# Where does light go after it enters a black hole?

Nothing passes through a black hole. Where do things go then? If light enters a black hole, what happens to it? Do things go to a white hole and resume in another universe?

• It just sticks to the black hole, or goes in it. – PyRulez Aug 10 '15 at 14:58
• "What actually transpires beneath the veil of an event horizon? Decent people shouldn't think too much about that." -- Academician Prokhor Zakharov, For I Have Tasted The Fruit – Mason Wheeler Aug 10 '15 at 18:41
• @MasonWheeler What fruit was this Academician tasting at the time? – PyRulez Aug 11 '15 at 2:37

In 1974 Stephen Hawking published a paper that provides a theoretical basis for the thesis that black holes eventually may radiate away all the mass, light, and other energy they accumulate.

Evaporation of black holes has been called Hawking radiation. It takes place so slowly (at least until the black hole shrinks to a small size) that none has been measured to date, but researchers are searching for evidence of it. If they find any, then it's conceivable that all the matter, light, and other energy Hoovered up by a black hole may eventually find its way back into the universe as higher-entropy Hawking radiation.

Here's a link to an excellent discussion of Hawking radiation by mathematical physicist John Baez: http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html

It is believed, according to our most tested theory of gravity (General Relativity), that objects, such as material from other stars, particles such as electrons and also photons of light, may actually pass through the Event Horizon of a black hole.

For a large enough black hole, a person in a spaceship passing through the event horizon may not notice any difference from "normal" space travel. The mass of the black hole is very important in producing the effects you may encounter passing through it.

What happens to these objects mentioned above, after the passage through the event horzon, is currently unknown, as we cannot test or perform experments on or near black holes.

The above picture shows an artists impression of an accretion disc, that is a disc of matter moving around the black hole at very high speeds, that may eventually fall into the black hole. The large luminosity of Quasars, which we can detect on Earth, is believed to be a result of gas being accreted by supermassive black holes.

If you read this wikipedia article, Black Holes, you will find a lot more information on the various ideas regarding black holes and what effects they can have.

• When you say "may actually pass through the Event Horizon", you make the event horizon sound like some kind of magic barrier, when it's not locally any different from normal space, other than its proximity to a singularity. – Nick2253 Aug 10 '15 at 15:08
• @Nick2253 It is magic:) (sort of), but your point is perfectly valid and I have edited the post. Thanks very much for your comment, I hope it it does clarify the answer for the OP and I should have included it. – user81619 Aug 10 '15 at 15:17
• @Nick2253 Yup, a perfectly normal space, except that it's only ever one sided. Getting in is easy... when you're inside, there is no longer any path out (except back in time, of course :P). Thinking in terms of usual terrestrial topology, this is quite mind boggling :D Most people seem to have trouble wrapping their minds around this whole "time is just another part of spacetime and it distorts" thing (past me included). – Luaan Aug 10 '15 at 16:00
• a good analogy to the event horizon is a ship in a whirlpool (or bathtub drain!) (or an even closer analogy, an object in decaying orbit). Its moving along its happy way, not knowing that it could never produce enough thrust to make it out of the spiral. – Jason Coyne Aug 10 '15 at 18:59
• @JasonCoyne An important difference between this analogy and reality is that the reason escape is impossible is very different. In the analogy, it is simply because the ship does not have enough thrust to escape. However, in a black hole, the amount of thrust is entirely irrelevant. You cannot escape because, physically, every direction is down (assuming you move forward in time), which is due to counterintuitive warping of spacetime in such a strong gravity field. – Radon Rosborough Aug 10 '15 at 23:02

A short answer is that frames themselves are moving towards the black hole and light moves relative to a frame and hence it can be stuck.

Nothing passes through a black hole.

Things can enter a black hole, they can't can't exit without going faster than light.

Where do things go then?

The important part of that question is the word "where" you have to learn what that means. In general, and in general relativity, not just in the everyday situations when the word is quite intuitive.

The first thing to know is that your spatial coordinates can be relative, if you think something is at rest in your coordinate system and some else has a coordinate system that is moving relative to you then they might think the thing is moving.

