or ''space-time''? [closed]

My friend asked me,''which is more basic or fundamental? constancy of speed of light which makes space-time behave dynamically OR dynamical behavior of space-time that makes light to travel with ...
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75 views

General Relativity - Four Velocity Derivative Question

I am trying to get my head around a small point used in a book I am reading about General Relativity. The book states that because $u_au^a = c^2$ it follows that $u_a \nabla_b u^a = 0 $ The first ...
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23 views

Does relativity in any way “disprove” Newton? [duplicate]

I was reading a science magazine and it claimed that Einstein's theory of relativity disproves all of Newtonian physics. I was highly skeptical because all we have been learning at school is Newtonian ...
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Are there computerized ontologies for Relativity and (Experimental) Physics? How to contribute?

As machines and automation can generally assist humanity in physical activities, and even exceed mere human capabilities, so, increasingly, also in tasks of reasoning. A basic component of the latter ...
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What exactly is meant by “locating points in spacetime”, in the RT?

In this (presently quite popular) answer to another recent question of mine concerning foundations of the (Einstein's) theory of relativity, it is asserted that "Any observer can construct a ...
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Are the words “coincident” and “simultaneous” considered synonymous? Else, please explain the difference

In discussions, experimental and thought-experimental descriptions (especially concerning the Theory of Relativity) the words "coincident" and "simultaneous" appear, notably for instance in the ...