# Light curving around vortex simulating a black hole

• I
TL;DR Summary
OP is unconvinced that light cannot pass through a dense hole in a water vortex. They want to know how a water vortex has the same physics as what happens in a black hole, as claimed in a documentary.
So I just had a question. In a Netflix documentary, they showed these people trying to simulate the conditions of a black hole by having a vortex in a pool of water. they said that light cannot pass through the hole and the physics around is essentially adjacent to the event horizon and what happens to a black hole.

I guess I don’t really understand this. is the hole where light doesn’t pass not just a shadow cast from the inward curving water? how is the hole so dense that light doesn’t pass through? Could someone explain using how light interference and diffraction works with the vortex and how light must not pass through?

As you say, a water vortex does not behave like a black hole. I would say either you misunderstood what the documentary writers were saying or they misunderstood what they were supposed to say. Or they were documenting nutcases.

Did this documentary say who "these people" were?

Ibix
Assuming @Baluncore is referencing the same topic as the documentary, the reported experiment is about water waves picking up energy from orbiting a water vortex. Light waves travelling around a black hole can also pick up energy, and the paper thinks there's a useful mathematical analogy between the physics of water waves near a vortex and light waves near a black hole.

So light can pass quite happily through a water vortex. Water waves, however, are affected in a way the authors say is similar to light waves near a rotating black hole.

Last edited:
Ibix said:
As you say, a water vortex does not behave like a black hole. I would say either you misunderstood what the documentary writers were saying or they misunderstood what they were supposed to say. Or they were documenting nutcases.

Did this documentary say who "these people" were?
I may have strongly misunderstood then, but the people in the documentary kind of made it seem it was directed that way. One person specifically made the analogy of predicted behavior in a black hole (light bending, hawking radiation, superradiance) that is not experimentally verified and can be modeled with the vortex, and continues to say “in the end we see an effect that has been predicted without experimental confirmation. that’s real physics. It has been detected” which suggests the model with water was used to experimentally verify phenomena we see in a black hole - which to me doesn’t make sense how you can go ahead and make that comparison in such enthuasiasm?

Unless they didn’t mean that, and meant in a way we can compare water in the vortex and light with black holes (as you said) in some mathematically equivalent analogy. Although the tone and phrasing just feels very imprecise and misleading.

The documentary was Black Holes: The Edge of All We Know on Netflix.

I think that's probably a slightly exaggerated claim.

If you make a mathematical prediction like "there is superradiance around black holes" and we haven't yet tested it then there are two basic lines of attack. One is "general relativity makes incorrect predictions near black holes" and the other is "you made a mistake in your application of it". The latter is depressingly easy to do.

But if you can show that there are formal mathematical analogies between the maths of water waves around a water vortex and the maths of light near a black hole then you can do something towards countering the second line of attack. You can do an experiment and show that the predicted effect occurs - which at least means that you haven't messed up the maths.

So that these phenomena occur in water waves near vortices is an established fact. And I'd say this kind of approach is support for the existence of this equivalent phenomena around black holes. It's true to say that it shows that such phenomena aren't just an artefact of our analysis. But I would also say there's a bit of a gap between "they definitely occur in water wave situations that are mathematically the same as the maths of black holes" and "they definitely occur in black holes".

• Astronomy and Astrophysics
Replies
14
Views
2K
• Astronomy and Astrophysics
Replies
6
Views
2K
• Astronomy and Astrophysics
Replies
5
Views
1K
• Special and General Relativity
Replies
51
Views
446
• Astronomy and Astrophysics
Replies
15
Views
3K
• Special and General Relativity
Replies
4
Views
455
• Special and General Relativity
Replies
8
Views
989
• Astronomy and Astrophysics
Replies
129
Views
17K
• Special and General Relativity
Replies
67
Views
3K
• Special and General Relativity
Replies
57
Views
1K