Most images of black holes are illustrations. Here’s what our telescopes capture
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Because we found no better analogy, we named them "holes" despite the fact that they are basically the opposite: an object with enormous mass.
This influences our thinking and our language when we discuss problems concerning black holes. We talk about "inside the black hole", "light cannot escape the black hole" or "spitting things out of the black hole".
I think it is really interesting how these cognitive metaphors can sometime limit our ability to think about a problem, because they often restrict the properties of the described thing to the properties of the analogy.
Edit: also, another way of looking at it is: when we express any concepts in some concrete form, the particular representation we choose (and we have to choose something) brings its own baggage along with it. And, subsequently, we end up reasoning off of the baggage rather than the thing in itself.
When we think back on these things, we see technology, tools, & machines like tractors and looms. When we look forward we see robots and automation.
Practically this has 2 effects. First, we come back with a Jetsons lime picture, everything is the same as a regular hotel but the maid is a robot, the front desk is an ATM, the barman is a robot arm and the resteraunt is a Japanese conveyor belt. Robots are human shaped metaphors for technology, placeholders for unknowns.
Second, we think in terms of replacement. In reality, it's really more like efficiency. A tractor still uses a farmer, but requires a lot less time and effort to get a field ploughed. Again, there's a difference in how we think when looking back or forward.
> Because we found no better analogy, we named them "holes" despite the fact that they are basically the opposite
Which is false.
https://en.wikipedia.org/wiki/Philosophy_of_mathematics#Math...
On the other hand, mathematical abstractions of dimensionality not cognitive metaphors. A “hole” is a meaningful topological abstraction in various dimensions that corresponds to what we consider a colloquial “hole” in 3-dimensional space. If I tell you a Klein bottle is a bottle that has only one surface (no inside or outside) without any “hole”, I’m not making a metaphor, I’m describing a 4-dimensional object as closely as I can describe a mug or a vase in 3-dimensional space. Just because I’m not defining it through pure mathematics and we can’t immediately visualize it (in entirety) doesn’t mean it is a cognitive metaphor, because it perfectly corresponds to the actual concept of dimensionality (instead of being a clever figurative description of it).
Similarly, if we geometrically represent a black hole we can clearly see the curvature of space (from one angle), leading down to an event horizon, within which is a singularity. This representation corresponds to rolling a coin at an angle down a curvature, watching it spin at the bottom a bunch of times, then fall in. In fact, a shuttle falling out of orbit down to the event horizon would look just like this as it entered the whirl zoom of a black hole, before the unstable orbit failed and it just fell in.
Even further, there is a very cogent, topological sense of what “within” means here. Just as you can fall into a manhole in the middle of the street, you can fall into the event horizon of a black hole. If we represent a street as a 2-dimensional plane, a manhole in the street is the topological space in the plane that has no points - a mathematical hole. In three dimensions, a jelly donut has a topological hole, and the hole is also 3-dimensional (the missing points would be representable as 3-dimensional vectors). Information theoretically speaking, the inside of a black hole’s event horizon does not exist - it’s strictly an absence that we can only reason about heuristically. It would be a cognitive metaphor for me to say that we can only reason about the inside of the event horizon in the same way we can reason about the middle of a donut that tastes like nothing because there is “no donut”. But it is not a cognitive metaphor to call the black hole a “hole”, because fundamentally and literally it acts like one. The singularity inside the black hole is not a hole, but that’s different.
Our common sense of what it means to be a hole is literally described by the mathematical abstraction of a hole. When mathematical abstractions are actually just literal abstractions of what we already understand, they’re not cognitive metaphors. If you’d like an example of a cognitive metaphor insead of a pure abstraction, look at string theory. While our common sense of a hole literally corresponds to its 3-dimensional topological abstraction, our common sense of what we call a “string” has no precise mathematical correspondence. A string in the common sense of the word is a 3-dimensional object that is very long and thin. A string in the mathematical/physical sense is a one dimensional object, like a line. To call it a string is to invoke an intuition of something like a small thread that in some sense seems “barely” 3-dimensional.
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 analogous share the same structure but have representations of those structures in differing domains.
