They point out that as you approach a black hole, your proper time is advancing slower and slower when compared to an observer located far away from the black hole (like us, on Earth). That means that from the point of view of this far observer, nothing happens inside a black hole until an infinite amount of time. In other words, what is inside a black hole is separated from us not by a space-like frontier, but a time-like one. Events behind the event horizon are postponed to "the end of times".
Does it even make sense to say that those events exist, then? Are they any more real than fairy tales or mathematical equations? If not, we can make any speculation we want, including the existence of alien civilizations.
But the stuff that falls in is not purely described by its mass. It also has angular momentum, electrical charge, maybe a few other quantities such as charm and strangeness (speculating here). So the black hole must have a net angular momentum, electrical charge, charm, and strangeness. Electrical charge could be measured by holding an electrometer next to the black hole. And so forth.
So far I've described 5 quantities that "describe" a black hole. So my question is: How many more are there? What's the total number of numbers that accompany a particle as it gets sucked in?
Suppose it's a small number, like 11. Does that place an upper limit on the information content of a black hole, and hence the possibility of describing a "civilization" inside it?
The singularity is a prediction of GTR, but GTR breaks down there too. This is why black holes are so exciting in the context of new theories which are complementary to GTR!
It even preserves relative motion ! If A and B are both falling towards the event horizon, their relative movement doesn't change : by that I mean that if it was 2 moons, and someone launched a rocket from A towards B, it would take, say 10 hours to get to B. When both fall towards the black hole that time can still lengthen from the perspective of someone standing on A or B. It can shorten. It can stay the same. Depends on how it was changing before they started falling in.
So it could simply be that relativity stretches the space time near the event horizon enough for everything to fit in there, like that tent in Harry Potter. It looks weird from the outside, but if you're falling into the black hole it's the opposite: everything outside of that (very, very large) space near the event horizon is what looks weird. But if the black hole is big enough, it looks weird, but ... not very much.
The entirely weird thing is, you can choose initial conditions where all distances lengthen proportionally, in fact that's the more common scenario. So for all objects that are falling into the black hole, and objects not falling into it, all distances lengthen. Make the black hole big enough and the difference between things falling into it and things orbiting it or even moving away are very very small indeed.
And now you look at our universe, and that's exactly what you see. The value of the cosmological constant (ie. the universe is expanding, but ever slower and it will never actually stop expanding) can be explained by the assumption that we're falling into a very, very large black hole.
There is a TON of work done on BHs and the formulations to try to bend our knowledge of QM and 'classical' physics to them. If you really want to get into the nitty gritty of it start with this . It's a HARD road, but the proofs and knowledge in the field is mindblowingly cool. Go for it!
I've still got my copy of Jackson from grad school. ;-)
You'll probably find MTW a lot more approachable after working through a few less beastly texts.
That's exactly the right number to describe the macroscopic state of a no-hair black hole (defined as the horizon as seen from the outside) using a set of spatial coordinates: mass, charge, angular momentum for three spacelike axes, linear momentum for three spacelike axes, and centre-of-mass position for three spacelike points. Here I have already done a foliation of spacetime into a spatial slice with a constant timelike coordinate; it'd be normal to use coordinates in which the centre of mass is always at the origin, which fixes the last six components at zero.
On the other hand, if we omit the spacetime foliation, we are obliged to use tensor quantities for these variables; moreover, if the spacetime region we consider is not "sufficiently small", then no-hair looks much more conjectural. Foliating this region, we would expect at least the charge no-hair number not to change from one slice to another. For a black hole with a highly ionized accretion disk with nuclei or electrons liable to cross the event horizon at any time, this is a tough ask (and the subject of research in numerical relativity).
> charm and strangeness
The larger the black hole, the flatter the spacetime just outside the event horizon; astrophysical black holes' immediate neighbourhood is too flat to break quark confinement, so a surplus of colour charge seems unlikely. Conversely, a black hole small enough that tidal effects are very strong just outside the horizon is likely to be evaporating so violently that gravitational effects on hadrons just outside the horizon seems (to me, anyway) less interesting than scattering interactions.
> What's the total number of numbers that accompany a particle as it gets sucked in?
Good question. This is an area of research.
Any answer raises a second question: how does "balding" work?
