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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Chain longer than the distance the light travels in a second [duplicate]

I'm wondering what would happen if there was a cable that has 2400000000 meters (roughly 8 times the distance the light travels in a second) and if it had a pulley in the middle (see picture) and I ...
3
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1answer
20 views

Are orbits possible in de sitter space?

Since the de sitter space has constant positive curvature does that mean that objects can't orbit around other objects?
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17 views

Clocks at rest in Kruskal coordinates

The proper time in Kruskal coordinates is given by $$d\tau^2=\frac{4}{e^rr}(dT^2-dX^2)$$ (where r is measured in units of Schwarzschild radii). Since at $T=+/-\infty$, $dT^2-dX^2$ goes to zero for an ...
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1answer
53 views

The EFE - The 'Mother' of all equations of motion in the universe?

This question addresses the scope of general relativity. Scientists and engineers solve all sorts of physical problems in all sorts of fields and often to solve these problems a 'system' is defined ...
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41 views

Does Schwarzschild metric have cosmological horizon?

Since the space is not expanding in the Schwarzschild metric does that mean that there is no cosmological horizon? Also, what if the Schwarzschild metric was not asymptotically flat and we replace ...
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19 views

Is there a well defined way to shift Cauchy surfaces back and forth?

Let $S$ be a Cauchy surface on a four-dimensional connected Lorentzian manifold. Define $\Gamma(S)=\{\gamma:\mathbb{R}\to M\mid \gamma(0)\in S, \gamma$ is a unit speed geodesic passing orthogonally ...
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3answers
62 views

When a neutron star accumulates enough mass, does a black hole start at its center?

With less dense bodies, such as the earth and the sun, the center has less gravity/density (since there's an equal amount of mass surrounding the center, pulling out on it from all directions). Does ...
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26 views

Motion on a rolling Body [on hold]

How can an object can continue to run on a rolling circle along the X direction and maintain a balance with specific acceleration.
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0answers
40 views

Derivation of stress-energy tensor in curved space-time

I've a problem about calculating the components of the stress-energy tensor in general relativity. I've learned that if we have an action of the form : $$ S=\int (R+\mathcal L_{m})\sqrt{-g}d^4x $$ ...
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1answer
36 views

What's the physical meaning of a “sphere”, and a pair (or even a series) of “concentric spheres”, in a region with Schwarzschild geometry?

This accepted answer to the question "What is the physical meaning of the Eddington-Finkelstein coordinates?" (PSE/q/91724) prescribes to 1. Enclos[e] the origin of our Schwarzschild spacetime in ...
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0answers
33 views

What is the degrees of freedom of metric tensor?

As $g_{\mu\nu}$ can be taken to be symmetric, it contains 10 functions of spacetime in 4 dimensions. But, why we call these 10 functions as the degrees of freedom of the metric while they are the ...
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0answers
56 views

How to invert this metric?

Reading this article i find a result that i am not sure how to obtain (page 3 eq 3). It is about the inversion of a metric of the type $$ g_{\mu\nu}=Al_{\mu\nu}+BH_{\mu\nu}. $$ In order to invert ...
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0answers
20 views

Solve for distance travelled accounting for relativity [on hold]

I have: Initial speed $v_0$ Traveler time (in days) Constant Acceleration (in $m/s^2$) From that I want to solve for distance traveled, accounting for relativistic effects. I think I see how to ...
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0answers
31 views

Directional derivative of the Ricci scalar in the direction of a Killing vector field [on hold]

Do I need the Levi-Civita connection for the directional derivative in the direction of a Killing vector field of there curvature scalar to be 0? EDIT: Let $K$ be a Killing vector field and $R$ the ...
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0answers
29 views

What is the difference between cosmological horizon and event horizon?

