Large object entering a small BH

[Tio Barnabe]

What happens to a object much large than a BH when it enters the BH?

An specific scenario:

Suppose a scientist create a mini-bh on the lab and accidentally put his arm inside the BH. Would he lost his arm in this case?

 

[jbriggs444]

What happens to a object much large than a BH when it enters the BH?

An specific scenario:

Suppose a scientist create a mini-bh on the lab and accidentally put his arm inside the BH. Would he lost his arm in this case?

A black hole big enough to put your arm in would be more massive than the Earth. A black hole small enough to handle (say 100 tons) would be microscopic and would explode like a huge bomb in a fraction of a second due to Hawking radiation.

So I suppose you could say that yes, one way or another that arm is toast.

 

[Tio Barnabe]

would explode like a huge bomb in a fraction of a second due to Hawking radiation

Is there no way of creating a Schwarzschild Black Hole? That is, one which doesn't cease to exist.

 

[jbriggs444]

Is there no way of creating a Schwarzschild Black Hole? That is, one which doesn't cease to exist.

We have the impossibility of using a black hole big enough to put your arm in -- way too massive.
We have the impossibility of maintaining a black hole smaller than that -- it would explode too fast.
We have the technical challenge of being unable to assemble the material to form one in the first place.

What is the point of the question anyway?

 

[Tio Barnabe]

We have the impossibility of maintaining a black hole smaller than that -- it would explode too fast.

That's why I asked you whether there's a way of creating a Schwarzschild type black hole.

What is the point of the question anyway?

Because there's no talk in the literature about such a scenario.

 

[phinds]

That's why I asked you whether there's a way of creating a Schwarzschild type black hole.

In what way do you think he is NOT talking about a Schwartzchild BH?

Because there's no talk in the literature about such a scenario.

Probably because it's pointless, as jbriggs pretty much pointed out. Did you not understand what he said?

 

[pervect]

What happens to a object much large than a BH when it enters the BH?

An specific scenario:

Suppose a scientist create a mini-bh on the lab and accidentally put his arm inside the BH. Would he lost his arm in this case?

The scenario doesn't seem realistic. For instance, a black hole with the mass of the Earth would have a Schwarzschild radius of about 4 millimeters. How are you going to support a mass of a planet "in a lab", and how would you stick your hand in something that small? I'm having trouble finding anything that's plausible that matches your question to give detailed answers on what exactly goes wrong - the whole idea just doesn't seem well conceived.

If you're worried about lowering a mass into a larger black hole with, say, a very strong cable, the question seems better defined, and the answer is that the cable will eventually break as the tension in the cable increases without bound as you lower a test mass towards the event horizion of a more reasonable sized (and much more massive) black hole.

 

[russ_watters]

The scenario doesn't seem realistic. For instance, a black hole with the mass of the Earth would have a Schwarzschild radius of about 4 millimeters. How are you going to support a mass of a planet "in a lab", and how would you stick your hand in something that small?

Well, a tiny black hole would fall toward the center of the Earth and if your hand was in the way, would punch a small hole through it.

 

[Tio Barnabe]

In what way do you think he is NOT talking about a Schwartzchild BH?

This should be obvious from his response (a Schwarzschild black hole doesn't explode.)

Probably because it's pointless, as jbriggs pretty much pointed out. Did you not understand what he said?

Sure, I do.

The scenario doesn't seem realistic. For instance, a black hole with the mass of the Earth would have a Schwarzschild radius of about 4 millimeters. How are you going to support a mass of a planet "in a lab", and how would you stick your hand in something that small? I'm having trouble finding anything that's plausible that matches your question to give detailed answers on what exactly goes wrong - the whole idea just doesn't seem well conceived.

I imagined a BH of 30 cm radius. How can we construct it, well it's another story. The specific scenario I gave in the OP was just to give a picture of what I meant; you could replace the scientist and the lab by the scientist and other appropriate natural process which does have the power to create such a black hole.

