B What are the biggest misconceptions about black holes?

[Gregoriorosso]

I am surprised by many statements about bh - let’s take density.
In my normal life, it is defined as ρ = mass/volume
but what’s the volume of a bh ?
I guess
4/3 π R^3 ?
as R = is 2GM/c^2 it seems to me that ρ = 3c^6/32πGM^2
A bh with 10^9 solar masses (that is a large one, but not the largest)
has ρ = 8,15E-20 kg/cubic meter = the density of a good vacuum with 50 millions of hydrogen atoms / cubic meter.
Hundred times the density of the interstellar space.

 

[Ken G]

I am surprised by many statements about bh - let’s take density.
In my normal life, it is defined as ρ = mass/volume
but what’s the volume of a bh ?
I guess
4/3 π R^3 ?
as R = is 2GM/c^2 it seems to me that ρ = 3c^6/32πGM^2
A bh with 10^9 solar masses (that is a large one, but not the largest)
has ρ = 8,15E-20 kg/cubic meter = the density of a good vacuum with 50 millions of hydrogen atoms / cubic meter.
Hundred times the density of the interstellar space.

You are finding the density as if the mass were spread out throughout the region inside the event horizon. But general relativity says that the mass will not be so spread out-- everything inside the EH must fall to the center. We have no theory to say what the density at the center would be, if we do not believe it would be what GR says (infinite).

 

[Intresting]

Maybe a misconception is that nothing can escape a BH. If a photon or atom can be at two places at once http://bigthink.com/dr-kakus-universe/nobel-prize-awarded-to-two-quantum-physicists So in theory an atom could be inside the event horizon but also not have passed the event horizon. Proving it though would need some thinking.

 

[Vanadium 50]

That popularization is not correct. A particle can have an undefined position, but it can not be in two places at once.

 

[Intresting]

Well it has been proven that a particle can be at two places at once. Link for reference http://www.reuters.com/article/us-nobel-physics-quantum-idUSBRE8980V620121009

 

[Droidriven]

Rather, black holes are extremely dense with matter, which causes them to have such a massive gravitational force.

It isn't their density that gives them their massive gravity, it is their mass, they can be any size(in theory) but they are not as dense as commonly believed, immense density is another misconception. There "may" be areas such as the EH or the proposed singularity that are extremely dense but that density is not consistent through the entire mass. Regions of density with emptiness between the dense regions.

 

[Vanadium 50]

Well it has been proven that a particle can be at two places at once.

No, it has not. These are popularizations playing fast and loose with the truth.

 

[Droidriven]

Well it has been proven that a particle can be at two places at once. Link for reference http://www.reuters.com/article/us-nobel-physics-quantum-idUSBRE8980V620121009

The quantum superposition theory doesn't necessarily state that a particle can be in two places at once, the theory goes as far as to say that a particle can be in an infinite number of positions or any given point at any given time. There is no proof of this though. Only experiments that seem to provide evidence, still theory.

But even that is based on the conditions involved in observing the particle or even the observer themselves. In other words, what is observed by an individual under a certain set of circumstances at a given time, may not be what is observed by another individual in the same circumstances at the same time. No consistency, no measurability, only the observance of the event, which can change from moment to moment or observer to observer.

At least, that is my understanding of it.

 

[Gregoriorosso]

You are finding the density as if the mass were spread out throughout the region inside the event horizon. But general relativity says that the mass will not be so spread out-- everything inside the EH must fall to the center. We have no theory to say what the density at the center would be, if we do not believe it would be what GR says (infinite).

They told me that the density of water is approximately 1 kg/liter. That was long time ago, when I didn’t know the density of the atomic nuclei of oxygen or hydrogen. Some years later I was informed that the density of water is much lower than the density of atomic nuclei, but the density of water at STP (should I add standard terrestrial space-time ?) is still 1 kg/liter.

 

[PAllen]

It isn't their density that gives them their massive gravity, it is their mass, they can be any size(in theory) but they are not as dense as commonly believed, immense density is another misconception. There "may" be areas such as the EH or the proposed singularity that are extremely dense but that density is not consistent through the entire mass. Regions of density with emptiness between the dense regions.

