Back when I was an undergrad in astrophysics one of my professors claimed that neutron stars are the most difficult objects to model in the universe. I believe him. In order to have a good model of a neutron star you need to use pretty much every branch of physics: general relativity, quantum field theory (both QED and QCD), fluid dynamics, and statistical mechanics. And these fields have important open questions in exactly the domains that you need to apply them. So you end up needing to solve not just one open problem, but a whole bundle of them, and they all conspire to work together to make each one harder than it would be on its own.
Consequently, not very much is actually known about neutron stars. One of the most important open problems in modeling neutron stars is just coming up with the most basic relationship you can, namely the "equation of state." This is simply a relationship between the density and the pressure within the neutron star. The equation of state of normal stars was effectively developed in the 19th century with the development of statistical mechanics. The equation of state of white dwarfs was developed by Chandrasekhar in the 1930s, not long after the development of quantum mechanics (and while Chandrasekhar was on a boat traveling from India to England!). But the equation of state for neutron stars is still unknown.
A major consequence of the equation of state is that it predicts the maximum mass of the object. In the case of a white dwarf, Chandrasekhar showed that the equation of state implied that the maximum mass had to be about 1.44 times the mass of the Sun (this is now appropriately known as the Chandrasekhar limit). But because the equation of state of neutron stars is not known, it is not known what the maximum mass of a neutron star is. Therefore, discovering more massive neutron stars is very important because it can possibly rule out certain equations of state. This, in turn, tells us a little bit more about what physical processes are going on in the neutron star.
Since the problem is so theoretically intractable astrophysicists usually have to neglect or simplify certain aspects of the physics considerably. If the equation of state that comes out of these assumptions can be ruled out, this means that one of these assumptions must be faulty, and some of the underlying physics that was neglected turns out to be important after all.
I worked on pulsars in grad school, and I always groused that I had to deal with ultrarelativistic plasma physics and pair-production in curved spacetime. There's a reason it's hard to come up with solid answers for these things.
Has anybody worked out yet how the polar "jets" work, or how they stay collimated over light years distant? How precisely would particle ejected need to be aimed to still be nearby other particles out that far? What could provide such precision?
For radio pulsars, I didn't think it was that mysterious. (Not like accretion-powered things, where I don't know how that works.) You've got an intense magnetic field that beams emission. At the low level any moving matter is pinned to a field line, like a bead on a wire, and that sets the direction.
You seem to be familiar with these things, so I hope you don't mind if I ask a tangential question: if dark matter is WIMPs, then there's a change that under sufficiently extreme conditions the WIMPs produce interactions with detectable outcomes. Maybe neutron stars offer just such conditions. Is anyone looking at neutron stars in search for evidence for dark matter?
That's a good question. I don't know of any specifics, but I wouldn't be surprised if someone used capture of WIMPs in neutron stars to provide constraints on the properties of wIMPs. I remember considering a project back when I was in grad school that would use Cepheid variables to provide constraints on the properties of axions. It seems plausible that you could do something similar for WIMPs and neutron stars.
Astronomical calculations never cease to amaze me.
I am the biggest astronomy nerd and watch loads of TV shows about it (the only thing apart from dinosaur stuff that I watch on TV actually!) and when scientists talk about the numbers involved in stellar objects it still blows my mind every time.
"Just a single sugar-cube worth of neutron-star material would weigh 100 million tons here on Earth, or about the same as the entire human population"
I struggle to visualize that...
On a side note, "'Neutron stars are as mysterious as they are fascinating', said Thankful Cromartie..." - Possibly the coolest name I've ever heard :)
Seems the PR people got the most important figure wrong, the measured mass is 2.14 +0.1 -0.09 M_sun, as per the preprint (and nature abstract[1]): https://arxiv.org/abs/1904.06759
The 2.17 M_sun figure is instead the hypothesised upper limit on the neutron star mass, derived from the equations of state that survive recent LIGO observations.
This neutron star would probably collapse into a black hole, but its fast rotations prevents it from doing so. It's like the movie "Speed", just with higher stakes...
If neutron stars can spin that fast, it's good that they don't fling fragments of themselves. Although, if they did, those fragments would likely turn to dust outside the star's gravity.
The more correct term is angular momentum. There are 'mechanisms' that can slow the angular momentum. Angular momentum is conserved, and as the star collapses from the original total mass/radius to its new much lower radius, resulting in a high spin rate. At some point the angualar momentum (or centrifugal force) perhaps will fall to some critical value where the star collapse further. I think this would be a valuable event in terms of figuring out the equation of state and the related unanswered questios the PO mentioned? It's been years since I studied phyisics at uni.... astronomy was always me fav.
