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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.
37
votes
How does gravity escape a black hole?
I think it's helpful to think about the related question of how the electric field gets out of a charged black hole. That question came up in the (now-defunct) Q&A section of the American Journal of …
- 20.1k
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 …
- 20.1k
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 …
- 20.1k
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 …
- 20.1k
25
votes
Redshifting of Light and the expansion of the universe
Other answers have covered the key points correctly, but I'll jump in too, maybe emphasizing a slightly different angle.
It's not just that energy is not conserved -- even defining the total energy o …
- 20.1k
21
votes
Accepted
Shape of the universe?
There are a bunch of questions here. Let me try to take them in order:
Is it possible that our Universe has the feature that if you travel far enough you return to where you started?
Yes. The stan …
- 20.1k
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 …
- 20.1k
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 …
- 20.1k
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 …
- 20.1k
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 …
- 20.1k
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 …
- 20.1k
11
votes
Newton's Bucket
The answers that have already been posted are correct, but @kakemonsteret raises a followup question in the comments that's worth addressing:
Lets say you are spinning somewhere in
outer space, …
- 20.1k
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 …
- 20.1k
11
votes
Accepted
What prevents the accumulation of charge in a black hole?
Coulomb repulsion it is. Specifically, if a black hole has a lot of charge, then particles with a high charge-to-mass ratio will be repelled. Anything that falls in will contribute "more mass than cha …
- 20.1k
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 …
- 20.1k