How do black holes swallow stars? (2021) | Hacker News Reader

How do black holes swallow stars? (2021)

astronomy.com

41 pointsmalshe25 comments

I do like the evolution of black hole illustrations. They change every few years, mostly based on depictions by hollywood, the latest being Interstellar. The current trend, the symetrical ufo-shape, is scientifically more accurate than the previous whirlpool but isnt perfect. It doesnt show the red/blue shifting. If you were actually looking at one with human eyes, you would probably see a rainbow effect. One side would be blue/white other red/black, with variations inbetween. But every attempt at such rendering just doesnt look as intimidating or magnificent.

It is like dinosaurs on TV. Over the years they have gone from naked lizards, to having feathers, to being almost furry, and now back to naked again as hollywood balances science with audience expectations.

This sort of thing. https://twitter.com/redshift3d/status/1156595374183854080?s=...

Incredibly beautiful - I wonder how (and how long) it will be that we can get better real images of black holes, the most extraordinary objects we know to exist.

It will be very tricky to get better images. Black holes are very small and very far away.

Current best bet is using our sun as gravitational lens, but with current technology it would take 2-3 decades to get satellites far enough (few times further than New Horizons is currently) for that to be possible.

Gravitational lensing is one option, but another is the Terrascope concept whereby the Earth's atmosphere would be used as a refractive lens. In theory, any atmosphere would do. It may even be possible to use Jupiter's outer atmosphere. As refraction is stronger than gravitational lensing, such schemes would not require multi-decade trips out focal points at edges of the solar system.

https://arxiv.org/abs/1908.00490

Can you explain why gravitational lensing is important?

It allows for star-sized telescoped, lenses bigger than stars that can focus objects very far away.

The best bet in our lifetimes will be by luck. There is a chance that gravitational lensing of a distant object might give us a glancing view of a black hole if thing line up just perfectly. But the chances of this happening, and that we actually are looking at the right place/time, are very very small.

They did do some test renders for Interstellar with better fidelity. You can see some in this article [1] or in the paper they wrote on their renderer. They were simulating film response rather than human retinal response, but it’s probably close enough.

[1] https://cerncourier.com/a/building-gargantua/

The idea that Interstellar "taught" us what black holes look like is way overblown and oversold. John-Pierre Luminet and others were drawing black holes back in the 1970s.

https://blogs.futura-sciences.com/e-luminet/2018/03/07/45-ye...

https://blogs.futura-sciences.com/e-luminet/2019/05/23/40-ye...

https://blogs.futura-sciences.com/e-luminet/2019/06/12/40-ye...

> The current trend, the symetrical ufo-shape, is scientifically more accurate than the previous whirlpool but isnt perfect. It doesnt show the red/blue shifting.

A bit of a technicality, but the frequency shifting would only occur for rotating black holes but not non-rotating black holes. However, as far as I understand, non-rotating black holes are assumed to not exist in reality.

If we are getting an image, it is a rotating black hole. The light we see came from somewhere and didn't fall straight into the event horizon. Therefore it imparted some rotational energy as it dipped in and out. Therefore, any black hole emitting anything more than hawking radiation has at least some angular velocity.

> Therefore, any black hole emitting anything more than hawking radiation

What else does a black hole emit besides Hawking radiation?

> Therefore it imparted some rotational energy as it dipped in and out.

You're bringing up an interesting point. So any photon passing by a Schwarzschild black hole should actually set the BH in motion/rotation. Then again, I would expect these effects to cancel each other out, given a somewhat random distribution of incoming photons?

> idea that Interstellar "taught" us what black holes look like is way overblown

Teachers don’t invent mathematics when they teach addition, they distribute known knowledge. Interstellar absolutely taught many people a more-accurate image of a black hole. They did not discover Doppler beaming or the accretion disk’s wrap around, sure, but I haven’t seen that claim made explicitly as much as heavily implied.

Taught was maybe a bad word choice. If you watch the promotional materials for Interstellar, you get a strong implication that they were the first to show what a black hole looks like, but that is not the case. People have picked up on that on that vibe and think that the tech behind the movie gave us the first pictures. There certainly was an evolution there, but it wasn't the start of it.

No doubt Interstellar popularized and helped re-energizze discussions about black holes, especially since it came out just before LIGO started making discoveries, which propelled things even further.

https://www.wolfram.com/customer-stories/academy-award-visua...

This has better background on it.

"This relativistic jet was aimed directly at Earth — and astronomers were staring directly into the beam."

Great. Something else to worry about.

Seriously, though: are jets like these generally detectable and would we know if there are any of them making their way out from the center of our own galaxy?

> Great. Something else to worry about.

Anthropic principle applies here - the universe is huge and there's already tons of dangerous things pointing at Earth. The only reason we've evolved enough to notice the danger is because we have protection from it.

We rather had protection. Anthropic principle tell us nothing about our future. Only about our past.

Well sure, but after many billions of years of the protection working and with no known reason to suspect that that's changed, it's not something I'm actively worried about.

Nope. They move at the speed of light. Our first detection of such a death start might be our oceans boiling. But the reality is that astrophysical jets come out at right angles from the rotational plane of the black hole. All the biggest black holes rotate in the same plane as their host galaxy. We are far away in the outer spiral arms, in the plane. Any dangerous jet will likely be at 90 degrees from us, missing earth by many thousands of light-years.

Nit: Relativistic jets move at significant fractions of the speed of light, but not at the speed of light.

The material of the jet is sub-light, but the radiation emitted by that material is lightspeed. If I throw a rock at you it is not at lightspeed. But if I heat first that rock to a billion degrees the light it is emitting will it will burn you before it hits you. The material of a jet is like a million-degree rock an AU across. We will be ashes long before any physical material hits earth.

> We will be ashes long before any physical material hits earth.

Earth is a moving target, so will physical material hit Earth after we have been incinerated by the radiation of the jet? Or can we get hit by a physical material after we had dodged radiation?

Is there a reason black holes only rotate in one plane? I would naively have expected a 3D rotation to result from their formation.

Nice write up, with a good explanation of how stars are “eaten” by black holes but the page is full of ads.

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