Most images of black holes are illustrations. Here’s what our telescopes capture(vox.com)
vox.com
Most images of black holes are illustrations. Here’s what our telescopes capture
https://www.vox.com/science-and-health/2018/1/8/16822272/black-hole-looks-like-what
166 comments
> 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 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.
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.
> 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 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.
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.
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 therefore very reasonably be considered ‘unphysical’.)
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.
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.
> 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.
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.
That is what GR predicts. However we know that GR is incomplete because it is incompatible with quantum field theory. I was trying to illustrate what can be relied upon to be true even if ultimately GR gets superseded by deeper theories: namely, we can rely on an event horizon forming at the schwartzchild radius whatever the ultimate fate of the in-falling matter may be (in an infinite future).
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.
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.
> we can rely on an event horizon forming at the schwartzchild radius whatever the ultimate fate of the in-falling matter may be (in an infinite future)
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).
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).
While I’m totally in agreement with the second part of your comment, the first part may not be true.
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.
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.
> We don’t actually know if there’s a singularity within the event horizon.
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.
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.
Agreed on all points, and the firewall is interesting, but I can’t see how it doesn’t somehow create a privileged frame of reference. It’s still very cool/terrifying, even in the already notoriously cool/terrifying realm of black holes.
The whole controversy about the firewall hypothesis is that I’d does create a privileged frame of reference, directly contradicting general relativity (that indirectly forms the basis for predicting the firewall’s existence).
>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.
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.
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.
Regarding your last point: that depends on your reference frame. From the outside it does appear to take "forever" to get into a black hole, but if you were the one falling in...it happens much quicker, certainly not taking an infinite amount of time.
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 hole, even for the object falling in.
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.
You keep repeating this misconception even though you have been repeatedly been corrected over and over again. What you don't seem to realize is that the choice of coordinates in GR is arbitrary and local. There are types of coordinates that are singular at the event horizon, and there are others which are not. This has nothing to do with infalling vs. the exterior observer. You seem to think that time is somehow fixed for the exterior observer, but that couldn't be further from the truth.
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.
[1] https://en.wikipedia.org/wiki/Gravitation_(book)
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.
[1] https://en.wikipedia.org/wiki/Gravitation_(book)
I can recommend:
Bernard F. Schultz, _A first course in general relativity_
It's accessible to undergrads! (who have studied SR, vector calculus, electrostatics).
Bernard F. Schultz, _A first course in general relativity_
It's accessible to undergrads! (who have studied SR, vector calculus, electrostatics).
> 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 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.
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.
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.
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.
> 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; 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.
No, they don't. See my other comments upthread.
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 time dilation, we can observe no black holes. So our telescopes will never see a black hole.
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.
> 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 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.
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.
> 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.
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.
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.
It's a consequence of the bending of spacetime around a singularity. All possible paths that light (or anything else) can take through spacetime lead towards the singularity.
So it's the same mechanism as gravitational lensing (or whatever it's called) that bends the path a photon takes when it passes something massive?
Yes, except here its much more extreme so that inside a certain distance (event horizon) spacetime is bent in such a way that all paths through it lead towards the singularity. That's the theory at least.
In spacetime there are paths of least resistance called geodesics, and an object left alone will bind to a geodesic determined by the distribution of moving masses in the spacetime. If we take two parallel geodesics in empty spacetime and draw (a section of each of) them like this ||. But let's consider if we put a massive object like a star (O) somewhere near the geodesics. We'll exaggerate in the diagrams: O<| vs |>O vs >O< vs | O | etc, depending on where we put the star in relation to the two geodesics. Note that if they are close enough, they bend towards the star.
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.
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.
[deleted]
A better way of thinking about the event horizon is that it is the boundary beyond which events happening cannot be assigned a 'when' in our universe - i.e. the object falling 'in' to the black hole is never seen to pass this horizon, but to the object it seems that they do, however those events happen to it after an infinite time in our universe, i.e. never, due to time dilation. The contents of a black hole are the events that occur outside/after our universe relative to that boundary. I found these explanations on an awesome PBS youtube video [0] about black holes and space-time.
0. https://youtu.be/vNaEBbFbvcY
0. https://youtu.be/vNaEBbFbvcY
> from an observers POV nothing can actually fall into the black hole
This is not correct. See my other comments upthread.
This is not correct. See my other comments upthread.
Also, did we forget of LIGO's recent observations of black hole mergers, in perfect agreement with the GR calculations? Nothing else (known, or proposed) can match that.
