What keeps earth from collapsing in on itself? (2010) | Hacker News Reader

What keeps earth from collapsing in on itself? (2010)

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Electron degeneracy pressure: https://en.wikipedia.org/wiki/Electron_degeneracy_pressure

Stars above 8 solar masses can turn into neutron stars or black holes because during their life they have two forces counteracting gravitational collapse: electron degeneracy pressure and thermal pressure from fusion. When the fusion stops you’re left with a massive body where the electron degeneracy pressure alone isn’t sufficient to counteract gravity. https://en.wikipedia.org/wiki/Chandrasekhar_limit.

You get a neutron star instead of a black hole when the gravitational collapse can be overcome by the strong force and neutron degeneracy pressure (the same forces that keep the nuclei of atoms from collapsing).

See this more general description of the fundamental forces: https://en.wikipedia.org/wiki/Fundamental_interaction

Interesting! I recall being taught that it was the electromagnetic repulsion between the electron shells of atoms, but that was probably in high school. I took quantum mechanics in college but don't remember touching on the particular question of stellar collapse. Somewhat surprising that this question wasn't really answered until the late 60s.

I was going to say the same thing about neutron degeneracy pressure, so I'll add a thought experiment that I haven't seen discussed or answered yet online. This is probably a really old debate, and is maybe settled, but not in layman's terms:

Imagine we have a neutron star that's 1 atom away from collapsing into a black hole. What does that mean? There are 2 edge cases of interest: 1) when an event horizon begins forming at the core, and 2) when it swallows the surface.

1) Starting with the core event horizon, we can imagine dropping an atom on the surface of the star, and pressure passing the neutron degeneracy pressure limit at its center. A tiny black hole appears in the core, roughly the width of a neutron, and all of the neutrons start slipping down the drain into it, making the horizon bigger and bigger until the star is eventually swallowed.

Except that time passes more slowly in a deep gravity well. It crawls almost to a halt at the center of a neutron star, and halts completely when the event horizon at the core forms. Time may even seem to be running backwards inside the event horizon from our frame of reference.

So my feeling is that this is the point where quantum mechanics stops and relativity takes over. We can treat the event horizon as a black box trying to evaporate outwards with Hawking radiation as hard as the star is trying to collapse into it. In other words, the event horizon feels solid to the star around it.

The test of this is to see how Hawking radiation for the smallest black hole compares to the pressure at the center of a neutron star at the brink of collapse. I'd predict them to either be off by orders of magnitude or exactly equal.

2) Carrying that idea forward, as we add more atoms, the event horizon grows bigger and bigger until it finally covers the surface and we see the star red shift redder and redder until it approaches black.

What happens when we pass the tipping point and 1 final atom causes the event horizon to surround the entire star? Is it like a one-way door, where the black hole stays that way forever?

Well, we can test that by waiting for as much Hawking radiation energy to evaporate as the mass of 1 atom.

Then the neutron star red shifts less and comes back into view. In other words, time gets faster again, the event horizon retreats, and the star blue shifts back into being visible. The longer we wait, and the more Hawking radiation is emitted, the smaller the event horizon gets until it finally disappears completely and the star goes back to being a neutron star.

By this logic, with time running slower and slower, then faster and faster, is it more likely that the neutrons within the event horizon suddenly turn into pure energy and collapse into each other to a singularity? Or is it more likely that they stay how they are, frozen in time, then go back to how they were when the energy representing their mass leaks out in Hawking radiation and then back into the form of a neutron?

This is the central question not being adequately addressed IMHO.

Now, I don't think that I have broken any rules of physics talking about black hole formation this way. But I'd vote for the latter case where nothing happens to the neutrons, the event horizon just moves outwards and inwards within the neutron star as it gains and loses mass. This may happen in real life when they suck in mass from a neighboring star.

What happens if a white dwarf or neutron star gains a lot of mass this way quickly? Any potential energy remaining in atoms which haven't been reduced to neutrons yet quickly falls beneath the electron degeneracy pressure, goes through the high-pressure slowed space on and within the neutron star, fuses instantly and explodes. Which is called a nova.

So even in the center of an ordinary star like our sun, the high-pressure slowed space presses atoms close enough together to fuse. The rate of fusion is proportional to how slow that space is. The slower the space, the faster the fusion.

Going beyond edge case 2, if we add a lot of matter and it falls beneath the event horizon, it's going to take a long time to evaporate back out, potentially trillions of years. Which is why most black holes that formed naturally probably have so much mass that we'll never see them blue shift back to our frame of reference.

