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Most images of black holes are illustrations. Here’s what our telescopes capture

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Re: Most images of black holes are illustrations. Here’s what our telescopes capture

#141

Post by coryfklein »

Sir_Cmpwn wrote: post #75
blackrock wrote: post #38

Earlier quoted context omitted.

I would prefer if black holes were called "Dark Stars". Because that really is what they are. A weird star that is so massive, its gravitational field distorted the fabric of its local space, that not even light can escape it. It is theoretically possible that the black hole is still performing fusion, and emitting light and heat, like a regular star. But its gravitational field is so intense, that even light cannot…

It's a lot more complicated than this and a lot of your assumptions are not true. So far as our universe is concerned, the inside of a black hole literally doesn't exist and events in there literally don't happen. Once you get inside, spacetime is warped so severely that a lot of our fundamental assumptions about how space and time work change. It's not reasonable to assume that normal processes observed outside of t…

> So far as our universe is concerned, the inside of a black hole literally doesn't exist and events in there literally don't happen.

Except our universe _is_ affected by what happens inside a black hole via Hawking Radiation [1], where energy from inside the black hole does make it out and interacts with the rest of the universe.

[1] https://en.wikipedia.org/wiki/Hawking_radiation

Re: Most images of black holes are illustrations. Here’s what our telescopes capture

#142

Post by westoncb »

dsacco wrote: post #136
westoncb wrote: post #135

Earlier quoted context omitted.

> Information theoretically speaking, the inside of a black hole’s event horizon does not exist There's your metaphor. If you didn't need to qualify it that way, you could perhaps get away with saying otherwise—but with the qualification, your telling us about a different domain in which something has the same 'structure' as a (physical) hole. That's exactly how analogies/metaphors work: two things which are analogou…

> There's your metaphor. No, it’s not. That’s the definition, much like information theoretic death is death. Here is a metaphor: from our perspective on a street, we cannot perceive anything within a manhole, just like from outside a black hole we cannot perceive anything inside it. Here is not a metaphor: a black hole is a hole in space, with a singularity inside of it.

> That’s the definition, much like information theoretic death is death

Information theoretic 'death' implies actual death (assuming I understand your 'death' metaphor in 'information theoretic death' ), but that's not the same as being it.

> Here is not a metaphor: a black hole is a hole in space, with a singularity inside of it.

No, you've just removed the linguistic cues for introducing a metaphor, while still making as heavy use of metaphor as ever.

Re: Most images of black holes are illustrations. Here’s what our telescopes capture

#143

Post by InclinedPlane »

coryfklein wrote: post #140
InclinedPlane wrote: post #133

Earlier quoted context omitted.

Neither dark star nor black hole are apt "models" for the objects. These are phenomena of space-time, not just of gravity. Light cannot escape a black hole because escaping a black hole is impossible. And it's impossible because the space-time inside an event horizon has no trajectories that go forward in time which go farther away from the singularity. Black holes are like pocket universes with one-way roads connect…

> escaping a black hole is impossible Except for Hawking Radiation [1] "black holes that do not gain mass through other means are expected to shrink and ultimately vanish. Micro black holes are predicted to be larger emitters of radiation than larger black holes and should shrink and dissipate faster." [1] https://en.wikipedia.org/wiki/Hawking_radiation

Hawking Radiation comes from outside the event horizon.

Re: Most images of black holes are illustrations. Here’s what our telescopes capture

#144

Post by dahart »

InclinedPlane wrote: post #133
dahart wrote: post #81

Earlier quoted context omitted.

