# Q: How bad would it be if we accidentally made a black hole?

Physicist: Not too bad!  Any black hole that humanity might ever create is very unlikely to harm anyone who doesn’t try to eat it.

Black holes do two things that make them (potentially) dangerous: they eat and they pop.  For the black holes we might reasonably create on Earth, neither of these is a problem.

Home-grown black holes: not a serious concern.

The recipe for black holes is literally the simplest recipe possible; it’s “get a bunch of stuff and put it somewhere”.  In practice, you need at least 3.8 Sun’s worth of stuff and the somewhere is anywhere smaller than a few dozen km across.  That last bit is important: the defining characteristic of black holes isn’t their mass, it’s their density.

For a given amount of mass the same amount of gravity “flows” through every containing surface.  In this picture, the same total gravity points through both outer surfaces if they contain the same total mass.  But if all of the mass is concentrated in a tiny place (as it is on the right), then the gravity through the smaller surface must be stronger in order to equal the weaker gravity through the larger surface.  Fun fact: this can be used to derive the inverse square law of gravitation and/or is a consequence of it.

If you’re any given distance away from a conglomeration of matter, it doesn’t make much difference how that matter is arranged.  For example, if the Sun were to collapse into a black hole (it won’t), all of the planets would continue to orbit around it in exactly the same way (just colder).  The gravitational pull doesn’t start getting “black-hole-ish” until you’re well beyond where the surface of the Sun used to be.  Conversely, if the Sun were to swell up and become huge (it probably will), then all of the planets will continue to orbit it in exactly the same way (just hotter).

To create a new black hole here on Earth, we’d probably use a particle accelerator to slam particles together and (fingers crossed) get the density of energy and matter in one extremely small region high enough to collapse.  This is wildly unreasonable.  But even if we managed to pull it off, the resulting black hole wouldn’t suddenly start pulling things in any more than the original matter and energy did.

For comparison, if you were to collapse Mt. Everest into a black hole it would be no more than a few atoms across.  It’s gravity would be as strong as the gravity on Earth’s surface within around 10 meters.  If you stood right next to it you’d be in trouble, but you wouldn’t fall in if you gave it a wide berth.  In fact, that’s why mountain climbers aren’t particularly bothered by Everest’s mass; even if you’re literally standing on it, you can’t get within more than a few km of most of its mass (fundamentally, Mt. Everest is a big, spread out, pile of stuff).

But the amount of material used in particle accelerators (or any laboratory for that matter) is substantially less than the mass of Everest.  They’re “particle accelerators” after all, not “really-big-piles-of-stuff accelerators”.  The proton beams at the LHC have a mass of about 0.5 nanograms and when moving at full speed have a “relativistic mass” of about 4 micrograms (because they carry about 7500 times as much kinetic energy as mass).  4 micrograms doesn’t have a scary amount of gravity, and if you turn that into a black hole, it still doesn’t.  A black hole that small probably wouldn’t even be able to eat individual atoms.  “Probably” because we’ve never seen a black hole anywhere near that small.

The other thing that black holes do is “pop”.  Black holes emit Hawking radiation.  We haven’t measured it directly, but there a some good theoretical reasons to think that it’s a thing.  Paradoxically, the smaller a black hole is, the more it radiates.  “Natural” black holes in space (that are as massive as stars) radiate so little that they’re completely undetectable (hence the name: black hole).  The itty-bitty black holes we might create would radiate so fast that they’d be exploding (explosion = energy released fast).  The absolute worst case scenario at CERN (where all of the 115 billion protons in each of the at-most 2,808 groups moving at full speed are all piled up in the same tiny black hole) would be a “pop” with the energy of a few hundred sticks of dynamite.

That’s a good sized boom, but not world ending.  More to the point; this is exactly the same amount of energy that was put into the beams in the first place.  This boom isn’t the worst case scenario for black holes, it’s the worst case scenario for the LHC in general (cave-ins and eldritch horrors notwithstanding).  It is this “pop” that would make a tiny black hole a hazard.  The gravitational pull of a few micrograms of matter, regardless of how it is arranged, is never dangerous; you wouldn’t get pulled inside out if you ate it.  However, you wouldn’t get the chance, since any black hole that we could reasonably create would already be mid-explosion.

A black hole with a mass of a few million tons would blaze with Hawking radiation so brightly that you wouldn’t want it on the ground or even in low orbit.  It would be “stable” in that it wouldn’t just explode and disappear.  This is one method that science fiction authors use for powering their amazing fictional scientific devices.

The kind of black holes that we might imagine, that are cold (colder than the Sun at least), stable, and happily absorbing material, have a mass comparable to a continent at minimum.  Even then, it would be no more than a couple millimeters across.  These wouldn’t be popping or burning things with Hawking radiation.  The real danger of a black hole of this size isn’t the black hole itself, so much as the process of creating them (listen, I’m making a black hole, so I need to crush all of Australia into a singularity real quick).

