"Dark matter" is just a word for the missing mass we can detect gravitationally, but can't otherwise detect. We know there's something out there from it's effects on other normal matter. But it doesn't seem to have any other effect apart from gravitation.
What the authors are suggesting is that this "dark matter" effect is actually the result of many small black holes orbiting galaxies. That they aren't exotic particles, just something we already know about in an unexpected place.
That's such a good explanation! I chose the spreadsheet life instead of the space life, but I especially love difficult ideas explained simply!
Is the unexpected element of this that it is weird to think that very small black holes interact with each other? What does it mean that they don't collapse into each other and rather stay individual?
I'm not certain- I too chose the spreadsheet life, but am a space nerd. Someone correct me where I'm wrong!
I think it has to do with the number of black holes that would be required, the rate that black holes evaporate naturally(?), and our current best theories of the history and mechanisms of the universe. Like maybe in a universe where all these black holes form, we wouldn't expect to see [something we definitely do see]. So you need to adjust more variables to make that fit the puzzle, but now more things we do see don't make sense.
It's a bit like a big Sudoku, but with way more constraints!
Black holes of any realistic size evaporate very slowly, and will be absorbing more mass than they will be radiating for most of the lifetime of the universe.
You’re talking billions of billions of billions of years before most of them even start shrinking.
The same reason stars or planets don't collide for billions of years. Space is mostly empty and heads-on collisons are rare.
So objects usually need to be captured into mutual orbit first via 3-body interactions and then slowly move closer to each other via further perturbations or gravitational waves.
According to https://phys.org/news/2016-09-cold-black-holes.html a black-hole greater than the mass of the moon would have an apparent temperature less than that of the cosmic background radiation, so it wouldn't have started shrinking yet. And it'd be very hard to see. Temperature is inversely proportional to mass ( https://en.wikipedia.org/wiki/Hawking_radiation ) so heavier black holes are even colder.
I don't know how far out LIGO can detect small-mass black-hole mergers, nor how likely these mergers would be. Nor do I know how detectable micro-lensing events would be.
Reasonable sized black holes evaporate extremely slowly. A black hole the mass of the moon would take 10^45 years to evaporate, so none would have evaporated yet in the lifetime of this universe which is about 1.4*10^10 years old. See https://en.wikipedia.org/wiki/Hawking_radiation. Only black holes around 10^11 kilos or less will have evaporated by now. Couldn't think of any objects around that size to compare to, but there are some here: https://en.wikipedia.org/wiki/Orders_of_magnitude_(mass)
Of course a black hole’s temperature is inversely proportional to its mass, so a tiny black hole would be terrifically hot. Ironically a much more massive black hole is undetectable unless it’s lensing something, or “feeding” on something. A small black hole would have a very high temperature just from Hawking radiation.
Aren‘t those the most energy dense objects known to us? Not sure how large the black holes in the ‚Kugelblitz‘ idea are, but supposedly if we could harvest the energy output of a significant fraction of the sun, we could manufacture them and then use it to travel to other stars rather easily... or destroy entire planets, as those two things unfortunately go hand in hand (in that regard Star Wars may be more realistic than other SciFi funnily enough).
Vehicles are too easily weaponized. The amount of energy it takes to move a human and a cabin is more than enough energy to destroy a small building. People have driven their cars into houses and businesses for as long as cars have existed. Not to mention aircraft. And driving cars into cars is a too common tragedy.
The odd part about Star Wars is the relative weakness of their weapons. Star Trek have "warp core breaches in a bottle" AKA the photon torpedo, which is much better at damaging starships than ramming.
So next question how does this account for the required spin? won't black holes that small would be spinning beyond what is possible through GR? can these black holes be just normal ones or would they be a "naked black hole/singularity"?
Yes. But it wouldn't be an "anti black hole" or an "antimatter black hole", just a "black hole" like any other.
But as far as we know there are no "anti stars" out there that could collapse and form a black hole. We haven't observed any at least (don't ask me how we could tell an antimatter star from a regular one - but we could tell if one met a regular one).
I haven't studied astronomy as much as I would like, so this is probably a stupid question, but is it possible that something other than star collapses caused a bunch of anti-matter in the early universe to form black holes, both explaining dark matter and answering the question of "where did all the antimatter go?"
