Talk:Event horizon
Add topicLatest comment: 14 days ago by Johnjbarton in topic Ambiguous use of "beyond which ..."
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Ambiguous use of "beyond which ..."
[edit]The first two paragraphs of the introduction use the term "beyond which ...", referring to the event horizon. That is ambiguous in that "beyond" is relative to the position of a hypothetical observer (either within or outside of the event horizon). Wouldn't the term "within which" be a better substitute, being absolute? BMJ-pdx (talk) 09:25, 17 July 2026 (UTC)
- The event horizon is relative to the position of an observer, so "beyond" is not ambiguous. The unfortunate simplification made in many discussions of black holes is to talk about the "black hole's event horizon", but such a thing is only meaningful for observers at infinity as implied by the mathematics behind such discussions. Spatial diagrams of black holes are not like maps where we can imagine visiting the various localities depicted. They are records of photons emitted in the past just now arriving for observation. The event horizon marks the points where that emission time was infinitely far back in the observer's time.
- You can see some of this in Fig. 1 of
- Rindler, Wolfgang (1956-12-01). "Visual Horizons in World Models". Monthly Notices of the Royal Astronomical Society. 116 (6). [Also reprinted in General Relativity and Gravitation, 34, 133–153 (2002), doi: 10.1023/A:1015347106729]: 662–677. doi:10.1093/mnras/116.6.662. ISSN 0035-8711.
- It has three "particles" A, B, C. Consider C to be a fiery asteroid falling into a black hole. E1 is a photon emitted by the asteroid before it crosses A's event horizon. E2 is a photon emitted later as far as C is concern. "A" never sees it: it is beyond A's event horizon. But B would see E2 and E3 emitted even later, while missing events after E4, beyond B's event horizon.
- A term like "within which" implies a closed surface but these horizons need not enclose. For example a black hole is not closed along the time dimension. I know that black hole event horizons are depicted as spherical surfaces projected on to 2 dimensions for diagrams but that is misleading. You can see this in the spacetime diagrams in the article: the spacetime is divided into non-overlapping regions of infinite extent. Johnjbarton (talk) 19:30, 17 July 2026 (UTC)