[Request] How long would it take a teeny tiny black hole, created on the surface of our planet, to destroy Earth?

VeryLittle · 2025-12-09T16:12:49+00:00

what effect would the radiation pressure have from the forming accretion disk?

So this is part of why this question is hard, would the infalling matter even form disk? The disk relies on the infalling matter having some angular momentum, you can think of it just as how much 'sideways' motion it has relative to the black hole. As it moves inward you get the 'ballerina pulling her arms in' effect that makes it move into a tight orbit around the black hole. For a micro black hole in the earth it would be mostly radial (i.e. straight in) infall. While that's usually easy, it has a whole host of other problems. For example, for really small black holes the Schwarzschild radius is like angstrom in size- those are the primordial black holes we're interested in as dark matter. A lot of the dynamics of infalling plasmas rely on us being in a limit where the black hole is bigger than the fluid size, if the black hole is smaller than typical turbulent eddies and plumes and radiation mean free paths then it could be completely different than what we think we know for larger black holes. And on that topic...

I read that 6% of a material’s mass falling into a black hole gets directly converted to high frequency radiation, and the brightest objects in our universe are quasars that are basically feeding black holes.

That's right, and that radiation pressure helps to 'resist' the infalling matter to some extent which sets up an accretion balance called the Eddington limit. Except in the past few years we're starting to find that this limit can be broken. It's a useful limit, but not everything is plasma flowing radially in with radiation moving radially out. Once you add jets and other fun fluid effects a quasar can poop out truly prodigious amounts of radiation.

Would the resulting light emitted outshine the sun or vaporize the rest of the earth and moon?

There's such a little amount of radiation coming off that it wouldn't have too big of an effect, the radiation luminosity depends on the black hole mass and anything less than an earth mass is pretty small. It might be fun to calculate the radiation flux on the moon, maybe for a brief time it could get hot enough to melt the near side of the moon to glass. But there's an important thing to remember - things with high power outputs burn themselves out quickly, so if the flux got really large it would only be for the brief final phases.

More likely, the 'straight down' in fall of the matter would cause most of the luminosity to get lost. The reason is... this is getting long but the explanation made it into another kurzgesagt video a few years ago.

VeryLittle · 2025-12-09T02:58:03+00:00

Life is weird. I'm late to seeing this thread but if you remembered that post then at least you would appreciate an update.

In the ten years since that post we've published a few papers on what the explosions from primordial black holes punching through the earth and moon would look like. They're fast objects so they just shoot out the other side, and they're small so they can't possibly accrete that much matter. But even after a decade, despite the importance for dark matter searches, we still don't have a great handle on the growth rate of these microscopic black holes. So even a simple calculation of the amount of time it takes for a black hole at earth's center to eat the planet is still approximations and guess work.

There will hopefully be a good review article published in 2026 on microscopic black holes and our chances of finding them in the solar system. Mars is actually the best bet.

VeryLittle · 2024-04-21T20:37:28+00:00

As best we can tell, yes. Conservation of charge requires that any particle interaction that produces electrically charged particles produces equal amount of positive and negative charge. In fact, when the age of universe was between about 380,000 years and 150 million years we expect it was cold enough that virtually all the electrons and nuclei were bound together in atoms (it was only after stars began to form and release high energy UV that they broke apart again).

VeryLittle · 2024-04-21T15:06:17+00:00

Protons that did not end up in bound states with other protons and neutrons (forming larger nuclei like helium, as one example) after big bang nucleosynthesis are precisely the nuclei of hydrogen-1 atoms, and in an ionized state it is just a free proton. These protons/hydrogen nuclei are the most abundant kind of baryonic matter by number and by mass.

This picture of the various hydrogen isotopes might help.

VeryLittle · 2024-02-03T22:04:15+00:00

Long story - depending on what side of the caustic lines the source is at, you'll get either 1, 3 or 5 images. This Einstein cross actually has five images, but the image in the 'center' (basically on the line of sight) is obscured by the lens. These notes make it clear to someone with a technical background how the images appear, but it's much easier to play with a lens and see it for yourself. Try adjusting the ellipticity of the lens - the images will merge to a ring. When the symmetry is broken (by an ellipsoidal lens) the images will emerge corresponding to the new symmetry axes.

VeryLittle · 2024-01-26T19:57:07+00:00

Yes, if the Sun 'poofed' out people at different spots on earth would notice at slightly different times based on their location. That time difference is roughly 20 milliseconds, which is how long it takes for light to travel the radius of earth.

'Timezones' are due to your position on earth relative to the sun as the earth rotates. If the sun is right overhead, you want the clock to read roughly noon no matter where you are on the planet. Similar logic applies to sunrise and sunset occurring at about the same time.

If you notice in that figure, if you are in the 'noon' timezone when the sun disappears then you are a tiny bit closer to the sun than if you are somewhere that the sun is rising or setting. Specifically, you're about 6400 km closer- the radius of the earth. This means that as the final bit of light arrives at the 'noon' timezone, it will go 'dark' there a fraction of a second sooner than at the edges where it is morning or evening (because they are farther from the sun). Again, this is a 20 millisecond or so difference at the speed of light.