An example is a train, you might describe things with a coordinate system based on the front of the train, and someone sitting in the train would be at rest, but to someone on the ground, the front of the train is moving and so is the person sitting on the train. You might think the person on the ground is right and the person on the train is wrong, but to someone sitting near the sun, the earth itself is moving around the sun so the person on the ground says things are at rest when they are going around the sun. And someone in a different part of the galaxy thinks the sun is going around the center of galaxy so things at rest to the sun are not at rest to the galactic center. And even the galactic center is not the end of the story, someone in the Andromeda galaxy sees the whole Milky Way galaxy rushing towards it (on a collision course in fact).

So you can do two things. You can describe things locally, such as how far object A is from object B or you can pick a local frame and describe the coordinates in that local frame and learn how to get distances from those points.

There are lots of possible frames. In some of them the laws of physics hold, these are called inertial frames, and they cover very very very small regions of spacetime, so they only work for a small interval of times and a small range of positions, if you move a ways or persist a ways then you have to switch to a new frame every so often.

Outside a black hole, there is a surface called the event horizon. The inertial frames themselves pass through the event horizon (or alternatively the event horizon is an invisible surface that moves across the frame, motion is relative so there is no real difference). There are still many different inertial frames, for any particle there is an inertial frame that is momentarily moving at the same velocity as the particle (or alternatively, the particle is at rest in that frame, motion is relative so there is no real difference). Since there are many different velocities, there are many different inertial frames. But in each frame that has the event horizon surface at its origin, the surface in that frame that represents the event horizon is expanding outwards. And it expands outwards at speed c.

Since light moves at sped c relative to the frame and the event horizon moves outwards at speed c, if the event horizon has a head start you never catch up, so you can cross it one way (it is rushing towards you at the speed of light) but not the other way (it is rushing away from you at the speed of light).

If this wasn't general relativity and I told you there was as surface expanding at the speed of light and the two sides of the surface are the outside and inside of some region you might think that region is simply getting larger. That's understandable. But how would you know if something is getting larger. Time itself is affected by gravity, what if time slowed down inside the region so things in there though thousands of years were passing, they might think light was traveling thousands of lightyears but to you it wouldn't look like it went very far. It is a much more subtle question when distance and time can be related to coordinates in different ways in different places.

It is possible that the area of the surface stays the same even if it is expanding radially outwards in every inertial frame. This requires spacetime to be curved, and the time part too, not just space. And the word curvature is a technical word, it doesn't for instance say the surface of a cylinder is curved, it is not talking about bending in some larger space it is talking about the geometry of curve one the surface. For instance the surface of the earth is curved and we can tell that because if you travel south from the north pole you end up at the south pole no mater which path you take, the curves heading away from teh north pole simply don't get as far away from each other as they would on a flat plane, when you send a bunch of people along a path traveling 1m away from the north pole, they form a circle that is not $2\pi (1m)$ in circumference, but instead form one that is smaller. A similar thing happens outside a black hole, a surface could expand outwards but the circumference of the surface can be smaller than you'd expect. The event horizon is such a surface, it can expand in any local frame, but the whole surface area never gets larger.

Since we call the region on one side "the outside", and that surface is expanding in your frame at the speed of light, you simply stay on the "inside".

If light enters a black hole, what happens to it? Do things go to a white hole and resume in another universe?

If the light is heading in and then if it bounces off of something or scatters around the insides of the black hole or somehow get near the event horizon surface again, in any inertial frame that event horizon is expanding at the speed of light and has a head start. You won't cross because you just never get to it. If you were heading in and immediately bounced back you could stay on it. That surface is actually the cutoff: if you scatter before you get to it you can stay out and if you go through it then you will never be able to catch up and right at it is your last chance to stay right on it.

There are models of black holes that have white holes "inside" the event horizon too, but that is because they have two universe that have always had event horizons and the two outside universes are joined together the the event horizon. Even then light that goes through an event horizon merely meets light coming in from the other universe (remember light that just came in is moving one way, the light moving the other way is either from the other universe or it entered much earlier and turned around nearby and now just got here). And light from both universes are equally stuck in the sense that they are equally getting farther away from the event horizon. the way they are connected is the incoming light from one side of the event horizon is outgoing light for the other side of the event horizon, so imagine two event horizons, one for each universe, incoming on one being outgoing on the other. Now rightly the event horizon for one universe has a head start against any actual light coming in from the other universe.

