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

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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 ...
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Do matter that falls in a black hole go to the future and the end of the universe?

Black holes are black because everything that enters don't exists anymore. When you pass the event horizon, the time stops for you, all the future of the universe has passed for you and the universe ...
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The Twin Paradox using reference frame following the ship

The Twin Paradox is undoubtedly one of the most discussed things in special relativity and have a tendency to confuse most of us. Classically, it's resolved by either stick to one of the three ...
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Relativistic velocity in Special Theory of Relativity [on hold]

Prove that, 0.5 m*v^2 , where m = M / (1- (v^2 / c^2))^0.5 ["M" is original mass, and "m" is mass with velocity] doesn't equal to the the Kinetic Energy of a particle moving at relativistic velocity. ...
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51 views

Einsteins thought experiment on travelling with a light wave

This might be better suited to the science.history.SE; but I thought I would try here first. Einstein reportedly considered a thought experiment where one considers travelling alongside a light wave; ...
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Does deceleration (or negative acceleration) feel like acceleration?

This may seem like a dumb question, but I'm having a hard time trying to intuit the answer. The question is in relation to Einstein's famous thought experiment which says that it is impossible for a ...
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110 views

Why does gravity affect time?

So Special Relativity states that for all non-accelerating objects of matter the laws of physics are the same. I'm confused on why this law of physic applies to objects in acceleration and gravity ...
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45 views

Help with relativity simultaneity paradox

To begin I will explain briefly a classic thought experiment used to demonstrate simultaneity; There is a 100m long train rushing towards a 100m tunnel at a fraction of the speed of light, from an ...
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Ligth clock with spaceships side-by-side

In the reference frame of an observer, two spaceships travel in a straight direction (e.g. x axis) at a very high velocity and side-by-side; the distance between them is always d (km) = c (km/s) x 0.1 ...
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If there is no definite speed in the universe, only relative speeds, how does energy increase when velocity approaches the speed of light?

Is the concept of energy increasing as it approaches the speed of light based on the fact that this is only true relative to the observer? Lets say, there's a scenario where a person in a rocket ...
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How relativity affects projectile motion?

If For example; I'm in a train traveling on earth at .99c, and I drop a ball from a 1m height inside that train how long would it take to hit the ground from the ball's frame of reference? I can see ...
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How do we know the speed of light is constant and spacetime dilates rather than vice versa?

Some conspiracy nut was telling me that Einstein was BS and there's a giant conspiracy that he's wrong but scientists would loose all their jobs if they admitted it. Of course this is all baloney, ...
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Why the notation of Lorentz Transform has to be like this?

I have a confusion about the notation used for Lorentz Transformation ($\Lambda^{\mu}{}_{\nu}$). I think Lorentz transform is not a tensor because it transforms a vector from one coordinate frame to ...
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Relativistic Energy

The question stated: By what percentage does your rest mass increase when you climb 30m to the top of a ten-story building? New to the concept of relativistic energy, I was a bit confused with ...
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3answers
104 views

Using Lorentz transformations

The question states that two particles in a high-energy accelerator experiment approach each other head-on with a relative speed of $0.890 \, c$. Both particles travel at the same speed on a straight ...
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Clarification of Type D space-time

I've asked this question on MathOverflow but received no feedback, so I thought I'd try better luck here. I've read the following formulation of the Goldberg-Sachs theorem in Chandrasekhar's ...
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Does the accelerated rate of expansion of the Universe have any effect on the speed of light in vacuum?

So I was just wondering about this as I finished reading about Michelson-Morley's experiment which disproved the ether theory. My question is since the Universe is constantly expanding and that too ...
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Model of spherical wave fronts in relativity

In relativity, the 3-surface formed by light rays emanating from an event as they evolve in time has the shape of a hypercone wich is flat. I have difficulties seeing how can a spherical wave front, ...
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Why the nonexistence of a “one sentence layman phrase” for the de Broglie relation $p=\hbar k$? [closed]

(My question seems most likely will be considered a duplicate of OP (and possibly 1, 2, 3), but it turns out to be WAY TOO LONG as a comment in OP, and the system has deleted the corresponding chat ...
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Time in general relativity

A physical quantity is introduced by its operational definition. In general relativity we use a differential manifold to describe the 4-dimensional space-time and, to identify a point in it, we use a ...
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115 views

Can you tell your absolute speed in space?

Normally in relativity your speed can only be known relative to another object, given that as one approaches light speed more energy is required to accelerate faster, based on the energy consumption ...
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Can everything be described without anything needing to actually “bend”? [closed]

Is space bending because gravity actually causes small particles to move differently? If large source of gravity is somewhere are particles extending towards it, creating a "bend" in space? So "bend" ...
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would a clock at Absolute zero have no mass

since Absolute zero corresponds to the theoretical state in which particles have no energy at all. and m = E/c^2 Then would an unwound clock have zero mass if it was cooled ,theoretically , to ...
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Are we all moving at $c$? [duplicate]

Is it true to say that all matter in the universe is travelling with velocity c through spacetime, but that for baryonic matter most of that velocity is through the time dimensions rather than the XYZ ...
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Question on applying the force transformation in a specific case

When I try to apply the force transformation (the 3 vector one) to the describe following situation, I find a result that I can't make sense of. Hopefully someone can tell me what I'm doing wrong. ...
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Can we see the photon sphere from outside? [duplicate]

This question came to my mind when i saw the movie interstellar. In the movie there is a scene with a black hole and a sphere of light around it, what i assume to be the photon sphere. You can see it ...
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Effects of relativity on New Horizons spacecraft

How much would an atomic clock on board the new horizons spacecraft drift in comparison to an atomic clock on the surface of the earth near the equator after the spacecraft's 10 year journey to Pluto? ...
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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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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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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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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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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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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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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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3answers
98 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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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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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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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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Perfect fluid and Cauchy momentum equation

The stress-energy tensor of a perfect fluid is given by $$T^{\mu\nu}=\left(\rho+pc^{-2}\right)u^\mu u^\nu+pg^{\mu\nu}$$ The divergence of the stress-energy tensor is zero: $\nabla_\mu T^{\mu\nu}=0$. ...
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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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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 ...