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Scientists discover the highest energy gamma-rays ever from a pulsar

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Re: Scientists discover the highest energy gamma-rays ever from a pulsar

#51

Post by raattgift »

ben_w wrote: post #49
raattgift wrote: post #40

Earlier quoted context omitted.

What motivates your first factor? 0.782343 MeV is the free neutron beta decay; where in the solar system are the free neutrons minutes after they are magically teleported to terrestrial ground zero as a something like a (degenerate, possibly ultra-relativistic) Fermi gas? I think most attempts to arrive at an answer will end up somewhere between half and virtually all of them being "not very close" (~ light-minutes)…

> I think most attempts to arrive at an answer will end up somewhere between half and virtually all of them being "not very close" (~ light-minutes) away Mean free path of free neutrons moving past normal matter is only in the order of centimetres, exactly how many centimetres depends on the neutron energy and the specific nuclei it's interacting with, but still order of centimetres. Given the relative masses, I can…

I don't know enough about neutron physics to comment usefully on your mean free path logic, but I do know that solar eruptive activity can launch relativistic neutrons at Earth which can be detected even at sea level using scintillators, and that mountaintop detection has been around since the early 1980s. Shibata 1994, Propagation of Solar Neutrons https://sci-hub.se/https://doi.org/10.1029/93JA03175>, §4.2.1 (Fig 3) higher energy neutrons get further into the atmosphere, so I don't think the atmosphere is much of a barrier for the comparable (MeV-GeV) teleported neutron-star neutrons.

We seem to agree that free neutrons don't stay free neutrons when they slam into the solid earth.

I too wanted to think about neutrons as a non-self-interacting gas, but that just doesn't work: Meyer 1994, https://ned.ipac.caltech.edu/level5/Sept01/Meyer/Meyer3.html (Paragraph beginning with, "Only the strong gravity of the neutron star keeps such matter from exploding apart." Cold in this context is partly explained in the preceding paragraph; in inner regions the matter is a degenerate gas meaning the particle kinetic energy becomes dependent on the density or equivalently pressure becomes independent of temperature; even at enormous pressures, degenerate gases don't hold much thermal energy -- that was practically all radiated away when the NS was young. Our teleporting (of inner region matter) therefore engages a very low-entropy r-process.

Outer regions are just too complicated and varied for a HN comment. The crust is thin -- a few to a few hundred metres or so compared to an NS radius of ~ 10km. It's also much less dense, so is a small fraction of the NS mass, and thus maybe not a target for our teleportation. Here's a 180-page open access review: https://link.springer.com/article/10.12942/lrr-2008-10 Pesky electrons and protons complicating things.

Re: Scientists discover the highest energy gamma-rays ever from a pulsar

#52

Post by tomohelix »

FluffyClouds wrote: post #21
tomohelix wrote: post #14

Earlier quoted context omitted.

I agree. I actually think that is a very common thought, at least from what I have seen with my colleagues who are mostly scientists in engineering and biotech. It could be because of our background but the kind of complexity and seemingly genius engineering solutions in some biological systems just boggle the mind and make us question how it happened. We often handwave them with evolution and sure we can come up wit…

We are a tad beyond handwaving, but with all the information overload it's hard to recognize scientific breakthroughs anymore. Anyway thermodynamics did it. https://arxiv.org/abs/1203.3271 Most people won't like this answer, because it's too impersonal. Thus the search for a more inspiring origin story continues. There is no drama in thermodynamics, no love, no revenge, although there are explosions.

That is a great paper! Yes, from my engineering school I know if we dig deep enough, it would all be thermodynamics. But I was not sure if anyone dig that deep yet. This paper seems to be deeper than what I thought was done. Strange how I didn't see it, seems very relevant especially with all the LLM rage these days.

Re: Scientists discover the highest energy gamma-rays ever from a pulsar

#53

Post by ben_w »

raattgift wrote: post #51
ben_w wrote: post #49

Earlier quoted context omitted.

> I think most attempts to arrive at an answer will end up somewhere between half and virtually all of them being "not very close" (~ light-minutes) away Mean free path of free neutrons moving past normal matter is only in the order of centimetres, exactly how many centimetres depends on the neutron energy and the specific nuclei it's interacting with, but still order of centimetres. Given the relative masses, I can…

I don't know enough about neutron physics to comment usefully on your mean free path logic, but I do know that solar eruptive activity can launch relativistic neutrons at Earth which can be detected even at sea level using scintillators, and that mountaintop detection has been around since the early 1980s. Shibata 1994, Propagation of Solar Neutrons https://sci-hub.se/https://doi.org/10.1029/93JA03175 >, §4.2.1 (Fig…

Thanks for all three links; it's getting late here, so this is only going to touch on the first part of your message.

> Shibata 1994, Propagation of Solar Neutrons https://sci-hub.se/https://doi.org/10.1029/93JA03175>

If I'm reading that figure right, at sea level the attenuation is at least a factor of 2000 for all energies they're graphing. That sounds about right to me.

I realise now that I may have been unclear in intent previously: if you look at figure 2, and then consider a typical solid or liquid's cross sectional mass density, hopefully that explains why I was speaking of neutron mean free path of centimetres — 100g/cm^2 is 1m of water.

However this is just the initial condition, and I don't think this scenario is one where the atmospheric density can be accurately approximated as constant over time.

Re: Scientists discover the highest energy gamma-rays ever from a pulsar

#54

Post by swayvil »

tomohelix wrote: post #4
asicsp wrote: post #2

> These dead stars are almost entirely made up of neutrons and are incredibly dense: a teaspoon of their material has a mass of more than five billion tons, or about 900 times the mass of the Great Pyramid of Giza Five billion tons packed within a teaspoon volume!? Incredible.

