B What are the biggest misconceptions about black holes?

[43zombiegit]

Hey guys, I'm currently doing a project on black holes and need some input on what you believe to be the biggest misconceptions about black holes, thanks! You can read more about the guidelines of the project at: http://www.aqa.org.uk/subjects/projects/aqa-certificate/EPQ-7993

 

[ProfuselyQuarky]

I personally think that the largest misconception is the fact that black holes do not just suck up everything that comes there way like a giant vacuum. Also, black holes are anything but empty. A lot of people think both these things. Rather, black holes are extremely dense with matter, which causes them to have such a massive gravitational force. I know people who have sometimes said that if the sun turned to a black hole, Earth would be sucked up and we would all die. No, that's not exactly true. If the black hole had the same mass as our sun right now, Earth would still orbit in its same path just as always (of course we would all still die a freezing death, but that's different matter).

 

[43zombiegit]

I personally think that the largest misconception is the fact that black holes do not just suck up everything that comes there way like a giant vacuum. Also, black holes are anything but empty. A lot of people think both these things. Rather, black holes are extremely dense with matter, which causes them to have such a massive gravitational force. I know people who have sometimes said that if the sun turned to a black hole, Earth would be sucked up and we would all die. No, that's not exactly true. If the black hole had the same mass as our sun right now, Earth would still orbit in its same path just as always (of course we would all still die a freezing death, but that's different matter).

Thanks for the input, do you mind if I use your contribution for my project?

 

[ProfuselyQuarky]

Thanks for the input, do you mind if I use your contribution for my project?

Not so sure what you mean by "use your contribution", but sure. None of this is my own information or anything—it’s just what I think to be the biggest misconception. If you do some research, you’ll find much more and more PF members will probably add there own input, as well.

There's a lot you can find on the internet. You just have to look for it.

 

[Chronos]

One of my favorites: a black hole is impossible because an infinite amount of time is required to form an event horizon.

 

[phinds]

Another is that there is a contradiction that since nothing can escape a black hole, it can't have any gravitational effect since gravity could not escape it.

 

[43zombiegit]

Another is that there is a contradiction that since nothing can escape a black hole, it can't have any gravitational effect since gravity could not escape it.

Isn't that just a theory, not a misconception though?

 

[43zombiegit]

One of my favorites: a black hole is impossible because an infinite amount of time is required to form an event horizon.

Can you elaborate? Why does an event horizon take an infinite amount of time to form?

 

[phinds]

Isn't that just a theory, not a misconception though?

I don't even know what you mean about that being a theory.

 

[43zombiegit]

We

I don't even know what you mean about that being a theory.

Well I'm not sure how to say it but is it really a misconception?

 

[ProfuselyQuarky]

Another is that there is a contradiction that since nothing can escape a black hole, it can't have any gravitational effect since gravity could not escape it.

Can you elaborate? Why does an event horizon take an infinite amount of time to form?

Your probably thinking of gravitational pull as something between two objects. Well, if we had one body of mass in open space without any object "nearby", would it still have gravitational force? Read this: https://briankoberlein.com/2015/08/21/how-does-gravity-escape-a-black-hole/

 

[phinds]

Well I'm not sure how to say it but is it really a misconception?

Well, do you think it's CORRECT? It is something that people have come here and asked about so, yes, it definitely IS a misconception.

 

[rootone]

I think one of the biggest misconceptions is that there is some kind of physical object called a 'singularity' at the centre of a black hole.
What the term 'singularity' actually means is that our current best theories are inapplicable for the centre of a black hole.
We don't really have much of a clue what happens to matter there, and attempting to extrapolate from theories which are otherwise sound produces nonsense results.

 

[Chronos]

Remember Bob, Sally and the black hole? Bob volunteers[?] to cross the event horizon while Sally stays on the ship and watches. Bob never makes it, Sally watches as Bob slows to a halt and freezes upon reaching the EH due to time dilation. So, how can a black hole form when infalling matter takes an eternity to cross the EH?

 

[ShayanJ]

black holes are extremely dense with matter

I don't think that's true. For example for a Schwarzschild black hole, the light cones inside the Schwarzschild radius are pointing towards the center. Let's imagine there is some matter at a radius $R<R_S$, then it has to move faster than light in order to stay there, otherwise it will just "hit the singularity" like anything else.
Now if, instead we imagine that there is some sphere of matter concentric with the event horizon, with a radius $R<R_S$, the metric inside it will be different from Schwarzschild's but still continuity of metric requires that the metric on its surface is Schwarzschild's and so still its surface should move faster than light in order to stay there. But we know that can't happen and so we need to accept that the inside of a black hole has to be empty, at least according to GR.

