Submitted by WittyUnwittingly t3_zys5ho in space

I have a relatively firm handle on all of the weird spacetime mechanics going on at/around a black hole, but something has been bugging me.

We're told that once an object crosses the event horizon, the singularity at the center becomes an unavoidable eventuality - there is no way to move in a way that puts it further from the singularity than it was before. So, what would two observers falling toward the "shared center" from opposite sides see? Their eventual collision at the singularity would be apparent, but would they be able to interact at any other point? Can they even know that the other is there?

If you framed this from a 3D geometry perspective, it's obvious that photons from one observer would never reach the other (they're all falling in toward the center, so nothing is traveling from one observer, past the center, and to the other observer). However, if you were to map all points "equidistant" from the center onto a sort of 1-dimensional spatial degree of freedom, then you'd conclude that whichever observer is closer to the center would be able to observe infalling photons from the one further out. Is this treatment of spacetime not valid here (is this not "1+3 relativistic spacetime?"), or do we just not know enough yet to give a definitive answer?

Also, don't pull any punches. Rigorous mathematical proofs and obtuse technical documents will be appreciated just as much here as ELI5s.

I figure the answer will be speculative, which is why I used the term "best guess."



You must log in or register to comment.

Most-Hawk-4175 t1_j27m7nq wrote

I think a PBS spacetime show talked about something like this. Of course the answer is ridiculously complicated but I seem to remember you would basically see everything frozen in spacetime including other observers in the black hole.


kimthealan101 t1_j28kn82 wrote

So you don't die. You just stop


Xentavious_Magnar t1_j28p6me wrote

Relative to people outside. Inside the event horizon, all world lines converge on the singularity, so you'll end up there eventually. How long that would subjectively take for the person inside is a fun question that I don't know the answer to.

Edit: also assuming people outside could see you, which they can't because any light bouncing off of you will also follow a world line into the singularity and never make it to them.


gladfelter t1_j29b3gs wrote

Seems to me you couldn't think either since any conscious thought would require a round trip in your brain, and the party of the trip going away from the singularity wouldn't make it.


Nerdcoreh t1_j2bfjjj wrote

In my understanding not YOU would stop but theTIME around you. So if you”fell” in a black hole you wouldnt notice the slowness of the time because that would be your normal (implying you are not dead by that time). Your brain functions would be just fine from your perspective


wokeupatapicnic t1_j2br80d wrote

Mostly correct. You’d see the universe progressively move fast and faster towards infinite motion until the “light-death” of the universe. Again, that is provided you were able to perceive and maintain thought during the process. But yes, you would not feel like you were moving in slowmo or anything, but everything outside of the BH would begin happening faster and faster and faster as you blinked out of existence


HouseOfZenith t1_j2bsewm wrote

That would be scary. When we develop space travel we should throw people in them.


paloprint t1_j2c967q wrote

I’ve always said I’d volunteer. That’s one hell of a way ticket. But your name will be forever.


big_black_doge t1_j2bd35l wrote

Time continues at its normal pace for all observers. What would happen is that you would not see the other person on the other side of the horizon ever again. They would both fall into the center after being spaghettified.


undergroundsilver t1_j29cu7c wrote

I think you would die from the massive gravity pulling you to the center, probably flattened like a meat pancake


beef-o-lipso t1_j28qeg9 wrote

No thought, no awareness?


MoogProg t1_j28ugg5 wrote

Only spaghettification and a timeless orbit until the eventual accretion of your particles feeding the singularity removes your information from existence*

*That last part is being debated and some suggest information is not lost.


[deleted] t1_j28zfyu wrote

I'm not studied enough to say much about it, but I believe some recent studies strongly suggest information is preserved. However that doesn't make much of a difference to the spaghettified spacefarer.


spymaster1020 t1_j29part wrote

I've heard of this theory before,something about the information being spread out on the event horizon. But what about when black holes decay through hawking radiation? Does that carry away the information?


lawblawg t1_j2angnj wrote

Yes, that’s the best solution we have. The information is encoded in quantum fluctuations in the shape of the event horizon, and Hawkins radiation is both caused by those fluctuations and carries that information away with it.


33ff00 t1_j2ashy6 wrote

What type of information? What’s that mean here?


MaelstromFL t1_j28v2y4 wrote

IF! You will more than likely be torn to shreds before you reach the horizon...


lawblawg t1_j2anlfg wrote

It depends on the size of the black hole. For a supermassive black hole like Sagittarius A*, the average density is less than liquid water, and the tidal forces at the event horizon are negligible.


kimthealan101 t1_j2b6m2i wrote

I just wanted to find an easy way to die. What is easier than just hanging out at an event horizon?


capmap t1_j2bqex5 wrote

No. That's only from an outsiders' perspective.


graveybrains t1_j2a5mwn wrote

Except the subjective experience of time never stops, no matter how dilated it gets, so the first thing you’d likely experience crossing the event horizon is the evaporation of the black hole, or the end of the universe, whatever those might look like.

