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Comment by billpg | Hacker News Reader

Exactly how terrified should I be at this news?

Really super terrified, right?

Your reaction should be "astronomy fun fact, that will have no direct effects on anyone's life." This black hole is so dim and distant that it required a super powerful telescope to even notice it exists. It will have no effect on life on earth, except by adding to scientific understanding of the cosmos.

Moreover, black holes aren't death machines or anything. This black hole puts out much less light and energy than an equivalent-mass star. It's quite safe.

A lot of people seem to have the impression that black holes are like vacuum cleaners, sucking up anything that comes near.

If the Sun were replaced with a black hole of equal mass, apart from the heat and light going out, we wouldn't notice anything. All the planets would still orbit more-or-less the same.

The scary thing would be a rogue, massive black hole hurtling through our solar system at a fraction of c. Such an event could destabilize orbits enough to make the remaining time on Earth quite terrifying. But, good news: if such an event should ever occur, we would be utterly powerless to change a damned thing, so we can rest easy knowing that panic is a waste of energy better spent on hedonistic pursuit.

Sorry. I can't hear you over all the panic screaming.

>> It's quite safe.

Famous last words...

The closest one has 10 solar masses. Gravitationally there is zero difference between a black hole and a regular star except that the black hole has a way smaller radius. So for us it doesn't matter _at all_.

Yes. If the Sun somehow instantly collapsed into a 1 solar mass black hole, it would have no effect on Earth's orbit. The Schwarzschild radius of a 1 solar mass black hole is a bit under 3km, so the event horizon would be 3km in radius. For comparison, the Sun's current equatorial radius is about 696,000km.

To fall into the former Sun black hole would be much more difficult than falling into the Sun as it is now. You'd pretty much have to make a deliberate effort to "land" on the event horizon.

I recently read "A Brief History of Black Holes" by Dr. Becky Smethurst and I recommend it if you're interested in these things. She wrote that the math indicates you have no reason to fear a black hole consuming you, or this solar system.

Black holes are just massive objects, no different from any other "massive object" in the universe. It's pretty easy / common for stuff to orbit massive objects, rather than fall into them. Furthermore I vaguely remember her mentioning another force involved with pushing things away from black holes, such that physicists of today aren't able to reach consensus on the idea that maybe one day all mass in the universe will be in one black hole or another, it seems there's other forces involved that may prevent that from happening (though there are hypothesis around that time is a cycle of all things falling into a universal singularity which then explodes into a big bag and the cycle continues).

In short though, no, you shouldn't be terrified. There has been an ongoing theory for a while that there's a micro black hole somewhere near pluto causing our predictions for its orbit to be just ever so slightly off. https://www.newscientist.com/article/mg24933280-100-is-there... If it's there, it's been there basically forever, and its event horizon, if it's a micro black hole, is hilariously small.

https://www.goodreads.com/en/book/show/61324554

Also as others have said, if it's coming directly for earth, it would take at least 1500 years to get here, which is plenty of time for humans to do something about it. Almost certainly you'll already be dead anyway.

Well, merely disturbing the Earth’s orbit would be deadly enough.

Not terrified at all. I see no reason to be.

Unless it starts moving closer...

With it being 1560 light years away, we'll (parent, you, me etc) be long gone before it's a concern for Earth.

It's like worrying about a blind grizzly bear in a zoo on the other side of the planet moving closer to you.

Well, if it had choosen our planet as a direct destination for (providing it has means to travel at light speed) collision, we'd only have 1560 years to escape.

There are a couple of comically improbable conditions that would happen only in fantasy to start threating us

On the other hand, if it had chosen our planet as its destination 1559 years ago, then we would have no way of being aware of this fact until next year

But then we wouldn't be seeing it at a distance of 1560 ly right now.

In principle it could be approaching us at near the speed of light (for example, if it was hit by something big going near the speed of light from the other direction a few days after the presently observed light was emitted) and we would never know it until it's just about too late.

The same, of course, could be said for any other celestial body, currently observed or otherwise.

> a few days after the presently observed light was emitted) and we would never know it until it's just about too late.

Right, good point! I shouldn't have skipped my morning coffee.

> if it was hit by something big

For the record, I assume you mean not a literal "hit" but some sort of binary interaction with another very massive object that ends up transferring (a lot of) momentum to the black hole.

It's 10 solar masses. If it were hit by something big (100 solar masses, say) traveling at 0.9 c, then the result would be headed our way at a bit less than 0.9 c. At 1560 light years away, that would take 1733 years, so it wouldn't get here for another 173 years.

But why bother with this black hole? If you want to go that route, why not just imagine that there could be a 100-solar-mass something headed directly at us at 0.9 c, and it will show up tomorrow?

For anyone who didn't immediatly get why that is (me for example). I asked GPT4 to explain. Here is the explanation.

The reason we wouldn't be able to know until it's too late is due to the nature of light and the speed at which information can travel.

When we observe celestial objects like stars or black holes, we do so by detecting the light they emit or the effects they have on nearby objects. The speed of light is approximately 299,792 kilometers per second (km/s) in a vacuum. This speed is the fastest anything can travel, including information.

In the hypothetical scenario presented in the comments, if a black hole or any celestial body was approaching Earth at near the speed of light, the light (or information) about its approach would be traveling towards us at almost the same speed as the object itself. This means that by the time we receive and process the information about its approach, the object would already be dangerously close to us.

In simpler terms, imagine you and a friend are racing towards a finish line, and your friend is just slightly slower than you. If you start the race at the same time, you'll reach the finish line almost simultaneously. In this analogy, you represent the light (information) and your friend represents the celestial object. Since the information about the approaching object is only slightly ahead of the object itself, we wouldn't have much time to react or prepare once we become aware of the threat.

In the case of the black hole discussed in the comments, if it were moving towards us at near the speed of light, we wouldn't know it until it was almost upon us, making it too late to effectively respond or take any meaningful action to protect ourselves.

Human comment: Near light speeds are a fascinating topic, that even though I claim basic understanding it always defies intuition.

If you should be terrified, you'd see it show up everywhere. At least as much news coverage as COVID got. If something happened that was likely to end the whole world, and it was detectable by many people, then you wouldn't only hear about it on one random internet forum.

This is pure clickbait.

How is it clickbait? The headline is not in any way misleading.

The HN title has been changed.

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