# Are black holes, real holes?

#### eirhnh

Hello to all physics community!

I have a question concerning black holes. Are black holes, real holes?
And which is the difference between black holes and wormholes?

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#### Drakkith

Staff Emeritus
2018 Award
Re: Black holes

It is difficult to explain unless you know a few basic concepts, such as what Mass is. As such, it is not a "hole" exactly, but a very massive object who's gravity pulls things in if they get too close, even light. The effect is almost exactly like you being pulled down to Earth all the time, but the strength is much much greater. The details can all be found in the 2 articles Joncon linked above.

#### Cbray

Re: Black holes

It is difficult to explain unless you know a few basic concepts, such as what Mass is. As such, it is not a "hole" exactly, but a very massive object who's gravity pulls things in if they get too close, even light. The effect is almost exactly like you being pulled down to Earth all the time, but the strength is much much greater. The details can all be found in the 2 articles Joncon linked above.
Quick question - That it can pull light in, and because light bends due to gravity, does that mean a black hole would have to have an infinity depth?

#### eirhnh

Re: Black holes

Thank you all for your replies :D

#### Drakkith

Staff Emeritus
2018 Award
Re: Black holes

Quick question - That it can pull light in, and because light bends due to gravity, does that mean a black hole would have to have an infinity depth?
I'm not sure honestly. I don't really know what you mean by "infinity depth".

#### Whateverworks

I am not a expert on black holes but I think I have an answer - please correct me if wrong.

A black hole is not a hole in a 2-dimensional surface which is the most common use of a hole. A black hole is a 3-dimensional hole. Because if its gravitation it pulls everything down into the hole and nothing escapes not even light which has an insane speed at 3x10^5 km/s. The actual content of a black hole has collapse to what we call the singularity that's where in Einsteins theory we divide by zero. So Einsteins theory fail at the center of a black hole - that's why we know there's something beyond Einsteins theory's. The region of space around it with in which where you fall you will never come back out its called the event horizon (EH). So when you think of the size of a black hole it is not the matter that occupies it, it's just the region of space where if you cross that boundary you will never come out because the fabric of the universe have curved back on itself and there is no trajectory you can take to come out of that region. So you can say that a black hole has a infinity "depth".

Hope I am correct !

/WeW

#### Drakkith

Staff Emeritus
2018 Award
That looks good to me WeW.

#### WannabeNewton

I guess you can call it a hole because a singularity is something that is not part of the 4 - manifold that represents space - time.

#### Nabeshin

Because if its gravitation it pulls everything down into the hole and nothing escapes not even light which has an insane speed at 3x10^5 km/s.
Not sure of your intent here, but this is the kind of rhetoric which often leads people to the notion that black holes 'suck in' everything surrounding them. In reality, far away from the black hole the gravitational field behaves precisely like any other object.

The actual content of a black hole has collapse to what we call the singularity that's where in Einsteins theory we divide by zero. So Einsteins theory fail at the center of a black hole - that's why we know there's something beyond Einsteins theory's.
I definitely wouldn't go so far as to say that the existence of singularities implies there is a theory beyond GR. It certainly suggests that something is wrong, and since we know GR is not a quantum theory that's the most promising avenue to explore, but it does not imply. It could be the case that singularities actually exist, but are always hidden behind event horizons (cosmic censorship hypothesis).

#### Gestahl

No they're not "holes" but appeared to express the properties of a hole when they were first being discovered. They are just a very large amount of matter collapsing on a relatively small point in space due to the phenomena of gravity. Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature through stellar nucleosynthesis, or because a star that would have been stable receives extra matter in a way that does not raise its core temperature. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight.

If the Sun were to collapse and form a black hole, the Schwarzschild radius (event horizon) would be ~3 km. If Earth were to collapse and form a black hole, the Schwarzschild radius would be a mere 2 cm! Now keep in mind that in both of these examples, the black holes are keeping the same mass as the Sun and Earth respectively. So looking only at the effects of gravity alone, if both Sun and Earth were to collapse into black holes it would have no effect on the solar system (again, gravitationally speaking). The moon would continue to orbit what is now a 2cm size of seemingly invisible mass along with the remainder of the solar system.

#### Zentrails

Re: Black holes

Quick question - That it can pull light in, and because light bends due to gravity, does that mean a black hole would have to have an infinity depth?
The GR equations predict black holes to be of either infinite volume or infinite density, both of which are obvious absurdities. In other words, the theory really can't be used to describe what a black hole is like, regardless of what nonsense you may read in this forum to the contrary.

At a suitable distance, a black hole's physics is not distinguishable from any other massive object, including (according to Hawking) radiating exactly like any other black body radiator.

#### Lord Challen

Einsteins theory we divide by zero.
/WeW
Einstein had a theory about dividing by Zero? Really? And I thought I was sort of out on a limb with that idea. Now I don't feel so stupid. I've been saying that for years and people just stop listening.

