Thread

1. anigbrowl · 1790380260 · focus · HN ↗

   Pause for a moment to reflect on how outrageous this assertion is. You can’t see into the box at all. Nevertheless, holography says that you can learn exactly what’s happening everywhere in the box without any access to the interior. Observing the surface alone is enough. In this sense, the amount of stuff that fills a box is the same as the amount of paint that covers it. That’s a violation of logic and geometry.

   The breathless tone of this article obscures rather than illuminates its subject. It sounds as if the author describes a box with some particles bouncing around inside, and every time a particle bounces off the wall of the box the outside glows briefly indicating the location and intensity of the collision. The author is amazed that by repeatedly measuring this, you can draw inferences about what's going on in the box.

   I can't see what's 'outrageous' about this. You need a lower surface which registers information about activity inside a higher-dimensional volume, some way to accurately read that information, and the time/patience to repeat the measurement many times. Isn't this how radar works, or feeling your way around a pitch-dark room using only the 2-dimensional surface of your hands (or shins)? We know from computer science that you can mathematically encode any structure of arbitrary dimension into a binary tree. One might as well ask how it's possible that our complex 3 dimensional world can be contained in the flat surface of a mirror, film strip, or camera sensor.

   1. pimlottc · 1790381917 · focus · HN ↗

      It’s not just drawing inferences, though; it’s saying you can determine exactly what’s happening in the box. But the pigeonhole principle would suggest there’s far more possible states in the 3-dimension volume than can be unique expressed on the 2-dimensional surface. That’s the part that defies common sense.

      1. senderista · 1790382804 · focus · HN ↗

         Is it any weirder than the fact that there is a bijection between the unit interval and the unit square?

         1. dnautics · 1790394126 · focus · HN ↗

            Yeah because you can prove that any such bijection cannot be a topological homeomorphism, for example. So necessarily some "nice to have" properties must drop out.

            1. zmgsabst · 1790428932 · focus · HN ↗

               Those drop out in holography too — eg, information local within the interior can get smeared out across the surface.

               1. senderista · 1790442514 · focus · HN ↗

                  That is exactly what I was wondering about in my other comment: is physical nonlocality somehow equivalent to topological noncontinuity?

                  1. dnautics · 1790461930 · focus · HN ↗

                     I am not a physicist but i presume that is the hope of some who pursue holographic modes
      2. ReflectedImage · 1790383246 · focus · HN ↗

         A 3d volume can contain a number of states equal to it's 2d dimensional surface.

         All the extra states you would assume the 3d volume can contain are actually a single state called Black Hole.

         There is a limit to how much you can fill a 3d volume before it's a Black Hole basically.

         1. api · 1790384538 · focus · HN ↗

            That's the clearest description of how this dimension collapse via gravity thing works that I've ever heard.

            I really think we live on the 3D event horizon of a 4D black hole, folks.

            1. rbanffy · 1790415535 · focus · HN ↗

               Why stopping at four dimensions?

               1. api · 1790419158 · focus · HN ↗

                  Oh sure. The parent universe could be the 4D event horizon of a 5D hole. It’s holes all the way up. If you draw little heads on them I guess they’re like round turtles so if this is true the turtles people weren’t as far off as we thought.

                  Since all the event horizons “touch” at N-1 dimensions I wonder if this is isomorphic with certain forms of string theory? Maybe there’s 11 levels of holes?

                  BTW this isn’t my idea. There’s papers on black hole cosmology. You can make the math work but no way to test it. Yet?

                  1. mjanx123 · 1790421943 · focus · HN ↗

                     Legs, also little legs.
               2. tclancy · 1790435839 · focus · HN ↗

                  The people at Gillette are working on five.

                  1. junkblocker · 1790452596 · focus · HN ↗

                     That's old news. They solved five a while ago.

                     1. rbanffy · 1790455879 · focus · HN ↗

                        I heard they were researching up to 18, but most of the blades were curved and really short.
         2. b800h · 1790391386 · focus · HN ↗

            Is that result proven?

            1. nbernard · 1790403296 · focus · HN ↗

               Depends on what hypotheses you are willing to accept, but, as far as I know, it is generally accepted by physicists: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Bekenstein_bound" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Bekenstein_bound
            2. geocar · 1790405504 · focus · HN ↗

               No.
            3. nextaccountic · 1790447753 · focus · HN ↗

               That&#x27;s a purely mathematical result, the top comment of this thread has a link to the paper

               I don&#x27;t think it has been experimentally verified or anything

         3. brotchie · 1790392461 · focus · HN ↗

            This is by far the most succinct, understandable, correct, description of the holographic principle. I don’t know all articles don’t just lead with this.
         4. bell-cot · 1790426195 · focus · HN ↗

            &gt; a single state called Black Hole.

