[HN Gopher] What sonic black holes say about real ones
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What sonic black holes say about real ones
Author : Phithagoras
Score : 38 points
Date : 2022-01-18 05:27 UTC (2 days ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| JackFr wrote:
| Absolute pure amateur here with no expertise or special
| knowledge, but could this work lead to new insights about the
| emergence of turbulent flows, or is thaat currently very well
| understood?
| Phithagoras wrote:
| Fluid mech on the whole has a lot left to solve.
|
| https://news.ycombinator.com/item?id=30011260 looks at this a
| bit.
|
| Author confirmed that superradiance, backreaction, and quasi-
| normal radiance are all observable in the fluid analog. These
| observations will likely prove useful for fluid mech as well as
| astrophys
| phreeza wrote:
| I think it's one of the situations where the fundamentals are
| well understood (Navier Stokes equation) but the emergent
| behavior is so complex that many aspects remain poorly
| understood.
| ncmncm wrote:
| My current question about black holes is:
|
| It is usually reported that the electron is a dimensionless
| point, with no extent. But it has mass. Why is it not, then, a
| black hole? A black hole with the mass of an electron would have
| an event horizon, which is not a dimensionless point. If it is a
| black hole, how does the charge "get out" to affect the outside
| world? If a black hole were given a charge, e.g. by injecting a
| stock of electrons, would it respond to magnetic fields, as an
| electron does, without being able to exchange photons with
| anything?
|
| Or, if the electron is not a black hole, why isn't it?
|
| An electron and positron annihilate on contact producing a pair
| of photons, which seems incompatible with their being black
| holes, unless maybe anti-particles have negative gravitational
| mass, so that contact gives the pair zero gravitation. Does it
| make sense to have positive inertial mass, but negative
| gravitational mass? I don't think that other particles (e.g.
| neutral, or positively-charged) smashed into electrons are known
| ever to get absorbed, as we might expect if it were a black hole.
|
| If the electron and positron are both dimensionless points, yet
| somehow not black holes, how do their paths ever succeed in
| intersecting? If there is some minimum distance where they
| annihilate, why isn't that the size of the particle?
| karlicoss wrote:
| Cool question, never thought about it! I'm not a physicist, but
| hopefully will be able to point at the right direction.
|
| Guess the short and boring answer is that we don't really know
| if GR applies at the scales of electrons (or to be more
| specific, how to apply it), so perhaps the notion of black hole
| doesn't even make sense, at least conventional sense.
|
| If you try speculating about it, there are some interesting
| points to ponder:
|
| 1. We know that black holes should evaporate via Hawking
| radiation. If you substitute electron's mass in the formula
| here [1], you get that electron mass black hole should
| evaporate in 6 * 10^-107 seconds. Clearly we don't see
| electrons doing that :) (however note that this formula assumes
| Schwarzschild solution, more about it further)
|
| 2. When you reason that if you narrow down electron's radius
| sufficiently, at some point is has to become a black hole, you
| kind of implicitly assume that it behaves according to the
| Schwarzschild metric [2]. This minimum radius at which you'd
| reason it's a black hole, would be the Schwarzschild radius.
| However, electron also has charge and angular momentum. Spin is
| quite different from a bunch of stellar stuff rotating around
| its axis, and I don't remember the derivation of GR solutions
| for spinning matter, but if we do speculate here anyway, still
| feels like we'd need to use the Kerr-Newman metric, which takes
| charge and angular momentum into the account [3]. If you use
| that metric, it's not the case anymore that any object with
| sufficiently small radius could form an event horizon -- if it
| has enough charge or angular momentum, it won't, so the
| electron won't be a black hole! However in a sense it only
| makes things works -- it exposes a naked singularity which we
| also don't seem to observe. P.S. the article also references
| "black hole electron" [4], didn't know it was a thing!
|
| 3. P.S. I also found this [5], which argues that even if you
| assume Schwarzschild radius for an electron, at this scale it's
| possible that the electroweak symmetry is restored, and the
| electron starts appearing massless. Can't say I'm fully able to
| follow this argument, so will just leave it here
|
| [1]
| https://en.wikipedia.org/wiki/Hawking_radiation#Black_hole_e...
|
| [2] https://en.wikipedia.org/wiki/Schwarzschild_metric
|
| [3]
| https://en.wikipedia.org/wiki/Kerr%E2%80%93Newman_metric#Ove...
|
| [4] https://en.wikipedia.org/wiki/Black_hole_electron
|
| [5]
| https://www.reddit.com/r/Physics/comments/7fd7pg/if_an_elect...
| alsonote wrote:
| Electrons' mass is a little strange, in my (limited)
| understanding. The Standard Model predicts that electrons
| should be massless, but they gain mass (or maybe easier to say,
| "massiness") through interactions with the Higgs field. This
| effectively creates a constant "drag" on electrons, giving them
| a kind of inertia or "mass". I think this video does a good job
| explaining it, much better than I could:
| https://youtu.be/kixAljyfdqU
| andrewflnr wrote:
| It's extremely aggravating that this article assumes the
| existence of Hawking radiation directly implies the real loss of
| information in black holes. There's been some work, probably
| covered in this magazine, on how information may escape black
| holes _via_ Hawking Radiation.
|
| Ed: for instance, https://www.quantamagazine.org/netta-
| engelhardt-has-escaped-... or https://www.quantamagazine.org/the-
| most-famous-paradox-in-ph...
| motohagiography wrote:
| This triggered my complete layman understanding as well, as
| this idea of information being destroyed, yet not annhialited
| via its anti-particles, or just being added to the mass of the
| singularity sounded like a logical misattrubution. there are so
| many places for it to "go." Everything else has turbulunce,
| especially during energy transtions, intuitively there seems
| like no obvious reason to expect gravity is exempt.