 

[Vanadium 50]

I imagined a BH of 30 cm radius.

Such a black hole will weigh more than 30x the mass of the earth.

How can we construct it, well it's another story.

But your question now boils down to "if we suspend the laws of physics, what do the laws of physics say will happen?" This is an unanswerable question.

 

[Tio Barnabe]

"if we suspend the laws of physics, what do the laws of physics say will happen?"

I would never ask such a question. Maybe it sounds like I did, but in fact, it's not what I meant.

However, it's true that one must take care when distinguishing between what is theoretically possible from what is actually observed.

 

[PeterDonis]

Maybe it sounds like I did, but in fact, it's not what I meant.

Sorry, but when you say,

I imagined a BH of 30 cm radius

you meant a BH of 30x the mass of the Earth, whether you realized it or not. That's what the laws of physics say the mass of a BH that size will be. You don't get to specify the size and the mass of a BH; one determines the other.

The specific scenario I gave in the OP was just to give a picture of what I meant

But that picture is not realizable, so all it says to us is that what you meant is not realizable, hence not discussible.

you could replace the scientist and the lab by the scientist and other appropriate natural process which does have the power to create such a black hole

Then you need to make that replacement and give us a scenario that can be discussed. For example: we have a BH 30 cm across (and hence about 30x the mass of the Earth) floating in deep space, far away from all other gravitating bodies. What happens if an astronaut falls towards it and gets close enough that his arm passes through the horizon?

If you want to discuss that scenario, then say so, and we will. Or give us some other scenario that makes sense.

 

[Ibix]

As you get closer to any mass you need to apply more and more force just to stay at the same altitude. Near a small black hole, the increase in the force from moving an atom's width closer can be more than even an atomic bond can provide. Then you get ripped apart along a surface at that altitude.

An Earth-mass black hole is tiny. Note that a naive calculation (an underestimate) suggests that the walls of the lab would experience an inward force upwards of ten billion gravities. So to ask "what would happen to my hand" you have to first explain why the rest of you isn't following it down.

 

[phinds]

a Schwarzschild black hole doesn't explode.

Yes, it does, exactly as he SAID it would. You said you understand what he said but clearly you do not.

 

[PeterDonis]

Yes, it does

Careful. The model of a BH that explodes is not the classical Schwarzschild model. So @Tio Barnabe was correct in what he said. What he was missing is that there is no way of creating a "real" black hole that is exactly described by the classical Schwarzschild model, because any "real" black hole will be subject to quantum effects, which will cause it to evaporate (and if it is small enough, "evaporate" means "explode").

 

[PeterDonis]

Is there no way of creating a Schwarzschild Black Hole? That is, one which doesn't cease to exist.

No. See my response to @phinds just now.

 

[phinds]

Careful. The model of a BH that explodes is not the classical Schwarzschild model. So @Tio Barnabe was correct in what he said. What he was missing is that there is no way of creating a "real" black hole that is exactly described by the classical Schwarzschild model, because any "real" black hole will be subject to quantum effects, which will cause it to evaporate (and if it is small enough, "evaporate" means "explode").

Thanks for that correction.

 

[russ_watters]

Then you need to make that replacement and give us a scenario that can be discussed. For example: we have a BH 30 cm across (and hence about 30x the mass of the Earth) floating in deep space, far away from all other gravitating bodies. What happens if an astronaut falls towards it and gets close enough that his arm passes through the horizon?

If you want to discuss that scenario, then say so, and we will. Or give us some other scenario that makes sense.

I know it isn't my thread, but I'd propose a black hole entering our solar system on an overtaking course toward Earth and that unlucky scientist's lab...

My thoughts on what would happen are that:
1. The Earth and black hole would accelerate toward each other in freefall, so no one on Earth would feel anything until it got very close.
2. Tidal forces would rip much of the Earth apart as the black hole punched a hole through it and came out the other side.

