# Holographic principle and string theory

#### masbt

Hi. I am no physicist but I am trying to know something about the holographic principle.
As I understand, everything is happening in a surface and our three-dimensionality is an illusion of our senses.
On the other side, string theory assume the existence of 11 dimensions.
Are the two theories related in some way or are completely independent?

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#### atyy

Superstring theory requires 10 spacetime dimensions (not 11).

In the most famous example of the holographic conjecture, although it is commonly said that the boundary is 4 dimensional and the bulk is 5 dimensional, the bulk is a 10D string theory, in which the 5 dimensions that are less frequently mentioned are a 5 dimensional sphere. When people want to stress that the bulk is a 10D string theory, they write the boundary as CFT4, and the bulk as AdS5 X S5.

#### masbt

Thanks for your answer but i don't get it. Too technical for me.

#### atyy

Ok let's try a different explanation.

Our everyday world has 3 dimensions of space and 1 dimension of time, so it is 4 dimensional. According to string theory, our 4 dimensional world is really 10 dimensional, with 6 hidden dimensions.

According to the holographic conjecture, this 10 dimensional world of 4 everyday dimensions and 6 hidden dimensions is represented by a holographic theory in 3 dimensions. So if just look at the large dimensions, we can say the 4 everyday dimensions are represented by a holographic theory in 3 dimensions.

In other words, the holographic principle about our 4 dimensions being represented by a 3 dimensional theory is consistent with the idea that our universe is 10 dimensional.

Of course, one has to bear in mind this is all conjectural. The holographic conjecture can produce a universe whose gravity is locally similarly to ours, which is why it is interesting, but so far no one knows how to make the holographic principle produce a universe that matches ours in detail.

#### Doug Huffman

Gold Member
Leonard Susskind explains it rather rigorously in his Stanford U. lecture series streamed and his Theoretical Minimum.

#### masbt

Thanks again. I only wanted to know if the two theories are in some way compatible, and for what you say, this seems to be the case.
Thanks Doug for this info, I am going to check. I read The Black Hole War by Leonard Susskind some time ago and even though I didn't understand too much, I learned some neat and cool things like the event horizon growing 1 planck square every time a bit of information is added.

#### haushofer

I would recommended the books by Brian Greene. Very nice for laymen (and experts :P ) :)

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