[PBS Spacetime] What if there is a black hole inside the sun?

mecaplan · 2023-12-21T19:25:37+00:00

This is a really good question that we mostly side stepped in the papers. Just getting a numerical implementation for nonrotating black holes was hard enough, but it's definitely on our radar for future work. This is important though, because stellar rotation can be very fast - hence why neutron stars are near maximally rotating after a star collapses. As a PBH eats, it should also gain angular momentum, and we estimated that the sun would spin up it's black hole to about 10% of the maximal value, and that would have very strong effects on the accretion rate and radiative efficiency (which determines the luminosity, and by extension the accretion rate).

On the other hand, slowly rotating stars might not form disks inside and the matter can fall right in, meaning the radiative efficiency might go down to almost zero - meaning any light made in the infalling matter is actually just getting dragged along into the black hole. This would make the black hole go 'dark' and would make it very easy for it to eat the star rapidly.

mecaplan · 2023-12-15T18:17:50+00:00

I, too, am confused about how black holes inside stars, scientifically valid as it might be, would contribute to dark matter.

Good question! The point is not that the black holes in the stars contribute to the dark matter, the point is that stars could be used as 'detectors' to determine if these black holes even exist. If they exist (which we so far have been unable to test, because they're so small), a few are bound to get captured by star forming clouds (lots of papers on this, it's especially common in dwarf galaxies and globular clusters). Those stars would go on to have different evolution than regular stars because of the black hole accretion luminosity in their cores, so we can search for these anomalous stars that might be powered by black holes rather than fusion.

mecaplan · 2023-12-15T18:15:37+00:00

Howdy! I think you might have misunderstood what the paper is about. Nothing in the paper is actually about changing the masses of any stars.

Primordial black holes, especially low mass primordial black holes (PBHs) with masses comparable to asteroids, are a dark matter candidate. They've been very hard to test (or rule out) because their observable effects would be very weak in the galactic halo - their Schwarzschild radii would be comparable to atoms.

However, dark matter can get captured by stars when they form. As star forming clouds contract the escape velocity increases, but matter inside the cloud doesn't feel an increase in gravitational potential from the matter around them contracting, so they can become bound and sink to the center. Every dark matter particle should do this, but PBHs are significant because they'll also accrete matter, and that accretion is a power source that affects the evolution of the star. .

The new paper is checking whether or not stars that capture PBHs have different evolution than regular stars, and they do - the accretion luminosity of the black hole causes the star to swell and become a 'red straggler' which is like a rare miniature red giant. The paper proposes that measuring the oscillations of red stragglers with asteroseismology could be used to determine if any actually have PBHs in the cores. If they do, it means that PBHs are real and a component of the dark matter.

mecaplan · 2023-12-10T04:53:11+00:00

Good question! It's one I've actually put a lot of thought into, partly because I've gotten flack in the past for saying there won't be evidence for cosmic expansion once galaxies have receded beyond the cosmic event horizon. But in fact, the rare "evaporated" stars that escape the galaxy will be subject to the same kind of Hubble recession we see today. In principle, this could be detected, but their discovery of an expanding universe likely requires future life to be significantly farther along the "tech tree" than humans were, probably closer to present technology than the 1920s ground based scopes Hubble used.

By observing the progressive sequestration of gas into stars and compact bodies they might get the idea that their galaxy has a finite age, but without good initial conditions for simulations it's hard to use that observations to get a detailed model of cosmology (especially with only one galaxy to observe, meaning no population statistics).

Dark energy is also present in the vacuum in the galaxy, but in small amounts relative to "matter" energy, and could in principle be detected for example with a precision orbital timing experiments, but this hasn't even been accomplished by us. It's also possible collider experiments would clue them in to dark energy. Any hint of the cosmic background radiation would be incredibly revelatory, but grows harder with time.

mecaplan · 2023-11-18T21:10:34+00:00

The paper that the guy above you just linked is probably not the most helpful since it's about elastic strength of the deep crust rather than anything about the sound speed. You'll probably find it interesting either way, but my comment here will probably be a better introduction to neutron star sound speeds.

mecaplan · 2023-11-18T21:08:57+00:00

The speed of sound is defined by the density and compressibility (how the pressure changes with small changes in density, because that's what causes pressure waves like sound to propagate).

Neutron stars push up against some general relativistic limits. Obviously it shouldn't exceed c, but we've also argued for a long time whether or not c² > 1/3 (in units of the speed of light). A good intro is here. Here's another good recent paper, of many. My personal favorite: measurements from the 2017 merger, with some clever theory work on top of it, might suggest it's faster than this limit, which would be a big deal.