 

[PAllen]

I know it isn't my thread, but I'd propose a black hole entering our solar system on an overtaking course toward Earth and that unlucky scientist's lab...

My thoughts on what would happen are that:
1. The Earth and black hole would accelerate toward each other in freefall, so no one on Earth would feel anything until it got very close.
2. Tidal forces would rip much of the Earth apart as the black hole punched a hole through it and came out the other side.

If, in this scenario, we are talking about the 30 cm BH with 30 Earth masses, an approach towards Earth would far more likely result initially in Earth orbiting the BH. This is a catastrophic scenario, in that the Earth would soon be shredded into an accretion disk, with most of it ultimately absorbed.

 

[Tio Barnabe]

Thank you guys.

So, it seems that even a 30 cm BH would cause the scientist (and the lab, and the Earth) to break up inside it. I don't understand, since we are told that a BH doesn't suck things up*. I thought a object only gets trapped inside a BH (and, thus, is destroyed) when it passes the event horizon. Is not it so?

* Therefore, I thought there would be no problem in having a 30 cm BH on a lab... (of course, apart from the other difficulties.)

 

[nikkkom]

Thank you guys.

So, it seems that even a 30 cm BH would cause the scientist (and the lab, and the Earth) to break up inside it. I don't understand, since we are told that a BH doesn't suck things up*. I thought a object only gets trapped inside a BH (and, thus, is destroyed) when it passes the event horizon. Is not it so?

BH does not suck things up. Objects orbit BH just like they orbit any other massive body. Orbits can be all kinds: elliptical, hyperbolic, and so on. When such orbit takes object to about 3 BH radii, it almost inevitably falls into BH (relativistic corrections change orbit so much that it end up partially inside BH).

When objects (lab walls etc) are mere meters from the center of a body with 30 Earth masses (the "30 cm BH"), their orbital speeds and accelerations are very large. If we take individual bits of a wall as "objects", these accelerations are stronger than wall tensile strength, and it will be immediately shredded into bits, which then fall into the BH. A few of them may circle the BH a few times.

 

[jbriggs444]

So, it seems that even a 30 cm BH would cause the scientist (and the lab, and the Earth) to break up inside it. I don't understand, since we are told that a BH doesn't suck things up*. I thought a object only gets trapped inside a BH (and, thus, is destroyed) when it passes the event horizon. Is not it so?

No, it is not so.

A black hole has gravity, just like any other object with an equivalent mass. A 30 cm black hole has the same gravity as a planet with about 30 times the Earth's mass. For comparison, that puts it somewhere between the mass of Saturn and that of Uranus.

Obviously, a 30 cm black hole is very much smaller than either. The inverse square law still applies (approximately -- we are getting into relativistic effects). The attractive acceleration of gravity near the black hole would be vastly greater than the acceleration of gravity near the surface of Saturn or Uranus. @Ibix calculated that it would be tens of billions of g's.

You may be familiar with the tides. They are caused by the fact that the pull of the moon's gravity is slightly stronger on the side of the Earth nearer to the moon and slightly weaker on the side opposite the moon. This is due to the inverse square law. The effect is like a gentle pull, stretching the Earth very slightly on the axis that is aligned with the moon. While gravity scales with the inverse square of distance, the tidal effect (the change in gravity with distance) scales with the inverse cube of distance. The moon is far away, its gravity is weak and the inverse cube law means that its tidal effect is almost negligible.

However, if you were within a few meters of a 30 cm black hole then one side of your body would be more strongly attracted (say 11 billion g's) and the other side of your body would be less strongly attracted (say 10 billion g's). So while both sides of your body were being snatched into the black hole at about 10.5 billion g's, the two sides would be pulled apart at about 1 billion g's.

This "pulling apart" effect is well known and even has a cutesy name: "spaghettification".

 

[Tio Barnabe]

Thanks @nikkkom and @jbriggs444 for your clarifying responses.