Do the terms cosmological horizon and event horizon mean the same thing?
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0answers
24 views

Proof of transformation between contravariant and covariant components of vector [on hold]

What is the proof of the relation ${g^{ir}g_{rj}=\delta^i_{j}}$ where ${g}$ is the metric tensor and ${\delta}$ is the Kronecker delta. EDIT:The operations is known as equivalent to raising and ...
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3answers
65 views

Can't we consider that EmDrive is Pushing against Space itself? [on hold]

When something move through space it pushes against something else in space with equal force. However, EmDrive warps space around. So can't we say it's pushing against space itself, and thus momentum ...
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2answers
81 views

Why can black hole evaporate if objects need infinity time to reach event horizon (as seen by a distant observer)?

I am new to black hole, but have a question about it: Object needs infinity time to reach event horizons (as seen by a distant observer). Particle-antiparticles separates, one of them drops into ...
2
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1answer
42 views

Shouldn't dark matter distort light and there for be directly detectable?

It is an established fact that gravity bends space time and therefore distorts light. We know that Dark matter acts gravitationally. So, just for giggles, lets say I had a sphere of dark matter ...
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0answers
29 views

Variational principle conformal time in cosmology [on hold]

I am studying mimetic gravity in which physical metric is written as $$g_{\mu \nu}= \tilde g^{\alpha \beta}\partial_{\alpha}\phi\partial{\beta}\phi \tilde g_{\mu \nu}$$ The action of ...
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2answers
43 views

Principle of equivalence

Einstein's principle of equivalence states that one can not distinguish between a freely falling nonrotating system in a gravitational field and a uniformly moving system in the absence of a ...
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1answer
145 views

Certain doubts regarding the movie 'Interstellar' [on hold]

In the movie, or rather a scientific fiction, Interstellar, we saw in the last part, that the actor falls into the black hole, travels in time, which is a dimension inside the black hole, and then, ...
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1answer
47 views

Time Dilation In Between Two Objects

Suppose an object A is exactly halfway in between two identical objects B and C, so the magnitude of the cumulative acceleration on object A is $0$. Objects A, B, and C have no velocity relative to ...
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0answers
26 views

Free fall and gravitational time dilation

An observer 'A' at rest in a gravitational field has some amount of time dilation relative to an observer 'B' far away from the gravitational field. Would the clock of an observer 'C' in free fall at ...
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1answer
46 views

Does spaghettification really happen with conditions found close to a black hole?

This website tries to explain why the term spaghettification doesn't actually occur when something gets close to a black hole. The argument of the author is that the equations we use to predict the ...
2
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1answer
51 views

Derivation of geodesic deviation equation from two neighbouring geodesics

I'm stuck trying to follow Foster and Nightingale's derivation of the geodesic equation from two neighbouring geodesics $x^{a}\left(u\right)$ and $\tilde{x}^{a}\left(u\right)$ joined by a ...
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2answers
73 views

Natural Philosophy [closed]

My question is an extension of the celebrated question on the moon’s existence if unobserved. “do we still have tides on earth if the moon is unobserved?”
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2answers
76 views

Gravitational waves in general relativity

After reading some concepts of general relativity,is it true that the universe communicate with its different constituents (stars, galaxies, etc...) via gravitational wave? If so, how is that? Do ...
5
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1answer
49 views

Charged versus rotating black holes as different kinds of wormholes

I've heard that a maximally extended charged black hole can be a traversable wormhole to the same universe whereas a maximally extended uncharged rotating black hole can only be a wormhole to ...
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2answers
185 views

Can something (again) ever fall through the event horizon?

Since I am more confused by the answers given in this site to the many variants and duplicates of this question, with some arguing that from the point of view of the falling observer, it happens in ...
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1answer
34 views

What is the meaning of t=infinity at the black hole horizon?

Looking at the Kruskal diagram for black holes, it seems to me that at the horizon, all free falling objects cross the horizon at the same coordinate radius and time. Does this not mean that all ...
1
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1answer
44 views

Can I say that the two-body problem is also a vacuum solution for the special case of two orbiting black holes?

