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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Isn't the aether existant

Before you say I'm wrong consider this, Einstein is supposively the first person to get completely get rid of the various aether models that were proposed. But didn't Einstein actually prove them ...
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22 views

Variation of quadratic term in modified Einstein-Hilbert actions

In the context of mimetic gravity at some point one try to add to an already modified Einstein-Hilbert action also a term like $$ S_\chi=\int\,d^4x\,\sqrt{-g}\frac{1}{2}\gamma\chi^2,\qquad(\star) $$ ...
3
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0answers
41 views

Angle sum of triangle in Schwarzschild solution

Curvature of space is often intuitively explained as angles of a triangle not adding up to 180 degrees. My questions concerns that. Suppose you have a perfectly spherical star of uniform density - so ...
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1answer
34 views

Does spacetime curvature increase when an attractor's potential energy is converted to kinetic energy?

Imagine an asymptotically flat spacetime with nothing but two stars at a certain distance. They fall into each other and form one big star, so their potential energy is converted to kinetic energy. ...
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1answer
48 views

Mass and Schwarzschild Radius [duplicate]

Do free massless particles have a Schwarzschild radius? I'm curious about the mass in the equation for the Schwarzschild radius. I know that you can calculate a Schwarzschild radius for any massive ...
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4answers
163 views

Is a black hole's mass uniformly distributed?

If you were to fly around a black hole, would the gravitational pull be uniform and centered on the singularity, regardless of your relative location? If yes, how can this be consistent with models ...
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1answer
39 views

Can you recover the values of spacetime intervals $s^2$ from given causal relations between events?

Given a suitable set $\mathcal S$ of events together with their (pairwise) causal relations, i.e. for each pair of distinct events $\mathsf A, \mathsf B \in \mathcal S$ the assignment whether ...
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0answers
23 views

Short Gamma Ray Bursts from black hole mergers

I have read in passing that short gamma ray bursts can be caused by the merger of 2 black holes in a binary system. I have Googled but can't seem to find any good sources describing the phenomenon ...
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1answer
53 views

Can you recover a spacetime from its null geodesics?

So, I know that you can learn a lot about a spacetime from its causal structure, but can one completely recover the metric of a spacetime, just knowing the equations for the null geodesics in it? If ...
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1answer
45 views

What force causes massive objects to bend space? [duplicate]

The visualization of gravity as shown by this video is pretty good at explaining how massive objects bend space, and such bending causes objects around it to fall towards it (a.k.a: gravity). ...
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1answer
41 views

A question about the physics involved in tracking satellites such as those used in the GPS system [duplicate]

I know that besides the effects of Newton's theory of Gravitation on the satellite's motion, one has to take account of the retardation of the satellite's clocks when compared to earth-fixed clocks. ...
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1answer
46 views

“Proper mass” and “gravitational binding energy” in general relativity

I'm reading Robert Wald's "General Relativity" and after the discussion of the Schwarzschild Solution it goes on to talk about interior, static, spherically symmetric solutions. Wald says that "M" in ...
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0answers
20 views

What component of the stress–energy tensor contains the kinetic energy of heat? [duplicate]

As I understand it, the component $T_{00}$ of the stress-energy tensor contains the energy density (which equals the mass density), $T_{0i}$ are the impulse flows (intuitively speaking, the ...
2
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1answer
53 views

Gravitational redshift in a general stationary metric

Suppose you have a general metric $g_{\mu \nu}(t,r,\theta,\phi)$ which don't depend explicitly on $t$ coordinate, i.e a stationary metric. Light travels along a geodesic from A (at which the frequency ...
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0answers
35 views

Does the effect of spaghettification mean that (speculative) traversable wormholes can only exist from supermassive black holes? [on hold]

Assumption that wormholes really exist. Even if humanity could produce wormholes from quantum black holes. These would be never traversable due to the effect of spaghettification? Or does the exotic ...
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0answers
29 views

How to explain the homogeneity of the universe through a physic model [duplicate]

If in general, two objects homogenize themself by combining it's parts, why the horizon effect, based on a big bang model, excludes such an important physical interaction process? I wonder if this ...
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3answers
115 views

If time is relative, how could time pass? [duplicate]

EDIT: I appreciate people who answered below. But it does not answer the question, so I will clarify my questions: -It seems like everyone is saying that time passing is actualized by physical ...
3
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3answers
136 views

What is the binding energy of a neutron star?