Here's something that just popped into my head: If we could instanly beam a sugar-cube of Neutron Star material to earth (using the same measurements as in the article) what would happen to it?
Would it just fall right through whatever it's sat on or would it instantly expand into something less dense?
With the confining pressure of gravity gone, it would instantly and violently expand. The intense 10^32 pascals of degeneracy pressure drop quickly with expansion, but even just assuming it can expand 5 ml at that pressure, the energy released would be 5e26 joules. That's about 1000 times the asteroid that made the dinosaurs extinct. It would certainly imply the end of all multicellular life.
If somehow you could keep it confined, it would drop through whatever it was put on, through the entire Earth, and pop up at the other side. I suppose it would keep oscillating and eventually settle in the core.
It will cool into conventional matter eventually, but it would pass through a lot of exotic phases in between. I'm not sure we can even really answer that question, but here's something to give you an idea of what I mean: https://en.wikipedia.org/wiki/Quark%E2%80%93gluon_plasma I'm not sure it would necessarily pass through that, but it would at least make a decent pass at it in those first few nanoseconds.
If kmm's guesstimate of 5e26 joules is accurate, we're well beyond "extinguishing multicellular life" and in the realm of eliminating Earth entirely, which requires a minimum of 5.97e24 joules: https://en.wikipedia.org/wiki/Gravitational_binding_energy 1% efficiency of transfer to the Earth is probably attainable.
I think you might have mistaken the Earth's mass in kg with the gravitational binding energy. That last one is 2.5e32 J. So about a millionth of the neutron sugar cube.
Not a physicist, but: "expansion" is probably understating it. My guess would be something analogous to an insanely powerful neutron bomb. Interesting thought experiment, would love to see someone do the actual maths.
A 10cm3 cube of neutron star would weigh about a billion tonnes, comparable to a cubic km of water ice.
I wonder how all that mass would behave if left to expand with all the exotic physics involved - if it would just sort of... vaporize the planet.
Randall Munroe (author of xkcd) addresses this question in his excellent book "What If?", covering what would actually happen (basically a nuclear explosion), what would happen if it was somehow kept magically at the same density without exploding (it will end up oscillating around then settling at the Earth's core), and finally and most interestingly what it would be like if you could keep it near the surface and attempted to approach it.
As I understand it, there is some speculation by observers at LIGO/VIRGO that two neutron stars colliding with each other may eject neutron-degenerate matter into space before they coalesce into a single new compact object.
Not sure what that ejecta would look like...but if I had to guess, it would be release a shitload of energy as it came out from under compression, and would eventually condense into normal matter, much of it comprised of the sort of heavy elements not easily created via stellar nucleosynthesis.
Imagine a cloud of incandescent radioactive gold dust weighing as much as multiple earths, and you've got the idea.
"Mr. President, we finally finished our gravity gun project! It only fires inwards, so we have to convince the enemy to invade the Earth's core first".
This newly detected neutron star is 333,000 times the mass of the Earth and 2.17 times the mass of the sun. But the star is only about 15 miles across. This is close to the limit of how much mass a compact object can contain before it crushes itself into a black hole.
Assuming you are starting from orbit wouldn't you have to give that coin quite a "toss" to get it to change its orbit so as to hit the surface? No atmosphere to help...
I'm curious as well, for example to imagine how the event would unfold, as it would not be the direct remnant of a supernova. For an observer, would the neutron star seem to just disappear? Where would be the horizon and what would it look like as it suddenly appears?
I'm not a physicist, but I _think_ to my understanding what you would see is a collapse that would slow down and get more and more red-shifted, as the photons from the collapse take longer and longer to get to you (and are robbed of more and more energy, hence the red-shift).
So what you would see is more of a slow fadeout as the object shrinks and gets redder and dimmer, until you can't see anything at all. In the limit as the object gets smaller and the photons get dimmer, you end up with something that emits no light and is the size of the black hole silouette.
Saying that's what you would see is a bit like saying that if you push something it will keep moving forever. It's the physicist's answer, assuming ideal conditions.
In reality, the initial collapse probably won't include all the mass of the object, including any dust or gas orbiting nearby it. So your lovely "fading from sight" black hole will be there, but obscured by the raging maelstrom (always wanted to use that word) of material now circling the plug-hole around it and heated to $DEITY knows what temperature.
Imagine a far future game where bored teenagers grab the family spaceship, find a neutron star just below the blackhole limit, and throw things into it until it makes the switch. How fun would that be?!