So, when we read that "scientists created a mini-black hole in the lab", what is it, really?

 

[nikkkom]

Thanks @nikkkom and @jbriggs444 for your clarifying responses.

So, when we read that "scientists created a mini-black hole in the lab", what is it, really?

An attempt on being sensationalist?

 

[Ibix]

So, when we read that "scientists created a mini-black hole in the lab", what is it, really?

If you can provide a reference for that we can investigate. But "nonsense written by a journalist who didn't understand what they were told" is quite likely.

 

[russ_watters]

So, it seems that even a 30 cm BH would cause the scientist (and the lab, and the Earth) to break up inside it. I don't understand, since we are told that a BH doesn't suck things up*. I thought a object only gets trapped inside a BH (and, thus, is destroyed) when it passes the event horizon. Is not it so?

* Therefore, I thought there would be no problem in having a 30 cm BH on a lab... (of course, apart from the other difficulties.)

I think you may be missing two things:

1. Black holes are reeeeeeaaaaly massive.

2. For any two massive objects not to hit each other they must be in orbit around each other (or on an escape trajectory). The speed required for a circular orbit at 3 meters distance (the size of a small lab) in this case is 64,000 km/sec. Or put another way, a 1kg framed photo on the wall is pulled toward this black hole with a force of a quadrillion Newton's!

This may also be a reflection of the common movie myth that out in space you can just stand still or go anywhere at any speed. But every trajectory except an escape trajectory is an orbit of some kind, around something. You can't just sit quietly above Earth, much less a black hole - you're either orbiting, fighting to stay in place or falling (or a combination of the three).

Along those lines, your thought is related to a common sci-fi power source using a small black hole confined on a spaceship (the Romulins, for example). Energy is generated by throwing stuff into the black hole and capturing the radiation it throws back. The problem is that you couldn't possibly hold even a small black hole inside a spaceship without it imploding the ship! And even if we invoke a ficticious artificial/anti-gravity (which most have), that would also defeat the purpose of the black hole.

 

[jbriggs444]

If you can provide a reference for that we can investigate. But "nonsense written by a journalist who didn't understand what they were told" is quite likely.

Google chased this to an article which references: https://arxiv.org/ftp/arxiv/papers/1510/1510.00621.pdf

As I understand it, that paper deals with an attempt to confirm the prediction of Hawking evaporation.

 

[Ibix]

That looks to be condensed matter physics. They seem to do a lot of stuff that is closely analogous to things like black holes and negative masses without actually being them in the sense relativity means. Note that they talk about an "analogue black hole".

It's not a collapsed star type black hole floating around in the lab. It's something in the Bose-Einstein condensate that looks like a black hole to phonons (not photons!) propagating in the condensate, if my limited understanding is correct.

 

[Nugatory]

I thought a object only gets trapped inside a BH (and, thus, is destroyed) when it passes the event horizon. Is not it so?

"Trapped" and "destroyed" are different things. Once an object passes the event horizon it is trapped. However, depending on the size of the object and the size of the black hole, tidal forces may be strong enough even well outside the event horizon to tear an object apart. Conversely, a small object falling into a large black hole may not experience serious tidal forces until well after it passed the horizon.

 

[russ_watters]

Thanks @nikkkom and @jbriggs444 for your clarifying responses.

So, when we read that "scientists created a mini-black hole in the lab", what is it, really?

Based on your initial question, it is probably a misunderstanding of something like this:
https://www.google.com/amp/s/amp.livescience.com/27811-creating-mini-black-holes.html

...a misunderstanding of scale. If you create a tiny black hole by smashing a couple of subatomic particles together, "microscopic" isn't even a strong enough word to describe just how tiny it would be.

Again, small black holes are reeeeaaaly dense, so a black hole of small mass would be reeeealy tiny or a black hole the size of a bowling ball would be reeeeaally massive and generate huge gravitational/tidal accelerations.

