Throwback Thursday: What would you see as you fell into a black hole? (Synopsis)

They say ‘A flat ocean is an ocean of trouble. And an ocean of waves… can also be trouble.’ So, it’s like, that balance. You know, it’s that great Oriental way of thinking, you know, they think they’ve tricked you, and then, they have.” -Nigel Tufnel

When you travel towards an object like a moon, planet or star, the closer you get, the larger it appears. Halve the distance and its angular size doubles; reduce the distance to a quarter and it appears four times as large. But for black holes, their gravitation is so strong that relation no longer holds as you approach the event horizon.

Image credit: SXS team; Bohn et al 2015. Image credit: SXS team; Bohn et al 2015.

Instead, the region of "blackness" increases much faster than you'd expect, eventually taking over a full half of the sky as you crossed the event horizon and causing all the light-paths to contract down to a point behind you the instant before you hit the singularity. Even less intuitive is what happens to the gravitational field, which would show the singularity in all directions once you crossed over the event horizon.

Image credit: Cetin Bal. Image credit: Cetin Bal.

Go get the whole story on today's remarkable Throwback Thursday!

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Is a singularity even possible in a true physical sense? The matter and light is trapped inside a black hole, sure but they probably still exist in some "spaceless" form. I wonder if the modern physics can speculate as to the actual structure of the matter inside - any particles can still exist or is all "quantum" nature of the matter get's "dissolved" so to speak (pure energy?)..

@Alex #1

I've been asking that same question about singularities being possible. More and more I am getting a hedge about no one really knowing before or ongoing research, then a personally held belief by whatever source I'm asking that 'no, there is no such thing a a real singularity'.

There is a perfect example of that exact hedge/no answer linked to in Ethan's article. Towards the bottom, there is a link with the text 'it would only take around 20 seconds to reach the singularity '. If you click on that link you can find that author's disclaimer on singularities.

It reads:

** A singularity is something inside a model that tells us that the model does not properly describe the physics. General relativity has singularities because it does not include quantum mechanics. So, "singularity" in the above article is basically the middle of the black hole; I do not claim there are true singularities.

@Ethan: In your article, you write a couple of times that within the event horizon, the singularity (or whatever, see above comments) is "downhill in all directions."

Back when I took GR in grad school, almost three decades ago, I learned that in the Schwarzchild metric, as you approach the event horizon from the outside, the 't' axis "bends" to point toward the horizon. As you cross it, the 't' axis become "radial," pointing at the singularity.

Is this just the same statement as yours? Since the singularity is directly in the future (i.e., along the 't' axis), there's no spatial direction you can move to escape it.

By Michael Kelsey (not verified) on 04 Jun 2015 #permalink

An apparent paradox? Here it goes: If all directions lead to singularity inside the event horizon then effectively you are surrounded by it which is indistinguishable from being a part of "the singularity" already as soon as you cross the e. h. There should not be any further "20 sec" delay to reach it. Besides, for the external observer, you would never (= infinite time) completely reach the e. h. - that also points to the impossibility of this extra "singularity trip". Maybe this paradox is only a consequence of the limitations of the Einstein's physics (without quantum effects)

For a sufficiently massive black hole, the gravitational pull toward the singularity at the cusp of the event horizon would be Earthlike ~10 m/s/s. But if all directions are now toward the singularity, which part of an object of a human size is closer? Isn't it whichever parts entered first? So the gravitational gradient is based upon the time the parts of you entered? So "spagettification" isn't being pulled apart in space but in time?

By Roger Shrubber (not verified) on 04 Jun 2015 #permalink

@Alex #4

When crossing the Event Horizon you are not yet at the singularity because you have not been compressed to infinite density, but there are no longer any straight lines through space that lead out of the black hole. The space inside the Event Horizon is so warped that even a straight line pointing directly back toward the direction you came from now twists around to point toward the singularity. Once you pass the Event Horizon all paths are, in the words of Private Hudson, an express elevator to hell, doing down.

