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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12 views

How Space Curvature is proportional to Mass in G. Relativity?

In General Relativity, Einstein established that the gravity is due to the curves produced by objects in space. We all know that the gravity is proportional to Mass. The picture Einstein painted ...
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1answer
65 views

Is it possible to define time without light? [on hold]

IS time an illusion? I have tried thought experiment after TE and quite frankly I can't find any instance where time is not defined wholly on its own. It is always a measurement of the interim ...
4
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1answer
74 views

Light clocks measure conformal time - detailed argument

Let us assume that an observer is stationary at the origin in expanding space. We assume the FRW metric near the origin is given by: $$ds^2=-dt^2+a(t)^2dr^2$$ Let us assume that the observer ...
1
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1answer
39 views

Sign of matter Lagrangian term in curved space

In field theory the (matter) Lagrangian $\mathcal{L}_m$ is uncertain upto an overall constant multiplying factor (i.e. $\mathcal{L}_m$ and $a\mathcal{L}_m$ yield the same field equation(s) on ...
8
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2answers
985 views

Black hole with two singularities?

I hope this question isn't too naive, but would it theoretically be possible to have a black hole with 2 singularities (or 2 black holes at the same location). If this is possible, would there be any ...
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1answer
56 views
1
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51 views

Schwarzschild metric circular orbits and kepler's 3rd law

I have been looking at the Schwarzschild metric presented to me as the following within lectures: ...
4
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1answer
68 views

How GR, QFT, or string theory address the one-directional feature of time?

It seems to me today's theoretical relativistic physics treat time and space on equal footing, with manifold diffeomorphism structure decoded in metric. However an obvious difference is that time is ...
30
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10answers
16k views

How exactly does curved space-time describe the force of gravity?

I understand that people explain (in layman's terms at least) that the presence of mass "warps" space-time geometry, and this causes gravity. I have also of course heard the analogy of a blanket or ...
1
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1answer
112 views

Can two distinct events occur at precisely the same moment in time?

I am writing a simulation and am having difficulty resolving the order in which two distinct forces occur. The simulation will give different results if the forces are applied to the state in ...
3
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2answers
130 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 ...
1
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0answers
19 views

Accretion disks on neutron star binaries

Why does hydrogen gas from accretion disks not constantly get sucked onto/into a neutron star or into a black hole? I understand that some gets sucked into the black hole and some may come down and ...
0
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0answers
20 views

Who orbits who? Earth or Sun [duplicate]

We always say that the earth orbits the sun, but how can we prove this? Could we not say that the sun orbits the earth or that the earth orbits Mars?
5
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3answers
162 views
+50

Causality principle and Entropy (Second Law)

I was reading about the light cone in relativity and I got to the point where in order to avoid paradoxes one can introduce the causality principle: Causality Principle: For every inertial ...
1
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0answers
17 views

FRW Metric maximally symmetric, derivation, $R=3K$ or $R=6K$ confusion, two different texts

I'm looking at Tod and Hughston Introduction to GR and writing the metric in the two forms; [1]$$ ds^{2}=dt^{2}-R^{2}(t)(\frac{dr^{2}}{1-kr^{2}}+r^{2}(d\theta^{2}+sin^{2}\theta d\phi^{2})) $$ [2] $$ ...
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0answers
15 views

Photon sphere of Sagrittarius A* , calculation correct? [on hold]

For Kerr Parameter a=0.52, my result for the photon sphere of Sagrittarius A* is ca. 19 Mio. km. (direct orbit and retrograd) Equation Calculation (direct orbit) I am not sure about the arccos ...
0
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2answers
47 views

Is there any physical meaning for the inverse metric?