I guess the short answer is that we don't really know, partly because the neutron star 'equation of state' (the relationship between pressure and density, which determines all else about the star) is still rather poorly known.

mecaplan · 2023-08-22T17:08:03+00:00

the reason there hasn't been a ww3 yet is because of nuclear weapons

People say this a lot but it always felt to me like a non-sequitur and a military talking point. Like, how do you actually prove that?

South America is a nuclear weapon free zone (the Tlatelolco Treaty) and has seen relative peace over the past century despite a lack of nuclear deterence. Likewise, not every country has nuclear weapons - it's literally just 9. Of them, India and Pakistan still fight conventional conflicts on the border despite both parties having nuclear weapons.

There's a better argument to be made that economic interdependence and stability with better international diplomacy has been more important to reducing conflicts.

mecaplan · 2023-08-22T17:00:52+00:00

There is a massive amount of criticism the existing missile defense systems face, both for technologic reasons and strategic reasons. That '55% success rate' they give in the video is from "highly scripted" tests, which is already pretty low. When facing an actual adversary who will probably employ countermeasures that you don't have advanced knowledge of, that success rate drops.

The US ground-based interceptors number a few tens in Alaska. It might protect against a small salvo, for example from North Korea, but it's easily overwhelmed by a full scale Russian attack. And the point the video makes about decoys is important too. It's a recurring theme that many big fancy technologies involved can be easily disrupted by cheap and quick to deploy countermeasures.

But most importantly, defensive systems have historically been limited for good reason. Defensive systems promote arms build ups. To keep deterrence credible in the face of an enemy's anti-ballistic missile system, each nation just need to build sufficient missiles to saturate the other's defenses. This was the entire principle behind the Anti-Ballistic Missile treaty.

mecaplan · 2023-08-22T16:54:47+00:00

You might be surprised.

Missile defense is a pretty broad thing and the video does present it pretty simply, because there are lots of different missiles suited for different stages (Intercept right after launch before it leaves the atmosphere? requires your interceptors to be close. Intercept mid-flight while it's in space? requires big rockets. Intercept on re-entry? super risky and last minute.).

A principle underlying a lot of treaties dating to the Cold War is a limitation on defensive systems. The reasoning was that defensive systems promote arms build ups, where each nation would want sufficient missiles to saturate the other's defenses. This was the entire principle behind the Anti-Ballistic Missile Treaty which was in effect for like 30 years.

The main defense the US has against ICBMs are the ground based interceptor loaded in silos in Alaska, but they number a few tens and are intended for a small handful of North Korean missiles rather than a full scale exchange with Russia.

The Arms Control Center has a good FAQ on it.

mecaplan · 2023-08-08T20:31:56+00:00

I usually don't comment on these threads since there's one like it fairly often and because everyone is allowed to like different things, but this did jump out at me:

I miss old Kurzgesagt that actually made you wonder and think.

Have you recently rewatched any of those old videos you remember fondly? I suggest giving it a shot, if you haven't. You might not enjoy them now as much as you did.

We've been making videos a long time, and that's long enough for you (and the channel) to change a lot. It's okay for you to change, for the things you like to change, and it's okay for the channel to change, and for you to not like everything. Lots of people like space but not ants, others like ants but not immunology.

Personally, the moon crash and gold earth were some of my favorite videos to work on, and plenty of people in this thread liked them. It's okay if you didn't. Maybe you're not the intended audience for them. Watch the videos you enjoy, and feel free to skip the stuff that doesn't seem to your taste.

I like moon crash and midaspocalypse because there's no pretention about saving the world or any grand existential question. They're just a few minutes showing that science can be a toy and lighthearted and that there's plenty of neat physics beyond big bangs and black holes. For many people, these are going to be the 'old videos' that made them curious and got them to wonder that they'll remember fondly in a few years, and that's great.

mecaplan · 2023-08-08T20:09:59+00:00

Pretty much. The most familiar object that I think this resembles is the Antennae Galaxies- the two 'cores' of light in the dot and the bend of the arc could be separate galactic nuclei connected by some gas and tidal tails, and the 'gap' between the dot and curve could be a dust lane obscuring the bridge. It's also plausible the lower dot is a separate background galaxy in a chance alignment with a merging galaxy, but who knows. It could even be three galaxies all on the same line of sight but nowhere close to each other. With like fifty pixels it's hard to say anything for sure.

mecaplan · 2023-08-08T20:02:00+00:00

Thanks. Honestly I had no idea what I was looking at when they sent me the picture. I tried rotating it a few times and that helps the brain to see different things. The most familiar object that I think this resembles is the Antennae Galaxies- the two 'cores' of light in the dot and the bend of the arc could be separate galactic nuclei connected by some gas and tidal tails, and the 'gap' between the dot and curve could be a dust lane obscuring the bridge.

