A theory that describes how matter interacts dynamically with 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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Big Bang…or…Everywhere Stretch?

Recently I watched a minute-physics video that suggested that a better name for the beginning of time would be "Everywhere stretch" because there wasn't a space-time singularity that formed where the ...
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1answer
86 views

Is dark matter moving faster than light? [closed]

My way of thinking is as follows: Consider a balloon. Let this be blown into a nearly elliptical sphere. Now, the temperature inside the balloon is increased by some means; this leads to the ...
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2answers
304 views

Before Einstein came up with General Relativity, were there serious reasons to doubt Newton's theory?

Before Einstein came up with General Relativity, was there any serious reason to doubt Newton's theory (and its various developments)? I only know about the discrepancy in Mercury's orbit, which may ...
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2answers
53 views

Expanding universe and speed of light [duplicate]

Galaxies are moving away from us proportional to the distance between us and them , but nothing can travel faster than light, so even the farmost galaxies should be travelling away from us along with ...
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1answer
134 views

2 + 1 dimensional gravity as an exactly soluble system [closed]

In Gomez's article (Higher spin part 2)http://arXiv:1307.3200, I face to gauge algebras. Gomez says we have some algebras for ISO(2,1)and SO(2,2),SO(3,1). and he tells us only ISO(d-1,1) and only ...
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2answers
93 views

Once I calculate the Riemann curvature tensor, what do I do with it?

I am considering the Schwarzschild metric. I have calculated my Christoffel symbols and am able to calculate the Riemann tensor (I think). In short, I have done a bunch of work to find this thing ...
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3answers
282 views

Do photons lose energy due to gravitational redshift? If so, where does the lost energy go? [duplicate]

In the gravitational redshift, the frequency of photons radiated from some source is reduced. As the energy of a photon is given by $\hbar\omega$, if the frequency is reduced where is the lost energy? ...
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0answers
41 views

How can matter reach the singularity of a black hole if time stops at the event horizon? [duplicate]

Black holes are said to be composed of two parts: A singularity at the center, where all the mass of the black hole is condensed in a point of zero volume. A black and empty ball delimited by the ...
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3answers
104 views

Acceleration due to Gravity: Free Fall [duplicate]

If we are in a free fall which implies we are accelerating at 9.8 m/sec every sec. And let's say that we are falling into a pit that has enormous depth. So isn't this be possible that we may ...
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41 views

Super-luminal separation

Suppose there are are 5 bodies separating in space (due to it's expansion), and let us consider that they can emit light. 1st body moves in the -x direction, at the speed of light. 2nd body moves ...
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1answer
62 views

What is the Kerr factor for Sagittarius A*?

I have searched for it, but everything what I found is that A0620-00 (the current closest black-hole to Earth) is a slow spinner with Kerr factor $a=0.12$. How about the Kerr factor for Sagittarius ...
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1answer
34 views

transformations between 1st and 2nd order formalism in pure gravity

I am currently studying about 1st order formalism and I was wandering if the gauge transformation in the vielbein can be mapped to the coordinate transformation of the metric ( pure 2+1 gravity), ...
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1answer
65 views

Connection between two Petrov classification schemes

For the Weyl scalars of all spacetimes, at any point, possess one special structure, the so called principal null directions. Consider a general null tetrad $\{ l_a,n_a,m_a,\overline{m}_a \}$, we ...
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2answers
78 views

Schwarzschild: Proof that $\{r<2m\}$ is a black hole

I saw the following proof to show that $\{r<2m\}$ is a black hole in the Schwarzschild metric. Consider the Schwarzschild metric: $$ g=-V(r)\text d t^2 + \frac{1}{V(r)}\text d r^2 + r^2 \text d ...
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2answers
139 views

Why doesn't the light get out?

You're standing on a gedanken planet holding a laser pointer straight up. The light doesn't curve round, or slow down as it ascends, or fall down. It goes straight up. Now I wave my magic gedanken ...
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2answers
50 views

Gravitoelectromagnetism higher-order effects

See http://en.wikipedia.org/wiki/Gravitoelectromagnetism#Higher-order_effects According to the Higher-order effects... "Gravitomagnetic arguments also predict that a flexible or fluid toroidal mass ...
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0answers
58 views

Gauge invariant quantities [closed]

In the context of cosmological perturbation one write the most general perturbed metric as $$ ...
2
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0answers
62 views

What's the physical meaning of gravitational shock wave?

