Black holes question... Communicating with the outside

[Oualawouzou]

Hello everybody,

I know all sorts of weird things happen when you deal with extreme situations such as what goes on beyond the event horizon of a black hole, and it's ever harder as a layman to wrap my head around this phenomenon, so I hope I'll make sense and not annoy our resident physicists...

So, I was wondering the other day if it is at all possible to send information from inside the event horizon to outside it. I figured that since even light cannot escape the black hole, communicating through any kind of light or radio wave is out of the question. But what about "physical vibrations", so to speak? Would two cans connected by a string be able to relay information through the event horizon? I figured not, but I'm at a loss to explain why. The only explanation I can come up with is that it'd be too simple.

Thanks!

 

[Solus]

I'd say no, because the string would come apart... Is that a simple enough explanation for you?

 

[Oualawouzou]

At which point does it come apart? I think I remember reading somewhere that, according to what we know, it could take several seconds (if not more) for objects to desintegrate once past the EH.

 

[RecoveringYuppy]

Well, the string wouldn't survive. But ignoring that, the string is held together at the lowest level by the exchange of particles travelling at the speed of light or less. Those particles won't be able to cross the event horizon so the string can't be continuous across the horizon.

 

[RecoveringYuppy]

At which point does it come apart? I think I remember reading somewhere that, according to what we know, it could take several seconds (if not more) for objects to desintegrate once past the EH.

Depends on the object and the black hole. Small, dense black holes have tidal forces that will rip things apart at a distance. Large, nondense black holes have negligible tidal forces with no reason to assume the object is ripped apart merely because it crossed inside the horizon.

 

[Oualawouzou]

I see, so particles break down at the moment they cross the horizon... Thanks!

 

[RecoveringYuppy]

I see, so particles break down at the moment they cross the horizon... Thanks!

Well, not so much the particles as objects made from the particles.

Though I have to wonder about something I said earlier: Not sure about whether crossing the event horizon might distrupt the object as it's crossing (since pieces inside can't maintain contact with pieces outside). But I am sure about the part about objects being able to approach a very large black hole all the way to the horizon without experiencing disruptions due to gravity or tidal forces.

 

[Solus]

If I had a black hole available to me, I could run some tests but at best all we can do is speculate. Interesting phenomena and they were always the stuff of science fiction until recently.

 

[aggle-rithm]

I see, so particles break down at the moment they cross the horizon... Thanks!

A simpler way to look at it is that in order to get past the event horizon, a message must travel faster than light, and the acoustical message in a can-and-strings setup travels much slower. (This is ignoring the fact that the can and string would be quickly spagettified in most situations.)

 

[Ziggurat]

Once you're past the event horizon, the singularity is no longer simply located at some different location from you: the sigularity is actually your future, and the event horizon is your past. To send a signal out of the event horizon, you need to be able to time travel. Going faster than the speed of light is equivalent to time travel, BTW, but thinking in these terms I think makes it a little more clear why it's an absolute impossibility.

 

[Ziggurat]

Though I have to wonder about something I said earlier: Not sure about whether crossing the event horizon might distrupt the object as it's crossing (since pieces inside can't maintain contact with pieces outside). But I am sure about the part about objects being able to approach a very large black hole all the way to the horizon without experiencing disruptions due to gravity or tidal forces.

If the black hole is large enough, then tidal forces for a falling object could be small, and anyone falling across the event horizon won't necessarily notice the moment they cross the event horizon. But that moment marks their doom anyways. And while a falling object can stay intact across the horizon, as you correctly surmized, a stationary object cannot. The force required to keep an object stationary in a gravitational field diverges to infinity at the event horizon.

 

[Yllanes]

Just as a curiousity: it is possible to show that for a spherically symmetric, uncharged BH the time of fall from the horizon to the singularity is just 10^-5 M seconds, where M is the mass of the BH in solar masses.

 

[petra10]

black holes are great.No-one really knows what they are about,opinions and theries change all the time.So maybe anything is possible with black holes.

 

[Dumbledore]

If you could use a device that is based on quantum entanglement for communication you could communicate out of the black hole assuming that you and the device were not destroyed by the black hole, the receiver for the device, the particles that it is entangled with, would have to be outside the black hole. Quantum entanglement allows for communication between two particle instantaneously, not at the speed of light, therefore such a device should be able to defy the gravitational effects of the black hole. Stating the obvious such devices do not exist as of yet.

 

[andyandy]

is at all possible to send information from inside the event horizon to outside it?

