10 things you should know about black holes (Synopsis)

Earlier this week, Stephen Hawking shook up the world when he announced that he had uncovered the solution to the black hole information paradox at a conference in Stockholm. When particles fall into (or create) a black hole, information is encoded on the black hole's surface, but when the black hole decays into radiation, that information appears to be lost, as the radiation is thermal.

Image credit: E. Siegel. Image credit: E. Siegel.

But perhaps the information is stored on the event horizon, and can be encoded into the outgoing radiation thanks to the interplay of gravitation and matter. Details should be forthcoming in a paper to be released next month by Hawking, Malcom Perry and Andrew Strominger.

Image credit: Dana Berry/NASA, of a neutron star (L) and a black hole (R), via http://www.nasa.gov/mission_pages/swift/bursts/short_burst_oct5.html. Image credit: Dana Berry/NASA, of a neutron star (L) and a black hole (R), via http://www.nasa.gov/mission_pages/swift/bursts/short_burst_oct5.html.

Sabine Hossenfelder -- attending the conference -- has the full story.

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While it should have been obvious, I didn't realize until reading this article that there is not a single moderate or large black hole that has lost even a single atom's worth of mass to Hawking Radiation in the history of our universe to this point. Even if a black hole was completely cut off from being able to absorb any matter for the past 13 billion years, it would still absorb more energy from the CMB than it could lose in Hawking Radiation.

The article shows models of how ever heavier objects, such as the sun or a neutron star, bend spacetime.The model for a black hole is similar and just bends spactime deeper. I think this model misses an essential point. In black holes spacetime is bent to such extremes that light starts running in circles or even spirals inwards. This means that inside a black hole, events get disconnected from outside events. If I understand correctly, this disconnection happens at the event horizon. Shouldn't this disconnection between inside and outside be somehow visualized in the model?

An interesting point made is that time starts running very slow at the event horizon. Does this also imply that time in the universe starts running extremely fast from the point of view of the event-horizon? Would you see galaxies spinning up to mulitple rotations per second when you are close to the event horizon?

By A Blankert (not verified) on 26 Aug 2015 #permalink

@Ethan and/or @Sabine: Do either of you know if the talks at the HR Conference are going to be posted to the program (http://global.unc.edu/hawking-radiation-conference-program/)? The popular-press reports are all well and good, but I'd really prefer to see the actual slides (and some of the other talks sound interesting, too, even if 't Hooft and Misner are over my head).

By Michael Kelsey (not verified) on 26 Aug 2015 #permalink

@Ethan & Sabine

"The idea is that black holes have a way to store information which has so far been neglected. This information is stored on the black hole horizon ..."

am confused a bit... according to Sabine this is latest news and the paper should be out in September. But I distinctly remember watching lecture/s on youtube about this a year or more ago.. I mean, wasn't the "war" between Hawking and Susskind been about this in the first place.. and they came to "conclusion" some 4-5 years ago.. that's when I first heard of event horizon storing information about blackhole on it's limit. So, question is, what is new? what has changed since then?

By Sinisa Lazarek (not verified) on 26 Aug 2015 #permalink

AFAIK, the only difference is that Stephen has detailed and presented a model to explain what "information" you get back from this event horizon, and it's about as useful as the "information" you get from an entangled pair that one person reads the orientation of and has "sent" to the other location instnataneously.

IOW, no useful information. Just a state conveying nothing new.

I recall reading some other theory about this (involving holograms projecting the information onto the event horizon or something, or information being stored holographically on the surface of the black hole) in the past year- sadly can't recall where or who.
I appreciate that these kind of explanations are necessarily pretty high level in the articles which I'm likely to find legible, but I'm not sure what exactly is new here compared to what I've read previously. Apart from maybe what I read previously didn't have a solution for getting the "information" back out.
Is there some significant new nuance that does not translate well to explanations to folks like me?

By Andrew O'Byrne (not verified) on 27 Aug 2015 #permalink

I have a question about list article number three:

3. What happens at the horizon?

Somebody crossing the horizon doesn’t notice anything different in their immediate surroundings...