The misconception I want to mention is exactly what triggered the above response. Its actually not that a black hole's gravity is so much stronger than other things and that's the reason for its strange properties. The reason for such properties is the strange causal structure of the spacetime region past the event horizon of black holes.

 

[Vanadium 50]

I think this is not going to work well - a list of misconceptions is not the way to learn something. If someone tried to explain baseball to you by explaining the ground rule double, the difference between interference and obstruction, and the infield fly rule, would you understand how the game is played?

 

[martinbn]

I think one of the biggest misconceptions is that there is some kind of physical object called a 'singularity' at the centre of a black hole.

I agree with this, but the next part

What the term 'singularity' actually means is that our current best theories are inapplicable for the centre of a black hole.
We don't really have much of a clue what happens to matter there, and attempting to extrapolate from theories which are otherwise sound produces nonsense results.

is a bit of a misconception itself.

 

[Ken G]

I think the biggest misconception about black holes is that physicists agree on what they are. I know of no example of mainstream physics literature where the experts in the field disagree more completely than on the topic of what goes on inside an event horizon.

 

[Tom MS]

Hey guys, I'm currently doing a project on black holes and need some input on what you believe to be the biggest misconceptions about black holes, thanks! You can read more about the guidelines of the project at: http://www.aqa.org.uk/subjects/projects/aqa-certificate/EPQ-7993

I think the biggest is that all black holes give off no light. Although they themselves don't, the infalling material can create x-ray radiation and other bursts. Especially with black holes at the center of the galaxy.

 

[nipoleon]

Probably the greatest misconceptions about black holes comes from what people see in movies.
That it's possible to pass through a black hole and come out the other side into another time or place. That black holes have infinite gravity and are like bottomless pits and violate dimensional space.
Black holes do not have infinite gravity, they have enough gravity to not allow light to escape but that's not infinite gravity. An object with infinite gravity would suck in the whole universe.

 

[Demystifier]

Another is that there is a contradiction that since nothing can escape a black hole, it can't have any gravitational effect since gravity could not escape it.

Even though it is a very common misconception, I have never seen a simple explanation of why exactly it is wrong. Two simple possible explanations come to my mind. One is that pure gravity is non-linear, so that the source of outside gravity is outside gravity itself. Another is that the outside gravity is a static remnant of the field created before the creation of horizon. Would you agree with one of those explanations? Or is there a better one?

 

[nipoleon]

Under Newton gravity was a force.
Einstien proved that gravity is an effect. The effect of the bending of space by mass.
We feel gravity because the material of the surface of the Earth prevents us our falling towards the center of the Earths mass.
Everything in the universe follows the curvature of space in 4 dimensions, space and time ( enen light ). The greater the mass of an object, the more pronounced the curve ( or warping ) of space around it.

 

[phinds]

Even though it is a very common misconception, I have never seen a simple explanation of why exactly it is wrong. Two simple possible explanations come to my mind. One is that pure gravity is non-linear, so that the source of outside gravity is outside gravity itself. Another is that the outside gravity is a static remnant of the field created before the creation of horizon. Would you agree with one of those explanations? Or is there a better one?

I agree that the gravitational field of a BH is static and is formed as the BH is being formed so the EH is irrelevant. As more matter moves in then, at every point of its existence, everything farther away from the BH that the matter is sees that particular matter as contributing to the gravity field of the BH and of course this persists even after it enters the EH, so this whole "escapes the EH" is nonsense.

 

[Ken G]

It depends on whether one uses general relativity for gravity or not. Certainly that is our only current theory of gravity that has been verified at a high level of precision, but for some reason most elementary-particle theorists seem to doubt it is a correct description of the situation. They imagine a theory where gravity is carried by a force-carrying virtual graviton. They also imagine that, if one takes the virtual particle picture for the force carrier, virtual particles are not bound by the usual rules (indeed, virtual particles do not need to even propagate outward from the source of the force, they can propagate inward for the case of attractive forces, as gravity is). Of course, we are free to reject the graviton idea if we like, or we are free to reject the virtual particle picture. Every "why" answer we can give must always be in the context of some chosen theory.

 

[BiGyElLoWhAt]

Even though it is a very common misconception, I have never seen a simple explanation of why exactly it is wrong. Two simple possible explanations come to my mind. One is that pure gravity is non-linear, so that the source of outside gravity is outside gravity itself. Another is that the outside gravity is a static remnant of the field created before the creation of horizon. Would you agree with one of those explanations? Or is there a better one?

