- > Prior to this new finding, all the black holes that have been identified have also had a companion star—they are discovered due to their impact on light emitted by their companion star. Without such a companion star, it would be very difficult to see a black hole.
It seems like we think there's many more of these black holes, but we just can't see them
- Lone stars are actually the exception, so not radically more as you might think. But there are also binary black holes.
- This only covers stellar black holes. (Note that this black hole is believed to be a stellar black hole.) Those statistics could change quickly if you add to it a currently unknown number of primordial black holes that arose around the Big Bang.
If those primordial black holes are mostly on their own, and are both numerous and small, they make a potential candidate for dark matter. They could also be potentially small enough to be evaporating in our current era. This has been suggested as a potential source of a very high energy neutrino that was found in February. See https://www.livescience.com/space/black-holes/evidence-for-s....
(Note that this is just a single observation. We are a very long way from being able to obtain strong experimental evidence for such speculative theories.)
- PBHs can't be too small, since they'd have evaporated by now, but if they're large enough to last several times longer than our current universe age then perhaps there could be lots and we not be able to see them.
- My understanding is that they can't be _ultramicroscopic_, but they can be quite small. According to the Wikipedia:
> A primordial black hole with an initial mass of around 10^12 kg would be completing its evaporation today
And according to this:
https://www.omnicalculator.com/physics/schwarzschild-radius
Mass: 10e12 kg → Schwarzschild radius of 1.5e-14 m which is smaller than a hydrogen atom (5.3e−11 m).
I wonder what would be the mass that'd keep a black hole in thermal equilibrium with the current background radiation...
(Edit. whoops, minus sign in a wrong place made the calculations haywire, fixed now.)
- > I wonder what would be the mass that'd keep a black hole in thermal equilibrium with the current background radiation...
About 4.5e22 kg, i.e. about 60% the mass of the Moon.
Note that over time, the CMB will cool and the equilibrium will be broken.
- How does a black hole this small stay together. I assumed it was gravity pushing against the atomic forces.
Is it a case of once you black, you never go back?
- Exactly. Once your object reaches the right radius it's done. It will 'evaporate' eventually. But it won't revert back.
An interesting result is that you could, and this is very much beyond our engineering, manufacture black holes like this.
- Very cool. Is there a place I can read up on this?
- https://en.wikipedia.org/wiki/Schwarzschild_radius The principle is simple. If something crosses it's Schwarzschild radius, it becomes a black hole. So if you had a very, very, obscenely large particle collider, you could start smashing heavy nuclei with enough force to get them within that radius. Then you would have to 'trap' the new micro-black hole in some way (possibly give it a charge) and 'feed' it more matter very, very quickly, before it evaporates.
The evaporation is a bit trickier, also known as Hawking radiation. It has not been observed yet, but it's one of those cases when we are pretty sure something like it must be going on. Otherwise we have even bigger hole in our understanding of the universe.
Theory is more difficult, but the practical effect is relatively obvious. Once the 'surface temperature' of black hole reaches over that of microwave background, it will begin to evaporate. The process is slow at first, but non-linear. As black hole evaporates, it loses mass, but the temperature actually rises.
Eventually, the process should end with a gamma ray burst.
There is a lot of unknown in that.
Right now we are very, very far from doing any of this. We would need to achieve much higher strength of magnetic fields before we could even consider this. It may turn out, that it's easier just to "catch" an existing micro-black hole. If they exist that is.
- One way would be to make a "Kugelblitz". It works by concentrating a huge amount of energy in a very small space. Because of E=mc2, concentrating energy does the same thing as concentrating mass, and could create a black hole from which even the photons that created it can't escape, at least in theory.
In practice, probably impossible, or at least well beyond our current technological capabilities.
- Nobody really knows for black holes of these sizes, because we don't have a theory of quantum gravity, but I think that answer according to general relativity would that the curvature is steep enough that escaping would require going faster than c, the universal speed limit, so it doesn't happen.
- Black holes are not held together by gravity. The geometry of spacetime inside the event horizon doesn't allow anything to move away from the singularity. All possible worldlines move closer to the singularity.
- Except some radiation does leave, evidently. As far as I know, no one knows how or why. I hope we find out before I die. It’s such a fascinating, confusing aspect of black holes
- >Except some radiation does leave, evidently
Well, not evidently. Rather, theoretically. We don't have any evidence of this whatsoever, but quantum field theory predicts that it should happen. On the other hand, we don't have a unified theory of quantum physics and general relativity, so it may well turn out to be pure fantasy; an artifact of the intersection of two incomplete theories.
- The radiation you talk about, Hawking radiation, does not come from inside of the black hole. It is caused by the black hole but it is formed from beyond its event horizon.
- Well, I’ve exposed myself. I don’t have a PhD in physics. In fact, I don’t know much about physics. Including the things I comment about on Hacker News (my bad).
I’m not sure how I made it all these years thinking the radiation came from inside.
I see now that it’s hypothesized to be some crazy interplay of particles and antiparticles within vacuums in curved space time which leads to the black hole absorbing a lesser charged particle while the higher charged particle is emitted as radiation… Which incurs some sort of mass debt for the hole due to the rules of vacuums? I will have to read about it for a few years before it makes any sense. In the meantime I’ll avoid saying dumb things about it on the internet
- You didn't say anything dumb whatsoever. Your explanation of it comes from Hawking himself but it was more for illustrative purposes rather than a rigorous description of it.
