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- Hacker News
- Ok so imagine we set up two sets of babushca dolls. Each larger one has two smaller ones nested inside. The total mass of the two smaller ones between the two sets is equal in mass and they are configured to have the same centre of gravity. But crucially the individual masses of the inner dolls are different between the two sets. So maybe the innermost one is made of a denser material in set A and the middle one a less dense material and in set B vice versa. And let's also say in each case the dolls are all locked into position relative to the outermost one, so there's no "play" if you move it around to infer a difference from.
From an outside mass/gravity point of view these are similar right? Yet the holographic universe theory seems to say we should be able to differentiate between the two sets of dolls purely based on space-time observations of the surface of the outer doll, despite each system having identical gravitational properties when viewed externally?
But doesn't that suggest a contradiction between this theory and what we have observed about gravity?
by captainbland - I think the box explanation is misleading. There isn't literally a box whose surface you measure. The idea is that the state of a higher-dimensional gravitational system can, in some cases, be completely described by a theory in one fewer dimension.by tananaev
- Some assumptions of holographic principle either do not match with standard model of cosmology or predict yet another consequences. This model acknowledges expansion of the Universe, observed via Hubble in 1990s and since then only more facts. Information limit of volume is measured by how much information (written into matter) can be put into volume before it becomes black hole and forcibly occupy as much volume as required to preserve area limits. But that expansion of the Universe, how does it interact with black holes?
If the space expands, that helps escaping black hole. And effective horizon of black hole should be smaller than without accounting for space expansion. And it is possible to put little more information than area permits. This is hard to verify since distances where space expansion starts being noticeable, are orders of magnitude bigger than radius of known most massive black hole. But consistent description of reality should combine them somehow.
There may be alternative solution. Area limit may turn out to be more fundamental than space expansion, and big radius black holes will have stronger gravitation to overcome space expansion.
> With gravity, mass plays the role of charge. It bends space-time around it, and it is always positive. There is no negative mass
Equations have not just mass, but also pressure, and space expansion is theoretically described by dark energy that has positive mass and negative pressure overcoming its positive mass.
by OCTAGRAM - PBS Space Time, could watch it all day, sometimes listen in the background all day
* https://www.youtube.com/watch?v=DoCYY9sa2kU
* https://www.youtube.com/watch?v=klpDHn8viX8
by ck2 - As a non expert who's just curious about science but also tries to talk like a normal person, from what I can tell holography really seems like it's now the leading candidate to be the next big conceptual revolution in physics.
There's not necessarily any breakthrough right around the corner and we shouldn't rush to coronation preemptively, but there's a lot of physics "voting with their feet" for holography as the article says, and I think it's officially time to start getting hyped. It could have sanity-restoring answers to questions of quantum weirdness, albeit at the rather expensive cost of giving up even more on our day to day intuitions about space and time in favor of an apparently informational substrate.
String theory had a generation of great science communicators writing its books and singing its praises, but holography doesn't yet have equivalent public champions, and I hope that's something that changes sooner than later.
by glenstein - Mathematician, not physicist, but it seems reasonable to me that you could encode a sufficiently constrained 3D space on a 2D boundary. And if you have such an encoding, it also doesn't seem insane that some things might be more easily modeled on the 2D boundary than the 3D space.
If you can convert back and forth between a 2D representation and a 3D representation, and different phenomena are more easily modeled in each, does it matter which is "real"? Unless of course you can come up with a specific prediction and experiment to test it.
- 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.
by anigbrowl - 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.
by qnleigh