It seems like we think there's many more of these black holes, but we just can't see them
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.)
> 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.)
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.
Is it a case of once you black, you never go back?
An interesting result is that you could, and this is very much beyond our engineering, manufacture black holes like this.
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.
In practice, probably impossible, or at least well beyond our current technological capabilities.
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.
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
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.
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.
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.
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.
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.
A BH needs to be truly tiny for it to be hotter than the CMB
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.
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.
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.
Earlier article about first discovery: https://iopscience.iop.org/article/10.3847/1538-4357/ac739e/...
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.
A small, lone black hole could be on an intersecting trajectory with us within a few years and we’d be completely oblivious.
Still not super likely, but I would think far more likely than a direct hit by a black hole.
Is this what would happen if we got slurped into a black hole? I was hoping for something more exciting …
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
I’d probably welcome the quicker demise tbh
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.
With all that said, 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.
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.
But any specific random example, is often brutally hard to see.
Have you seen the Walking Dead?
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.
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).
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.
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.
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.
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.
Apparently the Roman Space Telescope will be great at detecting these, if it doesn't get cancelled.
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.
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)
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.
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.
It's hard if you aim at the Sun. So don't do that.
You just have to kill your starting orbital velocity relative to the Sun (the efficient way is to fly away out from Earth to higher orbit and then kill your orbital velocity rather than just immediately killing the orbital velocity you get from Earth at Earth's orbital distance -- we can launch craft with enough delta-V to do the former, but not quite the latter, IIRC, with current technology.
> Things don't just get sucked in.
They do unless they have a sufficient component of their velocity at right angles to the Sun to avoid doing that, but that's a solvable problem. You don't hit the Sun (easily) by thrust at the Sun, you hit it by thrusting at right angles to it, in the direction opposite whatever component of velocity you currently have orthogonal to the Sun, and gravity will take care of the rest.
(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.
Your comment would be just fine without the first sentence.
Not sure how you got from me responding to two of your posts in this thread that I was "targeting" you for something in some other thread.
This 3 day old comment was not flagged until just now, what I said is not a swipe at all.
Somehow, I can't edit a 5-minute-old post.
Looking over account histories is standard moderation; of course we do that.
Most things that approach a black hole aren't trying to hit it.
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.
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?
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.
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:
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?
> 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'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.
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.
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.
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.
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.
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.
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.
--Douglas Adams
Mind you, this is a pop science, handwavy explanation.
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.
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.
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!
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.
Found a couple of videos on it too