Science: Reality, including humans, may be a hologram projection

For a general audience 

An unproven but now accepted possibility is that our perception of a 3 dimensional spatial reality, length, width, depth) (plus the non-spatial time dimension) arises from a 2 dimensional universe (just length and width in curved space). In other words, we and what we perceive as the universe might be a holographic projection of a 2 dimensional reality. This idea arose about 25 years ago.

The hologram hypothesis was applied to concerns that when matter got sucked into black holes, the information inherent in the matter was lost by being crushed into oblivion by gravity. Information here means everything needed to be known to perfectly reassemble the exact thing that fell into the black hole, including the exact location of every single atom in the thing. One hypothesis arose that as matter fell in, all of its information content was conserved at the black hole’s event horizon. Once this understanding arose, it was immediately obvious that it could apply to the entire universe. That led to the thought that actual reality is two dimensional and the geometry of space is curved instead of flat. 

At present, existing data says the geometry of space in our universe is flat . This is different from saying space is not flat because it has three dimensions, length, width, and depth.  

Flat space geometry means Ω0 = 1 in the image below

The mind blower here is about geometry, not spatial dimensions or time. The geometry of space could look like one of these:

The Goldilocks scenario for our universe, Ω0 = 1, is the most plausible based on current data. Most cosmological evidence points to the universe’s density as being just right — the equivalent of around six protons per 1.3 cubic yards — and that it expands in every direction without curving positively or negatively. In other words, the universe is flat. (Perhaps this will come as some consolation to anyone, i.e., flat Earthers, disappointed by Earth’s roundness.)

The mind blower analogy

Start walking along the edges of a room square room until you get back to your starting place. It takes four 90 degree turns to get back to start. That is flat space, i.e., Euclidean geometry. But in a universe where Ω0 < 1 (gravity eventually pulls all matter in the universe back into a single point), space is curved into a sphere. Start walking from a point on the equator of Earth to the North Pole, turn 90 degrees and walk back to the equator, and then turn 90 degrees again and walk back to the starting point. Only three 90 degree turns were needed to get back to start. That is curved space. 😵‍💫😶‍🌫️

Researchers still cannot prove that there is a 2 dimensional universe and what we perceive is a 4 dimensional projection via quantum entanglement. Physicists and cosmologists are still trying to figure out experiments that would prove or disprove the hologram hypothesis. 

Wikipedia: The holographic principle is an axiom in string theories and a supposed property of quantum gravity that states that the description of a volume of space can be thought of as encoded on a lower-dimensional boundary to the region, such as a light-like boundary like a gravitational horizon [e.g., a black hole’s event horizon].

Germaine’s warranty is void 

if this hologram sticker is removed

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For science wonks

dS (de Stitter space) = our 4-D (four dimensional) universe (length, width, depth, time)

CFT (conformal field theory) = 2-D (or other dimensional) space where angles between lines and curves are always preserved (at least length and width)

Scientific American writes:

A quarter of a century ago a conjecture shook the world of theoretical physics. It had the aura of revelation. “At first, we had a magical statement ... almost out of nowhere,” says Mark Van Raamsdonk, a theoretical physicist at the University of British Columbia. The idea, put forth by Juan Maldacena of the Institute for Advanced Study in Princeton, N.J., suggested something profound: that our universe could be a hologram. Much like a 3-D hologram emerges from the information encoded on a 2-D surface, our universe’s 4-D spacetime could be a holographic projection of a lower-dimensional reality.

How could a theory that included gravity be the same as a theory that had no place for gravity? How could they describe the same universe? But the duality has largely held up. In essence, it argues that we can understand what happens inside a volume of spacetime that has gravity by studying the quantum-mechanical behavior of particles and fields at that volume’s surface, using a theory with one less dimension, one in which gravity plays no role.

.... spacetime may not be fundamental—it may be something that emerges from quantum entanglement in a lower-dimensional system. These advances all involve the theoretically plausible spacetime of anti–de Sitter space, which is not the de Sitter space that describes our universe. But physicists are optimistic that they’ll one day arrive at a duality that works for both. If that were to happen, it could lead to a theory of quantum gravity, which would combine Einstein’s general relativity with quantum mechanics. It would also imply that our universe is in truth a hologram.

The idea took some time to sink in. “There were hundreds, thousands of papers, just checking [the duality] because at first, it [seemed] so ridiculous that some nongravitational quantum theory could actually just be the same thing as a gravitational theory,” theoretical physicist Mark Van Raamsdonk says. But AdS (anti-de Sitter space )/CFT (conformal field theory) held up to scrutiny, and soon theorists were using it to answer some confounding questions.

The connection between entanglement entropy in the CFT and the geometry of spacetime in the AdS led to another important result—the notion that spacetime on the AdS side emerges from quantum entanglement on the CFT side, not just in black holes but throughout the universe. The idea is best understood by analogy. Think of a very dilute gas of water molecules. Physicists can’t describe this system using the equations of hydrodynamics because the dilute gas does not behave like a liquid. But suppose the water molecules condense into a pool of liquid water. Now those very same molecules are subject to the laws of hydrodynamics. “You could ask, originally, where was that hydrodynamics? It just wasn’t relevant,” Van Raamsdonk says. 

Something similar happens in AdS/CFT. On the CFT side, you can start with quantum subsystems—smaller subsets of the overall system you’re describing—each with fields and particles, without any entanglement. In the equivalent AdS description, you’d have a system with no spacetime. Without spacetime, Einstein’s general relativity isn’t relevant .... But when the entanglement on the CFT side starts increasing, the entanglement entropy of the quantum subsystems begins to correspond to patches of spacetime that emerge in the AdS description. These patches are physically disconnected from each other. Going from patch A to patch B isn’t possible without leaving both A and B; however, each individual patch can be described using general relativity.

Now, increase the entanglement of the quantum subsystems in the CFT even more, and something intriguing happens in the AdS: the patches of spacetime begin connecting. Eventually you end up with a contiguous volume of spacetime. “When you have the right pattern of entanglement, you start to get a spacetime on the other side,” Van Raamsdonk says. “It’s almost like the spacetime is a geometrical representation of the entanglement. Take away all the entanglement, and then you just eliminate the spacetime.” Engelhardt agrees: “Entanglement between quantum systems is important for the existence and emergence of spacetime.” The duality suggested that the spacetime of our physical universe might simply be an emergent property of some underlying, entangled part of nature.

Van Raamsdonk credits the AdS/CFT correspondence for making physicists question the very nature of spacetime. If spacetime emerges from the degree and nature of entanglement in a lower-dimensional quantum system, it means that the quantum system is more “real” than the spacetime we live in, in much the same way that a 2-D postcard is more real than the 3-D hologram it creates. “That [space itself and the geometry of space] should have something to do with quantum mechanics is just really shocking,” he says.