All Questions
40 questions
2
votes
2
answers
180
views
Can a body escape a black hole by being thrusted? [duplicate]
I am told many time that nothing can escape black-hole because black-holes escape velocity is more than speed of light. But we know object don't necessarily have to exceed speed of light to escape a ...
- 41
1
vote
1
answer
71
views
Is pp-wave spacetime strongly causal?
Is pp-wave spacetime, strongly casual?
If not, where is it on the causal ladder?
- 1,280
1
vote
1
answer
48
views
A question on the causal hierarchy/ladder and the existence of CTC
What does the existence of CTC imply for the Causal Structure of the spacetime?
Can a strongly causal spacetime have any CTC[without fluctuating the metric]?
Is there any such example?
Can someone ...
- 1,280
1
vote
3
answers
158
views
When a curve is future (past) inextendible?
Future (past) endpoint: We say that $p\in M$ is a future (past) endpoint of a curve $\lambda$ if for every neighborhood $O$ of $p$ there exists a $t_0$ such that
$\lambda(t)\in O$ for all $t>t_0$ (...
- 27
0
votes
0
answers
23
views
Is it possible for a black hole singularity to interact gravitationally with other celestial bodies (if we analyze it using the concept of gravitons)? [duplicate]
It is known to all that the travelling speed of gravitons (the propagation speed of gravitational field) is not instant. So for black holes, the gravitons (the gravitational field) generated by the ...
2
votes
1
answer
98
views
How much time does it take for the gravitons generated by a black hole singularity to travel before exerting gravity forces on other celestial bodies?
It is known to all that the travelling speed of gravitons (the propagation speed of gravitational field) is not instant. So for black holes, the gravitons (the gravitational field) generated by the ...
1
vote
2
answers
78
views
If a star passes near another star will that star feel gravitational influence immediately or with a delay? [duplicate]
If a star passes near another star will that star feel gravitational influence immediately or with a delay? Assuming that the distance is a large number and the stars are very massive?
- 2,275
-3
votes
1
answer
54
views
If gravity can catch up a light photon (speed=$c$) and change its wavelength is it faster than light?
Why can gravity catch up a light photon (speed=$c$) and change its wavelength? How can that be logic although nature shows somehow it is... Maybe gravity has different speed than gravitational waves ...
- 2,275
0
votes
1
answer
43
views
When the Earth suddenly stops pulling [closed]
Let's say the Earth stops attracting objects to itself all of a sudden. So, I was wondering what would happen to a person standing on the surface of the Earth? (Neglect the effect of the rotation of ...
0
votes
7
answers
422
views
If light and gravity travels at the same speed then why light cant escape the gravitational pull of black holes?
Speed of gravity is infinite?
If fastest thing is light then how universe is spreading faster than the speed of light?
5
votes
3
answers
499
views
What motivated Einstein to formulate general relativity?
I never really fully understood what motivated general relativity or why the Newtonian concept of gravity was considered problematic.
One thing I always hear is that it is because it doesn't address ...
- 59
2
votes
0
answers
35
views
How can black holes emit detectable magnetic fields and gravity if nothing can escape the event horizon? [duplicate]
How can black holes emit detectable magnetic fields and gravity if nothing can escape the event horizon?
- 439
-2
votes
1
answer
326
views
What does it mean that gravity is '"local"? [closed]
What is "local" defined to be? Why don't larger systems affect smaller ones? ie. Don't we need to consider the gravitational pull from all other objects in the universe? Is this "...
- 167
0
votes
2
answers
905
views
Does the effect of field travels faster than light? [duplicate]
As I was going through some work of my college level courses of fundamental physics, the formula caught my attention $F={G}\frac{Mm}{r^s}$. "r" seems can be any value; there would still be ...
- 3
3
votes
2
answers
277
views
Is general relativity the simplest possible theory of gravitation?
I can't find this, but i've seen that GR is the only possible theory of gravity if you assume causality and principle of equivalence?
- 864
-1
votes
2
answers
556
views
Can gravity act through time? [closed]
I know, that gravity acts through time and space, as it propagates at the speed of light, through space.
I mean to discuss whether gravity has an effect in spacetime, along the time axis. Effects of ...
- 11
5
votes
1
answer
178
views
Gravity at light speed
my question in regards to light. Since gravity warps space time, and we have discovered gravitational waves. Would the light at the front of the wave, be traveling faster than lightspeed? Essentially ...
- 51
12
votes
2
answers
800
views
Do the forces between Earth and Sun point in the directions that the Earth and the Sun were 8 minutes ago?
No information can travel faster than light, so does the force between Earth and Sun point in the direction that the Earth was 8 minutes ago or in the direction that the Earth is now?
- 431
4
votes
2
answers
117
views
How do we know the speed of gravity? [duplicate]
I have read things like if the sun disappeared, it would take 8 minutes or so before we knew this, cf. e.g. this Phys.SE post.
However, in real life the sun will never disappear and if it exploded, ...
- 2,288
4
votes
5
answers
642
views
Is it theoretically possible to escape a classical black hole with force?
A stone being thrown from the surface must have a velocity of $11.2$ km/sec if it wants to escape Earth's gravity. However, a stone that has a constant force being applied on it need not have an ...
- 165
-1
votes
1
answer
180
views
Gravitational field of sun [closed]
Let there be a solar system without Earth.