There's a simpler way of seeing that it's metaphor still, though. You have chosen some characteristics of holes and arbitrarily decided that they are the ones which define it—but if we want to say it's literally a hole and not metaphorically, then all the attributes which familiar physical holes have should apply. For instance, there should be an interior surface, and things inserted into it should be retractable. Your description treats the singularity and event horizon as two distinct objects, which provides a kind of solution to the second—but it seems like those two things aren't as readily separable as, for instance, if we had a hole in the ground and it was filled with a powerful acid: in that case it's clear which is hole and which is thing filling it. Perhaps I'm mistaken, but I'd bet that the way in which the singularity 'fills' a black hole cannot be anything more than metaphorically.
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.
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.
"Hole" and "in" are pretty good metaphors for what a black hole is. It's hard to have any particularly accurate metaphors for what a black hole is because time dilation, the speed of light, curved space, etc. just aren't a part of life for any humans.
Although not specifically in this context, Ludwig Wittgenstein said:"The limits of my language mean the limits of my world."
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 escape it. So, from the outside, the black hole appears dark. Hence, a dark star.
Perhaps if you are inside the Event Horizon of the black hole, then you can still see the light from the fusion of the star. But if you are outside of the Event Horizon, then you will only see the star as pitch black, since light can't even escape it.
Would it be black? Right above the event horizon is flaming relativistic matter.
It’s extremely improbable that any “classical” (non exotic-quantum) processes are going on within the horizon because it’s very clear that the strength of the gravitational field would far exceed the hydrostatic pressure generated by fusion reactions. It far exceeds the degeneracy forces that prevent neutron stars from collapsing further under their own mass.
[0] https://en.wikipedia.org/wiki/Dark_star_(Newtonian_mechanics...
I don't think that's a useful picture. It's closer to the truth to say that from the outside perspective, all the information and energy sits on the border and there is not even an inside to speak about. In other words, it's the edge of the universe and what's past the event horizon is another universe. Not like a star at all.
However, for fun, I once thought of a science fiction scenario for a novel.
I thought a Black Hole, can be used as the instrument for a wormhole in space. An Einstein-Rosen Bridge, that connects our universe to another universe, in the multiverse.
But there are a lot of plot holes in this idea. If light cannot escape the black hole, then you somehow need to go faster than light, to traverse the wormhole, and to escape it, when you cross over to the other side. So, once you exit the black hole, then you land at the other side, and there is a whole other universe there, with its own set of stars.
But, this will also allow other people from another universe, to cross into our universe, via this black hole.
Then how does such a black hole form?
It starts with a very massive star, more massive than a Neutron Star, that roams around our universe. Then, in an adjacent universe, another very massive star is wandering around also. Think of our universe as a bubble, and the adjacent universe is another bubble. These two massive stars, will distort the fabric of their bubbles, at the edges. The two stars will gravitationally attract each other, and they will basically punch a hole through each bubble. And when they collide, the mass from each star will form the structure of the wormhole. They won't explode on contact, but instead, it will now form a bridge between two universes.
For millions of years, they will dance around each other, until they finally find a stable orbit, where both are spinning around each other.
Think of this as a binary star system, where the two stars are rotating around each other, so fast, that it forms this virtual wall. This virtual wall, is the structure of the Einstein-Rosen bridge. And in the middle, it is gravitationally neutral.
And this wormhole structure, will now allow regular spacecraft, that can travel faster than light, to traverse the wormhole, and emerge on the other side.
Next, I just have to add the other parts of the story, like the bad guy, the love story and love triangle, and some good guy that is searching for himself. And with that, I'll have a blockbuster novel that I can turn into a live action movie.
Third, probably least interesting, how does one get into BH travelling FTL, if that makes you go back in time?
Are you sure that's known? What kind of experiment would settle that?
No experiment that I know of could confirm this because everything will depend on extrapolation from less extreme and finite scenarios.
Before we get to the black holes part... why didn't they have timestamped signals from the surface of that water-planet? Why didn't they go into a polar orbit (as opposed to equatorial orbit) around it so as to minimise the cumulative time dilation around the black hole it was orbiting? What kind of specific impulse were they supposed to have to be able to take off from it after they'd landed.