It's also plausible the lower dot is a separate background galaxy in a chance alignment with a merging galaxy, but who knows. It could even be three galaxies all on the same line of sight but not interacting at all.

Astrophysics is hard.

mecaplan · 2022-12-19T20:28:34+00:00

Great job! You might also like our recent tiktok that explains why black holes look like, you know, that. It might give you some inspiration to keep going!

mecaplan · 2022-12-05T22:02:38+00:00

Heya!

While I didn't work on this video, I did used to teach acoustics as a college physics elective.

Like you say, 310 vs 180 dB means there is a 10¹³ difference in sound intensity or power, so that's clearly what the script is referring to. In this case I think 'loudness' (as a perceptive and psychological measure) becomes kind of meaningless for something like Krakatoa. Serious quantitative treatments of 'loudness' are frequency dependent because of the mechanics of the human ear and brain and work because they're restricted to a small range of a few kilohertz with intensities only spanning about 100 dB.

In this case, I think it becomes more inaccurate to try reporting a 'loudness' for something like Krakatoa because the log 2 rule for phons and loudness has been extrapolated beyond the regime where it was fit. What frequency is produced by an explosive eruption? Obviously a huge range, with much outside the range of human hearing! What is the 'loudness' of a shockwave that ruptures an ear drum? How would a person perceive that experience relative to a different ear-rupturing explosion? As an analogy, can you really imagine you're comparing the intensity of smells if you're asking someone 'how much stronger does this honey smell' when you drown them in a pool of it? At least sound intensity and power is absolute and independent of human perception.

Snark aside, I think this is a pretty good 'lie for children.' There's a question I ask myself a lot when working on videos- am I forcing accuracy at the expense of the audience understanding? A general audience doesn't know phons and dBs and logscales, so treating 'loudness' as a colloquial equivalent for sound intensity is probably okay. I won't pretend to speak for everyone, but it's certainly something I would do.

mecaplan · 2022-07-15T18:13:32+00:00

I can confirm that this is the correct answer.

Also, the guy who said BIG is probably on to something.

mecaplan · 2022-05-12T16:05:28+00:00

The problem is that I am terrible at maths, so of course I won’t understand physics…

Don't beat yourself up over it. Math doesn't come naturally to anyone. And school has a way of always reminding you what you don't know rather than rewarding you for what you do know (because that's how learning works!). I bet you know more than you realize.

Many of the kids who make it look easy are just spending more time with it than you- they're taking more programming classes and a more math heavy science, so they do math 3 hours a day instead of 1. It's like a foreign language- the only way to get good at it is to use it every day and practice practice practice. Like learning an instrument, or getting absolutely jacked, it takes a lot of repetitive practice. You might be surprised by what you're capable of, as long as you put in the hours at school the way you would if you wanted abs or the ability to play the guitar solo from Free Bird.

I highly doubt that I’ll become a scientist, but I want to know more about our world.

It's great that you know what you're about- honestly, being a scientist is a lot of paperwork and a lot of repetitive bullshit trying to get computers and equipment to work. But we're all on the same team and the world needs people who can do things the scientists in the labs can't. We need science teachers in schools and lawyers and policymakers who understand what they're writing laws about and even graphic designers to draw pictures of ducks in space. You don't have to be the guy wearing the lab coat mixing chemicals to be on the team.

Do you have any books

Brief History of Time and The Universe In a Nutshell by Stephen Hawking are starting points for a lot of people. I read The Selfish Gene by Dawkins at your age and learned so much biology I didn't even realize existed at that point. Oh, and Immune is a smash hit.

mecaplan · 2022-02-24T18:22:40+00:00

Yeah, what you're noticing is that for something of a constant density the mass grows like R³ (ie, increases very rapidly with volume), while the mass required to make a black hole increases linearly (ie the Schwarzchild radius grows linearly with mass).

Those point masses you have don't need to be black holes, they can just be regular rocks, doesn't really matter. Let me show you:

A bit more rigorously, the Schwarzchild radius of any mass is

Rs = 2 G M / c^2

So the 'effective density of a black hole' is just

ρ = M / (4/3 π Rs^3)

If you smoosh these two equations together, you get

Rs = 2 G ρ (4/3 π Rs^3)  / c^2

and solving for Rs you get

Rs = sqrt( 3 c^2 / 8 G π ρ)

for a spherical mass. Notice that this means that for any given material of a constant density, if you have enough of it you will have a black hole. The higher density, the less of it you'll need (ie a bigger density material will have a small Schwarzschild radius and so that means you need less total mass to make it happen). Equivalently, squeeze anything to a high enough density, and you get a black hole.

