Puteulanus luna said:
It sounds more impressive than it is. When people hear "the Universe might be a hologram", they think of the hologram of the type that Arnold Rimmer or Star Trek have - a manufactured state of existence. Unfortunately the holographic principle is much less exciting than that (to the layman) and instead is the idea that all information contained within the volume of an object can be read on the surface of that object.
Think of a bath of water. Now if we wanted to know the exact temperature of that bath then we'd have to track the energy and position of every single molecule within it then use different formulas to see how they all add up. This is a colossal waste of time; instead we can use a fairly simple mechanism that we call temperature that will get the average temperature of the water in the bath.
But here's the point to it - if we look at the average temperature of the bath as a whole then how much information about the bath in all of its specific peaks and troughs of heat on a molecular level have we lost?
When we determine a high level of information that is being hidden such as in the bath, we call this a high entropy system.
So now think of a small sink of water, a small bath of water and a swimming pool of water. You'd probably suggest that the small sink has the lowest entropy and the swimming pool has the highest entropy because there's a large amount of space for the hidden information to hide in. This is only logical, more "stuff" creates more "information" right?
Now think of a black hole - a black hole carries maximum entropy (hidden information) because it is this massive, dense thing that swallows a bunch of information. But it still has a volume just like the swimming pool which is where the information "is" so to speak. The holographic principle is the somewhat surprising result that all of the entropy in a black hole can be measured just by looking at the surface of it rather than having to get inside the thing and get your calculator out. It can be measured because the entropy is being reflected of "projected" onto the black hole's surface.
There's no reason for this to be limited to black holes alone so the principle is the same across the Universe - that bathtub's entropy can potentially be measured just by looking at the surface area rather than the full volume of it.
But here's the very important bit - debateably, this isn't actually a law of physics. The black hole and the bathtub aren't actually projecting anything at all in reality. It's just that we as evolved apes on a non-descript rock have brains that can only build the world around them in 3 dimensions so we see it as a 3 dimensional object as we are incapable of viewing it in any other context. The Universe doesn't care about how many dimensions we can see, it is just what it is and there are many dimensions that we just cannot see or fathom because of the way our biology works. There's nothing special about 3 dimensional existence - the holographic principle is the name given to the evidence backing the idea that just because we see things in 3 dimensions doesn't mean that the Universe is in 3 dimensions.
Carl Sagan can explain it better.
[video]https://www.youtube.com/watch?v=N0WjV6MmCyM[/video]