I like the latter, personally. An outside observer can never observe matter cross the event horizon, so I think we would necessarily have to measure the source of the gravitational field source as approaching the EH. So perhaps not necessarily exactly what you said, in the sense that the field can still be generated, but that the infalling matter "lodges" itself near the event horizon, and this can still happen even after the EH has been created. I'm no expert on GR by any means, but seeing as how both relativities are observer based, I think all of our observations need to be consistent in and of each other. If we "see" matter near the event horizon, we should also measure a gravitational field source near the event horizon.
Perhaps the bulk of the field, i.e. the quantity of matter that caused the creation of the EH, would follow what you were saying. Perhaps that's what you meant. I don't see any reason why it can't increase, however. (by the addition of matter along the EH)
Edited-*poorly worded post is poorly worded*

I agree that the gravitational field of a BH is static and is formed as the BH is being formed so the EH is irrelevant. As more matter moves in then, at every point of its existence, everything farther away from the BH that the matter is sees that particular matter as contributing to the gravity field of the BH and of course this persists even after it enters the EH, so this whole "escapes the EH" is nonsense.

Are the fields necessarily static? I think that's an assumption in the solution of a lot of metrics, as it completely eliminates the dependence of the metric on 1/4 of the coordinates, but I don't see any reason why this should be necessarily for say, a binary black hole system.

 

[phinds]

Are the fields necessarily static?

I really should have said that I think that applies only in the absence of in-falling matter, but I'm no expert on it so even that could be wrong.

 

[anorlunda]

Another is that there is a contradiction that since nothing can escape a black hole, it can't have any gravitational effect since gravity could not escape it.

Isn't that also true with electric charge? If there was a net charge in the BH, the fields outside the EH would be the same as if the object was not a BH? Is that true or a misconception?

 

[BiGyElLoWhAt]

Considering the force carrier of EM is the photon, and light can't escape the BH, I would think yes, but I'm not 100%. Perhaps there's a reason why this isn't the case.

 

[phinds]

Black holes can have a charge:

https://en.wikipedia.org/wiki/Charged_black_hole

 

[Dakota]

That black holes are black, not colorless.

 

[jerromyjon]

a list of misconceptions is not the way to learn something.

I agree, but could this be more of a "forget what you think you know, it is LIKELY to be one of these and this is what is wrong with it..." More like clearing the slate before you start?

 

[jerromyjon]

That black holes are black, not colorless.

Hi and welcome to PF!
REALLY black...

 

[jerromyjon]

are you saying they're black

I'm colorless blind... Black and colorless are the same thing to me... White is all visible colors...

 

[newjerseyrunner]

I'm colorless blind... Black and colorless are the same thing to me... White is all visible colors...

Why would you define white as all visible colors but define black as no colors at all? I would define black as no visible colors and colorless to mean no light at all.

I would use this terminology for myself:
Full spectrum - light in all frequencies at once
White - light in all wavelengths between 390 and 700nm
Black - no wavelengths between 390 and 700nm
Colorless - no wavelengths at in any part of the spectrumMy biggest irk about black hole misconceptions is about micro black holes, that if one were accidentally created in a lab on earth, it'll trigger a runaway expansion and devour the planet.

 

[Vanadium 50]

I agree, but could this be more of a "forget what you think you know, it is LIKELY to be one of these and this is what is wrong with it..." More like clearing the slate before you start?

Would you start teaching baseball by explaining that it's not really one base on an overthrow, it's actually two bases? (The base the runner is going to, plus one)

 

[phinds]

Why would you define white as all visible colors but define black as no colors at all? I would define black as no visible colors and colorless to mean no light at all.
.

Exactly.

 

[jerromyjon]

Black - no wavelengths between 390 and 700nm
Colorless - no wavelengths at in any part of the spectrum

Yes I like it! Another obscure clarification would be that it is NOT "invisible" as in see-thru or cloaked, it's colorless which is blacker than any black around it...

Would you start teaching baseball by explaining that it's not really one base on an overthrow, it's actually two bases? (The base the runner is going to, plus one)

Is that 4 "balls" makes a "walk" and if there is a person on first base they walk to second and so on? I am not a big sports fan but I think I remember the rules from my youth. I'm not sure what you mean by 2 bases? Is that just saying you're going to teach a different "interpretation", if you will pardon the poor analogy, where you describe going from home to first as "two bases"?

 

[jerromyjon]

Why would you define white as all visible colors but define black as no colors at all? I would define black as no visible colors and colorless to mean no light at all.