"If you prevent that from ever happening" is the key part here. There's no magic that can prevent that - there are a number of factors (e.g. heat pressure, rotation, neutron degeneracy pressure, etc), all of which we can estimate, and which generally increase as the star shrinks/collapses.
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.
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.
> 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.
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.
I'll take your comment in good faith, but there are very good reasons to believe black holes exist and that, for instance, the center of our galaxy contains a very massive black hole and not a neutron star. This is basic general relativity, which only has (physical and theoretical) evidence in support of it. While quantum gravity is still very much uncertain, that would (most likely) only have to do with understanding the possible singularity inside black hole. Otherwise, defining a black hole as "a thing with an event horizon" is purely a general relativity prediction, consistent with all of the other predictions and observations.
(Disclaimer: theoretical physicist)
(Disclaimer: theoretical physicist)
OP's claim is that black holes infinite amount of time to form, because matter would take an infinite amount of time to fall into it.
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?
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?
Signals take an infinte amount of time to reach a distant observer from just outside the event horizon. Which means we cannot see the event horizon or what is beyond, and we cannot also 'see' the event horizon being formed, or any matter crossing the event horizon. When the star collapses, light takes progressively longer to reach an outside observer, and eventually becomes too dim to detect. The same thing happens with an object falling into a black hole. But since the gravitational effect can still be felt, we cannot also say that the black hole never formed. At all events, a region of spacetime that emits no discernable signals has formed. You can call it a black hole or speculate further about what it is, but the phenomenon is still right there.
>we cannot also 'see' the event horizon being formed
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.
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.
What is meant when we say we expect to see a black hole, or event horizon, forming? If we mean that we can no longer detect any signal from particular region, then to that extent, we can assert that what we call as a black hole exists there. We're defining it by its negatives, but I can't see how one can say that even those negatives don't exist.
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.
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.
Put simply, matter certainly falls into a black hole in finite time. Black holes merge, as LIGO recently observed gravitational waves from, if that helps give some "direct" evidence.
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.
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.
Maybe they never finish forming (keep collapsing forever). But once you have an event horizon, you have a black hole, right?
You skipped a step. Yes, the general relativity prediction of a black hole is solid once it exists. But you have to construct that black hole in the first place. And it's that part that's on shaky ground.
There are indeed people who closely study the process of forming a black hole from collapsing matter, hoping to understand things like the singularity and event horizon. I have not ever heard of any good results that show that a black hole for some reason cannot be formed, but I'm sure people are investigating. This would be very surprising if found, and there doesn't seem to be any evidence against black holes actually forming. So the mainstream, and as far as we can tell, correct, view is that black holes can, do, and have formed.
You seem to have missed the part where podiki offered the object at the center of the galaxy as evidence that a black hole currently exists. So you're arguing against the evidence when you say (over and over and over) that black holes can't form.
This seem to be a contrarian view. Do you have any sources you can share with us?
> Keep in mind as well that no observation has yet distinguished between a neutron star and 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.
- --
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.
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.
- --
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 afraid that's not really accurate. The most massive neutron star observed is just under 2 solar masses, which is difficult to explain if event horizons do not exist. Quark and strange stars are hypothetical objects intermediate in mass between neutron stars and black holes, not suggested replacements for the latter.
It is true we don't fully understand neutron stars and we certainly don't understand what happens inside the event horizon.
It is true we don't fully understand neutron stars and we certainly don't understand what happens inside the event horizon.
Aren't the detected gravitational waves evidence that black holes exist?
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?
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?
Gravitational waves are evidence, two extremely large objects merged, not that a black hole was formed.
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).
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).
This is not correct, the nature of gravitational waves emitted by the merging of two objects is different in its final stage, depending whether the objects form a black hole or not. Until a black hole is formed, the nature of the emission is the same for all objects (e.g. black holes or neutron stars), but if a black hole forms, the final emission (ringdown) is different.
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.
[1] https://arxiv.org/abs/1710.05938
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.
[1] https://arxiv.org/abs/1710.05938
But they can tell the distance between centers of mass and total mass. Haven't we observed that distance shrink until it reaches their Schwarzschild radius?
(I really don't know the answer.)
(I really don't know the answer.)
See my parallel comment, ringdown is different for black hole remnants vs. something else, so in principle we can tell if it resulted in a black hole or not. However, for objects of suitable mass (a.i. small) the current detectors don't work very well at the frequency required to make this assessment accurately, so secondary evidence is required.