If you could hang out for a while, you’d see the black hole growing around you as it’s gravity leaves space more and more distorted. The event horizon would be the point where the black hole wraps all the way around and closes behind you.

At that point there is no physical way out, and any direction you can travel is going to be inward.

…I may have seen too much PBS as a kid.


WorstMedivhKR t1_j2amk4i wrote

You'd be spaghettified and die long before the evaporation of the black hole or hitting the singularity or the end of the universe.


graveybrains t1_j2at6ky wrote

Depends. A black hole the size of Sagittarius A* probably wouldn’t spaghettify you, but I don’t remember why


jaydfox t1_j2b665c wrote

If you double the mass of a black hole, its gravity doubles. But its radius also doubles. Gravity decreases with the square of the radius, so at double the radius, you have 1/4th the gravity. So double mass of the black hole, and you get 1/4th of double the gravity, or 1/2 the gravity at the event horizon. But the tidal forces that cause spaghettification decrease with the cube of the distance, so you get 1/2 the gravity and 1/4th the spaghettification. Make a black hole 10 times bigger, and you'll get 1/10th the gravity and 1/100th the spaghettification at the event horizon.

The black hole Sagittarius A* is about a million times more massive than a typical (stellar) black hole, so it's gravity at the event horizon is a million times smaller, and the spaghettification will be a trillion times smaller. Not sure if it's enough to survive being spaghettified near the event horizon, but a trillion times less stretching can't hurt. Black holes a thousand times more massive than Sagittarius A* exist in other galaxies, so they'd be even easier (a million times easier) to survive falling into. (Actually, there are a few known black holes about 10,000 times bigger or more than Sagittarius A*.)


graveybrains t1_j2bkolq wrote

This guy is PBS!

Thank you 😁


jaydfox t1_j2bqat6 wrote

Haha, no, but I do watch a fair amount or space channels (PBS Space Time, Dr. Becky, Anton Petrov, etc.).

One thing worth mentioning is that you'll eventually be spaghettified, no matter how big the black hole is. If a stellar mass black hole has an event horizon of about 3 km (I think, not positive), then a 1 million stellar mass black hole would have an event horizon of about 3 million km. But only when you get within 300 km of the singularity, would the tidal forces be as strong as at the event horizon of the stellar mass black hole. For the largest known black hole (66 billion solar masses, off the top of my head, but maybe there's a bigger one), the event horizon would be about 200 billion km in radius, about 2% of a light-year (so 0.04 light-years in diameter). But the tidal forces wouldn't be as strong as at the event horizon of a stellar mass black hole, until you were about 12,000 km from the singularity. This is due to the rapid decay of tidal forces with distance.

So in that sense, the person you were replying to was right. You will be spaghettified before you get anywhere near the hypothetical singularity, no matter how big. But with a big enough black hole, you'd get pretty deep into the black hole before you get ripped apart. Long enough to make interesting observations. Which is what the OP was asking about. What would you see if you fell into a large enough black hole? Long after you've crossed the event horizon, and long before you get close enough to the singularity to be ripped apart by tidal forces. I assume you would see stuff that fell in right after you, and stuff that fell in right before you. (If not, that implies you wouldn't even see your feet after they went in, if you went in feet first. And I seem to recall several different authors saying you would hardly notice yourself pass through the event horizon.) How much longer before and after? I assume the cosmic background radiation behind / above you would be blue shifted from microwave into infrared, visible, eventually ultraviolet, xrays, etc. Whether you get ripped apart before witnessing that, I don't know. I'd like to know.


WorstMedivhKR t1_j2dsjjw wrote

You will eventually be spaghettified regardless. In a large enough black hole it's after you cross the event horizon but before hitting the singularity.


Ryunah t1_j27r4kj wrote

Yeah, I remember that too. I definitely remember them mentioning something about being frozen once you crossed into the hole. That you wouldn't be able to move and that time would completely stop or something like that. You'd never make it to the other side.


WorstMedivhKR t1_j2amc26 wrote

That's what it looks like to a distant external observer. Except even then the person gets redshifted to invisibility pretty quickly. If you're inside time still passes "normally" locally.


fromadifferentplanet t1_j2az8xl wrote

There's a great episode of Star Talk Radio in its earlier years and they cover all this.


Theometer1 t1_j2a7b6d wrote

Seen a video on it as well it said for the observer they would see you die but in your perspective you crossed into the black hole but can never leave it.


Spaventoo t1_j2ayxn2 wrote

Is that because black holes are a localized end of the universe?


