61
votes
Can light become a satellite of a black hole?
No, light cannot form stable, bound orbits around a black hole.
The possible trajectories of light around a Schwarzschild black hole are conveniently parameterised in terms of the "impact ...
- 135k
49
votes
Accepted
Explain to a non-physicist what goes wrong when trying to quantize gravity
“What exactly goes wrong when trying to quantise gravity?”
There is no problem specific to quantum gravity! I know this isn’t conventional way to look at it, but the physics is actually not ...
- 514
44
votes
Accepted
How does a laser from Earth manage to hit the Moon with precision?
Commenters on the first version of this answer (preserved in the edit history) did me the favor of finding the literature that I hadn't read for years, which contains a succinct answer to your ...
rob♦
- 91.9k
39
votes
Would weightlessness (i.e. in thrill rides, planes, skydiving, etc.) be different on a Flat Earth?
If we assume a flat disk sans gravity accelerating upwards at one $g$ (which is what the FEers propose), then the physics of thrill rides and parachutes and whatnot would actually be the same. If the ...
- 1,409
38
votes
Accepted
How does spacetime curve around an object in superposition?
The short answer is we do not know.
Our best theory of gravity, general relativity, tells us that the curvature of spacetime follows Einstein's equation,
$$ G_{\mu\nu} = \frac{8\pi G}{c^4}T_{\mu\nu},$$...
- 12.6k
31
votes
Accepted
Does the gravitational field have a gravitational field?
Yes. One interpretation of the fact that Einstein's equations of general relativity are non-linear is that "gravity gravitates." In other words, the gravitational field itself carries energy,...
- 52.1k
28
votes
Explain to a non-physicist what goes wrong when trying to quantize gravity
I think, like a lot of technical questions, you can get different answers by "zooming in" to different levels of technical detail.
Here is highest level, least technical, shortest ...
- 52.1k
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. ...
- 393
28
votes
Accepted
Given gravitational lensing, why would black holes visually appear black?
There is still a black disk because no photon can reach the observer directly from the event horizon. Even though light may circle the black hole many times, resulting in multiple images of the same ...
- 12.1k
27
votes
A study on the speed of gravity
It is well known that General Relativity predicts that gravitational waves exist [1] and that they travel at the speed of light. But, until the construction of LIGO, these were not experimental facts. ...
- 24.5k
27
votes
If gravity is not a force, what makes massive objects spheroid?
The basic idea is still the same: the gravitational influence of the parts on each other tends to pull them together. The only difference is that now you don't have a force pulling them together, but ...
- 22.4k
27
votes
Accepted
How long is a second on the moon?
The time dilation is mainly gravitational and, to a first approximation, calculated by the Schwarzschild metric, which is
$$c^2\mathrm{d}\tau^2 = -\left(1-\frac{r_S}{r}\right)c^2\mathrm{d}t^2 + \left(...
- 12.1k
27
votes
Why does Roy Kerr claim that the Kerr black hole does not contain a singularity?
As far as I can tell, Kerr's point is that the singularity theorem doesn't specify whether or not a collapsing object will collapse to a "real" singularity. As a reminder, singularity ...
- 16.6k
25
votes
Why does my perpetual CMBR rocket ship not work? Why does the CMBR have infinite energy?
So this works even a little bit better than you think because of something called relativistic beaming, which would cause all of the CMB radiation to be coming at you from a point “ahead of you”—i.e. ...
- 38.5k
23
votes
How does a laser from Earth manage to hit the Moon with precision?
As Rob mentions, when reflecting a laser off the Moon you do need to compensate by ~1.4 arc-seconds for the light-time delay. But the whole point of the Lunar Laser Ranging experiments (LLR) is to ...
- 12.4k
22
votes
What would one use a theory of quantum gravity for?
The two standard answers are:
Resolving the singularity at the Big Bang
Resolving the singularity in Black Holes
Both of these have large spatial curvature within very small regions.
- 21.5k
22
votes
Is quantum gravity research implying that gravity is actually a force and not spacetime curvature according to GR?
Disclaimer: I am mainly a relativist, and I think science has benefitted much more by thinking of gravity as a geometrical entity than as a "force". It is impossible to answer this question ...
- 22.4k
22
votes
Is quantum gravity research implying that gravity is actually a force and not spacetime curvature according to GR?
First, the concept of "force" is not as easy as it might seems. It is inherited from Newtonian mechanics in which it is defined as a vector sourcing the motion of the particles. In a sense, ...
- 1,152
20
votes
Given gravitational lensing, why would black holes visually appear black?
In order for a light ray to reach you that looks like it came from the direction of the "black disc", a light ray would have to curve away from the black hole. That doesn't happen; the ...
- 135k
20
votes
Accepted
Is this an actual photo of frame dragging?
It is not frame dragging, it is a visualisation of the linear polarisation present in 230 GHz light emerging from the vicinity of Sgr A* at the centre of the Milky Way. But getting an answer took some ...
- 135k
19
votes
How long is a second on the moon?
It's basically due to gravitational time dilation. Ratio of proper times of Moon vs Earth is :
$$\tag 1\large \frac{t_{moon}}{t_{earth}} = \frac {{\sqrt {1-{\frac {2Gm}{rc^{2}}}}}}{{\sqrt {1-{\frac {...
- 14.4k
19
votes
Accepted
Why it is the mass instead of the mass distribution used in Schwarzschild metric?
Birkhoff's theorem tells us that:
any spherically symmetric solution of the vacuum field equations must be static and asymptotically flat. This means that the exterior solution (i.e. the spacetime ...
- 359k
18
votes
Accepted
Does the cosmological constant entail a mass for the graviton?
No, you don't get a mass term in the propagator, and there is no graviton mass for GR with a cosmological constant.
If you linearize about Minkowski space, $g_{\mu\nu}=\eta_{\mu\nu}+h_{\mu\nu}$, then ...
- 52.1k
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, ...
- 15.6k
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 ...
- 3,430
17
votes
Accepted
Does the equivalence principle only apply for the gravitational field of an infinitely sized body?
You have noted something really important, but there is a caveat in the formulation of the equivalence principle that avoids your remark.
The equivalence principle states that locally a gravitational ...
- 22.4k
17
votes
Aren't places where geodesics end singularities?
Do not confuse geodesics (the abstract geometrical things) with the paths of particles. Particles follow geodesics, but the fact that a particle is created at some point and destroyed at another doesn'...
- 10.1k
17
votes
Accepted
Do the Einstein Field Equations force the metric to be Lorentzian?
The Einstein equations alone do not enforce any particular signature. For example, in the derivation of the Schwarzschild metric you require that the metric is asymptotically flat... but not just any ...
- 5,157
16
votes
Can light be somehow confined to create a kugelblitz?
At high enough intensities light forms particle-antiparticle pairs which carry energy away from the region in which you're trying to form a black hole:
https://arxiv.org/abs/2405.02389
This constrains ...
- 9,236
16
votes
If gravity is not a force, what makes massive objects spheroid?
First, even in general relativity (GR), there's nothing wrong with taking a weak-field limit where GR "looks like" Newtonian gravity, and in that limit you can think of gravity just the same ...
- 52.1k
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