Now what happen if we place the Earth suddenly on its actual position does the gravitational force of sun acts on it immediately because of its pre ...
- 327
3
votes
0
answers
203
views
Graviton propagator, and Gauss-Bonnet gravity
Let's say we consider Einstein's Lagrangian from GR. In linearized gravity, we would expand the Ricci scalar to quadratic order in the perturbation parameter to find the propagator.
My question is as ...
- 1,202
4
votes
2
answers
567
views
Why can't we insert gravity in the special relativistic lagrangian?
I am a math student and I have taken four-five lessons about special relativity in a course about Lagrangian and Hamiltonian mechanics, so be patient with me if my question is stupid. My teacher says ...
- 241
0
votes
0
answers
63
views
"Speed" of gravity? [duplicate]
This is probably going to be a poorly written question, since I know really a little of physics.
Imagine a object of small mass, let's say a book, is floating in the universe in absence of any ...
- 101
0
votes
0
answers
33
views
What does it mean be gravity propagates with $c$? [duplicate]
I know that gravity propagates with speed of light. Does it mean that if Earth's orbit will disturb after 2 years if a supermassive black hole suddenly appears 2 light years away? Is this related to ...
-1
votes
2
answers
1k
views
Is the future predetermined and fixed? [closed]
I just watched a YouTube video which explained that life as a sequence of events is a geometric object in the four-dimensional spacetime and that the future is not only predetermined, but it already ...
- 109
0
votes
1
answer
141
views
Is the curvature of spacetime in General Relativity instantaneous? [duplicate]
Assume one initially has perfectly flat Minkowski vacuum. At some point in this Gedankenexperiment a point mass spontaneously appears at some location in Minkowski creating a Schwarzschild geometry. ...
- 1
2
votes
1
answer
466
views
Can forces from outside the observable universe be observed through a distant object? [duplicate]
I recently saw a video on dark flow from PBS space time on youtube, and it said that the flow had to be a constant motion, and not accelerating, because the object likely that caused the flow using ...
- 121
2
votes
2
answers
1k
views
How does the Earth know when to send a graviton to a newly born particle?
Pardon if this is a silly queston, but I'm reading this for the first time. It says that the force we perceive between two objects is an effect of the exchange of the force carrier particles. Even ...
- 1,113
1
vote
2
answers
251
views
How high can a light-beam (or apple) travel when pointed (thrown) out from the event horizon? [duplicate]
I am a bit confused at the idea that information cannot be sent out from behind the event horizon of a black-hole. Consider the following figure: (source This link)
An apple thrown up from the ...
- 3,443
0
votes
2
answers
830
views
What is the difference between gravitational waves and gravitational distortions in spacetime?
I understand that gravitational waves can be caused by accelerating masses (e.g. The gravitational waves that were detected by two very massive black holes merging earlier this year) and that they are ...
- 45
6
votes
5
answers
458
views
Can the mass within the event horizon of a black hole interact gravitationally with the mass outside the event horizon?
If so, gravitons and their fields, unlike photons, must be able to cross the event horizon freely in both directions. If not, the observed mass of a black hole must depend only on the particles ...
- 61
-1
votes
1
answer
55
views
How can the speed and the mechanics of light be similar to the speed and the mechanics of gravity? [duplicate]
How can the speed and the mechanics of light be similar to the speed and the mechanics of gravity?
Most say that light cannot escape a black hole. Then gravity cannot escape a black hole either, so ...
- 211
0
votes
1
answer
305
views
Is gravity instantaneous? [duplicate]
I want to know if (hypothetically) a star appears out of nowhere at a certain distance (say 20 light seconds) away from me, how long will it take for me to get the feel of it's gravity? Will I know it ...
- 141
0
votes
2
answers
63
views
Is the influence of gravity greater than light? [duplicate]
As the influence of gravity is infinite throughout the universe.is the influence of its force on a body very far away faster than the speed of light.suppose a star dies...is the influence of its ...
0
votes
3
answers
276
views
What is the reasoning behind the idea that light cannot escape from a black hole? [duplicate]
According to the definition, light cannot escape from a black hole.
How did scientists deduce that light cannot escape from a black hole?
- 181
10
votes
2
answers
626
views
Speed of gravity in cosmological codes and ephemeris generation
There are few questions in Phys.SE concerning the speed of gravity, and the answers are traditionally that the speed of gravity equals to the speed of light. But in that case I have three more ...
- 547
1
vote
2
answers
99
views
Solar Catastrophe [duplicate]
Consider all of sudden the sun vanishes. What would happen to planetary motion. Will it continue to move in elliptical path or move in a tangential to the orbit immediately after sun vanishes or move ...
- 598
8
votes
6
answers
9k
views
Gravity on supermassive black hole's event-horizon
$M =$ black hole mass
Gravitation is about $r^{-2}$
Schwarzschild radius, $r_{\text{S}}$, is $\propto M$
So, more massive black holes have weaker gravitation at their event horizon.
Consider a black ...
- 99
141
votes
11
answers
21k
views
How fast does gravity propagate?
A thought experiment: Imagine the Sun is suddenly removed. We wouldn't notice a difference for 8 minutes, because that's how long light takes to get from the Sun's surface to Earth.
However, what ...
- 3,950