Skip to main content

New answers tagged

1 vote

Geodesic variation and the (Riemann) curvature tensor -- what about uniform gravitational fields?

I am not sure about the correctness of my answer, so please be lenient. However, I suppose the crucial part here is to realise that this is just one special case of initial data where the geodesic ...
Octavius's user avatar
  • 705
0 votes

What is the geometry of light cones if space is curved/non-Euclidean?

You seem are asking for the conformal infinities of curved spacetimes. Specifically for the $S^1 \times S^3$ topology known as $AdS_5$ boundary and for certain choice of coordinates, it can be ...
Mariana Lozano's user avatar
1 vote
Accepted

Motivation for pure Yang-Mills Lagrangian

Toffomat's answer is a great answer. Let me try to add a little bit more physical motivation for what it means to "decouple the ghosts." SUMMARY: The requirement of gauge invariance comes ...
Luke Pritchett's user avatar
2 votes

Motivation for pure Yang-Mills Lagrangian

You can look at this from various angles, but the Lagrangean as you write can be (basically) fixed by basic QFT considerations: The $A_\mu^a$ are the basic degrees of freedom of the theory. This the ...
Toffomat's user avatar
  • 4,477
1 vote

Motivation for pure Yang-Mills Lagrangian

Classical Yang Mills theory is a generalisation of classical EM. More precisely, it generalises the gauge structure group from an abelian group like $U(1)$ to non-abelian group like $SU(2)$ or $SU(3)$....
Mozibur Ullah's user avatar
3 votes

Theoretically, can perfectly flat space exist in the universe?

No, since you're not going to be able to evacuate the vacuum energy. Thus you cannot get a perfectly flat space theoretically. However, at most humanly probeable scales today, it would appear flat.
Mozibur Ullah's user avatar
5 votes
Accepted

Theoretically, can perfectly flat space exist in the universe?

In principle, you would have flat spacetime in the hollow center of a spherically symmetric mass/energy distribution. So you could imagine rearranging the contents of the Universe into such a ...
Sten's user avatar
  • 6,012
2 votes
Accepted

Mathematical description of higher spin gauge theories

The answer depends on what kind of consistent higher spin theories you are interested in. There are several examples: in $3d$ simple higher spin gravities are just Chern-Simons matter theories (note ...
John's user avatar
  • 806
0 votes

Is spacetime flat inside a spherical shell?

Recently, it has been shown that an alternative set of solutions exist for the Einstein field equations inside a thin spherical shell. The solutions were initially found by Jun Ni and have since been ...
MattEdwards's user avatar
0 votes

Commutator of covariant derivatives to get the curvature/field strength

I want to add on to Mike's answer, because there's some subtlety here about whether we're talking about a commutator of Lie algebra elements or of operators. We should really think of $A_\mu$ as the ...
A. Kriegman's user avatar
  • 1,262
0 votes

Understanding Wormholes Geometrically

Unfortunately the folding paper is all you’re really going to get. If you were to embed our four-dimensional spacetime in something of higher dimensions, you would be able to see a definite “wormhole”-...
controlgroup's user avatar
3 votes

How do black holes move if they are just regions in spacetime?

The black hole is a region around a singularity. It's the singularity that creates the "gravity well" around it that we call a "black hole" and the singularity itself can move like ...
Alex's user avatar
  • 31
3 votes

How do black holes move if they are just regions in spacetime?

The renowned John Archibald Wheeler (Princeton) once (the 1960's) said "Matter tells space-time how to curve, space-time tells matter how to move" This expresses the intimate, "...
JonesTheAstronomer's user avatar
28 votes

How do black holes move if they are just regions in spacetime?

Black holes are not just regions of space time. There was once a star there. According to the law of conservation of mass and energy, the star is still there. The star has spin and angular momentum. ...
Russell Hankins's user avatar
18 votes

How do black holes move if they are just regions in spacetime?

Adding a picture to illustrate what some other answers here already have said. Suppose you throw a ball straight up into the air, and then it falls back down. You've probably seen a picture like this, ...
Solomon Slow's user avatar
  • 14.8k
18 votes

How do black holes move if they are just regions in spacetime?

Remember, we are talking about spacetime, not just space. In space by itself, the black hole is a ball-shaped region. But it extends far into the past and future, so it's more like a kind of tube. So ...
Mark Foskey's user avatar
  • 3,440
3 votes

How do black holes move if they are just regions in spacetime?

First of all, a point to note is that a black hole is not a "body" in of itself. Black holes are just solutions to point masses in general relativity, analogous to point masses in newtonian ...
Ronny's user avatar
  • 166
3 votes

How do black holes move if they are just regions in spacetime?

A black hole has all the properties of an object with mass. has gravitational influence on other objects, it responds to the gravitational field of other objects (e.g a bigger black hole) and ...
KDP's user avatar
  • 5,359
9 votes

How do black holes move if they are just regions in spacetime?

How do black holes move if they are just regions in spacetime? It might sound strange but nothing really moves in spacetime. Say, for example, we have two black holes orbiting each other in the frame ...
S.G's user avatar
  • 2,515
1 vote

How do black holes move if they are just regions in spacetime?

how can black holes even move? Imagine yourself in a powerful ship hovering outside a black hole (BH) event horizon. You start to accelerate in a particular direction then get to a constant velocity ...
joseph h's user avatar
  • 30k
3 votes

How do black holes move if they are just regions in spacetime?

If black holes are just regions in spacetime, how can it bend the spacetime around it, so it can remain a black hole, if there is no matter to continuously bend it? There is no matter, but there is ...
Níckolas Alves's user avatar
2 votes

If space has a positive curvature, is the expansion of the universe caused by time, not "dark energy"?

General relativity doesn't work like that. Space doesn't need a place to put itself. It is the place. In many cosmologies with positive spatial curvature, the universe recollapses, so it passes ...
benrg's user avatar
  • 27.4k

Top 50 recent answers are included