# What's between the branes?

So I'm doing my best to explain M Theory to some friends... Mind you, I'm no expert... When we finally got past the obstacle of more than three dimensions... which can be difficult for folks who aren't turned on by this stuff to understand... we talked about strings, with many puzzled faces to be sure... it's difficult to get folks past the idea that everything may not be made of little balls of stuff... then puzzled looks seemed to clear when I advanced to branes, as it seemed easier for folks to comprehend reality woven into a 2D sheet, rather than a bunch of vibrating strings in 11D space, go figure... then I forwarded the "traditional" illustration of many branes in a stack, billowing and occasionally colliding, and what current thinking says are the ramifications of those collisions... and up came a question for which I had no information, so I'm bringing it here...

If all of "our" 11 dimensions, (as well as "our" physics) are woven into our brane, and all of the dimensions and physics of the next brane are woven into it, and so forth... What's in-between the branes? It ain't space, it ain't time... So what do we call the stuff, or lack thereof in this case, that separates the branes in the first place?

• The space between (or outside) the branes is called the bulk or the ambient space. Commented Oct 30, 2015 at 8:42
• I had a feeling somebody had formally called it something... So if it's not part of anybody's brane, spacetime, or universe, what is it? Does it have properties?
– Hep
Commented Oct 30, 2015 at 8:59

I would guess that you're thinking of the brane world ideas. If so, the branes you're describing are not eleven dimensional. They are surfaces of dimension lower than eleven embedded in the eleven dimensional space. For example in this context example our universe would be a four dimensional brane.

• I see, so our brane is woven from the standard four dimensions, then how does the string exist within that brane with all 11?
– Hep
Commented Oct 30, 2015 at 8:12
• @Hep: the ends of the string are in our brane, or the ends of the string can be in two different branes, but the length of the string between the ends extends out into the 11D spacetime. Commented Oct 30, 2015 at 8:19
• What of the closed strings in our brane? Do those only exist in 4 dimensions? My understanding is that strings, closed or open, crossing branes or not, exist in 11D in our brane. Can you clarify?
– Hep
Commented Oct 30, 2015 at 8:39
• Closed strings propagate in all eleven dimensions. They are not confined to any brane, though they may intersect a brane. Open strings are not confined to a brane either, but their ends lie on a brane. Commented Oct 30, 2015 at 9:08
• So you're saying there are no strings, closed or open, which lay completely within our, or any other brane?
– Hep
Commented Oct 30, 2015 at 9:13