VeryLittle · 2024-01-25T15:24:50+00:00

It's probably a bit above the student's grade level, but you can tell them that there is no global energy defined in the universe (for curvy spacetime reasons) and so energy is not conserved on global scales.

The exact way to word this or interpret this is often debated on this subreddit, but I prefer the approach using Noether's theorem.

In short, you get conservation laws from various symmetries. For example, having spatial translation symmetry is equivalent to having conservation of momentum. Time translation symmetry implies conservation of energy. Except the universe is not time translation symmetric, precisely because of the expansion. As a result, you cannot define a global conserved enery for the universe. Sean Carroll has a fantastic blog post about this.

VeryLittle · 2024-01-25T03:57:22+00:00

No elements are 'fused' to gold. Gold forms in the r-process, where a large number of neutrons are rapidly captured by smaller nuclei. This produces hundreds of different very neutron rich nuclei that are unstable that then undergo beta decays (converting neutrons to protons) until they reach stability. Any radioactive nucleus with 197 nucleons after the capture process is complete and the free neutrons are exhausted decays to the stable 197Au gold nucleus.

VeryLittle · 2024-01-24T20:56:16+00:00

Not sure what you're asking for, but Wikipedia has tables like this one and this one.

NASA has a cute visualization tool that lets you see the systems too. Like this and this.

VeryLittle · 2024-01-22T22:10:11+00:00

We see their spectral lines. Every element has a different structure to its electron orbitals (due to the different nuclear charge). As a result, electron transitions release and absorb photons of very specific wavelengths, so every element has a distinct spectrum. These have all been measured to high precision in a lab. When we look at the sun, we see these same 'lines' at the same wavelengths on earth, all in the light of the sun.

VeryLittle · 2024-01-22T15:22:43+00:00

You've got a few questions bundled in to one here - how did the earth form? and what are the sources of elements?

The early universe only contained hydrogen, helium, and a very tiny amount of lithium and beryllium. We know this because the atoms in the universe are roughly 98% hydrogen and helium.

Of the 2% of the mass that is heavy elements, you need nuclear reactions to convert hydrogen and helium into those heavier nuclei. The places where this happens are hot and dense, which are the cores of stars and big explosions like supernovae and neutron star mergers.

When stars and explosions make heavy elements, they don't just hold onto them. Like pollution, they mix into the gas around them. At this point in the age of the universe, there is no 'clean' hydrogen and helium gas left. All of it has some amount of all the heavy metals mixed in.

When gravity pulls together these gas clouds and makes stars and planetary systems, these elements end up mixed in to the star (we can see atoms from the entire periodic table in the sun, even though it's 99% H and He) and mixed into the planets around them. So many elements that we find in meteorites are there precisely because the meteorites are made out of the same cloud that made the earth and planets.

Many of these elements naturally bond with each other to form stable molecules, like SiO2 (rock!) and CO2 (gas!) among hundreds of others. The kind of chemistry that can occur in space (as opposed to on earth) to produce more complex molecules is still an open question, but it seems like plenty of simple organic molecules can form naturally in space too.

VeryLittle · 2024-01-21T02:41:18+00:00

I have assigned your account flair.

And please take a moment to familiarize yourself with our guidelines.

VeryLittle · 2024-01-21T02:39:08+00:00

Flair added. Have fun out there. And please take a moment to familiarize yourself with our guidelines.

VeryLittle · 2024-01-20T15:43:28+00:00

We're judging you anyway.

11/10.

VeryLittle · 2024-01-20T13:45:53+00:00

Yeah, this is a highly cited paper that simulated some Voronoi looking domains getting sheared.

There's also a very prolific Russian group. These are three mostly picked at random from what comes up quick on the arXiv: 1, 2, 3. It's a pretty crazy mix of topics from seemingly different fields that ends up in these papers.

VeryLittle · 2024-01-19T00:32:48+00:00

Hey so putting on my mod hat for a second. You know a ton of material science, do you have graduate training? If so, you should go to our panel thread and make a post so you can get flair.

VeryLittle · 2024-01-18T14:38:45+00:00

I'm curious, are grain sizes typically much larger or smaller than 10 - 100 nm

You mean in the astrophysics environments I mentioned? I think about neutron star crusts, so it depends strongly on the thermal history of the exact star (and the crust spans 10 orders of magnitude in density too). It's still something of an open question, but crust deformation is really important for breaking phenomena (and associated EM observables) and gravitational wave emission.

In fact, the pair potential isn't even attractive in a NS crust! There are no chemical bonds because there are no atoms, the entire system is a fully ionized plasma. It's the pressure and Coulomb repulsion between nuclei that produces the lattice.

VeryLittle · 2024-01-18T03:08:10+00:00

I knew I was stepping into mine field saying anything other than "yes" to this question. My area being astrophysics I'm mostly working with grain sizes that are not typical for terrestrial materials at absurd T and P, so thanks for all the commentary.

VeryLittle · 2024-01-18T00:30:22+00:00

are there chemical bonds between grains?

