MARK VAN RAAMSDONK: It’s truly amazing, so.
BRIAN GREENE: So, I think we’ll sort of step through that now, if that works for you. So we have a little, you can sort of walk us through what we’re having here.
MARK VAN RAAMSDONK: So we’re looking at some kind of universe. There’s a black hole in this universe. And then what’s on the outside is this hologram; this is the actual mathematical description in our modern way of understanding.
So this red around the outside has all of the information that is telling us what kind of geometry is in there –
BRIAN GREENE: So that’s Gerard’s hologram.
MARK VAN RAAMSDONK: The information. That’s Gerard’s hologram. On the outside, you’ve got that hologram in a particular kind of physical configuration. And that’s coding for the fact that there’s this black hole, and maybe some stars in there in the spacetime.
BRIAN GREENE: Yeah. And then if we go on and go to the second black hole in the story. OK, so we show–
MARK VAN RAAMSDONK: All right, now we’ve got two separate black holes. And basically that’s going to be encoded by some other information. So you change up the information and now you’ve got two black holes.
BRIAN GREENE: Yup, and then if we add to the story a certain kind of entanglement, say, so…
MARK VAN RAAMSDONK: So here what we did was we turned that situation into one where you have a wormhole connecting behind the two black holes. And the remarkable thing is, in order to do that in the holographic set, in the holographic description, in the outside description, what we actually, you know, we have to do something fundamentally quantum mechanical. What we had to do is actually add in a whole bunch of entanglement between different parts of the hologram. And that was what achieved getting this, this wormhole.
BRIAN GREENE: So, just to summarize, because this is a deep and utterly stunning idea, you’re saying that entanglement in the holographic description, the red description, is, in the interior description, nothing but a wormhole connecting two black holes.
MARK VAN RAAMSDONK: That’s right, which is, sort of a classical thing that would have been covered by Einstein’s kind of classical understanding of gravity. It’s just a geometrical connection saying you could get from here to here, and that property is entirely, according to this – or according to our current understanding, due to quantum entanglement between different parts of the hologram.
BRIAN GREENE: And, moreover, if you find that you can actually generalize this, that it actually even holds without a black hole in space. So take us from here.
MARK VAN RAAMSDONK: Yeah, so I – this was I guess 2009, I was thinking about that. It seemed crazy, and then one of the things that you realize if you start reading about entanglement and about just our description in these theories of just empty space, is that even when you’re describing empty space, you still have entanglement in the hologram.
In the holographic description, there’s lots of entanglement. And then you sort of ask yourself, well wait, if that entanglement in the previous story was creating a connection between the two black holes, could all of this entanglement there, in this picture–could that have something to do with the fact that the space is sort of connected up into one nice smooth, empty universe?
BRIAN GREENE: That space has threads. In some sense, we call it the fabric of space, is it’s somehow threaded in some manner.
MARK VAN RAAMSDONK: Could that be related to this entanglement?
BRIAN GREENE: And then you were able to mathematically study that by mathematically cutting the entanglement lines on the outside.
MARK VAN RAAMSDONK: Right. So it’s what we call a thought experiment. You just sort of take your math – your description of this and you say, well what happens if I cut those threads of entanglement. What happens if–
BRIAN GREENE: So if we cut some of them—
MARK VAN RAAMSDONK: – I take the left half of the hologram and the right half of the hologram, and I remove the entanglement between those two sides? There’s an effect. You remove entanglement in the hologram, and then the spacetime starts splitting up, and it, you know, you could actually imagine even more than this.
So you’ve got a ball of clay, and you’re pulling it apart, and it’s getting further and further apart, and the middle is pinching off, and so you could keep doing that. You say, well what would happen if I took away even more entanglement, and took away even more entanglement, and then in this model, you know, now you’ve got your space and it’s split into four pieces.
And I still got a little bit of entanglement, but I’m going to take that away, and what happens in this description is that the big nice empty universe that you thought you were describing just splits up into millions of tiny bits. And once you’ve got no more entanglement there in this description, you’ve got no more spacetime at all. And so you get to, you know, if this is all right, you get to this incredibly dramatic conclusion that maybe you’ve just understood what space actually is, and it’s actually fundamentally quantum mechanical that space is somehow a manifestation of quantum entanglement in the underlying hologram system.