I think you’re confusing the lack of knowledge of whether or not singularities really form, with whether or not black holes form. There are very clearly black holes, but of course we can have no way of knowing if they contain singularities. Quark stars could exist, it not with the masses observed in Sag A*. Quark degeneracy pressure isn’t going to support the mass of millions, or even billions of suns. It might be that something other than a singularity is within the event horizon, but there will still be an event horizon. Given that, we’ll never observe the interior of a black hole and be able to communicate those findings, so it’s purely academic at that point.
As for time dilation... no. In no way could that magically overcome collapse.
As for time dilation... no. In no way could that magically overcome collapse.
What's a black hole without a singularity? A neutron star? Is there a distinction you are making? To me a black hole and a singularity are the same thing. Are you using a different definition?
> 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.
> 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.
> What's a black hole without a singularity?
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?
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?
A better definition is a black hole is something with an event horizon.
(for your last comment, see my previous response)
(for your last comment, see my previous response)
> something with an event horizon.
... 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).
... 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).
> 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.
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.
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.
Thank you all for the various comments made here, both for and against "black holes".
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.
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.
Source for the ring of fire image at the bottom of the page: http://eventhorizontelescope.org/simulations-gallery under "> Accretion Disk"
Magnificent. I’ve been totally entranced by black holes since I first read A Brief History of Time as a kid. Years and years of trying to wrap my head around them, and they’re still just about the closest thing I get to a spiritual moment.
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.
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.
If nothing can escape black holes how is it they “spew” things?
They also (are believed to, based on famous calculations by Hawking) radiate like a black body with a temperature related to their mass. And this leads to them losing mass over time, eventually evaporating. (Leaving what is a subject of much debate.)
I believe the "spew" that booleandilemma was referring to were the twin perpendicular "jets" that black holes often seem to eject, for which the explanation is probably some sort of angular momentum/near miss close to the event horizon sort of thing.
The things aren't coming from inside the black hole. They're coming from a region near, but outside, the horizon.
The jets shown in the article come from the accretion disk [1], a disk of matter orbiting the black hole, formed as matter 'spirals' in.
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
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
I love black holes
Between two pillars they lie
Dark and mysterious
It pulls me in
Its gravity is so strong
And out comes
A new born star.
Between two pillars they lie
Dark and mysterious
It pulls me in
Its gravity is so strong
And out comes
A new born star.
Black holes are an excellent example for so called "cognitive metaphors".
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.
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.
> 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 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.
https://youtu.be/MTY1Kje0yLg
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.
https://youtu.be/MTY1Kje0yLg
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 connecting them to our Universe, this is why light doesn't leave the inside of the event horizon, the path between there and here only goes one way, unlike the rest of space-time.
"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.
"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.
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.
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.
> 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
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.
Hawking radiation takes energy away from the inside of a black hole.
Certainly, but the radiation still comes from outside the event horizon.
Sounds like the confusion here comes from labeling matter outside the event horizon as "not the black hole". With this wording you can say that nothing escapes the black hole by forever shrinking the region of what constitutes the black hole.
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.
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.
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 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.
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.
Gravitational collapse overwhelms fusion pressure much before the black hole stage is reached, i.e., neutron stars. Also general relativity seems to indicate unambiguously that the in-falling matter collapses to a singularity, which obviously cannot have any kind of structure or process inside it. I guess only a quantum theory of gravity can clear up this enigma.
(Not to mention that once one has crossed the Schartzchild Radius time dilation becomes infinite from the perspective of the outer universe, and time freezes, so any process becomes frozen in its tracks, at least from the perspective of external observers.)
What happens in the Swartzchild Radius, stays in the Swartzchild Radius.
Yes, but that process is never truly reached since time has to slow down infinitely for that to happen.
Why does the perspective of outside observers matter at that point? Aren't we talking about what happens inside?
True, for the in-falling person, the outside observer's perspective doesn't matter and vice versa. But what happens inside is, (and probably will always be), entirely theoretical, but we can observe the space just outside the event horizon.
This is untrue, gravitational collapse does not overwhelm fusion pressure, fusion stops, that's why neutron stars are created.
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 the event horizon are still happening inside (plus, black holes form from supernovas, which happen precisely because fusion ceased in the star (simplified explanation)).
