# The Naked Scientists Forum

### Author Topic: Do things fall into or onto a black hole?  (Read 369 times)

#### Lord Antares

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##### Do things fall into or onto a black hole?
« on: 21/10/2016 11:36:52 »
I always hear people talking about object falling into the black hole, possibly going through a wormhole and exiting in another universe etc.

But isn't this fundamentally impossible? Black holes are completely solid, comprised of matter. Falling ''into'' a black hole is worse than saying falling into the earth from an airplane because the black hole is considerably more dense.

(And this is another thing. I'm supposed to say the black hole is infinitely more dense than the earth because it has infinite density. In principle, this is very easy to understand because the space inside it is completely filled up with 0 cracks inside, therefore infinite density; But isn't this mathematically unacceptable since there should be such a thing as ''the largest possible value before infinity'' in this case if I'm not mistaken)

Therefore, wouldn't something fall ONTO a black hole? I am aware that it would get shredded to bits before it could even reach it, but we're speaking hypothetically, of course. This is why wormholes never made sense to me. It would be pretty funny if in xxxx years we could come up with such an advanced technology, that we would be able to send a spacecraft which resists all of the black hole's forces to try to enter it, and the spacecraft just slammed into the black hole, killing everyone inside.

I would be grateful if someone more knowledgeable corrected me :)
« Last Edit: 24/10/2016 23:03:00 by chris »

#### Janus

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##### Re: Falling ''into'' a black hole
« Reply #1 on: 21/10/2016 17:55:20 »
I always hear people talking about object falling into the black hole, possibly going through a wormhole and exiting in another universe etc.

But isn't this fundamentally impossible? Black holes are completely solid, comprised of matter. Falling ''into'' a black hole is worse than saying falling into the earth from an airplane because the black hole is considerably more dense.

(And this is another thing. I'm supposed to say the black hole is infinitely more dense than the earth because it has infinite density. In principle, this is very easy to understand because the space inside it is completely filled up with 0 cracks inside, therefore infinite density; But isn't this mathematically unacceptable since there should be such a thing as ''the largest possible value before infinity'' in this case if I'm not mistaken)

Therefore, wouldn't something fall ONTO a black hole? I am aware that it would get shredded to bits before it could even reach it, but we're speaking hypothetically, of course. This is why wormholes never made sense to me. It would be pretty funny if in xxxx years we could come up with such an advanced technology, that we would be able to send a spacecraft which resists all of the black hole's forces to try to enter it, and the spacecraft just slammed into the black hole, killing everyone inside.

I would be grateful if someone more knowledgeable corrected me :)

The only place "infinite density" could exist, would be at the singularity, which, for a non-rotating black hole is a mathematical point at its center. (and there is some debate as to whether such a singularity does actually reside there.)     What we typically consider the "surface" of a black hole is the event horizon, which is not a physical object at all, but rather is a boundary that defines where the escape velocity equals that of the speed of light.

A black hole isn't a solid object, it is more of a region of extremely curved space-time, with the singularity being a point of infinite curvature.

Now you may have noted that I said that for a non-rotating black hole, the singularity would be a point.  In a rotating black hole however, the singularity would be ring-shaped.  In such a case, it could be possible to fall into the black hole, and pass through the ring while avoiding the region of infinite curvature.  It is passing through the center of this ring singularity which is theorized could result in your traversing a wormhole.

#### evan_au

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##### Re: Falling ''into'' a black hole
« Reply #2 on: 21/10/2016 19:12:30 »
Quote from: Lord Antares
I'm supposed to say the black hole is infinitely more dense than the earth because it has infinite density.
If you measure the mass of a black hole (by the orbits of stars orbiting around it), you will find that it is finite.
And if you measure the diameter of the event horizon, you will find that it is also finite, and non-zero.
Since density = Mass/Volume, the effective density of a black hole is finite.

As Janus says, the singularity at the center of a black hole is assumed to have infinite density. This led Einstein to doubt the existence of black holes for some time, because to him, if there is an actual infinity, there is an error in your calculation.