This sort of thing pops up a lot in lists like these, as well as in all sorts of science fiction, but I could have sworn that in a previous 'Ask Ethan' article, it was suggested that any given lump of matter would evaporate as it crossed the event horizon because the forces and the particles mediating the forces that bond molecules, atoms and even the quarks that make up individual protons can't cross from one side to the other, and therefore immediately zip off on individual arcs directly to the center as soon as they're on the other side.

Is one or the other of these explanations not the full picture, and if not, what is?

I could have sworn that in a previous ‘Ask Ethan’ article, it was suggested that any given lump of matter would evaporate as it crossed the event horizon...

Then you would be swearing to a complete fiction, Adam. One you've made up. NOTHING like that was said.

If you want to find out, why didn't you go back and recheck what "you could have sworn" was said?

@Adam #7

I remember the article you were referring to. It was an 'Ask Ethan' titled something like "can you climb out of a black hole?"

I think you misunderstood what @Ethan was saying. He was saying that even at the subatomic level, nothing can move farther away from a singularity. If one end of a rope was immovably anchored outside a black hole, any part of the rope crossing the event horizon would snap off. That is not to say the whole assembly can't stay intact for a few seconds if it were all collectively flying towards the singularity, or at least until the gravitational tidal forces spaghettified it .

It was probably the spaghettification one.

This is why I prefer to let Adam work out what the hell he's going on about rather than everyone guessing about what he might have been misremembering then "correcting" him on something that he doesn't even know he was misremembering.

It's a waste of time for everyone else, and the only one who can resolve it is the one who is asking everyone else to do the work.

Thanks Denier, that turned out to be exactly the post I was looking for: Ask Ethan #81. I've reread the post, and it was just how I remembered, complete with nifty picture of gluons keeping a neutron stuck together and the following quote:

"Now, one of these quarks is going to be closer to the singularity at the center of the black hole than another, and another will be farther away. For an exchange of forces to happen — and for a neutron to be stable — a gluon will have to travel, at some point, from the closer quark to the farther quark. But even at the speed of light (and gluons are massless), that’s not possible! All null geodesics, or the path an object moving at the speed of light will travel along, will lead to the singularity at the center of the black hole. Moreover, they will never get farther away from the black hole’s singularity than they are at the moment of emission.

That is why a neutron inside of a black hole’s event horizon must collapse to become part of the singularity at the center."

What I'm not seeing is how this allows someone to stay in any kind of coherent shape no matter how far away from the center of the singularity you are or how gently you are able to ease into it.

What I’m not seeing is how this allows someone to stay in any kind of coherent shape no matter how far away from the center of the singularity you are or how gently you are able to ease into it.


Have you tried to work it out and see? Or have you not bothered?

Who says someone would be in a coherent shape near a singularity, Adam?

The event horizon is a long way away from the singularity in a supermassive black hole. A small one will rip you apart and likely well before you reach the event horizon. Assuming you are able to breathe and not boil or freeze to death out there.

There's nothing special about the event horizon to something very near it, or just inside. Unless you pick a specially sized one, in which case the problem isn't in the event horizon, it's in the set up.

@Adam #12

Let me start with the following disclaimer: The rules of Classical Mechanics break down at the Event Horizon and even academics who have spent their professional lives studying this stuff don't agree on exactly how things work on the singularity side of an Event Horizon. My thinking is also often unorthodox so take what I say with a grain of salt. You've been warned.

My previous description of an item staying intact is based on the theory that gravity is probabilistic in nature. At the event horizon, the probability of movement towards the center of mass becomes 100%. Not only can gluons not travel farther away from the singularity, they can't even travel sideways. The same goes for every subatomic component. Nothing breaks down or flies apart because it can't. It all falls in the exact formation it was in when it crossed the event horizon.

Denier, thanks again for the response.

I don't see how everything would all fall in the same formation though. Here's my current takeaway from 'Ask Ethan 81' (which is all I have to go by on this topic).

From what I can see, as soon as any part of an object passes through the event horizon, even a really big one where the object had been able to resist spaghettification to that point, all the massless force mediating particles would zip off to the singularity at the speed of light, leaving behind a cloud of unbound elementary particles with mass (quarks, electrons, etc.), each following a separate orbit towards the center. The original shape would diffuse into a cloud, since each particle had it's own momentum (Brownian motion, vibration due to temperature, etc.) and the cloud would be increasingly spaghettified with no forces left to oppose it.

