A theory that describes how matter produces and responds to the geometry of space and time. It was first published by Einstein in 1915 and is currently used to study the structure and evolution of the universe, as well as having practical applications like GPS.

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In GR, how do particles know how to fall in instead of out of a gravitational well?

The geodesic equation (let's suppose that we're talking about massive particles, so I'll parameterize the path by proper time $\tau$) $\frac{d^2 x^\mu}{d \tau^2} + \Gamma^\mu_{\rho \sigma}\frac{d ...
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2answers
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How can a point-like particle “feel” gravity, if locally the curvature of spacetime is always flat?

I imagine a point-like particle can only experience the local properties of spacetime. But locally there is no curvature and no gravity, as it is often stated that Locally, as expressed in the ...
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3answers
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The Pioneer anomaly finally explained?

Pioneer 10 & 11 are robotic space probes launched by the NASA in the early 1970's. After leaving our solar system, an unusual deceleration of both spacecrafts has been measured to be approximately ...
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1answer
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Does the recent Gravity Probe - B mission mean both the mass of an object and the spin of an object affect time?

I'm a non-engineer interested in the recent GP-B mission results: http://www.engadget.com/2011/05/06/nasa-concludes-gravity-probe-b-space-time-experiment-proves-e/#disqus_thread Is it correct that ...
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343 views

How close can two extremal black holes with the same charge can get?

Here's a puzzle I have been pondering over. If we have two extremal black holes with the same charge, the electrostatic repulsion between them ought to cancel the gravitational attraction between ...
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2answers
263 views

Can the equivalence principle be tested to high precision in a human-sized lab falling through the horizon of a black hole, in principle?

Can the equivalence principle be tested to high precision in a human-sized lab falling through the horizon of a black hole, in principle? By "human-sized lab" I mean a lab the size of the ...
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1answer
181 views

Testing General Relativity

Ever since Einstein published his GR theory in 1916, there have been numerous experimental tests to confirm its correctness--and has passed with flying colors. NASA and Stanford have just announced ...
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293 views

Gravitational wave energy

Electromagnetic energy can be related to it's frequency via $E=h\nu$. Is there a comparable relationship between gravitational wave energy and frequency?
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239 views

Do all of our discoveries of black holes in nature depend on the validity of GR?

In the question Is there a black hole in the centre of the Milky Way? the answer by Motl seems to all but say the existence of that black hole is a fact (see also Evidence for black hole event ...
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714 views

Can colliding gravitational waves create a black hole?

Whether gravitational waves are real or just a coordinate freedom was argued in the early days of GR. Eventually the conclusion was that they were real. And if they are 'real' then I'm curious if... ...
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3answers
613 views

Anti-matter repelled by gravity - is it a serious hypothesis? [duplicate]

Possible Duplicate: Why would Antimatter behave differently via Gravity? Regarding the following statement in this article: Most important of these is whether ordinary gravity attracts ...
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623 views

Why is there a flux of radiation in the Hawking effect but not in the Unruh effect? (and other questions)

This question is slightly related to this one Do all massive bodies emit Hawking radiation?, which I think was poorly posed and so didn't get very useful answers. There are several questions in this ...
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1answer
512 views

How will the Twin Paradox become, for Time Dilation, if no acceleration was ever involved?

I think one catch in Twin Paradox was about the big acceleration that can turn back the traveling twin from light speed outward bound, to become light speed inward bound. What if there is strictly no ...
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3answers
814 views

Does the Unruh effect violate Mach's principle?

Mach's principle says that it is impossible to tell if something is accelerating unless there is something else in the universe to compare that motion to, which seems reasonable. However, if you had ...
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4answers
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Is there a fundamental reason why gravitational mass is the same as inertial mass?

The principle of equivalence - that, locally, you can't distinguish between a uniform gravitational field and a noninertial frame accelerating in the sense opposite to the gravitational field - is ...
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2answers
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Is dark matter repulsive to dark matter? Why?

