jb manchak @manchak.bsky.social · 28/09/2026thinking about gödel spacetime while on a run in tokyo… 120
Reposted by jb manchakJim Weatherall @jamesowenweatherall.com · 16/09/2026Just launched! LPS is running a new certificate program on Science for Policy and the Public. It's an introduction to modern philosophy of science aimed at people who communicate about and advise on science. Check it out! ce.uci.edu/programs/soc... #philsci #philsky #academicskyce.uci.eduScience for Policy and the Public | UC Irvine DCEThe Science for Policy and the Public Certificate is an interdisciplinary certificate program designed to provide learners with foundational knowledge in... 0187
Reposted by jb manchakHourly Cosmos @hourlycosmos.bsky.social · 15/09/2026Glare - From Gordan Ugarković (ugordan.bsky.social) - flic.kr/p/hdKCPZ 014132
jb manchak @manchak.bsky.social · 05/09/2026From today’s trail run to Crystal Cove… #running #trailrunning 091
jb manchak @manchak.bsky.social · 19/08/2026This would be a cool! Something to add to the story to give some dramatic tension: An observer at p has no way of knowing that her past I^-(p) actually contains a Cauchy surface. Will her future prediction materialize??? Read on to the next chapter to find out... link.springer.com/article/10.1...link.springer.comIs Prediction Possible in General Relativity? - Foundations of PhysicsHere we briefly review the concept of “prediction” within the context of classical relativity theory. We prove a theorem asserting that one may predict one’s own future only in a closed universe. We t... 040
jb manchak @manchak.bsky.social · 18/07/2026From today's trail run in Bommer Canyon... #trailrunning #running 0111
jb manchak @manchak.bsky.social · 12/07/2026😂 just me for this one. but she'll be speaking at the "tween logic=dad, you're supes embarrassing me, and beyond" conference next week... 020
jb manchak @manchak.bsky.social · 12/07/2026w progeny during an evening hike in budapest (logic, relativity, and beyond 2026 conference starts tomorrow). 150
jb manchak @manchak.bsky.social · 30/06/2026from today's trail run to crystal cove... #trailrunning 0100
jb manchak @manchak.bsky.social · 28/06/2026I asked Geroch, Wald, and few others this question and, although they had no doubt that it was true, couldn't supply a proof. Sbierski finally did and he was kind enough to credit me in the write up... Trying to generalize these results now... 110
jb manchak @manchak.bsky.social · 28/06/2026See p. 33 for a wild ride: arxiv.org/pdf/2208.07752arxiv.org 110
jb manchak @manchak.bsky.social · 28/06/2026Here's one that works (but is highly non-trivial to show!): X=can be properly extended by a single point to a maximal spacetime. In other words, take any maximal spacetime you like and remove a single point. There is only one extension (up to iso) of this spacetime: the one you started with! 110
jb manchak @manchak.bsky.social · 28/06/2026Corona Del Mar after a run! Looking for conditions X such that: If an extendible spacetime satisfies X, then it has a unique (up to iso) extension. Statement is false if X=globally hyperbolicity. Take the gh t<0 portion of Minkowski spacetime. Uncountably many extensions (with holes, etc). 210
jb manchak @manchak.bsky.social · 28/06/2026thinking about conditions to ensure the uniqueness of spacetime extensions... 2140
jb manchak @manchak.bsky.social · 25/06/2026from yesterday's trail run to laguna beach... #trailrunning 0120
jb manchak @manchak.bsky.social · 25/06/2026I talked to one of the site administrators recently. It's been happening on and off for a while now and they don't know why! 120
jb manchak @manchak.bsky.social · 12/06/2026any other philosophers out there who are first generation college grads? (i am also one!) 130
jb manchak @manchak.bsky.social · 10/06/2026Yes! The downside is that things get can get complicated and any desired local properties (e.g. being a vacuum solution to EE) of the original spacetime are not necessarily preserved. The beauty of your first duplication idea is that the resulting spacetime is locally isometric to the original. 010
jb manchak @manchak.bsky.social · 03/06/2026Dropping connectedness can be fun (and sometimes useful) to explore though! The "mother" spacetime is what Thomas Barrett and I call the (topologically disconnected) disjoint union of all (smooth, Hausdorff) Lorentzian manifolds... 110
jb manchak @manchak.bsky.social · 03/06/2026Should have added: the cutting and pasting is needed to ensure that the resulting spacetime is topologically connected. This is usually taken to be a necessary condition for a (standard) GR model... 110
jb manchak @manchak.bsky.social · 01/06/2026Yes! Your idea amounts to a simple proof: Cinsider two copies of the same H spacetime. Cut the same slit is each and (excluding slit boundary points) identify the top edge of one with the bottom edge of another. The resulting structure is a GR spacetime that is Buddha but not Heraclitus. 110
