Sending signals out of a black hole

While I was reading along in this thread, admittedly with a comprehension level approximating that of a 2 year old reading 100 Years of Solitude, I arrived at a ridiculously good solution to getting at the answer.

But I fear it may not be possible. Here’s my cunning plan:

I’ve got an uncrushable probe that can take video and measure temperatures. We used to use them back in the day when exploring gas giant planets. It comes with a ridiculously long Monster® data cable reinforced with rigidium–an unstretchable, unbreakable fiber which doubles as a Very Good EM shield. OK? So we can just “lower” the probe into the black hole (or launch it toward the core of the black hole if it’s really driving you nuts to think about these things as 2-dimensional or funnel-shaped features) and nothing’s gonna break.

Question. When it starts transmitting data back up the line, does the signal have to overcome gravitational effects of the black hole?

What a silly question. Since you’re using magic, it doesn’t have anything to do with physics as we know it, so of course the signal doesn’t have to overcome gravitational effects.

You could have a long wire cross the event horizon of an extremely large blank hole and have it not experience catastrophic failure. But the transmission of information out of the hole is impossible.

However in your hypothetical, magical equipment can magically transmit whatever it wants.

Apologies for couching the question in such a manner as to head off the inevitable and insightful observations of, “your probe would be crushed, dude” and “the geometric increase in cumulative weight of the probe and data cable would snap your data cable long before you even got close to the event horizon so no answer for you, Lucy.”

So…gravity is affecting the transmission of the signal back up the wire, is that it for certain? Or is there another reason that I’m to silly to hear?

Yes. In order to travel up the wire past the event horizon, the signal would have to travel faster than the speed of light.

For very large black holes, I understand the event horizon can be a long way out and crossing it may be quite uneventful (i.e. the object crossing it doesn’t get torn apart) - so what’s the deal with objects that are big enough that only part of them crosses the event horizon?

In orbit around such a black hole, if I cast out a long piece of wire and dip a very small part of the end across the event horizon, then try to pull it back out, what happens?

I can’t remember who it was on this board who summed up threads like this as “I have a magic box that violates the laws of physics. Doesn’t that violate the laws of physics?”.

I think it’s just the nature of this place–sometimes you have to put magic into the scenario just so the particular “what if” can be addressed. Otherwise the thread collapses into an ever-increasing mass of reasons why you couldn’t get the answer using $3.46 worth of off-the-shelf merchandise at Radio Shack and none of that really gives you the information you’re looking for. I mean, FFS why even bother asking astronomical questions at all when the sad truth is even $15.34 worth of Radio Shack stuff, or even twice that from Best Buy, won’t allow you to get off the ground, let alone into space, let alone anywhere near another star before you asphyxiate or die of old age or both. Same goes for earth’s core questions, or questions about reasonably-priced apartments in San Francisco.

Yes, the signal is subject to the same laws of physics as all other mass and radiation entering the black hole is, and a signal cannot be conveyed back through the event horizon. But that is the least of your problems, now: your “unstretchable, unbreakable fiber” has violated basic tenets of relativity and continuum mechanics and will probably introduce entirely new and catastrophic phenomena, which is rarely a good idea.


You don’t understand. Give us a scenario that would require a quadrillion dollars, ten trillion years, matter from the core of a neutron star, a cosmic string, and three magnetic monopoles, and we’ll try to answer it to the best of our ability. But give us a scenario that requires magic, and the best we can do is say “I dunno, it’s magic”.

I think there’s two ways to look at this:

First, from the point of view of the outside observer, time slows down as the probe nears the EH. And the result of this is - from the observer’s perspective - the probe never enters the black hole. It’s not even a matter of invincible rope or FTL electrons. The outside observer will never see the probe touch the EH. It can only approach asymptotically close. (again, from their perspective).

Second, from the point of view of the probe, it can go right through the EH and will keep functioning as well as anything can in that environment. (The reality is that no material in the universe can hold itself together here, but we’ve postulated something that cannot break. OK, fine.) So all the electrons in the wire must be free to move or no signal could be sent, right? Thus, they are sucked down into the bottom of the wire and that’s where they’ll stay. Unobtanium can keep them in, but no force in the universe can push them up outside the black hole.

From either perspective, there will be no communication from inside the black hole.

Could the mass or spin of a black hole be manipulated (at least conceptually) to alter gravitational waves emitted by it, as a binary code?

At any time before you cross the event horizon, sure. After that, no matter what you try, the redshift will kill you: Any signal you could attempt would take an infinite time to get out.

EDIT: By “the redshift will kill you”, I mean it’ll kill your attempts at communication, not that it’ll kill you yourself. Other things will do that.

I think you’re all working too hard to deconstruct OP’s question, and missing the point in the process. Only eburacum45 in Post #5 seems to have gotten it right.

All of OP’s “magical” scenarios are simply an attempt to ask one particular question, isolated from all the other effects of black holes. He wants to know if a signal from inside can get out. He didn’t specifically ask anything about “escape velocity” – maybe he doesn’t know to ask that?