Oh, I cringed. I cringed so hard. Poor Kip Thorne.
https://en.wikipedia.org/wiki/Lee_Smolin#Fecund_universes
His idea is that production of black holes is the universe producing offspring, and our perception that a black hole is constrained within a coordinate space in our own universe, is an illusion.
No, it is not. In GR, once critical density is reached, gravitational collapse is inevitable: https://en.wikipedia.org/wiki/Penrose–Hawking_singularity_th....
It's possible this doesn't hold true in some theory of quantum gravity, but you'd get some exotic matter, not "stars still performing fusion, and emitting light and heat, like a regular star".
Please note that no fusion is happening in white dwarfs, nor in neutron stars.
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.
Either way, we do not know what processes occur inside because we lack any theory which could describe it. This certainly doesn't mean nothing happens in there or nothing exists in there.
>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.
It would probably be easier to get this across if I substituted "So far as our universe is concerned" with "From our perspective outside of the event horizon". Remember that a singularity doesn't just "pull" light so hard that it can't escape - they warp spacetime around them, so much so that the space and time within the event horizon ceases to be meaningful from our reference frame. The only way to go is down - literally. If you turn 360 degrees in a black hole you will never face the outside.
Also, we definitely have theories that describe what could happen inside an event horizon. General relativity, for example. It's from these that we can establish that space and time "switch" within, for example. We can solve general relativity for the conditions within an event horizon and make some pretty strong predictions about it (it's from these predictions that I conclude elsewhre in this discussion that fusion is unlikely).
Common misunderstanding. All mass has acceleration from gravity, but stuff like orbits can still happen due to existing velocity.
More importantly, even if objects were to be pulled together, there space expanding means they might still be futher apart, if the space between objects grows faster then they move togeather. So the heat death looks like it will happen anyways.
All of these inaccurate descriptions assume we have the concept already in mind to recursively look up. Describing more accurately let's the reader get a clearer picture and thats where real concepts get conceived.
I like heavy/dark star/planet for a vague description. I also think the fact that humans rely on visuals now and we can't get more than a single angle of one of these we can show anyone a 3D view of how it's not just nothing.
Strictly speaking, the defining character is enormous density, not mass. And the black hole is arguably a name for an effect of the object, not the object itself; the object itself is (or is in the process of becoming; verb tenses get weird when time gets weird) infinitesimally small, but the “black hole” generally refers to the space bounded by the event horizon.
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 term. "Dark stars" is one term they were called before the term "black hole".
http://curious.astro.cornell.edu/about-us/86-the-universe/bl...
Wikipedia says the name black hole was adopted because it was catchy, which is not because of the value of the analogy. "Dark star" might be a better analogy, and it existed before black hole.
https://en.m.wikipedia.org/wiki/Black_hole#Etymology
Having said that, black holes are actually black, and they are literally gravity holes in space. A hole in the ground on earth is only a hole because of gravity, it's a thing you can fall into. Because black holes are things you can fall into if you get too close, because it's doing the exact same thing as a hole in the ground, I'd argue that black hole is not a very good example of a cognitive metaphor.
Note also that the best way to demonstrate the intuitive effect of gravity is to show how it acts like a hole. A black hole visualized the same way is a very, very deep pit.
"Dark star" is also a misleading term in that it implies the object is a star or something physical at all. In fact to us it doesn't matter what the object is inside the black hole, and its structure is essentially unknown to us (at present), because we can't interact with it. We can only interact with the space-time phenomenon that it created, the event horizon.
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."
In the end, matter/energy that was inside the black hole can eventually exist in a region that is no longer referred to as part of the black hole.
^ This being my non-physicist layman interpretation, I'm happy to be enlightened if I'm wrong.
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.
No, they actually are holes. We don’t need analogies or metaphors to describe black holes. Their mathematical properties are quite complicated to define and we have to reason about what happens around or inside them using figurative thought experiments, but their action does correspond to our intuitive sense of what a hole is.