Even fully classically we have this problem: if we drop a thin uniform spherical shell of neutral matter of mass M into a Schwarzschild black hole (so there's no rotation or charge or quantum-anything to consider) how do we distinguish that black hole from an identical setup except we drop in two concentric shells of 1/2 M each?
If in some future region of spacetime when the shells are inside the BH (black hole) we can distinguish between BH with one shell vs BH with two shells, then no-hair is wrong. If we cannot distinguish, then classical information is lost.
This is black hole thermodynamics because we have a relationship between macrostates (the no-hair values) and microstates (the set of values that migrated from outside the BH to inside the BH), and we can define entropy in a Boltzmannian way using that relationship. If no-hair is accurate such that we can throw in an huge number of shells each with a tiny fraction of the mass of our single-shell example, then that black hole's entropy is enormous.
The quantum picture is in some ways "just" a complication of this fully classical information loss problem. If we can throw in whole molecules / bits-of-dust, atoms and ions of various masses, free electrons, photons, neutrinos, and so forth and still see a "no-hair" set of macroscopic variables, then a reasonable-size BH's entropy is enormous, and it gets much more enormous if we only increase the BH mass while keeping the other no-hair values always zero (and letting the others vary does not help much).
If in our universe we find a black hole which we can comfortably describe with a tiny number of variables (e.g. a no-hair black hole), we should expect that it will have lots of hidden microstates thanks to things having infallen (and indeed, for astrophysical black holes, lots of particles from the stellar remnant). Did these bald away in the past? Or, if black holes evaporate, will these hidden microstates be revealed during that process?
> what's inside
Good question. There's a wide variety of guesses by theoreticians.
As we increase our number of observations of BH-BH, BH-NS, and NS-NS (NS for neutron star) mergers where we get decent gravitational wave signals, we can exclude many possible answers to these problems.
> an upper limit on the information content of a black hole
No-hair black holes' event horizons are determined by the no-hair variables alone; their entropy limit is related to the surface area of the horizon.
> a "civilization" inside it
We don't know without a full answer to "what's inside". The "Eldar" idea is that for an extremely massive black hole with an improbably large charge (how does one prevent such an object from drawing in matter of the opposite charge?) there may be stable orbits in an interior region. I think even that is a really big stretch.
So how do black holes gain any mass then?
Well, from our point of view all incoming mass gets stuck very close to the event horizon. That's not too surprising considering the amount of information of a black hole is proportional to its surface area. So from a far away observer a black hole is more like a sticky sphere than an hollow ball.
At that point we probably can't tell more without diving into the math, and it's beyond my abilities. Yet the gross idea does not seem absurd to me.
It is also important to distinguish between the implications of a model black hole formed from the uniform collapse of a perfectly spherical dust cloud, from infinity, in an otherwise empty universe, with no charge or angular momentum, from what happens in nature.
So in that case, you wouldn't be seeing the "future of the universe" just the "future" (I guess) of the space around the black hole.
I kind of see blackholes in space as vortexes in a lake. Anything that gets "trapped" in the vortex moves much faster, including the water (space-time) itself. It doesn't impact the rest of the lake, except for the things that are on a collision course with the vortex, and then get swallowed by it (and spit back out?).
So I had the idea that smaller black holes are at the center of the sun, the earth and so on, being the principle source of gravity and the "movement" that we see is just us falling into different black holes at the same time, which are also falling into each other. So micro black holes must be at the center of massive particles too. The world line of a photon on the other hand is just the intersection of two event horizons as they grow, so you get a wave model. And that's why you have entanglement: circles have two intersections, so if your model is two dimensional, you get two entanglements. But you can have vastly more complicated geometries and thus assembles of entangled particles.
I don't know the "standard model" well enough to take the analogy any further, not to mention string theory and all that jazz.
>So micro black holes must be at the center of massive particles too.
If an object has a schwarzschild radius smaller than its own radius, then it isn't a black hole. That literally describes all non-black-hole objects with mass. That's just the standard manifestation of gravity.
>The world line of a photon on the other hand is just the intersection of two event horizons as they grow, so you get a wave model. And that's why you have entanglement: circles have two intersections, so if your model is two dimensional, you get two entanglements.
