54 votes
Accepted

Why doesn't a black hole have linear momentum?

In relativity the covariant properties are tensors. The linear momentum you are referring to is a 3-vector and therefore is not covariant. In particular its magnitude is not a scalar invariant and ...
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50 votes

Why would black hole rip me apart?

You don't need GR to see this effect. It's due to tidal forces. Suppose you are 2 meters tall. Then the force of the Earth on your feet is $GMm/r^2$, and the force on your head is $GMm/(r+2)^2$. The ...
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  • 16.9k
49 votes

Where does the kinetic energy of the orbiting black holes go after the merger?

The masses used in the gravitational wave analysis are those that the black holes would have had in their own frames in isolation. The best way to think about this is that the given masses of 36 &...
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43 votes
Accepted

Why would black hole rip me apart?

The problem is that when you are falling, all of you can't be in the same inertial frame. That is, whilst your centre of mass might be inertial, parts of your body will be feeling accelerating forces ...
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  • 113k
35 votes

When an object crosses a black hole event horizon, does the entire object cross the event horizon "all at once?"

These two quotes seem like they contradict each other. Which one is correct? They do contradict. Please be aware that comments cannot be downvoted so they often serve as a haven for content that an ...
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  • 69.1k
34 votes

Why are there three bright spots in the first picture of Sagittarius A*?

According to the imaging data release paper (Akiyama et al. 2022, ApJL, 930, L14), about the only thing that can be reliably taken from this image (which is a kind of time-averaged composite based on $...
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  • 113k
33 votes
Accepted

Shouldn't black holes exert the same gravitational force as an object of similar mass but lower density?

The parameter you're not considering is the distance. The Earth is an object with the mass of the Earth $m_E$ and the radius of the Earth $r_E$ (duh). If you take a black hole with mass $m_E$, then ...
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33 votes
Accepted

Wouldn't Miller's planet be fried by blueshifted radiation?

Miller's world would be fried by a strong flux of extreme ultraviolet (EUV) radiation. The cosmic microwave background (CMB) would be blueshifted by gravitational time dilation and then would be very ...
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  • 113k
30 votes
Accepted

Flywheel half-way through the event horizon of a black hole vs the equivalence principle

since classically no object can escape the black hole once it passes the event horizon, it seems as though the flywheel should break as it passes through the event horizon, because for every piece ...
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  • 69.1k
28 votes

If electrons were just positrons moving backwards in time, then shouldn't we see them coming out of black holes?

The short answer to your question is that positrons are not really electrons moving backward in time, and the premise of your argument doesn't work. However, something like what you are saying, is ...
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  • 35.5k
25 votes
Accepted

Can a Black Hole be moved by lasers?

A laser carries linear momentum, so when a laser is fired into a black hole, this linear momentum is transferred to the black hole, causing it to accelerate. Of course, for a realistic astrophysical ...
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  • 8,357
25 votes
Accepted

When an object crosses a black hole event horizon, does the entire object cross the event horizon "all at once?"

Dale is right. Falling feet-first through an event horizon means the horizon sweeps over you in the feet-to-head direction at the speed of light, which means your feet cross first. The event of your ...
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  • 18k
24 votes
Accepted

How close does a photon have to get to a black hole to do a full loop?

The motion of a photon in a Schwarzschild spacetime is described by $$ \frac{1}{L^2} \dot{r}^2 + V _{\text{eff}} (r) = \frac{1}{b^2}\,, $$ where $$V _{\text{eff}}(r) = \frac{1}{r} \left(1 - \frac{2GM}{...
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22 votes

Why are there three bright spots in the first picture of Sagittarius A*?

I don't think anybody knows. Take details of these images with a great deal of skepticism. Just reconstructing any image from the EHT is extremely difficult. There probably are hot spots running ...
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  • 5,516
21 votes

If the escape velocity at the event horizon is the speed of light does it mean that slower bodies won't move away at all?

Your arguments make sense in pre-relativistic Physics, but they don't apply to actual black holes, which are described by General Relativity. In this answer, I'll try to briefly sketch the difference. ...
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18 votes

Will a neutron star always collapse into a black hole in the future?

