Detecting matter falling into a Black Hole

1. May 5, 2015

Logan5

Hello to all !

I wanted to ask a question in this forum. I am french, and I have discussed this topic in a french physics forum, but with no clear conclusion. I hope I'll have another insights in this forum, which seems very well frequented.

I know the underlying subject has been discussed in this forum already (with no clear conclusion either !) : can an infalling object in a black hole ever enter the horizon before the end of the universe, or before the BH evaporates ? But I'll try to formulate the problem in a new way, trying to avoid traditionnal pitfalls and shortcomings, and particulary without using the simultaneity concept (or the question WHEN the object enter the horizon ?), wich is very tricky in this context.

Here is a mind experience and the question :

Let a regular BH, generating a Schwartzschild metric. Let an observer O at fixed Schw. distance from the BH, in near Minkowski metric and time. The observer sees an infalling object passing near it at his Minkowski time T1 (radial free fall in the BH). Mass of object and observer is negligible compared to the mass of the BH.

The object will disappear from the detecting instruments of O (which does not mean that the object have crossed the horizon) at observer/Minkowski time T2. When (at whatever moment) the object crosses the horizon, the horizon is disturbed, the BH "loose his hair" and emit gravitational waves.

The observer O detects the gravitational waves at his time T3. The question is : what is this time T3 ?

Here are some possible answers :

1. Never (T3 = infinite)
2. At some finite Minkowski time T3. In this case what is the formula which gives T3 ?
3. The GW will not be detectable (for example infinitly expanded), or GW are detectable but with no clear signal at horizon cross, so this mind experience cannot give clue about horizon crossing (this answer is fundamentally different from answer 1).
4. This mind experience has some flaw in his description or assumptions so there is no answer (please kindly explicit the flaws).
5. GR is not sufficient to give the answer. We must take into account quantum or tunnel effects when matter reaches planck distance from the horizon.
My own answer is something like 3. But in this case, I wonder what is the exact signification of articles like http://kipac.stanford.edu/kipac/black-holes-eating-stars-and-making-waves (there are a lot of such articles). If the answer is 3, we can only detect matter approaching horizon, but we can never say that matter have crossed the horizon and have been "eated" by BH.

The final question if the answer is 3 (or 1 BTW) is : what are the facts and measures, or mind experience, which can invalidate the "frozen star"model (Lifchits model for example) of BH, with matter infinitly approaching horizon but never crosses it in our referential ?

2. May 5, 2015

phinds

The EH is not a physical thing. Matter falling into the BH is not aware of the existence of the EH. The extra gravity of the BH due to an unfailing particle is measured as increasing as soon as the infalling object is closer to the BH than the observer is because the "gravity of a BH" for the purposes of any observer is the gravity of everything inside the sphere centered on the BH and reaching out to the observer (assume for simplicity that there are no other object inside that sphere other than the BH).

3. May 5, 2015

ShayanJ

This is wrong. EH is surely physical. But it can't be detected locally.

4. May 5, 2015

phinds

Oh? Why do you think the EH is "surely" physical, particularly if it can't be detected locally? If it can't be detected where it is, how can it be physical?

5. May 5, 2015

ShayanJ

"it can't be detected locally" is much different from "it can't be detected". Its true that an infalling observer doesn't notice anything strange when passing the horizon, but hey, if the horizon wasn't physical, what would be the meaning of the observer passing it?
The horizon is the surface of infinite redshift, the border of no return, and most importantly, the border after which you're doomed to be sunk in the singularity.(Don't take me wrong, that's not what I wish for you.)

6. May 5, 2015

phinds

The link you gave is beyond me but I don't see how "can't be detected locally" does not extrapolate to "can't be detected". There is nothing physical there. Yes, there are events that occur there because it is the surfice of infinite redshift and consequently LOOKS to be physical to a remote observer but that is an "optical illusion" exactly as is the fact that the remote observer can't see the infalling object fall in. The object doesn't care about that remote observer.

7. May 5, 2015

Logan5

Thanks for these first answer which tends towards answer 3 also (I think, but this is not clear). If the mind experience is correctly described, there must be an experimental answer, whatever it is.

what are the facts and measures, or mind experience, which can invalidate the "frozen star"model (Lifchits model for example) of BH, with matter infinitly approaching horizon but never crosses it in our referential ?

8. May 5, 2015

ShayanJ

OK, I said I didn't wish for you to fall in the singularity but it seems I should throw you into a black hole for explaining things to you.
Let's imagine you invent a curvaturometer, a device that somehow measures the curvature of spacetime in your vicinity. I know your vicinity looks Minkowskian but let's imagine this device can do the measurement by somehow using two far enough events to detect the curvature.(I won't be surprised if someone pops in here and argues somehow that this device is impossible to build but I won't stop because I'd like to hear that argument.)
Now you and I go near a black hole with our spaceship. I give you the device and then throw you out of the spaceship(sorry, but there is a price to pay for learning). Now let's imagine you have a jetpack too. At first you desperately fire your jetpack to escape the blackhole. Let's imagine you manage to get a bit far. If you look at your device, you'll see that as you get near the blackhole, the curvature increases and as you get further, the curvature decreases. But then your jetpack doesn't work for a moment and you fall into the blackhole. As you pass the horizon, you notice nothing strange. Suddenly your jetpack starts working. You know you're in trouble so you try to escape using the jetpack. But then you realize that it doesn't matter which way you go, the device is saying that the curvature increases in all directions. It doesn't matter which direction you go, you will hit the singularity. I think that's much different from outside the horizon.