In particular, a black hole consists of an event horizon, around which various stable and unstable orbits are possible, and within which is a theoretical singularity. The event horizon perfectly corresponds to the mathematical concept of a hole, which is a pure abstraction of our common sense of a hole, not a metaphor. A hole is a lack of points in a dimensional space, which means everything inside the event horizon is as much a hole as a manhole in a street is a hole (information theoretically speaking, the inside of a black hole is nothing). To say a black hole is a hole is not a cognitive metaphor, because space around the event horizon actually does curve down to something that is a physical hole. Instead, here are two examples of cognitive metaphors:
1. A donut is a 3-dimensional space with a topological hole in the middle of it. Suppose I define the mathematical properties of taste and equip it as the only sense you have for investigation. Then the hole of a donut tastes like nothing because it is nothing, in the same sense that we can know nothing about the inside of a black hole because it is nothing.
2. Roll a quarter at an angle down a spherical curvature with a hole in the center. The quarter will gradually descend down the curvature, with each revolution about the center happening faster and faster. Finally, it will simply drop it. This is analogous to deteriorating your orbit around a black hole, until you enter various unstable orbits and finally fall into the event horizon.
Obviously these cognitive metaphors, while instructive, are imperfect. For instance, we can see a quarter drop into the hole, but we’d never actually see a shuttle fall into the event horizon. On the other hand, the event horizon is a hole in the same sense that a manhole is a hole. Comparing it to a manhole is another cognitive metaphor; calling it a hole isn’t, because it is one.
i.e. all those photos of super massive objects could be neutron stars, and we would not be able to tell the difference.
They are assumed to be black holes because of the mass, but if there is something in the law of physics that prevents black holes from forming we would not know.
We do not have a theory on quark degenerative pressure for example, which could possibly exert enough pressure to prevent black holes from forming (you would get quark stars instead, with no event horizon).
There are also time dilation issues that might make black holes impossible.
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 at the center of our galaxy is about 3 million solar masses).
> We do not have a theory on quark degenerative pressure for example, which could possible exert enough pressure to prevent black holes from forming
No amount of pressure can prevent a black hole from forming, if the mass is large enough. Even for hypothetical quark matter, the equation of state can't get any stiffer than the assumed equation of state that leads to the 2.7 solar mass upper limit for neutron stars. Relativity sets limits to how much an object's pressure can resist gravity, regardless of the source of the pressure. That's where the maximum mass limit comes from.
> There are also time dilation issues that might make black holes impossible.
No, it doesn't. It just means that light signals from very near the horizon take a long time to get out.
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 are talking assuming the black hole is already there, and saying quark degeneracy can't resist that, but you forget the black hole has to form first.
> No, it doesn't. It just means that light signals from very near the horizon take a long time to get out.
It also means mass takes a long (infinite) time to get in. So the black hole may never form.
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 hole, even for the object falling in.
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 form since it would take an infinite amount of time for light signals from the newly formed black hole to reach you. However, to any observers that fell into the black hole (and so were in the same frame of reference), the black hole would form in finite time.
So an observer falling in would just see it fizzle away as they get closer to it.
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 time dilation, we can observe no black holes. So our telescopes will never see a black hole.
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 falling into it in the usual sense.
In the same way, a density wave can 'travel' faster than the speed of sound (or light) in a medium because the wave is a mathematical construct that emerges from a physical situation.
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.
Now we just have to bind an object to one of these ten geodesics shown schematically above.
The strong equivalence principle stems from the observations by Galileo et al. that objects of different weights and configurations fall at the same rate (if one can eliminate air drag and so on). Any object may bind to an available geodesic, whether it's a feather, a bowling-ball, a beam of light, or a moon. One has to do work to move an object off a geodesic [1].
That light binds to geodesics and geodesics are determined by proximity to mass was tested by Eddington et al. during the 1919 solar eclipse, where they observed something similar to the |>O diagram above. Gravitational lensing works the same way.
As we increase the mass of O, the closer geodesics are more and more bent towards O. So for a lighter star: |)o
Black holes are much more massive (and yet more compact) than O, so there are geodesics more bent towards the black hole (because of the mass) and and more geodesics closer to the black hole's centre of mass. The closer geodesics can be bent around the black hole, possibly several times.
