# Tagged Questions

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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### How Come gravity doesn't affect itself?

If gravity is this "unexplainable force" that pulls everything to the center of a planet or stellar remnant you stand upon, why doesn't gravity pull itself? If gravity affects anything with energy, ...
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### Relativity and Galaxy Rotation Speed

If time travels slower nearer gravity wells, why can't the galaxy rotation speeds being faster on the outer edges than the inner areas be explained by relativity? What necessitates dark matter?
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### How energy would be consumed for bending spacetime?

If we could assume that relativity theory is correct about spacetime bending. Can we calculate energy used for moving 1 kg of object in 1 meter by changing the shape of spacetime (simulate gravity)? ...
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### About the use of Newtonian Relations for the movement of stars in the Galaxy [duplicate]

From a General Relativity point of view Gravity is given as the result of spacetime curvature interacting with energy-mass density. To get to the Newtonian limit one needs to take a) ...
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We have a covariant derivative of a covariant tensor: $$A_{\mu ; \nu} = A_{\mu , \nu} - \Gamma^{\alpha}_{\mu \nu} A_{\alpha}$$ The covariant derivative of a contravariant tensor is: $$... 1answer 106 views ### 'Hovering' light rays on the edge of a black hole According to Prof. Hawking, light rays will 'hover' on the edge of a black hole. If this is true, and the light 'stops' on the edge, how can the electric/magnetic fields which, constitute the light, ... 0answers 53 views ### Why Newton's gravitational constant remains unchanged in relativity though gravity is not a force? I know that Einstein described gravity as a curvature of spacetime. So, It is not a "force" but why Einstein had to accept Newton's gravitational force constant? 2answers 59 views ### Can black holes grow via accretion of dark matter particles? I'm assuming that the answer to the question in the title is a resounding yes. Since Baryonic matter and dark matter interact via gravitational forces. If this is the case how is information not lost ... 1answer 63 views ### For gravitational wave from twin stars, how was the tidal effect counted? As the primary indirect evidence, the work on calculating the rotational slow down earned the 1993 Nobel prize. However, I cannot find any where mention how the work deal with the tidal effect. Are ... 1answer 60 views ### Gravitational waves and it's interaction with matter I have been reading an article on gravitational waves here. There, it is written that the gravitational wave, unlike the electromagnetic waves, interact very weakly with matter. The principle of LIGO ... 0answers 49 views ### What is basic tensor algebra in teleparallel equivalent of general relativity? Teleparallel gravity represents a viable alternative to general relativity where gravitation comes from torsion rather that curvature. The theory is based on a new modified connection, and the ... 1answer 117 views ### What is the metric of a constant electromagnetic (pure electric or pure magnetic) field? For example, imagine a magnetic field B_x directing in \hat{x} direction filling all the space. What is its associated metric field? I can construct the electromagnetic stress-energy tensor for ... 2answers 75 views ### Straight line null geodesics in Minkowski, De Sitter and Schwarzschild I'm trying to understand which part of the following metric determines whether photons travel on a "straight" line (thinking of (t,r,\theta,\phi) as a flat background), the metric I'm considering ... 2answers 79 views ### Total derivatives in GR Without gravity we can easily switch between terms in a Lagrangian, such as \partial\phi\partial\bar{\phi} and \phi\Box\bar{\phi}, since total derivative vanishes. But in GR we have additional ... 0answers 29 views ### Integral curves of vector field are geodesics [migrated] Say we have a Riemannian manifold (M, g) with vector field X obeying the following: g(X, X) = 1; and the 1-form \varphi(Y) = g(Y, X) is d-closed, d\varphi = 0. Does it necessarily ... 1answer 1k views ### How much energy can be extracted by lowering something into a black hole? [duplicate] If an object is in orbit around a star, the object has gravitational potential energy that could possibly be extracted. For example, when we perform gravitational slingshots around Jupiter, our ... 1answer 76 views ### At a center of Gödel's universe A few quick questions clarifying a picture about Gödel's universe, they bug me badly! Taken from here. So Gödel's universe is made out of dust particles. All of them have angular velocity. Do this ... 1answer 75 views ### What happens to objects sucked into a black hole after the black hole evaporates away? Suppose an object falls into a black hole that's so massive that it wouldn't get torn apart at the event horizon. What happens to it after the black hole evaporates away? According to the theory ... 0answers 57 views ### Why is the frame dragging effect trillions times as strong around a spinning superconducter? It´s a bit of a forgotten experiment, but why measured Tajmar and his team a frame dragging effect in the surrounding space of a spinning niobium superconductor, that was even measurable in the lab? ... 2answers 228 views ### Coordinate Singularity in Metric Suppose I have some metric$$ds^2=g(t)dt^2+\frac{1}{r}dr^2$$which has a singularity at r=0. However, if I make the coordinate transformation u=\frac{1}{r}, then I get:$$ds^2=g(t)dt^2+r^3 ...
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Here is the twin paradox with a twist. Scenario 1: An observer (A) leaves from the equator of the earth and travels with an acceleration of $9.8\,\mathrm{m/s^2}$ in a north direction ( i.e. in the ...
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### What made Einstein think that gravity was caused by the curvature of spacetime?

What observation/thought experiment led him to think this?
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### Testing General Relativity using radioactivity on the Moon

I was reading a question involving an ultracentrifuge to test General Relativity. Instead of using an atomic clock the asker posited using radioactive decay as the metric to evaluate time dilation ...
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### An argument that massive particles don't redshift?

I start with the spatially flat FRW metric in conformal co-ordinates: $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)$$ This metric has the following non-zero Christoffel symbols: \begin{eqnarray} ...
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### Unknown functions in Schwartzchild Metric

I was reading through MTW and they made a big deal about how we were able to make an astute choice of coordinates to eliminate unknown functions from the most general form of the metric for a ...
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### Is there such thing as imaginary time dilation?

When I was doing research on General Relativity, I found Einstein's equation for Gravitational Time Dilation. I discovered that when you plugged in a large enough value for $M$ (around $10^{19}$ ...
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We know that the curvature of spacetime is gravity itself and it is not a force.so,why do we feel our weight in a curve spacetime but not in a straight(I mean not curve) space time like zero gravity ...
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### Deriving Birkhoff's Theorem

I am trying to derive Birkhoff's theorem in GR as an exercise: a spherically symmetric gravitational field is static in the vacuum area. I managed to prove that $g_{00}$ is independent of t in the ...
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### Do gravitational waves propagate backwards in time?

Gravitational waves are spacetime waves, which stretch and squeeze both space and time. Since relativity puts space and time (almost) on an equal footing, it seems to me that since gravitational waves ...
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### Negative Mass's Effect on Gravitational Time Dilation [duplicate]

When I was playing around with the equations for Gravitational Time Dilation, I discovered that when a negative value was plugged in for $M$, the equation gave the exact same answer, and it would ...
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### Is It Possible to look at our own past if we find a giant reflecting galaxy as distant as needed for a telescope? [duplicate]

Can a telescope catch light reflected from object distant away so that we see our own planet hundreds/thousands/millions years ago. Another suggestion is that we send the giant mirror as far as needed ...
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