At any rate, even taking your description which certainly has merit, it is still not the case that the radiation comes from the inside of the black hole, from beyond the event horizon. Rather it's that just outside of the event horizon a virtual particle anti-particle pair is produced which has a combined energy of zero. One way their energies can add up to zero is for one to have positive energy and the other to have negative energy. The explanation then goes that the virtual particle with negative energy enters the black hole and the virtual particle with positive energy escapes, which results in the mass of the black hole decreasing. But both of these particles were formed outside of the black hole, not beyond the event horizon.
So yes the black hole loses mass, and yes for illustrative purposes one can think about a thought experiment involving the production of virtual particle anti-particles, but the key principle is that nothing escaped the black hole in the sense of coming from within the event horizon.
- Oh noes my friend.. Its not about negative energy, but background (void) temperature. The pair of particles (matter / anti-matter) spawn, cooling down the vacoom. Once they anihilate again, they change to energy (various photon emisions) increasing background temperature again. Thats why empty space is not at 0 kelvin. Every time such particle is absorbed by blackhole, it just increases it energy (and possibily mass). Not sure where that evaporation comes from ;)
- There is no singular "it". I am responding to someone else who came to understand Hawking radiation through a thought experiment that Hawking himself described for pedagogical purposes.
A brief description of that thought experiment involving negative energy can be found here along with the appropriate citation coming from Hawking's "A Brief History of Time":
https://en.wikipedia.org/wiki/Negative_energy#Hawking_radiat...
>Virtual particles can exist for a short period. When a pair of such particles appears next to a black hole's event horizon, one of them may get drawn in. This rotates its Killing vector so that its energy becomes negative and the pair have no net energy. This allows them to become real and the positive particle escapes as Hawking radiation, while the negative-energy particle reduces the black hole's net energy. Thus, a black hole may slowly evaporate.
- Stephen Hawking had a pretty good idea as to how and why - https://en.wikipedia.org/wiki/Hawking_radiation
This is not to be confused with the infrared light that’s relatively easy to detect, which isn’t leaving the black hole but is just light emitted from matter heating up due to tidal forces near the black hole.
I don’t think humanity is ever really going to know what a black hole actually is, simply because they’re too far away and the energy levels required for experiments are many many orders of magnitude beyond what we can generate on Earth. But I do expect if we could get up close and study one, we’d simply discover a lot of fascinating new physics over top of some kind of matter not that far off from neutron stars.
A lot of people don’t really get past the pop science articles about “singularities” to realize that those are just failures of our theories. We don’t know how anything behaves when gravity is strong enough to affect the quantum level, because QM has some kind of silly techniques for avoiding it under normal circumstances (called renormalization) that no longer work.
At extremely high energy levels, my guess as a guy on the internet is that the universe looks less like magical portals to Matthew McConaughey’s bookshelf, but more like regular boring space crap that destroys our notions of stuff like time, matter, energy, causality etc. And in fairness, neutron stars already kind of do this, but nobody cares for some reason.
- [flagged]
- I thought there were too many constraints to make PBHs a significant contributor?
- My understanding is that we can rule out having too many really small ones (we'd see them evaporating) and we can rule out too many stellar sized ones (gravitational lensing), but we haven't ruled out black holes in size ranges of a moon to a planet.
But I don't track this field. So there could well be research that I don't know about which puts bigger constraints on it.
- Is black hole evaporation experimentally confirmed in any way?
- The only thing that I've seen cited as potential evidence either way is that one neutrino, with an energy too high for other mechanisms that we have thought of.
But it is just a single neutrino. And it may be produced by a mechanism we haven't yet thought of.
The black holes that we know about are large. Large black holes are supposed to emit so little radiation that we'd never be able to detect it.
- Large black holes do have significant accretion disks that do impact their visibility and can and will spew forth vast amounts of radiation. Anything caught in their event horizon gets pulled in and is never seen again. Anything near it bumps into other things in orbit around it, which lights it up like a candle.
Note that there is also a 'weight class' of black holes that are significantly large that their event horizon would be visible to the naked eye, but which don't have an accretion disk. We currently don't know enough to make scientific estimates on how many of each weight class exists.
- That is light coming from the accretion disk, and things falling it. The light does not come from the black hole itself.
- Yes and no. The mechanism for evaporation is a straightforward implication of matter/antimatter pair production in the vicinity of the event horizon. This general phenomenon (minus the event horizon) is observed every day in particle accelerators. But the signal would be too small to detect from any black hole we do know about.
- That depends on how you're counting.
About 2/3 of star systems have only one star.
About 2/3 of stars exist in a system with multiple stars.
However, the higher the mass, the more likely it is to be in a multiple-star system. For stars with a mass high enough to form a black hole ... hm, Wikipedia says "at least 80%" but makes it unclear which statistic is being measured.
- > But there are also binary black holes.
A binary black hole system will keep any other companions quite far from the binary, and perhaps not even allow companions at any distance. What is the density of lone black holes and black hole binaries that we could have in -say- our galaxy before we would notice them frequently in the same way as in TFA? Well, presumably LIGO would sense them, or could if they knew what to look for.
- LIGO does sense them, or at least the infrequent merges.
- Yes, the mergers. I'm just wondering about binaries that are millions of years from merging.
- Fun to realize there's barely any way for us to detect if there's one heading for us right this very moment.
- Thankfully the speed of light is slow enough to where that's not a huge concern... that is, unless the Nemesis hypotheses is actually a black hole and not a brown dwarf or planet.
- A type-1a supernova peer would produce this effect, leaving only the black hole (or the oversize star that would become it). I don't know any other types where the star is completely destroyed.
- Astrophysical Journal article: https://iopscience.iop.org/article/10.3847/1538-4357/adbe6e
Earlier article about first discovery: https://iopscience.iop.org/article/10.3847/1538-4357/ac739e/...
- Thanks, the original article is unreadable on mobile due to the ads.