I've been wondering if we can attribute any physical meaning to the inverse metric. I mean when we talk about the metric itself, there are lots of insights we can have towards its role in spacetime, ...
5
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0answers
110 views
+50

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 ...
0
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1answer
90 views

$SO(p,1)$ transformation on black p-branes [on hold]

I'm working on Blackfolds at the moment and I'm trying to boost-rotate( $SO(1,p)$ ) the p-brane $$ds^2=ds^2_{Sch}+\sum_{i=1}^p dz_i^2$$ Where $ds^2_{Sch}$ is the Schwarzschild metric in $N$ ...
2
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3answers
227 views

Vacuum-ether and spacetime

In the past you could not give an explanation for various phenomena in which there was an action at a distance, like magnetism or gravity, that occurred in a vacuum; For this reason, ether was ...
3
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0answers
142 views

Curvature and spacetime

Suppose that it is given that the Riemann curvature tensor in a special kind of spacetime of dimension $d\geq2$ can be written as $$R_{abcd}=k(x^a)(g_{ac}g_{bd}-g_{ad}g_{bc})$$ where $x^a$ is a ...
5
votes
2answers
114 views

General Relativity visualization software

As I am approaching the study GR, I was wondering if there are softwares that allow a quick visualization of custom metrics, curvature, and particle motion even in the limited context of 2D space. ...
1
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1answer
101 views

If a point r lies in the boundary of the chronological future of another point p, why does the chronological future of r belong to that of p?

I am studying the global causality of the spacetime. Here, I come across a problem. Suppose a point $r\in \partial I^+(p)$. $I^+(p)$ is the chronological future of a different point $p$ in ...
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0answers
35 views

Local translations included in covariant general coordinate transformation

It is known that if we use the constraint $R_{\mu\nu}(P^{a})=0$ , i.e. the curvature of local translations vanishes, then we can modify general coordinate transformations (gct) to a covariant ...
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3answers
101 views

Why would the curvature in space-time attract photons which have zero mass? [closed]

I know that we are surrounded by dark matter, does the dark matter affects space time ???and I know the reason for gravity according to explanations in general relativity. But all the explanation ...
2
votes
1answer
88 views

How physical are vacuum solutions to Einstein's equations?

It is shown in any standard textbook on general relativity how you might get black holes and gravitational radiation, both of which are vacuum solutions to field equations, i.e., such that ...
1
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0answers
29 views

What is the speed of gravity? [duplicate]

Hypothetically, let`s say that one day our Sun is suddenly removed from the galaxy. For us, on earth, would take up to 8.3 minutes to notice that the sun was removed, since the distance from the earth ...
0
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1answer
55 views

How can electromagnetic waves be affected by gravity if photons have no mass? [duplicate]

I have found answers that say that gravity interferes with the oscillating electric and magnetic fields of the waves and others that say that since gravity is a bend in space-time, in which case the ...
0
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0answers
45 views

How do I obtain the metric tensor of a constant electric or magnetic field? [on hold]

Consider the empty space with a constant magnetic or electric field. From the Faraday or the Maxwell tensor that describes this situation, how do I obtain its metric tensor? I could try the Einstein ...
0
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1answer
51 views

Lorentz contraction in continuously accelerating rod

I was having a little thought experiment about Lorentz contraction, and I couldn't really figure out what would actually happen. Note that I'm not looking for a answer 'this effect is barely ...
2
votes
1answer
43 views

Local translations in curved spacetime

A global Poincare transformation on a scalar field induces $$\delta(a, \lambda)\phi(x) = [a^{\mu}+\lambda^{\mu\nu}x_{\nu}]\partial_{\mu}\phi(x). \tag{11.46}$$ In curved spacetime we replace $a^{\mu} ...
0
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1answer
47 views

Is our locally measured time actually conformal time?

The FRW metric at the origin $r=0$, with $c=1$, is given by: $$ds^2=-dt^2+a(t)^2dr^2$$ Now one can change variables so that near the origin the FRW metric is approximated by the Minkowski metric ...
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0answers
53 views

using tetrads to glue local currents into global currents

According to John Baez it is possible to take a locally conserved tensor $\nabla_\mu\: T^{\mu\nu}(x)=0\ \ \ \ \ \mbox{(locally)}$ and convert it to a globally conserved tensor by "patching" ...
0
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0answers
31 views

Perturbations and Chronology condition [on hold]

Let $(M,g)$ be the quotient of the 2-dimensional Minkowski space-time by the discrete group of isometries generated by the map $f(t, x) = (t + 1, x + 1)$. Show that $(M, g)$ satisfies the ...
7
votes
3answers
327 views

Recommended books for a “relativity for poets” class?