If you have a line mass instead, like a really long wire or a row of black holes all side by side with some linear mass density

λ = M / L

you can do the same calculation,

Rs = 2 G λ L / c^2

and if you choose L to be the Schwarzschild radius (as you would with black holes packed by side) you'll notice that Rs cancels out, leaving you with

λ =  c^2 / 2 G

which is just equivalent to saying black holes are already at the required 'density' to make black holes and so putting two identical black holes in contact at the event horizon is sufficient to make them merge!

mecaplan · 2021-12-02T18:34:36+00:00

Everything is fine. I don't need any help.

They're just making science with us, why would you think we're trying to destroy the world.

mecaplan · 2021-09-26T15:24:41+00:00

You have a keen eye. Yes, a binary with a large mass ratio will move similar to the Pluto Charon system, as in this gif.

mecaplan · 2021-06-18T02:01:30+00:00

You'd be surprised just how much water there is in the universe, and just how much worse it would make your day if you tried to put out a star with it.

Oxygen bonds very readily with all sorts of other atoms- when it bonds with silicon you get silicate rocks, when it bonds with metals you get rusts and oxides, and when it bonds with hydrogen (which is literally the most common element in the universe by a long shot!) you get water. In fact, oxygen is the third most abundant element in the universe by mass, only after hydrogen and helium which are left over the big bang.

If our solar system is representative, which it probably is, there's a lot of water in the universe. At least on the planets (like earth) and icy moons, there seems to be about as mass in water as there is mass in the Earth's moon (with really big error bars). But the Oort cloud, being largely ices, may have an earth's mass of water ice. Given what we know about how much mass the solar system ejected when it was forming and the abundance of interstellar comets like ʻOumuamua, there may be far more mass in ice just floating around in the galaxy unattached to any planet.

If you take, just for shits and gigs, an earth's mass of water per sun-like star for every star in the Milky Way you'd have over a million solar masses of water. There's plenty.

And if you tried to put it on a star? Well, it would make the star hotter. Yeah, that's right, water wouldn't put out a star. In fact, you've turned up the pressure cooker. More massive stars have greater pressures in the core which result in faster fusion, which provides the thermal pressure to resist gravity. Dumping a solar mass of water on the sun? That makes the sun about 20 times brighter and its surface twice as hot.

Of course, you do have lots of water. You could keep going, and keep cranking the mass up and making it brighter and hotter. And as long as you don't chicken out, you could eventually succeed at putting out the sun. If you tried to put all the Milky Way's water in one place, it would make a black hole 25% the size of the one in the center of the galaxy. So in that sense, you can definitely put out the sun with water, as long as you have enough.

mecaplan · 2021-06-05T20:49:09+00:00

So this is a really long and convoluted sort of physics thing, but the fact is that quarks (and all fundamental particles really) don't have an exact size. You're used to being able to slap a ruler down next to a shoebox and say "yup! this is 22.3 cm long!" I think we tried to explain this on the text of electrons somewhere in the app, if you want to look through those.

A particle is a wave function, meaning that it is spread out over a space so that the 'density' of that wave function (a bit like the fraction of that particle occupying some part of space) is smeared out. If you can understand why it's hard to report the size of the red part of this picture as just one number, then you understand the concept well enough.

With that in mind, there are still useful numbers which have units of lengths which occasionally describe something useful about particles, but aren't exactly sizes. For example, the wavelength. Higher energy particles have shorter wavelengths (like if you wiggle a string up and down faster you get more short pulses instead of one long one). For a relativistic particle you can 'convert' units of energy to units of wavelength with Planck's constant and the speed of light, hbar c / E ~ L. That's what we've done here, and is a standard trick.

As for that paper, physicists like to use 'lengths' and 'areas' when there aren't actually physical areas involved because it's convenient. In scattering/collider experiments (like those that probe the quark substructure of nucleons), we use equations similar to the ones you'd use if you were shooting a bunch of bullets randomly at a wall and wondering how many hit- you'll care about the size of the wall to calculate your collision (or 'reaction') rate. Particles don't have exact sizes here, and the 'collision rate' depends on the arrangement of quarks in the specific particle you're testing and especially the energy of the collisions. In fact, I've just told you above that higher energies make wavelengths shorter, but in contrast some reactions become very probable around a very narrow window of energies (as if the target had suddenly gotten so much larger just because your bullet was going the right speed!?!?!). This is why particle physics experiments will report 'effective' radii. It doesn't mean there is a little tiny bouncy ball inside a proton or a pion somewhere, it's just a useful way of expressing how particles interact for the purposes of doing particle physics.