jb manchak @manchak.bsky.social · 01/06/2026Roughly: Imagine a universe that is (locally) impermanent at each spacetime event but is cyclical in the sense that it returns to the same impermanent state every 17 years. Buddha but not Heraclitus.… 010
jb manchak @manchak.bsky.social · 29/05/2026Let's say a *Buddha spacetime* (impermanence at every event) is one with no local Killing fields. A *Heraclitus spacetime* (can't step into the same river twice) is one with no local isometries between any distinct points. Theorem. Every Heraclitus spacetime is Buddha but not the other way around. 151
jb manchak @manchak.bsky.social · 24/05/2026Not sure who needs to hear this today but determinism is not a mysterious metaphysical notion. If you or a loved one are struggling with haecceities, hang in there and read this paper. You got this! (new work with Hans Halvorson and @jamesowenweatherall.com) onlinelibrary.wiley.com/doi/10.1111/...onlinelibrary.wiley.comDeterministic TheoriesDeterminism is (roughly) the thesis that the past determines the future. But efforts to define it precisely have exposed deep methodological disagreements. Standard possible-worlds formulations of de... 081
jb manchak @manchak.bsky.social · 19/05/2026I laughed out loud when I read this: "But I feel like a beginner in almost all areas of physics except, possibly, classical general relativity." David is "possibly" a beginner in classical general relativity in the same sense that I am possibly a fish! 030
jb manchak @manchak.bsky.social · 19/05/2026There is a new interview with David Malament that was just posted to the AIP website. Check it out! repository.aip.org/node/151302repository.aip.org 111
jb manchak @manchak.bsky.social · 29/04/2026my new bumper sticker: leibnizian spacetime+heraclitus matter is more deterministic than leibnizian spacetime itself. 030
Reposted by jb manchakOpen Culture @openculture.bsky.social · 17/04/2026A Newly Discovered Recording Lets You Hear Delta Blues Legend Robert Johnson in Stunning Clarityopenculture.comA Newly Discovered Recording Lets You Hear Delta Blues Legend Robert Johnson in Stunning ClarityGreat swathes of rock music since the nineteen-sixties would never have existed, we're sometimes told, were it not for the recordings of Robert Johnson. Certainly the likes of Keith Richards, Eric Cla... 8440631273
jb manchak @manchak.bsky.social · 16/03/2026Had a lovely time on the "Theories of Everything" podcast! #philsci #philsky #physics www.youtube.com/watch?v=iGOG...youtube.comTime Travel in Physics: “We Still Don't Know”YouTube video by Curt Jaimungal 1133
jb manchak @manchak.bsky.social · 19/02/2026I wrote a Galilean dialogue w James Read on the hole argument and determinism. It is essentially a fruitful email exchange we had after we each had published papers diametrically opposed to each other. It was a fun project! philosophyofphysics.lse.ac.uk/articles/10....philosophyofphysics.lse.ac.ukDialogue Concerning the Two Chief Views on Spacetime Rigidity | Philosophy of Physics 060
jb manchak @manchak.bsky.social · 06/01/2026Given your interests and your posts here, I think you'd love being a philosopher of physics! There are lots of grad students at LPS who switch out of math/physics after realizing it's not where their passion is. But with good technical skills (like you), they often pick things up very quickly... 110
jb manchak @manchak.bsky.social · 14/12/2025The Bobs are indeed paragons of clarity! I was fortunate to have Bob G on my dissertation committee. I am perhaps a bit biased but the pinnacle of GR clarity is David Malament (also at Chicago during the golden age). His 2012 book is my bible. I don't believe in God but I do believe in David. #wwdd 010
jb manchak @manchak.bsky.social · 14/12/2025Wiki is wrong! Wald (1984) will never lead you astray... 110
jb manchak @manchak.bsky.social · 14/12/2025S spacetime is usually understand to be stationary, e.g. Wald in his book. This means there is a global timelike KF. The K extension doesn't have a global timelike KF for the reasons you give. So Wald and anyone else who claims S is stationary must also take S to be just the exterior region. 100
jb manchak @manchak.bsky.social · 14/12/2025It's also good that you picked up on the difference between a global timelike KF and the stronger condition of a global timelike isometry. Imagine Minkowski spacetime with the t<0 region deleted. Still has a global timelike KF but there is no global timelike isometry! 110
jb manchak @manchak.bsky.social · 14/12/2025Exactly! The Killing vector field d/dt becomes spacelike inside the EH. As you say: can't flow along d/dt into the singularity. So you are right that the Kruskal extension is not stationary (bc no global timelike KF) but S spacetime is (bc global timelike KF). 310
jb manchak @manchak.bsky.social · 13/12/2025Hi Rochelle! S spacetime (M,g) has a timelike isometry! Write the metric g in (t,r, theta, phi) coordinates. Now consider the map f: M --> M defined by f(t,r,theta,phi)=(t+1,r, theta, phi). Notice that none of the metric components depend on t and d(t+1)=dt. So f*(g)=g and f is an isometry. 110