- We've known about black holes for decades, but it's kind of mind-blowing that this is the first time we've confirmed one that's truly isolated. Makes you wonder how many more are just quietly drifting through space, totally invisible unless they happen to photobomb some poor background star
- The crazy thing is the answer is quite possibly TRILLIONS.
- Does anyone else find this...unsettling? Floating around in the void of space, alone, is an almost invisible monster that can gobble planets and stars.
- It can't really "gobble" anything any more than a star "gobbles" things that fly into its photosphere or a planet "gobbles" things that crash into it.
Unless you cross its event horizon, its gravity works just like any other celestial object. Maybe at worst it slingshots you off in a different direction.
- I think the concern is that if a star was headed in our direction we’d see it coming. We don’t see one, so we know there is no anticipated threat.
A small, lone black hole could be on an intersecting trajectory with us within a few years and we’d be completely oblivious.
- If I was going to be afraid of an invisible killer, I would be far more concerned with gamma-ray bursts. It covers a much larger area than that a black hole and could crisp our planet like some kind of sci-fi intergalactic superweapon laser beam. And some might consider getting turned to ash in mere moments a mercy compared to the potential of a near "miss" that would give half the planet instant cancer and completely fuck our weather patterns beyond any comprehension.
Still not super likely, but I would think far more likely than a direct hit by a black hole.
- A mass of 6x to 7x our sun (size of this object) would start messing with solar system orbits well before it got here. Not that that would be much better for us!
- Yeah it wouldn't just sneak up on us. We would have years and years to worry and hypothesize before finally just dying.
- Barely enough time to recruit a bunch of lovably gruff leatherneck astronauts to drill a hole in it and blow it up with a nuclear bomb.
- Wouldn't it be quicker to train some drilling experts how to be astronauts though?
- > finally just dying
Is this what would happen if we got slurped into a black hole? I was hoping for something more exciting …
- We would most likely freeze to death. As the black hole crossed the asteroid belt we would be pulled away from the sun as it started to compete with the black hole's gravity. Depending how fast the black hole was moving we might die over a few months or we might freeze to death in a few days. Probably there are paths where the earth would briefly be pulled into an elliptical orbit, and then we would be burnt to a crisp as we circled back close to the sun.
- This is the setting of one of my favorite short stories, A Pail Full of Air.
Which I am delighted to note, since the last time I referenced it, appears to have fallen out of copyright, so I can link straight to it on Project Gutenberg: https://www.gutenberg.org/cache/epub/51461/pg51461.txt
- In a chaos such as that we would get killed by the weather before we even realized what was happening.
- I thought you ended up behind a bookshelf.
- Well, we hit a little snag when the universe sort of collapsed on itself. But dad seemed cautiously optimistic.
- It seems hard to see a way that life forms survive https://en.wikipedia.org/wiki/Spaghettification.
- As if there were any different actions that could be taken to avoid a star vs a black hole.
I’d probably welcome the quicker demise tbh
- It’s less about actions we could take and more about knowing we don’t have to worry about colliding with a star for the moment.
- You probably wouldn’t want society to know we were about to collide with a star-sized mass, visible or not.
- > wouldn’t want society to know we were about to collide with a star-sized mass
I may be misunderstanding the distances involved but wouldn't such a collision take centuries if not thousands of years to play out? For the most part it would just look like we had 2 suns, one of which gets a few millimeters bigger (to the naked eye) every year.
- Yes, and the weather would ruin us long before any exciting cosmological collision took place.
- ah dammit I didn't think of that you're right it would be more of a disturbed orbit, weird ass days and nights but hey atleast there's a chance the scientists could have close up real life simulation of the three body problem
- We’d see the lensing soon enough, but couldn’t do anything about it
- Fortunately space is really, really, really, really, really, really, REALLY big. The chance of this happening is so infinitesimal we might as well worry about spontaneously transforming into a whale or potted flower manifested a mile above the surface of our planet.
- If the result were both the same as well as inevitable, does it really matter whether you saw it coming?
- I wonder if a few solar mass black hole would bend light far enough around it that it would show up at some point.
With all that said, maybe it's better off if we were completely oblivious.
- I wonder what the chances are if there's a few of them within the Boötes void. It's pretty big.
- > maybe it's better off if we were completely oblivious.
even that would be a slow death I suppose. Don’t think the Earth would just vanish instantly.
- I'd wager such an encounter is way more likely to result in any of:
1) the Earth being flung out of the Sun's orbit
2) planetary orbits becoming disrupted such that an encounter with another planet over the coming years or millennia becomes likely,
2.1) which could eventually have the same "flinging away from the Sun" effect,
2.2) or (unlikely, but possible) result in a collision
2.3) or result in the Earth being shredded into asteroids
2.4) or other planets suffering that fate and then showering the Earth with dangerously-large asteroids over a period of decades or centuries until it's nearly, or actually (think: outright crust liquefaction from impacts) lifeless.
than the Earth actually getting swallowed up, by at least an order of magnitude.
IOW, the most-likely "we're all dead" outcomes for us, from a close encounter with a massive rogue anything really, including a black hole, might take years and years to play out.
- Sounds like a plot of Seveneves by Neal Stephenson
- Schwarzschild radius of a 10 stellar mass black hole is ~20 miles. It would need to be pretty close in order to resolve optically.
- Yet they detected a lone black hole in the article. Maybe detection isn't guaranteed though?
- Detection typically requires exceptionally rare circumstances - which if looking at a dataset the size of the visible universe, typically turns up several examples if we look hard enough.
But any specific random example, is often brutally hard to see.
- If we saw a star coming toward us...
Have you seen the Walking Dead?
- No, how does that relate?