Xethinus t1_j27xzlx wrote

So... im happy to give this a try. I'm happy to be corrected if there are better guesses.

Let's assume an observer can survive all the effects of a black hole, and can "see" in every direction.

In the up direction, the observer sees the rest of the universe pass in an instant, blue shifted to numbers that don't make sense for any physical model.

In every other direction, nothing.

Space and time have swapped places, and you can time travel now, but you can only travel in one direction through space, and it's down. Same goes for every photon. Down. It can't orbit anything, can't go sideways. Only down. The observer on the other side have the same problem. You might get a glimpse of them just before you both pass the event horizon, but now, it's only down. Your worldline, just as it was always pointing forward in time above the horizon, will always point down below it.

You can't see what's below, as photons are only traveling in one direction at this point, and can't go up to meet the observer above.

I have a theory that an observer wouldn't even make it that far, as hawking radiation would annihilate anything that just passed the event horizon immediately from its own perspective. This would be because an indefinite amount of time passes on the outside to produce hawking radiation, giving no time at all to an internal observer to get very far past the event horizon. This may be a cosmic necessity, given the extreme nature of black holes.

Edit: I think this is my first gold! I don't know what that means yet, but thanks!


WittyUnwittingly OP t1_j27zdpk wrote

>hawking radiation would annihilate anything that just passed the event horizon immediately from its own perspective

This is perhaps the most interesting point I've read all night.

Thinking about an observer's "perception" from within a black hole circles me back to all of the same problems we have with time/FTL travel, which makes sense.

An observer from "inside" a black whole should be able to perceive all of the photons arriving at the event horizon after they did simultaneously, but "infinite blue shift" should imply that any causality information would be lost (Think of a binary pulse... Physically, a 1 would be unable to arrive before a 0 or vice versa)

Yeah man... That makes total sense, because from the perspective of the observer falling in toward the black hole, they would arrive at the singularity at the same instant that it evaporates due to hawking radiation at the end of the universe. This fits very neatly with some of the other "causality protection" conjectures.


Xethinus t1_j2806q2 wrote

Oh. If you keep my amateur theory consistent, there is no actual singularity. Any nothing would ever approach it.

To the observer, all of this happens at the same time, while they pass the event horizon. The center of the black hole would take an infinite amount of time to approach.

Black holes are really annoying, because most of their calculations result in "undefined" or "zero" and there's not much in between.


WittyUnwittingly OP t1_j280ddz wrote

Well, you can use the word "annoying" if you'd like, but I would call it a rather elegant reconciliation of a lot of the problems I was having.

For example, if one were to have a mastery of mechanics such that they could dip in and out of an event horizon, how could there be any continuity of what they perceived versus what they did? No need for any of that with this explanation, because it necessarily cannot happen.


superVanV1 t1_j2952g4 wrote

not unless we achieve FTL travel.
though under current models of acheiving that via the warping of space, sticking a spacetime bubble into a spacetime singularity may cause "issues"


WittyUnwittingly OP t1_j29s599 wrote

Well, even if you took an Alcubierre drive into a singularity, once you get there, there would be no path you could choose to get back out, regardless of what speed you can go. Even if you could somehow distort space enough to "get back," when would you arrive? At the end of the universe?

Seems like your best bet for something that can go into and come out of blackhole, is something that does not obey the normal laws of causality, and can come out of a black hole before it goes in.


superVanV1 t1_j2a3qxv wrote

Which we have no way to model for, since current mathematics still just returns a big ‘ol error sign when trying to calculate beyond the event horizon. It’s entirely possible that physics just.ceases to operate properly at that point


Kevskates t1_j2c1vse wrote

All this talk of the limits of physics make me feel like we’re in a video game trying to explain the invisible edge of the map


paloprint t1_j2cbew3 wrote

Like when McConaughey touched hands with Brand. Corny I know. He’s was going out they were going in.


wokeupatapicnic t1_j2brwdu wrote

Pretty sure you just described the Firewall theory. I think Hawking proved that or at least was able to suggest that, this is not the case, but it’s worth looking into on your own if you’re interested!


WorstMedivhKR t1_j2apghs wrote

This is what actually falling into a simple Schwarzschild black hole would look like. The physicist behind this, Andrew Hamilton, also has some for more realistic types of black holes but they are harder to understand. Notice that locally nothing special happens when crossing the "true" event horizon, and there is still an outside view of the universe. The only way you get a tunnel effect upward with darkness in every other direction is if you use a great deal of energy just outside the true event horizon to accelerate against the gravity of the black hole.

The ending of this video is hitting the singularity.


lawblawg t1_j2apya0 wrote

So you’re quite correct on almost everything, except for the bit about being annihilated by Hawking radiation. This is the firewall problem. The event horizon cannot be defined objectively in general relativity; rather the event horizon is defined relative to an observer at a specified location. That’s because if the event horizon was a defined location, it would dictate a universal reference frame, which violates relativity.