It’s not impossible, but we’d have no way of knowing because the result would be just another black hole. What I can say with some confidence is that unless some wildly unexpected physics emerges, there’s no way antimatter only occurred in dense clumps and collapsed before it could annihilate. So could there have been a freak occurrence yielding a black hole from a dense cloud of antimatter? Unlikely, yet possible, but it would still do nothing to explain why matter ended up dominating antimatter.
To be more technically correct. "Dark matter" is the word we use to describe the thing that would explain why galaxies and the stars within them do not move in accordance with the gravitational model we believe explains what we see.
We assume that our gravitational model is correct and that an additional item is needed to bring the model into agreement with the observations made. When a model seems to work so well, we are generally unable to go back and look at the basis (and assumptions) on which our theories are made. This applies in all areas of our lives, including all of our science. That is the nature of who people are.
That’s just not true, it’s an artifact of the Bohr model of the atom which doesn’t take the wave-particle duality of the electron into account. The wavefunction of the electron is “smeared” around the atom, which is not empty.
The black hole as dark matter theory is also largely discredited (MACHO theory for MAssive Compact Halo Object) because gravitational lensing studies don’t support it.
While you can argue they are still flawed, they do clearly show that the electrons have fairly large probabilities of being observed pretty much anywhere inside the "radius" of the atom.
Those are simply the electron orbitals which describe the probability of finding the respective election at a given point. That doesn't imply "smearing" any more than a picture with a long exposure does. Atoms are still mostly empty space.
I am really confused by what you are saying. It sounds like you think that electrons are point particles with charge that have some position somewhere. This is frequently a convenient model, but the last 100 years have showed pretty convincingly that the "reality" is that electrons are wavefunctions (or if you want to be pedantic, excitations of a particular quantum field).
At this point, arguing about "are atoms mostly empty space" becomes a futile semantic argument. The model you picked in order to defend this statement is indeed useful in some limits, but it has been proven wrong. Imagining electrons as clouds has proven to be much closer to reality.
It's hard to abandon the old idea, when the new idea, I mean, it's there, everywhere and nowhere at the same time, if you don't look, isn't particularly convincing, and the retort that it doesn't need to be convincing if you (read: I) just aren't smart enough, because it's counterintuitive, then I have a counterintuitive disregard for your cleverness.
I have to concede, though, that "empty" is not a particularly meaningful physical concept, quite the opposite. Disregarding the Aether theory, only to replace it with fields of potentials, that is kind of running in circles.
A model is fundamentally wrong. That's what it means to have a model. At that "clouds" is no more helpful then "empty". The fact of the matter is that no one has ever seen these things, no nucleus, no electrons, as far as I know.
Yet, you haven't pointed out what was actually wrong with "Those are simply the electron orbitals which describe the probability of finding the respective election at a given point".
Edit: Perhaps you intended to imply that this description is overly idealized (not to say simple).
Regarding the "smearing" concept, how does that square with the formation of neutron stars and white dwarfs? Those objects are incredibly dense because they're composed mostly (?) of nuceei or neutrons, respectively.
The plots I linked to are for a single atom. If you have many other atoms around it, the smearing becomes different. For instance, in metals some of the electrons are completely smeared over the whole macroscopic piece of metal (this is a bit handwavy, but not incorrect phrasing). Chemical bonds also happen due to deforming and sharing of those "smeared" clouds between neighboring atoms.
If you have too much external pressure (gravity), those smeared electron clouds come too close to the nucleus, react with it, and the only thing left is neutrons.
As a non-astrophysicist, just scanning the wikipedia entry [1] it appears to me that gravitational microlensing studies must be fiendishly difficult, you probably need massive amounts of data and of compute power to get to highly confident conclusions. Is my impression accurate?
If that is so, than the MACHO theory should not be discredited that quickly. It is after all the only theory that does not require any new physics in order to explain the missing mass.
The articles I read about LIGO detecting collisions between black holes said they were medium sized, meaning several solar masses. I kind of assumed that they wouldn't be able to detect anything orders of magnitude less than solar mass. Is that not correct?
Isn't this misinterpreting the results of that experiment? The Geiger-Marsden experiments established that most of the mass in an atom is concentrated within the nucleus, where all positive charge is located. It does not try to establish whether or not the area outside the nucleus is empty or not.
The (oversimplified) notion that the matter we typically deal with is mostly empty space is a direct consequence of that experiment, not an artifact of the Bohr model.
The wavefunction of the electron is “smeared” around the atom, which is not empty.
The wave function may be smeared out over space, but that does not mean that there is an electron everywhere. Electrons are never at two places, never has anyone looked and found the same electron at two different places at the same time.