When it comes to cosmic scales, it is literally unimaginable by the puny human mind. I don't know about others but I frankly can't really comprehend what does the mass of an entire mountain condensed into a single pebble mean. And there are stars full of this stuff. And there are stars spinning so fast its magnetic waves would rip the iron out of my blood and destroy the physics that hold my atoms together just becau…

Your ideas about the world are drawn from what you see. Maybe you see something they didn't see. Maybe they see something you didn't see.

Re: Scientists discover the highest energy gamma-rays ever from a pulsar

#55

Post by swayvil »

ourmandave wrote: post #30
hollander wrote: post #15

Earlier quoted context omitted.

> I frankly can't really comprehend what does the mass of an entire mountain condensed into a single pebble mean. Maybe turn it around. On a cosmic level, that mountain is nothing but a fluff of cottonwool, the earth a hot air balloon.

Or turn around again. Zoom out far enough and maybe there's a view where the pulsar is comparably no more that a lit sparkler. Note: This isn't an argument for god or whatever. Just thinking on scales where our "laughing in the rain" is a nightmare for ants.

>This isn't an argument for god or whatever...

It better not be. This is a proper orthodox forum.

Re: Scientists discover the highest energy gamma-rays ever from a pulsar

#56

Post by raattgift »

ben_w wrote: post #53
raattgift wrote: post #51

Earlier quoted context omitted.

I don't know enough about neutron physics to comment usefully on your mean free path logic, but I do know that solar eruptive activity can launch relativistic neutrons at Earth which can be detected even at sea level using scintillators, and that mountaintop detection has been around since the early 1980s. Shibata 1994, Propagation of Solar Neutrons https://sci-hub.se/https://doi.org/10.1029/93JA03175 >, §4.2.1 (Fig…

Thanks for all three links; it's getting late here, so this is only going to touch on the first part of your message. > Shibata 1994, Propagation of Solar Neutrons https://sci-hub.se/https://doi.org/10.1029/93JA03175 > If I'm reading that figure right, at sea level the attenuation is at least a factor of 2000 for all energies they're graphing. That sounds about right to me. I realise now that I may have been unclear…

I wouldn't sweat it, and I don't know enough about the nuclear physics to keep up (and we haven't even been talking about the neutrino energy in beta- decays, the gamma spectrum, or what becomes of the electrons; resonances go way over my head). This isn't really a gravitational problem (but...footnote [1]), so I'm not so useful here.

So, more for the original questioner than for us:

What's inside a neutron star stays inside a neutron star. Unless of course the NS is destroyed via e.g. collision, tidal disruption, or infall pushing it over a mass limit like Tolman-Oppenheimer-Volkoff. Sci-Fi teleporters don't exist, and there's no basis to think they ever will.

The closest neutron stars are between hundreds and a thousand light-years away and IIRC all the close ones are isolated (in the sense of no stellar multiplicity; they have no binary partner(s)).

Consequently what ben_w and I have been yakking about is inaccessible to experiment (we can't generate the relevant pressures, and artificial neutron sources are not very bright yet (pardon the BrightnESS pun, https://europeanspallationsource.se/about>)).

It's not accessible to astronomical observation either. The closest physical phenomenon I can think of is an NS mass ejection (for which there is an ample and active academic literature), and that's far from a close match. At least in some parts of the spectrum we can see a large NS mass ejection -- large meaning somewhere around 10% of the mass of the sun -- but there's practically no hope to see just a spoonful, and not hurled into a close-by planet's atmosphere or even that of a noncompact companion star.

So the answer to the question ultimately is -- if we imagined the magical arrival of a small ball of NS matter on Earth at rest on the Earth's surface -- "complex nuclear physics" is in the details of the practically-instantaneous kaboom, and a lot of that complexity is because the Earth is not the practical vacuum around a neutron-star/neutron-star collision that ejects a lot more than a spoonful of material.

- --

footnote [1]: I mean, one can think of it in terms of Raychaudhuri's equation (and that's where I started, in fact): the initial radial divergence of the acceleration vector from the sudden release of pressure dominates, causing the bits to tend to fly away beyond the hope of recollapse. But the solid earth (and as the thread involved, considerations of nuclear interactions even in the atmosphere) generates enormous shear via contact forces, so some of the energy-density of the NS matter will stick around, and in due course what wasn't ejected "to infinity" settles back to a basically round Earth (hydrostatic equilibirium returns). From this perspective comparing the NS matter with an asteroid impact makes sense to me, but probably undersells the nuclear fallout.

Re: Scientists discover the highest energy gamma-rays ever from a pulsar

#57

Post by johndunne »

AnimalMuppet wrote: post #32
johndunne wrote: post #27

Earlier quoted context omitted.

Black hole's aren't matter, they're pure gravitational binding energy. A neutron star becomes a black hole when the neutrons pushing against each other can't push back at the gravitational forces (neutron degeneracy pressure) and the neutrons do something we're not sure of... but whatever happens, they're crushed down into something smaller than a neutron star; into a singuality and we see the result.. a black hole.…

> Black hole's aren't matter, they're pure gravitational binding energy. Could you expand on this a bit? What exactly do you mean, and what is your basis for saying that it's true?

Black holes are not made of matter… the matter has collapsed into pure energy. The form of energy is a mystery (it’s inside the event horizon) but since the gravitational field persists, it’s often referred to as pure gravitational binding energy. I graduated physics at Manchester Uni and I’ve still got a ‘preference’ to be as correct as possible when talking about BH’s and what they’re ‘made’ of. Kip Thorne also often refers to the stuff BHs are made of as ‘gravitational binding energy’ so I thinks it’s safe to do the same.
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