74
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Accepted
Can gravitational waves pass through a black hole?
No, gravitational waves cannot pass through a black hole.
A gravitational wave follows a path through spacetime called a null geodesic. This is the same path that would be followed by a light ray ...
- 1,699
55
votes
Accepted
If light has no mass, why is it affected by gravity?
Another way to answer this question is to apply the Equivalence Principle, which Einstein called his "happiest thought" (so you know it has to be good). The equivalence principle says that ...
- 5,336
48
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Can Newton's gravity equation explain why black holes are so strong?
No you can't and the behaviour of bodies with mass and of light is completely different near a compact, massive object if you use Newtonian physics rather than General Relativity.
In no particular ...
- 158k
42
votes
Accepted
Does General Relativity really predict Black Holes?
Well, yes, but we must be careful with the meaning of "predict".
The Schwarzschild solution, developed by Karl Schwarzschild in 1916 [1], is the first closed-form, explicit solution of ...
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33
votes
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Maximum spin rate of a black hole?
Since I like math, let's throw some math into this. I'll try to keep it as simple as possible though.
Kerr Black Holes
A rotating black hole is known as a Kerr Black Hole (named after Roy Kerr who ...
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32
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If light has no mass, why is it affected by gravity?
There are a couple of ways one could approach your question:
Black holes are regions of space that have been deformed by a sufficiently concentrated mass. Light waves/particles always travel in a ...
- 1,199
30
votes
Accepted
Is there a better formula for gravitation, besides Newton's?
Going from Newton's theory to Einstein's theory is not simple. It's not like you can just add a term to Newton's gravity, like $\textbf{F}=-{GmM \over r^3}\textbf{r} + \textbf{f}(\textbf{r})$ and ...
- 4,481
29
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Is it possible to detect gravitational lensing of stars behind the Moon?
Measuring the gravitational deflection of light by the Moon is just out of reach of current observational techniques.
The angular deflection caused by the lensing of a distant background object by a ...
- 158k
28
votes
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How would "dark matter", subject only to gravity, behave?
What you describe is the standard paradigm in cosmological physics, so it has been studied extensively. The basic consequence of dark matter not having significant nongravitational interactions is ...
- 4,777
25
votes
Accepted
Was the Sun's gravitational lensing observed in other solar eclipses than the one in 1919?
Yes, observations of this kind are within the technical scope of amateur astronomers. Several groups succeeded in replicating the experiment during the 2017 eclipse that crossed the USA.
For example ...
- 127k
22
votes
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How do two black holes merge?
The "event horizon" is defined as the point (or surface) from within which light rays can never (ever) reach a distant observer. To find the location of the event horizon implies that you ...
- 158k
21
votes
Is there anything different about the gravitation around a non-spinning black hole and a neutron star of the same mass?
Birkhoff's theorem is very useful: in general relativity, if you are in vacuum and there is a spherically symmetric gravitational field, then it will be the Schwarzschild solution. This solution only ...
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20
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Is there anything different about the gravitation around a non-spinning black hole and a neutron star of the same mass?
If they were spinning they would be distinguishable (in principle), otherwise not.
Astrophysical black holes and neutron stars are expected to spin. In the case of a neutron star that automatically ...
- 158k
17
votes
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Have I nearly found the event horizon of a black hole?
This is the Newtonian model of gravity. It is a very good model, it is used for accurate calculating the motion of objects in the solar system to a very high degree of accuracy.
However, for very ...
- 127k
17
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Doubt regarding size and shape of black hole images published by the Event Horizon Telescope (EHT)
The detail you seek is contained in Johansson (2014).
The photon ring around a black hole is not the event horizon. It is the projection of unstable photon orbits that are able to loop around the ...
- 158k
16
votes
Accepted
Latest cosmological parameters
Cosmological parameters are measured in a variety of ways, and their values will depend on which measurements you trust the most. The paper you link to (Planck Collaboration et al. 2016) with the 2015 ...
- 39.1k
16
votes
How do we know that objects that appear in duplicate or triplicate, etc. due to strong gravitational lensing aren't actually multiple objects?
Perhaps this isn't the case for every scenario, but I can think of at least two instances where this can be determined:
Stars
In the case of stars, it's pretty straightforward to get the spectra of ...
- 3,404
15
votes
Accepted
Is the age of the universe relative to an observer's location in that universe?
You are labouring under the misapprehension that how far we can see directly gives the age of the universe. Whilst it is true that the oldest light we can see was emitted some 13.7 billion years ago, ...
- 158k
15
votes
Accepted
Is Webb or any near-future telescopes like ELT capable of observing redshift changes to confirm General Relativity?
The effect whereby, as the universe expands, the redshift of an object changes with time is known as redshift drift.
A galaxy at a fixed co-moving distance will have a redshift that changes with time ...
- 158k
14
votes
Can Newton's gravity equation explain why black holes are so strong?
I am not an expert in physics and the explanation of the others is excellent. However, I noticed a flaw in your reasoning which they did not address.
You have written:
Considering the Newton's Law of ...
- 241
13
votes
Why doesn't the vertical light beam get out of a black hole?
There is no "up" direction within the event horizon.
Most people get fixated on the speed of light, or energy or whatever. They're like, if light was faster, could it escape the black hole? If my ...
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13
votes
Does a merging massive binary black hole ‘emits’ more than one gravitational wave?
We can currently only detect gravitational radiation when it is extremely intense: in the last fraction of a second. For example the first gravitational wave detection lasted less 0.15 seconds. The ...
- 127k
13
votes
Accepted
Is there a closest free-return trajectory to a black hole?
The trajectory of a ballistic body, whether in Newtonian or Relativistic physics depends on the initial energy and angular momentum.
The difference is that in Newtonian physics, if the mass is compact ...
- 158k
12
votes
Do tidal forces on moons cause them to emit gravitational waves?
Yes, but the effect is tiny
Tim Rias (in a comment below) calculates it to be $10^{-18}$ Watts. (see a paper about doing a similar calculation for neutron stars)
Bodies that have spherically ...
- 127k
12
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Concerning a binary system of stars/planets/black holes could one of them be ejected before eventually merging or colliding?
Not in Newtonian gravity with particles. This situation is soluble with stable elliptical orbits, so any examples would have to depend on either Relativity, or that the bodies are not particles.
If ...
- 127k
12
votes
Accepted
Newtonian vs General Relativistic light deflection angle
The $R$ in the formula is the closest approach of the light ray to the centre of the Sun. If a star were seen right at the limb of the Sun, then $R \simeq R_\odot$ and the GR deflection angle would be ...
- 158k
11
votes
Can gravitational waves pass through a black hole?
Gravitational waves should be lensed by massive objects in a very similar way to light.
Light rays (and by extension, gravitational waves) from a distant object, that pass within 1.5 times the ...
- 158k
11
votes
Is there anything different about the gravitation around a non-spinning black hole and a neutron star of the same mass?
Technically, there would be minute differences due to the gravitational field generated by the mass of the spacecraft tidally deforming the neutron star leading a small response in the gravitational ...
- 2,740
11
votes
Accepted
Why is the ring in this simulation of Sgr A* off center?
The photon ring around a non-spinning Schwarzschild black hole is perfectly circular and centered on the black hole.
The photon ring around a spinning Kerr black hole is almost circular (except for ...
- 158k
10
votes
Accepted
Confused about rubber sheet analogy!
The rubber sheet only is not meant to be a qualitative model, it gives one concept and one concept only: Mass causes curvature of spacetime.
You can't get any more than that from the rubber sheet. ...
- 127k
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