Is there any connection here besides that an event horizon and the sum of all possible paths of a photon in a given amount of time are both spheres?
If there were a micro-black-hole inside of a particle, its event horizon would have to be within the particle, or else the particle would just be indistinguishable from a black hole. The particle wouldn't be like a normal particle with an invisible spherical event horizon surrounding it and affecting its interactions.
I think you meant to say ether.
This is not an analogy as much as an example of an external-vs-internal observer problem. When you close the (opaque, insulated) door of your fridge, observers inside will see the (filament of the incandescent) light significantly dim, and if the door stays closed long enough, will see the light thermalize with rest of the internal volume. Someone standing outside the fridge might not even see the initial dimming; indeed, that observer may only ever see the light as "on" (rather than "heating from cold" or "cooling from hot").
 We could talk about naked singularities a bit: this usually means that there is at least one outside-the-black-hole observer for which the shape of the horizon is such that the centre of mass-energy of the BH is outside the horizon, rather than an observer for which there is no horizon at all. However, even these scantily clad BHs don't arise in realistic universes described by General Relativity. Fully naked singularites (where at least one observer exists which does not see any horizon at all) require an alternative theory of gravitation, or conditions extremely unlike those anywhere in our universe.
 Consider the observation of a supermassive black hole at the edge of the observable universe. From our view here around Earth, we see a race between a very bright star about to cross the black hole's event horizon and the black hole about to cross our Hubble horizon. Observatory A sees the star vanishing behind the horizon just in time; Observatory B sees the BH cross out of observability before the star vanishes behind the BH horizon. A and B have (very slightly) different Hubble horizons focused on them , and also with a (n also slightly) different radial distance to the BH horizon. "B" can never directly see the same coincidence of events that "A" sees; should "B" deny the infalling?
 Maybe this is illustrative of observer-centred observables? Glories (an optical phenomenon similar to rainbows) are so observer-specific that you and your handheld camera will have different ones (and each of your eyes will have different ones). As noted in the "From the air" subsection, we can tell what seat a photographer of a glory from a plane must have been sitting in. https://www.atoptics.co.uk/droplets/gloim1.htm
Likewise, we can determine the location in spacetime of an observer of a star-into-black-hole event from that observer's detailed description.
My immediate, naive instinct is that by crossing this limit, the frame dragging effect, would be extremely powerful, to the point where it might increase the radius where Hawking radiation is formed/emitted and increase the rate of Hawking radiation to avoid passing the limit. A sort of self limiting process to prevent breaking the speed limit of c.
But in the time it took to write this out, I remembered angular momentum and realised that notwithstanding the additional angular momentum of the infalling mass, the conservation of angular momentum would just make the event horizon slower as it expands. Which makes the superluminal event horizon unlikely in my mind.
The biggest issue, aside from the model, is that time dilation is something which only matters when two observers “compare clocks.” Neither observer alone ever experiences a difference. The crew of a 99.9% lightspeed ship doesn’t experience time dilation... until they return home. It makes no sense to talk about the effects of time dilation from the point of view of a one-way trip to the event horizon.
That's not true. They see the universe around them moving much faster.
Time dilation has nothing to do with "returning home".
Common sense dictates that the probe and the sample would require equal fuel to return to earth; but to an observer riding this cigar-rock, why would the universe cut our probe some slack if we changed its kinetic energy rather than the observer?
That has to do with the experience of their frame of reference. Time does appear to “slow down” for them, rather everything else will seem to “speed up.” You can infer the difference, but you can’t sctually communicate that or compare with anyone else until you decelerate. In the extreme case of a gravitational event horizon, there will be no ability to ever communicate again. The fact that external observers will see you infinitely redshifted doesn’t imply anything about your experience of subjectively falling past the horizon. Both are valid frames of reference, but ultimately will develop spacelike separation which prohibits further communication.
As it relates to the issue st hand, you can’t make accurate statements about mass never passing through the EH based on observations from a distant from of reference.
So, if you don't sense anything, you don't sense time dilation either?