I teach physics at a community college and have developed a new course titled "Relativity for Poets," which I will be teaching for the first time in spring 2015. As implied by the title, it's a ...
4
votes
2answers
179 views

How do we age if we tunneled to Earth's core?

Scenario Suppose there exists an advanced technology that can hypothetically transport living humans to study the center of the Earth, as they goes deeper underground most of the Earth's mass would ...
1
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1answer
88 views

Covariant derivative of Levi-Civita tensor [on hold]

I'm currently studying Carroll's GR book Spacetime & Geometry, and ran into some trouble understanding the text at page 99 which says: By using metric compatibility $$(\nabla_{\alpha}g)_{\mu ...
0
votes
1answer
45 views

Does one *feel* the electrostatic force while not resisting it?

Suppose that I put lots of big magnets around me, or say, that I charge myself up to a macroscopic charge. Now, suppose that there's a huge magnet in front of me (or a huge object with opposite ...
2
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1answer
101 views

Question about universe expansion

In general relativity, we cannot determine the global structure of the universe (since it is not flat), therefore all measurements and observations are only meaningful locally. In particular, we can ...
3
votes
3answers
159 views

How can space be euclidean when light bends?

I have read people arguing that tridimensional space sections of space time continuum (whatever its number of dimensions) appears to be euclidean from empirical evidence. I cannot reconcile it with my ...
4
votes
1answer
132 views

Does the definition of the SI unit “second” require that possible perturbation of primary frequency standards should be measured?

The definition of the SI unit "second" is stated as The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground ...
0
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2answers
106 views

why do x Schwarzschild radii equal time dilation effects of speed of light going y times faster than an object^2?

let me walk you through the math. First you start with the gravitational time dilation formula where: $$ T_1=T\sqrt{1-\frac{2GM}{rc^2}} $$ and rather than entering $r$ for the radius we replace $r$ ...
3
votes
1answer
136 views

Regarding the possibility of Closed Timelike Curves

I've been looking a lot at Closed Timelike Curves, and how if a theory allows for these curves it doesn't respect causality. I understand that about the curves themselves (Grandfather Paradox), but ...
0
votes
3answers
65 views

What will be the gravitational pull of the Sun if the Sun contracts to a black hole?

What will be the gravitational pull of the Sun if the Sun contracts to a black hole, and would the Sun be a Schwarzschild black hole or not?
0
votes
0answers
46 views

Gravitational electromagnetic clock effect [closed]

Suppose an orbiting object is moving around a rotating black hole. There is a massive gravitational acceleration that cause gravito-electro-magnetic clock effect. This effect says that object moving ...
2
votes
1answer
44 views

Gravity of photons in different reference frames

I know that photons have gravity because they contribute to the stress energy tensor, but this means that observers in different reference frames experience a different gravitational force from the ...
1
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0answers
49 views

Is the Weyl Postulate correct?

The Weyl postulate in cosmology states that worldlines do not intersect but it can be shown in GR that using Raychaudhuri equation that geodesics can intersect if there is curvature so I'm really ...
3
votes
2answers
72 views

Why is $p_\phi$ conserved in a Schwarzschild orbit?

This arises from the question What is the relationship between $a$ and $m$, which I'm afraid I answered just by looking it up in Schutz's book. However Schutz (as he frequently does) glosses over ...
1
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0answers
39 views

Problem on finding the spatial components of canonical momenta in Fierz-Pauli Lagrangian

My problem is about finding the spatial components of canonical momenta $\pi_{i j}=\frac{\partial \mathcal{L}}{\partial \dot{h}_{i j}}$ corresponding to the Fierz-Pauli Lagrangian. I am using the ...
6
votes
1answer
130 views

Non-trivial scalar quantity

Is there any scalar quantity made of only the Christoffel symbols, determinant of a metric and tensors, not derivatives? In other words, can we construct a scalar quantity which cannot be written in ...