In this paper link.aps.org/doi/10.1103/PhysRevD.50.3666 the authors discuss the gravitational shock wave metric produced by massless particle: $ds^2=-du(dv+4p\,\text{ln}(\rho^2)\delta(u)du)+dx^2+dy^2$ ...
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1answer
64 views

Exotic matter out of a “squeezed” vacuum?

In the light of the current warp field experiments by NASA I found this paper: http://arxiv.org/pdf/1005.5682v1.pdf Could this actually lead to the creation of exotic matter or at least strengthen ...
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1answer
37 views

Definition of conformal time in de Sitter spacetime

I'm trying to follow the calculations in http://arxiv.org/abs/hep-th/0201158v2 The aim is to rederive the expressions (2.16), (2.17) for the power spectrum in de Sitter spacetime. In order to do so, ...
2
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1answer
276 views

Gravity's Rainbow

What is it? I've seen it in context of a few research papers such as Absence of black holes at LHC due to gravity's rainbow and Thermodynamics of black holes in gravity’s rainbow formalisms My best ...
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1answer
50 views

Lapse and shift in ADM decomposition

Poisson in Relativist's Toolkit and also other authors in various papers state explicitly that after one does the 3+1 decomposition, the lapse and shift $N$ and $N^a$ are non-dynamical variables, and ...
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0answers
45 views

Connection one-form and suppressed indices

I am reading Sean Carroll's notes on GR, which states (Page 91): Using our freedom to suppress indices on differential forms, we can write the defining relations for these two tensors as: $$ T^a ...
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0answers
21 views

Calculate Total Energy Of Universe [duplicate]

Physicist using $mc^2$ to calculate positive energy and $-MmG/R$ to calculate negative energy to calculate total nergy of the universe but I heard that one of them is relativly invariant and other ...
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1answer
36 views

Basic question about curved and flat indices, and the Dirac matrices on $S^5$

In discussing the Kaluza-Klein formalism for Type IIB Supergravity on $S^5$, or the AdS5xS5 compactification, one requires Killing spinors on $S^5$. I read that the Dirac matrices on $S^5$ satisfy ...
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0answers
33 views

Does the rotation of the Earth affect time?

If the Earth were to spin from east to west instead of west to east, how would that affect time or our perception of it?
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1answer
75 views

Study the center and interior of a black hole using quantum entanglement

Having just watched the latest in a long line of space movies; Interstellar tries to combat the problem of studying the center of a black hole and understanding what's going on inside. My question is, ...
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2answers
72 views

Allowable spacetime deformations [closed]

What deformations are possible with spacetime? By 'deformation' I am referring to the kind of change in spacetime caused by the presence of a mass which deforms spacetime sufficiently to deflect ...
7
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3answers
180 views

Can the Unruh effect be confirmed by the LHC?

Two short questions regarding the Unruh effect. There are related answers on this forum and on wikipedia, but I am looking for confirmation of my own intuitive assumptions, so a straightfoward yes or ...
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0answers
38 views

Light rays in linearized General Relativity

In General Relativity, particles follow geodesics in space-time, obeying $$\ddot x^a + \Gamma^a_{bc}\dot x^b\dot x^c=0,$$ where $\Gamma^a_{bc}$ are the Christoffel symbols, expressed in terms of the ...
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1answer
36 views

What observation(s)--if any--confirm that the types & concentrations of energy, which are influenced by gravity, are the same ones that cause gravity?

General relativity allows various forms of energy to participate in the gravitational force. What observation(s)--if any--confirm general relativity's notion that the various types & ...
1
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1answer
61 views

Real-world evidence that non-massive entities (or even: antiparticles), and their behaviors, are sources of gravity?

The theory of general relativity tells us that non-massive entities, and their behaviors, are possible sources of gravity. Mass isn't needed, the theory says. What's the real-world evidence that ...
2
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1answer
78 views

Does spacetime have symmetric curvature around an object? If yes, then why do planets revolve around the Sun in elliptical orbits?

Does spacetime have symmetric curvature around an object? If yes, then why do planets revolve around the Sun in elliptical (as opposed to circular) orbits?
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2answers
143 views

How confident are we that mass is not being lost in the universe?