What happens if nobody looks is up for debate, whether you think the electron has no position, whether you think it is guided along its path by a pilot wave, whether you think it split into zillions of copies in different universes, or whatever.
But the electron or many electrons filling out its entire orbit is not compatible with any of that or any experimental evidence. At least as far as I know, I am not a physicist.
I think the other comment addresses this somewhat and, of course, there is the endless verbiage online on all levels - you can't really explain the observed phenomena by treating the particle as little ball with a somewhat uncertain location.
> But the electron or many electrons filling out its entire orbit is not compatible with any of that or any experimental evidence.
On the contrary, imagining the electron as a smeared cloud (a complex-valued wave function with a bunch of specific properties), is pretty much the only way to explain chemical bonds.
The confusion stems from semantics. Sure, nobody claims that an electron has been detected at two spots simultaneously, not because it does not happen, rather because to say that you need the premise that the electron is a point particle with a position.
Sure, it is useful to claim that the electron is a point particle - in plenty of circumstances it does look like one. But modern physics pretty conclusively has demonstrated that it is actually something described by a wave function, that is most certainly smeared over space.
I am a physicist, so I will leave the discussion of whether "smeared" is a good word for it to the philosophers and linguists. It is a good enough word for me, when I try to derive properties of atoms.
I am not sure whether or not I want to agree with that, it sounds a bit like the claim that there are fields and not particles, just for single instead of many particle physics.
The full claim is "there are quantum fields", which is a different mathematical object from fields (or particles). To some it sounds unsatisfactory, to others it sounds profound, and yet others are happy to just shut up and use the model to do calculation that just work.
The full claim is "there are quantum fields", which is a different mathematical object from fields (or particles).
Sure, but the crucial points is, quantum fields are just mathematical tools, the universe is filled with particles, not with quantum fields stretching across all of spacetime. The fact that it is under some circumstances convenient to mathematically treat a large collection of particles as a quantum field does not make that quantum field a real thing, just as describing the traffic flow on roads with fluid dynamics as a mathematical modeling tool does not mean there actually exists a car field spanning across all the roads.
[...] yet others are happy to just shut up and use the model to do calculation that just work.
This certainly seems to works in many circumstances, but I think it is not really good enough to treat the point raised in this thread. When you are discussing whether an atom is mostly empty space, you have to be more careful. Just because your mathematical model uses objects that fill the entire atom does not necessarily mean that this is true for the actual atom, it could just be an artifact of your mathematical model.
I can not argue with you about that, because I consider it more a question for philosophers, not scientists.
However, please also accept that for many people (me included) the model of quantum fields is much more real, and the model of point particles is the one that is just a mathematical illusion.
There is no one-to-one correspondence between quantum fields and physical states due to gauge freedom and therefore quantum fields can not be real things. No experiment can yield the value of a quantum field at some point in space and time without fixing a gauge.
We invented quantum fields to make locality obvious but this is not possible without making the description redundant and so we just declared that one physical state corresponds to an entire equivalence class of mathematical states.
So I really don't think this is a matter of philosophy, they are not equivalent models.
I've read this a lot about gravitational lensing, but I'm not clear on whether we would be seeing lensing if most black holes were sand-grain-sized or smaller.
It is actually when the primordial black holes are too large, that we can't find them with lensing, we know the mass density of dark matter and if the black holes are large on average, then we would not see them with lensing because lensing events would be too rare. If the black holes are smaller, then there are a lot of lensing events and we can detect them. At somewhat larger scales one expects to see black holes due to interactions with stars.
That leaves a rather small window in between were black holes as dark matter works, but nobody has postulated a plausible mechanism to only generate black holes in a narrow mass range, so the idea can be made to work, but fell out of fashion because the wimp miracle looked theoretically more attractive.
The wave function is a mathematical construct that doesn't actually have anything to say about the actually of a particle. It is, in essence, the probability of where you might find the entity if you make measurements.
As such to say that the electron is "smeared" around the atom is not saying anything meaningful about the electron. All we can say is that we don't know where it is. To attribute a dispersal property to the electron (or any other particle for that matter) is not doing anything helpful to discover or understand what an electron is.
I doubt we could detect the lensing of these black holes so close to the galaxies they orbit. I think we would, perhaps, eventually see something falling into one.
edit: someone pointed out the gas in the galactic halo is not dense enough to form accretion disks around PHBs, so, no. We wouldn't see anything glowing.
