Our eyes see depth because light from different distances arrives slightly out of phase. Flat photos that only store light intensity don’t show depth but holograms show depth because they store the phase differences that encode it, so holograms can show 3D images. A hologram is made by splitting laser light and letting the half that shines on the object interfere with a matched reference half to give an interference pattern (Figure 3.24). Light later shone on that flat pattern recreates the original 3D image as a holograph.
The holographic principle is that everything we know about the universe is essentially a hologram, or more precisely:
Everything physically knowable about a volume of space can be encoded on a surface surrounding it (Bekenstein, 2003).
This principle, which is widely accepted in physics, is that all the information we receive about the world can be encoded on a flat surface just like a hologram. The information in a space seems to depend on its volume but if one were to pack smaller and smaller memory chips into a space to get more information in it, the end result would be a black hole whose entropy depends on its surface area not its volume. Since black holes have more entropy than anything else for the same volume it follows that the information about any physical object can be encoded on a two-dimensional surface. The holographic principle is maintained by the behavior of black holes (Bekenstein, 2003).
Quantum realism interprets the holographic principle as follows. A virtual world must be observed from some angle so the act of observing uses up one of the three dimensions of space. If an observation is an information transfer, as proposed here, that leaves only two dimensions to carry out the transfer. So the physical world registered at a point can always be painted on the surface of a sphere around it because that is the structure that delivers it. Quantum realism thus requires the holographic principle and equally that it applies to our world supports quantum realism.
So is our 3D universe really two-dimensional? The holographic principle is a consequence of how physical reality presents not how it operates. A 3D world that presents as an image must be delivered across two dimensions but space still has three degrees of freedom. The holographic principle implies that physical reality is virtual not that it is two dimensional. Equally to say that the physical world is “like a hologram” is misleading, as this is no Star Trek hologram we can enter and leave at will because our bodies are its images. If this “hologram” was ever switched off, the continuity of physical reality would immediately stop and the only way to recover it would be to start it all again from scratch.