The large scale universe projected onto a two-dimensional boundary
There is an active field of research in cosmology and physics seeking to explain the cosmos in terms of a radical idea: we live in a universe with some of the
properties of a hologram:
At first glance, there is not the slightest doubt: to us, the universe looks three dimensional. But one of the most fruitful theories of theoretical physics in the last two decades is challenging this assumption. The "holographic principle" asserts that a mathematical description of the universe actually requires one fewer dimension than it seems. What we perceive as three dimensional may just be the image of two dimensional processes on a huge cosmic horizon.
That's a mind-being principle and the math behind it is a fearsome thing, pulling together rigorous work on everything from event horizons to string theory to the quantum information paradox. It's not easy to describe some of the ramifications that emerge in general terms.
But if you drift below the fold, thanks to no small amount of help from Jennifer Ouellette, one of the best hard-science writers in the world today, we'll at least try. And we'll do that without bringing up hyper-advanced mathematics!
For starters it doesn't mean we really live in a hologram, title notwithstanding. It certainly doesn't mean that some similar construct was built by exo-cosmic aliens for reasons unknown to house us and everything else we know to exist. In simplest terms it means that the three-dimensional universe may share a property at best grossly similar to the one found in two-dimensional holograms.
The principle arises in part from something physicists call the information paradox. Quantum information is a subtle concept. We could in theory encode the quantum properties, attributes like spin or rest mass, of every particle in existence in a computer and gain insight into the future behavior of the entire universe to an unprecedented degree of accuracy. In reality, that deterministic calculation quickly grows beyond any hope of actually carrying it out, even in simplified model systems with only a few hundred particles. But a paradox arises in physics because that info would seem to be stored away when particles fall into a black hole.
There are a lot of black holes in our universe. Billions and billions, to coin a phrase. Large stars can end their life as one, those dark remnants can in turn merge with one another in the center of galaxies and grow into super massive black holes. There are more billions of rather splendid galaxies shining brightly in our cosmic era and cosmologists think most of them contain such monsters. Big or little, if there's one thing black holes excel at, it's gobbling up matter and energy by the gigaton. That represents a lot of information.
But Stephen Hawking famously proposed that black holes evaporate their mass away over time and eventually pop out of existence. The implication is that the information that fell into them would follow suit. This takes a long time to begin in earnest for stellar mass black holes and above. So the quantum information is locked up in a metaphorical vault until the heat death of the universe begins—if you subscribe to that fate instead of the Big Crunch or the Big Rip—and then the black hole begins evaporating until it and the information stuck inside winks away into nothingness.
That may not sound profound, but it's considered a catastrophic problem in physics on par with violating the conservation of matter and energy. The holographic principle grew out of work by physicists, chiefly Gerard 't Hooft, Leonard Suskind, and Juan Maldacena, that that info could be retrievable if it were encoded onto a two-dimensional boundary. The basic idea is this boundary encoding principle can be extended, under the rubric of advanced physics anyway, to the larger universe. That has some weird and wonderful implications.
Science writer Jennifer Ouellette, who posted this fascinating review of how the principle is being analyzed using tensors, explained via email that it could mean the elegant, continuous schematic of flowing, curving space-time familiar to generations of science junkies is actually "stitched together by qubits encoded in a two-dimensional boundary. Which is pretty radical stuff if you think about it!"
Since this is superficially analogous to how a hologram encodes the dimension of depth onto a flat surface, the principle got the name it has. Of course, depth in a real hologram is a clever trick, easily confirmed by trying to stick your finger in it. The familiar holograms you see on a credit card are also a static image. Stuff in the universe moves around, a lot. A better analogy might be to a dynamic video game, which also encodes depth on a two-dimensional surface. Alas, an MMORPG principle or first-person shooter principle lacks the same kind of pizazz.
One thing that's attractive about the holographic principle, what makes some physicists sit back and hope that surely something like it is true, is it offers the possibility of tying together a number of otherwise disparate theories and conjectures. It works well with string theory, an idea that might help inform scientists about what the unification of the twin pillars of modern physics, general relativity, and quantum mechanics would eventually look like. That unification has been the Holy Grail of physics for the better part of a century. Any progress at all, even a tiny advance, would be tremendously exciting.
Back to cold hard reality for a moment: in science, we often learn as much from ideas that don't pan out as we do from those that pass muster. Like any proposed scientific explanation that unites a suite of observations into a single cohesive framework, the holographic principle will have to make valid testable predictions before it can graduate from an incredibly fascinating idea to a full blown, accepted theory. It's not easy to test this particular proposal, and so far the only tests devised have come up inconclusive at best.
Either way, we won't be making shortcuts to exotic new worlds through space-time in the foreseeable future using the holographic principle. To span the distances between us and our closer stellar neighbors in anything remotely approaching practical periods of time will take tremendous advances in propulsion, spacecraft design, and fault-tolerant, reliable machines. All of which are excellent topics for future science posts, right here on Daily Kos.