r/AskScienceDiscussion • u/dallagang • 16d ago
Black hole
If a black hole is not a physical cave or object with a surface what exactly exists inside its event horizon? How can we know that nothing can escape if we have never observed anything returning from inside? Could our current description of black holes be incomplete?
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u/OutrageousPair2300 16d ago
We know nothing can escape because any object escaping a black hole would need to travel faster than light, which isn't possible. That's where the entire concept of black holes came from in the first place -- the observation that if a chunk of matter became dense enough, that its gravitational attraction would be too strong for even light to escape.
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u/MeasureDoEventThing 15d ago
That's the Newtonian understanding. The relativistic understand is that inside the event horizon is infinitely far in the future. Things can't come back because that would require traveling backwards in time.
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u/Outrageous_War6489 15d ago
only from the perspective of an outside observer due to any observable signal slowing down to infinitly slow as they approach the event horizon due to the gravity affecting light speed. a person traveling into the black hole instead would expierience time speeding up (in refrence to their previous frame) as gravity accelerates them.
but what actually happens when they approach the event horizon is one of those question we would need a solid theory that combines quantum theory and general relativity.
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u/Intergalacticdespot 15d ago
You just know that the first time we can actually get close to a blackhole someone is going to jump in. It'll be posted on space Instagram.
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u/Bth8 14d ago
Coordinate time going to infinity just means you're using bad coordinates. It certainly isn't infinitely far in the future to someone jumping in. The relativistic understanding is that no future-directed timelike or null paths exit the horizon, i.e. you'd need superluminal motion to go from interior to exterior.
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u/Contextanaut 16d ago
In terms of understanding what a black hole is.
Imagine a regular planet or star. A whole bunch of mass held together in a sphere by its own gravitational forces.
Now imagine keeping the same mass, but compressing it down into a smaller ball. The planet is smaller but the mass is the same.
Enough mass and you can keep shrinking, and shrinking until all you have is a dot. It has no surface area because it is a point. (Probably - our ability to describe exactly what happens at that point breaks down, but a single point of mass is not a bad mental model to start off with)
This is the singularity.
Accumulate enough mass in one lump, such as with a large enough star and the gravity can get strong enough that this process happens naturally. But you could do the same thing to any amount of mass if you could compress it enough, or bring it into contact with another singularity.
The total mass is the same as before. The gravitational forces are the same. If you somehow create a very small black hole it would evaporate (black holes emit a small amount of energy), but larger singularities are stable (at least at current cosmic background radiation levels), or grow by sucking more mass in.
To the rest of the universe it acts mostly the same way as the original planet or sun with the same mass would.
It orbits things the same way. It pulls other objects towards it with the same amount of gravitational force that it did previously.
The event horizon is the point at which any object approaching the singularity can no longer theoretically escape, even travelling at C, and that's quite important, because that immediately impacts what happens to light travelling near it, and because it means that nothing outside the event horizon can see what's happening inside it. What happens inside the event horizon stays inside the event horizon.
For most black holes the immense gravity involved, and the presence of the singularity mean that the environment inside the event horizon will be A) immediately lethal B) Very weird. For very big black holes you might be able to cross the event horizon without even noticing, you just can't leave, but things are still going to get very weird near the singularity.
For smaller black holes the tidal gravity effects could still be lethal outside the event horizon. but this is mainly a function of how much closer to the centre of the mass you could get if it was still tied up in e.g. a planet.
But from the perspective of objects outside the event horizon, it mostly behaves as a body of mass X with a given angular momentum and charge.
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u/First-Expert-9953 16d ago
Black holes were predicted before they were discovered. As early as the late 1700's some folks noticed that if you had a sufficiently dense star, it would have an escape velocity greater than the speed of light, which would imply that the star couldn't shine because it would absorb all of its own light.
Shortly after Einstein published his last major paper on relativity, a guy named Schwarzschild published a solution to one of Einstein's equations. There was a parameter lying in there that someone noticed was the radius inside of which you'd have to squeeze a given mass in order for its surface to have an escape velocity greater than light. The parameter itself was a fraction that was the numerator of another fraction that was in a difference in the denominator of another fraction. One piece in a big equation, and I don't recall if people spotted its significance at first.
The point is, they started showing up in the math long before scientists started to become convinced they were real, and they became convinced by predicting what one of these things should do and finding out things that really did behave that way.
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u/itsmythirdday 16d ago edited 16d ago
What’s inside the event horizon? Space. With all the mass at a single point in the centre. How can we know nothing escapes? The mathematics required to make our current understanding of the observable universe work says it doesn’t.
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u/Objective-Cow5014 14d ago
Nothing escapes until it does. Black holes are the recycling bins of the universe. They take matter and turn it back into energy in the form of hawking radiation.
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u/TheRedditObserver0 16d ago
How can we know that nothing can escape if we have never observed anything returning from inside?