Additionally there are "no return" geodesics that twist into circular orbits around the black hole. There is an innermost stable circular orbit (ISCO) too.
Finally, there are "no return" geodesics that lead past the ISCO and into the region covered by the event horizon. @ | could be a diagram where we replace O in )O | with a black hole.
Light can bind to any of these "no return" geodesics just like any other object like a feather or a bowling ball.
- --
[1] Strictly speaking, our universe is 1+3 Lorentzian with extremely high experimental confidence. One dimension is timelike and the other three spacelike. This lets us sort geodesics into three types: spacelike, timelike, and null (or lightlike). In normal empty space light (and any other massless particle) always moves along a null geodesic, and moving it off a null geodesic is energetically impossible. Likewise, in normal empty space, massive particles always move along timelike geodesics, and while (with a lot of work) you can move them onto timelike geodesics that look more and more lightlike, it's energetically impossible to push it onto a lightlike geodesic.
Distinguishing between lightlike and timelike is best done with respect to some coordinates, intervals, and using a tiny bit of calculus. The Euclidean distance for an object only moving in one spatial direction is ds^2 = dx^2. The spacetime interval for an object only moving in the timelike direction is ds^2 = c^2dt^2. If we let it move in the x direction, it's ds^2 = c^2dt^2 - dx^2. For light, and units of lightseconds in x and seconds in t, we have ds^2 = 0, thus "null". If ds^2 > 0, the interval is timelike. If between every two points on a geodesic the interval is lightlike, the geodesic is lightlike. If between every two points on a geodesic the interval is timelike, the geodesic is timelike: an object bound to such a geodesic does not travel as far in space over a given time as light does.
The most lightlike but still timelike geodesic is available to ultra-relativistic massive objects. So if we define an event horizon as the surface below which all lightlike geodesics lead inward, we have also forced ultra-relativistic massive objects inwards on their almost-lightlike geodesics.
Putting this more colloquially, if you are inside the event horizon, even if you could accelerate to the speed of light, you aren't getting out.
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.
This is not correct. See my other comments upthread.
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.
No, they don't. See my other comments upthread.
This is really, really trivial and really basic GR. I don't really know what to suggest apart from MTW[1], except maybe this basic course from http://theoreticalminimum.com/courses/general-relativity/201.... If you understand what Penrose diagram are, and how to compute them, the answer is immediately obvious.
Bernard F. Schultz, _A first course in general relativity_
It's accessible to undergrads! (who have studied SR, vector calculus, electrostatics).
An active main-sequence star can be very large without collapsing, as the nuclear reaction inside sustains its size. But for a neutron star the limit is much lower - under a certain mass limit, the stable size is still sufficiently large, but above a certain mass a neutron star can not be larger than the expected event horizon, there's nothing sufficient to prevent the collapse from happening, and it will collapse to a black hole.
A star has internal fusion/fission that keeps it's mass from collapsing. Once a supermassive star runs out of energy, the entire thing comes rushing centerwards (and some of it gets explodified as supernova)
>You are talking assuming the black hole is already there, and saying quark degeneracy can't resist that, but you forget the black hole has to form first.
Blackholes aren't magic. To form, a certain matter density must be reached in a certain volume of space (which can be surprisingly low; the blackhole at the center of the milkyway is not much more dense than water at normal atmospheric pressure). Once you have reached this specific point, you get a black hole.
From an outside perspective, being pulled into a blackhole looks like being redshifted out of existence and torn apart, for the infalling object, nothing happens (unless the BH is small enough).
The black horizon around a black hole is not a simple line, it's a smort of smudge leading up to the real border; the closer you get the more redshifted everything becomes. Thus, for the naked observer, it might appear as though the object has been swallowed even though it might not have yet crossed the event horizon.
Second, and this is surprisingly subtle: we often discuss black-hole formation in terms of mass, but it's really a matter of relative density: the 2.7 solar mass limit only pertains when one is considering a mass in a void, if for example one found oneself in an infinite universe with an undisturbed uniform density of (say) 2.7 solar masses per metre cubed the tug of gravity would be uniform in all directions and there would be no impetus to initiate gravitational collapse towards any specific point. Once perturbed, however, average density somewhere would rise, and the collapse would begin.