Cool. Thanks for that little nugget.

#### Drakkith

Staff Emeritus
2018 Award
Einstein had a theory about dividing by Zero? Really? And I thought I was sort of out on a limb with that idea. Now I don't feel so stupid. I've been saying that for years and people just stop listening.

Cool. Thanks for that little nugget.
Yeah. His theory was that his theory failed to apply at the singularity because you had to divide by 0.

#### CosmicEye

Hang on, what is GR if I can ask.

The reason why it is called a black 'hole' is becuase since it swallows light. Theoretically if we could actually see it, it would just be black because no light is coming from it, thus looking like a hole in space.

Personally I dont believe in wormholes, atleast in a BH. How could you go somewhere else in the U if you reach the center if you couldnt escape it in either direction. Also Infinite density is stupid because infinity is not a actual number. If it swallows stars then it gets that much more mass. Even if its 10^100 kg/m^3, its still not 'infinite'. Infinite depth is mind blowing too.

BH are very strange things and seem to void the current laws of physics. How fun.

#### Drakkith

Staff Emeritus
2018 Award
Hang on, what is GR if I can ask.

The reason why it is called a black 'hole' is becuase since it swallows light. Theoretically if we could actually see it, it would just be black because no light is coming from it, thus looking like a hole in space.

Personally I dont believe in wormholes, atleast in a BH. How could you go somewhere else in the U if you reach the center if you couldnt escape it in either direction. Also Infinite density is stupid because infinity is not a actual number. If it swallows stars then it gets that much more mass. Even if its 10^100 kg/m^3, its still not 'infinite'. Infinite depth is mind blowing too.

BH are very strange things and seem to void the current laws of physics. How fun.
GR is General Relativity.

And "infinite" density is used because once you reach the point that a black hole forms from matter, the force is SO strong that it overcomes all repulsive effects and the matter collapses in on itself. As it collapses the mass is increased in density which then increases the force even more. It turns into a runaway effect of increasing density making increased attraction which makes even more density and so forth and so forth. At least I think that is correct.

#### CosmicEye

What do you mean matter collapses in on itself, like the electrons impact its nucleus in the atom?

#### Drakkith

Staff Emeritus
2018 Award
What do you mean matter collapses in on itself, like the electrons impact its nucleus in the atom?
When a star collapses the force of gravity causes wierd things to happen.

For stars that are of similar mass to the Sun, they simply use up their fuel, stop producing energy in their cores and shrink down to White Dwarfs. A white dwarf is not held up by the gas pressure of a normal star, but of electron degeneracy pressure. This means that the atoms in the star are so close that the only thing keeping them from getting closer together is the fact that only 2 electrons can occupy the same point in space at the same time, and both need to have opposite spins.

For a star that collapses into a Neutron Star, gravity makes it more favorable for the Electrons to be absorbed into Protons to turn both into a Neutron than for the degeneracy pressure to keep holding up the core. Normally a neutron will decay into a Proton, Electron, and anti-neutrino within about 15 minutes (the half life of a neutron) unless it is bound inside a nucleus. The extreme gravity acts similar to the strong force inside a nucleus, as it makes it more favorable for the Neutron NOT to decay. So at this point the star is held up by Neutron degeneracy pressure, as Neutrons are fermions (as are Electrons) and also obey the rule that they can't occupy the same spot as other Neutrons.

Past a Neutron star, there are hypothesized "exotic" stars, such as a Quark star. It is hypothesized that when the mass of a star is higher than a certain amount, instead of collapsing into a Neutron star, gravity causes the Neutrons to decompose into their individual Quarks and for the core to be held up by Quark degeneracy pressure.

That is what I mean by collapse.

#### dcorbett

"Black holes", physically, are extremely dense masses. How can one possibly fall into a "Black hole" ? Can one practice falling into a black hole by jumping into a pavement from a 10 storey building?

#### Chronos

Gold Member
A black hole merely represents the point where our theories fail.

#### alexg

The Black Hole is the event horizon. What's inside of that is unknown.

#### alexg

John Wheeler first came up with the term Black Hole in 1967. The French were reluctant to use the term because apparently in french Black Hole has nasty sexual connotations.

#### Drakkith

Staff Emeritus
2018 Award
"Black holes", physically, are extremely dense masses. How can one possibly fall into a "Black hole" ? Can one practice falling into a black hole by jumping into a pavement from a 10 storey building?
Unlikely. Whatever might be behind the event horizon, you'd still have to make it that far. Jumping from a building doesn't rip you to shreds in either the accretion disk or from the gravitational force.

#### SHISHKABOB

I guess it depends, then, on what we mean by "black hole"

if we mean the event horizon, then those exist, I'm pretty sure, because we have observed them

but then if we mean the singularity, then we have this problem?

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