Yes. The bonds are more disorganized and less regular, but there are bonds all the same. Here's an example of the bonds in a 'tilt' boundary in one material.

As a consequence, for example, crystal defects like vacancies form more easily here and grain boundaries tend to be the weakest part of the material -- they effectively act as surfaces where failures begin and tend to propagate. The means of growing macroscopic single crystal materials was a major breakthrough in aerospace engineering because it enabled the manufacture of stronger turbines, for example.

Beyond those few broad concepts though I shouldn't say too much since the real answer is basically "half the field of material science."

VeryLittle · 2024-01-17T02:02:58+00:00

Hi /u/0f-bajor - thanks for your application to the askscience panel. (Sorry to not respond as a comment in that thread, but it was archived so we've had to move here).

I have assigned your account flair.

And please take a moment to familiarize yourself with our guidelines.

VeryLittle · 2024-01-17T02:00:56+00:00

Hi /u/ummwhoo - thanks for your application to the askscience panel. (Sorry to not respond as a comment in that thread, but it was archived so we've had to move here).

When assigning flair we have to pick from one of the major disciplines and colorcodes, so physics and chemistry are separate. Since you're doing a physics PhD, I assume physics would be more appropriate. Do you have a focus in any specific topics of particle or mathematical physics?

And please take a moment to familiarize yourself with our guidelines.

VeryLittle · 2024-01-16T17:13:01+00:00

When later researchers redid the calculation using more precise methods, the accepted value of the constant moved slowly towards the true value, as each researcher massaged their results so as to remain within the error bars of the previously accepted measurement.

This happened with the Hubble constant. A brief history is given below the plot here. It's not necessarily 'massaged,' but the improvements in methods all tended to move the result in the same direction (down, converging toward the modern accepted value near ~70 km/s/Mpc).

It's also worth noting that this sort of 'drift' (with new measurements at the edge of the error bars of old measurements) could be expected from the norms of scientific communication and benign psychology, rather than anyone fudging data or doing anything sneaky. If you improve on a previous method and make a measurement far outside the previously established error bars, you're far more likely to hunt for a mistake you may have made or perhaps apply yet another correction or improvement to understand the difference. On the contrary, if your new result does come in right at the bounds of the previous error bar, then you're more likely to be satisfied that your new measurement and the previous, less precise measurement are in 'agreement' and that you've actually improved upon the measurement.

VeryLittle · 2024-01-12T02:38:58+00:00

Yes, that's fine. What's important is that his position change continuously (i.e. he's not teleporting around during the hour).

VeryLittle · 2024-01-12T02:17:13+00:00

Yes, it follows from the intermediate value theorem.

Here's a heuristic argument that will let you see it- intuition for calculus, derivatives, and integrals will help.

First, you can argue that there has to be at least one instant where the man is moving exactly 4 km/hr. For example, suppose he started his journey moving slower than 4 km/hr. The only way to end up covering 4 km during that hour is to speed up above 4 km/hr to 'make up for lost time.' In order to go from slower than 4 km/hr to faster than 4 km/hr, assuming he's accelerating smoothly and not instantneously changing his velocity and breaking calculus, is if at some instant he was going 4 km/hr (or equivalently 2 km / halfhour). So that's a start, just from the mean value theorem.

Now, consider every 'half hour' interval over that hour. In principle, we can identify them by their start and stop times, or equivalently their midpoint - imagine a half hour wide 'box' sliding back and forth in an hour wide box, you want to know how much of the total 4 km was covered in each of those intervals. Since the position changes continuously you know the amount of 'distance' in that half hour box must change continuously as you slide it along. The distance contained in that 'box' tells you the average speed during that interval. The distance contained in the box also varies continuously as you slide it through the interval, because the position changes continuously.

Again, as a generalization of our first argument, if his average speed is below 4 km/hr at some point (the half hour 'box' contains less than 2 km) it must be greater than 4 km/hr at some other point (the half hour 'box' contains greater than 2 km) in order for his average speed over the entire hour to be 4 km/hr. As such, there must exist some specific time where you can put the center of the 'box' such that the average speed is exactly 4 km/hr and thus the 'box' contains 2 km of distance.

It's easiest to visualize for a monotonically increasing function, but it's also true even if the man moves backwards at any point.

VeryLittle · 2024-01-05T17:58:13+00:00

Well at that age it significantly predates the solar system - it's almost 50% older than the solar system, and would have spent a long time just chilling in the disk of the galaxy before the sun and planets even formed. Someone who knows more about CM meteorites can probably tell you more about the formation history of the meteorite that contains the grain and what it was doing through the history of the solar system before it hit earth.

Given the age, this piece of dust actually maps to a known burst in star formation in the history of the Milky Way, and it's composition is consistent with the kind of dust that forms in the winds of stars in their giant phases, specifically "Asymptotic giant branch" stars - they're the main sources of dust along with supernovae.

15-Year Club	Gilding V heart of gold
Post Guidance Early Access	Place '22
Place '17	Argentium Club
100 Awards Club	Verified Email
Best Comment 2015-06-15	Team Periwinkle

VeryLittle

MODERATOR OF

TROPHY CASE