From the point of view of the object being hurled into the black hole, things proceed more or less normally: you accelerate as you fall, but light behind you becomes curiously more blue and brighter. You eventually reach the event horizon, which ought to be called EH-sub-0, because it is only the first event horizon. It might be helpful to think of a black hole's interior as infinitely layered event horizons, event horizons "all the way down." You'd note a restriction of movement -- "up" (away from the black hole) is no longer a possibility; every direction is some variation of down, perhaps down and to the left, down and east, whatever, but always down. Eventually tidal forces take over depending on the size of the thing -- you might notice them before or after the outmost event horizon, and "spaghettification" occurs even as you are pummeled with X-rays and gamma radiation from behind (millennia of impacting photons blueshifted and jammed into a smaller timeline).
From the outside, however, your astronaut or thrown Cylume lightstick becomes more and more red, and dim, slowly approaching that event horizon but you'll never see it get there as it now emits infrared and not much of that. You switch on your FLIR and you can see it, for a while, but it grows dimmer and eventually disappears off of that. Eventually it emits very weak radio waves, and you lose track of the thing, but even if you spent a million years building longer and longer antennae, you'd never see it hit.
The effects you see of a black hole are 1) gravitational lensing (photons bending in their trajectory around the exterior of a black hole, just above the outermost event horizon), 2) the formation of an accretion disc (as matter swirls into it, growing hot from friction and compression), 3) absolute blackness if you managed to get a "transit" of something particularly large across a light path, 4) other knock-on gravitational effects, like disturbed orbits.
You'd notice if you were doing calculations as you fell. Locally, nothing particularly special happens at the event horizon, as far as I understand; just, past that point, all future directions lead to the centre of the black hole. (Plenty of past directions still point outwards, so you wouldn't even see anything particularly special unless you were paying close attention to, say, the apparent shape of the black hole.)
Could the gravity not be the parabolic reflector? The actual black hole itself is creating the "dent" in spacetime and the gravitational lensing is "pushing" the forces in a parabolic manner. Since that gravitational lensing keeps things focused for a very huge distance, it could keep the beam collimated, no?
Sorry, I love this topic, but I don't know much about physics. This might be a really dumb question.
The lensing happens in all directions.
And wouldn’t the density of in-falling matter quickly exceed what is physically possible?
2) No. What's physically possible is the ridiculousness of a neutron star's core. If you surrounded an event horizon with something of that density with the snap of your fingers (say one hemisphere), well, just as suddenly you would have a local amount of mass that would be quite formidable and the event horizon would seamlessly expand in that direction. You wouldn't see it, just watch your half-neutron-star shell wink out as you have "too great" a concentration of mass in a particular volume. See #1.
Probably. We don't know for sure because quantum effects might change things, and we don't yet have a theory of quantum gravity. The event horizon of a black hole is pretty much the one place in the universe where the effects of quantum gravity are most likely to manifest themselves, so one should hedge one's bets when making predictions in their vicinity.
> It has never made sense to me, since we see effects of black holes all the time.
Falling into a black hole is different from falling into a regular gravitational field. All kinds of weird shit happens before you reach the event horizon. Among other things, tidal forces rip you apart, heat you up, and turn you into a plasma. That plasma emits radiation, and that is what you see (because all that happens outside the event horizon).
That’s not correct: It’s a supermassive black hole at the center of a galaxy; the tidal forces will be weak at the event horizon.
The more sensitive whatever tool you are using to detect the photons is, the longer you can watch, and something that approaches infinite sensitivity would be able to see you for a time period that also approaches infinity, but outside of the realm of the theoretical, anything falling in to the black hole will wink out of existence in a fairly large hurry once it reaches the event horizon.
But things like accretion disks and these relativistic jets are happening outside of the event horizon, so they're not subject to these same concerns to begin with. For example, the accretion disk of Sagittarius A*, the supermassive black hole at the center of our galaxy, has an accretion disk that is roughly 1/100th of a light year, or about 25 times the size of our solar system. The event horizon, however, is only about 16 million miles - or a roughly 1/6th the distance between the sun and the earth. (These numbers are based on our current best estimates - and those estimates have changed frequently over the past 20 years as we get better data, but the general scale should be quite accurate)
> So if you, watching from a safe distance, attempt to witness my fall into the hole, you'll see me fall more and more slowly as the light delay increases. You'll never see me actually get to the event horizon. My watch, to you, will tick more and more slowly, but will never reach the time that I see as I fall into the black hole. Notice that this is really an optical effect caused by the paths of the light rays.
> This is also true for the dying star itself. If you attempt to witness the black hole's formation, you'll see the star collapse more and more slowly, never precisely reaching the Schwarzschild radius.
I don't understand everything but it seems that the falling guy tends towards the event horizon, getting slower and slower relative to the observer, and never reaching it from the observer's point of view?
Edit: this applies when observing something falling into the black hole. It doesn't apply to faraway objects that deviate because of the black hole's gravity well, so we can observe most of a black hole's effects.
The wiki page isn't very clear about how the jet is created: https://en.wikipedia.org/wiki/Tidal_disruption_event
Here’s a nice simulation showing this in action (article in description): https://youtu.be/g1aW8TDOm4A
The YouTube channel Kurzgesagt also has some pretty good illustrations and animations.