If the object information is smeared on the event horizon as it enters the black hole, as is conjectured in order to prevent the information paradox, then all the information aleady exists outside the black hole - it has already been sent.....

interestingly from the viewpoint of someone outside the black hole, the object would appear to stop at the event horizon - frozen in time....




*waits to be corrected*

 

[RecoveringYuppy]

Only if they can see extremely long wavelengths.

 

[tracer]

Now hold on just a cotton-pickin' minute here!

Long long ago, back when I was studying undergraduate physics, I was told that the Event Horizon (a.k.a. the Schwarzchild Radius) was the distance from the center of the black hole at which the escape velocity became equal to c. Farther out than the Schwarzchild Radius, the escape velocity is less than c; inside the Schwarzchild Radius, it's greater than c.

But if that's the case, shouldn't a beam of light (or an object trravelling at nearly the speed of light) shot straight upward from just inside the Schwarzchild Radius be able to get a little ways up past the Radius before it loses all its energy?

If so, couldn't such a signal be captured by a "relay station" just outside the Schwarzchild Radius, and then re-sent at full energy to another relay station higher up, and so on until it reaches open space and escapes?

 

[RecoveringYuppy]

Ignore my previous version of this post if you saw it. This is the case (post 17) that confused me earlier and hasn't been addressed (at least to my understanding) by subsequent posts.

I believe I'm rephrasing the question in post 17: If the event horizon is the point of no return, where no information from inside can be communicated to the "outside" then how does an "object" remain an object as it crosses the event horizon? For a sufficiently large black hole, will an object retain it's composition as it passes the horizon or not? In what sense would it take 10-5 M seconds to reach the singularity? How do atoms of an object inside the horizon retain their connections to atoms outside the horizon such that an object is not destroyed as it crosses the horizon (assuming the case of large BH where tides don't destroy the bonds).

 

[Yllanes]

I believe I'm rephrasing the question in post 17: If the event horizon is the point of no return, where no information from inside can be communicated to the "outside" then how does an "object" remain an object as it crosses the event horizon? For a sufficiently large black hole, will an object retain it's composition as it passes the horizon or not?

For a sufficiently large BH, tidal forces are small at the horizon and an object can keep its composition. A falling observer wouldn't notice when he passed the horizon. Let me put it in another way. The minkowskian metric is ds²=dx² - dt². Time is negative. The Schwarzschild metric is

[latex]\footnotesize\[
\mathrm{d}s^2 = -\left(1-\frac{2M}{r}\right)\mathrm{d} t^2 +\left(1-\frac{2M}{r}\right) \mathrm{d}r^2 + r^2 \mathrm{d}\Omega^2.\]
[/latex]

Outside the Schwarzschild radius, 1-2M/r >0, and t is time. However, inside the horizon, 1 - 2M /r <0, and the minus sign in the metric is in dr. This can be made precise to show that inside the horizon, at the point (t₀, x₀) your past is not t < t₀. The past inside the horizon is r > r₀. In the same way that for us time always increases, for someone inside the horizon distance to the singularity always decreases. The horizon is your past once you have crossed it. You can't go back not because there is some kind of physical barrier there, but for exactly the same reason that you cannot go back to yesterday now.

In what sense would it take 10-5 M seconds to reach the singularity?

An observer in free fall would measure 10^-5M seconds since he crossed the horizon and until he gets to r = 0. Free fall maximises the proper time, so an oberver firing his rockets or whatever to try to get away would actually fall even faster. Of course, as somebody else said, for an outside observer the time to reach the horizon is infinite, you never see anything crossing it.

How do atoms of an object inside the horizon retain their connections to atoms outside the horizon such that an object is not destroyed as it crosses the horizon (assuming the case of large BH where tides don't destroy the bonds).

Do you mean something like this?

 

[69dodge]

I believe I'm rephrasing the question in post 17: If the event horizon is the point of no return, where no information from inside can be communicated to the "outside" then how does an "object" remain an object as it crosses the event horizon? For a sufficiently large black hole, will an object retain it's composition as it passes the horizon or not? [...] How do atoms of an object inside the horizon retain their connections to atoms outside the horizon such that an object is not destroyed as it crosses the horizon (assuming the case of large BH where tides don't destroy the bonds).

The "connection" between an object's atoms is never instantaneous anyway. What one atom does now, can have an effect on another atom only some time later---specifically, after light has had enough time to pass between them. But an object falling through a black hole's event horizon always falls through it very quickly---exactly at the speed of light, in fact. By the time an atom inside would have affected an atom outside, the outside atom has fallen inside already. So it can be affected as usual, without any signal escaping from the black hole.