But, I'm probably wrong. Another interpretation is that the slower space at the center of a neutron star is actually longer. So we can imagine the distance to the core getting further away as the star gains mass. When the event horizon forms, the space there starts falling in away from us faster than the speed of light. Meaning that Hawking radiation may not be strong enough to hold the neutrons out. The gravity gradient passes 90 degrees and they fall straight in as if nothing is beneath them.

If that's the case, then the neutrons fall through that pin prick rip in spacetime, maybe turning into pure energy. Their energy comes out the other side where time is running backwards from our frame of reference, so space is ballooning out at the speed of light, possibly with one less spatial dimension, although my gut feeling is that the z axis is just flipped so the number of dimensions probably stays constant.

This would be a white hole and look very much like the birth of our universe in the big bang. Inflation would be the period when spacetime is still flowing faster than the speed of light within the event horizon, until it reaches the radius of the event horizon in that frame of reference and slows below the speed of light again.

The child universe within the black hole would last until the parent black hole evaporates. Inside, it would look like there's an outward pressure causing the child universe to expand (dark energy), powered by the Hawking radiation of the parent black hole. This would push matter outside the child universe's visible universe, whose radius is equal to parent black hole's event horizon radius from our frame of reference, causing the rate of expansion to increase. Eventually more and more matter would escape, lowering the mass of the child universe until the edge of the universe rushes in, pulling an observer in the child universe apart in reverse spaghettification as it gets pulled back into our universe when the parent black hole evaporates completely and explodes. Which might be our universe's fate in trillions of years.

Again, I'm probably wrong about this stuff. But I'm approaching it as a software developer who just thinks of the problem as a bunch of edge cases to test. It would be helpful to know the improper steps in my thought process so that a working solution can be found.

Sorry this got so long, it might have been better to put it on a wiki somewhere.

Edit: added speculation about the source of dark energy.

Edit 2: the time direction in the white hole paragraphs doesn't seem quite right. For example, if time is running backwards inside, then I might have the inflation and reverse spaghettification timestamps reversed.

Apologies for replying to my own post, but I should have included sources. Here is the best stuff I've found so far:

https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g

https://www.youtube.com/playlist?list=PL39_ud5aKSvkl_wj5BIkR...

https://jila.colorado.edu/~ajsh/insidebh/waterfall.html

There is an episode of How the Universe Works (I think) where they talk about the multiverse looking like an infinite block of swiss cheese, and each black hole having a child universe inside with its own physics, sealed off from the parent universes. So we're in the black hole where the rules of physics allow us to exist to realize it. In other words, the multiverse used black holes to evolve the rules of physics until consciousness could exist. It's another interpretation of the anthropic principle so isn't really testable, but I thought it was elegant. Wish I could find it.

Edit: some places to start:

https://www.youtube.com/watch?v=KePNhUJ2reI

https://www.youtube.com/watch?v=3DhDSjgw3j8

Lee Smolin's 1997 book, "The Life of the Cosmos" was where I first learned about the idea of universes evolving through black holes. I don't recollect if it was his idea, but it is certainly creative.

The event horizon does not expand until it consumes the neutron star, because the Schwarzchild radius is smaller than the radius of the neutron star. The neutron star collapses into the smaller black hole.

The matter cannot be considered neutrons anymore at that point, because it is too dense. Once neutron degeneracy pressure is overcome, they’re not individual neutrons anymore.

Once the black hole forms, the original matter never comes out again, even if it loses mass to Hawking radiation. The neutron star never comes back into view.

Hey you're probably right. I'm regretting what I wrote after rereading it. I'm just trying to find stepping stones to get from our sense of everyday reality to a mental model of what happens when a black hole forms. Like doing discovery on a big software project that someone else wrote. But it's all speculation and hearsay that has little to do with actual cosmology, because I'm a programmer not a physicist. This fringe stuff would have a better place in a book or sci fi. Too much streaming and social media, not enough study. Sorry if my words mislead anyone.

What allows neutron stars to overcome the degeneracy pressure? Does the electron get so close to the proton that it gets absorbed and then the pressure disappears?

Yes, essentially. Degeneracy pressure arises as compression forces electrons into higher energy states to avoid being forced into the same quantum state (Pauli Exclusion principle). But at a certain point the gravitation pressure becomes so strong that protons capture electrons, producing a neutron and an electron neutrino, and relieving the electron degeneracy pressure. See: http://csep10.phys.utk.edu/OJTA2dev/ojta/c2c/neutron/neutron...

What happens to the neutrino at that point? Does it stay with the neutron star?

Initially they’re trapped but they play a major role in the supernova explosion that accompanies stellar collapse: https://indico.cern.ch/event/657167/contributions/2677895/at...

Yes but after exceeding ~2 solar masses (Tolman–Oppenheimer–Volkoff limit) the neutron star will undergo further collapse to form a black hole.