Really I was going to say white would be all wavelengths from not quite infinite all the way to up to God knows where but I knew that would be over-dramatic and besides the point at hand...

 

[nikkkom]

There is a misconseption that "BHs are invisible because they emit no light (except hawking radiation which is far tooo weak to detect for macroscopic BH)".

It is true that BHs don't emit light. However, that does not make them invisible. Example: a really black cat does not emit any visible light (to the limit of detection by unaided human eye), but no one would say that "black cat is invisible". Black objects (cats or BHs) are easily visible when they obscure other objects. A real invisible object would pass EM radiation through itself (example: glass in water).

 

[virgil1612]

Remember Bob, Sally and the black hole? Bob volunteers[?] to cross the event horizon while Sally stays on the ship and watches. Bob never makes it, Sally watches as Bob slows to a halt and freezes upon reaching the EH due to time dilation. So, how can a black hole form when infalling matter takes an eternity to cross the EH?

Sally never sees Bob getting beyond the EH. Bob falls all right onto the BH, just that Sally never sees it.

 

[votingmachine]

How about the basic misconception of the existence of black holes at all? The recent gravity wave discovery shows two very large masses colliding, and from what I gather, is the first truly strong evidence that black holes definitively exist.

I also think the size examples are the biggest thing that seems to get missed. The example of our Sun collapsing to a minute radius was given. But a 1 kilogram mass at a small enough radius will have a gravitational attraction that has a light speed escape velocity. It will still behave as a 1 kilogram mass. The ordinary conception of a black hole is considering a large mass:

If I was better with posts I would start a quote here .......

When the body is outside of the gravitational pull, its kinetic energy and potential energy will be 0, so if we equate them

(1/2)mv^2 = (GMm)/r

and the rearrange for v we get an expression for the escape velocity:

v = square root(2GM/r)

Where M is the mass of the planet or body, and r is the radius you are taking off from. The formula contains no mass of the escaping object, if you wanted to get a space shuttle off the Earth you would have to get it to the same speed as if you wanted to get a pebble off the earth, the difference being the amount of energy it would take getting something as heavy as a space shuttle up to the right speed.

Cambridge scientist John Michell argued that if you made the value of M big enough in the escape velocity formula, then you could get a value for v that was bigger than the speed of light. We wouldn’t be able to see these objects as no light would be able to reach us, and, as nothing can travel faster than light, no objects would be able to escape their pull once they were close enough. This is a Black Hole.

And end the quote here .........

http://physicsforidiots.com/space/black-holes/

But the escape velocity is also large as the radius is small. So a small enough radius for our Sun is a black hole. An electron mass should be a black hole at some radius (sorry, I am just guessing here, but it seems to follow from Newtonian math).

So I also think the biggest misconception would be that people tend to only consider the existence of large black holes, when any mass that might compress into the limiting radius should count (ignore the ridiculous 1 kg example, which would have a radius you can calculate, and then calculate a density that would be rather large. The ordinary space within and between atoms stops such a thing). The reasoning that a black hole could be stable and microscopic was the (acceptably impossible) risk raised against the LHC, if I recall correctly.

But again, the misconception that they are proven to exist should be considered ... although I am not going to claim a review of literature that adds to the evidence that they exist ... I'm going by the recent comments that were around the gravity wave detection.

 

[votingmachine]

Is that 4 "balls" makes a "walk" and if there is a person on first base they walk to second and so on? I am not a big sports fan but I think I remember the rules from my youth. I'm not sure what you mean by 2 bases? Is that just saying you're going to teach a different "interpretation", if you will pardon the poor analogy, where you describe going from home to first as "two bases"?

He is talking about an overthrow. Say a ball is grounded to the Shortstop, and he makes a quick but wild throw into the stands. The throw would ordinarily have resulted in the runner out at first base, as the runner was moving between home-plate and 1st base. The runner cannot just run around the bases and score, and the 1st baseman cannot go into the stands and retrieve the ball, and throw to second base. The rule says runner gets 1st base, and then second. 1st base, because they did not throw him out there, and 2nd because otherwise, he is free to keep going, until the other team retrieved the ball and tagged him out, or he scored. The rule stops play with the ball going into the stands, and the runner gets the base he was going to, plus 1 more. Other runners on the base path also get a bonus base ... so a base-runner from 2nd running to 3rd would get awarded 3rd and home when the throw went into the stands at 1st base.

It is a strange rule to try to phrase.

 

[phinds]

How about the basic misconception of the existence of black holes at all? The recent gravity wave discovery shows two very large masses colliding, and from what I gather, is the first truly strong evidence that black holes definitively exist.