> Because we found no better analogy, we named them "holes" Are you sure about that? This sounds specious. This requires knowing who named them and why. It sounds like a logical explanation, but history and human events don't follow logic and can't be derived. > We talk about [...] "light cannot escape the black hole" So, light not escaping certain "dark stars" was observed and was a concept before "black hole" was a…

Neither dark star nor black hole are apt "models" for the objects. These are phenomena of space-time, not just of gravity. Light cannot escape a black hole because escaping a black hole is impossible. And it's impossible because the space-time inside an event horizon has no trajectories that go forward in time which go farther away from the singularity. Black holes are like pocket universes with one-way roads connect…

Agree with all of this, and I appreciate the explanation, but "black hole" is just a name and not a model. We need a shortcut and not a thesis to talk about it efficiently, right? The name really only needs to identify, and not convey perfect and complete understanding, no? It's nice that the analogy is decent, but there is no name that could capture and communicate the complete meaning, that requires further explanation. True for most/all scientific subjects and physical phenomena, I think.

Given your description, "hole" still seems like a great and very literal analogy to me, if we had to pick a single word. It's capturing a sense of going in but not out, and it's also capturing a sense of darkness, and of going into or down via gravity as well. A one-way road that you can drive in but not out, and is downhill, and dark from the outside, sounds like a 'hole'.

We could call it a pocket universe, or a spacetime existence prison, or a one-way road to infinity, but I'd have to agree with John Wheeler, that "black hole" is brief and catchy, captures the essence of what we know about them in 9 letters, in a way that is accessible to non-physicists.

BTW, isn't gravity a phenomenon of spacetime? I don't understand your differentiation.

Re: Most images of black holes are illustrations. Here’s what our telescopes capture

#145

Post by pdonis »

ars wrote: post #10
pdonis wrote: post #6

Earlier quoted context omitted.

> all those photos of super massive objects could be neutron stars Not if they are more massive than 2.7 times the mass of the Sun. That's the maximum mass for a neutron star (more precisely, it's the upper limit of the range of possible maximum masses, assuming the stiffest possible equation of state). Most of the objects referred to in the article are more massive than that, in some cases much more (the black hole…

> No amount of pressure can prevent a black hole from forming And yet suns with far more mass than is needed to make a black hole exist. Because simple heat pressure keeps the black hole from forming. Remember that the black hole doesn't just suddenly appear - to have enormous gravity the density must go up. If you prevent that from ever happening you can stop the black hole from ever starting in the first place. You…

> And yet suns with far more mass than is needed to make a black hole exist. Because simple heat pressure keeps the black hole from forming.

Ok, if you're going to quibble over irrelevancies, let me restate more carefully: in situations where kinetic pressure is negligible, no amount of pressure can prevent a black hole from forming if sufficient mass is present. And kinetic pressure is always temporary, because it depends on having a heat source, and all heat sources eventually run out.

> to have enormous gravity the density must go up. If you prevent that from ever happening you can stop the black hole from ever starting in the first place.

You can't prevent it from ever happening. You can only prevent it from happening for as long as a heat source is available. And that will never be forever. See above.

> It also means mass takes a long (infinite) time to get in.

No, it doesn't. The proper time for an object to free-fall to the horizon, and on inward to the singularity, is finite. Outgoing objects and light behave differently from ingoing objects and light in the presence of gravity.

Re: Most images of black holes are illustrations. Here’s what our telescopes capture

#146

Post by pdonis »

qubex wrote: post #69
ars wrote: post #10

Earlier quoted context omitted.

> No amount of pressure can prevent a black hole from forming And yet suns with far more mass than is needed to make a black hole exist. Because simple heat pressure keeps the black hole from forming. Remember that the black hole doesn't just suddenly appear - to have enormous gravity the density must go up. If you prevent that from ever happening you can stop the black hole from ever starting in the first place. You…

First and foremost: GR guarantees that an event horizon will form. Oddball exotic states of matter (quark stars, fuzz balls, whatever) might form in proper time within the event horizon, halting collapse before it reaches the singularity state of infinite compaction. (Since time freezes at the event horizon, these states would occur infinitely far in the future from the perspective of any external observer and might…

> Oddball exotic states of matter (quark stars, fuzz balls, whatever) might form in proper time within the event horizon, halting collapse before it reaches the singularity state of infinite compaction.

