Don't mind me. Just illustrating a text on the river model for black holes. Some fish swimming at the "speed of fish", in a river whose flow speed gets faster from right to left. H is the "river horizon". An external observer will never see a fish that is to the left of H swim to the right. 🧪🔭⚛️ - ThreadSky

mpoessel.de • 97 days ago

Don't mind me. Just illustrating a text on the river model for black holes. Some fish swimming at the "speed of fish", in a river whose flow speed gets faster from right to left. H is the "river horizon". An external observer will never see a fish that is to the left of H swim to the right. 🧪🔭⚛️

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moistenedtart.bsky.social•97 days ago

I am a person who knows nothing about this, and so I’m going to ask a very uninformed question, if there’s no possibility for an external observer to ever see a fish swimming left to right then why talk about it?

mpoessel.de•97 days ago

Fish do swim to the left or to the right in the water. It is just that relative to the river bank, the flow is so fast to the left of H that even a fish swimming at maximal speed (what I call the "speed of fish") to the right will still be carried to the left, because the water is flowing faster 1/

mpoessel.de•97 days ago

there than the fish can swim. That is fairly exactly analogous to what happens near a black hole: In a way, if you cross the horizon, space is "flowing" into the black hole so fast that not even light going the other way can ever make it outside the horizon. 2/2

moistenedtart.bsky.social•97 days ago

That is so very cool! Thank you so much for explaining that☺️
I write speculative fiction so I’m always interested in this sort of thing even if I’m incredibly ignorant about it. 🙏🏻

mpoessel.de•97 days ago

You're welcome! I should have explained more instead of just dumping the picture here. And the simplified picture with the river works very well, I think. (I didn't come up with it, there are articles about this like https://arxiv.org/abs/gr-qc/0411060.)

moistenedtart.bsky.social•97 days ago

I have another silly question, that’s probably basic physics, I assume black holes are named that way because they don’t let light out again.
My question is, why does light travel at all?

moistenedtart.bsky.social•97 days ago

That’s okay!
I’m glad you shared it because having physics explained in these ways is very accessible to lay folks like myself who still want to learn but find it difficult to approach!
I’ll take a look at that link.

oldebill.bsky.social•97 days ago

Douglas Adams would bow to you, sir.

mpoessel.de•97 days ago

[blushes] I am but a mere cobbler of vector graphics elements in the presence of the master!

kwsteve.ca•97 days ago

If the river flows faster than the fish can swim, can we infer that it is possible for particle matter to travel faster than the speed of light?

mpoessel.de•97 days ago

No. Faster-than-light travel in this picture would be fish swimming *faster* than the usual speed-of-fish. They don't, relative to the river. In general relativity, you will never measure relative speeds faster than light. At the horizon, infalling speeds approach c.

kwsteve.ca•97 days ago

Thank you, Markus. 🍻

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