So the event horizon cannot be accompanied by a firewall of deadly Hawking radiation. The currently accepted solution is that Hawking radiation is emitted from quantum fluctuations which are not only uncertain in energy but are also uncertain in location. And those fluctuations are redshifted or blueshifted relative to the observer, and so relativity is preserved.


cbusalex t1_j295xdr wrote

> You might get a glimpse of them just before you both pass the event horizon

At the very least, you'd see them (well, a very distorted image of them) at 90 degrees left and right as you cross the photon sphere.


piousflea84 t1_j29tbq7 wrote

Is this the “firewall hypothesis” of everything inside a black hole piling up in an infinitely hot infinitely thin surface just inside of the event horizon?


phantagom t1_j282iws wrote

With general relativity, the gravitational field is encoded in the curvature of spacetime. This means that the path that light follows is determined by the curvature of spacetime, which is itself determined by the distribution of mass and energy in the universe.
So with a black hole, the spacetime around the black hole is highly curved due to the presence of the black hole's mass. This means that light that is emitted by one observer will follow a path that is determined by the curvature of spacetime, which will generally not be a straight line.
It is possible to consider a "1+3" decomposition of spacetime, where space is divided into a set of spatial coordinates and time is treated as a separate coordinate. However, this decomposition is not unique, and different choices of coordinates can give rise to different perspectives on the same physical situation.
With a black hole, it is possible to consider a set of coordinates that are "equidistant" from the center of the black hole, as you described. However, these coordinates would not necessarily be a good choice for describing the physics of the system, as they would not be well-behaved at the event horizon of the black hole.
In general, the physics of a system should not depend on the choice of coordinates used to describe it. Therefore, it is generally not valid to draw conclusions about the behavior of a physical system based on a particular choice of coordinates.


WittyUnwittingly OP t1_j2832t5 wrote

>this decomposition is not unique, and different choices of coordinates can give rise to different perspectives on the same physical situation

Is this formally "loss of causality information" and somewhat equivalent to "infinitely blueshifted" or am I way off?

>the physics of a system should not depend on the choice of coordinates used to describe it.

Are there better choices by which to describe what I'm asking about?


phantagom t1_j285l7s wrote

No, "loss of causality information" and "infinitely blueshifted" is not the same thing.
The event horizon of a black hole acts as a "one-way membrane" that separates the region inside the black hole (the interior) from the region outside (the exterior). Once an object or particle crosses the event horizon, it becomes trapped inside the black hole and cannot escape. This means that, from the perspective of an observer outside the black hole, the interior of the black hole is causally disconnected from the exterior. This means that there is no way for an observer outside the black hole to receive information about what is happening inside the black hole, or to affect events that are occurring inside the black hole. This is what is meant by "loss of causality information."
The phenomenon of an object being infinitely blueshifted as it approaches the event horizon of a black hole is a consequence of the extreme gravitational forces present in the region. more extreme. In this sense, it is possible to say that the object is "infinitely blueshifted" as it approaches the singularity. However, this is not the same thing as a loss of causality information, as the object is not able to transmit any information about its experience to an outside observer once it crosses the event horizon.

The choice of coordinates used to describe a physical system is a matter of convenience, and different choices of coordinates can give rise to different perspectives on the same physical situation. It is generally more useful to choose coordinates that are well-behaved (e.g. that do not diverge or become singular at certain points) and that are adapted to the symmetries of the system (e.g. cylindrical coordinates for a cylindrically symmetric system).


WittyUnwittingly OP t1_j29tbcy wrote

Sorry. I did not mean to imply that blueshifted light observed from within an event horizon is what most mainstream scientists mean when they say "loss of causality information"

I understand that debate relatively well, and it pertains to recovery of information about what fell into the black hole from the outside.

I'm asking: is there something fundamental that I'm overlooking with my description of the "infinitely blueshifted" light? Seems to me that from the inside of a black hole you would be unable to deduce anything about the order of events outside using the photons falling in, because from your perspective, they would be arriving all at the same time.


phantagom t1_j2a17xx wrote

It is true that from the perspective of an observer inside the event horizon of a black hole, the photons falling into the black hole would appear to be infinitely blueshifted and would arrive at the singularity at the center of the black hole all at the same time. This is a consequence of the extreme gravitational forces present in the region, which cause the photons to be blueshifted and to follow highly curved paths as they fall towards the singularity.
However, from the perspective of an observer outside the event horizon, the photons falling into the black hole would not be infinitely blueshifted, and would not all arrive at the same time. From this perspective, the photons would follow paths that are determined by the curvature of spacetime, which would generally not be straight lines.


