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Cool, and absolutely terrifying.

On the flip side, you wouldn't be any less dead if you fell into Jupiter and that's a lot easier to get to

Falling into Jupiter is an eye-opening experience tho, esp. if you fall into the "big eye" (pun intended).

Just the sheer massive size of a turbulent, harsh planet that covers all the horizon... The world becomes flat much faster near such a planet.

We're already on a planet that covers the horizon; some say there's more beyond the horizon. Would Jupiter look subjectively bigger, if you couldn't also see the Earth for comparison?

I guess it wouldn't, but it probably would be a much longer fall until either pressure is crushing you or your heatshield fails.

Crossing into the event horizon of a black hole has a very profound poetry to it in that you will never be able to communicate back your findings. I find the romance of the destruction of knowledge very intriguing - falling into a black hole is similar to shooting a mathematician in the head in that regard.

Great way to go, all things considered.

Yes you'd be dead.

I am prepared to put down MTW and dream

Except that with a black hole this small, you'll be pulled apart by the tidal forces long before you even reach the event horizon. If you want to make it across the event horizon intact, you need a much larger black hole, like the one in the centre of our galaxy.

Which conversely means that supermassive black holes allow you to pass through the event horizon unharmed.

Not sure if you meant it that way but information actually can't be created or destroyed in quantum mechanics. This is known as the no-hiding theorem [1] which has been experimentally proven and which leads to the black hole information paradox. [2]

1. https://en.wikipedia.org/wiki/No-hiding_theorem

2. ttps://en.wikipedia.org/wiki/Black_hole_information_paradox

So, is it kind of like a cryptographic hash function?

Somewhat. Like a hash function, it projects some large state space (the hash input/you) onto a different state space (hash digest/2d-surface of a BH). But a hash digest actually has lost information, i.e. it's guaranteed by pigeonhole principle that some two inputs have the same digest, which means you lose the information about which one it was. Whereas a black hole scrambles you, but none of the "bits" are actually erased, just smeared out and projected onto the surface.

This is not scary. Yes the event horizon will expand the more matter falls into its gravity well, but otherwise a black hole acts like a normal planet orbiting the Sun.

Is there a theoretical limit on the minimum mass of a black hole?

It's counter-intuitive to me that something could get dense enough to form a black hole and not have more mass than the Sun, or at least enough mass to steal Pluto.

Yeah, the event horizon the size of a Planck unit.

Again, unless you get super close, it acts identical to a regular planet of the same mass.

Do we actually have a theory that predicts a theoretical minimum size of a Planck length?

Black holes are an artifact of gravity and the theory of relativity. The planck unit comes up as a result of quantum mechanics. Without a working theory of quantum gravity, we do not have a description of what happens in scenerios where both theories are relevent.

There's an approximation that's expected to hold, semiclassical gravity, where both theories are relevant. This is the background against which calculations of Hawking radiation and black hole evaporation are made.

I don't think so. With the various colliders there was talk of black holes but they would be small and super short lived. The question may be more interesting if we talk about stable black holes at least by human time scales.

Small black holes evaporate fairly quickly from Hawking radiation. The rate increases dramatically the smaller the mass. Since the only time primordial black holes would have formed was in the Big Bang there is a lower limit based on the black hole surviving 13.8 billion years. iirc the number is on the order of 10^12 kilograms.

Well since the mass of the Earth is ~ 5x10e24 seems we're still in the realm of possibility

Yes, Tolman–Oppenheimer–Volkoff limit would estimate a star around 1.5-2.5 sun masses could result in a stellar black hole.

We don't know much about stars collapsing, and the mass wasted in the process to have a more accurate number at this point.

That's only true for black holes created from the collapse of stars. Primordial black holes would have been created in the early universe and be much much smaller.

Ted Kazinski was concerned that high energy particle physics would create a blackhole large enough to become self sustaining. If I remember correctly, high energy physics creates tiny black holes during particle collisions, but they "evaporate" faster than they can absorb mass.

