141 votes
Accepted

Would touching a black hole of a small mass (the mass of an apple) cause you to spiral in and get dead?

This is just a quick calculation that shows what would happen to a black hole with a mass equal to the mass of an apple: It is shown that not only the builder of this black hole but also the whole ...
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  • 4,408
130 votes
Accepted

Is there such thing as imaginary time dilation?

Nice discovery! The formula for time dilation outside a spherical body is $$\tau = t\sqrt{1-\frac{2GM}{c^2r}}$$ where $\tau$ is the proper time as measured by your object at coordinate radius $r$, $...
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  • 26k
122 votes
Accepted

What does it mean when people say "Physics break down"?

"Physics breaks down" is a bad way of saying what people are trying to say. It's the sort of thing that sounds cool at first, but then it starts misleading people. What scientists mean is &...
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101 votes
Accepted

Why are planets not crushed by gravity?

In very simple terms which I hope you will understand. The gravitational force of attraction depends on mass and distance. For the atoms which make up the Earth there are two forces acting on them, ...
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  • 79.7k
77 votes

Why is matter-antimatter asymmetry surprising, if asymmetry can be generated by a random walk in which particles go into black holes?

Congratulations on finding a method for baryogenesis that works! Indeed, it's true that if you have a bunch of black holes, then by random chance you'll get an imbalance. And this imbalance will ...
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  • 95.5k
77 votes
Accepted

Can you exit the event horizon with a rocket?

It is often said that the escape velocity at the event horizon is the speed of light, but while this is true in a sense it is not very useful. The problem is that the speed is an observer dependent ...
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74 votes
Accepted

Thought experiment - would you notice if you fell into a black hole?

This is a great question, because it's a subtle variation on the usual question about spaghettification and supermassive black holes, and shows somewhat deeper thinking. So let's assume the black ...
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  • 14.8k
74 votes
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If the speed of light is constant, why can't it escape a black hole?

The speed $c$ that is constant is so when measured locally relative to a freefalling frame (i.e. one for which all points follow spacefime geodesics wrt to the metric $g$). Local means that the frame'...
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62 votes
Accepted

Indirectly breaking the speed of light

A black hole of mass $10^{36}$ kg would have a Schwarzschild radius (the distance from the center to the event horizon) of about $1.5\times 10^{9}$ m. So your choice of "orbital" distance is ...
57 votes
Accepted

Why don't merging black holes disprove the no-hair theorem?

No. The no-hair conjecture applies to stable solutions of the Einstein-Maxwell equations. In the case of merging black holes, it applies to the end state of the merger into a single quiescent black ...
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  • 50k
57 votes
Accepted

Do "almost black holes" exist?

The conformal limit For simplicity, consider non-rotating compact objects. A non-rotating object with mass $M$ becomes a black hole when its radius $R$ is $$ R = 2\frac{GM}{c^2} \tag{1} $$ where $G$ ...
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56 votes

If iron can’t undergo fusion, does that mean a black hole is mostly iron?

If we are talking about stellar-sized black holes, then the object that collapses to form a black hole will have a high concentration of iron (and other iron-peak elements like manganese, nickel and ...
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  • 113k
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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53 votes

Would touching a black hole of a small mass (the mass of an apple) cause you to spiral in and get dead?

No. A black hole created from something like an apple will still have the gravitational pull of an apple, so it's not going to suck you into it. If you swiped it it would pass through your hand, ...
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  • 6,214
52 votes
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Why did the neutron star merger signal last for so much longer than the black hole merger signals?

It is not that the merger of two neutron stars takes longer, the inspiral and merger of a pair of neutron stars just spends a longer time in the frequency range where LIGO is most sensitive. Let me ...
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  • 8,357
51 votes
Accepted

Why would a black hole explode?

The expression for the power emitted as Hawking radiation is $$ P = \frac{\hbar c^6}{15360 \pi G^2 M^2} = 3.6\times10^{32} M^{-2}\ \text{W} = -c^2 \frac{dM}{dt},$$ where the term on the far right hand ...
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  • 113k
50 votes
Accepted

Why was M87 targeted for the Event Horizon Telescope instead of Sagittarius A*?

Of course they targeted Sgr A* as well. I think though that this is a more difficult target to get good images of. The black hole is about 1500 times less massive than in M87, but is about 2000 times ...
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  • 113k
50 votes

Can an object tethered to a spaceship be pulled out of event horizon?

In General Relativity, no amount of force, exerted through a tether or in any other way, can extract an object from the interior of a black hole. There are no “tricks” to get around this fact, any ...
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  • 50k
50 votes

What does it mean when people say "Physics break down"?

Let me give an example of a very, very mild case of 'theory breaks down'. Boyle's law is stated as follows: $$ P_1V_1 = P_2V_2 $$ Expressing that for a given quantity of gas the pressure and volume ...
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  • 15.8k
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
Accepted

Can a neutron star become a black hole via cooling?

No (or at least not much). One of the essential properties of stars that are largely supported by degeneracy pressure, is that this pressure is independent of temperature and that is because although ...
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  • 113k
49 votes
Accepted

What happens to an astronaut (with a long rope trailing behind him), when he crosses the event horizon of a black hole?

Dale's answer is correct, but I want to further emphasize that nothing special happens in the vicinity of an event horizon. It's just like any other region of spacetime. Here's an analogy. Suppose you'...
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  • 18k
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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48 votes
Accepted

Are black holes very dense matter or empty?

The phrase black hole tends to be used without specifying exactly what it means, and defining exactly what you mean is important to answer your question. The archetypal black hole is a mathematical ...
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48 votes
Accepted

Why is the information paradox restricted to black holes?

(The answers by Mark H and B.fox were posted while this one was being written. This answer says the same thing in different words, but I went ahead and posted it anyway because sometimes saying the ...
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47 votes
Accepted

What is the evidence for a supermassive black hole at the center of Milky Way?

Black holes cannot be seen because they do not emit visible light or any electromagnetic radiation. This is not absolutely correct in the sense that visible light is emitted during the capture of ...
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