# Holographic principle and holograms

Holographic principle or Maldacena's duality is a theory that says that the volume of space can be described by just looking the information encoded on a boundary to the region of that space.

Holography uses light to recreate the three dimensional objects by scattering it off their surfaces (therefore using the information of the boundary of the object to recreate it in a sense).

Recently I read that a group of Japanese scientists gave numerical calculations that support the idea of the holographic principle (you need to google: 'simulations back up theory that universe is a hologram', nature doesn't allow linking without paying).

Since then all I've seen in the news is this (paraphrasing):

"We all live in a hologram" ; "We are not real" ; etc.

You get the picture.

Now, I may not have really big knowledge of these topics, but isn't this kinda wrong? I mean, yes, theories of duality tells us that we can extract a lot of information of the space time purely from looking at the boundary of it (especially if the space time is asymptotically flat). But other than that, there doesn't seem to be much contact points between these two terms.

Or am I wrong?

I was under the impression that the holographic principle is just one of the theories that could be used to understand the quantum aspect of gravity. So how would that mean that we live in a hologram or that we are holograms?

A lot of people are also claiming this, and this kinda grinds my gears since they have no background in physics (quite in fact they have no connection to physics and don't understand the mathematics that stand underneath it all), and are seemingly making these pompous claims.

How would you explain this?

• I don't think the "we are not real" part needs much physics to deal with. If you have a principle that tells you that X and Y are equivalent, it makes no sense to treat either of them as more or less "real" than the other. – Stan Liou Dec 14 '13 at 9:04
• As I understand it, the computation by the Japanese group confirms (but does not prove) the AdS/CFT duality for the special case of $AdS_2 \times S^8$. It doesn't have any direct bearing on whether $AdS_5 \times S^5$ is a good description of our universe, though it is evidence that it could be. – John Rennie Dec 14 '13 at 9:18
• What interests me is: is it ok to say that we are living in a hologram (Susskind likes to say so if I recall correctly)? In my opinion these two are very different things (holographic principle and holograms). But people think they are somehow connected. Or am I wrong and they really are connected? – dingo_d Dec 14 '13 at 9:27
• The word hologram originally meant the diffraction grating and/or the image formed by shining a laser through it. However is has come to mean any $n$ dimensional system that is dual to an $n$+1 dimensional system. It is in this sense that Susskind says we are living in a hologram since he believes our 4D flat space is dual to a 5D AdS space. – John Rennie Dec 14 '13 at 14:46
• Ok, so technically I'm not wrong XD – dingo_d Dec 14 '13 at 15:14