Per classical GR, the event horizon has exactly zero density for a BH more than a millisecond old, and soon the near central density approaches infinite. Of the hypothetical quantum gravity approaches, none that I know of posits any unusually high density at or near the horizon. For example, the firewall hypothesis has high temperature at the horizon, not high density. Generally, what these post GR theories propose is that you have 'near vacuum' outside some ball of finite radius (which may be inside or approximately at the horizon). What the density (gradient) of this ball is, is not well predicted by these (unverified) theories.

 

[Intresting]

The quantum superposition theory doesn't necessarily state that a particle can be in two places at once, the theory goes as far as to say that a particle can be in an infinite number of positions or any given point at any given time. There is no proof of this though. Only experiments that seem to provide evidence, still theory.

But even that is based on the conditions involved in observing the particle or even the observer themselves. In other words, what is observed by an individual under a certain set of circumstances at a given time, may not be what is observed by another individual in the same circumstances at the same time. No consistency, no measurability, only the observance of the event, which can change from moment to moment or observer to observer.

At least, that is my understanding of it.

A lot of our understanding on black holes are theory. Hawkins Radiation theorises that particles can escape. Does anyone find it interesting that a BH will evaporate over a large scale of time yet time stands still within a BH, so if time stands still within a BH how can a BH evaporate. The maths needed to prove any of it still needs to be done. I like to keep an open mind on theories unless, proven wrong beyond a doubt. An open mind let's in knowledge as opposed to a closed mind.

 

[rootone]

...if time stands still within a BH...

Does it?

 

[Intresting]

Does it?

Read this article http://www.space.com/10702-black-hole-kerr-state-spacetime.html

 

[Droidriven]

Read this article http://www.space.com/10702-black-hole-kerr-state-spacetime.html

That article only confirmed what I was going to say.

It states that time "in effect" stands still, not that it actually stands still. Note that it states that nothing in Kerr spacetime changes "over time", it is stationary, nothing moves, nothing changes but time keeps on. If it didn't, then there wouldn't be such a thing as "over time" in Kerr spacetime, time continues, it is just different somehow. And then only when the Black Hole has reached Kerr state.

 

[Intresting]

That article only confirmed what I was going to say.

It states that time "in effect" stands still, not that it actually stands still. Note that it states that nothing in Kerr spacetime changes "over time", it is stationary, nothing moves, nothing changes but time keeps on. If it didn't, then there wouldn't be such a thing as "over time" in Kerr spacetime, time continues, it is just different somehow. And then only when the Black Hole has reached Kerr state.

Thats a given that not all BH's are super massive like BH's at the center of galaxies or mature BH's. As micro BH's disappear as instantaneously as it appears, in theory at least. I should have worded my opinion more carefully and assumed that you would get the gist off it.
"It states that time "in effect" stands still" can be viewed as semantics to suit an ideology. . http://hubblesite.org/explore_astronomy/black_holes/encyc_mod3_q15.html the link is one way of looking at it. If you where at point B it would seem to be at a normal time but from point A, point B has in effect stopped. so from point A, point B should not evaporate overtime as in effect it is stationery, since we do not have physics to describe the inside of black holes, the official answer is “we don’t know” and can only speculate.

 

[phinds]

If you where at point B it would seem to be at a normal time but from point A, point B has in effect stopped. so from point A, point B should not evaporate overtime ...

This is not correct. Just because A sees what he knows is an illusion, that doesn't mean the evaporation doesn't happen.

 

[Intresting]

This is not correct. Just because A sees what he knows is an illusion, that doesn't mean the evaporation doesn't happen.

So a super massive black hole distorting space time to breaking point is also an illusion?

 

[phinds]

So a super massive black hole distorting space time to breaking point is also an illusion?

No, and I can't imagine why you conflate the two.

 

[ProfuselyQuarky]

It isn't their density that gives them their massive gravity, it is their mass, they can be any size(in theory) but they are not as dense as commonly believed, immense density is another misconception. There "may" be areas such as the EH or the proposed singularity that are extremely dense but that density is not consistent through the entire mass. Regions of density with emptiness between the dense regions.

With density comes mass . . .

 

[Droidriven]

With density comes mass . . .