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.
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.
We don’t actually know if there’s a singularity within the event horizon. GTR predicts it, but that’s in the context of the breakdown of the predictive power of the theory. Until/Unless we have a viable theory of quantum gravity what is inside an EH is speculative. Quark stars or any form of conventional matter are right out obviously, but Fuzzballs or some other novel “structure” can’t be ruled out yet.
Having said that, I’m not a string theory adherent, I’m just pointing out that within the event horizon we need a complementary theory to augment GTR and QM.
Not if we take quantum effects into account, no. I was describing what classical GR predicts. I agree that GR also predicts that it should break down in the regime close to the singularity.
It's also worth noting that there is a school of thought among physicists that says that quantum effects are non-negligible even at the horizon of a black hole of stellar mass or larger. This is the "firewall" debate that is currently ongoing. I personally don't find the arguments in favor of a "firewall" convincing, but it is an area of ongoing debate.
Furthermore, whereas you and I apparently know what a null surface is, I was trying to illustrate what infinite time dilation means without resorting to very specialist knowledge.
This is not necessarily true if the quantum "firewall" speculations end up panning out (I think they're unlikely, as I posted elsewhere in this thread, but it's an open area of debate).
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.
As for time dilation... no. In no way could that magically overcome collapse.
> Quark degeneracy pressure isn’t going to support the mass of millions, or even billions of suns.
Why not? If the star is very large the density (and thus gravity) never get high enough.
For example, if you consider the milky way as a single object, then that's an object resisting the mass of billions of suns.
The mass is irrelevant, what matters is the density.
> As for time dilation... no. In no way could that magically overcome collapse.
It can keep mass from ever reaching the surface of the black hole (singularity). At least as far as we are concerned. It takes mass an infinite time to actually get to the surface, so the black hole can never grow.
(for your last comment, see my previous response)
... where the vast majority of that something's mass and charge are within the event horizon. To keep Birkhoff happy, and to avoid confusion with event horizons that pop up where a black hole clearly isn't, maybe add that in a small region of spacetime mass and charge inside an event horizon is a black hole if after stationarization electromagnetic and gravitational perturbations in the Schwarzschild metric are small compared to those in e.g. the Minkowski or Robertson-Walker metrics.
I'm tempted to go the other direction: a physically reasonable arrangement of stress-energy can source a black-hole-like metric and that a black hole metric (i.e., an exact solution for a mathematical black hole, like Kerr-Newman) can usefully approximate such that there is a good match between the geodesics structure around a mathematical black hole and the behaviour of observed matter in the vicinity of a black hole candidate.
A practical probe of the null geodesics structure around a BH candidate is surface emissions: we routinely detect them directly and through analyses of radiative efficiency for neutron stars and white dwarfs (whether or not these compact objects are accreting), and a detection of surface emission from a compact BH candidate would rule it out as a source of a BH metric. If all BH candidates have surface emissions, then BH metrics are a poor choice of modelling tool. However candidates which survive this probe and other tests of near-horizon geodesics structure might as well be called black holes, even if we have reasons to hope (or even doubt) that its mass and charge within the apparent horizon is concentrated in the singularity point.
You'll note here that I'm taking a "quacks like a duck" view of astrophysical black holes; I'd go even further and want to be agnostic about event horizon vs trapping surface and so forth. Around BH-like objects, arranged by increasing ease of observation, there will be a predictable (set of) ISCO(s) below which free-falling trajectories always decay or plunge inward (or outward if retrograde); there will be very strong gravitational lensing; there will be characteristic outflows from Penrose-like mechanisms; and a characteristic efficiency in conversion of accretion matter into radiation compared to matter accreting onto non-BH objects of similar mass. These all depend on whether the exterior region near the candidate object is like that of an exact BH solution, and when you have all of the above, and no evidence to the contrary, it is pretty safe to assume there is something very similar to an event horizon dividing the near-exterior region from the non-exterior region(s).