You couldn't actually do that. Time dialation means the thrown object would never actually hit the neutron star from your POV and the black hole would never form.
2. What would be medieval would be a superstitious reaction to units of measurement as if they were purity religion cult signifiers rather than simple rational concepts to be converted between at will, depending on the task at hand.
Who do you think ruled Britain until 410AD and give the English their mile, which is simply the word for "1000" in Latin?
And I think that the press release is in error here. There was no radius measurement proposed in the Nature Astronomy paper. However, the radius in the press release is far to large. See:
Something on the order of 6-10 miles for a 2.14 M neutron star is much more appropriate.
Notice that this measurement may rule out some of our models (mostly the gray lines on the left of the chart) that don't predict neutron stars this massive.
The "correct" conversion to (modern) miles would be 20, both because 30 has only one significant digit and because it makes more sense to round 18.x to 20 than to 15.
OP here. I got that number from the CNN article [1], which they probably quoted for their US audience. It is neither medieval, nor meant to be accurate to the decimals.
I love HN comments, but this reminds me of the flame war threads that has very little relationship to the original subject :) Cheers!
I'd be more inclined to use leagues, which, when used at sea, in the English-speaking world, are usually three nautical miles or five point five five six kilometers.
So about 5.39 leagues across.
Since we use ships when we go to space and sea, I like the idea that space ships are naval.
When I read the miscorrection of kilometres I was hoping someone would convert the various quantities to fit the oft' overlooked furlong–firkin–fortnight [1] measurement system.
That's not quite right, we do know what black holes are made up of: the remains of a star. Well, if they are reasonably small they are made out of the remains of a star, if they are too big they can contain the remains of lots of stars and also gas, dust and maybe bits of planets, but exactly how we get the biggest black holes isn't clear.
The issue is rather that we don't know what all that star stuff becomes once it goes into the black hole, though there are theories. The problem is that the defining feature of a black hole, its even horizon, by necessity means there is no way of knowing what theory is correct since nothing happening inside is supposed to matter to us.
So before we can make any real progress on the interior of black holes we thus need to figure out exactly in what sense the event horizon leaks information, if it does.
In contrast, while we don't really know what happens to matter inside a neutron star, its surface remains visible to us and so we have been able to discard many different theories about what goes inside it. There is still debate about the precise equation of state, which correspond to different internal structure the various models represent and so different ideas of how a neutron star is made up.
Honestly, I don't know how much difference there are between the various models, but pretty crazy sounding ideas are still popping up, so there must be at least some space left.
> The issue is rather that we don't know what all that star stuff becomes once it goes into the black hole, though there are theories. The problem is that the defining feature of a black hole, its even horizon, by necessity means there is no way of knowing what theory is correct since nothing happening inside is supposed to matter to us.
Right. As far as "an observer at infinity" is concerned, all that in-falling matter gets asymptotically closer to the event horizon and never crosses it.
One way to reframe the situation which I quite like: Black holes have no insides.
I once figured, could most massive stars have a black hole at the center? With thr crazy time dilations around a BH, I imagine the implosion would capture the escaping image of the prior star. I imagine a microscopic black hole at the core of thessun that immediately evaporates, but immediately looks like an eternity for us
No, you need something releasing a lot of energy in the middle of a star, otherwise it collapses down on itself and stops being a star, the event horizon doesn't provide any supporting pressure.
Also, the process that compresses matter to the point it collapses to a black hole is very destructive; there is no reasonable way to form a black hole in the middle of the star.
> So before we can make any real progress on the interior of black holes we thus need to figure out exactly in what sense the event horizon leaks information, if it does.
So we need to find the Meltdown/Specter-like vulnerabilities of black holes? I wonder how many problems that would cause the universe.
Nothing comes out of a black hole, so we can't get observations from the inside.
(There are nit-picky theoretical ways in which a black hole can evaporate, keyword "Hawking Radiation", but as far as I know that hasn't been observed, because it's too weak and too far away. But even then we learn about its surface/event horizon, not its inside).
This is a "numerical observation", so a simulation. And not of a black hole, but of a model of a black hole in one dimension in a Bose-Einstein-Condensate.
Nobody has created a black hole in a lab and measured its radiation.
I specifically stated we have not seen it coming off of a black hole, but dismissing this as just a simulation is silly and reductionist. To do the same in a counter-argument, if I can accurately model fluid dynamics, I don't have to go drop a log in my bathtub to observe the effects.