The disconnect between what you would see falling into a black hole and what someone outside the black hole would see when you fell into the black hole are not a limitation of Einstein's physics, they are a direct consequence of Einsteins well tested physics.

The phenomena has been used to as a conservation of information. Normally you would consider anything to have been thrown into a black hole to be erased from the universe. Leonard Susskind theorized that outside observers see items falling toward a black hole get progressively slower, redder, and dimmer but nothing ever reaches the Event Horizon. As such the Event Horizon itself was keeping a record of everything that ever fallen into the black hole.

So you fall in and are crushed to oblivion, but the Event Horizon maintains a copy of all your information at the very edge between the Black Hole and our side of the Universe.

There's no "personally held belief" nor is there hedging.

You're demanding a type of answer people cannot give and then re-interpreting this as "personally held belief" or "hedging".

Singularities cannot exist in the framework of physics we can apply.

End of story.

What people are trying to tell you is what may be there *instead of* the singularity. Some "thing" that isn't a load of infinities and infinitesimals.

Precisely what that is is not known.

This is different from hedging or personal belief.

But what about being inside the event horizon, of a truly ginormous black hole? A black hole big enough where the infall time is billions of years?
Assume there are galaxies around you, just as in our universe.
What does spacetime look like then? Would it look like a universe with a Big Crunch?
I found a partial answer:

From measurements made in the 1920s of the velocities of distant galaxies, it was found that all galaxies are flying away from us at a rate that depends only on their distance from us. No matter which direction we look in the sky this is true. We assume therefore that the Universe is isotropic, meaning rotationally invariant. There is no special or "preferred" direction to space.
Inside a black hole the situation is quite different - all objects are propelled toward the singularity at the center of the black hole. There is a preferred direction in space in the vicinity of a black hole. So, at this time we can say that we do not live inside of a black hole.

Nice article, interesting perspective. You write: "If you halved the distance between you and a star, the star’s angular size would appear twice as large" - the angle's dependence is actually arctan (1/distance).

There are some similarities to a wormhole, I think. As you approached and went in, all the light from your entry location would compress down to a small angle or point behind you. To left and right, your sky would become black or show reflected images of stars from entry and exit (think of a photon 'circling' around the drain), probably with fewer reflected images as you turn your head closer to a right angle to the wormhole's direction. What you would see directly in front would depend on what's beyond your exit point, but if it was also a sky full of stars, that would also be compressed down to a small angle or point.

I am not sure about what would see if you shined a light left or right 90 degres. Would you see the back of your own head, as the light describes a circle around the wormhole's curved space? Or does a wormhole also "pull" light (towards the narrowest, center point) like a black hole?

Nice and interesting article, but I've two questions :

1. It only speaks about how the black hole bends space, not about how it bends time. It's the whole spacetime that is curved by mass, how would the bending of time be noticed by the traveler ?

2. Wouldn't there be a point where, before reaching the event horizon, the gravity difference between your head and your feet is so high (or between your left and right side) that you would be literally torn apart ? You wouldn't need then only an indestructible ship, but also like being in an indestructible robotic body.

It's layperson-questions time again;-)

What would occur if two "perfect" spacecraft entered a black hole simultaneously, a short distance apart but at equal distances from the event horizon (by analogy two spacecraft entering Earth's atmosphere simultaneously, both at the same distance from the surface of Earth as they proceeded)? What would their crews and instruments observe during the same trip as y'all have been discussing here?

And, what would crews & instruments observe aboard a cluster of a few dozen spacecraft that flew in an approximately spherical formation, with one at the center and the rest arranged as if on the surface of a sphere a short distance from the central one? How if at all, would radio communication break down between them?

Lastly, I'm probably mistaken but it appears there's a paradox here: on one hand, the singularity is a logical outcome of GR, but on the other hand, it doesn't necessarily arise out of QM. Is that correct, and is that part of the set of discrepancies between GR and QM that remain to be resolved?