Neutrons which constitute a neutron star have a rest mass that is greater when separated from the star because they are bound with a certain potential energy. This potential energy causes the system ...
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4answers
3k views

Why do physicists trust black hole physics?

Based on popular accounts of modern physics and black holes (articles, video lectures), I have come to understand the following: Black holes are predicted by General Relativity, a classical theory ...
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1answer
56 views

Does an object traveling near the speed of light create a gravitaional field? [duplicate]

Does a particle traveling near the speed of light create an observable/measureable gravitational field around it? I know most elementary particles travel near the speed of light and have no ...
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1answer
51 views

What $f(R)$ models pass most of the known constraints?

In most papers and talks about $f(R)$ gravity authors repeatedly state that the model proposed by Starobinsky 2007 $$ f(R)=R+\lambda\,R_{0} \bigg[\bigg(1+\frac{R^{2}}{R_{0}^{2}}\bigg)^{-n}-1\bigg] ...
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1answer
40 views

3-cylinder surface element (Poisson's “A Relativist's Toolkit”)

From Poisson's "A Relativist's Toolkit": he introduces the non-dynamical term $$ S_0=\frac{1}{8\pi}\int_{\partial\Omega}\epsilon K\sqrt{\lvert h\rvert}d^3x $$ in the GR action, where $h$ is the ...
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3answers
89 views

Can a Dyson Sphere around a Black Hole be built so that it would not radiate significant IR?

This is related to this question If the sphere surrounded a BH and used it as a heat dump (as well as extracting energy from it by dropping in mass) could its exterior be engineered to match the ...
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0answers
49 views

Particle energy in conformal FRW spacetime?

Let us start with the flat-space FRW metric in Cartesian co-ordinates for simplicity: $$ds^2=-dt^2+a(t)^2(dx^2+dy^2+dz^2).$$ In GR the energy of a particle is given by: \begin{eqnarray*} ...
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2answers
86 views

How to define $\delta{g_{\mu\nu}}$?

In general relativity, when deriving the field equation using the variational principle we use $\hat{g}_{\mu\nu}=g_{\mu\nu}+\delta{g_{\mu\nu}}$. Does $\delta{g_{\mu\nu}}$ mean the measurement of how ...
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0answers
55 views

What is the status of the area theorem in AdS?

The area theorem, by Hawking (in "Gravitational Radiation from Colliding Black Holes", also found in Wald), states that the area of an event horizon cannot decrease with time. As I understand, there ...
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1answer
45 views

Distance between two galaxies of different redshift

Let $Q_1$ and $Q_2$ two different objects in the Universe (we can think to two galaxies or quasars), that we observe from the Earth at different angular position $(\alpha_1,\delta_1)$, ...
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2answers
77 views

Special and general relativity and space time

I have a question. I'm reading The Elegant Universe and it's talking about the special and general theory of relativity. One of the things it mentions is that time and the three dimensions of space ...
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2answers
174 views

Why general relativity over other similar theories?

Since all theories where gravity is seen as a result of space curvature automatically satisfies the equivalence principle, why should one prefer GR over all other alternatives? Surely you can make the ...
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2answers
49 views

If the measurements of a clock above the earth depend on orientation, then what measurements are correct?

Take a clock in space above the earth (assuming a Schwarzchild spacetime) that works by relaying a light signal a small distance radially; ticking each time the light signal returns. Compare this to ...
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0answers
23 views

Does every particle in a neutron star accretion disc undergo Thomas precession?

Assuming, based on this wikipedia article Accretion Discs That accretion discs surrounding neutron stars are diffuse (I take this to mean they are composed of a gas and / or plasma). That the ...
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0answers
33 views

Are there any experimental data for light bending angle? [duplicate]

The gravitational lensing formula is given by equation $a=4M/r$. Is there any experimental data for this equation. I am trying to plot a curve for this equation and the real experimental data and ...
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0answers
76 views

The Riemannian Curvature in a solid sphere [migrated]

Is the Riemannian Curvature at the centre of a solid sphere zero?
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1answer
67 views

The relativity of gravity: If mass is relative how much gravity do I experience?