I disagree. There has been 10year (or so) project that mapped the trajectories of the stars right at the center of the Milky Way and they show the existence of an object so massive and so small that it can't be anything but a black hole, so I don't think the recent LIGO results were the first strong evidence.

EDIT: I DO agree that the existence of BH's has been called into question, right from when they were first proposed as real objects (Einstein thought they were a mathematical fiction, as I recall).

 

[votingmachine]

I disagree. There has been 10year (or so) project that mapped the trajectories of the stars right at the center of the Milky Way and they show the existence of an object so massive and so small that it can't be anything but a black hole, so I don't think the recent LIGO results were the first strong evidence.

EDIT: I DO agree that the existence of BH's has been called into question, right from when they were first proposed as real objects (Einstein thought they were a mathematical fiction, as I recall).

I'm not able to disagree. The main reason I raised it was I read that the LIGO results were important confirmation. I know of some other measured things that also are considered strong support of the existence of black holes. If I had not read that around the LIGO results, I would have regarded black holes as proven with a great deal of certainty. It could easily be that the story I read exaggerated the current lack of evidence ... and I clearly have not gone looking to see all the supporting evidence. A reporter might easily report that LIGO was the first really strong confirmation of a theory other than gravity waves. Observational confirmation of one theory might be mistaken for another.

The misconception I raised would be whether Black Holes are strongly predicted by theory, and so far confirmed by weak evidence, or whether they unambiguously exist, based on strong evidence. There probably is not a misconception there ... people conceive that they exist, and the evidence supports it enough to say it is not just a theoretical construct (a mathematical fiction).

 

[nikkkom]

"Unambiguous" is subjective.
There are people who don't believe that stars are powered by fusion... (the "electric universe" crowd)

 

[votingmachine]

"Unambiguous" is subjective.
There are people who don't believe that stars are powered by fusion... (the "electric universe" crowd)

I'm just trying to retract what I said without sounding like an idiot ... but never mind what I sound like .. really, it was nothing the OP should be running with.

 

[MikeL#]

'the biggest misconceptions about black holes'
Evapourating black holes suggest anti-matter has negative matter but is this really true?

1. I think anti-matter has positive mass - because when a positron and electron annihilate they emit 0.5MeV + 0.5MeV photons (exploited by PET-CT scanners) with a huge positive energy. Surely if the positron had negative mass then there would be no 1.0MeV of photon energy created?

2. But when describing evaporation of black holes (e.g. paragraph 2 and 3 of https://en.wikipedia.org/wiki/Hawking_radiation#Overview ), 'virtual' matter [e.g. electron] and corresponding anti-matter [positron] are created out of 'the vaccuum' at event horizon. If the electron is ejected, then the positron is absorbed into the black hole which loses mass. In this case the positron mass is negative. (But if the electron were absorbed then the black hole would gain mass - so I have misunderstandood something here.)

3. So does anti-matter have positive or negative mass-energy or both? Does it depend on whether positron is real or 'virtual'?

 

[phinds]

...

2. But when describing evaporation of black holes (e.g. paragraph 2 and 3 of https://en.wikipedia.org/wiki/Hawking_radiation#Overview ), 'virtual' matter [e.g. electron] and corresponding anti-matter [positron] are created out of 'the vaccuum' at event horizon. If the electron is ejected, then the positron is absorbed into the black hole which loses mass. In this case the positron mass is negative. (But if the electron were absorbed then the black hole would gain mass - so I have misunderstandood something here.)

As Hawking himself has said, this whole business of "virtual particles" causing Hawking radiation is not correct. It was just the only way he could find to describe in English something that really can only be described in the math, so your statement here is based on an incorrect premise and therefore is not anything you need to be concerned about.

 

[MikeL#]

Thanks phinds for clearing that up - so I can tell the friend who asks me:
1. Real anti-matter (like positrons) definitely has positive mass/energy and not negative.
2. Black holes do not suggest anti-matter has negative mass - that would be misinterpreting QFT etc.

 

[phinds]

Thanks phinds for clearing that up - so I can tell the friend who asks me:
1. Real anti-matter (like positrons) definitely has positive mass/energy and not negative.
2. Black holes do not suggest anti-matter has negative mass - that would be misinterpreting QFT etc.

"anti matter" has positive mass and is gravitationally attracted to normal matter just as normal matter is. The "anti-" just means it has some other quantum characteristic that is the opposite of normal matter. For example, positrons have a positive charge instead of the negative charge of electrons but that has nothing to do with mass.