Thread

Gravity seems holographic. What does that mean for reality?

300 points · 234 comments · ibobev

qnleigh · 1790394181 · focus · HN ↗
Susskind's original paper is shockingly readable, at least the first section. It doesn't use a lot of fancy math or derivations to justify holography. Mostly it uses basic concepts from undergraduate physics to show how the idea of holography is consistent and makes sense, despite it seeming incredibly counterintuitive at first glance.
For instance, figure 3 shows how you can't hide a black hole behind another black hole, which has implications for how a 3D universe can be completely encoded in a 2D region.
<a href="https://arxiv.org/html/hep-th/9409089v2" rel="nofollow">https://arxiv.org/html/hep-th/9409089v2
1. lhd1 · 1790397677 · focus · HN ↗
  Much of Susskind's papers are like that. A few simple equations motivated by dimensional analysis, a couple of Penrose diagrams, and the rest speculation and insight.
2. jmvoodoo · 1790435370 · focus · HN ↗
  Thank you. This is far easier to understand than the OP.
3. westurner · 1790436384 · focus · HN ↗
  "The World as a Hologram" (1994) <a href="https://arxiv.org/html/hep-th/9409089v2" rel="nofollow">https://arxiv.org/html/hep-th/9409089v2 .. citations: <a href="https://scholar.google.com/scholar?cites=12883331061830328784&as_sdt=5,43&sciodt=0,43&hl=en" rel="nofollow">https://scholar.google.com/scholar?cites=1288333106183032878...
  But can there be holograms within the outer hologram?
4. godwinson__4-8 · 1790438287 · focus · HN ↗
  I recommend when sharing arxiv links to link to the abstract. That way your reader can decide for themselves whether they want to bother with either a PDF or HTML version, especially since the HTML views are still experimental and have occasional display issues across devices.
  Link to the abstract of that paper: <a href="https://arxiv.org/abs/hep-th/9409089v2" rel="nofollow">https://arxiv.org/abs/hep-th/9409089v2
  Thanks for sharing!
  1. kzrdude · 1790452350 · focus · HN ↗
    Fortunately, it's very simple to navigate between pdf, html, and abstract, just put "abs" in the URL where it says pdf and so on.
5. Obscurity4340 · 1790451465 · focus · HN ↗
  What shape is space? Like, how far does space extend "above" and "below" where the Earth hangs in space? What stuff is there in both areas?
  My concept of space is informed by the hammock analogy where gravity is the result of the bodies' mass/density causing the fabric to sag and draw in approaching objectz
  1. pas · 1790496589 · focus · HN ↗
    space is 3D, but the information content can be encoded on a 2D hologram. general relativity works, mass-energy is curving spacetime, it just happens to do it in a way, that these curves and slopes and ... the whole 3D geometry can be flattened to 2D without loss of information.
    Obscurity4340 · 1790596779 · focus · HN ↗
    So is there stuff below and above Earth
    qnleigh · 1790612696 · focus · HN ↗
    Yes, lots and lots of stuff. For all we know, space may extend infinitely in all directions.

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