This is slightly more complicated. First of all, you haven't given a frame of reference. If you claim someone were moving at 0.99c then you have already set the frame of reference. And they would have to gain near infinite mass and would die. You seem to assume a restricted frame of reference though, inside the spaceship. So, a point of reference inside the spaceship would see light moving with c inside the spaceship. And would assume his own point of reference as the origin of the inertial frame of reference. So baring any outside measurement, how do you know the spaceship is moving with 0.99c and in which frame of reference?
Recently, I worked out the ISS orbit using the Schwartzchild metric as an approximation of Earth. It's cool to see the orbital period pop out and agree with real life! It's then just a small step to calculate the time dilation experienced by ISS astronauts.
The answer is kinda easy if I can make up my own intrinsic definition. The mass is the mass of the stuff around the black hole. A black hole is a singular point, it can't have mass, don't be silly.
I have heard "the theoretical minimum" suggested in the past, but I haven't heard from anyone who has actually used it to go from zero to say, general relativity, only people who think it sounds good. It also seems like a steep investment -- I just want to have a decent enough grasp to separate sense from nonsense and hold a picture in my mind.
One related thing I have wished existed was a good place to ask questions about and discuss topics like this while learning them, but it seems like in theoretical physics, everyone (including myself) has an infinite number of different stupid questions and misconceptions, to the point that we drown each other out, and answers to our questions often have little cross-applicability.
So, would love ideas for either!
You say you haven't heard from anyone who has used it to go from zero to say, general relativity. You might want to look into the adult alumni of the class.
EDIT: You might also find another HN post interesting, it's currently at the top of HN classic: "Learning classical mechanics through Haskell", here: https://news.ycombinator.com/item?id=16453192
(And a direct link to "Classical Mechanics via Scheme", https://mitpress.mit.edu/sites/default/files/titles/content/...)
Nobody has ever discovered a way to truthfully map physics concepts into English sentences that you can understand without having to have explored the entire definition tree.
This route provides a gentle, productive ramp as you make sure you really want to dive deep.
I've done that and, ideally, I'd like to now buy some college physics books. However, I may have to delay this next step for quite a while due to some other higher priority goals... We will see if I get back to it. There is too much interesting shit and important things to do in the world, heh.
It's a little bit like The Little Schemer in that it often asks you a question and expects you to try to answer it before turning the page (or turning it upside down). The problems tend to go like "if you did this, would that (A) increase (B) decrease (C) stay the same" or "(A) increase by one (B) double (C) quadruple" -- to minimize technical machinery while still engaging you with genuine problems, not vague analogies.
The same author has a book on relativity which looked very good from the first couple of chapters I read.
I think teachers often welcome any question. On coursera for instance teachers are usually available to answer students (via a mail-like interface though, don't expect to chat with them via Skype or anything). There are several courses on general relativity.
It's incomplete but there's some good stuff there.
For Example: http://astro.cornell.edu/undergraduate-studies.html
Is there something about it that I have missed?
This is mind blowing...shatters my misconception of blackholes....is there any more books on this?
If I understand correctly competing theories disagree on more complicated descriptions, those that ask different questions or want to describe more details.
It's just that one of these point of view is ours, and from this point of view, the other point of view does not correspond to any event that will actually happen, like ever.
In other words, the "happening of events" is a relative concept. Just like simultaneity. Not only two events can happen at the same time or not depending on who is observing them, but the very happening of an event can also depend on who is observing (or not observing) it.
That "happening of events" is very close to a definition one can give to the concept of reality. So one might conclude that reality is a relative concept.
Take a black hole that’s been slurping matter in and it’s all at the event horizon because as you say, it takes forever to cross. But as that mass builds and builds doesn’t the event horizon move? Take the scene to the limit. Say a black hole never absorbs anything. The amount of mass parked just above the horizon becomes so great that another black hole forms. Now you have two overlapping horizons, which can’t stay that way. What really happens is that the density of mass inside the radius near the black hole is sufficient to be a black hole of a larger diameter and then it just... is.
I think that what this means is that you never get to the event horizon, but the horizon comes to you.
To external observers, of course nothing can be observed as falling through the event horizon because not even light can escape past the event horizon. So no light from beyond the horizon of the probe can ever reach the observers. But that doesn't mean things can't go past the EH normally, or that here is some other magic happening like
> I think that what this means is that you never get to the event horizon, but the horizon comes to you.