 

[PAllen]

Thank you guys.

So, it seems that even a 30 cm BH would cause the scientist (and the lab, and the Earth) to break up inside it. I don't understand, since we are told that a BH doesn't suck things up*. I thought a object only gets trapped inside a BH (and, thus, is destroyed) when it passes the event horizon. Is not it so?

* Therefore, I thought there would be no problem in having a 30 cm BH on a lab... (of course, apart from the other difficulties.)

It looks like several replies have clarified most of this, but I'll just add one more point. If the Earth were captured by a passing 30 cm BH (which is necessarily 30 times as massive as earth), at a distance comparable to several times the Earth moon distance, then the fact that one of the orbital partners is a BH would, indeed be irrelevant. You would just have a binary system stable for ordinary time scales (millions of years). However, the scenario I was responding to was an attempt to hit the Earth with the BH. In this case, while initially you might imagine the BH punching a hole in the earth, the rest of the mass would be captured in orbit, and then shredded to an accretion disk due to extreme tidal forces. A direct hit is hard to manage, but my point was that a near miss (e.g. hundreds of miles) would be equally catastrophic.

 

[pervect]

If you are more than 10x the Schwarzschild radius (for your 30cm radius black hole, that's 3 meters) you can apply Newton's formulae with only small errors. As previously mentioned, the mass of the BH would be about 30 Earth masses. At 3 meters, a Newtonian calculation gives you about 0.4 million billion times Earth's gravity (4*10 ^ 15 meters/sec^2 acceleration).

So a more reasonable question would be if you were at a location where the gravity of the BH was 1g so you could sit comfortably in your lab without being compressed into a flat puddle of goo, what happens if you lower an ultra-strong cable into the BH. Your lab would have to be about 6 Earth radii away for that, make it 24,000 miles or so. The answer to the revised question that is that if the cable has a finite tensile strength, it will break.

 

[Tio Barnabe]

So let me see if I got the point

A black hole doesn't suck up things by itself, but as it is a concentration of mass, it has gravity and, therefore, will atract other masses as any other body would do. Correct?

So, we can say that while on the outside of a black hole, (say) a scientist would be atracted to the BH as he would be by any other massive body, but once that scientist passes the event horizon, then GR predicts that he will not be able to go out anymore, this time not because of gravity, but because the space-time path inside the BH doesn't allow for this to happen. Correct?

 

[Ibix]

You are making a distinction between gravity and curvature that doesn't exist. Gravity is the curvature of spacetime (or, at least, so says GR). So a long distance from a black hole your path curves towards the hole because of spacetime curvature, and close to (or inside) the hole your path curves towards the singularity because of spacetime curvature. The definition of "inside the hole" is where the curvature is such that there are no outward-moving paths (not timelike ones anyway).

All pervect is saying is that more than ten Schwarzschild radii away from the black hole, Newtonian gravity is a good approximation and you can save yourself a lot of maths by using it. Nothing is different about the physics of gravity close to or far away from the black hole - it's all curvature. But the maths is horrible, and there's no significant loss of precision in using a simpler theory.

 

[jbriggs444]

So let me see if I got the point

A black hole doesn't suck up things by itself, but as it is a concentration of mass, it has gravity and, therefore, will atract other masses as any other body would do. Correct?

That is the Newtonian approximation. It is also what General Relativity predicts when masses are smallish and distances are largish. Newton models this "gravity" as a force. General Relativity models it as straight paths in curved space-time. Even outside the horizon.

So, we can say that while on the outside of a black hole, (say) a scientist would be atracted to the BH as he would be by any other massive body, but once that scientist passes the event horizon, then GR predicts that he will not be able to go out anymore, this time not because of gravity, but because the space-time path inside the BH doesn't allow for this to happen. Correct?

Once you are inside the event horizon, you are definitely in a regime where the Newtonian approximation is not at all accurate.