Black Hole On/Off Switch??

In this story, I’m reading along looking for the on/off switch the headline talks about.

I’ve always wondered what causes black holes to “mysteriously” stop feeding, so I was reading with some interest.

Now, is it just me, or is the story simply saying:

Black holes stop sucking matter in when there’s no matter within it’s gravitational field to grab.

Ummm… ok, now this seemed like something incredibly obvious to me from the very first PBS special on black holes I saw years and years ago.

So what’s the big “story” in this???

I mean, the way I’m reading it, the thing’s always on, it’s just out of matter to grab. Send more matter it’s direction and it will get sucked in. No matter, no radiation from matter getting sucked in.

So what am I missing?


What you’re missing is this:

The same way an explosion is used to put out an oil well fire, because it evacuates oxygen from an area.

I just took that to mean that the supernova tossed a bunch of matter out into the black hole’s gravity well, and the matter that doesn’t get into that well, pushes everything else in in the area out of range in it’s explosive force.

I mean, a black hole doesn’t need a spark like an oil fire. Once sufficient matter gathers to form a black hole, that’s all there is to it, nothing’s going to cause there to be less matter there, so nothing’s going to cause there to be less gravitational pull, so what’s turning anything on or off other than the presence of matter to be pulled in???


Yeah, except that once the well fire is finally out, it’s not going to spontaneously start again when the oxygen comes back.

Well, if Hawking radiation DOES cause black holes to emit radiation, the black hole will eventually evaporate if it isn’t constantly feed with new matter. As I recall, the theory is that the evaporation accelerates as the BH gets smaller, so it sort of “pops” at the end.

So it sounds like you understood the article just fine.

That’s exactly the point. The matter from the supernova leaves at a significant fraction of the speed of light. What doesn’t get sucked up pushes everything else out of the vicinity. Leaving a vacuum. Meaning that there is nothing else for the black hole to suck up, once the local area is empty.

Nothing. That’s the point. The violent ejection of matter and radiation from the supernova pushes everything else away. If there is nothing left, there is nothing to fall into the black hole.

Yeah, it’s a simplified analogy, not a computerized model. Oil well fires go out because the oxygen is thrown out of the way by an explosion. The same way the black hole stops “feeding” because its fuel was thrown out of the way by an exploding star.


From what you’re saying, I’ve got the right idea.

So the truth of the situation is that there is no “on/off switch” for black holes, a black hole will keep sucking in matter and leaking radiation from the process until all available matter has been pulled in and there’s nothing else within range of it’s gravity well.

The Supernova situation may beause this to happen prematurely as the force of the explosion pushes available matter out of the range of the black hole.

Would that be a correct paraphrase?

If so, then the title of the article is definitely misleading.


That’s precisely what I got out of the article. I guess it’s possible we’re both completely missing the point. :slight_smile: As for being deceptive, I’ve come to expect that from journalists. After all, “Black Holes Suck” isn’t really the kind of headline that’s going to sell papers. To the public at large, black holes are old news and unexciting.

remember that light too will get sucked in - so it will never go totally w/o eating.

Hawking radiation is thermal in nature. The temperature of a black hole caused by gravitational collapse is less than the 2.7[sup]o[/sup] background radiation. So even if a black hole is in the dark, it will still grow and its temperature decreases as mass increases. Black holes formed by collapse will not evaporate due to Hawking radiation until the universe expands enough for the background radiation to be cooler than a black hole. It’s not gonna happen soon.