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

phys.org

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

#31

Post by euroderf »

c4mpute wrote: post #18
nullandvoid wrote: post #17

I have a question - what would happen if you were to drop one of these spoons of matter that weigh 5 billion tonnes onto the floor right now, where would it stop?

It would sink into the center of the earth and then oscillate around the center until friction stops it dead center. Materials in the earth's crust and mantle are not strong enough to stop that mass from sinking ever deeper. But that assumes that this spoon of matter were stable in that state, which it isn't. That kind of density can only be held up with some force like gravity keeping up the pressure. The gravity of…

35th-century science fair project gone very, very wrong.

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

#32

Post by AnimalMuppet »

johndunne wrote: post #27
Eddy_Viscosity2 wrote: post #22

Earlier quoted context omitted.

I've heard that black hole matter is stable at any size, so how much difference in density is there between neutron star matter and black hole matter where it crosses the threshold of stability from its own gravity?

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?

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

#33

Post by Jedd »

mikub wrote: post #8
tomohelix wrote: post #4

Earlier quoted context omitted.

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…

I find all this really incredible as well. But at the same time I also find it really 'mindblowing' that I am able to think about why and how all of this stuff was made, and while thinking about it, there will always come a point where you have to at least 'think' about an creator. Not that I think it was made especially for us, but it's still very strange. Somewhere I read when you think about the universe it is lik…

I suspect it's been used in many places - we need to summon that quote investigator! - but I did make a note of this when I read Surface Detail by Iain M. Banks:

"All you ever were was a little piece of the universe, thinking to itself."

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

#34

Post by colechristensen »

nullandvoid wrote: post #17

I have a question - what would happen if you were to drop one of these spoons of matter that weigh 5 billion tonnes onto the floor right now, where would it stop?

Taken out of the gravity keeping it together it would instantly explode in a sort of small supernova. There would be no survivors. It is not immediately obvious if there would be a planet left afterwards.

If it somehow stayed bound together it would start a sort of elliptical orbit inside the planet slowly losing energy and punching a long spiraling hole towards the center of the planet.

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

#35

Post by op00to »

automatic6131 wrote: post #28
op00to wrote: post #26

Earlier quoted context omitted.

It is now that I recommend you read Dragons Egg, a hard sci-fi book about a species that evolved on the surface of a neutron star. I am contractually obligated to mention this any time I hear neutron stars mentioned.

That must have inspired Stephen Baxter's Flux, which is about artificial humans, living just inside the skin of a neutron star.

thank you for the recommendation! :)

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

#36

Post by itishappy »

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.

That's us! We're not bathed in pulsar brilliance, we're picking up the sparse but energetic photons it emits from far far away. We cannot see the "highest energy pulsar" with our naked eyes.

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

#37

Post by itishappy »

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…

> When it comes to cosmic scales, it is literally unimaginable by the puny human mind.

One of my favorite mind-benders is that on some level, we embody cosmic scales of our own. The "biggest length" is our observable universe is 10^26 m. The "smallest length" is the Plank length at 10^-35 m. That puts the "middle length" at about a millimeter, or (very roughly) human scale.

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

#38

Post by stronglikedan »

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…

> what does the mass of an entire mountain condensed into a single pebble mean

I find it easier to comprehend in the subatomic context, picturing the space between particles like the space between stars and planets on the cosmic scale. Mash all the stars and planets together in, say, our solar system, and the entire solar system would only be about the size of the sun. Then repeat that for all the solar systems in the Milky Way, and the entire Milky Way would only be about the size of the black hole at the center. Relatively, of course!

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

#39

Post by nine_k »

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.

Matter is mostly empty space. Atomic nuclei are small, the electron clouds around them are pretty far away. From this point if view, ordinary matter is more like foam, or aerogel, with very little matter spread over a large volume.

Neutron stars are made if matter with very little empty space embedded into it. This is why they are so small.

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

#40

Post by raattgift »

ben_w wrote: post #20
c4mpute wrote: post #18

Earlier quoted context omitted.

It would sink into the center of the earth and then oscillate around the center until friction stops it dead center. Materials in the earth's crust and mantle are not strong enough to stop that mass from sinking ever deeper. But that assumes that this spoon of matter were stable in that state, which it isn't. That kind of density can only be held up with some force like gravity keeping up the pressure. The gravity of…

Very large asteroid, too big for a nuke: https://www.wolframalpha.com/input?i=%280.782343+MeV+%2F+neu... ~89 billion megatons of TNT equivalent

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) away, and that's assuming one corrects for the differences in escape velocities. (The equatorial escape velocity of a spinning neutron star is in tenths of the speed of light, thus the sobriquet "relativistic star"). Without this correction, it is likely the bulk of the expanding drop of Fermi gas just exits the atmosphere in milliseconds (timed by terrestrial stopwatches), with time dilation extending the mean lifetime of the free neutrons in the drop comparably to the extended lifetime of atmospheric muons from cosmic rays. The bulk of the beta decays happen at a distance from terrestrial ground zero best measured in astronomical units.

If we play Star Trek transporter games such that the neutrons arrive at ground zero at rest in local East-North-Up coordinates, you'd want to know the internal kinetic energy (KE) density of the (pure-)neutron star, which will be in the range of 20-40 for x in 10^{x} J m^-3. The 10^25ish or even 10^30ish joules of KE will be released from our several cm^3 spoonful practically all at once and practically omnidirectionally from ground zero (so again, most free neutron decays happen at ~ AU distances from ground zero because they'll zip right through the atmosphere). The expansion of the suddenly unpressurized gas of neutrons will make a mess, particularly the fraction that slams into and through the ground. Part of the mess is neutron scattering physics, and I have no expertise there, but I would guess there wouldn't be any free neutrons near ground zero (and probably not within the solid Earth) in ~minutes.

Additionally, one might compare the R-process https://en.wikipedia.org/wiki/R-process> for kilonovas in which a binary neutron star collision ejects high-neutron-density matter which decompresses pretty spectacularly, forming lots of heavy elements.

To summarize, I think the free neutron decay timescale (mean lifetime ~ 15 minutes, multiply by ln 2 if you prefer half-life) is simply too long after the neutron star material is teleported to Earth: any free neutrons that haven't been absorbed into heavy nuclei likely will be millions of kilometres away from ground zero when they decay.

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