9. May 5, 2015

nitsuj

A shadow can move ftl, it is not a physical thing, though i can go in it. Remember the joke, "what gets bigger the more you take out of it?" Well a "hole" is a region. An event horizon, like a hole are regions...same with "elsewhere" section of a light cone. Not "physical" things themselves, but simply regions.

Another, as you mentioned, is borders. Though that's just a boundary of a region, not an edge. Like the edge of a cliff.

10. May 5, 2015

ShayanJ

That's not what I meant!!!

11. May 5, 2015

ShayanJ

I don't know enough to answer the whole first post. But about this question its easy. Just calculate the proper time it takes for the infalling particle to pass the horizon. Its finite. The distant observer never sees the infalling observer pass the horizon, but for the infalling observer herself, it takes a finite time.

12. May 5, 2015

nitsuj

Ah sorry, I thought it was a proper retort to...

"....if the horizon wasn't physical, what would be the meaning of the observer passing it?"

just because an EH not physical itself, doesn't mean the region of isn't physically significant, like shade on a hot day.

13. May 5, 2015

ShayanJ

I meant, if the horizon wasn't physical and only an illusion/mathematical artifact or whatever, then people(physicists) wouldn't talk as such that it actually exists. It does exist but not as a balloon that can be pierced!

14. May 5, 2015

phinds

Sorry, you're going to have to hit me on the head with a hammer and THEN throw me out the airlock. I still don't get it. Yes, there is an effect that occurs but I don't get how that makes the EH physical.

15. May 5, 2015

Logan5

I know that. But if "this question" is "what are the facts and measures, or mind experience, which can invalidate the "frozen star"model (Lifchits model for example) of BH, with matter infinitly approaching horizon but never crosses it in our referential ?", this is not an easy question.

Your observation seems to lead to the answer "no facts and measures, no mind experience, can invalidate etc.." so the Lifchits model (frozen star) is possible. But Lifchits model is rejected. So I don't see this question as easy.

16. May 5, 2015

phinds

I don't agree w/ this argument at all. People talk about the "observable universe" all the time and that is bounded by a sphere that is not physical but only exists as a mathematical construct. For me, it's a spherical surface of about 49 billion light years radius and centered on my left eyeball (when I've got my right eye closed) but it does not exist physically.

17. May 5, 2015

ShayanJ

OK, I think we have a matter of different definitions here. By physical, I mean its a property of spacetime, something really existing out there, not something that depends on our interpretations, mathematical methods, illusions or errors. Now what do you mean by physical?

18. May 5, 2015

ShayanJ

OK, its not easy. But I didn't support the frozen star model! I said it takes a finite proper time for anything to fall into the blackhole and that confirms the fact that the star isn't frozen.

19. May 5, 2015

phinds

I mean something that has physical substance, something that is made up of fundamental particles, something that can be detected locally and remotely (at least, if you have the right equipment)

Here's a good dictionary-type definition: physical = of or relating to things perceived through the senses as opposed to the mind; tangible or concrete.

Is it your belief that physicists in general do not subscribe to that definition but use yours instead?

20. May 5, 2015

ShayanJ

OK, by your definition of physically given below(but only the first part of it) neither the horizon nor the boundary of the observable universe are physical. But by my definition, they are physical because they are well-defined in terms of physics and don't depend on humans to survive! They are out there(if our models are correct) whether we exist or not.
Your definition of physical has two parts that are not equivalent:

1-something that has physical substance, something that is made up of fundamental particles.
2-something that can be detected locally and remotely.

If what I described in post #8 is a valid thought experiment, then you can use it to determine where was the horizon. But not locally. You should at first be out of the horizon then pass the horizon and find out that you did, and then remember when was the first time you noticed the change. So you did detect the horizon but nonlocally. Also you detected something not made of particles, so the above two parts of your definition aren't equivalent.

Last edited: May 5, 2015
21. May 5, 2015

phinds

You make a good point, but I think we need a different word for what you are describing so that "physical" stays with the dictionary definition.

22. May 5, 2015

jimgraber

The gravitational wave signal will increase with an inverse power law as it gets closer to the center of the black hole. (It goes to infinity, or would go to infinity, at the center, not the horizon. It is still finite at the horizon.) But as it gets close to the horizon, it begins to decrease exponentially and is very soon arbitrarily close to zero, but it never reaches zero, in the classical approximation. So you get a totally negligible tail of radiation extending to future infinite time. But the peak of the radiation arrives very close to the time you would expect from your Minkowski approximation.

This is basically your answer 2. The formula for T3 can be computed arbitrarily precisely, but to a very good approximation, it will be equal to the flat space answer.

Best,

Jim Graber

23. May 5, 2015

Logan5

But it is possible (even probable ?) that, at a finite proper time Tau1, while approaching the EH, the external universe ends, or the BH himself evaporates, or all observers has decayed. So for all practical purposes the BH can be a frozen star, no ?

24. May 5, 2015

Logan5

Thank you for your answer. Can you give me a source where I can see this formula and how it can be obtained ? What is typical T3 for a dozen Solar Mass BH and say 1000 LY distance ?

25. May 5, 2015

ShayanJ

This is different from the frozen star model!