Careless_Implement12 t1_j28iutg wrote

One you cross the event horizon, you are traveling faster than the speed of light. The spacetime you occupy is traveling faster than the speed of light.. any information generated would not get from you to the observer next to you because it would all be heading towards the singularity, faster than the speed of light, faster than the speed of causality.


WorstMedivhKR t1_j2ap0bt wrote

That's not true, you never locally exceed the speed of light pre-singularity in a black hole. You also don't notice anything special when crossing the event horizon, there is always an apparent event horizon ahead of you even when you have crossed the "true" event horizon already.


ScrubbyOldManHands t1_j2a1pwu wrote

I thought it was more that space time itself has been bent so far that all directions head towards the singularity. So you can't see anything from beyond the event horizon because space time does not extend from it.


The_Frostweaver t1_j27me3n wrote

If one of them is closer to the center than the other the one deeper in could theoretically see light from the person who entered later but keep in mind light is spiraling down wards in a way that would likely create extreme distortion and if the black hole has any sort of disk that disk is likely producing enough light to overwhelm the few photons coming from the person falling in.

I imagine you would be looking for a dark spec a single pixel wide obscenely stretched against a background illuminated like a sun (the disk of stuff just outside the event horizon).


WittyUnwittingly OP t1_j27msnl wrote

I swear, I'm usually more professional than this, but the vivid imagery of the last paragraph elicited only one response from my brain:

"So you're saying there's a chance!"

More seriously though, it is valid to assume that an observer inside the black hole would have some sort of visual field that correlated with the infalling spacetime in all directions? (I. E. The inner observer could "see" photons from the outer observer regardless of the initial entry points in 3D space)

Setting aside all of the other problems with causality and information procurement, wouldn't that make black holes an optimal place from which to observe the rest of the universe?


triffid_hunter t1_j27oe2k wrote

How can an object cross the event horizon when it's a surface where time basically stops, and movement is distance ÷ time?


WittyUnwittingly OP t1_j27p9cq wrote

There is no issue with crossing the event horizon from your own perspective, is there?

Time stopping at the event horizon is a relativistic effect, so from the perspective of an observer falling toward the center, there would not be any sense of time stoppage. Right?

There would be all sorts of issues with causality and such, but that's all part of "what it looks like," which is what I'm asking about.


triffid_hunter t1_j27pxdf wrote

This is the heart of the ongoing firewall debate; basically one set of physics says there should be a maelstrom of particles flying around at insane temperatures at the event horizon, but different physics says you shouldn't notice anything at all when approaching or crossing an event horizon because it's a non-local phenomena.


WittyUnwittingly OP t1_j27rssk wrote

Oh wow, I did not realize that was this, but now I am connecting the dots. Thank you!


The-Temple-Of-Iron t1_j27rr6l wrote

I wonder if the concept of time in a singularity affecting the temperature/energy of those particles may play a hand in solving this? I'm keen on science but took me 3 tries to pass algebra so take that with a big grain of sodium chloride :-)


triffid_hunter t1_j27tf5e wrote

I don't think temperature depends on time…


The-Temple-Of-Iron t1_j27z5j2 wrote

Temperature is a measurement of vibrations in particles essentially. That is wholly dependent on time passing. Mathematically time stops in a singularity. If that is so then, in my incredibly layman-style interpretation, Temperature is physically the same as absolute 0 K. Would you like to explain what you mean? I love learning. I'm very curious. Or you can make condescending statements, or rather half-statements, without providing any explanation and then downvoting my curiosity. Your choice, I suppose, but I was eager to see an intelligent conversation on this. You have offered no conversation nor any intelligence. I would enjoy it if you did.


triffid_hunter t1_j2810g5 wrote

> Temperature is a measurement of vibrations in particles essentially.

That's the entry-level understanding of it in matter, but the scientists have come up with new better understandings based on entropy - which is how we end up with negative temperatures that are hotter than any positive temperature, and exist in lasers.

> Mathematically time stops in a singularity. If that is so then, in my incredibly layman-style interpretation, Temperature is physically the same as absolute 0 K.

Nope - particles' momentum is related to temperature, and they keep their momentum if you stop time - they can't move anywhere because no time is passing, but their velocity is still non-zero.

This is quite distinct from particles inhabiting the lowest possible energy state (ie being at absolute zero) where they don't move (or do weird stuff) even though time is passing


The-Temple-Of-Iron t1_j2816ac wrote

Thank you! That is really interesting and I'm definitely about to go down a rabbit hole on this. I appreciate that!


WittyUnwittingly OP t1_j281je9 wrote

So I'm not sure about all this, but I do have an MS in optics, so I do have a firm grasp on optical communication and dispersion and such things.