- I'm pretty sure it would be something like even if a star was coming and we saw it what the fuck could we even do it would just create mass panic, hysteria and would make everyone hella religious depending on how early we detect it though we might have some time to make good memories before we die. atleast I would prefer a black hole smashing into us unbeknownst to us instead of a known star. Also no one would want to show up to jobs and stuff
- Fritz Leiber, "A Pail of Air": https://www.gutenberg.org/cache/epub/51461/pg51461-images.ht...
- That brings me memories of Cosmos 1999. The moon left Earth's orbit to outer space because explosions, but being slingshoted away because a nearby massive enough object passing by looks like a more possible scenario, not explored enough by sci-fi.
- The basic premise of the show that an explosion at a nuclear waste dump could produce enough energy to push the Moon out of the Solar System to wander the galaxy is an interesting product of its time. Concerns over the power of nuclear explosions was high and casual access to knowledge about the plausibility of such a scenario was somewhat limited.
There's a fan driven update called Space: 2099 that improves some of the more dated aspects of the show, including showing the Moon enter some type of portal or wormhole to make suspension of disbelief easier. While the Special Edition releases of Star Wars often suffered from updating certain aspects, especially special effects, the Space: 2099 changes were generally good for the show. Too bad they're unable to fund raise enough and get permission to do the entire series.
- Arguably, the information was there. People just chose to be ignorant.
- > Cosmos 1999
Space: 1999. Do you happen to be french or polish?
In Germany they called it "Mondbasis Alpha". As I child I really liked this series and it's predecessor UFO made by the same team (Gerry and Sylvia Anderson of Thunderbirds fame).
- He technically did not say the invisible monster was a black hole.
- Space between stars is amazingly empty. It's almost unlikely that we would bump into another star (or black hole). For instance, if the whole Milky way were the size of the US (east to west), our sun would be smaller than 1/20th the size of human hair and the closest star to us would be at least a football field away. (Credits for this analogy: Kurzgesagt app https://play.google.com/store/apps/details?id=org.kurzgesagt... https://apps.apple.com/us/app/universe-in-a-nutshell/id15263...)
- > gobble planets and stars
Direct interaction isn't needed for havoc. A supermassive object sweeping by the Solar System could destabilize Jovian orbits. In the Nice model, Neptune flung Kuiper belt asteroids sunward, gifting the inner planets with a late heavy bombardment.
Rogue gas giants, brown dwarfs accelerated to relativistic speeds, giant asteroids approaching from the Sun's direction, Carrington Events, an ill-directed gamma ray, etc. So many ways life on Earth can see its 250 million remaining years cut short, and those are only a few of the cosmic threats we can imagine.
A black hole with a Schwarzschild radius of 20 km would weigh about 6.8 Solar masses. It wouldn't even need to get super close to affect the Solar System.
- But if you do see it gobble planets and stars, they you know Remina is on the way! ;-)
- Where is the 250 million years come from?
- Perhaps a reference to Pangea Proxima?
https://en.wikipedia.org/wiki/Pangaea_Proxima
Life might very well exist on earth even through those conditions, but not to the extent we have today.
- No more unsettling than space in general is. It’s pretty hostile to life. We’re not just making turns around the orbital racetrack setup around the Sun, we’re also flying through space following the gravitational trail of the Sun as it races forward without a destination.
I was playing with Universe Sandbox over the weekend trying to figure out how to terraform Venus. Changing its axial rotation period to a day to match the Earth while I screwed around with its chemistry was enough to cause Europa and some of the other famous moons of Jupiter and Saturn as well as Charon to yeet themselves outside of the solar system within about 10 or 20 years of simulated time.
- Why would changing the rotation speed of Venus have any noticeable effect on the outer planets? That sounds more like a limitation of the model than anything else. Especially over such a short time! 20 years is nothing to the orbit of Charon.
- Probably, but if a Venus-sized mass showed up in the inner solar system because the Sun just picked it up along the way, it might not be instant death but we’re probably in for a rough time. It doesn’t have to be a black hole that does us in, it could be something much smaller that still strips the Moon away or causes Earth to readjust its own position in a way we, as in life, but also maybe we as in humans or we as in mammals just don’t like very much in a very short amount of time temporally speaking, and we couldn’t do anything about it anymore than we could do anything about a black hole because we’re just not the captains of this ship. We’re just some homegrown stowaways.
But for what it’s worth, it’s also just so incredibly unlikely it’s not a scenario worth thinking about either, and thinking about it too much just invites existential dread.
- It would be nice to get a rigorous estimate on how big and nearby a black hole could be before we'd notice it with routine sky surveys or orbital deviations. A 6-solar-mass black hole only has a radius of around 18km or 11 miles. How often will one pass in front of a star precisely enough for OGLE and MOA to detect it, as they did with this one?
Apparently the Roman Space Telescope will be great at detecting these, if it doesn't get cancelled.
- There are many theoretical astronomical risks. For example, if we happened to come into the path of a relatively nearby gamma-ray burst, it could eliminate all life. Given that life has existed on the earth for quite some time, the 'Lindy effect' suggests that the sum of these presumably-constant risks is small. We are much more likely to become extinct due to an anthropogenic cause.
- A gamma ray burst is one of the possible hypotheses for the cause of the Ordovician mass extinction event, one of 5 big ones Earth has had. No idea why the Great Oxidation Event isn't included there as it was also one of the deadliest mass extinction events - plants and their vile poisonous oxygen killing off basically everything else.
So I don't think the 'Lindy Effect' would apply as species are mostly perishable, just on longer timeframes. Humanity is hopefully the exception, but absence of evidence of other advanced intelligences in the universe doesn't paint the most promising picture there either. On the other hand we're already on the cusp of colonizing other planets and once that process begins the odds of humanity ever going extinct will approach zero. On the other hand at greater distances "humanity" will likely splinter fairly quickly (relative to on a geologic or even species survival timeline) into numerous distinct species.