But sure, if you want, we can discount that specific study and focus on others that use different methods to detect it that aren't "just simulations"
Also, because we understand neutron degeneracy pressure, but don't yet know what (if any) quantum effect would prevent an actual singularity from being reached once the upper limit of a neutron star was breached. There is speculation that quark matter and quark stars may exist but no evidence of it yet.
Personally I believe the Universe as we know it is the insides of a black hole, and that the predicted "singularity" is wrong. There is no reason why there would be a singularity there, anyone looking at an object smaller than its Schwarzschild radius will just see a black hole since that equation comes from calculating the curvature outside a homogeneous sphere and not outside a singularity.
That way the space expansion in the origin of the universe could just be the black hole forming and quickly eating more mass of its surrounding and thus expanding. Also would explain a lot of things related to lack of anti-matter and such since the part of the outer universe our blackhole ate wouldn't necessarily have an even distribution.
>Personally I believe the Universe as we know it is the insides of a black hole, and that the predicted "singularity" is wrong.
Well, regardless of whether or not our universe is contained within a black hole, basically no one thinks there is literally a singularity of infinite density inside of black holes. It's just what relativity states (and we already know relativity isn't a complete explanation, especially in regards to some of these incredibly extreme situations) and treating black holes like they contain incredibly massive quantities of mass while basically being a point-particle works for all of our day to day purposes.
String theory and loop quantum gravity both explain black holes without needing a singularity, and every astrophysicist that I've spoken to a read comments from discussing this topic has basically said "Yeah, it's not a singularity, we just don't know what it is, and that whole singularity thing is close enough"
It's only 4600 light years away. If it collapses in to blackhole, it would be the closest blackhole to the earth? In few 10s of thousands of year, can it devour solar system?
This is correct. It is a common misconception that the gravity an object exerts on it's surrounding increases when it collapses into a black hole.
What actually happens is that the object shrinks
- according the to the currently accepted models into a single dimensional point. The mass now being confined to a singularity, the gradient of the gravity as you get near becomes extreme, reaching infinity at the singularity. The gradient never could get so extreme before because the mass was diffused over a much bigger volume.
Imagine replacing the Sun with a black hole of equal mass. Apart from the lack of visible light, the gravity we here on Earth would experience would be exactly the same as before, as would every single planet that is currently orbiting. What would change is the distribution of mass inside the sun. The event horizon would be at about 6kms from the Singularity. You could now hypothetically approach the Sun to within ~10km from the center and still stay outside of the event horizon, but the gravity gradient at that point would be unimaginable. The difference in force applied to a the feet and the head of a hypothetical astronaut in orbit would disintegrate them into atoms, which themselves would possibly be squished into spindles.
With the sun as it is currently, even if you could somehow get to within 10kms of the center, the majority of the mass of the sun would be actually outside of this sphere so the net gravity you'd experience would be negligible (compared to the black hole scenario).
Not a physicist, but isn't the whole point of black hole that they can grow.
The way I picture it, is once you reach the black-hole state, there is a regime change which make it more easy to gulp matter in. I guess how big and how fast it grows depends how much mass there is nearby to concentrate.
Once formed they tend to an equilibrium with their surrounding dictated by the balance mass-in/evaporation-out.
Not really: (stellar) black holes have less mass than whatever object they were born from, and the mass is the only way by which they interact with matter.
It's true that they don't have a stellar wind that pushes gas away, but they are born in the void left by their progenitor star, and once they start accreting there will be radiation pushing things again.
Hawking radiation for a 1 solar mass black hole represents less than 1 atom of material lost per billion years (~10^67 years vs ~10^57 atoms) They gain more than that even without a noticeable accretion disk inside our current galaxy.
It’s only very very long time frames after black holes collect this material that they start losing mass on average.
A black hole is a... hole, in terms of gravity[0]. Things fall into it. Unless you're not in a stable orbit, or being washed into it by gas (like water going down a drain), it's not going to 'suck' you into it.
Yes I was aware of this misconception, but I was thinking that there could be strange effects, like tide effect ripping away the stars passing nearby. I was also thinking of the possibility of the hole getting wider transforming once stable orbit into non-stable one which will end-up inside the black hole.
From what I've gathered, evaporation seems to be a really slow process. So I figured that it only gets bigger. The remaining question is how fast.
We know there are very big black holes, though they were formed when conditions were different. I was wondering if there were possibility of a small one slowly by accretion growing into one, and was thinking maybe that if you are already very close to the black hole limit, maybe the neighborhood is also crowded and you could start a snow-ball effect.
