Spacetime and geometry

What Hacker News says

1. captainclamOct 2026
   I was not familiar with the mathathon, and I suppose I am ignorant of the circumstances in which Tao allegedly bullied students into changing the event. I don't personally have any reason to doubt the veracity of the "Joint Statement about Mathathon", but perhaps you have some insight that I don't. I will say that I would imagine that any input from Tao, even in passing, probably has outsized influence on the organizational direction of the event due to his preeminence, but I'm not so sure that necessarily rises to the level of bullying.

   But from what I gather, the theme of the statement is to expand the scope of participation to include work that is quite literally the /opposite/ of gatekeeping, i.e. taking proofs that are popularly understood to be opaque or difficult to understand (from any source, AI, human, or otherwise) and developing a new way to communicate them that is more accessible to more people.

   I do see how it may come off as a slight to the original mathematicians who developed those proofs. But developing new ways to communicate already established concepts should not diminish the original works. There are probably much better communicators/teachers of relativity than Einstein ever was. No one out there thinks any less of Einstein because of Sean Carrol's "Spacetime and Geometry" textbook.

   And this to me reads as only embracing the use of AI in mathematics, which is not gatekeeping behavior. Tao continues to emphasize that once proofs become effectively automatable, deeply comprehending and being able to transmit that understanding to others will become an increasingly important aspect of the field, the natural way for mathematicians to use the time that was once occupied by developing the proofs themselves.

   Literally all of this is about expanding the accessibility of mathematics, which once again, I perceive to be quite the opposite of gatekeeping. Maybe we have different definitions of that term.

   Also I have not seen Amadeus, so no comment about Salieri.

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2. raattgiftJun 2025
   tl;dr The farrrr-from-the-horizon part of Schwarzschild spacetime is just not like our spacetime. Only near and outside the horizon (or better, in the absence of a horizon) does Schwarzschild become a decent physical approximation for anything in our universe.

   Schwarzschild infinity is unphysical, while your notion of t(Earth) is physical because we can associate a worldline with the planet's centre of mass (COM), hold the COM at the spatial origin of a system of spacetime coordinates, and use whatever "timestamps" we like on the time axis. But we could decide that t(Earth)=infinity could be yesterday, or tomorrow, or a billion years ago, or a couple billion years from now; if we count of seconds before or after t(Earth)=infinity, we still have t'(Earth)=infinity, so it's not a very good choice of coordinate.

   I think you have a misunderstanding that is probably beyond my ability to help you with, since we can't do interactive blackboard work in HN comments. The root of your problem seems to be mis-identifying the local time at Earth with the Schwarzschild time at infinity in the Schwarzschild solution. We aren't at infinity to any known black hole: between us and the most distant black holes we know of is expanding spacetime not found in Schwarzschild's solution; betwee us and the nearest black holes is substantially and lumpily curved spacetime and plenty of matter unlike Schwarzschild's unique pointlike mass surrounded by non-lumpy matterless vacuum; none of the astrophysical black holes are infinitely old today (whereas Schwarzchild black holes are infinitely old at every time, otherwise the spacetime would not be static); and in general exact solutions of the Einstein Field Equations -- even ones that are not eternal -- do not superpose cleanly with solutions for other black holes (and crucially there are no black hole mergers in Schwarzschild), ordinary stars, galaxies, clusters, and expanding spacetime. As an example: hover just above the apparent horizon of Sagittarius A*. Look at a stellar black hole in our galaxy. What do you make of infallers plunging towards the smaller black hole? What do you make of the evolution of mass of the stellar black hole, from your vantage point hugging an SMBH's horizon?

   Short of taking a series of courses or finding an informal short-term tutor to walk you through particular things (you can find either at your local tertiary education school, like a community college or university), there are plenty of good textbooks on General Relativity. You seem to have found Wald's, which is probably the most rigorously and densely mathematical of several popular teaching choices, and it does not seem to have helped you. I'd guess you'd be better off with e.g. Carroll's Spacetime and Geometry or Wheeler's Gravity and Spacetime.

   There is also the Israel-Darmois thin shell method, which is technically annoying but lets us cut the central part of an e.g. Schwarzschild solution and paste it into a cosmology populated with other such pasted-in subregions. We can then trace light rays from e.g. a quasar, across early expanding space to a SMBH or elliptical galaxy acting as a gravitational lens, and then across later expanding space to an approximation of our neighbourhood, adapting the rays at each shell boundary. Although there is very definitely a subregion of black hole solution in that kind of approach, the asymptotically flat part of Schwarzschild is cut away along with its distant infinities. One can compare this cutting and pasting to the Hill sphere of influence of Jupiter and those of its satellites, for example, if one were interested in a navigational plan like Juno's or JUICE's.

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3. consilientJan 2023
   Google won't help you here. Popsci stuff is useless at best, and more often actively misleading. You need a GR textbook. An Introduction to General Relativity, Spacetime, and Geometry (https://fma.if.usp.br/~mlima/teaching/PGF5292_2021/Carroll_S...) is relatively accessible.
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4. jperrasMay 2021
   Sean Carroll is incredible, and his textbook on Spacetime & Geometry was the only reason I understood anything in my graduate-level class on general relativity. There's truly an art to be able to explain incredibly complex topics in an easy(er) to understand way.
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5. andrepdDec 2019
   For general relativity: I never miss an opportunity to recommend "Spacetime and Geometry" by Sean Carrol. The only book I needed for my GR course (plus Misner Thorne and Wheeler for occasional reference).
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6. andrepdJun 2018
   I'm not sure what I would recommend as an introduction to relativity. If you're mathematically minded and have a lot of time on your hands, and actually want to learn the physics, a superb book is "Spacetime and Geometry" by Sean Carrol. As for a layman introduction, I'm not sure what to recommend.

   As succinctly as I can put it, in our current understanding spacetime is a 4-dimensional space whose curvature is related to the local energy-momentum content. In turn, the motion of particles in freefall is dictated by this curvature.

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