B What are the biggest misconceptions about black holes?

[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.

 

[1oldman2]

Just wondering what everyone makes of this?
http://www.nasa.gov/image-feature/computer-simulated-image-of-a-supermassive-black-hole

 

[Greg Bernhardt]

Just wondering what everyone makes of this?

Kinda looks like a simple radial blur photoshop filter

 

[phinds]

Kinda looks like a simple radial blur photoshop filter

Sure does. Also, the area around it (outside the blurred area) seems too uniformly distributed.

 

[1oldman2]

Sure does. Also, the area around it (outside the blurred area) seems too uniformly distributed.

Funny but the "shop effect" was my first thought also. I thought I'd throw it on the table for discussion since it's from NASA and not just another You tube crackpot deal.
Here is another release on the BH in question.
http://www.nasa.gov/feature/goddard/2016/behemoth-black-hole-found-in-an-unlikely-place

 

[rootone]

I think it's impressive art but not sure if it's any more than that

 

[Flyx]

I think the biggest misconception is 'How can black holes not let light escape when photons have no mass?'

 

[Flyx]

Just wondering what everyone makes of this?
http://www.nasa.gov/image-feature/computer-simulated-image-of-a-supermassive-black-hole
View attachment 99129

I'm not sure about the black circle in the middle? Wouldn't gravitational lensing make it not possible to actually 'see' the black hole?

 

[PAllen]

That image seems correct and consistent with other calculations (the background assumed is presumably overly uniform to simplify the calculation). For example, here is a simulation by one the largest, most reputable, numerical relativity groups:

 

[1oldman2]

I'm not sure about the black circle in the middle? Wouldn't gravitational lensing make it not possible to actually 'see' the black hole?

I believe its meant to be more a representation of an area rather than an actual object. I have to concede nearly complete ignorance of the subject myself, I am able to learn a lot however by posting a question (#51 would be an example) and following the comments on the thread.

 

[1oldman2]

That image seems correct and consistent with other calculations (the background assumed is presumably overly uniform to simplify the calculation). For example, here is a simulation by one the largest, most reputable, numerical relativity groups:

I could watch that simulation all day long, It would be interesting to see either simulation animated from a rotating viewpoint.