The "infinitely blue shifted" light that would be incident on you from everything "outside" would not contain any causal information (a pulse that was originally 101, would now have all of those in superposition - the original message could have been 110, 101, or 011, and you wouldn't be able to tell the difference).

You'd be receiving all of the radiative energy that the black hole ever gobbles up after you instantaneously. So, any temperature calculation you'd do would just yield infinity, including a speculative black body radiation calculation (infinitely blue shifted).


wokeupatapicnic t1_j2bsr06 wrote

Absolute 0 is unreachable, in the sense that it breaks the laws of quantum physics. It violates the uncertainty principle.

I realize that black holes violate a lot of fundamental ideas in physics, but the general attempt is to find rational ways to rectify those violations, not simply accept them. Hawking’s work was based on solving many of those discrepancies.


s1ngular1ty2 t1_j288o2v wrote

You can 100% cross it. You only appear to stop to outside observers. You don't actually stop.


PandaEven3982 t1_j29r42t wrote

I think I'd lose consciousness right around 5G of acceleration. If I wake up, I'll send email. :-)


SaulsAll t1_j27peqy wrote

I think each observer would need to "turn around" and point their vision back toward the event horizon. There, each would see the entire sphere of visibility - including the other person on the other side of the singularity - condensed into a tiny bit of view. Everything else would look black.


WittyUnwittingly OP t1_j27rhje wrote

Would this not make a black hole the ideal place from which to observe the outside universe? (Assuming of course that we can solve the information procurement problem)

All of the information incident on the entire 3d surface of the event horizon presented simultaneously to the observer inside. There would be time information too, but it's beyond me to speculate what that could be at this point. Sounds like a really nifty vantage point.


SaulsAll t1_j28ica9 wrote

I think the information would be severely warped and condensed, though. It's like saying the best way to see an IMAX movies is by shrinking the screen down to the size of a postage stamp. Sure all the info is closer in terms of angular resolution, but discerning individual parts becomes much harder.

Or just the difference between looking at the sky at night versus looking at a tiny section with the Hubble.


WittyUnwittingly OP t1_j29q7yi wrote

Oh yeah, you would need some otherworldly spectroscopy to actually get any usable information from it, but that doesn't mean the information isn't there.


The-Temple-Of-Iron t1_j27rw3v wrote

Is this not unlike the "what's outside the universe" question? I'd love to hear comparisons.


WittyUnwittingly OP t1_j27smbh wrote

Reading an article about superluminal spacetime ( is what prompted this question.

I see what you're saying, though. From the perspective of an observer within the event horizon, talking about something outside of the event horizon doesn't make any sense. Since I'm sitting comfortably outside of a blackhole (at least, from my perspective), it makes sense to speculate about the "inside."


s1ngular1ty2 t1_j288swm wrote

There is no outside the universe. Just stop. The universe is everything that exists.


The-Temple-Of-Iron t1_j29c522 wrote

I didn't argue that. Think you missed my point. Geeze it must be so awesome to be as smart as you. How do you do it?


Kear_Bear_3747 t1_j280sba wrote

The person on the outside wouldn’t be able to see the person on the inside they would only see warped space from the other side of the event horizon until they cross the threshold.

The person on the inside would see have a heavily redshifted and distorted view.


LookingForDialga t1_j28azpq wrote

My first guess would be that they will be able to see each other because light doesn't need to follow a radial geodesic.

The only way to make sure is to write Schwarzschild metric, find the path of the two falling observers (you can assume radial geodesic) and then find all the possible null geodesics that cross a point of the trajectory of observer 1 and see if any of them also crosses the trajectory of the other observer.


s1ngular1ty2 t1_j28c8zw wrote

You'd actually use a penrose diagram and plot it out on there.


uselessopinionman t1_j2933if wrote

this is all very theoretical but here is my best guess.

lets call the two observers A and B. and lets assume they are not negatively impacted by the physics of where they are at. if they were on opposite sides of the black hole, they would not be able to see one another as the black hole which is solid would block the view. But lets say the black hole is transparent. In this case you will still see nothing due to the gravity well. If you look towards the center of the black hole, it would be absolute darkness as no photons are able to reach your eyes due to the pull towards the center. if you turned 180 degrees away from the center, you would be blinded by the amount of light coming at you from the event horizon, this will be caused by the inverse gravitational lenseing that would occur on the inner "surface" (not an actual surface but an appox of the event horizon). if you turned 90 degrees to the left or right and look along the perimeter of the event horizon you would see a distorted image of observer B on both sides of you. While the gravity prevents the light from escaping the black hole it does not prevent it from orbiting the black hole like a satellite. so some of the light leaving observer B would travel parallel to the event horizon perhaps perpetually. looking up or down in the same manner would give similar results but, looking up you would see the top of obsever B's head. if you look down you would see observer B's feet. crazy thing is if observer B was not there, you would see images of your self. if you looked to your right you would see the left side of your self. look down and see the bottom of your feet ect.

