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View Full Version : Is it meaningful to say "Nothing can escape a black hole" ?


am77494
09-13-2017, 07:05 PM
Okay - I understand the Hawking radiation phenomenon and this is not a question about that.

Classically, it is said that the gravitational pull of black holes is so strong that even light cannot escape from a black hole (event horizon). This kind of brings an image of a photon trying to leave a black hole but pulled back in a spiral maybe.

Is this a true picture ? How and do we know that light (EM radiation) exists inside a black hole but is unable to leave ?

If I understood singularity correctly, I would say that we can't (not don't) know what happens inside a black hole except that we can observe externally Hawking radiation and gravitational effects. Is this an appropriate discription ?

Chronos
09-13-2017, 08:45 PM
It's misleading to say that the escape speed of a black hole is greater than c. That calculation does give you the correct value for the size of the event horizon, but it's because you've got multiple errors that just happen to cancel each other out.

A better description is that there simply isn't any path from inside of the black hole to outside of it. I can tell you how to get from here to tomorrow: That's easy, just wait 24 hours. But I can't tell you how to get from here to yesterday. There's just no path from here to there. In exactly the same way, there's a path (actually, a bunch of them) from outside the hole to inside of it, but there is no path from inside the hole to outside of it. We don't know if there's light inside a black hole, but even if there is, it can't get out, because there just isn't any path to yesterday.

glowacks
09-16-2017, 04:15 PM
It's misleading to say that the escape speed of a black hole is greater than c. That calculation does give you the correct value for the size of the event horizon, but it's because you've got multiple errors that just happen to cancel each other out.

A better description is that there simply isn't any path from inside of the black hole to outside of it. I can tell you how to get from here to tomorrow: That's easy, just wait 24 hours. But I can't tell you how to get from here to yesterday. There's just no path from here to there. In exactly the same way, there's a path (actually, a bunch of them) from outside the hole to inside of it, but there is no path from inside the hole to outside of it. We don't know if there's light inside a black hole, but even if there is, it can't get out, because there just isn't any path to yesterday.

This is exactly what I was going to say when I saw the question.

General Relatively says that mass warps space-time, and this warping is what causes objects to appear to travel in curves when moving under only gravitational effects. They are always actually moving in perfectly straight lines - it's just that the geometry of space is curved. So with a black hole, there is so much mass in a small enough space, it causes enough curvature of space-time such that there is no path that starts inside the event horizon that leads somewhere outside the event horizon.

am77494
09-16-2017, 05:18 PM
We don't know if there's light inside a black hole

Bolding mine.

Thanks for the reply Chronos. Is it true that "we don't know" or is it more "we can't know" ?

Similar to - "We don't know" what was before the big bang or We can't know what was before the big bang.

DPRK
09-16-2017, 05:24 PM
What if I shone a light at a black hole? Then there would (temporarily) be some light inside that hole.

Chronos
09-16-2017, 05:57 PM
Well, we can make models that describe black holes. And we can test those models in great detail, in the region outside the event horizon. And we can extrapolate those models across the event horizon, and see what the extrapolated model says there. But we can never check whether those extrapolations are valid. Decide for yourself whether that means we "know", "don't know", or "can't know".

msmith537
09-16-2017, 10:12 PM
What if I shone a light at a black hole? Then there would (temporarily) be some light inside that hole.

It's my understanding that in a very large black hole (like the supermassive kind found at the center of most (all?) galaxies, there isn't anything to indicate you've crossed the event horizon. Compared to smaller black holes where the rapid increase in tidal forces would literally rip you into spaghetti.

You basically drift across the event horizon inevitably toward the singularity with no way to get back. It might look something like this:
https://www.youtube.com/watch?v=xBAHRjPeiIg

The analogy I've heard most often is like drifting down a river towards a waterfall. The boat has some theoretical chance of getting back up stream up until it crosses the event horizon of the waterfall.

I can't remember what happens if you lower an astronaut on a rope into a black hole. You certainly can't fish them out again. But I don't remember what the astronaut experiences if he tries to climb back up.



"can't know" is probably a more accurate term since no information can escape a black hole. I mean aside from traveling into the past and using the power of love to Morris Code a message to your daughter through her book case.

DPRK
09-16-2017, 10:34 PM
How was that video generated – is it accurate or merely an artist's impression? The question is about light inside the black hole; presumably some light is falling in from nearby stars, but could you see your own feet?

Asympotically fat
09-17-2017, 09:26 AM
In layman's terms the definition of a black hole is a region of space nothing can escape from, so it certainly is meaningful to say that nothing can escape from it.

Visualizing how it works though is more difficult. One way of thinking of it is that the event horizon expands in space with a speed of c, but that space is contracting so that for a faraway observer it appears static.

Blue Blistering Barnacle
09-17-2017, 09:42 AM
In layman's terms the definition of a black hole is a region of space nothing can escape from, so it certainly is meaningful to say that nothing can escape from it.

Visualizing how it works though is more difficult. One way of thinking of it is that the event horizon expands in space with a speed of c, but that space is contracting so that for a faraway observer it appears static.

