Tagged Questions
0
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0answers
41 views
Ricci tensor question
Suppose the Ricci tensor $R_{a}^{\;\;b}$, considered as a linear map on $T_{m}M$ at an even $m$, has only real eigenvalues. What are the five possible Jordan forms of $R_{a}^{\;\;b}$? Show that the ...
1
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0answers
56 views
Newman-Penrose tetrad question
I have a question/exercise relevant to students of mathematical relativity:
Let $\left \{ l^{a},n^{a},m^{a},\bar{m}^{a} \right \}$ be a Newman-Penrose tetrad, where only the direction of $l^{a}$ is ...
1
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0answers
82 views
Weyl's axisymmetric static solution
I'm currently doing a course on general relativity, and I'm struggling with the following exercise - I would greatly appreciate the help anyone might offer. It is as follows:
Weyl's solution to the ...
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3answers
102 views
Black hole accretion of dark energy
Dark energy physically can be interpreted as either a fluid with positive mass but pressure the negative of its density (pressure has units of energy/volume, and energy is mass), or a property of ...
3
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3answers
310 views
Why do clocks measure arc-length?
Apologies in advance for the long question.
My understanding is that in GR, massive observers move along timelike curves $x^\mu(\lambda)$, and if an observer moves from point $x^\mu(\lambda_a)$ to ...
1
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1answer
133 views
Why is energy-momentum 4-vector so much easier to explore/observe than spacetime 4-vector
I have read that spacetime 4-vector is quite difficult to observe/explore and that energy-momentum 4-vector is much more appropriate for CERN etc.
Why is that? Could anyone give me a brief ...
0
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0answers
123 views
What facts and examples should I use in a debate with a Relativity Denier? [closed]
I have been asked to 'defend Science' at a Creationist event in Minneapolis, MN, and the guy I'll be debating is against 'Einsteinian Relativity', or as he characterizes it, the idea that there is no ...
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1answer
202 views
Why are we talking about space curvature as if we know what space is? [closed]
1) Why are we talking about space curvature as if we know what space is?
Every question about gravity seems to evoke an answer involving "space curvature" which seems like an undefined placeholder ...
0
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1answer
101 views
Light orbiting a massive body [duplicate]
Possible Duplicate:
Can a photon be made to orbit a known (or undiscovered theoretical) body?
How massive would a black hole have to be for light to orbit it at 1km away from the ...
5
votes
1answer
141 views
Confused about indices of the Ricci tensor
In an intro to GR book the Ricci tensor is given as:
$$R_{\mu\nu}=\partial_{\lambda}\Gamma_{\mu \nu}^{\lambda}-\Gamma_{\lambda \sigma}^{\lambda}\Gamma_{\mu \nu}^{\sigma}-[\partial_{\nu}\Gamma_{\mu ...
3
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3answers
136 views
Having trouble seeing the similarity between these two energy-momentum tensors
Leonard Suskind gives the following formulation of the energy-momentum tensor in his Stanford lectures on GR (#10, I believe):
$$T_{\mu \nu}=\partial_{\mu}\phi \partial_{\nu}\phi-\frac{1}{2}g_{\mu ...
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2answers
156 views
length contraction question
we know from eintein's theory of relativity that lets say, a ruler is travelling to a speed if light, then we can say that the ruler (from our view as observers) has shorten. but why, lets say we have ...
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2answers
99 views
The relativistic mechanics of a battery that is being charged and accelerated at the same time
This might be an interesting question:
Let's attach a battery into one end of an electric cable. Then we rotate the battery around, with accelerating speed, using 100 Watts of power, while the ...
1
vote
2answers
139 views
Finding the correct units for the energy-momentum tensor?
I'm trying to understand the energy-momentum tensor $T^{\mu\nu}$ but I'm confused about the units. My textbook says the components of $T^{\mu\nu}$ are $\mathrm{Jm^{-3}}$. Four-momentum is is given ...
2
votes
1answer
212 views
Thought experiment about acceleration
Case 1: two people wake up in spaceships accelerating at 1g. They can measure or observe anything inside the room but not outside. They couldn't determine if they were on a spaceship or on earth. ...
5
votes
1answer
189 views
Are objects in a gravitational well shortened?
Bob is in a gravitational potential well, he moves a long vertical stick up and down a distance of 1 meters. Alice observes the upper end of the stick, at upper location.
There is the phenomenon of ...
1
vote
4answers
263 views
Is there a universal rest frame of reference?
I am still struggling with C being a constant and what that implies.
So can an experiment be done to find the resting state for the universe?
Take a device with an observer and a light source and two ...
2
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4answers
314 views
Reducing General Relativity to Special Relativity in limiting case
I understand that general relativity is applicable to gravitational fields and special relativity is applicable to case when there is no gravity. But is there a derivation on how to reduce General ...
6
votes
3answers
294 views
Getting back out of an Alcubierre warp bubble
Does the theory on paper provide a way for hypothetical travelers to get back out of the bubble that has gotten them close to their distant destination by compressing all the space in front of them ...
5
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1answer
370 views
Length contraction in rotating frame?
If an observer were to rotate around a point at near light speeds, what sort of length contraction would he observe the universe undergo?
2
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1answer
327 views
How will the Twin Paradox become, for Time Dilation, if no acceleration was ever involved?
I think one catch in Twin Paradox was about the big acceleration that can turn back the traveling twin from light speed outward bound, to become light speed inward bound.
What if there is strictly no ...
10
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4answers
989 views
Newton's Bucket
This thought experiment is originally due to Sir Isaac Newton. We have a sphere of water floating freely in an opaque box in intergalactic space, held together by surface tension and not rotating with ...
8
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3answers
566 views
Special Relativistic approximation to GR
Some time ago I was talking to a professor in college about some of the fundamental aspects and origin of General Relativity. I was surprised to learn, in fact, that a pretty good approximation to GR ...