I recently went to a talk from the author of https://arxiv.org/abs/1709.06576, which claims that PBH's can't make up the majority of the dark matter because the merger rate would be much higher than what LIGO observes.
Particles like that would be ordinary matter. In the quantities necessary to produce the effects attributed to dark matter, we would be able to detect them easily.
Even if they remained "dark" - not emitting any light of their own - they would block or otherwise interfere with the light coming from everything behind them.
The reason PBHs can be a dark matter candidate is because they can compress enormous amounts of matter into a very small space, and are difficult to detect when they're not interacting with ordinary matter.
black holes do not emit appreciable amounts of radiation themselves. Their accretion discs do. That in turn requires mass influx large enough to form an accretion disk with enough friction to radiate. The gas in the galactic halo and for that matter even the interstellar gas in the center of the galaxy[0] is just too thin on average to provide that. We only see them when they're draining mass from a binary companion or swallowing a gas cloud.
"Could my one fudge factor be used to legitimize my other fudge factor?" "Could this legitimize my career as philosopher of fudge factors?" Lots of important questions...
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[ 0.21 ms ] story [ 144 ms ] threadWhat the authors are suggesting is that this "dark matter" effect is actually the result of many small black holes orbiting galaxies. That they aren't exotic particles, just something we already know about in an unexpected place.
Is the unexpected element of this that it is weird to think that very small black holes interact with each other? What does it mean that they don't collapse into each other and rather stay individual?
I think it has to do with the number of black holes that would be required, the rate that black holes evaporate naturally(?), and our current best theories of the history and mechanisms of the universe. Like maybe in a universe where all these black holes form, we wouldn't expect to see [something we definitely do see]. So you need to adjust more variables to make that fit the puzzle, but now more things we do see don't make sense.
It's a bit like a big Sudoku, but with way more constraints!
You’re talking billions of billions of billions of years before most of them even start shrinking.
So objects usually need to be captured into mutual orbit first via 3-body interactions and then slowly move closer to each other via further perturbations or gravitational waves.
According to https://what-if.xkcd.com/129/ a moon-mass black hole would be as big as a grain of sand.
I don't know how far out LIGO can detect small-mass black-hole mergers, nor how likely these mergers would be. Nor do I know how detectable micro-lensing events would be.
The odd part about Star Wars is the relative weakness of their weapons. Star Trek have "warp core breaches in a bottle" AKA the photon torpedo, which is much better at damaging starships than ramming.
But as far as we know there are no "anti stars" out there that could collapse and form a black hole. We haven't observed any at least (don't ask me how we could tell an antimatter star from a regular one - but we could tell if one met a regular one).
We assume that our gravitational model is correct and that an additional item is needed to bring the model into agreement with the observations made. When a model seems to work so well, we are generally unable to go back and look at the basis (and assumptions) on which our theories are made. This applies in all areas of our lives, including all of our science. That is the nature of who people are.
The black hole as dark matter theory is also largely discredited (MACHO theory for MAssive Compact Halo Object) because gravitational lensing studies don’t support it.
While you can argue they are still flawed, they do clearly show that the electrons have fairly large probabilities of being observed pretty much anywhere inside the "radius" of the atom.
At this point, arguing about "are atoms mostly empty space" becomes a futile semantic argument. The model you picked in order to defend this statement is indeed useful in some limits, but it has been proven wrong. Imagining electrons as clouds has proven to be much closer to reality.
I have to concede, though, that "empty" is not a particularly meaningful physical concept, quite the opposite. Disregarding the Aether theory, only to replace it with fields of potentials, that is kind of running in circles.
A model is fundamentally wrong. That's what it means to have a model. At that "clouds" is no more helpful then "empty". The fact of the matter is that no one has ever seen these things, no nucleus, no electrons, as far as I know.
Yet, you haven't pointed out what was actually wrong with "Those are simply the electron orbitals which describe the probability of finding the respective election at a given point".
Edit: Perhaps you intended to imply that this description is overly idealized (not to say simple).
If you have too much external pressure (gravity), those smeared electron clouds come too close to the nucleus, react with it, and the only thing left is neutrons.
As a non-astrophysicist, just scanning the wikipedia entry [1] it appears to me that gravitational microlensing studies must be fiendishly difficult, you probably need massive amounts of data and of compute power to get to highly confident conclusions. Is my impression accurate?