A black hole is just another object with a very very powerful gravitational field.
Near the horizon a heartbeat takes eons from the perspective of someone watching you fly into the black hole. Assuming of course that you haven’t already been torn to shreds by the high energy particles and gamma rays traveling perpendicular to your path...
Incorrect. You would still be relativisticly separated and this would be measurable especially at infinity. You would be accelerated by 4 billion more years and would have that much more relativistic acceleration in signals you sent out. This could be monitored and the outside observers would be able to tell the difference. The second person would be more blue shifted compared to you (clock ticking faster).
> Does this mean everything that falls into a black hole crosses the event horizon (from their perspective) at the same moment, a moment which only exists from the crossers' perspective, as it is always infinitely far away from a perspective outside the event horizon?
No, their clocks tick away just like they always did. They cross the event horizon just as they thought they would. They do 'see' the universe's clocks accelerate along though. But they do cross it nonetheless.
Note: By 'see' I mean that the rest of the universe blue shifts to infinity and the Lorentz transformation causes the incoming radiation to be directed to directly in front of you. Only when you cross the event horizon, the incoming radiation from the 'outside' turns into an infinitely energetic (infinite blue shift) beam of radiation coming directly at you. In your reference frame, you vaporize.
This is my BHs are soooo crazy. When we run the numbers, the Quantum Mechanical effects really do affect things. Infinitely small wavelength light isn't something we can really handle, the Plank Length should come into play. But General Relativity don't care. This is why we really need Quantum Gravity, because nothing makes sense.
I also really need one of these:
Granted, there is a bit of tree falling conundrum here, but that does not make the question less interesting (arguably, it even makes it all the more so).
The key is the Relative in Relativity.
Truthfully, there is no clear answer as to what a black hole has for an interior, or if it really has one. Holographic theorists would argue that the event horizon is the black hole, String theorists would argue that there is an interior, but no singularity. We just don’t know.
Thank you. To me, this has been obvious since I learned about the time slowing required by GR. Since then, I was unable to understand why people are talking about things "falling into" black holes. (There's an episode of SG-1 where a team is trapped by a black hole, and times keep slowing, but they never realize that this means that team WILL NEVER DIE.)
Really good book
A similar theme is in "Pushing Ice" by Alastair Reynolds.
Someone has built machines that capture beings and then use near-light speed travel to align civilization in time.
>They would also be brightly illuminated by the central singularity and by photons trapped in the same orbit.
That sounds creepily like a typical description of heaven. Timeless, ageless, godlike beings all in a region of intense uniform brightness.
Maybe the background radiation is the part of the hawking radiation that falls inward. And we perceive it infinite but it’s the dilatated space between the hevent horizon and the naked singularity at the center. And the big bang was the supernova that left the black hole.
I love animations where it zooms out farther and farther into space, until it gets to the point where galaxies start to resemble molecular structures.
And then of course The Simpson's did a couch gag where exactly this happens and they zoom out and out into space until it finally zooms out of Homer's own head.
In short, our universe looks more like a white hole than a black hole.
If we’re in a simulation, maybe it’s the final result of the computation. If we’re in a black hole, maybe it’s in the past of the universe within, but in the future of the one without. All good material for a short story, one of these days…
I’m not sure why I’m being so heavily down voted for stating this fact you can find in any cosmology textbook.
This is true...
> meaning we exist inside of a black hole as big as the observable universe.
This is an unwarranted jump. I'd recommend looking at the reference  from Sean Carroll.
If by "black hole" you mean an object sourcing a Kerr-Newman-like metric, then you have the problem that the distribution of matter in the distribution we see is not at all like the interior region of such a metric. The metric one can infer from the bulk distribution of the visible matter is best described by a Robertson-Walker metric.
Under time-reversal, galaxies in R-W fall through each observer-centric horizon, break up into dust and gas that in turn combines into denser higher-energy and lower-entropy states.
While this might seem a little like a "white hole" (defined as a time-reversed BH), since under time-reversal a stellar BH has Hawking radiation rush into its vicinity becoming relatively dense at the time of the bust-up of the BH horizon, the difference is on the other side of that point a much lower entropy neutron star or similar body emerges from the high-entropy BH. Even stepping back a bit, macroscopically, galaxies are a lot more structured than either Hawking radiation; nor is Hawking radiation more structured than the cold relic fields crossing into optical detectability of an observer in time-reversed RW or time-reversed asymptotic de Sitter space.