This is a thoughtful comment but IMO your phrasing is too strong and potentially misleading.

As far as science is concerned, the jury is still out on stellar nucleosynthesis. The leading theory - that gravitational collapse causes fusion near the core - may be entirely incorrect, and there is mounting evidence suggesting so including a plethora of modern observations that the theory fails to explain.

For example the so-called ‘coronal heating’ problem has existed for 75 years, since the solar corona was first demonstrated to contain plasma with temperatures of 1 million degrees kelvin and above, much higher than the photospheric surface temperature of approximately 6000 K [1].

As another stunning counterexample, current gravitational models do not explain observed properties of the solar wind like spatial variation and periodicity in time (thanks voyager 1 & 2), among many other things.

See [2] for a dense intro to more unresolved problems in solar physics.

Alternative (non mainstream) theories like “plasma cosmology” are gaining traction in the scientific community for this reason. Note though that many papers do not use that particular phrase to describe their work.

Unfortunately (for intellectually curious folks), the contributors to Wikipedia pages on these physics topics are extremely … biased. Reading [3] may leave you with the impression that “plasma cosmology” has been completely debunked by the scientific community, when in fact nothing could be further from the truth ([4][5] are examples to support this claim).

[1] https://royalsocietypublishing.org/doi/full/10.1098/rsta.201...

[2] https://www.ias.ac.in/article/fulltext/joaa/029/01-02/0003-0...

[3] https://en.m.wikipedia.org/wiki/Plasma_cosmology

[4] https://link.springer.com/article/10.1007/s00159-013-0062-7

[5] https://agupubs.onlinelibrary.wiley.com/doi/pdfdirect/10.102...

I'm certainly no physicist. I'm just recounting the explanation I remember from high school quantum mechanics.

Shorter answer, with simpler vocabulary:

The Earth is basically a solid hunk of rock, with a core of dense metals like iron. Solid rock and iron just don't collapse under pressures or temperatures that are even slightly sane. (Unless they are hollow, have an empty space under them, or are falling down into something less dense - like an undersea landslide does.) Sure, the pressures and temperatures in the middle of the Earth are high - but we're pretty much talking pressures that some C-list college physics lab could maintain for days on end, and temperatures lower than an old arc welder you can buy on eBay could do forever.

Stars that actually collapse - not just cool off, shrink, and fade away - have pressures and temperatures inside that are a million miles from sane. We're talking "a hundred-billion dollar atom smasher, that is 15 miles wide, could do that - but only to a speck of dust, and for a trillionth of a second". Vs. a star that is about to collapse has had millions of cubic miles of stuff under those pressures and temperatures, for years.

Under those sorts of conditions, atoms get smashed. Really smashed. We're talking the difference between taking a box of packing peanuts and lightly patting 'em down (inside the Earth), and putting that box of packing peanuts into a big stamping press in a steel mill (inside the collapsing star).

The statement about particle colliders is not really correct. The relativistic heavy ion collider, where I used to work, is about 2.4 miles in circumference, and reaches temperatures in the 10s of trillions of degrees in its collisions. This is > 1000 times hotter than a supernova. The environment in these collisions is made to reproduce the state of the universe fractions of a second after the Big Bang, and aren’t really comparable to any normal stellar activity.

Any thought on what sort of hardware would actually be necessary to reproduce the temperatures / pressures / densities of a sleepy little 1.4 solar mass red supergiant pre-supernova iron core, for a full 10e-12 seconds, in the full volume of a visible speck of dust?

(At least in Wikipedia's version of things, the temperature inside a nice, fresh hot-off-the-Collapse'O'Matic-grill neutron star is "from around 10e11 to 10e12 kelvins". Not quite your comfort zone, but far closer:)

Why is the earth's crust so stable? It has supported life for billions of years, yet it is just 12 mi thick [1] and floats on top of liquid rock that extends for thousands of miles -- the part of earth we are most familiar with is a tiny fraction of what our planet really is. You could imagine that turbulence in the liquid below would cause currents that blast through the crust with catastrophic consequences, and that this would happen regularly. But it doesn't -- we have a small number of (super) volcanos, but they are pretty rare. I find this unnerving and remarkable. Our perception of reality -- in this case the earth beneath us -- is skewed.

[1] https://en.wikipedia.org/wiki/Structure_of_Earth

"Liquid" does not describe the Earth's mantle - https://en.wikipedia.org/wiki/Structure_of_Earth#Mantle It is more like cold pitch - https://en.wikipedia.org/wiki/Pitch_drop_experiment

Agree that "rock made weak and ductile under intense heat and pressure" would probably be better than "liquid", but common language and daily experience do not apply. Whatever the dynamic processes are doing, they must have symmetry and be stable -- hot plumes of stuff don't punch through the contents in random locations.

something that is counter intuitive is that although the mantle is often described as liquid, that description is only valid on large scales (both space and time). for comparison, the mantle is significantly less liquid than cold asfault. it's just subject to huge pressures.