I think I saw in a video that if dark matter wasn't repulsive to dark matter, it would have formed dense massive objects or even black holes which we should have detected. So, could dark matter be ...
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1answer
781 views

Time dilation when falling into black hole

I know that if one astronaut falls into a black hole, then a distant observer will see him take an infinite amount of time to reach the event horizon (provided the observer can see light of ...
3
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2answers
370 views

General Relativity Paradox: Holding a string across a gravitational gradient

The paradox I envision involves two objects that exist in very different gravitational potentials -- one very high (eg: close to a black hole) and one very low (eg: far away from a black hole). Now, ...
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Can a non-euclidean space be descripted through an euclidean of higher dimension? so why use non-euclidean?

If you draw a big triangle in earth 2D surface you will have an aproximated spherical triangle, this will be a non euclidean geometry. but from a 3D perspective, for example the same triangle from ...
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Alcubierre Drive - Clarification on relativistic effects

On the Wikipedia article on the Alcubierre drive, it says: Since the ship is not moving within this bubble, but carried along as the region itself moves, conventional relativistic effects such as ...
3
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1answer
471 views

Math and Wormholes

Hopefully this is the correct forum for this. I felt that Physics Overflow may not be the correct place. I had a student approach me ask me what kinds of mathematics goes into the study of wormholes. ...
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3answers
455 views

Curvature of Conical spacetime

Inspired by: Angular deficit The 2+1 spacetime is easier for me to visualize, so let's use that here. (so I guess the cosmic string is now just a 'point' in space, but a 'line' in spacetime) Edward ...
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Is special relativity an exact description of reality?

In discussing relativity with a (somewhat mathematical) friend the other day, I ran into a problem showing why special and/or general relativity could be considered as exact descriptions of reality ...
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0answers
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A question about the relativity of time [duplicate]

Possible Duplicate: Invariant spacetime - distance - Circular Motion I understand that the closer something travels to the speed of light, that time will stretch by a factor, and distance ...
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Invariant spacetime - distance - Circular Motion

I understand that the closer something travels to the speed of light, that time will stretch by a factor, and distance will compress by the same factor. My question is, if something travels in a ...
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0answers
351 views

Calculation of the non-Gaussity parameter for primordial cosmological perturbations by the ADM Formalism

Maldacena has used the ADM Formalism in one of his papers (http://arxiv.org/abs/astro-ph/0210603) in computing the the three point correlation function (i.e the non-Gaussianity) parameter for ...
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6answers
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Why can't light escape from a classical black hole?

Photons do not have (rest) mass (that's why they can move at speed of "light"). So, my question is how the gravity of classical$^1$ black hole can stop light from escaping? -- $^1$ We ignore ...
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1answer
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Scale set by cosmological constant

Following on Jim Graber's answer to: Can "big rip" rip apart an atomic nucleus? If the cosmological constant is large enough, even the ground state of a hydrogen atom can be affected. So ...
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2answers
225 views

Can a trapped surface be formed by a mass configuration outside of that trapped surface?

Can a trapped surface be formed without any massive bodies inside that trapped surface, but only by a configuration of massive bodies surrounding the trapped volume?
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2answers
277 views

Does throwing a watch into the air cause it to gain or lose time?

Suppose I'm on a non rotating planet. I have two identical, perfect watches. I synchronize them. Then I throw one of them into the air and catch it. Does the one I throw into the air gain or lose time ...
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3answers
725 views

Maximum time difference between clocks in a gravity field

From Surely you must be joking, Mr Feynman. You blast off in a rocket which has a clock on board, and there's a clock on the ground. The idea is that you have to be back when the clock on ...
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1answer
493 views

Perturbation of a Schwarzschild Black Hole

If we have a perfect Schwarzschild black hole (uncharged and stationary), and we "perturb" the black hole by dropping in a some small object. For simplicity "dropping" means sending the object on ...
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699 views

Can the Big Rip really rip apart an atomic nucleus?