> 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
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
Obviously the inside of a black hole does exist on account of the fact that it is able to warp spacetime in an externally detectable manner. If the interiors of black holes didn't exist they would be indistinguishable from empty space, and not seem so remarkable as to be called "black holes."
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.
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.
From my earlier comment:
>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).
>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).
The inside of a black hole doesn’t exist as far as our universe is concerned. Even the ‘gravity’ isn’t ‘emanated’ by the mass ‘inside’ the event horizon; rather it is a ‘recursively’ generated field (remember: gravity is generated by mass-energy curving space-time, gravity is a form of energy, and so gravity begets gravity in an entirely self-sustaining manner).
This is a misconception of spacetime. The object exists, it just consumes faster than it dissipates mater. Just because there's no discoverable interaction between the consumed matter doesn't mean it doesn't happen, which is all time really illustrates.
From our external perspective, they never happen. The time they use to happen in doesn't exist.
So, in our reference frame, black holes never grow from original collapse and remain in that same state, tiny mass and size, no matter what falls into it. Wait, no. It loses mass through particle emission, which is assumed to actually happen. This puzzle was worth investigating when I learned this topic (not sure I understood all math and concepts correctly though).
I mean, the real answer is "black holes are very complicated" and a full explanation is out of scope for a HN comment. Hawking radiation requires a lot of background knowledge to understand (and I have to admit I still don't fully understand it). The pop-sci explanation of particle/anti-particle pairs appearing on either side of the event horizon is incorrect.
I would argue that calling them "dark stars" is just as misleading and "limiting thinking". In fact, I think you are a victim of this in your post: you incorrectly ascribe nuclear fusion (which is typical for many stars) to a black hole.
> 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.
Would it be black? Right above the event horizon is flaming relativistic matter.
It is my understanding that black holes create new stars. It's also my understanding that small mass will eventually be drawn towards bigger mass. Does this mean the eventual "heat death of the universe" won't actually happen as stray light/heat/RF/particles/whatever will always get drawn in back towards a mass like a blackhole and reformed?
> small mass will eventually be drawn towards bigger mass
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.
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.
The term “Dark Star” actually predates the term “Black Hole” by a very long margin (first recorded usage 1783) [0] for classical Newtonian objects whose mass is so large that the escape velocity at their surface exceeds the finite speed of light. Black Holes are very different beasts (self-sustaining concentrations of space-time curvature), despite their popular explanation as being “objects so massive space cannot escape”.
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...
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 believe the term "frozen star" was also used for a time to describe black holes. I'm no physicist but I'm not sure this is an appropriate term either: blackholes are supposed to be really cold inside but really hot from the outside.
I think frozen star refers to the fact that from an outside observer's perspective, the star seems to stop collapsing at the last possible moment, suspended in time as it were, because light from the moment of black hole formation takes an infinite amount of time to reach the observer.
> It is theoretically possible that the black hole is still performing fusion, and emitting light and heat, like a regular star.
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.
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.
To me, the concept that there is another universe, beyond the Event Horizon, is a little silly.
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.
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.
Ah, that story when realistic physics happened and she loves him. First question, lasers or blasters? Second, is good guy’s dad bad guy?
Third, probably least interesting, how does one get into BH travelling FTL, if that makes you go back in time?
Third, probably least interesting, how does one get into BH travelling FTL, if that makes you go back in time?
Lee Smolin proposed a really compelling concept in "Life of the Cosmos" - that the reason our universe has the universal constants that it does, is the product of natural selection at the cosmological level.
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.
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.
I personally subscribe to this theory. I also think it's plausible that the development of intelligent life may be a part of the universe's reproductive process and may end up facilitating it in some way
> From the outside perspective, all the information and energy sits on the border
Are you sure that's known? What kind of experiment would settle that?
Are you sure that's known? What kind of experiment would settle that?
Since at the event horizon (schardzchild radius) time dilation becomes infinite, essentially time ceases there. That's why it's called an “event horizon”: because events (four-dimensional coordinates) become disconnected there. As time is frozen (from an external observer's perspective) stuff ceases to move and just “piles up” there. From the in-falling matter's perspective, however, the transition occurs in finite proper time. Once it has crossed the horizon, it finds itself inside the hole where the direction of space pointing towards the horizon (inwards) becomes ‘timelike’ (in the sense that its flow cannot be arrested and it is unidirectional) and time becomes ‘spacelike’ (in the sense that it could be negotiated at will).
No experiment that I know of could confirm this because everything will depend on extrapolation from less extreme and finite scenarios.