Now let's say the I am on a spaceship. The spaceship is not accelerating, i.e., it is not firing its rockets. Most of the ship's mass is in the back of the ship. Let's say it is moving arbitrarily ...
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1answer
77 views

When is Einstein summation implied by Lorentz indices?

I would like to ask if it is possible to find out whether Einstein summation is used in an equation. For example, $$A^{\mu \nu} = 1$$ can either mean $\sum_{\mu\nu} A^{\mu \nu}=1$ or $A^{\mu \nu}=1$ ...
2
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1answer
95 views

Speed of light and warp drives in general relativity

Velocities can be a tricky thing in general relativity. A cool concept seemly consistent with the Einstein field equations) is an Alcubierre drive, described by the Alcubierre metric. However, I ...
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0answers
45 views

Compactly generated vs. compactly constructed causality violating region?

I am currently trying to grasp the nuance between a compactly generated future Cauchy horizon (as per Hawking's chronological protection conjecture) and a compactly constructed causality violating ...
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0answers
40 views

Dark matter and ghost free

I have to study this article http://arxiv.org/abs/1311.3111 The title is Dark Matter as a ghost free conformal extension of Einstein theory What does ghost free mean? Thank you,
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1answer
60 views

What is the value of the variation stress energy tensor?

If we are living in a portion of space-time where the metric is very close to flat space and we know that the stress energy tensor is negligible at this portion of space-time is it ok to assume that ...
2
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1answer
69 views

Naked singularity: how would it behave?

We know that the theory of cosmic censorship prevent singularities from existing without an event horizon that hides them from the Universe. Now let's assume that this theory is somehow false, and a ...
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1answer
62 views

Understanding the nature of metric tensor [closed]

The metric tensor for a flat spatial manifold gives us length on object, or separation between two space points. Similarly, $g_{\mu \lambda} dx{^\mu} dx{^\lambda}$ gives separation between two space ...
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1answer
51 views

Time and gravity relation

I was reading about relativity when I crossed by the applications of relativity in real life, and they said that as the elevation increase time increase so gravity decrease Can someone explain to me, ...
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4answers
106 views

Particle energy in general relativity?

Assume that a particle is moving with four-velocity $u^\mu$ through a spacetime with metric $g_{\mu\nu}$ ($-+++$ signature). Let us also assume that there exists a time-like vector field ...
4
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1answer
90 views

Curvature of spacetime and the equivalence principle

Assuming the Einstein equivalence principle, formulated as following: The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the ...
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3answers
118 views

General Relativity 2-Body Closed Form

Is there a closed form solution in general relativity to the 2-body orbit problem?
3
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1answer
104 views

How long can a particle survive inside of the horizon of a black hole?

By examining the causal structure of a Schwarzschild black hole, one can see that a particle in region II is unable to escape to $r=\infty$. Such diagrams do not show, however, how long a particle ...
17
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0answers
223 views

How does one correctly interpret the behavior of the heat capacity of a charged black hole?

Consider the Reissner-Nordström metric $$ds^2=-f(r)dt^2+f^{-1}(r)dr^2-r^2d\Omega^2\hspace{2cm} f(r)=1-\frac{2M}{r}+\frac{M^2q^2}{r^2}$$ where I defined the charge-to-mass ratio $q:=Q/M$, which ...
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0answers
35 views

Normal of a null surface and null junction conditions in general relativity

I am trying to use the null junction formalism in general relativity (as explained in eg http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.43.3763&rep=rep1&type=pdf, "Junctions and thin ...
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0answers
57 views

Gravitational lensing experimental data [duplicate]

Is there any experimental data for the gravitational lensing equation $$ \alpha=\frac{4M}{b} $$ where $b$ is the distance of closest approach, $M$ is the mass of the object. I tried Google but ...
6
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3answers
267 views

Pendulum moving faster than speed of light

In classical mechanics, the period $T$ of a pendulum is given by $$ T = 2\pi\sqrt{\frac{l}{g}},$$ where $g$ is the gravitational field and $l$ the length of the rope attaching the bob to the pivot. ...