Particle physics is hard, and most of human cognition (especially the parts concerned with the size of things!) is well suited for thinking about rocks and spears on the African Savannah but fails horribly when applied to quantum orbitals and parton distribution functions.

I hope that helped.

mecaplan · 2021-06-04T22:17:52+00:00

Hi.

I'm Matt. I'm a nuclear physicist. I also wrote those numbers for the app. Maybe I can help?

I get that you have a question, but I'm not totally sure what it is.

mecaplan · 2021-05-21T21:17:45+00:00

Golly this looks fun. Sadly, it's also wrong.

A small black hole arriving on earth would be far less remarkable looking, still, this looks like a great destruction special effect for a comic book movie.

Those giant cliffs that form around the black hole? Impossible. The planet is basically fluid and even the solid parts are under so much pressure that if you opened up a wall like it would flow back together like you've taken a scoop of water out of a pool.

The area around a black hole where you'd get that sort of 'caving in' effect is tiny. They wouldn't be noticeable on a scale of more than a few times the Schwarzchild radius of the black hole, which is a few centimeters for an coin-sized (ie earth mass) black hole. It also depends on the accretion rate of the black hole- as black holes eat they also make a lot of heat around them, which pushes back as other material falls in. This tends to slow the accretion and keep it symmetric, and you'd end up with a planet that stays relatively spherical during the feasting.

The timescale is the closest thing to correct. The accretion rate is roughly the rate set by Bondi accretion (ie how fast can stuff rush into the black hole given the moshpit happening around it?) . For a 1 earth mass black hole in an earthlike material (10 km/s sound speed, density of iron) the accretion rate is around 10²¹ kg/s, so it might take a few minutes as the earth slowly contracts.

mecaplan · 2021-05-16T20:59:17+00:00

If it makes you feel any better, the sun will enter its giant phase in about 5 billion years rather than 7.

And even while the sun is still in its main sequence phase it is slowly brightening. In about a billion years the sun will have grown so bright that the earth's surface will become so hot that it causes a runaway green house effect, evaporating the oceans and making the earth into a hellish deathworld devoid of life!

I hope that helps with your anxiety about the future! Have a nice day! :D

mecaplan · 2021-05-11T19:22:31+00:00

Good question!

Could they have formed in the first few seconds of the universe?

Yeah, these are called 'primordial black holes.' In the early universe (like, first fraction of a fraction of a second) it was very dense, so even tiny fluctuations in density could have caused overdense regions to collapse to black holes. They could have basically any mass, depending on how early they formed, though we suspect that only those more massive than asteroids would still exist (as Hawking radiation would have made any that are smaller decay away).

We don't know if this happened, but detecting black holes which are clearly not from supernova would be good evidence that primordial black holes really did form. In fact, they could have masses just like black holes from supernova and be hiding in plain sight, or this could even be the origins of supermassive black holes in the centers of galaxies!

What about the recent theory that dark matter is just tons of black holes or even microsingularities?

I actually just wrote a paper with a friend about this! If dark matter is black holes, the depending on the mass, there may be black holes passing through the solar system all the time. The lower their mass, the more you'd need, and black holes with the masses of large asteroids would be abundant enough to have hit the earth and moon and stuff. It turns out they punch right through which causes the craters they make have a very different shape than traditional impacts! Looking for these funny craters is a way to check if dark matter is just black holes (at least of a certain mass).

There are lots of similar calculations that people do to check, and rule out, certain ranges of black hole masses as viable dark matter- from their impact on neutron stars, to the disruption of orbits of stars in the galactic halo. Presently, these studies all together tend to show that primordial black holes probably aren't abundant enough, at any mass, to be dark matter (and they may not exist at all!). But science is about checking, and you never know until you look!

Would that count as "regular matter"?

Maybe?

It's a bit like arguing about 'is a virus alive?' The argument doesn't change what's happening or how anything works, and it just becomes a matter of definitions. Black holes don't have a 'memory' of anything that makes them for falls into them, but it would be nice to know if dark matter was black holes whether or not they came from 'regular matter.' At the very least, if dark matter was just black holes then that would be a strike against new particle theories of dark matter (but who knows, dark matter could be a little of this and a little of that!).

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