No experiment that I know of could confirm this because everything will depend on extrapolation from less extreme and finite scenarios.
You could go into the black hole yourself but you'd need to communicate the results of your experiment by twitching the second hand on your daughter's watch who, conveniently, works for NASA.
Interstellar infuriated me worse than the average “space opera” Star Wars/Star Trek-type crap because it spread the pretence of being scientifically accurate and plausible.
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.
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.
In the first place it is just ridiculous to envision settling humanity on to a planet so close to a supermassive black hole & experiencing such extreme time dilation. It was foolish to have sent an astronaut there at all, let alone the later party going to check on her. But then, Kip Thorne confessed that Miller's Planet was essentially something the studios demanded and he wasn't given an option to exclude it. He wasn't even given an option to make the time dilation less extreme.
> 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.
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.
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.
> Because we found no better analogy, we named them "holes" despite the fact that they are basically the opposite: an object with enormous mass.
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.
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.
does it make sense to you that a "hole" has mass, electric charge, and angular momentum and is completely characterized by these properties?
Notice how an electron hole also has all those properties. That's because those properties are due to specific symmetries of various fields (in space), or symmetries of space itself. Those properties are not "in" the particle, they do not belong to it, rather they are constraints over what kind of interactions can happen in all space that is causally-connected.
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Great comment -- but while I agree w/ your point about metaphors, I respectfully mildly disagree about this one in particular. For context, I associate it with the popular analogy of timespace as an elastic "sheet" of sorts, with marble-like objects resting on (and sinking into) its surface, with their mass dictating how "deep" their gravity "well" is. With that metaphor in mind, black holes really do approximate holes in the sheet.
But even how we illustrate a sheet of spacetime we show it dip when it should stay flat and pinch, closing the gap between atoms as they get more dense.
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.
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.
I don't like the elastic sheet analogy because it doesn't work without gravity, the very thing it's attempting to explain.
I like it because it isn't quite as self-referential as it at first seems: the notion of gravity that it requires is not the notion of gravity it is explaining: it uses the colloquial and accessible idea of gravity (i.e. "things fall downwards") to describe a more precise idea of gravity (i.e. "things fall towards each other in accordance with mass").
This has always bothered me as well, and I'm surprised that this is the first time I've seen anyone else mention it!
I think this is something that deserves much greater general awareness. I think by default we're pretty unaware of the kind of analogizing/metaphoring that takes place when we conceptualize anything—but it's completely pervasive. I see it as a kind of subtle 'dust on the lens' of the microscope which we use to observe things generally. It's an (in a certain sense) inescapable artifact due to the fact the we conceptualize with a definite thing with its own properties and quirks and limitations, which is the human brain. Anyway, I see it as a kind of 'bias' (best I can think of atm, but that term isn't really broad enough to capture the idea) which we could watch for and in doing so make gains in objectivity.
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.
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.
One that I think comes up often is the robots/automation metaphor. Technology has reduced 90% or more of farm labour needs, multiplied industrial output many times, etc. We've seen a lot of automation, but it's been vague economics.
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.
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: an object with enormous mass.
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.
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.
A black hole can literally be considered severely warped spacetime. The high gravity is a consequence of the severe warping, and as for the internal contents, they don't matter to external observers and physics (so far) has very little to say about it.
No, black holes are topological holes. It's a mathematical abstraction, not merely a cognitive metaphor.
I'm pretty sure every "mathematical abstraction" is a "cognitive metaphor".
if you are unaware, this is extremely controversial.
https://en.wikipedia.org/wiki/Philosophy_of_mathematics#Math...
https://en.wikipedia.org/wiki/Philosophy_of_mathematics#Math...
No, not really. Symmetries of an n-gon (a 2-dimensional figure with n sides) are just permutations of the n numbers representing the sides. A cognitive metaphor is to call certain types of symmetries “rotations” or “reflections”, because we can’t really describe all symmetries through a metaphor like that. The symmetries are not literally operated by picking up your figure and flipping it over.
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.
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.
> 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 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.
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.
> 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.
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.
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.
I don't know what a cognitive metaphor is and whether it's applicable to mathematical abstractions. I was responding to the claim that:
> Because we found no better analogy, we named them "holes" despite the fact that they are basically the opposite
Which is false.
> Because we found no better analogy, we named them "holes" despite the fact that they are basically the opposite
Which is false.
Very true.