Some scenarios describing the fate of the matter vs dark energy tug of war on the universe involve the acceleration of the universe increasing to the point that it ends up ripping apart even atoms. ...
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0answers
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net displacement and path dependence

reading the paper about spacetime swimming by Wisdom (something related to this has been previously asked here) can't help but think that there is more to this than what is on the paper. Basically ...
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3answers
427 views

How were non-Euclidean manifolds applied to physics before Einstein?

In the letter of introduction to Einstein's 1916 paper on General Relativity, he writes, "The mathematical tool sthat are necessary for general relativity were readily available in the 'absolute ...
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2answers
464 views

Decomposing geodetic/de Sitter effect into Thomas precession and spatial curvature

According to Rindler the geodetic effect can be considered as consisting of Thomas precession combined with the effect of moving through curved space. Wolfgang Rindler (2006) Relativity: special, ...
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1answer
537 views

Why is GR ghost-free?

I wonder how one can show that general relativity is ghost-free? By ghost I mean the negative norm state that breaks the unitarity. I think it is a well-known "fact" but I just couldn't find any ...
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1answer
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Angular deficit

If one starts with a flat piece of paper, removes a wedge, and tapes the paper together, you get a cone. The angle of the removed wedge is called the "angular deficit". Now if this is done in 3 ...
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2answers
779 views

Intrinsic angular momentum in classical mechanics

Please note, I am only interested in classical mechanics discussion on this. Please do not involve quantum mechanics. Inspired by this question: Is Angular Momentum truly fundamental? My question ...
11
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1answer
726 views

Can GR be derived by postulating a maximum force?

This paper was published in a peer review journal, and claims the answer is yes. http://arxiv.org/abs/physics/0607090 The derivation in the paper seems more like dimensional analysis hand-waving in ...
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2answers
940 views

Space-like Killing vector of Robertson-Walker metric?

In the book "Kinetic theory in the expanding Universe" (J. Bernstein, 1988, Camb. Univ. Press), it was stated that "for nonstationary Robertson-Walker matrixes [sic] there is no spacelike ...
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How do wormholes work?

Firstly, I understand that we have no observational evidence for 'wormholes'. They are theorised solutions to general relativity equations. That said, if macroscopic wormholes do exist---how do they ...
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3answers
347 views

How can you tell if a critical energy density is actually a black hole?

Here's a question inspired by Edward's answer to this question. It's my understanding that the average energy density of a black hole in its rest frame is $\rho_\text{BH}(A)$, a function of surface ...
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2answers
949 views

Is there an energy density limit in GR?

I am speaking about GR with classical fields and energy. One question, spread over three increasingly strict situations: Is there an energy density limit in GR? (literally, can the energy density ...
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1answer
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535 views

Brachistochrone problem in general relativity

This question Brachistochrone Problem for Inhomogeneous Potential has the obvious extension. Namely the same question, when gravity is treated according to general relativity. To make it specific ...
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1answer
446 views

Does change of coordinate system require acceleration?

This question came about from a side discussion that arose on this: Does GR provide a maximum electric field limit? Can we change our choice of coordinate system completely independent of physical ...
9
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1answer
698 views

What all is needed to solve for the metric in GR?

Einstein's field equations are: $R_{ab} - {1 \over 2}g_{ab}\,R + g_{ab} \Lambda = {8 \pi G \over c^4} T_{ab}$ And since the Ricci curvature tensor is "less information" than the Riemann curvature ...
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No hair theorem for black holes and the baryon number

The no hair theorem says that a black hole can be characterized by a small number of parameters that are visible from distance - mass, angular momentum and electric charge. For me it is puzzling why ...
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1answer
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Is GR vacuum equation unique?

The title question would be too long if I tried to specify it clearly. So let me be more clear. Consider the class of theories having the following properties: The langrangian density is only ...