There is also the dark-matter/dark-energy issue if it exists, when you are a neutron star, you can pass freely but once a black hole you should collect it as you go.
> Black holes have the same gravitational effects as any other equal mass in their place. They will draw objects nearby towards them, just as any other planetary body does, except at very close distances to the black hole. If, for example, the Sun were replaced by a black hole of equal mass, the orbits of the planets would be essentially unaffected. A black hole can act like a "cosmic vacuum cleaner" and pull a substantial inflow of matter, but only if the star from which it formed was already having a similar effect on surrounding matter.
The article refers to a neutron star being a giant atomic nuclei. How did this happen? The mass of the neutron star and the star before the collapse are similar. so why did the matter change into atomic nuclei? This makes me think that gravity is not the only thing at play here; density also matters. Is my this correct? So, is gravity dependent on the density? I guess I am confused.
The mass of the neutron star is _not_ the same as the entire star before it. The neutron star is (mostly) core material from its parent star, the rest of the matter was likely flung out upon that stars death. Gravity is not the only thing at play, strong nuclear forces between atoms prevent the star from collapsing under its own gravity, as well as the angular momentum of the stars spin. The neutron star is one giant atomic nuclei in the sense that all of its atoms are incredibly densely packed together. Imagine a styrofoam ball that you put under immense pressure from all angles (say by putting it deep under water). The air/free space in the styrofoam is squeezed out, and this in a sense is the same thing that happens to the plasma inside the parent star's core as it collapses. All of the atoms are squeezed together, and a balancing act is created whereby the incredible density of the core is trying to pull the matter inwards (into a black hole), while the strong nuclear force/momentum are pushing back to prevent the star from collapsing further.
There's not really a way to convey it in human terms. It's as if the magnetic field flowing through your body had the same energy as if you ate a slug of mixed lead and anti-lead with the same volume as your body, and it was undergoing matter/antimatter annihilation while it was in your stomach.
The main reason is that while black holes are more "extreme" objects, they are actually physically very simple. The matter collapses to a singularity (as far as we know), and so the physics is just described by the Einstein field equations with a correction for quantum fluctuations near the event horizon. Remarkably, the entire state of the black hole can be described by only the mass, charge, and angular momentum of the black hole.
By contrast, although neutron stars are less "extreme", they end up in a regime where the physics becomes much more difficult. The gravity is strong enough that you still need general relativity, but because the object is extended rather than a singularity, there's also a bunch of "stuff" which needs to be described as well. This means that you need to add in quantum electrodynamics and quantum chromodynamics. Furthermore, it's thought that at these densities you end up with some sort of a superfluid which means you need fluid dynamics and statistical physics to model its behavior, all of which is extremely difficult.
That said, there is another sense in which we could say that black holes are actually more mysterious than neutron stars. Although the physics of neutron stars is very complicated, we at least know what all the fundamental equations are. In the case of black holes if we want to understand physics near the singularity we would need a theory of quantum gravity which we currently don't have.
It is entirely possible that there could be different types of neutron stars, although the same physics will describe them all. As an analogy, white dwarfs all have the same equation of state, but based on the mass of the white dwarf the electrons can either be relativistic or non-relativistic. (This is not really a true dichotomy, it's just a spectrum.) There could plausibly be something similar with neutron stars.
Consequently, not very much is actually known about neutron stars. One of the most important open problems in modeling neutron stars is just coming up with the most basic relationship you can, namely the "equation of state." This is simply a relationship between the density and the pressure within the neutron star. The equation of state of normal stars was effectively developed in the 19th century with the development of statistical mechanics. The equation of state of white dwarfs was developed by Chandrasekhar in the 1930s, not long after the development of quantum mechanics (and while Chandrasekhar was on a boat traveling from India to England!). But the equation of state for neutron stars is still unknown.
A major consequence of the equation of state is that it predicts the maximum mass of the object. In the case of a white dwarf, Chandrasekhar showed that the equation of state implied that the maximum mass had to be about 1.44 times the mass of the Sun (this is now appropriately known as the Chandrasekhar limit). But because the equation of state of neutron stars is not known, it is not known what the maximum mass of a neutron star is. Therefore, discovering more massive neutron stars is very important because it can possibly rule out certain equations of state. This, in turn, tells us a little bit more about what physical processes are going on in the neutron star.
Since the problem is so theoretically intractable astrophysicists usually have to neglect or simplify certain aspects of the physics considerably. If the equation of state that comes out of these assumptions can be ruled out, this means that one of these assumptions must be faulty, and some of the underlying physics that was neglected turns out to be important after all.