After reading about the latest super-massive black hole in Nature 518, 512–515 (26 February 2015), I couldn't help but wonder if the accelerating expansion is a result of mass being lost. Would a ...
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2answers
95 views

Why are orbits around black holes stable?

Black hole theory involves space (or space-time), itself, being sucked into the black-hole, with the event horizon marking the point at which space/space-time is moving faster than the speed of light. ...
6
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223 views

What are Galileons good for?

Lately I've seen many papers (for example "The galileon as a local modification of gravity"; 292 total hits on the arXiv) on types of field theories known as Galileons, and I'm wondering what the ...
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1answer
142 views

Gravity in $d$ spacetime dimensions

Given the following action $$S=\frac{1}{16\pi G}\int d^4x \sqrt {-g}(R+aR^2+bR_{\mu\nu}R^{\mu\nu}+cR_{\mu\nu\lambda\sigma}R^{\mu\nu\lambda\sigma}),$$ which is in 4D. How to we generalise this ...
2
votes
1answer
89 views

Can energy bend space? [duplicate]

I know mass bends the space around it and I also remember matter can be converted into energy and vice versa, so my question is: can energy interact with space in a similar fashion as matter does?
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3answers
752 views

Gödel's solutions to Einstein's relativity equations and their consequences

Gödel gave certain solutions to Einstein's relativity equations that involved a rotating universe or something unusual like that; that predicted stable wormholes could exist and therefore time travel, ...
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1answer
33 views

How to prove that the nonlinear completion of free massless spin-2 action must be Einstein-Hilbert action?

There is a saying that the nonlinear completion of free massless spin-2 action in Minkovski spacetime (that is Fierz-Pauli action) must be Einstein-Hilbert action up to Lovelock invariants. I find a ...
2
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5answers
141 views

A ball in the gravity potential field of a black hole — seems a paradox

As illustrated in the following diagram (A, B, C, D are 4 specified space points, and C is close to a black hole), a small ball at distance of a black hole is stationary (suppose now it's mass is m0) ...
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0answers
50 views

Friedmann Equations Derivation [closed]

I was trying to understand how one goes from the following system of Friedmann equations \begin{cases} \frac{\dot{a}^2}{a^2}+\frac{k}{a^2} = \frac{8\pi\rho(t)}{3} + \frac{\Lambda}{3}\\ ...
2
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1answer
65 views

Heuristics for the Hawking mass

I have the following definition of Hawking Mass. Given a spacelike 2-surface $S$ embedded in a 3+1-dimensional Lorentzian Manifold $L$, $$ M(S) := \sqrt{ \frac{\text{Area}(S)}{16 \pi}} \left(1- \frac ...
2
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1answer
62 views

Non-linearity and self-coupling of gravity

I have heard that non-linearity of Einstein's field equations has to do with the fact that gravity self-couples. What does non-linearity have to do with self-coupling?
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4answers
2k views

Will an object always fall at an infinite speed in a black hole?

Most of you if not everybody will agree that the stronger the gravitational pull, the faster an object will fall. For example, on a planet with 50 times the gravity of Earth, any object will hit the ...
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0answers
39 views

How one can solve Friedman equations of General relativity numerically?

How one can solve Friedman equations numerically subjected to any initial condition?
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1answer
25 views

Black holes in a denser media

In Crown Glass, the speed of light is 1.52 times the speed of light in vacuum. Consider the following hypothetical situation: Hypothetical Situation If a black hole is surrounded by a giant crown ...
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4answers
135 views

Normal Vectors to these Hypersurfaces on a Lorentzian Manifold

With respect to the coordinates $(x^{0},x^{1},x^{2},x^{3})=(v,r,\theta,\phi)$, we have the following components of the metric tensor: $\begin{bmatrix} g_{00} & g_{01} & g_{02} & ...
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1answer
54 views

Bianchi Identity using null tetrad

I'm currently looking at the Newman-Penrose Formalism, and trying to understand where there sets of equations come from. For that, I need to know how I can write the second Bianchi identity for the ...
5
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3answers
87 views

Has anyone tried Michaelson-Morley in an accelerated frame?

After doing much more digging than I thought I had to do, I found out that the speed of light is NOT invariant in an accelerated reference frame. Has anyone done any experiments to confirm this? In ...