Quote
(anything) would get shredded to bits before it could even reach (a black hole)
This is true of stellar-mass black holes; the tidal forces are so great that you would be pulled apart (the technical term is "spaghettification").

However, for a supermassive black hole, such as the one assumed to exist at the center of our galaxy, the tidal forces at the event horizon are rather gentler, and an astronaut could safely reach the event horizon intact. But they would be traveling at such a high speed that if they blinked they would miss the whole event.

See: https://en.wikipedia.org/wiki/Spaghettification
https://en.wikipedia.org/wiki/Sagittarius_A*#Central_black_hole

#### Lord Antares

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##### Re: Falling ''into'' a black hole
« Reply #3 on: 21/10/2016 20:36:02 »

The only place "infinite density" could exist, would be at the singularity, which, for a non-rotating black hole is a mathematical point at its center.

why would it be a singularity (i.e. of infinitesimal size/smallest possible size) if mathematically, to get the same density you could just take the volume of all the atoms that make up the ''singularity'' and arrange them in a sphere. I don't mean in a way that a normal object is made, constructed by atoms side by side; I mean to make one consistent object/sphere whose volume would be exactly equal to the volume of all the atoms it contains. Surely, following the same logic, this would result in infinite density, or maximum density if you will. Wouldn't this result in a black hole as well, rather than having to somehow shrink beyond that into a singularity?

However, this assumes that atoms are incompressible. If they were, this would be undeniably true. So by infinite density, I meant that it's the smallest possible shape containing normal density and volume of all the atoms, but in the meantime I found out that atoms are in fact compressible and do not have a ''volume'' as I thought they did. Is this true? This goes beyond my knowledge so I can't comment on that.

By the way, if such a thing as Planck length exists, doesn't that contradict the notion of singularity since it would be smaller than that?
It's very hard to believe its existence as its value would, in theory, be ''the first number after zero'', which is mathematically inexplicable.

#### evan_au

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##### Re: Falling ''into'' a black hole
« Reply #4 on: 22/10/2016 01:34:10 »
Quote from: Lord Antares
in the meantime I found out that atoms are in fact compressible and do not have a ''volume'' as I thought they did. Is this true?
Atoms are mostly empty space, with a very small and very dense nucleus at the center.

Atoms are slightly compressible, but this is resisted by the electrons of the atoms repelling each other, and by the quantum nature of electron orbitals.

But in the enormous pressures found within a supernova explosion, atoms can be crushed to the density of a nucleus. In this case, they are no longer atoms, since there are no separate protons and electrons, but they are combined into neutrons (with the emission of a neutrino), forming a neutron star.

The neutron star has far higher density than any arrangement of atoms.

With a sufficiently large star undergoing a supernova, the gravity is so great that it will collapse past the neutron star stage into a black hole.

#### Lord Antares

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##### Re: Falling ''into'' a black hole
« Reply #5 on: 22/10/2016 09:57:58 »

With a sufficiently large star undergoing a supernova, the gravity is so great that it will collapse past the neutron star stage into a black hole.

Yes but then atoms ARE compressible beyond the neutron level, no? You are just saying they are.
By the way, is this a proven fact? Because, as I understand it, a singularity isn't a universally accepted fact. If it isn't, I choose not to accept it because it breaks the laws of physics.

And the alternative is easy. If there is no such thing as a singularity, then the black hole has a size and it is equal to the sum of sizes of the smallest possible units within (neutrons as you say).

#### yor_on

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##### Re: Falling ''into'' a black hole
« Reply #6 on: 24/10/2016 21:37:49 »
No, it doesn't break any laws by becoming a singularity. To break it, it needs to tell us what it does. And it refuses, Even Hawking radiation becomes 'useless' for knowing what that elusive pimpernel does. It's a shady character.

#### The Naked Scientists Forum

##### Re: Falling ''into'' a black hole
« Reply #6 on: 24/10/2016 21:37:49 »