> asfault

asphalt?

N. K. Jemisin wrote a triple Hugo winning trilogy[1] about what could happen to humanity with a sliiightly less stable crust, if you want some more unnerving ;)

[1] https://en.wikipedia.org/wiki/N._K._Jemisin#Broken_Earth_ser...

Yet it's thankfully not that stable as is Venus's crust, otherwise we'd have much more carbon in atmosphere, so persistent hothouse effect and surface temperatures of 100+ Celsius.

My favorite new thing I learned from this was:

> while larger stars will form neutron stars (which are essentially giant atomic nuclei)

I had never heard that description before, and it's fascinating to think about!

Neutron stars behave somewhat like giant nuclei, but only somewhat. We don’t even really good models of how their cores behave, maybe it’s a quark-gluon plasma, but we just don’t have a good enough model of matter to know.

Short answer, it's a solid hunk of rock. Stars are not. Stars are a mass of incandescent gasses; a gigantic nuclear furnace where hydrogen is smelt into helium at a temperature of millions of degrees.

Gaseous planets and stars will collapse into something solid (or explode). Not all stars collapse to explode though. They have to be 1.4 times the size of our Sun to do that. https://en.wikipedia.org/wiki/Chandrasekhar_limit

Actually, the sun is a miasma of incandescent plasma. Forget what they told you before.

https://tmbw.net/wiki/Why_Does_The_Sun_Really_Shine%3F

I see what you did there.

https://www.youtube.com/watch?v=3JdWlSF195Y [They Might be Giants - Why does the sun shine?]

Fascinating. Definitely very different environments. Stars are not places where we could live, but here on Earth there would be no life without the light our sun gives.

> Stars are not.

Stars core pretty much are. Or rather, "solid" is a bad name because it is a specific name for material that is much more compressible than what is there.

Mass, not size.

Trying to keep it simple for those who would ask the question in the first place and head off the follow-up: "What's the difference between size and mass?"

When discussing compression, the difference between size and mass is kind of important. :)

Since we're on this subject, one question I've never seen answered is why neutron stars have such intense magnetic fields. I mean in the normal universe we inhabit, certain chemical elements are magnetic while others are not. How can neutron star matter have magnetism when there are no electrons in orbitals anymore? I mean that seems like a good question, how a relatively well understood phenomenon like pulsars seems to depend on electromagnetic behavior that can't be explained by Maxwell's field equations.

My take before reading the article was that “earth is already collapsed”. After reading the article, this still sounds like a fine explanation. Am I wrong?

Weird. Why would anybody expect Earth to collapse at all, or to wait billions of years to collapse, or to collapse, but Jupiter not collapse first? Or not wonder why their own head doesn't?

Why? Because

* people need to be educated, and this article does a good job

* different forces dominate at different sizes

For a skull, gravitational forces of a human head are tiny. Collapse is prevented because of skull rigid structure that supports itself: inward forces are resisted by resistance to deformation and pressure (similar to semi-spherical cellars and rounded bridges)

Earth has large gravitational forces, but resistance to compresson of its solid inner core and liquid iron outer core prevents collapse.

Stars are much bigger, and the larger gravitational forces are resisted by an additional force, next to resistance of compression gasses:

Nuclear reactions produce radiation in the form of photons that are streaming out from the star's interior creating outward pressure forces.

To simplify even more, both the Earth and your head would eventually collapse due to gravity if it weren't for electromagnetic forces pushing the atoms apart. I would be in free fall towards the Earth's center right now if it weren't for the atoms in my couch pushing the atoms in my butt upwards.

All I can say is better them than me.

Why wouldn't they expect it? It's a perfectly valid question to tease out curious aspects from our reality.

Why ask 'why' at all, about anything? In particular, why did you ask this question?

There is a plausible Darwinian story to be told about why generalized curiosity about causes abets fitness, but no-one would have come up with that explanation without asking quite a few questions about why things are as they are.

> There is a plausible Darwinian story to be told about why generalized curiosity about causes abets fitness…

HN has served it’s purpose this morning I can now go about my day (first order, of course: feed the cats). Bravo.

It's a kids'/popular science q&a site. I don't think it lines up well with the audience here, but it's a fine question for that site.

Yeah, the question might stem from someone overemphasising that stars don't live forever which lead to the generalisation that everything in astronomy collapses.

The Earth is the most dense celestial body in the solar system that weighs more than 10^20 kg.

Matter is mostly empty space, so enough for a collapse.

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