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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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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2answers
499 views

What is the physical meaning of the Eddington-Finkelstein coordinates?

What is the physical meaning of the Eddington-Finkelstein coordinates? I want to see a some physical process (experimental) that could explain the many transformations of coordinates into this ...
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0answers
25 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
27 views

Variational principle conformal time in cosmology

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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8answers
6k views

Is gravity just electromagnetic attraction?

Recently, I was pondering over the thought that is most of the elementary particles have intrinsic magnetism, then can gravity be just a weaker form of electromagnetic attraction? But decided the ...
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0answers
50 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
16 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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2answers
74 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 ...
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1answer
51 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
44 views

Meaning of “physical” and “gravitational” metrics

I've recently been reading some notes (following a paper by J.D. Bekenstein, titled "The Relation between Physical and Gravitational Geometry": http://arxiv.org/abs/gr-qc/9211017) on alternative ...
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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 ...
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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
30 views

Directional derivative of the Ricci scalar in the direction of a Killing vector field

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
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 ...
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0answers
23 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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0answers
24 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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3answers
58 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
852 views

If you are carrying a magnet, can you tell when you cross the event horizon of a black hole?

By the no hair theorem, a black hole is completely characterized by it's mass, charge and angular momentum. Therefore dipole and higher pole magnetic fields are completely expelled when a black hole ...
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1answer
39 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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3answers
220 views

Gravity - Force or Result?

I am no Physicist, but I enjoy reading about Physics. However reading about leading theories such as M-Theory and others they speculate about the existence of the Graviton. In my past reading of ...
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2answers
180 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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2answers
41 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 ...
2
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1answer
132 views

Closed timelike curves in the spin-2 gravity formalism

Let's say we take some topologically trivial CTC spacetime, like the Gödel metric: $$ds^2 = -dt^2 - 2e^{\sqrt{2}\Omega y} dt dx - \frac{1}{2}e^{\sqrt{2}\Omega y} dx^2 + dy^2 + dz^2$$ And then I ...
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4answers
4k views

Why does a flat universe imply an infinite universe?

This article claims that because the universe appears to be flat, it must be infinite. I've heard this idea mentioned in a few other places, but they never explain the reasoning at all.
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0answers
46 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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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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1answer
43 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 ...
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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1answer
139 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
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 ...
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1answer
46 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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4answers
4k views

That 10km/day error predicted if GPS satellite clocks not corrected for relativity

Some authorities have stated publicly and without explanation that if the theories of Special and General Relativity were not taken into account in the design of the GPS (by building the satellite ...
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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 ...
5
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1answer
48 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
74 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 ...
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2answers
72 views

Natural Philosophy [on hold]

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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1answer
33 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 ...
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 ...
4
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2answers
110 views

Gravity in 2d space and inverse linear law

In our three-dimensional universe, gravity obeys the inverse square law. In a four-dimensional universe, gravity would be expected to obey the inverse cube law et cetera. In a two-dimensional ...
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2answers
51 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 ...
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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?
11
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1answer
596 views

How to thoroughly distinguish a coordinate singularity and a physical singularity

In a course on general relativity I am following at the moment, it was shown that the singularity $r=2M$ in the Schwarzschild solution is a consequence of the choice of coordinates. Introducing ...
13
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2answers
426 views

Definition of stress at the microscale

Take, for simplicity, a Lennard-Jones fluid below the critical temperature, which is to say that there is a phase separation into fluid and gas and thus an interface is formed. The macroscale picture ...
2
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1answer
78 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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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 ...
6
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1answer
177 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 ...
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1answer
587 views

Can negative energy explain black holes?

I think black holes are peculiar and I want to know if there is a relation to negative energy. Does negative energy have positive mass and therefore can explain the nature of black holes? IS there any ...