TLDR: you would not be able to see across the hole as it would be darkness, looking behind you would be blindingly bright, and looking up, down, left, or right you would see 2 distorted fuzzy dim copys of observer B along with everything else that has ever come into this space.


Cyoarp t1_j2ah9pi wrote

They wouldn't have time to recognize each other. For one thing specification of course would have killed any living observers on contact with the event horizon, the other thing to consider though is that they'll be moving towards each other faster than the speed of light. If two objects are moving near the speed of light and they happen to be moving in opposite directions then they are moving towards each other at faster than the speed of light.

What this means is they'll never know that each other existed. The closer to the singularity you are the faster you move when you get up to relativistic speeds the slower time moves and the more gravity present the slower time moves. Once you cross the event horizon gravity in the speed of your motion or going to mean that you can't actually ever observe anything in the other side of the black hole.


WorstMedivhKR t1_j2apvf8 wrote

> If two objects are moving near the speed of light and they happen to be moving in opposite directions then they are moving towards each other at faster than the speed of light.

That's not correct.


Cyoarp t1_j2aq3y3 wrote

Yes it is. I have checked this over multiple times and even consulted multiple physicists.

But let me clarify. They move towards each other at greater than the speed of light from a third party's reference frame. From the objects own reference frame nothing can move faster than the speed of light.

What I mean to say is that from your reference for frame you are moving the speed of light you will not observe anything outside of your reference frame and time will not move at all as far as you can tell

Which is why I specifically said that from their own reference frames the two objects will never observe each other because there will never be a time where they will observe each other meeting because time dilation does not allow them to do so.

(This isn't the best way to say this but I am driving. If this isn't clear enough let me know and I will better explane)

However from a third party's reference point they will be moving towards each other at faster than the speed of light or if they're going half the speed of light then they will be going towards each other at the speed of light.


WorstMedivhKR t1_j2e2ko9 wrote

Ok, that's correct, but still from either observer's perspective the other person is not traveling faster than the speed of light, due to relativity it is very different. It is true that they won't see each other though even if they enter at the same time, so long as the entry point is different, for reasons I don't really understand mathematically since I haven't actually taken GR but have to do with the geometry of the black hole and that the singularity doesn't actually appear small, it appears to grow larger and larger as you approach it paradoxically. So it's more like free falling onto a huge black planet would look like.

> Geometrical intuition, bolstered by pictures like this one would suggest that the center of the Schwarzschild black hole is a point. That intuition is misleading. If you and a friend fall into a black hole at the same time but at different locations (in latitude and longitude), you do not approach each other as you approach the singularity. Rather, the diverging tidal force channels the parts of your body along the inward radial direction. Far from meeting your friend at the singularity, you cannot even put out your arms to touch her.

> “The” singularity is not a point. Rather, it is a 3-dimensional spatial boundary where general relativity commits suicide. New physics, presumably quantum gravity in some form, must replace general relativity at singularities. What that new physics is remains a profound unanswered question.


Cyoarp t1_j2enn6x wrote

First off is Specifically SAID they would never observe each other. That was the entire point of my post. Second off NO F-ING SHIT it's like falling into a planet. Did you think black holes are actually hollow?

I am sorry, I shouldn't be angry. It is fine that you don't known stuff. The reason I am upset is that you didn't know what you were talking to about but you tried to correct me. Next time, if what someone is saying doesn't make sense to you and you aren't an expert in the subject, maybe try asking questions. The answers will tell you if the person knows more than you do or if they are just full of shit.

PLUS that way if they do turn out to be full of shit you get to be the reasonable guy who really embarrassed them when you figure out why they are wrong. And if they turn out to be right you learned something and gain a lot of respect and karma.


FrostyAcanthocephala t1_j2ak7xf wrote

Hawking had an idea about infalling objects being smeared on the event horizon.


WittyUnwittingly OP t1_j2alj4i wrote

That would be what is referred to as "soft hair," yes?


FrostyAcanthocephala t1_j2b1tdc wrote

I had never seen that expression before. I suppose it would be. Thanks for pointing me to that.


WittyUnwittingly OP t1_j2b6qy9 wrote

Just recently reconciled those two concepts myself. Lots of "the same stuff but with different words" in black hole physics.


FrostyAcanthocephala t1_j2begkz wrote

What amazes me is that people like Hawking and Einstein did a lot of this just by making thought experiments.


Paradox68 t1_j27y027 wrote

If you’ve crossed the event horizon, presuming you’re not already dead from the immense pressure, wouldn’t you only be able to see photos from behind you? I.e. everything from every angle your body is not blocking to the event horizon would just be pitch black.


