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Results tagged with general-relativity
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user 1241
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.
4
votes
Accepted
Does the Big Bang need a cause?
I don't think I agree with your first sentence. Our simplest theoretical models, based on classical general relativity, say that there was a singularity in the past, but few if any cosmologists take t …
34
votes
Accepted
Detection of the Electric Charge of a Black Hole: How can an electromagnetic field escape th...
A charged black hole does produce an electric field. In fact, at great distances (much larger than the horizon), the field strength is $Q/(4\pi\epsilon_0 r^2)$, just like any other point charge. So me …
8
votes
Is spacetime simply connected?
Benjamin Horowitz's answer covered a lot of the key points, but it's worth adding that the question of the topology of the universe has been investigated by astrophysical observations. If the universe …
14
votes
Effect of gravitation on light
Michael Luciuk's answer is right, but there's an even stronger reason for rejecting this hypothesis: refraction in the corona would be wavelength-dependent, but the gravitational bending due to the Su …
10
votes
Accepted
Does gravitation of a sphere equal gravitation of a point?
Actually, the same result is true in general relativity: any spherically symmetric mass distribution gravitates in exactly the same way as a point mass.
Here's the more precise statement: any spheri …
20
votes
Accepted
Gravitational time dilation at the earth's center
The rule I mentioned in another question, that the time dilation factor is $1+\Delta\Phi/c^2$, applies here. The derivation (found in various textbooks) depends only on the assumptions that fields are …
9
votes
Voyager local time dilation (caused by gravity)
There is in principle a "gravitational blueshift" for signals traveling from Voyager to us. The data rate we receive will be higher than the data rate transmitted by a factor $(1+\Delta\Phi/c^2)$, whe …
14
votes
In GR, how do particles know how to fall in instead of out of a gravitational well?
If you take any solution to the geodesic equation, the time-reversal of that will also be a solution. If one describes a rock falling down in the Earth's gravitational field, the other will describe a …
14
votes
How will the Twin Paradox become, for Time Dilation, if no acceleration was ever involved?
You say that both twins are "exactly 20 years old." I assume you mean that they are both 20 years old at the same time. But part of the point of special relativity is that a phrase like "at the same t …
32
votes
Accepted
Is dark matter repulsive to dark matter? Why?
Lubos Motl's answer is exactly right. Dark matter has "ordinary" gravitational properties: it attracts other matter, and it attracts itself (i.e., each dark matter particle attracts each other one, as …
5
votes
General Relativity Paradox: Holding a string across a gravitational gradient
It certainly is possible for two observers to remain at rest near a black hole, as long as they are both outside the horizon. They can stretch a string from one to the other, and they can bounce light …
26
votes
Accepted
Can a non-Euclidean space be descripted through an Euclidean space of higher dimension? So w...
You have to add more than one dimension, in general. Mathematicians have studied in great detail the question of how many extra dimensions you need in order to embed a curved manifold in a flat one. O …
6
votes
Curvature of Conical spacetime
Imagine a triangle on the original
paper, and now after piecing it
together, wouldn't an observer think
the lines are now curved?
The answer is no: lines that were straight on the original …
11
votes
How does the curvature of spacetime induce gravitational attraction?
I'm a bit worried about getting a reputation for citing myself too much, but I'll go for it anyway. (In my defense, I always admit it when I'm doing it!)
John Baez's and my pedagogical paper The Mea …
12
votes
Accepted
Brachistochrone problem in general relativity
Update: I made a number of mistakes in the original version of this post, although I think all the big ideas are right. I tried to fix everything, but I wouldn't be at all surprised if I've made addit …