The scientific media has been buzzing this week about the possibility that our Universe is holographic. All across the internet I have seen headlines claiming that we live in only two dimensions, and that everything we see and sense is an illusion. And while both the media coverage is definitely overhyping what should be a minor scientific paper, the entire topic of a holographic universe is an interesting subject that is worth exploring.

Let me first make it clear that I have read the original academic paper that spawned this coverage, and what the authors are claiming is far from the definitive proof that is being reported. At best they have shown that existing data from astronomy and cosmology experiments does not disprove the existence of a holographic Universe, but it certainly does not provide strong evidence for its existence either.

So exactly what is a holographic Universe?

This idea has its origins in theoretical black hole research from the 1970s. Bekenstein and Hawking independently studied the properties of black holes when the laws of quantum mechanics were included. Black holes arise in the study of general relativity as objects which are so dense that nothing can escape from them - not even light. However this raises interesting questions about what happens to information that falls into the black hole. Quantum mechanics claims that information can never be created or destroyed, and general relativity claims that no information can escape from a black hole, and furthermore that no measurement made outside of a black hole can reveal any information about its contents aside from its total mass, angular momentum, and electrical charge. 

As it turns out, when quantum mechanics is applied to a classical black hole we find that the black hole itself has thermodynamic properties. (We must use classical black holes, because at this time no one has been able to construct a consistent theory of quantum gravity that would be necessary to study the properties of quantum black holes.) A black hole will emit thermal radiation as if its surface has a non-zero temperature, and the entropy of a black hole will grow as its size increases. In effect,the laws of quantum mechanics suggest that any information that falls into the black hole will in some sense be stored on the surface of the black hole - although exactly how this happens in nature is still an mystery.

However this raises another very interesting question. Suppose that you store information on some form of medium, such as paper books, or computer memory chips, or USB flash drives, and suppose you keep all of these objects in a box that is one meter by one meter by one meter in size. Once the box is filled, you need to obtain a second box, and then a third, and so forth as each gets filled with information. 

Then in a cube that is two meter on each side, you can store eight times as much information. If the cube is three meters on each size, you can store nine times as much information. The amount of information that can be stored depends on the third power of its scale, which is what is expected when information scales as volume. 

But now consider what happens when these boxes of information are compressed to form a black hole. As I mentioned above, in a black hole the amount of information that is stored inside the black hole depends on the surface area of the black hole. And assuming that the density of the medium storing the information is constant, that means that as our boxes of information are dropped into the black hole, the information going in will increase with the volume going in and yet the information inside the black hole will increase with area.  Information cannot be destroyed, so where is it going?

One answer is that it didn't exist in the first place. Perhaps through some law of nature that we do not yet understand, those original boxes of information contained a lot of duplicate information. Or perhaps there exists some limit at which we cannot put more information into those boxes. Maybe we cannot store information at the same density as we currently believe. We just do not know.

And that is where the concept of a holographic Universe comes into the argument. As we keep dumping our boxes of information into the black hole, that information is taken out of the Universe. If we keep growing this black hole by dumping things into it, then eventually the entire Universe will be inside our black hole. But that means that all of the information stored in our Universe - all the planets, the stars, the galaxies, and every memory of every life form - can be written on a two dimensional sphere that surrounds our Universe. The vast majority of information in our Universe does not exist. 

In essence, all of our Universe is stored on a cosmic horizon that surrounds everything. It is the difference between seeing the real world that we live in, and viewing a film of the world. We exist, and we do live in three dimensional space, but in some sense we are just a holographic projection.

That is the holographic Universe. At this point we do not know whether this theory is true or not, but this recent article shows us that this seemingly bizarre idea cannot be dismissed so easily with experimental data. All we can do is continue to do more experiments, and continue to wonder at the fundamental nature of space itself.