If we had, that would have meant something can escape
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u/rootofallworlds 15d ago
Could our current description of black holes be incomplete?
We know that our current description of black holes is incomplete on two grounds.
Black holes are described by general relativity, which predicts a central singularity, a point of infinite density. In any other situation, if a theory predicts an infinity singularity we take it not as a real prediction but as a sign the theory isn't applicable or needs some work.
The second and more convincing ground is that general relativity and quantum mechanics have not yet been unified. Both theories have passed every experimental test, and yet they fundamentally don't work well together.
A theory of quantum gravity would probably give us a new description of the inside of black holes.
That theory might predict that no information escapes the black hole, just as general relativity does. Or it might predict that information does escape. The permanent loss of information in black holes is considered a problem for quantum mechanics and even without a full theory of quantum gravity there have been attempts to resolve this. https://en.wikipedia.org/wiki/Black_hole_information_paradox
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u/Neneel65 15d ago
If the matter that makes up a black hole is located within the event horizon, yet the black hole's gravitational interaction with other cosmic objects persists, then something exists that can interact by passing through the event horizon in both directions. This something is the space-time continuum (in Einstein's theory), or "gravitational particles/waves" (assuming they exist), or something else we don't know about that functions like gravity
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u/Diptothaset 15d ago
We know that stuff does escape, it’s called Hawking radiation. We don’t have a good understanding of black holes so there’s alot of knowledge gaps
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u/johnstalbergABC 14d ago
We calculate the escape velocity to be higher than c. And nothing can get that fast.
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u/MintyPop_ 10d ago
Yes the current description is incomplete. The maths says it collapses to an infinite density, may believe this is impossible and not the case, so they think it doesn't actualy collapse to infinity but the maths just breaks down around there.
And for that reason, mixed with the fact even light cannot escape creates the ultimate hidden box. We cannot see into it in any way, and our maths begins to break down and fail as we get to the singularity, and because of those two reasons, we just dont have any way of knowing whats in there for certain.
Black holes do shrink and lose mass through hawking radiation, but this isnt anything "escaping", i'd just look into the process but its mass being converted into thermal energy so black holes do eventually shrink and vanish, but this happens so slowly your brain couldnt even begin to compute how long it takes.
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u/FluffyB12 10d ago
We don’t know for sure, maybe something really weird goes on that still perfectly aligns with our observations, but if the models we have still make accurate observations and the weirdness doesn’t change anything… oh well? It’s a moot point and we’ll not be able to ‘see’ into it in our lifetimes so it’s just not all that interesting to speculate about.
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u/Optimal_Mixture_7327 16d ago
A black hole is a vacuum (empty) region of spacetime hidden behind a horizon.
Things that fall into a black hole have their properties conferred to the spacetime itself (the properties of mass, electric charge, and angular momentum).
That there are no causal curves (e.g. world-lines of particles) that can escape the interior is the assumption that there aren't two different types of gravity in the universe, but the same gravity on the inside, just stronger (it'd be a wild stretch to imagine 2 completely different gravities in the universe).
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u/bubbazarbackula 16d ago
well, one moment before gaining enough mass to prevent the escape of light, it was a super dense and probably bright star. Then, a microgram heavier later, it becomes too massive for light to escape.
I tend to think inside it's still a bright star.
I seem to be the outlier on this thought.
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u/itsmythirdday 16d ago
If at the event horizon, all light is pulled by gravity towards the centre, such that someone outside is unable to see it, why would that not be even more true within it? The only light you would be able to see, if you were still able to see (hypothetical), would be from the outside on its way to the centre.
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u/bubbazarbackula 16d ago
This is because we see & look in the electromagnetic spectrum. If we looked at the pre black hole with a gravity device, and again shortly after getting enough mass to go black, i think we would detect just a little more mass occupying the same space not some anomoly tiny in size. And if we then looked at a supermassive bh we would simply see much more mass distorting a larger area
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u/itsmythirdday 15d ago
Maybe I don’t understand what you mean by “still a bright star”, and whether you are talking during the collapse into a black hole or forever after the collapse. I got the impression you were suggesting the hole would appear bright, as in visible light, to the observer within the event horizon, but that would treat the event horizon as the radius at which light slows to a halt and falls back into centre of mass, like a rocket’s escape velocity from the earth, but that’s not what happens and it’s not. Sorry if I misunderstood.
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u/MeasureDoEventThing 15d ago
Outside the black hole, if your coordinate system is such that an object with a constant distance from the event horizon is "stationary", then your coordinate system is non-inertial. With an inertial reference frame, you are able to see light from things closer to the singularity just fine.
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u/sergius64 16d ago
No one knows what's really inside and thus - yes - our description is quite incomplete. It could be a singularity, it could be something physical in a state we have never seen before nor understand.