Physical observability of horizons · HackerPapers discussions

Physical observability of horizons — mentioned by raattgift

Fri, 30 Oct 2026 12:57:14 GMT

> external observers would never Consider two strongly-interacting freefallers related by a boost colliding and sticking together (like a black tshirt and a floof of white pet fur) close to the apparent horizon. Consider a freefaller and an accelerated observer meeting close to the apparent horizon. The freefaller here could be distant starlight and the accelerated observer a powerful rocket. Are you saying these coincidences are forbidden in the vicinity of an apparent horizon (which is locally detectable), or are you distinguishing between the apparent horizon and the (global, but not…

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Physical observability of horizons — mentioned by raattgift

Wed, 28 May 2025 17:18:59 GMT

I agree with you so much that I can't resist a nitpick. I'll also amplify your points in the last two paragraphs and justify "the popular scientists". > the event horizon moves outwards to meet it The event horizon is a global surface that might even be measurable locally in principle, and certainly might not be where one thinks. The event horizon is determined by what's inside and outside it, and arguably it's only "dynamical" if one takes a 3+1 view of the spacetime containing it. Local measurements can determine which side of an apparent horizon the measurer is on. That's arguably more…

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Physical observability of horizons — mentioned by raattgift

Tue, 03 Dec 2024 02:00:44 GMT

> (classically) ... photons Uhhh... one of those words should go. Let's keep it fully classical and drop "photon": we're interested in gravitational effects rather than quantum ones (uncertainty, fluctuations, tunnelling, details about scattering and more). Really what we want is something to illuminate (pardon the pun) interesting null geodesics, so a thin collimated beam -- a pencil of light -- will do. The relevant surface here is the apparent horizon, which can be measured by infalling apparatuses, and not the event horizon, the location of which is determined by the configuration of the…

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Physical observability of horizons — mentioned by raattgift

Thu, 24 Aug 2023 20:24:14 GMT

Yay, I love Big Crunch. BC is such a tasty sugary addictive treat. It generates lots of black holes in the (re-)collapsing universe just like Chocolate Frosted Sugar Bombs generate lots of black holes in Calvin's tooth enamel. I think I mostly agree with you, loosely, that we could be in a BC cosmos but I also think it's not great science communication to say so because "could" is doing a lot of work there. :-) Let's start by ignoring the fact that all the new z > 11 (!!!) galaxies JWST is spotting lately do not seem to be rushing inwards from the horizon[*], because I think those…

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Physical observability of horizons — mentioned by raattgift

Thu, 17 Aug 2023 12:57:28 GMT

Thanks for asking this good (and pretty difficult) question, rather than trying to make an assertion; cf. a sibling comment of mine. tl;dr: that the body is all attached for some time after crossing the horizon is [a] likely and [b] sufficent to justify the notion of stimulation in the feet being processed in the brain, when falling feet-first within a black hole. Firstly and basically the whole of [b], I'm just gonna assume that nerve impulses are the result of chemicals spat within neurons and between them, all very slowly compared to the speed of light, and all quite local (one chemical…

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Physical observability of horizons — mentioned by raattgift

Wed, 22 Feb 2023 19:06:58 GMT

Preamble: we are largely on the same page. I thought I'd draw your (or our mutual readers') attention to a couple details where we may differ a little. > [link to wikipedia's de Sitter-Schwarzschild page] Let's call it Schwarzschild-de Sitter (SdS, for short, and for ease of literature-searching). More generally there is the McVittie family of metrics of a massive object in a dynamical spacetime. SdS is the limiting case of McVittie where the spacetime is stationary and the central mass is compact, spherically symmetric, 0-angular-momentum, and uncharged. (One could alternatively say that…

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Physical observability of horizons — mentioned by raattgift

Sat, 18 Feb 2023 19:37:47 GMT

> It is the observer at infinity that never sees you fall into the black hole We don't even need an observer to be at infinity, thanks to the expansion of the universe. With some future telescope our descendants may observe something on a trajectory to enter a black hole in an early-universe galaxy that is just crossing that observer's (cosmological) horizon. I think it's relevant to raise this since the article at the top is about embedding black-hole-like collapsed stars in an expanding universe and the research which directly discusses the observable consequences. > real physics is local…

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Physical observability of horizons — mentioned by raattgift

Sat, 18 Feb 2023 18:19:53 GMT

That's only current thinking in SYM/string/D-brane circles, surely? Strominger & Vafa (at the link) is explicitly about N=5 AdS_2 x S^3. Ok, nice that you get unitarity above all in that, but it's not at all clear that the picture corresponds with our universe. I do not see how it could possibly correspond with the universe of the linked article. Croker, Weiner et al. (the authors of the topic paper) are keen first and foremost on their spinning black hole interior solution, which is wholly classical and found at <https://arxiv.org/abs/2107.06643>. In the more recent topic paper they argue…

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