I don’t see how everything would all fall in the same formation though

I don't see how you insist they can't.

all the massless force mediating particles would zip off to the singularity at the speed of light

Yup, that mistake will make it happen.

You're wrong. The massless force mediating particles ALWAYS zip along at the speed of light.

If they don't, why wouldn't they move through the event horizon as a coherent body?

@Adam #16

The force mediators are not actual particles that can zip off like photons. All force mediators, with the possible exception of gravity, are virtual particles. They are wavy lines on a Feynman diagram that allow physicists to describe interactions. A better way to think of what @Ethan was saying is to conceptualize that no information other than gravity can propagate away from the singularity. When you start picturing virtual particles as teeny-tiny bullets of matter you get yourself in trouble, and that goes for interactions in normal space as well.

With regards to the actual matter in whatever it was that fell in, you can draw a straight line from each particle going directly towards the singularity and that is where it all goes. Nothing poofs out into a cloud because that would introduce a bunch of paths that were not shortest distance. The individual masses of the subatomic particles don't matter because everything falls at the same rate in a gravitational field.

Hope that helps.

Thanks again Denier! I had allowed myself to think of the force mediating gluons as little bullets (probably because of the article and the gif diagram), so thank you very much for pointing out that they're virtual particles. From what I can read up on, they're perturbations in whatever field they're mediating that can be thought of as extremely transient real particles for working out QCD problems. I have a couple of questions for you though.

Virtual particles or not, if information can't cross the event horizon, what's keeping particles connected to each other as they cross over? Are they still close enough to snap back together on the other side? How, if information being transmitted from one particle to the other can't travel in certain directions?

I'm also interested in your statement that once matter gets inside the event horizon it just drops strait down. I've never heard that before. So a particle coming in at an angle wouldn't follow along on its previous orbit towards the center? Is this another way that the statement that 'Somebody crossing the horizon doesn’t notice anything different in their immediate surroundings' falls apart? It's even easier for me to imagine a macroscopic object falling in at an angle disintegrating on contact with an event horizon as if being rubbed across a cheese grater.

Actually, could you explain what you meant about 'everything falls at the same rate in a gravitational field'? Acceleration due to gravity varies by distance...

By the way, Denier, thanks for the responses!

@Adam #19

When things cross the horizon, the constituent particles of matter are not bound together as they are in normal space. On the subatomic scale, it does fall apart in a manner of speaking. However on a macro-scale, everything stays intact other than spaghettification. On the interior of a black hole, nothing is stronger than gravity. Gravity rules, and overrules, all. As stated earlier, you can draw a line between each subatomic particle and the singularity. That keeps everything in the same macroscopic order and falling in formation.

The straight line toward the singularity is another fun concept to wrap your head around and further drives home the point of what I'm talking about. The space inside an event horizon is so twisted that all lines point directly to the singularity. There is no right, left, forward, back, up, whatever. Information cannot travel from one neighbor particle to the next because there is no path to reach it. No direction exists that is not toward the singularity.

There is another sphere farther out called the Innermost Stable Circular Orbit (ISCO), and it is possible to hit that sphere at an angle, but when you cross the Event Horizon it is mathematically impossible to fall in any direction other than in a straight line directly toward the singularity.

The dramatization of what an astronaut would see as he fell into a black hole is mostly meant to illustrate the relativistic time differences between two different perspectives at different spots in the gravity well, but a real astronaut were falling into a real black hole he would be having a very bad day even before he hit the Event Horizon, and crossing the Event Horizon would be non-survivable even if the lump of matter that cross the Event Horizon was in astronaut shape.

With regard to my statement on gravity, it wasn't meant as a rebuke to tidal forces which are demonstrably very real. It was more a reference to the Galileo-Lean Tower of Pisa experiment.

Thanks again Denier!

I'll have to look into the straight line idea (vs an orbit ending at the singularity), but otherwise what you're saying meshes with my understanding of things from post 16. Thanks for giving me some food for thought!