I suppose a black hole might be kind of like an onion. Light (and stuff) can always curve or fall inward, but never outward.

An invulnerable astronaut might see her feet if she dove in headfirst (but who could do such a thing- unimaginable!*), but not if she went in feet first?



*riffing on In the Suicide Mountains by John Gardner
https://www.goodreads.com/book/show/687793.In_the_Suicide_Mountains

DPRK
09-17-2017, 10:15 AM
The classical textbook understanding is that if the black hole were supermassive enough that you would not notice anything special upon crossing the event horizon, and you freely fell in, and you brought along your own light source, then, sure, you could see your feet, again assuming the hole to be big enough so that you have time to do anything before being killed.

But what is the environment inside a black hole really like? And nothing gets out beyond an event horizon, so what exactly happens to information that enters the black hole?

Blue Blistering Barnacle
09-17-2017, 02:07 PM
The classical textbook understanding is that if the black hole were supermassive enough that you would not notice anything special upon crossing the event horizon, and you freely fell in, and you brought along your own light source, then, sure, you could see your feet, again assuming the hole to be big enough so that you have time to do anything before being killed.

I'm just wondering what makes the event horizon so special. If its impossible for light to go outward at that point, wouldn't it be even harder to go outward when you are further in?


But what is the environment inside a black hole really like? And nothing gets out beyond an event horizon, so what exactly happens to information that enters the black hole?

I guess this is the nub of the whole post. We don't really know, and as mentioned above, while we can try to model it, we can't ever really know what it's like in the interior, even in principle. As opposed to lots of other impossible stuff, like The Fantastic Voyage or Dragon's Egg (life on a neutron star), which while also based on models is at least knowable in principle.

As to what happens to the information, I am no expert, but last I heard, a consensus was emerging that the information is stored on the surface of the event horizon, perhaps to someday return as imprints on the Hawking radiation. Or maybe it's stored just above the surface (time dilation) and has never really left the universe.

Trinopus
09-17-2017, 02:21 PM
I'm just wondering what makes the event horizon so special. If its impossible for light to go outward at that point, wouldn't it be even harder to go outward when you are further in? . . .

Well, in a way. Linguistically, nothing can be "harder" than "impossible," but in math, you can have larger and larger "imaginary" solutions to equations. It's "harder" to go five times the speed of light than it is to go twice the speed of light (maybe: some tachyon equations suggest it might be easier!)

Imagine those old word problems -- you're going from Denver to St. Louis -- let's say the distance is 400 miles. You have to make the trip in an hour. (I recommend a jet aircraft.) For the first leg of the journey, you've been going 300 mph, and you've covered 300 miles. How fast do you have to go to get to St. Louis on time?

Trick question: you can't. You'd have to go infinitely fast to do it. Now say you've done the first leg at 300 mph, and you've covered 330 miles. You *really* can't do it, because you're six minutes *past* the deadline.

Which of the two is "harder" to do? Go infinitely fast, or go backward in time?

Some black hole trajectories put you into that kind of "impossible" math.

Blue Blistering Barnacle
09-17-2017, 04:10 PM
Well, in a way. Linguistically, nothing can be "harder" than "impossible," but in math, you can have larger and larger "imaginary" solutions to equations. It's "harder" to go five times the speed of light than it is to go twice the speed of light (maybe: some tachyon equations suggest it might be easier!)

Imagine those old word problems -- you're going from Denver to St. Louis -- let's say the distance is 400 miles. You have to make the trip in an hour. (I recommend a jet aircraft.) For the first leg of the journey, you've been going 300 mph, and you've covered 300 miles. How fast do you have to go to get to St. Louis on time?

Trick question: you can't. You'd have to go infinitely fast to do it. Now say you've done the first leg at 300 mph, and you've covered 330 miles. You *really* can't do it, because you're six minutes *past* the deadline.

Which of the two is "harder" to do? Go infinitely fast, or go backward in time?

Some black hole trajectories put you into that kind of "impossible" math.

Well, isn't any trajectory with increasing distance from center impossible at event horizon or closer? Or is this not true, and may some trajectories temporarily arc away from center (like a suborbital launch)? Or is this not knowable?

Trinopus
09-17-2017, 04:29 PM
Alas, now, you're way beyond my pay grade. I'm sure math has been done on trajectories inside the event horizon, but I don't know any details, and I couldn't suss the equations.

I was just having fun with the "harder" part of the question, which is almost factually answerable!

Chronos
09-17-2017, 04:33 PM
According to the extrapolations of our models, you can't increase your r from anywhere inside the hole. But again, we can't check those extrapolations.

Blue Blistering Barnacle
09-17-2017, 06:49 PM
I propose that an invulnerable astronaut in-falling within a black hole feetfirst would be unable to see or feel their feet. (Also, blood would get stuck in the feet, never to return.)

That is, assuming that biological processes of any sort are possible. Even a robot astronaut would be unable to function if signals can't go both ways.

DPRK
09-17-2017, 07:15 PM
The astronaut's head and feet and everything in between are inexorably falling inward towards the crunchy center, so there is nothing preventing photons originating near the feet from reaching the head, even as all paths have decreasing radius.

