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Zihan Yan

@physics-cat

Theoretical Physics PhD student @ Cambridge Black Hole Thermodynamics and Quantum Gravity https://inspirehep.net/authors/2757519

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16.11.2024
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Latest posts by Zihan Yan @physics-cat

If I got enough money, I’ll start my own non-profit theoretical physics institute! Let’s do it together then

28.02.2026 02:12 👍 0 🔁 0 💬 1 📌 0

Me. Want to learn QFT properly

04.02.2026 02:36 👍 2 🔁 0 💬 0 📌 0
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Academia is brutal. This year I’m shortlisted by 5 places for postdoc positions, got high up on the waitlist, and then got no offer. The first-round offer holders all had other offers, but that cannot be traded. Academia has no safety net. I’m probably out. This is more cruel than hearing nothing…

03.02.2026 16:06 👍 1 🔁 0 💬 0 📌 0

Just passed my PhD viva!

27.01.2026 12:52 👍 1 🔁 0 💬 0 📌 0

27啦!下周就要博士论文答辩,悬而未决的postdoc申请也即将尘埃落定,不知道还能在象牙塔里面待多久。新的一岁,新的开始,新的探索,新的希望。

25.01.2026 01:32 👍 1 🔁 0 💬 0 📌 0

Looks super interesting! Will apply!

06.09.2025 21:49 👍 1 🔁 0 💬 0 📌 0
Preview
Generalised focusing theorem and dynamical horizon entropy in diffeomorphism-invariant theories I summarise recent progress on light-ray focusing and horizon thermodynamics in general diffeomorphism-invariant theories of gravity coupled to bosonic matter. In pure gravity and with scalar or vecto...

That’s the story! If you’re curious about generalised focusing, entropy, or horizon dynamics—let’s discuss!
Full paper: arxiv.org/abs/2509.00628 (8/8)

03.09.2025 14:20 👍 2 🔁 0 💬 0 📌 0

Next steps: extend these results to semiclassical & nonlinear regimes.
Excited to see how “entropic geometry” reshapes our view of spacetime & gravity. (7/8)

03.09.2025 14:20 👍 0 🔁 0 💬 1 📌 0

For higher-spin fields, the theorem holds conditionally. I propose a “higher-spin focusing condition” as a criterion for physical consistency. (E.g. 3D sl(3,R) higher-spin black holes satisfy it ✅) (6/8)

03.09.2025 14:20 👍 0 🔁 0 💬 1 📌 0

Key insight: Geometrically, light rays don’t always focus under positive energy in general gravity theories. But in entropic geometry—measuring separation via Wall entropy, not area—they do. Gravity remains attractive! (5/8)

03.09.2025 14:20 👍 2 🔁 0 💬 1 📌 0

This generalised expansion decreases monotonically under the null energy condition. Even more: it equals the change in Wall entropy density, which satisfies the 1st & 2nd laws. So it’s a natural higher-order/dynamical generalisation of BH/Wald entropy. (4/8)

03.09.2025 14:20 👍 1 🔁 0 💬 1 📌 0

We extend this to all diffeo-invariant gravities with bosonic matter.
On perturbed Killing horizons, the linearised equation of motion gives a generalised Raychaudhuri equation → a notion of “generalised expansion.” (3/8)

03.09.2025 14:20 👍 0 🔁 0 💬 1 📌 0

In GR, the focusing theorem says: with positive energy (NEC), light rays converge. This simple idea reveals the attractive nature of gravity, and underpins Hawking’s area theorem & Penrose’s singularity theorem. (2/8)

03.09.2025 14:20 👍 0 🔁 0 💬 1 📌 0
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🚀 My first conference paper is out!
arxiv.org/abs/2509.00628
Based on my GR24 talk, it summarises 2 years of work with my supervisor Aron Wall on generalised focusing theorems & horizon thermodynamics in diffeo-invariant gravity. A short thread 👇(1/8)

03.09.2025 14:20 👍 2 🔁 0 💬 1 📌 0

Happy New Year! 新年快乐!All the best to our 2025!

31.12.2024 22:37 👍 0 🔁 0 💬 0 📌 0

I hope I can grow a better vision for theoretical physics in the new year… I always feel I’m so naïve…

27.12.2024 00:38 👍 0 🔁 0 💬 0 📌 0
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It was fun to explore the intricacies of higher-spin fields from the perspectives of gravitational focusing and horizon thermodynamics. There are so many open questions to address in the future…

arxiv.org/abs/2412.07107

11.12.2024 03:51 👍 0 🔁 0 💬 0 📌 0

Wow this is super useful!

24.11.2024 09:40 👍 1 🔁 0 💬 0 📌 0
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New departmental cat of DAMTP, Cambridge

17.11.2024 17:50 👍 1 🔁 0 💬 0 📌 0

Just noticed I was already on Bluesky way before I joined 😂😂😂

16.11.2024 14:17 👍 1 🔁 0 💬 0 📌 0

Thanks very much!

16.11.2024 10:23 👍 1 🔁 0 💬 1 📌 0

🥂🥂🥂

16.11.2024 10:23 👍 1 🔁 0 💬 0 📌 0

Hi! I’m new here! Glad to chat about black holes and quantum gravity or anything about physics and mathematics!

16.11.2024 10:11 👍 7 🔁 0 💬 1 📌 0