Terminology note: the Chandrasekhar limit $M_C \approx 1.4 M_\text{sun}$ is for electron-degenerate matter. The analogous limit for neutron-degenerate matter, $M_\text{TOV} \sim 2.5 M_\text{sun}$, is ...
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  • 74.4k
15 votes

If electrons were just positrons moving backwards in time, then shouldn't we see them coming out of black holes?

If electrons were just positrons moving backwards in time, then shouldn't we see them coming out of black holes? If you time reverse an electron falling into a black hole you don’t get a positron ...
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  • 69.1k
15 votes

Does an accelerating charged black hole emit EM radiation?

Yes, it will radiate. The light does not come from inside the black hole. The accelerated charge of the black hole affects the electromagnetic field around but outside the event horizon. You could ask ...
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  • 35.5k
15 votes

How can black holes evaporate into photons if they contain no anti-matter?

Hawking radiation is not only in the form of photons. Fermions, scalars, other spin-1 particles, and even gravitons also contribute (for example, see https://www.sciencedirect.com/science/article/pii/...
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  • 35.5k
14 votes
Accepted

Can a Kerr black hole be viewed as a Schwarzschild black hole by changing the frame of reference?

No. A coordinate transform cannot change the angular momentum (or mass) of a Kerr black hole. In particular, the Schwarzschild black hole is spherically symmetric. The Kerr black hole is not. By the ...
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13 votes

How intense are gravitational fields, where we have been able to test General Relativity?

Quantifying how strong a gravitational field is, is notoriously tricky. The two main quantities that people use to qualify the strength of the gravitational field probed in an expertiment are: The ...
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  • 8,357
13 votes

How close to a black hole can an object orbit elliptically?

A bound elliptical orbit around a Schwarzschild black hole must have $r > 2 r_s$ at all times (where $r_s = 2 M$ is the Schwarzschild radius). Deriving this result is a good exercise for students ...
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13 votes

Why can't the information inside a black hole be reconstructed from what's left outside?

I'll first give an anecdote exemplifying the issue, and later I'll give the problem in more technical terms. I'm doing it like this because I'm not sure of what pieces of Physics you are acquainted ...
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13 votes

How can primoridal black holes form with less mass than the Chandrasekhar limit?

Primordial black holes were never stars at any stage in their lifecycle, so those mass limits do not apply to them. See this Wikipedia article.
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  • 35.5k
12 votes
Accepted

Question about light orthogonal to an event horizon

If the black hole is of sufficient mass, the light would be pulled back towards the black hole If the light was emitted from outside the event horizon, then the black hole is necessarily not of ...
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  • 34.6k
12 votes
Accepted

Is there any *global* timelike Killing vector in Schwarzschild geometry?

The are only four Killing vectors of Schwarzschild. They are $\partial_t$ and the three rotational Killing vectors. No linear combination of these is globally timelike within the horizon, so there is ...
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12 votes

Time dilation effects at the center of a binary black hole system

Lets suppose that the orbit is very large, so we could apply the linearized theory at the center. The spacetime metric is then $$\tag{1} ds^2 \approx (1 + 2 \phi) \, dt^2 - (1 - 2 \phi) (dx^2 + dy^2 +...
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  • 6,499
12 votes
Accepted

Would our orbit really remain the same if the sun were a black hole of equal mass?

It depends what you mean by 'Would remain the same'. If you are asking about the exact precise orbit, the answer is yes, it would change, since the Schwarzschild (and also Kerr solution of a rotating ...
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  • 676
12 votes

Where does all the energy in black holes go?

Energy inside black holes doesn't "go" anywhere. Energy and mass are the same thing ($E=mc^2$). There's a "no hair" theorem that says that black holes can be completely described ...
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  • 6,214
12 votes
Accepted

Could bound and stable clusters of 6 or 8 neutrons be a good candidate for dark matter?

Let’s grant you the existence of one or more stable isotopes of “neutronium,” with atomic mass six or eight, $\rm^6_0n$ or $\rm^8_0n$. Now we need to re-do our calculations for big-bang ...
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  • 74.4k

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