| PicassoCTs wrote:
| I have a (probably stupid) question - are the tunnels
| (https://www.indiatimes.com/technology/science-and-future/ear...)
| the solar system is currently in - connected to black hole at the
| heart of our galaxy?
|
| Like do black holes extend enormous magnetic tendrils into the
| surrounding space similar to neutron stars?
| dmitrybrant wrote:
| Neither black holes nor neutron stars extend any kind of
| magnetic tendrils. There can be jets of matter that are ejected
| from the vicinity of a black hole (due to the Penrose process
| or something similar), and those jets can have electric charge,
| but that's not what your linked study is about.
| [deleted]
| neom wrote:
| Is there a good word that describes the somewhat uncomfortable
| feeling one gets in their stomach when thinking about the
| magnitude and complexity of reality? Is humbled a feeling? I
| can't thinking of anything good in English, wondering if any
| other languages have this word (like how Saudade is in
| Portuguese)
| elorant wrote:
| The Greek word "deos" I think describes what you feel. A
| combination of admiration and fear, but also to recognize the
| grandeur of a higher power. It's frequently used for spiritual
| purposes. Awe is the English translation.
| Phithagoras wrote:
| Some might call it the sublime:
|
| "fascinated but also a bit unsettled" "existentially
| insignificant but strangely exalted" "delightful horror" and a
| "sort of tranquility tinged with terror."
|
| from https://erraticus.co/2018/12/04/sublime-experience-kant-
| scho...
| neom wrote:
| This is a fantastic article, thanks for sharing, I hadn't
| remotely considered sublime in this context, probably because
| they're my favourite band...sublime is great word in this
| context.
| prof-dr-ir wrote:
| Blaise Pascal needed a few sentences:
|
| "When I consider the short duration of my life, swallowed up in
| the eternity before and after, the small space which I fill, or
| even can see, engulfed in the infinite immensity of spaces
| whereof I know nothing, and which know nothing of me, I am
| terrified, and wonder that I am here rather than there, for
| there is no reason why here rather than there, or now rather
| than then. Who has set me here? By whose order and design have
| this place and time been destined for me? The eternal silence
| of these infinite spaces frightens me."
| krastanov wrote:
| Existential dread? It can be a pretty exciting feeling. I like
| how the Kurtzgesagt educational channel on YouTube has it
| permeate their cosmology/astronomy videos. They even addressed
| it explicitly in their "nihilistic optimism" video.
| kadoban wrote:
| If you like that feeling, read Stephen Baxter's work.
| Existential dread is the main character in a lot of it, it's
| quite something. The Xeelee Sequence especially comes to
| mind.
| krastanov wrote:
| My go to is works by Greg Egan (e.g. Diaspora and Schild's
| Ladder and Permutation City). Thanks for the suggestion, I
| will check out Baxter.
| kadoban wrote:
| Always glad to share. I'll check out Greg Egan, I've
| heard the name a bunch of times but never actually got to
| his work yet.
| arrow7000 wrote:
| He is my all time favourite science fiction author. If
| it's existential dread you're after he delivers it in
| spades.
| credit_guy wrote:
| You can flip this, and look like the bright side: all that huge
| complexity means lots of things to discover. Nobel prize winner
| Frank Wilczek writes in his last book "Fundamentals" that
| learning physics is like being born again.
|
| Kids have a lot of fun playing with water when they are around
| 3-5. Fluid dynamics is complex, and it takes our brains a long
| time to observe and get an intuition of it. All that
| observation, experimentation, (subconscious) hypothesis
| formulation, testing, adjustment, retesting is a rewarding
| experience.
|
| After we are about 7, we pretty much finish understanding the
| world around us. Leaves fall from trees, but not straight,
| sometimes they go this way and that because of the wind -> all
| this would have produce a sense of wonder in someone who's only
| 3 years old, but when you are 7 it's just old news. We switch
| to learning about the human world, via school. Sometimes it's
| fun, sometimes not, but whatever.
|
| But when you encounter quantum mechanics, or special
| relativity, or generalized relativity, or any number of topics
| in Physics, your intuition fails you big time. You need to go
| through a new learning process, just like when you were 3 years
| old. And that's a tremendous lot of fun.
|
| Now, here's the sad part: a person like Frank Wilczek probably
| knows everything about physics. All the ins and outs of all the
| physical theories out there, quantum field theory, quantum
| chromodynamics (he's basically one of its inventors), general
| relativity, you name it. For him the number of times he can
| experience the euphoria of such an intellectual rebirth
| dwindles by the day. But who knows, maybe that's what it looks
| like from our vantage point, maybe the world at the frontiers
| of knowledge is still full of wonders for Frank.
| tejtm wrote:
| terrible beauty
| undershirt wrote:
| "Scientism"
| _Microft wrote:
| Maybe you mean "awe"?
|
| https://en.wikipedia.org/wiki/Awe
| neom wrote:
| I was thinking about awe, it's probably the most accurate,
| but when you go to something like "saudade" - awe doesn't
| feel "deep" enough (not sure that makes sense)? Then again, I
| don't know that we tend to have particularly "emotionally
| deep" words in English... Awe is good though.
| _Microft wrote:
| Thanks for making me look up "saudade", it is quite
| different from what I had expected!
|
| Wikipedia has articles in different languages on it.
| stronglikedan wrote:
| No, it's deeper than mere awe. I've felt it once, when
| contemplating what eternal life would be like. I can see how
| people would feel it while contemplating the scale of the
| universe.
| hammock wrote:
| I read the wikipedia article for sonic black holes before reading
| this article, and got a decent understanding. Then I came back to
| this article and just wanted to comment that the graphics are
| really good and nailed my understanding in an even better way.
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