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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250 views

Confusion about the concept of cosmological horizon

This question was inspired by this one By definition we cannot see any event happening beyond the cosmological horizon. Let us assume that the expansion rate of the universe is such that the radius ...
10
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5answers
1k views

How does “curved space” explain gravitational attraction? [duplicate]

They say that gravity is technically not a real force and that it's caused by objects traveling a straight path through curved space, and that space becomes curved by mass, giving the illusion of a ...
1
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2answers
111 views

Should dark matter play an important role in designing a quantum theory of gravity?

Since dark matter "accounts" for most of the matter of the universe, and its effect are inferred from the gravitational effects, should it play an important role in designing an unified theory between ...
0
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0answers
27 views

String spreading?

What is 'string spreading' (in the context of string theory)? Does it have implications for the observability of post-classical effective field theory of gravity terms near macroscopic black hole ...
4
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4answers
284 views

Relation of General Relativity to Dark Matter and Dark Energy

I was reading an elementary book on dark matter (in fact, a historical perspective) and there were mentioned how the scientific community react to the idea of dark matter proposed as a solution to ...
2
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1answer
65 views

What does Kaluza-Klein theory say about the attraction/repulsion of opposite/same charges?

Since Kaluza-Klein theory is made out of general relativity - a gravitational theory in 4 dimensions which is only attractive, then how does it takes into account the attraction/repulsion of opposite/...
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4answers
172 views

Which tensor describes curvature in 4D spacetime?

I heard these two statements which don't work together (in my mind): In 4D spacetime the curvature is encoded within the Riemann tensor. He holds all the information about curvature in spacetime. ...
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4answers
270 views

Why is “dark matter” theory accepted? Why wasn't general relativity rejected?

Dark matter was made up to account for unexplained effects such as gravitational lensing, the speed of expansion of the universe, or the rate of rotation of certain galaxies. However, as Feynman ...
1
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1answer
178 views

Null geodesic equations

If one is constrained to the $xt$ plane, one can define the intersection with that plane of the null hypersurfaces originating at some point $P$ as $$ g_{tt} \frac{d P^t}{d \lambda}\frac{d P^t}{d \...
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0answers
34 views

Can gravity affect light speed? [duplicate]

Can gravity affect light speed, for example slow it down ?
0
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0answers
50 views

Questions arising from the presentation of “Schild's ladder” in “Gravitation” (Misner, Thorne, Wheeler)

MTW, p. 248, presents a description of "Schild's ladder" construction which I quote here extensively enough for referencing some subsequent questions. A. Transport any sufficiently short stretch ...
0
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0answers
38 views

How is speed of light constant if it bends? [duplicate]

Gravitational lensing causes light to be bent.If light changes direction how can it be travelling with constant speed?Where am I wrong.Thanks for any help.
2
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1answer
113 views

Spacetime curvature effect on chemistry

Do current chemistry / astrophysics / stellar chemistry calculations include the effects of the curvature of spacetime on chemical reactions? For example, the heat transfer from a point closer to the ...
0
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1answer
61 views

Notation: tetrad indices

I am trying to understand the meaning of upper and lower indices as used in the Newman-Penrose formalism. The tetrad is $\lbrace l^{a},n^{a},m^{a},\overline{m}^{a}\rbrace$, where the upper index ...
2
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0answers
228 views

Stephen Wolfram claims to deduce the field equations from cellular automata, has anyone seen the actual mathematics?

In his new blog post Stephen Wolfram claims that he can derive general relativity from cellular automata. OK, so one can derive Special Relativity from simple models based on networks. What about ...
0
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0answers
52 views

Bending of light in a gravitational field & the principle of equivalence

I have a few conceptual issues following a standard thought experiment to argue why light bends in a gravitational field and I'm hoping I can clear them up here. Consider an observer in a lift in ...
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0answers
45 views

What would happen, gravitationally, to ships passing by each other at high speeds vs high accelerations?