Intriguingly, though, Chronos reminds us that this is all theoretical– you could always go see for yourself but would not be in a position to communicate your findings afterwards. Is that all you meant, or was there more to it than that?

Blue Blistering Barnacle
09-17-2017, 07:46 PM
No. I had neglected the consideration that the head (eyes) was rushing to where the feet had been.

msmith537
09-17-2017, 08:18 PM
How was that video generated – is it accurate or merely an artist's impression? The question is about light inside the black hole; presumably some light is falling in from nearby stars, but could you see your own feet?

Not sure. Certainly not from actual footage.:D


Another video I found indicated that for all intents and purposes "nothing" happens beyond the event horizon. You appear to be frozen in time while red-shifted into oblivion at the edge of the event horizon.

DPRK
09-17-2017, 10:31 PM
Another video I found indicated that for all intents and purposes "nothing" happens beyond the event horizon. You appear to be frozen in time while red-shifted into oblivion at the edge of the event horizon.
Well, according to the encyclopedia survey of numerical relativity (https://en.wikipedia.org/wiki/Numerical_relativity), one simulation technique does indeed involve
"excising" problematic regions inside event horizons, which is valid for the precise reason you mention: for all intents and purposes nothing happening inside the hole matters.

Yet it still seems like black holes are real enough, and their interiors are real places.
(Impossible to observe, but is it that mind-bending? One is not likely to observe the core of the sun, either.)

Der Trihs
09-17-2017, 10:59 PM
How was that video generated – is it accurate or merely an artist's impression? The video (https://www.youtube.com/watch?v=xBAHRjPeiIg) says it was created on something called Space Engine (http://spaceengine.org/). Which claims scientific accuracy, but who knows?

Chronos
09-18-2017, 05:49 AM
(Impossible to observe, but is it that mind-bending? One is not likely to observe the core of the sun, either.)
We do observe the core of the Sun, via neutrinos and via helioseismic waves that pass through it. It's a very murky sort of observation, but the tech is constantly improving. The closest equivalent with a black hole is observing the "ringdown" of a merger in gravitational waves, and even that is likely analyzed using models that don't even consider the internal regions.

DesertDog
09-18-2017, 11:34 AM
The video (https://www.youtube.com/watch?v=xBAHRjPeiIg) says it was created on something called Space Engine (http://spaceengine.org/). Which claims scientific accuracy, but who knows?When Gargantua, the black hole featured in Interstellar, was rendered, Kip Thorne (https://www.space.com/27539-interstellar-black-hole-physics-video.html), executive producer and their their cosmological consultant was as surprised as anyone.

"Is that what a black hole looks like?"

"Well, we cranked the math you gave us through our imaging software and that's what came out."

"...Can I borrow that?"

The prediction of Hawking radiation (https://en.wikipedia.org/wiki/Hawking_radiation) allows for photons, among other things, to escape the other side of the event horizon but the effect is too subtle to be observed directly.

Lemur866
09-18-2017, 11:51 AM
Right, and for stellar mass black holes the temperature of their Hawking radiation is below 3K, which means that they absorb more from the background radiation than they emit. It's going to be a long long long time before the universe cools enough for large black holes to even begin to evaporate.

DPRK
09-18-2017, 01:43 PM
Here (https://en.wikipedia.org/wiki/Fuzzball_(string_theory)), for instance, is described an (obviously non-classical) idea where the interior of a black hole is crammed full of strings. (Thus no classical singularities.) Anyone familiar with that one?

Chronos
09-18-2017, 02:50 PM
Quoth Lemur866:

Right, and for stellar mass black holes the temperature of their Hawking radiation is below 3K, which means that they absorb more from the background radiation than they emit. It's going to be a long long long time before the universe cools enough for large black holes to even begin to evaporate.
Not really very long at all-- A mere trillion years or so would do it.

And the fact that a trillion years can be considered "mere" tells you something about just how long black holes last.

DPRK, nobody would be particularly surprised if, once we eventually get a working theory of quantum gravity, it predicted that there weren't any singularities after all. But the String Model is far from being a working theory of quantum gravity. In the state it's in right now, it'd be surprising if you couldn't support any given hypothesis in the string model.

Melbourne
09-18-2017, 07:12 PM
According to the extrapolations of our models, you can't increase your r from anywhere inside the hole. But again, we can't check those extrapolations.

So black holes are by definition spherical?

Chronos
09-18-2017, 07:46 PM
Well, I've been assuming we're talking about non-rotating ones, for simplicity's sake, and those are spherical (though by theorem, not by definition). But rotating ones are more common, and those are a bit squashed in shape.

Trinopus
09-19-2017, 06:19 PM
So black holes are by definition spherical?

If they're rotating, then they are ellipsoidal.

Whack-a-Mole
09-20-2017, 11:33 AM
Physicist Brian Cox did a BBC special on the science of Doctor Who (the TV show).

There is a segment that I think answers the OPs question pretty well.

Watch the first clip for the background setup needed for the second clip which is an explanation for the OP:

1) https://www.youtube.com/watch?v=jNvNgsY2mu4

2) https://www.youtube.com/watch?v=KIKXTziwOGY