So to answer your question with my limited understanding, yes, there's a lower limit to stable black holes. Beyond the critical mass, they simply evaporate in a matter of seconds.

> Beyond the critical mass, they simply evaporate in a matter of seconds.

For a sense of scale, a black hole with a mass of 2.2 * 10^5 kg will evaporate in roughly 1 second. A 1 kg black hole would evaporate in roughly 100 attoseconds.

"Evaporate", meaning "vaporize everything for miles around", right?

And for the 10^5 kg one, "vaporize the entire planet" if one is nearby?

Schwarzschild radius and the Compton wavelength both combine to say that in theory, the minimum mass required for a quantum black hole is the Planck mass(ish). They might not be possible or they might require more energy than what we can produce today.

https://link.springer.com/article/10.1140/epjc/s10052-017-54...

This article looks interesting and will probably go into more detail if you are wanting to know more. I will read it tomorrow. Great question!

It's a little complicated... Hawking radiation establishes the minimum size of stable black hole in our universe. If the temperature of the event horizon is higher than the temperature of the CMB, than a black hole will lose mass by Hawking radiation, which raises is temperature further and so on. Using a knowledge of expansion of the universe (and thus the cooling of the CMB) plus the basic math of hawking radiation, you can pretty easilly calculate the minimum size of a black hole that you should see.

Agree with cool, but not with terrifying. According to the paper, the mass would be ~5-15 M⊕ at a distance of 300-1000AU, so not a threatening object in any way.

wait, 2020 is not over yet...

I assume M-plus means Mass of earth?

Yes. Similarly, M with a dot in the circle is the sun.

And at 5M⊕ would have a diameter of about 9cm (or at 10M⊕ about the size of a bowling ball).

How would we ever find that? Gravitational lensing of the background stars?

Do we stand a chance of uncovering it at all?

Edit: from the abstract,

> The observational constraints on a PBH in the outer Solar System significantly differ from the case of a new ninth planet. This scenario could be confirmed through annihilation signals from the dark matter microhalo around the PBH

Wild.

Cool, and absolutely terrifying.

Sibling comments are questioning this.

But ...

what if the black hole's orbit around the sun isn't something approximately circular?

What if the orbit resembles a long-period comet? Highly elliptical? What if it puts in an appearance near the sun every 100,000 years? How would we know? What in the historical record could we study?

They have a pretty good idea of what the orbit would look like to explain the eccentric orbits of TNOs. It's not circular, but it's not that eccentric: https://en.wikipedia.org/wiki/Planet_Nine#/media/File:Planet...

somehow it never occurred to me that black hole could actually orbit what does it mean for the event horizon to move one side is fairly predictable as space gets swallowed as the black hole advances but the space at the rear side would just sort of pop out of the event horizon or be stretched from the previous position

what would happen to the space behind a black hole moving at relativistic speed?

Bet there some paper I cannot understand about it somewhere

No more terrifying than any gravity well.

Yes the cool thing about black holes is that the terrible things inside them can never get out. It's like a prison-planet with the most terrible criminals in there, but they can never escape. Laws of Physics prohibit that

Less scary than most, since the paper predicts it would be somewhere in the range of 5cm diameter. Very difficult to hit, even if you were trying.

Acrophobia, anyone?

I don't see how this is more terrifying than, say, have a sun in the neighbourhood.

Much less terrifying than a 5-Earth-mass planet. Such a planet would be quite easy for a spacecraft to crash into. But if all that mass is the size of a tennis ball, it would be very easy to miss. Unless your aim was perfect you'd just slingshot around it.

A fun way to not be terrified -

"What would happen if the Moon were replaced with an equivalently-massed black hole?"

https://what-if.xkcd.com/129/

Also need one for lava, but looking up quicksand is one of the few that might make it worse for you.

In what sense? It poses no threat to us.

its hung about out there for a few millennia, I doubt it's going to be a problem for us in a the entire lifespan of the human race...

it's not more terrifying than a planet or a sun, it's just more dense and invisible to the eye :)

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