Of course, and with mass, comes gravity.

But that is relative as well. Two objects of equal mass with one being denser would from a distance equally pull on each other, but surface gravity acceleration would cause the denser object to have a higher gravity at its surface, a difference in in local gravity only, not its gravity as a whole(so to speak).

 

[Droidriven]

So a super massive black hole distorting space time to breaking point is also an illusion?

The Black Hole isn't an illusion but what you observe of said Black Hole in undistorted spacetime is an illusion because what you see and what is happening are relatively different.

 

[phinds]

Of course, and with mass, comes gravity.

Tell me about it. My gut is now attracting items from around the room. I've GOT to cut down on that coke.

 

[ProfuselyQuarky]

Tell me about it. My gut is now attracting items from around the room. I've GOT to cut down on that coke.

So maybe you should go to a different room where, rather than Coke, the veggies+plain water are the ones that gravitate towards you.

 

[phinds]

So maybe you should go to a different room where, rather than Coke, the veggies+plain water are the ones that gravitate towards you.

Yeah, but "should" and "will" are, sadly, different.

 

[PAllen]

Thats a given that not all BH's are super massive like BH's at the center of galaxies or mature BH's. As micro BH's disappear as instantaneously as it appears, in theory at least. I should have worded my opinion more carefully and assumed that you would get the gist off it.
"It states that time "in effect" stands still" can be viewed as semantics to suit an ideology. . http://hubblesite.org/explore_astronomy/black_holes/encyc_mod3_q15.html the link is one way of looking at it. If you where at point B it would seem to be at a normal time but from point A, point B has in effect stopped. so from point A, point B should not evaporate overtime as in effect it is stationery, since we do not have physics to describe the inside of black holes, the official answer is “we don’t know” and can only speculate.

You should be aware that there are classical GR solutions describing formation, then shrinkage of a BH with true horizon that forms and ultimately vanishes. These are not exactly equivalent to Hawking radiation; they are a classical analog. For example, glue the Oppenheimer-Sneider collapse some time past horizon formation to a matched outgoing Vaidya solution. The result is as I described. A somewhat different approach is presented in: https://arxiv.org/abs/gr-qc/0506126, which can readily be taken as a clasical model of a forming, then evaporating BH (where, in some sense, the final evaporation prevents singularity formation). The classically unconventional feature is the that outgoing vaidya metric violates all the 'energy condition', but it is well known that Hawkging radiation must, as well - so this is good feature of classical analog model.

 

[1oldman2]

While this link deals with gravitational waves as much as BH's I found the merger simulation fascinating, Hope this doesn't stray from the topic too much.

 

[Intresting]

Under the classical theory of general relativity, once a black hole is created, it will last forever since nothing can escape it. However, if quantum mechanics is also considered, it turns out that all black holes will eventually evaporate as they slowly leak Hawking radiation. We just don't know.
A fun doco to watch

 

[PAllen]

Under the classical theory of general relativity, once a black hole is created, it will last forever since nothing can escape it. However, if quantum mechanics is also considered, it turns out that all black holes will eventually evaporate as they slowly leak Hawking radiation. We just don't know.
A fun doco to watch

It partly depends on what you mean by classical general relativity. If you choose this to include energy conditions (e.g. the dominant energy condition), this is true (and I often consider this a definition of 'truly classical GR'). However, if there were such a thing as classical scalar fields, the energy conditions are violated, and you could have purely classical evaporation of BH as I indicated in my prior post.

 

[haael]

One misconception I have seen in movies is that event horizon is some actual surface or membrane, usually one-way. And when you are inside the hole, you can no longer feel gravity, as if you were trapped in a bubble. And it's the horizon that prevents you from escaping, like a solid wall. And that the "horizon" may be eventually broken given enough power or determination and you can escape through the crack in the horizon.

 

[Flyx]

One misconception I have seen in movies is that event horizon is some actual surface or membrane, usually one-way. And when you are inside the hole, you can no longer feel gravity, as if you were trapped in a bubble. And it's the horizon that prevents you from escaping, like a solid wall. And that the "horizon" may be eventually broken given enough power or determination and you can escape through the crack in the horizon.

I've never heard this before?