# B Information from a black hole

1. Jul 25, 2017

### Torog

I read recently that no information comes out of a black hole. Doesn’t the mass information exit a Black hole with ease? Say I am orbiting a black hole and looking away. If while looking away the black hole gobbled up another star I could tell immediately that something had changed inside the hole by the shift in my orbit. Isn’t this solid information on what is inside? Another question: How fast does this information travel?

2. Jul 25, 2017

### George Jones

Staff Emeritus
How could you tell that the black hole ate a star? Why not an equal mass in pure gold? Or a gazillion Justin Biebers?

3. Jul 25, 2017

### Staff: Mentor

The mass that fell into the black hole didn't was hanging around outside the event horizon and contributing to shifts in your orbit before it crossed the horizon. Thus, these observed shifts don't tell us anything about what's changed inside the black hole, they tell us about the distribution of mass at some time in the past, before the mass fell through the horizon.

4. Jul 25, 2017

### kimbyd

The full statement about what happens with information "leaving" a black hole comes from the "no hair" theorem. This theorem proves that in General Relativity, a black hole can only have three properties: mass, rotation, and electric charge. No other properties are possible. You can't have, for example, a black hole with hair (hence the name of the theorem).

This means that when an object enters a black hole, its mass, angular momentum, and charge are added to the black hole's properties, but no other information about the swallowed object can ever exit the black hole again.

5. Jul 25, 2017

### anorlunda

Black holes has properties mass, electric charge, and angular momemtum, all of which can change in time, and all of which can be detected remotely from outside the event horizon. Does that mean information leaks from inside? I don't want to say. Ordinary language descriptions of space and time become very difficult in that neighborhood. For example, as @Nugatory just said, before something changes inside the BH, something outside must fall in (bringing mass, charge, and momemtum in with it.) That clouds the question.

The so-called black hole information paradox is not associated with those properties.

6. Jul 26, 2017

### Torog

Information waves continue to exit the black hole as “ringdown” after a mass goes beyond the event horizon. This seems to be confirmed by the recent LIGO experiments. But also as I understand it time almost stops beyond the event horizon. So why do we still hear what is going on inside?

7. Jul 26, 2017

### kimbyd

Why do you think this is information about the mass leaving the black hole?

8. Jul 26, 2017

### Staff: Mentor

This is not correct. A correct statement would be that, for an object of negligible mass falling into a black hole, and sending light signals outward as it falls, the signals sent outward from closer and closer to the horizon take longer and longer to get back out to an observer at some fixed altitude far away. (And no light signals emitted by the object will get out at all from at or below the horizon.)

However, this description, as just noted, applies to an object of negligible mass (compared to the mass of the hole) falling in. The situation you are talking about--where the object that gets "eaten" by the hole adds significant mass to the hole--is more complicated and the simple description given above does not apply.

We don't. Gravitational waves emitted when a black hole "eats" a large object come from outside the horizon. They are telling you, heuristically, about asymmetries in the infall process, which are radiated away to leave behind a perfectly symmetrical hole (i.e., a hole with no "hair", no externally measurable properties besides mass, angular momentum, and electric charge).

9. Jul 27, 2017

### Torog

It seems as if the ringdown happens after one black hole joins another and all the mass is beyond an event horizon. The ringdown could be noise as the masses settle down beyond the event horizon.

10. Jul 27, 2017

### Staff: Mentor

No, it isn't. It is the "settling down" of the final hole itself into a symmetrical configuration, i.e., one with no "hair". In other words, it's the radiating away of any "hair" that is left over after the holes have merged. The "hair" is outside the horizon, so nothing has to escape from the horizon for it to radiate away.

11. Jul 27, 2017

### Torog

I can know the mass of a black hole from the outside and when two black holes merge, say 14 and 7 solar masses, and I can tell what the internal mass of the black hole is after the fact. I get a nice clear signal, the mass makes a big distortion of the local spacetime. Why can I “see” the mass when it is stable first at 14 solar masses and then at 21 solar masses and not the be able to detect the waves coming from the internal mass as it wiggles and jiggles into a stable state?

And why is there an eye next to my post? Is the post interesting or are you worried I might not obey the rules?

12. Jul 27, 2017

### DrGreg

13. Jul 27, 2017

### kimbyd

As I understand it, the sequence is this:
1. Two black holes spiral towards one another. We can discover the masses of each by measuring their gravitational effect.
2. The two black holes collide, forming a new black hole with a new horizon. The space-time outside the horizon, however, is disturbed greatly by the violent event.
3. The disturbance in space-time outside the newly merged black hole radiates away as gravitational radiation, leaving a symmetrical black hole and a smooth space-time behind.

14. Jul 27, 2017

### Staff: Mentor

You can detect those waves; those are the gravitational waves that come from the black hole merger.

15. Jul 28, 2017

### Torog

Now I'm embarrassed. My paranoia showing through!

16. Jul 28, 2017

### Torog

Can I then say that the event horizon around the black hole is transparent to gravity waves?

17. Jul 28, 2017

### jbriggs444

The event horizon is not a stationary surface surrounding a fixed object. It is (locally) moving outward at the speed of light. Gravitational waves (if any) cannot pass through it in the outbound direction because they cannot catch up to it. Transparency does not enter in.

18. Jul 28, 2017

### Torog

From outside the black hole we can detect the change in mass after two black holes merge. Isn’t the detection of the change in gravity a type of gravity wave only slower. As I said above if I can detect the total mass beyond the event horizon why can’t I detect the masses settling down (as seen by LIGO) from my point of view, beyond the event horizon.

19. Jul 28, 2017

### Staff: Mentor

No. The gravitational waves come from outside the horizon. See post #10.

20. Jul 28, 2017