Although not specifically in this context, Ludwig Wittgenstein said:"The limits of my language mean the limits of my world."
Although not specifically in this context, Ludwig Wittgenstein said:"The limits of my language mean the limits of my world."
"Hole" is accurate. The "object" with enormous mass would be the singularity, whereas "black hole" refers to the nature of spacetime near the singularity, for which "hole" is an apt name from our reference frame. Also, we don't talk about "spitting things out of the black hole".
On the topic of how languages and metaphors affect (and might even form the fundamental substrate of) our thinking, I highly recommend Douglas Hofstadter (he of Godel, Escher, Bach fame) 's book Surfaces and Essences (2010).
Is it really an [excellent] example of that? We don't use much of the language we use for holes in general for 'black holes'. The term may have been influenced by 'Black hole of Calcutta', itself a metaphorical use.
So for example is it more accurate to say that things fall...on to...rather than in to a black hole? That’s fascinating!
Eh, that's taking it too far as the parent does.
"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.
"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.
Reminds me of one of my favorite moments[1] in Red Dwarf:
> 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?
1. https://www.quotes.net/show-quote/66989
> 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?
1. https://www.quotes.net/show-quote/66989
No mention of gravitational lensing (except in a dead comment here)? 20 years ago those images were in all the pop-sci media as evidence of black holes. They seem as strong evidence as x-ray sources.
It bugs me how almost never in astronomy-related animated gifs authors never tell explicitly how long is 1 frame.
In that blue x-ray burst image, am I looking at 1000 years per frame, 1 second? 1 minute?
Really frustrating
In that blue x-ray burst image, am I looking at 1000 years per frame, 1 second? 1 minute?
Really frustrating
I found this gif particularly amazing because it did. And the others a little underwhelming.
https://cdn.vox-cdn.com/uploads/chorus_asset/file/9984865/st...
https://cdn.vox-cdn.com/uploads/chorus_asset/file/9984865/st...
Saying we have not "seen" a BH is like saying we haven't seen the core of any other star. We haven't, we cannot and never will. Such things are hidden behind the massively bright regions they power (corona, acretion disks ect). We must calculate the interior of a BH as we calculate the interior of jupiter. That doesnt make thier existance any less real or profound.
If nothing can escape a black hole how can it have massive jets?
1. Energy does actually escape black holes
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!
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!
The jets come from outside of the hole.
If the speed of light of this universe was raised significantly, is there some point where black holes would be impossible to form?
I assume if the speed of light were lowered black holes would become far more common?
I assume if the speed of light were lowered black holes would become far more common?
>If the speed of light of this universe was raised significantly, is there some point where black holes would be impossible to form?
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.
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.
In the Death's End by Cixin Liu, there is a speed of light weapon that lowers the local speed of light to almost nothing, effectively taking an enemy out of the game until the end of the universe.
In Redshift Rendesvous (don't recall the author), there was an environment where the speed of light was just above running speed - maybe 15 m/s. You observed relativistic effects just walking around.
There was a game like this, I wonder if such a universe can be simulated in VR? Perhaps it would allow for a better understanding of relativity?
Yeah but if you do that, then you'll get a universe that's not this universe. Speed of light affects much of physics and causality.
Hard to imagine how different the resulting universe might be, what if the speed of light and causality were say 10x higher than what it is now? Or what if the speed gradually increased over time? Would we just evaporate?
I think not, though am just speculating as a layman when I say that. But physicists have theorised this in much detail:
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. :-)
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. :-)
I would have thought in such a strange universe there could at least exist some structures capable of knowing joy, but that thought has been thoroughly eradicated.
Where could I get that last image 1600x900. Wanna use it as my background picture because this looks incredible!!
serious question: the accretion disk lies flat in the same plane as the earth... so how do we get an image showing the black hole "top-down" ? do they take a guess at what the far side must look like, then rotate the perspective ?
either way, how is this new imagery any less synthesized than the old imagery ?
either way, how is this new imagery any less synthesized than the old imagery ?
Every time I read articles like this I feel bad about my career choice. How could anything I do compare to the fascination of space exploration? The universe we live in is nothing short of mind-boggling.
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'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.
But then who would deliver the ads?? /s
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...
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...
My favorite video about observing black holes: https://youtu.be/iphcyNWFD10
Enjoy :)
Enjoy :)
When I first loaded the article all of the images were black, and I thought it was just a joke. Then after about 30 seconds they started filling in with actual pictures...
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.