We have no way, even in theory, to compress a mountain of material into a volume the size of a virus.  Nature compresses matter into black holes by parking a star on it.  That seems to be far and away the best option, so if we want to create black holes the “easiest” way may be to collect some stars and throw them in a pile.  But by the time you’re running around grabbing stars, you may as well just find an unclaimed black hole in space and take credit for it.

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### 20 Responses to Q: How bad would it be if we accidentally made a black hole?

1. Nice read.. I loved it..

2. Idran says:

This might be a little nitpicky, but it’s something I’ve wondered before about the impact of the sun’s volume on orbital dynamics and how it’s usually described as nil. Wouldn’t it only be exactly the same if the sun were a perfectly rigid and spherical body at all sizes? That is, as the sun’s volume increases and its density lowers, other bodies in the solar system would have a greater impact on its shape, and as the sun’s volume decreases and its density increases, other bodies would have a lesser impact on its shape. Wouldn’t the change in mass distribution result in some extremely minor changes in overall orbital dynamics as time goes forward?

The Sun’s not a perfect sphere, planets make it wobble and change shape, and planets would make it wobble and change shape more if it had bigger volume/less if it had lower volume. Would this really have no impact on the orbital dynamics of the solar system whatsoever, even if it’s only to a degree of inches or feet over the course of thousands or millions of years?

3. The Physicist says:

@Idran
You’re absolutely right. An effect similar to that responsible for the recession of the Moon is likely to show up with an especially fluffy Sun. The effect would be (is) extremely small. For the post, the important thing is that the overall strength would be the same.

4. Xuoran says:

How exactly do you calculate the mass of a continent?

5. Jules Morrison says:

Does a small, hot black hole give you basically free matter-energy conversion without needing antimatter? It sounds like you could feed it stuff, and harvest a proportional amount of Hawking radiation.

6. Mike says:

Ive watched a series that explained if the Sun were to expand, the planets would not orbit as they are now. It explained that with less density it would effectively have less gravity and the planets would start drifting. Widen their orbits essentially. If the Sun were to expand very slowly over billions of years and hypothectically dissolve or dilute itself within the vacuum of space, the planets should fling out to be “rogue planets”.

7. Jeff says:

This is the most awesome black hole explanation ever. I have known lots of people who freak out at the idea of mini black holes. I plan on pointing them here in the future.

I am intrigued about the part where you say we are only actually affected by the gravity in our immediate area, within a few kilometers. So does this mean an asteroid the size of MT Everest would exert a similar pull on me to the Earth? How much pull is the mass in, say China, actually exerting on me in Colorado?

Would an Everest mass black hole even be macroscopic? Would it even have enough of an affect to cause damage to me if it was embedded in my body? It seems like it would have to be so small I would not even notice it.

8. Nino Porcino says:

You did not consider the fact that a man-created black hole, even if very small, would not stand on the floor like any other earth object. It would sink in the ground, moving toward the center of earth attracted by earth’s gravity. And once reaching the center, the high pressure around the black hole would carry to it more and more mass, swallowing the whole planet in the end.

9. The Physicist says:

@Nino Porcino
A not-mountain-sized black hole has a lifetime of nearly zero. For those the “pop” is the only real issue. By the time a black hole is big enough to last long enough to sink to the Earth’s core, it already has a mass on par with the Earth’s.

10. Jeff says:

Swallow what? As he said, it is so small it probably can’t even shallow atoms. The amount of matter it would consume is absurdly small. The earth would be swallowed by the sun long before it ever even became macroscopic, nevermind threaten the Earth.

11. The Physicist says:

@Jeff
The strength of the gravity due to a chunk of mass is given by Newton’s universal law of gravitation. In order for something to have a noticeable pull it needs to be either really massive (like Earth) or really close. You don’t notice the pull of a mountain because, although it might be fairly close, it isn’t particularly massive. Most of the Earth is a lot farther away than whatever mountain you might find yourself on, but it’s a lot more massive.
China is somewhere in the neighborhood of 7,500 miles from Colorado (as the crow digs), so that’s the distance you’d plug into Newton’s law. The mass of China is a little harder to nail down; you’d have to determine how deep into the ground that China extends.
A black hole with the mass of Earth is about half a centimeter across. A black hole with the mass of a single mountain is invisibly small.

12. nemo says:

So, a bit disappointing that we couldn’t make a black hole ’cause, yeah, that whole power generation thing seems pretty cool. I was wondering about the particle accelerator method. Could the black hole generated have a charge? Could we keep it zipping around the accelerator force-feeding it a stream of particles faster than it could evaporate until it stabilised, then keep it in a magnetic box?