- The anthropic principle is at odds with that. Lindy's doesn't reason about itself.
- No, not me anyway. we are all floating (falling) in the (nigh) void of space (equally) alone ... which is great protection from all the monsters everywhere!
Deliberately hitting things in space is hard, accidentally, more-so.
Consider the chance of our sun getting whacked when the entire Andromeda galaxy gets here ... billions or more likely trillions to one. The chance of a single mass in our own galaxy getting us should be less than that.
edit: as far as I know the only difference between getting gobbled by a black hole v.s. anything else is our atoms won't get to continue their evolution into larger atoms in this universe. (or maybe see it as our atoms get to complete their evolution in this universe)
- It can't gobble planets and stars any more than any stars of the same mass.
- I'm trying to picture intersecting paths though. Does a faster moving black hole cause more or less damage to a target?
Imagine a black hole on the quite small end, intersecting the core of a planet. Unlike regular matter, it can't really produce bow shock through collisions, right? All the target matter in the direct path just "falls in" and in elastically reduces the black hole momentum a tiny bit?
Some matter outside the direct path could be accelerated towards the black hole but slingshot behind it, rather than into it. So this material could produce an impressive wake, with material spraying outward from the collision path and interacting with the remainder of the target.
But, all this visible chaos comes from gravity rather than more direct kinetic interactions, right? If the black hole is moving faster, doesn't the target's material gets less gravitational acceleration as it spends less time in the near field? So, if the blackhole is moving very fast, does it bore a smaller hole and have less interaction with the target? Or do other effects of relativity make this more convoluted to think about?
I'm imagining a cylindrical plug of a planet "instantaneously" disappearing, and then the remainder of the planet collapsing inward to fill the void, bouncing off itself, and ringing like a bell.
- > Does a faster moving black hole cause more or less damage to a target?
When a black hole accretes matter, the matter can create tremendous radiation before it crosses the event horizon due to the atoms experiencing many effects such as rapid nuclear fusion and becoming new forms of matter such as neutronium. The precise amount of energy released depends on spin, charge, and size of the black hole, and the speed at which the matter approaches the black hole.
If a tiny black hole (Let's say 10cm across) ripped through the earth at significant speed it would be like the center of the planet momentarily became the center of a star and (hand waving a bunch of assumptions) the total energy could easily be greater than the gravitational binding energy of the planet. The planet would explode.
- For sure - The largest threat would be traveling by our solar system close enough to throw off the orbits of earth or any of our nearby neighbors.
- There's something existentially creepy about a massive, invisible object just wandering the galaxy, indifferent to everything
- In practical terms, not so very far away from the NEAs that we have no idea of, and which we notice after they've just skimmed by the Earth.
- I don't. If the sun were replaced by a black hole of equal mass next Tuesday at noon, the only thing we would notice is that it suddenly got very dark and very cold. We would continue orbiting the thing while freezing to death over the next few days.
- It's really, really hard to fire something into the Sun. We can't do it. The same goes for black holes. Things don't just get sucked in. They usually end up in orbit instead.
- That's not true.
(1) We can launch a rocket that has a payload that hits the sun: it's just costly to do because the rocket starts out orbiting the sun, so we would have to expend delta-v to neutralize the tangential component of the initial velocity.
(2) If you are in a space ship heading towards the sun, it is easy to hit the sun as long as you can steer, and even if you can't steer, if you are heading squarely at it and no planet gets near you to change your velocity, you will hit it.
- You are missing the point. Most things that approach a black hole end up orbiting it because they are not aimed directly at its center.
- Are you worried? :-) I bit like bringing sunscreen to the apocalypse no? :-)
- The size of the black hole described in the paper is ~ 20 km, so it is tiny. Even we have millions of such objects (and most likely we do), the chance of hitting something, given the enormous size of the galaxy is negligible.
- The black hole in the paper is also ~7 solar masses. If that passed between the Earth and the Moon it would rip apart the earth from just the tidal forces.
- Well, if present anywhere in the solar system it would also completely fuck our orbits all to hell too eh?
- the chance of that is negligible.
- Would we notice if it was to point in our direction?
- Relax, it’s probably traveling in a straight path at constant velocity.
- What I don't understand is how big bang could exist if such relatively "small" mass concentration creates black holes?
- Excellent question.
Gravity pulls things in by causing space-time to accelerate in a particular direction. In other words we accelerate towards the Earth at 9.8 meters per second per second because that is what space-time itself does. The space-time that is in our frame of reference accelerates down, carrying us with it. The floor pushes up on us, causing us to accelerate up. Balancing things out so that we remain where we are.
A dense mass will cause flat space-time to start falling in. Enough mass, densely enough, will cause it to fall in so fast that not even light can escape. This is a black hole.
However the Big Bang wasn't a flat space-time. The space-time that was the structure of the universe was moving apart extremely quickly. There was more than enough mass around to create a black hole today. But what it did is cause the expansion rate to slow. Not to stop, reverse, and fall back in on itself into a giant black hole.
- > Gravity pulls things in by causing space-time to accelerate in a particular direction.
Ok, how does this sketch work for a low-ellipticity eccentric orbit?
> "The space-time that is in our frame of reference"
Isn't throwing out general covariance (and manifold insubstantivalism) rather a high price for a simplification of Einsteinian gravitation?
