# B What are the biggest misconceptions about black holes?

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1. Apr 10, 2016

### 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

Last edited by a moderator: Apr 13, 2016
2. Apr 10, 2016

### 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).

3. Apr 10, 2016

### 43zombiegit

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

4. Apr 10, 2016

### ProfuselyQuarky

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.

5. Apr 10, 2016

### Chronos

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

6. Apr 10, 2016

### 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.

7. Apr 10, 2016

### 43zombiegit

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

8. Apr 10, 2016

### 43zombiegit

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

9. Apr 10, 2016

### phinds

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

10. Apr 10, 2016

### 43zombiegit

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

11. Apr 10, 2016

### ProfuselyQuarky

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/

12. Apr 10, 2016

### phinds

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.

13. Apr 10, 2016

### 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.

14. Apr 10, 2016

### 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?

15. Apr 10, 2016

### ShayanJ

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.

16. Apr 10, 2016

### Vanadium 50

Staff Emeritus
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?

17. Apr 11, 2016

### martinbn

I agree with this, but the next part

is a bit of a misconception itself.

18. Apr 11, 2016

### 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.

19. Apr 12, 2016

### Tom MS

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.

20. Apr 12, 2016

### 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.

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