> 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.
> 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.
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.
> 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.
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.
> 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).
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.
> 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.
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.
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.
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.
> 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.
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.
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.
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.
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.
> 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.
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.
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.
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[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.
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.
> 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.
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.