PBS Space Time explains what happens as you approach and cross a black hole's event horizon, using Penrose diagrams and light cones as the tool for understanding spacetime there.
Why Hacker News points people here: Commenters call it a short, clean, visual explanation of the infalling-particle question and of Penrose diagrams.
“I think this is the cleanest explanation with actual visual representation.”HN comment, 2020
“Here's a really excellent and short video which attempts to explain this in ten minutes.”
Short, excellent ten-minute explanation of infalling particle at event horizon.
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The particle goes through the event horizon in finite time according to its own clock (and will then hit the singularity in finite term however it tries to travel). Think of the view of it by an external observer as being 'frozen' at the event horizon as kind of optical illusion caused by the extreme warping of spacetime. For a non-evaporating black hole (as described by general relativity), the 'last' photon coming off it will indeed be at infinity (or with infinite redshift). If (when) the black hole evaporates, the external observer will see that and of course there will be no more photons from the infalling particle. These two different views are hard to reconcile from our human perspective of how the world works, but they really do come from the mathematics of general relativity. Here's a really excellent and short video which attempts to explain this in ten minutes. (His explanation of how a Penrose diagram works is fleshed-out in more detail in the Susskind lectures I mentioned above). PBS Spacetime - What happens at the Event Horizon this video
Recommendsme_me_meOct 2020
“I think this is the cleanest explanation with actual visual representation.”
Cleanest explanation with visual representation.
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this video I hope it helps. I think this is the cleanest explanation with actual visual representation. Edit: Sorry this one is even better. How Time Becomes Space Inside a Black Hole | Space Time link
RecommendsneomMay 2021
“PBS Space Time to the rescue”
Event horizon explainer among PBS Space Time black hole videos.
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PBS Space Time to the rescue: What Happens at the Event Horizon? - this video What’s On The Other Side Of A Black Hole? - link How Time Becomes Space Inside a Black Hole - link The Black Hole Information Paradox - link
Cites itwillis936May 2025
“A fun tool to think around such things are Penrose diagrams.”
Penrose diagrams as tool for thinking about distant future.
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A fun tool to think around such things are Penrose diagrams. Personally I'm a little dubious of strong claims of what will happen in the distant future since we have such incomplete models of physics today. It takes GUTs to predict the future. this video