This is important: the densest phase of a universe like ours is (extremely) low entropy, while the densest phase of the black hole is (extremely) high entropy, especially if we measure just at their respective horizons. Entropy here is Boltzmannian: the log relationship between the observed macrostates and microstates that can produce them. (We could even test this directly by comparing the structures in the relic fields with Hawking radiation if we were to find sufficiently low-mass astrophysical black holes; we can however get some details from the ringing modes of BH mergers).
Additionally, we can consider curvature. The extremely low bound on our universe's Weyl curvature allowed by indirect and direct gravitational wave observations (so far) completely precludes a Kerr-Newman-like solution, where Weyl curvature will be very large. Our imaging of ever older galaxies does not reveal them to be spaghettified.
Finally, it's worth noting that while Kerr-Newman and Robertson-Walker can have optical horizons so can many other solutions. For example, there are optical horizons in the Gödel solution but we obviously aren't in a Gödel universe. There are also many solutions which cannot have optical horizons; a torus or Klein bottle universe is clearly different from a Euclidean one, although all of those can be everywhere flat and horizon-free. So I don't think one should promote the presence of optical horizons into the defining characteristic of a metric.
> We are in a black hole.
This is straightforwardly wrong. Solve the null geodesics in the near-horizon for any observer you care to conjecture, and you'll see.
Could it be that these type III civilizations are simply consuming so much energy that that in itself becomes the black holes?
The KARDASHEV Scale explained.
Such civilizations might be like “simple germs”, their self-sustaining black hole vessels interacting and stirring things together; perhaps the cores of the most “interesting” galaxies are sentient, waiting for one femto-tech civ to arise from a trillion slime-ball planetoids — to make first Contact...
The more general possibility of “intersecting” reality at right angles has been taken up in a few additional works I can think of: in Count to Infinity and Excession.
[ ... snip ... ]
> In theory, highly advanced aliens could live on such planets, being unobservable from outside while exploiting the high energies and large time dialtions available.
reminds me of Frederick Pohl's Heechee Saga.
Also, isn't the assumption that there is a "big crunch" (also a good name for a cereal if it doesn't exist)? We're not sure whether there is a big crunch or heat death? This still seems plausible whether there is a big crunch or heat death - either way a black hole may be the only way for a civilization to stay alive at "the end".
The black hole wouldn’t be older than the universe. The black hole is just older than the last, most recent big bang, and the big bang doesn’t explain anything anymore.
It’s disproven as an event of creation. The big bang just becomes different parlance for turtles all the way down.
what if Buddha was right in that there are thousand fold universe systems each with their own unique civilizations?
If there are blackholes older than the universe itself, what if that suggested the universe was cyclical and that we may be in some gazillion-th iteration?
what if big bang was just a universe eventually being consumed by a monstrous blackhole that collapses itself?
What about the strange UFO occurences that pentagon acknowledged, are we getting visits from aliens within our universe or from another universe? Have they figured out how to travel between multi-verse, that is if you believe in it?
No proof, no way to find these answers but it really makes me wonder, especially when particles behave strangely like being coupled regardless of distance...but how could Buddha have known that blackholes existed?
Anyways, just some things to ponder and gawk about...
The Pentagon has never acknowledged the existence of extraterrestrial craft, or of UFOs as being anything but hoaxes and misidentified, mundane phenomena.
>but how could Buddha have known that blackholes existed?
He didn't, any more than the Norse knew that wormholes existed when describing the Bifrost bridge or the World Tree. Reading modern scientific meaning into ancient religious ideas is a common way to attempt to validate religion, but doing so does a disservice both to the religion and to science.
The Buddha may have had many insights, but none of them involved the relationship between spacetime and gravity.
We don't know if the footage is aliens, but we do know that the footage is real.
Yes there are. Answering such questions is why physicists are searching so hard for a unified micro/macro theory. Black holes are both quantum mechanical (at their singularities) and relativistic.
> how could Buddha have known that blackholes existed?
Doesn't look like a description of black hole, honestly.