So here’s the isolated answer that I think he’s looking for, to elaborate a bit on eburacum45’s response. Chronos, Stranger On A Train, et al., is the following correct? And do you think it goes right to the question that OP is asking?

The strength of a gravitational field increases as you cram more mass into a closed region (volume) of space. The stronger a gravitational field is, the faster an object must get moving to escape from orbit there. The speed required to escape from, say, the surface of a planet is called the “escape velocity”.

It happens that, when you cram enough matter into a small enough volume, the necessary escape velocity can exceed c, the speed of light. When that happens, nothing can escape, since nothing can get moving fast enough. As since we now know that even light itself is subject to gravity, it follows that even light (or any other radiant energy) cannot escape if doing so would require going faster than light.

As you get farther and farther from the center of gravity, the strength of the gravitational field gets weaker, and the necessary escape velocity decreases. Thus, an object at some distance above the mass could escape. So, you have a spherical volume of space, of a certain radius, surrounding the dense mass, within which the escape velocity might be greater than c, and outside of which the escape velocity is less than c. Thus, this radius defines a sort of boundary. Anything within that distance of the mass is stuck there, and anything outside that distance can get away.

That boundary is called the “event horizon”.

Now, can the real physicists of this thread answer if the above explanation is accurate (at least as far as it goes), from a strictly technical point of view? Or even from a relatively lay point of view? Does this explanation give a good isolated answer to OP’s isolated question, free of all the other obvious physical problems with sending objects into a black hole and attempting to retrieve anything?

I’m not sure what is unclear about the answers that Grey, Chronos, and I have provided, which would seem to clearly state that no information, matter, or energy that resides within the event horizon. The o.p. added in the complication of “a ridiculously long Monster® data cable reinforced with rigidium–an unstretchable, unbreakable fiber which doubles as a Very Good EM shield” which doesn’t change anything about the physics of the black hole but applies a component that is not physically possible–not just beyond structural capabilities of existing materials, but actually in violation of basic principles of relativity.

Your explanation is mostly correct in an informal and limited sense, although the strength of the gravity field doesn’t change with distance, only the gravitational potential of the position does. The effect of the distortion created by a black hole isn’t just a “gravitational effect” in the Newtonian sense; according to relativity, space is actually stretched in spacelike dimension within a black hole such that the radius (the distance to reach the singularity and corresponding time from the subjective observer) may be infinite, or at least, very very long (perhaps longer than the lifetime of the universe). So the complication of an “unstretchable” tether doesn’t even make sense in the context of the question, hence the lack of a straightforward answer without additional qualifications. I’m sorry that the physics are more complicated than the conception of the o.p., but to gloss over that point woudl be doing a disservice in terms of providing a complete answer.


Look, imagine a cone resting, point down on a table. We’re going to say that to the left of the point is towards the singularity. We’re now going to say that to the right is the brains of the super magical probe. The edges of the cone are the light paths available to a transmission. That’s going to be our ultimate signal speed.

Now, since we’re talking about an event horizon question let’s place the tip of the cone on the table and let the mass/energy of the singularity bend space and tip our cone over so it opens to the left.

Remember the right hand side of the cone? Well now it’s pointing straight up - no information is moving towards your probe.

Move the cone to the left (inside the event horizon) and the cone will tip even more to the left and again no information crosses the event horizon.

Look at these sketches to get a sense of what I’m badly trying to explain.

This is puzzling me too. In the past I’ve heard it said that space-time is only gently curved near the event horizon of a super-massive black hole - so that this crossing would be ‘quite uneventful’ as Mangetout states. But if I were halfway across, not only could I not feel my feet, but the atoms of the lower part of my body could have no physical communication with the upper part. In other words, I would fall in half.

This dissociation would occur as soon as I dipped my toe into the horizon - far from being uneventful, crossing the event horizon would surely cause me to fall apart into monatomic dust or something, even if I were made of unobtanium. This is not what I would call uneventful.

Is the, for lack of a better word, thickness of an event horizon known? When you cross the EH are you crossing an immediate sharp boundary measured in nanometers, or is the EH more gradual and stretching for hundreds of kilometers for example?

As you are falling in, nerve signals from your feet will still travel to your head. They just won’t get there until your head is in, too.

And the event horizon is defined as a surface, not an object or region, so it has no thickness. That said, the standard definitions might (or might not, nobody knows all the details) be inadequate in a context of quantum gravity.

By which I take it that the unrealistic aspect of “dipping your feet across the event horizon of a very large black hole” is the idea that you would be doing so slowly. This may be a kinder, gentler event horizon, but it is still an event horizon and in the absence of magic braking thrusters, you are going to be crossing it with some speed. Any of your atoms may be briefly disconnected from its neighbours by the event horizon, but not for long enough for them to notice.

Is that a fair assessment?