> the Big Bang wasn't a flat space-time
Sure, it's a set of events in a region of the whole spacetime. If we take "Big Bang" colloquially enough to include the inflationary epoch, always assuming GR is correct, then at every point in that "Big Bang" region of the whole spacetime there is a small patch -- a subregion -- of exactly flat spacetime. However, these small patches must be small because most choices of initially-close pairs of test objects can only couple to timelike curves that wildly spread in one direction (and focus in the other).
I don't know how to understand your two final sentences: how do you connect the period just before the end of inflation and the expansion history during the radiation and matter epochs?
- First, the idea of describing it that way comes from Veritasium. Take complaints to him. See https://www.youtube.com/watch?v=XRr1kaXKBsU for the video where he does it.
Yes I know it is handwavy and misleading. But I consider it less misleading than most attempts at visualizing it.
> Ok, how does this sketch work for a low-ellipticity eccentric orbit?
At what point in the orbit does it not work as a description of what's going on locally where the orbiting body is?
> Sure, it's a set of events in a region of the whole spacetime. If we take "Big Bang" colloquially enough to include the inflationary epoch, always assuming GR is correct, then at every point in that "Big Bang" region of the whole spacetime there is a small patch -- a subregion -- of exactly flat spacetime. However, these small patches must be small because most choices of initially-close pairs of test objects can only couple to timelike curves that wildly spread in one direction (and focus in the other).
No, there is no requirement of any region of locally flat spacetime existing. It is required (outside of singularities) that, when measuring things to first order, things are flat. However in curved space-time, the curvature can be theoretically revealed in any region, no matter how small, by measurements that are sufficiently precise to show the second order deviations from flatness that we call curvature.
> I don't know how to understand your two final sentences: how do you connect the period just before the end of inflation and the expansion history during the radiation and matter epochs?
I'm just referring to the fact that the Hubble parameter is believed to have been higher in the early universe than it is today. I'm not referring to periods such as the hypothesized inflation where the behavior is not described by GR.
- > See https://www.youtube.com/watch?v=XRr1kaXKBsU for the video where he does it.
I'd be grateful if you take me to the time stamp, because in a casual watch of the video there was nothing like your:
The closest thing I noticed was just after the 10 minute mark, where he points at Christoffel symbols and essentially says that you can choose a set of accelerated coordinates such that to remain at the spatial origin you have to undergo proper acceleration. "Your acceleration must be equal to this curvature term ... in curved spacetime you have to accelerate just to stand still". Which is totally fine, and even finer if he made it clear that you are "standing still" at some spatial coordinate after an arbitrary splitting of spacetime into space + time. But I don't know how or if those approx 90 seconds connect to what I quoted from you above.Gravity pulls things in by causing space-time to accelerate in a particular direction. In other words we accelerate towards the Earth at 9.8 meters per second per second because that is what space-time itself does.(Even finer still if he removed the coordinates and completed the equation: see e.g. slide 20/50 at https://slideplayer.com/slide/12694784/ - his whiteboard is the term labelled in cyan, adapted. approx 10m45s "You don't have to worry about the details here. The point is...". The point is that I'm not his intended audience, and his presentation is fine enough, so there's no good reason for me to take you up on your suggestion to contact him.)
> At what point in the orbit does it not work as a description of what's going on locally where the orbiting body is?
I don't know that it doesn't work - it's just that to me it's such an odd way of putting it that the consequences of your "describing it that way" are unclear to me. An obvious probe is solving for an eccentric orbit.
Shoot a slower timelike observer on a ~secant line hyperbolic trajectory across the quasicircular orbit, comparing proper-time-series accelerometer and chronometer logs from their first kiss before the latter's periastron to their last kiss after. How does your "accelerated spacetime" vary by position and initial velocity? How does it work as we take v->c?
- I don't know enough physics to know whether the parent knows what they are talking about, but there is one piece of math that makes me think they do not.
> at every point in that "Big Bang" region of the whole spacetime there is a small patch -- a subregion -- of exactly flat spacetime
Can explain how you get a non-empty region of exactly flat space time around every point?
Patching together curved things out of not curved things happens all of the time. The Earth looks flat around the point you are standing. I'm worried that just because it looks flat in my city doesn't mean it is actually flat in my city, if I measure carefully.
- > I don't know enough about physics
I'll try to keep this understandable, but can expand or ELI5 bits of it if that would help you.
Physically, local flatness is a statement about the local validity of Special Relativity. Practically, a failure of the local validity of Special Relativty -- a Local Lorentz Invariance violation (often abbreviated LLI violation or local LIV or local LV) -- would be apparent in stellar physics and the spectral lines of white dwarfs and neutron stars and close binaries of them. Certainly we haven't been able to generate local LIV in our highest-energy particle smashers, so the Lorentz group being built into the Standard Model is on pretty safe footing.
(For example, we need tests of Special Relativity -- and notably those of the Standard Model, which bakes in the group theory of Special Relativity -- to work for material bodies in free-fall, even if that free-fall is an elliptical path around and close to a massive object. That's everything from our atomic-clock navigation satellites to gas clouds and stars near our galaxy's central black hole or distant quasars.)
It wasn't a piece of math, which would involve writing out an Einstein-Cartan or Palatini action that let one break out the local Lorentz transformations and diffeomorphisms into a mathematical statement, as one can find in modern (particularly post-Ashtekar in the late 1980s) advanced graduate textbooks. Nobody wants that scribbled out in pseudo-LaTeX here on HN. :-)
Here is an interesting and very slightly contrarian (they do arrive at Theorem 1: it and most of the following text explaining it is beautifully stated orthodoxy -- and note Corollary 4) view by a pair of philosophers of mathematics (they both have also done physics, they are not cranks) at <https://philosophyofphysics.lse.ac.uk/articles/10.31389/pop....> (their rather orthodox part 2 is at <https://philosophyofphysics.lse.ac.uk/articles/10.31389/pop....>).