WittyUnwittingly OP t1_j27ykc7 wrote

Yes, but it kinda seems like you would be able to see all of the photons "behind" you that will ever hit the event horizon after you've been there.

I.e. Immediately after crossing the event horizon, you'd be able to witness the end of the outside universe. However, the relationship between you and all of the rest of the stuff once it crosses the event horizon too, is what I'm finding tricky to define.

Thinking about it like this, it would seem that if you managed to cross the event horizon, from your perspective, everything else will now be inside the black hole with you, with only a difference in one-dimensional space quantifying your "times of entry."

The harder I think about it, the less clear the answer becomes.


WorstMedivhKR t1_j2aqomj wrote

No, you can see ahead of you everything that fell in before you, because it's all time dilated and "frozen" as it approaches the singularity.


DanishWeddingCookie t1_j28k5zt wrote

Once you pass the event horizon doesn’t time stop? And wouldn’t that mean all brain function stop as well, such that even if you could see things your brain couldn’t interpret it?


WorstMedivhKR t1_j2aql5g wrote

No, time does not stop on crossing the event horizon. The view of the external universe above actually speeds up.


s1ngular1ty2 t1_j288lmn wrote

You can't see them, no paths for light lead to them. All paths lead to the center. Light can not go from them to you or vise versa. The only light you can see is from where you came from.

You'd see something like this.

Note, all real black holes spin and drag space with them in a tremendous vortex of warped space time like you see in this video.

Also all the light you see is from the accretion disk being bent around the black hole into your eyes from all sides of the black hole. The light you see may look strange but it is accurate and from the accretion disk. You see it everywhere because of how the space is warped near the black hole. Light from all sides of the black hole can reach you because of the warped space.


SmoloTHEKloWn t1_j2bc1rb wrote

I think Stargate tv series did a really good example of this and what would happen.
A matter of time S2:E15.


Loud_Focus_7934 t1_j2beqis wrote

The only thing that happens at a black hole is it atomizes matter than the atoms either stick to it or are ejected.

I don't understand why there's so much speculation on this. Nothing magical is happening.


AholeBrock t1_j2bnjwz wrote

You are assuming there is another side to look back from but idk why you would assume that


capmap t1_j2bq8ln wrote

For a black hole of sufficient size such as a SMBH, GR states there's a gentle fall (at light speed, lol) until the inner event horizon. QM states you're destroyed at the event horizon by a firewall of radiation.

I'm not going to profer which one is accurate and then go on a riff on that one theory when any matter have already been vaporized.


MSU_Dawg0529 t1_j2c2lcq wrote

You mean before you get ripped into trillions of atoms?


novembergosh t1_j2cbut5 wrote

I think they wouldn’t see each at all as the light does not escape from singularity to reflect off of them…


bitemy t1_j2chq9n wrote

The amount of speculative mumbo-jumbo here is amusing.

The short answer, of course, is that they would both be dead from spaghetti if Acacian. The title forces of the insane amount of gravity would tear them to shreds.

The amount of gravity around a black hole is staggeringly humongous. Just imagine how strong gravity would have to be to bend light.

Everything inside the event horizon is presumably moving straight inward at nearly the speed of light.


MrZorg58 t1_j27q9na wrote

One would think you were dead, if not, you'd most likely be blinded by nothing but light.


MrZorg58 t1_j27qhjt wrote

When you fall into a black hole, you die, you become a noodle, stretched over millions of miles. You can't live like that. Imagine getting too close to one, and your head up facing it, suddenly your head starts getting longer, faster than your feet are, your dead at this point, and now you just keep stretching out, but always moving towards the black hole.


WittyUnwittingly OP t1_j27qvez wrote

Sure, but I was looking for more of an "information theory" explanation. For the sake of thought, we can reduce our "observers" to single particles and what they "see" is simply what photons are incident on them.

The basics of tidal forces and whatnot are inconsequential when you're discussing what things look like beyond the event horizon. We've already suspended disbelief to get past that point.


LordGoldenEagle t1_j28j6ov wrote

Are astronomers going down a rabbit hole with black holes? Every astronomy programme or article bangs on about black holes. To regular folk they are not interesting. Obviously if you are super intelligent they are fascinating and intellectually challenging. But is cosmology getting side tracked by them?


Tetbudertruk t1_j28nnba wrote

You too kxcv##-3353774474748----8---+_8---(7+7+447-


Quarkchild t1_j2bypoh wrote

When you say you have a firm handle, have you done any math???


WittyUnwittingly OP t1_j2c6vwy wrote

I have an MS in optics and photonics and a BS in nuclear engineering, and have worked in an optics research lab. Many of the formal derivations I've had to do are directly applicable to this topic. I also teach math... So, yes!