What sonic black holes say about real ones

https://www.quantamagazine.org/what-sonic-black-holes-say-about-real-ones-20161108/

by Phithagoras • 5 years ago

47 31 5 years ago

JackFr• 5 years ago

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?

phreeza• 5 years ago

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.

Phithagoras2• 5 years ago

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

hammock• 5 years ago

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.

neom3• 5 years ago

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)

krastanov2• 5 years ago

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.

kadoban2• 5 years ago

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.

_Microft2• 5 years ago

Maybe you mean "awe"?

https://en.wikipedia.org/wiki/Awe

neom3• 5 years ago

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.

stronglikedan• 5 years ago

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.

Phithagoras2• 5 years ago

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...

neom3• 5 years ago

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.

elorant• 5 years ago

The Greek word "δέος" 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.

tejtm• 5 years ago

terrible beauty

handojin• 5 years ago

fearful symmetry

undershirt• 5 years ago

“Scientism”

prof-dr-ir• 5 years ago

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."

credit_guy• 5 years ago

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.

PicassoCTs• 5 years ago

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• 5 years ago

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.

andrewflnr• 5 years ago

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• 5 years ago

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.

ncmncm• 5 years ago

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?

alsonote• 5 years ago

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

karlicoss• 5 years ago

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...

Ygg2• 5 years ago

dimensionless point

According to QM. Electrons/positrons aren't a particle but much closer to a probability cloud.

Black holes are fundamentally modified space time. So much, much different than particles.

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?

Charge is one of few properties that black holes preserve.

If the electron and positron are both dimensionless points, yet somehow not black holes, how do their paths ever succeed in intersecting?

Same way two clouds interact. No black holes necessary. Analogy isn't perfect. Two interacting clouds don't obliterate instantly to produce different clouds.

ianai• 5 years ago

Think of a black hole as [edit] a region of [/edit] the entire universe curling 'up' to an edge. Nothing within the universe escapes the edge, but the curl of spacetime has a way of pulling additional matter-energy into it.

What sonic black holes say about real ones

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