No, this is not correct. Once an event horizon forms, everything inside the horizon will hit the singularity, because the singularity is not a place in space, it's a moment of time, which is in the future (and the not too distant future for holes of reasonable size--the time from horizon to singularity for a black hole of 10 solar masses is about 100 microseconds) for everything inside the horizon.

> time freezes at the event horizon

No, it doesn't. This is a common pop science misconception, but it's still a misconception. The correct statement is that the horizon is a null surface: a surface generated by outgoing light rays. To someone falling through the horizon, the horizon looks like any other surface generated by light rays, and nothing unusual happens there.

Re: Most images of black holes are illustrations. Here’s what our telescopes capture

#147

Post by pdonis »

monocasa wrote: post #120
saagarjha wrote: post #19

Earlier quoted context omitted.

> But it's things from the outside that are falling in, and that make a black hole. Since they take an infinite amount of time to fall in, the black hole never forms in the first place. See this excellent answer on Physics StackExchange: https://physics.stackexchange.com/questions/5031/can-black-h... . In a nutshell (if I'm interpreting this correctly), yes, to an outside observer you never observe the black hole for…

Except Hawking radiation means that it doesn't exist for an infinite amount of time. So an observer falling in would just see it fizzle away as they get closer to it.

Even in the presence of Hawking radiation, this statement is not correct. An observer falling in would fall right through the horizon and be destroyed in the singularity; then, much, much later (something like 10^70 years for a black hole of 10 solar masses), the hole would be fully evaporated away by Hawking radiation and some of the light rays in that outgoing Hawking radiation would carry information about the observer falling through the horizon 10^70 years before.

Re: Most images of black holes are illustrations. Here’s what our telescopes capture

#148

Post by pdonis »

dragonwriter wrote: post #27
ars wrote: post #16

Earlier quoted context omitted.

That's true. But it's things from the outside that are falling in, and that make a black hole. Since they take an infinite amount of time to fall in, the black hole never forms in the first place. (I'm less sure about this paragraph, but I believe that) An object falling in also never sees a black hole, since the other things falling in are dilated relative to him, so there's still not enough mass to make a black hol…

> But it's things from the outside that are falling in, and that make a black hole. Since they take an infinite amount of time to fall in, the black hole never forms in the first place. They only take an infinite time to fall in once a black hole has formed ; OTOH, unless I misunderstand, the time would asymptotically approach infinity up to that point, which seems to have a similar effect.

> They only take an infinite time to fall in once a black hole has formed

No, they don't. See my other comments upthread.

Re: Most images of black holes are illustrations. Here’s what our telescopes capture

#149

Post by pdonis »

ars wrote: post #25
short_sells_poo wrote: post #22

Earlier quoted context omitted.

You seem to misunderstand the way an event horizon works. From the infalling object's (and the hole's) reference frame, the object crosses the event horizon in finite time and falls into the singularity. It is only from an observer's perspective that the object never seems to cross the horizon.

No, that's exactly what I understand. But we don't care about the infalling object. We care about the observer, because any infalling object initially is an observer, and because we (us humans) are observing the black holes. Since from an observers POV nothing can actually fall into the black hole, no black hole can form. The fact that an infalling object reaches the black hole makes no difference to us. Because of t…

> from an observers POV nothing can actually fall into the black hole

This is not correct. See my other comments upthread.

Re: Most images of black holes are illustrations. Here’s what our telescopes capture

#150

Post by pdonis »

criddell wrote: post #98
teamonkey wrote: post #28

Earlier quoted context omitted.

> Since from an observers POV nothing can actually fall into the black hole, no black hole can form. The event horizon isn't really a physical boundary in that sense. It's the mathematical boundary at which, according to general relativity, a particle must have velocity equal to the speed of light in order to escape. A density change inside the star can change the size and shape of that boundary without things fallin…

> a particle must have velocity equal to the speed of light in order to escape This has always confused me. Does a photon have mass? I've always thought the answer is no and so I don't understand why even light can't escape from a black hole.

> Does a photon have mass?

It has zero rest mass, but a photon is never at rest. Photons have energy, and anything that has energy is subject to gravity in general relativity.

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