What happens to TIME inside a black hole? (And why us physicists can’t agree)
From Dr. Becky.
Watch this video on YouTube ↗
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In a black hole, time might end, or loop, or become a place in space, or become wildly unpredictable. And us physicists cannot agree which one actually happens.
I think most people when they’re taught about black holes are taught a simplified version, that you fall in, and you end up in the middle crushed into an infinitely small point, a singularity. Which as you’re falling towards does weird things to time.
But that describes a version of black holes that don’t exist in the real Universe (the Schwarzschild solution).
Real black holes spin, and spin changes everything. Including the nature of the singularity itself, which becomes a donut shape (the Kerr solution).
At least, an infinitely thin donut. Which does even weirder things to time, perhaps even making time travel possible.
I’m going to walk you through first what is a black hole, what happens to time in a non-rotating black hole? What actually happens to time in a more realistic rotating black hole? And why physicists cannot agree on what’s going on, and hope that if we only had a theory of quantum gravity we might be able to solve this...
Papers mentioned:
Hamilton & Avelino (2010) - https://arxiv.org/pdf/0811.1926
Porras-Valverde & Forbes (2026) - https://arxiv.org/pdf/2610.27061
Wagner (2021) - https://itp.uni-frankfurt.de/~hees/gen-phys/derivation-of-the-kerr-metric-Wagner.pdf
Einstein (1915) - https://etienneklein.fr/wp-content/uploads/2016/01/Relativit%C3%A9-g%C3%A9n%C3%A9rale.pdf
Penrose (2002) - https://the-center-of-gravity.com/documents/145/Roger-Penrose_Gravitational-Collapse-The-Role-of-General-Relativity.pdf
Shah (2025) - https://nhsjs.com/2025/investigating-ring-singularities-at-the-centre-of-kerr-black-holes/
Previous videos:
https://www.youtube.com/watch?v=jRLNVvsxfFo
00:00 - Introduction
02:16 - I - What is a black hole?
06:07 - II - What happens to time inside a static black hole?
08:32 - III - What actually happens to time inside a realistic spinning black hole?
15:11 - IV - Why physicists can’t agree
20:03 - Bloopers
Video filmed on a Sony ⍺7 IV
Video edited by Martino Gasparrini: martino.freelance@gmail.com
Video produced by Marina Hui & Dr Becky Smethurst
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📚 My book, "A Brief History of Black Holes", out NOW in hardback, paperback, e-book and audiobook (which I narrated myself!): http://lnk.to/DrBecky
---
👕 My merch, including JWST designs, are available here (with worldwide shipping!): https://dr-becky.teemill.com/
---
🎧 Royal Astronomical Society Podcast that I co-host: podfollow.com/supermassive
---
🔔 Don't forget to subscribe and click the little bell icon to be notified when I post a new video!
---
👩🏽💻 I'm Dr. Becky Smethurst, an astrophysicist at the University of Oxford. I love making videos about science with an unnatural level of enthusiasm. I like to focus on how we know things, not just what we know. And especially, the things we still don't know. If you've ever wondered about something in space and couldn't find an answer online - you can ask me! My day job is to do research into how supermassive black holes can affect the galaxies that they live in. In particular, I look at whether the energy output from the disk of material orbiting around a growing supermassive black hole can stop a galaxy from forming stars.
http://drbecky.uk.com
Free UPDF download: https://updf.com/download/?utm_source=Youtube-drbecky-2610-liki&utm_medium=en&utm_campaign=liki202610&packageKey=overseapromotion
OCR with UPDF: https://updf.com/ocr-pdfs/?utm_source=Youtube-drbecky-2610-liki&utm_medium=en&utm_campaign=liki202610&packageKey=overseapromotion
In a black hole, time might end, or loop, or become a place in space, or become wildly unpredictable. And us physicists cannot agree which one actually happens.
I think most people when they’re taught about black holes are taught a simplified version, that you fall in, and you end up in the middle crushed into an infinitely small point, a singularity. Which as you’re falling towards does weird things to time.
But that describes a version of black holes that don’t exist in the real Universe (the Schwarzschild solution).
Real black holes spin, and spin changes everything. Including the nature of the singularity itself, which becomes a donut shape (the Kerr solution).
At least, an infinitely thin donut. Which does even weirder things to time, perhaps even making time travel possible.
I’m going to walk you through first what is a black hole, what happens to time in a non-rotating black hole? What actually happens to time in a more realistic rotating black hole? And why physicists cannot agree on what’s going on, and hope that if we only had a theory of quantum gravity we might be able to solve this...
Papers mentioned:
Hamilton & Avelino (2010) - https://arxiv.org/pdf/0811.1926
Porras-Valverde & Forbes (2026) - https://arxiv.org/pdf/2610.27061
Wagner (2021) - https://itp.uni-frankfurt.de/~hees/gen-phys/derivation-of-the-kerr-metric-Wagner.pdf
Einstein (1915) - https://etienneklein.fr/wp-content/uploads/2016/01/Relativit%C3%A9-g%C3%A9n%C3%A9rale.pdf
Penrose (2002) - https://the-center-of-gravity.com/documents/145/Roger-Penrose_Gravitational-Collapse-The-Role-of-General-Relativity.pdf
Shah (2025) - https://nhsjs.com/2025/investigating-ring-singularities-at-the-centre-of-kerr-black-holes/
Previous videos:
https://www.youtube.com/watch?v=jRLNVvsxfFo
00:00 - Introduction
02:16 - I - What is a black hole?
06:07 - II - What happens to time inside a static black hole?
08:32 - III - What actually happens to time inside a realistic spinning black hole?
15:11 - IV - Why physicists can’t agree
20:03 - Bloopers
Video filmed on a Sony ⍺7 IV
Video edited by Martino Gasparrini: martino.freelance@gmail.com
Video produced by Marina Hui & Dr Becky Smethurst
---
📚 My book, "A Brief History of Black Holes", out NOW in hardback, paperback, e-book and audiobook (which I narrated myself!): http://lnk.to/DrBecky
---
👕 My merch, including JWST designs, are available here (with worldwide shipping!): https://dr-becky.teemill.com/
---
🎧 Royal Astronomical Society Podcast that I co-host: podfollow.com/supermassive
---
🔔 Don't forget to subscribe and click the little bell icon to be notified when I post a new video!
---
👩🏽💻 I'm Dr. Becky Smethurst, an astrophysicist at the University of Oxford. I love making videos about science with an unnatural level of enthusiasm. I like to focus on how we know things, not just what we know. And especially, the things we still don't know. If you've ever wondered about something in space and couldn't find an answer online - you can ask me! My day job is to do research into how supermassive black holes can affect the galaxies that they live in. In particular, I look at whether the energy output from the disk of material orbiting around a growing supermassive black hole can stop a galaxy from forming stars.
http://drbecky.uk.com