So what about the white hole? The white hole is in some sense in the distant past. Anything that comes in through either event horizon arrives too late to affect the white hole. No matter how early they came through. The white hole had the ability to affect the outsides but already did so earlier than any event on the outside. Basically when you specified the conditions of the the outside of the black hole at a certain time, that time is too far from the white hole and too late to affect it.

The white hole is singularity, the theory is actually broken there, and you shouldn't treat it too seriously. Yes it looks like it is in the past of everything outside, but it doesn't have a past itself and it is completely broken there.

But you most definitely do not end up in the white hole, nothing that is outside ever ends up in the white hole. So what does the white hole do? You can think of it as something that kept the outsides of the two universes apart. Since the event horizons is eternal, you can think of the white whole as where the event horizons themselves came from (but that is farther away ago then any outside time).

I don't expect this answer to be fully satisfying, but there is a lot to learn and hopefully this is the rough bones of the theory.

• Using the words "never", "eternal", "earlier", "before" means the reader must understand that even these descriptions are tethered to frames of reference (understanding) that must change when one crosses the event horizon. – qneill Aug 10 '15 at 22:35
• You answered this in a much more comprehensive manner than I did. I did not address each of the points raised by the OP. Rereading my answer, I can't say I am happy simply referring the OP on to Wikipedia, but I do wish there was a system for asking the OP what level of textbook or study they are at. – user81619 Aug 11 '15 at 0:40
• @AcidJazz It's OK to have many answers at different levels because the answers aren't just for the OP. Sometimes my answers really only spell out the breadth of issues involved more like a map than an education. But that can happen if the question was a bigger question than the OP realized it was. – Timaeus Aug 11 '15 at 0:52

No one knows what happens inside the black hole (I mean inside the event horizon). But inside the event horizon space becomes unidirectional (like time in real world) and therefore whatever enters into it must hit the singularity inside (at least according to classical general relativity). But again no one knows what happens at that singularity. By definition (classical) theory breaks down there. Till date, only "science fictions" work there, as far as I know!

• Well, the theory doesn't quite break down; it just gets kind of wibley-wobley, timey-wimey... – Luaan Aug 10 '15 at 16:02

Nothing special is happening!

Think of a black hole as the accumulation of mass which is exceeding a certain limit. The same laws of gravity are applying before and after exceeding the limit. That means: Mass particles keep on being attracted. They are becoming part of the mass of the black hole. Electromagnetic waves will equally be attracted by the mass of the black hole, and the light will be absorbed by the mass, in the same way as light is absorbed by any mass particle. At my knowledge mass will not stop absorbing light when it is part of a black hole.

The particularity is that everything is happening behind the event horizon. So we will never be able to observe what is going on inside.

• "Nothing special is happening!" Its amazing how science zaps people's sense of wonder. – developerwjk Aug 10 '15 at 22:26
• @developerwjk That's a good point and as scientists we need to be careful that we don't convey an interjection like "Nothing special is happening!" in a belittling or trivializing way. But do be aware that when people in physics make that interjection, it's not needfully trivializing the subject. Very often (and this is how I heard Moonraker's statement as I read it) it's an exclamation that conveys excitement: I've heard it said many times when the speaker is brimming with enthusiasm and is about to tell you, "hey, this is subtle and hard, but if you look at it this way, you suddenly ... – WetSavannaAnimal Aug 12 '15 at 1:40
• @developerwjk ".... understand that the question you've been asking is too complicated - let me show you a simpler question, answer it and prove it amounts to the same thing!". And that is the opposite of trivializing something. Listen to Brian Cox and his summary at the end of Einstein grasping the equivalence principle, that ultimately leads to GTR: what Einstein described as the happiest moment of his life was understanding "nothing special is happennning. To fall is natural. To fall is to be in the simplest motion state possible". That's ... – WetSavannaAnimal Aug 12 '15 at 1:45
• @developerwjk a "trivialization" that shook physics to its very foundations. – WetSavannaAnimal Aug 12 '15 at 1:45

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