Uh, no. What's keeping the Milky Way from collapsing is the fact that it's rotating, not its density. Any cloud of matter that is initially non-rotating will eventually collapse due to gravity, no matter what its initial density was.
A black hole with a lumpy interior. (It does not have to be always lumpy).
Consider two black holes of the type you envisage, where the mass and charge of each is entirely at the singularity point of each black hole. Let the black holes merge. How do two singularities become one singularity?
(Disclaimer: theoretical physicist)
Personally, as a layman on the subject of black holes, what I understand is that mass does take infinite amount of time to fall into an event horizon. But I don't know about the formation process of how/when that event horizon is formed.
Perhaps you can help me, and him, understand why his claim is false--that it would happen in finite time?
I think what you're saying here is the confusing part.
If no observer in the universe can ever witness a black hole being formed, it stands to reason they cannot exist (within the reference frame of observers outside it).
Now I'm not asserting this as true or not, because I wouldn't assert such contrarian conclusions on a subject I have not properly studied; rather I am just highlighting what appears to be the confusing bit here.
Also its gravitational effects can be detected, so that is another reason why merely claiming a black hole doesn't exist, or never forms, seems to rather miss the mark.
Yes, studying the exact formation for a black hole is a subject under investigation. From what little I've heard in the past, there is nothing surprising happening, but I guess people are hoping that could lead to some understanding. Not sure what they do, numerical GR I suppose, maybe compare to alternative gravity theories to see if any differences are predicted.
It is true we don't fully understand neutron stars and we certainly don't understand what happens inside the event horizon.
I mean, don't the waves measure a merge up to the point that both black holes are inside a single Schwarzschild radius? And if so, isn't that very strong evidence that the things merging are black holes?
EDIT: If our instruments were precise enough and we could remove the noise, we could theoretically detect gravitational waves of two Sun sized stars merging (2 solar masses shouldn't form a star capable of becoming black hole).
(I really don't know the answer.)
The first LIGO-Virgo events detected black hole mergers which resulted in black holes.
The last LIGO-Virgo event detected two neutron star mergers. Unfortunately the sensitivity of the apparatus is not good enough to determine if the merge resulted in a black hole, or a neutron star based purely on gravitational wave measurements. However, we have secondary evidence[1] that the remnant was a black hole.
No, that's wrong.
Narayan, R. and McClintock, J.E., New Astronomy Reviews, 51, 733–51, 2008 [arxiv: https://arxiv.org/abs/0803.0322 ] has a beautiful chart (Fig. 8) comparing the quiescent bolometric luminosity of star-to-accreting-neutron star binaries and star-to-accreting-black hole binaries. Observed NS binaries are brighter than observed BH (candidate) binaries.
This is one test for whether a compact accretor in a binary is a BH rather than an NS.
This technique was used by Bennett et al., 2002 and Mao et al., 2002 to find several candidate BHs.
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Bennett, D.P., Becker, A.C., Quinn, J., et al. (2002). Gravitational microlensing events due to stellar-mass black holes. Astrophysical Journal, 79: 639–59.
Mao, S., Smith, M.C., Woz ́niak, P., et al. (2002). Optical Gravitational Lensing Experiment OGLE-1999-BUL-32: the longest ever microlensing event—evidence for a stellar mass black hole? Monthly Notices of the Royal Astronomical Society, 329: 349–54.
I'm thankful that I can at least marvel at nebula pictures taken by Hubble. However, I wish they'd switch goals from getting a few people to another planet to getting imaging equipment there that is capable of forming a VR experience we all could share.
I completely agree, but with the arts as my object of fixation. I could never live the swanky life I lead without my current job, but damn I would have been a good potter. Luckily, there is still time...
In that blue x-ray burst image, am I looking at 1000 years per frame, 1 second? 1 minute?
Really frustrating
https://cdn.vox-cdn.com/uploads/chorus_asset/file/9984865/st...
I assume if the speed of light were lowered black holes would become far more common?
If the speed of light was infinite, yes, but a lot of physics relies on the speed of light being finite. Pretty much everything breaks (e.g. mass) if you have an infinite speed of light.