A two body system is one where two bodies orbit each other. In the case of two orbiting black holes, since an individual black hole is described by a vacuum solution, can I say that the two-body ...
0
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1answer
40 views

How can black holes be observed to grow? [duplicate]

If, from the reference frame of an observer at rest outside a black hole, it takes an infinite amount of time for an object to be observed to fall into a black hole, how can black holes ever be ...
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0answers
29 views

How to calculate backreaction in AdS space?

This might be a very straight forward and basic question in GR. I am interested in calculating backreaction due to certain matter field (say, scalar) in AdS space. Should I put the energy-momentum ...
0
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1answer
37 views

Equations for Big Bang expansion, distance and energy

On the back page of the CPEP poster “The History and Fate of the Universe” a table gives distance scale (size), temperature, and energy (radiation) for seven expansion times (10-43 sec to now). ...
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0answers
47 views

How would you describe what the affine parameter is in layman's terms? [duplicate]

I've been trying to learn it from other sites, but I'm not well-versed enough in mathematics to understand.
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2answers
52 views

Black Hole horizons and the Equivalence Principle?

In the reference frame of a freefalling observer, does crossing the event horizon not cause a contradiction between two classical principles that supposedly apply at the event horizon: the equivalence ...
1
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1answer
36 views

Do contractions with Dirac matrices involve a metric?

When figuring out where the spacetime metric enters an equation it is often useful to write all vector indices as covariant indices and write out the inverse metrics that are needed to contract them, ...
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2answers
76 views

Is there anything else than spacetime? [closed]

Can we say that the only thing that exists is the spacetime and everything in it is expressed as curvature?
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0answers
22 views

Shape of the universe [duplicate]

I have just started learning GR (but have some rudimentary knowledge on differential geometry) and came across this statement: "the universe is flat with only a 0.4% margin of error". I have read ...
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0answers
25 views

Expanding Electric Field, Magnetic Field in post-Newtonian Gravitational Potential

I've cross-posted this question from Mathematics since Physics is probably better suited for the nature of the question. I've ...
1
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0answers
30 views

Null geodesic equation with affine parameters [duplicate]

A photon's geodesic equation is defined by re-parameterizing the geodesic equation to some parameter other than proper time. This is done because $ds=0$ for the photon. Again if we use affine ...
2
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1answer
48 views

Movement of bodies in space, affected only by gravity

I have been extensively studying General Relativity for some time now. Recently I asked myself a question which I can't answer. If the gravitational metric is determined by the Energy content of the ...
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0answers
49 views

Modeling of Big Bang [duplicate]

Well, I’m just an ordinary High School passed student. I had passion towards space-time- relativity and other things related with physics :D I have been watching theories of physics like Big Bang from ...
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0answers
52 views

About the entanglement and space time construction [closed]

It seems lots of people now believe that spacetime geometry is emergent from entanglement. But according to papers, usually they only talk about how the geometry of 'space' is related with ...
0
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0answers
32 views

What is the scale factor of a hyperbolic universe?

I wanted to derive the solution to this question from the Friedmann equations myself but I ran into some trouble. I was working in natural units where $c=G=1$, then, for brevity, I defined ...
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0answers
63 views

Why are gravitational forces always attractive? [duplicate]

In my curiosity, gravitational forces, unlike electric forces, is always attractive, because of the empirical evidence. However, why is it so? In what model or theory can this phenomena be explained ...
2
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1answer
79 views

Classical conformal invariance

So I am trying to understand classical conformal invariance. So we move gently from general coordinate invariance to Weyl invariance to conformal invariance, and now they start out with this thing ...
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1answer
52 views

If spacetime is folded upon itself

Say there are two black holes about a parsec apart. You theoretically fold spacetime upon itself so that the singularities of the black holes are "touching". ...
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1answer
80 views

Why curvature produced by electric field in spacetime is so small?

The electric field due to charge is $E=\frac{Q}{r^2}$. Why is the curvature in spacetime produced by this electric field so small? Let's say we want to calculate curvature(in spacetime) produced by ...