The choice quote from part 1: "[our] final interpretation says that every spacetime is locally approximately flat in the sense that near any point of any spacetime (or near sufficiently small segments of a curve), there exists a flat metric that coincides with the spacetime metric to first order at that point (or on that curve) and approximates it arbitrarily well," [emphasis mine].
You might prefer to emphasise "approximately" in that quote, but the approximation is much better than that of, say, a square millimetre of your floor.
Next, from a historical perspective: General Relativity was built with making gravitation Special-Relativistic, following Poincaré's 1905 argument about the finite-speed propagation of the gravitational interaction. Einstein (and others) had several false starts marrying gravitation and Special Relativity in various ways before ultimately arriving at spacetime curvature. (At that point, in the 1920s, one finally had the vocabularly to describe Special Relativity's Minkowski spacetime as flat; the Lorentz group theory came later). But making sure Special Relativity didn't break on around Earth -- where it had been tested aggressively for two decades -- was terribly important to Einstein. Additionally, he did not want to break what Newtonian gravitation got right. The mathematics follow somewhat from this compatibility approach where Newtonian gravitation and Special Relativity are correct in the limit where masses are moving very slowly compared to the speed of light and are not compact like white dwarfs or denser objects.
The regions in which there is no hope in many many human lifetimes for finding a deviation from Local Lorentz Invariance are huge (there are interplanetary tests with space probes in our solar system, and interstellar tests using pulsar timing arrays), even if General Relativity turns out to be slightly wrong. This is an area which invites frequent experimental investigation: <https://duckduckgo.com/?t=ffab&q=local%20lorentz%20invarianc...>.
Finally, it is precisely your intuition that big curvature must be built up from small curvature that is the point of investigating local LIV. So far, and to great precision, those intuitions are wrong. Nature builds up impressive spacetime curvature (e.g. in white dwarfs and neutron stars) without showing any signs of softening the local validity of Special Relativity (i.e., the interactions of matter within those compact stars). And that's part of why quantum gravitation is nowhere near decided.
- > The choice quote from part 1: "[our] final interpretation says that every spacetime is locally approximately flat in the sense that near any point of any spacetime (or near sufficiently small segments of a curve), there exists a flat metric that coincides with the spacetime metric to first order at that point (or on that curve) and approximates it arbitrarily well," [emphasis mine].
This statement is a mathematical conclusion from GR of a similar nature to noting that for any point on a sphere, there is a map projection onto the plane where distances on the sphere coincide to first order to distances on the plane.
This no more or less means that space-time is locally flat than it means that a sphere is locally flat. To a first order approximation, it is. But when we calculate the curvature tensor, we find that it isn't flat at all.
- How do you propose to measure the ultralocal Riemann curvature? I agree with you that it can be there in an exact solution, but I don't really agree that it's necessarily physical (there are many many exact solutions which aren't, for starters). Even with a broader definition of local, a Synge 5-point like process is not going to find a failure of parallel transport in a sufficiently small region. Would you be satisfied with "effectively flat" (in an EFT sense) or even "FAPP flat"?
- > Nature builds up impressive spacetime curvature (e.g. in white dwarfs and neutron stars) without showing any signs of softening the local validity of Special Relativity (i.e., the interactions of matter within those compact stars).
I.e., you need to be near huge, dense masses, or on/in them to see LIV violations, but we can't see them from observing those masses.
> And that's part of why quantum gravitation is nowhere near decided.
There are also problems with quantizing curved spacetime.
- > You need to be near huge, dense masses, or on/in them to see LIV violations
A sufficient LLI violation in a compact object is likely to lead to a difference in pressure/contact line broadening, thanks to a modified dispersion relation. Ok, there's optical depth issues there, but looking at metal-rich WDs is a start (it gets you to your "or on...them", at least). Neutrino fluxes probably carry some Lorentz-symmetry-related information from multmessenger events too.
Additionally, binaries and multiples might show various equivalence breakdowns if there are LLI violations, with enhanced ellipticities or periastron precessions (by altering orbital polarization and spin precession parameters in the PPN).
But also there are plenty of theories which slightly violate local Lorentz-invariance in the Newtonian limit, bulding up over distances, and PTA data and GRB data are already constraining those.
- I don't understand this answer. By GR there is no possible flat space-time around a dense mass no? BC the energy will curve the space-time. Saying that the space-time was expanding very quickly is also describing the shape of the space-time. Isn't it kind of circular to say that big bang doesn't end in a singularity b/c it is curved out? You can still ask why it's curved out with so much energy and whether it is compatible with GR? But I guess the answer if GR was holding near big bang must just be that there's some solution which is compatible with GR with so much energy in a small place which doesn't end in singularity.
- The Schwarzschild solution is the unique distribution in GR for nonrotating mass in a small area, in a universe that is asymptotically flat at a long distance from the mass. This is not a flat universe, but most of it is pretty darned close to flat.
As for describing the shape of space-time, that's what GR does. What we can think of as the "shape" is actually described by something called the metric. GR says that the metric satisfies a differential equation. If the universe starts close to flat, things are moving slowly, and there is a low density of mass, the solutions to this equation create an effect that, to first order, matches Newtonian gravity. But the full theory has solutions with all sorts of bizarre things in it, like waves traveling through space, made up of fluctuations in the very structure of space-time. We call those gravity waves.
And yes, those solutions do include things like expanding universes. And the effect of gravity within an expanding universe is to slow the rate of expansion.