If that is so, than the MACHO theory should not be discredited that quickly. It is after all the only theory that does not require any new physics in order to explain the missing mass.
[1] https://en.m.wikipedia.org/wiki/Gravitational_microlensing
No gamma ray signals as hoped/expected from evaporating PBH’s.
LIGO is not seeing PBH mergers at a rate that would support the numbers required.
Micro lensing studies don’t support it.
Is it’s possible, but it’s much less likely than other theories which allow for less fine tuning of parameters.
There’s a very good graphic that shows what I’m talking about. https://astrobites.org/wp-content/uploads/2017/08/constraint...
From: https://astrobites.org/2017/08/31/could-dark-matter-be-black...
It's the direct result of the Geiger–Marsden experiments.
The wave function may be smeared out over space, but that does not mean that there is an electron everywhere. Electrons are never at two places, never has anyone looked and found the same electron at two different places at the same time.
What happens if nobody looks is up for debate, whether you think the electron has no position, whether you think it is guided along its path by a pilot wave, whether you think it split into zillions of copies in different universes, or whatever.
But the electron or many electrons filling out its entire orbit is not compatible with any of that or any experimental evidence. At least as far as I know, I am not a physicist.
https://en.wikipedia.org/wiki/Double-slit_experiment
On the contrary, imagining the electron as a smeared cloud (a complex-valued wave function with a bunch of specific properties), is pretty much the only way to explain chemical bonds.
The confusion stems from semantics. Sure, nobody claims that an electron has been detected at two spots simultaneously, not because it does not happen, rather because to say that you need the premise that the electron is a point particle with a position.
Sure, it is useful to claim that the electron is a point particle - in plenty of circumstances it does look like one. But modern physics pretty conclusively has demonstrated that it is actually something described by a wave function, that is most certainly smeared over space.
I am a physicist, so I will leave the discussion of whether "smeared" is a good word for it to the philosophers and linguists. It is a good enough word for me, when I try to derive properties of atoms.
Sure, but the crucial points is, quantum fields are just mathematical tools, the universe is filled with particles, not with quantum fields stretching across all of spacetime. The fact that it is under some circumstances convenient to mathematically treat a large collection of particles as a quantum field does not make that quantum field a real thing, just as describing the traffic flow on roads with fluid dynamics as a mathematical modeling tool does not mean there actually exists a car field spanning across all the roads.
[...] yet others are happy to just shut up and use the model to do calculation that just work.
This certainly seems to works in many circumstances, but I think it is not really good enough to treat the point raised in this thread. When you are discussing whether an atom is mostly empty space, you have to be more careful. Just because your mathematical model uses objects that fill the entire atom does not necessarily mean that this is true for the actual atom, it could just be an artifact of your mathematical model.
However, please also accept that for many people (me included) the model of quantum fields is much more real, and the model of point particles is the one that is just a mathematical illusion.
We invented quantum fields to make locality obvious but this is not possible without making the description redundant and so we just declared that one physical state corresponds to an entire equivalence class of mathematical states.
So I really don't think this is a matter of philosophy, they are not equivalent models.
That leaves a rather small window in between were black holes as dark matter works, but nobody has postulated a plausible mechanism to only generate black holes in a narrow mass range, so the idea can be made to work, but fell out of fashion because the wimp miracle looked theoretically more attractive.
As such to say that the electron is "smeared" around the atom is not saying anything meaningful about the electron. All we can say is that we don't know where it is. To attribute a dispersal property to the electron (or any other particle for that matter) is not doing anything helpful to discover or understand what an electron is.
edit: someone pointed out the gas in the galactic halo is not dense enough to form accretion disks around PHBs, so, no. We wouldn't see anything glowing.
Try these lectures on for size:
[0]: linked directly to primordial black holes
https://youtu.be/vPHfvMQd73A?t=31m39s
[1]: previous lecture
https://www.youtube.com/watch?v=NJhANpBtUyQ
I recently went to a talk from the author of https://arxiv.org/abs/1709.06576, which claims that PBH's can't make up the majority of the dark matter because the merger rate would be much higher than what LIGO observes.
Even if they remained "dark" - not emitting any light of their own - they would block or otherwise interfere with the light coming from everything behind them.
The reason PBHs can be a dark matter candidate is because they can compress enormous amounts of matter into a very small space, and are difficult to detect when they're not interacting with ordinary matter.
[0] https://www.sciencenews.org/article/dozen-new-black-holes-fo...