            Quibble: A black hole still has a charge, mass, momentum vector, and spin vector. Which, yes, still vastly reduces the number of states needed on the surface of the enclosing container.

         5. thaumasiotes · 1790435538 · focus · HN ↗

            &gt; All the extra states you would assume the 3d volume can contain are actually a single state called Black Hole.

            How can that be? Don&#x27;t different black holes have different masses?

            1. ReflectedImage · 1790456351 · focus · HN ↗

               Well the density goes down the larger the black hole so the extra states are still missing.

               For exactly how that works, I&#x27;m a Comp Sci not a Physicist.

               1. thaumasiotes · 1790551292 · focus · HN ↗

                  &gt; Well the density goes down the larger the black hole so the extra states are still missing.

                  Huh? If the correlation between density and mass is perfect, that means two things:

                  1. There is an upper limit on the mass of the black hole, set by the volume of the box containing it.

                  2. The black hole&#x27;s possible mass and density occupy a one-dimensional space and not a two-dimensional space.

                  Now, it appears to be true that the (possibly nonlinear) correlation between mass and density is perfect; for any mass a black hole can have only one density. But that doesn&#x27;t make the space of possibilities finite. It certainly won&#x27;t make it into a single point. And in fact black holes appear to have three independent physical dimensions: their mass, their electric charge, and their spin.

         6. [deleted] · 1790441686 · focus · HN ↗

            [deleted]

         7. __david__ · 1790441775 · focus · HN ↗

            Does that mean that all the black holes of a particular volume have the same interior? Or does it mean that the black hole state is the case where you can’t tell what the interior is?

            1. idiotsecant · 1790451896 · focus · HN ↗

               Classical understanding, yes. Black holes have no &#x27;hair&#x27;. You can measure charge spin mass that&#x27;s it
         8. oulipo · 1790540226 · focus · HN ↗

            &gt; There is a limit to how much you can fill a 3d volume before it&#x27;s a Black Hole basically.

            Which is exactly the part that &quot;defies common sense&quot; and is intersting

      3. Alex3917 · 1790385419 · focus · HN ↗

         &gt; That’s the part that defies common sense.

         Does it? Isn&#x27;t this basically the same as how cellular companies can figure out exactly where someone is calling from as long as their phone is pinging 3+ towers? Just connect the towers into a box and you have the same result.

         1. EthanHeilman · 1790387778 · focus · HN ↗

            Build an object out of legos and then reconstruct the shape of what you built by only looking at it from the sides and above.

            1. zelphirkalt · 1790411522 · focus · HN ↗

               Seems simple enough in many cases. But I think the difference is in the generality of it. Being able to do it for any interior arrangement of parts.

               1. EthanHeilman · 1790457106 · focus · HN ↗

                  Agreed, that was what I was trying to say. You can build a lego structure that you can not infer by just looking at from the top, bottom and sides.
      4. anigbrowl · 1790386542 · focus · HN ↗

         I&#x27;m not sure if I agree, but you certainly articulated it far more clearly than the original writer.
      5. rbanffy · 1790415458 · focus · HN ↗

         If the events inside the box can’t influence events outside the box directly, only by changing the surface of the box, it’s fair to say they don’t happen at all, and all that happens is the change of the surface.
      6. larksimian · 1790432938 · focus · HN ↗

         2d observed over time is also 3d. 3d over time would be 4d.

         You still have less information than if you could observe the full 3d spatial volume over time, because presumably you won&#x27;t know in perfect detail qnd precision what all the particles are doing internally?

         Or does it not work like this?

      7. thaumasiotes · 1790435470 · focus · HN ↗

         If you&#x27;re willing to use continuous dimensions, the surface is the same size as the volume; I don&#x27;t see why the pigeonhole principle would present any problems.

         Interestingly enough, I believe there is a result that space-filling curves cannot be one-to-one, but the implication there is just that, by virtue of the continuity of the one-dimensional curve, it contains more points than the two-dimensional space that it fills.

Unofficial Hacker News client; not affiliated with Y Combinator.