All Questions
588
questions
9
votes
3
answers
6k
views
Why is Spacetime described as flat even though we live in 3 dimensions of space?
I’ve always heard and seen diagrams that show spacetime as being “flat” or in 2 dimensions with curvature. How does this correspond to the 3 spacial dimensions that we perceive to exist in?
- 135
0
votes
0
answers
53
views
Question on gravity and spacetime curvature [duplicate]
In General Theory of Relativity, it is explained that the fabric of reality i.e. spacetime bends around objects with mass, and that curvature causes other objects to come close to/ fall towards the ...
1
vote
3
answers
2k
views
Why does mass make curvature in spacetime? [duplicate]
According to Einstein's general relativity theory, matter with mass makes curvature in spacetime. The greater the mass, the curvature in spacetime will be greater.
My questions:
Why will mass make ...
- 19
2
votes
1
answer
135
views
How do you relate $\Omega_{k}$, the curvature term in the FLRW metric, to the radius of curvature?
I have assumed, for reasons a bit too detailed to go into here, that if $\Omega_{k}$, the curvature term in the FLRW metric, is equal to 1, then the radius of curvature is equal to 13.8 billion light ...
- 371
4
votes
6
answers
1k
views
How does general relativity theory explain gravitational pull? [duplicate]
I watched some videos on YouTube that explain why gravity is not a force,
according to general relativity theory.
I can wrap my head around the idea that spacetime can be curved due to a massive mass,
...
- 141
2
votes
2
answers
387
views
Is there a relation between spacetime curvature and radiation?
To my understanding, the curvature of spacetime is determined by the stress-energy tensor.
I was wondering if we could calculate some of those components using radiation.
Is it possible that objects ...
1
vote
3
answers
108
views
Where is the normal force that pushes us up comes from if gravity is not a force according to general relativity?
https://youtu.be/XRr1kaXKBsU?t=530
I was watching this video and at this point he said that since gravity is not a force as per GR, we are left with only these normal forces pushing you up that ...
- 21
3
votes
0
answers
84
views
Why are departures from flat spacetime geometry small on scales smaller than the Hubble radius?
In Chapter 5 of Baumann's cosmology book where he discusses structure formation starting from Newtonian perturbation theory, Baumann mentions at the beginning that
Newtonian gravity is a good ...
- 394
4
votes
2
answers
638
views
Characterising Minkowski spacetime as a flat manifold with some other property?
It is known that flat manifolds can be characterized as follows
If a pseudo-Riemannian manifold $M$ of signature $(s,t)$ has zero Riemann
curvature tensor everywhere on $M$, then the manifold is ...
- 1,588
3
votes
3
answers
164
views
How do we know if spacetime is bent?
For example I'm at certain location in outer space.
How do I know if the spacetime in front of me is bent, e.g. by some dark matter?
- 131
13
votes
3
answers
2k
views
In general relativity, why is Earth able to accelerate?
I was told and convinced that gravity is not a force, and in free fall you're an inertial frame and experience no force, and when on the surface of Earth you would be accelerating upwards.
What I ...
- 320
2
votes
3
answers
221
views
What is Dirac's reasoning when showing the curvature vanishing implies we can choose rectilinear coordinates?
In section 12 of Dirac's book "General Theory of Relativity" he is justifying the name of the curvature tensor, which he has just defined as the difference between taking the covariant ...
- 43
0
votes
1
answer
105
views
Can dark matter be explained by defects of spacetime?
Dark matter is believed to be a substance of unknown origin with mass that is distributed in space.
Can the same observed effects be explained by an intrinsic curvature of regions of space without ...
- 158
0
votes
0
answers
144
views
Does this theorem holds out for spacetime?
The theorem:
Let $F$ and $C$ be two finite geometric figures (those defined by two continuous functions in a given region $D$), where $F$ belongs to an $n$-dimensional Euclidean space and $C$ is the ...
- 495
1
vote
0
answers
39
views
When is the Weyl tensor applied on null vectors a null vector?
Let $C^{\rho}_{~\alpha \beta \gamma}$ be the Weyl tensor of a spacetime $(M,g)$, that is a solution to Einstein's equation. Let $X^\alpha, Y^\alpha, Z^\alpha$ be null vector fields, i.e. $X_\alpha X^\...
- 384