13. Anders Gustafson says:

Is the planck mass the minimum rest mass a black hole could have or would it be possible for a black hole to have a rest mass that’s less than the planck mass?

14. Brigid says:

I’ve just found this website. I love it! I’m a stay-at-home mum. But I’m a secret budding physicist. If someone had told my 15yr old self that in 20 years time I’d be a housewife with three kids and a passion for physics on the side, I’d have been horrified! Physics was the most boring dull subject in school. Just goes to show how rubbish some teachers can be.

15. Richard says:

Creating a “black hole” should be this issue anyhow ! It’s the extremely powerful magnetic fields that would have to be present in order to create a black hole ! These technologies have a far greater chance of causing problems ! Just look at the “Hadron” collider in Geneva ! Accidents can & do happen !

16. Entertaining as it may be to contemplate the exotic characteristics of black holes, what is supposed to happen to bodies falling into them, etc., the truth is that nobody would ever survive to provide empirical evidence to support the falling predictions; and evidence to support the other predictions is extremely indirect and inconclusive. At least a few physicists will admit that black holes are still very theoretical objects.

By contrast, it is entirely within our technological reach to arrange to observe a test mass falling into an ORDINARY hole through an ORDINARY body of matter. Yet we’ve never done this.

Galileo proposed the idea in 1632. Since the experiment involves the undisturbed and collision-free motion of two bodies of matter, the apparatus needed to carry it out may be called a Small Low-Energy Non-Collider.

Instead of investing so much ink, bandwidth, and mental energy on predictions that can never be tested, on exotic objects that reside perhaps entirely on paper and in the mind, why don’t we extend our exploration of gravity in the real physical world where we have not yet looked?

Why don’t we build and operate a Small Low-Energy Non-Collider? As scientists, are we not obliged to abide by the empirical spirit of Galileo, to make sure the standard prediction for his experiment is correct (or discover that it is not)?

17. Hill Strong says:

The theoretical entity known as a “black hole” is just that, a theoretical entity. No such entity can exist in our universe, at least, not according to any theory that proposes such an entity. Using the basis by which such entities are supposed to come into existence, gravity, it is also the reason by which these entities can never come into existence in our universe, if one takes that the universe came into existence a finite time ago.

The very theories that suppose the existence of “black holes” also posit that at increasing levels of gravity time dilation effects come into play. Not only that, but these theories posit that at the event horizon (as far as the universe at large is concerned) time halts.

Hence, at increasing densities under the influence of gravity, the cause for the increase of gravity will (as far as we are concerned at a position in the universe at large) also cause that process to be slowed as far as the universe at large is concerned.

Effectively, any process that uses gravity to attempt to create a “black hole” will be slowed to a standstill as far as the universe at large is concerned. The process can never complete in finite time. either the universe will collapse back in on itself or will have suffered complete heat death before the first “black hole” even get close to forming.

Since the event horizon is a function of gravity, any body (including subatomic bodies of sufficient density) that approaches the conditions for an event horizon must be subject to the time dilation effects due to gravity.

Changing the perspective (point of view) of the observer to just above the event horizon doesn’t change any of the above. If the observer looks outwards, what does she see? What is the universe at large doing from her perspective? Well if she is undergoing time dilation effects, she will be seeing the universe at large speeding up to such an extent, that the she will see the full life cycles of stars, galaxies, etc, passing by in seconds. The closer she gets to the event horizon, she will either see the collapse of the universe or the heat death of the universe before she passes the event horizon.

In conclusion, by the very tenets of the theories predicting “black holes”, no such entity can ever form in the universe in any finite time period. It would require that the universe has existed for an eternity first. This is direct opposition of the considered view that the universe is of finite age.

18. Gene Bird says:

What the physicist ignores in this case is that the black hole would do what everything does – fall toward the Earth’s center – eating mass along the way and growing bigger (or “popping” if it couldn’t eat fast enough to make up for the Hawking radiation – radiation WHICH IS ONLY THEORICIZED AND HAS NOT BEEN OBSERVED!). Once at the center of the earth … all that nickel would begin dropping into the black hole. Then everything above that, that now has no support would drop in … slowly hollowing out the Earth until the crust crumbles and falls too! ANY BLACK HOLE WOULD DESTROY THE EARTH. And why do they think LHC might create one? Because gravity might get stronger at small distances and high energies … it is expected to, in fact. Gravity might only seem weak because it is dispersed into tiny, rolled dimensions … ask Harvard’s Lisa Randall about this.

19. JeffDenver says:

Even if the LHC black holes were completely stable, it would not matter. There is a limit to how much stuff a black hole can eat at once. It’s like draining the ocean through a bathtub… It doesn’t matter how much suction is used. It won’t happen quickly. From what I understand these micro black holes would only consume a few photons a year. You could live with millions of them in your gut your entire life and never even know they were there.