Consider this scenario: Two identical space ships, the SS Observer and the SS Accelerator. In scenario A, the SS Accelerator is accelerated up to near C, stops accelerating, then flies past the SS ...
3
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0answers
52 views

How does GR determine the topology of spacetime? [duplicate]

The crux of GR is the action $$ S=\int _\mathcal M d^n x \sqrt{|g|}\,R $$ Varying this and setting $\delta S=0$ gives you the Einstein field equations. However, that only determines the metric, not ...
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1answer
67 views

Eccentric binary black holes

Comparable-mass binary black hole inspirals and mergers are expected to be an important source of gravitational wave signals for current and future ground-based detectors. It is generally expected ...
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0answers
33 views

Dark Energy Density Paramater For Unlimited Expansion

Suppose that the universe is dominated by matter and dark energy. Let $\Omega_m$ and $\Omega_\Lambda$ denote the respective relative density parameters. Given $\Omega_m$, what does $\Omega_\Lambda$ ...
60
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8answers
5k views

Turbulent spacetime from Einstein equation?

It is well known that the fluid equations (Euler equation, Navier-Stokes, ...), being non-linear, may have highly turbulent solutions. Of course, these solutions are non-analytical. The laminar flow ...
2
votes
1answer
214 views

Riemann tensor with 2nd and last indice the same will vanish?

I calculated that Riemann tensors are antisymmetric with respect to 2nd and last indice,as the symmetry properities of $R_{\rho\nu\sigma\mu}$ goes. $$R^{\omega}_{\ \ \ \nu\sigma\mu}=g^{\rho\omega}R_{\...
1
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1answer
99 views

Geodesic equation (free particle)

How to find a coordinate system whose geodesic equation does not have the "Christoffel symbol" term? (i.e. free particle - generalized Newton's second law.)
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0answers
17 views

Acceleration measured when hovering over a non-rotating black hole [duplicate]

I’m wondering what would be the acceleration theoretically measured (for example by an accelerometer) locally by an astronaut whose (ideally pointlike) spacecraft is hovering steadly over a non ...
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2answers
71 views

Minkowski geometry definition

The general relativity is based on Minkowski geometry definition with its special properties. The general relativity cant be approved wihout Minkowski geometry definition. Why Minkowski geometry is "...
4
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1answer
91 views

In what manner does momentum of a particle with mass decrease due to spatial expansion?

I've read that the momentum of particles declines due to the universe's expansion. In particular, that $p \propto \frac{1}{a}$, where $a$ is the scale factor. For light, this momentum reduction ...
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1answer
27 views

A book containing a large subset of known exact solutions to the EFEs

I am looking for a good book which contains the known exact solutions to the EFEs. A "library" of sorts. I am aware that it's not really possible to get all of them in a single book, but at least one ...
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1answer
130 views

Traveling through time, reaching the past - POSSIBLE? [closed]

I was obsessed in studying the "faith" and I've made several great discoveries. I don't want to publish anything yet, but I have a semi-proven (proven partially) / theories / so I am curious on bones. ...
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1answer
25 views

Vacuum solutions to the EFEs

I was wondering if there are any papers in which authors have compared cosmological observations to those phenomena predicted by vacuum solutions to the EFEs? Obviously there is matter in the ...
2
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1answer
122 views

Are all maximally symmetric spacetimes constant curvature spacetimes?

A $d$ dimensional maximally symmetric spacetime is a spacetime with the maximum allowed number of Killing vectors. This number is $\frac{d(d+1)}{2}$. Constant curvature spacetimes are spacetimes ...
8
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1answer
795 views

The final parsec “problem”

Many and perhaps all galaxies seem to contain supermassive black holes of about $10^7 M_\odot$ at their centres. Determining their origins is of great astrophysical interest. In what I understand to ...
2
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0answers
35 views

Why is it useful to study spatially inhomogeneous spacetimes? [closed]

There are many scholars who have dedicated their efforts towards finding solutions to the Einstein Field Equations that are spatially inhomogeneous. Current cosmological observations indicate that on ...
2
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0answers
28 views

In causal dynamical triangulation, what equation(s) give the distribution of angles of the triangles?