(Of these questions, the ones I'm most interested in answers to, are those pertaining to observations among a formation of many spacecraft entering a black hole while remaining in formation as long as possible.)

(I first read about black holes when I was about 12 and they were still considered speculative. But the idea of something infinitely dense that could capture light was very interesting, and suggested one half of a possible mechanism for a steady-state universe, this at a time when the steady-state theory had not been completely abandoned.)

Cool article! Slight nitpick: above it was stated: "... taking over a full half of the sky as you crossed the event horizon". Actually, the blackness would take up full half of the sky as you crossed the photon sphere, which is 1.5 times further out than the event horizon.

By Robert Nemiroff (not verified) on 05 Jun 2015 #permalink


If stars, etc are falling in alongside you, you should still see them, I think. Just like you would see your own hand.

@kilobug #11

1 - So long as you were entering the black hole in free fall, you wouldn't notice much in the seconds it takes you to be doomed. You certainly won't see the universe end or anything like that.

2 - The Spaghettification distance depends on the size of the black hole. You would be tidally ripped apart well outside the Event Horizon when falling into a small black hole, but you'd fall through the Event Horizon of a super-massive black hole like Sgr A* at the center of the Milky Way quite intact.

Aye, the scenario of a big black hole was discussed at least once before on BH discussions on this site.

Use the search engine.

Trying to answer yourself means you have more chance remembering it (since your efforts give more hooks to the recollection) than if it's dumped on your lap as a fait accompli.

"on one hand, the singularity is a logical outcome of GR, but on the other hand, it doesn’t necessarily arise out of QM. Is that correct, and is that part of the set of discrepancies between GR and QM that remain to be resolved?"

All "yes" response.

Though singularities don't occur in GR, they appear only in newtonian gravity. GR curvature goes all undefined, as does most of maths, when you throw infinities.

Quantum mechanics gives a reason why the MATHS won't let you have infinite densities (single valued-ness of eignefunctions being basically concluding this).

Quantum Gravity give a reason for no singularities actually appearing.

@G #12

The crews of each spaceship would see themselves hit the singularity first. That would be the case even if you entered the black hole in formation immediately behind another spaceship. You'd still see your ship hit the singularity first.

Here is an article roughly analogous to what you are getting at. Instead of space ships, they use a single train and what each of the passengers would see.

click here for article

Trying to answer yourself means you have more chance remembering it (since your efforts give more hooks to the recollection) than if it’s dumped on your lap as a fait accompli.

What do you know about my memory, and what is your source for that knowledge?
I would appreciate a link, if there are knowledgeable posts about a cosmological-size black hole.

"What do you know about my memory,"

A hell of a lot. Medicine knows a hell of a lot how memories are formed.

A specialist will know far more than me, but then again, I take my cue from those specialists.

Not to mention, if you're talking about YOUR SPECIFIC MEMORIES, then nobody brought that up until you whined.

Yeah, doesn't work, though.

Real black hole interiors gain mass, not lose it. And their internal advancement is different.

It's "like" a black hole interior.

But it doesn't work except as "analogous situation". When you work on it, you have a hell of a lot of special pleading about why it "works" despite it not being what you'd get.

Denier: most interesting. Each sees themselves enter first. What's the mechanism for that: persistence of photons outside the event horizon?, or something else? BTW, your link didn't work, probably better to copy & paste the URL.

I think you can light up a black hole by shining a cosmic torch on it, none of the photons will be able to escape and lo! you have a bright incandescent and no longer black hole

the mechanism for that is that light takes time to move from the emitter to the receiver.

Photons to leave the event horizon. At or outside that radius, they'll go to infinity if they don't get intercepted first.

Within, they have a light-like curve that never gets to infinity, but that doesn't exclude other things inside the event horizon. Like another observer who fell in...