>I assume if the speed of light were lowered black holes would become far more common?
Yep. It might clarify things a bit to say that the "speed of light" is a misleading name - it's really more about being the speed of causality, the speed at which things happen.
https://en.wikipedia.org/wiki/Variable_speed_of_light
I would imagine that a universe that doesn't obey basic causality will not exist for long, even if it comes into existence, nor will it support complex structures that can observe and reflect (like life). Other universes' physics and evolution might be very different, but whatever its laws, on a macro scale I'd imagine they follow internal consistency. So yes, we might evaporate, but it would be well predicted by the science of that universe. It won't happen inexplicably. :-)
> Well, the thing about a black hole - its main distinguishing feature - is it's black. And the thing about space, the colour of space, your basic space colour, is black. So how are you supposed to see them?
For reasons we aren't 100% sure about yet (though we suspect it is due to angular momentum and energy transfer through some process [3]), matter (in the form of hot plasma) from this accretion disk can be launched out along the axis of rotation at relativistic speeds (close to the speed of light) in the form of astrophysical jets [2], which we can observe to due the radiation they emit. These jets play a large part in galaxy evolution, and as shown in the article can be huge, extending far beyond the galaxy itself.
1: https://en.wikipedia.org/wiki/Accretion_disk 2: https://en.wikipedia.org/wiki/Astrophysical_jet 3: https://link.springer.com/article/10.1007/s10509-009-9984-y
Enjoy :)
2. The jets are not matter originating inside the black hole. They are are matter that is accelerated by the black hole then slung away. It’s a similar mechanism to how we accelerate some of our spacecraft (by flying them close to the mooon or Jupiter) but with much higher energy levels!
either way, how is this new imagery any less synthesized than the old imagery ?
For those who assume that General Relativity is "fact", there is an extension of classical mechanics that gives the same predictions without the space-time curvature aspects. Came out a number of years ago.
For those who believe gravity (and relative density) are the causes of the theoretical entity "black hole", a question to think about in relation to fields, what happens as you move towards the centre of any mass? A second question is related to electromagnetic fields strength and approaching atomic nuclei? The same question applies to "neutron stars".
For those who see want a change of reference between the external universe and the reference point of approaching the "event horizon", what does the observer see of the external universe when falling towards an "event horizon"?
The term "dark star" long predates the GR model and is an entity that is quite different to the "black hole" entity of Einstein's GR theory.
Due to inconsistencies between Einstein's SR theory and his GR theory, which model is more correct, if either are correct?
All of our models and theories are simplifications of the explanation of how our universe works and as such will be subject to change. They all have limits of applicability.
Too often, it is assumed that certain things happen in a specific way because a theory or model has some applicability in explaining observations. What a lot of intelligent people forget is that if you cannot observe some feature of the universe then the explanation of what is going on in that feature is only speculation and belief not "fact", irrespective of how "good" your theory is, including any predictions it makes.
At any time, there are competing theories about specific subjects and each will have it proponents and opponents. Which of them is more correct is not the question, the question is are they useful models to help in understanding the universe around us?
There is nothing wrong with pushing that a specific model or theory is more applicable than another. What we must be careful of is believing that the theories and models we push and believe in are "truth". When we do this, observations that disagree with or theories and models will (due to human nature) be discarded as irrelevant or faulty.
It is interesting to note that there are many observations that have been made that are no longer reported because they do not match the predicted outcomes of the consensus theories and models. This is a shame as we then lose our ability to expand our understanding of the universe about.
It is useful to remember that mathematics is a useful tool to help in building our explanations of what we observe, but every field of mathematics has specific premises, axioms etc that are simplifications or generalisations that are not actually matched by the universe around us.
This is a fantastic piece, and even though some images are more visually striking, for me the ones that gives me the most chills are the time-lapse of stellar motion around Sagittarius A. Those are stars* being whipped around like toys. Stars. Over 97% of the mass of the solar system is just Sol, and these stars are more massive, and look at them move!
Chills.
Between two pillars they lie
Dark and mysterious
It pulls me in
Its gravity is so strong
And out comes
A new born star.