I know very little about this topic, but going on what I learned in my one semester of QM, there has to be some Schrodinger-like equation they are using to get the distribution of angles of triangles ...
2
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0answers
95 views

Gravitational mass defect

In nuclear physics we have a mass defect by the binding energy of the nuclides. A similar effect appears in the theory of gravitation induced by the gravitational binding energy, which reduces the ...
3
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2answers
272 views

Is the popular explanation given for gravity in General Relativity misleading? [duplicate]

In most popular explanations of General Relativity, both in print and film/television, gravity is demonstrated using an example of a 2 dimensional plane being flat, then when putting a heavy object in ...
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1answer
49 views

Why only binary star system produce ripples in space-time (gravitational waves)?

I saw a statement being made in a video, but it didn't explain why. It originally said, "stars orbiting each other does not create gravitational waves, binary system of two massive stars or black hole ...
6
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1answer
229 views

The cosmological constant as a Lagrange multiplier?

The cosmological constant $\Lambda$ can be introduced into the gravitational action like this : \begin{equation} S = \frac{1}{2 \kappa} \int_{\Omega} (R - 2 \Lambda) \sqrt{-g} \; d^4 x + \text{matter ...
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1answer
73 views

How can light follow curved spacetime?

At the speed of light, there is supposedly infinite length contraction so the entire universe is a single zero-dimensional point (or an infinitely thin 2D plane perpendicular to the direction of light'...
0
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2answers
82 views

Are there continuous spacetime events?

What is the formal definition of an event? According to Wikipedia, "an event is a point in spacetime (that is, a specific place and time) and the physical situation or occurrence associated with it." ...
2
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1answer
88 views

Time independent Kerr metric

The Kerr metric expressed in terms of polar coordinates $r,\theta,\phi$, such that $x = r\sin(\theta)\cos(\phi)$, $y = r\sin(\theta)\sin(\phi)$, $z = r\cos(\theta)$. Then the Kerr metric is given as \...
0
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1answer
53 views

Galileo 5 and 6 satellites testing gravitational time dilation

In today's Guardian there an article titled "Satellite launch accident provides unexpected test of Einstein’s theory" - http://www.theguardian.com/science/across-the-universe/2015/nov/25/satellite-...
5
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3answers
622 views

G4v Gravity Theory: Why does this get rid of Dark Energy?

Earlier this year, Carver Mead of CalTech published a paper which seems to be garnering a lot of attention: http://arxiv.org/abs/1503.04866 http://www.npl.washington.edu/AV/altvw180.html http://...
1
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1answer
77 views

Numerical relativity in causally pathological spacetimes

To perform numerical relativity simulations one almost universally adopts the so called "3+1" approach: spacetime is divided up into spacelike slices, each representing a "moment in time". After some ...
2
votes
3answers
148 views

From the photons perspective

Probably been answered but couldn't find the answer. From the perspective of a photon: - For the photon to travel from body A to body B would take 0 secs. - The universe would be one point in "...
7
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1answer
291 views

Does frame dragging apply to linear motion?

Firstly I will admit I do not understand the real cause of rotational frame dragging and some of the math heavy explanations are too complicated for me. To me frame dragging looks like unsubstantiated ...
1
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2answers
61 views

Light and Gravity

The gravitational force does not affect the speed of light rather affects the frequency of light (hence light changes colors, red to blue and vice-versa). I know this has been verified but I have 2 ...
3
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0answers
43 views

Sources for black hole geodesic orbits

I am looking for good sources that discuss both Kerr and Schwarzschild particle orbits (geodesics). Most sources write down the geodesic equations, constants of motion and the Hamiltonian, but do not ...
1
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1answer
138 views

Is max speed of causality (light) proven experimentally?

Is it possible to have an alternative theory which is consistent with experiments supporting GR, but which doesn't mandate a speed limit on causality? In other words, I understand there is ...
0
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0answers
27 views

Are only measurements registered as events?

When we measure an electron or some other system behaving quantum mechanically, we observe something. Now, this can be called as an event in relativity terms meaning if we live in 1-D spacetime, only ...
1
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1answer
63 views

Is there a way to write the Lorentz force in terms of one field, $L$, and one charge, $X$?

I have heard that physicists like to write electromagnetism as one force (the Lorentz force) and define it as $\vec{F_L}\left(q, \vec{v}, \vec{E}, \vec{B}\right) = q\left(\vec{E} + \vec{v} \times \vec{...