- The metric you're referring to, oddly enough is a mapping from flat spacetime to curved. This is why the Schwarzschild and Kerr solutions to the EFE have `r` values that are in flat spacetime and yield spacetime intervals. The metric is symmetric, so you can also map back from curved to flat spacetime.
- > By GR there is no possible flat space-time around a dense mass no?
In standard cosmology in the super early universe there wasn't _a_ mass, like a point mass -- there was lots of mass-energy everywhere (not a point anything but a huge swath of space, and very dense), pulling on everything, yes, but at the same time stuff was flying apart with more momentum than the gravity of all the stuff because that gravity was pulling in all directions (therefore causing the gravitational potential to be huge but the net gravitational pull in any direction to be zero) but the pressure was pushing in all directions, so it all could fly apart after all.
- While not a direct answer in itself, I think it's noteworthy that the big bang already requires esoteric physics, like inflation [1], that have no known explanation, source, or parallel anywhere else in existence -- and in fact contradict everything we know about known physics. It's an entirely ad-hoc hypothesis that was largely developed to solve numerous other problems with a big bang, in particular our universe not fitting what would be expected following a "normal" big bang, such as the Horizon Problem. [2]
And so looking for logical explanations for the big bang is already a nonstarter. At this point it remains highly dependent upon ad hoc constructions.
- One of the theories is that the properties of the Higgs Field changed and so the laws of physics changed. And that if they ever change again that we'll likely be dead before we know to be afraid, since the change would propagate through the universe at the speed of light. We wouldn't even see the stars blink out before the molecules in our bodies stopped being the molecules in our bodies.
- "There is another theory which states that this has already happened."
--Douglas Adams
- Well, imagine that the big bang is really something more akin to a white hole, which is a black hole running backwards: all the stuff has outward momentum, and lots of it, so it can't get pulled back. In standard cosmology the big bang happens not at a point (so it's not really like a white hole) but everywhere, but there's also really strong repellent forces that cause "inflation", which defeats the mass-energy's gravitational pull's attempt to collapse the whole thing. But still, perhaps the thought of a white hole might help you bridge the gap.
Mind you, this is a pop science, handwavy explanation.
- The layman's answer is that since everywhere was very dense there wasn't a gravitational pull to one direction or another since it all cancelled out.
- Except that this answer does not make sense. General Relativity predicts that if you fill flat space-time with matter, it will start to contract due to gravity. It is not uniform density by itself that prevented the early Universe from forming a giant black hole.
In fact one of the proposed cosmological models for our universe is that it has sufficient density to some day reverse its expansion and then fall in on itself into a giant black hole. See https://en.wikipedia.org/wiki/Big_Crunch for more.
- My theory with zero study, math, or proper explanation is that this is exactly how the universe operates.
It explodes outward until the explosion energy is cancelled out by gravity, wherein the universe then collapses on itself. The moment in which the last bit of matter and energy is consumed by the massive black hole that forms, it's enough to cause another explosion.
- My experiences with psychedelics agree with this baseless theory =)
- Because space expanded faster than the speed of light.
- I'm sure I'm not the only one that's thought of this, but could this be "dark matter"? Is the universe simply filled with these rouge black holes?
- > rouge black holes
They makeup much of the stylish universe in the cosmos ;-)
Just kidding, I know you meant rogue.
I would assume we'd see a lot of more tricks of light bending if they did. Light lensing was used to confirm relativity by looking for multiple super novae signatures from the same event, which passed by large black holes on their way here!
- Depends on the size, position, and number of black holes, right? We see lensing currently because of super massive black holes that we know about. But if there's a bunch that are basically as massive as our sun (or less) then we are dealing with event horizons ~3km or less. It'd be pretty hard to spot those as the diffraction would be rounding errors.
- My logical leap is that if blackholes constitute our measure of dark matter (25% of proportion) or dark energy (68% of proportion) that they'd attract each other and glob together. Also true of gravstars.[0]
- But are there enough of them that we're really the rogue matter that is abnormal?
- These would be 'primordial' black holes: https://en.m.wikipedia.org/wiki/Primordial_black_hole
- This would certainly be some of it but the awkward fact is that we've positively identified so very little of the matter that makes the universe the shape that it appears to us to be that we could double the known matter in the universe with black holes and we'd still only be a 10th of the way there.
However if we could eliminate the false signals from invisible (singularity) matter I am hopeful that will give us a clearer idea of whatever the rest is.
- I can't remember who I heard talk about this, but scientists have considered this. I think there was a good reason for why it doesn't seem to match observations.
- If a significant portion of dark matter was made of these we would see a lot more gravitational lens distorions of distant objectes. There are further hard limits on how much baryonic dark matter there can be from big bang nuceleo synthesis. I think that would also put limits on contributions from lone black hole contribution.
- Probably not the main component of dark matter, but definitely a spooky and fascinating piece of the cosmic jigsaw
- That was my first thought too. A cursory search indicates we would see a lot more gravitational lensing in our observations if that was the case.
Found a couple of videos on it too
- There are some theories that primordial black holes could be dark matter. It's not a mainstream view though.
- You're not the only one. This is a fairly old idea.
- I’m suprised this is a surprise to many - shouldn’t this be equally likely as those black holes we can only see since they’re not done yet consuming everything in their reach?
- You'd be "lone" too if you ate all your neighbours.
- More like it eats the dust that brushes pass the front door, but neighbours are safe.
- TIL there’s a Nancy Grace telescope
- Planned to launch in 2027: https://en.wikipedia.org/wiki/Nancy_Grace_Roman_Space_Telesc...
- Unusable website with too many popups.
- Perhaps the links in https://news.ycombinator.com/item?id=43755028 will be better.
- "Lone Black Hole [games|jeans| ...]
... sounds like a cool brand :)
- [flagged]
