In Plato’s “The Allegory of the Cave,” prisoners are chained inside a cave, able to look at only shadows cast on the wall by puppeteers standing in front of a fire behind them. For these prisoners, the shadows—not the real objects that cast those shadows—represent their entire reality.
It’s only when prisoners are released from the cave that they understand the true nature of reality—that what they called a “book” or a “man” were really just shadows of the real objects projected onto a wall.
Plato’s point is that even for us, outside the cave, we may not perceive reality accurately. For example, we may confuse the name or image of an object in our mind with the actual object itself.
Physicists, however, are starting to see that “shadows” like those experienced by Plato’s prisoners may be a much more accurate representation of reality than the philosopher realized, and could help tie together previously unconnected physics concepts like spacetime, gravity and quantum entanglement.
This realization stems from The Holographic Principle, a concept proposed in the 1990s by a group of theoretical physicists that included Gerardus ’t Hooft and Leonard Susskind. Their assertion was that all of the information contained within a region of space can be represented on the boundary of that region. So the contents of a three-dimensional space would be described on the two-dimensional boundary that forms the border of that space.
If this is difficult to imagine, picture a room whose walls are covered in mirrors. The complete information about the three-dimensional objects within the room is present in their two-dimensional reflections—“holograms” or “shadows”—in the mirrors. That means you can actually reconstruct those objects based upon their reflections.
The Holographic Principle also holds true for a five-dimensional spacetime, with its information represented in four dimensions on its own boundary. Seen this way, our world is a four-dimensional “shadow” on the wall of that five-dimensional “cave.” If you move away from the wall where our reality exists, you enter a fifth dimension.