The object, meanwhile, does fall in from its perspective.
Or do I have that backwards...
If you were falling into a wormhole, you would die of old age before getting there while outside observers would see you fall in quickly.
The far away observer sees the falling one infinitesimally approach the event horizon, but never cross.
The more sensitive the instrument, the longer you can observe, and this doesn't really have a limit - as sensitivity approaches infinity, so does the length of time you could continue to observe the object.
But for practical purposes, we would not see a black hole as some sort of weird psuedo-magnet with all sorts of junk stuck to the edge of the event horizon.
For example, lets say you dropped a beacon that flashes every second into a black hole. As it approached the event horizon, you'd see the flashes only happen every 2 seconds, 4 seconds, minute, hour, decade, etc. Meanwhile, the length of those flashes are getting longer at the same rate, while producing the same number of photons, so the light gets dimmer and dimmer.
I guess you're assuming zero velocity by that person. But if there is velocity, wouldn't the fall not be straight towards but in an ever shrinking/decaying orbit trajectory?
I should have remembered the film Interstellar, where an observer on a spacecraft orbiting a massive planet (itself orbiting a black hole) feels decades pass whereas the landing party to the massive planet experience about an hour go by.
Even wording this sentence is difficult! I'm reminded of the Persian word for time, which literally means "the thing that passes".
> since we see effects of black holes all the time.
We do not, we see super massive objects, or black holes in progress, but no actual black holes.
This is incorrect. They necessarily do not have an event horizon that light cannot escape until they are a black hole, so as they have finished forming into one by the time we can no longer observe past the event horizon. We can also still witness the black hole growing even after that point. [1]
1. https://www.forbes.com/sites/startswithabang/2020/01/11/ask-...
It basically say that yes, it will take an infinite amount of time, but because the mass is able to arrive at the black hole before it becomes a black hole, it can first arrive at the black hole, and then after that the event horizon will grow to encompass the new mass.
I'm sorry, but that's just ..... I don't even know how to respond. The mass arriving and the black hole changing mass happen at the exact same time, as the mass arrives, the time dilation of the pending black hole increases, and the mass never actually arrives. These things don't happen sequentially, they happen simultaneously.
1) Again, it cannot have this event horizon until it becomes a black hole. That is one of the defining features of a black hole. We will be able to see photons escaping through the collapse period until it is a black hole, and once photons can't escape, it is already a black hole.
2) Time does not flow universally with a single reference frame. It appearing to take infinitely long for something to fall into the black hole from our point of view does not mean that it does so for the thing falling into the black hole or for the black hole itself. Time flows normally for the observer, and if they are falling into, say, a supermassive black hole with limited tidal forces, and in a spaceship that can protect from the radiation, etc. from the accretion disk and everything else, they will not notice anything particularly different about the low of time as they fall.
In fact, for black holes of sufficient size and with sufficient charge or rotational velocity, there's actually a second event horizon past the first that could even potentially support stable orbits, even those of planets or entire solar systems. It's likely only a theoretical thing - we don't know a mechanism that would generate enough rotational velocity or charge for a black hole large enough for there to be enough room to push this second event horizon out far enough from the singularity to make this practical. But time flows differently for both parties, and the increase in mass in the black hole is immediate for the black hole itself, as is the increase in the event horizon's radius, as it would be for anything falling into it.
Keep in mind that from the POV of the infalling object they experience the entire lifetime of the universe! The two POV's are complimentary - just like it takes until the end of time to fall in (from the POV of the infalling object), it also means I never see the black hole form.
i.e. from both POV's the black hole only forms at the end of time (which of course is never, since time never ends).
>Because from my POV here on Earth, just like I can't see the final object fall in a make a black hole, the same way from my POV the black hole also doesn't exist.
Again, until there is an event horizon it is not a black hole. Once it has the event horizon, it is a black hole. Once it at the point that you are concerned about this, it is already a black hole. If you are in a situation where your (misunderstood) scenario is occurring, it is already a black hole.
(I am speaking purely of the gravitational event horizon here, not a causality event horizon a la the observable universe, etc.)
>just like it takes until the end of time to fall in (from the POV of the infalling object)
It does not take until the end of time to fall in. If you were to be launched into a black hole that lacked strong enough tidal forces to spaghettify you, you would go past the event horizon at basically the same speed you were going right before you crossed. If you are going straight towards the center of the black hole, you will be accelerating under the gravitational forces.
>it also means I never see the black hole form.
Again, even if what you were saying is true (and it isn't), by definition the black hole has already formed when it collapses into a singularity (or something closely approximating our understanding of the singularity) and gains this event horizon. Not one atom or quark or any other type of particle more needs to fall in past this point for the black hole to have formed, because it already has. If it somehow formed in some sort of theoretical vacuum where even virtual particles do not pop into existence and never even had a particle or anti-particle appear and fall in, it would still be a fully formed black hole.
>which of course is never, since time never ends
Not really related to the current discussion, but we don't actually know if this is the case or not, either from a philosophical or hard science perspective. There are plenty of cosmological models where time ends, or the emergent property we call time stops being an emerging property because of some other reason, etc.
I think spaghettification is my favorite word, maybe it's tied with defenestrate.